THE HUMAN-COMPLEMENT C9 GENE - IDENTIFICATION OF 2 MUTATIONS CAUSING DEFICIENCY AND REVISION OF THE GENE STRUCTURE

The ninth component of human complement (C9) is the last of the termin al complement components creating the membrane attack complex. C9 is a single-chain serum protein that is encoded by a gene located on chrom osome 5p. Deficiency of terminal complement components is generally as sociated with recurrent neisseria infections. We studied a previously described Swiss family with inherited C9 deficiency. To identify the g enetic basis of C9 deficiency, we developed an approach using exon-spe cific PCR and direct DNA sequencing. As a cause of C9 deficiency, we f ound two different point mutations, both generating TGA stop codons in the coding sequence. One mutation, a C to A exchange, was detected in exon 2 at cDNA position 166, the other, a C to T exchange, was locate d in exon 4 (cDNA position 464). In family studies of three first-degr ee relatives with heterozygous C9 deficiency, we demonstrated that the true mutations are segregating independently. Therefore, these mutati ons are sufficient to explain the complete deficiency of both the prob ands studied. DNA sequencing of the exon-intron junctions revealed a n umber of revisions regarding the boundaries between exons 4, 5, and 6 as well as between exons 10 and 11. No additional introns were detecte d in exons 6 and 10. Furthermore, DNA marker studies were conducted us ing known polymorphisms of the C6, C7, and C9 genes, confirming the li nkage of the observed C9 mutations with defined haplotypes.