Role of oxidatively induced DNA lesions in human pathogenesis

Abstract

Genome stability is essential for maintaining cellular and organismal homeostasis, but it is subject to many threats. One ubiquitous threat is from a class of compounds known as reactive oxygen species (ROS), which can indiscriminately react with many cellular biomolecules including proteins, lipids, and DNA to produce a variety of oxidative lesions. These DNA oxidation products are a direct risk to genome stability, and of particular importance are oxidative clustered DNA lesions (OCDLs), defined as two or more oxidative lesions present within 10 bp of each other. ROS can be produced by exposure of cells to exogenous environmental agents including ionizing radiation, light, chemicals, and metals. In addition, they are produced by cellular metabolism including mitochondrial ATP generation. However, ROS also serve a variety of critical cellular functions and optimal ROS levels are maintained by multiple cellular antioxidant defenses. Oxidative DNA lesions can be efficiently repaired by base excision repair or nucleotide excision repair. If ROS levels increase beyond the capacity of its antioxidant defenses, the cell's DNA repair capacity can become overwhelmed, leading to the accumulation of oxidative DNA damage products including OCDLs, which are more difficult to repair than individual isolated DNA damage products. Here we focus on the induction and repair of OCDLs and other oxidatively induced DNA lesions. If unrepaired, these lesions can lead to the formation of mutations, DNA DSBs, and chromosome abnormalities. We discuss the roles of these lesions in human pathologies including aging and cancer, and in bystander effects.

Fig. 1

ROS have different origins. ROS can arise following exposure to ionizing radiation or light (IR, UV), drugs and other chemicals such as metals. Enzymes, oxygen metabolism and apoptosis also account for ROS production. Finally, the inflammatory responses involving the immune system and bystander signaling also utilize ROS. When ROS enter the nuclear cell compartment, they interact with DNA creating lesions ranging from base or sugar modifications to abasic sites (represented by red stars) and SSBs. ROS-induced DNA lesions can appear in an isolated or clustered form and they are primarily repaired by two BER subpathways: the short-patch and the long-patch pathways. The short-patch or single-nucleotide pathway is initiated by a DNA glycosylase (hOGG1 or hNTH1) that cleaves and removes the altered base, giving an abasic site. This abasic site is then processed by an endonuclease (APE1) allowing DNA polymerase β to process the next step, catalyzing the elimination of the 5′-sugar phosphate residue and filling the gap with a nucleotide. Finally, the nick is sealed by the ligase III/XRCC1 complex. To simplify, only the branch of the short-pathway utilizing a monofunctional glycosylase is represented. SSBs can be repaired by the long-patch pathway (replacing approximately 2–12 nucleotides). This subpathway is dependant on PCNA and FEN1. It contains many of the same factors as the short-patch pathway but in contrast to the short-patch subpathway, DNA synthesis is thought to be mediated by several DNA polymerases including polymerases β, δ and ε. nt: nucleotide.

Fig. 2

Oxidative stress can lead to DNA DSB formation. (A) A DSB can arise when two SSBs form close to each other (1), when topos cleave next to a SSB (2) and when ROS-induced DNA damage interferes with both DNA replication and transcription (3). A DSB is generated during DNA repair when excision of a modified base takes place near an unrepaired SSB. Oxidative DNA lesions can also interfere with reversible topo cleavage complexes during DNA replication and RNA transcription. In such cases, DNA/RNA polymerase forks run off the DNA to generate DSBs. Finally, DSBs can also appear when transcription and replication forks collide directly with SSBs or other ROS-induced lesions. Rarely, interference during DNA repair by BER also leads to DSB formation (4). (B) Representative image showing DNA DSBs induced by oxidative stress on mouse chromosomes. Blue: DNA; red: FISH signal indicating telomeres; green: DSBs visualized by γ-H2AX foci. White arrows indicate chromosomes containing a DSB.

Fig. 3

The role of oxidative stress in human pathogenesis. Many studies suggest that ROS have a role in normal aging, cancer, infertility, and a wide variety of age-related diseases such as some neurological diseases, type 2 diabetes, autoimmune and cardiovascular diseases. In healthy cells, the steady-state level of the ROS-induced lesions depends on the relative rates for their formation and repair. Chronic exposure to ROS and/or deficiencies in DNA repair processes or redox machinery can result in persistent DNA lesions. Accumulation of DNA lesions can lead to point mutations and/or chromosomal aberrations via the formation of DNA DSBs leading to the development of disease. Diseased cells, in turn, develop increased ROS production and/or decreased efficiency of DNA repair processes or redox systems. Red arrows represent the possible negative feedback of pathology on ROS production. : increase of ROS production. T: decrease of DNA repair or redox capacity.