Characterization of Mucosal Dysbiosis of Early Colonic Neoplasia

Aberrant crypt foci (ACF) are the earliest morphologically identifiable lesion in the colon that can be detected by high-definition chromoendoscopy with contrast dye-spray. Although frequently associated with synchronous adenomas, their role in colorectal tumor development, particularly in the proximal colon, is still not clear. The goal of this study was to evaluate the profile of colon-associated bacteria associated with proximal ACF and to investigate their relationship to the presence and subtype of synchronous polyps present throughout the colon. Forty-five subjects undergoing a screening or surveillance colonoscopy were included in this retrospective study. Our study cohort included a total of 16 subjects with no identifiable proximal lesions (either ACF or polyp), 14 subjects with at least 1 ACF but no polyp(s), and 15 subjects with both at least 1 ACF and a synchronous proximal polyp(s) detected at colonoscopy. Bacterial cells adherent to the epithelia of ACF and normal mucosa were visualized by in situ hybridization within confocal sections. Bacterial DNA isolated from biopsies was used to construct PCR amplicon libraries targeting the V4 hypervariable region of the 16S rRNA gene, which were then sequenced on the Illumina platform. ACF showed significantly greater heterogeneity in their bacterial profiles compared to normal mucosa. Interestingly, one of the bacterial community structures we characterized was strongly correlated with the presence of synchronous polyps. The observed dysbiosis is more prominent within the colonic epithelium that also harbors synchronous polyps. Finally using DNA-mass spectrometry to evaluate a panel of colorectal cancer hot-spot mutations present in the ACF, we found that several APC gene mutations (R1450*, R876*, S1465fs*3) were positively associated with the presence of Instestinibacter sp., whereas KRAS mutations (G12V, G12D) were positively correlated with Ruminococcus gnavus. This result indicates a potential relationship between specific colon-associated bacterial species and somatically acquired CRC-related mutations. Overall, our findings suggest that perturbations to the normal adherent mucosal flora may constitute a risk factor for early neoplasia, demonstrating the potential impact of mucosal dysbiosis on the tissue microenvironment and behavior of ACF that may facilitate (or impede) their progression towards more advanced forms of neoplasia.