Up and downregulation of p16Ink4a expression in BRAF-mutated polyps/adenomas indicates a senescence barrier in the serrated route to colon cancer

Abstract

P16Ink4a is an important factor in carcinogenesis and its expression can be linked to oncogene-induced senescence. Oncogene-induced senescence is characterized by growth arrest and occurs as a consequence of oncogene activation due to KRAS or BRAF mutation. It has been shown that the induction of p16 Ink4a in premalignant lesions and its loss during malignant transformation is an important mechanism in the carcinogenesis of several tumours. Loss of p16 Ink4a is often caused by CDKN2A promoter hypermethylation. This mechanism of gene silencing is associated with the CpG island methylator phenotype (CIMP) in colorectal carcinomas, which is characterized by widespread promoter methylation. In particular, colorectal carcinomas with BRAF mutations have been shown to be strongly associated with CIMP. Also, BRAF mutations are strongly correlated with the serrated route to colorectal cancer. In this study, we investigated p16 Ink4a expression and promoter methylation in BRAF-mutated serrated lesions of the colon. P16 Ink4a expression was found to be upregulated in premalignant lesions and was lost in invasive serrated carcinomas. P16 Ink4a expression and Ki67 expression were mutually exclusive, indicating that p16 Ink4a acts as cell cycle inhibitor. Additionally, progression of malignant transformation in serrated lesions was accompanied by increasing methylation of the CDKN2A promoter. Therefore, our data provide evidence for oncogene-induced senescence in the serrated route to colorectal cancer with BRAF mutation and upregulation of p16 Ink4a expression appears to be a useful indicator of induction of senescence. Loss of p16 Ink4a expression occurs during malignant transformation and is caused mainly by aberrant methylation of the CDKN2A promoter.

A clinicopathological and molecular analysis of 200 traditional serrated adenomas

“…In our more recent opinion, rendering a diagnosis of sessile serrated adenoma with dysplasia in this situation will imply a significantly greater degree of malignant risk than is warranted. True sessile serrated adenomas with dysplasia have been demonstrated to show Wnt pathway activation, 23 loss of mismatch repair function, 32 CDKN2A silencing, 24 and sometimes TP53 mutation. 33 In contrast, from our data, ordinary traditional serrated adenomas arising in a sessile serrated adenoma do not have these advanced features.…”

“…40 In contrast, increased cytoplasmic or nuclear p16 staining indicates increased production of functional protein. 24 Normal colonic mucosa is p16 negative, whereas the basal crypts and the ectopic crypt formations of ordinary traditional serrated adenomas show weak and patchy p16 staining; advanced areas frequently show strong p16 expression. This incremental pattern is postulated to represent increasing efforts to control cell proliferation by upregulation of CDKN2A expression.…”

“…14 Positive P16 required either cytoplasmic or nuclear staining with a score of Z3 in any compartment. 24 Positive Ki67 required nuclear staining with a score of Z9 in any compartment.…”

Suppression of p16 induces mTORC1-mediated nucleotide metabolic reprogramming

“…p16 is upregulated during oncogene-induced senescence (Braig et al, 2005;Collado and Serrano, 2010;Michaloglou et al, 2005). Loss of p16 may be an early event in the progression from senescent benign lesions to cancer (Bennecke et al, 2010;Bennett, 2016;Caldwell et al, 2012;Kriegl et al, 2011;Michaloglou et al, 2005;Shain et al, 2015). It is unknown whether p16 regulates nucleotide synthesis in this context.…”

CpG island methylator phenotype and its association with malignancy in sporadic duodenal adenomas

“…Low rates of p16 can also be seen in colorectal adenomas but is dramatically higher in CRC. [39][40][41][42] In CRC, p16 methylation was detected in about 30% samples, and was related to lower 5-y overall survival. 43 This suggests that p16 methylation is a somewhat later event in the development of both colorectal and duodenal adenomas.…”