Epigenetic regulation of telomere length in mammalian cells by the Suv39h1 and Suv39h2 histone methyltransferases

Abstract

Telomeres are capping structures at the ends of eukaryotic chromosomes composed of TTAGGG repeats bound to an array of specialized proteins 1-3 . Telomeres are heterochromatic regions. Yeast and flies with defects in activities that modify the state of chromatin also have abnormal telomere function 4-6 , but the putative role of chromatin-modifying activities in regulating telomeres in mammals is unknown. Here we report on telomere length and function in mice null with respect to both the histone methyltransferases (HMTases) Suv39h1 and Suv39h2 (called SUV39DN mice). Suv39h1 and Suv39h2 govern methylation of histone H3 Lys9 (H3-Lys9) in heterochromatic regions 7 . We show that primary cells derived from SUV39DN mice have abnormally long telomeres relative to wild-type controls. Using chromatin immunoprecipitation (ChIP) analysis, we found that telomeres were enriched in diand trimethylated H3-Lys9 but that telomeres of SUV39DN cells had less dimethylated and trimethylated H3-Lys9 but more monomethylated H3-Lys9. Concomitant with the decrease in H3-Lys9 methylation, telomeres in SUV39DN cells had reduced binding of the chromobox proteins Cbx1, Cbx3 and Cbx5, homologs of Drosophila melanogaster heterochromatin protein 1 (HP1). These findings indicate substantial changes in the state of telomeric heterochromatin in SUV39DN cells, which are associated with abnormal telomere elongation. Taken together, the results indicate epigenetic regulation of telomere length in mammals by Suv39h1 and Suv39h2.The N-terminal tails of histones are subjected to post-translational modifications, including acetylation, methylation and phosphorylation, generating an extensive repertoire of chromatin structures 8,9 . Binding of factors that specifically recognize these modified histones mediate cellular responses 8,9 . Heterochromatic regions are enriched in methylated H3-Lys9, which creates a binding site for the chromobox proteins 9 , mediators of heterochromatin formation and epigenetic gene regulation. In D. melanogaster, HP1 is also required for telomere capping 4,5 . To investigate the impact of H3-Lys9 methylation on telomere length regulation and telomere function, we studied embryonic stem (ES) cells and mouse embryonic fibroblasts (MEFs) from SUV39DN mice 7 , which have less methylation of H3-Lys9 at the pericentric heterochromatin compared to wildtype mice 7 . First, we used terminal restriction fragment (TRF) analysis to estimate telomere length in these cells. Two independent SUV39DN ES cell cultures showed abnormally long telomeres, which were not present in wild-type ES cells (Fig. 1a). This finding was confirmed using MEFs (passage 1-2) derived from SUV39DN and wild-type littermate embryos (Fig. 1a). TRF analysis of increasing passages of the SUV39DN MEFs (D43) showed that the long telomeres in these MEFs were stable (Fig. 1b).Next, we carried out quantitative fluorescence in situ hybridization (Q-FISH) using a telomere-specific peptide nucleic acid probe. We determined the average telomere length for each ES-cell and ME...

Dicer independent small RNAs associate with telomeric heterochromatin

“…This is consistent with the observations that Suv39h1/h2 (À/À) cells, which have aberrantly long telomeres, have drastically lower level of tel-sRNAs (Fig. 3A) and higher TERT activity (Garcia-Cao et al 2004). Future studies on the biogenesis and functions of tel-sRNAs will be key, to understand the role played by tel-sRNAs in the telomere length control.…”

“…Increasing evidence indicates that there is a functional association between heterochromatin formation and telomere-length homeostasis. For example, mutations in the histone methyltransferases, such as Suv39h1/h2 or Suv4-20, lead to drastically reduced H3K9me3 or H4K20me3, respectively, which are concomitant with loss of telomere length control and genome instability (Garcia-Cao et al 2004;Benetti et al 2007b). Conversely, mutations in telomere-associated proteins and telomerase (TERT) can bring about changes to the telomeric chromatin structures and/or epigenetic status.…”

Cancer and aging: the importance of telomeres in genome maintenance

“…This epigenetic change results in decreased binding of the HP1 (heterochromatin protein-1) family of proteins to telomeric chromatin, abnormal telomere elongation and genomic instability [44]. These findings support the idea that telomeres are heterochromatic structures and subject to epigenetic regulation [45].…”

“…Epigenetic regulation is also believed to be responsible for the Telomere Position Effect (TPE) -the transcriptional silencing of genes near the telomeres [45][46][47].…”

2009 Nobel Prize in Physiology or Medicine: telomeres and telomerase

“…These heterochromatic marks are proposed to negatively control telomere length and telomere recombination (reviewed by Blasco, 2007;Schoeftner and Blasco, 2009). In particular, decreased heterochromatin protein 1 binding to telomeres, as well as a decrease in the density of trimethylation of histone 3 at lysine 9 and trimethylation of lysine 20 of histone 4 in cells deficient for the Suv39 H1 and H2 histone methyltransferases, leads to substantial telomere elongation (Garcia-Cao et al, 2004). Furthermore, loss of subtelomeric DNA methylation also results in telomere length increase (Gonzalo et al, 2006).…”