Inhibition of Excision of Oxidatively Generated Hydantoin DNA Lesions by NEIL1 by the Competitive Binding of the Nucleotide Excision Repair Factor XPC-RAD23B

Treatment of Human HeLa Cells with Black Raspberry Extracts Enhances the Removal of DNA Lesions by the Nucleotide Excision Repair Mechanism

As demonstrated by us earlier and by other researchers, a diet containing freeze-dried black raspberries (BRB) inhibits DNA damage and carcinogenesis in animal models. We tested the hypothesis that the inhibition of DNA damage by BRB is due, in part, to the enhancement of DNA repair capacity evaluated in the human HeLa cell extract system, an established in vitro system for the assessment of cellular DNA repair activity. The pre-treatment of intact HeLa cells with BRB extracts (BRBE) enhances the nucleotide excision repair (NER) of a bulky deoxyguanosine adduct derived from the polycyclic aromatic carcinogen benzo[a]pyrene (BP-dG) by ~24%. The NER activity of an oxidatively-derived non-bulky DNA lesion, guanidinohydantoin (Gh), is also enhanced by ~24%, while its base excision repair activity is enhanced by only ~6%. Western Blot experiments indicate that the expression of selected, NER factors is also increased by BRBE treatment by ~73% (XPA), ~55% (XPB), while its effects on XPD was modest (<14%). These results demonstrate that BRBE significantly enhances the NER yields of a bulky and a non-bulky DNA lesion, and that this effect is correlated with an enhancement of expression of the critically important NER factor XPA and the helicase XPB, but not the helicase XPD.

Nucleotide Excision Repair: From Molecular Defects to Neurological Abnormalities

Nucleotide excision repair (NER) is the most versatile DNA repair pathway, which can remove diverse bulky DNA lesions destabilizing a DNA duplex. NER defects cause several autosomal recessive genetic disorders. Xeroderma pigmentosum (XP) is one of the NER-associated syndromes characterized by low efficiency of the removal of bulky DNA adducts generated by ultraviolet radiation. XP patients have extremely high ultraviolet-light sensitivity of sun-exposed tissues, often resulting in multiple skin and eye cancers. Some XP patients develop characteristic neurodegeneration that is believed to derive from their inability to repair neuronal DNA damaged by endogenous metabolites. A specific class of oxidatively induced DNA lesions, 8,5′-cyclopurine-2′-deoxynucleosides, is considered endogenous DNA lesions mainly responsible for neurological problems in XP. Growing evidence suggests that XP is accompanied by defective mitophagy, as in primary mitochondrial disorders. Moreover, NER pathway is absent in mitochondria, implying that the mitochondrial dysfunction is secondary to nuclear NER defects. In this review, we discuss the current understanding of the NER molecular mechanism and focuses on the NER linkage with the neurological degeneration in patients with XP. We also present recent research advances regarding NER involvement in oxidative DNA lesion repair. Finally, we highlight how mitochondrial dysfunction may be associated with XP.

Recognition and repair of oxidatively generated DNA lesions in plasmid DNA by a facilitated diffusion mechanism

The oxidatively generated genotoxic spiroiminodihydantoin (Sp) lesions are well-known substrates of the base excision repair (BER) pathway initiated by the bifunctional DNA glycosylase NEIL1. In this work, we reported that the excision kinetics of the single Sp lesions site-specifically embedded in the covalently closed circular DNA plasmids (contour length 2686 base pairs) by NEIL1 are biphasic under single-turnover conditions ([NEIL1] ≫ [SpDNApl]) in contrast with monophasic excision kinetics of the same lesions embedded in147-mer Sp-modified DNA duplexes. Under conditions of a large excess of plasmid DNA base pairs over NEIL1 molecules, the kinetics of excision of Sp lesions are biphasic in nature, exhibiting an initial burst phase, followed by a slower rate of formation of excision products The burst phase is associated with NEIL1-DNA plasmid complexes, while the slow kinetic phase is attributed to the dissociation of non-specific NEIL1-DNA complexes. The amplitude of the burst phase is limited because of the competing non-specific binding of NEIL1 to unmodified DNA sequences flanking the lesion. A numerical analysis of the incision kinetics yielded a value of φ ≍ 0.03 for the fraction of NEIL1 encounters with plasmid molecules that result in the excision of the Sp lesion, and a characteristic dissociation time of non-specific NEIL1-DNA complexes (τ-ns ≍ 8 s). The estimated average DNA translocation distance of NEIL1 is ∼80 base pairs. This estimate suggests that facilitated diffusion enhances the probability that NEIL1 can locate its substrate embedded in an excess of unmodified plasmid DNA nucleotides by a factor of ∼10.

Excision of Oxidatively Generated Guanine Lesions by Competitive DNA Repair Pathways

The base and nucleotide excision repair pathways (BER and NER, respectively) are two major mechanisms that remove DNA lesions formed by the reactions of genotoxic intermediates with cellular DNA. It is generally believed that small non-bulky oxidatively generated DNA base modifications are removed by BER pathways, whereas DNA helix-distorting bulky lesions derived from the attack of chemical carcinogens or UV irradiation are repaired by the NER machinery. However, existing and growing experimental evidence indicates that oxidatively generated DNA lesions can be repaired by competitive BER and NER pathways in human cell extracts and intact human cells. Here, we focus on the interplay and competition of BER and NER pathways in excising oxidatively generated guanine lesions site-specifically positioned in plasmid DNA templates constructed by a gapped-vector technology. These experiments demonstrate a significant enhancement of the NER yields in covalently closed circular DNA plasmids (relative to the same, but linearized form of the same plasmid) harboring certain oxidatively generated guanine lesions. The interplay between the BER and NER pathways that remove oxidatively generated guanine lesions are reviewed and discussed in terms of competitive binding of the BER proteins and the DNA damage-sensing NER factor XPC-RAD23B to these lesions.

Base and Nucleotide Excision Repair Pathways in DNA Plasmids Harboring Oxidatively Generated Guanine Lesions

The base and nucleotide excision repair pathways (BER and NER, respectively) are two major mechanisms that remove DNA lesions formed by the reactions of genotoxic intermediates with cellular DNA. We have demonstrated earlier that the oxidatively generated guanine lesions spiroiminodihydantoin (Sp) and 5-guanidinohydantoin (Gh) are excised from double-stranded DNA by competing BER and NER in whole-cell extracts [Shafirovich, V., et al. (2016) J. Biol. Chem. 321, 5309-5319]. In this work we compared the NER and BER yields with single Gh or Sp lesions embedded at the same sites in covalently closed circular pUC19NN plasmid DNA (cccDNA) and in the same but linearized form (linDNA) of this plasmid. The kinetics of the Sp and Gh BER and NER incisions were monitored in HeLa cell extracts. The yield of NER products is ∼5 times greater in covalently closed circular DNA than in the linearized form, while the BER yield is smaller by ∼20-30% depending on the guanine lesion. Control BER experiments with 8-oxo-7,8-dihydroguanine (8-oxoG) show that the BER yield is increased by a factor of only 1.4 ± 0.2 in cccDNA relative to linDNA. These surprising differences in BER and NER activities are discussed in terms of the lack of termini in covalently closed circular DNA and the DNA lesion search dynamics of the NER DNA damage sensor XPC-RAD23B and the BER enzyme OGG1 that recognizes and excises 8-oxoG.

The involvement of nucleotide excision repair proteins in the removal of oxidative DNA damage

The six major mammalian DNA repair pathways were discovered as independent processes, each dedicated to remove specific types of lesions, but the past two decades have brought into focus the significant interplay between these pathways. In particular, several studies have demonstrated that certain proteins of the nucleotide excision repair (NER) and base excision repair (BER) pathways work in a cooperative manner in the removal of oxidative lesions. This review focuses on recent data showing how the NER proteins, XPA, XPC, XPG, CSA, CSB and UV-DDB, work to stimulate known glycosylases involved in the removal of certain forms of base damage resulting from oxidative processes, and also discusses how some oxidative lesions are probably directly repaired through NER. Finally, since many glycosylases are inhibited from working on damage in the context of chromatin, we detail how we believe UV-DDB may be the first responder in altering the structure of damage containing-nucleosomes, allowing access to BER enzymes.