Germline epimutation of MLH1 in individuals with multiple cancers

Abstract

Epigenetic silencing can mimic genetic mutation by abolishing expression of a gene. We hypothesized that an epimutation could occur in any gene as a germline event that predisposes to disease and looked for examples in tumor suppressor genes in individuals with cancer. Here we report two individuals with soma-wide, allele-specific and mosaic hypermethylation of the DNA mismatch repair gene MLH1. Both individuals lack evidence of genetic mutation in any mismatch repair gene but have had multiple primary tumors that show mismatch repair deficiency, and both meet clinical criteria for hereditary nonpolyposis colorectal cancer. The epimutation was also present in spermatozoa of one of the individuals, indicating a germline defect and the potential for transmission to offspring. Germline epimutation provides a mechanism for phenocopying of genetic disease. The mosaicism and nonmendelian inheritance that are characteristic of epigenetic states could produce patterns of disease risk that resemble those of polygenic or complex traits.Epigenetic silencing is a stable but reversible alteration of gene function mediated by histone modification, cytosine methylation, the binding of nuclear proteins to chromatin and interactions among these 1,2 . It does not require, or generally involve, changes in DNA sequence. Errors in the elaborate apparatus of epigenetic silencing possessed by higher eukaryotes can lead to 'epimutation' 3-5 , which we define as epigenetic silencing of a gene that is not normally silenced, or epigenetic activation of a gene that is normally silent. Germline epimutations are known in plants, examples being methylation and transcriptional silencing of the gene Lcyc in toadflax (Linaria vulgaris) 6 , paramutation in maize 5,7 and the clark kent alleles of SUPERMAN in Arabidopsis thaliana 8 . Similar phenomena in mammals could be an unrecognized source of phenotypic effects, which might manifest as disease. The increasingly detailed understanding of the genetics of human disease suggests a strategy to identify epimutations: screen for methylation of known disease-associated genes in affected individuals who do not carry a mutation in the relevant gene. Tumor suppressors are good candidates for this strategy because there is a clear relationship between their inactivation and the development of cancer. Germline mutations in tumor suppressor genes can predispose to cancer 9 . Tumor suppressors are also commonly methylated (and inactivated) in the course of neoplastic progression 10 , but the causal relationship between hypermethylation and tumorigenesis has not been established.We hypothesized that some individuals are predisposed to develop cancer because they carry germline epimutations of tumor suppressor genes. We selected 94 individuals for this study: 18 with hyperplastic polyposis 11 , 11 with personal histories of colorectal cancer and 65 with a family history of colorectal cancer but without deleterious germline changes in MSH2, MLH1 or APC 12 . We screened a subset of 44 individuals for promoter met...

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

“…No methylation was detected in their peripheral blood lymphocytes (data not shown), suggesting that methylation was somatically acquired and localized, as opposed to constitutional, such as that found in carriers of MLH1 epimutations. [34][35][36][37] In the tumours of each of these seven cases, at least one additional gene within 3p22 was concomitantly methylated with MLH1 ( Figure 6). In five cases, the tumours were BRAF V600E mutant and in four the tumours were CIMP þ (Table 6).…”

Epigenetic changes in virus-associated human cancers

“…Hypermethylation of tumor suppressor genes is generally associated with transcriptional repression to give rise to reduced expression or even gene silencing. Furthermore, hypermethylation of tumor suppressor genes can be an early event in cancer development [29][30][31] suggesting that both epigenetic modifications and genetic mutations can contribute to the disfunction of tumor suppressor genes, leading ultimately to tumor formation.…”

Patients with an unexplained microsatellite instable tumour have a low risk of familial cancer

“…Patients with a tumour with MSI and somatic hypermethylation of the MLH1 promoter rarely carry a germline mutation in the MMR system, although rare exceptions have been reported. A few families have been described in which Lynch syndrome patients display hypermethylation of the MLH1 promoter in tumour as well as in non-tumour tissue (Gazzoli et al, 2002;Miyakura et al, 2004;Suter et al, 2004;Hitchins et al, 2005;Valle et al, 2007a). In addition, a family was recently described in which the susceptibility to tumours is caused by germline methylation of the MSH2 promoter (Chan et al, 2006).…”