HLA-B27 is positive in 90-95% of patients with ankylosing spondylitis  (Tab. 1). There are now considerable epidemiological and transgenic animal model data indi-
Table 1 - Association of HLA-B27 with spondyloarthritides
Population of Population of Ankylosing Sacroilitis/ Reactive
Western/Central Scandinavian spondylitis spondylitis with arthritis
European countries inflammatory countries bowel disease or psoriasis
cating a direct role for HLA-B27 in disease pathogenesis, rather than a closely linked gene, but final proof for this is still lacking in humans . It is also clear now that one copy of HLA-B27 (heterozygosity) is sufficient to get the disease. Recent studies of families and twins affected by AS suggest a polygenic model of genetic susceptibility . The concordance rates for monozygotic twins are about 63%, for dizygotic twins about 12.5% and for B27-positive dizygotic twins about 27%. The susceptibility to AS has been estimated to be >90% genetically determined and it has been suggested that there might be, because of this, a rather ubiquitous environmental factor [3, 5].
Most striking is the direct relationship between the prevalence of SpA and the prevalence of HLA-B27 in the general population. Thus, populations with a higher HLA-B27 prevalence have a higher prevalence of SpA and AS. HLA-B27 is present throughout Eurasia (2-15%), but it is absent among genetically unmixed populations of natives in South America, Australia and of equatorial and southern Africa . However, HLA-B27 has a high prevalence (25-50%) among the native peoples of the circumpolar arctic and subarctic regions of Eurasia and North America. The best investigations about the prevalence of SpA are available for AS. Its prevalence has been estimated between 0.2-1.4% in the western world: 0.2% in Holland  (HLA-B27 background 8%; AS in HLA-B27-positives: 2%); 0.86% in Germany  (HLA-B27 background 9%; AS in HLA-B27-positives 6.4%), and 1.4% in Norway  (HLA-B27 background 14%; AS in HLA-B27 positives 6.7%).
However, there are also exceptions from this close correlation demonstrating that some environmental or genetical factors are different in some places of the world. For example, Japan and Lebanon both have a low prevalence of HLA-B27 of 1% and 2-3%, respectively, but 90% of AS patients are HLA-B27-positive in Japan but only 28% in Lebanon. This means that there must be a strong genetical or environmental difference leading to AS in B27-negatives in Lebanon but not in Japan or in other parts of the world. Furthermore, in West Africa (the Gambia), AS is rare despite the presence of B27 (2-3%) in the population .
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