Epigenetic inactivation of RUNX3 in preneoplastic lesions of colorectum

Abstract

B6

Background:
Based on widely prevalent epigenetic silencing of RUNX3 gene in invasive malignancies and precancerous gastric lesions, we hypothesized that RUNX3 inactivation by promoter methylation in colorectal polyps is an early molecular event in colorectal carcinogenesis.
Design and settings:
We examined RUNX3 protein expression by immunohistochemical analysis and promoter hypermethylation by methylation-specfic PCR (MSP) in 50 sporadic colorectal polyps, including 17 hyperplastic polyps (HPs), 14 serrated adenomas (SAs) and 19 sporadic traditional adenomas (sTAs), as well as 19 familial adenomatous polyposis (FAP) samples of 10 patients showing aberrant crypt foci (ACF) (n=91), small adenomas (SAds) (n=40) and large adenomas (LAds) (n=13).
Results:
Compared to normal colon (2/12, 16%) and sTAs (3/17, 18%), HPs (15/19, 79%) and SAs (8/14, 57%) displayed significant inactivation of RUNX3 (p<0.05). Promoter hypermethylation of RUNX3 was significantly higher in colorectal polyps (52/68, 94%) compared to normal colon (2/12, 16%) (p=0.001). Serrated polyps such as HPs (17/19, 85%) and SAs (12/14, 89%) were significantly methylated compared to sTAs (7/17, 44%) (p=0.004). In FAP, RUNX3 inactivation was more frequently seen in ACF (78/91, 86%), SAds (25/40, 62%) and LAds (6/13, 45%) compared to normal mucosa (0/19, 0%) in the same samples (all p<0.05). RUNX3 hypermethylation was observed in 16 of total 19 (84%) FAP cases. Overall, RUNX3 methylation correlated with lack of protein expression in sporadic (27/36, 75%) (p=0.022) and FAP (15/16, 94%) (p=0.004) polyps.
Conclusions: Epigenetic inactivation of RUNX3 due to promoter hypermethylation in colorectal polyps constitute an early event in colorectal cancer (CRC) progression. Frequent RUNX3 inactivation in serrated polyps compared to traditional colorectal adenomas differentiates the two distinctive pathways of colorectal carcinogenesis.