Michel Charbonneau

Telomeres, the ends of linear chromosomes, form a cap composed of specific telomeric DNA associated with specialized proteins that ensures efficient protection against chromosome degradation and, consequently, overall genome instability. Three main complexes have been implicated in chromosome end protection, namely telomerase, shelterin, and the CST complex (CTC1/Cdc13-STN1-TEN1). In a recent study, we uncovered mutants of the S. cerevisiae CST in which damage was sensed both by the two major spindle checkpoints and the major DNA damage checkpoint. In the best-studied of these mutants, stn1-sz2 , the stability and organization of the mitotic tubulin spindle was profoundly affected, as were the organization of the centrosomes and the kinetochores. Here, we find that the stn1-sz2 mutant is hypersensitive to deletion mutants of two members of the kinetochores, Ctf19 and Mcm21, which correlated with mis-localization of their associated subunit in the COMA complex, Okp1, during anaphase. Defects in interactions between the kinetochores and the tubulin spindle are entirely sensed by the SAC (spindle assembly checkpoint). Here, we find that some of the stn1-sz2 damage is recognized by the Mad2 checkpoint protein independently from the SAC. The data presented here contribute to a better understanding of the damage in stn1-sz2 , the prototype of novel mutants of telomeric CST characterized by mitotic-spindle-damage in addition to the classical telomeric-DNA-damage.

Telomeres, the ends of linear chromosomes, form a cap composed of specific telomeric DNA asso-ciated with specialized proteins that ensures efficient protection against chromosome degrada-tion and, consequently, overall genome instability. Three main complexes have been implicated in chromosome end protection, namely telomerase, shelterin, and the CST complex (CTC1/Cdc13-STN1-TEN1). In a recent study, we uncovered mutants of the S. cerevisiae CST in which damage was sensed both by the two major spindle checkpoints and the major DNA damage checkpoint. In the best-studied of these mutants, stn1-sz2, the stability and organization of the mitotic tubulin spindle was profoundly affected, as were the organization of the centrosomes and the kineto-chores. Here, we find that the stn1-sz2 mutant is hypersensitive to deletion mutants of two mem-bers of the kinetochores, Ctf19 and Mcm21, which correlated with mis-localization of their asso-ciated subunit in the COMA complex, Okp1, during anaphase. Defects in interactions between the kinetochores and the tubulin spindle are entirely sensed by the SAC (spindle assembly check-point). Here, we find that some of the stn1-sz2 damage is recognized by the Mad2 checkpoint pro-tein independently from the SAC. The data presented here contribute to a better understanding of the damage in stn1-sz2, the prototype of novel mutants of telomeric CST characterized by mitotic-spindle-damage in addition to the classical telomeric-DNA-damage.

The conserved and essential Cdc13/CTC1-Stn1-Ten1 telomeric complex (CST) ensures chromosome stability by protecting telomere ends and regulating telomerase accessibility. In a recent study, we uncovered mutants of the S. cerevisiae CST, in which damage was sensed by the two major G2/M spindle checkpoints (one is Bub2-dependent and the other one Mad2-dependent), as well as the major G2/M DNA damage checkpoint (Mec1-dependent). In this study, we found, by fluorescence microscopy, that the stability of the mitotic tubulin spindle was profoundly affected in the best-studied of these mutants, stn1-sz2. Additional data from genetic analyses suggested the potential involvement of Stu1 and Stu2, as well as Slk19, in these defects. Throughout this study, we compared the phenotypes of stn1-sz2 with those of cdc13-1, the best-studied CST mutant, which also serves as a prototype of telomere-damage-characterized CST mutants. We propose that stn1-sz2 represents the prototype of cst mutants characterized by tubulin spindle damage. These newly described phenotypes potentially represent the basis for identifying new functions of the CST telomeric complex. These functions might consist of ensuring correct chromosome segregation through the stabilization of the mitotic spindle.

Telomeres, the ends of eukaryotic linear chromosomes, are composed of repeated DNA sequences and specialized proteins, with the conserved telomeric Cdc13/CTC1-Stn1-Ten1 (CST) complex providing chromosome stability via telomere end protection and the regulation of telomerase accessibility. In this study, SIZ1, coding for a SUMO E3 ligase, and TOP2 (a SUMO target for Siz1 and Siz2) were isolated as extragenic suppressors of Saccharomyces cerevisiae CST temperature-sensitive mutants. ten1-sz, stn1-sz and cdc13-sz mutants were isolated next due to being sensitive to intracellular Siz1 dosage. In parallel, strong negative genetic interactions between mutants of CST and septins were identified, with septins being noticeably sumoylated through the action of Siz1. The temperature-sensitive arrest in these new mutants of CST was dependent on the G2/M Mad2-mediated and Bub2-mediated spindle checkpoints as well as on the G2/M Mec1-mediated DNA damage checkpoint. Our data suggest the existence of yet unknown functions of the telomeric Cdc13-Stn1-Ten1 complex associated with mitotic spindle positioning and/or assembly that could be further elucidated by studying these new ten1-sz, stn1-sz and cdc13-sz mutants.

Telomeres, the ends of eukaryotic linear chromosomes, are composed of repeated DNA sequences and specialized proteins, with the conserved Cdc13/CTC1-Stn1-Ten1 (CST) telomeric comple providing chromosome stability via telomere end-protection and regulation of telomerase accessibility. In the present study, SIZ1 , coding for a SUMO E3 ligase, and TOP2 (Top2 is a SUMO target for Siz1 and Siz2) were isolated as extragenic suppressors of temperature-sensitive mutants of Saccharomyces cerevisiae CST. ten1 - sz, stn1 - sz and cdc13 - sz mutants were next isolated on the basis of being sensitive to intracellular Siz1 dosage. In parallel, strong negative genetic interactions between mutants of CST and septins were identified, septins being noticeably sumoylated through action of Siz1. The temperature-sensitive arrest in these new mutants of CST was dependent on the G2/M Mad2-mediated and Bub2-mediated spindle checkpoints as well as on the G2/M Mec1-mediated DNA damage checkpoint. Our data suggest the existence of yet unknown functions of the telomeric Cdc13-Stn1-Ten1 complex related with mitotic spindle positioning and/or spindle assembly that could be further elucidated by studying these new ten1 - sz, stn1 - sz and cdc13 - sz mutants.

All cancer cells need to maintain functional telomeres to sustain continuous cell division and proliferation. In human diffuse gliomas, functional telomeres are maintained due either to reactivation of telomerase expression, the main pathway in most cancer types, or to activation of a mechanism called the alternative lengthening of telomeres (ALT). The presence of IDH1/2 mutations (IDH-mutant) together with loss of ATRX expression (ATRX-lost) are frequently associated with ALT in diffuse gliomas. However, detection of ALT, anda fortioriits quantification, are rarely, if ever, measured in neuropathology laboratories. We measured the level of ALT activity using the previously described quantitative “C-circle” assay and analyzed it in a well characterized cohort of 104 IDH-mutant and ATRX-lost adult diffuse gliomas. We report that in IDH-mutant ATRX-lost anaplastic astrocytomas, the intensity of ALT was inversely correlated with age (p < 0.001), the younger the patient, the higher the intensity of ALT. Strikingly, glioblastomas having progressed from anaplastic astrocytomas did not exhibit this correlation. ALT activity level in the tumor did not depend on telomere length in healthy tissue cells from the same patient. In summary, we have uncovered the existence, in anaplastic astrocytomas but not in glioblastomas with the same IDH and ATRX mutations, of a correlation between patient age and the level of activity of ALT, a telomerase-independent pathway of telomere maintenance.

Specialized telomeric proteins have an essential role in maintaining genome stability through chromosome end protection and telomere length regulation. In the yeast Saccharomyces cerevisiae, the evolutionary conserved CST complex, composed of the Cdc13, Stn1 and Ten1 proteins, largely contributes to these functions. Here, we report genetic interactions between TEN1 and several genes coding for transcription regulators. Molecular assays confirmed this novel function of Ten1 and further established that it regulates the occupancies of RNA polymerase II and the Spt5 elongation factor within transcribed genes. Since Ten1, but also Cdc13 and Stn1, were found to physically associate with Spt5, we propose that Spt5 represents the target of CST in transcription regulation. Moreover, CST physically associates with Hmo1, previously shown to mediate the architecture of S-phase transcribed genes. The fact that, genome-wide, the promoters of genes down-regulated in the ten1-31 mutant are prefentially bound by Hmo1, leads us to propose a potential role for CST in synchronizing transcription with replication fork progression following head-on collisions.

Summary: Specialized telomeric proteins have an essential role in maintaining genome stability through chromosome end protection and telomere length regulation. In the yeast Saccharomyces cerevisiae , the evolutionary conserved CST complex, composed of the Cdc13, Stn1 and Ten1 proteins, largely contributes to these functions. Here, we report the existence of genetic interactions between TEN1 and several genes coding for transcription regulators. Molecular assays confirmed this novel function of Ten1 and further established that it regulates the occupancies of RNA polymerase II and the Spt5 elongation factor within transcribed genes. Since Ten1, but also Cdc13 and Stn1, were found to physically associate with Spt5, we propose that Spt5 represents the target of CST in transcription regulation. Moreover, CST physically associates with Hmo1, previously shown to mediate the architecture of S phase-transcribed genes. The fact that, genome-wide, the promoters of genes down-regulated in the ten1 - 31 mutant are prefentially bound by Hmo1, leads us to propose a potential role for CST in synchronizing transcription with replication fork progression following head-on collisions.

All tumors have in common to reactivate a telomere maintenance mechanism to allow for unlimited proliferation. On the other hand, genetic instability found in some tumors can result from the loss of telomeres. Here, we measured telomere length in colorectal cancers (CRCs) using TRF (Telomere Restriction Fragment) analysis. Telomeric DNA content was also quantified as the ratio of total telomeric (TTAGGG) sequences over that of the invariable Alu sequences. In most of the 125 CRCs analyzed, there was a significant diminution in telomere length compared with that in control healthy tissue. Only 34 tumors exhibited no telomere erosion and, in some cases, a slight telomere lengthening. Telomere length did not correlate with age, gender, tumor stage, tumor localization or stage of tumor differentiation. In addition, while telomere length did not correlate with the presence of a mutation in BRAF (V-raf murine sarcoma viral oncogene homolog B), PIK3CA (phosphatidylinositol 3-kinase catalytic subunit), or MSI status, it was significantly associated with the occurrence of a mutation in KRAS. Interestingly, we found that the shorter the telomeres in healthy tissue of a patient, the larger an increase in telomere length in the tumor. Our study points to the existence of two types of CRCs based on telomere length and reveals that telomere length in healthy tissue might influence telomere maintenance mechanisms in the tumor.