An amino acid motif in HLA-DRß1 distinguishes patients with uveitis in juvenile idiopathic arthritis

A.J.W. Haasnoot; M.W. Schilham; S.S.M. Kamphuis; P.C.E. Hissink Muller; A. Heiligenhaus; D. Foell; R.A. Ophoff; T.R.D.J. Radstake; A.I. Den Hollander; T.H.C.M. Reinards; S. Hiddingh; N. Schalij-Delfos; E.P.A.H. Hoppenreijs; M.A.J. van Rossum; C. Wouters; R.K. Saurenmann; N. Wulffraat; R. ten Cate; J.H. de Boer; S.L. Pulit; J.J.W. Kuiper

Through interrogation of imputed HLA types and amino acids, we have identified the amino acid serine at position 11 in the HLA-DRB1 gene as strongly associated to increased risk of uveitis in female patients with JIA. This association indicates that, though JIA with uveitis and JIA without uveitis share clinical features and genetic risk factors (and in particular, HLA-specific genetic risk factors 20 ) , there is genetic architecture specific to uveitis. 4, 42, 43 Specifically, we use conditional testing and likelihood ratio tests to show that the previously reported signals in JIA at position 13 is a distinct signal from the uveitis signal at position 11 in HLA-DRB1. These findings suggest that the two phenotypes are likely to have biological features and progressions that are partially distinct. 44 Association testing initially revealed either serine or aspartic acid at position 11 in HLA-DRB1 as the most-associated variant, a signal we determined was primarily driven by serine. However, the perfect linkage disequilibrium between serine, tyrosine (position 10), and threonine (position 12) makes it impossible to disentangle the three amino acids in a statistical framework; a subset of the amino acids, or all three (that is, the YST-motif) may be key to disease onset. Importantly, though all three residues are located at the bottom of the HLA-DRB1 peptide-binding groove, only serine-11 is positioned towards, and, thus most likely interacting with, binding peptide epitopes (Figure 2) . Previous work has identified serine-11 as the strongest risk factor in seronegative rheumatoid arthritis (RA). Some JIA patients, including those with uveitis, might be categorized as seronegative RA by the time they reach adulthood, and seronegative RA is considered to genetically mirror the uveitisprone JIA categories. 20 In contrast, serine-11 is highly protective against seropositive RA, a biologically distinct form of RA in which uveitis is not common (<1% of cases). 45, 46 Sex-specific analyses in the MHC further revealed the HLA-DRB1 signal was unique to female samples: the interaction of sex and the HLA-DRB1 serine-11 genotype was significant, and the association signal at serine-11 was entirely absent when the analysis was restricted to male samples only. The reduced number of cases (N = 56 male JIA-uveitis cases and N = 102 male non-uveitis JIA controls) and therefore reduced power in the maleonly analysis may explain the absence of a signal at serine-11. However, we had 98.5% power to detect an association of p < 0.05 at serine-11 in the male-only analysis, assuming the odds ratio in males was the same as in females (OR = 3.30). Assuming a more modest effect in males (OR = 2.00), we still had 75.2% power to detect a signal at p < 0.05. To detect potentially more subtle effects in males (OR < 2), larger case collections will be necessary to improve power. Nonetheless, the interaction test of sex and serine-11 provides compelling evidence for a sexually-dimorphic signal in HLA-DRB1.