Jonction d'extrémités non homologues

Résumé

vignette|La jonction d'extrémités non homologues. La jonction d'extrémités non homologues (en anglais Non-Homologous End-Joining ou NHEJ) est un mécanisme de réparation de l'ADN qui permet de réparer des lésions provoquant des cassures double brin (CDB). C'est un mécanisme non-conservatif (contrairement par exemple à la réparation par recombinaison) c'est-à-dire qu'il ne restaure pas la séquence initiale de l'ADN; mais seulement la continuité de l'ADN endommagé par une cassure double brin. Cette réparation conduira ainsi au changement de l'information génétique, en général une délétion, et donc possiblement à l'apparition d'une mutation pour le gène concerné si la cassure survient à l'intérieur d'un gène. Cette réparation est possible grâce à l'intervention de la protéine Ku70/80 (dimère entre deux sous-unités : Ku70 et Ku80) qui va interagir avec les deux extrémités d'ADN résultant de la cassure. Cette protéine va ensuite agir par essais et erreurs pour tenter d'établir une inte

Source officielle

https://fr.wikipedia.org/wiki/Jonction_d'extr%C3%A9mit%C3%A9s_non_homologues

À propos de ce résultat

Cette page est générée automatiquement et peut contenir des informations qui ne sont pas correctes, complètes, à jour ou pertinentes par rapport à votre recherche. Il en va de même pour toutes les autres pages de ce site. Veillez à vérifier les informations auprès des sources officielles de l'EPFL.

Publications associées (29)

Publications associées

Chargement

Personnes associées (3)

Personnes associées

Chargement

Unités associées (1)

Unités associées

Chargement

Concepts associés (35)

Concepts associés

Chargement

Cours associés (8)

Cours associés

Chargement

Séances de cours associées (20)

Séances de cours associées

Chargement

The shelterin proteins protect telomeres against activation of the DNA damage checkpoints and recombinational repair. We show here that a dimer of the shelterin subunit TRF2 wraps similar to 90 bp of DNA through several lysine and arginine residues localized around its homodimerization domain. The expression of a wrapping-deficient TRF2 mutant, named Top-less, alters telomeric DNA topology, decreases the number of terminal loops (t-loops), and triggers the ATM checkpoint, while still protecting telomeres against non-homologous end joining (NHEJ). In Top-less cells, the protection against NHEJ is alleviated if the expression of the TRF2-interacting protein RAP1 is reduced. We conclude that a distinctive topological state of telomeric DNA, controlled by the TRF2-dependent DNA wrapping and linked to t-loop formation, inhibits both ATM activation and NHEJ. The presence of RAP1 at telomeres appears as a backup mechanism to prevent NHEJ when topology-mediated telomere protection is impaired.

Cell Press2016

Chargement

The telomeric DNA damage response: Insights into telomere compaction and identification of novel damage signaling factors

Aleksandra Vancevska

Telomeres are dynamic nucleo-protein structures capping the ends of all eukaryotic chromosomes. Together with telomerase, they counteract replication-dependent telomere attrition. Additionally, they disguise the linear ends of the chromosome from the DNA damage response (DDR) machinery. Otherwise, the chromosome end would be recognized as a DSB and would elicit a deleterious DDR signal. Both of these functions contribute greatly to the maintenance of genome stability. Telomeres are composed of repetitive DNA sequences, protein complex dubbed shelterin and telomeric repeat-containing RNAs. In addition to the core protein complex, other proteins are important for proper telomere structure and function. Our laboratory has a longstanding interest in the discovery of novel factors that have indispensable roles in telomere biology. For that purpose, previous lab members developed a Quantitative Telomeric Chromatin Isolation protocol (QTIP) and detected binding of new proteins to long telomeres, SMCHD1 and LRIF1. Their function at telomeres is not described but they are shown to function in higher-order chromatin organization and genome-wide DDR. Understanding their role in telomere biology is important because the list of players involved in DDR at telomeres is far from complete and we know very little about how chromatin structure affects DDR activation. Thus, this thesis provides insights into the DDR at telomeres by studying the functions of SMCHD1 and LRIF1 and describes the implementation of a novel microscopy-based method to study the role of these proteins and shelterins in early steps of the DDR. Firstly, we describe crucial roles for SMCHD1 and LRIF1 in DDR activation at telomeres lacking shelterin protein TRF2. Removal of TRF2 leads to activation of the ATM kinase and elicits a DDR giving rise to persistent chromosome fusions. We show that LRIF1 and SMCHD1 removal leads to attenuation of ATM activation and subsequent DDR defect as well as impairment in non-homologous end joining (NHEJ) at unprotected telomeres. Considering that these phenotypes are rescued by removal of TPP1 and activation of the ATR kinase we propose that these proteins mainly participate in DDR signaling operating at uncapped telomeres. They are among the rare ones described to act early in the DDR cascade upon TRF2 removal. Stimulated by the discovery that SMCHD1 and LRIF1 function in X-chromosome compaction, we sought to test if they would function in remodeling telomeric chromatin. To do that, we have implemented super-resolution microscopy method (STORM) to measure sizes and shapes of normal human telomeres and ones lacking SMCHD, LRIF1, and shelterin. We have shown, by examination of thousands of telomeres, that removal of shelterin proteins leads to decompaction of only a small subset of telomeres. This decompaction is not required for efficient DDR activation, as the DDR was also efficiently elicited from compacted TRF2-depleted telomeres. Thus, we propose that DDR is triggered by changes at the molecular level in protein recruitment upon telomere deprotec-tion. In addition, transient removal of SMCHD1 and LRIF1 did not affect the compaction state of telo-meres as they do in the X-chromosome. Overall, we have described two novel factors that are important for DDR at uncapped telomeres and excluded the need for decompaction as the initial step of DDR at uncapped telomeres and excluded the need for decompaction as the initial step of DDR activation.

EPFL2018

Chargement

Difficulties to replicate telomeres - the ends of our chromosomes - can cause telomere shortening andgenome instability. These difficulties are due to the repetitive DNA sequence and distinct structures at telomeresthat challenge the semi-conservative DNA replication machinery. Among the proteins that help toovercome the obstacles to telomere replication are factors known to be mutated in genetic diseases (forinstance WRN, BLM, RTEL1, CTC1), but we suspect that there are still factors and pathways unknown.DNA replication defects at telomeres manifest as smeary or multiple telomeric fluorescence in situ hybridization(FISH) signals on metaphase chromosomes - commonly called telomere fragility. To identify proteinsthat protect human telomeres from fragility, we depleted 71 proteins (including 8 controls) from HeLa cellsusing siRNA pools and analysed their metaphase chromosomes for fragile telomeres. The majority of theseproteins had been identified by QTIP-iPOND - a proteomic analysis of chromatin near telomere replicationforks. Among the 71 proteins that were depleted in the siRNA screen, we identified 29 novel proteins thatprevent telomere fragility and are therefore crucial to maintain genome stability. The telomere fragilityscreen validated QTIP-iPOND as a technique to identify telomere replication proteins. The hits identified inour telomere fragility screen may for example have functions in regulating telomeric chromatin compositionand compaction, regulating transcription of the telomeric long non-coding RNA TERRA and its associationwith telomeric chromatin, dealing with replication stress and stalled replication forks, modulating DNAdamage signalling and repair, modulating sister chromatid cohesion, and preventing oxidative stress.One of the 29 novel proteins identified in the telomere fragility screen is chromobox 8 (CBX8) whose functionat telomeres was investigated in further detail in this study. CBX8 is a component of the canonical polycombrepressive complex 1 (PRC1), an epigenetic regulator of gene expression. However, CBX8 also hasfunctions that are independent of the PRC1 complex. We show that human CBX8 prevents telomere fragilitycaused by RNA-DNA hybrids which could stall replication forks and induce homologous recombination.CBX8 depletion or knockout did however not affect telomeric RNA-DNA hybrid levels. We hypothesize thatCBX8 may be involved in repair activities at stalled replication forks that arise when the replisome encounterspersistent RNA-DNA hybrids. Moreover, we found that CBX8 localizes to telomeric repeat-binding factor2 (TRF2)-deficient telomeres in a manner that is dependent on RNA-DNA hybrids and ATM signalling.We demonstrate that H3K27me3 (trimethylated histone 3 lysine 27) marks, which have previously beensuggested to help recruitment of CBX proteins to chromatin via the CBX chromodomain, are dispensable forCBX8 recruitment to TRF2-deficient telomeres. Furthermore, we found that CBX8 promotes non-homologousend joining (NHEJ)-mediated telomere fusions of dysfunctional telomeres. We therefore hypothesizethat CBX8 is involved in repair activities that influence the DNA repair pathway choice at TRF2-deficient telomeres.

EPFL2022

Chargement

EPFL2022

The telomeric DNA damage response: Insights into telomere compaction and identification of novel damage signaling factors

Aleksandra Vancevska

EPFL2018