The etiology of RP is unknown. There appears to be a genetic susceptibility, with an association described with HLA-DR4 in Caucasian patients (56% patients vs. 26% healthy controls) (3). Unlike rheumatoid arthritis, however, no specific HLA-DR4 subtype allele has been found (4). A separate study of Caucasian patients with RP found additional associations with DQA1 *0103, DQA1 *0301, and DQB1 *0601, with 59% of patients expressing one or more alleles compared with 38% of controls (5). No association with the HLA type I MHC gene or non-MHC genes has been demonstrated. The exact role these associated genes play in the pathogenesis is unclear, although transgenic mouse models expressing certain DQ Class II alleles develop severe RP following type II collagen immunization.
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