Immune-Array Analysis in Sporadic Inclusion Body Myositis Reveals HLA-DRB1 Amino Acid Heterogeneity Across the Myositis Spectrum

Abstract

Objective: Inclusion body myositis (IBM) is characterized by a combination of inflammatory and degenerative changes affecting muscle. While the primary cause of IBM is unknown, genetic factors may influence disease susceptibility. To determine genetic factors contributing to the etiology of IBM, we conducted the largest genetic association study of the disease to date, investigating immune-related genes using the Immunochip.

Methods: A total of 252 Caucasian patients with IBM were recruited from 11 countries through the Myositis Genetics Consortium and compared with 1,008 ethnically matched controls. Classic HLA alleles and amino acids were imputed using SNP2HLA.

Results: The HLA region was confirmed as the most strongly associated region in IBM (P = 3.58 × 10^-33 ). HLA imputation identified 3 independent associations (with HLA-DRB1*03:01, DRB1*01:01, and DRB1*13:01), although the strongest association was with amino acid positions 26 and 11 of the HLA-DRB1 molecule. No association with anti-cytosolic 5'-nucleotidase 1A-positive status was found independent of HLA-DRB1*03:01. There was no association of HLA genotypes with age at onset of IBM. Three non-HLA regions reached suggestive significance, including the chromosome 3 p21.31 region, an established risk locus for autoimmune disease, where a frameshift mutation in CCR5 is thought to be the causal variant.

Conclusion: This is the largest, most comprehensive genetic association study to date in IBM. The data confirm that HLA is the most strongly associated region and identifies novel amino acid associations that may explain the risk in this locus. These amino acid associations differentiate IBM from polymyositis and dermatomyositis and may determine properties of the peptide-binding groove, allowing it to preferentially bind autoantigenic peptides. A novel suggestive association within the chromosome 3 p21.31 region suggests a role for CCR5.

Figure 1

Manhattan plots of the inclusion body myositis (IBM) analysis. Red line represents genome‐wide level of significance (P < 5 × 10⁻⁸); blue line represents suggestive significance (P < 2.25 × 10⁻⁵). Shown is the analysis of 252 patients with IBM and 1,008 matched controls. A, Manhattan plot of the total Immunochip analysis. B, Manhattan plot of the IBM analysis with the major histocompatibility complex (MHC) region (chromosome 6 25–35) removed for visualization purposes. Color figure can be viewed in the online issue, which is available at http://onlinelibrary.wiley.com/journal/doi/10.1002/art.40045/abstract.

Figure 2

Regional association plot of the chromosome 3 (Chr3) p21.31 region in inclusion body myositis. The plot shows strength of association (−log₁₀[P]) against chromosomal position. The most strongly associated single‐nucleotide polymorphism (SNP) is colored purple, with other SNPs colored by the degree of linkage disequilibrium (r²). Local recombination rates estimated from the HapMap population of Utah residents with ancestry from northern and western Europe are plotted against the secondary y‐axis, showing recombination hotspots across the region.

Figure 3

Locations of positions 26 and 11 of HLA–DRB1 within DR β‐chain 1. Positions 26 and 11 are independently associated with inclusion body myositis. Arrows indicate the locations of the risk‐conferring amino acids Tyr²⁶ and Ser¹¹ within the β‐sheet floor of DR β‐chain 1. Color figure can be viewed in the online issue, which is available at http://onlinelibrary.wiley.com/journal/doi/10.1002/art.40045/abstract.