Irritable bowel syndrome with diarrhea and non-celiac gluten sensitivity are characterized by increased zonulin serum levels

Background & Aims: Zonulin, the human analogue of the Vibrio cholerae zonula occludens toxin, is an endogenous modulator of epithelial tight junctions (TJs) and is overexpressed in disorders in which TJ dysfunction is central, including celiac disease (CD). We hypothesized that loss of intestinal barrier function secondary to the activation of the zonulin pathway by food-derived environmental triggers or changes in gut microbiota, may be a key factor involved in the pathogenesis of irritable bowel syndrome (IBS) and non-celiac gluten sensitivity (NCGS). Methods: A total of 15 patients with IBS-D, 15 patients with NCGS, 15 patients with CD and 15 healthy controls (HC) were studied. ELISA assay was used to evaluate zonulin serum levels; total protein amount was evaluated spectrophotometrically and used to normalize data. Clinical data were recorded for each patient including: anti-transglutaminase (TTG) antibodies, anti-deamidated gliadin peptide (DGP) antibodies, IgE, abdominal symptoms and bowel habit. The expression of zonulin gene was evaluated in paraffin-embedded colonic biopsy samples (20 IBS and 10 HC) using quantitative real-time RT-PCR assay. Results: Zonulin serum levels were significantly different among the four groups (p<0.0001). CD patients showed significantly higher zonulin serum levels compared to HC (0.033± 0.004 vs 0.007 ± 0.001 ng/mg total proteins, p<0.0001) and to IBS-D patients (0.012 ± 0.002 ng/mg total proteins, p<0.001). NCGS zonulin serum levels were significantly higher than HC (0.030 ± 0.006 vs 0.007 ± 0.001, p<0.05) and IBS-D (p<0.05). IBS-D patients showed higher zonulin serum level than HC without reaching the statistical significance (p= 0.1). Zonulin levels were positively correlated with the titer of anti-DGP antibodies (r:0.6; p<0.05,) and anti-TTG antibodies (r:0.6; p<0.05). mRNA expression of zonulin was increased in the colonic mucosa of patients with IBS in comparison with HC although the difference did not reach the statistical significance (p=0.08). Conclusions: Zonulin serum levels increase not only in CD but also in NCGS and IBS-D patients. Interestingly, zonulin serum levels correlate with anti-TTG and anti-DGP antibody titers. Our data suggest that a zonulindependent TJ dysfunction inducing "leaky gut" may play a role in the pathophysiology of NCGS and IBS. Further study are necessary to demonstrate the possible role of zonulin as a biomarker in pathologies with permeability alteration like NCGS and IBS.