MicroRNA-31 expression in relation to BRAF mutation, CpG island methylation and colorectal continuum in serrated lesions

The CpG island methylator phenotype (CIMP) is a distinct form of epigenomic instability. Many CIMP-high colorectal cancers (CRCs) with BRAF mutation are considered to arise from serrated pathway. We recently reported that microRNA-31 (miR-31) is associated with BRAF mutation in colorectal tumors. Emerging new approaches have revealed gradual changes in BRAF mutation and CIMP-high throughout the colorectum in CRCs. Here, we attempted to identify a possible association between miR-31 and epigenetic features in serrated pathway, and hypothesized that miR-31 supports the colorectal continuum concept. We evaluated miR-31 expression, BRAF mutation and epigenetic features including CIMP status in 381 serrated lesions and 222 non-serrated adenomas and examined associations between them and tumor location (rectum; sigmoid, descending, transverse and ascending colon and cecum). A significant association was observed between high miR-31 expression and CIMP-high status in serrated lesions with BRAF mutation (p=0.0001). In contrast, miR-31 was slightly but insignificantly associated with CIMP status in the cases with wild-type BRAF. miR-31 expression in sessile serrated adenomas (SSAs) with cytological dysplasia was higher than that in SSAs, whereas, no significant difference was observed between traditional serrated adenomas (TSAs) and TSAs with high-grade dysplasia. The frequency of miR-31, BRAF mutation CIMP-high and MLH1 methylation increased gradually from the rectum to cecum in serrated lesions. In conclusion, miR-31 expression was associated with CIMP-high status in serrated lesions with BRAF mutation. Our data also suggested that miR-31 plays an important role in SSA evolution and may be a molecule supporting the colorectal continuum.