Telomeres, crisis and cancer.

Eukaryotic chromosomes terminate in specialized nucleic acid-protein complexes known as telomeres. Disruption of telomere structure by erosion of telomeric DNA or loss of telomere binding protein function activates a signal transduction program that closely resembles the cellular responses generated upon DNA damage. Telomere dysfunction in turn induces a permanent proliferation arrest known as senescence. Senescence is postulated to perform a tumor suppressor function by limiting cellular proliferative capacity, thus imposing a barrier to cellular immortalization. Genetic or epigenetic silencing of components of the DNA damage pathway, allows cells to proliferate beyond senescence limits. However, these cells eventually reach a stage of extreme telomere dysfunction known as crisis that is characterized by cell death and the concomitant appearance of cytogenetic abnormalities. Telomeric crisis produces significant chromosomal instability, a hallmark of human cancer, and may thus be relevant to carcinogenesis by increasing the occurrence of genetic alterations that would favor neoplastic transformation. The following review examines the relationship of telomere function during crisis in accelerating chromosomal instability and cancer.