C6 HAPLOTYPES - ASSOCIATIONS OF A DDE-I SITE POLYMORPHISM TO COMPLEMENT DEFICIENCY GENES AND THE MSP-I RESTRICTION-FRAGMENT-LENGTH-POLYMORPHISM (RFLP)

Complement C6 has a common charge polymorphism designated A and B with gene frequencies of 0.65 and 0.35. The probable molecular basis for t his is a Glu (C6A) for Ala (C6B) substitution at amino acid position 9 8, and is detected by digestion with the restriction enzyme Dde I of a polymerase chain reaction (PCR)-amplified fragment of genomic DNA. C6 A was found to be Dde I-positive and C6B corresponds to Dde I-negative . We have applied our Dde I A/B polymorphism genotyping method to the investigation of CG-deficient individuals with complete (C6Q0) and sub -total deficiency (C6SD) protein phenotypes, including members of four families. We have also investigated the RFLP detected by digestion of genomic DNA with the enzyme Msp I, which is due to a polymorphic site located in the 5' section of the gene, the variable sequence of which has yet to be determined. Sixteen out of seventeen unrelated C6Q0 sub jects were found to be genotypically Dde I B/Msp I-negative; the remai ning subject was heterozygous at both the loci under investigation. Th e C6SD phenotype was found to be associated with the Dde I A/Msp I-pos itive genotype in two families with combined C6/C7 subtotal deficiency and two with C6SD. It can be concluded that the two forms of C6 defic iency, C6Q0 and C6SD, arose independently on two different C6 allelic backgrounds. These associations have allowed the genotyping of the rar e families that contain both types of deficiency. We have also defined a number of normal C6 Dde I/Map I haplotypes in Caucasians and Cape C oloured populations.