Homocystinuria due to Cystathionine Beta-Synthase Deficiency
National Organization for Rare Disorders, Inc.
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Homocystinuria is a rare metabolic condition characterized by an excess of the compound homocystine in the urine. The condition may result from deficiency of any of several enzymes involved in the conversion of the essential amino acid methionine to another amino acid (cysteine)--or, less commonly, impaired conversion of the compound homocysteine to methionine. Enzymes are proteins that accelerate the rate of chemical reactions in the body. Certain amino acids, which are the chemical building blocks of proteins, are essential for proper growth and development.
In most cases, homocystinuria is caused by reduced activity of an enzyme known as cystathionine beta-synthase (CBS). Infants who develop homocystinuria due to CBS deficiency (which is also known as classical homocystinuria) may fail to grow and gain weight at the expected rate (failure to thrive) and have developmental delays. By approximately age three, additional, more specific symptoms and findings may become apparent. These may include partial dislocation (subluxation) of the lens of the eyes (ectopia lentis), associated "quivering" (iridodonesis) of the colored region of the eyes (iris), severe nearsightedness (myopia), and other eye (ocular) abnormalities. Although intelligence may be normal in some cases, many children may be affected by progressive mental retardation. In addition, some may develop psychiatric disturbances and/or episodes of uncontrolled electrical activity in the brain (seizures). Affected individuals also tend to be thin with unusually tall stature; long, slender fingers and toes (arachnodactyly); and elongated arms and legs ("marfanoid" features). In addition, affected individuals may be at risk for the development of blood clots that can become lodged within certain large and small blood vessels (thromboembolisms), potentially leading to life-threatening complications. Homocystinuria due to deficiency of CBS deficiency is inherited as an autosomal recessive trait. The disorder is caused by changes (mutations) of a gene on the long arm (q) of chromosome 21 (21q22.3) that regulates the production of the CBS enzyme.
CLIMB (Children Living with Inherited Metabolic Diseases)
176 Nantwich Road
Crewe, Intl CW2 6BG
Tel: +44 870 7700 325
Fax: +44 870 7700 327
March of Dimes Birth Defects Foundation
1275 Mamaroneck Avenue
White Plains, NY 10605
Arc (a national organization on mental retardation)
1010 Wayne Ave
Silver Spring, MD 20910
National Institute of Neurological Disorders and Stroke (NINDS)
P.O. Box 5801
Bethesda, MD 20824
Save Babies Through Screening Foundation
P.O. Box 42197
Cincinnati, OH 45242
Belgian Association for Metabolic Diseases (BOKS)
Alice Nahonlann 7
Genetic and Rare Diseases (GARD) Information Center
PO Box 8126
Gaithersburg, MD 20898-8126
PO Box 241956
Los Angeles, CA 90024
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