The makings of TERRA R-loops at chromosome ends

Telomeres protect chromosome ends from nucleolytic degradation, uncontrolled recombination by DNA repair enzymes and checkpoint signaling, and they provide mechanisms for their maintenance by semiconservative DNA replication, telomerase and homologous recombination. The telomeric long noncoding RNA TERRA is transcribed from a large number of chromosome ends. TERRA has been implicated in modulating telomeric chromatin structure and checkpoint signaling, and in telomere maintenance by homology directed repair, and telomerase - when telomeres are damaged or very short. Recent work indicates that TERRA association with telomeres involves the formation of DNA:RNA hybrid structures that can be formed post transcription by the RAD51 DNA recombinase, which in turn may trigger homologous recombination between telomeric repeats and telomere elongation. In this review, we describe the mechanisms of TERRA recruitment to telomeres, R-loop formation and its regulation by shelterin proteins. We discuss the consequences of R-loop formation, with regard to telomere maintenance by DNA recombination and how this may impinge on telomere replication while counteracting telomere shortening in normal cells and in ALT cancer cells, which maintain telomeres in the absence of telomerase.

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

RADX: A novel single-stranded DNA-binding protein regulating telomere recombination

Anna-Sophia Briod

Telomeres are specialized nucleoprotein structures present at the ends of linear chromosome. The telomeric DNA part is comprised of 5-15 kilo base pairs of double stranded TTAGGG repeats and it contains at the 3' end a single-stranded G-rich overhang of 50-400 nucleotides. Telomeric DNA associates with a large number of proteins, which inhibit DNA damage checkpoint activation, DNA repair activities and nucleolytic degradation. To better understand the complex roles which telomeres play in genome stability, premature aging and cancer development we explored the telomeric proteomic environment with BioID. We employed a CRISPR/Cas9 based knock-in approach to integrate the promiscuous biotin ligase BirA at the genomic loci of the telomeric components TRF1, TRF2 and POT1. Thus, we expressed the fusion proteins from native promoters aiming at retaining native expression levels. Upon incubation with Biotin, the promiscuous biotinylase can label proximal proteins, which can be selectively purified and identified via mass spectrometry. Our BioID results indicated that TRF1, TRF2 and POT1 share a large number of common protein partners. In addition to already known telomere components, we identified a significant number of novel telomeric proteins. Among those newly identified proteins, we found RADX, an RPA-like single-stranded DNA-binding-protein, which counteracts RAD51 at stalled replication forks. Even though RADX had so far only been characterized for its role in DNA replication we observed colocalization of ectopically expressed 3xFlag-RADX with telomeres throughout the cell cycle. The colocalization decreased when the RADXs single-stranded (ss)DNA-binding domain was mutated. We therefore hypothesize that RADX binds the single-stranded G-rich telomeric strand either as displaced strand in the t-loop or as single-stranded telomeric overhang. Chromatin-immunoprecipitation experiments suggest that RADX plays a distinct role in telomere replication, since upon treatment with the replication stress inducing reagent hydroxyurea we observed increased binding of RADX specifically to telomeres. Notably, RADX depletion on its own did not alter telomere integrity and telomere length but upon co-depletion with POT1, telomere fragility, sister-chromatid associations and telomere length increased strikingly. POT1 and POT1/RADX depletions lead to increased associations of the recombinase RAD51 with telomeres and consequently silencing of RAD51 rescued telomere integrity and elongation implying the regulatory role of RADX for RAD51. In summary my thesis describes the proteomic microenvironment of different telomeric proteins, identifies the novel telomeric protein RADX and characterizes its role at telomeres. My findings provide important insights into how homologous recombination and RAD51 loading is suppressed at telomeres.

EPFL2019

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