Free energy profiles of base flipping in intercalative polycyclic aromatic hydrocarbon-damaged DNA duplexes: energetic and structural relationships to nucleotide excision repair susceptibility

Relative genotoxicity of polycyclic aromatic hydrocarbons inferred from free energy perturbation approaches

Utilizing molecular dynamics and free energy perturbation, we examine the relative binding affinity of several covalent polycyclic aromatic hydrocarbon - DNA (PAH-DNA) adducts at the central adenine of NRAS codon-61, a mutational hotspot implicated in cancer risk. Several PAHs classified by the International Agency for Research on Cancer as probable, possible, or unclassifiable as to carcinogenicity are found to have greater binding affinity than the known carcinogen, benzo[a]pyrene (B[a]P). van der Waals interactions between the intercalated PAH and neighboring nucleobases, and minimal disruption of the DNA duplex drive increases in binding affinity. PAH-DNA adducts may be repaired by global genomic nucleotide excision repair (GG-NER), hence we also compute relative free energies of complexation of PAH-DNA adducts with RAD4-RAD23 (the yeast ortholog of human XPC-RAD23) which constitutes the recognition step in GG-NER. PAH-DNA adducts exhibiting the greatest DNA binding affinity also exhibit the least RAD4-RAD23 complexation affinity and are thus predicted to resist the GG-NER machinery, contributing to their genotoxic potential. In particular, the fjord region PAHs dibenzo[a,l]pyrene, benzo[g]chrysene, and benzo[c]phenanthrene are found to have greater binding affinity while having weaker RAD4-RAD23 complexation affinity than their respective bay region analogs B[a]P, chrysene, and phenanthrene. We also find that the bay region PAHs dibenzo[a,j]anthracene, dibenzo[a,c]anthracene, and dibenzo[a,h]anthracene exhibit greater binding affinity and weaker RAD4-RAD23 complexation affinity than B[a]P. Thus, the study of PAH genotoxicity likely needs to be substantially broadened, with implications for public policy and the health sciences. This approach can be broadly applied to assess factors contributing to the genotoxicity of other unclassified compounds.

The nucleotide excision repair proteins through the lens of molecular dynamics simulations

Mutations that affect the proteins responsible for the nucleotide excision repair (NER) pathway can lead to diseases such as xeroderma pigmentosum, trichothiodystrophy, Cockayne syndrome, and Cerebro-oculo-facio-skeletal syndrome. Hence, understanding their molecular behavior is needed to elucidate these diseases' phenotypes and how the NER pathway is organized and coordinated. Molecular dynamics techniques enable the study of different protein conformations, adaptable to any research question, shedding light on the dynamics of biomolecules. However, as important as they are, molecular dynamics studies focused on DNA repair pathways are still becoming more widespread. Currently, there are no review articles compiling the advancements made in molecular dynamics approaches applied to NER and discussing: (i) how this technique is currently employed in the field of DNA repair, focusing on NER proteins; (ii) which technical setups are being employed, their strengths and limitations; (iii) which insights or information are they providing to understand the NER pathway or NER-associated proteins; (iv) which open questions would be suited for this technique to answer; and (v) where can we go from here. These questions become even more crucial considering the numerous 3D structures published regarding the NER pathway's proteins in recent years. In this work, we tackle each one of these questions, revising and critically discussing the results published in the context of the NER pathway.

Multiscale computational investigations of the translesion synthesis bypass of tobacco-derived DNA adducts: critical insights that complement experimental biochemical studies

Among the numerous agents that damage DNA, tobacco products remain one of the most lethal and result in the most diverse set of DNA lesions. This perspective aims to provide an overview of computational work conducted to complement experimental biochemical studies on the mutagenicity of adducts derived from the most potent tobacco carcinogen, namely 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (nicotine-derived nitrosaminoketone or NNK). Lesions ranging from the smallest methylated thymine derivatives to the larger, flexible pyridyloxobutyl (POB) guanine adducts are considered. Insights are obtained from density functional theory (DFT) calculations and molecular dynamics (MD) simulations into the damaged nucleobase and nucleoside structures, the accommodation of the lesions in the active site of key human polymerases, the intrinsic base pairing potentials of the adducts, and dNTP incorporation opposite the lesions. Overall, the computational data provide atomic level information that can rationalize the differential mutagenic properties of tobacco-derived lesions and uncover important insights into the impact of adduct size, nucleobase, position, and chemical composition of the bulky moiety.

Chemical Composition and Antioxidant Activity of Steam-Distilled Essential Oil and Glycosidically Bound Volatiles from Maclura Tricuspidata Fruit

Essential oil obtained from Maclura triscuspidata fruit has been reported to have functional properties. This study aimed at determining chemical compositions and antioxidant activities of steam-distilled essential oil (SDEO) and glycosidically bound aglycone fraction (GBAF) isolated from fully ripe M. triscuspidata fruit. SDEO was isolated by simultaneous steam distillation and extraction (SDE). GBAF was prepared by Amberlite XAD-2 adsorption of methanol extract, followed by methanol elution and enzymatic hydrolysis. Both fractions were analyzed by gas chromatography–mass spectrometry (GC–MS). A total of 76 constituents were identified from both oils. Apart from fatty acids and their esters, the SDEO contained p-cresol in the highest concentration (383.5 ± 17.7), followed by δ-cadinene (147.7 ± 7.7), β-caryophyllene (145.7 ± 10.5), β-ionone (141.0 ± 4.5), n-nonanal (140.3 ± 20.5), theaspirane A (121.3 ± 4.5) and theaspirane B (99.67 ± 9.05 µg/g). Thirteen carotenoid-derived compounds identified in the SDEO are being isolated from M. triscuspidata fruit for the first time. Out of the 22 components identified in GBAF, 14 were present only in the glycosidically bound volatiles. Antioxidant activity of the GBAF was higher than that of SDEO. These results suggest that glycosidically bound volatiles of M. triscuspidata fruit have a good potential as natural antioxidants.

Molecular Mechanism, Dynamics, and Energetics of Protein-Mediated Dinucleotide Flipping in a Mismatched DNA: A Computational Study of the RAD4-DNA Complex

DNA damage alters genetic information and adversely affects gene expression pathways leading to various complex genetic disorders and cancers. DNA repair proteins recognize and rectify DNA damage and mismatches with high fidelity. A critical molecular event that occurs during most protein-mediated DNA repair processes is the extrusion of orphaned bases at the damaged site facilitated by specific repairing enzymes. The molecular-level understanding of the mechanism, dynamics, and energetics of base extrusion is necessary to elucidate the molecular basis of protein-mediated DNA damage repair. The present article investigates the molecular mechanism of dinucleotide extrusion in a mismatched DNA (containing a stretch of three contiguous thymidine-thymidine base pairs) facilitated by Radiation sensitive 4 (RAD4), a key DNA repair protein, on an atom-by-atom basis using molecular dynamics (MD) and umbrella-sampling (US) simulations. Using atomistic models of RAD4-free and RAD4-bound mismatched DNA, the free energy profiles associated with extrusion of mismatched partner bases are determined for both systems. The mismatched bases adopted the most stable intrahelical conformation, and their extrusion was unfavorable in RAD4-free mismatched DNA due to the presence of prohibitively high barriers (>12.0 kcal/mol) along the extrusion pathways. Upon binding of RAD4 to the DNA, the global free energy minimum is shifted to the extrahelical state indicating the key role of RAD4-DNA interactions in catalyzing the dinucleotide base extrusion in the DNA-RAD4 complex. The critical residues of RAD4 contributing to the conformational stability of the mismatched bases are identified, and the energetics of insertion of a β-hairpin of RAD4 into the DNA duplex is examined. The conformational energy landscape-based mechanistic insight into RAD4-mediated base extrusion provided here may serve as a useful baseline to understand the molecular basis of xeroderma pigmentosum C (XPC)-mediated DNA damage repair in humans.

Repair-Resistant DNA Lesions

The eukaryotic global genomic nucleotide excision repair (GG-NER) pathway is the major mechanism that removes most bulky and some nonbulky lesions from cellular DNA. There is growing evidence that certain DNA lesions are repaired slowly or are entirely resistant to repair in cells, tissues, and in cell extract model assay systems. It is well established that the eukaryotic DNA lesion-sensing proteins do not detect the damaged nucleotide, but recognize the distortions/destabilizations in the native DNA structure caused by the damaged nucleotides. In this article, the nature of the structural features of certain bulky DNA lesions that render them resistant to NER, or cause them to be repaired slowly, is compared to that of those that are good-to-excellent NER substrates. Understanding the structural features that distinguish NER-resistant DNA lesions from good NER substrates may be useful for interpreting the biological significance of biomarkers of exposure of human populations to genotoxic environmental chemicals. NER-resistant lesions can survive to replication and cause mutations that can initiate cancer and other diseases. Furthermore, NER diminishes the efficacy of certain chemotherapeutic drugs, and the design of more potent pharmaceuticals that resist repair can be advanced through a better understanding of the structural properties of DNA lesions that engender repair-resistance.

Nucleotide Excision Repair Lesion-Recognition Protein Rad4 Captures a Pre-Flipped Partner Base in a Benzo[a]pyrene-Derived DNA Lesion: How Structure Impacts the Binding Pathway

The xeroderma pigmentosum C protein complex (XPC) recognizes a variety of environmentally induced DNA lesions and is the key in initiating their repair by the nucleotide excision repair (NER) pathway. When bound to a lesion, XPC flips two nucleotide pairs that include the lesion out of the DNA duplex, yielding a productively bound complex that can lead to successful lesion excision. Interestingly, the efficiencies of NER vary greatly among different lesions, influencing their toxicity and mutagenicity in cells. Though differences in XPC binding may influence NER efficiency, it is not understood whether XPC utilizes different mechanisms to achieve productive binding with different lesions. Here, we investigated the well-repaired 10R-(+)-cis-anti-benzo[a]pyrene-N²-dG (cis-B[a]P-dG) DNA adduct in a duplex containing normal partner C opposite the lesion. This adduct is derived from the environmental pro-carcinogen benzo[a]pyrene and is likely to be encountered by NER in the cell. We have extensively investigated its binding to the yeast XPC orthologue, Rad4, using umbrella sampling with restrained molecular dynamics simulations and free energy calculations. The NMR solution structure of this lesion in duplex DNA has shown that the dC complementary to the adducted dG is flipped out of the DNA duplex in the absence of XPC. However, it is not known whether the "pre-flipped" base would play a role in its recognition by XPC. Our results show that Rad4 first captures the displaced dC, which is followed by a tightly coupled lesion-extruding pathway for productive binding. This binding path differs significantly from the one deduced for the small cis-syn cyclobutane pyrimidine dimer lesion opposite mismatched thymines [ Mu , H. , ( 2015 ) Biochemistry , 54 ( 34 ), 5263 - 7 ]. The possibility of multiple paths that lead to productive binding to XPC is consistent with the versatile lesion recognition by XPC that is required for successful NER.

Recognition of Damaged DNA for Nucleotide Excision Repair: A Correlated Motion Mechanism with a Mismatched cis-syn Thymine Dimer Lesion

Mammalian global genomic nucleotide excision repair requires lesion recognition by XPC, whose detailed binding mechanism remains to be elucidated. Here we have delineated the dynamic molecular pathway and energetics of lesion-specific and productive binding by the Rad4/yeast XPC lesion recognition factor, as it forms the open complex [Min, J. H., and Pavletich, N. P. (2007) Nature 449, 570–575; Chen, X., et al. (2015) Nat. Commun. 6, 5849] that is required for excision. We investigated extensively a cis-syn cyclobutane pyrimidine dimer in mismatched duplex DNA, using high-level computational approaches. Our results delineate a preferred correlated motion mechanism, which provides for the first time an atomistic description of the sequence of events as Rad4 productively binds to the damaged DNA.

Adenine versus guanine DNA adducts of aristolochic acids: role of the carcinogen-purine linkage in the differential global genomic repair propensity

Computational modeling is employed to provide a plausible structural explanation for the experimentally-observed differential global genome repair (GGR) propensity of the ALII-N(2)-dG and ALII-N(6)-dA DNA adducts of aristolochic acid II. Our modeling studies suggest that an intrinsic twist at the carcinogen-purine linkage of ALII-N(2)-dG induces lesion site structural perturbations and conformational heterogeneity of damaged DNA. These structural characteristics correlate with the relative repair propensities of AA-adducts, where GGR recognition occurs for ALII-N(2)-dG, but is evaded for intrinsically planar ALII-N(6)-dA that minimally distorts DNA and restricts the conformational flexibility of the damaged duplex. The present analysis on the ALII adduct model systems will inspire future experimental studies on these adducts, and thereby may extend the list of structural factors that directly correlate with the propensity for GGR recognition.

Structural and energetic characterization of the major DNA adduct formed from the food mutagen ochratoxin A in the NarI hotspot sequence: influence of adduct ionization on the conformational preferences and implications for the NER propensity

The nephrotoxic food mutagen ochratoxin A (OTA) produces DNA adducts in rat kidneys, the major lesion being the C8-linked-2′-deoxyguanosine adduct (OTB-dG). Although research on other adducts stresses the importance of understanding the structure of the associated adducted DNA, site-specific incorporation of OTB-dG into DNA has yet to be attempted. The present work uses a robust computational approach to determine the conformational preferences of OTB-dG in three ionization states at three guanine positions in the NarI recognition sequence opposite cytosine. Representative adducted DNA helices were derived from over 2160 ns of simulation and ranked via free energies. For the first time, a close energetic separation between three distinct conformations is highlighted, which indicates OTA-adducted DNA likely adopts a mixture of conformations regardless of the sequence context. Nevertheless, the preferred conformation depends on the flanking bases and ionization state due to deviations in discrete local interactions at the lesion site. The structural characteristics of the lesion thus discerned have profound implications regarding its repair propensity and mutagenic outcomes, and support recent experiments suggesting the induction of double-strand breaks and deletion mutations upon OTA exposure. This combined structural and energetic characterization of the OTB-dG lesion in DNA will encourage future biochemical experiments on this potentially genotoxic lesion.

The relationships between XPC binding to conformationally diverse DNA adducts and their excision by the human NER system: is there a correlation?

The first eukaryotic NER factor that recognizes NER substrates is the heterodimeric XPC-RAD23B protein. The currently accepted hypothesis is that this protein recognizes the distortions/destabilization caused by DNA lesions rather than the lesions themselves. The resulting XPC-RAD23B-DNA complexes serve as scaffolds for the recruitment of subsequent NER factors that lead to the excision of the oligonucleotide sequences containing the lesions. Based on several well-known examples of DNA lesions like the UV radiation-induced CPD and 6-4 photodimers, as well as cisplatin-derived intrastrand cross-linked lesions, it is generally believed that the differences in excision activities in human cell extracts is correlated with the binding affinities of XPC-RAD23B to these DNA lesions. However, using electrophoretic mobility shift assays, we have found that XPC-RAD23B binding affinities of certain bulky lesions derived from metabolically activated polycyclic aromatic hydrocarbon compounds such as benzo[a]pyrene and dibenzo[a,l]pyrene, are not directly, or necessarily correlated with NER excision activities observed in cell-free extracts. These findings point to features of XPC-RAD23B-bulky DNA adduct complexes that may involve the formation of NER-productive or unproductive forms of binding that depend on the structural and stereochemical properties of the DNA adducts studied. The pronounced differences in NER cleavage efficiencies observed in cell-free extracts may be due to differences in the successful recruitment of subsequent NER factors by the XPC-RAD23B-DNA adduct complexes, and/or in the verification step. These phenomena appear to depend on the structural and conformational properties of the class of bulky DNA adducts studied.

Structural basis for the recognition of diastereomeric 5',8-cyclo-2'-deoxypurine lesions by the human nucleotide excision repair system

The hydroxyl radical is a powerful oxidant that generates DNA lesions including the stereoisomeric R and S 5′,8-cyclo-2′-deoxyadenosine (cdA) and 5′,8-cyclo-2′-deoxyguanosine (cdG) pairs that have been detected in cellular DNA. Unlike some other oxidatively generated DNA lesions, cdG and cdA are repaired by the human nucleotide excision repair (NER) apparatus. The relative NER efficiencies of all four cyclopurines were measured and compared in identical human HeLa cell extracts for the first time under identical conditions, using identical sequence contexts. The cdA and cdG lesions were excised with similar efficiencies, but the efficiencies for both 5′R cyclopurines were greater by a factor of ∼2 than for the 5′S lesions. Molecular modeling and dynamics simulations have revealed structural and energetic origins of this difference in NER-incision efficiencies. These lesions cause greater DNA backbone distortions and dynamics relative to unmodified DNA in 5′R than in 5′S stereoisomers, producing greater impairment in van der Waals stacking interaction energies in the 5′R cases. The locally impaired stacking interaction energies correlate with relative NER incision efficiencies, and explain these results on a structural basis in terms of differences in dynamic perturbations of the DNA backbone imposed by the R and S covalent 5′,8 bonds.

DNA origami deposition on native and passivated molybdenum disulfide substrates

Maintaining the structural fidelity of DNA origami structures on substrates is a prerequisite for the successful fabrication of hybrid DNA origami/semiconductor-based biomedical sensor devices. Molybdenum disulfide (MoS₂) is an ideal substrate for such future sensors due to its exceptional electrical, mechanical and structural properties. In this work, we performed the first investigations into the interaction of DNA origami with the MoS₂ surface. In contrast to the structure-preserving interaction of DNA origami with mica, another atomically flat surface, it was observed that DNA origami structures rapidly lose their structural integrity upon interaction with MoS₂. In a further series of studies, pyrene and 1-pyrenemethylamine, were evaluated as surface modifications which might mitigate this effect. While both species were found to form adsorption layers on MoS₂ via physisorption, 1-pyrenemethylamine serves as a better protective agent and preserves the structures for significantly longer times. These findings will be beneficial for the fabrication of future DNA origami/MoS₂ hybrid electronic structures.

Role of structural and energetic factors in regulating repair of a bulky DNA lesion with different opposite partner bases

Extensive molecular modeling with molecular dynamics simulations and van der Waals energy analyses were used to elucidate the striking finding that a mutagenic benzo[a]pyrene-derived DNA lesion, the base-displaced intercalated 10R-(+)-cis-anti-B[a]P-N(2)-dG (G*), manifests large differences in nucleotide excision repair (NER) efficiencies in DNA duplexes, which depend on the identities of the partner base opposite G*. The nature of the partner base causes marked differences in the extent of its major groove extrusion and dynamics, as well as energetic stability of the intercalation pocket that parallels the relative NER efficiencies.