bims-numges Biomed News
on Nucleotide metabolism and genome stability
Issue of 2021‒01‒24
twenty-five papers selected by
Sean Rudd
Karolinska Institutet


  1. Nat Cell Biol. 2021 Jan 18.
    Verma P, Zhou Y, Cao Z, Deraska PV, Deb M, Arai E, Li W, Shao Y, Puentes L, Li Y, Patankar S, Mach RH, Faryabi RB, Shi J, Greenberg RA.
      The response to poly(ADP-ribose) polymerase inhibitors (PARPi) is dictated by homologous recombination (HR) DNA repair and the abundance of lesions that trap PARP enzymes. It remains unclear, however, if the established role of PARP in promoting chromatin accessibility impacts viability in these settings. Using a CRISPR-based screen, we identified the PAR-binding chromatin remodeller ALC1/CHD1L as a key determinant of PARPi toxicity in HR-deficient cells. ALC1 loss reduced viability of breast cancer gene (BRCA)-mutant cells and enhanced sensitivity to PARPi by up to 250-fold, while overcoming several resistance mechanisms. ALC1 deficiency reduced chromatin accessibility concomitant with a decrease in the association of base damage repair factors. This resulted in an accumulation of replication-associated DNA damage, increased PARP trapping and a reliance on HR. These findings establish PAR-dependent chromatin remodelling as a mechanistically distinct aspect of PARPi responses and therapeutic target in HR-deficient cancers.
    DOI:  https://doi.org/10.1038/s41556-020-00624-3
  2. Mol Cell. 2021 Jan 08. pii: S1097-2765(20)30957-6. [Epub ahead of print]
    Adolph MB, Mohamed TM, Balakrishnan S, Xue C, Morati F, Modesti M, Greene EC, Chazin WJ, Cortez D.
      The RAD51 recombinase forms nucleoprotein filaments to promote double-strand break repair, replication fork reversal, and fork stabilization. The stability of these filaments is highly regulated, as both too little and too much RAD51 activity can cause genome instability. RADX is a single-strand DNA (ssDNA) binding protein that regulates DNA replication. Here, we define its mechanism of action. We find that RADX inhibits RAD51 strand exchange and D-loop formation activities. RADX directly and selectively interacts with ATP-bound RAD51, stimulates ATP hydrolysis, and destabilizes RAD51 nucleofilaments. The RADX interaction with RAD51, in addition to its ssDNA binding capability, is required to maintain replication fork elongation rates and fork stability. Furthermore, BRCA2 can overcome the RADX-dependent RAD51 inhibition. Thus, RADX functions in opposition to BRCA2 in regulating RAD51 nucleofilament stability to ensure the right level of RAD51 function during DNA replication.
    Keywords:  DNA curtain; DNA damage response; DNA repair; double-strand break; electron microscopy; fork reversal; replication stress
    DOI:  https://doi.org/10.1016/j.molcel.2020.12.036
  3. J Biol Chem. 2020 Dec 25. pii: S0021-9258(17)50673-0. [Epub ahead of print]295(52): 17935-17949
    Ladds MJGW, Popova G, Pastor-Fernández A, Kannan S, van Leeuwen IMM, Håkansson M, Walse B, Tholander F, Bhatia R, Verma CS, Lane DP, Laín S.
      The tenovins are a frequently studied class of compounds capable of inhibiting sirtuin activity, which is thought to result in increased acetylation and protection of the tumor suppressor p53 from degradation. However, as we and other laboratories have shown previously, certain tenovins are also capable of inhibiting autophagic flux, demonstrating the ability of these compounds to engage with more than one target. In this study, we present two additional mechanisms by which tenovins are able to activate p53 and kill tumor cells in culture. These mechanisms are the inhibition of a key enzyme of the de novo pyrimidine synthesis pathway, dihydroorotate dehydrogenase (DHODH), and the blockage of uridine transport into cells. These findings hold a 3-fold significance: first, we demonstrate that tenovins, and perhaps other compounds that activate p53, may activate p53 by more than one mechanism; second, that work previously conducted with certain tenovins as SirT1 inhibitors should additionally be viewed through the lens of DHODH inhibition as this is a major contributor to the mechanism of action of the most widely used tenovins; and finally, that small changes in the structure of a small molecule can lead to a dramatic change in the target profile of the molecule even when the phenotypic readout remains static.
    Keywords:  cell death; mitochondria; molecular modeling; molecular pharmacology; nucleoside/nucleotide biosynthesis; nucleoside/nucleotide transport; p53; tumor cell biology
    DOI:  https://doi.org/10.1074/jbc.RA119.012056
  4. J Biol Chem. 2021 Jan 19. pii: S0021-9258(21)00078-8. [Epub ahead of print] 100309
    Herrmann GK, Russell WK, Garg NJ, Yin YW.
      Mitochondral DNA is located in organelle that house essential metablic reactions and contain high reactive oxygen species. Therefore, mitochondrial DNA suffers more oxidative damage than its nuclear counterpart. Formation of a repair enzyme complex is beneficial to DNA repair. Recent studies have shown that mitochondrial DNA polymerase (Pol γ) and poly(ADP-ribose) polymerase 1 (PARP1) were found in the same complex along with other mitochondrial DNA repair enzymes and mitochondrial PARP1 level is correlated with mtDNA integrity. However, the molecular basis for the functional connection between Pol γ and PARP1 has not yet been elucidated because cellular functions of PARP1 in DNA repair are intertwined with metabolism via NAD+ (nicotinamide adenosine dinucleotide), the substrate of PARP1 and a metabolic cofactor. To dissect the direct effect of PARP1 on mtDNA from the secondary perturbation of metabolism, we report here biochemical studies that recapitulated Pol γ PARylation observed in cells and showed that PARP1 regulates Pol γ activity during DNA repair in a metabolic cofactor NAD+ (nicotinamide adenosine dinucleotide)-dependent manner. In the absence of NAD+, PARP1 completely inhibits Pol γ, while increasing NAD+ levels to a physiological concentration that enables Pol γ to resume maximum repair activity. Because cellular NAD+ levels are linked to metabolism and to ATP production via oxidative phosphorylation, our results suggest that mtDNA damage repair is coupled to cellular metabolic state and the integrity of the respiratory chain.
    Keywords:  ADP-ribosylation; DNA polymerase; DNA repair; DNA synthesis; post-translational modification (PTM); protein-DNA interaction; protein-protein interaction; western blot
    DOI:  https://doi.org/10.1016/j.jbc.2021.100309
  5. Nature. 2021 Jan 20.
    Liu L, Yan Z, Osia BA, Twarowski J, Sun L, Kramara J, Lee RS, Kumar S, Elango R, Li H, Dang W, Ira G, Malkova A.
      Break-induced replication (BIR) repairs one-ended double-strand breaks in DNA similar to those formed by replication collapse or telomere erosion, and it has been implicated in the initiation of genome instability in cancer and other human diseases1,2. Previous studies have defined the enzymes that are required for BIR1-5; however, understanding of initial and extended BIR synthesis, and of how the migrating D-loop proceeds through known replication roadblocks, has been precluded by technical limitations. Here we use a newly developed assay to show that BIR synthesis initiates soon after strand invasion and proceeds more slowly than S-phase replication. Without primase, leading strand synthesis is initiated efficiently, but is unable to proceed beyond 30 kilobases, suggesting that primase is needed for stabilization of the nascent leading strand. DNA synthesis can initiate in the absence of Pif1 or Pol32, but does not proceed efficiently. Interstitial telomeric DNA disrupts and terminates BIR progression, and BIR initiation is suppressed by transcription proportionally to the transcription level. Collisions between BIR and transcription lead to mutagenesis and chromosome rearrangements at levels that exceed instabilities induced by transcription during normal replication. Together, these results provide fundamental insights into the mechanism of BIR and how BIR contributes to genome instability.
    DOI:  https://doi.org/10.1038/s41586-020-03172-w
  6. EMBO J. 2021 Jan 20. e104509
    Li S, Wang H, Jehi S, Li J, Liu S, Wang Z, Truong L, Chiba T, Wang Z, Wu X.
      Break-induced replication (BIR) is a specialized homologous-recombination pathway for DNA double-strand break (DSB) repair, which often induces genome instability. In this study, we establish EGFP-based recombination reporters to systematically study BIR in mammalian cells and demonstrate an important role of human PIF1 helicase in promoting BIR. We show that at endonuclease cleavage sites, PIF1-dependent BIR is used for homology-initiated recombination requiring long track DNA synthesis, but not short track gene conversion (STGC). We also show that structure formation-prone AT-rich DNA sequences derived from common fragile sites (CFS-ATs) induce BIR upon replication stress and oncogenic stress, and PCNA-dependent loading of PIF1 onto collapsed/broken forks is critical for BIR activation. At broken replication forks, even STGC-mediated repair of double-ended DSBs depends on POLD3 and PIF1, revealing an unexpected mechanism of BIR activation upon replication stress that differs from the conventional BIR activation model requiring DSB end sensing at endonuclease-generated breaks. Furthermore, loss of PIF1 is synthetically lethal with loss of FANCM, which is involved in protecting CFS-ATs. The breast cancer-associated PIF1 mutant L319P is defective in BIR, suggesting a direct link of BIR to oncogenic processes.
    Keywords:  PIF1; break-induced replication; long track gene conversion; replication stress; short track gene conversion
    DOI:  https://doi.org/10.15252/embj.2020104509
  7. Nat Commun. 2021 01 20. 12(1): 482
    Williams JS, Tumbale PP, Arana ME, Rana JA, Williams RS, Kunkel TA.
      DNA ligase 1 (LIG1, Cdc9 in yeast) finalizes eukaryotic nuclear DNA replication by sealing Okazaki fragments using DNA end-joining reactions that strongly discriminate against incorrectly paired DNA substrates. Whether intrinsic ligation fidelity contributes to the accuracy of replication of the nuclear genome is unknown. Here, we show that an engineered low-fidelity LIG1Cdc9 variant confers a novel mutator phenotype in yeast typified by the accumulation of single base insertion mutations in homonucleotide runs. The rate at which these additions are generated increases upon concomitant inactivation of DNA mismatch repair, or by inactivation of the Fen1Rad27 Okazaki fragment maturation (OFM) nuclease. Biochemical and structural data establish that LIG1Cdc9 normally avoids erroneous ligation of DNA polymerase slippage products, and this protection is compromised by mutation of a LIG1Cdc9 high-fidelity metal binding site. Collectively, our data indicate that high-fidelity DNA ligation is required to prevent insertion mutations, and that this may be particularly critical following strand displacement synthesis during the completion of OFM.
    DOI:  https://doi.org/10.1038/s41467-020-20800-1
  8. J Biol Chem. 2020 Nov 23. pii: S0021-9258(20)00019-8. [Epub ahead of print]296 100033
    Hashimoto Y, Tanaka H.
      DNA replication is a major contributor to genomic instability, and protection against DNA replication perturbation is essential for normal cell division. Certain types of replication stress agents, such as aphidicolin and hydroxyurea, have been shown to cause reversible replication fork stalling, wherein replisome complexes are stably maintained with competence to restart in the S phase of the cell cycle. If these stalled forks persist into the M phase without a replication restart, replisomes are disassembled in a p97-dependent pathway and under-replicated DNA is subjected to mitotic DNA repair synthesis. Here, using Xenopus egg extracts, we investigated the consequences that arise when stalled forks are released simultaneously with the induction of mitosis. Ara-cytidine-5'-triphosphate-induced stalled forks were able to restart with the addition of excess dCTP during early mitosis before the nuclear envelope breakdown (NEB). However, stalled forks could no longer restart efficiently after the NEB. Although replisome complexes were finally disassembled in a p97-dependent manner during mitotic progression whether or not fork stalling was relieved, the timing of the NEB was delayed with the ongoing forks, rather than the stalled forks, and the delay was dependent on Wee1/Myt1 kinase activities. Thus, ongoing DNA replication was found to be directly linked to the regulation of Wee1/Myt1 kinases to modulate cyclin-dependent kinase activities because of which DNA replication and mitosis occur in a mutually exclusive and sequential manner.
    Keywords:  CDK; Wee1/Myt1; Xenopus egg extract; nuclear envelope breakdown (NEB); replication forks; replisome
    DOI:  https://doi.org/10.1074/jbc.RA120.015142
  9. J Biol Chem. 2020 Dec 11. pii: S0021-9258(17)50591-8. [Epub ahead of print]295(50): 16949-16959
    Shibata E, Dutta A.
      The origin recognition complex (ORC), composed of six subunits, ORC1-6, binds to origins of replication as a ring-shaped heterohexameric ATPase that is believed to be essential to recruit and load MCM2-7, the minichromosome maintenance protein complex, around DNA and initiate DNA replication. We previously reported the creation of viable cancer cell lines that lacked detectable ORC1 or ORC2 protein without a reduction in the number of origins firing. Here, using CRISPR-Cas9-mediated mutations, we report that human HCT116 colon cancer cells also survive when ORC5 protein expression is abolished via a mutation in the initiator ATG of the ORC5 gene. Even if an internal methionine is used to produce an undetectable, N terminally deleted ORC5, the protein would lack 80% of the AAA+ ATPase domain, including the Walker A motif. The ORC5-depleted cells show normal chromatin binding of MCM2-7 and initiate replication from a similar number of origins as WT cells. In addition, we introduced a second mutation in ORC2 in the ORC5 mutant cells, rendering both ORC5 and ORC2 proteins undetectable in the same cells and destabilizing the ORC1, ORC3, and ORC4 proteins. Yet the double mutant cells grow, recruit MCM2-7 normally to chromatin, and initiate DNA replication with normal number of origins. Thus, in these selected cancer cells, either a crippled ORC lacking ORC2 and ORC5 and present at minimal levels on the chromatin can recruit and load enough MCM2-7 to initiate DNA replication, or human cell lines can sometimes recruit MCM2-7 to origins independent of ORC.
    Keywords:  DNA replication; cancer; cell proliferation; chromatin; gene knockout
    DOI:  https://doi.org/10.1074/jbc.RA120.015450
  10. J Biol Chem. 2020 Dec 11. pii: S0021-9258(17)50614-6. [Epub ahead of print]295(50): 17251-17264
    Zahurancik WJ, Suo Z.
      In eukaryotic DNA replication, DNA polymerase ε (Polε) is responsible for leading strand synthesis, whereas DNA polymerases α and δ synthesize the lagging strand. The human Polε (hPolε) holoenzyme is comprised of the catalytic p261 subunit and the noncatalytic p59, p17, and p12 small subunits. So far, the contribution of the noncatalytic subunits to hPolε function is not well understood. Using pre-steady-state kinetic methods, we established a minimal kinetic mechanism for DNA polymerization and editing catalyzed by the hPolε holoenzyme. Compared with the 140-kDa N-terminal catalytic fragment of p261 (p261N), which we kinetically characterized in our earlier studies, the presence of the p261 C-terminal domain (p261C) and the three small subunits increased the DNA binding affinity and the base substitution fidelity. Although the small subunits enhanced correct nucleotide incorporation efficiency, there was a wide range of rate constants when incorporating a correct nucleotide over a single-base mismatch. Surprisingly, the 3'→5' exonuclease activity of the hPolε holoenzyme was significantly slower than that of p261N when editing both matched and mismatched DNA substrates. This suggests that the presence of p261C and the three small subunits regulates the 3'→5' exonuclease activity of the hPolε holoenzyme. Together, the 3'→5' exonuclease activity and the variable mismatch extension activity modulate the overall fidelity of the hPolε holoenzyme by up to 3 orders of magnitude. Thus, the presence of p261C and the three noncatalytic subunits optimizes the dual enzymatic activities of the catalytic p261 subunit and makes the hPolε holoenzyme an efficient and faithful replicative DNA polymerase.
    Keywords:  DNA binding; DNA polymerase; DNA replication; accessory subunits; eukaryotic DNA polymerase; exonuclease; function regulation; human DNA polymerase epsilon; nucleotide incorporation kinetics; polymerase fidelity; pre-steady-state kinetics; pre-steady-state kinetics enzyme mechanism; substrate specificity; the 3′→5′ exonuclease
    DOI:  https://doi.org/10.1074/jbc.RA120.013903
  11. J Biol Chem. 2020 Dec 25. pii: S0021-9258(17)50711-5. [Epub ahead of print]295(52): 18449-18458
    Lee S, Heo J, Park CJ.
      Replication protein A (RPA) is a eukaryotic ssDNA-binding protein and contains three subunits: RPA70, RPA32, and RPA14. Phosphorylation of the N-terminal region of the RPA32 subunit plays an essential role in DNA metabolism in processes such as replication and damage response. Phosphorylated RPA32 (pRPA32) binds to RPA70 and possibly regulates the transient RPA70-Bloom syndrome helicase (BLM) interaction to inhibit DNA resection. However, the structural details and determinants of the phosphorylated RPA32-RPA70 interaction are still unknown. In this study, we provide molecular details of the interaction between RPA70 and a mimic of phosphorylated RPA32 (pmRPA32) using fluorescence polarization and NMR analysis. We show that the N-terminal domain of RPA70 (RPA70N) specifically participates in pmRPA32 binding, whereas the unphosphorylated RPA32 does not bind to RPA70N. Our NMR data revealed that RPA70N binds pmRPA32 using a basic cleft region. We also show that at least 6 negatively charged residues of pmRPA32 are required for RPA70N binding. By introducing alanine mutations into hydrophobic positions of pmRPA32, we found potential points of contact between RPA70N and the N-terminal half of pmRPA32. We used this information to guide docking simulations that suggest the orientation of pmRPA32 in complex with RPA70N. Our study demonstrates detailed features of the domain-domain interaction between RPA70 and RPA32 upon phosphorylation. This result provides insight into how phosphorylation tunes transient bindings between RPA and its partners in DNA resection.
    Keywords:  DNA damage response; DNA-binding protein; fluorescence polarization; nuclear magnetic resonance (NMR); phosphorylation; protein-protein interaction; replication protein A
    DOI:  https://doi.org/10.1074/jbc.RA120.016457
  12. Crit Rev Biochem Mol Biol. 2021 Jan 18. 1-31
    Zhou ZX, Williams JS, Lujan SA, Kunkel TA.
      Ribonucleotides are the most abundant non-canonical nucleotides in the genome. Their vast presence and influence over genome biology is becoming increasingly appreciated. Here we review the recent progress made in understanding their genomic presence, incorporation characteristics and usefulness as biomarkers for polymerase enzymology. We also discuss ribonucleotide processing, the genetic consequences of unrepaired ribonucleotides in DNA and evidence supporting the significance of their transient presence in the nuclear genome.
    Keywords:  DNA repair; DNA replication; Ribonucleotide incorporation; genome stability; genome-wide sequencing
    DOI:  https://doi.org/10.1080/10409238.2020.1869175
  13. J Biol Chem. 2020 Dec 18. pii: S0021-9258(17)50657-2. [Epub ahead of print]295(51): 17802-17815
    Sullivan ED, Longley MJ, Copeland WC.
      Faithful replication of the mitochondrial genome is carried out by a set of key nuclear-encoded proteins. DNA polymerase γ is a core component of the mtDNA replisome and the only replicative DNA polymerase localized to mitochondria. The asynchronous mechanism of mtDNA replication predicts that the replication machinery encounters dsDNA and unique physical barriers such as structured genes, G-quadruplexes, and other obstacles. In vitro experiments here provide evidence that the polymerase γ heterotrimer is well-adapted to efficiently synthesize DNA, despite the presence of many naturally occurring roadblocks. However, we identified a specific G-quadruplex-forming sequence at the heavy-strand promoter (HSP1) that has the potential to cause significant stalling of mtDNA replication. Furthermore, this structured region of DNA corresponds to the break site for a large (3,895 bp) deletion observed in mitochondrial disease patients. The presence of this deletion in humans correlates with UV exposure, and we have found that efficiency of polymerase γ DNA synthesis is reduced after this quadruplex is exposed to UV in vitro.
    Keywords:  DNA polymerase γ; DNA replication; DNA structure; G-quadruplex; heavy-strand promoter; mitochondrial DNA (mtDNA); mitochondrial DNA damage; mtDNA deletion
    DOI:  https://doi.org/10.1074/jbc.RA120.015390
  14. DNA Repair (Amst). 2021 Jan 09. pii: S1568-7864(21)00001-X. [Epub ahead of print]98 103047
    Sadoughi F, Hallajzadeh J, Asemi Z, Mansournia MA, Alemi F, Yousefi B.
      Our genome bears tens of thousands of harms and devastations per day; In this regard, numerous sophisticated and complicated mechanisms are embedded by our cells in furtherance of remitting an unchanged and stable genome to their next generation. These mechanisms, that are collectively called DDR, have the duty of detecting the lesions and repairing them. it's necessary for the viability of any living cell that sustain the integrity and stability of its genetic content and this highlights the role of mediators that transduce the signals of DNA damage to the cell cycle in order to prevent the replication of a defective DNA. In this paper, we review the signaling pathways that lie between these processes and define how different ingredients of DDR are also able to affect the checkpoint signaling.
    Keywords:  CDK; CHK1; Cell cycle arrest; Checkpoint; Mec1; Rad53
    DOI:  https://doi.org/10.1016/j.dnarep.2021.103047
  15. Mol Cell. 2021 Jan 08. pii: S1097-2765(20)30951-5. [Epub ahead of print]
    Zhang JM, Genois MM, Ouyang J, Lan L, Zou L.
      Alternative lengthening of telomeres (ALT) is mediated by break-induced replication (BIR), but how BIR is regulated at telomeres is poorly understood. Here, we show that telomeric BIR is a self-perpetuating process. By tethering PML-IV to telomeres, we induced telomere clustering in ALT-associated PML bodies (APBs) and a POLD3-dependent ATR response at telomeres, showing that BIR generates replication stress. Ablation of BLM helicase activity in APBs abolishes telomere synthesis but causes multiple chromosome bridges between telomeres, revealing a function of BLM in processing inter-telomere BIR intermediates. Interestingly, the accumulation of BLM in APBs requires its own helicase activity and POLD3, suggesting that BIR triggers a feedforward loop to further recruit BLM. Enhancing BIR induces PIAS4-mediated TRF2 SUMOylation, and PIAS4 loss deprives APBs of repair proteins and compromises ALT telomere synthesis. Thus, a BLM-driven and PIAS4-mediated feedforward loop operates in APBs to perpetuate BIR, providing a critical mechanism to extend ALT telomeres.
    Keywords:  ALT; APB; BIR; BLM; PIAS4; PML; SUMO; phase separation; replication stress; telomere
    DOI:  https://doi.org/10.1016/j.molcel.2020.12.030
  16. Biochemistry. 2021 Jan 21.
    Varela FA, Freudenthal BD.
      DNA polymerases play vital roles in the maintenance and replication of genomic DNA by synthesizing new nucleotide polymers using nucleoside triphosphates as substrates. Deoxynucleoside triphosphates (dNTPs) are the canonical substrates for DNA polymerases; however, some bacterial polymerases have been demonstrated to insert deoxynucleoside diphosphates (dNDPs), which lack a third phosphate group, the γ-phosphate. Whether eukaryotic polymerases can efficiently incorporate dNDPs has not been investigated, and much about the chemical or structural role played by the γ-phosphate of dNTPs remains unknown. Using the model mammalian polymerase (Pol) β, we examine how Pol β incorporates a substrate lacking a γ-phosphate [deoxyguanosine diphosphate (dGDP)] utilizing kinetic and crystallographic approaches. Using single-turnover kinetics, we determined dGDP insertion across a templating dC by Pol β to be drastically impaired when compared to dGTP insertion. We found the most significant impairment in the apparent insertion rate (kpol), which was reduced 32000-fold compared to that of dGTP insertion. X-ray crystal structures revealed similar enzyme-substrate contacts for both dGDP and dGTP. These findings suggest the insertion efficiency of dGDP is greatly decreased due to impairments in polymerase chemistry. This work is the first instance of a mammalian polymerase inserting a diphosphate nucleotide and provides insight into the nature of polymerase mechanisms by highlighting how these enzymes have evolved to use triphosphate nucleotide substrates.
    DOI:  https://doi.org/10.1021/acs.biochem.0c00847
  17. J Biol Chem. 2020 Aug 21. pii: S0021-9258(17)50077-0. [Epub ahead of print]295(34): 12181-12187
    Howard MJ, Horton JK, Zhao ML, Wilson SH.
      DNA polymerase (pol) β catalyzes two reactions at DNA gaps generated during base excision repair, gap-filling DNA synthesis and lyase-dependent 5´-end deoxyribose phosphate removal. The lyase domain of pol β has been proposed to function in DNA gap recognition and to facilitate DNA scanning during substrate search. However, the mechanisms and molecular interactions used by pol β for substrate search and recognition are not clear. To provide insight into this process, a comparison was made of the DNA binding affinities of WT pol β, pol λ, and pol μ, and several variants of pol β, for 1-nt-gap-containing and undamaged DNA. Surprisingly, this analysis revealed that mutation of three lysine residues in the lyase active site of pol β, 35, 68, and 72, to alanine (pol β KΔ3A) increased the binding affinity for nonspecific DNA ∼11-fold compared with that of the WT. WT pol μ, lacking homologous lysines, displayed nonspecific DNA binding behavior similar to that of pol β KΔ3A, in line with previous data demonstrating both enzymes were deficient in processive searching. In fluorescent microscopy experiments using mouse fibroblasts deficient in PARP-1, the ability of pol β KΔ3A to localize to sites of laser-induced DNA damage was strongly decreased compared with that of WT pol β. These data suggest that the three lysines in the lyase active site destabilize pol β when bound to DNA nonspecifically, promoting DNA scanning and providing binding specificity for gapped DNA.
    Keywords:  DNA binding protein; DNA binding proteins; DNA damage; DNA polymerase; DNA repair; DNA–protein interaction; base excision repair (BER); facilitated diffusion; nonspecific DNA binding; processive search
    DOI:  https://doi.org/10.1074/jbc.RA120.013547
  18. JCI Insight. 2021 Jan 21. pii: 141518. [Epub ahead of print]
    Barghout SH, Aman A, Nouri K, Blatman Z, Arevalo K, Thomas GE, MacLean N, Hurren R, Ketela T, Saini M, Abohawya M, Kiyota T, Al-Awar R, Schimmer AD.
      TAK-243 is a first-in-class inhibitor of ubiquitin-like modifier activating enzyme 1 (UBA1) that catalyzes ubiquitin activation, the first step in the ubiquitylation cascade. Based on its preclinical efficacy and tolerability, TAK-243 has been advanced to phase 1 clinical trials in advanced malignancies. Nonetheless, the determinants of TAK-243 sensitivity remain largely unknown. Here, we conducted a genome-wide CRISPR/Cas9 knockout screen in acute myeloid leukemia (AML) cells in the presence of TAK-243 to identify genes essential for TAK-243 action. We identified BEN domain-containing protein 3 (BEND3), a transcriptional repressor and a regulator of chromatin organization, as the top gene whose knockout confers resistance to TAK-243 in vitro and in vivo. Knockout of BEND3 dampened TAK-243 effects on ubiquitylation, proteotoxic stress, and DNA damage response. BEND3 knockout upregulated the ABC efflux transporter breast cancer resistance protein (BCRP; ABCG2), and reduced the intracellular levels of TAK-243. TAK-243 sensitivity correlated with BCRP expression in cancer cell lines of different origin. Moreover, chemical inhibition and genetic knockdown of BCRP sensitized intrinsically resistant high-BCRP cells to TAK-243. Thus, our data demonstrate that BEND3 regulates the expression of BCRP for which TAK-243 is a substrate. Moreover, BCRP expression could serve as a predictor of TAK-243 sensitivity.
    Keywords:  Cancer; Drug screens; Oncology; Therapeutics; Ubiquitin-proteosome system
    DOI:  https://doi.org/10.1172/jci.insight.141518
  19. Nat Commun. 2021 01 18. 12(1): 422
    Larrue C, Guiraud N, Mouchel PL, Dubois M, Farge T, Gotanègre M, Bosc C, Saland E, Nicolau-Travers ML, Sabatier M, Serhan N, Sahal A, Boet E, Mouche S, Heydt Q, Aroua N, Stuani L, Kaoma T, Angenendt L, Mikesch JH, Schliemann C, Vergez F, Tamburini J, Récher C, Sarry JE.
      Drug tolerant/resistant leukemic stem cell (LSC) subpopulations may explain frequent relapses in acute myeloid leukemia (AML), suggesting that these relapse-initiating cells (RICs) persistent after chemotherapy represent bona fide targets to prevent drug resistance and relapse. We uncover that calcitonin receptor-like receptor (CALCRL) is expressed in RICs, and that the overexpression of CALCRL and/or of its ligand adrenomedullin (ADM), and not CGRP, correlates to adverse outcome in AML. CALCRL knockdown impairs leukemic growth, decreases LSC frequency, and sensitizes to cytarabine in patient-derived xenograft models. Mechanistically, the ADM-CALCRL axis drives cell cycle, DNA repair, and mitochondrial OxPHOS function of AML blasts dependent on E2F1 and BCL2. Finally, CALCRL depletion reduces LSC frequency of RICs post-chemotherapy in vivo. In summary, our data highlight a critical role of ADM-CALCRL in post-chemotherapy persistence of these cells, and disclose a promising therapeutic target to prevent relapse in AML.
    DOI:  https://doi.org/10.1038/s41467-020-20717-9
  20. Mol Cell. 2021 Jan 15. pii: S1097-2765(20)30956-4. [Epub ahead of print]
    Endres T, Solvie D, Heidelberger JB, Andrioletti V, Baluapuri A, Ade CP, Muhar M, Eilers U, Vos SM, Cramer P, Zuber J, Beli P, Popov N, Wolf E, Gallant P, Eilers M.
      The MYC oncoprotein globally affects the function of RNA polymerase II (RNAPII). The ability of MYC to promote transcription elongation depends on its ubiquitylation. Here, we show that MYC and PAF1c (polymerase II-associated factor 1 complex) interact directly and mutually enhance each other's association with active promoters. PAF1c is rapidly transferred from MYC onto RNAPII. This transfer is driven by the HUWE1 ubiquitin ligase and is required for MYC-dependent transcription elongation. MYC and HUWE1 promote histone H2B ubiquitylation, which alters chromatin structure both for transcription elongation and double-strand break repair. Consistently, MYC suppresses double-strand break accumulation in active genes in a strictly PAF1c-dependent manner. Depletion of PAF1c causes transcription-dependent accumulation of double-strand breaks, despite widespread repair-associated DNA synthesis. Our data show that the transfer of PAF1c from MYC onto RNAPII efficiently couples transcription elongation with double-strand break repair to maintain the genomic integrity of MYC-driven tumor cells.
    Keywords:  E3 ligase; HUWE1; MYC; PAF1c; RNAPII; double-strand break repair; histone H2B; ubiquitylation
    DOI:  https://doi.org/10.1016/j.molcel.2020.12.035
  21. Elife. 2021 Jan 20. pii: e61980. [Epub ahead of print]10
    DeVilbiss AW, Zhao Z, Martin-Sandoval MS, Ubellacker JM, Tasdogan A, Agathocleous M, Mathews TP, Morrison SJ.
      Little is known about the metabolic regulation of rare cell populations because most metabolites are hard to detect in small numbers of cells. We previously described a method for metabolomic profiling of flow cytometrically-isolated hematopoietic stem cells (HSCs) that detects 60 metabolites in 10,000 cells (Agathocleous et al., 2017). Here we describe a new method involving hydrophilic liquid interaction chromatography and high-sensitivity orbitrap mass spectrometry that detected 160 metabolites in 10,000 HSCs, including many more glycolytic and lipid intermediates. We improved chromatographic separation, increased mass resolution, minimized ion suppression, and eliminated sample drying. Most metabolite levels did not significantly change during cell isolation. Mouse HSCs exhibited increased glycerophospholipids relative to bone marrow cells and methotrexate treatment altered purine biosynthesis. Circulating human melanoma cells were depleted for purine intermediates relative to subcutaneous tumors, suggesting decreased purine synthesis during metastasis. These methods facilitate the routine metabolomic analysis of rare cells from tissues.
    Keywords:  mouse; regenerative medicine; stem cells
    DOI:  https://doi.org/10.7554/eLife.61980
  22. Biochimie. 2021 Jan 18. pii: S0300-9084(21)00009-2. [Epub ahead of print]
    Bezerra GA, Holenstein A, Foster WR, Xie B, Hicks KG, Bürer C, Lutz S, Mukherjee A, Sarkar D, Bhattacharya D, Rutter J, Talukdar A, Brown PJ, Luo M, Shi L, Froese DS, Yue WW.
      The folate and methionine cycles, constituting one-carbon metabolism, are critical pathways for cell survival. Intersecting these two cycles, 5,10-methylenetetrahydrofolate reductase (MTHFR) directs one-carbon units from the folate to methionine cycle, to be exclusively used for methionine and S-adenosylmethionine (AdoMet) synthesis. MTHFR deficiency and upregulation result in diverse disease states, rendering it an attractive drug target. The activity of MTHFR is inhibited by the binding of AdoMet to an allosteric regulatory domain distal to the enzyme's active site, which we have previously identified to constitute a novel fold with a druggable pocket. Here, we screened 162 AdoMet mimetics using differential scanning fluorimetry, and identified 4 compounds that stabilized this regulatory domain. Three compounds were sinefungin analogues, closely related to AdoMet and S-adenosylhomocysteine (AdoHcy). The strongest thermal stabilisation was provided by (S)-SKI-72, a potent inhibitor originally developed for protein arginine methyltransferase 4 (PRMT4). Using surface plasmon resonance, we confirmed that (S)-SKI-72 binds MTHFR via its allosteric domain with nanomolar affinity. Assay of MTHFR activity in the presence of (S)-SKI-72 demonstrates inhibition of purified enzyme with sub-micromolar potency and endogenous MTHFR from HEK293 cell lysate in the low micromolar range, both of which are lower than AdoMet. Nevertheless, unlike AdoMet, (S)-SKI-72 is unable to completely abolish MTHFR activity, even at very high concentrations. Combining binding assays, kinetic characterization and compound docking, this work indicates the regulatory domain of MTHFR can be targeted by small molecules and presents (S)-SKI-72 as an excellent candidate for development of MTHFR inhibitors.
    Keywords:  5,10-Methylenetetrahydrofolate reductase; Drug-development; Enzymatic inhibition; One-carbon metabolism; Small molecules
    DOI:  https://doi.org/10.1016/j.biochi.2021.01.007
  23. Bioorg Chem. 2021 Jan 05. pii: S0045-2068(20)31918-0. [Epub ahead of print]107 104620
    Joshi G, Sharma M, Kalra S, Gavande NS, Singh S, Kumar R.
      Xanthine oxidase (XO) has been primarily targeted for the development of anti-hyperuriciemic /anti-gout agents as it catalyzes the conversion of xanthine and hypoxanthine into uric acid. XO overexpression in various cancer is very well correlated due to reactive oxygen species (ROS) production and metabolic activation of carcinogenic substances during the catalysis. Herein, we report the design and synthesis of a series of 3,5-diaryl-4,5-dihydro-1H-pyrazole carbaldehyde derivatives (2a-2x) as xanthine oxidase inhibitors (XOIs). A docking model was developed for the prediction of XO inhibitory activity of our novel compounds. Furthermore, our compounds anticancer activity results in low XO expression and XO-harboring cancer cells both in 2D and 3D-culture models are presented and discussed. Among the array of synthesized compounds, 2b and 2m emerged as potent XO inhibitors having IC50 values of 9.32 ± 0.45 µM and 10.03 ± 0.43 µM, respectively. Both compounds induced apoptosis, halted the cell cycle progression at the G1 phase, elevated ROS levels, altered mitochondrial membrane potential, and inhibited antioxidant enzymes. The levels of miRNA and expression of redox sensors in cells were also altered due to increase oxidative stress induced by our compounds. Compounds 2b and 2m hold a great promise for further development of XOIs for the treatment of XO-harboring tumors.
    Keywords:  2-pyrazolines; 2D-QSAR docking model; Mitochondrial membrane permeability; Reactive oxygen species; Redox sensor; Xanthine oxidase
    DOI:  https://doi.org/10.1016/j.bioorg.2020.104620
  24. Cells. 2021 Jan 18. pii: E182. [Epub ahead of print]10(1):
    Pesi R, Allegrini S, Balestri F, Garcia-Gil M, Cividini F, Colombaioni L, Jordheim LP, Camici M, Tozzi MG.
      Cytosolic 5'-nucleotidase II (NT5C2) is a highly regulated enzyme involved in the maintenance of intracellular purine and the pyrimidine compound pool. It dephosphorylates mainly IMP and GMP but is also active on AMP. This enzyme is highly expressed in tumors, and its activity correlates with a high rate of proliferation. In this paper, we show that the recombinant purified NT5C2, in the presence of a physiological concentration of the inhibitor inorganic phosphate, is very sensitive to changes in the adenylate energy charge, especially from 0.4 to 0.9. The enzyme appears to be very sensitive to pro-oxidant conditions; in this regard, the possible involvement of a disulphide bridge (C175-C547) was investigated by using a C547A mutant NT5C2. Two cultured cell models were used to further assess the sensitivity of the enzyme to oxidative stress conditions. NT5C2, differently from other enzyme activities, was inactivated and not rescued by dithiothreitol in a astrocytoma cell line (ADF) incubated with hydrogen peroxide. The incubation of a human lung carcinoma cell line (A549) with 2-deoxyglucose lowered the cell energy charge and impaired the interaction of NT5C2 with the ice protease-activating factor (IPAF), a protein involved in innate immunity and inflammation.
    Keywords:  A549; ADF; AMPK; Cytosolic 5′-nucleotidase II; IPAF; NT5C2; energy charge; oxidative stress
    DOI:  https://doi.org/10.3390/cells10010182
  25. Cells. 2021 Jan 19. pii: E188. [Epub ahead of print]10(1):
    Di Iorio P, Ciccarelli R.
      Extracellular vesicles (EVs), mainly classified as small and large EVs according to their size/origin, contribute as multi-signal messengers to intercellular communications in normal/pathological conditions. EVs are now recognized as critical players in cancer processes by promoting transformation, growth, invasion, and drug-resistance of tumor cells thanks to the release of molecules contained inside them (i.e., nucleic acids, lipids and proteins) into the tumor microenvironment (TME). Interestingly, secretion from donor cells and/or uptake of EVs/their content by recipient cells are regulated by extracellular signals present in TME. Among those able to modulate the EV-tumor crosstalk, purines, mainly the adenine-based ones, could be included. Indeed, TME is characterized by high levels of ATP/adenosine and by the presence of enzymes deputed to their turnover. Moreover, ATP/adenosine, interacting with their own receptors, can affect both host and tumor responses. However, studies on whether/how the purinergic system behaves as a modulator of EV biogenesis, release and functions in cancer are still poor. Thus, this review is aimed at collecting data so far obtained to stimulate further research in this regard. Hopefully, new findings on the impact of adenine purines/related enzymes on EV functions may be exploited in tumor management uncovering novel tumor biomarkers and/or druggable targets.
    Keywords:  adenine-based compounds; cancer; exosomes/microvesicles; extracellular vesicles; purine metabolizing enzymes; purinergic receptors
    DOI:  https://doi.org/10.3390/cells10010188