Epigenetic features of human telomeres

Although subtelomeric regions in humans are heterochromatic, the epigenetic nature of human telomeres remains controversial. This controversy might have been influenced by the confounding effect of subtelomeric regions and interstitial telomeric sequences (ITSs) on telomeric chromatin structure analyses. In addition, different human cell lines might carry diverse epigenetic marks at telomeres. We have developed a reliable procedure to study the chromatin structure of human telomeres independently of subtelomeres and ITSs. This procedure is based on the statistical analysis of multiple ChIP-seq experiments. We have found that human telomeres are not enriched in the heterochromatic H3K9me3 mark in most of the common laboratory cell lines, including embryonic stem cells. Instead, they are labeled with H4K20me1 and H3K27ac, which might be established by p300. These results together with previously published data argue that subtelomeric heterochromatin might control human telomere functions. Interestingly, U2OS cells that exhibit alternative lengthening of telomeres have heterochromatic levels of H3K9me3 in their telomeres.