Classification of genetic profiles of Crohn's disease: a focus on the ATG16L1 gene.

Inflammatory bowel disease constitutes two related clinical entities, Crohn's disease (CD) and ulcerative colitis (UC), both of which have increased in prevalence over the last decade. Family and twin studies have strongly indicated that genetic factors play a large role in an individual's risk of developing inflammatory bowel disease. Despite this, it has proven difficult to isolate disease genes that confer susceptibility to this disease using classical candidate gene and linkage approaches, with the notable exception of the isolation of the caspase recruitment domain family, member 15 (CARD15) gene. However, over the last 2 years, genome-wide association (GWA) studies have become feasible, where modern high-throughput single nucleotide polymorphism (SNP) genotyping technologies can be applied to large and comprehensively phenotyped patient cohorts. Such approaches have enabled scientists to robustly associate specific variants with many complex diseases, including age-related macular degeneration, Type 2 diabetes, breast cancer and asthma. In the inflammatory bowel disease field, positive associations with CD and UC coming from GWA studies have been reported for an ever increasing number of genes. The most consistently and strongly associated variants have been in the CARD15, the interleukin 23 receptor (IL23R) and autophagy-related 16-like 1 (ATG16L1) genes. With respect to ATG16L1, the G allele of SNP rs2241880 has been shown in multiple association studies to confer strong risk for CD, although its association with UC remains more debatable. This SNP is in fact a common coding variant, specifically a threonine-to-alanine substitution at amino acid position 300 of the ATG16L1 protein (T300A), and appears to account for all of the disease risk conferred by this locus. This review addresses recent advances in GWA studies of inflammatory bowel disease, with specific focus on the growing evidence of the ATG16L1 gene's role in CD and how its protein product operating within the autophagic pathway makes autophagy an attractive therapeutic target for this debilitating disorder.