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Chapter 13 : Repair of Mitochondrial DNA Damage
Category: Microbial Genetics and Molecular Biology
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The cells of higher eukaryotes typically contain several thousand copies of mitochondrial DNA (mtDNA). This chapter first talks about mitochondrial mutagenesis and DNA damage in the mitochondrial genome. Several factors can facilitate preferential mtDNA damage. While a number of early studies provided suggestive hints about mtDNA repair, the first definitive study demonstrating active base excision repair (BER) in mitochondria in mammalian cells documented the formation and repair of N-methylpurines in an insulinoma cell line exposed to the naturally occurring nitrosamine streptozotocin. The chapter discusses loss of specific types of base damage from mtDNA and repair of oxidative damage. Other covered topics are enzymes for BER repair in mitochondrial extracts, short-patch BER of mitochondrial DNA, age-related studies of mitochondrial DNA repair, alternative excision repair pathway, and recombinational repair in mtDNA. The determination of the number and type of distinct DNA repair pathways that operate in mtDNA in mammalian cells remains an important challenge and that results obtained with lower eukaryotes cannot be extrapolated to higher organisms.
The mitochondrial genome comprises circular DNA molecules. The arrows indicate regions of replicating DNA. (Courtesy of David A. Clayton.)
The mitochondrial electron transport chain showing a schematic representation of mammalian electron transport complexes I to V. Electrons flow from NADH or succinate to complex I or II, respectively, and then to a ubiquinone (UQ) pool. Subsequently, electrons flow through complexes III and IV to the final acceptor, molecular oxygen. The flow of electrons is coupled to the movement of protons across the inner membrane in complexes I, II, and IV. The resulting proton gradient is harvested by complex V to generate ATP. (Adapted from reference 44 .)
In contrast to the nuclear ADA gene, there is no detectable NER (see chapters 8 and 9) of either CPD (A) or (6–4) photoproducts ([6-4] PP) (B) in mtDNA of human cells exposed to UV radiation. (Adapted from reference 55 .)
Loss of O6-methylguanine (O6-methyl dG) from rat liver nuclear DNA and mtDNA at various times after the administrationof N-nitrosodimethylamine. (Adapted from reference 51 .)
Kinetics of the removal of O6-ethylguanosine (O6-EtdGuo) from nuclear (continuous black line) and mitochondrial (broken black line) DNA from rat brain and kidneys after exposure of animals to ethylnitrosourea. Note that O4-ethyl-2’-deoxythymidine (O4-EtdThd) (gold line) is not removed from either DNA. (Adapted from reference 65 .)
Repair of 8-oxoG in mtDNA (A) and nuclear DNA (B) of liver cells from wild-type mice (+/+) but not from Ogg1 homozygous mutant mice (—/—). (Adapted from reference 72 .)
Frequency of spontaneous mitochondrial mutants in wild-type (WT) and isogenic Ogg1 mutant yeast strains. The numbers in parentheses reflect the number of independent cultures examined. (Adapted from reference 70 .)
Schematic representation of the role of mitochondria in human aging and age-related degenerative diseases. The electron transport system in the mitochondrial inner membrane, composed of protein subunits encoded by both mtDNA and nuclear DNA, is involved in ATP synthesis through coupling with respiration that consumes about 90% of the oxygen uptake of tissue cells. A fraction of the oxygen is incompletely reduced by one-electron transfer (mostly via ubisemiquinone) to generate the ROS and organic free radicals, which are usually disposed of by the coordinated function of antioxidant enzymes. However, if they escape, they may cause oxidative damage and mutation of the nearby mtDNA molecules. mtDNA with oxidative modification and/or mutation is transcribed and translated to produce defective protein subunits that are assembled to form a defective electron transport chain. The impaired chain not only works less efficiently in ATP synthesis but also generates more ROS, which will enhance the oxidative damage to various biomolecules in mitochondria. This “vicious cycle” is propagating in an age-dependent manner and results in the widely observed age-related accumulation of oxidative damage and mutation of mtDNA, which ultimately leads to a progressive decline in the bioenergetic function of tissue cells in the aging process. At the same time, free-radical scavenger enzymes and DNA repair systems for removal of oxidative damage by ROS and free radicals become less efficient. In addition, the high levels of oxidants can indirectly induce apoptosis by changing cellular redox potentials, depleting reduced glutathione, reducing ATP levels, and decreasing reducing equivalents such as NADH and NADPH. These changes can facilitate lipid peroxidation and the opening of permeability transition pores, leading to the subsequent release of cytochrome c and apoptosis inducing factor (AIF) (see chapter 23). Aging-related overproduction of mitochondrial ROS may thus lead to activation of apoptotic pathways. The accumulation of oxidatively damaged and mutated mtDNA molecules and defective mitochondria, together with enhanced apoptosis, acts synergistically to perpetuate a general decline of biochemical and physiological functions of tissue cells in the aging process. (Adapted from reference 85 .)
DNA repair proteins/activities detected in mitochondria