Inborn errors of metabolism

By : - Dr. Sanjeev

Phenylketonuria (PKU)• It is an autosomal recessive (two copies of an

abnormal gene) genetic disorder characterized by a deficiency in the hepatic enzyme phenylalanine hydroxylase (PAH).

• Necessary to metabolize the amino acid phenylalanine to the amino acid tyrosine.

• When PAH is deficient, phenylalanine accumulates and is converted into phenylpyruvic acid, phenyl – lactic acid and phenyl – acetic acid (also known as phenylketone), which is detected in the urine.

• Elevated blood phenylalanine and detection of phenylketones in the urine is diagnostic.

Metabolic pathways ---- Phenylalanine is a large, neutral amino acid

(LNAA). LNAAs compete for transport across the blood-brain barrier (BBB) via the large neutral amino acid transporter (LNAAT).

---- Excessive phenylalanine in the blood saturates the transporter. Thus, excessive levels of phenylalanine significantly decrease the levels of other LNAAs in the brain.

---- But these amino acids are required for protein and neurotransmitter synthesis, phenylalanine accumulation disrupts brain development, leading to mental retardation.

Symptoms Symptoms can be mild or severe :

– Mental retardation – Behavioral or social problems – Seizures, tremors or jerking movements in the arms

and legs – Hyperactivity – Stunted growth – Skin rashes (eczema) – Small head size (microcephaly) – Vomiting – A musty (smelling of decay) odor in the child's breath,

skin or urine, caused by too much phenylalanine in the body

– Fair skin and blue eyes, because phenylalanine cannot transform into melanin — the pigment responsible for hair and skin

Diagnosis

Criteria for PKU are : 1. On a normal diet, phenylalanine level in

excess of 20 mg/ dL/24 hours on two occasions

2. Blood tyrosine level more than 5 mg/dL and

3. Presence of abnormal urinary metabolites of phenylalanine detected by guthrie or ferric chloride test.

Treatment • Goal of PKU treatment is to maintain the blood

level of phenylalanine between 2 and 10 mg/dl – Mental retardation can be prevented if the baby is treated

with a special diet that is low in phenylalanine. This diet should be started as soon as possible after birth, ideally within the first seven to 10 days of life . 

– At first, the baby is fed a special formula that contains protein but no phenylalanine. Breast milk or infant formula is used to supply only as much phenylalanine as the baby needs and can tolerate.

– Later, certain vegetables, fruits, some grain products (for example, certain cereals and noodles) and other low-phenylalanine foods are added to the diet.

Cont..• No regular milk, cheese, eggs, meat, fish and other high

protein foods are ever allowed. Because protein is essential for normal growth and development, the child must continue to have one of the special formulas that is high in protein and essential nutrients, but contains little or no phenylalanine.

• Diet drinks and foods that contain the artificial sweetener (which contains phenylalanine) must be strictly avoided.

• All affected persons need regular blood tests to measure phenylalanine levels. Testing for babies may be as frequent as once a week for the first year of life, and then once or twice a month throughout childhood.

• Individuals with PKU must remain on a restricted diet throughout childhood and adolescence and generally for life.

Drugs used to manage PKU • Kuvan (sapropterin dihydrocholoride), the first

drug to help manage PKU . • Reduce blood phenylalanine levels in individuals

with PKU by increasing the activity of the PAH enzyme.

Kuvan is effective only in individuals who have some PAH activity. Individuals who take this drug must continue to follow a phenylalanine-restricted diet and have blood tests to measure phenylalanine levels.

New in PKU research

---- Researchers also are studying the benefits of a nutritional supplement called Tetrahydrobiopterin (BH4) in individuals with PKU.

---- Researchers also are exploring the possibility of treating PKU using gene therapy.

Cystinuria

Synonyms – Cistinuria – Cystine-Lysine-Arginine-Ornithinuria – Cystinuria with Dibasic Aminoaciduria

Disorder Subdivisions– Cystinuria, Type I – Cystinuria, Type II – Cystinuria, Type III – Hypercystinuria

Cystinuria– It is an inherited (get it from one's ancestors)

metabolic disorder characterized by the abnormal movement (transport) in the intestines and kidneys, of certain amino acids. These include cystine, lysine, arginine, and ornithine (LACO).

– Excessive amounts of undissolved cystine in the urine (cystinuria) cause the formation of stones (calculi) in the kidney, bladder, and/or ureter.

Cont.. Four subtypes of Cystinuria are

recognized. In Type I Cystinuria:

– Defect in the active transport of cystine and the dibasic amino acids lysine, arginine, and ornithine in the kidneys and small intestine.

– People who are carriers of the gene for this type of the disorder generally have no symptoms.

In Type II Cystinuria: – Cystine and lysine transport is severely impaired

in the kidneys and only somewhat impaired in the intestines.

Cont.. In Type III Cystinuria:

– Kidney transport of cystine and lysine is defective; intestinal transport is normal.

– People who are carriers of the gene for this variety of the disease typically have slightly elevated levels of cystine and lysine in the urine.

In Hypercystinuria:– There is generally a moderate elevation of

cystine in the urine; intestinal absorption of cystine and the dibasic amino acids is normal.

Causes– Cystinuria is caused by excessive levels of an amino

acid called cystine in the urine. – After entering the kidneys, most cystine normally

dissolves and goes back into the bloodstream. But persons with cystinuria have a genetic defect that interferes with this process. As a result, cystine builds up in the urine and forms crystals or stones, which may get stuck in the kidneys, ureters, or bladder.

– Cystinuria affects approximately 1 out of 10,000 people.

– Cystine stones are most common in young adults under age 40.

– Less than 3% of known urinary tract stones are cystine stones.

Cont..

• The high levels of the amino acid cystine in the urine lead to stone formation.

Symptoms

• Blood in the urine

• Flank pain or pain in the side or back – Usually on one side; rarely felt on both sides – Often severe – May get increasingly worse over days – Pain may also be felt in the pelvis, groin,

genitals, or between the upper abdomen and the back.

Exams and Tests

• The disorder is usually diagnosed after an episode of stones. Analysis of the stones shows they are made of cystine.

• Tests that may be done to detect stones and diagnose this condition include:– Abdominal CT scan, MRI, or ultrasound – Intravenous pyelogram (IVP) – 24-hour urine collection (shows high levels of cystine) – Urinalysis (may show cystine crystals)

Treatment Goal of treatment: • to relieve symptoms and prevent the

development of more stones. • Patient with severe symptoms may need to be

admitted to a hospital.

Treatment : - Involves drinking plenty of fluids, particularly water,

so that large amounts of urine are produced. - In some cases, fluids may need to be given through a

vein.– Medications may be prescribed to help dissolve the

cystine crystals. – Analgesics

• Surgery may be needed. Lithotripsy

Complications

– Bladder injury from stone – Ureteral obstruction – Kidney injury from stone – Urinary tract infection

Prevention

– Plenty of fluids to regularly produce a high amount of urine (allows stones and crystals to leave the body before they become large enough to cause symptoms).

Gaucher's disease

• It is an metabolic disorder with autosomal recessive inheritance that is caused by a deficiency in an enzyme called glucocerebrosidase which splits glucose from glucosylceramide.

• Enzyme acts on a fatty substance glucocerebroside (also known as glucosylceramide)

Cont..

Three types:

Type 1 : • Liver, spleen, bone involvement.

Type 2: • CNS involvement

Type 3:• Liver, spleen and CNS involvement

. Type I (or non-neuropathic type) - most common form. • Symptoms • Age: early in life or in adulthood • enlarged liver and grossly enlarged spleen (hepatosplenomegaly); the

spleen can rupture and cause additional complications. • Skeletal weakness and bone disease may be extensive. • Spleen enlargement and bone marrow replacement cause anemia,

thrombocytopenia and leukopenia. • Brain is not affected, but there may be lung and, rarely, kidney

impairment. • Bruise (due to low levels of platelets) and experience fatigue (due to

low numbers of red blood cells). • Depending on disease onset and severity, type 1 patients may live well

into adulthood. • Many patients have a mild form of the disease or may not show any

symptoms.

Cont..Type II (or acute infantile neuropathic Gaucher's disease) - Age :- typically begins within 6 months of birth • Symptoms include an enlarged liver and spleen, extensive

and progressive brain damage, eye movement disorders, spasticity, seizures, limb rigidity, and a poor ability to suck and swallow.

• Affected children usually die by age 2.

Type III (the chronic neuropathic form) - Age :- at any time in childhood or even in adulthood • It is characterized by slowly progressive but milder

neurologic symptoms compared to the acute or type 2 version.

• Major symptoms include an enlarged spleen and/or liver, seizures, poor coordination, skeletal irregularities, eye movement disorders, blood disorders including anemia and respiratory problems.

Signs and symptoms• Painless hepatomegaly and splenomegaly; the

size of the spleen can be 1500-3000 ml, as opposed to the normal size of 50-200 ml.

• Hypersplenism: the rapid and premature destruction of blood cells, leading to anemia, neutropenia and thrombocytopenia (with an increased risk of infection and bleeding)

• Neurological symptoms occur only in some types of Gaucher's: – Type II: serious convulsions, hypertonia, mental

retardation, apnea. – Type III: muscle twitches known as myoclonus,

convulsions, dementia, ocular muscle apraxia. • Osteoporosis: 75% develop visible bony

abnormalities due to the accumulated glucosylceramide.

Diagnosis

– Definitive diagnosis is made with genetic testing.

– Prenatal diagnosis is useful when there is a known genetic risk factor.

Treatment For type 1 and most type 3 patients:• Enzyme replacement treatment with intravenous

recombinant glucocerebrosidase (imiglucerase) can dramatically decrease liver and spleen size, reduce skeletal abnormalities, and reverse other manifestations.

• Successful bone marrow transplantation cures the non-neurological manifestations of the disease, because it introduces a monocyte population with active beta-glucosidase.

• Splenectomy (if the patient is anemic or when the enlarged organ affects the patient’s comfort).

• Blood transfusion.

Cont..

• Other patients may require joint replacement surgery to improve mobility and quality of life.

• Other treatment options include: - – antibiotics for infections,– antiepileptics for seizures, and– liver transplants.

Cont..• Substrate reduction therapy :- may prove to

be effective in stopping Type 2, as it can cross through the blood barrier into the brain. There is currently no effective treatment for the severe brain damage that may occur in patients with types 2 and 3 Gaucher disease.

• Gene therapy.• Miglustat oral drugs, • Isofagomine tartrate, is under development.

Alpha – 1 – antitrypsin deficiency

• Alpha 1-antitrypsin deficiency (α1-antitrypsin deficiency, A1AD or Alpha-1)

• Genetic disorder caused by defective production of alpha 1-antitrypsin (A1AT), leading to decreased A1AT activity in the blood and lungs, and deposition of excessive abnormal A1AT protein in liver cells.

• Function : - A1AD, a protease inhibitor (Pi), is synthesized in the liver and protects lung alveolar tissues from destruction by neutrophil elastase.

Pathophysiology– Phenotypes :- PiSS, PiMZ, PiZZ and PiSZ, – Blood levels of A1AT are reduced to between 40 and

60% of normal levels which is sufficient to protect the lungs from the effects of elastase in people who do not smoke.

– Individuals with the phenotype PiZZ, A1AT levels are less than 15% of normal, and patients are likely to develop emphysema at a young age; 50% of these patients will develop liver cirrhosis, because the A1AT is not secreted properly and instead accumulates in the liver.

– Cigarette smoke is especially harmful to individuals with A1AD. In addition to increasing the inflammatory reaction in the airways, cigarette smoke directly inactivates alpha 1-antitrypsin by oxidizing essential methionine residues to sulfoxide forms

Signs and symptoms• Shortness of breath • Recurrent respiratory infections• Asthma that does not respond to treatment• Emphysema during their thirties or forties

even without a history of significant smoking, though smoking greatly increases the risk for emphysema

• Cirrhosis and liver failure (15%)• Wheeze

Treatment

• Intravenous infusions of alpha-1 antitrypsin

• Therapy is not appropriate for liver-affected patients.

• In severe cases, liver transplantation may be necessary.

Wilson's disease

• Syn. : - Hepatolenticular degeneration

• Wilson's disease is a rare inherited disorder that affects about one in 30,000 people worldwide.

• In Wilson's disease, the body is unable to excrete excess copper.

• This manifests as neurological or psychiatric symptoms and liver disease.

Cont..

• The condition is due to mutations in the Wilson disease protein (ATP7B) gene.

• A single abnormal copy of the gene is present in 1 in 100 people, who do not develop any symptoms (they are carriers). If a child inherits the gene from both parents, they may develop Wilson's disease.

• Symptoms usually appear between the ages of 6 and 20 years, but cases in much older patients have been described.

• Wilson's disease occurs in 1 to 4 per 100,000 people. • Wilson's disease is named after Dr. Samuel Alexander

Kinnier Wilson (1878-1937), the British neurologist who first described the condition in 1912.

Pathophysiology• Copper acts as a cofactor for a

number of enzymes such as ceruloplasmin, cytochrome c oxidase, dopamine β-hydroxylase, superoxide dismutase and tyrosinase.

• Copper enters the body through the digestive tract. A transporter protein on the cells of the small bowel, copper membrane transporter (CMT1), carries copper inside the cells, where some is bound to metallothionein and part is carried by ATOX1 to an organelle known as the trans-Golgi network.

• Here, in response to rising concentrations of copper, an enzyme called ATP7A releases copper into the portal vein to the liver

• ATOX1 Copper transport protein

Cu = copper, CP = ceruloplasmin, green = ATP7A

Cont..• Liver cells also carry the

CMT1 protein, and metallothionein and ATOX1 bind it inside the cell, but here it is ATP7B that links copper to ceruloplasmin and releases it into the bloodstream, as well as removing excess copper by secreting it into bile.

• Both functions of ATP7B are impaired in Wilson's disease.

• Copper accumulates in the liver tissue; ceruloplasmin is still secreted, but in a form that lacks copper (termed apoceruloplasmin) and rapidly degraded in the bloodstream.

Cu = copper, CP = ceruloplasmin, green = ATP7B carrying copper.

Cont…

• When the amount of copper in the liver increased the proteins that normally bind it, it causes oxidative damage through a process known as Fenton chemistry; this damage eventually leads to chronic active hepatitis, fibrosis (deposition of connective tissue) and cirrhosis.

• The liver also releases copper into the bloodstream that is not bound to ceruloplasmin. This free copper precipitates throughout the body but particularly in the kidneys, eyes and brain.

Cont..• In the brain, most copper is

deposited in the basal ganglia, particularly in the putamen and globus pallidus (together called the lenticular nucleus); these areas normally participate in the coordination of movement as well as playing a significant role in neurocognitive processes such as the processing of stimuli and mood regulation.

• Damage to these areas, again by Fenton chemistry, produces the neuropsychiatric symptoms seen in Wilson's disease.

Cont…

– It is not clear why Wilson's disease causes hemolysis, but various lines of evidence suggest that high levels of free (non-ceruloplasmin bound) copper have a direct effect on either oxidation of hemoglobin, inhibition of energy-supplying enzymes in the red blood cell, or direct damage to the cell membrane.

Signs and symptoms

–Common sites of copper accumulation are the liver and the brain, and liver disease and neuropsychiatric symptoms are the main features that lead to diagnosis.

Liver disease

Liver disease – Tiredness, increased bleeding tendency or

confusion (due to hepatic encephalopathy) and portal hypertension.

– Esophageal varices (increased pressure on the portal vein )

– Splenomegaly – Ascites

Cont..

• About 5% of all patients are diagnosed only when they develop fulminant acute liver failure,

• This leads to abnormalities in protein production and metabolism by the liver.

• Protein metabolism leads to the accumulation of waste products such as ammonia in the bloodstream. When these irritate the brain, the patient develops hepatic encephalopathy (confusion, coma, seizures and finally life-threatening swelling of the brain).

Neuropsychiatric symptoms– Most patients initially have mild cognitive

deterioration as well as changes in behavior. – Specific neurological symptoms then follow, in

the form of parkinsonism (increased rigidity and slowing of routine movements) with or without a typical hand tremor, ataxia (lack of coordination) or dystonia (twisting and repetitive movements of part of the body).

– Seizures and migraine (more common) – Psychiatric problems : - behavioral changes,

depression, anxiety and psychosis.

Other organ systems

• Eyes: Kayser-Fleischer rings (KF rings) visible around the iris. They are due to copper deposition in Descemet's membrane of the cornea.

• visible on slit lamp examination.

• Sunflower cataracts, • KF rings occur in 66% of

cases, more often in those with neurological than with liver problems.

Kidney:– Renal tubular acidosis, a disorder of bicarbonate handling by the

proximal tubules leads to nephrocalcinosis (calcium accumulation in the kidneys), weakening of the bone (due to calcium and phosphate loss) and occasionally aminoaciduria (loss of amino acids, needed for protein synthesis).

Heart: – Cardiomyopathy (weakness of the heart muscle) is a rare but

recognized problem in Wilson's disease; it may lead to – Heart failure (fluid accumulation due to decreased pump function)

and – Cardiac arrhythmias (episodes of irregular and/or abnormally fast

or slow heart beat).

Hormones: – Hypoparathyroidism (failure of the parathyroid glands, leading to low

calcium levels), infertility and habitual abortion.

Investigations • Complete blood count (CBC) • Serum ceruloplasmin • Serum copper • Serum uric acid • Urine copper If there are liver problems, lab tests may find:• High AST and ALT • High bilirubin • High PTT • Low albumin

Other tests

– 24-hour urine copper test – Abdominal x-ray – Abdominal MRI – CT scan of the abdomen – Head CT scan – Head MRI – Liver biopsy – Genetic testing for ATP7B.

Diagnosis

• Gold standard or most ideal test is a liver biopsy.

Ceruloplasmin - Levels of ceruloplasmin are abnormally low

(<0.2 gram/liter) in 80-95% of cases.

- The combination of neurological symptoms, Kayser-Fleisher rings and a low ceruloplasmin level is considered sufficient for the diagnosis of Wilson's disease.

Serum and urine copper

• Serum copper is low but urine copper are elevated Urine is collected for 24 hours in a bottle :-• Levels above 100 μg/24h (1.6 μmol/24h) confirm

Wilson's disease, and • levels above 40 μg/24h (0.6 μmol/24h) are strongly

indicative.

In children, the penicillamine test :- • A 500 mg oral dose of penicillamine is administered, and

urine collected for 24 hours. If this contains more than 1600 μg (25 μmol), it is a reliable indicator of Wilson's disease.

Investigations

– Ceruloplasmin < 0.2 g/L– Serum copper < 12 µmol/L– Urine copper > 1.1 µmol / 24 h as an

isolated test; > 4 µmol is a more useful threshold

– Urine copper post-penicillamine > 25 µmol/24 h

Treatment• Goal of treatment is to reduce the amount of

copper in the tissues. • done by a procedure called chelation – medicine that

bind to copper and remove it through the kidneys or gut. Treatment must be lifelong.

Drugs used:• Penicillamine (Cuprimine, Depen) binds copper and

leads to increased release of copper in the urine. • Trientine (Syprine) binds the copper increases its release

through the urine. • Zinc acetate (Galzin) blocks copper from being absorbed

in the intestinal tract.

low-copper diet

Foods to avoid include:• Chocolate • Dried fruit • Liver • Mushrooms • Nuts • Shellfish • Drink distilled water because most tap water

flows through copper pipes.

• Liver transplant

Complications• Anemia • Central nervous system complications • Cirrhosis • Fatty liver • Hepatitis • Bone fractures • Jaundice • Muscle atrophy• Liver failure and damage to the central nervous

system (brain, spinal cord) are the most common and dangerous effects of the disorder. If not diagnosed and treated early, Wilson's disease is fatal.