Basic mechanisms of heterochromatin replication: the telomere connection – TELOCHROM

Constitutive heterochromatin (cHC), a key compartment of chromosomes essential for genome stability, is particularly difficult to replicate due to its dense organization and its enrichment in repetitive DNA. Recently a ground-breaking concept emerged from E. Gilson’s team (Partner 1) studies showing an unexpected role of a telomeric protein, TRF2, in the replication of the pericentromeric HC (PHC) regions. TRF2 is a member of the shelterin telomeric complex and protects chromosome ends from unwanted DNA damage response activation and illicit repair. It has also emerged as a multifunctional telomeric replication factor thanks to its capacity to recognize non-B DNA structures that can be formed on these repetitive sequences during replication (G4, chicken feet, DNA positive supercoils) and through the recruitment of several DNA targeting activities (helicases such as RTEL1 or WRN, nucleases such as APOLLO or MUS81-EME1). Our discovery of a key role of TRF2 in pericentromeric replication stems from the observation that TRF2 binds to PHC Satellite III DNA sequences during S phase, protects them from replicative DNA damage and is necessary for an efficient replication of PHC in terms of fork speed and origin firing (Mendez-Bermudez et al., Mol Cell 2018 and unpublished). TRF2 also protects other regions of the genome enriched in the H3K9me3 histone mark suggesting a much broader role of this protein in maintaining HC integrity and revealing the existence of a close functional relationship between telomeres and HC. Interestingly, both genomic regions are closely linked to cellular senescence and aging: one (telomeres) gradually shortening with age which ultimately leads to replicative senescence; the other (cHC) being subjected to profound structural changes at senescence. To decipher the mechanisms, the actors and the physiological consequences in cell senescence and aging of this telomere-cHC replication relationship, we designed a project named TELOCHROM that involves three teams, experts in telomere biology (Pr. E. Gilson, coordinator Dr. MJ Giraud-Panis) and in DNA replication (Pr. M. Debatisse and Dr. C. Chen). The aim of TELOCHROM is to Identify new pathways and telomere-associated actors of heterochromatin replication initiation and progression and their relationship with aging.
TELOCHROM is organized in 3 work-packages aimed at identifying the mechanisms and actors involved in TRF2-dependent regulation of cHC replication (WP1), deciphering the effect of these different actors in replication origins firing and in replisome composition (WP2) and understanding the consequences of the identified pathways and actors in cell senescence (WP3). All teams are involved in each WP according to their technical expertise: Partner 1 will bring its essential know how in telomeres, pericentromeres biology and DNA topology; Partner 2 is an expert in DNA replication on hard to replicate regions such as fragile sites; Partner 3 has great expertise in top of the art Next Generation Sequencing and the analyses of the data generated.
The originality and interest of the TELOCHROM project lies in: i) the innovative topic of a close functional link between telomeres and HC during replication; ii) the challenge of studying replication in highly repetitive, condensed and dynamic regions of the genome; iii) the top of the art technologies used (optical replication mapping, long read sequencing, Repli-seq, Topotools, Nascent Chromatin Capture); iv) the solid base of the project that stems from studies published recently or just submitted by the teams; v) the complementarity of the partners that have been successfully collaborating in recent years.
Overall, TELOCHROM will undoubtedly bring new and important data on the basic mechanisms of HC replication, replicative senescence and on how telomeres and HC drive cell fate.