Methylation epigenotypes and genetic features in colorectal laterally spreading tumors

Abstract

Aberrant DNA methylation plays an important role in genesis of colorectal cancer (CRC). Previously, we identified Group 1 and Group 2 methylation markers through genome-wide DNA methylation analysis, and classified CRC and protruded adenoma into three distinct clusters: high-, intermediate- and low-methylation epigenotypes. High-methylation epigenotype strongly correlated with BRAF mutations and these aberrations were involved in the serrated pathway, whereas intermediate-methylation epigenotype strongly correlated with KRAS mutations. Here, we investigated laterally spreading tumors (LSTs), which are flat, early CRC lesions, through quantitative methylation analysis of six Group 1 and 14 Group 2 methylation markers using pyrosequencing. Gene mutations in BRAF, KRAS and PIK3CA, and immunostaining of TP53 and CTNNB1 as well as other clinicopathological factors were also evaluated. By hierarchical clustering using methylation information, LSTs were classified into two subtypes; intermediate-methylation epigenotype correlating with KRAS mutations (p = 9 × 10-4) and a granular morphology (LST-G) (p = 1 × 10-7), and low-methylation epigenotype correlating with CTNNB1 activation (p = 0.002) and a nongranular morphology (LST-NG) (p = 1 × 10-7). Group 1 marker methylation and BRAF mutations were barely detected, suggesting that high-methylation epigenotype was unlikely to be involved in LST development. TP53 mutations correlated significantly with malignant transformation, regardless of epigenotype or morphology type. Together, this may suggest that two molecular pathways, intermediate methylation associated with KRAS mutations and LST-G morphology, and low methylation associated with CTNNB1 activation and LST-NG morphology, might be involved in LST development, and that involvement of TP53 mutations could be important in both subtypes in the development from adenoma to cancer. What's new? Although most colorectal cancers develop from adenomatous polyps, some arise via an alternative pathway involving non-protruding lateral extensions along the luminal wall. These laterally spreading tumors (LSTs) are molecularly not well characterized. Here the authors investigated epigenetic and genetic alterations in LSTs and correlated these with morphological appearances. While colorectal cancers showed high, intermediate, and low DNA methylation phenotypes, LSTs only showed intermediate and low methylation phenotypes, associated with unique genetic mutations and morphological phenotypes. These studies lay the groundwork for the molecular characterization of these untypical early colorectal cancer lesions. © 2014 UICC.