bims-numges Biomed News
on Nucleotide metabolism and genome stability
Issue of 2020‒08‒02
thirty-five papers selected by
Sean Rudd
Karolinska Institutet


  1. Cancers (Basel). 2020 Jul 25. pii: E2051. [Epub ahead of print]12(8):
    Moretton A, Loizou JI.
      Metabolism is a fundamental cellular process that can become harmful for cells by leading to DNA damage, for instance by an increase in oxidative stress or through the generation of toxic byproducts. To deal with such insults, cells have evolved sophisticated DNA damage response (DDR) pathways that allow for the maintenance of genome integrity. Recent years have seen remarkable progress in our understanding of the diverse DDR mechanisms, and, through such work, it has emerged that cellular metabolic regulation not only generates DNA damage but also impacts on DNA repair. Cancer cells show an alteration of the DDR coupled with modifications in cellular metabolism, further emphasizing links between these two fundamental processes. Taken together, these compelling findings indicate that metabolic enzymes and metabolites represent a key group of factors within the DDR. Here, we will compile the current knowledge on the dynamic interplay between metabolic factors and the DDR, with a specific focus on cancer. We will also discuss how recently developed high-throughput technologies allow for the identification of novel crosstalk between the DDR and metabolism, which is of crucial importance to better design efficient cancer treatments.
    Keywords:  DNA damage; DNA damage response; DNA repair; high-throughput technologies; metabolism
    DOI:  https://doi.org/10.3390/cancers12082051
  2. Nat Commun. 2020 Jul 30. 11(1): 3811
    Zhou W, Yao Y, Scott AJ, Wilder-Romans K, Dresser JJ, Werner CK, Sun H, Pratt D, Sajjakulnukit P, Zhao SG, Davis M, Nelson BS, Halbrook CJ, Zhang L, Gatto F, Umemura Y, Walker AK, Kachman M, Sarkaria JN, Xiong J, Morgan MA, Rehemtualla A, Castro MG, Lowenstein P, Chandrasekaran S, Lawrence TS, Lyssiotis CA, Wahl DR.
      Intratumoral genomic heterogeneity in glioblastoma (GBM) is a barrier to overcoming therapy resistance. Treatments that are effective independent of genotype are urgently needed. By correlating intracellular metabolite levels with radiation resistance across dozens of genomically-distinct models of GBM, we find that purine metabolites, especially guanylates, strongly correlate with radiation resistance. Inhibiting GTP synthesis radiosensitizes GBM cells and patient-derived neurospheres by impairing DNA repair. Likewise, administration of exogenous purine nucleosides protects sensitive GBM models from radiation by promoting DNA repair. Neither modulating pyrimidine metabolism nor purine salvage has similar effects. An FDA-approved inhibitor of GTP synthesis potentiates the effects of radiation in flank and orthotopic patient-derived xenograft models of GBM. High expression of the rate-limiting enzyme of de novo GTP synthesis is associated with shorter survival in GBM patients. These findings indicate that inhibiting purine synthesis may be a promising strategy to overcome therapy resistance in this genomically heterogeneous disease.
    DOI:  https://doi.org/10.1038/s41467-020-17512-x
  3. Oncogene. 2020 Jul 25.
    Shu Z, Li Z, Huang H, Chen Y, Fan J, Yu L, Wu Z, Tian L, Qi Q, Peng S, Wei C, Xie Z, Li X, Feng Q, Sheng H, Li G, Wei D, Shan C, Chen G.
      Ribonucleotide reductase (RNR) catalyzes the rate-limiting step of de novo synthesis of deoxyribonucleotide triphosphates (dNTPs) building blocks for DNA synthesis, and is a well-recognized target for cancer therapy. RNR is a heterotetramer consisting of two large RRM1 subunits and two small RRM2 subunits. RNR activity is greatly stimulated by transcriptional activation of RRM2 during S/G2 phase to ensure adequate dNTP supply for DNA replication. However, little is known about the cell-cycle-dependent regulation of RNR activity through RRM1. Here, we report that RRM1 is phosphorylated at Ser 559 by CDK2/cyclin A during S/G2 phase. And this S559 phosphorylation of RRM1enhances RNR enzymatic activity and is required for maintaining sufficient dNTPs during normal DNA replication. Defective RRM1 S559 phosphorylation causes DNA replication stress, double-strand break, and genomic instability. Moreover, combined targeting of RRM1 S559 phosphorylation and ATR triggers lethal replication stress and profound antitumor effects. Thus, this posttranslational phosphorylation of RRM1 provides an alternative mechanism to finely regulating RNR and therapeutic opportunities for cancer treatment.
    DOI:  https://doi.org/10.1038/s41388-020-01403-y
  4. J Biol Chem. 2020 Jul 31. pii: jbc.REV120.013746. [Epub ahead of print]
    Coggins SA, Mahboubi B, Schinazi RF, Kim B.
      Enzyme kinetic analysis reveals a dynamic relationship between enzymes and their substrates. Overall enzyme activity can be controlled by both protein expression and various cellular regulatory systems. Interestingly, the availability and concentrations of intracellular substrates can constantly change depending on conditions and cell types. Here, we review previously reported enzyme kinetic parameters of cellular and viral DNA and RNA polymerases with respect to cellular levels of their nucleotide substrates. This broad perspective exposes a remarkable co-evolution scenario of DNA polymerase enzyme kinetics with dNTP levels that can vastly change depending on cell proliferation profiles. Similarly, RNA polymerases display much higher Km values than DNA polymerases, possibly due to mM range rNTP concentrations found in cells (compared to µM range dNTP levels). Polymerases are commonly targeted by nucleotide analogue inhibitors for the treatments of various human diseases such as cancers and viral pathogens. Since these inhibitors compete against natural cellular nucleotides, efficacy of each inhibitor can be affected by varying cellular nucleotide levels in their target cells. Overall, both kinetic discrepancy between DNA and RNA polymerases and cellular concentration discrepancy between dNTPs and rNTPs present pharmacological and mechanistic considerations for therapeutic discovery.
    Keywords:  DNA polymerase; RNA polymerase; enzyme inhibitor; enzyme kinetics; nucleoside/nucleotide metabolism
    DOI:  https://doi.org/10.1074/jbc.REV120.013746
  5. Proc Natl Acad Sci U S A. 2020 Jul 27. pii: 202009237. [Epub ahead of print]
    Guo E, Ishii Y, Mueller J, Srivatsan A, Gahman T, Putnam CD, Wang JYJ, Kolodner RD.
      Synthetic lethality strategies for cancer therapy exploit cancer-specific genetic defects to identify targets that are uniquely essential to the survival of tumor cells. Here we show RAD27/FEN1, which encodes flap endonuclease 1 (FEN1), a structure-specific nuclease with roles in DNA replication and repair, and has the greatest number of synthetic lethal interactions with Saccharomyces cerevisiae genome instability genes, is a druggable target for an inhibitor-based approach to kill cancers with defects in homologous recombination (HR). The vulnerability of cancers with HR defects to FEN1 loss was validated by studies showing that small-molecule FEN1 inhibitors and FEN1 small interfering RNAs (siRNAs) selectively killed BRCA1- and BRCA2-defective human cell lines. Furthermore, the differential sensitivity to FEN1 inhibition was recapitulated in mice, where a small-molecule FEN1 inhibitor reduced the growth of tumors established from drug-sensitive but not drug-resistant cancer cell lines. FEN1 inhibition induced a DNA damage response in both sensitive and resistant cell lines; however, sensitive cell lines were unable to recover and replicate DNA even when the inhibitor was removed. Although FEN1 inhibition activated caspase to higher levels in sensitive cells, this apoptotic response occurred in p53-defective cells and cell killing was not blocked by a pan-caspase inhibitor. These results suggest that FEN1 inhibitors have the potential for therapeutically targeting HR-defective cancers such as those resulting from BRCA1 and BRCA2 mutations, and other genetic defects.
    Keywords:  DNA repair; DNA replication; cancer therapy; homologous recombination; synthetic lethality
    DOI:  https://doi.org/10.1073/pnas.2009237117
  6. Blood. 2020 Jul 31. pii: blood.2019003782. [Epub ahead of print]
    Okamoto Y, Abe M, Mu A, Tempaku Y, Rogers CB, Mochizuki AL, Katsuki Y, Kanemaki MT, Takaori-Kondo A, Sobeck AT, Bielinsky AK, Takata M.
      Fanconi anemia (FA) is a hereditary disorder caused by mutations in any one of 22 FA genes. The disease is characterized by hypersensitivity to interstrand crosslink (ICL) inducers such as mitomycin C (MMC). In addition to promoting ICL repair, FA proteins such as RAD51, BRCA2, or FANCD2 protect stalled replication forks from nucleolytic degradation during replication stress, which may have a profound impact on FA pathophysiology. Recent studies showed that expression of the putative DNA/RNA helicase SLFN11 in cancer cells correlates with cell death upon chemotherapeutic treatment. However, the underlying mechanisms of SLFN11-mediated DNA damage sensitivity remain unclear. Since SLFN11 expression is high in hematopoietic stem cells, we hypothesized that SLFN11 depletion might ameliorate the phenotypes of FA cells. Here we report that SLFN11 knockdown in the FA patient-derived FANCD2-deficient PD20 cell line improved cell survival upon treatment with ICL inducers. FANCD2-/-SLFN11-/- HAP1 cells also displayed phenotypic rescue, including reduced levels of MMC-induced chromosome breakage compared to FANCD2-/- cells. Importantly, we found that SLFN11 promotes extensive fork degradation in FANCD2-/- cells. The degradation process is mediated by the nucleases MRE11 or DNA2 and depends on the SLFN11 ATPase activity. This observation was accompanied by an increased RAD51 binding at stalled forks, consistent with the role of RAD51 antagonizing nuclease recruitment and subsequent fork degradation. Suppression of SLFN11 protects nascent DNA tracts even in wild type cells. We conclude that SLFN11 destabilizes stalled replication forks, and this function may contribute to the attrition of hematopoietic stem cells in FA.
    DOI:  https://doi.org/10.1182/blood.2019003782
  7. Nucleic Acids Res. 2020 Jul 30. pii: gkaa631. [Epub ahead of print]
    Sun X, Wang Y, Ji K, Liu Y, Kong Y, Nie S, Li N, Hao J, Xie Y, Xu C, Du L, Liu Q.
      Nuclear factor erythroid 2-related factor 2 (NRF2) is a well-characterized transcription factor that protects cells against oxidative and electrophilic stresses. Emerging evidence has suggested that NRF2 protects cells against DNA damage by mechanisms other than antioxidation, yet the mechanism remains poorly understood. Here, we demonstrate that knockout of NRF2 in cells results in hypersensitivity to ionizing radiation (IR) in the presence or absence of reactive oxygen species (ROS). Under ROS scavenging conditions, induction of DNA double-strand breaks (DSBs) increases the NRF2 protein level and recruits NRF2 to DNA damage sites where it interacts with ATR, resulting in activation of the ATR-CHK1-CDC2 signaling pathway. In turn, this leads to G2 cell cycle arrest and the promotion of homologous recombination repair of DSBs, thereby preserving genome stability. The inhibition of NRF2 by brusatol increased the radiosensitivity of tumor cells in xenografts by perturbing ATR and CHK1 activation. Collectively, our results reveal a novel function of NRF2 as an ATR activator in the regulation of the cellular response to DSBs. This shift in perspective should help furnish a more complete understanding of the function of NRF2 and the DNA damage response.
    DOI:  https://doi.org/10.1093/nar/gkaa631
  8. Cancers (Basel). 2020 Jul 28. pii: E2094. [Epub ahead of print]12(8):
    McAvera RM, Crawford LJ.
      Genomic instability is a hallmark of cancer cells which results in excessive DNA damage. To counteract this, cells have evolved a tightly regulated DNA damage response (DDR) to rapidly sense DNA damage and promote its repair whilst halting cell cycle progression. The DDR functions predominantly within the context of chromatin and requires the action of chromatin-binding proteins to coordinate the appropriate response. TRIM24, TRIM28, TRIM33 and TRIM66 make up the transcriptional intermediary factor 1 (TIF1) family of chromatin-binding proteins, a subfamily of the large tripartite motif (TRIM) family of E3 ligases. All four TIF1 proteins are aberrantly expressed across numerous cancer types, and increasing evidence suggests that TIF1 family members can function to maintain genome stability by mediating chromatin-based responses to DNA damage. This review provides an overview of the TIF1 family in cancer, focusing on their roles in DNA repair, chromatin regulation and cell cycle regulation.
    Keywords:  DNA damage; TRIM24; TRIM28; TRIM33; TRIM66; cancer; genome stability
    DOI:  https://doi.org/10.3390/cancers12082094
  9. Cell Rep. 2020 Jul 28. pii: S2211-1247(20)30955-4. [Epub ahead of print]32(4): 107974
    Zhang F, Yan P, Yu H, Le H, Li Z, Chen J, Liang X, Wang S, Wei W, Liu L, Zhang Y, Ji X, Xie A, Chen W, Han Z, Pu WT, Chen S, Chen Y, Sun K, Ge B, Zhang B.
      Attenuated DNA repair leads to genomic instability and tumorigenesis. BRCA1/BARD1 are the best-known tumor suppressors that promote homology recombination (HR) and arrest cell cycle. However, it remains ambiguous whether and how their E3 ligase activity regulates HR. Here, we demonstrate that upon genotoxic stress, BRCA1 together with BARD1 catalyzes the K48 polyubiquitination on LARP7, a 7SK RNA binding protein known to control RNAPII pausing, and thereby degrades it through the 26S ubiquitin-proteasome pathway. Depleting LARP7 suppresses the expression of CDK1 complex, arrests the cell at the G2/M DNA damage checkpoint, and reduces BRCA2 phosphorylation, which thereby facilitates RAD51 recruitment to damaged DNA to enhance HR. Importantly, LARP7 depletion observed in breast cancer patients leads to chemoradiotherapy resistance both in vitro and in vivo. Altogether, this study unveils a mechanism by which BRCA1/BARD1 control HR and cell cycle, and highlights LARP7 as a potential target for cancer prevention and therapy.
    Keywords:  7SK snRNP; BRCA1; DNA damage response; DNA repair; LARP7; RNAPII pausing; breast cancer; therapy resistance
    DOI:  https://doi.org/10.1016/j.celrep.2020.107974
  10. Nat Genet. 2020 Jul 27.
    Maciejowski J, Chatzipli A, Dananberg A, Chu K, Toufektchan E, Klimczak LJ, Gordenin DA, Campbell PJ, de Lange T.
      Chromothripsis and kataegis are frequently observed in cancer and may arise from telomere crisis, a period of genome instability during tumorigenesis when depletion of the telomere reserve generates unstable dicentric chromosomes1-5. Here we examine the mechanism underlying chromothripsis and kataegis by using an in vitro telomere crisis model. We show that the cytoplasmic exonuclease TREX1, which promotes the resolution of dicentric chromosomes4, plays a prominent role in chromothriptic fragmentation. In the absence of TREX1, the genome alterations induced by telomere crisis primarily involve breakage-fusion-bridge cycles and simple genome rearrangements rather than chromothripsis. Furthermore, we show that the kataegis observed at chromothriptic breakpoints is the consequence of cytosine deamination by APOBEC3B. These data reveal that chromothripsis and kataegis arise from a combination of nucleolytic processing by TREX1 and cytosine editing by APOBEC3B.
    DOI:  https://doi.org/10.1038/s41588-020-0667-5
  11. C R Biol. 2020 Jun 05. 343(1): 9-21
    Coquel F, Silva MJ, Técher H, Zadorozhny K, Sharma S, Nieminuszczy J, Mettling C, Dardillac E, Barthe A, Schmitz AL, Promonet A, Cribier A, Sarrazin A, Niedzwiedz W, Lopez B, Costanzo V, Krejci L, Chabes A, Benkirane M, Lin YL, Pasero P.
      DNA replication is an extremely complex process, involving thousands of replication forks progressing along chromosomes. These forks are frequently slowed down or stopped by various obstacles, such as secondary DNA structures, chromatin-acting proteins or a lack of nucleotides. This slowing down, known as replicative stress, plays a central role in tumour development. Complex processes, which are not yet fully understood, are set up to respond to this stress. Certain nucleases, such as MRE11 and DNA2, degrade the neo-replicated DNA at the level of blocked forks, allowing the replication to restart. The interferon pathway is a defense mechanism against pathogens that detects the presence of foreign nucleic acids in the cytoplasm and activates the innate immune response. DNA fragments resulting from genomic DNA metabolism (repair, retrotransposition) can diffuse into the cytoplasm and activate this pathway. A pathological manifestation of this process is the Aicardi-Goutières syndrome, a rare disease characterized by chronic inflammation leading to neurodegenerative and developmental problems. In this encephalopathy, it has been suggested that DNA replication may generate cytosolic DNA fragments, but the mechanisms involved have not been characterized. SAMHD1 is frequently mutated in the Aicardi-Goutières syndrome as well as in some cancers, but its role in the etiology of these diseases was largely unknown. We show that cytosolic DNA accumulates in SAMHD1-deficient cells, particularly in the presence of replicative stress, activating the interferon response. SAMHD1 is important for DNA replication under normal conditions and for the processing of stopped forks, independent of its dNTPase activity. In addition, SAMHD1 stimulates the exonuclease activity of MRE11 in vitro. When SAMHD1 is absent, degradation of neosynthesized DNA is inhibited, which prevents activation of the replication checkpoint and leads to failure to restart the replication forks. Resection of the replication forks is performed by an alternative mechanism which releases DNA fragments into the cytosol, activating the interferon response. The results obtained show, for the first time, a direct link between the response to replication stress and the production of interferons. These results have important implications for our understanding of the Aicardi-Goutières syndrome and cancers related to SAMHD1. For example, we have shown that MRE11 and RECQ1 are responsible for the production of DNA fragments that trigger the inflammatory response in cells deficient for SAMHD1. We can therefore imagine that blocking the activity of these enzymes could decrease the production of DNA fragments and, ultimately, the activation of innate immunity in these cells. In addition, the interferon pathway plays an essential role in the therapeutic efficacy of irradiation and certain chemotherapeutic agents such as oxaliplatin. Modulating this response could therefore be of much wider interest in anti-tumour therapy.
    DOI:  https://doi.org/10.5802/crbiol.10
  12. Genes Chromosomes Cancer. 2020 Jul 31.
    Kolb T, Khalid U, Simović M, Ratnaparkhe M, Wong J, Jauch A, Schmezer P, Rode A, Sebban S, Haag D, Hergt M, Devens F, Buganim Y, Zapatka M, Lichter P, Ernst A.
      In vitro assays for clustered DNA lesions will facilitate the analysis of the mechanisms underlying complex genome rearrangements such as chromothripsis, including the recruitment of repair factors to sites of DNA double-strand breaks. We present a novel method generating localized DNA double-strand breaks using UV-irradiation with photomasks. The size of the damage foci and the spacing between lesions are fully adjustable, making the assay suitable for different cell types and targeted areas. We validated this set-up with genomically stable epithelial cells, normal fibroblasts, pluripotent stem cells and patient-derived primary cultures. Our method does not require a specialized device such as a laser, making it accessible to a broad range of users. Sensitization by BrdU incorporation is not required, which enables analyzing the DNA damage response in post-mitotic cells. Irradiated cells can be cultivated further, followed by time-lapse imaging or used for downstream biochemical analyses, thanks to the high-throughput of the system. Importantly, we showed genome rearrangements in the irradiated cells, providing a proof of principle for the induction of structural variants by localized DNA lesions. This article is protected by copyright. All rights reserved.
    Keywords:  Chromothripsis; DNA-damage; genomic rearrangements
    DOI:  https://doi.org/10.1002/gcc.22890
  13. Cancers (Basel). 2020 Jul 28. pii: E2098. [Epub ahead of print]12(8):
    Reuvers TGA, Kanaar R, Nonnekens J.
      DNA damage-inducing therapies are of tremendous value for cancer treatment and function by the direct or indirect formation of DNA lesions and subsequent inhibition of cellular proliferation. Of central importance in the cellular response to therapy-induced DNA damage is the DNA damage response (DDR), a protein network guiding both DNA damage repair and the induction of cancer-eradicating mechanisms such as apoptosis. A detailed understanding of DNA damage induction and the DDR has greatly improved our knowledge of the classical DNA damage-inducing therapies, radiotherapy and cytotoxic chemotherapy, and has paved the way for rational improvement of these treatments. Moreover, compounds targeting specific DDR proteins, selectively impairing DNA damage repair in cancer cells, form a promising novel therapy class that is now entering the clinic. In this review, we give an overview of the current state and ongoing developments, and discuss potential avenues for improvement for DNA damage-inducing therapies, with a central focus on the role of the DDR in therapy response, toxicity and resistance. Furthermore, we describe the relevance of using combination regimens containing DNA damage-inducing therapies and how they can be utilized to potentiate other anticancer strategies such as immunotherapy.
    Keywords:  DDR modulators; DNA damage response; DNA damage-inducing therapies; DNA repair; cancer therapy; combination therapies; cytotoxic chemotherapy; radiotherapy
    DOI:  https://doi.org/10.3390/cancers12082098
  14. Cell Cycle. 2020 Jul 30. 1-11
    Smits VAJ, Alonso-de Vega I, Warmerdam DO.
      Chromatin plays a pivotal role in regulating the DNA damage response and during DNA double-strand break repair. Upon the generation of DNA breaks, the chromatin structure is altered by post-translational modifications of histones and chromatin remodeling. How the chromatin structure, and the epigenetic information that it carries, is reestablished after the completion of DNA break repair remains unclear though. Also, how these processes influence recovery of the cell cycle remains poorly understood. We recently performed a reverse genetic screen for novel chromatin regulators that control checkpoint recovery after DNA damage. Here we discuss the implications of PHD finger protein 6 (PHF6) and additional candidates from the NuA4 ATPase-dependent chromatin-remodeling complex and the Cohesin complex, required for sister chromatid cohesion, in DNA repair and checkpoint recovery in more detail. In addition, the potential role of this novel function of PHF6 in cancer development and treatment is reviewed.
    Keywords:  DNA damage; DSB repair; NHEJ; PHF6; checkpoint recovery; chromatin regulators
    DOI:  https://doi.org/10.1080/15384101.2020.1796037
  15. DNA Repair (Amst). 2020 Jul 18. pii: S1568-7864(20)30184-1. [Epub ahead of print]95 102935
    Powell BV, Bacurio JHT, Basu AK.
      6-Nitrochrysene (6-NC) is a potent mutagen in bacteria and carcinogenic in animals. It is the most potent carcinogen ever tested in newborn mouse assay. DNA lesions resulting from 6-NC modification are likely to induce mutations if they are not removed by cellular defense pathways prior to DNA replication. Earlier studies showed that 6-NC-derived C8-2'-deoxyadenosine adduct, N-(dA-8-yl)-6-AC, is very slowly repaired in human cells. In this study, we have investigated replication of N-(dA-8-yl)-6-AC in human embryonic kidney (HEK 293T) cells and the roles of translesion synthesis (TLS) DNA polymerases in bypassing it. Replication of a plasmid containing a single site-specific N-(dA-8-yl)-6-AC adduct in HEK 293 T cells showed that human DNA polymerase (hPol) η and hPol κ played important roles in bypassing the adduct, since TLS efficiency was reduced to 26 % in the absence of these two polymerases compared to 83 % in polymerase-competent HEK 293T cells. The progeny from HEK 293T cells provided 12.7 % mutants predominantly containing A→T transversions. Mutation frequency (MF) was increased to 17.8 % in hPol η-deficient cells, whereas it was decreased to 3.3 % and 3.9 % when the adduct containing plasmid was replicated in hPol κ- and hPol ζ-deficient cells, respectively. The greatest reduction in MF by more than 90 % (to MF 1.2 %) was observed in hPol ζ-knockout cells in which hPol κ was knocked down. Taken together, these results suggest that hPol κ and hPol ζ are involved in the error-prone TLS of N-(dA-8-yl)-6-AC, while hPol η performs error-free bypass.
    Keywords:  6-aminochrysene; Carcinogen-DNA adduct; Cellular replication; Mutagenicity; Nitroaromatic compounds; Translesion synthesis
    DOI:  https://doi.org/10.1016/j.dnarep.2020.102935
  16. Nat Commun. 2020 Jul 31. 11(1): 3839
    Keil JM, Doyle DZ, Qalieh A, Lam MM, Funk OH, Qalieh Y, Shi L, Mohan N, Sorel A, Kwan KY.
      Chromatin regulates spatiotemporal gene expression during neurodevelopment, but it also mediates DNA damage repair essential to proliferating neural progenitor cells (NPCs). Here, we uncover molecularly dissociable roles for nucleosome remodeler Ino80 in chromatin-mediated transcriptional regulation and genome maintenance in corticogenesis. We find that conditional Ino80 deletion from cortical NPCs impairs DNA double-strand break (DSB) repair, triggering p53-dependent apoptosis and microcephaly. Using an in vivo DSB repair pathway assay, we find that Ino80 is selectively required for homologous recombination (HR) DNA repair, which is mechanistically distinct from Ino80 function in YY1-associated transcription. Unexpectedly, sensitivity to loss of Ino80-mediated HR is dependent on NPC division mode: Ino80 deletion leads to unrepaired DNA breaks and apoptosis in symmetric NPC-NPC divisions, but not in asymmetric neurogenic divisions. This division mode dependence is phenocopied following conditional deletion of HR gene Brca2. Thus, distinct modes of NPC division have divergent requirements for Ino80-dependent HR DNA repair.
    DOI:  https://doi.org/10.1038/s41467-020-17551-4
  17. Essays Biochem. 2020 Jul 29. pii: EBC20200001. [Epub ahead of print]
    Li L, Tan W, Deans AJ.
      The Fanconi anemia (FA) pathway coordinates a faithful repair mechanism for DNA damage that blocks DNA replication, such as interstrand cross-links. A key step in the FA pathway is the conjugation of ubiquitin on to FANCD2 and FANCI, which is facilitated by a large E3 ubiquitin ligase complex called the FA core complex. Mutations in FANCD2, FANCI or FA core complex components cause the FA bone marrow failure syndrome. Despite the importance of these proteins to DNA repair and human disease, our molecular understanding of the FA pathway has been limited due to a deficit in structural studies. With the recent development in cryo-electron microscopy (EM), significant advances have been made in structural characterization of these proteins in the last 6 months. These structures, combined with new biochemical studies, now provide a more detailed understanding of how FANCD2 and FANCI are monoubiquitinated and how DNA repair may occur. In this review, we summarize these recent advances in the structural and molecular understanding of these key components in the FA pathway, compare the activation steps of FANCD2 and FANCI monoubiquitination and suggest molecular steps that are likely to be involved in regulating its activity.
    Keywords:  DNA synthesis and repair; Fanconi anemia; biochemistry; enzyme activity; protein structure; ubiquitin ligases
    DOI:  https://doi.org/10.1042/EBC20200001
  18. Environ Mol Mutagen. 2020 Jul 29.
    Pereira C, Smolka MB, Weiss RS, Brieño-Enríquez MA.
      In germ cells undergoing meiosis, the induction of double strand breaks (DSB) is required for the generation of haploid gametes. Defects in the formation, detection or recombinational repair of DSB often result in defective chromosome segregation and aneuploidies. Central to the ability of meiotic cells to properly respond to DSBs are DNA damage response (DDR) pathways mediated by DNA damage sensor kinases. DDR signaling coordinates an extensive network of DDR effectors to induce cell cycle arrest and DNA repair, or trigger apoptosis if the damage is extensive. Despite their importance, the functions of DDR kinases and effector proteins during meiosis remain poorly understood and can often be distinct from their known mitotic roles. A key DDR kinase during meiosis is Ataxia Telangiectasia and Rad3-related (ATR). ATR mediates key signaling events that control DSB repair, cell cycle progression, and meiotic silencing. These meiotic functions of ATR depend on upstream scaffolds and regulators, including the 9-1-1 complex and TOPBP1, and converge on many downstream effectors such as the checkpoint kinase CHK1. Here, we review the meiotic functions of the 9-1-1/TOPBP1/ATR/CHK1 signaling pathway during mammalian meiosis. This article is protected by copyright. All rights reserved.
    Keywords:  9-1-1 Complex; ATR; CHK1; DNA Damage Signalling; Double-stranded DNA Break; Homologous Recombination; Meiosis; Synapsis; TOPBP1
    DOI:  https://doi.org/10.1002/em.22401
  19. Biology (Basel). 2020 Jul 23. pii: E187. [Epub ahead of print]9(8):
    Chen PH, Tseng WH, Chi JT.
      Ferroptosis is a novel form of iron-dependent cell death characterized by lipid peroxidation. While the importance and disease relevance of ferroptosis are gaining recognition, much remains unknown about its interaction with other biological processes and pathways. Recently, several studies have identified intricate and complicated interplay between ferroptosis, ionizing radiation (IR), ATM (ataxia-telangiectasia mutated)/ATR (ATM and Rad3-related), and tumor suppressor p53, which signifies the participation of the DNA damage response (DDR) in iron-related cell death. DDR is an evolutionarily conserved response triggered by various DNA insults to attenuate proliferation, enable DNA repairs, and dispose of cells with damaged DNA to maintain genome integrity. Deficiency in proper DDR in many genetic disorders or tumors also highlights the importance of this pathway. In this review, we will focus on the biological crosstalk between DDR and ferroptosis, which is mediated mostly via noncanonical mechanisms. For clinical applications, we also discuss the potential of combining ionizing radiation and ferroptosis-inducers for synergistic effects. At last, various ATM/ATR inhibitors under clinical development may protect ferroptosis and treat many ferroptosis-related diseases to prevent cell death, delay disease progression, and improve clinical outcomes.
    Keywords:  ATM; ATR; DNA damage; MDM2; MDMX; ferroptosis; p53
    DOI:  https://doi.org/10.3390/biology9080187
  20. Cell Death Dis. 2020 Jul 30. 11(7): 602
    Zhang J, Jing L, Tan S, Zeng EM, Lin Y, He L, Hu Z, Liu J, Guo Z.
      Glioblastoma multiforme (GBM) is the most malignant primary brain tumor and has the highest mortality rate among cancers and high resistance to radiation and cytotoxic chemotherapy. Although some targeted therapies can partially inhibit oncogenic mutation-driven proliferation of GBM cells, therapies harnessing synthetic lethality are 'coincidental' treatments with high effectiveness in cancers with gene mutations, such as GBM, which frequently exhibits DNA-PKcs mutation. By implementing a highly efficient high-throughput screening (HTS) platform using an in-house-constructed genome-wide human microRNA inhibitor library, we demonstrated that miR-1193 inhibition sensitized GBM tumor cells with DNA-PKcs deficiency. Furthermore, we found that miR-1193 directly targets YY1AP1, leading to subsequent inhibition of FEN1, an important factor in DNA damage repair. Inhibition of miR-1193 resulted in accumulation of DNA double-strand breaks and thus increased genomic instability. RPA-coated ssDNA structures enhanced ATR checkpoint kinase activity, subsequently activating the CHK1/p53/apoptosis axis. These data provide a preclinical theory for the application of miR-1193 inhibition as a potential synthetic lethal approach targeting GBM cancer cells with DNA-PKcs deficiency.
    DOI:  https://doi.org/10.1038/s41419-020-02812-3
  21. Cancers (Basel). 2020 Jul 25. pii: E2054. [Epub ahead of print]12(8):
    Lee EK, Matulonis UA.
      The use of PARP inhibitors (PARPi) is growing widely as FDA approvals have shifted its use from the recurrence setting to the frontline setting. In parallel, the population developing PARPi resistance is increasing. Here we review the role of PARP, DNA damage repair, and synthetic lethality. We discuss mechanisms of resistance to PARP inhibition and how this informs on novel combinations to re-sensitize cancer cells to PARPi.
    Keywords:  BRCA; DNA damage repair; PARP inhibitor; PARP inhibitor resistance; homologous recombination; ovarian cancer; replication fork
    DOI:  https://doi.org/10.3390/cancers12082054
  22. Sci Rep. 2020 Jul 27. 10(1): 12506
    Min A, Kim K, Jeong K, Choi S, Kim S, Suh KJ, Lee KH, Kim S, Im SA.
      Breast cancer (BC) in patients with germline mutations of BRCA1/BRCA2 are associated with benefit from drugs targeting DNA damage response (DDR), but they account for only 5-7% of overall breast cancer. To define the characteristics of these tumors and also to identify tumors without BRCA mutation but with homologous recombination deficiency (HRD) is clinically relevant. To define characteristic features of HRD tumors and analyze the correlations between BRCA1/BRCA2 and BC subtypes, we analyzed 981 breast tumors from the TCGA database using the signature analyzer. The BRCA signature was strongly associated with the HRD score top 10% (score ≥ 57) population. This population showed a high level of mutations in DDR genes, including BRCA1/BRCA2. HRD tumors were associated with high expression levels of BARD1 and BRIP1. Besides, BRCA1/2 mutations were dominantly observed in basal and luminal subtypes, respectively. A comparison of HRD features in BC revealed that BRCA1 exerts a stronger influence inducing HRD features than BRCA2 does. It reveals genetic differences between BRCA1 and BRCA2 and provides a basis for the identification of HRD and other BRCA-associated tumors.
    DOI:  https://doi.org/10.1038/s41598-020-68176-y
  23. mBio. 2020 Jul 28. pii: e01017-20. [Epub ahead of print]11(4):
    Beaufay F, Quarles E, Franz A, Katamanin O, Wholey WY, Jakob U.
      Maintaining cellular iron homeostasis is critical for organismal survival. Whereas iron depletion negatively affects the many metabolic pathways that depend on the activity of iron-containing enzymes, any excess of iron can cause the rapid formation of highly toxic reactive oxygen species (ROS) through Fenton chemistry. Although several cellular iron chelators have been identified, little is known about if and how organisms can prevent the Fenton reaction. By studying the effects of cisplatin, a commonly used anticancer drug and effective antimicrobial, we discovered that cisplatin elicits severe iron stress and oxidative DNA damage in bacteria. We found that both of these effects are successfully prevented by polyphosphate (polyP), an abundant polymer consisting solely of covalently linked inorganic phosphates. Subsequent in vitro and in vivo studies revealed that polyP provides a crucial iron reservoir under nonstress conditions and effectively complexes free iron and blocks ROS formation during iron stress. These results demonstrate that polyP, a universally conserved biomolecule, plays a hitherto unrecognized role as an iron chelator and an inhibitor of the Fenton reaction.IMPORTANCE How do organisms deal with free iron? On the one hand, iron is an essential metal that plays crucial structural and functional roles in many organisms. On the other hand, free iron is extremely toxic, particularly under aerobic conditions, where iron rapidly undergoes the Fenton reaction and produces highly reactive hydroxyl radicals. Our study now demonstrates that we have discovered one of the first physiologically relevant nonproteinaceous iron chelators and Fenton inhibitors. We found that polyphosphate, a highly conserved and ubiquitous inorganic polyanion, chelates iron and, through its multivalency, prevents the interaction of iron with peroxide and therefore the formation of hydroxyl radicals. We show that polyP provides a crucial iron reservoir for metalloproteins under nonstress conditions and effectively chelates free iron during iron stress. Importantly, polyP is present in all cells and organisms and hence is likely to take on this crucial function in both prokaryotic and eukaryotic cells.
    Keywords:  chelator; cisplatin; iron regulation; oxidative damage; polyphosphate; stress response
    DOI:  https://doi.org/10.1128/mBio.01017-20
  24. Biochem J. 2020 Jul 31. 477(14): 2655-2677
    Fan L, Bi T, Wang L, Xiao W.
      DNA-damage tolerance (DDT) is employed by eukaryotic cells to bypass replication-blocking lesions induced by DNA-damaging agents. In budding yeast Saccharomyces cerevisiae, DDT is mediated by RAD6 epistatic group genes and the central event for DDT is sequential ubiquitination of proliferating cell nuclear antigen (PCNA), a DNA clamp required for replication and DNA repair. DDT consists of two parallel pathways: error-prone DDT is mediated by PCNA monoubiquitination, which recruits translesion synthesis DNA polymerases to bypass lesions with decreased fidelity; and error-free DDT is mediated by K63-linked polyubiquitination of PCNA at the same residue of monoubiquitination, which facilitates homologous recombination-mediated template switch. Interestingly, the same PCNA residue is also subjected to sumoylation, which leads to inhibition of unwanted recombination at replication forks. All three types of PCNA posttranslational modifications require dedicated conjugating and ligation enzymes, and these enzymes are highly conserved in eukaryotes, from yeast to human.
    Keywords:   schizosaccharomyces pombemyces ; DNA-damage tolerance; PCNA; sumoylation; ubiquitins
    DOI:  https://doi.org/10.1042/BCJ20190579
  25. Cell Death Dis. 2020 Jul 02. 11(7): 581
    Makino E, Fröhlich LM, Sinnberg T, Kosnopfel C, Sauer B, Garbe C, Schittek B.
      Rad51 is an essential factor of the homologous recombination DNA repair pathway and therefore plays an important role in maintaining genomic stability. We show that RAD51 and other homologous recombination repair genes are overexpressed in metastatic melanoma cell lines and in melanoma patient samples, which correlates with reduced survival of melanoma patients. In addition, Rad51 expression in melanoma cells was regulated on a transcriptional level by the MAPK signaling pathway with Elk1 as the main downstream transcriptional effector. Most strikingly, melanoma cells which developed resistance towards MAPK inhibitors could be efficiently targeted by Rad51 inhibitors similar to their sensitive counterparts, leading to DNA damage, G2/M arrest and apoptosis. Furthermore, the treatment of MAPK inhibitor resistant cells with Rad51 inhibitors enhances the susceptibility of these cells for MAPK inhibitor treatment in vitro and in vivo. These data indicate that Rad51 plays a critical role in the survival of metastatic melanoma cells and is a promising target for the therapy of melanoma irrespective of its MAPK inhibitor resistance status.
    DOI:  https://doi.org/10.1038/s41419-020-2702-y
  26. Nucleic Acids Res. 2020 Jul 31. pii: gkaa636. [Epub ahead of print]
    Chang HY, Lee CY, Lu CH, Lee W, Yang HL, Yeh HY, Li HW, Chi P.
      Microcephalin 1 (MCPH1) was identified from genetic mutations in patients with primary autosomal recessive microcephaly. In response to DNA double-strand breaks (DSBs), MCPH1 forms damage-induced foci and recruits BRCA2-RAD51 complex, a key component of the DSB repair machinery for homologous recombination (HR), to damage sites. Accordingly, the efficiency of HR is significantly attenuated upon depletion of MCPH1. The biochemical characteristics of MCPH1 and its functional interaction with the HR machinery had remained unclear due to lack of highly purified MCPH1 recombinant protein for functional study. Here, we established a mammalian expression system to express and purify MCPH1 protein. We show that MCPH1 is a bona fide DNA-binding protein and provide direct biochemical analysis of this MCPH family protein. Furthermore, we reveal that MCPH1 directly interacts with RAD51 at multiple contact points, providing evidence for how MCPH1 physically engages with the HR machinery. Importantly, we demonstrate that MCPH1 enhances the stability of RAD51 on single-strand DNA, a prerequisite step for RAD51-mediated recombination. Single-molecule tethered particle motion analysis showed a ∼2-fold increase in the lifetime of RAD51-ssDNA filaments in the presence of MCPH1. Thus, our study demonstrates direct crosstalk between microcephaly protein MCPH1 and the recombination component RAD51 for DSB repair.
    DOI:  https://doi.org/10.1093/nar/gkaa636
  27. Aging Cell. 2020 Jul 28. e13184
    White RR, Maslov AY, Lee M, Wilner SE, Levy M, Vijg J.
      Genomic instability is one of the hallmarks of aging, and both DNA damage and mutations have been found to accumulate with age in different species. Certain gene families, such as sirtuins and the FoxO family of transcription factors, have been shown to play a role in lifespan extension. However, the mechanism(s) underlying the increased longevity associated with these genes remains largely unknown and may involve the regulation of responses to cellular stressors, such as DNA damage. Here, we report that FOXO3a reduces genomic instability in cultured mouse embryonic fibroblasts (MEFs) treated with agents that induce DNA double-strand breaks (DSBs), that is, clastogens. We show that DSB treatment of both primary human and mouse fibroblasts upregulates FOXO3a expression. FOXO3a ablation in MEFs harboring the mutational reporter gene lacZ resulted in an increase in genome rearrangements after bleomycin treatment; conversely, overexpression of human FOXO3a was found to suppress mutation accumulation in response to bleomycin. We also show that overexpression of FOXO3a in human primary fibroblasts decreases DSB-induced γH2AX foci. Knocking out FOXO3a in mES cells increased the frequency of homologous recombination and non-homologous end-joining events. These results provide the first direct evidence that FOXO3a plays a role in suppressing genome instability, possibly by suppressing genome rearrangements.
    Keywords:  DNA damage; DSB repair; FOXO3a; aging; bleomycin; mutations
    DOI:  https://doi.org/10.1111/acel.13184
  28. Eur J Med Chem. 2020 Jul 15. pii: S0223-5234(20)30542-0. [Epub ahead of print]203 112570
    Zhao Y, Zhang LX, Jiang T, Long J, Ma ZY, Lu AP, Cheng Y, Cao DS.
      Poly(ADP-ribose) Polymerase 1 (PARP1), one of the most investigated 18 membered PARP family enzymes, is involved in a variety of cellular functions including DNA damage repair, gene transcription and cell apoptosis. PARP1 can form a PARP1(ADP-ribose) polymers, then bind to the DNA damage gap to recruit DNA repair proteins, and repair the break to maintain genomic stability. PARP1 is highly expressed in tumor cells, so the inhibition of PARP1 can block DNA repair, promote tumor cell apoptosis, and exert antitumor activity. To date, four PARP1 inhibitors namely olaparib, rucaparib, niraparib and talazoparib, have been approved by Food and Drug Administration (FDA) for treating ovarian cancer and breast cancer with BRCA1/2 mutation. These drugs have showed super advantages over conventional chemotherapeutic drugs with low hematological toxicity and slowly developed drug resistance. In this article, we summarize and analyze the structure features of PARP1, the biological functions and antitumor mechanisms of PARP1 inhibitors. Importantly, we suggest that establishing a new structure-activity relationship of developed PARP1 inhibitors via substructural searching and the matched molecular pair analysis would accelerate the process in finding more potent and safer PARP1 inhibitors.
    Keywords:  Cancer; DNA damage repair; Matched molecular pair; PARP1; PARP1 inhibitors; Structure-activity relationship
    DOI:  https://doi.org/10.1016/j.ejmech.2020.112570
  29. Mutat Res. 2020 Jul 17. pii: S0027-5107(20)30048-8. [Epub ahead of print]821 111715
    Blokhina YP, Buchwalter A.
      The genes that code for ribosomal RNA are present in hundreds of tandemly arrayed copies in the human genome. Ribosomal DNA repeats transcribe vast amounts of ribosomal RNA in order to meet the cell's relentless demand for ribosome production. Intrinsic features of ribosomal DNA repeats render them uniquely vulnerable to DNA damage. Sensing and repairing damage to ribosomal DNA involves dramatic spatial reorganization of the nucleolus, the phase-separated nuclear subdomain where ribosomes are made. We highlight recent advances in detecting the incidence of DNA damage and defining the mechanisms of DNA repair on these essential genes.
    Keywords:  DNA damage response; DNA repair; Nucleolus; Repeat arrays; Ribosomal DNA
    DOI:  https://doi.org/10.1016/j.mrfmmm.2020.111715
  30. Nucleosides Nucleotides Nucleic Acids. 2020 Jul 30. 1-12
    Jaramillo AC, Bergman AM, Comijn EM, Jansen G, Kaspers GJL, Cloos J, Peters GJ.
      Dexamethasone (DEX) is often used in the initial treatment of leukemia. Earlier we demonstrated that DEX decreased the activity of deoxycytidine kinase (dCK) which is essential for the activation of cytarabine (ara-C). Therefore we investigated the effect of DEX on the in vivo sensitivity of acute myeloid leukemia (AML) to ara-C and another deoxycytidine analog, gemcitabine, in the Brown Norway Myeloid Leukemia (BNML) rat model for AML, and its ara-C resistant variant B-araC, in relation to the effects on dCK activity. The antileukemic effect was evaluated as survival of the rats, while dCK activity was measured in leukemic spleen (completely consisting of BNML cells) with liver as representative normal tissue, 24 hr after treatment with ara-C or DEX with radioactive deoxycytidine (CdR) as a substrate. Treatment with ara-C increased life-span of BNML by 200%, which was not affected by DEX. Gemcitabine was ineffective. In the liver of BNML bearing rats DEX decreased dCK activity 33%, while ara-C increased dCK activity slightly (to 129%), but in the combination of ara-C/DEX dCK activity was also decreased. In the livers of Bara-C bearing rats dCK was 2.7-fold higher compared to BNML rats, which was increased 179% in the gemcitabine-DEX treated rats. In BNML leukemic spleens DEX decreased dCK activity 41% and gem/dex 46%, but ara-C increased dCK activity to 123%, but in the combination this effect was neutralized. In Bara-C spleens only ara-C/dex decreased dCK activity (32%). In conclusion; in an AML rat model DEX did not affect the antileukemic effect of ara-C, nor the dCK activity.
    Keywords:  Brown Norway Myeloid Leukemia; Cytarabine; acute myeloid leukemia; deoxycytidine kinase; gemcitabine
    DOI:  https://doi.org/10.1080/15257770.2020.1780441
  31. Nucleic Acids Res. 2020 Jul 27. pii: gkaa614. [Epub ahead of print]
    Meek K.
      As its name implies, the DNA dependent protein kinase (DNA-PK) requires DNA double-stranded ends for enzymatic activation. Here, I demonstrate that hairpinned DNA ends are ineffective for activating the kinase toward many of its well-studied substrates (p53, XRCC4, XLF, HSP90). However, hairpinned DNA ends robustly stimulate certain DNA-PK autophosphorylations. Specifically, autophosphorylation sites within the ABCDE cluster are robustly phosphorylated when DNA-PK is activated by hairpinned DNA ends. Of note, phosphorylation of the ABCDE sites is requisite for activation of the Artemis nuclease that associates with DNA-PK to mediate hairpin opening. This finding suggests a multi-step mechanism of kinase activation. Finally, I find that all non-homologous end joining (NHEJ) defective cells (whether deficient in components of the DNA-PK complex or components of the ligase complex) are similarly deficient in joining DNA double-stranded breaks (DSBs) with hairpinned termini.
    DOI:  https://doi.org/10.1093/nar/gkaa614
  32. Nucleosides Nucleotides Nucleic Acids. 2020 Jul 30. 1-10
    Peters GJ, Leyva A, Schwartsmann G.
      HL60 myeloid leukemia cells are extensively used as a differentiation model. We investigated a variant of HL60 which is resistant to differentiation induction (HL60-R) by standard differentiation inducers such as retinoic acid and dimethylsulfoxide (DMSO). To find an explanation for this resistance, we examined nucleotide (NTP) and deoxynucleotide (dNTP) pools in HL60-R and its parent cell line, sensitive to differentiation, HL60-S. We also explored whether these differences led to a difference in sensitivity to various antimetabolites. Drug sensitivity was measured with the tetrazolium (MTT) assay, while nucleotides were measured with anion-exchange HPLC. HL60-R cells were between 2- and 5-fold resistant to the antimetabolites 5-fluorouracil, Brequinar, hydroxyurea and N-(phosphonacetyl)-L-aspartate (PALA), but more sensitive to aza-2'-deoxycytidine (DAC), cytarabine and thymidine (5- to 10-fold). The NTP pools in both HL60 variants showed a normal pattern with ATP being the highest (2530-2876 pmol/106 cells) and CTP being lowest. However, UTP pools were 2-fold higher in the HL60-S cells (p < .01), while CTP and GTP pools were 30% higher (p < .01) compared to HL60-R cells. For the dNTP pools, larger differences were observed, with dATP (50 pmol/106 cells) being highest in HL60-R cells, but dATP was 4-fold lower in HL60-S cells. In HL-60-R, the triple combination retinoic acid, DMSO and DAC increased all NTPs almost 2-fold in contrast to HL60-S. Uridine increased UTP (1.4-fold), CTP (2-fold) and dCTP (1.4.-fold) pools in both cell lines, but thymidine increased only dTTP pools (4- to 7-fold), with a depletion of dCTP. PALA decreased UTP and CTP in both cell lines, but increased ATP (only in HL60-R). Hydroxyurea decreased dNTP especially in HL60-S cells. In conclusion, the pronounced differences in NTP and dNTP pools between HL60-S and HL60-R possibly play a role in the induction of differentiation and drug sensitivity.
    Keywords:  HL60 cells; deoxyribonucleotide pools; differentiation; pyrimidine antagonists; ribonucleotide pools
    DOI:  https://doi.org/10.1080/15257770.2020.1782933
  33. Elife. 2020 Jul 29. pii: e56428. [Epub ahead of print]9
    Paudel BP, Moye AL, Abou Assi H, El-Khoury R, Cohen S, Holien JK, Birrento ML, Samosorn S, Intharapichai K, Tomlinson CG, Teulade-Fichou MP, González C, Beck JL, Damha MJ, van Oijen AM, Bryan TM.
      Telomeric G-quadruplexes (G4) were long believed to form a protective structure at telomeres, preventing their extension by the ribonucleoprotein telomerase. Contrary to this belief, we have previously demonstrated that parallel-stranded conformations of telomeric G4 can be extended by human and ciliate telomerase. However, a mechanistic understanding of the interaction of telomerase with structured DNA remained elusive. Here, we use single-molecule fluorescence resonance energy transfer (smFRET) microscopy and bulk-phase enzymology to propose a mechanism for the resolution and extension of parallel G4 by telomerase. Binding is initiated by the RNA template of telomerase interacting with the G-quadruplex; nucleotide addition then proceeds to the end of the RNA template. It is only through the large conformational change of translocation following synthesis that the G-quadruplex structure is completely unfolded to a linear product. Surprisingly, parallel G4 stabilization with either small molecule ligands or by chemical modification does not always inhibit G4 unfolding and extension by telomerase. These data reveal that telomerase is a parallel G-quadruplex resolvase.
    Keywords:  biochemistry; chemical biology; chromosomes; gene expression; human
    DOI:  https://doi.org/10.7554/eLife.56428
  34. Proc Natl Acad Sci U S A. 2020 Jul 29. pii: 201922284. [Epub ahead of print]
    Sahu S, Wang Z, Jiao X, Gu C, Jork N, Wittwer C, Li X, Hostachy S, Fiedler D, Wang H, Jessen HJ, Kiledjian M, Shears SB.
      Regulation of enzymatic 5' decapping of messenger RNA (mRNA), which normally commits transcripts to their destruction, has the capacity to dynamically reshape the transcriptome. For example, protection from 5' decapping promotes accumulation of mRNAs into processing (P) bodies-membraneless, biomolecular condensates. Such compartmentalization of mRNAs temporarily removes them from the translatable pool; these repressed transcripts are stabilized and stored until P-body dissolution permits transcript reentry into the cytosol. Here, we describe regulation of mRNA stability and P-body dynamics by the inositol pyrophosphate signaling molecule 5-InsP7 (5-diphosphoinositol pentakisphosphate). First, we demonstrate 5-InsP7 inhibits decapping by recombinant NUDT3 (Nudix [nucleoside diphosphate linked moiety X]-type hydrolase 3) in vitro. Next, in intact HEK293 and HCT116 cells, we monitored the stability of a cadre of NUDT3 mRNA substrates following CRISPR-Cas9 knockout of PPIP5Ks (diphosphoinositol pentakisphosphate 5-kinases type 1 and 2, i.e., PPIP5K KO), which elevates cellular 5-InsP7 levels by two- to threefold (i.e., within the physiological rheostatic range). The PPIP5K KO cells exhibited elevated levels of NUDT3 mRNA substrates and increased P-body abundance. Pharmacological and genetic attenuation of 5-InsP7 synthesis in the KO background reverted both NUDT3 mRNA substrate levels and P-body counts to those of wild-type cells. Furthermore, liposomal delivery of a metabolically resistant 5-InsP7 analog into wild-type cells elevated levels of NUDT3 mRNA substrates and raised P-body abundance. In the context that cellular 5-InsP7 levels normally fluctuate in response to changes in the bioenergetic environment, regulation of mRNA structure by this inositol pyrophosphate represents an epitranscriptomic control process. The associated impact on P-body dynamics has relevance to regulation of stem cell differentiation, stress responses, and, potentially, amelioration of neurodegenerative diseases and aging.
    Keywords:  P bodies; cellular homeostasis; inositol; signaling
    DOI:  https://doi.org/10.1073/pnas.1922284117
  35. ACS Omega. 2020 Jul 21. 5(28): 17182-17192
    Gaurav K, Adhikary T, Satpati P.
      Thymidylate synthase is an enzyme that catalyzes deoxythymidine monophosphate (dTMP) synthesis from substrate deoxyuridine monophosphate (dUMP). Thymidylate synthase of Mycobacterium tuberculosis (MtbThyX) is structurally distinct from its human analogue human thymidylate synthase (hThyA), thus drawing attention as an attractive drug target for combating tuberculosis. Fluorodeoxyuridylate (F-dUMP) is a successful inhibitor of both MtbThyX and hThyA, thus limited by poor selectivity. Understanding the dynamics and energetics associated with substrate/inhibitor binding to thymidylate synthase in atomic details remains a fundamental unsolved problem, which is necessary for a new selective inhibitor design. Structural studies of MtbThyX and hThyA bound substrate/inhibitor complexes not only revealed the extensive specific interaction network between protein and ligands but also opened up the possibility of directly computing the energetics of the substrate versus inhibitor recognition. Using experimentally determined structures as a template, we report extensive computer simulations (∼4.5 μs) that allow us to quantitatively estimate ligand selectivity (dUMP vs F-dUMP) by MtbThyX and hThyA. We show that MtbThyX prefers deprotonated dUMP (enolate form) as the substrate, whereas hThyA binds to the keto form of dUMP. Computed energetics clearly show that MtbThyX is less selective between dUMP and F-dUMP, favoring the latter, relative to hThyA. The simulations reveal the role of tyrosine at position 135 (Y135) of hThyA in amplifying the selectivity. The protonation state of the pyrimidine base of the ligand (i.e., keto or enolate) seems to have no role in MtbThyX ligand selectivity. A molecular gate (consists of Y108, K165, H203, and a water molecule) restricts water accessibility and offers a desolvated dry ligand-binding pocket for MtbThyX. The ligand-binding pocket of hThyA is relatively wet and exposed to bulk water.
    DOI:  https://doi.org/10.1021/acsomega.0c01224