Telomere dysfunction: multiple paths to the same end

The molecular cloning of telomerase and telomere components has enabled the analysis and precise manipulation of processes that regulate telomere length maintenance. In mammalian cells and in other organisms, we now recognize that disruption of telomere integrity via any one of a number of perturbations induces chromosome instability and the activation of DNA damage responses. Thus, telomere dysfunction may represent a physiological trigger of the DNA damage or apoptotic response in an analogous fashion to other genotoxic insults that introduce chromosome breaks. Initial studies in mice lacking the murine telomerase RNA and in cells expressing a dominant negative version of the telomere binding protein TRF2 revealed a strong p53-dependent response to telomere dysfunction. Yet, telomere dysfunction exhibits p53-independent effects as well, an observation supported by p53-independent responses to telomere dysfunction in p53 mutant human tumor cell lines and mouse cells. As most tumors are compromised for p53 function, examination of this p53-independent response warrants closer attention. A better understanding of this p53-independent response may prove critical for determining the ultimate utility of telomerase inhibitors in the clinic. This review will summarize our current understanding of the molecular responses to telomere dysfunction in mammalian cells.