Genetic screens have laid much of the groundwork for our current understanding of biology, and mutagenesis screens in Caenorhabditis elegans have proven to be a particularly useful tool in determining the molecular components of biological processes. The Million Mutation Project (MMP) is a collection of mutagenized C. elegans strains that have been clonally propagated and whole-genome sequenced. Utilizing the MMP, I have performed a screen for touch insensitive mutants to assess the phenotypic coverage of the set, to obtain new alleles of known genes, and to potentially identify touch phenotype-causing mutations in genes that have not yet been linked to the touch response. In this thesis I first present my rationale for screening the MMP set for touch phenotypes, then review what has already been learned about genes required for the function of the neurons that sense gentle touch in C. elegans. I describe my approach to phenotyping the MMP set and present statistics on response distributions. As expected, most of the MMP strains that I determined to have strong touch insensitive phenotypes had mutations in genes identified in previous touch phenotype mutageneses. However, some of the phenotype-causing MMP alleles cause protein-coding changes in regions that were not known to be affected by previously characterized alleles. The genomic data from the MMP also allowed me to consider protein-altering mutations in known touch genes that did not result in a detectable phenotype. Finally, I address the set of strains for which we have not identified candidate causative mutations. Although I have not discovered any previously unknown touch genes through my screen of the MMP, it is still quite possible that the touch insensitive mutants I have identified will lead us to identify additional genes needed for gentle touch sensation. Ultimately, my screen has successfully demonstrated the utility of the MMP set and provided new insights as to the structure and function of the genetic determinants of gentle touch sensation in C. elegans.

The ability to efficiently and precisely modify the genome of living cells forms the basis of genetic studies and offers great potential to research and therapy. With its unprecedented ease of use and efficiency, CRISPR-Cas9 has revolutionized genome editing at a stunning pace. Functioning like a pair of molecular scissors, the RNA-guided endonuclease Cas9 can cleave genomic DNA to generate double-stranded breaks (DSBs). DSBs trigger the DNA damage response (DDR), that sets into motion multiple cellular processes that attempt to repair these lesions. One such cellular pathway, named homology-directed repair (HDR), enables researchers to make desirable changes precisely to genomic DNA sequences. HDR facilitates nearly any genomic DNA change, from the replacement of a single nucleotide to the insertion of several thousands of nucleotides. Thus, the precision, as well as versatility at modifying genomic DNA, make HDR a particularly promising repair pathway for genome editing. However, competition with other error-prone DSB repair pathways reduces the efficiency of HDR and results in the generation of an excess of undesirable mutations. In this thesis, I address these two challenges associated with CRISPR-Cas9 genome editing: i) low efficiency of HDR and ii) large deletion mutations generated upon repair of Cas9-induced DSBs. The first part of the thesis describes our study to identify genetic factors that stimulate HDR at Cas9 induced DSBs. Towards this goal, we individually express in human cells 204 open reading frames involved in the DDR and determine their impact on CRISPR-mediated HDR. From these studies, we identify RAD18 as a stimulator of CRISPR-mediated HDR. By defining the RAD18 domains required to promote HDR, we derive an enhanced RAD18 variant (e18) that stimulates HDR induced by CRISPR-Cas9 in multiple human cell types, including embryonic stem cells. Mechanistically, e18 suppresses the localization of the HDR-inhibiting factor 53BP1 to DSBs. Through this suppression of 53BP1, e18 promotes HDR at the expense of insertion and deletion mutations introduced by error-prone DSB repair pathways. Altogether, this study identifies e18 as an enhancer of CRISPR-mediated HDR and highlights the promise of engineering DDR factors to augment the efficiency of precision genome editing. In the second part of the thesis I describe our study of the genetic mechanisms regulating large deletions that are generated upon repair of Cas9-induced DSBs. We perform a pooled CRISPR screen to interrogate the effect of knocking out 610 DDR genes on the frequency of CRISPR-mediated long deletions. The screen identifies genes that consistently affect the frequency of long deletions when knocked-out in different experimental conditions. Thus, our study lays the foundations for uncovering the mechanisms regulating CRISPR-mediated long deletions and has the potential to aid in the development of new strategies to limit their generation.

The tumor suppressor protein p53, known as the ‘guardian of the genome’, transcriptionally regulates the expression of numerous genes, both coding and non-coding, in response to diverse forms of cellular stress. While numerous reports have been published characterizing the protein coding genes that are transcriptionally regulated by p53, the non-coding targets of p53 are less well-studied. In this thesis, high throughput transcriptome sequencing of cell lines was performed following treatment with different drugs in order to induce p53. Utilizing a combination of de novo transcriptome discovery and mapping to a comprehensive annotation of transcripts named the MiTranscriptome, an extensive catalog of long non-coding RNAs (lncRNAs) was identified. This set of lncRNAs, called p53LTCC (p53 LncRNA Transcriptome from Cultured Cells) are derived from an integrative analysis of RNA-Seq and ChIP-Seq data. It has been previously shown that while the mutation status of p53 may not be a significant predictor of cancer patient survival, a mutant p53 gene expression signature is associated with poor prognosis in many types of cancer. Moreover, the use of attractor metagenes has revealed that the increased expression of metagenes associated with epithelial-mesenchymal transition (EMT), mitotic instability (chromosomal/genomic instability) and lymphocyte infiltration are associated with poor prognosis. Since the p53 pathway is impaired in one way or the other in most tumors, a classifier based on a p53 metagene derived from our p53LTCC was developed that could differentiate between tumor and normal samples based on gene expression. Using machine learning approaches, diagnostic classifiers that could distinguish tumor and normal samples with a high degree of accuracy were developed. Also, while expression of individual long non-coding RNAs had low correlation with patient survival in different cancers, a lncRNA signature that was derived from the catalog of p53 targets had significant prognostic utility for cancer patient survival. Since p53 plays a central role in cancer etiology and it is mutated in over 50% of all cancers, we hypothesized that the lncRNA targets of p53 may have vital functions in effectuating the p53 pathway. Indeed, functional studies of two of the lncRNA targets of p53 showed that they play a role in p53-mediated regulation of cell cycle progression in response to DNA damage and are associated with the regulation of reactive oxygen species (ROS) levels in response to oxidative stress. Although the focus of the experimental studies was to elucidate the role of lncRNAs in the p53 pathway, careful analysis of the transcriptome sequencing results revealed insights into the role of different p53 targets (both coding and non-coding) in different contexts to enable a versatile response to diverse stresses. Not only were we able to identify novel targets of p53, the data showed that there are many p53 targets that are unique to each type of stress. There is also a core transcriptional lncRNA program that is activated by p53 regardless of the context. Finally, during the course of my computational studies, I made numerous observations from bioinformatics analysis of high throughput datasets from different sources that has allowed me to validate many of the experimental results derived by my colleagues (in cell-culture based assays) using cancer patient derived datasets. In order to streamline the workflow of such analysis, I have developed a tool for rapid exploratory data visualization of high throughput datasets for cancer genomics (REDVis) that enables users with minimal programming skills to quickly visualize gene expression, mutation, survival or other clinical, demographic or molecular characterization data for the analysis.

Over one billion people, more than 1/9th of the global population, are undernourished. Feeding the ever increasing population has to be the most important goal of plant sciences. Since cultivated areas are not likely to increase, I will need to produce more with what is available. This can be summarized in one word: yield. Unfortunately, wheat’s yield is expected to increase only 1.13% by 2019, a prediction that if converted into reality will likely indicate that I failed to cope with the world demographic increase. A new strategy to revolutionize wheat production is required, and some believe that this change might be represented by wheat hybrids. Achieving adequate commercial production of wheat hybrids has the potential to nearly double the yield of one of the world’s most important staple food. The first fundamental step toward this goal is to develop feasible methodologies to sterilize the male part of the complete wheat flowers. Two fertility-related genes are the primary target of this study, namely the species cytoplasm specific on chromosome 1D, and the desynaptic locus on chromosome 3B. This dissertation summarizes the important achievements obtained toward the cloning of the two loci by means of radiation hybrid functional analysis. Radiation hybrid is a technique that employs radiation to create genetic diversity along the targeted chromosome. Chapter 1 explains in details how this methodology can be applied to plants. The use of radiation hybrid mapping permitted creating a comprehensive map of wheat chromosome 3B, as discussed in Chapter 2, and then expanded the mapping information to identify the 2 Mb location of the desynaptic locus desw2, as discussed in Chapter 3. A similar approach on chromosome 1D allowed first to pinpoint the location of the species cytoplasm specific gene to a region of 2 Mb, as discussed in Chapter 4, and then ultimately to find a strong candidate for this locus, as discussed in Chapter 5. Now that the molecular locations of these genes have been unraveled by this study, their sequence can be streamlined into transformation to ultimately produce female wheat plants, and consequently hybrids.

DNA methylation is an epigenetic mechanism with roles that range from the fine tuning of transcription to genome wide dynamic acclimation to changing environments and regulation of developmental processes. While recent work has confirmed the presence and regulatory functions of DNA methylation in non-bilaterians, its role and distribution in Porifera has never been addressed. In this study, we performed whole genome bisulfite sequencing of the demosponge Amphimedon queenslandica and show that DNA methylation occurs mostly in CpG dinucleotides of coding regions. While high levels of gene-body methylation correlate positively with high expression and co-occur with the histone modification H3K36me3, they are not associated with amelioration of spurious transcription as found in other metazoans; nonetheless, per-exon methylation levels are predictive for exon retention suggesting a role in mRNA splicing. Additionally, analyses of Amphimedon and other sponges genomic data consistently revealed biased dinucleotide frequencies that suggest a long history of methylation-driven CpG conversion. Despite a genome wide loss of CpG dinucleotides, these are positively selected in exons and in methylated genes. These results indicate DNA methylation as a component of early metazoans regulome and challenge hypothesis on CpG methylation acting as a means for codon usage optimization.

No description available.

The strides made in malaria eradication efforts have been aided by a combination of vector control and chemoprevention. However, Plasmodium resistance to first-line artemisinin-based combination therapies (ACTs), and mosquito resistance to insecticides threatens the progress made. Innovative vector control measures, vaccines and antimalarial drugs with novel modes of action are key to disease eradication. High-throughput phenotypic screening of chemical libraries tested directly against all the stages of the Plasmodium lifecycle have been the mainstay of antimalarial drug discovery efforts and have identified compounds that are effective in parasite clearance. Unfortunately, these screens are handicapped in that they are unable to specify the actual compound targets in the Plasmodium parasites. As a result, many candidate hits have had to be re-screened in specific assays to determine putative mechanisms of antiplasmodial action. Predictably, this has elevated target-specific screens as the next frontier in drug discovery. This shift has been aided by a number of factors, including the cost effectiveness of these screens and the fact that target-specific screens do not always require specialized access to parasites. When combined with knowledge of the target’s structure, where known, target-specific screens have the potential to give lead compounds with impeccable potency and selectivity. This approach has already been successfully put to use, for example, in the identification of P. falciparum p-type ATPase 4 (PfATP4) and P. falciparum phosphatidylinositol 4-kinase (PfPI(4)K) inhibitors. The new challenge now is the identification of quality targets. Here, computational biology ‘omics’ tools have proved to be an invaluable resource. Two of the more commonly used of these tools are genomics and metabolomics. In-vitro evolution assays followed by whole genome sequencing analysis is a popular genomics approach and helps unveil novel target genes. Plasmodium parasites are exposed to sublethal doses of a compound until an upward shift in the half-maximal inhibitory concentration (IC50), indicative of resistant parasites, is observed in the culture. Sequenced genomes of the resistant parasite clones are compared to those of the drug-naive parent to reveal genetic changes, which include both single nucleotide polymorphisms (SNPs) and copy number variations (CNVs). While these genomic changes may point to genes encoding actual drug targets, they often reveal mediators of drug resistance or tolerance. Follow-up assays like SNP validation through gene editing are necessary to distinguish between actual targets, resistance mechanisms and random background mutations. Expectedly, genetic changes in uncharacterized Plasmodium genes are the bottle-necks in the identification of novel druggable targets. Even so, this genomics method has uncovered or reconfirmed novel antimalarial drug targets, including the proteasome, aminophospholipid-transporting P-type ATPase (PfAT-Pase2) and cGMP-dependent protein kinase (PfPKG). Metabolomic profiling and transcriptomics narrows down a compound’s mode of action. Here, parasites are treated with a compound of interest and the metabolites extracted and analyzed using liquid chromatography-mass spectrometry (LC-MS). The metabolomics fingerprint or metaprint is then compared to that of untreated parasites. While this method rarely provides the exact drug target, it narrows down the compound’s mode of action, which is valuable for target validation and characterization. The issue of non-specific or non-viable phenotype metabolite signals is easily filtered out by treating parasites with various drug concentrations and/or over a period of time. Other areas that limit the effectiveness of this tool and need to be addressed include the analysis of compounds that do not act through metabolic pathway disruption and potential host contamination. Nonetheless, metabolomics are a key player in drug discovery and have successfully been used in the study of pantothenamides (MMV689258) where the observed CoA analog buildup helped identify their mechanism of action in sequestering coenzyme A to block acetyl-CoA anabolism. Presented herein is a culmination of my graduate research in antimalarial drug discovery. Three independent projects are presented, and they all have either been published or are currently under reviewership. Chapter 1 is an introduction to malaria, a disease that has and continues to claim hundreds of thousands of lives, especially in my home continent of Africa. In chapter 2, I detail the experimental procedures used to generate the data presented in chapters 3-5. Chapter 3 is a detailed susceptibility profiling and metabolomic fingerprinting of Plasmodium falciparum asexual blood stages (ABS) to clinical and experimental antimalarials. This work, published in Cell Chemical Biology (2020), presents to the malaria research community a medium-throughput assay that can be utilized to identify new antimalarial lead compounds and novel assayable targets. Chapter 4 presents a detailed analysis of a novel ATP-binding cassette (ABC) transporter that confers pleiotropic antimalarial drug resistance in P. falciparum and that was first identified through in vitro evolution assays. This work is currently under review in Cell Chemical Biology. Chapter 5 presents work on an promising new preclinical compound, MMV688533, that provides single-dose cure and that was discovered using an innovative orthology-based screen by the Sanofi drug discovery team. In this chapter, I also present in detail the assays performed to better understand this compound’s mode of antiplasmodial action and the potential drivers of parasite resistance. This work has been accepted, pending minor textual revisions, in Science Translational Medicine. Finally in chapter 6, I summarize chapters 3-5 and share future follow-up work needed to strengthen and contextualize some of the experimental findings presented here.

Ciliates are important model organisms that have been used to study many aspects of cellular biology, including telomeres, histone modifications, and ribozymes. These unicellular eukaryotes house both a germline genome and a somatic genome in distinct nuclear structures within a single cell. One of their most unique features is their ability to undergo complex programmed genome rearrangements, during which their germline genome is fragmented and rearranged to form a new somatic genome. This rearrangement process results in a highly specialized somatic genome with many polyploid short chromosomes that are rich with genes. While all ciliates can undergo this developmental process, Oxytricha trifallax experiences particularly complex rearrangements that result in a more radically unconventional structure in its somatic genome. Much of the previous work studying Oxytricha has been focused on the complex rearrangements that it undergoes during sexual development and the mechanisms that allow it to perform these genome rearrangements events at the level of accuracy required for proper somatic function afterwards. Due to this particular focus on Oxytricha sexual development, the rest of Oxytricha’s unique biology has not been studied to the same degree. For my thesis I examined two aspects of Oxytricha biology that have not been well understood. In Chapter 1 I report the results of a proteomic survey of both types of nuclei found within the vegetative cell, the somatic macronucleus and the germline micronucleus. We performed mass spectrometry on enriched samples of both nuclear types and analyzed the enrichment of proteins between the two. Despite some mitochondrial contamination, we found that many categories of functional proteins were enriched in one of the two nuclei. We validated the appropriate nuclear localization of specific proteins from each subcategory through imaging Our results confirmed many previously predicted aspects of the two nuclei and provide a valuable resource for further studies on nuclear proteins in Oxytricha. In Chapter 2 I describe various features of a comparative analysis between the somatic genomes of multiple strains of Oxytricha trifallax. Previous work from the lab has focused primarily on the reference strains JRB310 and JRB510, which are most commonly used due to their ability to mate. We generated four new draft assemblies of the somatic genomes of strains JRB27, JRB39, SLC89, and SLC92. Many metrics demonstrate that these new assemblies are largely complete. Our analyses of these new strains revealed that there are numerous strain-specific chromosomes in Oxytricha that can encode genes. While they do not seem to encode core genes that would be missing otherwise, they are prime candidates for further examination to identify mating type-related genes.

<p> Three pieces of work are contained in this thesis. OGRe is a relational database that stores mitochondrial genomes of animals. The database has been operational for approximately five years and the number of genomes in the database has expanded to over 1000 in this period. However, sometimes, new genomes can not be added to the database because of small errors in the source ffies. Several improvements to the update method and the organizational structure of OGRe have been done, which are presented in the first part of this thesis. </p> <p> The second part of this thesis is a study on codon usage in mitochondrial genomes of mammals and fish. Codon usage bias can be caused by mutation and translational selection. In this study, we use some statistical tests and likelihood-based tests to determine which factors are most important in causing codon bias in mitochondrial genomes of mammals and fish. It is found that codon usage patterns seem to be determined principally by complex context-dependent mutational effects. </p> <p> The third part of this thesis is a phylogenetic study of 159 avian species obtained using mitochondrial rRNA sequences that were provided by Dr. van Tuinen. In this study, two methods are used: one considers sites of sequences as independently evolving; the other includes the secondary structure of rRNAs. Unfortunately, the amount of information in the rRNA sequences seems to be insufficient to determine the whole phylogeny of birds. However, our results make it clear that several traditionally defined orders are polyphyletic and therefore need to be redefined. </p> / Thesis / Master of Science (MSc)

This thesis presents novel methods for analysis of short viral sequences and identifying biologically significant regions based on their statistical properties. The first section of this thesis describes the ab initio method for identifying genes in viral genomes of varying type, shape and size. This method uses statistical models of the viral protein-coding and non-coding regions. We have created an interactive database summarizing the results of the application of this method to viral genomes currently available in GenBank. This database, called VIOLIN, provides an access to the genes identified for each viral genome, allows for further analysis of these gene sequences and the translated proteins, and displays graphically the distribution of protein-coding potential in a viral genome. The next two sections of this thesis describe individual projects for two specific viral genomes analyzed with the new method. The first project was devoted to the recently sequenced Herpes B virus from Rhesus macaque. This genome was initially thought to lack an ortholog of the gamma-34.5 gene encoding for a neurovirulence factor necessary for viability of the two close relatives, human herpes simplex viruses 1 and 2. The genome of Rhesus macaque Herpes B virus was annotated using the new gene finding procedure and an in-depth analysis was conducted to find a gamma-34.5 ortholog using a variety of tools for a similarity search. A profound similarity in codon usage between B virus and its host was also identified, despite the large difference in their GC contents (74% and 51%, respectively). The last thesis section describes the analysis of the Mouse Cytomegalovirus (MCMV) genome by the combination of methods such as sequence segmentation, gene finding and protein identification by mass spectrometry. The MCMV genome is a challenging subject for statistical sequence analysis due to the heterogeneity of its protein coding regions. Therefore the MCMV genome was segmented based on its nucleotide composition and then each segment was considered independently. A thorough analysis was conducted to identify previously unnoticed genes, incorrectly annotated genes and potential sequence errors causing frameshifts. All the findings were then corroborated by the mass spectrometry analysis.