Risk factors for CIMP sporadic colorectal cancer
Authors: Cynthia S. Spittle, Renata Coudry, Alison A. Evans
Journal: Cancer Research (2006)

Proc Amer Assoc Cancer Res, Volume 47, 2006

4602 

Recently, a subgroup of colorectal tumors has been described that exhibit the CpG island methylator phenotype (CIMP). The development of tumors via aberrant methylation of key genes rather than mutation has potentially important implications regarding prevention and treatment. We propose that unique risk factors exist for the development of specific subtypes of CRC. Frozen tissue from 103 primary, sporadic, stage II or III colorectal tumors was obtained from the Fox Chase Cancer Center Tumor Bank for epigenetic and genetic profiling. Sixty tumors were right-sided while 43 were left-sided. Methylation status of the MLH1 and CDKN2A gene promoters as well as MINT1, MINT 2 and MINT31 sequences and hotspot mutations in KRAS and BRAF were analyzed using pyrose quencing. Genotyping of MTHFR , MTR and MTRR polymorphisms was performed using TaqMan assays. In 103 samples, 34 (33.0%) were CIMP+, defined by hypermethylation of ≥2 of the 5 regions. Of the samples that were successfully analyzed, left-sided tumors were more likely to harbor KRAS mutations (12/42, 28%) than BRAF mutations (2/39, 5%). There was a nearly equal distribution of KRAS mutations (18/60, 30%) as BRAF (18/58, 31%) mutations in right-sided tumors. KRAS and BRAF mutations were mutually exclusive in that no tumors harbored mutations in both genes. In univariate analysis, CIMP+ status was significantly associated with age in years (72.1 + 7.8 CIMP+ vs. 67.9 + 8.7 CIMP-, p = 0.0002), location (left side 16% vs. right side 45.0%, p=0.003) and BRAF mutation status (mutant = 95.0% vs. wild-type =15.6%, p<<0.0001). Only 1 of 20 samples with BRAF mutation was not CIMP+. For the MTHFR \_1298 genotype, there was a trend for increasing probability of methylation with the presence of C alleles (p for trend = 0.02) and BRAF mutation (p = 0.03). No other association was found between mutation or methylation status and other polymorphisms. Because BRAF ’s role appeared to be largely confined to right-sided disease, we examined the risk factors for CIMP+ in right- and left- sided disease separately in multiple logistic models. For right-sided disease, only older age and presence of BRAF mutation (OR 89.3, 95% CI (7.1-502.2) were independently associated with CIMP+ status. The presence of BRAF mutation was in turn associated with the presence of the C allele of the MTHFR \_1298 polymorphism (OR 3.9, 95% CI 1.1-13.8). For left-sided disease, none of the mutations or polymorphisms tested was significantly associated with CIMP status. Epigenetic and genetic profiling provides evidence that unique subtypes of colorectal tumors exist that may develop through distinct pathways. In this study, CIMP+ tumors were associated with age, location and BRAF mutation. Moreover, BRAF mutations were largely confined to right-sided disease, suggesting that the colonic microenvironment may contribute differentially to specific epigenetic and genetic alterations. Further work is needed to ascertain the role of host genetic factors in CIMP+ disease.