bims-numges Biomed News
on Nucleotide metabolism and genome stability
Issue of 2020‒09‒06
37 papers selected by
Sean Rudd
Karolinska Institutet


  1. J Biol Chem. 2020 Sep 01. pii: jbc.RA120.013495. [Epub ahead of print]
    Gadgil RY, Romer EJ, Goodman CC, Rider SD, Damewood FJ, Barthelemy JR, Shin-Ya K, Hanenberg H, Leffak M.
      Short tandemly repeated DNA sequences, termed microsatellites, are abundant in the human genome. These microsatellites exhibit length instability and susceptibility to DNA double strand breaks (DSBs) due to their tendency to form stable non-B DNA structures. Replication-dependent microsatellite DSBs are linked to genome instability signatures in human developmental diseases and cancers. To probe the causes and consequences of microsatellite DSBs, we designed a dual fluorescence reporter system to detect DSBs at expanded (CTG/CAG)n and polypurine/polypyrimidine (Pu/Py) mirror repeat structures alongside the c-myc replication origin integrated at a single ectopic chromosomal site. Restriction cleavage near the (CTG/CAG)100 microsatellite leads to homology directed single strand annealing between flanking AluY elements, and reporter gene deletion that can be detected by flow cytometry. However, in the absence of restriction cleavage, endogenous and exogenous replication stressors induce DSBs at the (CTG/CAG)100 and Pu/Py microsatellites. DSBs map to a narrow region at the downstream edge of the (CTG)100 lagging strand template. (CTG/CAG)n chromosome fragility is repeat length dependent, while instability at the (Pu/Py) microsatellites depends on replication polarity. Strikingly, restriction-generated DSBs and replication-dependent DSBs are not repaired by the same mechanism. Knockdown of DNA damage response proteins increase (Rad18, Pol η, Pol κ) or decrease (Mus81) the sensitivity of the (CTG/CAG)100 microsatellites to replication stress. Replication stress and DSBs at the ectopic (CTG/CAG)100 microsatellite lead to break induced replication and high frequency mutagenesis at a flanking thymidine kinase gene. Our results show that non-B structure-prone microsatellites are susceptible to replication-dependent DSBs that cause genome instability.
    Keywords:  DNA repair; DNA replication; break-induced replication; cancer; genomic instability; microsatellite instability; mutagenesis; myotonic dystrophy; polycystic kidney disease; trinucleotide repeat disease
    DOI:  https://doi.org/10.1074/jbc.RA120.013495
  2. J Biol Chem. 2020 09 02. pii: jbc.RA120.015541. [Epub ahead of print]
    Rodriguez AA, Wojtaszek JL, Greer BH, Haldar T, Gates KS, Williams RS, Eichman BF.
      The NEIL3 DNA glycosylase maintains genome integrity during replication by excising oxidized bases from single-stranded DNA (ssDNA) and unhooking interstrand cross-links (ICLs) at fork structures. In addition to its N-terminal catalytic glycosylase domain, NEIL3 contains two tandem C-terminal GRF-type zinc fingers that are absent in the other NEIL paralogs. ssDNA binding by the GRF-ZF motifs helps recruit NEIL3 to replication forks converged at an ICL, but the nature of DNA binding and the effect of the GRF-ZF domain on catalysis of base excision and ICL unhooking is unknown. Here, we show that the tandem GRF-ZFs of NEIL3 provide affinity and specificity for DNA that is greater than each individual motif alone. The crystal structure of the GRF domain shows the tandem ZF motifs adopt a flexible head-to-tail configuration well-suited for binding to multiple ssDNA conformations. Functionally, we establish that the NEIL3 GRF domain inhibits glycosylase activity against monoadducts and ICLs. This autoinhibitory activity contrasts GRF-ZF domains of other DNA processing enzymes, which typically use ssDNA binding to enhance catalytic activity, and suggests that the C-terminal region of NEIL3 is involved in both DNA damage recruitment and enzymatic regulation.
    Keywords:  DNA binding protein; DNA damage; DNA glycosylase; DNA repair; DNA-protein interaction; GRF; X-ray crystallography; base excision repair (BER); interstrand DNA crosslink; nucleic acid enzymology; zinc finger
    DOI:  https://doi.org/10.1074/jbc.RA120.015541
  3. Int J Mol Sci. 2020 Sep 02. pii: E6383. [Epub ahead of print]21(17):
    Sharma A, Almasan A.
      Non-small cell lung cancer (NSCLC) represents ~85% of the lung cancer cases. Despite recent advances in NSCLC treatment, the five-year survival rate is still around 23%. Radiotherapy is indicated in the treatment of both early and advanced stage NSCLC; however, treatment response in patients is heterogeneous. Thus, identification of new and more effective treatment combinations is warranted. We have identified Ubiquitin-specific protease 14 (USP14) s a regulator of major double-strand break (DSB) repair pathways in response to ionizing radiation (IR) by its impact on both non-homologous end joining (NHEJ) and homologous recombination (HR) in NSCLC. USP14 is a proteasomal deubiquitinase. IR treatment increases levels and DSB recruitment of USP14 in NSCLC cell lines. Genetic knockdown, using shUSP14 expression or pharmacological inhibition of USP14, using IU1, increases radiosensitization in NSCLC cell lines, as determined by a clonogenic survival assay. Moreover, shUSP14-expressing NSCLC cells show increased NHEJ efficiency, as indicated by chromatin recruitment of key NHEJ proteins, NHEJ reporter assay, and increased IR-induced foci formation by 53BP1 and pS2056-DNA-PKcs. Conversely, shUSP14-expressing NSCLC cells show decreased RPA32 and BRCA1 foci formation, suggesting HR-deficiency. These findings identify USP14 as an important determinant of DSB repair in response to radiotherapy and a promising target for NSCLC radiosensitization.
    Keywords:  NSCLC; USP14; homologous recombination; non-homologous end-joining; radiosensitization
    DOI:  https://doi.org/10.3390/ijms21176383
  4. Curr Genet. 2020 Sep 04.
    Cerritelli SM, El Hage A.
      RNA/DNA hybrids are processed by RNases H1 and H2, while single ribonucleoside-monophosphates (rNMPs) embedded in genomic DNA are removed by the error-free, RNase H2-dependent ribonucleotide excision repair (RER) pathway. In the absence of RER, however, topoisomerase 1 (Top1) can cleave single genomic rNMPs in a mutagenic manner. In RNase H2-deficient mice, the accumulation of genomic rNMPs above a threshold of tolerance leads to catastrophic genomic instability that causes embryonic lethality. In humans, deficiencies in RNase H2 induce the autoimmune disorders Aicardi-Goutières syndrome and systemic lupus erythematosus, and cause skin and intestinal cancers. Recently, we reported that in Saccharomyces cerevisiae, the depletion of Rnr1, the major catalytic subunit of ribonucleotide reductase (RNR), which converts ribonucleotides to deoxyribonucleotides, leads to cell lethality in absence of RNases H1 and H2. We hypothesized that under replicative stress and compromised DNA repair that are elicited by an insufficient supply of deoxyribonucleoside-triphosphates (dNTPs), cells cannot survive the accumulation of persistent RNA/DNA hybrids. Remarkably, we found that cells lacking RNase H2 accumulate ~ 5-fold more genomic rNMPs in absence than in presence of Rnr1. When the load of genomic rNMPs is further increased in the presence of a replicative DNA polymerase variant that over-incorporates rNMPs in leading or lagging strand, cells missing both Rnr1 and RNase H2 suffer from severe growth defects. These are reversed in absence of Top1. Thus, in cells lacking RNase H2 and containing a limiting supply of dNTPs, there is a threshold of tolerance for the accumulation of genomic ribonucleotides that is tightly associated with Top1-mediated DNA damage. In this mini-review, we describe the implications of the loss of RNase H2, or RNases H1 and H2, on the integrity of the nuclear genome and viability of budding yeast cells that are challenged with a critically low supply of dNTPs. We further propose that our findings in budding yeast could pave the way for the study of the potential role of mammalian RNR in RNase H2-related diseases.
    Keywords:  DNA damage; R-loop; RNase H; Ribonucleotide; Ribonucleotide reductase; Topoisomerase 1
    DOI:  https://doi.org/10.1007/s00294-020-01086-8
  5. Gut. 2020 Sep 01. pii: gutjnl-2019-319970. [Epub ahead of print]
    Gout J, Perkhofer L, Morawe M, Arnold F, Ihle M, Biber S, Lange S, Roger E, Kraus JM, Stifter K, Hahn SA, Zamperone A, Engleitner T, Müller M, Walter K, Rodriguez-Aznar E, Sainz B, Hermann PC, Hessmann E, Müller S, Azoitei N, Lechel A, Liebau S, Wagner M, Simeone DM, Kestler HA, Seufferlein T, Wiesmüller L, Rad R, Frappart PO, Kleger A.
      OBJECTIVE: ATM serine/threonine kinase (ATM) is the most frequently mutated DNA damage response gene, involved in homologous recombination (HR), in pancreatic ductal adenocarcinoma (PDAC).DESIGN: Combinational synergy screening was performed to endeavour a genotype-tailored targeted therapy.
    RESULTS: Synergy was found on inhibition of PARP, ATR and DNA-PKcs (PAD) leading to synthetic lethality in ATM-deficient murine and human PDAC. Mechanistically, PAD-induced PARP trapping, replication fork stalling and mitosis defects leading to P53-mediated apoptosis. Most importantly, chemical inhibition of ATM sensitises human PDAC cells toward PAD with long-term tumour control in vivo. Finally, we anticipated and elucidated PARP inhibitor resistance within the ATM-null background via whole exome sequencing. Arising cells were aneuploid, underwent epithelial-mesenchymal-transition and acquired multidrug resistance (MDR) due to upregulation of drug transporters and a bypass within the DNA repair machinery. These functional observations were mirrored in copy number variations affecting a region on chromosome 5 comprising several of the upregulated MDR genes. Using these findings, we ultimately propose alternative strategies to overcome the resistance.
    CONCLUSION: Analysis of the molecular susceptibilities triggered by ATM deficiency in PDAC allow elaboration of an efficient mutation-specific combinational therapeutic approach that can be also implemented in a genotype-independent manner by ATM inhibition.
    Keywords:  DNA damage; drug resistance; pancreas; pancreatic cancer; pancreatic tumours
    DOI:  https://doi.org/10.1136/gutjnl-2019-319970
  6. DNA Repair (Amst). 2020 Aug 11. pii: S1568-7864(20)30197-X. [Epub ahead of print]95 102948
    Ruff SE, Logan SK, Garabedian MJ, Huang TT.
      The DNA damage response (DDR) is necessary to maintain genome integrity and prevent the accumulation of oncogenic mutations. Consequently, proteins involved in the DDR often serve as tumor suppressors, carrying out the crucial task of keeping DNA fidelity intact. Mediator of DNA damage checkpoint 1 (MDC1) is a scaffold protein involved in the early steps of the DDR. MDC1 interacts directly with γ-H2AX, the phosphorylated form of H2AX, a commonly used marker for DNA damage. It then propagates the phosphorylation of H2AX by recruiting ATM kinase. While the function of MDC1 in the DDR has been reviewed previously, its role in cancer has not been reviewed, and numerous studies have recently identified a link between MDC1 and carcinogenesis. This includes MDC1 functioning as a tumor suppressor, with its loss serving as a biomarker for cancer and contributor to drug sensitivity. Studies also indicate that MDC1 operates outside of its traditional role in DDR, and functions as a co-regulator of nuclear receptor transcriptional activity, and that mutations in MDC1 are present in tumors and can also cause germline predisposition to cancer. This review will discuss reports that link MDC1 to cancer and identify MDC1 as an important player in tumor formation, progression, and treatment. We also discuss mechanisms by which MDC1 levels are regulated and how this contributes to tumor formation.
    Keywords:  Cancer; DNA damage response; DNA damaging agents; MDC1; Transcriptional co-regulator; γ-H2AX
    DOI:  https://doi.org/10.1016/j.dnarep.2020.102948
  7. Molecules. 2020 Aug 28. pii: E3940. [Epub ahead of print]25(17):
    Bulygin AA, Kuznetsova AA, Vorobjev YN, Fedorova OS, A Kuznetsov N.
      Human apurinic/apyrimidinic (AP) endonuclease APE1 hydrolyzes phosphodiester bonds on the 5' side of an AP-site, and some damaged nucleotides such as 1,N6-ethenoadenosine (εA), α-adenosine (αA), and 5,6-dihydrouridine (DHU). To investigate the mechanism behind the broad substrate specificity of APE1, we analyzed pre-steady-state kinetics of conformational changes in DNA and the enzyme during DNA binding and damage recognition. Molecular dynamics simulations of APE1 complexes with one of damaged DNA duplexes containing εA, αA, DHU, or an F-site (a stable analog of an AP-site) revealed the involvement of residues Asn229, Thr233, and Glu236 in the mechanism of DNA lesion recognition. The results suggested that processing of an AP-site proceeds faster in comparison with nucleotide incision repair substrates because eversion of a small abasic site and its insertion into the active site do not include any unfavorable interactions, whereas the insertion of any target nucleotide containing a damaged base into the APE1 active site is sterically hindered. Destabilization of the α-helix containing Thr233 and Glu236 via a loss of the interaction between these residues increased the plasticity of the damaged-nucleotide binding pocket and the ability to accommodate structurally different damaged nucleotides. Nonetheless, the optimal location of εA or αA in the binding pocket does not correspond to the optimal conformation of catalytic amino acid residues, thereby significantly decreasing the cleavage efficacy for these substrates.
    Keywords:  5,6-dihydrouridine; AP endonuclease; active site plasticity; apurinic/apyrimidinic site; base excision repair; conformational dynamics
    DOI:  https://doi.org/10.3390/molecules25173940
  8. NAR Cancer. 2020 Sep;2(3): zcaa017
    Hwang T, Reh S, Dunbayev Y, Zhong Y, Takata Y, Shen J, McBride KM, Murnane JP, Bhak J, Lee S, Wood RD, Takata KI.
      DNA polymerase theta (POLQ)-mediated end joining (TMEJ) is a distinct pathway for mediating DNA double-strand break (DSB) repair. TMEJ is required for the viability of BRCA-mutated cancer cells. It is crucial to identify tumors that rely on POLQ activity for DSB repair, because such tumors are defective in other DSB repair pathways and have predicted sensitivity to POLQ inhibition and to cancer therapies that produce DSBs. We define here the POLQ-associated mutation signatures in human cancers, characterized by short insertions and deletions in a specific range of microhomologies. By analyzing 82 COSMIC (Catalogue of Somatic Mutations in Cancer) signatures, we found that BRCA-mutated cancers with a higher level of POLQ expression have a greatly enhanced representation of the small insertion and deletion signature 6, as well as single base substitution signature 3. Using human cancer cells with disruptions of POLQ, we further show that TMEJ dominates end joining of two separated DSBs (distal EJ). Templated insertions with microhomology are enriched in POLQ-dependent distal EJ. The use of this signature analysis will aid in identifying tumors relying on POLQ activity.
    DOI:  https://doi.org/10.1093/narcan/zcaa017
  9. Cell Cycle. 2020 Aug 30. 1-9
    Thongthip S, Conti BA, Lach FP, Smogorzewska A.
      Severe cellular sensitivity and aberrant chromosomal rearrangements in response to DNA interstrand crosslink (ICL) inducing agents are hallmarks of Fanconi anemia (FA) deficient cells. These phenotypes have previously been ascribed to inappropriate activity of non-homologous end joining (NHEJ) rather than a direct consequence of DNA ICL repair defects. Here we used chemical inhibitors, RNAi, and Clustered Regularly Interspaced Short Palindromic Repeat (CRISPR)-Cas9 to inactivate various components of NHEJ in cells from FA patients. We show that suppression of DNA-PKcs, DNA Ligase IV, and 53BP1 is not capable of rescuing ICL-induced proliferation defects and only 53BP1 knockout partially suppresses the chromosomal abnormalities of FA patient cells.
    Keywords:  53BP1; DNA-PKcs; FANCA; Fanconi anemia; KU70; KU80; LIGIV; mitomycin C; non-homologous end joining
    DOI:  https://doi.org/10.1080/15384101.2020.1810394
  10. J Hematol Oncol. 2020 Sep 03. 13(1): 118
    Topatana W, Juengpanich S, Li S, Cao J, Hu J, Lee J, Suliyanto K, Ma D, Zhang B, Chen M, Cai X.
      Synthetic lethality is a lethal phenomenon in which the occurrence of a single genetic event is tolerable for cell survival, whereas the co-occurrence of multiple genetic events results in cell death. The main obstacle for synthetic lethality lies in the tumor biology heterogeneity and complexity, the inadequate understanding of synthetic lethal interactions, drug resistance, and the challenges regarding screening and clinical translation. Recently, DNA damage response inhibitors are being tested in various trials with promising results. This review will describe the current challenges, development, and opportunities for synthetic lethality in cancer therapy. The characterization of potential synthetic lethal interactions and novel technologies to develop a more effective targeted drug for cancer patients will be explored. Furthermore, this review will discuss the clinical development and drug resistance mechanisms of synthetic lethality in cancer therapy. The ultimate goal of this review is to guide clinicians at selecting patients that will receive the maximum benefits of DNA damage response inhibitors for cancer therapy.
    Keywords:  Cancer therapy; DNA damage response inhibitors; DNA repair; PARP inhibitors; Synthetic lethality
    DOI:  https://doi.org/10.1186/s13045-020-00956-5
  11. Genes Dev. 2020 Sep 03.
    Yang Z, Takai KK, Lovejoy CA, de Lange T.
      TRF1 facilitates the replication of telomeric DNA in part by recruiting the BLM helicase, which can resolve G-quadruplexes on the lagging-strand template. Lagging-strand telomeres lacking TRF1 or BLM form fragile telomeres-structures that resemble common fragile sites (CFSs)-but how they are formed is not known. We report that analogous to CFSs, fragile telomeres in BLM-deficient cells involved double-strand break (DSB) formation, in this case by the SLX4/SLX1 nuclease. The DSBs were repaired by POLD3/POLD4-dependent break-induced replication (BIR), resulting in fragile telomeres containing conservatively replicated DNA. BIR also promoted fragile telomere formation in cells with FokI-induced telomeric DSBs and in alternative lengthening of telomeres (ALT) cells, which have spontaneous telomeric damage. BIR of telomeric DSBs competed with PARP1-, LIG3-, and XPF-dependent alternative nonhomologous end joining (alt-NHEJ), which did not generate fragile telomeres. Collectively, these findings indicate that fragile telomeres can arise from BIR-mediated repair of telomeric DSBs.
    Keywords:  BLM; G4; POLD3; POLD4; SLX1; SLX4; TRF1; break-induced replication; fragile telomere; telomere
    DOI:  https://doi.org/10.1101/gad.328575.119
  12. DNA Repair (Amst). 2020 Aug 23. pii: S1568-7864(20)30199-3. [Epub ahead of print]95 102950
    Shiloh Y.
      Research on the molecular pathology of genome instability disorders has advanced our understanding of the complex mechanisms that safeguard genome stability and cellular homeostasis at large. Once the culprit genes and their protein products are identified, an ongoing dialogue develops between the research lab and the clinic in an effort to link specific disease symptoms to the functions of the proteins that are missing in the patients. Ataxi A-T elangiectasia (A-T) is a prominent example of this process. A-T's hallmarks are progressive cerebellar degeneration, immunodeficiency, chronic lung disease, cancer predisposition, endocrine abnormalities, segmental premature aging, chromosomal instability and radiation sensitivity. The disease is caused by absence of the powerful protein kinase, ATM, best known as the mobilizer of the broad signaling network induced by double-strand breaks (DSBs) in the DNA. In parallel, ATM also functions in the maintenance of the cellular redox balance, mitochondrial function and turnover and many other metabolic circuits. An ongoing discussion in the A-T field revolves around the question of which ATM function is the one whose absence is responsible for the most debilitating aspect of A-T - the cerebellar degeneration. This review suggests that it is the absence of a comprehensive role of ATM in responding to ongoing DNA damage induced mainly by endogenous agents. It is the ensuing deterioration and eventual loss of cerebellar Purkinje cells, which are very vulnerable to ATM absence due to a unique combination of physiological features, which kindles the cerebellar decay in A-T.
    Keywords:  ATM; Ataxia-telangiectasia; Cerebellum; Genome stability
    DOI:  https://doi.org/10.1016/j.dnarep.2020.102950
  13. J Biol Chem. 2020 Sep 02. pii: jbc.RA120.015164. [Epub ahead of print]
    Ononye OE, Sausen CW, Balakrishnan L, Bochman ML.
      In S. cerevisiae, the Pif1 helicase functions in both nuclear and mitochondrial DNA replication and repair processes, preferentially unwinding RNA-DNA hybrids and resolving G-quadruplex structures. We sought to determine how the various activities of Pif1 are regulated in vivo Here, we report lysine acetylation of nuclear Pif1 and demonstrate that it influences both Pif1's cellular roles and core biochemical activities. Using Pif1 overexpression toxicity assays, we determined that the acetyltransferase NuA4 and deacetylase Rpd3 are primarily responsible for the dynamic acetylation of nuclear Pif1. Mass spectrometry analysis revealed that Pif1 was modified in several domains throughout the protein's sequence on the N-terminus (K118, K129), helicase domain (K525, K639, K725), and C-terminus (K800). Acetylation of Pif1 exacerbated its overexpression toxicity phenotype, which was alleviated upon deletion of its N-terminus. Biochemical assays demonstrated that acetylation of Pif1 stimulated its helicase, ATPase, and DNA binding activities while maintaining its substrate preferences. Limited proteolysis assays indicate that acetylation of Pif1 induces a conformational change that may account for its altered enzymatic properties. We propose that acetylation is involved in regulating of Pif1 activities, influencing a multitude of DNA transactions vital to the maintenance of genome integrity.
    Keywords:  DNA helicase; DNA repair; DNA replication; Esa1; Piccolo NuA4; Pif1; Rpd3; acetylation; lysine acetylation; toxicity
    DOI:  https://doi.org/10.1074/jbc.RA120.015164
  14. DNA Repair (Amst). 2020 Aug 20. pii: S1568-7864(20)30203-2. [Epub ahead of print]95 102954
    Arczewska KD, Krasuska W, Stachurska A, Karpińska K, Sikorska J, Kiedrowski M, Lange D, Stępień T, Czarnocka B.
      Oxidative stress (OS) is recognized as disturbance of cellular equilibrium between reactive oxygen species (ROS) formation and their elimination by antioxidant defense systems. One example of ROS-mediated damage is generation of potentially mutagenic DNA precursor, 8-oxodGTP. In human cells genomic 8-oxodGTP incorporation is prevented by the MutT homologue 1 (MTH1 or hMTH1 for human MTH1) protein. It is well established that malignant cells, including thyroid cancer cells, require hMTH1 for maintaining proliferation and cancerous transformation phenotype. Above observations led to the development of hMTH1 inhibitors as novel anticancer therapeutics. In the current study we present extensive analysis of oxidative stress responses determining sensitivity to hMTH1 deficiency in cultured thyroid cells. We observe here that hMTH1 depletion results in downregulation of several glutathione-dependent OS defense system factors, including GPX1 and GCLM, making some of the tested thyroid cell lines highly dependent on glutathione levels. This is evidenced by the increased ROS burden and enhanced proliferation defect after combination of hMTH1 siRNA and glutathione synthesis inhibition. Moreover, due to the lack of data on hMTH1 expression in human thyroid tumor specimens we decided to perform detailed analysis of hMTH1 expression in thyroid tumor and peri-tumoral tissues from human patients. Our results allow us to propose here that anticancer activity of hMTH1 suppression may be boosted by combination with agents modulating glutathione pool, but further studies are necessary to precisely identify backgrounds susceptible to such combination treatment.
    Keywords:  GPX1; Glutathione peroxidase 1; MTH1; NUDT1; Oxidative stress; hMTH1
    DOI:  https://doi.org/10.1016/j.dnarep.2020.102954
  15. J Cell Biol. 2020 Oct 05. pii: e201911049. [Epub ahead of print]219(10):
    Maranon DG, Sharma N, Huang Y, Selemenakis P, Wang M, Altina N, Zhao W, Wiese C.
      NUCKS1 (nuclear ubiquitous casein kinase and cyclin-dependent kinase substrate 1) is a chromatin-associated, vertebrate-specific, and multifunctional protein with a role in DNA damage signaling and repair. Previously, we have shown that NUCKS1 helps maintain homologous recombination (HR) DNA repair in human cells and functions as a tumor suppressor in mice. However, the mechanisms by which NUCKS1 positively impacts these processes had remained unclear. Here, we show that NUCKS1 physically and functionally interacts with the DNA motor protein RAD54. Upon exposure of human cells to DNA-damaging agents, NUCKS1 controls the resolution of RAD54 foci. In unperturbed cells, NUCKS1 prevents RAD54's inappropriate engagement with RAD51AP1. In vitro, NUCKS1 stimulates the ATPase activity of RAD54 and the RAD51-RAD54-mediated strand invasion step during displacement loop formation. Taken together, our data demonstrate that the NUCKS1 protein is an important new regulator of the spatiotemporal events in HR.
    DOI:  https://doi.org/10.1083/jcb.201911049
  16. Crit Rev Biochem Mol Biol. 2020 Sep 03. 1-13
    Guilliam TA, Yeeles JTP.
      In eukaryotes three DNA polymerases (Pols), α, δ, and ε, are tasked with bulk DNA synthesis of nascent strands during genome duplication. Most evidence supports a model where Pol α initiates DNA synthesis before Pol ε and Pol δ replicate the leading and lagging strands, respectively. However, a number of recent reports, enabled by advances in biochemical and genetic techniques, have highlighted emerging roles for Pol δ in all stages of leading-strand synthesis; initiation, elongation, and termination, as well as fork restart. By focusing on these studies, this review provides an updated perspective on the division of labor between the replicative polymerases during DNA replication.
    Keywords:  DNA polymerase; DNA replication; polymerase delta; replication fork; replisome
    DOI:  https://doi.org/10.1080/10409238.2020.1811630
  17. Mol Cell. 2020 Aug 24. pii: S1097-2765(20)30554-2. [Epub ahead of print]
    Meers C, Keskin H, Banyai G, Mazina O, Yang T, Gombolay AL, Mukherjee K, Kaparos EI, Newnam G, Mazin A, Storici F.
      DNA double-stranded breaks (DSBs) are dangerous lesions threatening genomic stability. Fidelity of DSB repair is best achieved by recombination with a homologous template sequence. In yeast, transcript RNA was shown to template DSB repair of DNA. However, molecular pathways of RNA-driven repair processes remain obscure. Utilizing assays of RNA-DNA recombination with and without an induced DSB in yeast DNA, we characterize three forms of RNA-mediated genomic modifications: RNA- and cDNA-templated DSB repair (R-TDR and c-TDR) using an RNA transcript or a DNA copy of the RNA transcript for DSB repair, respectively, and a new mechanism of RNA-templated DNA modification (R-TDM) induced by spontaneous or mutagen-induced breaks. While c-TDR requires reverse transcriptase, translesion DNA polymerase ζ (Pol ζ) plays a major role in R-TDR, and it is essential for R-TDM. This study characterizes mechanisms of RNA-DNA recombination, uncovering a role of Pol ζ in transferring genetic information from transcript RNA to DNA.
    Keywords:  DNA polymerase ζ; DNA repair; REV3; RNA recombination; RNA-mediated; RNA-templated; cDNA-mediated; homologous recombination; reverse transcriptase; translesion polymerase
    DOI:  https://doi.org/10.1016/j.molcel.2020.08.011
  18. Methods Mol Biol. 2021 ;2196 245-255
    Peng H, Zhang S, Chen X.
      DNA double-strand break (DSB) is one of the most deleterious types of DNA lesions threatening genome integrity. Cells have evolved several exquisite pathways to repair these breaks. Homologous recombination (HR) is an essential DSB repair mechanism that utilizes an intact homologous sequence as a template to repair DSBs with high fidelity. To initiate the HR repair, the 5'-ends of DSBs have to be nucleolytically cleaved by nucleases to generate 3'-single-strand DNA (ssDNA). Exposed 3'-ssDNA recruits the ssDNA binding protein complex RPA to activate the DNA damage checkpoint. RPA is subsequently replaced by Rad51 recombinase to form Rad51 nucleoprotein filament that catalyzes strand invasion and formation of the D-loop. Processing of 5'-ends (called resection) is a crucial step that determines the choice of repair pathways. Here we introduce an assay for monitoring the dynamics of resection at different locations from a site-specific DSB in yeast.
    Keywords:  DNA double-strand break; Homologous recombination; Resection; Southern blot
    DOI:  https://doi.org/10.1007/978-1-0716-0868-5_20
  19. Oncogene. 2020 Sep 03.
    Ray U, Raghavan SC.
      In the present day, it is possible to incorporate targeted mutations or replace a gene using genome editing techniques such as customisable CRISPR/Cas9 system. Although induction of DNA double-strand breaks (DSBs) by genome editing tools can be repaired by both non-homologous end joining (NHEJ) and homologous recombination (HR), the skewness of the former pathway in human and other mammals normally result in imprecise repair. Scientists working at the crossroads of DNA repair and genome editing have devised new strategies for using a specific pathway to their advantage. Refinement in the efficiency of precise gene editing was witnessed upon downregulation of NHEJ by knockdown or using small molecule inhibitors on one hand, and upregulation of HR proteins and addition of HR stimulators, other hand. The exploitation of cell cycle phase differences together with appropriate donor DNA length/sequence and small molecules has provided further improvement in precise genome editing. The present article reviews the mechanisms of improving the efficiency of precise genome editing in several model organisms and in clinics.
    DOI:  https://doi.org/10.1038/s41388-020-01445-2
  20. J Cell Biol. 2020 Nov 02. pii: e201908212. [Epub ahead of print]219(11):
    Valente LJ, Tarangelo A, Li AM, Naciri M, Raj N, Boutelle AM, Li Y, Mello SS, Bieging-Rolett K, DeBerardinis RJ, Ye J, Dixon SJ, Attardi LD.
      The mechanisms by which TP53, the most frequently mutated gene in human cancer, suppresses tumorigenesis remain unclear. p53 modulates various cellular processes, such as apoptosis and proliferation, which has led to distinct cellular mechanisms being proposed for p53-mediated tumor suppression in different contexts. Here, we asked whether during tumor suppression p53 might instead regulate a wide range of cellular processes. Analysis of mouse and human oncogene-expressing wild-type and p53-deficient cells in physiological oxygen conditions revealed that p53 loss concurrently impacts numerous distinct cellular processes, including apoptosis, genome stabilization, DNA repair, metabolism, migration, and invasion. Notably, some phenotypes were uncovered only in physiological oxygen. Transcriptomic analysis in this setting highlighted underappreciated functions modulated by p53, including actin dynamics. Collectively, these results suggest that p53 simultaneously governs diverse cellular processes during transformation suppression, an aspect of p53 function that would provide a clear rationale for its frequent inactivation in human cancer.
    DOI:  https://doi.org/10.1083/jcb.201908212
  21. Proc Natl Acad Sci U S A. 2020 Aug 31. pii: 202007455. [Epub ahead of print]
    Crowe JL, Wang XS, Shao Z, Lee BJ, Estes VM, Zha S.
      The DNA-dependent protein kinase (DNA-PK), which is composed of the KU heterodimer and the large catalytic subunit (DNA-PKcs), is a classical nonhomologous end-joining (cNHEJ) factor. Naïve B cells undergo class switch recombination (CSR) to generate antibodies with different isotypes by joining two DNA double-strand breaks at different switching regions via the cNHEJ pathway. DNA-PK and the cNHEJ pathway play important roles in the DNA repair phase of CSR. To initiate cNHEJ, KU binds to DNA ends and recruits and activates DNA-PK. Activated DNA-PK phosphorylates DNA-PKcs at the S2056 and T2609 clusters. Loss of T2609 cluster phosphorylation increases radiation sensitivity but whether T2609 phosphorylation has a role in physiological DNA repair remains elusive. Using the DNA-PKcs 5A mouse model carrying alanine substitutions at the T2609 cluster, here we show that loss of T2609 phosphorylation of DNA-PKcs does not affect the CSR efficiency. Yet, the CSR junctions recovered from DNA-PKcs 5A/5A B cells reveal increased chromosomal translocations, extensive use of distal switch regions (consistent with end resection), and preferential usage of microhomology-all signs of the alternative end-joining pathway. Thus, these results uncover a role of DNA-PKcs T2609 phosphorylation in promoting cNHEJ repair pathway choice during CSR.
    Keywords:  DNA-PKcs; T2609 autophosphorylation; alternative end joining; class switch recombination; nonhomologous end joining
    DOI:  https://doi.org/10.1073/pnas.2007455117
  22. Cell Rep. 2020 Sep 01. pii: S2211-1247(20)31065-2. [Epub ahead of print]32(9): 108080
    Chen J, Harding SM, Natesan R, Tian L, Benci JL, Li W, Minn AJ, Asangani IA, Greenberg RA.
      The DNA-dependent pattern recognition receptor, cGAS (cyclic GMP-AMP synthase), mediates communication between the DNA damage and the immune responses. Mitotic chromosome missegregation stimulates cGAS activity; however, it is unclear whether progression through mitosis is required for cancercell-intrinsic activation of anti-tumor immune responses. Moreover, it is unknown whether cell cycle checkpoint disruption can restore responses in cancer cells that are recalcitrant to DNAdamage-induced inflammation. Here, we demonstrate that prolonged cell cycle arrest at the G2-mitosis boundary from either excessive DNA damage or CDK1 inhibition prevents inflammatory-stimulated gene expression and immune-mediated destruction of distal tumors. Remarkably, DNAdamage-induced inflammatory signaling is restored in a RIG-I-dependent manner upon concomitant disruption of p53 and the G2 checkpoint. These findings link aberrant cell progression and p53 loss to an expanded spectrum of damage-associated molecular pattern recognition and have implications for the design of rational approaches to augment anti-tumor immune responses.
    Keywords:  ATR; DNA damage; RIG-I; anti-tumor immune response; cGAS; cell cycle checkpoint; inflammatory signaling; p53
    DOI:  https://doi.org/10.1016/j.celrep.2020.108080
  23. Cells. 2020 Aug 28. pii: E1980. [Epub ahead of print]9(9):
    Sayed IM, Chakraborty A, Abd El-Hafeez AA, Sharma A, Sahan AZ, Huang WJM, Sahoo D, Ghosh P, Hazra TK, Das S.
      Colorectal cancer (CRC) is the third most prevalent cancer, while the majority (80-85%) of CRCs are sporadic and are microsatellite stable (MSS), and approximately 15-20% of them display microsatellite instability (MSI). Infection and chronic inflammation are known to induce DNA damage in host tissues and can lead to oncogenic transformation of cells, but the role of DNA repair proteins in microbe-associated CRCs remains unknown. Using CRC-associated microbes such as Fusobacterium nucleatum (Fn) in a coculture with murine and human enteroid-derived monolayers (EDMs), here, we show that, among all the key DNA repair proteins, NEIL2, an oxidized base-specific DNA glycosylase, is significantly downregulated after Fn infection. Fn infection of NEIL2-null mouse-derived EDMs showed a significantly higher level of DNA damage, including double-strand breaks and inflammatory cytokines. Several CRC-associated microbes, but not the commensal bacteria, induced the accumulation of DNA damage in EDMs derived from a murine CRC model, and Fn had the most pronounced effect. An analysis of publicly available transcriptomic datasets showed that the downregulation of NEIL2 is often encountered in MSS compared to MSI CRCs. We conclude that the CRC-associated microbe Fn induced the downregulation of NEIL2 and consequent accumulation of DNA damage and played critical roles in the progression of CRCs.
    Keywords:  DNA damage; Fusobacterium nucleatum; NEIL2; base-excision repair; cancer development; colorectal cancer; enteroid; enteroid-derived monolayer; genomic instability; inflammation
    DOI:  https://doi.org/10.3390/cells9091980
  24. Nat Rev Drug Discov. 2020 Sep 03.
    Curtin NJ, Szabo C.
      The process of poly(ADP-ribosyl)ation and the major enzyme that catalyses this reaction, poly(ADP-ribose) polymerase 1 (PARP1), were discovered more than 50 years ago. Since then, advances in our understanding of the roles of PARP1 in cellular processes such as DNA repair, gene transcription and cell death have allowed the investigation of therapeutic PARP inhibition for a variety of diseases - particularly cancers in which defects in DNA repair pathways make tumour cells highly sensitive to the inhibition of PARP activity. Efforts to identify and evaluate potent PARP inhibitors have so far led to the regulatory approval of four PARP inhibitors for the treatment of several types of cancer, and PARP inhibitors have also shown therapeutic potential in treating non-oncological diseases. This Review provides a timeline of PARP biology and medicinal chemistry, summarizes the pathophysiological processes in which PARP plays a role and highlights key opportunities and challenges in the field, such as counteracting PARP inhibitor resistance during cancer therapy and repurposing PARP inhibitors for the treatment of non-oncological diseases.
    DOI:  https://doi.org/10.1038/s41573-020-0076-6
  25. DNA Repair (Amst). 2020 Jul 31. pii: S1568-7864(20)30190-7. [Epub ahead of print]95 102941
    Baddock HT, Yosaatmadja Y, Newman JA, Schofield CJ, Gileadi O, McHugh PJ.
      Unrepaired, or misrepaired, DNA damage can contribute to the pathogenesis of a number of conditions, or disease states; thus, DNA damage repair pathways, and the proteins within them, are required for the safeguarding of the genome. Human SNM1A is a 5'-to-3' exonuclease that plays a role in multiple DNA damage repair processes. To date, most data suggest a role of SNM1A in primarily ICL repair: SNM1A deficient cells exhibit hypersensitivity to ICL-inducing agents (e.g. mitomycin C and cisplatin); and both in vivo and in vitro experiments demonstrate SNM1A and XPF-ERCC1 can function together in the 'unhooking' step of ICL repair. SNM1A further interacts with a number of other proteins that contribute to genome integrity outside canonical ICL repair (e.g. PCNA and CSB), and these may play a role in regulating SNM1As function, subcellular localisation, and post-translational modification state. These data also provide further insight into other DNA repair pathways to which SNM1A may contribute. This review aims to discuss all aspects of the exonuclease, SNM1A, and its contribution to DNA damage tolerance.
    Keywords:  DCLRE1A; Interstrand crosslink repair; Nuclease; Pso2; SNM1A
    DOI:  https://doi.org/10.1016/j.dnarep.2020.102941
  26. J Cell Biol. 2020 Nov 02. pii: e201910149. [Epub ahead of print]219(11):
    Baumann C, Zhang X, De La Fuente R.
      The polycomb group protein CBX2 is an important epigenetic reader involved in cell proliferation and differentiation. While CBX2 overexpression occurs in a wide range of human tumors, targeted deletion results in homeotic transformation, proliferative defects, and premature senescence. However, its cellular function(s) and whether it plays a role in maintenance of genome stability remain to be determined. Here, we demonstrate that loss of CBX2 in mouse fibroblasts induces abnormal large-scale chromatin structure and chromosome instability. Integrative transcriptome analysis and ATAC-seq revealed a significant dysregulation of transcripts involved in DNA repair, chromocenter formation, and tumorigenesis in addition to changes in chromatin accessibility of genes involved in lateral sclerosis, basal transcription factors, and folate metabolism. Notably, Cbx2-/- cells exhibit prominent decondensation of satellite DNA sequences at metaphase and increased sister chromatid recombination events leading to rampant chromosome instability. The presence of extensive centromere and telomere defects suggests a prominent role for CBX2 in heterochromatin homeostasis and the regulation of nuclear architecture.
    DOI:  https://doi.org/10.1083/jcb.201910149
  27. Cells. 2020 Sep 01. pii: E2012. [Epub ahead of print]9(9):
    Dobler C, Jost T, Hecht M, Fietkau R, Distel L.
      DNA damage response inhibitors (DDRi) may selectively enhance the inactivation of tumor cells in combination with ionizing radiation (IR). The induction of senescence may be the key mechanism of tumor cell inactivation in this combinatorial treatment. In the current study the effect of combined IR with DDRi on the induction of senescence was studied in head and neck squamous cell carcinoma (HNSCC) cells with different human papilloma virus (HPV) status. The integrity of homologous recombination (HR) was assessed in two HPV positive, two HPV negative HNSCC, and two healthy fibroblast cell cultures. Cells were treated with the DDRi CC-115 (DNA-dependent protein kinase, DNA-pK; dual mammalian target of rapamycin, mTor), VE-822 (ATR; ataxia telangiectasia and Rad3-related kinase), and AZD0156 (ATM; ataxia telangiectasia mutated kinase) combined with IR. Effects on senescence, apoptosis, necrosis, and cell cycle were analyzed by flow cytometry. The fibroblast cell lines generally tolerated IR or combined treatment better than the tumor cell lines. The ATM and ATR inhibitors were effectively inducing senescence when combined with IR. The DNA-PK inhibitor was not an important inductor of senescence. HPV status and HR activity had a limited influence on the efficacy of DDRi. Induction of senescence and necrosis varied individually among the cell lines due to molecular heterogeneity and the involvement of DNA damage response pathways in senescence induction.
    Keywords:  ATM; ATR; DNA damage response inhibitor; DNAPK; HNSCC; homologous recombination; ionizing radiation; kinase inhibitor; radiosensitivity; senescence
    DOI:  https://doi.org/10.3390/cells9092012
  28. Sci Rep. 2020 Sep 02. 10(1): 14455
    Noh SE, Juhnn YS.
      DNA double-strand breaks (DSB) are formed by various exogenous and endogenous factors and are repaired by homologous recombination and non-homologous end joining (NHEJ). DNA-dependent protein kinase (DNA-PK) is the principal enzyme for NHEJ. We explored the role and the underlying mechanism of cAMP signaling in the NHEJ repair of DSBs resulted from gamma ray irradiation to non-small cell lung cancer (NSLC) cells. Activated cAMP signaling by expression of an activated stimulatory GTP-binding protein or by pretreatment with isoproterenol and prostaglandin E2, delayed the repair of DSBs resulted from gamma ray irradiation, and the delaying effects depended on protein kinase A (PKA). Activated cAMP signaling suppressed XRCC4 and DNA ligase IV recruitment into DSB foci, and reduced phosphorylation at T2609 in DNA-PK catalytic subunit (DNA-PKcs) with a concomitant increase in phosphorylation at S2056 in PKA-dependent ways following gamma ray irradiation. cAMP signaling decreased phosphorylation of T2609 by protein phosphatase 2A-dependent inhibition of ATM. We conclude that cAMP signaling delays the repair of gamma ray-induced DNA DSBs in NSLC cells by inhibiting NHEJ via PKA-dependent pathways, and that cAMP signaling differentially modulates DNA-PKcs phosphorylation at S2056 and T2609, which might contribute to the inhibition of NHEJ in NSLC cells.
    DOI:  https://doi.org/10.1038/s41598-020-71522-9
  29. Cell Rep. 2020 Sep 01. pii: S2211-1247(20)31085-8. [Epub ahead of print]32(9): 108096
    Rainey MD, Bennett D, O'Dea R, Zanchetta ME, Voisin M, Seoighe C, Santocanale C.
      DNA replication initiates from multiple origins, and selective CDC7 kinase inhibitors (CDC7is) restrain cell proliferation by limiting origin firing. We have performed a CRISPR-Cas9 genome-wide screen to identify genes that, when lost, promote the proliferation of cells treated with sub-efficacious doses of a CDC7i. We have found that the loss of function of ETAA1, an ATR activator, and RIF1 reduce the sensitivity to CDC7is by allowing DNA synthesis to occur more efficiently, notably during late S phase. We show that partial CDC7 inhibition induces ATR mainly through ETAA1, and that if ATR is subsequently inhibited, origin firing is unleashed in a CDK- and CDC7-dependent manner. Cells are then driven into a premature and highly defective mitosis, a phenotype that can be recapitulated by ETAA1 and TOPBP1 co-depletion. This work defines how ATR mediates the effects of CDC7 inhibition, establishing the framework to understand how the origin firing checkpoint functions.
    Keywords:  CRISPR-Cas9 screen; DNA replication; ETAA1; RIF1; cell cycle; checkpoint; functional genomics; kinase inhibitor; replication origins; replication stress
    DOI:  https://doi.org/10.1016/j.celrep.2020.108096
  30. Mol Cancer Ther. 2020 Sep 02. pii: molcanther.0423.2020. [Epub ahead of print]
    Dekhne AS, Hou Z, Gangjee A, Matherly LH.
      One-carbon (1C) metabolism encompasses folate-mediated 1C transfer reactions and related processes, including nucleotide and amino acid biosynthesis, antioxidant regeneration, and epigenetic regulation. 1C pathways are compartmentalized in the cytosol, mitochondria and nucleus. 1C metabolism in the cytosol has been an important therapeutic target for cancer since the inception of modern chemotherapy and "antifolates" targeting cytosolic 1C pathways continue to be a mainstay of the chemotherapy armamentarium for cancer. Recent insights into the complexities of 1C metabolism in cancer cells, including the critical role of the mitochondrial 1C pathway as a source of 1C units, glycine, reducing equivalents, and ATP, have spurred the discovery of novel compounds that target these reactions, with particular focus on 5,10-methylene tetrahydrofolate dehydrogenase 2 and serine hydroxymethyltransferase 2. In this review, we discuss key aspects of 1C metabolism, with emphasis on the importance of mitochondrial 1C metabolism to metabolic homeostasis, and its relationship to the oncogenic phenotype and therapeutic potential for cancer.
    DOI:  https://doi.org/10.1158/1535-7163.MCT-20-0423
  31. ACS Omega. 2020 Aug 25. 5(33): 20882-20889
    Webster P, Saito K, Cortez J, Ramirez C, Baum MM.
      Transporters are specialized integral membrane proteins, which mediate the passage of virtually all molecules through cell membranes. They are expressed in a broad range of human and animal tissues and play important roles in both normal and disease states. For these reasons, they are evaluated when developing and testing drugs. Two major families of drug transporters, the adenosine 5'-triphosphate-binding cassette and solute carrier transporters (SLC), have critical roles in the absorption, distribution, metabolism, and elimination of drugs. The SLC family contains known nucleoside transporters and therefore are important when nucleoside analogs are used as drugs to prevent or treat viral infections. In this study, we wanted to determine if it was possible to locate one member of the SLC family, the human concentrative nucleoside transporter 3 (CNT3) in human vaginal epithelial cells. The CNT3 protein has important roles in drug delivery, subsequent drug tissue distribution, and, hence, efficacy. Vaginal epithelial cells, taken from two human volunteers (one Caucasian and one African American), were labeled for light and electron microscopy, with a commercial antibody to a cytoplasmic domain of CNT3, the protein product of the SLC28A3 gene. Fluorescent secondary antibodies or protein A-gold were used to detect antibody binding. By electron microscopy, gold particle binding was quantified to determine labeling specificity. By light microscopy, positive labeling with anti-CNT3 antibodies was detected on human vaginal epithelial cells, but specificity to any intracellular structure was not easily determined, most likely a result of specimen preparation. Electron microscopy revealed that the CNT3 transporter protein was present predominantly on microvilli located on one side of some human vaginal epithelial cells. Quantification confirmed specific anti-CNT3 labeling over human vaginal epithelial cell microvilli. The CNT3 protein, present in the microvilli of human vaginal epithelial cells, may have a role in redistributing nucleoside homologues delivered to the vaginal tract. Transporter proteins such as CNT3 could shuttle nucleosides and their analogs through the vaginal epithelium to immune cells located in lower cell layers. Outer layers of cells, which are eventually shed from the epithelium, may remove accumulated nucleoside drug analogs from the vaginal tract.
    DOI:  https://doi.org/10.1021/acsomega.0c02329
  32. DNA Repair (Amst). 2020 Aug 20. pii: S1568-7864(20)30202-0. [Epub ahead of print]95 102953
    He L, Hu Z, Sun Y, Zhang M, Zhu H, Jiang L, Zhang Q, Mu D, Zhang J, Gu L, Yang Y, Pan FY, Jia S, Guo Z.
      The up-regulation of PRMT1 is critical to the cell growth and cancer progression of lung cancer cells. In our research, we found that PRMT1 is important to the DNA repair ability and drug resistance of lung cancer cells. To demonstrate the functions of PRMT1, we identified Flap endonuclease 1 (FEN1) as a post-translationally modified downstream target protein of PRMT1. As a major component of Base Excision Repair pathway, FEN1 plays an important role in DNA replication and DNA damage repair. However, the detailed mechanism of FEN1 up-regulation in lung cancer cells remains unclear. In our study, we identified PRMT1 as a key factor that maintains the high expression levels of FEN1, which is critical to the DNA repair ability and the chemotherapeutic drug resistance of lung cancer cells.
    Keywords:  DNA repair; Drug resistance; Flap endonuclease 1 (FEN1); Lung cancer cells.; Protein arginine methyltransferase 1 (PRMT1)
    DOI:  https://doi.org/10.1016/j.dnarep.2020.102953
  33. J Med Chem. 2020 Sep 03.
    Sun Y, Ke Y, Li C, Wang J, Tu L, Hu L, Jin Y, Chen H, Gong J, Yu ZQ.
      Floxuridine (FUdR, 5-fluoro-2-deoxyuridine) was widely used in patients with tumor. But the poor activity and severe side effect has been observed in the clinic, which resulted from increased degradation cleavage of FUdR to 5-FU by thymidine phosphorylase, and reduced transporter-mediated entry into cells. In this study, we have synthesized a series of L-aspartic acid β-ester and L-glutamic acid γ-ester of FUdR to improve the metabolic stability of FUdR and target FUdR to cancer cells via amino acid transporter ATB0,+ which was exclusively up-regulated in some cancerous tissue. The uptake mechanism, stability, in vitro/in vivo anti-proliferation action, pharmacokinetics and tissue distribution were studied. The combined results showed the unusual 5'-β-L-Asp-FUdR possessed a better tumor inhibition rate and a better metabolic stability than FUdR through a ATB0,+-mediated prodrug approach. The present study provided the first proof-of-concept of exploiting ATB0,+ for tumor-selective delivery of nucleoside analogues in the form of prodrug.
    DOI:  https://doi.org/10.1021/acs.jmedchem.0c00149
  34. Cell Metab. 2020 Sep 01. pii: S1550-4131(20)30421-6. [Epub ahead of print]32(3): 321-323
    Stuani L, Sarry JE.
      Metabolic dialogue between tumors and their microenvironment emerges as a key regulator of chemoresistance, the major barrier for the treatment of several cancers. In this issue of Cell Metabolism, van Gastel et al. decipher the pivotal role of stromal glutamine-derived aspartate to sustain pyrimidine biosynthesis in chemoresistant acute myeloid leukemia (AML) and thus state it as a target for anti-cancer therapy.
    DOI:  https://doi.org/10.1016/j.cmet.2020.08.008
  35. Proc Natl Acad Sci U S A. 2020 Sep 01. pii: 202008073. [Epub ahead of print]
    Davis L, Khoo KJ, Zhang Y, Maizels N.
      Interhomolog recombination (IHR) occurs spontaneously in somatic human cells at frequencies that are low but sufficient to ameliorate some genetic diseases caused by heterozygous mutations or autosomal dominant mutations. Here we demonstrate that DNA nicks or double-strand breaks (DSBs) targeted by CRISPR-Cas9 to both homologs can stimulate IHR and associated copy-neutral loss of heterozygosity (cnLOH) in human cells. The frequency of IHR is 10-fold lower at nicks than at DSBs, but cnLOH is evident in a greater fraction of recombinants. IHR at DSBs occurs predominantly via reciprocal end joining. At DSBs, depletion of POLQ caused a dramatic increase in IHR and in the fraction of recombinants exhibiting cnLOH, suggesting that POLQ promotes end joining in cis, which limits breaks available for recombination in trans These results define conditions that may produce cnLOH as a mutagenic signature in cancer and may, conversely, promote therapeutic correction of both compound heterozygous and dominant negative mutations associated with genetic disease.
    Keywords:  DNA nick; double-strand break; gene editing; gene therapy; polymerase
    DOI:  https://doi.org/10.1073/pnas.2008073117
  36. bioRxiv. 2020 Aug 28. pii: 2020.08.27.270819. [Epub ahead of print]
    Akinci E, Cha M, Lin L, Yeo G, Hamilton MC, Donahue CJ, Bermudez-Cabrera HC, Zanetti LC, Chen M, Barkal SA, Khowpinitchai B, Chu N, Velimirovic M, Jodhani R, Fife JD, Sovrovic M, Cole PA, Davey RA, Cassa CA, Sherwood RI.
      The adenosine analogue remdesivir has emerged as a frontline antiviral treatment for SARS-CoV-2, with preliminary evidence that it reduces the duration and severity of illness 1 . Prior clinical studies have identified adverse events 1,2 , and remdesivir has been shown to inhibit mitochondrial RNA polymerase in biochemical experiments 7 , yet little is known about the specific genetic pathways involved in cellular remdesivir metabolism and cytotoxicity. Through genome-wide CRISPR-Cas9 screening and RNA sequencing, we show that remdesivir treatment leads to a repression of mitochondrial respiratory activity, and we identify five genes whose loss significantly reduces remdesivir cytotoxicity. In particular, we show that loss of the mitochondrial nucleoside transporter SLC29A3 mitigates remdesivir toxicity without a commensurate decrease in SARS-CoV-2 antiviral potency and that the mitochondrial adenylate kinase AK2 is a remdesivir kinase required for remdesivir efficacy and toxicity. This work elucidates the cellular mechanisms of remdesivir metabolism and provides a candidate gene target to reduce remdesivir cytotoxicity.
    DOI:  https://doi.org/10.1101/2020.08.27.270819