A genome-wide association study of myasthenia gravis

Abstract

Importance: Myasthenia gravis is a chronic, autoimmune, neuromuscular disease characterized by fluctuating weakness of voluntary muscle groups. Although genetic factors are known to play a role in this neuroimmunological condition, the genetic etiology underlying myasthenia gravis is not well understood.

Objective: To identify genetic variants that alter susceptibility to myasthenia gravis, we performed a genome-wide association study.

Design, setting, and participants: DNA was obtained from 1032 white individuals from North America diagnosed as having acetylcholine receptor antibody-positive myasthenia gravis and 1998 race/ethnicity-matched control individuals from January 2010 to January 2011. These samples were genotyped on Illumina OmniExpress single-nucleotide polymorphism arrays. An independent cohort of 423 Italian cases and 467 Italian control individuals were used for replication.

Main outcomes and measures: We calculated P values for association between 8,114,394 genotyped and imputed variants across the genome and risk for developing myasthenia gravis using logistic regression modeling. A threshold P value of 5.0×10(-8) was set for genome-wide significance after Bonferroni correction for multiple testing.

Results: In the overall case-control cohort, we identified association signals at CTLA4 (rs231770; P=3.98×10(-8); odds ratio, 1.37; 95% CI, 1.25-1.49), HLA-DQA1 (rs9271871; P=1.08×10(-8); odds ratio, 2.31; 95% CI, 2.02-2.60), and TNFRSF11A (rs4263037; P=1.60×10(-9); odds ratio, 1.41; 95% CI, 1.29-1.53). These findings replicated for CTLA4 and HLA-DQA1 in an independent cohort of Italian cases and control individuals. Further analysis revealed distinct, but overlapping, disease-associated loci for early- and late-onset forms of myasthenia gravis. In the late-onset cases, we identified 2 association peaks: one was located in TNFRSF11A (rs4263037; P=1.32×10(-12); odds ratio, 1.56; 95% CI, 1.44-1.68) and the other was detected in the major histocompatibility complex on chromosome 6p21 (HLA-DQA1; rs9271871; P=7.02×10(-18); odds ratio, 4.27; 95% CI, 3.92-4.62). Association within the major histocompatibility complex region was also observed in early-onset cases (HLA-DQA1; rs601006; P=2.52×10(-11); odds ratio, 4.0; 95% CI, 3.57-4.43), although the set of single-nucleotide polymorphisms was different from that implicated among late-onset cases.

Conclusions and relevance: Our genetic data provide insights into aberrant cellular mechanisms responsible for this prototypical autoimmune disorder. They also suggest that clinical trials of immunomodulatory drugs related to CTLA4 and that are already Food and Drug Administration approved as therapies for other autoimmune diseases could be considered for patients with refractory disease.

Conflict of interest statement

Conflict of Interest Disclosures: None reported.

Figures

Figure 1. Myasthenia Gravis Cases and P Values From the Genome-Wide Association Study of Myasthenia Gravis

A, Quartile-quartile plot showing the distribution of expected vs observed P values for the US discovery cohort (972 myasthenia gravis cases and 1977 control individuals; λ = 1.036). B, Manhattan plot of genome-wide association results for all myasthenia gravis cases (972 cases and 1977 control individuals). C, Density plot of myasthenia gravis cases (n = 972) showing bimodal frequency distribution with disease being more common in younger women and older men. D, Plot of genome-wide association results for late-onset myasthenia gravis cases (737 cases and 1977 control individuals). E, Plot of genome-wide association results for early-onset myasthenia gravis cases (235 cases and 1977 control individuals). CTLA4 indicates cytotoxic T-lymphocyte–associated protein 4; HLA, human leukocyte antigen; and TNFRSF11A, tumor necrosis factor receptor 4 superfamily, member 11a, NFKB activator gene. The orange horizontal line depicts the Bonferroni-adjusted significance threshold (5.0 × 10−8). Single-nucleotide polymorphisms exceeding this threshold are indicated with red dots, whereas single-nucleotide polymorphisms with a P 5.0 × 10−8 are indicated with yellow dots.

Figure 2. Regional Association Plots Across Loci Identified by the Genome-Wide Association Study of Myasthenia Gravis

Regional association plots of the signals at the major histocompatibility complex (MHC) region (A) and CTLA4 (B) in all myasthenia gravis cases (972 cases and 1977 control individuals), as well as the MHC region in early-onset myasthenia gravis cases (C; 235 cases and 1977 control individuals), the MHC region in late-onset cases (D), and TNFRSF11A in late-onset myasthenia gravis cases (E; 737 cases and 1977 control individuals). Single-nucleotide polymorphisms are colored on the basis of their linkage disequilibrium with the labeled hit single-nucleotide polymorphism. Recombination rates estimated from the CEU HapMap population are represented by the blue line and genes are marked as arrows. Red diamonds represent the most associated single-nucleotide polymorphism in the discovery cohort and blue diamonds, P values for the combined discovery and replication cohorts. F, CTLA4 regulates T-cell activation by antigen-presenting cells (APCs). TCR indicates T-cell receptor.