Master's in Genetic Counseling (MGC)
Homocystinuria
Homocystinuria is an autosomal recessive disorder of methionine metabolism, leading to elevated excretion of the amino acid, homocystine. Although there are a variety of genetic and non-genetic causes of homocystinuria, the newborn screening test is designed to detect cystathionine β synthetase deficiency, which also causes elevated blood levels of methionine.
Clinical Features
Newborns with homocystinuria have no apparent symptoms associated with the disorder. During childhood, patients may experience dislocated ocular lenses as well as thromboembolic events that may lead to stroke or premature death. Additional features of homocystinuria include developmental delay, progressive mental retardation, scoliosis, osteoporosis, seizures, psychiatric disturbances, joint stiffness, arachnodactyly, fair skin and hair, and a marfanoid habitus. If an affected individual is not detected by newborn screening, the nonspecific features of the disorder prevent most individuals from being diagnosed until some of the more severe clinical features have developed. (OMIM)
Laboratory Tests
Screening for homocystinuria is performed by determining the level of methionine in the dried filter paper blood spot. Abnormal elevations of methionine may be suggestive of homocystinuria. A genetic/metabolic consultation is recommended for diagnostic testing and management of the disorder.
Treatment
Many individuals with cystathionine β synthetase deficiency will respond to a high dose of vitamin B6 (pyridoxine). For others, a diet restricted in methionine is required. In turn, special food supplements are then necessary to provide other amino acids, vitamins, and nutrients that are deficient in the methionine restricted diet. Antiplatelet agents may help prevent thromboembolic events. For specific treatment guidelines, please consult with a genetic/metabolic specialist.
Screening Practice Considerations
Detection of methionine levels depend on protein ingestion. False negative results may occur if the specimen is collected before 48 hours of age. In this circumstance, a repeat specimen is necessary.
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