Genetic aspects of ankylosing spondylitis.

There is substantial evidence strongly favouring a direct role for HLA-B27 in genetic susceptibility to ankylosing spondylitis (AS) and related spondyloarthropathies (SpA), although the underlying molecular basis has yet to be identified. HLA-B27 itself is a serologic specificity that encompasses 26 different alleles that encode 24 different subtypes - HLA-B2701 to B2725, with the exclusion of B2722. [The B2722 allele was deleted as an official WHO allele in April 2002, with a note that the reference cell has been shown to have the same sequence as B2706. Thus, from now on, with this deletion of B2722, there will be a "hole" among the HLA-B2701 to B2725 group of alleles]. The 24 HLA-B27 alleles (subtypes) seem to have evolved from the most widespread subtype, B2705. Two B27 alleles have been reported to lack association with AS: B2706 among Southeast Asian populations, and B*2709 among Sardinians. The distinction between the disease-associated subtypes and those that are not disease-associated may provide some clues to the actual role of HLA-B27 in disease pathogenesis. Genetic family studies in populations of European descent indicate that HLA-B27 contributes only 16 % of the total genetic risk for the disease. The genes in the Major Histocompatibility Complex (MHC) as a whole, that includes HLA-B27, account for about half of the genetic susceptibility for AS. This clearly indicates the presence of additional disease predisposing genes in the MHC region on chromosome 6, and genome-wide studies have identified many areas of interest on other chromosomes that may contain additional disease predisposing genes. Additional studies emanating from the recent mapping of the human genome is expected to lead to better understanding of the genetic basis of these and other rheumatic diseases. Genetic counselling and the use of HLA-B27 typing as an aid to diagnosis are also reviewed.