Complementation analysis of maple syrup urine disease in heterokaryons derived from cultured human fibroblasts

MAPLE syrup urine disease (MSUD) is an autosomal recessive disorder characterized by keto acidosis, convulsions, mental retardation and a maple syrup odour in the urine. The clinical phenotype varies among different families, with a severe early onset in the classical form of the disease and milder symptoms with delayed onset in variants. The biochemical defect is an inability to degrade the three branched-chain amino-acids (BCAA), leucine, isoleucine, and valine, as a result of a deficiency in decarboxylation of the branched-chain α-keto acids (BCKA). The enzyme defect is expressed in skin fibroblast cultures derived from affected patients, and the level of residual BCKA decarboxylase activity correlates with the severity of clinical symptoms. The heterogeneity in clinical presentation and the level of residual decarboxylase activity appear to be genetically determined¹.