Modern technological innovations have advanced our understanding of the genetic basis of spondyloarthritis. In
ankylosing spondylitis (AS), where the major histocompatibility complex (MHC) accounts for nearly half of the predisposition, most comes from
HLA-B27, for which 65 subtypes are now recognised, although other genes are also at work including
HLA-B60 (B*40:01). Other genes have been identified, including those involved in
peptide editing for loading onto class I MHC molecules (ERAP1) and
cytokine genes such as
interleukin 1A (IL-1A) and those involved in the Th17 network (IL-23R, an association seen primarily in Caucasians) and others. In acute
anterior uveitis, these associations are also seen as well as a region on chromosome 9p and genes whose confirmation is under way.
Psoriasis and
psoriatic arthritis fall into this disease spectrum, with the largest region of susceptibility coming from the MHC (most likely
HLA-C, ie, C*06:02 although additional influences are also being implicated), and most of the other
genetic susceptibility coming from genes involved in
cytokine production, specifically genes in the Th17 pathway (IL-12B, IL-23A and IL-23R, the latter, like in AS, not seen in Asians), genes in the nuclear factor κB pathway (TNFAIP3 and TNIP1) and genes in the Th2 pathway (IL-4 and IL-13). Given that more than half of patients with AS have evidence on colonoscopy of at least occult
inflammatory bowel disease (IBD), it is not surprising that shared genetic influences are operative. In IBD, genes important in the innate immune response (NOD2), autophagy (ATG6L1) and regulation of the
IL-23 pathway (IL-23R) play a role in
disease susceptibility.