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Objective: Poly(ADP-ribose) polymerase1 (PARP1) interacts and poly(ADP-ribosyl)ates telomere repeat binding factor 2 (TRF2), which acts as a platform to recruit a large number of proteins at the telomere. Since the discovery of TRF2-SLX4 interaction, SLX4 is becoming the key player in telomere length (TL) maintenance and repair by telomere sister chromatid exchange (T-SCE). Defective TL maintenance pathway results in a spectrum of diseases called telomeropathies like dyskeratosis congenita, aplastic anemia, fanconi anemia, cancer. We aimed to study the role of SLX4 and PARP1 on each other's telomere localization, T-SCE, and TL maintenance in human telomerase-negative osteosarcoma U2OS cells to understand some of the molecular mechanisms of telomere homeostasis., Materials and Methods: We checked the role of SLX4 and PARP1 on each other's telomere localization by telomere immunofluorescence. We have cloned full-length wild-type and catalytically inactive mutant PARP1 to understand the role of poly(ADP-ribosyl)ation reaction by PARP1 in telomere length homeostasis. TL of U2OS cells was measured by Q-FISH. T-SCE was measured by Telomere-FISH., Key Findings: We observed that SLX4 has no role in the telomere localization of PARP1. However, reduced localization of SLX4 at undamaged and damaged telomere upon PARP1 depletion was reversed by overexpression of exogenous wild-type PARP1 but not by overexpression of catalytically inactive mutant PARP1. PARP1 depletion synergized SLX4 depletion-mediated reduction of T-SCE. Furthermore, SLX4 depletion elongated TL, and combined insufficiency of SLX4 with PARP1 further elongated TL., Conclusion: So, PARP1 controls SLX4 recruitment at telomere by poly(ADP-ribosyl)ation reaction, thereby regulating SLX4-mediated T-SCE and TL homeostasis., (Copyright © 2021 Elsevier Inc. All rights reserved.)

Telomerase-negative cancer cells show increased telomere sister chromatid exchange (T-SCE) rates, a phenomenon that has been associated with an alternative lengthening of telomeres (ALT) mechanism for maintaining telomeres in this subset of cancers. Here we examine whether or not T-SCE can maintain telomeres in human cells using a combinatorial model capable of describing how telomere lengths evolve over time. Our results show that random T-SCE is unlikely to be the mechanism of telomere maintenance of ALT human cells, but that increased T-SCE rates combined with a recently proposed novel mechanism of non-random segregation of chromosomes with long telomeres preferentially into the same daughter cell during cell division can stabilize chromosome ends in ALT cancers. At the end we discuss a possible experiment that can validate the findings of this study.

Unequal sister chromatid recombination (uSCR) is elevated in budding yeast sgs1 mutants, which lack a homolog of the human BLM gene that causes Bloom syndrome. Examination of the mechanism responsible for elevated uSCR in sgs1 mutants showed that mutation of RAD51 also resulted in hyper-uSCR. Data from this study show that defects in the Rad51-Sgs1-dependent and Sgs1-dependent lesion-bypass pathways activate Rad59-Rad1- and Rad59-dependent pathways, respectively, resulting in uSCR. Moreover, the elevation of uSCR in sgs1 and rad51 mutants was dependent on MMS1, which encodes one of the components of the Mms22 module. Lastly, a putative role of Mms1 in the elevation of uSCR and a possible mechanism by which uSCR is elevated as a result of defective Sgs1 and Rad51 are discussed.

Saccharomyces cerevisiae RAD53 (CHK2) and CHK1 control two parallel branches of the RAD9-mediated pathway for DNA damage-induced G(2) arrest. Previous studies indicate that RAD9 is required for X-ray-associated sister chromatid exchange (SCE), suppresses homology-directed translocations, and is involved in pathways for double-strand break repair (DSB) repair that are different than those controlled by PDS1. We measured DNA damage-associated SCE in strains containing two tandem fragments of his3, his3-Delta5' and his3-Delta3'::HOcs, and rates of spontaneous translocations in diploids containing GAL1::his3-Delta5' and trp1::his3-Delta3'::HOcs. DNA damage-associated SCE was measured after log phase cells were exposed to methyl methanesulfonate (MMS), 4-nitroquinoline 1-oxide (4-NQO), UV, X rays and HO-induced DSBs. We observed that rad53 mutants were defective in MMS-, 4-NQO, X-ray-associated and HO-induced SCE but not in UV-associated SCE. Similar to rad9 pds1 double mutants, rad53 pds1 double mutants exhibited more X-ray sensitivity than the single mutants. rad53 sml1 diploid mutants exhibited a 10-fold higher rate of spontaneous translocations compared to the sml1 diploid mutants. chk1 mutants were not deficient in DNA damage-associated SCE after exposure to DNA damaging agents or after DSBs were generated at trp1::his3-Delta5'his3-Delta3'::HOcs. These data indicate that RAD53, not CHK1, is required for DSB-initiated SCE, and DNA damage-associated SCE after exposure to X-ray-mimetic and UV-mimetic chemicals.

Background: Propofol is frequently used for general anesthesia in children although little is known about possible genotoxic effects in humans. We investigated the formation of sister chromatid exchanges (SCE) in metaphase chromosomes of T-lymphocytes of children as a marker for possible genotoxocity following total intravenous anesthesia with propofol for minor surgical procedures., Methods: 40 children ASA classification I-III were included (ASA I n=34, ASA II n=5, ASA III n=1) in the study. Anesthesia was induced by propofol (3mg/kg) and alfentanil. Succinylcholine or rocuronium were administered for muscle relaxation. After tracheal intubation anesthesia was maintained by continuous propofol infusion at 12 mg/(kgh). Blood samples were drawn before induction and after termination of anesthesia. Following a 72 h cell culture period, 25 T-lymphocyte metaphases per blood sample for all children were analyzed for SCE frequencies., Results: Total intravenous anesthesia with propofol on children did not influence SCE rates in metaphase chromosomes of T-lymphocytes. No SCE differences could be detected between blood samples before initiation and after termination of anesthesia (Wilcoxon signed rank test). Slightly elevated SCE rates were obtained in T-lymphocytes of girls compared to boys, but these differences did not reach statistical significance., Conclusions: Propofol anesthesia under the chosen conditions did not induce the formation of SCE in children in vivo. No genotoxic effect of a short term exposure to propofol during pediatric anesthesia had been observed.

We studied the spontaneous sister chromatid exchange (SCE) frequencies in mice at different stages of development; early preimplantation: 2 days post conception (p.c.); late preimplantation: 4 days p.c.; post implantation: day 10 and 13 p.c. The SCE level in preimplantation embryos is 5 times higher than any of the stages following implantation. The explanation for such observations may include a direct impact of maternal circulatory system as a result of implantation or onset of expression of a set of embryonic genes. Here, we studied the expression and developmental profile of the three enzymes of oxygen radical metabolism (superoxide dismutase, glutathione peroxidase, and catalase) during development. Our results suggest that the onset and increase in the activity of these enzymes with in-utero differentiation, development and growth is not directly associated with the drop in SCEs/cell following implantation of the embryos. This unique developmental phenomenon may be mediated by the maternal circulatory system or expression of some other embryonic genes, possibly the genes involved in DNA repair.