Restriction fragment analysis of duplication of the fourth component of complement (C4A)

The two genes encoding the fourth component of complement ( C4A and C4B) reside between HLA- B and HLA-DR on human chromosome 6. Two kilobases downstream from each C4 gene lies a 21-hydroxylase gene ( CA21HA and CA21HB, respectively). Utilizing the method of Southern blotting and a 5′-end 2.4-kb Bam HI Kpn I fragment of the C4 cDNA, we have analyzed TaqI-digested DNA from four pedigrees with one or more extended haplotypes containing a C4A duplication, as demonstrated by protein electrophoresis and segregation analysis. Two C4A protein duplications ( C4A ∗2,A ∗3,C4B ∗QO and C4A ∗3,A ∗5,C4B ∗QO ) segregated with two large TaqI DNA restriction fragments (7.0 and 6.0). In pedigree Fi, one individual homozygous for HLA- A3, B35, C4, DR1, DQ1, BFF, C2 C, C4 A2,3, C4 BQO had TaqI 7.0- and 6.0-kb restriction fragments with equal hybridization intensities as measured by two-dimensional densitometry ( 7.0 6.0 kb = 0.83 , SD = 0.12, N = 7). A hybridization probe for the 21-hydroxylase gene also demonstrated equal gene dosage ( CA21HA CA21HB = 1.01 ). DNA from another individual ( Ma I-2) with a different C4A gene duplication ( C4A ∗3,A ∗5,C4B ∗QO ) also had equal densitometry measurements ( 7.0 6.0 kb = 1.07 ). We conclude that two extended haplotypes from unrelated pedigrees have two C4 genes and both C4 genes encode separate C4A alleles. These findings are compatible with a gene conversion event of C4 B to C4 A.