Peter D. Ennis

Comparison of the structure of HLA-Bw47 to HLA-B13 and its relationship to 21-hydroxylase deficiency

Adrenal 21-hydroxylase deficiency is strongly associated with HLA-Bw47. This rare HLA allele and the HLA-B13 allele are both found in positive genetic linkage disequilibrium with HLA-A3, -Cw6, -DR7 and also display serological cross-reactivity. To investigate the relationship between these two alleles at the structural level, the nucleotide sequences of the HLA-B13 and HLA-Bw47 genes have been determined. They differ by 28 nucleotides, resulting in 14 amino acid substitutions: 5 in the α1 domain, 8 in the α2 domain, and 1 in the transmembrane region. Comparison of HLA-Bw47 nucleotide sequence with other HLA-B sequences shows a segment of 228 by identical with B44 in the a 1 domain and a segment of 218 by identical with B27 in the a2 domain, but only a 91 by segment of identity with B13 in the al domain. The complex pattern of substitutions and their degree of divergence indicate that HLA-B13 and HLA-Bw47 alleles are not related by a simple mutational event.

Comparison of the Amino Acid Sequences Encoded by the HLA-Bw47 and HLA-B13 Genes

Congenital adrenal hyperplasia due to 21-hydroxylase deficiency (21-OHD) is a disorder of cortisol and aldosterone biosynthesis that is closely linked to the HLA-B locus (1,2). HLA-Bw47, a very rare antigen, is strongly associated with a deletion of 21-OHB gene among patients carrying 21-OHD. This allele frequently occurs on the HLA-A3;Cw6;DR7 haplotype which always includes a null allele at the locus encoding the fourth component of complement C4B. Moreover, the more common allele HLA-B13, which is not associated with the deletion of 21-OHB and C4B genes, shares the same pattern of genetic linkage disequilibrium. In addition, these HLA-B13 and HLA-Bw47 alleles show serological cross reactivity with alloantisera and have identical isoelectric points (3). On the basis of these structural similarities, we have postulated that Bw47 was the product of the B13 gene which became mutated during the genetic deletion of the 21-hydroxylase locus (4).