Human Genetic Variation Influences Vitamin C Homeostasis by Altering Vitamin C Transport and...

NU33CH03-Michels ARI 13 June 2013 18:21

Human Genetic VariationInfluences Vitamin CHomeostasis by AlteringVitamin C Transport andAntioxidant Enzyme FunctionAlexander J. Michels,1 Tory M. Hagen,1,2

and Balz Frei1,2

1Linus Pauling Institute, 2Department of Biochemistry and Biophysics, Oregon StateUniversity, Corvallis, Oregon 97331; email: [email protected],[email protected], [email protected]

Annu. Rev. Nutr. 2013. 33:45–70

First published online as a Review in Advance onApril 29, 2013

The Annual Review of Nutrition is online athttp://nutr.annualreviews.org

This article’s doi:10.1146/annurev-nutr-071812-161246

Copyright c© 2013 by Annual Reviews.All rights reserved

Keywords

ascorbate, SVCT, haptoglobin, GST, polymorphism, SNP

Abstract

New evidence for the regulation of vitamin C homeostasis has emergedfrom several studies of human genetic variation. Polymorphisms in thegenes encoding sodium-dependent vitamin C transport proteins arestrongly associated with plasma ascorbate levels and likely impact tissuecellular vitamin C status. Furthermore, genetic variants of proteins thatsuppress oxidative stress or detoxify oxidatively damaged biomolecules,i.e., haptoglobin, glutathione-S-transferases, and possibly manganesesuperoxide dismutase, affect ascorbate levels in the human body. Therealso is limited evidence for a role of glucose transport proteins. In thisreview, we examine the extent of the variation in these genes, theirimpact on vitamin C status, and their potential role in altering chronicdisease risk. We conclude that future epidemiological studies shouldtake into account genetic variation in order to successfully determinethe role of vitamin C nutriture or supplementation in human vitaminC status and chronic disease risk.

45

Ann

u. R

ev. N

utr.

201

3.33

:45-

70. D

ownl

oade

d fr

om w

ww

.ann

ualr

evie

ws.

org

by G

eorg

etow

n U

nive

rsity

on

08/1

1/13

. For

per

sona

l use

onl

y.


Contents

INTRODUCTION . . . . . . . . . . . . . . . . . . 46SODIUM-DEPENDENT

VITAMIN C TRANSPORTERS . . 47SVCT1 (SLC23A1) . . . . . . . . . . . . . . . . 49Genomic Characterization

of SLC23A1 . . . . . . . . . . . . . . . . . . . . 49Genetic Variation in SLC23A1

and Vitamin C Status . . . . . . . . . . . 52Genetic Variation in SLC23A1

and Chronic Disease Risk . . . . . . . 53SVCT2 (SLC23A2) . . . . . . . . . . . . . . . . 54Genomic Characterization

of SLC23A2 . . . . . . . . . . . . . . . . . . . . 54Genetic Variation in SLC23A2

and Vitamin C Status . . . . . . . . . . . 55Genetic Variation in SLC23A2

and Chronic Disease Risk . . . . . . . 56HAPTOGLOBIN . . . . . . . . . . . . . . . . . . . . 57

Function of Haptoglobin . . . . . . . . . . . 57Characterization of Human

Haptoglobin Genotypes . . . . . . . . . 57Haptoglobin Genetic Variation

and Vitamin C Status . . . . . . . . . . . 58GLUTATHIONE

S-TRANSFERASES . . . . . . . . . . . . . . . 59Functions of Glutathione

S-Transferase Isoforms. . . . . . . . . . 59Human GST Polymorphisms

and Vitamin C Status . . . . . . . . . . . 60OTHER GENETIC

INTERACTIONSWITH VITAMIN C . . . . . . . . . . . . . . 61Manganese Superoxide Dismutase

Polymorphisms and Vitamin CStatus . . . . . . . . . . . . . . . . . . . . . . . . . . 61

Glucose Transporters andVitamin C Transport . . . . . . . . . . . 61

CONSEQUENCES OF GENETICVARIATION . . . . . . . . . . . . . . . . . . . . . 62

SUMMARY . . . . . . . . . . . . . . . . . . . . . . . . . . 63FUTURE DIRECTIONS . . . . . . . . . . . . 64

INTRODUCTION

Maintaining adequate vitamin C (ascorbicacid) levels in the body is essential for collagen,catecholamine, and carnitine biosynthesis; ab-sorption of nonheme iron; and antioxidant pro-tection and hence plays a critical role in normalfunctioning of the body and optimum health(62). Numerous factors, both endogenous andexogenous, contribute to vitamin C body status,including vitamin C transporters that regulatethe vitamin’s bioavailability and plasma andtissue concentrations; enzymatic reactions inwhich vitamin C is used up by reducing redox-active metal cofactors; and a host of environ-mental factors and endogenous stresses, suchas oxidative stress, infection, and inflammation.

Thus, over the past decades, several pro-teins have been identified as critical regulatorsof vitamin C homeostasis. These proteins affectascorbate levels by different mechanisms, suchas vitamin C transport, transition metal regula-tion, enzymatic redox activity, and antioxidantprotection (Figure 1). However, much of thiswork has been performed in animal models andcell culture, the results of which often are diffi-cult to translate to humans or are confoundedby artifacts in study design. Controlled studiesof vitamin C status in humans are difficult toperform, challenged by design, and limited ininterpretation (56).

Despite these difficulties, new evidence forthe regulation of vitamin C status has emergedfrom studies of human genetic variations. Al-though a relatively new and burgeoning field,there is strong evidence that several geneticalterations, in the form of single-nucleotidepolymorphisms (SNPs), gene duplications, orgene deletions, affect ascorbate levels in the hu-man body. The phenotypes of these genomicchanges are supporting previous evidence of thephysiological functions of vitamin C and its rolein human health.

Although the list will undoubtedly growwith future research, this review focuses onvariations within the genes for which thereis direct evidence of alterations in vitamin Cplasma levels. These genes can be subdivided

46 Michels · Hagen · Frei

Ann

u. R

ev. N

utr.

201

3.33

:45-

70. D

ownl

oade

d fr

om w

ww

.ann

ualr

evie

ws.

org

by G

eorg

etow

n U

nive

rsity

on

08/1

1/13

. For

per

sona

l use

onl

y.


Vitamin C: includesascorbic acid and itsone- and two-electronoxidation products,ascorbyl radical anddehydroascorbic acid,respectively

Single-nucleotidepolymorphisms(SNPs): changes ingenetic structurebetween individualsdiffering by a singlenucleotide

Glutathione: acellular tripeptide witha reactive sulfhydrylgroup; it is a cellularantioxidant capable ofreducingdehydroascorbic acidto ascorbic acid

into two categories based on their interactionswith ascorbic acid. First, genes involved in thedirect transport and regulation of vitamin Cconcentrations in tissues and extracellularfluids are discussed. This discussion fo-cuses on the known polymorphisms in thesodium-dependent vitamin C transporters(SVCTs) of the SLC23 family (SLC23A1 andSLC23A2). The second category of genes isrelated to the antioxidant and redox activitiesof vitamin C and involves genetic variantsaffecting iron homeostasis and oxidative stress.Hence, we examine iron dysregulation throughhaptoglobin (HP) gene polymorphisms andgenetic variants impacting the activity of glu-tathione S-transferases (GSTs). In addition, ascandidates for future studies, we present somelimited data on the influence of genetic changesin manganese superoxide dismutase (SOD2)and glucose transport proteins [solute carrierfamily 2 (SLC2)] on ascorbate levels in humans.

SODIUM-DEPENDENTVITAMIN C TRANSPORTERS

The two sodium-dependent vitamin C trans-port proteins, SVCT1 and SVCT2, are specificfor the cotransport of sodium ions and ascorbicacid across cell membranes. Although sodium-dependent vitamin C transport in cells had beencharacterized decades before (35), the genesencoding these members of the solute carrierfamily 23 (SLC23A1 and SLC23A2) were notidentified until 1999 (86). Initially character-ized in rodents, human analogs were isolatedand mapped shortly afterward (26, 73, 91).

Transcriptional regulation of the SLC23genes controls the tissue distribution of SVCTsand is responsible for the maintenance ofvitamin C levels in nearly all cells, tissues, andextracellular fluids (Figure 2). The only cellswithout functional SVCT proteins are erythro-cytes, which selectively lose SVCT2 expres-sion during erythropoiesis (59). Consequently,ascorbate levels in erythrocytes are low, gener-ally in equilibrium with the surrounding bloodplasma.

Dependent on vitamin Ctransporter activity(SVCTs and GLUTs)

Influenced by antioxidantenzymes (MnSOD, GSTs,and haptoglobin)

CirculationDiet

Urine

Tissues

Glomerularfiltration

Nutrient interactions

Renalreabsorption

Antioxidant/enzyme functions(redox reactions)

Efflux

Absorption

Cellularuptake

Figure 1Vitamin C homeostasis. Vitamin C levels in the circulation and tissues areinfluenced by several regulatory mechanisms in the body. Proteins involved invitamin C homeostasis fall under the general-categories transporters (blue) thattransport vitamin C across cell membranes and those that modulate oxidativestress ( purple) and would interact with ascorbate due to its antioxidantproperties. Abbreviations: GLUT, glucose transporter protein; GST,glutathione S-transferase; MnSOD, manganese superoxide dismutase;SVCT, sodium-dependent vitamin C transporter.

Due to the direct interaction of SVCTswith ascorbic acid and their roles in absorptionand tissue accumulation of vitamin C, geneticalterations in SLC23A1 and A2 appear to havethe greatest effect on human vitamin C status ofall the genetic factors investigated to date. EachSLC23A1 and SLC23A2 is thought to haveemerged via gene duplication before vertebrateevolution and expansion (26). The divergentevolution of the two genes has led to distinctdifferences in the expression, regulation, andfunction of SVCT1 and SVCT2, and thereforethe two genes encoding these proteins areconsidered separately. It should be noted thatgenetic variations in both of these genes is thelatest area of research in this field, with theseand other polymorphisms having the greatestpotential for phenotypic effects that will impactvitamin C homeostasis and human health.

www.annualreviews.org • Vitamin C and Genetic Variation 47

Ann

u. R

ev. N

utr.

201

3.33

:45-

70. D

ownl

oade

d fr

om w

ww

.ann

ualr

evie

ws.

org

by G

eorg

etow

n U

nive

rsity

on

08/1

1/13

. For

per

sona

l use

onl

y.


Airway surface fluid(40–140 μM)

Respiratory epithelium

Brain and eye

Leukocytes(1–5 mM)

Erythrocytes(30–60 μM)

Platelets(1–4 mM)

Plasma(30–80 μM)

Endothelialcells

Bloodvessel

Nephron

Pituitary gland(40–50 mg/100 g)

Cerebrospinal fluid(50–220 μM)

Eye

Brain

Choroid plexus transportacross blood-brain barrier

Aqueous humor(0.5–2.4 mM)

Eye lens(25–30 mg/100 g)

Corneal epithelium(50–130 mg/100 g)

Tissues expressing

SVCT2

Tissues expressing SVCT1 and

SVCT2

Skin (1–3 mg/100 g)

Lung (7–10 mg/100 g)

Liver and pancreas (10–15 mg/100 g)

Kidney (5–15 mg/100 g)

Small intestine (10–30 mg/100 g)*

Reproductive organs (2–12 mg/100 g)

Proximal tubule reabsorption

Urinary excretion

Renal threshold(55–70 μM)

Glomerularfiltration of

blood

Brain (10–50 mg/100 g)

Eye (18–30 mg/100 g)

Heart (5–15 mg/100 g)

Adrenals (30–55 mg/100 g)

Skeletal muscle (3–4 mg/100 g)


Ann

u. R

ev. N

utr.

201

3.33

:45-

70. D

ownl

oade

d fr

om w

ww

.ann

ualr

evie

ws.

org

by G

eorg

etow

n U

nive

rsity

on

08/1

1/13

. For

per

sona

l use

onl

y.


Superoxide: reducedform of molecularoxygen (O2

·−); a freeradical and reactiveoxygen species

Solute carrier family2 (SLC2): includesSLC2A1–SLC2A14,encoding glucosetransportersGLUT1–GLUT14,respectively

Solute carrier family23 (SLC23): includesboth SLC23A1(SVCT1) andSLC23A2 (SVCT2). Agene identified asSLC23A3 has anundefined function

Apical: in renalphysiology, refers tothe side of the cellfacing the lumen(interior) of theproximal tubule

Hepatocyte nuclearfactor 1α (HNF1α):encoded by the geneHNF1A, it is atranscription factorhighly expressed in theliver

SVCT1 (SLC23A1)

SVCT1, encoded by the gene SLC23A1, ispredominantly responsible for high-capacityvitamin C transport across membranes. Foundon the apical side of polarized epithelial cells(8, 58), SVCT1 is highly expressed in the liver,kidney, lung, small intestine, and pancreas (86,91). Of these tissues, it is the expression ofSVCT1 in the kidney proximal tubule that isconsidered most important for the regulationof whole-body vitamin C status, as it is in-volved in the renal reabsorption of vitamin Cfollowing glomerular filtration of blood plasma(Figures 2 and 3a). Mice carrying a geneticknockout of SLC23A1 excrete large amounts ofascorbic acid in the urine and have low plasmaand tissue ascorbate levels (17). Because wecannot synthesize ascorbic acid in our body,loss of SVCT1 expression may cause a far moresevere vitamin C deficiency in humans than inascorbic acid–synthesizing animals. However,there are currently no reports of spontaneousloss of SVCT1 in humans.

Outside of an absence in renal reabsorp-tion, it is unclear what other impact the lossof SVCT1 expression may have in humans.SLC23A1 knockout mice experience increasedperinatal mortality, but with no apparent phys-iological or cell-specific phenotype (17). Un-like SVCT2, SVCT1 appears to be expressedin specific locations within tissues. For in-stance, SVCT1 expression in the kidney is lim-ited to the proximal tubule of the nephron(Figure 3a). It is possible that the loss ofSVCT1 would only impact ascorbate levels inspecific cellular environments or specific celltypes within tissues. Although SVCT1 expres-sion has been noted in the skin, bile duct,lung, pancreas, and reproductive organs (29,

45, 57, 91), it is not clear what consequencesa loss of SVCT1 would have in these tis-sues. Another possibility is that the apparentlack of a phenotype in SLC23A1-null animalsmay be obscured, as all cell types that ex-press SVCT1 also express SVCT2 (Figure 2).SVCT2 or glucose transporters (see below)may compensate for the lack of SVCT1, albeitwith a limited capacity. For example, althoughSVCT1 is expressed in the intestine, SLC23A1genetic knockout mice are still capable of ab-sorbing ascorbic acid from the diet (17).

Genomic Characterizationof SLC23A1

SLC23A1 consists of 16 kb found on humanchromosome 5 (26). The gene has 15 exons thatencode for a 598 amino acid protein. A non-functional splice variant with four extra aminoacids has been described in intestinal cells buthas not been well characterized (91). The lastintron and exon of SLC23A1 overlap with thepromoter and first exon and intron of the genefor polyadenylate-binding protein-interactingprotein 2 (PAIP2), which encodes a transcrip-tional repressor protein (26). No genetic inter-action between PAIP2 and SLC23A1 has beenreported.

The promoter region of SLC23A1 is small,with critical regulatory elements defined onlywithin a few hundred base pairs of the tran-scription start site. Two independent studieshave found functional binding sites for hep-atocyte nuclear factor 1 (HNF1) in this re-gion, and regulation of SVCT1 by HNF1α hasbeen demonstrated in HepG2 cells (63, 75).This would support regulation of the proteinby HNF4α as well. Although binding of othertranscription factors has been postulated, none

←−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−Figure 2Tissue distribution of vitamin C and its transport proteins in the human body. Ascorbate concentrations are from limited human dataand are represented as mg/100 g wet weight for tissues and molarity (mM or μM) for intracellular and extracellular fluids. Asteriskindicates that the tissue concentrations of ascorbate in the small intestine are heavily dependent on recent dietary intake.Sodium-dependent vitamin C transporter (SVCT) distribution is based on mRNA expression data in animals and human tissues.Redrawn with permission from Saunders/Elsevier, from A Michels and B Frei, Vitamin C, in Biochemical, Physiological, and MolecularAspects of Human Nutrition, third edition, ed. MH Stipanuk, MA Caudill, pp. 626–54. Copyright 2012 (62).


Ann

u. R

ev. N

utr.

201

3.33

:45-

70. D

ownl

oade

d fr

om w

ww

.ann

ualr

evie

ws.

org

by G

eorg

etow

n U

nive

rsity

on

08/1

1/13

. For

per

sona

l use

onl

y.


aSLC2

3A1

SA

GE

(ta

gs

pe

r m

illi

on

)

Nephron segments

80

70

60

50

40

30

20

10

0

Nephron segment

Glomerulus

Proximal convoluted tubule

Proximal straight tubule

Medullary thick ascendinglimb of Henle’s loop

Cortical thick ascendinglimb of Henle’s loop

Distal convoluted tubule

Cortical collecting duct

c

Min

or

all

ell

ic f

req

ue

ncy

Minor alelles of SNPs

0.200

0.180

0.160

0.140

0.120

0.100

0.080

0.060

0.040

0.020

0.000

C180T A652G A772G G790A

SLC23A1common type

A652G

d

Pla

sma

asc

orb

ate

(μ

M)

Dose of ascorbate (mg day–1)

00

20

40

60

80

100

500 1,000 1,500 2,000 2,500

Measured values (mean)

G790A

C180T

A772G

SLC23A1common type

A652G

G790A

C180T

A772G

Sham

SLC23A1deletion

b

Asc

orb

ate

(p

mo

l o

ocy

te–

1)

Time (min)

00

200

400

600

10 20 30 40

White

African American

African (Yoruba)

Figure 3SLC23A1 activity and human genetic variation. (a) Distribution of SLC23A1 in kidney nephron segments by Serial Analysis of GeneExpression (SAGE) libraries containing expression data from microdissected glomeruli and six different nephron segments; values areexpressed as tags per million. (b) Effect of SLC23A1 single-nucleotide polymorphisms (SNPs) on ascorbate transport. Xenopus laevisoocytes were microinjected with the following SLC23A1 cRNAs: common type; sham injected; human deletion construct; and SNPsA652G (rs34521685), G790A (rs33972313), A772G (rs35817838), and C180T (rs6886922). (c) Population prevalences of SLC23A1polymorphisms. Shown are averaged minor allelic frequencies of SLC23A1 genotypes in white (n = 1,874), African American (n =438), and African (n = 48) individuals, using pooled genotype data. (d ) Modeled effects of SLC23A1 polymorphisms on plasmaascorbate concentrations in humans. Values in healthy young women for common type SLC23A1 are measured and calculated fastingsteady-state plasma ascorbate concentrations. For women with SNPs, values are calculated from wild-type data comparisons totransport data in panel b. Adapted with permission of the American Society for Clinical Investigations, from Corpe CP, Tu H, Eck P,Wang J, Faulhaber-Walter R, Schnermann J, Margolis S, Padayatty S, Sun H, Wang Y, Nussbaum RL, Espey MG, Levine M. VitaminC transporter Slc23a1 links renal reabsorption, vitamin C tissue accumulation, and perinatal survival in mice. J. Clin. Investig.120(4):1069–83. Copyright 2010 (17).

have been identified. In some cells, SVCT1 ex-pression declines with increasing exposure toascorbate, but the regulatory factors mediatingthis response are unknown.

Over 150 SNPs have been identified inSLC23A1, although many of these have notbeen verified in large populations to deter-mine global allele frequencies. Prevalence of


Ann

u. R

ev. N

utr.

201

3.33

:45-

70. D

ownl

oade

d fr

om w

ww

.ann

ualr

evie

ws.

org

by G

eorg

etow

n U

nive

rsity

on

08/1

1/13

. For

per

sona

l use

onl

y.


Table 1 Phenotypic effects of single-nucleotide polymorphisms (SNPs) found in the human SLC23A1 gene

ReferenceSNP ID Location

Geneticvarianta References Alterations in phenotypeb

rs10063949 5′ UTR c.-97–487A>G

(27, 28, 85, 94, 97) Increased plasma and serum ascorbate levels in theBWHHS cohort but not in the EPIC cohort

rs6886922 Exon 3 c.180 C>T (17) Modeled in Xenopus oocytes: 40%–50% reduction inascorbate transport

rs34521685 Exon 7 c.652 A>G (17) Isoleucine-to-valine substitution at position 218 ofSVCT1. Modeled in Xenopus oocytes: 40%–50%reduction in ascorbate transport

rs35817838 Exon 8 c.772 A>G (17, 27) Methionine-to-valine substitution at position 258 ofSVCT1. Modeled in Xenopus oocytes: 75% reduction inascorbate transport

rs33972313 Exon 8 c.790 G>A (17, 27, 85) Valine-to-methionine substitution at position 264 ofSVCT1. Modeled in Xenopus oocytes: 40%–50%reduction in ascorbate transport. Strongly associatedwith decreased plasma and serum ascorbate levels inmultiple cohorts

rs11950646 Intron 9 c.1074–101A>G

(82, 94) Increased risk of follicular lymphoma observed in a UScohort

rs4257763 Intron 10 c.1179+109T>C

(9, 85) Decreased serum ascorbate levels in the BWHHS cohort

rs34063983 Intron 11 c.1310–41A>G

(27) None

rs6876106 Intron 13 c.1550–2088C>T

(94) None

rs6596473 Intron 13 c.1549+2515G>C

(9, 27, 82, 85, 94) Trend toward decreased plasma ascorbate levels in twostudies. Increased risk of follicular lymphoma in a UScohort

rs6596471 Intron 14 c.∗19+2088T>C

(85) None

aVariations listed relative to the coding region in the reference sequence NM_005847.4.bEffects of genetic polymorphism as described in the references provided.Abbreviations: BWHHS, British Women’s Heart and Health Study; EPIC, European Prospective Investigation into Cancer and Nutrition;SVCT, sodium-dependent vitamin C transport protein.

Nonsynonymous:DNA change in thecoding region of theprotein that results inan amino acid change

13 SNPs has been noted in Caucasian, AfricanAmerican, and, to a lesser degree, Asian popu-lations (9, 26). Overall, African American pop-ulations show the greatest diversity of SNPspresent in SLC23A1—many of which are notdetectable in Caucasians (9). However, as stud-ies into the genetic variations in SLC23A1 con-tinue, a different portrait of SVCT1 geneticvariability may emerge.

At least four of the SNPs present inSLC23A1 are found within the coding region

(exons 3, 7, and 8), which results in one syn-onymous and three nonsynonymous changesto the protein (Table 1). All nonsynonymouspolymorphisms produce a functional SVCT1protein, but each of these proteins displaysfunctional declines in ascorbate transportwhen expressed in Xenopus laevis oocytes (17)(Figure 3b). For example, the amino acidsubstitution encoded by the rs35817838 SNPresults in a methionine-to-valine missense mu-tation at position 258. Oocytes expressing this


Ann

u. R

ev. N

utr.

201

3.33

:45-

70. D

ownl

oade

d fr

om w

ww

.ann

ualr

evie

ws.

org

by G

eorg

etow

n U

nive

rsity

on

08/1

1/13

. For

per

sona

l use

onl

y.


particular variant demonstrate a 75% reductionin the rate of intracellular ascorbate accumula-tion (Figure 3b; A772G). This SNP is found invery low frequency in the general population,primarily in African Americans (Figure 3c;A772G). It is currently unclear if there areindividuals homozygous for the expressionof this SLC23A1 variant, and its impact onvitamin C homeostasis in humans is unknown.

Genetic Variation in SLC23A1and Vitamin C Status

SVCT1 is considered to play a central role inregulating whole-body vitamin C status and isinvolved in bulk vitamin C transport through-out the body (Figure 2). Although results fromSLC23A1 knockout mice suggest that absorp-tion of ascorbic acid may not be impaired bythe loss of SVCT1, the lack of renal reabsorp-tion results in reduced plasma levels of ascor-bate. Therefore, genetic variations in SLC23A1may lower the renal threshold for vitamin C andincrease its urinary excretion, limiting steady-state plasma and body vitamin C levels. Thereduced rate of ascorbate transport has beendemonstrated in Xenopus oocytes as describedabove (Figure 3b). It is expected that indi-viduals carrying these SNPs would have lowerplasma ascorbate levels even with high dietaryvitamin C intake (Figure 3d ).

In 2010, two studies were publisheddemonstrating the effects of SNPs in SLC23A1on circulating concentrations of vitamin C(Table 1). Lower serum ascorbate levels werefound in Caucasian and East Asian subjectsdisplaying the rs4257763 SNP variant (9).Similarly, this SNP was associated with lowerascorbate levels in subjects participating inthe British Women’s Heart and Health Study(BWHHS) (91). Strong trends toward adecline in circulating ascorbate levels were alsoobserved in both aforementioned studies (9,91) when a different intronic SNP (rs6596473)was examined (Table 1). Interestingly, thissame SNP was also associated with a decreasedrisk of follicular lymphoma (82), as furtherdiscussed below. Another SNP (rs10063949)

was also found in the BWHHS cohort to beassociated with a small but significant increasein circulating ascorbate levels, but this perallele effect was not observed in the EuropeanProspective Investigation into Cancer andNutrition (EPIC)-Norfolk cohort (85).

The most robust finding for the influence ofSLC23A1 genetic variation on plasma or serumascorbate levels was found by Timpson et al.(85) in a multiple cohort evaluation of morethan 15,000 individuals in the United King-dom. One of the SNPs examined (rs33972313)was associated with consistent, reproducibledeclines in circulating ascorbate levels acrossall study cohorts. As discussed below, this SNPencodes an amino acid change in the SVCT1protein that results in decreased vitamin Ctransport activity. Overall, in these humanpopulations, this genetic feature was associatedwith an approximately 6 μM lower plasmaor serum ascorbate concentration per allelepresent. These declines were noted despitedifferences in vitamin C intake, smoking status,alcohol consumption, geographical location,and socioeconomic status.

The rs33972313 variant is a nonsynony-mous SNP in SLC23A1 that results in avaline-to-methionine substitution at position264 of SVCT1 (17). The rs33972313 SNP isfound in Caucasians, but it is present at a higherfrequency in African American and YorubaAfrican populations (Figure 3c; G790A). Thisvariation results in an approximate 50% declinein the rate of ascorbate accumulation in cells(Figure 3b). It is postulated that the overall ef-fects of this mutation in humans would increaseexcretion of ascorbic acid from the body andalter the dose-response relationship betweenplasma ascorbate levels and dietary vitamin Cintake (Figure 3d ). However, no individualscarrying this variant or homozygous for thisallele have been examined to determine thestrength of this relationship with vitamin Chomeostasis.

To date, the association of SLC23A1genetic variation and plasma levels of vitaminC is the strongest evidence for the influence ofSVCT1 on human vitamin C status. Since the


Ann

u. R

ev. N

utr.

201

3.33

:45-

70. D

ownl

oade

d fr

om w

ww

.ann

ualr

evie

ws.

org

by G

eorg

etow

n U

nive

rsity

on

08/1

1/13

. For

per

sona

l use

onl

y.


Environmentalexposures: bothman-made and naturaltoxins present in theenvironment (smoke,asbestos, heavy metals,etc.)

regulation of plasma vitamin C homeostasisultimately influences the availability of ascorbicacid to all tissues and other fluids in the body,it is possible that further associations betweenthese SNPs in SLC23A1 and physiologicalfunction or chronic disease risk will be dis-covered. However, caution must be taken insuch analyses. Associations should not be basedmerely on genetic variation, but also on mea-sured plasma or serum levels of ascorbic acid,and possibly on urinary excretion rates. Dietaryrecords of vitamin C intake do not adequatelyreflect vitamin C plasma levels or body status(30, 43) and should not be used to evaluate thepossible role of vitamin C in chronic diseaserisk. Hence, any future studies into phenotypicchanges associated with SLC23A1 geneticvariation will require measuring the effects oncirculating vitamin C levels. Without this data,it will be difficult to draw any conclusions inrelation to vitamin C status.

Genetic Variation in SLC23A1and Chronic Disease Risk

Early studies of genetic variation in SLC23A1had focused on associations with disease risk.Because higher dietary intake of vitamin C hasbeen associated with lower risk of cancers ofthe digestive tract (13), and SVCT1 plays animportant role in ascorbic acid uptake fromthe diet, SNPs in SLC23A1 may play a role ingastrointestinal cancer development. However,neither incidence of gastric cancer (94) noradvanced colorectal adenoma (28) was relatedto SLC23A1 genotype. These results are notsurprising given the functional redundancyof vitamin C absorption, as both SVCT1 andSVCT2 are expressed in the digestive tract.

Since many of the genetic variations inSLC23A1 are strongly associated with de-creased plasma vitamin C levels, they might af-fect cancer development in tissues other thanthe gastrointestinal tract by altering the sup-ply of ascorbic acid in the circulation. Onlyone study supports this idea: Skibola et al. (82)observed an increased risk of follicular lym-phoma associated with two SNPs (rs6596473

and rs11950646), one of which (rs6596473) isassociated with decreased plasma ascorbate lev-els (9, 85) (Table 1). Interestingly, the in-creased cancer risk was observed in a US butnot a German cohort (82). The reason for thisdiscrepancy is not known but may be due to thesmall population size, lower lymphoma rate inthe European cohort, or interactions with dietand environmental exposures that were not as-sessed in the study.

Higher vitamin C intakes or plasma levelsare consistently associated with decreasedincidence and risk of death from cardiovasculardiseases (30, 32, 47, 65, 90, 95). Furthermore,several studies have shown that vitamin Csupplementation and higher plasma ascorbatelevels are associated with decreased bloodpressure, improved endothelial function,lowered circulating C-reactive protein levels,and increased serum high-density lipoproteincholesterol levels (5, 7, 31, 44). Given thesestrong associations with vitamin C status, itis surprising that no studies have investigatedthe effects of SNPs in SLC23A1 on cardiovas-cular function. There is a significant need todetermine the impact of genetic variation inthe SLC23 family on the risk of cardiovasculardiseases, including coronary artery disease,ischemic stroke, and hypertension.

Taken together, the current data suggestthat there is only a weak association, at best,between SLC23A1 polymorphisms and diseaserisk. However, it should be noted that in allof the aforementioned studies, plasma levelsof ascorbate were not measured, and dietaryvitamin C intake was evaluated in only two ofthe studies (28, 94). It is often assumed thatdietary vitamin C intake accurately reflectsvitamin C body status, but the presence ofSNPs in SLC23A1 may alter SVCT1-basedabsorption of vitamin C from the diet. Becausesteady-state levels of plasma ascorbate areaffected by polymorphisms in SLC23A1 (seeabove), the supply of ascorbate to tissues mayalso be affected. This suggests that in studies ofchronic disease risk, the influence of SLC23A1SNPs must be evaluated with regard to changesin measured, circulating levels of ascorbate.


Ann

u. R

ev. N

utr.

201

3.33

:45-

70. D

ownl

oade

d fr

om w

ww

.ann

ualr

evie

ws.

org

by G

eorg

etow

n U

nive

rsity

on

08/1

1/13

. For

per

sona

l use

onl

y.


microRNA: short,noncoding RNAinvolved in theregulation ofmessenger RNA levels

Furthermore, it must be noted that thestudies on SLC23A1 variants and cancer riskwere limited to Caucasian populations. Theonly nonsynonymous SNP in SLC23A1 foundat high frequency in Caucasian populations isthe rs33972313 variant, which—as mentionedabove—results in a valine-to-methionine sub-stitution at position 264 of SVCT1 and de-creased vitamin C transport activity (17). ThisSNP was not evaluated for cancer risk. Sinceother changes in the SVCT1 protein are foundat greater frequency in African and AfricanAmerican populations (Figure 3c), there maybe associations between risk for certain diseasesand genetic variations in SLC23A1 that have yetto be discovered.

SVCT2 (SLC23A2)

Encoded by the gene SLC23A2, SVCT2 is avitamin C transport protein found in nearlyevery tissue and cell of the body (Figure 2).SVCT2 is a high-affinity ascorbate transportprotein responsible for the accumulation ofvitamin C into tissues that have the highestconcentrations of ascorbate: the brain, eyes,and adrenals (86). SVCT2 is also the onlyvitamin C transport protein expressed inleukocytes (70) and platelets (79); however, itis notably absent from erythrocytes (59).

Mouse models in which SLC23A2 has beenablated survive until birth but die soon after-ward from cerebral hemorrhaging and respi-ratory failure due to lack of lung expansion(83). Thus, it is believed that the expressionof SVCT2 and adequate ascorbate levels in thebrain are critical for normal fetal development.Studies in animal models and human cell linessupport a role of redox signaling in the tran-scriptional regulation of SVCT2 (69, 70, 80).Intracellular ascorbic acid levels regulate the ex-pression of SVCT2 in many tissues, althoughthe brain appears to be an exception (61).

Genomic Characterizationof SLC23A2

SLC23A2 is approximately 10 times larger thanSLC23A1, exceeding 150 kbp on chromosome

20 in humans (26, 73). Corresponding to thisincreased gene size, larger 5′ and 3′ untranslatedregions (UTRs) and wider intronic sequenceshave been noted in SLC23A2. Two distinctpromoter regions exist in this gene, allowing foralternative sites for the initiation of transcrip-tion and different lengths of mRNA produced(76). SLC23A2 has 17 exons, with the alterna-tive starting exons labeled exon 1a and exon 1b.Transcription through both exons has been de-tected in human cells, but exon 1b predominatesin mature mRNA transcripts. Transcriptionalregulation of both promoter regions has beendescribed through KLF, Sp1, YY1, and factorsof the EGR family (71, 72), though the func-tional aspects of this regulation are unclear.Additional regulation may come in the formof microRNA binding to the 3′ UTR of thefull SLC23A2 transcript. A recently publishedstudy links the expression of miR-30b andmiR-30d with lowered SLC23A2 expression(33). These microRNAs have been implicatedin regulating p53 and catalase expression in re-sponse to hydrogen peroxide exposure (38, 50)and may mediate redox regulation of SVCT2expression.

Due to the relatively large size of the gene,variants within the genetic structure are rel-atively frequent and widespread. Over 2,200SNPs have been identified, and at least 250 ofthose are present in greater than 5% of all pop-ulations tested. Published reports have men-tioned 30 different SNPs that span the entirelength of the gene (26). Although some spe-cific variants have not been detected in differentgroups, many SNPs are present in individualsof different ethnic backgrounds.

Overall, most of the SNPs analyzed todate fall in intronic or untranslated regions ofSLC23A2 (Table 2) and do not directly af-fect the coding of the SVCT2 protein. SNPsthat do lie within the protein coding region ofthe gene have all been synonymous polymor-phisms, suggesting that amino acid changes inSVCT2 are generally unfavorable mutations.However, SNPs not involved in protein cod-ing are not precluded from impacting cellularSVCT2 levels. For example, one relatively rare


Ann

u. R

ev. N

utr.

201

3.33

:45-

70. D

ownl

oade

d fr

om w

ww

.ann

ualr

evie

ws.

org

by G

eorg

etow

n U

nive

rsity

on

08/1

1/13

. For

per

sona

l use

onl

y.


Table 2 Phenotypic effects of single-nucleotide polymorphisms (SNPs) found in the human SLC23A2 gene

ReferenceSNP ID Location Genetic varianta References Alterations in phenotypeb

rs1279683 Intron 1 c.-375–947 C>T (97) Associated with decreased plasma ascorbate levels andincreased risk of developing glaucoma

rs12479919 Intron 2 c.-282+1312 G>A (2, 27, 28, 94) Trend toward decreased risk of gastric cancer;considered part of a high-risk bladder cancerphenotype

rs2681118 Intron 2 c.-282+14050 C>A (27, 28, 94) Noners6139591 Intron 3 c.-155+80 C>T (9, 27, 28, 94) Trend toward increased risk of preterm delivery; no

association with serum ascorbate levelsrs2681116 Intron 3 c.-155+108 A>G (9, 27, 28, 94) None, including no association with serum ascorbate

levelsrs13037458 Intron 3 c.-155+224 T>G (28, 94) Noners4813725 Intron 3 c.-154–4623 G>A (27, 28, 94) Noners1776948 Intron 3 c.-154–17751 C>T (82) Noners1715365 Intron 4 c.109–5464 G>A (27, 28) Noners6133175 Intron 5 c.207+1767 T>C (82) Noners1776964 Exon 7 c.375 C>T (27, 28, 94) Noners4987219 Intron 9 c.642+453 G>C (15, 27, 28) Decreased risk of head and neck cancer with concurrent

HPV infectionrs1110277 Exon 12 c.1002 C>T (27, 28, 94) Noners16990301 Exon 17 c.∗2203 G>A (33) Limited interactions with regulating microRNA;

associated with a decreased expression of SVCT2 incells

rs35560557 Exon 17 c.∗2724 C>T (27, 28) None

aVariations listed relative to the coding region in the reference sequence NM_005116.6.bEffects of genetic polymorphism as described in the references provided.Abbreviations: HPV, human papillomavirus; SVCT, sodium-dependent vitamin C transport protein.

SNP (rs16990301) found in a Yoruba Africanpopulation appears to influence regulation bythe miR-30d microRNA, leading to decreasedSVCT2 mRNA levels (33).

Genetic Variation in SLC23A2and Vitamin C Status

Unlike variation in SLC23A1, genetic changesin SLC23A2 are not expected to have alarge impact on ascorbate homeostasis in thecirculation, as SVCT2 regulates the tissueaccumulation of the vitamin from the plasma.To date, only two studies have addressed thisquestion. The first found no correlation be-tween serum ascorbate levels and two commonSNPs (rs6139591 and rs2681116) in intron 3

of SLC23A2 (9). The second study observedsignificantly lower plasma ascorbate levels insubjects with variants of an SNP in intron 1(rs1279683) (97). Since this SNP lies in theintronic regions of the gene, it is difficult toexplain why it affects circulating ascorbatelevels. Because it lies on the 5′ end of the gene,it is possible that the rs1279683 polymorphismaffects the transcriptional regulation of SVCT2mRNA.

Studies are lacking on the possible effectsof genetic variation in SLC23A2 on cellularvitamin C status. No studies have looked atSVCT2 protein levels in human cell linesexpressing these SNPs, and no individuals car-rying these variant alleles have been evaluated


Ann

u. R

ev. N

utr.

201

3.33

:45-

70. D

ownl

oade

d fr

om w

ww

.ann

ualr

evie

ws.

org

by G

eorg

etow

n U

nive

rsity

on

08/1

1/13

. For

per

sona

l use

onl

y.


Haplotype: acombination of SNPs;haplotype analysis isused to determineadditive effects ofseparatepolymorphisms on anindividual’s phenotype

for tissue ascorbate levels. As indicated above,measuring plasma vitamin C may not be anaccurate evaluation of the effect of SLC23A2SNPs on the concentration of vitamin C intissues. Future studies should assess ascorbatelevels of circulating leukocytes or other cells,e.g., tissue biopsies or buccal cells, as a surrogateof tissues in subjects with genetic variations inSLC23A2.

Genetic Variation in SLC23A2and Chronic Disease Risk

Since vitamin C concentrations in the eyeare among the highest in the human body(Figure 2), changes in SVCT2 function mayhave a strong impact on eye health. Indeed, ina Mediterranean population, a polymorphismfound in intron 1 of SLC23A2 (rs1279683) wasstrongly associated with an increased risk ofdeveloping glaucoma (97). Not only was thisassociation statistically significant after adjust-ing for many confounding factors that couldimpact disease risk or alter vitamin C status, butglaucoma patients with the variant allele hadthe lowest plasma ascorbate levels of all subjectsenrolled in the study. Severity of cataracts hasalso been associated with vitamin C levels in theeye lens in humans. Although some studies haveobserved increased dietary vitamin C intake andincreased blood levels of vitamin C to be associ-ated with decreased risk of cataracts (22, 74, 87),no studies have been performed in conjunctionwith genetic variation in SLC23A2. Hence,this may prove an interesting avenue for futureresearch.

Vitamin C in tissues has been postulatedto affect relative cancer risk, but the overallsupport of this association for SLC23A2 islimited (Table 2). A variant in SLC23A2 ob-served within intron 2 (rs12479919) correlatedwith a trend (p = 0.06) toward a decrease inrisk of gastric cancer (94). In addition, geneticvariation at this site modified risk for bladdercancer, but only in association with SNPs in20 other “high-risk” genes (2). An SNP inintron 9 (rs4987129) was associated with adecreased incidence of head and neck cancer in

individuals testing positive for human papil-loma virus infection (15). Haplotypes withthe rs4987129 SNP and a SNP in exon 11(rs1110277) revealed that the combination ofthe two rare alleles reduced the risk of develop-ing advanced colorectal cancer (28). Similarly,other haplotype analyses suggested the in-volvement of intronic SNPs in a reduced risk ofdeveloping gastric cancer and non-Hodgkin’slymphoma (82). It should be noted that in eachof these studies investigating different types ofcancer, plasma or tissue ascorbate levels werenot assessed in association with the SNPs inSLC23A2. Therefore, it is not known whetherthese genetic variants may affect cancer risk dueto changes in vitamin C plasma or tissue levels.However, if these genetic variants indeedlead to lower tissue concentrations of vitaminC, a plausible mechanism for the increasedcancer risk may be increased expression ofhypoxia-inducible factor 1α (34).

Although not a disease state per se, the pres-ence of SLC23A2 SNPs in mothers was asso-ciated with an increased risk of spontaneouspreterm delivery (24–37 weeks gestational age),but the overall effects of the genetic variationwere not robust (27).

As mentioned above for SLC23A1, a ma-jority of the studies investigating disease riskdid not assess plasma ascorbate levels. SinceSVCT2 controls the transport of ascorbatefrom the bloodstream to tissues, circulatinglevels of ascorbate may influence the riskof disease in tissues. In light of the strongassociations between vitamin C levels andvascular function mentioned above and thefact that the heart and vascular endotheliumonly express SVCT2, it is interesting to notethat no studies have examined the impact ofgenetic variation in SLC23A2 on cardiovas-cular disease risk. In addition, no studies havebeen performed on SLC23A2 variation andbrain function. Because vitamin C is criticalfor normal brain development and functionand is thought to play a role in the preventionof neurodegenerative diseases, behavioral orfunctional associations with SLC23A2 SNPsshould be addressed in future studies.


Ann

u. R

ev. N

utr.

201

3.33

:45-

70. D

ownl

oade

d fr

om w

ww

.ann

ualr

evie

ws.

org

by G

eorg

etow

n U

nive

rsity

on

08/1

1/13

. For

per

sona

l use

onl

y.


Porphyrin: a carbon-and nitrogen-basedring that can bindmetals; heme (such asfound in hemoglobin)is an iron-containingporphyrin

CD136: also knownas macrophagestimulating 1 receptor,involved in response toinflammatory signals,injury, and invasivegrowth

HAPTOGLOBIN

Unlike the SVCT proteins, the interactionbetween haptoglobin and vitamin C is believedto be indirect, with iron dysregulation and thegeneration of reactive oxygen species actingas intermediaries. Both increased iron levelsand oxidative stress may lead to a decline incirculating ascorbate levels due to its antioxi-dant activity. Despite this indirect relationship,haptoglobin is the only circulating blood factorfor which there is a known, strong relationshipbetween genetic variation and vitamin Chomeostasis.

Function of Haptoglobin

Haptoglobin is a hemoglobin-binding proteinsynthesized in the liver and released into thecirculation. The primary role of haptoglobinis to bind free hemoglobin after it has beenreleased from damaged or lysed erythrocytes.It is believed that this interaction allows hap-toglobin to sequester the iron-porphyrin ringand prevent it from reducing molecular oxygento superoxide radicals and other reactive oxy-gen species (36). Experimental animals withouta functional copy of the haptoglobin gene (HP)suffer from extensive oxidative damage in tis-sues, especially the kidneys, where iron may bereleased from the porphyrin ring (55).

A majority of the haptoglobin-hemoglobincomplex is cleared from the circulation byparenchymal tissue in the liver. To a lesser de-gree, circulating macrophages also remove thiscomplex from the blood, as the CD136 recep-tor expressed by these cells can bind to conju-gated haptoglobin. This may be an importantmechanism for the clearance of haptoglobin-hemoglobin complexes trapped in capillaries ofperipheral tissues. In either case, after cellularuptake the complex is subjected to lysosomaldegradation and its iron atom is recycled foruse in other heme proteins (36).

Characterization of HumanHaptoglobin Genotypes

Found on chromosome 16, the human hap-toglobin gene is polymorphic, with two alleles

found in the population at relatively highfrequency. The ancestral allele (HP1) consistsof 5 exons, while the variant allele (HP2)contains 7 exons. The HP2 allele is thought tohave emerged by a duplication of exons 3 and4. The resulting protein has a larger α chainwith a duplication of the cysteine residue thatforms intermolecular disulfide bonds (36, 55).

Individuals homozygous for the HP1 allelehave traditionally been termed Hp1-1 phe-notype. The haptoglobin protein encoded byHP1 is an (αβ)2-tetramer, as the cysteine inthe α chain binds a cysteine residue in anadjacent α chain (Figure 4). On the otherhand, the α chain encoded by the HP2 al-lele has an additional cysteine residue thatbinds two other α chain cysteine residues.Therefore, heterozygous individuals (Hp2-1)can form haptoglobin proteins with three ormore α chains in a linear polymer: a core ofone or more Hp2 proteins that are capped ateach end by Hp1 subunits. In contrast, Hp2-2 individuals, homozygous for the HP2 allele,form larger haptoglobin protein complexes.The protein interactions between the cysteineresidues in the HP2 α chains require thatthese multimeric proteins are cyclical in nature(Figure 4).

The molecular interactions of the differenthaptoglobin proteins distinguish their geno-types. The haptoglobin protein encoded byHP2 binds poorly to hemoglobin compared toHP1. Compared to Hp1-1 individuals, indi-viduals expressing the Hp2-2 phenotype haveincreased circulating levels of iron, possiblyfree or still bound to the porphyrin ring ofhemoglobin. It is believed that this increasedredox-active iron can participate in the gener-ation of superoxide and hydroxyl radicals andis associated with increased oxidative damageto lipoproteins in Hp2-2 individuals (36,55). Thus, it would be expected that plasmavitamin C levels decline accordingly. TheHp2-2 phenotype is particularly detrimentalin diabetics, where glycosylated hemoglobinreduces the ability of haptoglobin to bind tothe protein. Thus, diabetic individuals carryingthe HP2 allele have a significantly increased risk


Ann

u. R

ev. N

utr.

201

3.33

:45-

70. D

ownl

oade

d fr

om w

ww

.ann

ualr

evie

ws.

org

by G

eorg

etow

n U

nive

rsity

on

08/1

1/13

. For

per

sona

l use

onl

y.


HP1 allele HP2 allele

Hp1–1 Hp2–2Hp2–1

α chain Variantα chain

β chain

Figure 4Haptoglobin phenotypes. Haptoglobin is translated as a single polypeptidechain that undergoes posttranslational processing and cleavage to form a lightchain (α subunit) and a heavy chain (β subunit). The α subunit of haptoglobinalso binds to other α chains through disulfide bridges (55). The haptoglobinprotein encoded by HP1 has a cysteine residue in the α chain that binds acysteine residue in an adjacent α chain. The α chain encoded by the HP2

variant allele has an additional cysteine residue and hence binds two other α

chain cysteine residues. The resulting phenotypes are a combination of thosetwo α chains forming a tetrameric protein (Hp1-1), or different sizes of linear(Hp2-1) or cyclic (Hp2-2) haptoglobin polymers.

Recommendeddietary allowance(RDA): in the UnitedStates the RDA forvitamin C is currently90 mg/day for men and75 mg/day for women

of cardiovascular disease and diabetes-relatedcomplications (55).

Beyond this more common allelic variation,there are reports of SNPs in the HP2 carri-ers that impact circulating haptoglobin levels(18). In addition, there are also individuals whohave a genetic loss of haptoglobin (HP0 allele),but these are found in very low frequencies(67).

Haptoglobin Genetic Variationand Vitamin C Status

As mentioned above, diabetic Hp2-2 individu-als demonstrate increased oxidative damage tolipoproteins, which is likely the result of in-creased redox-active iron levels. In vitro stud-ies have shown that the rate of ascorbic acidoxidation in human plasma containing freehemoglobin is decreased by the addition ofhaptoglobin from either Hp1-1 or Hp2-2 in-dividuals. This is presumably due to bindingof the redox-active iron in the porphyrin ringof hemoglobin by haptoglobin. However, therate of ascorbate oxidation in the presence ofhaptoglobin from Hp2-2 individuals is muchhigher than the oxidation rate observed withthe protein isolated from Hp1-1 carriers (51), inagreement with the observation that the Hp2-2protein binds hemoglobin with weaker affinity.

Several clinical studies have found a rela-tionship between circulating levels of ascorbicacid and haptoglobin phenotype. In twostudies, serum ascorbate levels were lowerin healthy Hp2-2 subjects compared to bothHp2-1 and Hp1-1 individuals (21, 66). Nosignificant differences in plasma vitamin Cconcentrations were noted between subjectswith the Hp2-1 and Hp1-1 phenotype. Thelower serum ascorbate levels in Hp2-2 subjectswere correlated with increased serum iron andan overall decrease in circulating levels of thehaptoglobin protein. This phenotype-specificdecline in plasma ascorbic acid was alsodemonstrated in individuals with peripheralartery disease (20) and HIV infection (66).

An additional study on haptoglobin phe-notype showed no overall difference in plasmaascorbate levels in Hp2-2 individuals versusthose carrying the HP1 allele (10). However,Hp2-2 individuals were more likely to haveplasma ascorbate levels in the deficient range(<11 μM) when consuming less than the USrecommended dietary allowance (RDA) of vita-min C compared to individuals with the Hp2-1or Hp1-1 phenotype. The authors suggestedthat this is indicative of a significant diet-geneinteraction between vitamin C and haptoglobin


Ann

u. R

ev. N

utr.

201

3.33

:45-

70. D

ownl

oade

d fr

om w

ww

.ann

ualr

evie

ws.

org

by G

eorg

etow

n U

nive

rsity

on

08/1

1/13

. For

per

sona

l use

onl

y.


Transferrin: aniron-carrying proteinfound in extracellularfluids

phenotype, with Hp2-2 individuals achievinglower steady-state plasma ascorbate concen-trations than Hp2-1 or Hp1-1 individualswith the same dietary intake of vitamin C.However, as mentioned above, assessment ofdietary intake of vitamin C is problematic, andmore carefully controlled studies are needed toestablish a relationship between dietary intakeof vitamin C, steady-state levels of plasmaascorbate, and haptoglobin phenotype.

It should be noted that most of the health ef-fects of haptoglobin polymorphisms have beendetermined in individuals with diabetes mellitus(55). However, phenotypic changes in plasmaascorbate levels have been determined primar-ily in healthy Hp2-2 individuals. It is possi-ble that, under normal conditions, ascorbateprovides adequate antioxidant protection to in-dividuals carrying the HP2 allele against ox-idative damage to lipoproteins and increaseddisease risk. It has been speculated that vita-min C supplementation may actually worseniron-catalyzed oxidative damage in Hp2-2 in-dividuals suffering from diabetes and should beavoided, but experimental evidence supportingthis notion is lacking (3).

Furthermore, it is unclear whether thechanges in vitamin C status associated with hap-toglobin genetic variation are directly relatedto changes in iron regulation. For example, it isnot known whether the haptoglobin phenotypeis as pronounced in individuals with low dietaryiron intake or chronic anemia. It is also possi-ble that iron dysregulation exacerbates the lossof vitamin C seen with the Hp2-2 phenotype.One study of Hp2-2 individuals from Zim-babwe found that a variant allele in the TF gene,encoding the iron-binding protein transferrin,further reduced circulating levels of ascorbate(46). It is also possible that genetic variations inhemoglobin or the CD136 receptor may com-pound any effects of the haptoglobin pheno-type, increasing the levels of circulating redox-active iron. Therefore, diet-gene interactionsin Hp2-2 versus Hp1-1 individuals may ex-tend beyond vitamin C regulation and shouldbe examined with respect to iron status aswell.

GLUTATHIONES-TRANSFERASES

Similar to haptoglobin, most of the interactionsbetween vitamin C and GSTs are thought to bemediated by reactive oxygen species. The ge-netic variations in GST may also impact ascor-bate levels through glutathione status, either bypreventing the oxidation of ascorbate by reac-tive oxygen species—hence “sparing” it—or byrecycling ascorbate from its oxidized form, de-hydroascorbic acid (DHA).

Functions of GlutathioneS-Transferase Isoforms

GSTs are phase II enzymes involved in thedetoxification of harmful electrophilic com-pounds of endogenous or exogenous origin.This is accomplished by the conjugation ofglutathione to a target molecule recognizedby the GST enzyme. Several members ofthe GST family exist, each with differentranges of substrate targets. Based on sequencehomology, eight distinct classes of solublehuman GSTs have been identified: alpha,kappa, mu, omega, pi, sigma, theta, and zeta. Itis believed that the various members emergedover time as a result of a progressive series ofgene duplication, recombination, and mutation(39, 60). Inactive GST pseudogenes andrepeated DNA sequences lie on chromosomesnear active GST members, suggesting theflexible nature of this genetic sequence. Thisflexibility is also seen in the protein activity, assubstrate specificities overlap between isoformsof different GST families.

As part of their response to toxic environ-mental factors, including the detoxification ofcarcinogens that otherwise may cause oxidativedamage, the GSTs are also induced underconditions of oxidative stress. Alpha-, pi-, mu-,and theta-class GSTs have been implicatedin detoxification of numerous reactive lipidoxidation products, such as α,β-unsaturatedaldehydes (39). Therefore, it is conceivablethat decreased GST activity results in lowerlevels of ascorbate due to its direct reaction


Ann

u. R

ev. N

utr.

201

3.33

:45-

70. D

ownl

oade

d fr

om w

ww

.ann

ualr

evie

ws.

org

by G

eorg

etow

n U

nive

rsity

on

08/1

1/13

. For

per

sona

l use

onl

y.


with products of lipid peroxidation (14, 64).Furthermore, changes in GST activity may alsoaffect glutathione levels, which in turn may alterascorbate levels through sparing or recyclingin cells and plasma, as explained above (98).

Multiple isoforms exist in the GST mu(GSTM1-5) and theta (GSTT1-2) families(39). Both GSTM1 and GSTT1 have deletionvariants occurring in relatively high frequen-cies in human populations from diverse ethnicbackgrounds. These are found on chromosome1 and chromosome 22, respectively. Othercommon variants are a missense mutation inGSTM1 and a deletion mutation in GSTM3.The GST pi class consists of a single memberencoded by the gene GSTP1, found on chro-mosome 11. Several SNPs have been describedin this gene, with the most commonly reportedbeing a missense mutation at position 105,resulting in an isoleucine-to-valine (Ile105Val)amino acid substitution. This mutation re-portedly affects the substrate specificity of theprotein.

In general, subjects with GSTM1 andGSTT1 deletions are considered to be at highrisk from exposure to environmental toxins.Since these two isoforms can also recognizelipid peroxidation products as substrates, a lossof activity may predispose the body to increasedoxidative stress.

Human GST Polymorphismsand Vitamin C Status

The relationship between plasma vitamin Cand GST genetic variation was first explored in2001 in a study of middle-aged men in Slovakia,which found that the GSTM1-null genotypewas associated with increased circulating levelsof ascorbate (25). This was also observed ina more recent study in a US cohort, whereindividuals with the highest vitamin C levelswere more likely to be GSTM1-null thanto have a functional copy of GSTM1 (6).However, this relationship between vitaminC and the presence of GSTM1 has not beenuniversally observed. Two studies, one ina Chinese cohort (96) and another with a

mixed population in Canada (11), showed nodifferences in vitamin C status associated withthe GSTM1 genotype. In addition, a study offactory workers in Slovakia reported an overalldecline in plasma vitamin C levels associatedwith the GSTM1-null genotype (42).

Although difficult to explain, these disparateresults may reflect a more complex relationshipof the GSTM1 genotype with vitamin Chomeostasis, involving, e.g., dietary vitaminC intake and environmental exposures. TheGSTM1-null genotype appeared to be associ-ated with increased plasma levels of ascorbatewhen the dietary intake of vitamin C also washigh (6, 25). Conversely, the populations withlow dietary intake of vitamin C and the GSTM1deletion showed lower plasma ascorbate levelsthan did individuals with functional GSTM1at the same intake level (11, 42). This suggestsa heretofore uncharacterized interactionbetween dietary vitamin C, plasma ascorbate,and the activity of GSTs, possibly involvinglipid peroxidation products or environmentaltoxins. There were many different types ofenvironmental exposure in these studies, fromcigarette smoking to hazardous working condi-tions. Together, these data would suggest thatfurther studies on genetic variation in GSTM1and vitamin C levels in plasma, tissues, and dietare warranted, with special attention to knownsubstrates for GST enzymes.

GSTP1 variation has a poorly defined rela-tionship with vitamin C status. One report inmiddle-aged men from Slovakia showed thatindividuals heterozygous for the Ile105Val vari-ant had lower circulating ascorbate levels thandid those homozygous for the valine substi-tution (25). However, this relationship withGSTP1 was not observed in a study of otherfactory workers in Slovakia (42). A more de-tailed study of bioavailability of vitamin C inJapanese women showed that individuals het-erozygous for the GSTP1 variant had lower ab-sorption after a single oral dose of ascorbic acid,despite having nearly identical plasma ascorbatelevels before the supplementation began (41).This would suggest that GSTP1 variations mayalter vitamin C bioavailability.


Ann

u. R

ev. N

utr.

201

3.33

:45-

70. D

ownl

oade

d fr

om w

ww

.ann

ualr

evie

ws.

org

by G

eorg

etow

n U

nive

rsity

on

08/1

1/13

. For

per

sona

l use

onl

y.


An additional study on vitamin C intake inpregnant women showed that GSTP1 genotypeand dietary vitamin C appeared to interactwith fetal development and exposure to envi-ronmental carcinogens. Although circulatinglevels of ascorbic acid were not measured in thisstudy, GSTP1 variation and vitamin C intakewere associated with differential effects ofbenzo(a)pyrene exposure (24). Overall, womenwith the Ile/Val or Val/Val variants of GSTP1exposed to high levels of benzo(a)pyrene wereat significantly higher risk of having childrenwith a low birth weight when their vitamin Cintakes were low. However, high vitamin Cintake negated the increased susceptibility ofthose carrying the GSTP1 polymorphism.

In contrast to other GSTs, a loss of GSTT1does not appear to impact circulating vitaminC levels in the general population. Although anearly study of individuals with the GSTT1-nullgenotype showed a decline in serum ascorbate(25), this was not seen in four other studypopulations (6, 11, 42, 96). One study reportedthat in individuals who consumed less thanthe RDA of vitamin C, GSTT1 gene deletionmay contribute to lower circulating levels ofascorbate (11), but the same was not observedin two other studies (6, 96). On the otherhand, isolated lymphocytes from individualsmissing a functional GSTT1 gene were moresusceptible to hydrogen peroxide-inducedDNA damage (92), suggesting that cellularascorbate levels, rather than those in theplasma, should be measured as a functionalconsequence of GSTT1 disruption.

In general, it appears that interactions be-tween vitamin C levels and genetic variation ofGSTs exist, but overall conclusions are difficultto reach. Future studies need to focus on theinteractions of GSTs not only with vitaminC bioavailability, but also the interaction ofthese polymorphisms with each other and withexposure to environmental toxins. Other classesof GSTs should also be examined besides dele-tions in GSTM1 and GSTT1, or variations inGSTP1. Genetic variation in other members ofthe mu and theta classes of the enzyme mightprovide further insight into vitamin C interac-

tions. GST omega has recently been postulatedto have DHA reductase activity (98), and futurework may address the effect of variation in theGSTO gene family on vitamin C recycling.

OTHER GENETICINTERACTIONSWITH VITAMIN C

Manganese Superoxide DismutasePolymorphisms and Vitamin C Status

Superoxide dismutase (SOD) is responsiblefor the removal of superoxide radicals by con-version to hydrogen peroxide and molecularoxygen and is thus considered one of themajor antioxidant enzymes in the body. ThreeSOD isoforms exist in humans. The isoformfound in mitochondria contains manganese asa cofactor (MnSOD) and is encoded by thegene SOD2 (16). A common polymorphismexists in SOD2 that causes a missense mutationat codon 16, causing an alanine-to-valinesubstitution. The valine-containing isoform ofMnSOD is thought to affect the mitochondrialtargeting sequence (84) and, hence, resistanceof mitochondria to oxidative stress.

Although many studies have found asso-ciations between vitamin C intake, MnSODpolymorphisms, and cancer risk (1, 37), onlyone has measured the effect of MnSODvariants on plasma vitamin C levels (23).Interestingly, healthy individuals homozygousfor the valine isoform had higher levels ofserum ascorbate. Conversely, those individualswith hypercholesterolemia and at least onecopy of the valine-containing MnSOD showeda significant decline in serum ascorbate levelscompared to subjects homozygous for thealanine variant. Further work is needed toconfirm and expand these observations before adefinite conclusion on the effect of SOD2 poly-morphisms on vitamin C status can be drawn.

Glucose Transporters andVitamin C Transport

In addition to the SVCTs, vitamin C ac-cumulation in cells may occur through thetransport of DHA by glucose transporter


Ann

u. R

ev. N

utr.

201

3.33

:45-

70. D

ownl

oade

d fr

om w

ww

.ann

ualr

evie

ws.

org

by G

eorg

etow

n U

nive

rsity

on

08/1

1/13

. For

per

sona

l use

onl

y.


(GLUT) proteins. Specifically, GLUT1,GLUT3, and GLUT10, the gene products ofSLC2A1, SLC2A3, and SLC2A10, respectively,have been shown to transport DHA (52, 78).Other GLUT proteins, such as GLUT2 andGLUT4, also have been shown to transportDHA, albeit at a lower rate (77, 89). In manycell types, the rate of transport of DHA throughGLUTs can exceed the transport of ascorbicacid through the SVCTs (93). Once broughtinto the intracellular environment, DHA israpidly reduced by a variety of nicotinamideadenine dinucleotide phosphate (NADPH)-or glutathione-dependent mechanisms toascorbic acid. Therefore, supplementing cellswith DHA can lead to increased intracellularascorbate. However, DHA is chemically un-stable, with a half-life of only a few minutes inaqueous solutions, and usually is not detectablein tissues or plasma except in times of oxidativestress. In addition, animal models unable toexpress SVCT1 or SVCT2 have severe phe-notypes due to ascorbate deficiency (17, 83),demonstrating that GLUT transporters cannotcompensate for the lack of SVCT-mediatedvitamin C transport. Thus, it is unclear what, ifany, contribution GLUT expression makes tocell and tissue vitamin C levels under normalphysiological conditions.

Human genetic variation in glucose trans-port follows a broad range of gene changes inthe SLC2 family. With respect to GLUT vari-ation and vitamin C homeostasis, the evidenceis scant. Erythrocytes from individuals withGLUT1 deficiency syndrome demonstrate adecreased ability to accumulate DHA ex vivo(48, 49), but whether this affects the concen-tration of ascorbate in erythrocytes in vivo isdoubtful. Arterial tortuosity syndrome is a ge-netic disorder that results in the developmentof elongated and twisted arteries, including theaorta, and is caused by several loss-of-functionmutations in SLC2A10 (GLUT10). It has beenpostulated that GLUT10 is a mitochondrialDHA transport protein since mitochondriaisolated from murine models expressing oneof these SLC2A10 genetic variants exhibitdecreased DHA uptake in vitro and increased

oxidative damage (52). However, there is noevidence to date that these genetic changes inGLUT10 result in altered circulating or tissuevitamin C levels in humans. Overall, geneticvariation in the various members of the SLC2family may affect vitamin C status, but morestudies are needed to support this notion.

CONSEQUENCES OF GENETICVARIATION

Pharmacokinetic studies have demonstratedthat plasma ascorbate rises in a linear fashionwith increasing doses of vitamin C untilplasma and tissues become saturated and amaximum steady-state level is reached (53,54) (Figure 3d ). However, a key aspect tounderstanding the impact of human geneticvariation on vitamin C homeostasis is thatthe relationship between dietary and plasmavitamin C levels may be altered. It is possiblethat those individuals expressing particularvariant gene products might require morevitamin C in the diet to achieve the sameplasma and tissue ascorbate levels as those seenin individuals without these genetic variants.Another possibility is that these individualssimply may not achieve the same maximumsteady-state level of plasma ascorbate evenwhen consuming large doses of vitamin C, asdescribed for SVCT1 variants (Figure 3d ).

It is likely that individuals with the lowestdietary intake of vitamin C are the mostsusceptible to the effects of genetic variation.The differences in plasma vitamin C levelsare relatively small between those consideredseverely deficient (<11 μM) and marginallydeficient (11–28 μM). Severe vitamin Cdeficiency is associated with clinical symptomsof scurvy, such as impaired wound healing andgingivitis, whereas marginally deficient plasmaascorbate levels are associated with early signsof scurvy, including malaise and fatigue, gingi-val inflammation, and bone abnormalities (30).In the presence of a genetic polymorphism, thenumber of individuals in the marginally andseverely deficient vitamin C categories mightbe increased compared to other members of


Ann

u. R

ev. N

utr.

201

3.33

:45-

70. D

ownl

oade

d fr

om w

ww

.ann

ualr

evie

ws.

org

by G

eorg

etow

n U

nive

rsity

on

08/1

1/13

. For

per

sona

l use

onl

y.


the population without this polymorphism, aspostulated for GST variations (11).

Numerous epidemiological studies have ob-served that individuals with the highest plasmaascorbate levels display a reduced risk of cardio-vascular diseases and cancer compared to indi-viduals with the lowest plasma ascorbate lev-els (30). For example, the EPIC-Norfolk studyconcluded that each 20-μM increase in plasmaascorbate level was associated with a 20% de-creased risk of all-cause mortality (47) and a9% relative reduction in risk of heart failure(68). To achieve these benefits, it is commonlyrecommended that dietary consumption of vi-tamin C–rich fruits and vegetables should be in-creased. Individuals with genetic variants mightrequire even higher dietary vitamin C intakesor vitamin C supplements to achieve maximalplasma and tissue ascorbate levels or may beunable to reach the same levels as individualswithout these genetic variants, even at very highvitamin C intakes (see Figure 3d ).

Human genetic variation is a confoundingfactor in epidemiological studies looking at vi-tamin C because it increases the variability in re-sponse to dietary intake or supplementation ofvitamin C and consequently reduces the statis-tical power to detect significant changes with anintervention. In order to overcome these differ-ences, a further subgrouping of the study pop-ulation(s) by the polymorphisms described inthis review may be necessary to detect specificeffects on vitamin C homeostasis or disease risk(30, 56). This has already been observed withhaptoglobin, where a beneficial effect of vitaminE supplementation is observed only in diabet-ics with the Hp2-2 phenotype, who are underincreased oxidative stress, but not the Hp1-1or Hp2-1 phenotypes (88). Future studies con-ducted without any regard to genetic variationin the population are likely to result in a poorassociation of vitamin C supplementation withdisease prevention.

SUMMARY

Understanding the complex network of inter-actions involved in the regulation of vitamin C

homeostasis in humans has proven difficult be-cause data from animal and cell culture studiescannot be easily extrapolated to humans. How-ever, studies into the normal genetic variationin the human population have proven to be auseful guide and, as evidenced by this review,are starting to provide meaningful informationabout the importance of several factors control-ling vitamin C levels in the human body.

Due to the specific and direct interactionwith ascorbic acid, genetic variation in theSLC23 family provides the best informationabout how alterations in SVCT-mediated vita-min C transport affect vitamin C homeostasis.The SNPs present in the protein-coding regionof SLC23A1, in particular, reveal the criticalrelationship of SVCT1 function with plasmaascorbate levels. The profound nature of thisgenetic effect cannot be overemphasized.Despite contributions from diet, environ-mental exposure, health status, and lifestyle, astatistically significant decline in steady-stateplasma ascorbate levels has been demonstratedfor one of these SLC23A1 polymorphisms(rs33972313), and two others (rs4257763 andrs6596473) showed nonsignificant but stillnotable reductions in plasma vitamin C levels.It also should be noted that rs4257763 isnot a rare allele found only in very specificpopulations but rather has been found inmany study populations. Therefore, there ispotential for SLC23A1 genetic changes tohave a widespread impact on human vitamin Chomeostasis. In addition, studies on variationin the gene coding SVCT2 strongly suggestan important role of vitamin C in diseaseprevention, especially in certain cancers andglaucoma. It is likely that the effect of SNPs inSLC23A2 will be evident in more tissues oncemore exacting studies are performed.

Although many of these genetic factors havebeen examined for their influence on vitamin Chomeostasis because of their known role in low-ering oxidative stress in the body, the observedinteractions with vitamin C reinforce the roleof ascorbic acid as a critical biological antioxi-dant in tissues and extracellular fluids. Geneticvariation in the haptoglobin gene demonstrates


Ann

u. R

ev. N

utr.

201

3.33

:45-

70. D

ownl

oade

d fr

om w

ww

.ann

ualr

evie

ws.

org

by G

eorg

etow

n U

nive

rsity

on

08/1

1/13

. For

per

sona

l use

onl

y.


this relationship directly, as variants in the HPgene show direct correlation with iron dysregu-lation and oxidative stress. The loss of ascorbatefrom the plasma reflects the increased demandsfor antioxidant protection the variant proteinshave placed on the system. Genetic variation inSOD2, leading to changes in MnSOD activity,likely will show similar, nonspecific interactionswith vitamin C due to its nature as an antioxi-dant enzyme.

Similar antioxidant-related interactions be-tween vitamin C and GSTs may occur, but,due to the pluripotent nature of glutathione,the relationship is more complex and currentlyincompletely understood. This uncertainty re-flects the complex interactions that govern vita-min C homeostasis, as a wide range of dietary,environmental, and lifestyle factors affect theinteraction between ascorbate, glutathione, andGST activity.

One topic only briefly touched on in thisreview is the impact that epigenetic regula-tion of these genes may have on vitamin Cstatus. MicroRNA regulation and methylation-dependent transcription of SLC23A2 werementioned above, but other types of epigeneticregulation may also apply to this gene and othergenes examined in this review. This type of ge-netic regulation of vitamin C status goes be-yond base-pair changes in DNA sequence andmay play an important role in vitamin C ho-meostasis in development and aging.

FUTURE DIRECTIONS

Early studies of the human SLC23 family in-vestigated variations in these genes and chronicdisease risk or health outcomes. However,genetic variation is only a single variable amongmany factors affecting vitamin C homeostasis.We now know that these studies were limitedby the nature of these interactions that affectthe interplay between dietary, circulating, andtissue levels of vitamin C. Without assessmentof vitamin C status, either by measuringplasma, serum, or cellular ascorbic acid con-centrations, such genetic studies are likely toresult in a null outcome. Measuring dietary

intake of vitamin C is an inaccurate measureof vitamin C body status, due to human recallerror, questionnaire validity, and variationsin food sources, storage, and preparation thataffect vitamin C content (4, 19, 40). Futurestudies must rely only on direct measurementof circulating or cellular vitamin C levels toaccurately reflect vitamin C status. Only whenheld to this rigorous standard can we expectto find robust interactions between vitaminC–related genetic variation and disease risk.

Furthermore, the need for more mechanis-tic studies is apparent. For example, changesin GST isoforms appear to have an effect onvitamin C levels in some studies, but whetherthis is due to increased oxidative stress and,hence, depletion of ascorbate by free-radicalscavenging is not clear. More mechanisticstudies are needed to better understand theinteraction between GST activity and ascor-bate function. Although genetic changes inSLC23A1 present in the coding region of theSVCT1 protein have been linked to a declinein transport activity, we know little about theeffects of changes in the noncoding regions ofthese genes. This is particularly true for SNPsin SLC23A2, which show only synonymouscodon mutations, intronic polymorphisms, andchanges in the untranslated regions of the gene.

The human genetic variations covered inthis review are by no means an exhaustive list offactors that can influence vitamin C homeosta-sis. Partially, this is due to the manner in whichthese genes were discovered. In the variousstudies reviewed, genetic variants in HP, GSTs,and SOD2 were not associated a priori withascorbic acid but rather were examined becauseof their known role in oxidative stress. BecauseSNPs in each of these genes were found tobe related to ascorbate status, these findingsnot only confirm the role of vitamin C asa powerful antioxidant in the human bodybut also suggest that genetic variations ofother antioxidant or oxidative stress–relatedgenes may affect vitamin C status. For ex-ample, genes other than GSTs involved inglutathione homeostasis, such as glutathioneperoxidase (GPX), glutathione reductase (GR),


Ann

u. R

ev. N

utr.

201

3.33

:45-

70. D

ownl

oade

d fr

om w

ww

.ann

ualr

evie

ws.

org

by G

eorg

etow

n U

nive

rsity

on

08/1

1/13

. For

per

sona

l use

onl

y.


Mature-onsetdiabetes of the young(MODY): ahereditary form ofdiabetes mellitus.MODY3 is associatedwith mutations inHNF1A

glutamate-cysteine ligase (GCLC or GCLM), orthe transcription factor Nrf2 (NFE2L2), shouldbe examined. Interactions of polymorphismsin some of these genes with vitamin C (12,81) have already been proposed to affect lungfunction. Other obvious candidates to explorefor a possible role in vitamin C homeostasisinclude SOD1 and SOD3 and genes involved iniron and heme metabolism, such as transferrin(TF ), ferritin (FT ), and heme oxygenase (HO).Perhaps with a focus on changes in vitamin Chomeostasis, an exploration of human geneticvariation will reveal more genes and enzymesthat may influence ascorbate levels in the body.

Along these lines, we have only scratchedthe surface of the protein-gene or RNA inter-actions for many of the genes discussed. We donot know if polymorphisms in the untranslatedregions of the SLC23 genes lead to a changein gene transcription due to changes in tran-scription factor binding, for instance. On theother hand, polymorphisms in the transcrip-tion factors themselves might lead to a changein expression. A genetic alteration in HNF1α,found in patients with mature-onset diabetesof the young (MODY) 3, would be expected tolead to a substantial decline in SVCT1 synthesisand the renal threshold for vitamin C reabsorp-tion (63). As HNF4α is an upstream regulatingfactor of HNF1α, the former is also a potentialtarget for SVCT1 regulation. If indeed geneticvariants of HNF1A and HNF4A are found to

play a role in vitamin C homeostasis, this wouldsuggest that vitamin C is regulated in the bodyas a carbohydrate in addition to an antioxidant.This intriguing approach to vitamin C geneticregulation has never been examined.

What is the overall impact of human geneticvariation in light of the myriad of factors thataffect vitamin C homeostasis? The answer tothis question is not immediately forthcoming.However, it is becoming increasingly clear thatthe optimum intake of vitamin C to supportnormal physiological function and promotegood health differs widely between individ-uals and hence needs to be addressed on anindividual basis. Not only are vitamin C levelsinfluenced by age, diet, lifestyle, and variousenvironmental exposures, but also—as evidentfrom this review—by multiple genetic varia-tions. These genetic variations may modify therelationships between ascorbic acid and each ofthe other variables, raising intriguing new ques-tions. For example, do GST deletions affect cel-lular ascorbate levels differently in smokers andnonsmokers? Are haptoglobin polymorphismsan important determinant of vitamin C levelsin the presence of iron deficiency? Do changesin SLC23A2 result in an altered relationshipbetween dietary intake and plasma and tissuelevels of vitamin C? Only as our knowledgeof vitamin C homeostasis grows can we makethese more complex associations to determinetheir effects on human health and disease.

DISCLOSURE STATEMENT

The authors are not aware of any affiliations, memberships, funding, or financial holdings thatmight be perceived as affecting the objectivity of this review.

LITERATURE CITED

1. Ambrosone CB, Freudenheim JL, Thompson PA, Bowman E, Vena JE, et al. 1999. Manganese superoxidedismutase (MnSOD) genetic polymorphisms, dietary antioxidants, and risk of breast cancer. Cancer Res.59:602–6

2. Andrew AS, Gui J, Sanderson AC, Mason RA, Morlock EV, et al. 2009. Bladder cancer SNP panel predictssusceptibility and survival. Hum. Genet. 125:527–39

3. Asleh R, Levy AP. 2010. Divergent effects of alpha-tocopherol and vitamin C on the generation ofdysfunctional HDL associated with diabetes and the Hp 2-2 genotype. Antioxid. Redox Signal. 12:209–17


Ann

u. R

ev. N

utr.

201

3.33

:45-

70. D

ownl

oade

d fr

om w

ww

.ann

ualr

evie

ws.

org

by G

eorg

etow

n U

nive

rsity

on

08/1

1/13

. For

per

sona

l use

onl

y.


4. Birlouez-Aragon I, Saavedra G, Tessier FJ, Galinier A, Ait-Ameur L, et al. 2010. A diet based on high-heat-treated foods promotes risk factors for diabetes mellitus and cardiovascular diseases. Am. J. Clin.Nutr. 91:1220–26

5. Block G, Jensen CD, Dalvi TB, Norkus EP, Hudes M, et al. 2009. Vitamin C treatment reduces elevatedC-reactive protein. Free Radic. Biol. Med. 46:70–77

6. Block G, Shaikh N, Jensen CD, Volberg V, Holland N. 2011. Serum vitamin C and other biomarkersdiffer by genotype of phase 2 enzyme genes GSTM1 and GSTT1. Am. J. Clin. Nutr. 94:929–37

7. Boekholdt SM, Meuwese MC, Day NE, Luben R, Welch A, et al. 2006. Plasma concentrations of ascorbicacid and C-reactive protein, and risk of future coronary artery disease, in apparently healthy men andwomen: the EPIC-Norfolk prospective population study. Br. J. Nutr. 96:516–22

8. Boyer JC, Campbell CE, Sigurdson WJ, Kuo SM. 2005. Polarized localization of vitamin C transporters,SVCT1 and SVCT2, in epithelial cells. Biochem. Biophys. Res. Commun. 334:150–56

9. Cahill LE, El-Sohemy A. 2009. Vitamin C transporter gene polymorphisms, dietary vitamin C and serumascorbic acid. J. Nutrigenet. Nutrigenomics 2:292–301

10. Cahill LE, El-Sohemy A. 2010. Haptoglobin genotype modifies the association between dietary vitaminC and serum ascorbic acid deficiency. Am. J. Clin. Nutr. 92:1494–500

11. Cahill LE, Fontaine-Bisson B, El-Sohemy A. 2009. Functional genetic variants of glutathione S-transferaseprotect against serum ascorbic acid deficiency. Am. J. Clin. Nutr. 90:1411–17

12. Canova C, Dunster C, Kelly FJ, Minelli C, Shah PL, et al. 2012. PM10-induced hospital admissionsfor asthma and chronic obstructive pulmonary disease: the modifying effect of individual characteristics.Epidemiology 23:607–15

13. Carr AC, Frei B. 1999. Toward a new recommended dietary allowance for vitamin C based on antioxidantand health effects in humans. Am. J. Clin. Nutr. 69:1086–107

14. Chavez J, Chung WG, Miranda CL, Singhal M, Stevens JF, Maier CS. 2010. Site-specific protein adductsof 4-hydroxy-2(E)-nonenal in human THP-1 monocytic cells: Protein carbonylation is diminished byascorbic acid. Chem. Res. Toxicol. 23:37–47

15. Chen AA, Marsit CJ, Christensen BC, Houseman EA, McClean MD, et al. 2009. Genetic variationin the vitamin C transporter, SLC23A2, modifies the risk of HPV16-associated head and neck cancer.Carcinogenesis 30:977–81

16. Church SL, Grant JW, Meese EU, Trent JM. 1992. Sublocalization of the gene encoding manganesesuperoxide dismutase (MnSOD/SOD2) to 6q25 by fluorescence in situ hybridization and somatic cellhybrid mapping. Genomics 14:823–25

17. Creation of theSLC23A1 knockoutmouse, linking renalreabsorption to SVCT1function.

17. Corpe CP, Tu H, Eck P, Wang J, Faulhaber-Walter R, et al. 2010. Vitamin C transporter Slc23a1links renal reabsorption, vitamin C tissue accumulation, and perinatal survival in mice. J. Clin.Investig. 120:1069–83

18. Cox SE, Doherty C, Atkinson SH, Nweneka CV, Fulford AJ, et al. 2007. Haplotype association betweenhaptoglobin (Hp2) and Hp promoter SNP (A-61C) may explain previous controversy of haptoglobin andmalaria protection. PLoS ONE 2:e362

19. Dehghan M, Akhtar-Danesh N, McMillan CR, Thabane L. 2007. Is plasma vitamin C an appropriatebiomarker of vitamin C intake? A systematic review and meta-analysis. Nutr. J. 6:41

20. Delanghe J, Langlois M, Duprez D, De Buyzere M, Clement D. 1999. Haptoglobin polymorphism andperipheral arterial occlusive disease. Atherosclerosis 145:287–92

21. Delanghe JR, Langlois MR, Boelaert JR, Van Acker J, Van Wanzeele F, et al. 1998. Haptoglobin poly-morphism, iron metabolism and mortality in HIV infection. AIDS 12:1027–32

22. Dherani M, Murthy GV, Gupta SK, Young IS, Maraini G, et al. 2008. Blood levels of vitamin C,carotenoids and retinol are inversely associated with cataract in a North Indian population. Investig.Ophthalmol. Vis. Sci. 49:3328–35

23. Duarte MM, Moresco RN, Duarte T, Santi A, Bagatini MD, et al. 2010. Oxidative stress in hypercholes-terolemia and its association with Ala16Val superoxide dismutase gene polymorphism. Clin. Biochem.43:1118–23

24. Duarte-Salles T, Mendez MA, Morales E, Bustamante M, Rodriguez-Vicente A, et al. 2012. Dietarybenzo(a)pyrene and fetal growth: effect modification by vitamin C intake and glutathione S-transferaseP1 polymorphism. Environ. Int. 45:1–8


Ann

u. R

ev. N

utr.

201

3.33

:45-

70. D

ownl

oade

d fr

om w

ww

.ann

ualr

evie

ws.

org

by G

eorg

etow

n U

nive

rsity

on

08/1

1/13

. For

per

sona

l use

onl

y.


25. First association ofGST polymorphismswith ascorbic acid levelsin humans.

25. Dusinska M, Ficek A, Horska A, Raslova K, Petrovska H, et al. 2001. Glutathione S-transferasepolymorphisms influence the level of oxidative DNA damage and antioxidant protection in hu-mans. Mutat. Res. 482:47–55

26. An in-depth geneticcharacterization ofSLC23A1 and SLC23A2,including the firstmention of geneticvariants.

26. Eck P, Erichsen HC, Taylor JG, Yeager M, Hughes AL, et al. 2004. Comparison of the genomicstructure and variation in the two human sodium-dependent vitamin C transporters, SLC23A1and SLC23A2. Hum. Genet. 115:285–94

27. Erichsen HC, Engel SA, Eck PK, Welch R, Yeager M, et al. 2006. Genetic variation in thesodium-dependent vitamin C transporters, SLC23A1, and SLC23A2 and risk for preterm delivery.Am. J. Epidemiol. 163:245–54

28. Erichsen HC, Peters U, Eck P, Welch R, Schoen RE, et al. 2008. Genetic variation in sodium-dependentvitamin C transporters SLC23A1 and SLC23A2 and risk of advanced colorectal adenoma. Nutr. Cancer60:652–59

29. Fischer H, Schwarzer C, Illek B. 2004. Vitamin C controls the cystic fibrosis transmembrane conductanceregulator chloride channel. Proc. Natl. Acad. Sci. USA 101:3691–96

30. Extensive review ofthe factors involved invitamin C research inhumans, includingevaluation of currentRDAs.

30. Frei B, Birlouez-Aragon I, Lykkesfeldt J. 2012. Authors’ perspective: What is the optimum intakeof vitamin C in humans? Crit. Rev. Food Sci. Nutr. 52:815–29

31. Frikke-Schmidt H, Lykkesfeldt J. 2009. Role of marginal vitamin C deficiency in atherogenesis: in vivomodels and clinical studies. Basic Clin. Pharmacol. Toxicol. 104:419–33

32. Gale CR, Martyn CN, Winter PD, Cooper C. 1995. Vitamin C and risk of death from stroke and coronaryheart disease in cohort of elderly people. BMJ 310:1563–66

33. Gamazon ER, Ziliak D, Im HK, LaCroix B, Park DS, et al. 2012. Genetic architecture of microRNAexpression: implications for the transcriptome and complex traits. Am. J. Hum. Genet. 90:1046–63

34. Gao P, Zhang H, Dinavahi R, Li F, Xiang Y, et al. 2007. HIF-dependent antitumorigenic effect ofantioxidants in vivo. Cancer Cell 12:230–38

35. Goldenberg H, Schweinzer E. 1994. Transport of vitamin C in animal and human cells. J. Bioenerg.Biomembr. 26:359–67

36. Goldenstein H, Levy NS, Levy AP. 2012. Haptoglobin genotype and its role in determining heme-ironmediated vascular disease. Pharmacol. Res. 66:1–6

37. Han J, Colditz GA, Hunter DJ. 2007. Manganese superoxide dismutase polymorphism and risk of skincancer (United States). Cancer Causes Control 18:79–89

38. Haque R, Chun E, Howell JC, Sengupta T, Chen D, Kim H. 2012. MicroRNA-30b-mediated regulationof catalase expression in human ARPE-19 cells. PLoS ONE 7:e4 2542

39. Hayes JD, Flanagan JU, Jowsey IR. 2005. Glutathione transferases. Annu. Rev. Pharmacol. Toxicol. 45:51–88

40. Henriquez-Sanchez P, Sanchez-Villegas A, Doreste-Alonso J, Ortiz-Andrellucchi A, Pfrimer K, Serra-Majem L. 2009. Dietary assessment methods for micronutrient intake: a systematic review on vitamins.Br. J. Nutr. 102(Suppl. 1):S10–37

41. Higasa S, Tsujimura M, Hiraoka M, Nakayama K, Yanagisawa Y, et al. 2007. Polymorphism of glutathioneS-transferase P1 gene affects human vitamin C metabolism. Biochem. Biophys. Res. Commun. 364:708–13

42. Horska A, Mislanova C, Bonassi S, Ceppi M, Volkovova K, Dusinska M. 2011. Vitamin C levels in bloodare influenced by polymorphisms in glutathione S-transferases. Eur. J. Nutr. 50:437–46

43. Jenab M, Riboli E, Ferrari P, Sabate J, Slimani N, et al. 2006. Plasma and dietary vitamin Clevels and risk of gastric cancer in the European Prospective Investigation into Cancer and Nutrition(EPIC-EURGAST). Carcinogenesis 27:2250–57

44. Juraschek SP, Guallar E, Appel LJ, Miller ER 3rd. 2012. Effects of vitamin C supplementation on bloodpressure: a meta-analysis of randomized controlled trials. Am. J. Clin. Nutr. 95:1079–88

45. Kang JS, Kim HN, Jung DJ, Kim JE, Mun GH, et al. 2007. Regulation of UVB-induced IL-8 andMCP-1 production in skin keratinocytes by increasing vitamin C uptake via the redistribution ofSVCT-1 from the cytosol to the membrane. J. Investig. Dermatol. 127:698–706

46. Kasvosve I, Delanghe JR, Gomo ZA, Gangaidzo IT, Khumalo H, et al. 2002. Effect of transferrin poly-morphism on the metabolism of vitamin C in Zimbabwean adults. Am. J. Clin. Nutr. 75:321–25


Ann

u. R

ev. N

utr.

201

3.33

:45-

70. D

ownl

oade

d fr

om w

ww

.ann

ualr

evie

ws.

org

by G

eorg

etow

n U

nive

rsity

on

08/1

1/13

. For

per

sona

l use

onl

y.


47. Khaw KT, Bingham S, Welch A, Luben R, Wareham N, et al. 2001. Relation between plasma ascorbicacid and mortality in men and women in EPIC-Norfolk prospective study: a prospective population study.European Prospective Investigation into Cancer and Nutrition. Lancet 357:657–63

48. Klepper J, Vera JC, De Vivo DC. 1998. Deficient transport of dehydroascorbic acid in the glucosetransporter protein syndrome. Ann. Neurol. 44:286–87

49. Klepper J, Wang D, Fischbarg J, Vera JC, Jarjour IT, et al. 1999. Defective glucose transport across braintissue barriers: a newly recognized neurological syndrome. Neurochem. Res. 24:587–94

50. Kumar M, Lu Z, Takwi AA, Chen W, Callander NS, et al. 2011. Negative regulation of the tumorsuppressor p53 gene by microRNAs. Oncogene 30:843–53

51. First associationwith the Hp2-2phenotype anddecreased levels ofplasma ascorbate.

51. Langlois MR, Delanghe JR, De Buyzere ML, Bernard DR, Ouyang J. 1997. Effect of haptoglobinon the metabolism of vitamin C. Am. J. Clin. Nutr. 66:606–10

52. Lee YC, Huang HY, Chang CJ, Cheng CH, Chen YT. 2010. Mitochondrial GLUT10 facilitates de-hydroascorbic acid import and protects cells against oxidative stress: mechanistic insight into arterialtortuosity syndrome. Hum. Mol. Genet. 19:3721–33

53. Levine M, Conry-Cantilena C, Wang Y, Welch RW, Washko PW, et al. 1996. Vitamin C pharmacoki-netics in healthy volunteers: evidence for a recommended dietary allowance. Proc. Natl. Acad. Sci. USA93:3704–9

54. Levine M, Wang Y, Padayatty SJ, Morrow J. 2001. A new recommended dietary allowance of vitamin Cfor healthy young women. Proc. Natl. Acad. Sci. USA 98:9842–46

55. Levy AP, Asleh R, Blum S, Levy NS, Miller-Lotan R, et al. 2010. Haptoglobin: basic and clinical aspects.Antioxid. Redox Signal. 12:293–304

56. Lykkesfeldt J, Poulsen HE. 2010. Is vitamin C supplementation beneficial? Lessons learned from ran-domised controlled trials. Br. J. Nutr. 103:1251–59

57. Macias RI, Hierro C, de Juan SC, Jimenez F, Gonzalez-San Martin F, Marin JJ. 2011. Hepatic expressionof sodium-dependent vitamin C transporters: ontogeny, subtissular distribution and effect of chronic liverdiseases. Br. J. Nutr. 106:1814–25

58. Maulen NP, Henriquez EA, Kempe S, Carcamo JG, Schmid-Kotsas A, et al. 2003. Up-regulation andpolarized expression of the sodium-ascorbic acid transporter SVCT1 in post-confluent differentiatedCaCo-2 cells. J. Biol. Chem. 278:9035–41

59. May JM, Qu ZC, Qiao H, Koury MJ. 2007. Maturational loss of the vitamin C transporter in erythrocytes.Biochem. Biophys. Res. Commun. 360:295–98

60. McIlwain CC, Townsend DM, Tew KD. 2006. Glutathione S-transferase polymorphisms: cancer inci-dence and therapy. Oncogene 25:1639–48

61. Meredith ME, Harrison FE, May JM. 2011. Differential regulation of the ascorbic acid transporter SVCT2during development and in response to ascorbic acid depletion. Biochem. Biophys. Res. Commun. 414:737–42

62. A broad overview ofvitamin C, with a reviewof ascorbate chemistry,biology, nutrition, andhealth effects ofsupplementation.

62. Michels AJ, Frei BB. 2012. Vitamin C. In Biochemical, Physiological, and Molecular Aspects of HumanNutrition, ed. MH Stipanuk, MA Caudill, pp. 626–54. St. Louis, Mo.: Elsevier/Saunders

63. Michels AJ, Hagen TM. 2009. Hepatocyte nuclear factor 1 is essential for transcription of sodium-dependent vitamin C transporter protein 1. Am. J. Physiol. Cell Physiol. 297:C1220–27

64. Miranda CL, Reed RL, Kuiper HC, Alber S, Stevens JF. 2009. Ascorbic acid promotes detoxification andelimination of 4-hydroxy-2(E)-nonenal in human monocytic THP-1 cells. Chem. Res. Toxicol. 22:863–74

65. Myint PK, Luben RN, Welch AA, Bingham SA, Wareham NJ, Khaw KT. 2008. Plasma vitamin Cconcentrations predict risk of incident stroke over 10 y in 20 649 participants of the European ProspectiveInvestigation into Cancer Norfolk prospective population study. Am. J. Clin. Nutr. 87:64–69

66. Na N, Delanghe JR, Taes YE, Torck M, Baeyens WR, Ouyang J. 2006. Serum vitamin C concentrationis influenced by haptoglobin polymorphism and iron status in Chinese. Clin. Chim. Acta 365:319–24

67. Park KU, Song J, Kim JQ. 2004. Haptoglobin genotypic distribution (including Hp0 allele) and associatedserum haptoglobin concentrations in Koreans. J. Clin. Pathol. 57:1094–95

68. Pfister R, Sharp SJ, Luben R, Wareham NJ, Khaw KT. 2011. Plasma vitamin C predicts incident heartfailure in men and women in European Prospective Investigation into Cancer and Nutrition—Norfolkprospective study. Am. Heart. J. 162:246–53


Ann

u. R

ev. N

utr.

201

3.33

:45-

70. D

ownl

oade

d fr

om w

ww

.ann

ualr

evie

ws.

org

by G

eorg

etow

n U

nive

rsity

on

08/1

1/13

. For

per

sona

l use

onl

y.


69. Portugal CC, da Encarnacao TG, Socodato R, Moreira SR, Brudzewsky D, et al. 2012. Nitric oxidemodulates sodium vitamin C transporter 2 (SVCT-2) protein expression via protein kinase G (PKG) andnuclear factor-κB (NF-κB). J. Biol. Chem. 287:3860–72

70. Qiao H, May JM. 2009. Macrophage differentiation increases expression of the ascorbate transporter(SVCT2). Free Radic. Biol. Med. 46:1221–32

71. Qiao H, May JM. 2011. CpG methylation at the USF-binding site mediates cell-specific transcription ofhuman ascorbate transporter SVCT2 exon 1a. Biochem. J. 440:73–84

72. Qiao H, May JM. 2011. Regulation of the human ascorbate transporter SVCT2 exon 1b gene by zinc-finger transcription factors. Free Radic. Biol. Med. 50:1196–209

73. Rajan DP, Huang W, Dutta B, Devoe LD, Leibach FH, et al. 1999. Human placental sodium-dependentvitamin C transporter (SVCT2): molecular cloning and transport function. Biochem. Biophys. Res.Commun. 262:762–68

74. Ravindran RD, Vashist P, Gupta SK, Young IS, Maraini G, et al. 2011. Inverse association of vitamin Cwith cataract in older people in India. Ophthalmology 118:1958–65.e2

75. Reidling JC, Rubin SA. 2011. Promoter analysis of the human ascorbic acid transporters SVCT1 and 2:mechanisms of adaptive regulation in liver epithelial cells. J. Nutr. Biochem. 22:344–50

76. Rubin SA, Dey S, Reidling JC. 2005. Functional analysis of two regulatory regions of the human Na+-dependent vitamin C transporter 2, SLC23A2, in human vascular smooth muscle cells. Biochim. Biophys.Acta 1732:76–81

77. Rumsey SC, Daruwala R, Al-Hasani H, Zarnowski MJ, Simpson IA, Levine M. 2000. Dehydroascorbicacid transport by GLUT4 in Xenopus oocytes and isolated rat adipocytes. J. Biol. Chem. 275:28246–53

78. Rumsey SC, Kwon O, Xu GW, Burant CF, Simpson I, Levine M. 1997. Glucose transporter isoformsGLUT1 and GLUT3 transport dehydroascorbic acid. J. Biol. Chem. 272:18982–89

79. Savini I, Catani MV, Arnone R, Rossi A, Frega G, et al. 2007. Translational control of the ascorbic acidtransporter SVCT2 in human platelets. Free Radic. Biol. Med. 42:608–16

80. Savini I, Rossi A, Catani MV, Ceci R, Avigliano L. 2007. Redox regulation of vitamin C transporterSVCT2 in C2C12 myotubes. Biochem. Biophys. Res. Commun. 361:385–90

81. Siedlinski M, Postma DS, van Diemen CC, Blokstra A, Smit HA, Boezen HM. 2008. Lung function loss,smoking, vitamin C intake, and polymorphisms of the glutamate-cysteine ligase genes. Am. J. Respir. Crit.Care Med. 178:13–19

82. Skibola CF, Bracci PM, Halperin E, Nieters A, Hubbard A, et al. 2008. Polymorphisms in the estrogenreceptor 1 and vitamin C and matrix metalloproteinase gene families are associated with susceptibility tolymphoma. PLoS ONE 3:e2816

83. Development of theSVCT2 knockoutmouse, originally namedSLC23A1, linkingperinatal survival withascorbate levels.

83. Sotiriou S, Gispert S, Cheng J, Wang Y, Chen A, et al. 2002. Ascorbic-acid transporter Slc23a1is essential for vitamin C transport into the brain and for perinatal survival. Nat. Med. 8:514–17

84. Sutton A, Imbert A, Igoudjil A, Descatoire V, Cazanave S, et al. 2005. The manganese superoxide dismutaseAla16Val dimorphism modulates both mitochondrial import and mRNA stability. Pharmacogenet. Genomics15:311–19

85. First association ofSLC23A1polymorphisms withplasma or serumascorbate levels inhuman volunteers.

85. Timpson NJ, Forouhi NG, Brion MJ, Harbord RM, Cook DG, et al. 2010. Genetic variation atthe SLC23A1 locus is associated with circulating concentrations of L-ascorbic acid (vitamin C):evidence from 5 independent studies with >15,000 participants. Am. J. Clin. Nutr. 92:375–82

86. Firstcharacterization of thegenes SLC23A1 andSLC23A2 and theirfunction as vitamin Ctransporters.

86. Tsukaguchi H, Tokui T, Mackenzie B, Berger UV, Chen XZ, et al. 1999. A family of mammalianNa+-dependent L-ascorbic acid transporters. Nature 399:70–75

87. Valero MP, Fletcher AE, De Stavola BL, Vioque J, Alepuz VC. 2002. Vitamin C is associated with reducedrisk of cataract in a Mediterranean population. J. Nutr. 132:1299–306

88. Vardi M, Levy AP. 2012. Is it time to screen for the haptoglobin genotype to assess the cardiovascularrisk profile and vitamin E therapy responsiveness in patients with diabetes? Curr. Diab. Rep. 12:274–79

89. Vera JC, Rivas CI, Fischbarg J, Golde DW. 1993. Mammalian facilitative hexose transporters mediatethe transport of dehydroascorbic acid. Nature 364:79–82


Ann

u. R

ev. N

utr.

201

3.33

:45-

70. D

ownl

oade

d fr

om w

ww

.ann

ualr

evie

ws.

org

by G

eorg

etow

n U

nive

rsity

on

08/1

1/13

. For

per

sona

l use

onl

y.


90. Voko Z, Hollander M, Hofman A, Koudstaal PJ, Breteler MM. 2003. Dietary antioxidants and the riskof ischemic stroke: the Rotterdam Study. Neurology 61:1273–75

91. Wang H, Dutta B, Huang W, Devoe LD, Leibach FH, et al. 1999. Human Na(+)-dependent vitamin Ctransporter 1 (hSVCT1): primary structure, functional characteristics and evidence for a non-functionalsplice variant. Biochim. Biophys. Acta 1461:1–9

92. Wilms LC, Claughton TA, de Kok TM, Kleinjans JC. 2007. GSTM1 and GSTT1 polymorphism in-fluences protection against induced oxidative DNA damage by quercetin and ascorbic acid in humanlymphocytes in vitro. Food Chem. Toxicol. 45:2592–96

93. Wilson JX. 2005. Regulation of vitamin C transport. Annu. Rev. Nutr. 25:105–2594. Wright ME, Andreotti G, Lissowska J, Yeager M, Zatonski W, et al. 2009. Genetic variation in sodium-

dependent ascorbic acid transporters and risk of gastric cancer in Poland. Eur. J. Cancer 45:1824–3095. Ye Z, Song H. 2008. Antioxidant vitamins intake and the risk of coronary heart disease: meta-analysis of

cohort studies. Eur. J. Cardiovasc. Prev. Rehabil. 15:26–3496. Yuan LH, Meng LP, Ma WW, Li S, Feng JF, et al. 2012. The role of glutathione S-transferase M1 and T1

gene polymorphisms and fruit and vegetable consumption in antioxidant parameters in healthy subjects.Br. J. Nutr. 107:928–33

97. Zanon-Moreno V, Ciancotti-Olivares L, Asencio J, Sanz P, Ortega-Azorin C, et al. 2011. Associationbetween a SLC23A2 gene variation, plasma vitamin C levels, and risk of glaucoma in a Mediterraneanpopulation. Mol. Vis. 17:2997–3004

98. Zhou H, Brock J, Liu D, Board PG, Oakley AJ. 2012. Structural insights into the dehydroascorbatereductase activity of human omega-class glutathione transferases. J. Mol. Biol. 420:190–203


Ann

u. R

ev. N

utr.

201

3.33

:45-

70. D

ownl

oade

d fr

om w

ww

.ann

ualr

evie

ws.

org

by G

eorg

etow

n U

nive

rsity

on

08/1

1/13

. For

per

sona

l use

onl

y.

NU33-FrontMatter ARI 20 June 2013 16:12

Annual Review ofNutrition

Volume 33, 2013Contents

Functional Organization of Neuronal and Humoral Signals RegulatingFeeding BehaviorGary J. Schwartz and Lori M. Zeltser � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 1

Extrarenal Vitamin D Activation and Interactions BetweenVitamin D2, Vitamin D3, and Vitamin D AnalogsGlenville Jones � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �23

Human Genetic Variation Influences Vitamin C Homeostasis byAltering Vitamin C Transport and Antioxidant Enzyme FunctionAlexander J. Michels, Tory M. Hagen, and Balz Frei � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �45

Vitamin D Biology Revealed Through the Study of Knockout andTransgenic Mouse ModelsSylvia Christakos, Tanya Seth, Jennifer Hirsch, Angela Porta, Anargyros Moulas,

and Puneet Dhawan � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �71

Vitamin E Trafficking in Neurologic Health and DiseaseLynn Ulatowski and Danny Manor � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �87

Cocoa and Human HealthSamantha Ellam and Gary Williamson � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 105

Novel Aspects of Dietary Nitrate and Human HealthEddie Weitzberg and Jon O. Lundberg � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 129

Prevention of Chronic Diseases by Tea: Possible Mechanismsand Human RelevanceChung S. Yang and Jungil Hong � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 161

Nutrient Deficiencies After Gastric Bypass SurgeryEdward Saltzman and J. Philip Karl � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 183

O-GlcNAc Cycling: A Link Between Metabolism and Chronic DiseaseMichelle R. Bond and John A. Hanover � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 205

Omega-3 Fatty Acids, Hepatic Lipid Metabolism, and NonalcoholicFatty Liver DiseaseE. Scorletti and C.D. Byrne � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 231

Fatty Acid–Regulated Transcription Factors in the LiverDonald B. Jump, Sasmita Tripathy, and Christopher M. Depner � � � � � � � � � � � � � � � � � � � � � � � 249

v

Ann

u. R

ev. N

utr.

201

3.33

:45-

70. D

ownl

oade

d fr

om w

ww

.ann

ualr

evie

ws.

org

by G

eorg

etow

n U

nive

rsity

on

08/1

1/13

. For

per

sona

l use

onl

y.

NU33-FrontMatter ARI 20 June 2013 16:12

Iron Nutrition and Premenopausal Women: Effects of Poor IronStatus on Physical and Neuropsychological PerformanceJames P. McClung and Laura E. Murray-Kolb � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 271

Evolutionary Perspectives on the Obesity Epidemic: Adaptive,Maladaptive, and Neutral ViewpointsJohn R. Speakman � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 289

Dietary Fat in Breast Cancer SurvivalNour Makarem, Urmila Chandran, Elisa V. Bandera, and Niyati Parekh � � � � � � � � � � � � 319

Quantifying Diet for Nutrigenomic StudiesKatherine L. Tucker, Caren E. Smith, Chao-Qiang Lai, and Jose M. Ordovas � � � � � � � � 349

The Role of Cost-Effectiveness Analysis in DevelopingNutrition PolicyLinda J. Cobiac, Lennert Veerman, and Theo Vos � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 373

African Lessons on Climate Change Risks for AgricultureChristoph Muller � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 395

Making, Baking, and Breaking: The Synthesis, Storage, and Hydrolysisof Neutral LipidsKelly V. Ruggles, Aaron Turkish, and Stephen L. Sturley � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 413

Indexes

Cumulative Index of Contributing Authors, Volumes 29–33 � � � � � � � � � � � � � � � � � � � � � � � � � � � 453

Cumulative Index of Article Titles, Volumes 29–33 � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 456

Errata

An online log of corrections to Annual Review of Nutrition articles may be found athttp://nutr.annualreviews.org/errata.shtml

vi Contents

Ann

u. R

ev. N

utr.

201

3.33

:45-

70. D

ownl

oade

d fr

om w

ww

.ann

ualr

evie

ws.

org

by G

eorg

etow

n U

nive

rsity

on

08/1

1/13

. For

per

sona

l use

onl

y.

Human Genetic Variation Influences Vitamin C Homeostasis by Altering Vitamin C Transport and...

Documents

Transcript of Human Genetic Variation Influences Vitamin C Homeostasis by Altering Vitamin C Transport and...