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RFTS-deleted DNMT1 enhances tumorigenicity with focal hypermethylation and global hypomethylation

ABSTRACT: Site-specific hypermethylation of tumor suppressor genes accompanied by genome-wide hypomethylation are epigenetic hallmarks of malignancy. However, molecular mechanisms that drive these linked changes in DNA methylation remain obscure. DNA methyltransferase 1 (DNMT1), the principle enzyme responsible for maintaining methylation patterns is commonly dysregulated in tumors. Replication foci targeting sequence (RFTS) is an N-terminal domain of DNMT1 that inhibits DNA-binding and catalytic activity, suggesting that RFTS deletion would result in gain of DNMT1 function. However, earlier data suggested that RFTS is required for DNMT1 activity. Here, we determined cellular consequences of RFTS deletion from DNMT1 in immortalized human bronchial epithelial cells. Compared to full-length DNMT1, ectopic expression of DNMT1- ΔRFTS caused greater malignant transformation and enhanced promoter methylation that silenced DAPK and DUOX1 gene expression and increased intensity of promoter methylation across the genome. Strikingly, DNMT1-ΔRFTS also produced genomic hypomethylation and demethylation of Satellite 2 repeat sequences. Because deletion of RFTS is sufficient to induce focal hypermethylation and global hypomethylation in parallel, evidence suggests that the RFTS domain is a target responsible for epigenetic changes in cancer.

ORGANISM(S): Homo sapiens

PROVIDER: GSE57829 | GEO | 2014/09/30

SECONDARY ACCESSION(S): PRJNA248201

REPOSITORIES: GEO

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Project description:Cancer is characterised by DNA hypermethylation and gene silencing of CpG island-associated promoters, including tumour suppressor genes The methyl-CpG-binding domain (MBD) family of proteins bind to methylated DNA and can aid in the meditation of gene silencing by interaction with histone deacetylases and histone methyltransferases. However the mechanisms responsible for eliciting CpG island hypermethylation in cancer, and the potential role that MBD may proteins play in modulation of the methylome remain unclear. Our previous work demonstrated that MBD2 preferentially binds to the hypermethylated GSTP1 promoter CpG island in prostate cancer cells. Here, we use functional genetic approaches to investigate if MBD2 plays an active role in promoting DNA methylation. First, we show that loss of MBD2 results in inhibition of both maintenance and spread of de novo methylation of a transfected construct containing the GSTP1 promoter CpG island in prostate cancer cells and Mbd2-/- mouse fibroblasts. De novo methylation was rescued by transient expression of Mbd2 in Mbd2-/- cells. Second, we show that MBD2 depletion triggers significant hypomethylation genome-wide in prostate cancer cells with concomitant loss of MBD2 binding at promoter and enhancer regulatory regions. Finally, CpG islands and shores that become hypomethylated after MBD2 depletion in LNCaP cancer cells show significant hypermethylation in clinical prostate cancer, highlighting a potential active role of MBD2 in promoting cancer specific hypermethylation. Importantly, co-immunoprecipiation of MBD2 reveals that MBD2 associates with DNA methyltransferase (DNMT) enzymes 1 and 3A. Together our results demonstrate that MBD2 plays a critical role in â??rewritingâ?? the cancer methylome at specific regulatory regions. LNCaP prostate cancer cell line clones with reduced MBD2 expression were establised by using shRNA to MBD2 and scrambled control clones were established with scrambled control shRNA. To interrogate methylation changes induced by MBD2 knock-down we profiled three stably transfected scrambled control clones and three MBD2 knockdown clones on Illumina HumanMethylation450K arrays. Differential methylation analysis was carried out to identified CpG sites hypo-/hyper-methylated as a result of MBD2 knockdown.

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RFTS-deleted DNMT1 enhances tumorigenicity with focal hypermethylation and global hypomethylation

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