DNA Oxidative Damage
Cellular DNA faces damage from various agents, especially oxidative damage induced by both endogenous and exogenous factors. Endogenous agents like reactive oxygen species (ROS) generated during cellular processes can include superoxide, hydrogen peroxide, singlet oxygen, hydroxyl radical, and peroxynitrite. Exogenous sources, such as ionizing radiation, ultraviolet light, and certain chemotherapeutics like bleomycin, also contribute to DNA damage. Understanding these mechanisms is crucial as they impact genomic stability and contribute to the initiation of various diseases.
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Categories of Oxidatively Modified DNA
DNA is vulnerable to damage from various agents affecting both its sugar-phosphate backbone and nucleobases. Among the four 2'-deoxynucleosides, dGuo exhibits the lowest reduction potential, making it highly susceptible to oxidation. Studies reveal non-specific reactions of solution-borne reactive oxygen species (ROS) at guanines, but DNA-mediated charge transfer chemistry demonstrates selective oxidation of guanine doublets or triplets. 8-oxo-7,8-dihydroguanine (8-oxoG) emerges as a prototypic oxidized G form, functioning as a biomarker for oxidative stress. Another oxidized form is 2,6-diamino-4-hydroxy-5-formamidopyrimidine (Fapy·G), believed to form from the C-8 hydroxyl radical adduct of G.
Fig. 1 Structure of (A) 8-oxoG and (B) Fapy·G (Delaney S., et al. 2012).
Hyperoxidized guanine products, like spiroiminodihydantoin (Sp) and guanidinohydantoin (Gh), result from further oxidation of 8-oxoG. The dose rate of oxidants influences the formation of these hyperoxidized products, emphasizing the complexity of the DNA oxidative damage process. While 8-oxoG is extensively studied, recent advancements in synthesizing oligonucleotides containing Fapy·G have enhanced research on this lesser-explored DNA lesion. Understanding these various forms of oxidatively damaged DNA is crucial for unraveling their biological consequences.
Genetic Effects of Oxidatively Damaged DNA
Genetic consequences of oxidatively damaged DNA, such as mutagenic and genotoxic potentials, hinge on replication dynamics. Lesions acting as blocks to DNA polymerases are considered toxic, while those causing incorrect base incorporation are mutagenic. 8-oxoG, originating from G, exhibits mutational properties when in an anti conformation, causing G → T transversions during replication. In vitro studies reveal various DNA polymerases' preferences for incorporating dCTP over dATP opposite 8-oxoG, leading to mutagenic outcomes.
Studies on hyperoxidized G lesions, such as Sp, Gh, Iz, Oz, Ca, Oa, Ua, and their in vitro replication properties, underline mutagenic and toxic potentials. Incorporation efficiencies and misinsertion frequencies vary among polymerases, with different lesions exhibiting diverse replication characteristics. In vivo experiments in E. coli underscore hyperoxidized G lesions' mutagenic nature, with G → T and G → C transversions prevalent. While hyperoxidized G lesions exhibit potent mutagenicity and toxicity in E. coli, their impact in mammalian cells remains unexplored.
Biological Implications of Oxidatively Damaged DNA
Exploration of the biological implications of oxidatively damaged DNA lesions has been limited compared to the extensive studies on 8-oxoG. Researchers have delved into the biological significance of 8-oxoG, highlighting its role in Huntington's disease, its potential impact on telomeres, its association with colorectal cancer, its involvement in gene expression regulation, and even proposing a prebiotic role in the repair of thymine dimers.
Role of 8-oxoG in Huntington's Disease
While DNA repair processes generally mitigate mutagenicity and toxicity, the repair of 8-oxoG within a cytosine-adenine-guanine (CAG) trinucleotide repeat (TNR) sequence has been linked to the expansion of repetitive sequences. In Huntington's disease (HD), this expansion is a pathogenic signature, and studies using an HD mouse model correlated the degree of TNR expansion with the level of 8-oxoG. OGG1, a glycosylase initiating base excision repair (BER), is implicated in HD by facilitating TNR expansion. The proposed 'toxic oxidation cycle' suggests that inefficient repair of the stem-loop hairpin formed by CAG repeats may lead to the reiteration of repair and expansion, contributing to the disease.
Telomeres' Role in Protecting Against Oxidative Damage
Telomeres, repetitive DNA sequences at chromosome ends, may serve as hotspots for oxidative damage due to their high guanine content. Experimental evidence suggests that telomeric DNA is susceptible to oxidative stress, and while not necessarily more prone to damage, repair may be less efficient in telomeres, leading to the accumulation of oxidative damage over time. Telomeric DNA's ability to mediate long-range charge transfer reactions could funnel oxidizing equivalents, providing a protective mechanism against genomic damage.
8-oxoG and Colorectal Cancer
Mutations in the MYH gene, involved in adenine removal from 8-oxoG: A base pairs, have been linked to colorectal cancer predisposition (MYH-associated polyposis or MAP). The Y165C and G382D MYH variants exhibit reduced activity, leading to an increased frequency of C → A mutations in a specific sequence context, 5'-GAA-3'. This highlights the critical role of 8-oxoG and MYH in preventing mutagenic events and underscores the link between DNA repair deficiencies and cancer susceptibility.
8-oxoG as a Regulator of Gene Expression
Oxidatively damaged DNA, particularly 8-oxoG, has been implicated in regulating gene expression. The localized production of H2O2 during histone demethylation, a process involving lysine-specific demethylase 1 (LSD1), can lead to 8-oxoG formation. This damage, repaired by OGG1, initiates base excision repair, causing chromatin relaxation necessary for transcription initiation. This controlled induction of oxidative DNA damage suggests a regulatory role for 8-oxoG in gene expression.
Prebiotic Role of 8-oxoG in Thymine Dimer Repair
In a novel perspective, it is proposed that, before the evolution of sophisticated enzyme cofactors, 8-oxoG might have served as a primordial flavin cofactor, catalyzing reactions similar to DNA repair enzymes. Experimental evidence supports 8-oxoG's ability to mimic the behavior of flavin cofactors and catalyze the repair of cyclobutane pyrimidine dimers, suggesting a prebiotic role in the early stages of DNA repair mechanisms.
In summary, 8-oxoG, while often considered a harmful DNA damage, plays diverse roles in biological processes, impacting genomic stability, disease initiation, gene expression regulation, and potentially serving as a primitive repair catalyst in the early evolution of life. Understanding these multifaceted roles enhances our grasp of the intricate interplay between DNA damage and cellular functions.
Reference
- Delaney S., et al. Chemical and biological consequences of oxidatively damaged guanine in DNA. Free Radical Research. 2012, 46(4): 420-441.
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