Genetic complementation among inherited deficiencies of methylmalonyl-CoA mutase activity: evidence for a new class of human cobalamin mutant.

Analysis of the genetic basis for inherited metabolic disorders in man has been aided significantly by the adaptation of human diploid somatic cells to growth in cell culture. One particularly informative application of this approach has been the use of complementation testing to assess the genetic basis for the clinical and biochemical heterogeneity which characterizes many inborn errors of metabolism [111]. The methylmalonicacidemias are a heterogeneous group of rare, recessively inherited disorders of methylmalonate and cobalamin (Cbl; vitamin B12) metabolism, due most commonly to blocks in the rearrangement of L-methylmalonyl-CoA to succinyl-CoA. This reaction is catalyzed by methylmalonyl-CoA mutase (methylmalonyl-CoA CoA-carbonyl mutase; E.C.5.4.99.2), a mitochondrial matrix enzyme requiring adenosylcobalamin (AdoCbl) as an essential cofactor [12]. Using complementation tests in Sendai virus-induced heterokaryons of human skin fibroblasts from patients with methylmalonicacidemia, Gravel et al. [4] demonstrated that mutant cell lines deficient in mutase activity comprise four complementation groups. Available cellular and biochemical evidence [12-21] suggests that (1) one complementation group (designated mut) is characterized by a primary lesion in the mutase apoenzyme; (2) a second group (cbl C) exhibits a pleiotropic deficiency of both mutase activity and N5-methyltetrahydrofolate:homocysteine methyltransferase (5-methyltetrahydropteroyl-L-glutamate:L-homocysteine S-methyltransferase; E.C.2. 1.1.13) activity and is defective in the synthesis of their respective Cbl coenzymes, AdoCbl and methylcobalamin (MeCbl); and (3) two complementation groups (cbl A and cbl B) have an apparently normal mutase apoenzyme, but are characterized by distinct defects in AdoCbl synthesis only. Gravel et al. concluded that the observed complementation among these four mutant classes was very likely intergenic in nature, indicating the existence of at least four distinct gene loci, mutations at which can lead to a deficiency of methylmalonyl-CoA mutase activity [4].