REPORT Genetic epidemiology of psoriasis and psoriatic arthritis

Psoriasis and psoriatic arthritis (PsA) are interrelated disorders, as most patients with PsA also have psoriasis. Thus it is not surprising that epidemiological and immuno-genetic studies have uncovered important links between these two disorders. Both disorders are highly heritable, and the prevalence of psoriasis is 19 times higher among first degree relatives of probands with PsA compared with the general population. Multiple human leucocyte antigen (HLA) associa-tions are shared between psoriasis and PsA, though the magnitudes of these associations differ between the diseases. Genome-wide linkage studies have noted overlapping regions of significance for these two disorders within and outside the major histocompatibility complex (MHC) region. Thus, exploration of the genetic basis of psoriasis will likely strengthen the contention of an underlying genetic suscept-ibility for PsA and vice versa. P soriasis and psoriatic arthritis (PsA) are clearly complex genetic disorders that result from an interplay between multiple genetic and environmental factors. Although the exact aetiologies of psoriasis and PsA are unclear, cumulative evidence implicates a substantive role for genetic factors in both these diseases with respect to disease susceptibility and expression.1 2 The genetic basis of psoriasis and PsA is supported by family based investigations, population based epidemiological studies, association studies with human leucocyte antigens (HLAs), genome-wide linkage scans, and candidate gene studies within and outside the major histocompatibility complex (MHC) region