Cobalamin deficiency hyperhomocysteinemia

Homocyst(e)ine CBS deficiency Cobalamin defects/defi-ciency MTHFR deficiency Methionine adenosyltrans-ferase deficiency Hyperhomocysteinemia Sulfite oxidase deficiency... 

Measurement of blood tHcy is usually performed for one of three reasons (1) to screen for inborn errors of methionine metabolism (2) as an adjunctive test for cobalamin deficiency (3) to aid in the prediction of cardiovascular risk. Hyperhomocysteinemia, defined as an elevated level of tHcy in blood, can be caused by dietary factors such as a deficiency of B vitamins, genetic abnormalities of enzymes involved in homocysteine metabolism, or kidney disease. All of the major metabolic pathways involved in homocysteine metabolism (the methionine cycle, the transsulfuration pathway, and the folate cycle) are active in the kidney. It is not known, however, whether elevation of plasma tHcy in patients with kidney disease is caused by decreased elimination of homocysteine in the kidneys or by an effect of kidney disease on homocysteine metabolism in other tissues. Additional factors that also influence plasma levels of tHcy include diabetes, age, sex, lifestyle, and thyroid disease (Table 21-1). 

Refsum, H., Yajnik, C.S., Gadkari, M., Schneede, J., Vollset, S.E., Orning, L., Guttormsen, A.B., Joglekar, A., Sayyad, M.G., Ulvik, A., and Ueland, P.M., 2001. Hyperhomocysteinemia and elevated methylmalonic acid indicate a high prevalence of cobalamin deficiency in Asian Indians. The American Journal of Clinical Nutrition. 74(2) 233-241. 

Regulation of homocysteine metabolism appears to be especially important in the central nervous system, presumably because of the critical role of methyl transfer reactions in the production of neurotransmitters and other methylated products. It has been known for decades that mental retardation is a feature of the genetic diseases, such as CBS deficiency, that cause severe hyperhomocysteinemia and ho-mocystinuria. Impaired cognitive function is also seen in pernicious anemia, which causes hyperhomocysteinemia due to deficiency of cobalamin (see Chapter 28). Hyperhomocysteinemia also may be linked to depression, schizophrenia, multiple sclerosis, and Alzheimer s disease. The molecular mechanisms underlying these clinical associations have not yet been delineated. 

Homocysteine-lowering therapy is lifesaving for patients with severe hyperhomocysteinemia due to CBS deficiency. Approximately 50% of patients respond to treatment with pharmacological doses (100 to 800 mg daily) of pyridox-ine (a form of vitamin B6). Adjunctive therapy may include betaine (2 to 6 g daily), folic acid (5 to 10 mg daily), or cobalamin (0.05 to 1.0 mg daily), or methionine restriction. Long-term homocysteine-lowering therapy substantially decreases cardiovascular risk in these patients. 

Deficiencies of methionine adenosyltransferase, cystathionine 8-synthase, and cystathionine )/-lyase have been described. The first leads to hypermethioninemia but no other clinical abnormality. The second leads to hypermethioninemia, hyperhomocysteinemia, and homo-cystinuria. The disorder is transmitted as an autosomal recessive trait. Its clinical manifestations may include skeletal abnormalities, mental retardation, ectopia lentis (lens dislocation), malar flush, and susceptibility to arterial and venous thromboembolism. Some patients show reduction in plasma methionine and homocysteine concentrations and in urinary homocysteine excretion after large doses of pyridoxine. Homocystinuria can also result from a deficiency of cobalamin (vitamin B12) or folate metabolism. The third, an autosomal recessive trait, leads to cystathioninuria and no other characteristic clinical abnormality. 

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