1
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Acton OJ, Sheppard D, Kunzelmann S, Caswell SJ, Nans A, Burgess AJO, Kelly G, Morris ER, Rosenthal PB, Taylor IA. Platform-directed allostery and quaternary structure dynamics of SAMHD1 catalysis. Nat Commun 2024; 15:3775. [PMID: 38710701 PMCID: PMC11074143 DOI: 10.1038/s41467-024-48237-w] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/03/2023] [Accepted: 04/25/2024] [Indexed: 05/08/2024] Open
Abstract
SAMHD1 regulates cellular nucleotide homeostasis, controlling dNTP levels by catalysing their hydrolysis into 2'-deoxynucleosides and triphosphate. In differentiated CD4+ macrophage and resting T-cells SAMHD1 activity results in the inhibition of HIV-1 infection through a dNTP blockade. In cancer, SAMHD1 desensitizes cells to nucleoside-analogue chemotherapies. Here we employ time-resolved cryogenic-EM imaging and single-particle analysis to visualise assembly, allostery and catalysis by this multi-subunit enzyme. Our observations reveal how dynamic conformational changes in the SAMHD1 quaternary structure drive the catalytic cycle. We capture five states at high-resolution in a live catalytic reaction, revealing how allosteric activators support assembly of a stable SAMHD1 tetrameric core and how catalysis is driven by the opening and closing of active sites through pairwise coupling of active sites and order-disorder transitions in regulatory domains. This direct visualisation of enzyme catalysis dynamics within an allostery-stabilised platform sets a precedent for mechanistic studies into the regulation of multi-subunit enzymes.
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Affiliation(s)
- Oliver J Acton
- Macromolecular Structure Laboratory, The Francis Crick Institute, 1 Midland Road, London, NW1 1AT, UK
- Structural Biology of Cells and Viruses Laboratory, The Francis Crick Institute, 1 Midland Road, London, NW1 1AT, UK
- AstraZeneca, The Discovery Centre, 1 Francis Crick Avenue, Cambridge, CB2 0AA, UK
| | - Devon Sheppard
- Macromolecular Structure Laboratory, The Francis Crick Institute, 1 Midland Road, London, NW1 1AT, UK
| | - Simone Kunzelmann
- Structural Biology Science Technology Platform, The Francis Crick Institute, 1 Midland Road, London, NW1 1AT, UK
| | - Sarah J Caswell
- Macromolecular Structure Laboratory, The Francis Crick Institute, 1 Midland Road, London, NW1 1AT, UK
- AstraZeneca, The Discovery Centre, 1 Francis Crick Avenue, Cambridge, CB2 0AA, UK
| | - Andrea Nans
- Structural Biology Science Technology Platform, The Francis Crick Institute, 1 Midland Road, London, NW1 1AT, UK
| | - Ailidh J O Burgess
- Macromolecular Structure Laboratory, The Francis Crick Institute, 1 Midland Road, London, NW1 1AT, UK
| | - Geoff Kelly
- The Medical Research Council Biomedical NMR Centre, The Francis Crick Institute, 1 Midland Road, London, NW1 1AT, UK
| | - Elizabeth R Morris
- Macromolecular Structure Laboratory, The Francis Crick Institute, 1 Midland Road, London, NW1 1AT, UK
- Department of Biosciences, University of Durham, Durham, DH1 3LE, UK
| | - Peter B Rosenthal
- Structural Biology of Cells and Viruses Laboratory, The Francis Crick Institute, 1 Midland Road, London, NW1 1AT, UK.
| | - Ian A Taylor
- Macromolecular Structure Laboratory, The Francis Crick Institute, 1 Midland Road, London, NW1 1AT, UK.
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2
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Orris B, Sung MW, Bhat S, Xu Y, Huynh KW, Han S, Johnson DC, Bosbach B, Shields DJ, Stivers JT. Guanine-containing ssDNA and RNA induce dimeric and tetrameric structural forms of SAMHD1. Nucleic Acids Res 2023; 51:12443-12458. [PMID: 37930833 PMCID: PMC10711556 DOI: 10.1093/nar/gkad971] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/11/2023] [Revised: 09/19/2023] [Accepted: 10/14/2023] [Indexed: 11/08/2023] Open
Abstract
The dNTPase activity of tetrameric SAM and HD domain containing deoxynucleoside triphosphate triphosphohydrolase 1 (SAMHD1) plays a critical role in cellular dNTP regulation. SAMHD1 also associates with stalled DNA replication forks, DNA repair foci, ssRNA and telomeres. The above functions require nucleic acid binding by SAMHD1, which may be modulated by its oligomeric state. Here we establish in cryo-EM and biochemical studies that the guanine-specific A1 activator site of each SAMHD1 monomer is used to target the enzyme to guanine nucleotides within single-stranded (ss) DNA and RNA. Remarkably, nucleic acid strands containing a single guanine base induce dimeric SAMHD1, while two or more guanines with ∼20 nucleotide spacing induce a tetrameric form. A cryo-EM structure of ssRNA-bound tetrameric SAMHD1 shows how ssRNA strands bridge two SAMHD1 dimers and stabilize the structure. This ssRNA-bound tetramer is inactive with respect to dNTPase and RNase activity.
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Affiliation(s)
- Benjamin Orris
- Department of Pharmacology and Molecular Sciences, Johns Hopkins University School of Medicine 725 North Wolfe Street Baltimore, MD 21205, USA
| | | | - Shridhar Bhat
- Department of Pharmacology and Molecular Sciences, Johns Hopkins University School of Medicine 725 North Wolfe Street Baltimore, MD 21205, USA
| | - Yingrong Xu
- Medicine Design, Pfizer, Groton, CT 06340, USA
| | | | - Seungil Han
- Medicine Design, Pfizer, Groton, CT 06340, USA
| | - Darren C Johnson
- Centers for Therapeutic Innovation (CTI), Pfizer, New York, NY 10016, USA
| | - Benedikt Bosbach
- Centers for Therapeutic Innovation (CTI), Pfizer, New York, NY 10016, USA
| | - David J Shields
- Centers for Therapeutic Innovation (CTI), Pfizer, New York, NY 10016, USA
| | - James T Stivers
- Department of Pharmacology and Molecular Sciences, Johns Hopkins University School of Medicine 725 North Wolfe Street Baltimore, MD 21205, USA
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3
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Egleston M, Dong L, Howlader AH, Bhat S, Orris B, Bianchet MA, Greenberg MM, Stivers JT. Deoxyguanosine-Linked Bifunctional Inhibitor of SAMHD1 dNTPase Activity and Nucleic Acid Binding. ACS Chem Biol 2023; 18:2200-2210. [PMID: 37233733 PMCID: PMC10596003 DOI: 10.1021/acschembio.3c00118] [Citation(s) in RCA: 2] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/24/2023] [Accepted: 05/10/2023] [Indexed: 05/27/2023]
Abstract
Sterile alpha motif histidine-aspartate domain protein 1 (SAMHD1) is a deoxynucleotide triphosphohydrolase that exists in monomeric, dimeric, and tetrameric forms. It is activated by GTP binding to an A1 allosteric site on each monomer subunit, which induces dimerization, a prerequisite for dNTP-induced tetramerization. SAMHD1 is a validated drug target stemming from its inactivation of many anticancer nucleoside drugs leading to drug resistance. The enzyme also possesses a single-strand nucleic acid binding function that promotes RNA and DNA homeostasis by several mechanisms. To discover small molecule inhibitors of SAMHD1, we screened a custom ∼69 000-compound library for dNTPase inhibitors. Surprisingly, this effort yielded no viable hits and indicated that exceptional barriers for discovery of small molecule inhibitors existed. We then took a rational fragment-based inhibitor design approach using a deoxyguanosine (dG) A1 site targeting fragment. A targeted chemical library was synthesized by coupling a 5'-phosphoryl propylamine dG fragment (dGpC3NH2) to 376 carboxylic acids (RCOOH). Direct screening of the products (dGpC3NHCO-R) yielded nine initial hits, one of which (R = 3-(3'-bromo-[1,1'-biphenyl]), 5a) was investigated extensively. Amide 5a is a competitive inhibitor against GTP binding to the A1 site and induces inactive dimers that are deficient in tetramerization. Surprisingly, 5a also prevented ssDNA and ssRNA binding, demonstrating that the dNTPase and nucleic acid binding functions of SAMHD1 can be disrupted by a single small molecule. A structure of the SAMHD1-5a complex indicates that the biphenyl fragment impedes a conformational change in the C-terminal lobe that is required for tetramerization.
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Affiliation(s)
- Matthew Egleston
- Department
of Pharmacology and Molecular Sciences, Johns Hopkins University School of Medicine, 725 North Wolfe Street, Baltimore, Maryland 21205, United States
| | - Linghao Dong
- Department
of Pharmacology and Molecular Sciences, Johns Hopkins University School of Medicine, 725 North Wolfe Street, Baltimore, Maryland 21205, United States
| | - A. Hasan Howlader
- Department
of Chemistry, Johns Hopkins University, 3400 North Charles Street, Baltimore, Maryland 21218, United States
| | - Shridhar Bhat
- Department
of Pharmacology and Molecular Sciences, Johns Hopkins University School of Medicine, 725 North Wolfe Street, Baltimore, Maryland 21205, United States
| | - Benjamin Orris
- Department
of Pharmacology and Molecular Sciences, Johns Hopkins University School of Medicine, 725 North Wolfe Street, Baltimore, Maryland 21205, United States
| | - Mario A. Bianchet
- Department
of Neurology and Department of Biophysics and Biophysical Chemistry, Johns Hopkins University School of Medicine, 725 North Wolfe Street, Baltimore, Maryland 21205, United States
| | - Marc M. Greenberg
- Department
of Chemistry, Johns Hopkins University, 3400 North Charles Street, Baltimore, Maryland 21218, United States
| | - James T. Stivers
- Department
of Pharmacology and Molecular Sciences, Johns Hopkins University School of Medicine, 725 North Wolfe Street, Baltimore, Maryland 21205, United States
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4
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Bowen NE, Tao S, Cho YJ, Schinazi RF, Kim B. Vpx requires active cellular dNTP biosynthesis to effectively counteract the anti-lentivirus activity of SAMHD1 in macrophages. J Biol Chem 2023; 299:104984. [PMID: 37390988 PMCID: PMC10374972 DOI: 10.1016/j.jbc.2023.104984] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/01/2023] [Revised: 06/21/2023] [Accepted: 06/22/2023] [Indexed: 07/02/2023] Open
Abstract
HIV-1 replication in primary monocyte-derived macrophages (MDMs) is kinetically restricted at the reverse transcription step due to the low deoxynucleoside triphosphates (dNTP) pools established by host dNTPase, SAM and HD domain containing protein 1 (SAMHD1). Lentiviruses such as HIV-2 and some Simian immunodeficiency virus counteract this restriction using viral protein X (Vpx), which proteosomally degrades SAMHD1 and elevates intracellular dNTP pools. However, how dNTP pools increase after Vpx degrades SAMHD1 in nondividing MDMs where no active dNTP biosynthesis is expected to exists remains unclear. In this study, we monitored known dNTP biosynthesis machinery during primary human monocyte differentiation to MDMs and unexpectedly found MDMs actively express dNTP biosynthesis enzymes such as ribonucleotide reductase, thymidine kinase 1, and nucleoside-diphosphate kinase. During differentiation from monocytes the expression levels of several biosynthesis enzymes are upregulated, while there is an increase in inactivating SAMHD1 phosphorylation. Correspondingly, we observed significantly lower levels of dNTPs in monocytes compared to MDMs. Without dNTP biosynthesis availability, Vpx failed to elevate dNTPs in monocytes, despite SAMHD1 degradation. These extremely low monocyte dNTP concentrations, which cannot be elevated by Vpx, impaired HIV-1 reverse transcription in a biochemical simulation. Furthermore, Vpx failed to rescue the transduction efficiency of a HIV-1 GFP vector in monocytes. Collectively, these data suggest that MDMs harbor active dNTP biosynthesis and Vpx requires this dNTP biosynthesis to elevate dNTP levels to effectively counteract SAMHD1 and relieve the kinetic block to HIV-1 reverse transcription in MDMs.
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Affiliation(s)
- Nicole E Bowen
- Department of Pediatrics, Center for ViroScience and Cure, School of Medicine, Emory University, Atlanta, Georgia, USA
| | - Sijia Tao
- Department of Pediatrics, Center for ViroScience and Cure, School of Medicine, Emory University, Atlanta, Georgia, USA
| | - Young-Jae Cho
- Department of Pediatrics, Center for ViroScience and Cure, School of Medicine, Emory University, Atlanta, Georgia, USA
| | - Raymond F Schinazi
- Department of Pediatrics, Center for ViroScience and Cure, School of Medicine, Emory University, Atlanta, Georgia, USA; Children's Healthcare of Atlanta, Atlanta, Georgia, USA
| | - Baek Kim
- Department of Pediatrics, Center for ViroScience and Cure, School of Medicine, Emory University, Atlanta, Georgia, USA; Children's Healthcare of Atlanta, Atlanta, Georgia, USA.
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5
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Orris B, Sung MW, Bhat S, Xu Y, Huynh KW, Han S, Johnson DC, Bosbach B, Shields DJ, Stivers JT. Guanine-containing ssDNA and RNA induce dimeric and tetrameric SAMHD1 in cryo-EM and binding studies. BIORXIV : THE PREPRINT SERVER FOR BIOLOGY 2023:2023.06.15.544806. [PMID: 37398126 PMCID: PMC10312740 DOI: 10.1101/2023.06.15.544806] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 07/04/2023]
Abstract
The dNTPase activity of tetrameric SAM and HD domain containing deoxynucleoside triphosphate triphosphohydrolase 1 (SAMHD1) plays a critical role in cellular dNTP regulation. SAMHD1 also associates with stalled DNA replication forks, DNA repair foci, ssRNA, and telomeres. The above functions require nucleic acid binding by SAMHD1, which may be modulated by its oligomeric state. Here we establish that the guanine-specific A1 activator site of each SAMHD1 monomer is used to target the enzyme to guanine nucleotides within single-stranded (ss) DNA and RNA. Remarkably, nucleic acid strands containing a single guanine base induce dimeric SAMHD1, while two or more guanines with ~20 nucleotide spacing induce a tetrameric form. A cryo-EM structure of ssRNA-bound tetrameric SAMHD1 shows how ssRNA strands bridge two SAMHD1 dimers and stabilize the structure. This ssRNA-bound tetramer is inactive with respect to dNTPase and RNase activity.
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Affiliation(s)
- Benjamin Orris
- Department of Pharmacology and Molecular Sciences, Johns Hopkins University School of Medicine 725 North Wolfe Street Baltimore, MD 21205
| | | | - Shridhar Bhat
- Department of Pharmacology and Molecular Sciences, Johns Hopkins University School of Medicine 725 North Wolfe Street Baltimore, MD 21205
| | | | | | | | - Darren C. Johnson
- Centers for Therapeutic Innovation (CTI), Pfizer, New York, NY 10016
| | - Benedikt Bosbach
- Centers for Therapeutic Innovation (CTI), Pfizer, New York, NY 10016
| | - David J. Shields
- Centers for Therapeutic Innovation (CTI), Pfizer, New York, NY 10016
| | - James T. Stivers
- Department of Pharmacology and Molecular Sciences, Johns Hopkins University School of Medicine 725 North Wolfe Street Baltimore, MD 21205
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6
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Tsai MHC, Caswell SJ, Morris ER, Mann MC, Pennell S, Kelly G, Groom HCT, Taylor IA, Bishop KN. Attenuation of reverse transcriptase facilitates SAMHD1 restriction of HIV-1 in cycling cells. Retrovirology 2023; 20:5. [PMID: 37127613 PMCID: PMC10150492 DOI: 10.1186/s12977-023-00620-z] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/24/2023] [Accepted: 04/06/2023] [Indexed: 05/03/2023] Open
Abstract
BACKGROUND SAMHD1 is a deoxynucleotide triphosphohydrolase that restricts replication of HIV-1 in differentiated leucocytes. HIV-1 is not restricted in cycling cells and it has been proposed that this is due to phosphorylation of SAMHD1 at T592 in these cells inactivating the enzymatic activity. To distinguish between theories for how SAMHD1 restricts HIV-1 in differentiated but not cycling cells, we analysed the effects of substitutions at T592 on restriction and dNTP levels in both cycling and differentiated cells as well as tetramer stability and enzymatic activity in vitro. RESULTS We first showed that HIV-1 restriction was not due to SAMHD1 nuclease activity. We then characterised a panel of SAMHD1 T592 mutants and divided them into three classes. We found that a subset of mutants lost their ability to restrict HIV-1 in differentiated cells which generally corresponded with a decrease in triphosphohydrolase activity and/or tetramer stability in vitro. Interestingly, no T592 mutants were able to restrict WT HIV-1 in cycling cells, despite not being regulated by phosphorylation and retaining their ability to hydrolyse dNTPs. Lowering dNTP levels by addition of hydroxyurea did not give rise to restriction. Compellingly however, HIV-1 RT mutants with reduced affinity for dNTPs were significantly restricted by wild-type and T592 mutant SAMHD1 in both cycling U937 cells and Jurkat T-cells. Restriction correlated with reverse transcription levels. CONCLUSIONS Altogether, we found that the amino acid at residue 592 has a strong effect on tetramer formation and, although this is not a simple "on/off" switch, this does correlate with the ability of SAMHD1 to restrict HIV-1 replication in differentiated cells. However, preventing phosphorylation of SAMHD1 and/or lowering dNTP levels by adding hydroxyurea was not enough to restore restriction in cycling cells. Nonetheless, lowering the affinity of HIV-1 RT for dNTPs, showed that restriction is mediated by dNTP levels and we were able to observe for the first time that SAMHD1 is active and capable of inhibiting HIV-1 replication in cycling cells, if the affinity of RT for dNTPs is reduced. This suggests that the very high affinity of HIV-1 RT for dNTPs prevents HIV-1 restriction by SAMHD1 in cycling cells.
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Affiliation(s)
- Ming-Han C Tsai
- Retroviral Replication Laboratory, The Francis Crick Institute, London, UK
- LabGenius, London, UK
| | - Sarah J Caswell
- Macromolecular Structure Laboratory, The Francis Crick Institute, London, UK
- AstraZeneca, Granta Park, Cambridge, UK
| | - Elizabeth R Morris
- Macromolecular Structure Laboratory, The Francis Crick Institute, London, UK
- Department of Biosciences, University of Durham, Durham, UK
| | - Melanie C Mann
- Retroviral Replication Laboratory, The Francis Crick Institute, London, UK
- Sartorius, Ulm, Germany
| | - Simon Pennell
- Structural Biology of DNA-Damage Signalling Laboratory, The Francis Crick Institute, London, UK
- MRC London Institute of Medical Sciences, London, UK
| | - Geoff Kelly
- The Medical Research Council Biomedical NMR Centre, The Francis Crick Institute, London, UK
| | - Harriet C T Groom
- Retroviral Replication Laboratory, The Francis Crick Institute, London, UK
- Department of Medicine, University of Cambridge, Cambridge, UK
| | - Ian A Taylor
- Macromolecular Structure Laboratory, The Francis Crick Institute, London, UK
| | - Kate N Bishop
- Retroviral Replication Laboratory, The Francis Crick Institute, London, UK.
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7
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Kapoor-Vazirani P, Rath SK, Liu X, Shu Z, Bowen NE, Chen Y, Haji-Seyed-Javadi R, Daddacha W, Minten EV, Danelia D, Farchi D, Duong DM, Seyfried NT, Deng X, Ortlund EA, Kim B, Yu DS. SAMHD1 deacetylation by SIRT1 promotes DNA end resection by facilitating DNA binding at double-strand breaks. Nat Commun 2022; 13:6707. [PMID: 36344525 PMCID: PMC9640623 DOI: 10.1038/s41467-022-34578-x] [Citation(s) in RCA: 9] [Impact Index Per Article: 4.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/21/2022] [Accepted: 10/29/2022] [Indexed: 11/09/2022] Open
Abstract
Sterile alpha motif and HD domain-containing protein 1 (SAMHD1) has a dNTPase-independent function in promoting DNA end resection to facilitate DNA double-strand break (DSB) repair by homologous recombination (HR); however, it is not known if upstream signaling events govern this activity. Here, we show that SAMHD1 is deacetylated by the SIRT1 sirtuin deacetylase, facilitating its binding with ssDNA at DSBs, to promote DNA end resection and HR. SIRT1 complexes with and deacetylates SAMHD1 at conserved lysine 354 (K354) specifically in response to DSBs. K354 deacetylation by SIRT1 promotes DNA end resection and HR but not SAMHD1 tetramerization or dNTPase activity. Mechanistically, K354 deacetylation by SIRT1 promotes SAMHD1 recruitment to DSBs and binding to ssDNA at DSBs, which in turn facilitates CtIP ssDNA binding, leading to promotion of genome integrity. These findings define a mechanism governing the dNTPase-independent resection function of SAMHD1 by SIRT1 deacetylation in promoting HR and genome stability.
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Affiliation(s)
- Priya Kapoor-Vazirani
- Department of Radiation Oncology and Winship Cancer Institute, Emory University School of Medicine, Atlanta, GA, 30322, USA
| | - Sandip K Rath
- Department of Radiation Oncology and Winship Cancer Institute, Emory University School of Medicine, Atlanta, GA, 30322, USA
| | - Xu Liu
- Department of Biochemistry, Emory University School of Medicine, Atlanta, GA, 30322, USA
| | - Zhen Shu
- Department of Radiation Oncology and Winship Cancer Institute, Emory University School of Medicine, Atlanta, GA, 30322, USA
| | - Nicole E Bowen
- Department of Pediatrics, Emory University School of Medicine, Atlanta, GA, 30322, USA
| | - Yitong Chen
- Department of Radiation Oncology and Winship Cancer Institute, Emory University School of Medicine, Atlanta, GA, 30322, USA
| | - Ramona Haji-Seyed-Javadi
- Department of Radiation Oncology and Winship Cancer Institute, Emory University School of Medicine, Atlanta, GA, 30322, USA
| | - Waaqo Daddacha
- Department of Biochemistry and Molecular Biology, Medical College of Georgia at Augusta University, Augusta, GA, 30912, USA
| | - Elizabeth V Minten
- Department of Radiation Oncology and Winship Cancer Institute, Emory University School of Medicine, Atlanta, GA, 30322, USA
| | - Diana Danelia
- Department of Radiation Oncology and Winship Cancer Institute, Emory University School of Medicine, Atlanta, GA, 30322, USA
| | - Daniela Farchi
- Department of Radiation Oncology and Winship Cancer Institute, Emory University School of Medicine, Atlanta, GA, 30322, USA
| | - Duc M Duong
- Department of Biochemistry, Emory University School of Medicine, Atlanta, GA, 30322, USA
| | - Nicholas T Seyfried
- Department of Biochemistry, Emory University School of Medicine, Atlanta, GA, 30322, USA
| | - Xingming Deng
- Department of Radiation Oncology and Winship Cancer Institute, Emory University School of Medicine, Atlanta, GA, 30322, USA
| | - Eric A Ortlund
- Department of Biochemistry, Emory University School of Medicine, Atlanta, GA, 30322, USA
| | - Baek Kim
- Department of Pediatrics, Emory University School of Medicine, Atlanta, GA, 30322, USA
| | - David S Yu
- Department of Radiation Oncology and Winship Cancer Institute, Emory University School of Medicine, Atlanta, GA, 30322, USA.
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