The aim of this thesis is to reveal the potential of mouse polyomavirus (MPyV) based virus-like particles (VLPs) as possible nanocarriers for directed delivery of therapeutic or diagnostic compounds to specific cells or tissues. We have chosen mouse polyomavirus VLPs because they do not contain viral DNA and are considered safe for utilization in bio-applications. In our research, we used a chemical approach for retargeting of MPyV based VLPs from their natural receptor to cancer cells. The chemical modification of the capsid surface exposed lysines by an aldehyde-containing reagent enabled conjugation of VLPs to selected molecules: transferrin and inhibitor of glutamate carboxypeptidase II (GCPII). Transferrin, as a transporter of iron to metabolically active cells, targeted VLPs to numerous types of cancer cells overexpressing the transferrin receptor. On the other hand, GCPII serves as a transmembrane marker specific for prostate cancer cells and conjugation of its inhibitor to VLPs resulted in successful recognition of these cells. Electron microscopy was used for visualization of modified VLPs and flow cytometry together with confocal microscopy for investigation of cell specific interactions and VLP uptake. Furthermore, we explored the influence of serum proteins on VLPs. The abundance of...

BK polyomavirus (BKV) is associated with urinary apparatus pathogenesis in kidney transplant recipient. Immune suppression triggers BKV reactivation that potentially causes polyomavirus associated nephropathy (PVAN), a major post-transplant problem causes graft rejection. Antiviral immunity plays the key role in limiting the viral replication but selection by the immune system or antivirals may cause the evolvement of escape variants with higher fitness. Mutation in VP1, the major capsid protein can allow BKV to escape neutralizing antibodies. In an attempt to detect co-infection and minor variants, BKV VP1 genomic region was amplified by PCR and analysed by deep sequencing from plasma samples of four kidney transplant recipients. BKV genotype I and IV was identified in patients and each patient was detected with one BKV genotype. Multiple point mutations and subsequent changes in amino acid were detected in majority, three out of four, of the patients.

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Yes / Virus-encoded miRNAs are emerging as key regulators of persistent infection and host-cell immune evasion. Merkel cell polyomavirus (MCPyV), the predominant aetiological agent of Merkel cell carcinoma (MCC), encodes a single miRNA, MCV-miR-M1, which targets the oncogenic MCPyV large T antigen (LT). MCV-miR-M1 has previously been shown to play an important role in establishment of long-term infection, however, the underlying mechanism is not fully understood. A key unanswered question is whether, in addition to auto-regulating LT, MCV-miR-M1 also targets cellular transcripts to orchestrate an environment conducive for persistent infection. To address this, we adopted an RNA-Seq-based approach to identify cellular targets of MCV-miR-M1. Intriguingly, bioinformatics analysis of transcripts that are differentially expressed in cells expressing MCV-miR-M1 revealed several genes implicated in immune evasion. Subsequent target validation led to the identification of the innate immunity protein, SP100, as a direct target of MCV-miR-M1. Moreover, MCV-miR-M1-mediated modulation of SP100 was associated with a significant decrease in CXCL8 secretion, resulting in the attenuation of neutrophil chemotaxis towards Merkel cells harbouring synthetic MCPyV. Based on these observations we propose that MCV-miR-M1 targets key immune response regulators to help facilitate persistent infection, which is a pre-requisite for cellular transformation in MCC. / Funded in part by a University of Bradford studentship to PA and a Royal Society research award to JRB.

Merkel cell carcinoma (MCC) is a highly aggressive skin cancer with rising global incidence. Merkel cell polyomavirus (MCV) was discovered in 2008 in 80% of MCC samples and since then a causal link between MCV and the majority of MCC cases has been established. microRNAs (miRNA, miR) are a family of small non-coding RNAs which play a key role in post-transcriptional regulation of gene expression and are considered significant players in disease and development in many species. Whilst the focus of MCV research has thus far been on the oncogenic MCV early proteins, large tumour (LT) and small tumour (sT) antigens, there is a knowledge gap regarding MCV miRNA and its functional significance in MCV pathogenesis. Given the emerging importance of viral miRNAs in virus-host interaction and pathogenesis, the aim of this doctoral research project was to investigate alterations in host cell transcripts induced by MCV miRNA and determine any functional significance these might have on virus-host cell interaction. RNA sequencing (RNA-Seq) in the presence and absence of MCV miRNA uncovered a multitude of downregulated cellular transcripts. Gene ontology analysis revealed that MCV miRNA targets transcripts associated with multiple cellular processes, however, regulation of immune response was overrepresented in our datasets. Validation of RNA-Seq data using MCV miRNA mimics and a synthetic, fully replicative MCV genome (MCVSyn) confirmed RNA-Seq data at mRNA and protein expression level for several targets, including the cytokine stimulating gene, SP100, and the neutrophil stimulator chemokine, CXCL8. Moreover, dual luciferase assays revealed that SP100 and MAPK10 (a member of mitogen-activated protein kinases (MAPK) family which is involved in regulation of CXCL8 expression) are directly and specifically targeted and downregulated by MCV miRNA. The MCV miRNA-dependent dysregulation of CXCL8 secretion is associated with impaired neutrophil migration, suggesting that the virus miRNA may be implicated in evasion of the host immune response.

Les virus sont utilisés depuis longtemps dans la recherche sur le cancer et ont grandement contribué à l’avancement des connaissances de même qu’à l’établissement de préceptes importants encore valables aujourd’hui dans le domaine. L’un des défis actuels est de mieux définir les étapes menant à la transition d’une cellule normale à une cellule transformée et c’est sur cette problématique que nous nous sommes penchés. Pour ce faire, nous avons tiré profit de l’utilisation de l’antigène grand-T du virus de polyome (PyLT), un virus capable d’induire des tumeurs chez les rongeurs. Cet oncogène viral à lui seul possède des propriétés intéressantes qui suggèrent que, en plus de l’immortalisation, il peut également contribuer aux événements précoces de la carcinogénèse. Ceci repose principalement sur la capacité de PyLT à induire des tumeurs en souris transgéniques et ce, avec une certaine latence ce qui suggère que des événements supplémentaires sont nécessaires. Ainsi, l’utilisation de PyLT dans un modèle de culture cellulaire permet de disséquer les changements qui lui sont attribuables. Dans un premier temps, l’établissement du profil d'expression génique associé à l'expression de PyLT dans un modèle murin nous a permis de sélectionner un bon nombre de gènes, parmi lesquels figurait Necdin. Nous avons choisi d’étudier Necdin plus en détail puisque peu d’attention était accordée à cette protéine dans le domaine du cancer, malgré que différentes données de la littérature lui suggèrent à la fois des fonctions suppresseurs de tumeur et oncogéniques. Nous avons démontré que, malgré sa fonction proposée de suppresseur de croissance, l’expression de Necdin n’est pas incompatible avec la prolifération dans la lignée cellulaire de souris NIH 3T3 et les cellules primaires humaines (IMR90), bien que l’inhibition de son expression par shARN confère un avantage prolifératif. Nous avons confirmé que Necdin est un gène cible de p53 induit par différents agents génotoxiques, toutefois son expression peut également être régulée de façon p53-indépendante. De plus, Necdin agit négativement sur l’arrêt du cycle cellulaire en réponse à l’activation de p53. Ceci suggère que Necdin est impliqué dans une boucle de régulation négative de la voie de p53 et que l’augmentation anormale de l’expression de Necdin pourrait contribuer à la perturbation la voie du suppresseur de tumeur p53. L’activation de p53 permet l’arrêt transitoire du cycle cellulaire en condition de stress, mais est aussi impliquée dans l’établissement d’un arrêt permanent nommé sénescence. La sénescence est un mécanisme de protection contre l’accumulation de mutations qui peut contribuer à l’initiation du cancer. Vu l’intéressante implication de Necdin dans la régulation de l’activité de p53, nous avons transposé les connaissances acquises du modèle murin à un modèle humain, plus adapté pour l’étude de la sénescence. La caractérisation de l’expression de Necdin dans des fibroblastes primaires humains à différents passages montre que les jeunes cellules en prolifération active expriment Necdin et que son niveau diminue avec l’établissement de la sénescence réplicative. Le même phénomène est observé lors de la sénescence prématurée provoquée par l’expression d’un oncogène et par l’exposition aux radiations ionisantes. De plus, dans des conditions normales de prolifération, la modulation de Necdin par des essais de gain et de perte de fonction n’affecte pas la durée de vie des cellules primaires. Toutefois, en condition de stress génotoxique dû à l’exposition aux irradiations, les cellules surexprimant Necdin présentent une radiorésistance accrue de la même façon que lorsque p53 est inactivé directement. Ce résultat en cellules humaines vient appuyer l’effet observé dans les cellules de souris sur l’impact qu’aura le niveau de Necdin sur la réponse de p53 en condition de stress. Un bref survol a été fait pour aborder de quelle façon nos résultats en culture cellulaire pouvaient se traduire dans des modèles de cancer chez l’humain. Nous avons caractérisé l’expression de Necdin dans deux types différents de cancer. D’abord, dans le cancer de l’ovaire, le niveau élevé de Necdin dans les tumeurs à faible potentiel de malignité (LMP) en comparaison aux cancers agressifs de l’ovaire de type séreux suggère que l’expression de Necdin se limite aux cellules de cancer LMP, qui présente généralement un p53 de type sauvage. Son expression est aussi retrouvée dans deux lignées cellulaires du cancer de l’ovaire non-tumorigéniques en xénogreffe de souris, dont l’une possède un p53 fonctionnel. De plus, la caractérisation de Necdin dans les lignées cellulaires du cancer de la prostate suggère une relation entre son expression et la présence de p53 fonctionnel. Dans le cancer de la prostate, tout comme pour le cancer de l’ovaire, Necdin semble être présent dans les lignées représentant un stade moins avancé de la maladie. L’utilisation de l’oncoprotéine virale PyLT nous a permis de révéler des propriétés intéressantes de Necdin. Nous proposons que dans certains contextes, l’expression constitutive de Necdin pourrait contribuer au cancer en retardant une réponse par p53 appropriée et possiblement en participant à l’augmentation de l’instabilité génomique. La fonction potentiellement oncogénique de Necdin quant à sa relation avec p53 que nous avons révélée requiert davantage d’investigation et les cancers caractérisés ici pourraient constituer de bons modèles à cette fin. / Viruses have been used extensively in cancer research and have contributed greatly to the advancement of knowledge as well as to the establishment of important concepts still valid in the field today. Currently, one of the challenges in cancer research is to better define the individual steps that contribute to the transition of a normal cell to a transformed cell. To address this important issue, we characterized a model system based on the expression of polyomavirus large-T antigen (PyLT), derived from a virus capable of inducing tumors in rodents. Importantly, PyLT is able to induce tumors in transgenic mice, although only after a latent period, suggesting that additional transforming events are necessary. Hence, the PyLT viral oncogene possesses several interesting properties, which suggest that, beside its role in immortalization, PyLT can contribute to the early events of carcinogenesis. Here, we used a cell culture model to dissect the early changes associated with the presence of PyLT. The establishment of a gene expression profile associated with PyLT expression in a mouse cell line model allowed us to select a number of genes whose levels were modulated by the presence of this viral oncogene. Among candidate genes, we chose to further study Necdin in more details because even if only a limited number of reports existed for this protein, there was evidence suggesting that Necdin displays either tumor suppressor or oncogenic functions within different contexts. We demonstrated that, despite the proposed growth suppressor function of Necdin, its expression was not incompatible with the proliferation in the mouse NIH 3T3 cell line and in human IMR90 primary cells. Nonetheless, the inhibition of Necdin expression by shRNA confered a proliferative advantage. We confirmed that Necdin was a p53 target gene inducible by different genotoxic stresses, although its expression was also regulated in a p53-independent manner. Moreover, Necdin acted negatively on cell cycle arrest in response to p53 activation. These results suggest that Necdin is involved in a negative feedback loop of the p53 pathway and that abnormal elevation of Necdin expression could contribute to the disruption of the p53 tumor suppressor pathway. p53 activation allows transitory cell cycle arrest under stress conditions and it is also involved in the establishment of a permanent growth arrest called senescence. Senescence represents a protective mechanism preventing the accumulation of mutations that could contribute to cancer initiation. As our research supports a role for Necdin in the regulation of p53 activity, we transposed the knowledge acquired from our mouse model to a human model more suitable to study senescence. The characterization of Necdin expression in human primary fibroblasts at different passages revealed that Necdin was expressed in actively proliferating young cells and its expression decreased gradually during the establishment of replicative senescence. The same phenomenon was observed during premature senescence induced by both oncogene expression and ionizing radiation exposure. Moreover, in normal growth conditions, Necdin modulation by gain- and loss-of-function assays did not affect the life span of primary cells. However, in a genotoxic stress conditions caused by irradiation, Necdin overexpressing cells presented an increase radioresistance comparable to when p53 was directly inactivated. These results in human cells supports the effect observed in mouse cells relative to the impact of Necdin levels on a p53 response under stress conditions. We initiated preliminary experiments to address whether our results in cell culture could be translated to human cancer models. We characterized Necdin expression in two types of human cancers. First, in ovarian cancer, we observed elevated levels of Necdin expression in low malignant potential serous ovarian cancers (LMP) when compared to aggressive serous ovarian cancers. Our results suggest that Necdin expression was limited to LMPs, which usually present a wild type p53 gene. Necdin expression was also found in two ovarian cancer cell lines, which were both non-tumorigenic in a mouse xenograft assay, and interestingly one of the cell line had a functional p53. Moreover, the characterization of Necdin expression in four prostate cancer cell lines also suggested a relationship between its expression and the presence of functional p53. In prostate cancer, as in ovarian cancer, Necdin expression seems to be detected in cell lines representing less aggressive forms of the disease. The use of the PyLT viral oncoprotein allowed us to reveal interesting properties for Necdin. We propose that, in some contexts, the constitutive expression of Necdin could contribute to cancer promotion by delaying appropriate p53 responses and possibly promoting genomic instability. The potential oncogenic function of Necdin, and its relationship with p53 as revealed by the research described in this dissertation, requires more investigation. Preliminary results suggest that human ovarian and prostate cancers could be good models to address the role of Necdin in carcinogenesis.

Novas abordagens clínicas para o diagnóstico e monitoramento de pessoas com doenças sistêmicas têm sido empregadas, através da utilização de fluidos biológicos orais, como a saliva e o fluido gengival crevicular (FGC). Alguns autores têm avaliado o potencial desses fluidos no diagnóstico e acompanhamento de doenças, por apresentarem vantagens tais como coleta não invasiva e segurança no manuseio. Até o presente momento, poucos trabalhos detectaram os poliomavírus humano BK (BKV) e JC (JCV) em saliva e nenhum trabalho procurou sua presença no FGC. Esses poliomavírus infectam assintomaticamente cerca de 80% da população geral, mantendo-se latente no trato urinário. No caso de imunossupressão mediada por células, pode ocorrer o aumento da replicação e indução de reação inflamatória. Uma das doenças causadas pela replicação do BKV é a nefropatia associada ao poliomavírus (NAP), caracterizada pela disfunção e perda do próprio rim ou do rim transplantado, enquanto a Leucoencefalopatia Multifocal Progressiva (LMP), causada pela replicação do JCV, infecta os oligodendrócitos, causando desmielinização. Métodos não invasivos para o screening dos poliomavírus podem facilitar a detecção de novos casos e a monitoração de casos previamente conhecidos. O objetivo deste estudo foi verificar a possibilidade de detecção e quantificação do BKV e JCV em fluidos orais (saliva, lavado bucal e FGC) de indivíduos com insuficiência renal crônica (IRC), transplantados renais (TR), e controles em relação ao sangue e urina, fluidos frequentemente usados para esse teste. Para tanto, foram incluídos no estudo 38 sujeitos, divididos em 3 grupos, sendo 14 indivíduos no grupo com IRC (GIR), 12 TR no grupo transplantado renal (GTR) e 12 indivíduos saudáveis no grupo controle (GC). No total, coletamos 283 amostras dos participantes, sendo 151 de FGC, 38 amostras de saliva, 38 de lavado bucal, 35 de soro e 21 amostras de urina. No GIR, 100% (14) dos indivíduos apresentaram positividade para BKV em pelo menos uma amostra analisada e 14% (2) foram positivos para JCV. No GTR, 91,7% (11) dos indivíduos foram positivos para BKV e 51,7% (5) foram positivos para JCV. Dentre os sujeitos do GC, 91,7% (11) foram positivos para BKV e 50% (6) para JCV, em pelo menos uma amostra testada. Não houve diferença de frequência de detecção viral entre os 3 grupos de participantes, com relação às amostras coletadas. As amostras de fluidos orais (saliva, lavado e FGC) exibiram alta prevalência de detecção, principalmente do BKV, com muitas amostras com níveis quantificáveis de carga viral. Concluímos que fluidos orais, especialmente saliva e lavado bucal, podem ser usados para o rastreamento do BKV e JCV. / New clinical approaches for diagnosis and monitoring of individuals with systemic diseases have been employed through the use of oral biological fluids such as saliva and gingival crevicular fluid (GCF). Some authors have evaluated the potential of these fluids in the diagnosis and monitoring of diseases, because they have advantages such as noninvasive collection and safe handling. To date, few studies have demonstrated the detection of human polyomavirus BK (BKV) and JC (JCV) in saliva and no study reached for its presence in GCF. These polyomavirus infect asymptomatically around 80% of general population, remaining latent in the urinary tract. In case of immunosuppression mediated by cells, there is increased inflammation and induction of replication. One of the diseases caused by BKV replication is polyomavirus associated to the nephropathy (PVAN), characterized by the dysfunction or loss of the kidney or transplanted kidney, while the progressive multifocal leukoencephalopathy (PML) is caused by replication of JCV, infects oligodendrocytes causing demyelination. Noninvasive screening could facilitate the detection of new cases and monitoring of cases previously known. The objective of this study was to investigate the possibility of BKV and JCV detection and quantification in oral fluids (saliva, mouthwash and GCF) of individuals with chronic kidney failure (CKF), kidney transplantation (KT), and controls compared with blood and urine, often used for this test. Therefore, we included 38 subjects, divided into 3 groups, being 14 individuals with CKF (KFG), 12 individuals with KT (KTG) and 12 healthy control individuals (CG). In a total, we collected 283 samples, being 151 of GCF, 38 of saliva, 38 of mouthwash, 35 of serum and 21 samples of urine. In the KFG, 100% (14) of the individuals were positive for BKV in at least one of the collected sample and 14% (2) were positive for JCV. In the KTG, 91.7% (11) were positive for BKV and 51.7% for JCV. Among the subjects of the CG, 91.7% (11) were positive for BKV and 50% (6) to JCV, in at least one tested sample. There was no difference in viral detection frequency between the 3 studied groups with respect to the collected samples. Oral fluids samples (saliva, mouthwash and GCF) exhibited high prevalence of detection, especially of BKV, and several samples showed detectable viral load. We conclude that oral fluids, especially saliva and mouthwash, can be used for the screening of BKV and JCV.

Novas abordagens clínicas para o diagnóstico e monitoramento de pessoas com doenças sistêmicas têm sido empregadas, através da utilização de fluidos biológicos orais, como a saliva e o fluido gengival crevicular (FGC). Alguns autores têm avaliado o potencial desses fluidos no diagnóstico e acompanhamento de doenças, por apresentarem vantagens tais como coleta não invasiva e segurança no manuseio. Até o presente momento, poucos trabalhos detectaram os poliomavírus humano BK (BKV) e JC (JCV) em saliva e nenhum trabalho procurou sua presença no FGC. Esses poliomavírus infectam assintomaticamente cerca de 80% da população geral, mantendo-se latente no trato urinário. No caso de imunossupressão mediada por células, pode ocorrer o aumento da replicação e indução de reação inflamatória. Uma das doenças causadas pela replicação do BKV é a nefropatia associada ao poliomavírus (NAP), caracterizada pela disfunção e perda do próprio rim ou do rim transplantado, enquanto a Leucoencefalopatia Multifocal Progressiva (LMP), causada pela replicação do JCV, infecta os oligodendrócitos, causando desmielinização. Métodos não invasivos para o screening dos poliomavírus podem facilitar a detecção de novos casos e a monitoração de casos previamente conhecidos. O objetivo deste estudo foi verificar a possibilidade de detecção e quantificação do BKV e JCV em fluidos orais (saliva, lavado bucal e FGC) de indivíduos com insuficiência renal crônica (IRC), transplantados renais (TR), e controles em relação ao sangue e urina, fluidos frequentemente usados para esse teste. Para tanto, foram incluídos no estudo 38 sujeitos, divididos em 3 grupos, sendo 14 indivíduos no grupo com IRC (GIR), 12 TR no grupo transplantado renal (GTR) e 12 indivíduos saudáveis no grupo controle (GC). No total, coletamos 283 amostras dos participantes, sendo 151 de FGC, 38 amostras de saliva, 38 de lavado bucal, 35 de soro e 21 amostras de urina. No GIR, 100% (14) dos indivíduos apresentaram positividade para BKV em pelo menos uma amostra analisada e 14% (2) foram positivos para JCV. No GTR, 91,7% (11) dos indivíduos foram positivos para BKV e 51,7% (5) foram positivos para JCV. Dentre os sujeitos do GC, 91,7% (11) foram positivos para BKV e 50% (6) para JCV, em pelo menos uma amostra testada. Não houve diferença de frequência de detecção viral entre os 3 grupos de participantes, com relação às amostras coletadas. As amostras de fluidos orais (saliva, lavado e FGC) exibiram alta prevalência de detecção, principalmente do BKV, com muitas amostras com níveis quantificáveis de carga viral. Concluímos que fluidos orais, especialmente saliva e lavado bucal, podem ser usados para o rastreamento do BKV e JCV. / New clinical approaches for diagnosis and monitoring of individuals with systemic diseases have been employed through the use of oral biological fluids such as saliva and gingival crevicular fluid (GCF). Some authors have evaluated the potential of these fluids in the diagnosis and monitoring of diseases, because they have advantages such as noninvasive collection and safe handling. To date, few studies have demonstrated the detection of human polyomavirus BK (BKV) and JC (JCV) in saliva and no study reached for its presence in GCF. These polyomavirus infect asymptomatically around 80% of general population, remaining latent in the urinary tract. In case of immunosuppression mediated by cells, there is increased inflammation and induction of replication. One of the diseases caused by BKV replication is polyomavirus associated to the nephropathy (PVAN), characterized by the dysfunction or loss of the kidney or transplanted kidney, while the progressive multifocal leukoencephalopathy (PML) is caused by replication of JCV, infects oligodendrocytes causing demyelination. Noninvasive screening could facilitate the detection of new cases and monitoring of cases previously known. The objective of this study was to investigate the possibility of BKV and JCV detection and quantification in oral fluids (saliva, mouthwash and GCF) of individuals with chronic kidney failure (CKF), kidney transplantation (KT), and controls compared with blood and urine, often used for this test. Therefore, we included 38 subjects, divided into 3 groups, being 14 individuals with CKF (KFG), 12 individuals with KT (KTG) and 12 healthy control individuals (CG). In a total, we collected 283 samples, being 151 of GCF, 38 of saliva, 38 of mouthwash, 35 of serum and 21 samples of urine. In the KFG, 100% (14) of the individuals were positive for BKV in at least one of the collected sample and 14% (2) were positive for JCV. In the KTG, 91.7% (11) were positive for BKV and 51.7% for JCV. Among the subjects of the CG, 91.7% (11) were positive for BKV and 50% (6) to JCV, in at least one tested sample. There was no difference in viral detection frequency between the 3 studied groups with respect to the collected samples. Oral fluids samples (saliva, mouthwash and GCF) exhibited high prevalence of detection, especially of BKV, and several samples showed detectable viral load. We conclude that oral fluids, especially saliva and mouthwash, can be used for the screening of BKV and JCV.

Les virus sont utilisés depuis longtemps dans la recherche sur le cancer et ont grandement contribué à l’avancement des connaissances de même qu’à l’établissement de préceptes importants encore valables aujourd’hui dans le domaine. L’un des défis actuels est de mieux définir les étapes menant à la transition d’une cellule normale à une cellule transformée et c’est sur cette problématique que nous nous sommes penchés. Pour ce faire, nous avons tiré profit de l’utilisation de l’antigène grand-T du virus de polyome (PyLT), un virus capable d’induire des tumeurs chez les rongeurs. Cet oncogène viral à lui seul possède des propriétés intéressantes qui suggèrent que, en plus de l’immortalisation, il peut également contribuer aux événements précoces de la carcinogénèse. Ceci repose principalement sur la capacité de PyLT à induire des tumeurs en souris transgéniques et ce, avec une certaine latence ce qui suggère que des événements supplémentaires sont nécessaires. Ainsi, l’utilisation de PyLT dans un modèle de culture cellulaire permet de disséquer les changements qui lui sont attribuables. Dans un premier temps, l’établissement du profil d'expression génique associé à l'expression de PyLT dans un modèle murin nous a permis de sélectionner un bon nombre de gènes, parmi lesquels figurait Necdin. Nous avons choisi d’étudier Necdin plus en détail puisque peu d’attention était accordée à cette protéine dans le domaine du cancer, malgré que différentes données de la littérature lui suggèrent à la fois des fonctions suppresseurs de tumeur et oncogéniques. Nous avons démontré que, malgré sa fonction proposée de suppresseur de croissance, l’expression de Necdin n’est pas incompatible avec la prolifération dans la lignée cellulaire de souris NIH 3T3 et les cellules primaires humaines (IMR90), bien que l’inhibition de son expression par shARN confère un avantage prolifératif. Nous avons confirmé que Necdin est un gène cible de p53 induit par différents agents génotoxiques, toutefois son expression peut également être régulée de façon p53-indépendante. De plus, Necdin agit négativement sur l’arrêt du cycle cellulaire en réponse à l’activation de p53. Ceci suggère que Necdin est impliqué dans une boucle de régulation négative de la voie de p53 et que l’augmentation anormale de l’expression de Necdin pourrait contribuer à la perturbation la voie du suppresseur de tumeur p53. L’activation de p53 permet l’arrêt transitoire du cycle cellulaire en condition de stress, mais est aussi impliquée dans l’établissement d’un arrêt permanent nommé sénescence. La sénescence est un mécanisme de protection contre l’accumulation de mutations qui peut contribuer à l’initiation du cancer. Vu l’intéressante implication de Necdin dans la régulation de l’activité de p53, nous avons transposé les connaissances acquises du modèle murin à un modèle humain, plus adapté pour l’étude de la sénescence. La caractérisation de l’expression de Necdin dans des fibroblastes primaires humains à différents passages montre que les jeunes cellules en prolifération active expriment Necdin et que son niveau diminue avec l’établissement de la sénescence réplicative. Le même phénomène est observé lors de la sénescence prématurée provoquée par l’expression d’un oncogène et par l’exposition aux radiations ionisantes. De plus, dans des conditions normales de prolifération, la modulation de Necdin par des essais de gain et de perte de fonction n’affecte pas la durée de vie des cellules primaires. Toutefois, en condition de stress génotoxique dû à l’exposition aux irradiations, les cellules surexprimant Necdin présentent une radiorésistance accrue de la même façon que lorsque p53 est inactivé directement. Ce résultat en cellules humaines vient appuyer l’effet observé dans les cellules de souris sur l’impact qu’aura le niveau de Necdin sur la réponse de p53 en condition de stress. Un bref survol a été fait pour aborder de quelle façon nos résultats en culture cellulaire pouvaient se traduire dans des modèles de cancer chez l’humain. Nous avons caractérisé l’expression de Necdin dans deux types différents de cancer. D’abord, dans le cancer de l’ovaire, le niveau élevé de Necdin dans les tumeurs à faible potentiel de malignité (LMP) en comparaison aux cancers agressifs de l’ovaire de type séreux suggère que l’expression de Necdin se limite aux cellules de cancer LMP, qui présente généralement un p53 de type sauvage. Son expression est aussi retrouvée dans deux lignées cellulaires du cancer de l’ovaire non-tumorigéniques en xénogreffe de souris, dont l’une possède un p53 fonctionnel. De plus, la caractérisation de Necdin dans les lignées cellulaires du cancer de la prostate suggère une relation entre son expression et la présence de p53 fonctionnel. Dans le cancer de la prostate, tout comme pour le cancer de l’ovaire, Necdin semble être présent dans les lignées représentant un stade moins avancé de la maladie. L’utilisation de l’oncoprotéine virale PyLT nous a permis de révéler des propriétés intéressantes de Necdin. Nous proposons que dans certains contextes, l’expression constitutive de Necdin pourrait contribuer au cancer en retardant une réponse par p53 appropriée et possiblement en participant à l’augmentation de l’instabilité génomique. La fonction potentiellement oncogénique de Necdin quant à sa relation avec p53 que nous avons révélée requiert davantage d’investigation et les cancers caractérisés ici pourraient constituer de bons modèles à cette fin. / Viruses have been used extensively in cancer research and have contributed greatly to the advancement of knowledge as well as to the establishment of important concepts still valid in the field today. Currently, one of the challenges in cancer research is to better define the individual steps that contribute to the transition of a normal cell to a transformed cell. To address this important issue, we characterized a model system based on the expression of polyomavirus large-T antigen (PyLT), derived from a virus capable of inducing tumors in rodents. Importantly, PyLT is able to induce tumors in transgenic mice, although only after a latent period, suggesting that additional transforming events are necessary. Hence, the PyLT viral oncogene possesses several interesting properties, which suggest that, beside its role in immortalization, PyLT can contribute to the early events of carcinogenesis. Here, we used a cell culture model to dissect the early changes associated with the presence of PyLT. The establishment of a gene expression profile associated with PyLT expression in a mouse cell line model allowed us to select a number of genes whose levels were modulated by the presence of this viral oncogene. Among candidate genes, we chose to further study Necdin in more details because even if only a limited number of reports existed for this protein, there was evidence suggesting that Necdin displays either tumor suppressor or oncogenic functions within different contexts. We demonstrated that, despite the proposed growth suppressor function of Necdin, its expression was not incompatible with the proliferation in the mouse NIH 3T3 cell line and in human IMR90 primary cells. Nonetheless, the inhibition of Necdin expression by shRNA confered a proliferative advantage. We confirmed that Necdin was a p53 target gene inducible by different genotoxic stresses, although its expression was also regulated in a p53-independent manner. Moreover, Necdin acted negatively on cell cycle arrest in response to p53 activation. These results suggest that Necdin is involved in a negative feedback loop of the p53 pathway and that abnormal elevation of Necdin expression could contribute to the disruption of the p53 tumor suppressor pathway. p53 activation allows transitory cell cycle arrest under stress conditions and it is also involved in the establishment of a permanent growth arrest called senescence. Senescence represents a protective mechanism preventing the accumulation of mutations that could contribute to cancer initiation. As our research supports a role for Necdin in the regulation of p53 activity, we transposed the knowledge acquired from our mouse model to a human model more suitable to study senescence. The characterization of Necdin expression in human primary fibroblasts at different passages revealed that Necdin was expressed in actively proliferating young cells and its expression decreased gradually during the establishment of replicative senescence. The same phenomenon was observed during premature senescence induced by both oncogene expression and ionizing radiation exposure. Moreover, in normal growth conditions, Necdin modulation by gain- and loss-of-function assays did not affect the life span of primary cells. However, in a genotoxic stress conditions caused by irradiation, Necdin overexpressing cells presented an increase radioresistance comparable to when p53 was directly inactivated. These results in human cells supports the effect observed in mouse cells relative to the impact of Necdin levels on a p53 response under stress conditions. We initiated preliminary experiments to address whether our results in cell culture could be translated to human cancer models. We characterized Necdin expression in two types of human cancers. First, in ovarian cancer, we observed elevated levels of Necdin expression in low malignant potential serous ovarian cancers (LMP) when compared to aggressive serous ovarian cancers. Our results suggest that Necdin expression was limited to LMPs, which usually present a wild type p53 gene. Necdin expression was also found in two ovarian cancer cell lines, which were both non-tumorigenic in a mouse xenograft assay, and interestingly one of the cell line had a functional p53. Moreover, the characterization of Necdin expression in four prostate cancer cell lines also suggested a relationship between its expression and the presence of functional p53. In prostate cancer, as in ovarian cancer, Necdin expression seems to be detected in cell lines representing less aggressive forms of the disease. The use of the PyLT viral oncoprotein allowed us to reveal interesting properties for Necdin. We propose that, in some contexts, the constitutive expression of Necdin could contribute to cancer promotion by delaying appropriate p53 responses and possibly promoting genomic instability. The potential oncogenic function of Necdin, and its relationship with p53 as revealed by the research described in this dissertation, requires more investigation. Preliminary results suggest that human ovarian and prostate cancers could be good models to address the role of Necdin in carcinogenesis.