Telomestatin is a potent G-quadruplex ligand that specifically interacts with the 3′ telomeric overhang, leading to its degradation and that induces a delayed senescence and apoptosis of cancer cells. Protection of Telomere 1 (POT1) was recently identified as a specific single-stranded telomere-binding protein involved in telomere capping and T-loop maintenance. We showed here that a telomestatin treatment inhibits POT1 binding to the telomeric overhang in vitro. The treatment of human EcR293 cells by telomestatin induces a dramatic and rapid delocalization of POT1 from its normal telomere sites but does not affect the telomere localization of the double-stranded telomere-binding protein TRF2. Thus, we propose that G-quadruplex stabilization at telomeric G-overhang inactivates POT1 telomeric function, generating a telomere dysfunction in which chromosome ends are no longer properly protected. (Cancer Res 2006; 66(14): 6908-12)

Cis-Diamminedichloroplatinum(II) (cisplatin) is among the most active antitumour agent used in human chemotherapy. The purpose of this review is to give an insight in several molecular mechanisms that mediate the sensitivity of cancer cells to this drug and to show how recent progress in our knowledge on some critical molecular events should lay the foundations of a more rational approach to anticancer drug design. Cisplatin is primarily considered as a DNA-damaging anticancer drug, mainly forming different types of bifunctional adducts in its reaction with cellular DNA. We will address the question of cellular activity disruption that cisplatin could cause through binding to more sensitive region of the genome such as telomeres. Cellular mechanisms of resistance to cisplatin are multifactorial and contribute to severe limitation in the use of this drug in clinics. They include molecular events modulating the amount of drug-DNA interaction, such as a reduction in cisplatin accumulation inside cancer cells or inactivation of cisplatin by thiol-containing species. Other important mechanisms acting downstream to the initial reaction of cisplatin with DNA, include an increase in adducts repair and a decrease in induction of apoptosis. Recently accumulating evidence suggest a role of the long patch DNA mismatch repair system in sensing cisplatin-damaged DNA and in triggering cell death through a c-Abl- and p73-dependent cascade; two other important pathways have been unravelled that are the mitogenactivated protein kinase cascade and the tumor suppressor p53. Several of these mechanisms underlying cisplatin resistance have been exploited to design new platinum derivatives. This issue will be covered in the present review.

The DNA damage-dependent poly(ADP-ribose) polymerase-2 (PARP-2) is, together with PARP-1, an active player of the base excision repair process, thus defining its key role in genome surveillance and protection. Telomeres are specialized DNA-protein structures that protect chromosome ends from being recognized and processed as DNA strand breaks. In mammals, telomere protection depends on the T₂AG₃ repeat binding protein TRF2, which has been shown to remodel telomeres into large duplex loops (t-loops). In this work we show that PARP-2 physically binds to TRF2 with high affinity. The association of both proteins requires the N-terminal domain of PARP-2 and the myb domain of TRF2. Both partners colocalize at promyelocytic leukemia bodies in immortalized telomerase-negative cells. In addition, our data show that PARP activity regulates the DNA binding activity of TRF2 via both a covalent heteromodification of the dimerization domain of TRF2 and a noncovalent binding of poly(ADP-ribose) to the myb domain of TRF2. PARP-2^−/− primary cells show normal telomere length as well as normal telomerase activity compared to wild-type cells but display a spontaneously increased frequency of chromosome and chromatid breaks and of ends lacking detectable T₂AG₃ repeats. Altogether, these results suggest a functional role of PARP-2 activity in the maintenance of telomere integrity.