How telomerase and Dna2 govern the fate of chromosome ends in Saccharomyces cerevisiae.

Abstract

Genome stability in eukaryotic cells relies heavily on their ability to differentiate between telomeres - natural ends of linear chromosomes - and double strand breaks (DSBs), pathological lesions which can occur throughout the stretch of chromosomal DNA. The former are to be protected from fusions, recombination and recognition by DNA damage response, whereas the latter are to be detected by the checkpoint system and repaired by end joining or homologous recombination. Due to the end replication problem, continually dividing cells are confronted with an additional necessity to maintain stable telomere length, which in most eukaryotes is fulfilled by telomerase. Telomerase is downregulated in human somatic cells to limit their replicative capacity and prevent malignization, forcing pre-malignant cells to seek ways of re-establishing telomere length homeostasis. Most cancers perform it by reactivating telomerase. Recently, Makovets group found a yeast model for telomerase reactivation through aneuploidy in cells with temperature-induced telomerase insufficiency. Given that aneuploidy is a common feature of cancers, a deeper mechanistic understanding of aneuploidy-driven telomerase reactivation in yeast may shed more light on cancer telomere biology. In telomerase-negative cancers telomere attrition is counteracted by alternative lengthening of telomeres (ALT). ALT is related to one of DSB repair pathways - break-induced replication (BIR), which operates on single-ended DSBs, such as those originating from broken replication forks. Phosphorylated Pif1 helicase has been reported to be essential for BIR, but the molecular mechanism of this requirement has not been ascertained. We found that the need for phosphorylated Pif1 in BIR is alleviated in yeast expressing an Nterminally truncated Dna2 nuclease/helicase. In my work I aim to gain insights into how yeast solve problems related to both natural chromosomal ends and DSBs by investigating mechanisms underlying aneuploidy-dependent telomerase reactivation in yeast cells with temperature-induced telomerase insufficiency and by studying the genetic interaction between Dna2 and Pif1 in BIR.