Beta-catenin Nuclear Labeling is a Common Feature of Sessile Serrated Adenomas and Correlates With Early Neoplastic Progression After BRAF Activation

Abstract

Recent observations indicate that some sessile serrated adenomas (SSAs) have aberrant β-catenin nuclear labeling, implicating the Wnt pathway in the molecular progression of SSAs to colorectal carcinoma. We sought to expand upon this finding by characterizing β-catenin expression in the full spectrum of serrated colorectal polyps, and correlating these findings with the genetic status of BRAF, KRAS and CTNNB1. Immunolabeling for β-catenin confirmed the presence of abnormal nuclear accumulation in SSAs, with 35/54 (67%) SSAs showing nuclear labeling compared to 0/12 hyperplastic polyps (HPs). Abnormal nuclear labeling was also identified in 4/11 (36%) traditional serrated adenomas (TSAs) (p=0.00001). When SSAs were further analyzed with respect to the presence or absence of conventional epithelial dysplasia, nuclear β-catenin labeling was seen in 8/27 (29%) SSAs without dysplasia (SSA) but in 27/27 (100%) of SSAs with dysplasia (SSADs) (p=0.000001). Sequencing of genomic DNA extracted from a subset of HPs, SSAs, SSADs, TSAs and tubular adenomas (TAs) failed to identify any CTNNB1 mutations to account for abnormal β-catenin nuclear labeling. However, abnormal nuclear labeling always occurred in the setting of a BRAF V600E mutation, indicating aberrant nuclear labeling occurs on a background of BRAF activation. Of interest, all six TSAs contained a KRAS mutation confirming that SSAs and TSAs are genetically distinct entities. These findings validate previous reports implicating activation of the Wnt signaling pathway in SSAs, and further indicate that Wnt pathway activation plays a role in the neoplastic progression of SSAs and TSAs to colonic carcinoma by mechanisms independent of CTNNB1 mutation.

Transgenic expression of oncogenic BRAF induces loss of stem cells in the mouse intestine, which is antagonized by β-catenin activity

“…13 BRAF-activated serrated human tumors show unique patterns of DNA methylation in CpG-rich regions 10,14,15 and strong nuclear β-catenin staining, indicative of tumorassociated activation of the Wnt/β-catenin pathway. 16,17 Mice with an oncogenic BRAF V637E knock-in mutation develop generalized serrated hyperplasia, thus corroborating an initiating role for oncogenic BRAF in serrated adenoma. 18 Aging BRAF V637E knock-in mice exhibit focal tumor progression to dysplastic serrated adenoma and metastatic carcinoma.…”

“…The resulting ISC depletion was counteracted by upregulation of Wnt/β-catenin signaling by Wnt3a ligand or by small-molecule inhibition of GSK3β in organotypic culture, or by transgenic expression of stabilized β-catenin in the mouse intestine. Both high MAPK and β-catenin activity has been noted in human SSA 16,17 and in BRAF-driven serrated intestinal tumors in the mouse. 18 Our data suggest that the concomitant activation of MAPK and β-catenin could be required to assure stem cell and consequently tumor tissue maintenance during progression of SSA, and our ISC marker analysis during serrated tumor progression in the mouse is in agreement with such a model.…”

Clinicopathologic and molecular features of sporadic early-onset colorectal adenocarcinoma: an adenocarcinoma with frequent signet ring cell differentiation, rectal and sigmoid involvement, and adverse morphologic features

“…Wnt signaling pathway abnormalities, particularly abnormalities of b-catenin, also likely have a role in CIMP-high colorectal carcinomas, as abnormal nuclear localization of b-catenin is frequently identified in sessile serrated polyps/adenomas. 32,33 Importantly, B60-80% of CIMP-H tumors harbor mutations in BRAF, and CIMP-H tumors rarely, if ever, demonstrate KRAS mutations. 31,34 In addition, CIMP-H colorectal carcinomas often demonstrate methylation of the p16 promoter, resulting in diminished or absent p16 protein expression.…”

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

“…shown associations of SSA/P and those with dysplasia or carcinoma with methylation or loss of protein expression for DNA repair genes, ie, MLH1, 1,4,6-9 a CpG island methylator phenotype, 3,4,6,8 BRAF mutations, 3,4,[6][7][8][9][10][11][12][13][14] and a lack of genetic alterations in CTNNB1 (the gene coding for b-catenin protein). 14 This pathway is thought to be distinct from the conventional adenoma-carcinoma pathway, where adenomas progress to invasive colorectal carcinomas through the influence of a series of genetic alterations including adenomatous polyposis coli (APC) and KRAS mutations.…”

“…14 This pathway is thought to be distinct from the conventional adenoma-carcinoma pathway, where adenomas progress to invasive colorectal carcinomas through the influence of a series of genetic alterations including adenomatous polyposis coli (APC) and KRAS mutations. 4,6,10,11,15,16 The WNT signaling pathway has a vital role in embryogenesis, 17 and its deregulation is also implicated in colorectal carcinogenesis.…”