Epigenetic-Genetic Interactions in theAPC/WNT, RAS/RAF, andP53Pathways in Colorectal Carcinoma

Abstract

Purpose: Early events in colorectal tumorigenesis include mutation of the adenomatous polyposis coli (APC) gene and epigenetic hypermethylation with transcriptional silencing of the O 6 -methylguanine DNA methyltransferase (MGMT), human mut L homologue 1 (hMLH1), and P16/CDKN2A genes. Epigenetic alterations affect genetic events: Loss of MGMT via hypermethylation reportedly predisposes to guanine-to-adenine or cytosine-to-thymine (G:C!A:T) transition mutations in KRAS and P53, and silencing of hMLH1 leads to high levels of microsatellite instability (MSI-H)/mutator phenotype, suggesting that epigenetic-genetic subtypes exist. Experimental Design: We evaluated the relationships of aberrant methylation of APC, MGMT, hMLH1, P16, N33, and five MINTs to mutations in APC, KRAS, BRAF, and P53 in 208 colorectal carcinomas. Results: We found that APC hypermethylation was age related (P = 0.04), in contrast to the other genes, and did not cluster with CpG island methylator phenotype (CIMP) markers. Hypermethylation of APC concurrently with either MGMT or hMLH1 was strongly associated with occurrence of G-to-A transitions in APC [odds ratio (OR), 26.8; P < 0.0002 from multivariable logic regression model], but C-to-T transitions had no associations. There was no relationship of hypermethylation of any gene, including MGMT, with G-to-A or C-to-T transitions in KRAS or P53, although APC hypermethylation was associated with P53 mutation (P < 0.0002). CIMP with MSI-H due to hMLH1 hypermethylation, or CIMP with loss of MGMT expression in nonM SI-H tumors, was associated with BRAF mutation (OR, 4.5; P < 0.0002). CIMP was also associated with BRAF V600E T-to-A transversion (OR, 48.5; P < 0.0002).Conclusions: Our findings suggest that the heterogeneous epigenetic dysregulation of promoter methylation in various genes is interrelated with the occurrence of mutations, as manifested in epigenetic-genetic subgroups of tumors.The development of most colorectal carcinomas (CRC) is thought to be initiated by inactivation of the APC/b-catenin/ Wnt signaling pathway, usually by mutation of one copy of the APC gene followed by a second event that inactivates the other allele (1-4). The second inactivating event is usually described as allelic deletion or mutation but can be epigenetic methylation of cytosines in CpG islands of the promoter region of APC that results in transcriptional silencing (5, 6). Altered APC has wideranging downstream effects on cell-cell adhesion, transcriptional regulation, chromosomal instability, cell migration, proliferation and cell cycle control, differentiation, and apoptosis (1-4).Somatic APC mutation is associated with the development of intraepithelial neoplasia (dysplasia) in aberrant crypt foci and early adenomas (7). The initiating alterations in APC in these early lesions persist whereas additional genetic and epigenetic events accumulate and drive tumor progression. Somatic mutation in the APC gene is present in as many as 80% of sporadic colorectal adenomas and carcinomas (7 -9), and a mutati...

The prognostic value of c‐Kit, K‐ras codon 12, and p53 codon 72 mutations in Egyptian patients with stage II colorectal cancer

“…16 Our reported frequency of K-ras overexpression and mutations (48.9% and 41.1%, respectively), together with the significant correlation with reduced OS, was comparable to that reported in some previous studies. In this context, Bazan et al, 16 Suehiro et al, 46 and Pajkos et al 47 reported a significant correlation between K-ras codon 12 mutations and aggressive forms of disease with reduced survival rates. In contrast, Akkiprik et al 7 and Bouzourene et al 15 demonstrated that the presence or absence of K-ras mutations was not correlated with prognosis in patients with stage II CRC.…”

The current value of determining the mismatch repair status of colorectal cancer: A rationale for routine testing

“…BRAF acts as direct effector of RAS and through the activation of MEK, promotes tumourogenesis (Sclafani et al, 2013). The BRAF pV600E (c.1799T > A) mutation occurs in 15-20% of CRCs and has important genetic, prognostic, and therapeutic implications (Koinuma et al, 2004;Suehiro et al, 2008). A recent literature review and meta-analysis indicates the incidence of BRAF mutation in sporadic MLH1 MSI CRC to be 63.50% (95% CI: 46.98% -78.53%) (Parsons et al, 2012) Likewise most CRCs containing activated BRAF mutation have CIMP, whereas CIMP and the BRAF mutation are uncommon in Lynch syndrome-associated cancers (≤1%) (Lagerstedt Robinson et al, 2007).…”

MGMT methylation is associated primarily with the germline C>T SNP (rs16906252) in colorectal cancer and normal colonic mucosa

“…14 MGMT methylation or loss of MGMT protein expression is reportedly associated with an increased frequency of mutations in the KRAS oncogene and P53 tumour-suppressor gene in colorectal cancer, particularly G4A transitions, consistent with a causative function of G4A mutagenesis in tumourigenesis. 11,12,[15][16][17][18] However, other studies have found no specific associations with this sequence change, 19,20 or only a weak association with the presence of a G4A mutation within one of the four cancer-associated genes KRAS, P53, APC and b-catenin. 21 Association studies between MGMT methylation, CIMP and microsatellite stability (MSS) status in colorectal cancer have also reported inconsistent findings.…”

“…21 Association studies between MGMT methylation, CIMP and microsatellite stability (MSS) status in colorectal cancer have also reported inconsistent findings. MGMT methylation has been correlated with MSS and low-level MSI, 17,20 though the existence of low-level MSI as an entity distinct from MSS remains controversial. Although MGMT methylation was initially included as one of the panel of markers to predict CIMP in some studies, [22][23][24][25] it has since been shown to be a poor marker for this epigenetic phenotype and may be independent of CIMP status altogether.…”