The Marseille Cancer Research Center celebrates its 50th anniversary ! -
Telomeres are nucleoprotein structures located at the ends of linear chromosomes. They consist of repeated sequences on which specific proteins bind to ensure their protection. Telomeres are a marker of aging, as they shorten with age. Premature shortening due to defects in maintenance mechanisms is related to aging, to genetic diseases called telomeropathies and to the development of cancers. In the laboratory, we are interested in the mechanisms that ensure the maintenance of telomeres, particularly those related to the replication of telomeres. Indeed, it is during this “critical” phase that telomeres are deprotected and vulnerable. Cells have an arsenal of proteins capable of limiting replicative stress on the telomeres and we explore these mechanisms. In the laboratory, we use human cell lines and the yeast Schyzosaccharomyces pombe. This yeast is a powerful tool since it has a telomeric structure similar to human telomeres.
Telomeres maintenance mechanisms and genome stability
Replication stress at telomeres in S. pombe
When a replication fork progresses towards telomeric sequences, it slows down and and stalls. Indeed, telomeric DNA secondary structures are obstacles which block replication forks (Maestroni et al., 2017). Forks retsart therefore requires the resolution of these structures by protein factors which facilitate the replication of terminal sequences. When a replication fork breaks, repair by homologous recombination mechanism can lead to loss of telomeric sequences and instability of the genome. Using the yeast S. pombe, we explore the mechanisms involved in the management of replicative stress at telomeres. Recently, we d that the Stn1-Ten1 complex plays an essential role in the replication of telomeric and subtelomeric regions, by stimulating DNA synthesis to avoid the accumulation of single-stranded DNA (Matmati et al. , Science Advances 2018). We showed that telomerase plays a key role in the replication of telomeres by repairing blocked or broken replication forks (Matmati et al., Cell reports 2020).
Telomeric functions of human RPA complex and telomeropathies
The RPA complex is an heterotrimer that binds single-stranded DNA. It is essential for replication, recombination and replication of DNA. We have shown that this complex is involved in the maintenance of telomeres in yeast (Schramke et al., Nature Genetics 2004; Luciano et al., EMBOj 2012). More recently, we showed that RPA prevents the formation of secondary DNA structures like G-quadruplexe at telomeres and thus facilitates the action of telomerase (Audry et al., EMBOj 2015). In collaboration with Caroline Kennengiesser (Bichat Hospital), Patrick Revy (Imagine Institute) and Carole Saintomé (Museum of Natural Histories), we have identified mutations in RPA genes in patients suffering from telomeropathy. Now, we investigate the impact of these mutations on telomeres in cell lines expressing the modified RPA subunit to determine in which mechanisms RPA is involved.
Telomeres maintenance in quiescent cells
Although cells do not divide, their genome can be damaged and needs to be repaired. This is particularly the case for telomeric sequences which are the preferred targets of free radicals (ROS). We showed that eroded telomeres are subject to strong rearrangements that are initiated by the production of non-coding RNA (TERRA) in quiescent cells (Maestroni et al., Nature Communications 2017). These rearrangements act as a molecular signal that prevents cells from dividing again. In addition, we have shown that telomeres are grouped at the nuclear membrane in quiescent cells and that this location plays an important role in their stability (Maestroni et al., Nucleic Acid research 2020). For this purpose, we use the yeast S. pombe which is a very advantageous model since the cells are easily kept in quiescence for several weeks.
