Upon activation in epidermal stem cells, the proto-oncogene c-Myc triggers their exit from the stem cell compartment resulting in an increase in progenitor cell proliferation and an induction in terminal differentiation. Whether c-Myc plays a direct transcriptional role in epidermal stem cell differentiation was unknown. The exploration of c-Myc's transcriptional roles at the epidermal differentiation complex (EDC), a locus essential for skin maturation demonstrated that binding of c-Myc to the EDC can simultaneously recruit and displace specific sets of differentiation-specific transcriptional regulators to EDC genes. Among these factors, Sin3A acts as a transcriptional co-repressor and was initially discovered via its direct interaction with Mxi1 and Mxd1, which are antagonists of the Myc family network. As such, I concentrated on the role of Sin3A as a potential opposing factor to c-Myc activity in the epidermis. To analyse the role of Sin3A in regulating epidermal stem cell fate in vivo, I generated a number of transgenic mouse models. To determine whether Sin3A functions in hair follicle stem cells, I inducibly deleted Sin3A in the hair follicle bulge, where quiescent stem cells reside. However, lack of Sin3A in the hair bulge did not cause any aberrant phenotype and I concluded that Sin3A is dispensable for hair follicle homeostasis. I next analysed a mouse model in which Sin3A is inducibly deleted in the basal layer of the epidermis. Deletion of Sin3A resulted in a severe disruption of epidermal homeostasis-namely due to increases in proliferation and differentiation. Further investigation demonstrated that this phenotype is driven by enhanced genomic recruitment of c-Myc to the epidermal differentiation complex and reactivation of c-Myc target genes involved in cellular proliferation. I found that Sin3A causes de-acetylation of the c-Myc protein to directly repress c-Myc’s transcriptional activity and is antagonistic to c-Myc in the interfollicular epidermis. I hypothesised that simultaneous deletion of Sin3A and c-Myc might return the skin to normality. Indeed, when Sin3A and Myc are concurrently deleted, proliferation and differentiation levels returned to normal. These results demonstrate how levels of Sin3A and c-Myc must be carefully balanced for epidermal homeostasis to be maintained. Decreased expression of Sin3A has been linked to tumour susceptibility in other tissues for example in non-small cell lung carcinoma making Sin3A a candidate tumour suppressor gene. I therefore considered that loss of Sin3A may lead to increased susceptibility to skin cancer. To investigate this I performed pilot experiments using UVB irradiation of skin that has one copy of Sin3A deleted in the basal layer of the epidermis. Under normal conditions, these mice have no identifiable phenotype, but pilot experiments demonstrated that after short term and long term UVB irradiation, they exhibit increased epidermal thickness and proliferation relative to controls. This recapitulated the phenotype observed when Sin3A is inducibly deleted in the interfollicular epidermis and further demonstrates the role of SinA as an inhibitor of proliferation in this tissue. Overall, these results demonstrate that an interplay between the opposing functions of Sin3A and c-Myc are necessary to ensure that there is balanced homeostasis in the interfollicular epidermis.

No description available.

The B cell population is one of the key components of the adaptive immune system, which protects the host from a tremendous variety of pathogens by producing antibodies. B cells develop from hematopoietic stem cells through a pathway known as B lymphopoiesis. This is a process accompanied by intensive gene expression reprogramming. By the end, genes appropriate for the B lineage are activated and those that are not are continuously repressed. The regulation of lineage gene expression is conferred by a network of transcriptional regulators. Although some key components have been defined, more factors, especially those orchestrating the repression of non-B lineage genes, remain to be identified. Chemically induced mutagenesis is a potent way of identifying genes with critical biological functions. Injection of n-ethyl-n-nitrosourea, a mutagen, has generated a unique point mutation in the mouse Gon4-like (Gon4l) gene that specifically causes a loss of peripheral B cells while maintaining the T cell population. The mutation is therefore named Justy for Just T cells. The goal of this thesis project is to analyze the Justy mice and provide insights into the mechanisms underlying the regulation of B lymphopoiesis. The work presented here demonstrates that the protein encoded by Gon4l is essential for early B lymphopoiesis, which is likely through the repression of non-B lineage genes. Gon4l protein contains conserved domains implicated in transcriptional repression and associates in a complex with the transcriptional repression mediators Yin Yang 1 and Sin3a/HDAC1, after these proteins are transiently expressed in cell lines. When bound to DNA, Gon4l is capable of repressing a nearby promoter and this function correlates with its ability to form a complex. Therefore, these results suggest that Gon4l may function as a transcriptional regulator by employing its associated co-factors in the identified complex. Lastly, a wide spectrum of tumors developed in Justy mice, indicating that Gon4l can also act as a tumor suppressor.

Les gènes de la famille ING : « INhibitor of Growth » (ING1-5) jouent un rôle crucial dans l'inhibition de la prolifération cellulaire, en régulant notamment le cycle cellulaire, l'apoptose et la sénescence. De plus, plusieurs études (portant majoritairement sur ING1) montrent que ces gènes sont fréquemment perdus dans de nombreux cancers. Ils pourraient donc être impliqués dans l'émergence et le développement de tumeurs. Ainsi, l'objectif de mon projet de thèse était d'étudier le gène ING2, afin d'évaluer son intérêt en cancérogénèse. Nous avons tout d'abord montré que l'expression d'ING2 (ARN et protéique) est perdue dans plus de la moitié des cancers bronchiques non à petites cellules, confortant ainsi un rôle d'ING2 comme gène suppresseur de tumeurs. Par ailleurs, nous avons montré que l'inhibition de l'expression d'ING2 conduit à des défauts de réplication et à une forte augmentation de l'instabilité génomique, mettant ainsi en évidence pour la première fois qu'ING2 est un gène suppresseur de tumeurs de type « caretaker ». Ceci permet aussi pour la première fois d'expliquer comment l'inactivation des ING, observée dans les tumeurs, pourrait contribuer à la cancérogénèse. Enfin, nous avons mis en évidence le premier mécanisme de régulation post-traductionnelle d'ING2. En effet, ING2 peut être sumoylée, et cette sumoylation est nécessaire pour son association avec le complexe de régulation Sin3A/HDAC afin de cibler ce dernier au niveau des promoteurs de gènes pour réguler leur expression. Ces travaux ont donc contribué à démontrer l'intérêt d'ING2 en cancérogénèse et à mieux comprendre ses fonctions suppressives de tumeurs. De plus, ils ont permis d'ouvrir plusieurs voies d'investigation sur les fonctions et les mécanismes de régulation des protéines ING.