Exon skipping in IVD RNA processing in isovaleric acidemia caused by pointmutations in the coding region of the IVD gene

Isovaleric acidemia (IVA) is a recessive disorder caused by a deficiency of isovaleryl-CoA dehydrogenase (IVD). We have reported elsewhere nine point mutations in the ND gene in fibroblasts of patients with IVA, which lead to abnormalities in IVD protein processing and activity. In this report, we d escribe eight IVD gene mutations identified in seven IVA patients that resu lt in abnormal splicing of IVD RNA. Four mutations in the coding region lea d to aberrantly spliced mRNA species in patient fibroblasts. Three of these are amino acid altering point mutations, whereas one is a single-base inse rtion that leads to a shift in the reading frame of the mRNA. Two of the co ding mutations strengthen pre-existing cryptic splice accepters adjacent to the natural splice junctions and apparently interfere with exon recognitio n, resulting in exon skipping. This mechanism for missplicing has not been reported elsewhere. Four other mutations alter either the conserved gt or a g dinucleotide splice sites in the IVD gene. Exon skipping and cryptic spli cing were confirmed by transfection of these mutations into a Cos-7 cell li ne model splicing system. Several of the mutations were predicted by indivi dual information analysis to inactivate or significantly weaken adjacent do nor or acceptor sites. The high frequency of splicing mutations identified in these patients is unusual, as is the finding of missplicing associated w ith missense mutations in exons. These results may lead to a better underst anding of the phenotypic complexity of IVA, as well as provide insight into those factors important in defining intron/ exon boundaries in vivo.