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The teratogenic effects of valproic acid (VPA) are well known, though its teratogenic mechanism remains unknown. VPA induces oxidative stress, which may lead to double strand breaks (DSBs) in DNA. Though the cell may repair this damage via homologous recombination (HR) and non-homologous end joining (NHEJ), repair is not always error-free; genomic instability may arise from gene deletions, amplifications, rearrangements, and loss of heterozygosity. Such alterations may underpin VPAʼs teratogenicity. The present study evaluated VPAʼs ability to induce NHEJ and HR and characterized the changes in expression of two proteins key to HR (RAD51) and NHEJ (XRCC4). Using pKZ1 transgenic mice (C57BL/6 genetic background), we sought to measure NHEJ events via X-gal staining. Although consistent staining was observed in adult male brain (positive control), no staining was observed in embryos 12 or 24 hours after in utero exposure to a teratogenic dose of VPA (500 mg/kg, maternal subcutaneous dose) on gestational day 9 (GD9). To determine whether the lack of staining observed in embryos was due to low/absent expression of key DSB-repair proteins, we measured mRNA/protein expression of RAD51 and XRCC4 in C57BL/6, GD9-exposed embryos and maternal brain. One hour after treatment, XRCC4 was increased at the protein level in brain and embryo. RAD51 was not increased in embryos and not detected in adult brain. These data suggest that embryos do possess the protein mediators of NHEJ and HR and that VPA-induced changes in expression of XRCC4 may influence the type of repair pursued, potentially affecting DSB repair fidelity (accuracy). Determination of fidelity of VPA-induced HR was attempted with the Chinese hamster ovary cell line (CHO33) using DNA sequencing; low template concentration and purity precluded successful sequencing of DNA from recombinant colonies and the assessment of fidelity. Overall, these data demonstrate that the lack of X-gal staining observed in pKZ1 embryos is not due to an underexpression of at least one key protein in the NHEJ pathway. Furthermore, a VPA-induced change in the the type of repair pathway pursued by the embryo may have teratological implications. / Thesis (Master, Pharmacology & Toxicology) -- Queen's University, 2012-10-15 11:06:30.613

Chromosomal replication is a source of spontaneous DNA double-strand breaks (DSBs). In E. coli, DSBs are repaired by homologous recombination using an undamaged sister template. During repair, the RecA protein polymerizes on single-stranded DNA generated at the site of the DSB and catalyses the search for sequence homologies on the undamaged sister template. This study utilized fluorescence microscopy to investigate the spatial and temporal dynamics of the RecA protein at the site of a replication-dependent DSB generated at the lacZ locus of the E. coli chromosome. The DSB was generated by SbcCD-mediated cleavage of a hairpin DNA structure formed on the lagging strand template of the replication fork by a long palindromic sequence. The tandem insertion of a recA-mCherry gene with the endogenous recA gene at the natural chromosomal locus produced no detectable effect on cell viability in the presence of DSB formation. During repair, the fluorescently-labelled RecA protein formed a transient focus, which was inferred to be the RecA nucleoprotein filament at the site of the replication-dependent DSB. The duration of the RecA focus at the site of the DSB was modestly reduced in a ΔdinI mutant and modestly increased in a ΔuvrD or ΔrecX mutant. Most cells underwent a period of extended cohesion of the sister lacZ loci after disappearance of the RecA focus. Segregation of the sister lacZ loci was followed by cell division, with each daughter cell obtaining a copy of the fluorescently-labelled lacZ locus. The RecA focus at the site of the DSB was observed predominantly between the mid-cell and the 1⁄4 position. In the absence of DSB formation, the lacZ locus exhibited dynamic movement between the mid-cell and the 1⁄4 position until the onset of segregation. Formation of the DSB and initiation of repair occurred at the spatial localization for replication of the lacZ locus while the downstream repair events occurred very close to the mid-cell. Genomic analysis of RecA-DNA interactions by ChIP-seq was used to demonstrate that the RecA focus at the lacZ locus was generated by the repair of the palindrome-induced DSB and not the repair of one-ended DSBs emanating from stalled replication forks at the repressor-bound operator arrays. This study has shown that the repair of a replication-dependent DSB occurs exclusively during the period of cohesion of the sister loci and the repair is efficiently completed prior to segregation of the two sister loci.

Afin de transmettre leurs informations génétiques d'une génération à l'autre, les organismes à reproduction sexuée doivent réduire de moitié leur contenu chromosomique pour former des gamètes haploïdes. Cette réduction se produit lors d'une division cellulaire appelée méiose, durant laquelle une étape de réplication est suivie de deux divisions successives, la méiose I et II. Au cours de la méiose I, les chromosomes homologues se séparent et leur bonne ségrégation dépend de la création entre eux d’un lien physique. En méiose c’est le processus de réparation appelé recombinaison homologue, qui à la suite de l’induction dans le génome de centaine de cassures double brin par la protéine Spo11, permet d’établir ce lien. Spo11 est l'orthologue méiotique de la sous-unité catalytique de la topoisomérase VI, TopoVIA. Comme TopoVI est composée de deux sous-unités, TopoVIA et TopoVIB, l’existence d’un orthologue méiotique de TopoVIB était une question posée depuis l'identification de Spo11. Au cours de ma thèse, j'ai contribué à identifier une nouvelle famille de protéine, que l’on a nommé TopoVIB-like, orthologue à TopoVIB et nécessaire à la formation des cassures double-brin d'ADN méiotiques(Robert et al, 2016). Ces protéines ont des domaines similaires à ceux de TopoVIB, à savoir un GHKL (impliqué dans la liaison et l'hydrolyse de l'ATP), un domaine transducteur et un domaine CTD. Nous avons démontré que chez la souris, SPO11 forme un complexe avec TOPOVIBL. De plus, nous avons démontré que cette protéine est nécessaire à la formation des CDB. Ces résultats suggèrent que chez la souris, les CDB méiotiques sont catalysées par un complexe TopoVI-like. Chez S. cerevisiae, il n'y a pas d'orthologue clair de TopoVIB, mais nous avons trouvé que la protéine Rec102, connue pour être nécessaire à la formation des CDB méiotiques, présente une homologie partielle avec le domaine transducteur des TopoVIB-like. Rec102 forme un complexe avec Rec104, une protéine également requise pour la formation des CDB. Ainsi, nous avons émis l'hypothèse que le complexe Rec102 / Rec104 était l'orthologue méiotique de TopoVIB chez la levure, interagissant avec Spo11 pour former un complexe de type TopoVI-like. Malgré l'importance de Spo11, son mode d'action est mal connu. Cette absence de données biochimiques est due à l’insolubilité de la protéine. Le but de ma thèse était de caractériser le mode d'action et la régulation du complexe TopoVI-like dans la formation des CDB méiotiques. Tout d'abord, biochimiquement, en purifiant in vitro une forme soluble du complexe TopoVI-like de levure composé de Spo11 / Rec102 / Rec104 / Ski8 (un partenaire direct de Spo11) en co-exprimant ces protéines dans deux systèmes d'expression, E. coli et S. cerevisiae. En utilisant E. coli, j'ai réussi à purifier un complexe soluble formé par Spo11 / Rec102 / Rec104 / Ski8 et en utilisant S. cerevisiae, j'ai purifié deux complexes différents, l'un formé par les quatre protéines, et un formé uniquement par Spo11 et Rec102. Néanmoins, les tests d'activité sur différents substrats d'ADN n'ont révélé aucune activité de coupure de l’ADN. Le deuxième objectif de ma thèse était d'étudier comment, chez la souris, TOPOVIBL régule l'activité de SPO11 en interagissant avec d'autres protéines nécessaires à la formation des CDB. En double hybride, j'ai prouvé que, comme chez la levure, l'orthologue méiotique de TopoVIB chez la souris interagissait avec REC114, une autre protéine nécessaire à la formation des CDB. La cartographie de cette interaction à l'échelle de l’acide aminé a conduit à l'identification d'un résidu sur TOPOVIBL essentiel pour l'interaction entre TOPOVIBL et REC114. Afin d'étudier in vivo le rôle de l'interaction entre TOPOVIBL et REC114, une souris mutante pour le résidu identifié de TOPOVIBL a été générée à l'aide de CRISPER-Cas9 et son phénotype a été analysé. / To properly transmit their genetic information from one generation to another, sexually reproductive organisms need to halve their genome to form haploid gametes. This reduction occurs during a special cell division called meiosis, which proceeds through one round of DNA replication followed by two successive divisions called meiosis I and II. During meiosis I homologous chromosomes segregate, and their proper segregation depends on the homologous recombination pathway that establishes a physical link between the homologues. During meiosis, homologous recombination events are triggered by the formation of DNA double strand break (DSB) catalyzed by the evolutionarily conserved Spo11 protein. Spo11 is the meiotic ortholog of the catalytic subunit of the TopoVI topoisomerase, TopoVIA. As TopoVI is composed of two subunits, TopoVIA and TopoVIB, the requirement for meiotic DSB formation of a B subunit was under investigation since the identification of Spo11. During my PhD, I contributed to the identification of a new family of protein, the TopoVIB-like family, ortholog to the Topoisomerase VI B subunit (TopoVIB) and required for meiotic DNA double strand break formation (Robert et al, 2016). These proteins share domains in part similar to the canonical TopoVIB which are a GHKL domain (involved in ATP binding and hydrolysis), a transducer domain and a CTD domain. We demonstrated that in mice, SPO11 forms a complex with TOPOVIBL. Biochemical characterization of this complex showed a structure compatible with an A2B2 organization. Furthermore, we demonstrated that this protein is required for meiotic DSB formation. These results suggest the existence, in mice, of a TopoVI-like complex that catalyzes the formation of meiotic DSB. In S. cerevisiae, there is no clear TopoVIB-like ortholog, but we found that the Rec102 protein, which is known to be required for the formation of meiotic DSB, shows a partial homology with the transducer domain of the TopoVIB-like proteins. Rec102 forms a complex with Rec104, a protein also essential for DSB formation. Thus, we hypothesized that the Rec102/Rec104 complex is the yeast meiotic ortholog of TopoVIB, interacting with Spo11 to form a meiotic TopoVI-like complex. Despite the importance of Spo11 little is known about its mode of action. This absence of biochemical data is due to the lack of solubility of the protein. The aim of my PhD was to characterize the mode of action and regulation of the TopoVI-like complex for meiotic DSB formation. First, biochemically, by purifying in vitro a soluble form of the yeast TopoVI-like complex composed by Spo11/Rec102/Rec104/Ski8. To achieve this objective, I co-expressed these proteins in two different expression systems, E. coli and meiotic culture of S. cerevisiae. Using E. coli I managed to purify a soluble complex formed by Spo11/Rec102/Rec104/Ski8, and using meiotic culture of S. cerevisiae, I purified two different complexes, one formed, by the four proteins, and one formed only by Spo11 and Rec102. Nevertheless, in vitro activity essays on different DNA substrates did not reveal any DNA cleavage activity. The second goal of my PhD was to study how in mouse, the activity of TOPOVIBL / SPO11 may be regulated by other proteins known to be required for DSB formation. Using Y2H experiment I was able to prove that, as in yeast, mouse TOPOVIBL interacts with REC114, a protein required for DSB formation. The mapping of this interaction at the amino-acid scale, leads to the identification of one residue on TOPOVIBL essential for the interaction between TOPOVIBL and REC114. In order to investigate in vivo the role of the interaction between TOPOVIBL and REC114, a mutant mouse carrying a mutation in the identified residue of TOPOVIBL was generated using CRISPER-Cas9, and its phenotype analyzed.

The goal of my research was to obtain a quantitative understanding of the mechanisms of DNA double-strand break (DSB) repair, and the activation of the tumor suppressor p53 in response to DSBs in human cells. In Chapter 2, we investigated how the kinetics of repair, and the balance between the alternate DSB repair pathways, nonhomologous end-joining (NHEJ) and homologous recombination (HR), change with cell cycle progression. We developed fluorescent reporters to quantify DSBs, HR and cell cycle phase in individual, living cells. We show that the rates of DSB repair depend on the cell cycle stage at the time of damage. We find that NHEJ is the dominant repair mechanism in G1 and in G2 cells even in the presence of a functional HR pathway. S and G2 cells use both NHEJ and HR, and higher use of HR strongly correlates with slower repair. Further, we demonstrate that the balance between NHEJ and HR changes gradually with cell cycle progression, with a maximal use of HR at the peak of active replication in mid-S. Our results establish that the presence of a sister chromatid does not affect the use of HR in human cells. Chapter 3 examines the sensitivity of the p53 pathway to DNA DSBs. We combined our fluorescent reporter for DSBs with a fluorescent reporter for p53, to quantify the level of damage and p53 activation in single cells. We find that the probability of inducing a p53 pulse increases linearly with the amount of damage. However, cancer cells do not have a distinct threshold of DSBs above which they uniformly induce p53 accumulation. We demonstrate that the decision to activate p53 is potentially controlled by cell-specific factors. Finally, we establish that the rates of DSB repair do not affect the decision to activate p53 or the dynamical properties of the p53 pulse. Collectively, this work emphasizes the importance of collecting quantitative dynamic information in single cells in order to gain a comprehensive understanding of how different DNA damage response pathways function in a coordinated manner to maintain genomic integrity.

DNA double-strand breaks (DSBs), where both strands of the DNA duplex are simultaneously fractured, are considered the most lethal type of DNA damage. The conserved Mre11-Rad50 DNA repair complex enables the catalytic activities of the Mre11 nuclease and the Rad50 ATPase to function together to coordinate the recognition and processing of DSBs prior to the recruitment of long-range end-resection machinery required to trigger the DSB repair by the homologous recombination (HR) pathway. Fast-scan atomic force microscopy (AFM) in fluid conditions was primarily used to explore the architectural arrangement, DNA binding and processing machinery of the Mre11-Rad50 complex from the thermophilic crenarchaeote Sulfolobus acidocaldarius. The structural analysis identified four distinct architectural arrangements and demonstrates the key role of the Rad50 zinc hooks in the oligomerisation of the complex. AFM imaging showed a dynamic and Velcro-like interplay between Mre11-Rad50 protein complexes and the DNA double-helix using the Rad50 coiled-coils in a novel mode of DNA binding. The complex appears to use the Rad50 zinc hook region to bind to and track along dsDNA for broken DNA-terminals. Furthermore, the present study shows that this archaeal complex can drive extensive ATP-dependent unwinding of DNA templates. It is the first time that such unwinding has been observed in a single molecule study. These observations reveal novel activities leading to the proposal of a new model for Mre11-Rad50 action during DSB repair. AFM was also used to visualise the structure and activity of the HerA-NurA protein complex, which has been predicted to combine the activity of the NurA nuclease and hexameric HerA-translocase to generate long single-stranded DNA overhangs essential for DSB repair by HR in archaea. The present data verify and clarify the presumed biological role of this complex. Overall, the present study provides new insights into the initial steps of DNA DSB repair by the HR pathway and, most importantly, the detection of the broken ends.

Les mitochondries sont des organites qui possèdent leur propre ADN (ADNmt), codant pour des gènes de la chaine respiratoire. La réparation des dommages dus aux ROS, une réplication défectueuse ou d’autres sources exogènes tels des agents chimiothérapeutiques ou des irradiations ionisantes peuvent générer des cassures double-brin (CDB) de l’ADNmt. L’ADNmt code pour des protéines essentielles à la production d’énergie, et des systèmes de maintenance de l’intégrité de ce génome efficaces sont donc nécessaires pour la viabilité des cellules. En effet des mutations de l’ADNmt sont présentes dans de nombreuses pathologies comme les myopathies mitochondriales, les cancers et les maladies neurodégénératives. Cependant les processus responsables de la maintenance de l’ADNmt suite à des CDB restent controversés.Pour élucider les mécanismes impliqués, nous avons généré des CDB mitochondriales en utilisant une lignée cellulaire humaine exprimant de manière inductible l’enzyme de restriction PstI liée à une séquence d’adressage mitochondrial. Nos résultats montrent, dans notre système, une première phase de dégradation de l’ADNmt lésé avec une cinétique rapide, n’impliquant pas l’autophagie ou l’apoptose, suivie de la ré-amplification d’ADNmt intact dans un deuxième temps. Contrairement à d’autres études nous n’avons pas pu détecter d’évènements de réparation des CDB mitochondriales générées. Nous avons ensuite cherché à identifier les protéines impliquées dans la dégradation de l’ADNmt lésé que nous observons, mais aucune nucléase testée ne semble responsable de ce processus. Des approches plus globales sont mises au point pour identifier de nouveaux acteurs, notamment un crible RNAi à grande échelle. Parallèlement nous nous intéressons aussi à une famille de phosphohydrolases, les Nudix, et à leur rôle protecteur en assainissant le réservoir de nucléotides libres. / Mitochondria are organelles that possess their own genome, the mitochondrial DNA (mtDNA). Repair of oxidative damages, defective replication, or various exogenous sources, such as chemotherapeutic agents or ionizing radiations, can generate double-strand breaks (DSBs) in mtDNA. MtDNA encodes for essential proteins involved in ATP production and maintenance of integrity of this genome is thus of crucial importance. Mutations in mtDNA are indeed found in numerous pathologies such as mitochondrial myopathies, neurodegenerative disorders or cancers. However, the mechanisms involved in mtDNA maintenance after DSBs remain unknown.To elucidate this question, we have generated mtDNA DSBs using a human inducible cell system expressing the restriction enzyme PstI targeted to mitochondria. Using this system, we could not find any support for DSBs repair of mtDNA. Instead we observed a loss of the damaged mtDNA molecules and a severe decrease in mtDNA content, followed by reamplification of intact mtDNA molecules. We have demonstrated that none of the known mitochondrial nucleases are involved in mtDNA degradation and that DNA loss is not due to autophagy, mitophagy or apoptosis but to a selective mechanism. Our study suggests that a still uncharacterized pathway for the targeted degradation of damaged mtDNA in a mitophagy/autophagy-independent manner is present in mitochondria, and might provide the main mechanism used by the cells to deal with DSBs. Global approaches are ongoing to identify proteins involved in degradation of damaged mtDNA following DSBs, mainly an RNAi screen targeting 80 nucleases. In parallel we are interested in a family of phosphohydrolases named Nudix and their putative protective role in sanitizing the nucleotides pool in mitochondria.

Depuis plus d'un siècle que les rayons X ont été découverts, les effets biologiques des radiations ionisantes ne sont pas encore entièrement expliqués. Pourtant, une description précise et la modélisation mathématique des événements physico-chimiques, moléculaires et cellulaires contribueraient significativement à l'effet des risques liés à une irradiation. Le groupe de Radiobiologie de l'UMR1052 Inserm (Lyon) a accumulé un nombre considérable de données sur la radiosensibilité individuelle et la réparation des dommages radioinduits de l'ADN qui nous permettent aujourd'hui de valider des modèles nouveaux qui sont souvent en contradiction avec les paradigmes actuels. En particulier, alors que les cassures double-brin de l'ADN semblent être les dommages clés de la létalité cellulaire, aucun protocole ni biomarqueur n'est considéré comme prédictif de la radiosensibilité. Le but de la thèse a donc été de déterminer les paramètres précis qui peuvent prédire la réponse aux radiations. Un grand nombre de protéines se relocalisent sous forme de foci nucléaire autour des sites de CDB. Dans une première étape, nous avons pu proposer une formule générale qui lie induction, reconnaissance et réparation des CDB, valable pour toutes les protéines relocalisantes après irradiation. Cette formule a été appelée « Formule de Bodgi ». La validité de cette formule a pu être vérifiée sur différents biomarqueurs et sur différents types de cellules de patients montrant des radiosensibilités différentes. Dans une deuxième étape, nous avons pu modéliser le processus de transit cytonucléaire de la protéine ATM. Nous avons pu alors apporter une interprétation nouvelle et cohérente des paramètres α et β du modèle linéaire-quadratique qui décrit la relation entre la dose de radiation et la survie cellulaire. Notre théorie s'est avérée également utile pour expliquer certaines autres énigmes de la radiobiologie, notamment le phénomène d'hypersensibilité aux faibles doses / More than a century after the discovery of X-rays, the biological effects of ionizing radiation are still not entirely explained. Nevertheless, a relevant description and a mathematical model of the physico-chemical, molecular and cellular events would significantly contribute to the evaluation of the related risks. The Radiobiology Group of the UMR1052 Inserm Unit (Lyon) has collected a considerable number of data concerning individual radiosensitivity and DNA damage repair, which allows us today to validate actual modeling approaches that are often contradicting actual paradigms. Particularly, while the DNA double-strand breaks (DSB) appear to be the key-damages of cell lethality, there is still no experimental protocol or biomarker that is considered to be predictive for radiosensitivity. The purpose of this thesis was to determine the precise parameters that can predict the response to radiation. An important number of proteins relocalize as nuclear foci in the DSB sites. As a first step, we proposed a general formula that links the DSB induction, recognition and repair, valid for all the relocalized proteins after irradiation. We called this formula the ‘’Bodgi’s formula’’. The validity of this model was verified with different biomarkers, but also on different cell types from patients showing different radiosensitivity. In a second step, we proposed a model for the whole process of the nucleo-shuttling of the ATM protein that occurs after irradiation. We provided a novel and coherent interpretation of the α and β parameters of the linear-quadratic model that describes the relation between radiation dose and cell survival. Our theory was also shown to be useful in explaining some other enigmas of radiobiology, including the hypersensitivity to lowdose phenomena

B cells play crucial roles in the development of the two human autoimmune diseases, type 1 diabetes (T1D) and systemic lupus erythematosus (SLE). In the past decade, numerous studies showed positive responses of B cell depletion therapies in these two diseases. However, the beneficial effects are temporary and accompanied with adverse events. In this dissertation, we aimed to identify novel targets for a better modulation of disease development using mouse models. These diseases have circulating autoantibodies that are mostly mutated with an IgG isotype, indicating B cells that are producing them have been through the process of affinity maturation. Activation-induced cytidine deaminase (AID) is a core enzyme that regulates somatic hypermutation (SHM) and class switch recombination (CSR), the two key mechanisms in affinity maturation. We showed that genetic ablation of AID significantly inhibited the development of TID in NOD mice. Homologous recombination (HR) pathway is important for the repair of AID-induced DNA double strand breaks during CSR. 4,4'-Diisothiocyano-2,2'-stilbenedisulfonic acid, also known as DIDS, is a small molecule that inhibits HR pathway and subsequently leads to apoptosis of class switching cells. DIDS treatment remarkably retarded the progression of TID, even when started at a relatively late stage, indicating the potential of this treatment for disease reversal. In both approaches, we observed a notable expansion of CD73+ B cells, which exerted an immunosuppressive role and could be responsible for T1D resistance. Next we examined the effect of targeting affinity maturation through these two approaches in lupus-prone mice. The genetic abrogation of AID in BXSB mice significantly ameliorated lupus nephritis and prolonged their lifespan. AID-deficient mice also exhibited improvement on disease hallmarks with increased marginal zone B cells and more normal splenic architecture. DIDS treatment notably reduced class switching when B cells were stimulated in vitro. However, the administration of DIDS did not strikingly alter the course of SLE in either BXSB mice or MRL/lpr mice. These findings demonstrated that affinity maturation could be a potential target for T1D and SLE, while further explorations into targeting other components in the repair pathway are warranted for SLE. Lastly, we assessed the effect of maternal AID modulation on the SLE development in the offspring using BXSB mouse model. Interestingly, the absence of maternal AID resulted in offspring that developed significantly more severe lupus nephritis compared to control. The offspring born to AID-deficient dams also exhibited elevated levels of pathogenic autoantibodies and exacerbated disease features. Therefore, the modulation of maternal AID could influence the SLE development in the offspring, and future investigations are needed to determine the underlying mechanisms responsible for the disease acceleration. / Doctor of Philosophy / The failure of the immune system to differentiate self from non-self leads to the development of autoimmune diseases. Type 1 diabetes (T1D) and systemic lupus erythematosus (SLE) are complex autoimmune diseases affecting millions of people in the world. Despite intensive research regarding these two diseases, no known cure is available indicating an imperative need for the development of novel therapies. With the importance of B cells in the pathogenesis of these two diseases, intensive research focused on whole B cell depletion therapies. However, these therapies exhibited high risks of infections as a result of depleting all the B cells. In this dissertation, we sought to selectively target specific B lymphocyte subsets that are crucial contributing factors in the development of T1D and SLE. While the effect of therapeutic treatment varied among different mouse models, the genetic manipulation of specific B cells successfully retarded the progression of both T1D and SLE and extended the lifespan of the mice. Further studies shed light on the possible mechanisms that are responsible for the disease inhibition. These data proved that targeting specific B cell compartment could be a potential disease management in T1D and SLE patients. In addition, using the established mouse model, we demonstrated the modulation of maternal factors significantly impact the SLE development in the offspring. Future experiments to identify the underlying mechanisms could provide more targets for the therapeutic development.

Résumé : La phase haploïde de la spermatogenèse (spermiogenèse) est caractérisée par une modification importante de la structure de la chromatine et un changement de la topologie de l’ADN du spermatide. Les mécanismes par lesquels ce changement se produit ainsi que les protéines impliquées ne sont pas encore complètement élucidés. Mes travaux ont permis d’établir la présence de cassures bicaténaires transitoires pendant ce remodelage par l’essai des comètes et l’électrophorèse en champ pulsé. En procédant à des immunofluorescences sur coupes de tissus et en utilisant un extrait nucléaire hautement actif, la présence de topoisomérases ainsi que de marqueurs de systèmes de réparation a été confirmée. Les protéines de réparation identifiées font partie de systèmes sujets à l’erreur, donc cette refonte structurale de la chromatine pourrait être génétiquement instable et expliquer le biais paternel observé pour les mutations de novo dans de récentes études impliquant des criblages à haut débit. Une technique permettant l’immunocapture spécifique des cassures bicaténaires a été développée et appliquée sur des spermatides murins représentant différentes étapes de différenciation. Les résultats de séquençage à haut débit ont montré que les cassures bicaténaires (hotspots) de la spermiogenèse se produisent en majorité dans l’ADN intergénique, notamment dans les séquences LINE1, l’ADN satellite et les répétions simples. Les hotspots contiennent aussi des motifs de liaisons des protéines des familles FOX et PRDM, dont les fonctions sont entre autres de lier et remodeler localement la chromatine condensée. Aussi, le motif de liaison de la protéine BRCA1 se trouve enrichi dans les hotspots de cassures bicaténaires. Celle-ci agit entre autres dans la réparation de l’ADN par jonction terminale non-homologue (NHEJ) et dans la réparation des adduits ADN-topoisomérase. De façon remarquable, le motif de reconnaissance de la protéine SPO11, impliquée dans la formation des cassures méiotiques, a été enrichi dans les hotspots, ce qui suggère que la machinerie méiotique serait aussi utilisée pendant la spermiogenèse pour la formation des cassures. Enfin, bien que les hotspots se localisent plutôt dans les séquences intergéniques, les gènes ciblés sont impliqués dans le développement du cerveau et des neurones. Ces résultats sont en accord avec l’origine majoritairement paternelle observée des mutations de novo associées aux troubles du spectre de l’autisme et de la schizophrénie et leur augmentation avec l’âge du père. Puisque les processus du remodelage de la chromatine des spermatides sont conservés dans l’évolution, ces résultats suggèrent que le remodelage de la chromatine de la spermiogenèse représente un mécanisme additionnel contribuant à la formation de mutations de novo, expliquant le biais paternel observé pour certains types de mutations. / Abstract : Germline mutations may arise from several endogenous and exogenous mechanisms in both male and female. However, recent next-generation sequencing (NGS) data confirmed that de novo mutations arise primarily in males. This observation suggests that specific spermatogenesis events are involved in the male mutation bias. One potential origin for male-driven mutations is the differentiation of spermatids into spermatozoa, which involves one of the most striking and global chromatin remodeling processes, where histone-bound chromatin is converted into highly condensed protaminated DNA toroid. Using pulse-field gel electrophoresis and comet assay on flow cytometry sorted cells, it was established that chromatin remodeling process is characterized by a transient surge in DNA double strand breaks (DSBs) in the whole population of murine spermatids, which get repaired by the end of spermiogenesis. Using a highly active nuclear extract and immunofluorescences, topoisomerases and markers of DNA repair systems were shown at these steps. Since haploid cells cannot rely on homologous recombination for templated DNA repair, it was hypothesized that this process may be genetically unstable and largely responsible for the observed male de novo mutations bias. Although very challenging, a method allowing the specific genome-wide mapping of DSBs using NGS was developed to establish the genomic distribution of DSBs during chromatin remodeling. It was shown that intergenic regions were enriched in DSBs, particularly LINE1, satellite DNA and simple repeats. Motif finding on potential hotspots showed that proteins from FOX and PRDM families may be implicated. Although homologous recombination cannot take place during spermiogenesis, an enrichment in BRCA1 motif was found, which is also known to be implicated in NHEJ and removal of topoisomerase adducts. Topoisomerase-like SPO11 motif was also enriched suggesting that the meiotic machinery may also be implicated during chromatin remodeling. Moreover, although DSBs tend to accumulate in intergenic regions, gene ontology analysis of hotspot-containing genes showed a marked enrichment in genes related to neurons and brain development. This result hence supports the fact that neurological disease associated mutations are also male biased and associated with advanced paternal age. Since DSB formation during spermiogenesis is conserved through evolution, these results suggest that chromatin remodeling in spermatids represents a significant component in the reported male de novo mutation bias.