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 V600EBRAF oncogene. Here we have investigated the pathogenesis of serrated CRCs by expressing V600EBraf 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 p16Ink4a. We show that tumour progression is associated with down-regulation of p16Ink4a through enhanced CpG methylation of exon 1 and knockout of Cdkn2a confirms this gene is a barrier to tumour progression. Our studies identify V600EBRAF as an early genetic driver mutation in serrated CRCs and indicate that, unlike APC-mutated cancers, this subtype arises by the bypassing of a V600EBraf driven oncogene-induced senescence programme.

Previous studies have shown heterogeneity of telomere length among chronic B cell malignancies and within CLL. Longer telomeres are a feature of germinal center-derived tumours and mean telomere length is shorter in unmutated than mutated CLL. The relative contributions of telomere length pre-transformation, germinal centre experience and tumour cell proliferation in determining telomere length remains uncertain. We have used a recently introduced real-time quantitative PCR method to measure the factor by which the ratio of telomere repeat copy number to single gene copy number differs between a sample and that of a reference DNA sample. We compared relative telomere length in 4 types of B cell tumour whose VH genes may be either unmutated or mutated. In 24 cases of mutated CLL mean relative length was 4.24 (range 0.16–24.13) compared to 0.81 (range 0.1–3.1) in 21 cases of unmutated CLL, p= 0.004. However, analysis of 8 mutated and 7 unmutated CLL cases using the poor prognosis Vh3-21 gene showed no significant difference between the two subsets. In non-CLL cases, the mean relative telomere length was 0.7 in 11 cases of B-PLL, 1.9 in 10 cases of MCL and 4.06 (range 0.2–22.7) in 29 cases of SMZL. In the latter 3 disorders there was again no significant difference in mean relative telomere length between mutated and unmutated cases. However, when SMZL patients were divided into 2 groups with relative telomere lengths above or below the median, 11/14 patients with stable disease had long telomeres whereas 10/15 patients with progressive disease had short telomeres, p= 0.025. We then compared 26 cases of stage A, mutated CLL, of whom 13 had stable disease for >10 years and 13 had progressive disease. There was no significant difference in telomere length at presentation between the 2 groups nor on follow up testing of samples from 7 patients from each group with a mean of 92 months between test samples. In contrast, 5 cases with stable, stage A, unmutated CLL for >8years had a longer mean relative telomere length of 2.15 (range 1.29–4.12) than the cohort of 21 unmutated cases with progressive disease described above p=0.002. These data confirm the differences in mean telomere length among different B cell tumours and between mutated and unmutated CLL, but suggest that mutational status per se does not influence telomere length, while in SMZL and unmutated CLL, rate of cell proliferation is reflected in telomere length.