Somatic mosaicism in a male with an exon skipping mutation in PDHA1 of the pyruvate dehydrogenase complex results in a milder phenotype

Pyruvate dehydrogenase complex (PDC) deficiency is commonly due to mutations of PDHA1 on the X chromosome. Milder phenotypic manifestations occur in heterozygous females than in hemizygous males with the same mutation, and females are more likely to survive with severe mutations. The boy described here had hypotonia, moderate developmental delay, tremors, normal growth and brain MRI, and normal to slightly elevated lactate. PDC activity was low in skin fibroblasts and skeletal muscle (27–37%) but normal in lymphocytes. PDHA1 cDNA from cultured fibroblasts revealed two populations, one normal, the other lacking exon 6 (c.511–603 del). Genomic DNA from fibroblasts contained both normal and mutant (g.592G→A) sequences within exon 6. Expression of minigene constructs containing exons 5, 6, and 7 with or without this mutation in 293T cells confirmed that the mutation alters splicing of exon 6. The mutant to wild-type DNA ratio varied substantially across tissues. Immunoblotting of fibroblast lysates detected only wild-type E1α protein. Immunocytochemistry of cultured skin fibroblasts showed a mosaic pattern with 60% of cells positive for E1α and 40% negative, consistent with PDC activity and DNA analysis. Karyotyping, FISH analyses, and genotyping revealed a 46XY male without chimerism. These data indicate somatic mosaicism for a mutation within exon 6 that causes exon skipping and production of a non-functional protein. The mutated 592G residue is conserved among all eukaryotes. Substituting A for G apparently alters normal splicing by creating a SRp40 exonic splice enhancer site. The milder phenotype in this male is accounted for by the mixture of normal cells and cells lacking E1α. Immunocytochemistry was a useful adjunct to molecular analysis for demonstrating mosaicism.