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Telomeres are key features of linear chromosomes that preserve genome stability and function. Variations in telomere status are critical for cell senescence, stem cell biology, and the development of cancer.

The team investigates in budding and fission yeast how telomeres are replicated and maintained and the cellular responses to telomere erosion. Our work focusses on the dynamics of telomere repair during replicative senescence and the role of the Nuclear Pore Complex in processing eroded telomeres and collapsed replication forks. We also study the mechanisms of telomere maintenance during quiescence using fission yeast as model.

The team is involved in several cancer programmes. In particular, we investigate the mechanism of Aternative Lengthening of Telomeres (ALT) in tumors of mesenchymal origin that are devoid of ATRX mutations.

In the recent years, we characterized a new mouse model (p21+/mTERT ) where Tert is expressed from p21 promoter that can be activated by p53 in response to telomere dysfunction. Our recent results indicate that that cellular senescence in lung cells as well as lung emphysema occurring in old mice are both suppressed in these mice. Protection against emphysema in old p21+/mTERT mice is associated with an increase proliferation of lung endothelial cells (EC) and is dependent of the catalytic activity of Tert. We investigate how p21-dependent expression of telomerase is able to mobilize stem/progenitor of lung cells providing insights on the mechanism by which telomerase expression can prevent emphysema in old mice.

Our team is also involved in an international program (e-Rare- REPETOMICS) aimed at understanding the mechanisms of instability of GGGGCC repeats in the C9orf72 gene whose amplification plays a role in the development of Amyotrophic Lateral Sclerosis and Frontotemporal Degeneration

TELOMERES
TEMPORAL AND SPATIAL REGULATION OF TELOMERIC FORK REPAIR PATHWAYS

The nuclear pore complex (NPC) are composed of 30 individual nucleoporins to form highly conserved macromolecular structures in the nuclear envelope. Its core function is the nucleo-cytoplasmic transport and RNA export, but several individual nucleoporins have been involved in DNA repair. Hard‐to‐repair DNA lesions, including irreparable double strand breaks (DSBs), eroded telomeres and collapsed replication forks relocate to NPC at which alternative repair pathways take place. We investigate in budding and fission yeast how eroded telomeres and replication forks stalled at telomere repeats are processed and the role of the NPC in promoting telomere repair and fork‐restart.

THE p21-mTERT KNOCK-IN MOUSE: AN IN VIVO MODEL OF SENESCENCE BYPASS AND MUCH MORE

A key regulator of cellular arrest in response to telomere shortening and DNA damage is the cyclin-dependent kinase inhibitor p21. p53-dependent upregulation of p21 is thought to be the primary event inducing replicative senescence. We asked whether aging could be delayed by abrogating telomere shortening in senescent cells by expressing telomerase “only when it is needed” and what would be the consequences of this conditional ectopic expression of telomerase. Indeed, previous studies revealed that overexpression of telomerase promotes cell proliferation and inflammation independently of its activity at telomeres. To this purpose, we have created a knock-in mouse model in which a cassette encoding mCherry-2A-mTert (telomerase) has been inserted after the first exon of p21 (p21-mTert mouse). While p21-driven expression of telomerase suppresses senescence in several tissues, we observed unexpected phenotypes associated to this ectopic telomerase expression. Our project aims to understand how the p21-driven expression of telomerase at the same time promotes the escape of senescence and deregulates signalling and metabolic pathways

ALTERNATIVE LENGTHENING OF TELOMERES (IN HIGH GRADE PAEDIATRIC OSTEOSARCOMAS)

Osteosarcomas are highly aggressive bone tumours that mainly occur in children and adolescents. Genetically, they are characterized by complex structural and numerical aberrations. Osteosarcomas are known for exhibiting a high frequency of ALT activation. Previous reports showed that ATRX gene mutation and/or loss of protein expression is detectable in only 30% of them. This discrepancy between a high level of ALT and a low proportion of ATRX inactivation led us to the hypothesis that ATRX-mediated ALT inhibition could be overridden in certain conditions. We investigate the mechanisms underlying ALT osteosarcomas.

TELOMERE MAINTENANCE
TELOMERE MAINTENANCE IN QUIESCENT FISSION YEAST CELLS (S. COULON GROUP)
COULON'S GROUP

Fission yeast Schizosaccharomyces pombe is a great model for cellular quiescence. Indeed, S. pombe can be experimentally maintained for weeks in quiescence in the absence of nitrogen. We took this opportunity to study the mechanism by which telomeres are maintained during quiescence. Indeed, although telomeres maintenance has been extensively studied in cycling cell, rare studies have been undertaken in quiescent cells. In this context, we investigate the stability of telomeres in quiescent cells, in particular the fate of short telomeres in quiescent cells in the presence and absence of telomerase. We characterize mechanisms of repair and elongation of short telomeres that are specific to post-mitotic cells.

CHROMATIN
THE MANY FACES OF SET1

By the same time as the group of Loraine Pillus (UCSD), we identified SET1 as the Saccharomyces cerevisiae gene encoding the yeast most closely related to SET domain proteins of multicellular organisms. Deletion of SET1 alleviated telomeric position effect, resulted in a mild shortening of telomeres and increased viability after DNA damage of checkpoint sensor mutants. Few years after, it was discovered by several other teams that Set1 was the catalytic subunit of a protein complex called Set1C or COMPASS (for Complex of Proteins Associated with Set1) that mediates H3 methylation at lysine 4 (H3K4). We had then an important contribution in collaboration with Francis Stewart’s Team in defining how each subunit of the Set1C was bound to the docking platform made by the catalytic Set1 subunit. Others and we showed that loss of individual Set1C subunits differentially affects Set1 stability, complex integrity, global H3K4 methylation patterns, and H3K4 methylation along active genes.

After having characterized the RNA Recognition Motifs of Set1, we uncovered in collaboration with Jaehoon Kim and Domenico Libri that Set1 directly binds RNA in vitro an in vivo. We discovered that Set1 binding to nascent transcripts is important to define the appropriate topology of Set1C distribution along transcription units and correlates with the efficient deposition of the H3K4me3 mark. We also reported in collaboration with Frank Holstege that H3K4 trimethylation loss on its own had little effect on steady-state mRNA expression levels and that the combined loss of H3K4me3 and H3K4me2 results in steady-state upregulation of a group of genes associated with Set1-mediated repression of 3’-end antisense transcription.

15 years ago, we demonstrated that inactivation of Set1 reduces the number of DSBs and impairs meiotic replication. This seminal paper was the first observation linking Set1 and H3K4 methylation to replication and to meiosis. In collaboration with Alain Nicolas and Valérie Borde, it was further shown that meiotic DSBs occur in regions showing higher H3K4me3 occupancy, although no higher transcript levels were detected near these DSB sites. We further reported that tethering of the PHD-containing protein, Spp1 to recombinationally cold regions was sufficient to induce DSB formation. Furthermore, we found that Spp1 physically interacted with Mer2, a key protein of the differentiated chromosomal axis required for DSB formation. Thus Spp1, by interacting with H3K4me3 and Mer2, promotes recruitment of potential meiotic DSB sites to the chromosomal axis. In collaboration with Lóránt Székvölgyi, we recently reported that spatial interactions of Spp1 and Mer2 occurred independently of Set1C. Our work led us to propose an enriched chromatin loop-axis model for the regulation of DSB formation that addresses how the meiotic DSB sites are mechanistically selected.

SET1 IN DNA REPLICATION

Cells respond to replication stress by signalling and repairing stalled replication forks. These mechanisms operate in the context of nascent chromatin and depend on the controlled resection of nascent DNA strands. However, how chromatin impacts on fork progression and stability remains poorly understood. We investigate how Set1C cooperate with chromatin remodellers to promote the remodelling of newly replicated chromatin to facilitate fork progression or restart.