Psoriasis is a chronic inflammatory skin disease. Initial disease onset typically occurs in adulthood, though it can occur in children, the elderly and any age in between. The clinical history usually consists of cycles of psoriatic flares followed by remissions. Psoriasis is common in Caucasians (2% - 3%), but is very rare in Asian populations and nearly absent from African populations.
There are several dermal phenotypes associated with psoriasis. The most common form, psoriasis vulgaris, is characterized by scaly, inflamed plaques targeting the knees, elbows, scalp and trunk. Other less common types include inverse psoriasis, sebopsoriasis, guttate psoriasis and pustular psoriasis. These target different body regions and have varying appearance. Approximately 10-30% of psoriasis patients develop psoriatic arthritis which progressively destroys joint tissue.
Genome-wide association studies (GWAS) have identified several loci associated with psoriasis vulgaris. The HLA class I region of the major histocompatibility complex (MHC), in particular the haplotype harboring HLA-Cw*0602 provides strongest evidence for association, and supports earlier association and linkage studies with alleles of the MHC. Other loci include those harboring genes involved in immune cell activation (IL23A, IL23R, TNFAIP3, TNIP1) and epidermal differentiation