A unified alternative telomere lengthening pathway in yeast survivor cells

Cancers develop when cells lose the ability to control the permitted number of cell divisions, which can lead to uncontrolled cell growth. Telomeres, consisting of DNA and protein complexes located on chromosome ends, represent an important regulator of cell divisions. With every cell division, telomeres become shorter until they reach a critically short length when they can no longer protect the chromosomes, and this leads to the termination of cellular divisions. Cancer cells resume their cell divisions by elongating their telomeres, which can be achieved by alternative lengthening of telomeres (ALT). ALT proceeds by recombination between shorter and longer telomeres, and which represents the main focus of this thesis. The goal of my thesis was to model ALT by using yeast Saccharomyces cerevisiae to 1) determine the frequency of ALT survivor formation; 2) to characterize molecular intermediates of ALT; 3) to determine the structure of ALT survivors; and 4) to determine the mechanism of ALT establishment. In this study I used population genetics principles to identify the frequency of ALT survivors to be 2.0x10-5 per senescent cell. I also calculated the frequency of ALT in several ALT mutants, which revealed, that contrary to the currently accepted model of ALT, proceeding via two separate pathways, ALT likely proceeds via a single unified pathway. Analysis of molecular intermediates revealed that this unified pathway of ALT proceeds via two consecutive steps, which was also supported by the “hybrid” structure of ALT survivors. Together, the obtained results allowed me to propose the molecular mechanism of ALT in yeast, and my approach can be used as a roadmap to characterize the mechanism of ALT in humans. These findings could pave the way for future studies of ALT in humans that will allow to identify a target for anti-cancer therapies that will specifically target cancers relying on ALT mechanism for survival.