Publications by authors named "Matthew J Longley"

37 Publications

Polymerase γ efficiently replicates through many natural template barriers but stalls at the HSP1 quadruplex.

J Biol Chem 2020 12;295(51):17802-17815

Mitochondrial DNA Replication Group, Genome Integrity and Structural Biology Laboratory, NIEHS, National Institutes of Health, Research Triangle Park, North Carolina, USA. Electronic address:

Faithful replication of the mitochondrial genome is carried out by a set of key nuclear-encoded proteins. DNA polymerase γ is a core component of the mtDNA replisome and the only replicative DNA polymerase localized to mitochondria. The asynchronous mechanism of mtDNA replication predicts that the replication machinery encounters dsDNA and unique physical barriers such as structured genes, G-quadruplexes, and other obstacles. In vitro experiments here provide evidence that the polymerase γ heterotrimer is well-adapted to efficiently synthesize DNA, despite the presence of many naturally occurring roadblocks. However, we identified a specific G-quadruplex-forming sequence at the heavy-strand promoter (HSP1) that has the potential to cause significant stalling of mtDNA replication. Furthermore, this structured region of DNA corresponds to the break site for a large (3,895 bp) deletion observed in mitochondrial disease patients. The presence of this deletion in humans correlates with UV exposure, and we have found that efficiency of polymerase γ DNA synthesis is reduced after this quadruplex is exposed to UV in vitro.
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http://dx.doi.org/10.1074/jbc.RA120.015390DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7762954PMC
December 2020

Ultrasensitive deletion detection links mitochondrial DNA replication, disease, and aging.

Genome Biol 2020 09 17;21(1):248. Epub 2020 Sep 17.

Genome Integrity and Structural Biology Laboratory, Mitochondrial DNA Replication Group, National Institute of Environmental Health Sciences, National Institutes of Health, Research Triangle Park, NC, 27709, USA.

Background: Acquired human mitochondrial genome (mtDNA) deletions are symptoms and drivers of focal mitochondrial respiratory deficiency, a pathological hallmark of aging and late-onset mitochondrial disease.

Results: To decipher connections between these processes, we create LostArc, an ultrasensitive method for quantifying deletions in circular mtDNA molecules. LostArc reveals 35 million deletions (~ 470,000 unique spans) in skeletal muscle from 22 individuals with and 19 individuals without pathogenic variants in POLG. This nuclear gene encodes the catalytic subunit of replicative mitochondrial DNA polymerase γ. Ablation, the deleted mtDNA fraction, suffices to explain skeletal muscle phenotypes of aging and POLG-derived disease. Unsupervised bioinformatic analyses reveal distinct age- and disease-correlated deletion patterns.

Conclusions: These patterns implicate replication by DNA polymerase γ as the deletion driver and suggest little purifying selection against mtDNA deletions by mitophagy in postmitotic muscle fibers. Observed deletion patterns are best modeled as mtDNA deletions initiated by replication fork stalling during strand displacement mtDNA synthesis.
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http://dx.doi.org/10.1186/s13059-020-02138-5DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7500033PMC
September 2020

Single-molecule level structural dynamics of DNA unwinding by human mitochondrial Twinkle helicase.

J Biol Chem 2020 04 25;295(17):5564-5576. Epub 2020 Mar 25.

Genome Integrity and Structural Biology Laboratory, NIEHS, National Institutes of Health, Research Triangle Park, North Carolina 27709. Electronic address:

Knowledge of the molecular events in mitochondrial DNA (mtDNA) replication is crucial to understanding the origins of human disorders arising from mitochondrial dysfunction. Twinkle helicase is an essential component of mtDNA replication. Here, we employed atomic force microscopy imaging in air and liquids to visualize ring assembly, DNA binding, and unwinding activity of individual Twinkle hexamers at the single-molecule level. We observed that the Twinkle subunits self-assemble into hexamers and higher-order complexes that can switch between open and closed-ring configurations in the absence of DNA. Our analyses helped visualize Twinkle loading onto and unloading from DNA in an open-ringed configuration. They also revealed that closed-ring conformers bind and unwind several hundred base pairs of duplex DNA at an average rate of ∼240 bp/min. We found that the addition of mitochondrial single-stranded (ss) DNA-binding protein both influences the ways Twinkle loads onto defined DNA substrates and stabilizes the unwound ssDNA product, resulting in a ∼5-fold stimulation of the apparent DNA-unwinding rate. Mitochondrial ssDNA-binding protein also increased the estimated translocation processivity from 1750 to >9000 bp before helicase disassociation, suggesting that more than half of the mitochondrial genome could be unwound by Twinkle during a single DNA-binding event. The strategies used in this work provide a new platform to examine Twinkle disease variants and the core mtDNA replication machinery. They also offer an enhanced framework to investigate molecular mechanisms underlying deletion and depletion of the mitochondrial genome as observed in mitochondrial diseases.
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http://dx.doi.org/10.1074/jbc.RA120.012795DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7186178PMC
April 2020

Mitochondrial single-stranded DNA binding protein novel de novo SSBP1 mutation in a child with single large-scale mtDNA deletion (SLSMD) clinically manifesting as Pearson, Kearns-Sayre, and Leigh syndromes.

PLoS One 2019 3;14(9):e0221829. Epub 2019 Sep 3.

Mitochondrial Medicine Frontier Program, Division of Human Genetics, Department of Pediatrics, Children's Hospital of Philadelphia, Philadelphia, PA, United States of America.

Mitochondrial DNA (mtDNA) genome integrity is essential for proper mitochondrial respiratory chain function to generate cellular energy. Nuclear genes encode several proteins that function at the mtDNA replication fork, including mitochondrial single-stranded DNA-binding protein (SSBP1), which is a tetrameric protein that binds and protects single-stranded mtDNA (ssDNA). Recently, two studies have reported pathogenic variants in SSBP1 associated with hearing loss, optic atrophy, and retinal degeneration. Here, we report a 14-year-old Chinese boy with severe and progressive mitochondrial disease manifestations across the full Pearson, Kearns-Sayre, and Leigh syndromes spectrum, including infantile anemia and bone marrow failure, growth failure, ptosis, ophthalmoplegia, ataxia, severe retinal dystrophy of the rod-cone type, sensorineural hearing loss, chronic kidney disease, multiple endocrine deficiencies, and metabolic strokes. mtDNA genome sequencing identified a single large-scale 5 kilobase mtDNA deletion (m.8629_14068del5440), present at 68% and 16% heteroplasmy in the proband's fibroblast cell line and blood, respectively, suggestive of a mtDNA maintenance defect. On trio whole exome blood sequencing, the proband was found to harbor a novel de novo heterozygous mutation c.79G>A (p.E27K) in SSBP1. Size exclusion chromatography of p.E27K SSBP1 revealed it remains a stable tetramer. However, differential scanning fluorimetry demonstrated p.E27K SSBP1 relative to wild type had modestly decreased thermostability. Functional assays also revealed p.E27K SSBP1 had altered DNA binding. Molecular modeling of SSBP1 tetramers with varying combinations of mutant subunits predicted general changes in surface accessible charges, strength of inter-subunit interactions, and protein dynamics. Overall, the observed changes in protein dynamics and DNA binding behavior suggest that p.E27K SSBP1 can interfere with DNA replication and precipitate the introduction of large-scale mtDNA deletions. Thus, a single large-scale mtDNA deletion (SLSMD) with manifestations across the clinical spectrum of Pearson, Kearns-Sayre, and Leigh syndromes may result from a nuclear gene disorder disrupting mitochondrial DNA replication.
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http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0221829PLOS
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6719858PMC
March 2020

Single-molecule DREEM imaging reveals DNA wrapping around human mitochondrial single-stranded DNA binding protein.

Nucleic Acids Res 2018 11;46(21):11287-11302

Genome Integrity and Structural Biology Laboratory, NIEHS, NIH, Research Triangle Park, NC 27709, USA.

Improper maintenance of the mitochondrial genome progressively disrupts cellular respiration and causes severe metabolic disorders commonly termed mitochondrial diseases. Mitochondrial single-stranded DNA binding protein (mtSSB) is an essential component of the mtDNA replication machinery. We utilized single-molecule methods to examine the modes by which human mtSSB binds DNA to help define protein interactions at the mtDNA replication fork. Direct visualization of individual mtSSB molecules by atomic force microscopy (AFM) revealed a random distribution of mtSSB tetramers bound to extended regions of single-stranded DNA (ssDNA), strongly suggesting non-cooperative binding by mtSSB. Selective binding to ssDNA was confirmed by AFM imaging of individual mtSSB tetramers bound to gapped plasmid DNA substrates bearing defined single-stranded regions. Shortening of the contour length of gapped DNA upon binding mtSSB was attributed to DNA wrapping around mtSSB. Tracing the DNA path in mtSSB-ssDNA complexes with Dual-Resonance-frequency-Enhanced Electrostatic force Microscopy established a predominant binding mode with one DNA strand winding once around each mtSSB tetramer at physiological salt conditions. Single-molecule imaging suggests mtSSB may not saturate or fully protect single-stranded replication intermediates during mtDNA synthesis, leaving the mitochondrial genome vulnerable to chemical mutagenesis, deletions driven by primer relocation or other actions consistent with clinically observed deletion biases.
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http://dx.doi.org/10.1093/nar/gky875DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6265486PMC
November 2018

Characterization of the human homozygous R182W POLG2 mutation in mitochondrial DNA depletion syndrome.

PLoS One 2018 29;13(8):e0203198. Epub 2018 Aug 29.

Genome Integrity and Structural Biology Laboratory, National Institute of Environmental Health Sciences, National Institutes of Health, DHHS, Research Triangle Park, NC, United States of America.

Mutations in mitochondrial DNA (mtDNA) have been linked to a variety of metabolic, neurological and muscular diseases which can present at any time throughout life. MtDNA is replicated by DNA polymerase gamma (Pol γ), twinkle helicase and mitochondrial single-stranded binding protein (mtSSB). The Pol γ holoenzyme is a heterotrimer consisting of the p140 catalytic subunit and a p55 homodimeric accessory subunit encoded by the nuclear genes POLG and POLG2, respectively. The accessory subunits enhance DNA binding and promote processive DNA synthesis of the holoenzyme. Mutations in either POLG or POLG2 are linked to disease and adversely affect maintenance of the mitochondrial genome, resulting in depletion, deletions and/or point mutations in mtDNA. A homozygous mutation located at Chr17: 62492543G>A in POLG2, resulting in R182W substitution in p55, was previously identified to cause mtDNA depletion and fatal hepatic liver failure. Here we characterize this homozygous R182W p55 mutation using in vivo cultured cell models and in vitro biochemical assessments. Compared to control fibroblasts, homozygous R182W p55 primary dermal fibroblasts exhibit a two-fold slower doubling time, reduced mtDNA copy number and reduced levels of POLG and POLG2 transcripts correlating with the reported disease state. Expression of R182W p55 in HEK293 cells impairs oxidative-phosphorylation. Biochemically, R182W p55 displays DNA binding and association with p140 similar to WT p55. R182W p55 mimics the ability of WT p55 to stimulate primer extension, support steady-state nucleotide incorporation, and suppress the exonuclease function of Pol γ in vitro. However, R182W p55 has severe defects in protein stability as determined by differential scanning fluorimetry and in stimulating function as determined by thermal inactivation. These data demonstrate that the Chr17: 62492543G>A mutation in POLG2, R182W p55, severely impairs stability of the accessory subunit and is the likely cause of the disease phenotype.
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http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0203198PLOS
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6114919PMC
February 2019

The C-terminal tail of the NEIL1 DNA glycosylase interacts with the human mitochondrial single-stranded DNA binding protein.

DNA Repair (Amst) 2018 05 6;65:11-19. Epub 2018 Mar 6.

University of South Alabama, Mitchell Cancer Institute, 1660 Springhill Avenue, Mobile, AL 36604, United States. Electronic address:

The 16.5 kb mitochondrial genome is subjected to damage from reactive oxygen species (ROS) generated in the cell during normal cellular metabolism and external sources such as ionizing radiation and ultraviolet light. ROS cause harmful damage to DNA bases that could result in mutagenesis and various diseases, if not properly repaired. The base excision repair (BER) pathway is the primary pathway involved in maintaining the integrity of mtDNA. Several enzymes that partake in BER within the nucleus have also been identified in the mitochondria. The nei-like (NEIL) DNA glycosylases initiate BER by excising oxidized pyrimidine bases and others such as the ring-opened formamidopyrimidine and the hydantoin lesions. During BER, the NEIL enzymes interact with proteins that are involved with DNA replication and transcription. In the current manuscript, we detected NEIL1 in purified mitochondrial extracts from human cells and showed that NEIL1 interacts with the human mitochondrial single-stranded DNA binding protein (mtSSB) via its C-terminal tail using protein painting, far-western analysis, and gel-filtration chromatography. Finally, we scrutinized the NEIL1-mtSSB interaction in the presence and absence of a partial-duplex DNA substrate using a combination of multi-angle light scattering (MALS) and small-angle X-ray scattering (SAXS). The data indicate that NEIL1 and homotetrameric mtSSB form a larger ternary complex in presence of DNA, however, the tetrameric form of mtSSB gets disrupted by NEIL1 in the absence of DNA as revealed by the formation of a smaller NEIL1-mtSSB complex.
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http://dx.doi.org/10.1016/j.dnarep.2018.02.012DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5911420PMC
May 2018

DNA polymerase β: A missing link of the base excision repair machinery in mammalian mitochondria.

DNA Repair (Amst) 2017 12 28;60:77-88. Epub 2017 Oct 28.

Genome Integrity and Structural Biology Laboratory, National Institutes of Health, NIEHS, 111 T.W. Alexander Drive, P.O. Box 12233, Research Triangle Park, NC 27709, USA. Electronic address:

Mitochondrial genome integrity is fundamental to mammalian cell viability. Since mitochondrial DNA is constantly under attack from oxygen radicals released during ATP production, DNA repair is vital in removing oxidatively generated lesions in mitochondrial DNA, but the presence of a strong base excision repair system has not been demonstrated. Here, we addressed the presence of such a system in mammalian mitochondria involving the primary base lesion repair enzyme DNA polymerase (pol) β. Pol β was localized to mammalian mitochondria by electron microscopic-immunogold staining, immunofluorescence co-localization and biochemical experiments. Extracts from purified mitochondria exhibited base excision repair activity that was dependent on pol β. Mitochondria from pol β-deficient mouse fibroblasts had compromised DNA repair and showed elevated levels of superoxide radicals after hydrogen peroxide treatment. Mitochondria in pol β-deficient fibroblasts displayed altered morphology by electron microscopy. These results indicate that mammalian mitochondria contain an efficient base lesion repair system mediated in part by pol β and thus pol β plays a role in preserving mitochondrial genome stability.
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http://dx.doi.org/10.1016/j.dnarep.2017.10.011DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5919216PMC
December 2017

Complementation of aprataxin deficiency by base excision repair enzymes in mitochondrial extracts.

Nucleic Acids Res 2017 Sep;45(17):10079-10088

Genome Integrity and Structural Biology Laboratory, DNA Repair and Nucleic Acid Enzymology Group, National Institutes of Health, National Institute of Environmental Health Sciences, Research Triangle Park, NC 27709, USA.

Mitochondrial aprataxin (APTX) protects the mitochondrial genome from the consequence of ligase failure by removing the abortive ligation product, i.e. the 5'-adenylate (5'-AMP) group, during DNA replication and repair. In the absence of APTX activity, blocked base excision repair (BER) intermediates containing the 5'-AMP or 5'-adenylated-deoxyribose phosphate (5'-AMP-dRP) lesions may accumulate. In the current study, we examined DNA polymerase (pol) γ and pol β as possible complementing enzymes in the case of APTX deficiency. The activities of pol β lyase and FEN1 nucleotide excision were able to remove the 5'-AMP-dRP group in mitochondrial extracts from APTX-/- cells. However, the lyase activity of purified pol γ was weak against the 5'-AMP-dRP block in a model BER substrate, and this activity was not able to complement APTX deficiency in mitochondrial extracts from APTX-/-Pol β-/- cells. FEN1 also failed to provide excision of the 5'-adenylated BER intermediate in mitochondrial extracts. These results illustrate the potential role of pol β in complementing APTX deficiency in mitochondria.
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http://dx.doi.org/10.1093/nar/gkx654DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5622373PMC
September 2017

Synergistic Effects of the T251I and P587L Mitochondrial DNA Polymerase γ Disease Mutations.

J Biol Chem 2017 03 2;292(10):4198-4209. Epub 2017 Feb 2.

From the Genome Integrity and Structural Biology Laboratory, NIEHS, National Institutes of Health, Research Triangle Park, North Carolina 27709

Human mitochondrial DNA (mtDNA) polymerase γ (Pol γ) is the only polymerase known to replicate the mitochondrial genome. The Pol γ holoenzyme consists of the p140 catalytic subunit (POLG) and the p55 homodimeric accessory subunit (POLG2), which enhances binding of Pol γ to DNA and promotes processivity of the holoenzyme. Mutations within impede maintenance of mtDNA and cause mitochondrial diseases. Two common mutations usually found in patients primarily with progressive external ophthalmoplegia generate T251I and P587L amino acid substitutions. To determine whether T251I or P587L is the primary pathogenic allele or whether both substitutions are required to cause disease, we overproduced and purified WT, T251I, P587L, and T251I + P587L double variant forms of recombinant Pol γ. Biochemical characterization of these variants revealed impaired DNA binding affinity, reduced thermostability, diminished exonuclease activity, defective catalytic activity, and compromised DNA processivity, even in the presence of the p55 accessory subunit. However, physical association with p55 was unperturbed, suggesting intersubunit affinities similar to WT. Notably, although the single mutants were similarly impaired, a dramatic synergistic effect was found for the double mutant across all parameters. In conclusion, our analyses suggest that individually both T251I and P587L substitutions functionally impair Pol γ, with greater pathogenicity predicted for the single P587L variant. Combining T251I and P587L induces extreme thermal lability and leads to synergistic nucleotide and DNA binding defects, which severely impair catalytic activity and correlate with presentation of disease in patients.
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http://dx.doi.org/10.1074/jbc.M116.773341DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5354484PMC
March 2017

Analysis of Translesion DNA Synthesis by the Mitochondrial DNA Polymerase γ.

Methods Mol Biol 2016 ;1351:19-26

Mitochondrial DNA Replication Group, Genome Integrity and Structural Biology Laboratory, National Institute of Environmental Health Sciences, National Institutes of Health, 111 T.W. Alexander Dr, Building 101, Rm E316, Research Triangle Park, NC, 27709, USA.

Mitochondrial DNA is replicated by the nuclear-encoded DNA polymerase γ (pol γ) which is composed of a single 140 kDa catalytic subunit and a dimeric 55 kDa accessory subunit. Mitochondrial DNA is vulnerable to various forms of damage, including several types of oxidative lesions, UV-induced photoproducts, chemical adducts from environmental sources, as well as alkylation and inter-strand cross-links from chemotherapy agents. Although many of these lesions block DNA replication, pol γ can bypass some lesions by nucleotide incorporation opposite a template lesion and further extension of the DNA primer past the lesion. This process of translesion synthesis (TLS) by pol γ can occur in either an error-free or an error-prone manner. Assessment of TLS requires extensive analysis of oligonucleotide substrates and replication products by denaturing polyacrylamide sequencing gels. This chapter presents protocols for the analysis of translesion DNA synthesis.
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http://dx.doi.org/10.1007/978-1-4939-3040-1_2DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5000860PMC
September 2016

Mitochondrial genome maintenance in health and disease.

DNA Repair (Amst) 2014 Jul 26;19:190-8. Epub 2014 Apr 26.

Laboratory of Molecular Genetics, National Institute of Environmental Health Sciences, National Institutes of Health, Research Triangle Park, Durham, NC 27709, USA.

Human mitochondria harbor an essential, high copy number, 16,569 base pair, circular DNA genome that encodes 13 gene products required for electron transport and oxidative phosphorylation. Mutation of this genome can compromise cellular respiration, ultimately resulting in a variety of progressive metabolic diseases collectively known as 'mitochondrial diseases'. Mutagenesis of mtDNA and the persistence of mtDNA mutations in cells and tissues is a complex topic, involving the interplay of DNA replication, DNA damage and repair, purifying selection, organelle dynamics, mitophagy, and aging. We briefly review these general elements that affect maintenance of mtDNA, and we focus on nuclear genes encoding the mtDNA replication machinery that can perturb the genetic integrity of the mitochondrial genome.
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http://dx.doi.org/10.1016/j.dnarep.2014.03.010DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4075028PMC
July 2014

A p.R369G POLG2 mutation associated with adPEO and multiple mtDNA deletions causes decreased affinity between polymerase γ subunits.

Mitochondrion 2012 Mar 4;12(2):313-9. Epub 2011 Dec 4.

Mitochondrial Research Group, Institute for Ageing and Health, The Medical School, Framlington Place, Newcastle University, Newcastle upon Tyne, NE2 4HH, UK.

Human mitochondrial DNA (mtDNA) polymerase γ (pol γ) is the sole enzyme required to replicate and maintain the integrity of the mitochondrial genome. It comprises two subunits, a catalytic p140 subunit and a smaller p55 accessory subunit encoded by the POLG2 gene. We describe the molecular characterization of a potential dominant POLG2 mutation (p.R369G) in a patient with adPEO and multiple mtDNA deletions. Biochemical studies of the recombinant mutant p55 protein showed a reduced affinity to the pol γ p140 subunit, leading to impaired processivity of the holoenzyme complex but did not show sensitivity to N-ethylmalaimide (NEM) inhibition, inferring a novel disease mechanism.
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http://dx.doi.org/10.1016/j.mito.2011.11.006DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3824628PMC
March 2012

Biochemical analysis of human POLG2 variants associated with mitochondrial disease.

Hum Mol Genet 2011 Aug 9;20(15):3052-66. Epub 2011 May 9.

Laboratory of Molecular Genetics, NIEHS, National Institutes of Health, DHHS, Research Triangle Park, NC 27709, USA.

Defects in mitochondrial DNA (mtDNA) maintenance comprise an expanding repertoire of polymorphic diseases caused, in part, by mutations in the genes encoding the p140 mtDNA polymerase (POLG), its p55 accessory subunit (POLG2) or the mtDNA helicase (C10orf2). In an exploration of nuclear genes for mtDNA maintenance linked to mitochondrial disease, eight heterozygous mutations (six novel) in POLG2 were identified in one control and eight patients with POLG-related mitochondrial disease that lacked POLG mutations. Of these eight mutations, we biochemically characterized seven variants [c.307G>A (G103S); c.457C>G (L153V); c.614C>G (P205R); c.1105A>G (R369G); c.1158T>G (D386E); c.1268C>A (S423Y); c.1423_1424delTT (L475DfsX2)] that were previously uncharacterized along with the wild-type protein and the G451E pathogenic variant. These seven mutations encode amino acid substitutions that map throughout the protein, including the p55 dimer interface and the C-terminal domain that interacts with the catalytic subunit. Recombinant proteins harboring these alterations were assessed for stimulation of processive DNA synthesis, binding to the p140 catalytic subunit, binding to dsDNA and self-dimerization. Whereas the G103S, L153V, D386E and S423Y proteins displayed wild-type behavior, the P205R and R369G p55 variants had reduced stimulation of processivity and decreased affinity for the catalytic subunit. Additionally, the L475DfsX2 variant, which possesses a C-terminal truncation, was unable to bind the p140 catalytic subunit, unable to bind dsDNA and formed aberrant oligomeric complexes. Our biochemical analysis helps explain the pathogenesis of POLG2 mutations in mitochondrial disease and emphasizes the need to quantitatively characterize the biochemical consequences of newly discovered mutations before classifying them as pathogenic.
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http://dx.doi.org/10.1093/hmg/ddr209DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3131046PMC
August 2011

Disease variants of the human mitochondrial DNA helicase encoded by C10orf2 differentially alter protein stability, nucleotide hydrolysis, and helicase activity.

J Biol Chem 2010 Sep 20;285(39):29690-702. Epub 2010 Jul 20.

Laboratory of Molecular Genetics, NIEHS, National Institutes of Health, Research Triangle Park, North Carolina 27709, USA.

Missense mutations in the human C10orf2 gene, encoding the mitochondrial DNA (mtDNA) helicase, co-segregate with mitochondrial diseases such as adult-onset progressive external ophthalmoplegia, hepatocerebral syndrome with mtDNA depletion syndrome, and infantile-onset spinocerebellar ataxia. To understand the biochemical consequences of C10orf2 mutations, we overproduced wild type and 20 mutant forms of human mtDNA helicase in Escherichia coli and developed novel schemes to purify the recombinant enzymes to near homogeneity. A combination of molecular crowding, non-ionic detergents, Mg(2+) ions, and elevated ionic strength was required to combat insolubility and intrinsic instability of certain mutant variants. A systematic biochemical assessment of the enzymes included analysis of DNA binding affinity, DNA helicase activity, the kinetics of nucleotide hydrolysis, and estimates of thermal stability. In contrast to other studies, we found that all 20 mutant variants retain helicase function under optimized in vitro conditions despite partial reductions in DNA binding affinity, nucleotide hydrolysis, or thermal stability for some mutants. Such partial defects are consistent with the delayed presentation of mitochondrial diseases associated with mutation of C10orf2.
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http://dx.doi.org/10.1074/jbc.M110.151795DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2943296PMC
September 2010

Purification and functional characterization of human mitochondrial DNA polymerase gamma harboring disease mutations.

Methods 2010 Aug 20;51(4):379-84. Epub 2010 Feb 20.

Laboratory of Molecular Genetics, National Institute of Environmental Health Sciences, National Institutes of Health, Research Triangle Park, NC 27709, USA.

More than 150 different point mutations in POLG, the gene encoding the human mitochondrial DNA polymerase gamma (pol gamma), cause a broad spectrum of childhood and adult onset diseases like Alpers syndrome, ataxia-neuropathy syndrome and progressive external ophthalmoplegia. These disease mutations can affect the pol gamma enzyme's properties in numerous ways, thus potentially influencing the severity of the disease. Hence, a detailed characterization of disease mutants will greatly assist researchers and clinicians to develop a clear understanding of the functional defects caused by these mutant enzymes. Experimental approaches for characterizing the wild-type (WT) and mutant pol gamma enzymes are extensively described in this manuscript. The methods start with construction and purification of the recombinant wild-type and mutant forms of pol gamma protein, followed by assays to determine its structural integrity and thermal stability. Next, the biochemical characterization of these enzymes is described in detail, which includes measuring the purified enzyme's catalytic activity, its steady-state kinetic parameters and DNA binding activity, and determining the physical and functional interaction of these pol gamma proteins with the p55 accessory subunit.
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http://dx.doi.org/10.1016/j.ymeth.2010.02.015DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2901396PMC
August 2010

Preparation of human mitochondrial single-stranded DNA-binding protein.

Methods Mol Biol 2009 ;554:73-85

Mitochondrial DNA Replication Group, Laboratory of Molecular Genetics, National Institute of Environmental Health Sciences, National Institutes of Health, Research Triangle Park, Query, NC, USA.

Defects in mtDNA replication are the principle cause of severe, heritable metabolic disorders classified as mitochondrial diseases. In vitro analysis of the biochemical mechanisms of mtDNA replication has proven to be a powerful tool for understanding the origins of mitochondrial disease. Mitochondrial single-stranded DNA-binding protein (mtSSB) is an essential component of the mtDNA replication machinery. To facilitate ongoing biochemical studies, a recombinant source of mtSSB is needed to avoid the time and expense of human tissue culture. This chapter focuses on the subcloning, purification, and initial functional validation of the recombinant human mitochondrial single-stranded DNA-binding protein. The cDNA encoding the mature form of the human mtSSB protein was amplified from a HeLa cDNA library, and recombinant human mtSSB was overproduced in Escherichia coli. A procedure was developed to rapidly purify milligram quantities of homogenous, nuclease-free mtSSB that avoids DNA-cellulose chromatography. We show that, similar to E. coli SSB, human mtSSB assembles into a tetramer and binds single-stranded oligonucleotides in a 4-to-1 protein:oligonucleotide molar ratio.
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http://dx.doi.org/10.1007/978-1-59745-521-3_5DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3953565PMC
October 2009

Disease mutations in the human mitochondrial DNA polymerase thumb subdomain impart severe defects in mitochondrial DNA replication.

J Biol Chem 2009 Jul 28;284(29):19501-10. Epub 2009 May 28.

Laboratory of Molecular Genetics, NIEHS, National Institutes of Health, Research Triangle Park, North Carolina 27709, USA.

Forty-five different point mutations in POLG, the gene encoding the catalytic subunit of the human mitochondrial DNA polymerase (pol gamma), cause the early onset mitochondrial DNA depletion disorder, Alpers syndrome. Sequence analysis of the C-terminal polymerase region of pol gamma revealed a cluster of four Alpers mutations at highly conserved residues in the thumb subdomain (G848S, c.2542g-->a; T851A, c.2551a-->g; R852C, c.2554c-->t; R853Q, c.2558g-->a) and two Alpers mutations at less conserved positions in the adjacent palm subdomain (Q879H, c.2637g-->t and T885S, c.2653a-->t). Biochemical characterization of purified, recombinant forms of pol gamma revealed that Alpers mutations in the thumb subdomain reduced polymerase activity more than 99% relative to the wild-type enzyme, whereas the palm subdomain mutations retained 50-70% wild-type polymerase activity. All six mutant enzymes retained physical and functional interaction with the pol gamma accessory subunit (p55), and none of the six mutants exhibited defects in misinsertion fidelity in vitro. However, differential DNA binding by these mutants suggests a possible orientation of the DNA with respect to the polymerase during catalysis. To our knowledge this study represents the first structure-function analysis of the thumb subdomain in pol gamma and examines the consequences of mitochondrial disease mutations in this region.
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http://dx.doi.org/10.1074/jbc.M109.011940DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2740576PMC
July 2009

Human DNA polymerase theta possesses 5'-dRP lyase activity and functions in single-nucleotide base excision repair in vitro.

Nucleic Acids Res 2009 Apr 2;37(6):1868-77. Epub 2009 Feb 2.

Laboratory of Structural Biology, NIEHS, National Institutes of Health, Research Triangle Park, NC 27709, USA.

DNA polymerase theta (Pol theta) is a low-fidelity DNA polymerase that belongs to the family A polymerases and has been proposed to play a role in somatic hypermutation. Pol theta has the ability to conduct translesion DNA synthesis opposite an AP site or thymine glycol, and it was recently proposed to be involved in base excision repair (BER) of DNA damage. Here, we show that Pol theta has intrinsic 5'-deoxyribose phosphate (5'-dRP) lyase activity that is involved in single-nucleotide base excision DNA repair (SN-BER). Full-length human Pol theta is a approximately 300-kDa polypeptide, but we show here that the 98-kDa C-terminal region of Pol theta possesses both DNA polymerase activity and dRP lyase activity and is sufficient to carry out base excision repair in vitro. The 5'-dRP lyase activity is independent of the polymerase activity, in that a polymerase inactive mutant retained full 5'-dRP lyase activity. Domain mapping of the 98-kDa enzyme by limited proteolysis and NaBH(4) cross-linking with a BER intermediate revealed that the dRP lyase active site resides in a 24-kDa domain of Pol theta. These results are consistent with a role of Pol theta in BER.
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http://dx.doi.org/10.1093/nar/gkp035DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2665223PMC
April 2009

DNA2 resolves expanding flap in mitochondrial base excision repair.

Mol Cell 2008 Nov;32(4):457-8

Laboratory of Molecular Genetics, National Institute of Environmental Health Sciences, Research Triangle Park, NC 27709, USA.

In a recent issue of Molecular Cell, Zheng et al. (2008) demonstrated that human DNA2, originally identified in yeast as a nuclear DNA replication and repair factor, functions exclusively in mammalian mitochondria in the recently discovered long-patch base excision repair pathway.
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http://dx.doi.org/10.1016/j.molcel.2008.11.007DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3967838PMC
November 2008

Long patch base excision repair in mammalian mitochondrial genomes.

J Biol Chem 2008 Sep 17;283(39):26349-56. Epub 2008 Jul 17.

Department of Biochemistry and Molecular Biology, University of Texas Medical Branch, Galveston, Texas 77555, USA.

The mitochondrial genome is highly susceptible to damage by reactive oxygen species (ROS) generated endogenously as a byproduct of respiration. ROS-induced DNA lesions, including oxidized bases, abasic (AP) sites, and oxidized AP sites, cause DNA strand breaks and are repaired via the base excision repair (BER) pathway in both the nucleus and mitochondria. Repair of damaged bases and AP sites involving 1-nucleotide incorporation, named single nucleotide (SN)-BER, was observed with mitochondrial and nuclear extracts. During SN-BER, the 5'-phosphodeoxyribose (dRP) moiety, generated by AP-endonuclease (APE1), is removed by the lyase activity of DNA polymerase gamma (pol gamma) and polymerase beta in the mitochondria and nucleus, respectively. However, the repair of oxidized deoxyribose fragments at the 5' terminus after strand break would require 5'-exo/endonuclease activity that is provided by the flap endonuclease (FEN-1) in the nucleus, resulting in multinucleotide repair patch (long patch (LP)-BER). Here we show the presence of a 5'-exo/endonuclease in the mitochondrial extracts of mouse and human cells that is involved in the repair of a lyase-resistant AP site analog via multinucleotide incorporation, upstream and downstream to the lesion site. We conclude that LP-BER also occurs in the mitochondria requiring the 5'-exo/endonuclease and pol gamma with 3'-exonuclease activity. Although a FEN-1 antibody cross-reacting species was detected in the mitochondria, it was absent in the LP-BER-proficient APE1 immunocomplex isolated from the mitochondrial extract that contains APE1, pol gamma, and DNA ligase 3. The LP-BER activity was marginally affected in FEN-1-depleted mitochondrial extracts, further supporting the involvement of an unidentified 5'-exo/endonuclease in mitochondrial LP-BER.
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http://dx.doi.org/10.1074/jbc.M803491200DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2546560PMC
September 2008

Progressive external ophthalmoplegia and vision and hearing loss in a patient with mutations in POLG2 and OPA1.

Arch Neurol 2008 Jan;65(1):125-31

Department of Pediatrics, University of Turin, Turin, Italy.

Objective: To describe the clinical features, muscle pathological characteristics, and molecular studies of a patient with a mutation in the gene encoding the accessory subunit (p55) of polymerase gamma (POLG2) and a mutation in the OPA1 gene.

Design: Clinical examination and morphological, biochemical, and molecular analyses.

Setting: Tertiary care university hospitals and molecular genetics and scientific computing laboratory.

Patient: A 42-year-old man experienced hearing loss, progressive external ophthalmoplegia (PEO), loss of central vision, macrocytic anemia, and hypogonadism. His family history was negative for neurological disease, and his serum lactate level was normal.

Results: A muscle biopsy specimen showed scattered intensely succinate dehydrogenase-positive and cytochrome-c oxidase-negative fibers. Southern blot of muscle mitochondrial DNA showed multiple deletions. The results of screening for mutations in the nuclear genes associated with PEO and multiple mitochondrial DNA deletions, including those in POLG (polymerase gamma gene), ANT1 (gene encoding adenine nucleotide translocator 1), and PEO1, were negative, but sequencing of POLG2 revealed a G1247C mutation in exon 7, resulting in the substitution of a highly conserved glycine with an alanine at codon 416 (G416A). Because biochemical analysis of the mutant protein showed no alteration in chromatographic properties and normal ability to protect the catalytic subunit from N-ethylmaleimide, we also sequenced the OPA1 gene and identified a novel heterozygous mutation (Y582C).

Conclusion: Although we initially focused on the mutation in POLG2, the mutation in OPA1 is more likely to explain the late-onset PEO and multisystem disorder in this patient.
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http://dx.doi.org/10.1001/archneurol.2007.9DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2364721PMC
January 2008

Modulation of the W748S mutation in DNA polymerase gamma by the E1143G polymorphismin mitochondrial disorders.

Hum Mol Genet 2006 Dec 6;15(23):3473-83. Epub 2006 Nov 6.

Laboratory of Molecular Genetics, National Institute of Environmental Health Sciences, National Institutes of Health, Research Triangle Park, NC 27709, USA.

DNA polymerase gamma (pol gamma) is required for replication and repair of mitochondrial DNA. Over 80 mutations in POLG, the gene encoding the catalytic subunit of pol gamma, have been linked with disease. The W748S mutation in POLG is the most common mutation in ataxia-neuropathy spectrum disorders and is generally found in cis with the common E1143G polymorphism. It has been unclear whether E1143G participates in the disease process. We investigated the biochemical consequences of pol gamma proteins containing W748S or E1143G, or both. W748S pol gamma exhibited low DNA polymerase activity, low processivity and a severe DNA-binding defect. However, interactions between the catalytic and accessory subunits were normal. Despite the benefits derived from binding with the accessory subunit, catalytic activities did not reach wild-type (WT) levels. Also, nucleotide selectivity decreased 2.1-fold compared with WT. Surprisingly, pol gamma containing only E1143G was 1.4-fold more active than WT, and this increased polymerase activity could be due to higher thermal stability for E1143G pol gamma. The E1143G substitution partially rescued the deleterious effects of the W748S mutation, as DNA binding, catalytic activity and fidelity values were intermediate for W748S-E1143G. However, W748S-E1143G had a notably lower change in enthalpy for protein folding than W748S alone. We suggest that when E1143G is in cis with other pathogenic mutations, it can modulate the effects of these mutations. For W748S-E1143G pol gamma, the benefits bestowed by E1143G include increased DNA binding and polymerase activity; however, E1143G was somewhat detrimental to protein stability.
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http://dx.doi.org/10.1093/hmg/ddl424DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1780027PMC
December 2006

Mutant POLG2 disrupts DNA polymerase gamma subunits and causes progressive external ophthalmoplegia.

Am J Hum Genet 2006 Jun 4;78(6):1026-34. Epub 2006 May 4.

Laboratory of Molecular Genetics, National Institute of Environmental Health Sciences, National Institutes of Health, Research Triangle Park, NC, USA.

DNA polymerase gamma (pol gamma ) is required to maintain the genetic integrity of the 16,569-bp human mitochondrial genome (mtDNA). Mutation of the nuclear gene for the catalytic subunit of pol gamma (POLG) has been linked to a wide range of mitochondrial diseases involving mutation, deletion, and depletion of mtDNA. We describe a heterozygous dominant mutation (c.1352G-->A/p.G451E) in POLG2, the gene encoding the p55 accessory subunit of pol gamma , that causes progressive external ophthalmoplegia with multiple mtDNA deletions and cytochrome c oxidase (COX)-deficient muscle fibers. Biochemical characterization of purified, recombinant G451E-substituted p55 protein in vitro revealed incomplete stimulation of the catalytic subunit due to compromised subunit interaction. Although G451E p55 retains a wild-type ability to bind DNA, it fails to enhance the DNA-binding strength of the p140-p55 complex. In vivo, the disease most likely arises through haplotype insufficiency or heterodimerization of the mutated and wild-type proteins, which promote mtDNA deletions by stalling the DNA replication fork. The progressive accumulation of mtDNA deletions causes COX deficiency in muscle fibers and results in the clinical phenotype.
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http://dx.doi.org/10.1086/504303DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1474082PMC
June 2006

DNA polymerase gamma in mitochondrial DNA replication and repair.

Chem Rev 2006 Feb;106(2):383-405

Laboratory of Molecular Genetics, National Institute of Environmental Health Sciences, Research Triangle Park, North Carolina 27709, USA.

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http://dx.doi.org/10.1021/cr040463dDOI Listing
February 2006

Mono-allelic POLG expression resulting from nonsense-mediated decay and alternative splicing in a patient with Alpers syndrome.

DNA Repair (Amst) 2005 Dec 21;4(12):1381-9. Epub 2005 Sep 21.

Laboratory of Molecular Genetics, National Institute of Environmental Health Sciences, National Institute of Health, Research Triangle Park, NC 27709, USA.

Alpers syndrome is an autosomal recessive mitochondrial DNA depletion disorder that affects children and young adults. It is characterized by a progressive, fatal brain and liver disease. This syndrome has been associated with mutations in POLG, the gene encoding the mitochondrial DNA polymerase (pol gamma). Most patients with Alpers syndrome have been found to be compound heterozygotes, carrying two pathogenic mutations in trans at the POLG locus. POLG is a nuclear-encoded gene whose protein product is imported into mitochondria, where it is essential for mtDNA replication and repair. We studied the skin fibroblasts of a patient with Alpers syndrome having the genotype E873stop/A467T. The E873stop mutation produces a premature termination codon (TAG) in exon 17. The A467T mutation produces a threonine to alanine substitution at a highly conserved site in exon 7. The allele bearing the stop codon (E873-TAG) is predicted to produce a truncated, catalytically inactive polymerase. However, only full-length pol gamma protein was detected by Western blot analysis. Here, we show that transcripts containing this stop codon undergo nonsense-associated alternative splicing and nonsense-mediated decay. More than 95% of the functional POLG mRNA was derived from the allele bearing the A467T mutation and less than 5% contained the E873stop mutation. These events ensured that virtually all POLG protein in the cell was expressed from the A467T allele. Therefore, the Alpers phenotype in this patient was a consequence of a single-copy gene dose of the A467T allele, and selective elimination of transcripts bearing the E873stop mutation.
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http://dx.doi.org/10.1016/j.dnarep.2005.08.010DOI Listing
December 2005

The common A467T mutation in the human mitochondrial DNA polymerase (POLG) compromises catalytic efficiency and interaction with the accessory subunit.

J Biol Chem 2005 Sep 16;280(36):31341-6. Epub 2005 Jul 16.

Laboratory of Molecular Genetics, NIEHS, National Institutes of Health, Research Triangle Park, North Carolina 27709, USA.

Among the nearly 50 disease mutations in the gene for the catalytic subunit of human DNA polymerase gamma, POLG, the A467T substitution is the most common and has been found in 0.6% of the Belgian population. The A467T mutation is associated with a wide range of mitochondrial disorders, including Alpers syndrome, juvenile spinocerebellar ataxia-epilepsy syndrome, and progressive external ophthalmoplegia, each with vastly different clinical presentations, tissue specificities, and ages of onset. The A467T mutant enzyme possesses only 4% of wild-type DNA polymerase activity, and the catalytic defect is manifest primarily through a 6-fold reduction in kcat with minimal effect on exonuclease function. Human DNA polymerase gamma (pol gamma) requires association of a 55-kDa accessory subunit for enhanced DNA binding and highly processive DNA synthesis. However, the A467T mutant enzyme failed to interact with and was not stimulated by the accessory subunit, as judged by processivity, heat inactivation, and N-ethylmaleimide protection assays in vitro. Thermolysin digestion and immunoprecipitation experiments further indicate weak association of the subunits for A467T pol gamma. This is the first example of a mutation in POLG that disrupts physical association of the pol gamma subunits. We propose that reduced polymerase activity and loss of accessory subunit interaction are responsible for the depletion and deletion of mitochondrial DNA observed in patients with this POLG mutation.
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http://dx.doi.org/10.1074/jbc.M506762200DOI Listing
September 2005

Consequences of mutations in human DNA polymerase gamma.

Gene 2005 Jul;354:125-31

Laboratory of Molecular Genetics, National Institute of Environmental Health Sciences, National Institutes of Health, P.O. Box 12233, Research Triangle Park, NC 27709, USA.

DNA polymerase gamma is responsible for replication and repair of the mitochondrial genome. Human DNA polymerase gamma is composed of a 140-kDa catalytic subunit and a 55-kDa accessory subunit. Mutations in the gene for the catalytic subunit (POLG) have been shown to be a frequent cause of mitochondrial disorders. To date over 40 disease mutations and 9 nonsynonymous polymorphisms in POLG have been found to be associated with autosomal recessive and dominant progressive external ophthalmoplegia (PEO), Alpers syndrome, sensory ataxia, neuropathy, dysarthria and ophthalmoparesis (SANDO), Parkinsonism, and male infertility. In this paper we review the literature of POLG mutations and discuss their impact on mitochondrial diseases. We also describe a public access web database to annotate POLG mutations for the research community.
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http://dx.doi.org/10.1016/j.gene.2005.03.029DOI Listing
July 2005

DNA precursor asymmetries in mammalian tissue mitochondria and possible contribution to mutagenesis through reduced replication fidelity.

Proc Natl Acad Sci U S A 2005 Apr 22;102(14):4990-5. Epub 2005 Mar 22.

Department of Biochemistry and Biophysics, Oregon State University, 2011 ALS, Corvallis, OR 97331-7305, USA.

The mutation rate of the mammalian mitochondrial genome is higher than that of the nuclear genome. Because mitochondrial and nuclear deoxyribonucleoside triphosphate (dNTP) pools are physically distinct and because dNTP concentrations influence replication fidelity, we asked whether mitochondrial dNTP pools are asymmetric with respect to each other. We report here that the concentrations of the four dNTPs are not equal in mitochondria isolated from several tissues of both young and old rats. In particular, in most tissues examined, mitochondrial dGTP concentrations are high relative to the other dNTPs. Moreover, in the presence of the biased dNTP concentrations measured in heart and skeletal muscle, the fidelity of DNA synthesis in vitro by normally highly accurate mtDNA polymerase gamma is reduced, with error frequencies increased by as much as 3-fold, due to increased formation of template T.dGTP mismatches that are inefficiently corrected by proofreading. These data, plus some published data on specific mitochondrial mutations seen in human diseases, are consistent with the hypothesis that normal intramitochondrial dNTP pool asymmetries may contribute to spontaneous mutagenesis in the mammalian mitochondrial genome.
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http://dx.doi.org/10.1073/pnas.0500253102DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC555996PMC
April 2005
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