Low birth weightZahra N. SohaniSupervisor: Dr. Sonia Anand

Outline of presentation• Background & brief methods of project

• Issues to consider for analysis

Low birth weight & type 2 diabetes – Epidemiological evidence

Hales & Barker reported an increase in incidence of IGT and T2DM in children born small

Systematic review of 31 studies (n=6090 diabetes cases) showed a 25% reduction in odds of T2DM per kg of weight gained

Another study comparing birth weight <2.5 kg with ≥2.5 kg found a 32% increase in odds of T2DM for those born with low birth weight

The hypothesesImpaired beta cell

function

fetal period is critical for pancreatic development

impairment in beta cell development leads to reduced insulin secretion, which results in reduced fetal growth

Beta cell genes are also associated with birth weight

Studies of human fetuses with growth restrictions have reported reduced pancreatic endocrine cell mass

Insulin resistance

in utero perturbations cause the fetus to develop peripheral insulin resistance, to divert nutrients to essential organs

results in permanent reductions in skeletal muscle glucose transporter number and/or function, causing extensive insulin secretion from the pancreas early in childhood, but eventual beta cell exhaustion

Children with IUGR show greater insulin response than those born with normal birth weight

Is there support for epigenetics?

Methylation is one of the commonly studied epigenetic marks addition of a methyl group to the 5-carbon position of a

cytosine residue located next to guanine

1. methylation can lead to the binding of methylated CpG binding proteins and transcriptional repressors to the methyl group which block transcription factor access

2. presence of the methyl group itself can inhibit binding of necessary transcription factors to initiate the process via steric hindrance

DNA methylation is erased throughout the genome and then re-established during embryogenesis & gametogenesis

Allows for ‘soft inheritance’

Human studies Inheritance of MLH1 epimutation (linked to certain

cancers) Germline epimutation of MSH2 in a family with

hereditary nonpolyposis colorectal cancer

Animal models model of rats demonstrated persistence of LBW for

three generations after re-introduction of normal nourishment to offsprings of malnourished mothers

QuestionsAre there differentially methylated regions

associated with birth weight?

Is there a difference in these regions between South Asians and white Europeans?

Cardio-metabolic profile in South Asians

South Asians have greater visceral adiposity, and experience metabolic abnormalities at lower a BMI and younger age than Europeans

South Asian babies show a thin-fat phenotype

South Asian newborns are born lighter even after generations in ‘western’ countries

Brief methods Phenotypes: birth weight, birth length, and percent

body fat

Genotypes: SNPs involved in beta cell function, insulin resistance, birth weight, adiposity, birth length, percent body fat, type 2 diabetes, and obesity in newborns and adults

Methylation/Expression: Differentially methylated regions for SNPs involved in beta cell function, insulin resistance, birth weight, adiposity, birth length, percent body fat, type 2 diabetes, and obesity

Look for gene expression differences if any DMRs are significantly associated with phenotype

Some current evidence

Three significant methylation studies investigating birth weight

Engel (2014) report Bonferroni-corrected associations for 19 CpGs with birth weight

Tarun (2012) identified 23 genes for which methylation collectively explained 70-87% of the variance in birth weight

Gordon (2012) found methylation in 8 genes to be associated with birth weight after correction for multiple testing

Issues to consider

Beta values versus M-values

Du BMC Bioinformatics 2010

Titration experiment of two samples known to be differentially methylated: 100:0, 90:10, 75:25, 50:50, 0:100

Methylation profile as established by beta values and M-values should correlate to titration profile

Beta-value method has a direct biological interpretation - it corresponds roughly to the percentage of a site that is methylated

M-value method is more statistically valid, but does not have an intuitive biological meaning