Linkage analysis of the C4A/C4B copy number variation and polymorphisms of the adjacent steroid 21-hydroxylase gene in a healthy population.

Genes encoding the steroid 21-hydroxylase (CYP21A2) and the complement component C4 proteins (C4A and C4B) are located in the MHC region in a strongly linked structure named RCCX module. Previous studies found that carriers of C4B gene deficiency (C4B*Q0) have higher risk for cardiovascular diseases. A potential explanation is that lacking the C4B gene may result in altered function of the neighboring CYP21A2 gene. Therefore we sequenced the CYP21A2 gene in 96 healthy individuals to identify polymorphisms and to characterize their linkage pattern. Fifty-three variations were detected including a new one which alters the TATA-box of the gene. Only three known mutations (V281L, Q318X and R479L) associated with congenital adrenal hyperplasia, were found in 7, 2 and 1 subjects, respectively. Linkage analysis revealed that some variations exhibit strong correlation with the C4 copy number polymorphism and constituents of the MHC III region. Rare alleles of three polymorphisms were identified as components of the 8.1 ancestral haplotype. Haplotyping and family study confirmed that the variant alleles of two intronic SNPs were constituents of haplotype blocks lacking the C4B gene. These results suggest that variations of CYP21A2 gene can be involved in disease associations of the 8.1 haplotype and the C4B*Q0 genotype.