A SPLICING MUTATION IN THE ALPHA-5(IV) COLLAGEN GENE OF A FAMILY WITH ALPORTS-SYNDROME

DNA sequence analysis of the alpha5(IV) collagen chain gene (COL4A5) was carried out between exon 47 and 51, which encode the noncollagenous (NC) domain, in eight Japanese families with Alport's syndrome. In one family with X-linked inheritance of the disease, a point mutation (G to C) was found at the 3' end of exon 49 in the COL4A5. This mutation converted the codon of a conserved methionine-1601 to the codon for isoleucine, and also altered the normal splicing process. The polymerase chain reaction (PCR) product amplified between exons 47 and 51 of cDNA in the affected male (hemizygote) of this family contained four fragments with various molecular weights, whereas that of a normal control contained one with the expected molecular weight. Sequence analysis of the PCR fragments of the male patient revealed various types of alternative splicing between the exons, reflecting the various sizes of PCR fragments. The PCR amplified product of the cDNA of the affected female (heterozygote), on the other hand, contained a fragment with the same molecular weight as the normal control. Sequence analysis of the PCR fragments of her cDNA revealed normal splicing and no point mutation at the 3' end of exon 49. These findings indicate that this point mutation at the consensus sequence not only converted the codon but also altered the splicing between these exons encoding the NC domain of the COL4A5. Resulting in missense of the alpha5(IV) chain, changing a large portion of the carboxyl terminal crosslinking NC domain, this mutation can alter the normal structure of the type IV collagen network. The absence of abnormal PCR fragments in the cDNA study of the heterozygote may be due to lyonization.