By 2030, the global colorectal cancer burden is projected to approximately double. This highlights the immediate need to expand our understanding of the etiological origins of colorectal cancer, so that novel therapeutic strategies can be identified and validated. The putative tumor suppressor gene RNF20 encodes a histone H2B mono-ubiquitin ligase and has been found altered/mutated in colorectal and numerous other cancer types. Several studies suggest that RNF20, and by extension mono-ubiquitinated histone H2B (H2Bub1), play important roles in maintaining genome stability in human cells. Indeed, hypomorphic RNF20 expression and/or function have been shown to underlie several phenotypes consistent with genome instability, making aberrant RNF20 biology a potential driver in oncogenesis. Through an evolutionarily conserved trans-histone pathway, RNF20 and H2Bub1 have been shown to modulate downstream di-methylation events at lysines 4 (H3K4me2) and 79 (H3K79me2) of histone H3. Accordingly, understanding the biology associated with RNF20, H2Bub1, H3K4me2, and H3K79me2 is an essential preliminary step towards understanding the etiological origins of cancer-associated RNF20 alterations and identifying a novel therapeutic strategy to selectively kill RNF20-deficient cancers. In this thesis, I employ single-cell imaging, and multiple biochemical techniques to investigate the spatial and temporal patterning and characterize the biology of RNF20, H2Bub1, H3K4me2 and H3K79me2 throughout the cell cycle. In addition, I employ the CRISPR-Cas9 genome editing system to generate RNF20-deficient HCT116 cells. Finally, I employ synthetic lethal strategies to selectively kill RNF20-depleted cells. In conclusion, the research chapters contained within this thesis have characterized putative drivers in cancer (Chapter 3), generated a valuable research reagent for CRISPR-Cas9 ii genome editing experiments (Chapter 4), and identified a novel therapeutic strategy to selectively kill certain cancer cells (Chapter 5). This thesis has increased our understanding of the etiological origins of cancer and generated novel reagents and treatments strategies that after further validation and clinical studies, could be employed to reduce morbidity and mortality rates associated with cancer. / October 2016

In prokaryotes, CRISPR/Cas9 system provides molecular immunity to bacteria and archaea against invading phages, conjugative plasmids and nucleic acids. CRISPR/Cas9 system has been adapted for targeted genome editing across diverse eukaryotic species for a variety of applications in basic and applied research. In this dissertation, I propose to adapt the CRISPR/Cas9 system to function as molecular immunity machinery against plant DNA viruses. Therefore, to test whether the CRISPR/Cas9 system is portable to plants, I produced plants stably over-expressing Cas9 and sgRNAs against single or multiple DNA viruses in Nicotiana benthamiana (N. benthamiana) and tomato (Solanum lycopersicum) plants. sgRNAs targeting the Cas9 endonuclease against different coding and non-coding viral sequences were tested in virus- interference experiments. I explored the possibility of generating robust interference against single and multiple DNA viruses. Subsequently, I studied the possibility of virus evasion of the CRISPR/Cas9 machinery and evolution of the virus escapees. Finally, I produced N. benthamaiana and tomato plants stably expressing the CRISPR/Cas9 machinery for developing durable virus resistance. Furthermore, developing effective viral-interference system in plants will help to understand the molecular underpinning of virus biology and host-defense mechanisms against plant viruses. In conclusion, my research project attempted to establish the efficacy and extend the utility of CRISPR/Cas9 system for viral interference in plants which promise exciting applications including producing engineered plants resistant to multiple viral infection.

Integration and expression of exogenous genetic material – in particular, transgenes – into the genomes of model organisms, cell lines or patients is widely used for the creation of genetically modified experimental systems and gene therapy. However, loss of transgene expression due to silencing is still a major hurdle which remains to be overcome. Judicious selection of integration loci can help alleviate the risk of silencing; in recent years the ability to efficiently and specifically target transgene integration has been improved by the advent of site-specific nucleases (SSNs). SSNs can be used to generate double strand breaks (DSBs) in a targeted manner, which increases the efficiency of homologous recombination (HR) mediated transgene integration into predetermined loci. In this work I investigate four human genomic loci for their potential to act as transgene integration sites which will support robust long term expression: the adeno-associated virus (AAV) integration site 1 (AAVS1); the human homologue of the mouse Rosa26 locus (hROSA26); the inosine monophosphate dehydrogenase 2 (IMPDH2) gene and the eukaryotic translation elongation factor 1 alpha 1 (EEF1A1) gene. I also investigate the potential of creating a novel drug-selectable transgene integration system at the IMPDH2 locus to allow for rapid and specific selection of correctly inserted transgenes. In addition to their ability to drive targeted transgene integration, SSNs can be harnessed to specifically disrupt gene function through indel formation following erroneous repair of the induced DSB. Using this strategy, I aimed to answer some important biological questions about eukaryotic translation elongation factor 1 alpha (eEF1A); eEF1A is responsible for providing aminoacylated tRNAs to the ribosome during the elongation phase of protein synthesis. Humans and other vertebrates express two isoforms, eEF1A1 and eEF1A2 (encoded by EEF1A1 and EEF1A2 respectively). During development eEF1A1 is replaced by eEF1A2 in some tissues. The reasons for this remain elusive, but one explanation may lie in the moonlighting functions of eEF1A1, which may not be shared by eEF1A2. Additionally, eEF1A2 can act as an oncogene, while there is no evidence that eEF1A1 is overexpressed in tumours. To begin to untangle these issues I targeted EEF1A1 using SSNs with the aim of making a cell line expressing only the eEF1A2 isoform. This work suggests that eEF1A1 may be essential even in the presence of eEF1A2, though further studies will be required to confirm this.

Mammalian neural stem cell (NSC) lines provide a useful experimental model for basic and applied research across stem cell and developmental biology, regenerative medicine and neuroscience. NSCs are clonally expandable, genetically stable, and easily transfectable - experimental attributes compatible with functional genetic analyses. However, targeted genetic manipulations have not been reported for mammalian NSC lines. Here, we deploy the CRISPR/Cas9 technology and demonstrate a variety of diverse targeted genetic modifications in both mouse and human NSC lines such as: targeted transgene insertion at safe harbour loci; biallelic knockout of neurodevelopmental genes; knock-in of epitope tags and fluorescent reporters; and engineering of glioma driver mutations at endogenous proto-oncogenes. Leveraging these new optimised methods, we explored gene editing to model the earliest stages of paediatric gliomagenesis in primary human NSCs. Our data indicate that oncogenic mutations in histone H3.3 play a role in NSC transformation and may operate through suppression of replication induced senescence. By comparing cellular responses of NSC cultures from different compartments of the developing brain, we also identify differences in susceptibility to distinct H3.3 mutations that mirror the disease etiology. Altogether, our findings indicate that CRISPR/Cas9-assisted genome editing in NSC lines is a versatile tool to explore gene function in CNS development and cancer biology. Our project resulted in the creation of valuable human cellular models of paediatric gliomagenesis, which will allow us to further our understanding of the disease and carry out cell based drug discovery projects.

Remarkable progress has been made in our ability to design and produce synthetic DNA binding domains (TALE or Cas9-based), which can be further functionalized into synthetic transcription factors (sTFs). This technology is revolutionizing our ability to modulate expression of endogenous mammalian genes. Forced expression of cDNAs encoding transcription factors (TFs) is widely used to drive lineage conversions. However, this process is often inefficient and unreliable. Multiplex delivery of sTFs pool to activate endogenous master regulators and extinguish the expression profile of the host cell type could be a potential solution to this problem. We have developed a novel, simple TALE assembly method that enabled us to produce and screen large numbers of TAL effectors and compare their activity to dCas9-based TFs. During this process, we constructed many new functionally validated sTFs. Our ultimate goal is to test whether combining synthetic transcriptional activators and repressors can efficiently reprogram fibroblasts to NS cells or alternatively ‘program’ NS cell differentiation to neurons. We performed analyses of the transcriptome and chromatin accessibility of both fibroblasts and neural stem cells to unravel their core TF networks and their epigenetic state. This will allow us in the future the targeted design of sTFs and synthetic chromatin modifiers for specifically changing cell identity.

The Integrated Stress Response (ISR) is a conserved eukaryotic translational and transcriptional program implicated in mammalian metabolism, memory and immunity. Although mainly considered to be a protective mechanism, prolonged and severe ISR can result in cell death. The ISR is activated by diverse stress pathways converged on phosphorylation of the alpha subunit of eukaryotic translation initiation factor 2 (eIF2) that inhibits the guanine nucleotide exchange activity of its partner eIF2B and attenuates overall rates of protein synthesis. Numerous mutations in eIF2B are linked to a fatal neurodegenerative disease of vanishing white matter. A new chemical inhibitor of the ISR (ISRIB), a bis-O-arylglycolamide, can reverse the attenuation of mRNA translation by phosphorylated eIF2 protecting mice from prion-induced neurodegeneration and traumatic brain injury. The work presented in this dissertation describes identification of mammalian eIF2B as a cellular target of ISRIB by implementing biochemical, biophysical, structural and chemogenetic methods. The herein reported cryo-electron microscopy-based structure of eIF2B uncovers a novel allosteric site on the translation factor capturing the ISRIB-binding pocket at the interface between its β and δ regulatory subunits. The extensive CRISPR/ Cas9-based screen for ISRIB-resistant and analogue-sensitive phenotypes revealed residues on the eIF2B dimer interface important for ISRIB binding. Based on the results reported in this dissertation along with the similar findings of others the potential molecular basis of ISRIB action, and its implication for the regulation of eIF2B's activity is broadly discussed. The identification of the ISRIB binding pocket away from the known interaction sites between eIF2B and eIF2 is also put into the context of a possible molecular mechanism of eIF2B's guanine exchange inhibition by phosphorylated eIF2. The work described in this dissertation provides new insight into the translational regulation and points to the importance of fine-tuning the activity of translation factors by small chemical molecules.

No description available.

GWAS (Genome wide association studies) have aided in the discovery of various novel variants associated with diabetes. However, a detailed study is required to uncover the role of these genes and to determine how their dysfunction affects pathophysiology. Previous work in the lab has been successful in establishing zebrafish as an efficient model to characterise the effects of these candidate genes. Consequently, efforts have been also made to establish zebrafish as an efficient model system for drug screening as well. The current POP (Proof of principle) study aims to find whether treatment with tolbutamide drug in zebrafish carrying MODY (Maturity onset diabetes of the young) mutations has the similar effects in humans. The study employed zebrafish carrying five (gck, hnf1a, hnf1ba, hnf1bb, pdx1) CRISPR induced MODY orthologues. The zebrafish larvae were supplemented with tolbutamide drug from 5dpf till 10dpf (day post fertilisation). At 10dpf, larvae were screened for various glycaemic traits, whole body glucose and lipids as well body size. CRISPR-CAS9- induced mutations were quantified using paired end sequencing. The results showed that treatment with tolbutamide had a significant effect on the hyperglycaemic outcome induced by hnf1bb, hnf1a, and pdx1 mutations which was in line with the known effects of the drug in humans. In conclusion, the POP study proved to be successful in leveraging zebrafish as an efficient model system for, in vivo characterisation of drugs and can likely help to identify novel targets for therapeutic interventions.

DNA-binding proteins bind to specific sequences to direct their activity to defined loci in the genome. Regulation of gene expression, for example, is dependent on the recognition of specific DNA sequences by transcription factors (TFs). These TFs receive input from cellular signals to control panels of genes to meet the needs of the cells. Critical to this function is the recognition and binding of TFs to the correct DNA sequence. The main focus of this thesis is to quantitatively determine how proteins, including TFs, distinguish DNA sequences, and to understand how DNA sequence affect their function. Primarily using the Glucocorticoid receptor (GR) as the model TF, I developed novel methods to measure the DNA binding specificity over long binding sites. These methods: 1) Distinguished the sequence specificity of GR and closely related androgen receptor (AR), which helped to both account for differential genomic localization between the two factors, and explained how GR can functionally substitute for AR in castration-resistant prostate cancer (Chapter II); 2) Explored the effect of DNA sequence on GR-regulated transcription through the specification of monomeric versus dimeric binding. Sequence motifs that bias GR binding toward the monomeric state were discovered (Chapter III); 3) Demonstrated a conserved role of intrinsic specificity in directing the degree of GR genomic occupancy in vivo in a fixed chromatin context (Chapter V); 4) Quantitatively modeled and decoupled the DNA binding and cleavage specificities of CRISPR-Cas9 system, providing a rapid pipeline to characterize the genome-editing reagents (Chapter IV). In summary, we showed here that DNA binding specificity is only the initial step in directing the activity of the bound protein. Beyond the affinity-based recruitment, DNA sequences can regulate the protein activity through alternative mechanisms, such as modulating the binding cooperativity, or directly serving as an allosteric ligand for protein function that is independent of DNA binding affinity.

Cells must employ a diversity of strategies to regulate the quantity and functionality of different proteins during development and adult homeostasis. Post-transcriptional regulation of gene transcripts by microRNAs (miRNAs) is recognized as an important mechanism by which the dosage of proteins is regulated. Despite this, the physiological relevance of direct regulation of an endogenous gene transcript by miRNAs in vivo is rarely investigated. PAX6 is a useful model gene for studying miRNA regulation directly. PAX6 is highly dosage-sensitive transcription factor that is dynamically expressed during development of the eye, nose, central nervous system, gut and endocrine pancreas, and is mutated in the haploinsufficiency disease aniridia. Several miRNAs have been implicated in regulating PAX6 in different developmental contexts. Notably, miR-7 appears to regulate Pax6 during specification of olfactory bulb interneurons in the ventricular-subventricular zone (V-SVZ) of the brain and during development of the endocrine pancreas. Here, we produced a bioinformatics tool to enable selective mutation of candidate microRNA recognition elements (MREs) for specific miRNAs while ensuring that new MREs are not inadvertently generated in the process. We then performed the first comprehensive analysis of the mouse Pax6 3’ untranslated region (3’UTR) to identify MREs that may mediate miRNA regulation of Pax6 and to identify miRNAs capable of interacting with the 3’UTR of Pax6. Using Pax6 3’UTR genetic reporter assay, we confirmed that two MREs for miR-7-5 located at 3’UTR positions 517 and 655 function together to regulate PAX6. We generated mice harbouring mutations in the Pax6 3’UTR that disrupt these miR-7-5p MREs, individually or in combination, to explore the biological relevance of miRNA regulation directly. PAX6 protein abundance was elevated in double miR-7-5p MRE mutants relative to wild type and single mutants in the ventral V-SVZ. However, this increase in PAX6 was not associated with an altered dopaminergic periglomerular neuron phenotype in the olfactory bulb. Our findings suggest that, in vivo, microRNA regulation can be mediated through redundant MRE interactions. This work also reveals that directly mutating predicted MREs at the genomic level is necessary to fully characterize the specific phenotypic consequences of miRNA-target regulation. / Graduate