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Muscle MRI characteristic pattern for late-onset TK2 deficiency diagnosis. J Neurol 2022; 269:3550-3562. [PMID: 35286480 PMCID: PMC9217784 DOI: 10.1007/s00415-021-10957-0] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/08/2021] [Revised: 12/29/2021] [Accepted: 12/30/2021] [Indexed: 11/13/2022]
Abstract
Background and objective TK2 deficiency (TK2d) is a rare mitochondrial disorder that manifests predominantly as a progressive myopathy with a broad spectrum of severity and age of onset. The rate of progression is variable, and the prognosis is poor due to early and severe respiratory involvement. Early and accurate diagnosis is particularly important since a specific treatment is under development. This study aims to evaluate the diagnostic value of lower limb muscle MRI in adult patients with TK2d. Methods We studied a cohort of 45 genetically confirmed patients with mitochondrial myopathy (16 with mutations in TK2, 9 with mutations in other nuclear genes involved in mitochondrial DNA [mtDNA] synthesis or maintenance, 10 with single mtDNA deletions, and 10 with point mtDNA mutations) to analyze the imaging pattern of fat replacement in lower limb muscles. We compared the identified pattern in patients with TK2d with the MRI pattern of other non-mitochondrial genetic myopathies that share similar clinical characteristics. Results We found a consistent lower limb muscle MRI pattern in patients with TK2d characterized by involvement of the gluteus maximus, gastrocnemius medialis, and sartorius muscles. The identified pattern in TK2 patients differs from the known radiological involvement of other resembling muscle dystrophies that share clinical features. Conclusions By analyzing the largest cohort of muscle MRI from patients with mitochondrial myopathies studied to date, we identified a characteristic and specific radiological pattern of muscle involvement in patients with TK2d that could be useful to speed up its diagnosis. Supplementary Information The online version contains supplementary material available at 10.1007/s00415-021-10957-0.
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Deoxythymidylate kinase, DTYMK, is a novel gene for mitochondrial DNA depletion syndrome. Clin Chim Acta 2019; 496:93-99. [DOI: 10.1016/j.cca.2019.06.028] [Citation(s) in RCA: 11] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/18/2019] [Revised: 06/28/2019] [Accepted: 06/30/2019] [Indexed: 12/12/2022]
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Bioavailability and cytosolic kinases modulate response to deoxynucleoside therapy in TK2 deficiency. EBioMedicine 2019; 46:356-367. [PMID: 31383553 PMCID: PMC6710986 DOI: 10.1016/j.ebiom.2019.07.037] [Citation(s) in RCA: 16] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/04/2019] [Revised: 07/10/2019] [Accepted: 07/15/2019] [Indexed: 12/21/2022] Open
Abstract
BACKGROUND TK2 is a nuclear gene encoding the mitochondrial matrix protein thymidine kinase 2 (TK2), a critical enzyme in the mitochondrial nucleotide salvage pathway. Deficiency of TK2 activity causes mitochondrial DNA (mtDNA) depletion, which in humans manifests predominantly as a mitochondrial myopathy with onset typically in infancy and childhood. We previously showed that oral treatment of the Tk2 H126N knock-in mouse model (Tk2-/-) with the TK2 substrates, deoxycytidine (dCtd) and thymidine (dThd), delayed disease onset and prolonged median survival by 3-fold. Nevertheless, dCtd + dThd treated Tk2-/- mice showed mtDNA depletion in brain as early as postnatal day 13 and in virtually all other tissues at age 29 days. METHODS To enhance mechanistic understanding and efficacy of dCtd + dThd therapy, we studied the bioavailability of dCtd and dThd in various tissues as well as levels of the cytosolic enzymes, TK1 and dCK that convert the deoxynucleosides into dCMP and dTMP. FINDINGS Parenteral treatment relative to oral treatment produced higher levels of dCtd and dThd and improved mtDNA levels in liver and heart, but did not ameliorate molecular defects in brain or prolong survival. Down-regulation of TK1 correlated with temporal- and tissue-specificity of response to dCtd + dThd. Finally, we observed in human infant and adult muscle expression of TK1 and dCK, which account for the long-term efficacy to dCtd + dThd therapy in TK2 deficient patients. INTERPRETATIONS These data indicate that the cytosolic pyrimidine salvage pathway enzymes TK1 and dCK are critical for therapeutic efficacy of deoxynucleoside therapy for Tk2 deficiency. FUND: National Institutes of Health P01HD32062.
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Blázquez-Bermejo C, Molina-Granada D, Vila-Julià F, Jiménez-Heis D, Zhou X, Torres-Torronteras J, Karlsson A, Martí R, Cámara Y. Age-related metabolic changes limit efficacy of deoxynucleoside-based therapy in thymidine kinase 2-deficient mice. EBioMedicine 2019; 46:342-355. [PMID: 31351931 PMCID: PMC6711114 DOI: 10.1016/j.ebiom.2019.07.042] [Citation(s) in RCA: 20] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/04/2019] [Revised: 07/05/2019] [Accepted: 07/15/2019] [Indexed: 01/01/2023] Open
Abstract
Background Thymidine kinase 2 (TK2) catalyses the phosphorylation of deoxythymidine (dThd) and deoxycytidine (dCtd) within mitochondria. TK2 deficiency leads to mtDNA depletion or accumulation of multiple deletions. In patients, TK2 mutations typically manifest as a rapidly progressive myopathy with infantile onset, leading to respiratory insufficiency and encephalopathy in the most severe clinical presentations. TK2-deficient mice develop the most severe form of the disease and die at average postnatal day 16. dThd+dCtd administration delayed disease progression and expanded lifespan of a knockin murine model of the disease. Methods We daily administered TK2 knockout mice (Tk2KO) from postnatal day 4 with equimolar doses of dThd+dCtd, dTMP+dCMP, dThd alone or dCtd alone. We monitored body weight and survival and studied different variables at 12 or 29 days of age. We determined metabolite levels in plasma and target tissues, mtDNA copy number in tissues, and the expression and activities of enzymes with a relevant role in mitochondrial dNTP anabolism or catabolism. Findings dThd+dCtd treatment extended average lifespan of Tk2KO mice from 16 to 34 days, attenuated growth retardation, and rescued mtDNA depletion in skeletal muscle and other target tissues of 12-day-old mice, except in brain. However, the treatment was ineffective in 29-day-old mice that still died prematurely. Bioavailability of dThd and dCtd markedly decreased during mouse development. Activity of enzymes catabolizing dThd and dCtd increased with age in small intestine. Conversely, the activity of the anabolic enzymes decreased in target tissues during mouse development. We also found that administration of dThd alone had the same impact on survival to that of dThd+dCtd, whereas dCtd alone had no influence on lifespan. Interpretation dThd+dCtd treatment recruits alternative cytosolic salvage pathways for dNTP synthesis, suggesting that this therapy would be of benefit for any Tk2 mutation. dThd accounts for the therapeutic effect of the combined treatment in mice. During the first weeks after birth, mice experience marked tissue-specific metabolic regulations and ontogenetic changes in dNTP metabolism-related enzymes that limit therapeutic efficacy to early developmental stages. Fund This study was funded by grants from the Spanish Ministry of Industry, Economy and Competitiveness, the Spanish Instituto de Salud Carlos III, the Fundación Inocente, Inocente, AFM Téléthon and the Generalitat de Catalunya. The disclosed funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
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Affiliation(s)
- Cora Blázquez-Bermejo
- Research Group on Neuromuscular and Mitochondrial Disorders, Vall d'Hebron Institut de Recerca, Universitat Autònoma de Barcelona, Barcelona, Spain; Biomedical Network Research Centre on Rare Diseases (CIBERER), Instituto de Salud Carlos III, Madrid, Spain
| | - David Molina-Granada
- Research Group on Neuromuscular and Mitochondrial Disorders, Vall d'Hebron Institut de Recerca, Universitat Autònoma de Barcelona, Barcelona, Spain; Biomedical Network Research Centre on Rare Diseases (CIBERER), Instituto de Salud Carlos III, Madrid, Spain
| | - Ferran Vila-Julià
- Research Group on Neuromuscular and Mitochondrial Disorders, Vall d'Hebron Institut de Recerca, Universitat Autònoma de Barcelona, Barcelona, Spain; Biomedical Network Research Centre on Rare Diseases (CIBERER), Instituto de Salud Carlos III, Madrid, Spain
| | - Daniel Jiménez-Heis
- Research Group on Neuromuscular and Mitochondrial Disorders, Vall d'Hebron Institut de Recerca, Universitat Autònoma de Barcelona, Barcelona, Spain
| | - Xiaoshan Zhou
- Division of Clinical Microbiology, Department of Laboratory Medicine, Karolinska Institute, Karolinska University Hospital, 141 86 Stockholm, Sweden
| | - Javier Torres-Torronteras
- Research Group on Neuromuscular and Mitochondrial Disorders, Vall d'Hebron Institut de Recerca, Universitat Autònoma de Barcelona, Barcelona, Spain; Biomedical Network Research Centre on Rare Diseases (CIBERER), Instituto de Salud Carlos III, Madrid, Spain
| | - Anna Karlsson
- Division of Clinical Microbiology, Department of Laboratory Medicine, Karolinska Institute, Karolinska University Hospital, 141 86 Stockholm, Sweden
| | - Ramon Martí
- Research Group on Neuromuscular and Mitochondrial Disorders, Vall d'Hebron Institut de Recerca, Universitat Autònoma de Barcelona, Barcelona, Spain; Biomedical Network Research Centre on Rare Diseases (CIBERER), Instituto de Salud Carlos III, Madrid, Spain
| | - Yolanda Cámara
- Research Group on Neuromuscular and Mitochondrial Disorders, Vall d'Hebron Institut de Recerca, Universitat Autònoma de Barcelona, Barcelona, Spain; Biomedical Network Research Centre on Rare Diseases (CIBERER), Instituto de Salud Carlos III, Madrid, Spain.
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Franzolin E, Salata C, Bianchi V, Rampazzo C. The Deoxynucleoside Triphosphate Triphosphohydrolase Activity of SAMHD1 Protein Contributes to the Mitochondrial DNA Depletion Associated with Genetic Deficiency of Deoxyguanosine Kinase. J Biol Chem 2015; 290:25986-96. [PMID: 26342080 PMCID: PMC4646252 DOI: 10.1074/jbc.m115.675082] [Citation(s) in RCA: 25] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/26/2015] [Indexed: 11/22/2022] Open
Abstract
The dNTP triphosphohydrolase SAMHD1 is a nuclear antiviral host restriction factor limiting HIV-1 infection in macrophages and a major regulator of dNTP concentrations in human cells. In normal human fibroblasts its expression increases during quiescence, contributing to the small dNTP pool sizes of these cells. Down-regulation of SAMHD1 by siRNA expands all four dNTP pools, with dGTP undergoing the largest relative increase. The deoxyguanosine released by SAMHD1 from dGTP can be phosphorylated inside mitochondria by deoxyguanosine kinase (dGK) or degraded in the cytosol by purine nucleoside phosphorylase. Genetic mutations of dGK cause mitochondrial (mt) DNA depletion in noncycling cells and hepato-cerebral mtDNA depletion syndrome in humans. We studied if SAMHD1 and dGK interact in the regulation of the dGTP pool during quiescence employing dGK-mutated skin fibroblasts derived from three unrelated patients. In the presence of SAMHD1 quiescent mutant fibroblasts manifested mt dNTP pool imbalance and mtDNA depletion. When SAMHD1 was silenced by siRNA transfection the composition of the mt dNTP pool approached that of the controls, and mtDNA copy number increased, compensating the depletion to various degrees in the different mutant fibroblasts. Chemical inhibition of purine nucleoside phosphorylase did not improve deoxyguanosine recycling by dGK in WT cells. We conclude that the activity of SAMHD1 contributes to the pathological phenotype of dGK deficiency. Our results prove the importance of SAMHD1 in the regulation of all dNTP pools and suggest that dGK inside mitochondria has the function of recycling the deoxyguanosine derived from endogenous dGTP degraded by SAMHD1 in the nucleus.
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Affiliation(s)
- Elisa Franzolin
- From the Department of Biology, University of Padova, 35131 Padova, Italy and
| | - Cristiano Salata
- Department of Molecular Medicine, University of Padova, 35121 Padova, Italy
| | - Vera Bianchi
- From the Department of Biology, University of Padova, 35131 Padova, Italy and
| | - Chiara Rampazzo
- From the Department of Biology, University of Padova, 35131 Padova, Italy and
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Garone C, Garcia-Diaz B, Emmanuele V, Lopez LC, Tadesse S, Akman HO, Tanji K, Quinzii CM, Hirano M. Deoxypyrimidine monophosphate bypass therapy for thymidine kinase 2 deficiency. EMBO Mol Med 2015; 6:1016-27. [PMID: 24968719 PMCID: PMC4154130 DOI: 10.15252/emmm.201404092] [Citation(s) in RCA: 72] [Impact Index Per Article: 8.0] [Reference Citation Analysis] [Abstract] [Key Words] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/20/2022] Open
Abstract
Autosomal recessive mutations in the thymidine kinase 2 gene (TK2) cause mitochondrial DNA depletion, multiple deletions, or both due to loss of TK2 enzyme activity and ensuing unbalanced deoxynucleotide triphosphate (dNTP) pools. To bypass Tk2 deficiency, we administered deoxycytidine and deoxythymidine monophosphates (dCMP+dTMP) to the Tk2 H126N (Tk2(-/-)) knock-in mouse model from postnatal day 4, when mutant mice are phenotypically normal, but biochemically affected. Assessment of 13-day-old Tk2(-/-) mice treated with dCMP+dTMP 200 mg/kg/day each (Tk2(-/-200dCMP/) (dTMP)) demonstrated that in mutant animals, the compounds raise dTTP concentrations, increase levels of mtDNA, ameliorate defects of mitochondrial respiratory chain enzymes, and significantly prolong their lifespan (34 days with treatment versus 13 days untreated). A second trial of dCMP+dTMP each at 400 mg/kg/day showed even greater phenotypic and biochemical improvements. In conclusion, dCMP/dTMP supplementation is the first effective pharmacologic treatment for Tk2 deficiency.
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Affiliation(s)
- Caterina Garone
- Department of Neurology, Columbia University Medical Center, New York, NY, USA Human Genetics Joint PhD Program, University of Bologna and Turin, Turin, Italy
| | - Beatriz Garcia-Diaz
- Department of Neurology, Columbia University Medical Center, New York, NY, USA
| | - Valentina Emmanuele
- Department of Neurology, Columbia University Medical Center, New York, NY, USA Pediatric Clinic University of Genoa IRCCS G. Gaslini Institute, Genoa, Italy
| | - Luis C Lopez
- Instituto de Biotecnología, Centro de Investigación Biomédica, Universidad de Granada Parque Tecnológico de Ciencias de la Salud, Armilla, Spain
| | - Saba Tadesse
- Department of Neurology, Columbia University Medical Center, New York, NY, USA
| | - Hasan O Akman
- Department of Neurology, Columbia University Medical Center, New York, NY, USA
| | - Kurenai Tanji
- Department of Pathology and Cell Biology, Columbia University Medical Center, New York, NY, USA
| | - Catarina M Quinzii
- Department of Neurology, Columbia University Medical Center, New York, NY, USA
| | - Michio Hirano
- Department of Neurology, Columbia University Medical Center, New York, NY, USA
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Sarig O, Goldsher D, Nousbeck J, Fuchs-Telem D, Cohen-Katsenelson K, Iancu TC, Manov I, Saada A, Sprecher E, Mandel H. Infantile mitochondrial hepatopathy is a cardinal feature of MEGDEL syndrome (3-methylglutaconic aciduria type IV with sensorineural deafness, encephalopathy and Leigh-like syndrome) caused by novel mutations in SERAC1. Am J Med Genet A 2013; 161A:2204-15. [PMID: 23918762 DOI: 10.1002/ajmg.a.36059] [Citation(s) in RCA: 34] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/14/2012] [Accepted: 04/19/2013] [Indexed: 12/17/2022]
Abstract
3-Methylglutaconic aciduria (3-MGCA) type IV is defined as a heterogeneous group of inborn errors featuring in common 3-MGCA and associated with primary mitochondrial dysfunction leading to a spectrum of multisystem conditions. We studied four patients who presented at birth with a clinical picture simulating a primary mitochondrial hepatic disorder consistent with the MEGDEL syndrome including 3-MGCA, sensorineural deafness, encephalopathy and a brain magnetic resonance imaging with signs of Leigh disease. All affected children displayed biochemical features consistent with mitochondrial OXPHOS dysfunction including hepatic mitochondrial DNA depletion in one patient. Homozygosity mapping identified a candidate locus on 6q25.2-6q26. Using whole exome sequencing, we identified two novel homozygous mutations in SERAC1 recently reported to harbor mutations in MEGDEL syndrome. Both mutations were found to lead to decreased or absent expression of SERAC1. The present findings indicate that infantile hepatopathy is a cardinal feature of MEGDEL syndrome. We thus propose to rename the disease MEGDHEL syndrome.
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Affiliation(s)
- Ofer Sarig
- Department of Dermatology, Tel Aviv Sourasky Medical Center, Tel Aviv, Israel
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Koczor CA, Torres RA, Fields EJ, Boyd A, He S, Patel N, Lee EK, Samarel AM, Lewis W. Thymidine kinase and mtDNA depletion in human cardiomyopathy: epigenetic and translational evidence for energy starvation. Physiol Genomics 2013; 45:590-6. [PMID: 23695887 DOI: 10.1152/physiolgenomics.00014.2013] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/22/2023] Open
Abstract
This study addresses how depletion of human cardiac left ventricle (LV) mitochondrial DNA (mtDNA) and epigenetic nuclear DNA methylation promote cardiac dysfunction in human dilated cardiomyopathy (DCM) through regulation of pyrimidine nucleotide kinases. Samples of DCM LV and right ventricle (n = 18) were obtained fresh at heart transplant surgery. Parallel samples from nonfailing (NF) controls (n = 12) were from donor hearts found unsuitable for clinical use. We analyzed abundance of mtDNA and nuclear DNA (nDNA) using qPCR. LV mtDNA was depleted in DCM (50%, P < 0.05 each) compared with NF. No detectable change in RV mtDNA abundance occurred. DNA methylation and gene expression were determined using microarray analysis (GEO accession number: GSE43435). Fifty-seven gene promoters exhibited DNA hypermethylation or hypomethylation in DCM LVs. Among those, cytosolic thymidine kinase 1 (TK1) was hypermethylated. Expression arrays revealed decreased abundance of the TK1 mRNA transcript with no change in transcripts for other relevant thymidine metabolism enzymes. Quantitative immunoblots confirmed decreased TK1 polypeptide steady state abundance. TK1 activity remained unchanged in DCM samples while mitochondrial thymidine kinase (TK2) activity was significantly reduced. Compensatory TK activity was found in cardiac myocytes in the DCM LV. Diminished TK2 activity is mechanistically important to reduced mtDNA abundance and identified in DCM LV samples here. Epigenetic and genetic changes result in changes in mtDNA and in nucleotide substrates for mtDNA replication and underpin energy starvation in DCM.
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Affiliation(s)
- Christopher A Koczor
- Department of Pathology, Emory University School of Medicine, Atlanta, Georgia, USA
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Huber-Ruano I, Errasti-Murugarren E, Godoy V, Vera Á, Andreu AL, Garcia-Arumi E, Martí R, Pastor-Anglada M. Functional outcome of a novel SLC29A3 mutation identified in a patient with H syndrome. Biochem Biophys Res Commun 2012; 428:532-7. [DOI: 10.1016/j.bbrc.2012.09.143] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/28/2012] [Accepted: 09/29/2012] [Indexed: 01/24/2023]
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Abstract
Because deoxyribonucleoside triphosphates (dNTPs) are the critical substrates for DNA replication and repair, dNTP pools have been studied in context of multiple basic biochemical processes. Over the last 12 years, interest in dNTPs, and specifically the mitochondrial dNTP pools, has expanded to biomedical science because several mitochondrial diseases have been found to be caused by dysfunctions of several enzymes involved in dNTP catabolism or anabolism. Techniques to reliably measure mitochondrial dNTPs should be sensitive and specific to avoid interference caused by the abundant ribonucleotides. Here, we describe detailed protocols to measure mitochondrial dNTPs from two specific samples, cultured skin fibroblasts and mouse liver. The methods can be easily adapted to other types of samples. The protocol follows a polymerase-based method, which is the most widely used approach to measure dNTP pools. Our description is based on the latest update of the technique, which minimizes the potential interference from ribonucleotides.
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Affiliation(s)
- Ramon Martí
- Laboratori de Patologia Mitocondrial, Institut de Recerca Hospital Universitari Vall D'Hebron, Universitat Autonoma de Barcelona, Barcelona, Spain
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Gandhi VV, Samuels DC. A review comparing deoxyribonucleoside triphosphate (dNTP) concentrations in the mitochondrial and cytoplasmic compartments of normal and transformed cells. NUCLEOSIDES NUCLEOTIDES & NUCLEIC ACIDS 2011; 30:317-39. [PMID: 21774628 DOI: 10.1080/15257770.2011.586955] [Citation(s) in RCA: 40] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/08/2023]
Abstract
The deoxyribonucleoside triphosphate (dNTP) pools that support the replication of mitochondrial DNA are physically separated from the rest of the cell by the double membrane of the mitochondria. Perturbed homeostasis of mitochondrial dNTP pools is associated with a set of severe diseases collectively termed mitochondrial DNA depletion syndromes. The degree of interaction of the mitochondrial dNTP pools with the corresponding dNTP pools in the cytoplasm is currently not clear. We reviewed the literature on previously reported simultaneous measurements of mitochondrial and cytoplasmic deoxyribonucleoside triphosphate pools to investigate and quantify the extent of the influence of the cytoplasmic nucleotide metabolism on mitochondrial dNTP pools. We converted the reported measurements to concentrations creating a catalog of paired mitochondrial and cytoplasmic dNTP concentration measurements. Over experiments from multiple laboratories, dNTP concentrations in the mitochondria are highly correlated with dNTP concentrations in the cytoplasm in normal cells in culture (Pearson R = 0.79, p = 3 × 10(-7)) but not in transformed cells. For dTTP and dATP there was a strong linear relationship between the cytoplasmic and mitochondrial concentrations in normal cells. From this linear model we hypothesize that the salvage pathway within the mitochondrion is only capable of forming a concentration of approximately 2 μM of dTTP and dATP, and that higher concentrations require transport of deoxyribonucleotides from the cytoplasm.
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Affiliation(s)
- Vishal V Gandhi
- Center for Human Genetics Research, Department of Molecular Physiology and Biophysics, Vanderbilt University Medical Center, Nashville, TN 37232-0700, USA
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Villarroya J, Lara MC, Dorado B, Garrido M, García-Arumí E, Meseguer A, Hirano M, Vilà MR. Targeted impairment of thymidine kinase 2 expression in cells induces mitochondrial DNA depletion and reveals molecular mechanisms of compensation of mitochondrial respiratory activity. Biochem Biophys Res Commun 2011; 407:333-8. [PMID: 21382338 PMCID: PMC7586248 DOI: 10.1016/j.bbrc.2011.03.018] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/01/2011] [Accepted: 03/03/2011] [Indexed: 11/27/2022]
Abstract
The mitochondrial DNA (mtDNA) depletion syndrome comprises a clinically heterogeneous group of diseases characterized by reductions of the mtDNA abundance, without associated point mutations or rearrangements. We have developed the first in vitro model to study of mtDNA depletion due to reduced mitochondrial thymidine kinase 2 gene (TK2) expression in order to understand the molecular mechanisms involved in mtDNA depletion syndrome due to TK2 mutations. Small interfering RNA targeting TK2 mRNA was used to decrease TK2 expression in Ost TK1(-) cells, a cell line devoid of endogenous thymidine kinase 1 (TK1). Stable TK2-deficient cell lines showed a reduction of TK2 levels close to 80%. In quiescent conditions, TK2-deficient cells showed severe mtDNA depletion, also close to 80% the control levels. However, TK2-deficient clones showed increased cytochrome c oxidase activity, higher cytochrome c oxidase subunit I transcript levels and higher subunit II protein expression respect to control cells. No alterations of the deoxynucleotide pools were found, whereas a reduction in the expression of genes involved in nucleoside/nucleotide homeostasis (human equilibrative nucleoside transporter 1, thymidine phosphorylase) and mtDNA maintenance (DNA-polymerase γ, mitochondrial transcription factor A) was observed. Our findings highlight the importance of cellular compensatory mechanisms that enhance the expression of respiratory components to ensure respiratory activity despite profound depletion in mtDNA levels.
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Affiliation(s)
- Joan Villarroya
- Institut de Recerca, Hospital Universitari de la Vall d'Hebron, Barcelona, Spain; Institut de Recerca l'Hospital de la Santa Creu i Sant Pau, Barcelona, Spain.
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Lythgow KT, Hudson G, Andras P, Chinnery PF. A critical analysis of the combined usage of protein localization prediction methods: Increasing the number of independent data sets can reduce the accuracy of predicted mitochondrial localization. Mitochondrion 2010; 11:444-9. [PMID: 21195798 PMCID: PMC3081538 DOI: 10.1016/j.mito.2010.12.016] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/07/2010] [Revised: 12/14/2010] [Accepted: 12/21/2010] [Indexed: 11/16/2022]
Abstract
In the absence of a comprehensive experimentally derived mitochondrial proteome, several bioinformatic approaches have been developed to aid the identification of novel mitochondrial disease genes within mapped nuclear genetic loci. Often, many classifiers are combined to increase the sensitivity and specificity of the predictions. Here we show that the greatest sensitivity and specificity are obtained by using a combination of seven carefully selected classifiers. We also show that increasing the number of independent prediction methods can paradoxically decrease the accuracy of predicting mitochondrial localization. This approach will help to accelerate the identification of new mitochondrial disease genes by providing a principled way for the selection for combination of appropriate prediction methods of mitochondrial localization of proteins.
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Affiliation(s)
- Kieren T Lythgow
- Institute of Human Genetics, Newcastle University, Central Parkway, Newcastle upon Tyne, NE1 3BZ, UK
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Morris GW, Laclair DD, McKee EE. Pyrimidine deoxynucleoside and nucleoside reverse transcriptase inhibitor metabolism in the perfused heart and isolated mitochondria. Antivir Ther 2010; 15:587-97. [PMID: 20587852 DOI: 10.3851/imp1567] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/19/2022]
Abstract
BACKGROUND The metabolism of pyrimidine deoxynucleosides and nucleoside reverse transcriptase inhibitors has been studied in growing cells. However, many of these drugs are associated with mitochondrial toxicities observed in non-replicating tissues, such as in the heart, where their metabolism has not been investigated. METHODS The aims of this study were twofold. The first was to investigate the metabolism of the thymidine analogues 3'-azido-3'deoxythymidine (AZT) and 2',3'-didehydrodideoxy-thymidine (d4T), and the deoxycytidine (dCyd) analogues 2'-deoxy-3'-thiacytidine (3TC) and 2',3'-dideoxycytidine (ddC) with regard to phosphorylation and breakdown. The second was to investigate their potential effects, singly or in combination with AZT, on metabolism of the naturally occurring deoxynucleosides in the perfused rat heart and in isolated heart mitochondria. RESULTS The analogue d4T was not metabolized in perfused heart or in isolated mitochondria, and had no effect on either thymidine or dCyd metabolism. The dCyd analogues were both phosphorylated in perfused heart to the triphosphate, but only at the limit of detection and they were not phosphorylated in isolated mitochondria. Neither ddC nor 3TC had any effect on thymidine or dCyd metabolism in either perfused heart or in isolated mitochondria. AZT has been previously shown to inhibit thymidine phosphorylation. When d4T, 3TC or ddC were given with AZT, only ddC caused a significant further decrease in thymidine phosphorylation. CONCLUSIONS These results indicate that with the exception of the competition between AZT and thymidine, there was little competition for phosphorylation among and between these other nucleoside reverse transcriptase inhibitors and the naturally occurring deoxynucleosides in cardiac tissue and isolated heart mitochondria.
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Dorado B, Area E, Akman HO, Hirano M. Onset and organ specificity of Tk2 deficiency depends on Tk1 down-regulation and transcriptional compensation. Hum Mol Genet 2010; 20:155-64. [PMID: 20940150 PMCID: PMC3000681 DOI: 10.1093/hmg/ddq453] [Citation(s) in RCA: 28] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/13/2022] Open
Abstract
Deficiency of thymidine kinase 2 (TK2) is a frequent cause of isolated myopathy or encephalomyopathy in children with mitochondrial DNA (mtDNA) depletion. To determine the bases of disease onset, organ specificity and severity of TK2 deficiency, we have carefully characterized Tk2 H126N knockin mice (Tk2-/-). Although normal until postnatal day 8, Tk2-/- mice rapidly develop fatal encephalomyopathy between postnatal days 10 and 13. We have observed that wild-type Tk2 activity is constant in the second week of life, while Tk1 activity decreases significantly between postnatal days 8 and 13. The down-regulation of Tk1 activity unmasks Tk2 deficiency in Tk2-/- mice and correlates with the onset of mtDNA depletion in the brain and the heart. Resistance to pathology in Tk2 mutant organs depends on compensatory mechanisms to the reduced mtDNA level. Our analyses at postnatal day 13 have revealed that Tk2-/- heart significantly increases mitochondrial transcript levels relative to the mtDNA content. This transcriptional compensation allows the heart to maintain normal levels of mtDNA-encoded proteins. The up-regulation in mitochondrial transcripts is not due to increased expression of the master mitochondrial biogenesis regulators peroxisome proliferator-activated receptor-gamma coactivator 1 alpha and nuclear respiratory factors 1 and 2, or to enhanced expression of the mitochondrial transcription factors A, B1 or B2. Instead, Tk2-/- heart compensates for mtDNA depletion by down-regulating the expression of the mitochondrial transcriptional terminator transcription factor 3 (MTERF3). Understanding the molecular mechanisms that allow Tk2 mutant organs to be spared may help design therapies for Tk2 deficiency.
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Affiliation(s)
- Beatriz Dorado
- Department of Neurology, Columbia University Medical Center, New York, NY 10032, USA
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16
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Origin of pyrimidine deoxyribonucleotide pools in perfused rat heart: implications for 3'-azido-3'-deoxythymidine-dependent cardiotoxicity. Biochem J 2009; 422:513-20. [PMID: 19558366 DOI: 10.1042/bj20082427] [Citation(s) in RCA: 23] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/17/2022]
Abstract
In adult non-replicating tissues such as heart, demand for dNTPs (deoxynucleoside triphosphates) is low but essential for mitochondrial DNA replication and nuclear DNA repair. dNTPs may be synthesized from salvage of deoxyribonucleosides or by reduction of ribonucleotides. We have hypothesized that the cardiac mitochondrial toxicity of the nucleoside analogue AZT (3'-azido-3'-deoxythymidine; known as zidovudine) is caused by inhibition of thymidine kinase 2 of the salvage pathway and subsequent TTP pool depletion. The extent to which this hypothesis has merit depends on how much the heart relies on thymidine phosphorylation for maintenance of the TTP pool. In the present study, we used isotopic tracing to demonstrate that both TTP and dCTP are solely synthesized by phosphorylation of thymidine and deoxycytidine respectively, with no evidence for synthesis from other precursors. We have also shown that UTP and CTP are synthesized by phosphorylation of uridine and cytidine respectively, with no detectable role for the de novo pyrimidine synthesis pathway. Lastly, we have demonstrated that AZT decreased the TTP pool by 50% in 30 min of perfusion, while having no effect on other dNTPs. In summary, the present study demonstrated that adult rat heart has a limited mechanism for dCTP and TTP synthesis and thus these pools may be more sensitive than replicating cells to drugs such as AZT that affect the salvage pathway.
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17
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Villarroya J, de Bolós C, Meseguer A, Hirano M, Vilà MR. Altered gene transcription profiles in fibroblasts harboring either TK2 or DGUOK mutations indicate compensatory mechanisms. Exp Cell Res 2009; 315:1429-38. [PMID: 19265691 DOI: 10.1016/j.yexcr.2009.02.018] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/13/2008] [Revised: 02/08/2009] [Accepted: 02/19/2009] [Indexed: 10/21/2022]
Abstract
Mitochondrial DNA (mtDNA) depletion syndrome (MDS) is an autosomal recessive disorder characterized by a reduced amount of mtDNA, which impairs synthesis of respiratory chain complexes. MDS has been classified into two main groups, the hepatocerebral form affecting liver and the central nervous system, and the myopathic form targeting the skeletal muscle. We have compared the molecular genetic characteristics of fibroblasts derived from two patients harboring TK2 mutations with two harboring mutations in DGUOK gene. Real-time PCR revealed mtDNA depletion in dGK-deficient fibroblasts (dGK-) but not in TK2-deficient cells (TK2-). Real-time RT-PCR and western blotting demonstrated significant differences in the expression of the human equilibrative nucleoside transporter 1 (hENT1) at the mRNA and protein levels. hENT1 transcript and protein were increased in quiescent control and TK2- fibroblasts relative to cycling cells. In contrast, hENT1 was stable in quiescent and cycling dGK- cells. Moreover, siRNA down-regulation of hENT1, but not of TK1, induced mtDNA depletion in TK2- fibroblasts indicating that hENT1 contributes to the maintenance of normal mtDNA levels in cells lacking TK2. Transcripts for thymidine phosphorylase, the mitochondrial transcription factor A (TFAM), and the polymerase gamma (Pol gamma), were reduced in dGK-, but not in TK2- cells while the mRNA expression of thymidylate synthase (TS) increased. Our results suggested differential gene expression in TK2 and dGK-deficient fibroblasts, and highlighted the importance of hENT1 as a compensatory factor in MDS disorder.
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Affiliation(s)
- Joan Villarroya
- Centre d'Investigacions en Bioquímica i Biologia Molecular (CIBBIM), Hospital Universitari Vall d'Hebron, Institut de Recerca, Plta 1 Lab 117, Pg. Vall d'Hebron, 119-129, 08035 Barcelona, Spain.
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18
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Frangini M, Rampazzo C, Franzolin E, Lara MC, Vilà MR, Martí R, Bianchi V. Unchanged thymidine triphosphate pools and thymidine metabolism in two lines of thymidine kinase 2-mutated fibroblasts. FEBS J 2009; 276:1104-13. [PMID: 19154348 DOI: 10.1111/j.1742-4658.2008.06853.x] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
Mitochondrial thymidine kinase (TK2) catalyzes the phosphorylation of thymidine in mitochondria. Its function becomes essential for dTTP synthesis in noncycling cells, where cytosolic dTTP synthesis via R1/R2 ribonucleotide reductase and thymidine kinase 1 is turned down. Mutations in the nuclear gene for TK2 cause a fatal mtDNA depletion syndrome. Only selected cell types are affected, suggesting that the other cells compensate for the TK2 deficiency by adapting the enzyme network that regulates dTTP synthesis outside S-phase. Here we looked for such metabolic adaptation in quiescent cultures of fibroblasts from two TK2-deficient patients with a slow-progressing syndrome. In cell extracts, we measured the activities of TK2, deoxycytidine kinase, thymidine phosphorylase, deoxynucleotidases and the amounts of the three ribonucleotide reductase subunits. Patient cells contained 40% or 5% TK2 activity and unchanged activities of the other enzymes. However, their mitochondrial and cytosolic dTTP pools were unchanged, and also the overall composition of the dNTP pools was normal. TK2-dependent phosphorylation of [(3)H]thymidine in intact cells and the turnover of the dTTP pool showed that even the fibroblasts with 5% residual TK2 activity synthesized dTTP at an almost normal rate. Normal fibroblasts apparently contain more TK2 than needed to maintain dTTP during quiescence, which would explain why TK2-mutated fibroblasts do not manifest mtDNA depletion despite their reduced TK2 activity.
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19
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Saada A. Mitochondrial deoxyribonucleotide pools in deoxyguanosine kinase deficiency. Mol Genet Metab 2008; 95:169-73. [PMID: 18723380 DOI: 10.1016/j.ymgme.2008.07.007] [Citation(s) in RCA: 31] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 07/15/2008] [Accepted: 07/15/2008] [Indexed: 11/23/2022]
Abstract
Mutated deoxyguanosine kinase (dGK), which catalyses the first step of the mitochondrial deoxypurine salvage pathway, accounts for a hepatocerebral variant of mitochondrial DNA (mtDNA) depletion syndromes. In order to elucidate the pathogenic mechanism of dGK deficiency, mitochondrial and cytoplasmic deoxyribonucleoside triphosphate (dNTP) pools in cycling and quiescent fibroblasts from a dGK deficient patient were measured. The mitochondrial dNTP pools were found to be imbalanced, mainly in quiescent cells due to decreased dGTP while mtDNA content and mitochondrial respiratory chain activities were concomitantly decreased. Supplementation of deoxyguanosine and deoxyadenosine normalized mitochondrial dNTP pools, mtDNA content and partially restored the MRC function. It is suggested that the cytoplasmic deoxycytine kinase supplemented with external substrates may compensate for the deficient dGK.
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Affiliation(s)
- Ann Saada
- Metabolic disease Unit, Hadassah Medical Center, Jerusalem 91120, Israel.
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20
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Pérez-Pérez MJ, Priego EM, Hernández AI, Familiar O, Camarasa MJ, Negri A, Gago F, Balzarini J. Structure, physiological role, and specific inhibitors of human thymidine kinase 2 (TK2): present and future. Med Res Rev 2008; 28:797-820. [PMID: 18459168 PMCID: PMC7168489 DOI: 10.1002/med.20124] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/05/2022]
Abstract
Human mitochondrial thymidine kinase (TK2) is a pyrimidine deoxynucleoside kinase (dNK) that catalyzes the phosphorylation of pyrimidine deoxynucleosides to their corresponding deoxynucleoside 5′‐monophosphates by γ‐phosphoryl transfer from ATP. In resting cells, TK2 is suggested to play a key role in the mitochondrial salvage pathway to provide pyrimidine nucleotides for mitochondrial DNA (mtDNA) synthesis and maintenance. However, recently the physiological role of TK2turned out to have direct clinical relevance as well. Point mutations in the gene encoding TK2 have been correlated to mtDNA disorders in a heterogeneous group of patients suffering from the so‐called mtDNA depletion syndrome (MDS). TK2 activity could also be involved in mitochondrial toxicity associated to prolonged treatment with antiviral nucleoside analogues like AZT and FIAU. Therefore, TK2 inhibitors can be considered as valuable tools to unravel the role of TK2 in the maintenance and homeostasis of mitochondrial nucleotide pools and mtDNA, and to clarify the contribution of TK2 activity to mitochondrial toxicity of certain antivirals. Highly selective TK‐2 inhibitors having an acyclic nucleoside structure and efficiently discriminating between TK‐2 and the closely related TK‐1 have already been reported. It is actually unclear whether these agents efficiently reach the inner mitochondrial compartment. In the present review article,structural features of TK2, MDS‐related mutations observed in TK2 and their role in MDS will be discussed. Also, an update on novel and selective TK2 inhibitors will be provided. © 2008 Wiley Periodicals, Inc. Med Res Rev, 28, No. 5, 797–820, 2008
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Leshinsky-Silver E, Michelson M, Cohen S, Ginsberg M, Sadeh M, Barash V, Lerman-Sagie T, Lev D. A defect in the thymidine kinase 2 gene causing isolated mitochondrial myopathy without mtDNA depletion. Eur J Paediatr Neurol 2008; 12:309-13. [PMID: 17951082 DOI: 10.1016/j.ejpn.2007.09.005] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 04/23/2007] [Revised: 08/28/2007] [Accepted: 09/02/2007] [Indexed: 11/24/2022]
Abstract
Isolated mitochondrial myopathies (IMM) are either due to primary defects in mtDNA, in nuclear genes that control mtDNA abundance and structure such as thymidine kinase 2 (TK2), or due to CoQ deficiency. Defects in the TK2 gene have been found to be associated with mtDNA depletion attributed to a depleted mitochondrial dNTP pool in non-dividing cells. We report an unusual case of IMM, homozygous for the H90N mutation in the TK2 gene but unlike other cases with the same mutation, does not demonstrate mtDNA depletion. The patient's clinical course is relatively mild and a muscle biopsy showed ragged red muscle fibers with a mild decrease in complexes I and an increase in complexes IV and II activities. This report extends the phenotypic expression of TK2 defects and suggests that all patients who present with an IMM even with normal quantities of mtDNA should be screened for TK2 mutations.
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22
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Ashley N, Adams S, Slama A, Zeviani M, Suomalainen A, Andreu AL, Naviaux RK, Poulton J. Defects in maintenance of mitochondrial DNA are associated with intramitochondrial nucleotide imbalances. Hum Mol Genet 2007; 16:1400-11. [PMID: 17483096 DOI: 10.1093/hmg/ddm090] [Citation(s) in RCA: 41] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/29/2022] Open
Abstract
Defects in mtDNA maintenance range from fatal multisystem childhood diseases, such as Alpers syndrome, to milder diseases in adults, including mtDNA depletion syndromes (MDS) and familial progressive external ophthalmoplegia (AdPEO). Most are associated with defects in genes involved in mitochondrial deoxynucleotide metabolism or utilization, such as mutations in thymidine kinase 2 (TK2) as well as the mtDNA replicative helicase, Twinkle and gamma polymerase (POLG). We have developed an in vitro system to measure incorporation of radiolabelled dNTPs into mitochondria of saponin permeabilized cells. We used this to compare the rates of mtDNA synthesis in cells from 12 patients with diseases of mtDNA maintenance. We observed reduced incorporation of exogenous alpha (32)P-dTTP in fibroblasts from a patient with Alpers syndrome associated with the A467T substitution in POLG, a patient with dGK mutations, and a patient with mtDNA depletion of unknown origin compared to controls. However, incorporation of alpha (32)P-dTTP relative to either cell doubling time or alpha (32)P-dCTP incorporation was increased in patients with thymidine kinase deficiency or PEO as the result of TWINKLE mutations compared with controls. The specific activity of newly synthesized mtDNA depends on the size of the endogenous pool diluting the exogenous labelled nucleotide. Our result is consistent with a deficiency in the intramitochondrial pool of dTTP relative to dCTP in cells from patients with TK2 deficiency and TWINKLE mutations. Such DNA precursor asymmetry could cause pausing of the replication complex and hence exacerbate the propensity for age-related mtDNA mutations. Because deviations from the normal concentrations of dNTPs are known to be mutagenic, we suggest that intramitochondrial nucleotide imbalance could underlie the multiple mtDNA mutations observed in these patients.
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Affiliation(s)
- Neil Ashley
- Mitochondrial Genetics Group, Nuffield Department of Obstetrics and Gynaecology, Level 3, Women's Centre,The John Radcliffe Hospital, Oxford OX3 9DU, UK
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23
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Abstract
Among the human diseases that result from abnormalities in mitochondrial genome stability or maintenance are several that result from mutations affecting enzymes of deoxyribonucleoside triphosphate (dNTP) metabolism. In addition, it is evident that the toxicity of antiviral nucleoside analogs is determined in part by the extent to which their intracellular conversion to dNTP analogs occurs within the mitochondrion. Finally, recent work from this laboratory has shown considerable variation among different mammalian tissues with respect to mitochondrial dNTP pool sizes and has suggested that natural asymmetries in mitochondrial dNTP concentrations may contribute to the high rates at which the mitochondrial genome undergoes mutation. These factors suggest that much more information is needed about maintenance and regulation of dNTP pools within mammalian mitochondria. This review summarizes our current understanding and suggests directions for future research.
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Affiliation(s)
- Christopher K Mathews
- Department of Biochemistry and Biophysics, Oregon State University, 2011 Agricultural and Life Sciences Bldg., Corvallis, OR 97331-7305, USA.
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24
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Abstract
Mitochondrial diseases are disorders caused by impairment of the mitochondrial respiratory chain, characterized by clinical-genetic heterogeneity and frequent multisystemic involvement. It is difficult to establish a precise genotype/phenotype correlation and obtain a definitive nosology. Today's genetic classification distinguishes disorders caused by defects in the mitochondrial genome (sporadic or maternally-inherited) from disorders caused by defects in the nuclear genome (autosomally-inherited). We report an updated classification, briefly review the main clinical syndromes and describe the most recent genetic knowledge.
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Affiliation(s)
- M Filosto
- Neurological Clinic, Section for Neuromuscular Diseases and Neuropathies, University Hospital Spedali Civili of Brescia, Brescia, Italy.
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25
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Lynx MD, McKee EE. 3'-Azido-3'-deoxythymidine (AZT) is a competitive inhibitor of thymidine phosphorylation in isolated rat heart and liver mitochondria. Biochem Pharmacol 2006; 72:239-43. [PMID: 16720018 PMCID: PMC1482733 DOI: 10.1016/j.bcp.2006.04.004] [Citation(s) in RCA: 47] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/15/2006] [Revised: 04/06/2006] [Accepted: 04/06/2006] [Indexed: 11/25/2022]
Abstract
Long-term use of 3'-azido-3'-deoxythymidine (AZT) is associated with various tissue toxicities, including hepatotoxicity and cardiomyopathy, and with mitochondrial DNA depletion. AZT-5'-triphosphate (AZTTP) is a known inhibitor of the mitochondrial DNA polymerase gamma and has been targeted as the source of the mitochondrial DNA depletion. However, in previous work from this laboratory with isolated rat heart and liver mitochondria, AZT itself was shown to be a more potent inhibitor of thymidine phosphorylation (IC50 of 7.0+/-1.0 microM AZT in heart mitochondria and of 14.4+/-2.6 microM AZT in liver mitochondria) than AZTTP is of polymerase gamma (IC50 of >100 microM AZTTP), suggesting that depletion of mitochondrial stores of TTP may limit replication and could be the cause of the mitochondrial DNA depletion observed in tissues affected by AZT toxicity. The purpose of this work is to characterize the nature of AZT inhibition of thymidine phosphorylation in isolated rat heart and rat liver mitochondria. In both of these tissues, AZT was found to be a competitive inhibitor of the phosphorylation of thymidine to TMP, catalyzed by thymidine kinase 2. The inhibition constant (Ki) for heart mitochondria is 10.6+/-4.5 microM AZT, and for liver mitochondria Ki is 14.0+/-2.5 microM AZT. Since AZT is functioning as a competitive inhibitor, increasing thymidine concentrations may be one mechanism to overcome the inhibition and decrease AZT-related toxicity in these tissues.
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Affiliation(s)
| | - Edward E. McKee
- Address correspondence to: Edward E. McKee, 1234 Notre Dame Ave., South Bend, IN 46617, Tel: 574-631-7193; Fax: 574-631-7821; E-mail:
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Lynx MD, Bentley AT, McKee EE. 3'-Azido-3'-deoxythymidine (AZT) inhibits thymidine phosphorylation in isolated rat liver mitochondria: a possible mechanism of AZT hepatotoxicity. Biochem Pharmacol 2006; 71:1342-8. [PMID: 16472780 PMCID: PMC1472706 DOI: 10.1016/j.bcp.2006.01.003] [Citation(s) in RCA: 41] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/08/2005] [Revised: 01/10/2006] [Accepted: 01/10/2006] [Indexed: 11/23/2022]
Abstract
3'-azido-3'-deoxythymidine (AZT) is a staple of highly active antiretroviral therapy (HAART). Prior to HAART, long-term use of high-dosage AZT caused myopathy, cardiomyopathy, and hepatotoxicity, associated with mitochondrial DNA depletion. As a component of HARRT, AZT causes cytopenias and lipodystrophy. AZT-5'-triphosphate (AZTTP) is a known inhibitor of the mitochondrial polymerase gamma and has been targeted as the source of the mitochondrial DNA depletion. However, in previous work from this laboratory with isolated rat heart mitochondria, AZT phosphorylation beyond AZT-5'-monophosphate (AZTMP) was not detected. Rather, AZT was shown to be a more potent inhibitor of thymidine phosphorylation (50% inhibitory concentration (IC50) of 7.0+/-1.0 microM) than AZTTP is of polymerase gamma (IC50 of >100 microM), suggesting that depletion of mitochondrial stores of TTP may limit replication. This work has investigated whether an identical mechanism might account for the hepatotoxicity seen with long-term use of AZT. Isolated rat liver mitochondria were incubated with labeled thymidine or AZT, and the rate and extent of phosphorylation were determined by HPLC analysis of acid-soluble extracts of the incubated mitochondria. The results showed that in the phosphorylation of thymidine to TMP, liver mitochondria exhibit a higher Vmax and Km than heart mitochondria, but otherwise heart and liver mitochondria display similar kinetics. AZT is phosphorylated to AZTMP, but no further phosphorylated forms were detected. In addition, AZT inhibited the production of TTP, with an IC50 of 14.4+/-2.6 microM AZT. This is higher, but comparable to, the results seen in isolated rat heart mitochondria.
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Affiliation(s)
- Matthew D. Lynx
- Department of Biological Sciences, University of Notre Dame, Notre Dame, IN 46556, United States
- Indiana University School of Medicine-South Bend, Biochemistry and Molecular Biology, 1234 Notre Dame Avenue, Room 147, Raclin-Carmichael Hall, South Bend, IN 46617, United States
| | - Alice T. Bentley
- Department of Biological Sciences, University of Notre Dame, Notre Dame, IN 46556, United States
- Indiana University School of Medicine-South Bend, Biochemistry and Molecular Biology, 1234 Notre Dame Avenue, Room 147, Raclin-Carmichael Hall, South Bend, IN 46617, United States
| | - Edward E. McKee
- Department of Biological Sciences, University of Notre Dame, Notre Dame, IN 46556, United States
- Indiana University School of Medicine-South Bend, Biochemistry and Molecular Biology, 1234 Notre Dame Avenue, Room 147, Raclin-Carmichael Hall, South Bend, IN 46617, United States
- * Corresponding author. Tel.: +1 574 631 7193; fax: +1 574 631 7821. E-mail address: (E.E. McKee)
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Hunsucker SA, Mitchell BS, Spychala J. The 5'-nucleotidases as regulators of nucleotide and drug metabolism. Pharmacol Ther 2005; 107:1-30. [PMID: 15963349 DOI: 10.1016/j.pharmthera.2005.01.003] [Citation(s) in RCA: 201] [Impact Index Per Article: 10.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Accepted: 01/07/2005] [Indexed: 11/19/2022]
Abstract
The 5'-nucleotidases are a family of enzymes that catalyze the dephosphorylation of nucleoside monophosphates and regulate cellular nucleotide and nucleoside levels. While the nucleoside kinases responsible for the initial phosphorylation of salvaged nucleosides have been well studied, many of the catabolic nucleotidases have only recently been cloned and characterized. Aside from maintaining balanced ribo- and deoxyribonucleotide pools, substrate cycles that are formed with kinase and nucleotidase activities are also likely to regulate the activation of nucleoside analogues, a class of anticancer and antiviral agents that rely on the nucleoside kinases for phosphorylation to their active forms. Both clinical and in vitro studies suggest that an increase in nucleotidase activity can inhibit nucleoside analogue activation and result in drug resistance. The physiological role of the 5'-nucleotidases will be covered in this review, as will the evidence that these enzymes can mediate resistance to nucleoside analogues.
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Affiliation(s)
- Sally Anne Hunsucker
- Lineberger Comprehensive Cancer Center, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599, USA
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28
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Lewis W, Haase CP, Miller YK, Ferguson B, Stuart T, Ludaway T, McNaught J, Russ R, Steltzer J, Santoianni R, Long R, Fiermonte G, Palmieri F. Transgenic expression of the deoxynucleotide carrier causes mitochondrial damage that is enhanced by NRTIs for AIDS. J Transl Med 2005; 85:972-81. [PMID: 15951836 DOI: 10.1038/labinvest.3700301] [Citation(s) in RCA: 29] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022] Open
Abstract
Nucleoside reverse transcriptase inhibitors (NRTIs) are antiretrovirals for AIDS with limiting mitochondrial side effects. The mitochondrial deoxynucleotide carrier (DNC) transports phosphorylated nucleosides for mitochondrial DNA replication and can transport phosphorylated NRTIs into mitochondria. Transgenic mice (TG) that exclusively overexpress DNC in the heart tested DNC's role in mitochondrial dysfunction from NRTIs. Two TG lines were created that overexpressed the human DNC gene in murine myocardium. Cardiac and mitochondrial structure and function were examined by magnetic resonance imaging, echocardiography, electrocardiography, transmission electron microscopy, and plasma lactate. Antiretroviral combinations (HAART) that contained NRTIs (stavudine (2', 3'-didehydro-2', 3'-deoxythymidine or d4T)/lamivudine/indinavir; or zidovudine (3' azido-3'-deoxythymidine or AZT)/lamivudine/indinavir; 35 days) were administered to simulate AIDS therapy. In parallel, a HAART combination without NRTIs (nevirapine/efavirenz/indinavir; 35 days) served as an NRTI-sparing, control regimen. Untreated DNC TGs exhibited normal cardiac function but abnormal mitochondrial ultrastructure. HAART that contained NRTIs caused cardiomyopathy in TGs with increased left ventricle mass and volume, heart rate variability, and worse mitochondrial ultrastructural defects. In contrast, treatment with an NRTI-sparing HAART regimen caused no cardiac changes. Data suggest the DNC is integral to mitochondrial homeostasis in vivo and may relate mechanistically to mitochondrial dysfunction in patients treated with HAART regimens that contain NRTIs.
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Affiliation(s)
- William Lewis
- Department of Pathology, Emory University School of Medicine, Atlanta, GA, USA.
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29
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Priego EM, Balzarini J, Karlsson A, Camarasa MJ, Pérez-Pérez MJ. Synthesis and evaluation of thymine-derived carboxamides against mitochondrial thymidine kinase (TK-2) and related enzymes. Bioorg Med Chem 2005; 12:5079-90. [PMID: 15351391 DOI: 10.1016/j.bmc.2004.07.036] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/27/2004] [Accepted: 07/16/2004] [Indexed: 11/21/2022]
Abstract
Based on the structure of our previously identified mitochondrial thymidine kinase (TK-2) inhibitors, three series of thymine-derived carboxamides have been synthesized and tested against TK-2 and related enzymes. The methodology employed has been a solution-phase parallel synthesis based on the coupling of three thymine-derived acids [4-(thymin-1-yl)butyric acid (I), [4-(thymin-1-yl)-butyrylamino]acetic acid (II) and 6-(thymin-1-yl)hexanoic acid (III)] with different commercially available primary amines that carry cyano and/or phenyl groups. The couplings were performed in good yields (from 60% to 90%), with the exception of those that incorporate the highly crowded triphenylmethylamine (e). From the new synthesized compounds, the N-trityl-6-(thymin-1-yl)hexanamide (IIIe) was the most active TK-2 inhibitor (IC(50)=19+/-2microM).
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Affiliation(s)
- Eva-María Priego
- Instituto de Química Médica (CSIC), Juan de la Cierva 3, E-28006 Madrid, Spain
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30
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McKee EE, Bentley AT, Hatch M, Gingerich J, Susan-Resiga D. Phosphorylation of thymidine and AZT in heart mitochondria: elucidation of a novel mechanism of AZT cardiotoxicity. Cardiovasc Toxicol 2005; 4:155-67. [PMID: 15371631 PMCID: PMC1472705 DOI: 10.1385/ct:4:2:155] [Citation(s) in RCA: 52] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 12/23/2003] [Revised: 02/16/2004] [Accepted: 02/18/2004] [Indexed: 11/11/2022]
Abstract
Antiretroviral nucleoside analogs used in highly active antiretroviral therapy (HAART) are associated with cardiovascular and other tissue toxicity associated with mitochondrial DNA depletion, suggesting a block in mitochondrial (mt)-DNA replication. Because the triphosphate forms of these analogs variably inhibit mt-DNA polymerase, this enzyme has been promoted as the major target of toxicity associated with HAART. We have used isolated mitochondria from rat heart to study the mitochondrial transport and phosphorylation of thymidine and AZT (azidothymidine, or zidovudine), a component used in HAART. We demonstrate that isolated mitochondria readily transport thymidine and phosphorylate it to thymidine 5'-triphosphate (TTP) within the matrix. Under identical conditions, AZT is phosphorylated only to AZT-5'-monophosphate (AZT-MP). The kinetics of thymidine and AZT suggest negative cooperativity of substrate interaction with the enzyme, consistent with work by others on mitochondrial thymidine kinase 2. Results show that TMP and AZT-MP are not transported across the inner membrane, suggesting that AZT-MP may accumulate with time in the matrix. Given the lack of AZT-5'-triphosphate (AZT-TP), it seems unlikely that the toxicity of AZT in the heart is mediated by AZT-TP inhibition of DNA polymerase gamma. Rather, our work shows that AZT is a potent inhibitor of thymidine phosphorylation in heart mitochondria, having an inhibitory concentration (IC)(50) of 7.0 +/- 0.9 microM. Thus, the toxicity of AZT in some tissues may be mediated by disrupting the substrate supply of TTP for mt-DNA replication.
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Affiliation(s)
- Edward E McKee
- Indiana University School of Medicine, South Bend Center for Medical Education, Notre Dame, IN 46556, USA.
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Activity profiles of deoxynucleoside kinases and 5'-nucleotidases in cultured adipocytes and myoblastic cells: insights into mitochondrial toxicity of nucleoside analogs. Biochem Pharmacol 2005; 69:951-60. [PMID: 15748706 DOI: 10.1016/j.bcp.2004.12.010] [Citation(s) in RCA: 33] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/19/2004] [Accepted: 12/28/2004] [Indexed: 10/25/2022]
Abstract
Nucleoside reverse transcriptase inhibitor (NRTI) treatment of HIV is associated with complications, including lipodystrophy (LD) and myopathy. Inhibition of mitochondrial DNA polymerase and depletion of mtDNA by NRTI triphosphates are believed to be key mechanisms in NRTI toxicity. Here, we determined the activities and mRNA levels of deoxynucleoside kinases (dNK) and 5'-nucleotidases (5'-NT) controlling the rate-limiting step in intracellular phosphorylation of NRTIs in cell models representing adipose, muscle tissue and peripheral blood cells using specific assays and Taqman RT-PCR. In vitro phosphorylation of 3'-azido-2',3'-dideoxythymidine (AZT) and 2',3'-didehydro-2',3'-dideoxythymidine (d4T) in extracts was also determined. 3T3-L1 adipocytes showed similar activity of mitochondrial thymidine kinase-2 (TK2) and deoxyguanosine kinase (dGK) but 3- to 36-fold lower levels of cytosolic deoxycytidine kinase (dCK), thymidine kinase-1 (TK1) and thymidine monophosphate kinase (TMPK) and higher levels of deoxyribonucleotidase activity compared to proliferating 3T3-L1. dCK, dGK and TK2 activities correlated with their mRNA levels in proliferating, resting and differentiating 3T3-L1. Differentiated L6 myoblasts had lower activities of cytosolic dNK's and TMPK, higher dGK and similar TK2 and deoxyribonucleotidases (dNT) activities compared to proliferating myoblasts. TK2 was the limiting dNK activity while dGK was predominant in adipocytes and myocytes. Activity profiles revealed limited capacity to phosphorylate dThd and dCyd in adipocytes and myocytes compared to proliferating cells and CEM lymphocytes. Phosphorylation of AZT and d4T was low in adipocytes and myocytes, and the presence of these analogs inhibited the phosphorylation of dThd by TK2 suggesting that mitochondrial toxicity of some NRTIs in adipocytes and myocytes is due to the depletion of normal mitochondrial dNTP pools.
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Abstract
Mitochondrial DNA (mtDNA) depends on numerous nuclear encoded factors and a constant supply of deoxyribonucleoside triphosphates (dNTP), for its maintenance and replication. The function of proteins involved in nucleotide metabolism is perturbed in a heterogeneous group of disorders associated with depletion, multiple deletions, and mutations of the mitochondrial genome. Disturbed homeostasis of the mitochondrial dNTP pools are likely the underlying cause. Understanding of the biochemical and molecular basis of these disorders will promote the development of new therapeutic approaches. This article reviews the current knowledge of deoxyribonucleotide metabolism in relation to disorders affecting mtDNA integrity.
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Affiliation(s)
- Ann Saada
- Metabolic Disease Unit, Shaare Zedek Medical Center, Jerusalem, Israel.
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Saada-Reisch A. Deoxyribonucleoside kinases in mitochondrial DNA depletion. NUCLEOSIDES NUCLEOTIDES & NUCLEIC ACIDS 2005; 23:1205-15. [PMID: 15571232 DOI: 10.1081/ncn-200027480] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/03/2022]
Abstract
Mitochondrial DNA (mtDNA) depletion syndromes (MDS) are a heterogeneous group of mitochondrial disorders, manifested by a decreased mtDNA copy number and respiratory chain dysfunction. Primary MDS are inherited autosomally and may affect a single organ or multiple tissues. Mutated mitochondrial deoxyribonucleoside kinases; deoxyguanosine kinase (dGK) and thymidine kinase 2 (TK2), were associated with the hepatocerebral and myopathic forms of MDS respectively. dGK and TK2 are key enzymes in the mitochondrial nucleotide salvage pathway, providing the mitochondria with deoxyribonucleotides (dNP) essential for mtDNA synthesis. Although the mitochondrial dNP pool is physically separated from the cytosolic one, dNP's may still be imported through specific transport. Non-replicating tissues, where cytosolic dNP supply is down regulated, are thus particularly vulnerable to dGK and TK2 deficiency. The overlapping substrate specificity of deoxycytidine kinase (dCK) may explain the relative sparing of muscle in dGK deficiency, while low basal TK2 activity render this tissue susceptible to TK2 deficiency. The precise pathophysiological mechanisms of mtDNA depletion due to dGK and TK2 deficiencies remain to be determined, though recent findings confirm that it is attributed to imbalanced dNTP pools.
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Affiliation(s)
- Ann Saada-Reisch
- Metabolic Disease Unit, Shaare Zedek Medical Center, Jerusalem, Israel
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Abstract
PET cellular proliferation imaging has its roots in a long history of in vitro cellular proliferation studies to characterize cancer and in the understanding of the biology of thymidine incorporation into DNA gained from these studies. PET imaging represents the logical translation of the in vitro work to measure in vivo tumor proliferation. Preclinical studies of [11C]-thymidine and other PET-labeled thymidine analogues set the stage for early clinical studies that provided very promising results. Recent progress in the application of [18F]-FLT, a clinically practical PET thymidine analogue, to patient studies sets the next stage for clinical PET cellular proliferation imaging. Further mechanistic studies of the imaging agents and well-designed clinical trials will be important in moving PET proliferation imaging into what is likely to be a significant role in the care of cancer patients by providing a quantitative measure of tumor response to cytotoxic or cytostatic therapy.
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Affiliation(s)
- David A Mankoff
- Division of Nuclear Medicine, Department of Radiology, University of Washington, 1959 Northeast Pacific Street, Room NN203, Box 356113, Seattle, WA 98195, USA.
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