Understanding the Implications of Checkpoint Adaptation for Genome Stability and DNA Repair

Abstract

To study the relationship between checkpoint adaptation (CA), genotoxin resistance, and aneuploidy, I have employed yeast as a model system and generated homozygous diploid rad52 deletion cells. These cells have a strong defect in homology-directed repair (HDR). Additionally, diploid budding yeast actively suppress their non-homologous end joining (NHEJ) pathway. Therefore, diploid rad52 mutants are defective in HR and NHEJ. The diploid state of the mutant allowed us to study genomic instability that occurs following CA. Yeast Rad52 participates in Rad51 filaments assembly and ssDNA strand exchange, wich in mammalian cells is executed by the BRCA2. This allowed me to use rad52 yeast mutants to model the response to genotoxins in BRCA2-deficient cancers. Despite a strong DNA repair defect, rad52 mutants were eventually able to form viable colonies following treatment with double-strand break-inducing genotoxins, X-rays and CPT (Figure 12). The colony formation required CA, as the re-growth of the adaptation-defective mutants was significantly impaired (Figure 12). These adapted colonies gained genotoxin resistance to the second round of genotoxic treatment (Figure 13). The adapted rad52 mutants underwent drastic genome alterations and became aneuploid. In this work, I have further analysed the relationship between the ploidy and genotoxin resistance, and conclude that the near-haploid phenotype confers resistance to CPT and X-rays in rad52 mutants. So far, the aneuploidy-associated phenotypes in budding yeast were studied using controlled experimental systems. I aimed to exploit aneuploidy-associated physiological disadvantages in checkpoint-adapted cells. I have characterized the aneuploidy-associated phenotypes in HDR defective mutants that became aneuploid following the genotoxic treatment. I show that these genotoxin-resistant rad52 aneuploids can be targeted pharmacologically (Figures 19-24). Furthermore, I employed the rad52 model to determine genetic requirements for the DNA repair and genotoxin resistance formation in repair-defective cells. I have analysed genetically the involvement of the genes that belong to the RAD52 epistasis group (Figure 26), and the genes required for the microhomology-medated end joining pathway in the post-adaptation DNA repair (Figure 30). I have found that the Mre11-Rad50-Xrs2 (MRX) complex confers the chemoresistance to CPT in naïve and checkpoint adapted rad52 mutants (Figures 27-29). These results suggest that the inhibition of Mre11 could potentially sensitize HDR-deficient human cells to CPT. Together, my results provide a connection between CA, aneuploidization, and genotoxin resistance in HDR-deficient yeast model. This provides a rationale to target genotoxin resistant cells based on their aneuploidy-associated stresses, and the genetic requirements for the MRX complex for the DNA repair.