Homocystinuria
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Homocystinuria

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Definition

Homocystinuria is an inherited disorder involving the metabolism of an amino acid called methionine (MET). Amino acids are the building blocks of protein. Homocystinuria occurs in approximately 1 in 200,000 people. It is more common in New South Wales, Australia, and Ireland.

People with homocystinuria lack enzymes that the body needs to properly break down the sulfur-containing amino acid methionine. A deficiency in any of several enzymes can lead to the disorder. In the most common form of the disorder, there is a deficiency of the enzyme cystathionine beta-synthase. Due to the enzyme deficiency, the body cannot properly metabolize MET and homocysteine. The result is impaired growth, development, and tissue repair. A form of the excess homocysteine appears in the urine and blood.

Causes

Homocystinuria is inherited as an autosomal recessive trait. This means that it occurs when a child inherits two defective genes—one from each parent.

Risk Factors

A risk factor is something that increases your chance of getting a disease or condition. A child is only at risk for this disorder if both parents are carriers of the faulty gene that causes it.

If both parents carry the faulty gene, for each child there is a:

  • 25% chance the child will be born with the disorder
  • 50% chance the child will be a carrier of the faulty gene

Carriers appear to have an increased risk of thromboembolic events and coronary artery disease.

Symptoms

The number and severity of symptoms vary among people. They include:

  • Nearsightedness and other visual problems
  • Flush across the cheeks, fair complexion
  • High-arched palate
  • Scoliosis
  • Seizures
  • Tall, thin build
  • Long limbs
  • High-arched feet (pes cavus)
  • Knock-knees (genu valgum)
  • Abnormal formation of the rib cage (pectus excavatum)
  • Protrusion of the chest over the sternum (pectus carinatum)
  • Mental retardation
  • Psychiatric disease
  • Osteoporosis may be noted on x-ray

Scoliosis

A Woman with Scoliosis

© 2008 Nucleus Medical Art, Inc.

Newborn infants appear normal, and early symptoms, if present at all, are vague and may occur as mildly delayed development or failure to thrive. Increasing visual problems may lead to diagnosis of this condition when the child, on examination, is discovered to have dislocated lenses and myopia.

Some degree of mental retardation is usually seen, but some affected people have normal IQs. When mental retardation is present, it is generally progressive if left untreated. Psychiatric disease can also result.

Homocystinuria has several features in common with Marfan syndrome including dislocation of the lens; a tall, thin build with long limbs; spidery fingers (arachnodactyly); and a pectus deformity of the chest.

The most serious complications of homocystinuria may be the development of blood clotting, which could results in a stroke , heart attack , or severe hypertension .

Diagnosis

Many states require that newborns be tested for homocystinuria before they leave the hospital. The test usually looks for high levels of MET. If the test is positive, blood or urine tests can be done to confirm the diagnosis. These tests can detect high levels of MET, homocystine, and other sulfur-containing amino acids. Tests to detect an enzyme deficiency (such as cystathionine synthetase) can be done as well.

If a child is not tested at birth, a doctor may later discover the disorder based on symptoms. At this point, the following may be done:

  • Blood tests to confirm the diagnosis
  • X-rays to look for bone problems
  • An eye exam to look for eye problems

Treatment

There is no specific cure for homocystinuria. However, treatment should begin as early as possible. Treatment may include medication and/or a special diet.

Medication

Many people respond to high doses of vitamin B6 (also known as pyridoxine). Slightly less than 50% respond to this treatment; those that do respond need supplemental vitamin B6 for the rest of their lives. A normal dose of folic acid supplement is helpful. Those that do not respond require a low methionine diet with cysteine supplementation, and occasionally treatment with trimethylglycine (a medication).

There is some evidence that vitamin C in relatively high dosage can improve blood vessel functioning in persons with homocystinuria. While data remains incomplete, this treatment might prove effective in reducing the risks of blood clotting and heart attacks.

Special Diet

A special diet may help people who don't respond to or don't respond fully to vitamin B6 treatment. Starting the diet early in life can help prevent mental retardation and other complications. In general, the diet:

  • Restricts foods with MET
  • Consists mainly of fruits and vegetables
  • Allows very little, if any, meats, eggs, dairy products, breads, and pastas
  • Is supplemented with:
    • Cysteine (an amino acid)
    • Folic acid

Prevention

Genetic counseling is recommended for prospective parents with a family history of homocystinuria.

Prenatal diagnosis of homocystinuria is available and is made by culturing amniotic cells or chorionic villi to test for the presence or absence of cystathionine synthase (the enzyme that is missing in homocystinuria).

If the diagnosis is made while a patient is young, a low methionine diet started promptly and strictly adhered to can spare some mental retardation and other complications of the disease. For this reason, some states screen for homocystinuria in all newborns. Check to see if your state screens for this condition.

RESOURCES:

March of Dimes
http://www.marchofdimes.com

National Organization for Rare Disorders, Inc.
http://www.rarediseases.org

US National Library of Medicine
http://www.nlm.nih.gov

CANADIAN RESOURCES:

Canada Health Portal
http://chp-pcs.gc.ca/CHP/index_e.jsp

Service Canada: Services for Families and Children
http://www.servicecanada.gc.ca/en/audiences/families/index.shtml

References:

Merck Manual of Diagnosis and Therapy . Simon and Schuster, Inc.; 2001.

National Organization for Rare Disorders, Inc. website. Available at: http://www.rarediseases.org .

Nelson Textbook of Pediatrics . 18th ed. WB Saunders; 2007.

US National Library of Medicine website. Available at: http://www.nlm.nih.gov .



Last reviewed February 2008 by Kari Kassir, MD

Please be aware that this information is provided to supplement the care provided by your physician. It is neither intended nor implied to be a substitute for professional medical advice. CALL YOUR HEALTHCARE PROVIDER IMMEDIATELY IF YOU THINK YOU MAY HAVE A MEDICAL EMERGENCY. Always seek the advice of your physician or other qualified health provider prior to starting any new treatment or with any questions you may have regarding a medical condition.

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