INCREASED NUMBERS OF SINGLE NUCLEOTIDE VARIANTS IN GENES RELATED TO BILE ACID KINETICS ARE ASSOCIATED WITH DIARRHEA-PREDOMINANT IRRITABLE BOWEL SYNDROME: A WHOLE EXOME SEQUENCING STUDY OF 526 PATIENTS AND 2950 CONTROLS

Background: Irritable bowel syndrome (IBS) is a common disorder with complex pathophysiological mechanisms. Prior studies suggest that 25-50% of patients with diarrhea-predominant IBS (IBS-D) have evidence of bile acid (BA) diarrhea (BAD). Studies of candidate genes involved in the BA pathway have identified single nucleotide variants (SNVs) in the synthesis of farnesoid X receptor (NR1H4, MALRD1/DIET1), proteins involved in hepatic re-uptake of BA (KLB, FGFR4), the rate-limiting enzyme of BA synthesis (CYP7A1), and BA receptors (GPBAR1/TGR5) that may be implicated in the pathophysiology of IBS-D and BAD. Aim: To explore the burden of allelic variants among known SNVs across these genes in IBS-D. Methods: Design: Case-control analysis of the genetic differences between those with diagnosis of IBS-D compared to age- and sex-matched controls. Genomic data were provided by the Mayo Clinic Tapestry Study for the target genes listed above along with links exomic database which was linked to the electronic medical record (EMR). Cases of IBS-D were defined according to ICD-10 code, and controls were those without documentation of the same ICD-10 diagnosis. SNVs with either low quality, read depth, or probability of a functional impact (e.g. synonymous and non-splice site intronic variants) were disregarded. The total burden of non-reference alleles between IBS-D and control patients was compared using ANOVA with α=0.05. Results: After data cleaning, our final sample included 526 patients with IBS-D and 2,950 controls. The mean (SD) age of the entire sample was 53.1 (15.8) years, and majority (78.9%) were female (Table 1). Three thousand and eighteen (95.5%) participants self-identified as White. Only ethnicity was statistically significantly different between the groups, with 4.6% and 9.4% identifying as Hispanic or Latino in the IBS-D and control groups, respectively. The summary of the genetic findings is shown in Table 2. Participants in the IBS-D group had, on average, a significantly greater genetic burden of homozygous alternate alleles in MALRD1 compared to controls (p=0.006). There was a greater proportion of heterozygote genotypes for key SNVs in CYP7A1 in controls compared to those with IBS-D. Other SNVs in candidate genes did not demonstrate a statistically significant difference in frequency of alternate alleles between IBS-D and controls. Conclusions: MALRD1 (previously DIET1), a gene that codes for an intestinal protein involved in enterohepatic circulation by regulating FGF-19 and CYP7A1 (BA synthesis), may contribute to pathophysiological mechanisms in IBS-D. Further studies are needed to explore specific genetic variants and their functional impact in MALRD1 and CYP7A1 in IBS-D that may elucidate disease mechanisms and potential therapeutic targets, such as with FGF-19 analogs or FXR agonists.