INTERALLELIC COMPLEMENTATION OF BETA-SUBUNIT DEFECTS IN FIBROBLASTS OF PATIENTS WITH PROPIONYL-COA CARBOXYLASE DEFICIENCY MICROINJECTED WITH MUTANT CDNA CONSTRUCTS

Propionic acidemia results from deficiency of propionyl-CoA carboxylas e (PCC) activity. PCC is a biotin-dependent, mitochondrial enzyme comp osed of alpha- and beta-subunits (structure, alpha(4) beta(4)), with t he alpha-subunit containing the biotin ligand. About two-thirds of fib roblast lines from patients with mutations in the PCCB (beta-subunit) gene show interallelic complementation in cell fusion experiments (the pccB and pccC subgroups of the pccBC major group defining beta-subuni t mutations, where pccB x pccC fusions show complementation). We previ ously identified the mutations in several pccB or pccC cell lines and suggested that point mutations or small, in-frame insertions or deleti ons were likely responsible for the complementation obtained between b eta-subunit defects. To test this hypothesis, we have introduced five different mutations (three pccB and two pccC) that fit these criteria into a PCC beta-subunit cDNA plasmid expressed from a cytomegalovirus promoter, The cDNA plasmids were microinjected into mutant fibroblasts and the cells were assayed by radioautographic detection of C-14-prop ionate incorporation into cellular macromolecules, Four different muta tions (Pro228Leu or dupKICK140 from pccB or Delta lle408 or Arg410Trp from pccC) complemented cells from complementation subgroups in a patt ern congruent with tile results obtained in cell fusion experiments, T he fifth mutation, Arg536Asn, which was found both ina complementing p ccB and a non-complementing pccBC cell line, failed to complement any of the mutant cell lines, The high proportion of mutant cell lines tha t show interallelic complementation, the diversity of mutations respon sible for it, and the extent of heteroallelism becoming apparent in af fected patients suggests that partial complementation between allelic mutations may contribute to the clinical heterogeneity observed in pro pionic acidemia due to beta-subunit defects.