Les cassures double-brin de l’ADN (CDB) sont des événements clés dans la réponse aux rayonnements ionisants qui, avec le profil génétique et épigénétique individuel, peuvent conditionner le devenir des tissus sains d’un individu exposé. À la suite des cassures de la molécule d’ADN et de la déstabilisation de la chromatine, une série de modifications post-traductionnelles des histones se produit, notamment la phosphorylation de la serine 139 de l'histone H2A.X (gamma-H2A.X), conduisant à la formation de foyers radio-induits. La réparation des CDB, et donc la disparition de ces foyers, a lieu dans les heures suivant l’exposition. Toutefois, une certaine proportion de ces foyers gamma-H2A.X persiste 24 heures après l’irradiation. La nature et le rôle de ces foyers persistants sont encore peu clairs. L’objectif de ce travail est d'explorer les caractéristiques de ces foyers persistants et leurs conséquences sur le devenir des cellules. Pour étudier la dynamique des foyers radio-induits, nous avons exposé des HUVEC synchronisées en phase G0/G1 à des doses de 1 et 5 Gy de rayons X. Les foyers radio-induits ont été étudiés à partir de 10 minutes et jusqu'à 7 jours après l'exposition par l’analyse de gamma-H2A.X et de l’association temporelle de la protéine 53BP1 et des CN-PML (corps nucléaires PML). L’impact des foyers persistants sur la prolifération cellulaire a également été exploré. Nous avons analysé en microscopie à fluorescence une moyenne de 4 000 cellules pour chaque condition à l'aide d'une analyse d’image permettant la détection automatique des noyaux et des foyers. L'analyse d'un grand nombre d‘évènements nous a permis de discriminer des sous-populations de cellules ou de foyers sur la base de différentes caractéristiques, telles que leur aire ou la phase du cycle cellulaire, et de mesurer leur représentativité dans l'ensemble de la population de cellules exposées. Ainsi, nous avons déterminé que les foyers gamma-H2A.X persistant ont une aire supérieure à 0,72 ± 0,11 µm² et qu’ils sont toujours colocalisés avec 53BP1. Plus de 70% des cellules exposées à 5 Gy ont au moins un foyer persistant 24 heures après l'exposition. De plus, ces foyers persistants sont observables au moins jusqu'à 7 jours après l’irradiation. Une association spatiale significative entre les CN-PML et les foyers gamma-H2A.X a été observée à partir de 10 minutes après l'exposition et 24 heures après l’exposition, environ 90% des foyers persistants sont associés à un CN-PML. De plus, la présence de foyers persistants ne bloque pas définitivement la prolifération des cellules. Cependant, la fréquence des foyers persistants est plus faible dans les cellules filles que dans les cellules irradiées, probablement en raison d'une certaine proportion de distribution asymétrique des foyers persistants entre les cellules filles. Nous avons également mesuré une corrélation positive entre la présence d'un foyer persistant et la probabilité de mauvaise ségrégation de l'ADN par l'observation de phénomènes de catastrophes mitotiques. Il semble donc que la structure formée après le passage d'un foyer persistant à travers les phases S et G2 soit susceptible d’empêcher la séparation correcte des chromatides sœurs du chromosome affecté. Nous suggérons donc que la nature des foyers persistants n’est pas la même avant et après la première division cellulaire due à une résolution anormale de l'anaphase. Ces assemblages chromosomiques atypiques résultants d’anaphases anormales pourraient être létaux pour la cellule ou entraîner un déséquilibre du dosage génique et une instabilité génomique accrue pouvant conduire à une mosaïque de phénotypes cellulaires. / The DNA double-stranded breaks (DSB) are key events in the cell response to ionizing radiation that may affect, with the individual genetic and epigenetic profile, the fate of healthy tissues of people exposed. Following initial breaks and chromatin destabilization, a set of post-translational modifications of histones occurs, including the phosphorylation of serine 139 of histone H2AX (gamma-H2A.X), which leads to the formation of ionizing radiation-induced foci (IRIF). DSB repair results in the disappearance of most IRIF within hours after exposure. However, a proportion of IRIF remains 24 hours upon irradiation. The nature and role of these persistent IRIF are still unclear. The goal of this work is to explore the characteristics of these persistent IRIF and their consequences on the cell behavior. To investigate the dynamic of IRIF in our model, we exposed G0/G1-phase synchronized HUVECs to 1 or 5 Gy of X-rays. IRIF were studied from 10 minutes up to 7 days after exposure by monitoring gamma-H2A.X foci, their temporal association with 53BP1 protein and PML NBs (Promyelocytic leukemia nuclear bodies), and their impact on cell proliferation. We analyzed a mean of 4 000 cells for each condition using an automated detection of nuclei and foci. The analysis of a large number of cells and foci allowed us to screen subpopulations of cells or foci through different characteristics, such as size, shape or cell cycle phase among others, and to weight their representativeness in the whole population of exposed cells. We identified that persistent gamma-H2A.X foci after irradiation had a size superior to 0.72 ± 0.11 µm² and always collocated with 53BP1. More than 70% of cells exposed to 5 Gy had at least one persistent IRIF 24 hours after exposure and we observed these persistent IRIF up to 7 days post irradiation. A significant spatial association between PML NBs and IRIF was observed from 10 minutes after exposure; at 24h post irradiation, around 90% of persistent IRIF were associated with PML NBs. Moreover we demonstrated that persistent IRIF did not block cell proliferation definitively. The frequency of IRIF was lower in daughter cells, probably due to a certain amount of asymmetric distribution of IRIF between them. We report a positive association between the presence of an IRIF and the likelihood of DNA missegregation by observation of mitotic catastrophes. Hence, the structure formed after the passage of a persistent IRIF across the S and G2 phases may impede the correct segregation of sister chromatids of the chromosome affected. Consequently, the nature of IRIF in the nucleus of daughter cells might differ before and after the first cell division due to an abnormal resolution of anaphase. The resulting atypical chromosomal assembly may be lethal or result in a gene dosage imbalance and possible enhanced genomic instability, and could lead to a patchwork of cell phenotypes.

It has been demonstrated that oocytes of several species acquire the capacity to complete meiotic maturation and support early embryonic development during the final stages of follicular growth. Aiming to investigate molecular differences between bovine embryos produced from oocytes derived from small (1 to 2-mm) and large (4 to 8-mm) follicles, two experiments were designed to evaluate by immunocytochemistry the presence on chromatin of three regulatory factors involved mainly in DNA transcription and repair. In the first experiment, we evaluated whether the expression pattern of the High- Mobility Group N2 (HMGN2) and acetylated histone H3 Lysine 14 (Ac.H3K14) were affected by the origin (from small vs. large follicles) and time of first cleavage (< 24 h vs. > 24 h) of parthenogenetically activated (PA) oocytes. Early (until 24 hr) and late (after 24 hr) cleaved embryos were fixed at 36, 50, 60, 70 and 80 h after PA and processed to detect HMGN2 or Ac.H3K14. The rate of nuclear maturation (81% vs. 59%), early cleavage (47% vs. 39%), and blastocyst (34% vs. 19%) were significantly higher (P<0.05) in oocytes from large compared to small follicles. The rate of Ac.H3K14 (61% vs. 38%) and HMGN2 (74% vs. 56%) positively stained nuclei at 60 h post PA was higher (P<0.05) in embryos derived from small compared to large follicles. However, more HMGN2 positively stained nuclei (94% vs. 75%; P<0.05) were detected in embryos from large follicles at 80 h post PA. We concluded that the temporal proportion of embryonic nuclei with positive signal to HMGN2 and Ac.H3K14 is affected by both follicle size and time to complete first cleavage of oocytes. In the second experiment, we investigated whether the expression pattern of the phosphorylated histone H2A.X (γH2A.X) protein, which is an indicator of DNA double-strand breaks, is different in embryos produced from oocytes derived from small or large follicles. Oocytes were PA or in vitro fertilized (IVF) for 18 h and then cultured. Cleavage was assessed at 24 and 36 h after PA and at 32 and 42 h after IVF. Cleaved embryos were fixed at 36 h after PA and 42 h after IVF, and then processed to detect the γH2A.X. Most of the cleaved embryos produced from PA and IVF oocytes had detectable amounts of γH2A.X, ranging from few foci to a complete diffuse staining of the nuclei. γH2A.X was detected in 64% vs. 76% (P<0.05) of nuclei in PA embryos and in 76% vs. 80% (P>0.05) of nuclei in IVF embryos produced from oocytes derived from small and large follicles, respectively. IVF embryos presenting less than 4 nuclei at 42 h showed higher rates (P<0.05) of γH2A.X positive nuclei (89% and 85%) than those with ≥4 nuclei (72% and 62%, for large and small follicles, respectively). We found that γH2A.X is highly detected but not differently expressed in early bovine embryos produced from PA and IVF oocytes derived from small and large follicles. In general, the present experiments demonstrate that HMGN2, Ac.H3K14 and γH2A.X proteins are expressed during early bovine embryogenesis. / Tem sido demonstrado que oócitos de várias espécies adquirem capacidade para completar a maturação meiótica e suportar o desenvolvimento embrionário durante os estágios finais do crescimento folicular. Com o objetivo de investigar diferenças moleculares entre embriões bovinos produzidos a partir de oócitos derivados de folículos pequenos (1-2 mm) e grandes (4-8 mm), dois experimentos foram delineados visando identificar por imunocitoquimica a presença de três fatores reguladores da cromatina envolvidos principalmente nos processos de transcrição e reparo do DNA. No primeiro experimento, foi investigado se o perfil de expressão das proteínas (do inglês) High-Mobility Group N2 (HMGN2) e histona H3 acetilada na lisina 14 (Ac.H3K14) seria alterado pela origem dos oócitos (folículos pequenos vs. folículos grandes) e pelo tempo necessário para realizar a primeira clivagem (<24 h vs. >24 h) após ativação partenogenética (AP). Embriões clivados cedo (até 24 h) e tarde (após 24 h) foram fixados às 36, 50, 60, 70 e 80 h após AP e processados para detectar a HMGN2 ou Ac.H3K14. Os percentuais de maturação nuclear (81% vs. 59%), clivagem cedo (47% vs. 39%) e blastocisto (34% vs. 19%) foram significativamente maiores (P<0,05) nos oócitos oriundos de folículos grandes em comparação aos de folículos pequenos. Os percentuais de núcleos positivos para Ac.H3K14 (61% vs. 38%) e HMGN2 (74% vs. 56%) às 60 h após AP foram maiores (P<0,05) nos embriões produzidos a partir de oócitos de folículos pequenos comparado aos de folículos grandes. Entretanto, um maior percentual de núcleos positivos para HMGN2 (94% vs. 75%; P<0,05) foi detectado em embriões produzidos com oócitos de folículos grandes às 80 h após a AP. Concluiu-se que a proporção de núcleos com sinal positivo para HMGN2 e Ac.H3K14 é dependente do tamanho dos folículos e do tempo transcorrido para os oócitos realizarem a primeira clivagem. No segundo experimento, foi investigado se o perfil de fosforilação da proteína histona H2A.X (γH2A.X), que é um indicador de rompimento da cadeia dupla do DNA, seria diferente em embriões produzidos a partir de oócitos oriundos de folículos pequenos ou grandes. Os oócitos foram submetidos à AP ou fertilização in vitro (FIV) por 18 h e, então, cultivados. A clivagem foi avaliada 24 e 36 h após a AP e 32 e 42 h após FIV. Os embriões clivados foram fixados 36 h após AP e 42 h após FIV, e então processados para detectar a presença da γH2A.X. A maioria dos embriões produzidos a partir de oócitos submetidos à AP e FIV apresentou quantidades detectáveis de γH2A.X, variando entre poucos focos até um sinal completamente difuso no núcleo. A γH2A.X foi detectada em 64% vs. 76% (P<0,05) dos núcleos dos embriões ativados e em 76% vs. 80% (P<0,05) dos núcleos dos embriões de FIV produzidos a partir de oócitos oriundos de folículos pequenos e grandes, respectivamente. Embriões oriundos de FIV que apresentaram número <4 núcleos às 42 h tiveram maiores percentuais (P<0,05) de núcleos positivos para γH2A.X (89% e 85%) do que os que apresentaram número ≥4 núcleos (72% e 62%, respectivamente para folículos pequenos e grandes). Foi demonstrado que a γH2A.X é altamente detectada mas não é diferentemente expressa em embriões bovinos produzidos a partir de oócitos oriundos de folículos pequenos e grandes e submetidos à AP ou FIV. Em geral, os experimentos demonstraram que as proteínas HMGN2, Ac.H3K14 e γH2A.X são expressas durante o desenvolvimento embrionário precoce em bovinos.