THE HLA-A3,CW6,B47,DR7 EXTENDED HAPLOTYPES IN SALT LOSING 21-HYDROXYLASE DEFICIENCY AND IN THE OLD-ORDER AMISH, IDENTICAL CLASS-I ANTIGENS AND CLASS-II ALLELES WITH AT LEAST 2 CROSSOVER SITES IN THE CLASS-III REGION

The HLA-B47,DR7 haplotype in congenital adrenal hyperplasia (CAH) due to 21-hydroxylase deficiency contains a deletion of most of the active CYP21 gene and the entire adjacent C4B gene. The C4A gene produces a protein which is electrophoretically C4A but anti-genically C4B. In th e Old Order Amish, the HLA-B47,DR7 haplotype contains no deletion, but is immunologically identical to the CAH haplotype in both areas flank ing the crossover region. We compared some of the genes in the MHC Cla ss II and Class III regions in the Amish and CAH-linked haplotypes to define further the relationships between the two. The complement facto r B (Bf) proteins differed, but no Bf RFLPs were identified, The compl ement factor 2 genes exhibited different BamHI RFLPs. Analyses of the tumor necrosis factor-alpha genes revealed the same NcoI restriction p atterns. The RD genes contained microsatellites of the same size. Port ions of the MHC Class II DR and DQ, and Class III CYP21 and C4 alleles were sequenced. The exon 2 sequences of DQ2 and DR7 were identical in the two haplotypes. In the Amish haplotype, both CYP21 and C4 gene pa irs were present and functionally normal. The CAH haplotype had two se quence crossovers: from CYP21P to CYP21 in the 7th intron, and from C4 A to C4B between codons 1106 (exon 26) and 1157 (exon 28). A model is proposed which accounts for the CAH-linked mutant haplotype arising fr om a nonmutant homologue via three crossings-over.