Background and Aim: Conventional colorectal cancer develops following the adenoma-carcinoma sequence of mutational events. We aimed to characterize the heterogenicity of mutational alterations and immune cell infiltrates in colorectal polyp subtypes obtained from patients undergoing colonoscopy. We examined relationships of these molecular features with histopathology variables used for disease risk assessment.
Methods: Polyp-, adjacent-, healthy-mucosal biopsies and blood were collected from 63 patients during colonoscopy at Royal North Shore Hospital, Sydney. Indications for colonoscopy included rectal bleeding, FOBT-positive, scheduled surveillance, positive family history of bowel neoplasia. Patients with familiar syndromes or inflammatory bowel disease were excluded. Molecular profiling included hybridization capture whole-exome sequencing (WES) targeting 200X depth, and multiplexed IHC immune cell phenotyping.
Results: In 63 low grade polyp biopsies (48 Tubular/villous adenomas, 10 Hyperplastic, 5 Sessile serrated lesions (SSL)), molecular profiling revealed that APC, KRAS, BRAF, ROS1, TP53, and CTNNB1, were the top 6 driver genes most frequently mutated in colorectal polyps. Mean mutational burden of 63 polyps was 3.5/Mb (range 2.12-6.45/Mb). SSL had higher mean mutational burden 4.0/Mb, while hyperplastic polyps had lower mean mutational burden (3.2/Mb). Interestingly, some small tubular adenomas (<10mm) had relatively high mutational burdens, compromising tumour suppressor functions and could be considered to show elevated risk of malignant transformation.
Immunophenotyping of polyps showed CD3+ T cell density was significantly increased compared to paired normal mucosa. In contrast, significant decreases in CD68+ macrophage and CD11c+ dendritic cell densities occurred in polyp tissue. Polyp mutational burden showed significant associations with changes in macrophage density.
Conclusion: In-depth WES and molecular characterization provides a comprehensive view of genetic background in early colorectal polyps. This information may be useful to identify at-risk disease which would be missed by conventional histopathology review.