GENETIC-BASIS OF HUMAN-COMPLEMENT C4A DEFICIENCY - DETECTION OF A POINT MUTATION LEADING TO NONEXPRESSION

The fourth component of the human complement system (C4) is coded for by two genes, C4A and C4B, located within the MHC. Null alleles of C4 (C4Q0) are defined by the absence of C4 protein in plasma. These null alleles are due either to large gene deletions or to nonexpression of the respective genes. In a previous study, evidence was obtained for n onexpressed defective genes at the C4A locus, and for gene conversion at the C4B locus. To further characterize the molecular basis of these nonexpressed C4A genes, we selected nine pairs of PCR primers from fl anking genomic intron sequences to amplify all 41 exons from individua ls with a defective C4A gene. The amplified products were subjected to single-stranded conformation polymorphism (SSCP) analysis to detect p ossible mutations. PCR products exhibiting a variation in the SSCP pat tern were sequenced directly. In 10 of 12 individuals studied, we dete cted a 2-bp insertion in exon 29 leading to nonexpression due to the c reation of a termination codon, which was observed in linkage to the h aplotype HLA-B60-DR6 in seven cases. In one of the other two individua ls without this mutation, evidence was obtained for gene conversion to the C4B isotype. The genetic basis of C4A nonexpression in the second individual is not yet known and will be subject to further analysis.