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Homocystinuria

Introduction

Homocystinuria is heredity defect caused by deficiency of an enzyme (cystathionine synthetase) responsible for digesting methionine, an amino acid. In absence or decreased level of the enzyme may cause the rise of homocysteine  in the blood and may excrete the high amount of homocysteine in the urine. This will lead to muscle problem, problems associated with central nervous system.

• Two metabolic conversions are associated with homocysteine and one is remethylation and another is transsulfuration.
• Women generally have lower values of homosysteine than men.
• Classic form of homocysteine is caused be deficiency of cystathionine synthase, an ezyme required for metabolic breakdown of methionine.
• Homocystinuria can also be caused by insufficient vitamin B-12. In such case methylmalonic aciduria will be present.
• Homocystinuria can also be caused by the deficiency in methylenetetrahydrofolate reductase, as folate and vitamin B-12 both are required to remethylation of homocysteine.

Causes:

In homocystinuria, a child inherits the defective gene from the parents. If both the parents are carriers of homocystinuria, a 25% chance is that the child will inherit defective gene from both the parents and will have homocystinuria.

Symptoms:

Homocystinuria is more common in male children than in females.

Typical symptoms include

• Nearsightedness
• Dislocation of lens of eyes.
• Pale and pink skin
• Rashes
• Fragile hairs
• Long limbs
• High arched feet
• At the time of birth child is usually healthy and problem related to central nervous system starts with in one year.
• Developmental delays and progressive mental retardation is observed within 2 to 3 years.
• Osteoporosis in spine may be observed during childhood
• Psychiatric disease begin to develop in some of the children.

Some of the features are similar to Marfan syndrome such as dislocated lens, long limbs, chest and spinal deformities but other symptoms including mental retard ness and cerebral symptoms are absent in Marfan syndrome.

Diagnosis:

If the physical feature examination suspects the possibility of homocystinuria, the following tests should be carried out to confirm or rule out homocystinuria.

• Skeletal X ray should be carried out to confirm spinal osteoporosis.
• An amino acid test should be carried out in blood and urine to access levels of methinine and homocyteine. The normal range of methionine level is less than 1 mg/dl.
• A liver biopsy and enzyme assay should be carried out to establish deficiency of enzymatic activity of cystathionine synthase.
• Ophthalmic examination should reveal nearsightedness or dislocated lens or any other problem such as glaucoma and cataracts.
• Skin biopsy should be carried out to confirm the reduced activity of cystathionine beta synthase.
• In new born babies, a screening test semiquantitative bacterial inhibition assay is carried out to measure the methionine concentration in dried blood spot.

Treatment:

• The diagnosis should be carried out at very early stage.
• If homosystinuria is diagnosed in infants, should be given methionine restricted cysteine supplemental diets.
• Some people may be given higher doses of vitamin B-6.
• Some people may require regular medicine.

Progonosis:

• No specific cure exists for homocystinuria.
• Vitamin B-6 supplements improve the symptoms in half of the affected people.
• Half of the patients are at increased risk of heart problems due to blood clots and shorter life span.

Prevention:

• Prospective parents with a family history of homocystinuria, should consult with the health care provider.
• During pregnancy Chorionic villi sampling (CVS) should be carried out for confirming or ruling out homocystinuria.

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Homocystinuria

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