The Influence of Single, Tandem, and Clustered DNA Damage on the Electronic Properties of the Double Helix: A Theoretical Study

The Influence of Clustered DNA Damage Containing Iz/Oz and OXOdG on the Charge Transfer through the Double Helix: A Theoretical Study

The genome-the source of life and platform of evolution-is continuously exposed to harmful factors, both extra- and intra-cellular. Their activity causes different types of DNA damage, with approximately 80 different types of lesions having been identified so far. In this paper, the influence of a clustered DNA damage site containing imidazolone (Iz) or oxazolone (Oz) and 7,8-dihydro-8-oxo-2'-deoxyguanosine (OXOdG) on the charge transfer through the double helix as well as their electronic properties were investigated. To this end, the structures of oligo-Iz, d[A1Iz2A3OXOG4A5]*d[T5C4T3C2T1], and oligo-Oz, d[A1Oz2A3OXOG4A5]*d[T5C4T3C2T1], were optimized at the M06-2X/6-D95**//M06-2X/sto-3G level of theory in the aqueous phase using the ONIOM methodology; all the discussed energies were obtained at the M06-2X/6-31++G** level of theory. The non-equilibrated and equilibrated solvent-solute interactions were taken into consideration. The following results were found: (A) In all the discussed cases, OXOdG showed a higher predisposition to radical cation formation, and B) the excess electron migration toward Iz and Oz was preferred. However, in the case of oligo-Oz, the electron transfer from Oz2 to complementary C4 was noted during vertical to adiabatic anion relaxation, while for oligo-Iz, it was settled exclusively on the Iz2 moiety. The above was reflected in the charge transfer rate constant, vertical/adiabatic ionization potential, and electron affinity energy values, as well as the charge and spin distribution. It can be postulated that imidazolone moiety formation within the CDL ds-oligo structure and its conversion to oxazolone can significantly influence the charge migration process, depending on the C2 carbon hybridization sp2 or sp3. The above can confuse the single DNA damage recognition and removal processes, cause an increase in mutagenesis, and harm the effectiveness of anticancer therapy.

The Influence of Oxidized Imino-Allantoin in the Presence of OXOG on Double Helix Charge Transfer: A Theoretical Approach

The genome is continuously exposed to a variety of harmful factors that result in a significant amount of DNA damage. This article examines the influence of a multi-damage site containing oxidized imino-allantoin (OXIa) and 7,8-dihydro-8-oxo-2'-deoxyguanosine (OXOdG) on the spatial geometry, electronic properties, and ds-DNA charge transfer. The ground stage of a d[A1OXIa2A3OXOG4A5]*d[T5C4T3C2T1] structure was obtained at the M06-2X/6-D95**//M06-2X/sto-3G level of theory in the condensed phase, with the energies obtained at the M06-2X/6-31++G** level. The non-equilibrated and equilibrated solvent-solute interactions were also considered. Theoretical studies reveal that the radical cation prefers to settle on the OXOG moiety, irrespective of the presence of OXIa in a ds-oligo. The lowest vertical and adiabatic ionization potential values were found for the OXOG:::C base pair (5.94 and 5.52 [eV], respectively). Conversely, the highest vertical and adiabatic electron affinity was assigned for OXIaC as follows: 3.15 and 3.49 [eV]. The charge transfers were analyzed according to Marcus' theory. The highest value of charge transfer rate constant for hole and excess electron migration was found for the process towards the OXOGC moiety. Surprisingly, the values obtained for the driving force and activation energy of electro-transfer towards OXIa2C4 located this process in the Marcus inverted region, which is thermodynamically unfavorable. Therefore, the presence of OXIa can slow down the recognition and removal processes of other DNA lesions. However, with regard to anticancer therapy (radio/chemo), the presence of OXIa in the structure of clustered DNA damage can result in improved cancer treatment outcomes.

The Influence of Spirodi(Iminohydantoin) on Charge Transfer through ds-DNA Containing 8-OXO-dG: A Theoretical Approach

Genetic information stored in a DNA base sequence is continuously exposed to harmful factors. It has been determined that 9 × 10⁴ different DNA damage events occur in a single human cell every 24 h. Of these, 7,8-dihydro-8-oxo-guanosine (^OXOG) is one of the most abundant and can undergo further transformations towards spirodi(iminohydantoin) (Sp). Sp is highly mutagenic in comparison to its precursor if not repaired. In this paper, the influence of both Sp diastereomers 4R and 4S as well as their anti and syn conformers on charge transfer through the double helix was taken into theoretical consideration. In addition, the electronic properties of four modelled double-stranded oligonucleotides (ds-oligos) were also discussed, i.e., d[A₁Sp₂A₃^oxoG₄A₅] * [T₅C₄T₃C₂T₁]. Throughout the study, the M06-2X/6-31++G** level theory was used. Solvent-solute non-equilibrated and equilibrated interactions were also considered. The subsequent results elucidated that the 7,8-dihydro-8-oxo-guanosine:cytidine (^OXOGC) base pair is the settled point of a migrated radical cation in each of the discussed cases, due to its low adiabatic ionization potential, i.e., _~5.55 [eV]. The opposite was noted for excess electron transfer through ds-oligos containing anti (R)-Sp or anti (S)-Sp. The radical anion was found on the ^OXOGC moiety, whereas in the presence of syn (S)-Sp or syn (R)-Sp, an excess electron was found on the distal A₁T₅ or A₅T₁ base pair, respectively. Furthermore, a spatial geometry analysis of the discussed ds-oligos revealed that the presence of syn (R)-Sp in the ds-oligo caused only a slight deformation to the double helix, while syn (S)-Sp formed an almost ideal base pair with a complementary dC. The above results are in strong agreement with the final charge transfer rate constant, as calculated according to Marcus' theory. In conclusion, DNA damage such as spirodi(iminohydantoin), especially when becoming part of clustered DNA damage, can affect the effectiveness of other lesion recognition and repair processes. This can lead to the acceleration of undesired and deleterious processes such as carcinogenesis or aging. However, in terms of anticancer radio-/chemo- or combined therapy, the slowing down of the repair machinery can result in increased effectiveness. With this in mind, the influence of clustered damage on charge transfer and its subsequent effect on single-damage recognition by glycosylases justifies future investigation.

FapydG in the Shadow of OXOdG—A Theoretical Study of Clustered DNA Lesions

Genetic information, irrespective of cell type (normal or cancerous), is exposed to a range of harmful factors, which can lead to more than 80 different types of DNA damage. Of these, oxoG and FapyG have been identified as the most abundant in normoxic and hypoxic conditions, respectively. This article considers d[AFapyGAOXOGA]*[TCTCT] (oligo-FapyG) with clustered DNA lesions (CDLs) containing both the above types of damage at the M06-2x/6-31++G** level of theory in the condensed phase. Furthermore, the electronic properties of oligo-FapyG were analysed in both equilibrated and non-equilibrated solvation–solute interaction modes. The vertical/adiabatic ionization potential (VIP, AIP) and electron affinity (VEA, AEA) of the investigated ds-oligo were found as follows in [eV]: 5.87/5.39 and −1.41/−2.09, respectively. The optimization of the four ds-DNA spatial geometries revealed that the transFapydG was energetically privileged. Additionally, CDLs were found to have little influence on the ds-oligo structure. Furthermore, for the FapyGC base-pair isolated from the discussed ds-oligo, the ionization potential and electron affinity values were higher than those assigned to OXOGC. Finally, a comparison of the influence of FapyGC and OXOGC on charge transfer revealed that, in contrast to the OXOGC base-pair, which, as expected, acted as a radical cation/anion sink in the oligo-FapyG structure, FapyGC did not significantly affect charge transfer (electron–hole and excess–electron). The results presented below indicate that 7,8-dihydro-8-oxo-2′-deoxyguanosine plays a significant role in charge transfer through ds-DNA containing CDL and indirectly has an influence on the DNA lesion recognition and repair process. In contrast, the electronic properties obtained for 2,6-diamino-4-hydroxy-5-foramido-2′deoxypyrimidine were found to be too weak to compete with OXOG to influence charge transfer through the discussed ds-DNA containing CDL. Because increases in multi-damage site formation are observed during radio- or chemotherapy, understanding their role in the above processes can be crucial for the efficiency and safety of medical cancer treatment.

The 2Ih and OXOG Proximity Consequences on Charge Transfer through ds-DNA: Theoretical Studies of Clustered DNA Damage

Genetic information is continuously exposed to harmful factors, both intra- and extracellular. Their activity can lead to the formation of different types of DNA damage. Clustered lesions (CDL) are problematic for DNA repair systems. In this study, the short ds-oligos with a CDL containing (R) or (S) 2Ih and ^OXOG in their structure were chosen as the most frequent in vitro lesions. In the condensed phase, the spatial structure was optimized at the M062x/D95**:M026x/sto-3G level of theory, while the electronic properties were optimized at the M062x/6-31++G** level. The influence of equilibrated and non-equilibrated solvent-solute interactions was then discussed. It was found that the presence of (R)2Ih in the ds-oligo structure causes a greater increase in structure sensitivity towards charge adoption than (S)2Ih, while ^OXOG shows high stability. Moreover, the analysis of charge and spin distribution reveals the different effects of 2Ih diastereomers. Additionally, the adiabatic ionization potential was found as follows for (R)-2Ih and (S)-2Ih in eV: 7.02 and 6.94. This was in good agreement with the AIP of the investigated ds-oligos. It was found that the presence of (R)-2Ih has a negative influence on excess electron migration through ds-DNA. Finally, according to the Marcus theory, the charge transfer constant was calculated. The results presented in the article show that both diastereomers of 5-carboxamido-5-formamido-2-iminohydantoin should play a significant role in the CDL recognition process via electron transfer. Moreover, it should be pointed out that even though the cellular level of (R and S)-2Ih has been obscured, their mutagenic potential should be at the same level as other similar guanine lesions found in different cancer cells.

The Usefulness of Autoradiography for DNA Repair Proteins Activity Detection in the Cytoplasm towards Radiolabeled Oligonucleotides Containing 5′,8-Cyclo-2′-deoxyAdenosine

Autoradiography of 32P-radiolabeled oligonucleotides is one of the most precise detection methods of DNA repair processes. In this study, autoradiography allowed assessing the activity of proteins in the cytoplasm involved in DNA repair. The cytoplasm is the site of protein biosynthesis but is also a target cellular compartment of synthetic therapeutic oligonucleotide (STO) delivery. The DNA-based drugs may be impaired by radiation-induced lesions, such as clustered DNA lesions (CDL) and/or 5′,8-cyclo-2′-deoxypurines (cdPu). CDL and cdPu may appear in the sequence of STO after irradiation and subsequently impair DNA repair, as shown in previous studies. Hence, the interesting questions are (1) is it safe to combine STO treatment with radiotherapy; (2) are repair proteins active in the cytoplasm; and (3) is their activity different in the cytoplasm than in the nucleus? This unique study examined whether the proteins involved in the DNA repair are affected by the CDL while they are still present in the cytoplasm of xrs5, BJ, and XPC cells. Double-stranded oligonucleotides with bi-stranded CDL were used (containing AP site in one strand and a (5′S) or (5′R) 5′,8-cyclo-2′-deoxyadenosine (cdA) in the other strand located 1 or 4 bp in both directions). The results have shown that the proteins involved in the repair were active in the cytoplasm, but less than in the nucleus. The general trends aligned for cytoplasm and nucleus—lesions located in the 5′-end direction inhibited the course of DNA repair. The combination of STO with radiotherapy should be applied carefully, as unrepaired lesions within STO may impair their therapeutic efficiency.

Recognition of a tandem lesion by DNA bacterial formamidopyrimidine glycosylases explored combining molecular dynamics and machine learning

The combination of several closely spaced DNA lesions, which can be induced by a single radical hit, constitutes a hallmark in the DNA damage landscape and radiation chemistry. The occurrence of such a tandem base lesion gives rise to a strong coupling with the double helix degrees of freedom and induces important structural deformations, in contrast to DNA strands containing a single oxidized nucleobase. Although such complex lesions are known to be refractory to repair by DNA glycosylases, there is still a lack of structural evidence to rationalize these phenomena. In this contribution, we explore, by numerical modeling and molecular simulations, the behavior of the bacterial glycosylase responsible for base excision repair (MutM), specialized in excising oxidatively-damaged defects such as 7,8-dihydro-8-oxoguanine (8-oxoG). The difference in lesion recognition between a simple damage and a tandem lesion featuring an additional abasic site is assessed at atomistic resolution owing to microsecond molecular dynamics simulations and machine learning postprocessing, allowing to extensively pinpoint crucial differences in the interaction patterns of the damaged bases. Our results reveal substantial changes in the interaction network surrounding the 8-oxoG upon addition of an adjacent abasic site, leading to the perturbation of the intercalation triad which is crucial for lesion recognition and processing. The recognition process might also be impacted by a more constrained MutM-DNA binding upon tandem damage, as shown by the machine learning post-processing. This work advocates for the use of such high throughput numerical simulations for exploring the complex combinatorial chemistry of tandem DNA lesions repair and more generally local multiple damaged sites of the utmost significance in radiation chemistry.

How (5'S) and (5'R) 5',8-Cyclo-2'-Deoxypurines Affect Base Excision Repair of Clustered DNA Damage in Nuclear Extracts of xrs5 Cells? A Biochemical Study

The clustered DNA lesions (CDLs) are a characteristic feature of ionizing radiation’s impact on the human genetic material. CDLs impair the efficiency of cellular repair machinery, especially base excision repair (BER). When CDLs contain a lesion repaired by BER (e.g., apurinic/apyrimidinic (AP) sites) and a bulkier 5′,8-cyclo-2′-deoxypurine (cdPu), which is not a substrate for BER, the repair efficiency of the first one may be affected. The cdPus’ influence on the efficiency of nuclear BER in xrs5 cells have been investigated using synthetic oligonucleotides with bi-stranded CDL (containing (5′S) 5′,8-cyclo-2′-deoxyadenosine (ScdA), (5′R) 5′,8-cyclo-2′-deoxyadenosine (RcdA), (5′S) 5′,8-cyclo-2′-deoxyguanosine (ScdG) or (5′R) 5′,8-cyclo-2′-deoxyguanosine (RcdG) in one strand and an AP site in the other strand at different interlesion distances). Here, for the first time, the impact of ScdG and RcdG was experimentally tested in the context of nuclear BER. This study shows that the presence of RcdA inhibits BER more than ScdA; however, ScdG decreases repair level more than RcdG. Moreover, AP sites located ≤10 base pairs to the cdPu on its 5′-end side were repaired less efficiently than AP sites located ≤10 base pairs on the 3′-end side of cdPu. The strand with an AP site placed opposite cdPu or one base in the 5′-end direction was not reconstituted for cdA nor cdG. CdPus affect the repair of the other lesion within the CDL. It may translate to a prolonged lifetime of unrepaired lesions leading to mutations and impaired cellular processes. Therefore, future research should focus on exploring this subject in more detail.

Redox Properties of Native and Damaged DNA from Mixed Quantum Mechanical/Molecular Mechanics Molecular Dynamics Simulations

The redox properties of two large DNA fragments composed of 39 base pairs, differing only by an 8-oxoguanine (8oxoG) defect replacing a guanine (G), were investigated in physiological conditions using mixed quantum mechanical/molecular mechanical (QM/MM) molecular dynamics simulations. The quantum region of the native fragment comprised 3 G-C base pairs, while one G was replaced by an 8oxoG in the defect fragment. The calculated values for the redox free energy are 6.55 ± 0.28 eV and 5.62 ± 0.30 eV for the native and the 8oxoG-containing fragment, respectively. The respective estimates for the reorganization free energy are 1.25 ± 0.18 eV and 1.00 ± 0.18 eV. Both reactions follow the Marcus theory for electron transfer. The large difference in redox potential between the two fragments shows that replacement of a G by an 8oxoG renders the DNA more easily oxidizable. This finding is in agreement with the suggestion that DNA fragments containing an 8oxoG defect can act as sinks of oxidative damage that protect the rest of the genome from assault. In addition, the difference in redox potential between the native and the defect DNA fragment indicates that a charge transfer-based mechanism for the recognition of DNA defects might be feasible, in line with recent suggestions based on experimental observations.