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Objectives: The genetic contribution to psoriatic disease is substantial with a dominating influence of the HLA region. The profile of HLA class I genotypes likely contributes to shaping clinical phenotypes. Herein we aimed to explore such genotypes in cohorts of closely characterised subsets of psoriatic disease with special focus on psoriatic arthritis mutilans (PAM), a severe and rare form of psoriatic arthritis (PsA)., Methods: Cohorts of patients with the diagnosis of psoriasis vulgaris with or without arthritis (n=1217), psoriasis without arthritis (n=534), psoriatic arthritis without mutilating disease (n=337) and psoriatic arthritis mutilans (n=63) were collected and genotyped for HLA class I and II genes, with standardised methodologies. Cases were compared with a healthy control population (n=2468). Case-only and case-control association tests were performed to address the hypothesis of genetic contribution to clinical phenotypes., Results: The presence of HLA-B*27 was strikingly increased in PAM (45%) compared with PsA without mutilating disease (13%) and with healthy controls (13%). However, within the PAM population, HLA-B*27 did not correlate with clinical markers such as number of mutilating joints, radiographic scoring, disease duration and age of disease onset., Conclusions: HLA-B*27 emerges as an important genotype marker for PAM.

HLA-DRB1 shared epitope (SE) alleles are the strongest genetic determinants for autoantibody positive rheumatoid arthritis (RA). One of the key regulators in expression of HLA class II receptors is MHC class II transactivator (CIITA). A variant of the CIITA gene has been found to associate with inflammatory diseases.We wanted to explore whether the risk variant rs3087456 in the CIITA gene interacts with the HLA-DRB1 SE alleles regarding the risk of developing RA. We tested this hypothesis in a case-control study with 11767 individuals from four European Caucasian populations (6649 RA cases and 5118 controls).We found no significant additive interaction for risk alleles among Swedish Caucasians with RA (n = 3869, attributable proportion due to interaction (AP) = 0.2, 95%CI: -0.2-0.5) or when stratifying for anti-citrullinated protein antibodies (ACPA) presence (ACPA positive disease: n = 2945, AP = 0.3, 95%CI: -0.05-0.6, ACPA negative: n = 2268, AP = -0.2, 95%CI: -1.0-0.6). We further found no significant interaction between the main subgroups of SE alleles (DRB1*01, DRB1*04 or DRB1*10) and CIITA. Similar analysis of three independent RA cohorts from British, Dutch and Norwegian populations also indicated an absence of significant interaction between genetic variants in CIITA and SE alleles with regard to RA risk.Our data suggest that risk from the CIITA locus is independent of the major risk for RA from HLA-DRB1 SE alleles, given that no significant interaction between rs3087456 and SE alleles was observed. Since a biological link between products of these genes is evident, the genetic contribution from CIITA and class II antigens in the autoimmune process may involve additional unidentified factors.

Primary Sjögren's syndrome (SS) shares many features with systemic lupus erythematosus (SLE). Here we investigated the association of the three major polymorphisms in IRF5 and STAT4 found to be associated with SLE, in patients from Sweden and Norway with primary SS. These polymorphisms are a 5-bp CGGGG indel in the promoter of IRF5, the single nucleotide polymorphism (SNP) rs10488631 downstream of IRF5 and the STAT4 SNP rs7582694, which tags the major risk haplotype of STAT4. We observed strong signals for association between all three polymorphisms and primary SS, with odds ratios (ORs) >1.4 and P-values <0.01. We also found a strong additive effect of the three risk alleles of IRF5 and STAT4 with an overall significance between the number of risk alleles and primary SS of P=2.5 x 10(-9). The OR for primary SS increased in an additive manner, with an average increase in OR of 1.78. For carriers of two risk alleles, the OR for primary SS is 1.43, whereas carriers of five risk alleles have an OR of 6.78. IRF5 and STAT4 are components of the type I IFN system, and our findings emphasize the importance of this system in the etiopathogenesis of primary SS.