TGR5 sequence variation in primary sclerosing cholangitis

TGR5, the G-protein-coupled bile acid receptor 1 (GPBAR1), has been linked to inflammatory pathways and homeostasis, and could therefore be involved in primary sclerosing cholangitis (PSC), a chronic inflammatory bile duct disease associated with ulcerative colitis (UC). In addition, the TGR5 gene is localized at chromosome 2q35, close to a genetic variant associated with both PSC and UC in recent genome-wide association studies. This provided a strong rationale for a recent study searching for novel genetic variants of TGR5 in PSC. In the study, resequencing of TGR5 was performed in 267 PSC patients and 274 healthy controls. Six nonsynonymous mutations were identified in addition to 16 other novel single-nucleotide polymorphisms. In experimental studies, five of the nonsynonymous mutations were found to reduce or abolish TGR5 function by affecting localization or activity. Fine-mapping of the chromosome 2q35 locus in large patient panels revealed an overall association between a common TGR5 single-nucleotide polymorphism and PSC and UC, but strong linkage disequilibrium precluded demarcation of TGR5 from neighboring genes. In conclusion, the data from the experimental evaluation of novel nonsynonymous mutations represent an important insight into the structural biology of TGR5. In addition, the combination of the known roles of the receptor, the genetic associations and the mutations which affect receptor function, suggests that TGR5 variation may contribute to PSC and UC susceptibility. However, this conclusion needs to be strengthened in further research.