Epigenetic regulation of telomeres in human cancer

Hypomethylationofrepeatedelementsinthegenomeisacommon feature of human cancer, however, the directconsequences of this epigenetic defect for cancer biologyare still largely unknown. Telomeres are specializedchromatin structures at the ends of eukaryotic chromo-somesformedbytandemrepeatsofG-richsequencesandassociated proteins, which have an essential role inchromosome end protection and genomic stability. Telo-meric DNA repeats cannot be methylated, however, theadjacent subtelomeric DNA is heavily methylated inhumans. Here, we show that the methylation status ofsubtelomeric DNA repeats negatively correlates withtelomere length and telomere recombination in a largepanel of human cancer cell lines. These findings suggestthattumortelomerelengthandintegritycanbeinfluencedby epigenetic factors. Finally, we show that treatment ofhuman cancer cell lines with demethylating drugs resultsinhypomethylationofsubtelomericrepeatsandincreasedtelomere recombination, which in turn may facilitatetelomere elongation. All together, these findings suggestthattumortelomerelengthandintegritycanbeinfluencedby the epigenetic status of cancer cells.Oncogene(2008) 27,6817–6833; doi:10.1038/onc.2008.289;published online 1 September 2008Keywords: telomeres; subtelomeres; DNA methylation;epigenetic; telomerase; human cancerIntroductionGlobal DNA hypomethylation and in particular hypo-methylation of DNA repetitive sequences in the genome(subtelomeric and pericentric repeats) are well-knowncharacteristics of human cancer (Fraga et al., 2005).However, little is known on the direct consequences ofthese epigenetic defects other than they are associated toloss of acetylation at Lys16 (AcH4K16) and trimethyla-tion at Lys20 of histone 4 (trimH4K20) at theseDNA repetitive sequences (Fraga etal., 2005), suggest-ing a change in the chromatin structure at these regionstowards a less compacted state. Interestingly, globalDNA hypomethylation and loss of trymethylation atLys20 of histone 4 (H4K20) can result from abrogationof the Retinoblastoma family of tumor suppressorproteins, which are frequently mutated in cancer, thushighlighting the potential relevance of these epigeneticdefects for cancer biology (Gonzalo etal., 2005).Telomeres are heterochromatic structures at the endsof chromosomes, which in vertebrates consist of tandemrepeats of the TTAGGG sequence bound by an array ofspecialized proteins that form a protective structureknown as shelterin (reviewed in Blackburn, 2001; deLange, 2005; Blasco, 2007). Telomeres are essential forchromosome end protection (telomere capping) andchromosomal stability (de Lange, 2005). Next totelomeric repeats are located the so-called subtelomericrepeats, which are also heterochromatic (Gonzalo etal.,2006; Benetti etal., 2007b; reviewed in Blasco, 2007). Inparticular, both telomeres and subtelomeres are enrichedin histone marks characteristic of constitutive hetero-chromatin, including trimethylated H4K20 and H3K9, aswell as are bound by the heterochromatin protein 1isoforms (HP1a,HP1b and HP1g) (Garci´a-Cao et al.,2004; Gonzalo etal., 2005, 2006; Benetti etal., 2007b;reviewed in Blasco, 2007). In addition, subtelomericDNA repeats are heavily methylated both in humans andmice (Steinert etal., 2004; Fraga etal., 2005; Gonzaloet al., 2006). These epigenetic marks at telomeres andsubtelomeres are characteristic of a compacted chromatinstate, and have been demonstrated to act as negativeregulators of telomere length. In particular, abrogation ofthe corresponding histone methyltransferases and DNAmethyltransferases in genetically modified mice, results inabnormally elongated telomeres (Blasco, 2004; Garci´a-Cao etal., 2002, 2004; Blasco, 2004; Gonzalo etal., 2005,2006; Gonzalo and Blasco, 2005; Benetti etal., 2007a).These findings suggested that the chromatin state atmammalian telomeres and subtelomeres is important fortelomere length control. However, evidence for epigeneticfactors controlling telomere length in the context ofhuman adult development or during human disease is stilllacking.Mammalian telomeres can be elongated at least bytwo independent mechanisms. The main mechanism fortelomere elongation is the enzyme telomerase (reviewedin Blackburn, 2001; Collins and Mitchell, 2002;Blasco, 2005). Telomerase is a reverse transcriptase thatsynthetizes telomere repeats denovoat the chromosome