Chromosome X | EPFL Graph Search

Le chromosome X est l'un des deux chromosomes sexuels de l'être humain et de certains animaux (l'autre chromosome sexuel étant le chromosome Y). Il fait partie du système XY de détermination sexuelle. Caractéristiques du chromosome X humain

Nombre de paires de base :
Nombre de gènes : 931
Nombre de gènes connus : 766
Nombre de pseudogènes : 380
Nombre de variations des nucléotides (polymorphismes nucléotidiques) :
L'image ci-dessous illustre la richesse génétique du chromosome X à comparer avec le chromosome Y. La zone en vert localise le centre de l'inactivation de l'X.

Inactivation du chromosome X ou lyonisation Les hommes ont un seul chromosome X et les femmes deux chromosomes X. Ce chromosome X supplémentaire n'aboutit pas à une « surproduction » des protéines codées par ce chromosome. Par exemple, les facteurs de coagulations comme le facteur VIII est codé par un gène situé sur le chromosome X. Les femmes ne possèdent pas pour autant

Identification and characterization of proteins that protect human telomeres from fragility

Anna Christina Näger

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

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.