The genetics of psoriasis: a complex disorder of the skin and immune system

In the last few years, molecular genetics analyses have permitted novel insights into psoriasis, a disease characterized by uncontrolled proliferation of keratinocytes and recruitment of T cells into the skin. The disease affects approximately 1-2% of the Caucasian population and can occur in association with other inflammatory diseases such as Crohn's disease and in association with human immunodeficiency virus (HIV) infection. Given that psoriasis has characteristics of an autoimmune disease, it is not surprising that HLA studies revealed an association with certain alleles, notably HLA-Cw6. Despite this HLA component, psoriasis in some families is inherited as an autosomal dominant trait with high penetrance. Loci at chromosome 17q25 and 4q have been identified following genome-wide linkage scans of large, multiply affected families. In the case of at least the susceptibility locus at 17q25, the development of psoriasis does not require the presence of HLA-Cw6. Sib-pair analyses have confirmed the association with HLA-Cw6, confirmed the existence of a locus at 17q25 and identified other possible susceptibility loci. Two independent groups have reported a third region on chromosome 20p. Despite these findings, the extent of genetic heterogeneity and the role of environmental triggers and modifier genes is still not clear. The precise role of HLA also still needs to be defined. The isolation of novel susceptibility genes will provide insights into the precise biochemical pathways that control this disease. Such pathways will also reveal additional candidate genes that can be tested for molecular alterations resulting in disease susceptibility.