Nucleases are an essential component of the prokaryotic CRISPR-Cas immunity as well as repair mechanisms within prokaryotic organisms. To better understand the adaptation step of CRISPR-Cas immunity, I have characterized three Cas4 proteins from hyperthermophilic archaea: SSO0001 and SSO1391 from Sulfolobus solfataricus and Pcal_0546 from Pyrobaculum calidifontis. All three proteins have metal-dependent 5′ to 3′ exonuclease and endonuclease activities, while SSO1391 also demonstrates 3′ to 5′ exonuclease activity. Site-directed mutagenesis confirmed that the conserved RecB motif residues are important for the nuclease activity in all three proteins. SSO0001 and Pcal_0546 also exhibit ATP-independent unwinding and cleavage of splayed arm substrates. Structural analysis of SSO0001 showed it is a toroidal decamer with a [4Fe-4S] cluster and Mn2+ ion bound in the active site located inside the internal tunnel. Our results show that Cas4 proteins have the ability to create 3'-DNA overhangs which may contribute to the addition of novel CRISPR spacers.

Powerful mechanisms of genetic interference in both unicellular and multicellular organisms are based on the sequence-directed targeting of DNA or RNA by small effector RNAs. In many bacteria and almost all archaea, RNAs derived from clustered, regularly interspaced, short palindromic repeat (CRISPR) loci are involved in an adaptable and heritable gene-silencing pathway. Resistance to phage infection is conferred by the incorporation of short invading DNA sequences into the prokaryotic genome as CRISPR spacer elements separated by short repeat sequences. A central aspect to this pathway is the processing of a long primary transcript (pre-crRNA) containing these repeats by crRNA endonucleases to generate the mature effector RNAs that interfere with phage or plasmid gene expression. Here we describe a structural and functional analysis of the CasE endonuclease of T. thermophilus a member of the Ecoli CRISPR sub-type. High resolution X-ray structures of CasE bound to repeat RNAs model both the pre-and post-cleavage complexes associated with processing the pre-crRNA. These structures establish the molecular basis of a specific CRISPR RNA recognition and suggest the mechanism for generation of effector RNAs responsible for gene-silencing.

Cachexia is a multifactorial metabolic wasting syndrome that affects a large percentage of cancer patients and results in the involuntary loss of skeletal muscle and adipose tissue. The consequences of this condition include metabolic imbalances and fatigue, which are strongly associated with poor prognosis. While the specific mechanism for skeletal muscle wasting is still undefined, LIF secreted by C26 colon carcinoma cells has recently be found to induce atrophy in treated myotubes. The purpose of this study is to determine the necessity of LIF for inducing atrophy in mouse myotubes by producing a knockout of Lif in C26 cells using CRIPSR-Cas9. Media was collected from these cells and used to treat myotubes. Measurements of myotube diameters were made and atrophy was compared between myotubes that received medium from C26 and C26Lif-/- cells. A dosage of recombinant mouse LIF was also added to LIF-deficient medium in order to determine if LIF alone was sufficient to induce atrophy. At study endpoint, myotubes that were treated with media taken from C26 cells showed significant signs of atrophy compared to myotubes that were treated C26Lif-/- media. LIF was also shown to be sufficient to induce myotube atrophy on its own, with atrophy being rescued in myotubes that received a dosage of LIF added to C26Lif-/- media. These results demonstrate that LIF is required for atrophy to be induced in mouse myotubes treated with media taken from cancer cells, and can do so independent of other secreted factors.

Viruses are ubiquitous in natural growth environments and cause severe losses to crop yields, globally. Approximately 30% of plant viruses described to date are grouped within the family Potyviridae, making it one of the largest plant virus families. Furthermore, certain potyvirus species can cause devastating diseases in several agriculturally and economically important crops. Hence, gaining insight into potyvirus resistance and recovery mechanisms in plants is an important research focus. This thesis firstly explores how environmental cues can modulate the activity of a central form of viral defence, namely RNA silencing. Specifically, high temperatures and low light intensities were found to increase the efficacy of viral RNA silencing in Arabidopsis, resulting in recovery from infection by Turnip Mosaic Virus. The biological context and potential for agricultural exploitation of these phenomena are discussed. Secondly, this thesis explores the ability to engineer resistance alleles using the latest genome editing techniques. Specifically, resistance to Turnip Mosaic Virus was successfully engineered in Arabidopsis by CRISPR/Cas9-induced deletion of a known susceptibility factor eIF(iso)4E. Biotechnological methods to implement this proof of concept research in crop species were also investigated.

Several techniques have recently been described for precise mutagenesis of selected target sites in the genome. This thesis establishes the method of gene targeting by CRISPR/Cas system in D. melanogaster and compares it with gene targeting using TALENs. To test the mutagenesis systems, we choose an endogenous gene encoding concentrative nucleoside transporter gene (CNT1). We have received two mutants containing large deletions affecting the N-terminal part of the CNT1 gene. We show that CRISPR/Cas is useful tool for targeted gene disruption in D. melanogaster.

8q24.21 is a highly amplified region in cancer and associated with many epithelial cancer such as bladder, breast, colorectal and prostate cancer. The proto-oncogene c-myc is located in this region and surrounded by many lncRNAs genes such as PCAT family, CCAT family, PRNCR1. In this study, we used CRISPR-Cas9 constructs to knock out PCAT1, PRNCR1, CASC8, CASC11 and also the sequences between PCAT1-CASC11 and CASC8-CASC11in the prostate cancer cell PC3. The transfected cells with CRISPR-Cas9 targeting CASC11 gene had less proliferation ability comparing with the transfected cells with CRISPR-Cas9 targeting PCAT1, PRNCR1 or CASC8. The role of CASC11 in cancer progression and development is obscure. In our study, The CASC11 Knockout efficiency was 90% compare to the control cell. Furthermore, the study showed the importance of CASC11 in cell proliferation by significantly decreasing in the forming colonies and the growth rate comparing to the control. Also, MMP2, MMP3 and MMP9 expression levels were detected in the transfected cell by using real time PCR and the result revealed the crucial role for CASC11 in metastasis and migration. The slug and vimentin expression levels were reduced in the transfected and the double transfected clones which indicate the possible role of CASC11 in epithelial mesenchymal transition and cell motility. Taken together, our study revealed that the lncRNA CASC11 plays important roles in prostate cancer progression and metastasis by promoting the cell proliferation and migration.

Loss-of-function genetic screens are a powerful approach to identify the genes involved in biological processes. For nearly a century, forward genetic screens in model organisms have provided enormous insight into many cellular processes. However, the difficulty in generating and recovering bi-allelic mutations in diploid cells severely hindered the performance of forward genetic screens in mammalian cells. The development of a retroviral gene-trap vector to mutagenise the human near-haploid KBM7 cell line transformed forward genetic screens in human cells. The re-purposing of the microbial CRISPR/Cas9 system now offers an effective method to generate gene knockouts in diploid cells. Here, I performed a head-to-head comparison of retroviral gene-trap mutagenesis screens and genome-wide CRISPR knockout screens in KBM7 cells. The two screening approaches were equally effective at identifying genes required for the endoplasmic reticulum (ER)-associated degradation of MHC class I molecules. The ER-resident enzyme HMG-CoA reductase (HMGCR) catalyses the rate-limiting step in the cholesterol biosynthesis pathway and is targeted therapeutically by statins. To maintain cholesterol homeostasis, the expression of HMGCR is tightly regulated by sterols transcriptionally and post-translationally. Sterols induce the association of HMGCR with Insig proteins, which recruit E3 ubiquitin ligase complexes to mediate degradation of HMGCR by the ubiquitin proteasome system. However, the identity of the E3 ligase(s) responsible for HMGCR ubiquitination is controversial. Here, I use a series of genome-wide CRISPR knockout screens using a fluorescently-tagged HMGCR exogenous reporter and an endogenous HMGCR knock-in as an unbiased approach to identify the E3 ligases and any additional components required for HMGCR degradation. The CRISPR screens identified a role for the poorly characterised ERAD E3 ligase RNF145. I found RNF145 to be functionally redundant with gp78, an E3 ligase previously implicated in HMGCR degradation, and the loss of both E3 ligases was required to significantly inhibit the sterol-induced degradation and ubiquitination of HMGCR. A focused E3 ligase CRISPR screen revealed that the combined loss of gp78, RNF145 and Hrd1 was required to completely block the sterol-induced degradation of HMGCR. I present a model to account for this apparent complexity.

The ability to precisely manipulate the genome in a targeted manner is fundamental to driving both basic science research and development of medical therapeutics. Until recently, this has been primarily achieved through coupling of a nuclease domain with customizable protein modules that recognize DNA in a sequence-specific manner such as zinc finger or transcription activator-like effector domains. Though these approaches have allowed unprecedented precision in manipulating the genome, in practice they have been limited by the reproducibility, predictability, and specificity of targeted cleavage, all of which are partially attributable to the nature of protein-mediated DNA sequence recognition. It has been recently shown that the microbial CRISPR-Cas system can be adapted for eukaryotic genome editing. Cas9, an RNA-guided DNA endonuclease, is directed by a 20-nt guide sequence via Watson-Crick base-pairing to its genomic target. Cas9 subsequently induces a double-stranded DNA break that results in targeted gene disruption through non-homologous end-joining repair or gene replacement via homologous recombination. Finally, the RNA guide and protein nuclease dual component system allows simultaneous delivery of multiple guide RNAs (sgRNA) to achieve multiplex genome editing with ease and efficiency. The potential effects of off-target genomic modification represent a significant caveat to genome editing approaches in both research and therapeutic applications. Prior work from our lab and others has shown that Cas9 can tolerate some degree of mismatch with the guide RNA to target DNA base pairing. To increase substrate specificity, we devised a technique that uses a Cas9 nickase mutant with appropriately paired guide RNAs to efficiently inducing double-stranded breaks via simultaneous nicks on both strands of target DNA. As single-stranded nicks are repaired with high fidelity, targeted genome modification only occurs when the two opposite-strand nicks are closely spaced. This double nickase approach allows for marked reduction of off-target genome modification while maintaining robust on-target cleavage efficiency, making a significant step towards addressing one of the primary concerns regarding the use of genome editing technologies. The ability to multiplex genome engineering by simply co-delivering multiple sgRNAs is a versatile property unique to the CRISPR-Cas system. While co-transfection of multiple guides is readily feasible in tissue culture, many in vivo and therapeutic applications would benefit from a compact, single vector system that would allow robust and reproducible multiplex editing. To achieve this, we first generated and functionally validated alternate sgRNA architectures to characterize the structure-function relationship of the Cas9 protein with the sgRNA in DNA recognition and cleavage. We then applied this knowledge towards the development and optimization of a tandem synthetic guide RNA (tsgRNA) scaffold that allows for a single promoter to drive expression of a single RNA transcript encoding two sgRNAs, which are subsequently processed into individual active sgRNAs.

Parkinson’s disease is a highly prevalent multifactorial neurodegenerative disorder caused by a complex cascade of interactions between various genetic and environmental factors. Due to this, the majority of cases are termed idiopathic. However, about 10% of PD cases are due to defined genetic factors. Interestingly, both idiopathic and familial cases of PD share mitochondrial dysfunction as a central component in the pathology of the disease. The mitochondrial protease, presenilin-associated rhomboid-like (PARL), is one such Parkinson's disease-linked gene, and is associated with diverse processes including mitochondrial dynamics, active inhibition of unnecessary apoptosis and mitophagy in Drosophila and yeast. Here, I investigated the role of the two zebrafish parl paralogs, parla and parlb, through stable CRISPR-Cas9 mediated mutagenesis. I injected wild type embryos with sgRNAs targeting parla and parlb loci, successfully producing indel mutations in parlb and multi-exon deletions in parla at mutation efficiencies of 74% and 40%, respectively. Through whole mount in situ hybridization experiments against th1, I saw no change in dopaminergic (DA) neuron development displayed by parlb mutants compared to wild types. Injection of parla splice blocking morpholinos into parlb mutants indicates that proper DA neuron development may depend principally on Parla function and loss of both Parla and Parlb function increases larval mortality. These results suggest a negative epistatic relationship between the parl paralogs as seen by the more severe phenotype observed in the loss of both Parla and Parlb function compared to the individual effects.

Les systèmes CRISPR-Cas confèrent aux bactéries une immunité adaptative contre les éléments génétiques mobiles jouant ainsi un rôle important dans l’évolution bactérienne. Cependant, moins de la moitié des génomes bactériens encodent des systèmes CRISPR-Cas ; cela, malgré la protection qu’ils confèrent et leur haut taux de transfert horizontal. Des hypothèses telles que le coût des phénomènes d’auto-immunité ou de posséder des défenses adaptatives plutôt qu’innées ont été mises en avant pour expliquer ce paradoxe. Je propose une nouvelle hypothèse complémentaire : le contexte génétique jouerait un rôle important dans la fixation d’un système CRISPR-Cas après son transfert. Plus précisément, j’ai étudié comment les interactions entre les systèmes de réparation de l’ADN et les CRISPR-Cas influencent la distribution de ces derniers. Pour cela, j’ai d’abord examiné finement la distribution des systèmes CRISPR-Cas dans les génomes bactériens. J’ai ensuite analysé les co-occurences des systèmes de réparation de l’ADN et des CRISPR-Cas et démontré l’existence d’associations positives et négatives entre eux. Enfin, je me suis concentrée sur une des associations négatives découvertes pour valider mes prédictions expérimentalement et comprendre les mécanismes moléculaires sous-jacents. Mes travaux permettent de mieux comprendre les interactions complexes entre systèmes de réparation de l’ADN et CRISPR-Cas et démontrent la nécessite d’accommodation des CRISPR-Cas à un contexte génétique pour être sélectionnés et maintenus dans les génomes bactériens. / CRISPR-Cas systems confer bacteria and archea an adaptative immunity against phages and other invading genetic elements playing an important role in bacterial evolution. Only 47% of bacterial genomes harbor a CRISPR-Cas system despite their high rate of horizontal transfer. Hypothesis such as the cost of autoimmu- nity or the trade off between a constitutive or an inducible defense system have been put forward to explain this paradox. I propose that the genetic background plays an important role in the process of maintaining a CRISPR-Cas system af- ter its transfer. More precisely I hypothesized that CRISPR-Cas systems interact with DNA repair pathways. To test this idea, we detected DNA repair pathways and CRISPR-Cas systems in bacterial genomes and studied their co-occurences. We report both positive and negative associations that we interpret as poten- tial antagonistic or synergistic interactions. We then focused on one interaction to validate our result experimentally and explored molecular mechanisms behind those interactions. My findings give insights on the complex interactions between CRISPR-Cas systems and DNA repair mechanisms in bacteria and provide a first example on the necessity of accommodation of CRISPR-Cas systems to a specific genetic context to be selected and maintained in bacterial genomes.