The human complement C9 gene: structural analysis of the 5 ' gene region and genetic polymorphism studies

C9 is the last of the human complement components creating the membrane att ack complex. The single chain serum protein is encoded by a gene located on chromosome 5p13 that is composed of 11 exons. With the aid of inverse PCR, the hitherto unknown regions flanking exon 1 and the 3' part of exon 11 (3 'UTR) have been sequenced. A computer-based analysis of the 300-bp region l ocated just upstream of the AUG start codon showed homologies to known DNA modules which affect the transcriptional regulation of certain genes. The m ost striking of these is a sequence that may substitute the missing TATA bo x in initiating C9 transcription. In the 3'UTR, three successive polyadenyl ation signals were found. Although the C9 protein is invariant, four differ ent single nucleotide polymorphisms (SNPs) have been observed at the DNA le vel by exon-specific PCR and direct sequencing. None of them changes the am ino acid composition of the mature protein. Due to a C --> T transition in exon 1 at cDNA position 17, the fifth amino acid of the leader peptide may be either an arginine or a tryptophane. Using either PCR/RFLP analysis (exo ns 1 and 11) or allele-specific PCR (intron 1 and exon 4), each polymorphis m can be characterized without sequencing. All of the exon 1, intron I and exon 11 variants could be detected in small population samples of European, Thai or South American Indian origin. In contrast, the exon 4 C variant wa s observed only once in a European. The first three SNPs can be combined to designate eight different 'C9 alleles'. Of these, six have actually be fou nd. These data provide strong evidence that several mutation and recombinat ion events occurred in the course of C9 gene evolution.