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Hormesis is a low-dose phenomenon that has been reported to occur, to different extents, in animals, plants, and microorganisms. However, a review of the literature shows that only a few reports describe it in humans. Also, the diversity of experimental protocols and cellular models used makes deciphering the mechanisms of hormesis difficult. In humans, hormesis mostly appears in the 20 to 75 mGy dose range and in nontransformed, radioresistant cells. In a previous paper by Devic et al, a biological interpretation of the adaptive response (AR) phenomenon was proposed using our model that is based on the radiation-induced nucleoshuttling of the ATM protein (the RIANS model). Here, we showed that the 20 to 75 mGy dose range corresponds to a maximum amount of ATM monomers diffusing into the nucleus, while no DNA double-strand breaks is produced by radiation. These ATM monomers are suggested to help in recognizing and repairing spontaneous DNA breaks accumulated in cells and contribute to reductions in genomic instability and aging. The RIANS model also permitted the biological interpretation of hypersensitivity to low doses (HRS)-another low-dose phenomenon. Hence, for the first time to our knowledge, hormesis, AR, and HRS can be explained using the same unified molecular model.

Our understanding of the molecular and cellular response to ionizing radiation (IR) has progressed considerably. This is notably the case for the repair and signaling of DNA double-strand breaks (DSB) that, if unrepaired, can result in cell lethality, or if misrepaired, can cause cancer. However, through the different protocols, techniques, and cellular models used during the last four decades, the DSB repair kinetics and the relationship between cellular radiosensitivity and unrepaired DSB has varied drastically, moving from all-or-none phenomena to very complex mechanistic models. To date, personalized medicine has required a reliable evaluation of the IR-induced risks that have become a medical, scientific, and societal issue. However, the molecular bases of the individual response to IR are still unclear: there is a gap between the moderate radiosensitivity frequently observed in clinic but poorly investigated in the publications and the hyper-radiosensitivity of rare but well-characterized genetic diseases frequently cited in the mechanistic models. This paper makes a comprehensive review of semantic issues, correlations between cellular radiosensitivity and unrepaired DSB, shapes of DSB repair curves, and DSB repair biomarkers in order to propose a new vision of the individual response to IR that would be more coherent with clinical reality.

Purpose: Xeroderma Pigmentosum (XP) is a rare, recessive genetic disease associated with photosensitivity, skin cancer proneness, neurological abnormalities and impaired nucleotide excision repair of the UV-induced DNA damage. Less frequently, XP can be associated with sensitivity to ionizing radiation (IR). Here, a complete radiobiological characterization was performed on a panel of fibroblasts derived from XP-group D patients (XPD). Materials and methods: Cellular radiosensitivity and the functionality of the recognition and repair of chromosome breaks and DNA double-strand breaks (DSB) was evaluated by different techniques including clonogenic cell survival, micronuclei, premature chromosome condensation, pulsed-field gel electrophoresis, chromatin decondensation and immunofluorescence assays. Quantitative correlations between each endpoint were analyzed systematically. Results: Among the seven fibroblast cell lines tested, those derived from three non-relative patients holding the p.[Arg683Trp];[Arg616Pro] XPD mutations showed significant cellular radiosensitivity, high yield of residual micronuclei, incomplete DSB recognition, DSB and chromosome repair defects, impaired ATM, MRE11 relocalization, significant chromatin decondensation. Interestingly, XPD transduction and treatment with statins and bisphosphonates known to accelerate the radiation-induced ATM nucleoshuttling led to significant complementation of these impairments. Conclusions: Our findings suggest that some subsets of XPD patients may be at risk of radiosensitivity reactions and treatment with statins and bisphosphonates may be an interesting approach of radioprotection countermeasure. Different mechanistic models were discussed to better understand the potential specificity of the p.[Arg683Trp];[Arg616Pro] XPD mutations.

BRCA1 is one of the two major breast cancer susceptibility genes. The numerous binding partners of BRCA1 allow it to participate to several cellular pathways which globally contribute to its cell surveillance capacity. The team in which I performed my PhD identified a new binding partner of BRCA1, the Poly(A)-Binding Protein 1 and, consequently, a new function of this tumor suppressor, namely, the translation regulation. Moreover, recent studies suggest that under conditions dangerous for the cell and potentially oncogenic, such as a genotoxic stress, protein synthesis is strongly altered. My thesis work was aimed at demonstrating that this new function of BRCA1 contributes, like its nuclear functions, to its role of tumor suppressor. During my thesis, I identified the mRNAs "targets" of BRCA1 by the technique of immunoprecipitation of the ribonucleoprotein complexes (RIP), I validated that these mRNAs "targets" of BRCA1 are associated to it for their translational control by realizing a comparative analysis of the contents of the polysomes of MCF-7 mammary epithelial cells transiently expressing an interfering RNA directed against BRCA1 by the technique that I have set up in the laboratory, the polysomal profiles. Subsequently, I established the conditions of genotoxic stress inducing the cytoplasmic localization of BRCA1, and in collaboration with clinicians of the Center Léon Bérard Hospital, I allowed the acquisition of tumor samples. This will allow us to identify among these targets new diagnostic markers or new therapeutic targets for breast cancers deficient in BRCA1, BRCA1 est l'un des deux gènes majeur de prédisposition au cancer du sein. Les multiples partenaires protéiques de BRCA1 lui confèrent de nombreuses fonctions par lesquelles elle peut assurer la surveillance de l'intégrité cellulaire. L'équipe dans laquelle j'ai mené ma thèse a identifié un nouveau partenaire protéique de BRCA1, la protéine de liaison au poly(A) des ARN messagers PABP1, et a ainsi mis en lumière l'implication de BRCA1 dans la régulation de la traduction. De plus, de récents travaux suggèrent que dans des conditions dangereuses pour la cellule et potentiellement oncogéniques, comme par exemple un stress endommageant l'ADN (génotoxique), la synthèse protéique est fortement altérée. Mon travail de thèse a eu pour objectif de démontrer que cette nouvelle fonction de BRCA1 contribue, comme ses fonctions nucléaires, à son rôle de suppresseur de tumeur. Durant ma thèse, j'ai identifié les ARNm « cibles » de BRCA1 par la technique d'immunoprécipitation des complexes ribonucléoprotéiques (RIP), j'ai validé que ces ARNm « cibles » de BRCA1 lui sont associés pour leur régulation traductionnelle en réalisant une analyse comparative du contenu des polysomes de cellules épithéliales mammaires MCF-7 exprimant de façon transitoire un ARN interférent dirigé contre BRCA1 par la technique que j'ai mise en place au laboratoire, les profils polysomiques. Par la suite, j'ai établi les conditions de stress génotoxique induisant la localisation cytoplasmique de BRCA1, et en collaboration avec des cliniciens du Centre Léon Bérard j'ai permis l'acquisition d'échantillons tumoraux. Ceci nous permettra d'identifier parmi ces cibles de nouveaux marqueurs diagnostiques ou encore de nouvelles cibles thérapeutiques pour les cancers du sein déficientes en BRCA1