Mutational characterization of the bile acid receptor TGR5 in primary sclerosing cholangitis.

TGR5, the G protein-coupled bile acid receptor 1 (GPBAR1), has been
linked to inflammatory pathways as well as bile homeostasis, and
could therefore be involved in primary sclerosing cholangitis (PSC)
a chronic inflammatory bile duct disease. We aimed to extensively
investigate TGR5 sequence variation in PSC, as well as functionally
characterize detected variants. Complete 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. To investigate the impact
from the nonsynonymous variants on TGR5, we created a receptor
model, and introduced mutated TGR5 constructs into human epithelial
cell lines. By using confocal microscopy, flow cytometry and a
cAMP-sensitive luciferase assay, five of the nonsynonymous
mutations (W83R, V178M, A217P, S272G and Q296X) were found to
reduce or abolish TGR5 function. Fine-mapping of the previously
reported PSC and UC associated locus at chromosome 2q35 in large
patient panels revealed an overall association between the TGR5
single-nucleotide polymorphism rs11554825 and PSC (odds ratio =
1.14, 95% confidence interval: 1.03-1.26, p = 0.010) and UC (odds
ratio = 1.19, 95% confidence interval 1.11-1.27, p = 8.5 x 10(-7)),
but strong linkage disequilibrium precluded demarcation of TGR5
from neighboring genes. Resequencing of TGR5 along with functional
investigations of novel variants provided unique insight into an
important candidate gene for several inflammatory and metabolic
conditions. While significant TGR5 associations were detected in
both UC and PSC, further studies are needed to conclusively define
the role of TGR5 variation in these diseases.