Analysis of Translesion DNA Synthesis by the Mitochondrial DNA Polymerase γ

Structure and function relationships in mammalian DNA polymerases

DNA polymerases are vital for the synthesis of new DNA strands. Since the discovery of DNA polymerase I in Escherichia coli, a diverse library of mammalian DNA polymerases involved in DNA replication, DNA repair, antibody generation, and cell checkpoint signaling has emerged. While the unique functions of these DNA polymerases are differentiated by their association with accessory factors and/or the presence of distinctive catalytic domains, atomic resolution structures of DNA polymerases in complex with their DNA substrates have revealed mechanistic subtleties that contribute to their specialization. In this review, the structure and function of all 15 mammalian DNA polymerases from families B, Y, X, and A will be reviewed and discussed with special emphasis on the insights gleaned from recently published atomic resolution structures.

Enzymology of mitochondrial DNA repair

The mitochondrial genome encodes proteins essential for the oxidative phosphorylation and, consequently, for proper mitochondrial function. Its localization and, possibly, structural organization contribute to higher DNA damage accumulation, when compared to the nuclear genome. In addition, the mitochondrial genome mutates at rates several times higher than the nuclear, although the causal relationship between these events are not clearly established. Maintaining mitochondrial DNA stability is critical for cellular function and organismal fitness, and several pathways contribute to that, including damage tolerance and bypass, degradation of damaged genomes and DNA repair. Despite initial evidence suggesting that mitochondria lack DNA repair activities, most DNA repair pathways have been at least partially characterized in mitochondria from several model organisms, including humans. In this chapter, we review what is currently known about how the main DNA repair pathways operate in mitochondria and contribute to mitochondrial DNA stability, with focus on the enzymology of mitochondrial DNA repair.

Principal Aspects Regarding the Maintenance of Mammalian Mitochondrial Genome Integrity

Mitochondria have emerged as key players regarding cellular homeostasis not only due to their contribution regarding energy production through oxidative phosphorylation, but also due to their involvement in signaling, ion regulation, and programmed cell death. Indeed, current knowledge supports the notion that mitochondrial dysfunction is a hallmark in the pathogenesis of various diseases. Mitochondrial biogenesis and function require the coordinated action of two genomes: nuclear and mitochondrial. Unfortunately, both intrinsic and environmental genotoxic insults constantly threaten the integrity of nuclear as well as mitochondrial DNA. Despite the extensive research that has been made regarding nuclear genome instability, the importance of mitochondrial genome integrity has only recently begun to be elucidated. The specific architecture and repair mechanisms of mitochondrial DNA, as well as the dynamic behavior that mitochondria exert regarding fusion, fission, and autophagy participate in mitochondrial genome stability, and therefore, cell homeostasis.

The Role of DNA Repair in Maintaining Mitochondrial DNA Stability

Mitochondria are vital double-membrane organelles that act as a "powerhouse" inside the cell and have essential roles to maintain cellular functions, e.g., ATP production, iron-sulfur synthesis metabolism, and steroid synthesis. An important difference with other organelles is that they contain their own mitochondrial DNA (mtDNA). Such powerful organelles are also sensitive to both endogenous and exogenous factors that can cause lesions to their structural components and their mtDNA, resulting in gene mutations and eventually leading to diseases. In this review, we will mainly focus on mammalian mitochondrial DNA repair pathways that safeguard mitochondrial DNA integrity and several important factors involved in the repair process, especially on an essential pathway, base excision repair. We eagerly anticipate to explore more methods to treat related diseases by constantly groping for these complexes and precise repair mechanisms.