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Constitutional Mosaic Epimutations - a hidden cause of cancer?

ABSTRACT: Silencing of tumor suppressor genes by promoter hypermethylation is a key mechanism to facilitate cancer progression in many malignancies. While promoter hypermethylation can occur at later stages of the carcinogenesis process, constitutional methylation of key tumor suppressors may be an initiating event whereby cancer is started. Constitutional BRCA1 methylation due to cis-acting germline genetic variants is associated with a high risk of breast and ovarian cancer. However, this seems to be a rare event, restricted to a very limited number of families. In contrast, mosaic constitutional BRCA1 methylation is detected in 4-7% of newborn females without germline BRCA1 mutations. While the cause of such methylation is poorly understood, mosaic normal tissue BRCA1 methylation is associated with a 2-3 fold increased risk of high-grade serous ovarian cancer (HGSOC). As such, BRCA1 methylation may be the cause of a significant number of ovarian cancers. Given the molecular similarities between HGSOC and basal-like breast cancer, the findings with respect to HGSOC suggest that constitutional BRCA1 methylation could be a risk factor for basal-like breast cancer as well. Similar to BRCA1, some specific germline variants in MLH1 and MSH2 are associated with promoter methylation and a high risk of colorectal cancers in rare hereditary cases of the disease. However, as many as 15% of all colorectal cancers are of the microsatellite instability (MSI) "high" subtype, in which commonly the tumors harbor MLH1 hypermethylation. Constitutional mosaic methylation of MLH1 in normal tissues has been detected but not formally evaluated as a potential risk factor for incidental colorectal cancers. However, the findings with respect to BRCA1 in breast and ovarian cancer raises the question whether mosaic MLH1 methylation is a risk factor for MSI positive colorectal cancer as well. As for MGMT, a promoter variant is associated with elevated methylation across a panel of solid cancers, and MGMT promoter methylation may contribute to an elevated cancer risk in several of these malignancies. We hypothesize that constitutional mosaic promoter methylation of crucial tumor suppressors may trigger certain types of cancer, similar to germline mutations inactivating the same particular genes. Such constitutional methylation events may be a spark to ignite cancer development, and if associated with a significant cancer risk, screening for such epigenetic alterations could be part of cancer prevention programs to reduce cancer mortality in the future.

SUBMITTER: Lonning PE

PROVIDER: S-EPMC6551830 | biostudies-literature | 2019 Mar

REPOSITORIES: biostudies-literature

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Constitutional Mosaic Epimutations - a hidden cause of cancer?

Lønning Per E PE Eikesdal Hans P HP Løes Inger M IM Knappskog Stian S

Cell stress 20190322 4

Silencing of tumor suppressor genes by promoter hypermethylation is a key mechanism to facilitate cancer progression in many malignancies. While promoter hypermethylation can occur at later stages of the carcinogenesis process, constitutional methylation of key tumor suppressors may be an initiating event whereby cancer is started. Constitutional <i>BRCA1</i> methylation due to <i>cis</i>-acting germline genetic variants is associated with a high risk of breast and ovarian cancer. However, this ...[more]

PMID: 31225507

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Project description:The application of genome sequencing in patients with intellectual disability and congenital anomalies (ID/CA) typically identifies causative mutations only in a minority of cases. We hypothesized that some cases of ID/CA are caused by epigenetic aberrations that dysregulate normal genome function, and that these would be missed by conventional sequencing approaches. We performed Illumina 450k DNA methylation profiling in ~500 individuals with ID/CA, who were negative for causative mutations by microarray, exome- and/or whole genome sequencing. We screened patient methylation profiles for epimutations absent in ~1,500 controls, and validated these by bisulfite sequencing, revealing that epimutations represent large methylation changes specifically on one allele. Parental studies show that approximately half represent de novo events, a rate significantly higher than that seen in controls. We identified seven recurrent events, two of which (FMR1, MEG3) have known disease associations, validating our method for detecting pathogenic epimutations. From large-scale sequencing, population and expression studies we conclude that epimutations are: (i) Frequently associated with extreme outlier and mono-allelic gene expression, with an impact comparable to loss-of-function mutations; (ii) Generally conserved across multiple tissues within an individual, validating the use of blood DNA to study ID/CA; (iii) Can occur secondary to cis-linked regulatory mutations, providing a rationale for interpreting non-coding genetic variants; (iv) Occur sporadically with a remarkably high de novo rate, and (v) Exhibit non-Mendelian inheritance, likely being reset between generations by epigenetic reprogramming during embryogenesis. We conclude that epimutations underlie 5-10% of patients with ID/CA who are refractory to conventional mutation screening, and propose that epigenome profiling represents a promising method for the study of human disease that complements sequence-based approaches.

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Constitutional Mosaic Epimutations - a hidden cause of cancer?

Publications

Constitutional Mosaic Epimutations - a hidden cause of cancer?

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