Distinct WNT/β-catenin signaling activation in the serrated neoplasia pathway and the adenoma-carcinoma sequence of the colorectum

Abstract

Sessile serrated adenoma/polyp (SSA/P) is considered as an early precursor in the serrated neoplasia pathway leading to colorectal cancer development. The conventional adenoma-carcinoma sequence is associated with activation of the WNT signaling pathway, although its role in serrated lesions is still controversial. To clarify differences in WNT signaling activation in association with MLH1 methylation or BRAF/KRAS mutations between serrated and conventional routes, we performed β-catenin immunostaining, methylation-specific PCR for MLH1 and WNT signaling associated genes such as AXIN2, APC, and MCC and secreted frizzled-related proteins (SFRPs), and direct sequencing of BRAF/KRAS in 27 SSA/Ps, 14 SSA/Ps with high-grade dysplasia and 9 SSA/Ps with submucosal carcinoma, as well as 19 conventional adenomas, 26 adenomas with high-grade dysplasia and 25 adenomas with submucosal carcinoma. Nuclear β-catenin labelings were significantly lower in the serrated series than in their adenoma counterparts, and a significant increment in those labelings was found from SSA/Ps to those with high-grade dysplasia or submucosal carcinoma. The frequency of MLH1 and SFRP4 methylation was significantly higher in SSA/P series, as compared with corresponding adenoma series. AXIN2 and MCC were more frequently methylated in SSA/Ps with high-grade dysplasia and those with submucosal carcinoma than in adenoma counterparts. Stepwise increment of AXIN2 and MCC methylation was identified from SSA/Ps through those with high-grade dysplasia to those with submucosal carcinoma. A significant correlation was seen between nuclear β-catenin expression and methylation of AXIN2 or MCC in the SSA/P series. BRAF mutation was more frequent, whereas KRAS mutation was less frequent in the SSA/P series as compared with the adenoma series. There was an inverse association of BRAF mutation with AXIN2 methylation in SSA/P series. In conclusion, WNT/β-catenin signal activation mediated by the methylation of SFRP4, MCC, and AXIN2 may make different contributions to colorectal neoplasia between the serrated and conventional routes.