BRAF mutation is associated with DNA methylation in serrated polyps and cancers of the colorectum

Abstract

Background and aims: Mutations in BRAF have been linked with colorectal cancers (CRC) showing high level microsatellite instability (MSI-H). However, the distribution of BRAF mutations in MSI-H cancers remains to be clarified with respect to precursor lesions and the CpG island methylator phenotype (CIMP). Methods: Forty three hyperplastic polyps (HP), nine mixed polyps (MP), five serrated adenomas (SA), 28 conventional adenomas (AD), 18 hereditary non-polyposis colorectal cancers (HNPCC), and 127 sporadic CRC (46 MSI-H and 81 non-MSI-H) were collected from patients undergoing colectomy for either CRC or hyperplastic polyposis. Twenty five of 57 serrated lesions were derived from four patients with hyperplastic polyposis. HP were further subdivided according to recently documented morphological criteria into 27 classical HP and 16 variant lesions described as ''sessile serrated adenoma'' (SSA). All tumours were screened for BRAF activating mutations. Results: The BRAF mutation was more frequent in SSA (75%) and MP (89%) than in classical HP (19%), SA (20%), and AD (0%) (p,0.0001), and also in sporadic MSI-H cancers (76%) compared with HNPCC (0%) and sporadic non-MSI-H cancers (9%) (p,0.0001). The BRAF mutation was identified more often in CIMPhigh serrated polyps (72%) and CIMP-high CRC (77%) than in CIMP-low (30%) and CIMP-negative (13%) polyps (p = 0.002) as well as CIMP-low (18%) and CIMP-negative (0%) CRC (p,0.0001). Conclusions: The BRAF mutation was frequently seen in SSA and in sporadic MSI-H CRC, both of which were associated with DNA methylation. Sporadic MSI-H cancers may originate in SSA and not adenomas, and BRAF mutation and DNA methylation are early events in this ''serrated'' pathway.

Methylation of the 3p22 region encompassing MLH1 is representative of the CpG island methylator phenotype in colorectal cancer

“…40 Previous observations between the BRAF V600E mutation and methylation of multiple genes at distinct loci in conjunction with CIMP þ have been reported. 14,16,[47][48] One other study has also reported the close association between long-range epigenetic silencing of the 2q14 region and CIMP þ in sporadic colorectal cancer. 24 Taken together, the independent findings of long-range epigenetic silencing of both the 3p22 and 2q14 regions in the context of CIMP þ provide further support for a common mechanism governing epigenetic dysregulation that presents with the overlapping features of long-range epigenetic silencing and CIMP in the development of sporadic colorectal cancer.…”

“…Unfortunately, no tissue was available from these polyps to determine if they demonstrated a similar molecular profile to support this notion. However, other groups have demonstrated extensive methylation of multiple genes in conjunction with the BRAF V600E mutation in the serrated polyps of colorectal cancer cases, 47,57 as well as in the normal colonic mucosa of individuals with hyperplastic polyposis or colorectal cancer, suggesting that in rare cases these molecular defects may serve as a field defect in colorectal neoplasia. 57,58 In summary, our findings show that concomitant methylation of the 3p22 region is a consistent finding in CIMP þ colorectal cancers, and occurs predominantly, but not exclusively in microsatellite unstable tumours.…”

CpG island methylator phenotype-low (CIMP-low) colorectal cancer shows not only few methylated CIMP-high-specific CpG islands, but also low-level methylation at individual loci

“…To compare the frequencies of methylation index 4-10 (or [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] in CACNA1G, for example, we limited the denominator to tumors with CACNA1G methylation index 44 (methylation positive) ( Table 1). This was because, compared to CIMP-high tumors, CIMP-low tumors had a higher proportion of tumors negative for CACNA1G methylation (methylation index o4), which, if included in the denominator, would have by itself decreased Table 1).…”

“…1,2 A subset of colorectal cancers have been shown to exhibit widespread promoter methylation, which is referred to as the CpG island methylator phenotype (CIMP). 1,3 CIMPhigh colorectal tumors have a distinct clinical, pathologic and molecular profile, such as associations with proximal tumor location, female sex, poor tumor differentiation, inactive WNT/b-catenin (CTNNB1) and high BRAF and low TP53 mutation rates, [4][5][6] independent of microsatellite instability status. [7][8][9][10] Although controversial, CIMP may have prognostic implications in colorectal cancer.…”

Survival in stage II/III colorectal cancer is independently predicted by chromosomal and microsatellite instability, but not by specific driver mutations

“…Sporadic MSI + tumors occur more frequently proximal to the splenic fl exure and tend to show poor diff erentiation, mucinous histology and increased peritumoral lymphocytic infi ltration ( 4 ). Th ese tumors usually have near-diploid karyotypes and harbor a distinct set of driver mutations in genes such as BRAF and TGFBR2 ( 5,6 ). MSI + cancers also oft en exhibit a CpG island methylator phenotype ( 7 ).…”