Telomere replication: poised but puzzling

SNM1B/Apollo in the DNA damage response and telomere maintenance

hSNM1B/Apollo is a member of the highly conserved β-CASP subgroup within the MBL superfamily of proteins. It interacts with several DNA repair proteins and functions within the Fanconi anemia pathway in response to DNA interstrand crosslinks. As a shelterin accessory protein, hSNM1B/Apollo is also vital for the generation and maintenance of telomeric overhangs. In this review, we will summarize studies on hSNM1B/Apollo's function, including its contribution to DNA damage signaling, replication fork maintenance, control of topological stress and telomere protection. Furthermore, we will highlight recent studies illustrating hSNM1B/Apollo's putative role in human disease.

Tetrahymena telomerase holoenzyme assembly, activation, and inhibition by domains of the p50 central hub

The eukaryotic reverse transcriptase, telomerase, adds tandem telomeric repeats to chromosome ends to promote genome stability. The fully assembled telomerase holoenzyme contains a ribonucleoprotein (RNP) catalytic core and additional proteins that modulate the ability of the RNP catalytic core to elongate telomeres. Electron microscopy (EM) structures of Tetrahymena telomerase holoenzyme revealed a central location of the relatively uncharacterized p50 subunit. Here we have investigated the biochemical and structural basis for p50 function. We have shown that the p50-bound RNP catalytic core has a relatively low rate of tandem repeat synthesis but high processivity of repeat addition, indicative of high stability of enzyme-product interaction. The rate of tandem repeat synthesis is enhanced by p50-dependent recruitment of the holoenzyme single-stranded DNA binding subunit, Teb1. An N-terminal p50 domain is sufficient to stimulate tandem repeat synthesis and bridge the RNP catalytic core, Teb1, and the p75 subunit of the holoenzyme subcomplex p75/p19/p45. In cells, the N-terminal p50 domain assembles a complete holoenzyme that is functional for telomere maintenance, albeit at shortened telomere lengths. Also, in EM structures of holoenzymes, only the N-terminal domain of p50 is visible. Our findings provide new insights about subunit and domain interactions and functions within the Tetrahymena telomerase holoenzyme.

Compromised genomic integrity impedes muscle growth after Atrx inactivation

ATR-X syndrome is a severe intellectual disability disorder caused by mutations in the ATRX gene. Many ancillary clinical features are attributed to CNS deficiencies, yet most patients have muscle hypotonia, delayed ambulation, or kyphosis, pointing to an underlying skeletal muscle defect. Here, we identified a cell-intrinsic requirement for Atrx in postnatal muscle growth and regeneration in mice. Mice with skeletal muscle-specific Atrx conditional knockout (Atrx cKO mice) were viable, but by 3 weeks of age presented hallmarks of underdeveloped musculature, including kyphosis, 20% reduction in body mass, and 34% reduction in muscle fiber caliber. Atrx cKO mice also demonstrated a marked regeneration deficit that was not due to fewer resident satellite cells or their inability to terminally differentiate. However, activation of Atrx-null satellite cells from isolated muscle fibers resulted in a 9-fold reduction in myoblast expansion, caused by delayed progression through mid to late S phase. While in S phase, Atrx colocalized specifically to late-replicating chromatin, and its loss resulted in rampant signs of genomic instability. These observations support a model in which Atrx maintains chromatin integrity during the rapid developmental growth of a tissue.

Human Stn1 protects telomere integrity by promoting efficient lagging-strand synthesis at telomeres and mediating C-strand fill-in

Telomere maintenance is critical for genome stability. The newly-identified Ctc1/Stn1/Ten1 complex is important for telomere maintenance, though its precise role is unclear. We report here that depletion of hStn1 induces catastrophic telomere shortening, DNA damage response, and early senescence in human somatic cells. These phenotypes are likely due to the essential role of hStn1 in promoting efficient replication of lagging-strand telomeric DNA. Downregulation of hStn1 accumulates single-stranded G-rich DNA specifically at lagging-strand telomeres, increases telomere fragility, hinders telomere DNA synthesis, as well as delays and compromises telomeric C-strand synthesis. We further show that hStn1 deficiency leads to persistent and elevated association of DNA polymerase α (polα) to telomeres, suggesting that hStn1 may modulate the DNA synthesis activity of polα rather than controlling the loading of polα to telomeres. Additionally, our data suggest that hStn1 is unlikely to be part of the telomere capping complex. We propose that the hStn1 assists DNA polymerases to efficiently duplicate lagging-strand telomeres in order to achieve complete synthesis of telomeric DNA, therefore preventing rapid telomere loss.

Effects of a halogenated G-quadruplex ligand from the pyridine dicarboxamide series on the terminal sequence of XpYp telomere in HT1080 cells

Non-canonical four-stranded structures called G-quadruplexes can form among telomere repeats during its replication. Small molecule ligands able to interact and to stabilize G-quadruplexes were shown to disrupt the binding of essential telomeric components, such as POT1 and to trigger a telomeric dysfunction associated with a delayed growth arrest in tumor cells. We describe here the chemical synthesis and the G-quadruplex binding properties of three halogenated analogs of the 360A ligand that belongs to the 2,6 pyridine dicarboxamide series. 360A is now commonly used as a benchmark both for biophysical and cellular assays as this compound was shown to display a potent affinity and selectivity for telomeric G-quadruplex DNA over duplex DNA and to induce delayed growth inhibition in HT1080 tumor cell line. Two biophysical assays indicate that, in most cases, the presence of the halogen atom seems to slightly improve the interaction with the telomeric quadruplex. For stability reasons, the bromo derivative (360A-Br) was selected for the cellular assays. Since POT1 participates to the fine tuning of the C-strand end resection during telomere replication, we investigated the effect of 360A-Br to alter the terminal nucleotide composition of XpYp telomere in HT1080 cells using C-STELA. HT1080 cells treated for up to 24 days with 360A-Br presented some minor but significant variations of C-strand terminal nucleotide composition, also observed with a partial siRNA depletion of POT1. The relevance of these minor modifications of the telomeric C-strand resection induced by 360A-Br in HT1080 cells are discussed.

CTC1 deletion results in defective telomere replication, leading to catastrophic telomere loss and stem cell exhaustion

The proper maintenance of telomeres is essential for genome stability. Mammalian telomere maintenance is governed by a number of telomere binding proteins, including the newly identified CTC1-STN1-TEN1 (CST) complex. However, the in vivo functions of mammalian CST remain unclear. To address this question, we conditionally deleted CTC1 from mice. We report here that CTC1 null mice experience rapid onset of global cellular proliferative defects and die prematurely from complete bone marrow failure due to the activation of an ATR-dependent G2/M checkpoint. Acute deletion of CTC1 does not result in telomere deprotection, suggesting that mammalian CST is not involved in capping telomeres. Rather, CTC1 facilitates telomere replication by promoting efficient restart of stalled replication forks. CTC1 deletion results in increased loss of leading C-strand telomeres, catastrophic telomere loss and accumulation of excessive ss telomere DNA. Our data demonstrate an essential role for CTC1 in promoting efficient replication and length maintenance of telomeres.