Society: AGA

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.

<b>Table 1. Demographics of Tapestry participants by diagnosis of diarrhea-predominant irritable bowel syndrome. Estimates are mean (SD) or n (%).</b>

Table 1. Demographics of Tapestry participants by diagnosis of diarrhea-predominant irritable bowel syndrome. Estimates are mean (SD) or n (%).

<b>Table 2. Number of SNVs in genotypes related to candidate genes associated with BAD per participant in Tapestry database, according to ICD-10 diagnosis of diarrhea-predominant irritable bowel syndrome or control. Presented as mean (SD) [range].</b>

Table 2. Number of SNVs in genotypes related to candidate genes associated with BAD per participant in Tapestry database, according to ICD-10 diagnosis of diarrhea-predominant irritable bowel syndrome or control. Presented as mean (SD) [range].

Background: Patients with irritable bowel syndrome (IBS) often report gastrointestinal symptoms after consuming wheat/gluten-containing foods. It is, however, unclear whether gluten is the main driver of symptoms, as other immunogenic peptides, such as amylase trypsin inhibitors (ATI), poorly digestible fiber (inulin, part of FODMAP) or even the nocebo effect may contribute to symptom generation.

Aim: To evaluate whether whole wheat (containing ATIs) or purified gluten alone triggers symptoms compared to nocebo in patients with IBS adopting a gluten-free diet (GFD).

Method: We conducted a double-blind, randomized, nocebo-controlled, crossover study in adult IBS patients (Rome IV criteria) who previously perceived symptomatic improvement while on a GFD. Participants were challenged for 7 days with whole wheat, purified gluten, and nocebo (gluten-free flour) added to low-FODMAP cereal bars. Each challenge was followed by a 2-week washout. Patients remained on a GFD throughout the study. Diet adherence was assessed by a dietitian and stool gluten immunogenic peptides (GIP; Biomedal, Spain). Symptoms were assessed by IBS Symptom Severity Score (IBS-SSS); an increase >50 points was considered significantly worse. Blood samples were collected to assess immune markers and celiac genotype. Statistical comparisons used Friedman rank sum tests and paired Wilcoxon signed rank tests.

Results: Twenty-nine IBS patients (27 female, mean age 42 years) were enrolled in the study; 1 dropped. Similar proportions of patients reacted symptomatically to wheat (11/28, 39.3%), gluten (10/28, 35.7%), and nocebo (8/28, 28.6%). However, there was an overall increase in IBS symptoms after wheat (+39.5 on IBS-SSS; p=0.030) but not after gluten (+27.5; p=0.051) or nocebo (+5.5; p=0.236) challenges (Figure 1). Ten participants experienced moderate symptoms (IBS-SSS >175) during baseline and did not worsen during the challenges. Median GIP levels were 0.58 after wheat, 0.43 after gluten, and 0.09 µg/g after nocebo; p<0.001. Four out of 28 patients had GIP levels 0.10 µg/g after the gluten/wheat challenges. TNF-α, IL-8, and IFN-γ levels were low and did not increase after challenges. Celiac genes were present in 19/24 patients (HLA DQ2 n=10, DQ8 n=2, DQ7 n=9).

Conclusions: IBS patients self-reporting wheat/gluten sensitivity had worse symptoms after whole wheat, but not after purified gluten or nocebo challenges. However, similar proportions of patients reacted to each challenge, suggesting that central mechanisms play an important role in symptom genesis. One third of patients had persistently high symptoms during a GFD and challenges, suggesting that other mechanisms are underlying their IBS. Furthermore, negative GIP in some patients suggests a lack of adherence to the study protocol, highlighting the complexity and challenges of dietary intervention studies.

Figure 1. IBS-SSS scores before and after diet challenges.

Figure 1. IBS-SSS scores before and after diet challenges.

Detlef Schuppan

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