Role of PML in telomere metabolism of normal and cancer cells

Telomeres are specialized structures that cap chromosomes ends, protecting them from degradation and processing by the DNA-repair machinery. In normal cells, telomeres become progressively shorter during replication, and this leads to replicative senescence. The majority of human tumors (approximately 90%) express the ribonucleic complex telomerase, that prevents this continuous DNA loss, thus endowing cells with immortal growth properties. In the remaining 10% of tumors telomerase is not expressed, and telomeric DNA is preserved through alternative mechanisms (Alternative Lengthening of Telomeres, ALT). An hallmark of all ALT tumor cell lines is the presence of nuclear structures that localize exclusively at telomeres (ALT-associated PML nuclear bodies, APBs). Here we show that PML (ProMyelocitic Leukemia protein), a nuclear protein that is an essential component of PML nuclear bodies (PML-NBs), colocalizes with telomeres not only in telomerase-positive tumor cell lines, but also in normal cells, although at few telomeres. Interestingly, in normal cells telomere-specific DNA-damage induced recruitment of PML to damaged telomeres. Furthermore, PML depletion led to the formation of Telomere Dysfunction-Induced Foci (TIFs) and consequent growth inhibition in normal and ALT cell lines. Preliminary cytogenetic characterization of PML-depleted fibroblasts revealed diffused genomic defects, suggesting that PML is a key regulator of telomere maintenance and whole genomic stability. Unpublished data from our lab have shown that DNA-PKcs, an enzyme involved in DNA repair and telomere stability, is part of the PML-macromolecular protein complex. To investigate the mechanism involvement of PML activity in telomere maintenance we studied by confocal microscopy its interaction with the active phosporylated form of DNA-PKcs in ALT cell lines and in normal cells. As expected, not only the two proteins interacted at telomeres, but PML was essential for the correct localization of DNA-PKcs at the chromosome ends. Additionally, telomeric-specific damage in normal cells increased the interaction between DNA-PKcs and PML. Altogether these findings shed light on a Telomeres are specialized structures that cap chromosomes ends, protecting them from degradation and processing by the DNA-repair machinery. In normal cells, telomeres become progressively shorter during replication, and this leads to replicative senescence. The majority of human tumors (approximately 90%) express the ribonucleic complex telomerase, that prevents this continuous DNA loss, thus endowing cells with immortal growth properties. In the remaining 10% of tumors telomerase is not expressed, and telomeric DNA is preserved through alternative mechanisms (Alternative Lengthening of Telomeres, ALT). An hallmark of all ALT tumor cell lines is the presence of nuclear structures that localize exclusively at telomeres (ALT-associated PML nuclear bodies, APBs). Here we show that PML (ProMyelocitic Leukemia protein), a nuclear protein that is an essential component of PML nuclear bodies (PML-NBs), colocalizes with telomeres not only in telomerase-positive tumor cell lines, but also in normal cells, although at few telomeres. Interestingly, in normal cells telomere-specific DNA-damage induced recruitment of PML to damaged telomeres. Furthermore, PML depletion led to the formation of Telomere Dysfunction-Induced Foci (TIFs) and consequent growth inhibition in normal and ALT cell lines. Preliminary cytogenetic characterization of PML-depleted fibroblasts revealed diffused genomic defects, suggesting that PML is a key regulator of telomere maintenance and whole genomic stability. Unpublished data from our lab have shown that DNA-PKcs, an enzyme involved in DNA repair and telomere stability, is part of the PML-macromolecular protein complex. To investigate the mechanism involvement of PML activity in telomere maintenance we studied by confocal microscopy its interaction with the active phosporylated form of DNA-PKcs in ALT cell lines and in normal cells. As expected, not only the two proteins interacted at telomeres, but PML was essential for the correct localization of DNA-PKcs at the chromosome ends. Additionally, telomeric-specific damage in normal cells increased the interaction between DNA-PKcs and PML. Altogether these findings shed light on a novel role for PML in the regulation of telomere metabolism, and are the basis for future investigations on its role in both aging and cancer.