DNA methyltransferases control telomere length and telomere recombination in mammalian cells

Abstract

Here, we describe a role for mammalian DNA methyltransferases (DNMTs) in telomere length control. Mouse embryonic stem (ES) cells genetically deficient for DNMT1, or both DNMT3a and DNMT3b have dramatically elongated telomeres compared with wild-type controls. Mammalian telomere repeats (TTAGGG) lack the canonical CpG methylation site. However, we demonstrate that mouse subtelomeric regions are heavily methylated, and that this modification is decreased in DNMT-deficient cells. We show that other heterochromatic marks, such as histone 3 Lys 9 (H3K9) and histone 4 Lys 20 (H4K20) trimethylation, remain at both subtelomeric and telomeric regions in these cells. Lack of DNMTs also resulted in increased telomeric recombination as indicated by sister-chromatid exchanges involving telomeric sequences, and by the presence of 'alternative lengthening of telomeres' (ALT)-associated promyelocytic leukaemia (PML) bodies (APBs). This increased telomeric recombination may lead to telomere-length changes, although our results do not exclude a potential involvement of telomerase and telomere-binding proteins in the aberrant telomere elongation observed in DNMT-deficient cells. Together, these results demonstrate a previously unappreciated role for DNA methylation in maintaining telomere integrity.

Subtelomeric DNA hypomethylation is not required for telomeric sister chromatid exchanges in ALT cells

“…Recent studies have shown that mammalian telomeres and subtelomeres display epigenetic features characteristic of heterochromatin, and that alterations of these epigenetic marks are associated with telomere deregulation and changes in telomere dynamics (Blasco, 2007;Michishita et al, 2008). Notably, a lack of DNA methyltransferases (DNMTs) in mouse embryonic stem cells alters the DNA methylation status of subtelomeric regions, leading to telomere elongation and increased T-SCE frequency (Gonzalo et al, 2006). Similar observations were made for chromosome ends of mouse cells lacking histone methyltransferases (Benetti et al, 2007b).…”

“…Hypomethylation of subtelomeric DNA in ALT cancer cell lines Given recent reports indicating that a defect in DNA methylation of mouse chromosome ends may facilitate T-SCE (Gonzalo et al, 2006), a hallmark of ALT cells, we analysed the methylation status of subtelomeric DNA isolated from 11 human cancer cell lines relying on either telomerase or ALT pathways for telomere maintenance.…”

“…In particular, an association between hypomethylation of subtelomeric DNA and increased T-SCE frequency (Gonzalo et al, 2006;Benetti et al, 2007a), a hallmark of ALT cells (Londono-Vallejo et al, 2004), was observed in murine cells. The role of DNA methylation in telomere exchanges is, however, still controversial, as mutations in human DNMT3b affect telomere length but not T-SCE (Yehezkel et al, 2008).…”

Telomere and telomerase in stem cells

“…Even using highly sensitive assays, no telomerase activity has been detected in hMSCs so far, and there may be a mechanism of telomere maintenance other than telomerase, such as alternative lengthening of telomeres, in hMSCs. Recent observation of subtelomeric DNA hypomethylation facilitating telomere elongation in mammalian cells suggests that such epigenetic modification of chromatin may occur in hMSCs (Gonzalo et al, 2006). On the other hand, mMSCs with their telomerase activity knocked-down completely failed to differentiate into adipocytes or chondrocytes, even in early passages (Liu et al, 2004).…”

Live-cell imaging reveals the dynamics and function of single-telomere TERRA molecules in cancer cells

“…Telomeres are enriched in heterochromatic marks [3,4]. Nevertheless, they are transcribed into a long noncoding RNA called TERRA (telomeric repeat-containing RNA) [5,6].…”