The Smc5/6 complex and the difficulties cutting the ties of twin sisters

Replication stress as a source of telomere recombination during replicative senescence in Saccharomyces cerevisiae

Replicative senescence is triggered by short unprotected telomeres that arise in the absence of telomerase. In addition, telomeres are known as difficult regions to replicate due to their repetitive G-rich sequence prone to secondary structures and tightly bound non-histone proteins. Here we review accumulating evidence that telomerase inactivation in yeast immediately unmasks the problems associated with replication stress at telomeres. Early after telomerase inactivation, yeast cells undergo successive rounds of stochastic DNA damages and become dependent on recombination for viability long before the bulk of telomeres are getting critically short. The switch from telomerase to recombination to repair replication stress-induced damage at telomeres creates telomere instability, which may drive further genomic alterations and prepare the ground for telomerase-independent immortalization observed in yeast survivors and in 15% of human cancer.

Telomerase-independent paths to immortality in predictable cancer subtypes

The vast majority of cancers commandeer the activity of telomerase - the remarkable enzyme responsible for prolonging cellular lifespan by maintaining the length of telomeres at the ends of chromosomes. Telomerase is only normally active in embryonic and highly proliferative somatic cells. Thus, targeting telomerase is an attractive anti-cancer therapeutic rationale currently under investigation in various phases of clinical development. However, previous reports suggest that an average of 10-15% of all cancers lose the functional activity of telomerase and most of these turn to an Alternative Lengthening of Telomeres pathway (ALT). ALT-positive tumours will therefore not respond to anti-telomerase therapies and there is a real possibility that such drugs would be toxic to normal telomerase-utilising cells and ultimately select for resistant cells that activate an ALT mechanism. ALT exploits certain DNA damage response (DDR) components to counteract telomere shortening and rapid trimming. ALT has been reported in many cancer subtypes including sarcoma, gastric carcinoma, central nervous system malignancies, subtypes of kidney (Wilm's Tumour) and bladder carcinoma, mesothelioma, malignant melanoma and germ cell testicular cancers to name but a few. A recent heroic study that analysed ALT in over six thousand tumour samples supports this historical spread, although only reporting an approximate 4% prevalence. This review highlights the various methods of ALT detection, unravels several molecular ALT models thought to promote telomere maintenance and elongation, spotlights the DDR components known to facilitate these and explores why certain tissues are more likely to subvert DDR away from its usually protective functions, resulting in a predictive pattern of prevalence in specific cancer subsets.