Doctor of Philosophy in Medical Genetics (PhD)
Investigating the function and regulation of the histone variant H2A.Z in Saccharomyces cerevisiae
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RNA splicing mutants have been broadly implicated in genome stability, but mechanistic links are often unclear. Two predominant models have emerged: one involving changes in gene expression that perturb other genome maintenance factors and another in which genotoxic DNA:RNA hybrids, called R-loops, impair DNA replication. Recent efforts in whole genome sequencing have identified splicing factor mutations in several cancers, suggesting that splicing disruption may be a common mechanism involved in oncogenesis. To understand how splicing factor mutations contribute to genetic instability (GIN) in budding yeast, I selected strains with mutations in core snRNP complexes involved in establishing the splicing reaction and characterized GIN phenotypes to find that mitotic defects, and in some cases R-loop accumulation, are causes of GIN. I observed evidence of R-loop induced DNA damage in some cases, while all splicing mutants tested caused GIN through aberrant splicing of the TUB1 transcript, the protein product of which, α-tubulin, is critical in forming the mitotic spindle. GIN is exacerbated by loss of the spindle-assembly checkpoint protein Mad1, and moreover, removal of the intron from the α-tubulin gene TUB1 restores genome integrity. To gain functional insights to how HSH155 could influence GIN in the context of cancer progression, I studied five cancer-associated SF3B1 point mutations in the yeast ortholog HSH155. While the splicing activity in Hsh155 and SF3B1 were conserved, I did not observe measurable phenotypes in the yeast mutant strains. Thus, I used isogenic NALM6 human leukemia cell lines to investigate how a specific SF3B1 hotspot mutation, H662Q contributes to GIN. My data indicate that GIN occurs in two ways: 1) by induction of R-loop-mediated replication stress either directly or indirectly through suboptimal expression of an R-loop modulating factor, and 2) aberrant splicing of the multifunctional protein DYNLL1, which may potentially perturb double strand break repair pathway choice. The results of my study show how differing penetrance and selective effects on the transcriptome in yeast and human splicing factors contribute to GIN through R-loop accumulation and altered gene expression, adding to a growing body of evidence that splicing factors play a key role in genome maintenance across species.
Chromosome instability (CIN) is characterized by an increased rate of the unequal distribution of DNA between daughter cells. Such changes in chromosome structure or number can occur due to both mitotic defects leading to aneuploidy and DNA damage-induced chromosome rearrangements. Previous large-scale screens for CIN genes in the model organism Saccharomyces cerevisiae identified DIS3, which encodes a catalytic component of the core RNA exosome complex, as a novel CIN gene. Mutations in human DIS3 have been identified in roughly 11% of multiple myeloma (MM) cases. I sought to recapitulate MM-associated point mutations at conserved sites in yeast cells, in order to understand the mechanism of emergent CIN in MM. I have found that MM-associated DIS3 exonuclease mutations do increase the frequency of CIN. A temperature sensitive DIS3 mutant accumulates DNA:RNA hybrids, however analysis of DNA damage foci by microscopy revealed no increase in double-strand breaks in any of the tested strains. Yeast DIS3 exonuclease mutants experience growth retardation, temperature sensitivity, and an altered cell cycle. Microarray analysis of one MM mutant has additionally demonstrated downregulation of cell cycle components, consistent with the potential for mitotic defects, in addition of upregulation of a host of metabolic pathways. Further, genetic interaction profiling by synthetic genetic array indicates MM-associated DIS3 mutations synthetically interact with rRNA processing proteins, as well as a host of mitotic regulators and metabolic pathways, particularly those involved in spindle and kinetochore function. Further, I verify that DIS3 mutants have a functional spindle assembly checkpoint, and are in fact resistant to microtubule poisons. Finally, I discover that the fitness defects induced by these mutations can be abrogated through culturing on media containing only a non-fermentable carbon source, suggesting that growth on poor carbon sources may also rescue CIN.Together, these results demonstrate extensive phenotypic consequences of MM-associated point mutations in DIS3, and support a model for CIN in DIS3 mutants involving defects in mitotic progression.
Gene-gene or gene-drug interactions are typically quantified using fitness as a readout because the data is continuous and easily measured in high-throughput. However, to what extent fitness captures the range of other phenotypes that show synergistic effects is usually unknown. Using Saccharomyces cerevisiae, and focusing on a matrix of DNA repair mutants and genotoxic drugs, I quantified 76 gene-drug interactions based on both mutation rate and fitness and find that these parameters are not connected. Independent of fitness defects I identified seven cases of synthetic hypermutation, where the combined effect of the drug and mutant on mutation rate was greater than predicted. One example occurred when yeast lacking RAD1 were exposed to cisplatin and I characterized this interaction using whole-genome sequencing. Our sequencing results indicate mutagenesis by cisplatin in rad1∆ cells depended almost entirely on interstrand crosslinks at GpCpN motifs. Interestingly, our data suggest that the 3’ base in this motif templates the addition of the mutated base. This result differs from cisplatin mutation signatures in XPF-deficient C. elegans and supports a model in which translesion synthesis polymerases perform a slippage and realignment extension across from the damaged base. Accordingly, DNA polymerase zeta activity was essential for mutagenesis in cisplatin-treated rad1∆ cells. Together these data reveal the potential to gain new mechanistic insights from non-fitness measures of gene-drug interactions and extend the use of mutation accumulation and whole-genome sequencing analysis to define DNA repair mechanisms.