Giant cell arteritis and polymyalgia rheumatica can be differentiated by distinct patterns of HLA class II association

Objective. To determine whether patients with polymyalgia rheumatica (PMR) and giant cell arteritis (GCA) exhibit identical HLA class II associations. Methods. A case-control association study was performed on a population sample from Lugo, in Northwestern Spain. DNA samples were available for 128 patients and 145 ethnically matched controls. Within the patient group 26 exhibited both PMR and GCA, 75 PMR alone, and 27 GCA alone. HLA-DRB1, DQA1, and DQB1 phenotypes were defined by molecular based techniques. Results. HLA-DRB1 *0401 was associated with GCA regardless of PMR status, although this only reached statistical significance in the total GCA group. This was also seen for DRB1 *0101, *0102, although the association was less strong. Patients with PMR without GCA were not associated with DRB1 *0401 or *0101, *0102, but exbibited a significant association with DRB1 *13, *14. Non-significant increases in DQA1 and DQB1 phenotype frequencies appeared to reflect known patterns of linkage disequilibrium with the HLA-DRB1 alleles associated with GCA and PMR groups. An association was observed between the presence of the RA DRB1 shared epitope (SE) and GCA but not with PMR in the absence of GCA. This association was primarily accounted for by the presence of a single copy of the SE, and homozygosity for the SE did not confer additional risk. A high frequency of SE-bearing DRB1 alleles was observed in patients with GCA with jaw claudication or visual manifestations, although the sample size of these subgroups was small. Conclusion. PMR and GCA in a Northwestern Spanish population have distinct HLA class II associations. HLA is unlikely to account for the observed high level of overlap in these patients, and other etiological factors may be involved.