Evidence for heritable predisposition to epigenetic silencing of MLH1

Abstract

Epigenetic silencing of MLH1 is the most common cause of defective DNA mismatch repair in endometrial and colorectal cancers. We hypothesized that variation in the MLH1 gene might contribute to the risk for MLH1 methylation and epigenetic silencing. We undertook a case-control study to test for the association between MLH1 variants and abnormal MLH1 methylation. Eight MLH1 SNPs were typed in the normal DNA from women with endometrial carcinoma. For these studies, the cases were women whose cancers exhibited MLH1 methylation (N 5 98) and the controls were women whose cancers had no MLH1 methylation (N 5 219). One MLH1 SNP, rs1800734, located in the MLH1 CpG island at 293 from the translation start site, was significantly associated with MLH1 methylation as were age at diagnosis and patient body mass index. In validation experiments, a similar-sized cohort of colorectal carcinoma patients (N 5 387) showed a similar degree of association with the 293 SNP; a smaller cohort of endometrial carcinomas (N 5 181) showed no association. Combining all 3 cohorts showed an odds ratio of 1.61 (95% CI: 1.20-2.16) for the AA or AG vs. GG genotype at the 293 SNP. Identification of risk alleles for MLH1 methylation could shed light on mechanisms of epigenetic silencing and may ultimately lead to new approaches to the prevention or treatment of malignancies associated with MLH1 inactivation. Epigenetic silencing of MLH1 is the most common cause of defective DNA mismatch repair. [2][3][4][5][6][7][8] In our large series of endometrial carcinomas, >20% have MLH1 promoter methylation.1 We previously showed that MLH1 methylation is associated with increased age at diagnosis.1 Furthermore, MSI1 endometrial cancers appear to be associated with lower body mass index (BMI).9 As is the case for endometrial cancers, MSI with MLH1 methylation is associated with older age at diagnosis in colorectal cancers. 6,10 MSI1 colorectal cancers tend to be right-sided tumors.11 To date, there have been no genetic factors associated with the risk for epigenetic silencing of MLH1 in tumors.Murrell et al. reported an association between polymorphisms in the IGF2 gene and Beckwith-Wiedemann syndrome, an overgrowth syndrome in which loss of imprinting of IGF2 and KCNQ1 is frequently observed.12 Variants in the IGF2 differentially methylated region were found to be significantly associated with sporadic Beckwith-Wiedemann syndrome in which loss of maternal allele-specific methylation of the differentially methylated region KvDMR1 in the KCNQ1 gene was observed.12 This was the first evidence that sequence variation within a gene may be causally associated with abnormalities in methylation (loss of methylation).Unlike the loss of methylation in Beckwith-Wiedemann syndrome, the MLH1 methylation defect in endometrial cancer is a gain of methylation in sequences that are normally unmethylated. We hypothesized that germline variation in the MLH1 locus might contribute to the risk for MLH1 methylation. We undertook a case-control association study to test for MLH1...

MLH1 methylation screening is effective in identifying epimutation carriers

“…14,17,20,21 It is intriguing that the A allele at rs1800734 associates with somatic MLH1 promoter methylation and increased risk of MSI CRC. 23,[31][32][33][34][35] In addition, it has been shown that this polymorphism modifies the efficiency of MLH1/EPM2AIP1 transcription. 36 It is difficult to translate these findings into specific recommendations for these patients and their relatives.…”

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

“…Nevertheless, in sporadic endometrial and colorectal cancer populations in which an association with this SNP variant has been reported, these have been weak, with modest odds ratios. 7 Thus, a minor elevation in the risk of MLH1 methylation or microsatellite instability observed in some cancer populations may not be replicated in others. This is in contrast to the c.-56C4T SNP (rs16906252) of the DNA repair gene MGMT, which correlates strongly with the presence of MGMT promoter methylation in both colorectal carcinoma and non-neoplastic tissues, illustrating that cis-acting elements can have an instrumental role in gene methylation.…”

“…6 The c.-93G4A SNP (rs1800734) within the MLH1 promoter has been associated with an increased risk of microsatellite instability or MLH1 methylation in some colorectal and endometrial cancer populations, but not in others. [7][8][9][10] However, these associations have been disputed on the basis that linkage disequilibrium with pathogenic mutations could not be ruled out in populations with a high incidence of familial cancer. 11 Recently, promoter reporter assays showed that the A allele of this SNP conferred reduced transcriptional activity compared with the G allele, consistent with the notion that this allele might predispose to promoter methylation.…”

MLH1 −93G>A promoter polymorphism and risk of mismatch repair deficient colorectal cancer

“…Consistent with this model, Chen and colleagues reported an association between the MLH1 293A allele and methylation of the MLH1 promoter in colorectal and endometrial cancer. 22 There is also evidence that promoter methylation and transcriptional silencing of other loci occurs in an allele-specific manner during the pathogenesis of colorectal cancer. 23 However, we cannot exclude the possibility that the MLH1 293G>A polymorphism is in linkage disequilibrium with a highly penetrant germline MLH1 mutation for CRC, although the available evidence would argue against this.…”