Doctor of Philosophy in Genome Science and Technology (PhD)
Defining and manipulating the effect of cell-cell variation on drug response
Corey Nislow has been a very great supervisor and mentor. From my first day in UBC till now, he has always supported me. He provided an enabling and supportive environment for me to succeed in my PhD study and research. Importantly, he believes in me and answers whenever I needed his attention and/or support. In fact, I never regretted having him as my supervisor. He's the best!
The Yeast Knockout (YKO) collection has provided functional annotations from thousands of genome-wide screens. As an unintended consequence however, ~90% of gene annotations are derived from a single genotype. The nutritional auxotrophies in the YKO are of particular concern as they have phenotypic consequences. To address this issue, repaired ‘prototrophic’ versions of the YKO collection have been constructed; the first by introducing an ARS-CEN plasmid carrying wildtype copies of the auxotrophic markers (Plasmid-Borne, PBprot), and the second by backcrossing (Backcrossed, BCprot) to a strain wildtype for the auxotrophies. To systematically assess the impact of the auxotrophies, genome-wide fitness profiles of the prototrophic and auxotrophic YKO collections were compared across a diverse set of drug and environmental conditions. Comparative fitness profiling for the prototrophic collections revealed genotypic and strain-construction-specific phenotypes. The PBprot collection exhibited fitness defects associated with plasmid maintenance, while the BCprot collection’s fitness profiles were compromised due to strain loss resulting from nutrient selection steps during strain construction. The repaired prototrophic versions of the YKO collection did not restore wildtype behaviour and had additional experimental liabilities. Neither prototrophic collection compensated for gaps in gene annotation resulting from the auxotrophic YKO genetic background. To remove marker bias and expand the experimental scope of current deletion libraries, construction of a bona fide prototrophic collection from a wildtype strain will be required.
Hypoxia, the state of reduced oxygen, is a microenvironment found in many solid tumours and is correlated with an increased risk in patient mortality. This is due to an increase in resistance to radiotherapy and chemotherapy as well as a decrease in drug efficacy. The mechanisms and cellular factors (gene products) associated with this reduced chemotherapeutic efficacy in hypoxia remains unclear. This research looks to identify cellular processes and pathways that cancerous cells are able to exploit in order to survive and thrive in this microenvironment. The eukaryotic model baker’s yeast Saccharomyces cerevisiae combined with a genome-wide approach was used to screen the yeast knockout collection for specific genotypes that are sensitive to the hypoxic environment alone, and in combination with commonly used chemotherapeutics. Pathways and processes identified in these screens include transcriptional regulation, cytoskeleton maintenance, ribosomal biogenesis, macromolecular complex assembly and the heat shock response. The combination of heat and hypoxia was found to result in a synergistic effect that drastically affected cell fitness. DNA-damaging chemotherapeutics screened in hypoxic conditions showed reduced efficacy. Genotypes most sensitive to drugs in the hypoxic environment fall into Gene Ontology (GO) terms categorized in the response to the specific mechanism of the drug. This includes DNA repair processes such as homologous repair, post-replicative repair and mismatch repair. The mechanistic specificity uncovered in these screens suggests that the hypoxic environment exacerbates drug-specific stresses, and the identified genotypes highlight gene products and pathways critical for these responses. Cell survival and success in this microenvironment therefore requires adaptations to these exacerbated stresses, a phenomenon successfully accomplished by resistant tumour cells. This research contributes to our understanding of cellular biology under this cancer microenvironment, and provides data to highlight the challenges in using chemotherapeutics to treat tumours.
As of 2007, over 30 million hectares are affected by salinization resulting in poor crop yield and a reduction of food production. Reversing salinization of soil is an expensive and long term process. The bioengineering of plants to better cope with salinization of the soil is an ongoing research effort. Hortaea werneckii is an extremely halotolerant (salt tolerant) black yeast and can grow in the absence of salt or in almost saturating conditions (5M NaCl). Its natural ecological niche is the solar salterns of Slovenia which have range of environmental extremities such as the salt concentration, low oxygen, and high UV intensity. Recently it was discovered that this yeast has had recent genome duplication and 90% of the proteins exist in duplicate. The whole genome duplication and the extreme NaCl tolerance of H. werneckii provide an interesting model to investigate molecular mechanisms involved in salt stress. In this study, H. werneckii’s genome assembly is improved (increased contiguity) and used for subsequent molecular experiments such as MNase-seq and RNA-seq. These experiments were used to examine differences of gene expression and the corresponding chromatin architecture across a range of saline conditions to determine important molecular mechanism in salt tolerance. H. werneckii increases respiration in response to salt stress exemplified by the upregulation of mitochondrial associated genes and antioxidant defense genes. Additionally, H. werneckii genes encoding zinc transporters and genes involved in glycerol assimilation were increased in response to high salt. The chromatin landscape of some of these genes differs from other yeasts such as S. cerevisiae. Using next generation sequencing and third generation sequencing a more complete picture of H. werneckii’s mechanisms of salt tolerance has been obtained while also creating an extensive data-base for future research.