V600EBraf induces gastrointestinal crypt senescence and promotes tumour progression through enhanced CpG methylation of p16INK4a

EMBO Mol Med. 2010 Nov;2(11):458-71. doi: 10.1002/emmm.201000099.

Abstract

The majority of human colorectal cancers (CRCs) are initiated by mutations arising in the adenomatous polyposis coli (APC) tumour suppressor gene. However, a new class of non-APC mutated CRCs has been defined that have a serrated histopathology and carry the (V600E)BRAF oncogene. Here we have investigated the pathogenesis of serrated CRCs by expressing (V600E)Braf in the proliferative cells of the mouse gastrointestinal tract. We show that the oncogene drives an initial burst of Mek-dependent proliferation, leading to the formation of hyperplastic crypts. This is associated with β-catenin nuclear localization by a mechanism involving Mapk/Erk kinase (Mek)-dependent, Akt-independent phosphorylation of Gsk3β. However, hyperplastic crypts remain dormant for prolonged periods due to the induction of crypt senescence accompanied by upregulation of senescence-associated β-galactosidase and p16(Ink4a). We show that tumour progression is associated with down-regulation of p16(Ink4a) through enhanced CpG methylation of exon 1 and knockout of Cdkn2a confirms this gene is a barrier to tumour progression. Our studies identify (V600E)BRAF as an early genetic driver mutation in serrated CRCs and indicate that, unlike APC-mutated cancers, this subtype arises by the bypassing of a (V600E)Braf driven oncogene-induced senescence programme.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Aging*
  • Amino Acid Substitution / genetics
  • Animals
  • Cell Nucleus / chemistry
  • Colorectal Neoplasms / pathology
  • Colorectal Neoplasms / physiopathology*
  • Cyclin-Dependent Kinase Inhibitor p16 / biosynthesis*
  • Cyclin-Dependent Kinase Inhibitor p16 / metabolism
  • DNA Methylation
  • Gastrointestinal Tract / pathology
  • Gastrointestinal Tract / physiopathology*
  • Gene Expression Profiling
  • Glycogen Synthase Kinase 3 / metabolism
  • Glycogen Synthase Kinase 3 beta
  • Hyperplasia / pathology
  • Mice
  • Mutation, Missense*
  • Proto-Oncogene Proteins B-raf / genetics*
  • Proto-Oncogene Proteins B-raf / metabolism*
  • Up-Regulation
  • beta Catenin / metabolism

Substances

  • Cdkn2a protein, mouse
  • Cyclin-Dependent Kinase Inhibitor p16
  • beta Catenin
  • GSK3B protein, human
  • Glycogen Synthase Kinase 3 beta
  • Gsk3b protein, mouse
  • Proto-Oncogene Proteins B-raf
  • Glycogen Synthase Kinase 3