Description de nouvelles voies impliquées dans l'apparition de télomère - sénescence réplicative dépendante dans Saccharomyces cereviviae

Linear chromosomes end with special regions, the telomeres, which ensure the integrity and the stability of the genome. In eukaryotes, telomeres also determine cell proliferation potential by triggering replicative senescence. This occurs upon telomere shortening in the absence of telomerase. In Saccharomyces cerevisiae, it is likely mediated by the first telomere in the cell that reaches a critically short length. This shortened telomere subsequently activates a DNA-damage-like response. How the signaling is modulated in terms of telomeric structure and context is largely unknown. During my thesis, I aimed at understanding the influence of the chromatin environment on the senescence signal starting at the shortest telomere. By comparing two sets of strains in which the shortest telomere either harbors naturally occurring subtelomeric elements or lacks these elements altogether, we show that a subtelomeric region comprising an X element counteracts the establishment of senescence. This effect is likely not due to differential Rad51-mediated homology directed repair activities at both types of telomeres. Furthermore, TERRA transcription is induced at both types of critically short telomeres, although levels are elevated in the absence of natural subtelomeric elements. Together, our results demonstrate that transcription from a telomere-proximal region greatly increases when the shortest telomere reaches a critical length, regardless of the presence of a native subtelomere or a dedicated TERRA promoter. This transcription at short telomere is intriguingly reminiscent of the transcripts found at double-strand breaks in other organisms.Les chromosomes linéaires se terminent par des régions particulières, les télomères, qui assurent l'intégrité et la stabilité du génome. Chez les eucaryotes, les télomères déterminent également le potentiel de prolifération de cellules en déclenchant la sénescence réplicative. Ce signal se produit lors du raccourcissement des télomères en l'absence de la télomérase. Chez Saccharomyces cerevisiae, il est probablement médié par le premier télomère de la cellule qui atteint une taille courte critique. Ce télomère raccourci, active ensuite une réponse de dommage à l’ADN. Comment la signalisation est modulée en termes de structure et du contexte télomérique est largement inconnue. Au cours de ma thèse, j’ai cherché à comprendre l'influence de l'environnement chromatinien sur le signal de la sénescence à partir du télomère le plus court. La comparaison de deux souches dans lesquelles le télomère le plus court contient les éléments sous-télomériques naturels ou non, nous montre qu'une région sous-télomérique comprenant un élément X s'oppose à la mise en place de la sénescence. Cet effet n'est probablement pas dû à des différences de réparation par récombinaison homologue dépendante de Rad51 aux deux types de télomères. De plus, la transcription de TERRA est induite dans les deux types de télomères courts, bien que les niveaux soient plus élevés en l'absence d'éléments sous-télomériques naturels. Ensemble, ces résultats démontrent que la transcription à partir d'une région proximale du télomère augmente considérablement lorsque le télomère le plus court atteint une taille critique, indépendamment de la présence d'un sous-télomère natif ou d’un promoteur TERRA dédié. Cette transcription au télomère court est similaire à la transcription trouvée aux cassures double-brin chez d'autres organismes

Ddx19 links mRNA nuclear export with progression of transcription and replication and suppresses genomic instability upon DNA damage in proliferating cells

International audienceThe DEAD-box Helicase 19 (Ddx19) gene codes for an RNA helicase involved in both mRNA (mRNA) export from the nucleus into the cytoplasm and in mRNA translation. In unperturbed cells, Ddx19 localizes in the cytoplasm and at the cytoplasmic face of the nuclear pore. Here we review recent findings related to an additional Ddx19 function in the nucleus in resolving RNA:DNA hybrids (R-loops) generated during collision between transcription and replication, and upon DNA damage. Activation of a DNA damage response pathway dependent upon the ATR kinase, a major regulator of replication fork progression, stimulates translocation of the Ddx19 protein from the cytoplasm into the nucleus. Only nuclear Ddx19 is competent to resolve R-loops, and down regulation of Ddx19 expression induces DNA double strand breaks only in proliferating cells. Overall these observations put forward Ddx19 as an important novel mediator of the crosstalk between transcription and replication

A subtelomeric region affects telomerase-negative replicative senescence in Saccharomyces cerevisiae

Abstract In eukaryotes, telomeres determine cell proliferation potential by triggering replicative senescence in the absence of telomerase. In Saccharomyces cerevisiae, senescence is mainly dictated by the first telomere that reaches a critically short length, activating a DNA-damage-like response. How the corresponding signaling is modulated by the telomeric structure and context is largely unknown. Here we investigated how subtelomeric elements of the shortest telomere in a telomerase-negative cell influence the onset of senescence. We found that a 15 kb truncation of the 7L subtelomere widely used in studies of telomere biology affects cell growth when combined with telomerase inactivation. This effect is likely not explained by (i) elimination of sequence homology at chromosome ends that would compromise homology-directed DNA repair mechanisms; (ii) elimination of the conserved subtelomeric X-element; (iii) elimination of a gene that would become essential in the absence of telomerase; and (iv) heterochromatinization of inner genes, causing the silencing of an essential gene in replicative senescent cells. This works contributes to better delineate subtelomere functions and their impact on telomere biology