Mutation screening of apical sodium-dependent bile acid transporter (SLC10A2): novel haplotype block including six newly identified variants linked to reduced expression

Abstract

The apical sodium-dependent bile acid transporter (SLC10A2) plays a key role in the reabsorption of luminal bile acids into the enterohepatic circulation. Rare variations in SLC10A2 have been reported to be associated with Crohn's disease, primary bile acid malabsorption and familial hypertriglyceridemia; however, variants associated with reduced SLC10A2 expression have not been reported to date. In this study, we have performed a sequence analysis of SLC10A2 using genomic DNA of 93 individuals. A new haplotype structure was identified including ten variants with complete linkage disequilibrium (LD' = 1.0, r (2) = 1.0) of which six polymorphisms were novel. The sequence variants were confirmed in three independent cohorts (n = 1,290) by a recently established MALDI-TOF MS iPLEX assay. Remarkably, haplotype carriers with the minor allele exhibited significant reduced ileal SLC10A2 expression on mRNA levels (2.6-fold, P = 0.0009) and protein levels (2.4-fold, P = 0.0157). In future studies a single tag SNP selected of this haplotype block will provide reliable genetic testing to investigate systemically the influence of the SLC10A2 haplotype for disease susceptibility and/or drug response.

Bile acid malabsorption in inflammatory bowel disease

“…The 129 C>T variant, representing a novel haplotype block (possibly associated with the decreased ileal expression of SLC10A2 21 ) was typed in 87% of the control subjects and 91% of the patients. Due to the low frequency of a minor allele, no significant effect on the serum C4 levels could be observed (data not shown).…”

“…20 Recently, a haplotype block associated with the lower expression of ISBT has been described. 21 Upon stimulation with BA, ileal enterocytes secrete fibroblast growth factor 19 (FGF19), which subsequently downregulates BA synthesis in the hepatocytes. 22 Thus, FGF19 represents an additional negative feedback regulatory mechanism for the production of BA.…”

Bile acid transporters

“…Although dysfunctional mutations were not found in the ASBT gene from patients with adult-onset forms of idiopathic bile acid malabsorption ( 190 ), aberrant regulation of the ASBT may still contribute to the phenotype in a subset of those patients ( 172 ). Indeed, a recent genetic study identifi ed an ASBT haplotype associated with signifi cantly reduced ileal expression of ASBT mRNA and protein ( 191 ). Other disorders associated with intestinal bile acid malabsorption that could potentially involve the ASBT include hypertriglyceridemia ( 192,193 ), idiopathic chronic diarrhea ( 194 ), chronic ileitis ( 195 ), gallstone disease ( 196,197 ), postcholecystectomy diarrhea, Crohn's disease (198)(199)(200)(201)(202), and irritable bowel syndrome ( 203 ).…”

Upregulation of hepatic bile acid synthesis via fibroblast growth factor 19 is defective in gallstone disease but functional in overweight individuals

“…21,22,28 Primer sequences used for amplifications and specific PCR conditions are listed in Supplementary Table S2. At the end of the cycling program, a dissociation curve was calculated.…”