A Point Mutation Creating a 3' Splice Site in C8A Is a Predominant Cause of C8α-γ Deficiency in African Americans.

C8α-γ deficiency was examined in four unrelated African Americans. Two individuals were compound heterozygotes for a previously reported point mutation in exon 9. mRNA from the remaining six C8A alleles contained a 10 nt insertion between nt 992 and 993 corresponding to the junction between exons 6 and 7. This suggested that C8α-γ deficiency in these individuals was caused by a splicing defect. Genomic sequencing revealed a G→A point mutation in intron 6, upstream of the exon 7 acceptor site. This mutation converts a GG to an AG, generates a consensus 3' splice site that shifts the reading frame, and creates a premature stop codon downstream. To verify that the point mutation caused a splicing defect, we tested wild-type and mutant mRNA substrates, containing 333 nt of the C8α intron 6/exon 7 boundary, in an in vitro splicing assay. This assay generated spliced RNA containing the 10 bp insertion observed in the C8α mRNA of affected patients. In addition, in mutant RNA substrates, the new 3' splice site was preferentially recognized compared with wild-type. Preferential selection of the mutant splice site likely reflects its positioning adjacent to a polypyrimidine tract that is stronger than that adjacent to the wild-type site. In summary, we have identified a G→A mutation in intron 6 of C8A as a predominant cause of C8α-γ deficiency in African Americans. This mutation creates a new and preferred 3' splice site, results in a 10 nt insertion in mRNA, shifts the reading frame, and produces a premature stop codon downstream.