Emerging patterns of genetic overlap across autoimmune disorders

Autoimmune disorders (AIDs), which as a group affect approximately 8.5% of individuals worldwide [1], are responsible for a substantial amount of disability and morbidity. Some AIDs are organ specific (for example, type 1 diabetes (T1D) targets the pancreas, autoimmune thyroid disease (AITD) attacks the thyroid gland), whereas others can affect multiple organs and/or be associated with systemic manifestations. Systemic lupus erythematosus (SLE) is the prototypic systemic AID that can affect multiple organs and can also be associated with significant systemic manifestations, morbidity and early mortality [2]. Most AIDs, including rheumatoid arthritis (RA), ankylosing spondylitis (AS), inflammatory bowel disease (IBD) and multiple sclerosis (MS) have a predilection for specific organs (for example, the synovial joints in RA and the gastrointestinal tract in IBD) but are also associated with manifestations outside the primary target organ. Reasons for the diverse manifestations exhibited by different AIDs remain unclear, but recent progress in elucidating genetic susceptibility loci for this group of disorders promises to shed light on this important issue.Although AIDs encompass a broad range of phenotypic manifestations and severity, several features suggest that they share common etiologic factors. For example, most AIDs are characterized by female predominance, and many are associated with the production of autoantibodies (for example, anti-citrullinated-peptide antibodies are observed among 70 to 80% of RA patients). These shared disease features, in conjunction with epidemiologic evidence that demonstrates the clustering of multiple AIDs within individuals and families, strongly implicate shared etiologic factors, including shared genetic loci.Familial clustering of autoimmune disorders has been long recognized and supports a role for shared genetic predisposition. For example, family studies have documented the clustering of certain autoimmune diseases among the