Deletion polymorphism upstream of IRGM associated with altered IRGM expression and Crohn's disease

Abstract

Mark Daly, Ramnik Xavier and colleagues report that a 20-kb deletion polymorphism upstream of IRGM is associated with altered IRGM expression and Crohn's disease. They also show that manipulation of IRGM levels in cells modulated the efficiency of autophagic responses to internalized bacteria, suggesting a mechanism by which the deletion variant might influence disease risk. Following recent success in genome-wide association studies, a critical focus of human genetics is to understand how genetic variation at implicated loci influences cellular and disease processes. Crohn's disease (CD) is associated with SNPs around IRGM1,2, but coding-sequence variation has been excluded as a source of this association2. We identified a common, 20-kb deletion polymorphism, immediately upstream of IRGM and in perfect linkage disequilibrium (r2 = 1.0) with the most strongly CD-associated SNP, that causes IRGM to segregate in the population with two distinct upstream sequences. The deletion (CD risk) and reference (CD protective) haplotypes of IRGM showed distinct expression patterns. Manipulation of IRGM expression levels modulated cellular autophagy of internalized bacteria, a process implicated in CD. These results suggest that the CD association at IRGM arises from an alteration in IRGM regulation that affects the efficacy of autophagy and identify a common deletion polymorphism as a likely causal variant.