Serrated pathway and APC (conventional)-type colorectal polyps: molecular-morphologic correlations, genetic pathways, and implications for classification

Abstract

This review addresses the genetic mutations and cell signaling pathway alterations in colorectal premalignant polyps, focusing on the link between molecular changes and morphologic features. Biallelic APC (adenomatous polyposis coli) mutations are directly responsible for the specific and characteristic cytologic features of dysplastic cells in conventional tubular adenomas. Sessile serrated adenomas (SSAs) are the precursor lesions of the serrated neoplasia pathway. The BRAF activating mutation and hypermethylation of SLC5A8, which mediates short chain fatty acid transport, may be the important events in the genesis of SSAs. Intracellular butyrate inhibits histone deacetylase, allowing histone hyperacetylation and, eventually, transcriptional activation of specific genes. Decreased p21(WAF1/CIP1) and activation of the mitogen-activated protein kinase pathway may be the key intermediary alterations. Progressive loss of cell cycle control and decreased and altered cytoplasmic differentiation produce the characteristic constellation of morphologic changes of SSAs and traditional serrated adenomas.