Detection of the thietane precursor in the UVA formation of the DNA 6-4 photoadduct (original) (raw)
Related papers
Chemistry - A European Journal, 2015
The main chromophore of (6-4) photoproducts, namely, 5-methyl-2-pyrimidone (Pyo), is an artificial noncanonical nucleobase. This chromophore has recently been reported as a potential photosensitizer that induces triplet damage in thymine DNA. In this study, we investigate the spectroscopic properties of the Pyo unit embedded in DNA by means of explicit solvent molecular-dynamics simulations coupled to time-dependent DFT and quantum-mechanics/ molecular-mechanics techniques. Triplet-state transfer from the Pyo to the thymine unit was monitored in B-DNA by probing the propensity of this photoactive pyrimidine analogue to induce a Dexter-type triplet photosensitization and subsequent DNA damage. Scheme 1. Molecular formula of the (6-4)PP and dPyo subunits.
UVA-induced cyclobutane pyrimidine dimers in DNA: a direct photochemical mechanism?
Organic & Biomolecular Chemistry, 2010
The carcinogenic action of UVA radiation is commonly attributed to DNA oxidation mediated by endogenous photosensitisers. Yet, it was recently shown that cyclobutane pyrimidine dimers (CPD), well known for their involvement in UVB genotoxicity, are produced in larger yield than oxidative lesions in UVA-irradiated cells and skin. In the present work, we gathered mechanistic information on this photoreact ion by comparing formation of all possible bipyrimidine photoproducts upon UVA irradiation of cells, purified genomic DNA and dA 20 :dT 20 oligonucleotide duplex. We observed that the distribution of photoproducts, characterized by the sole formation of CPD and the absence of (6-4) photoproducts was similar in the three types of samples. The CPD involving two thymines represented 90 % of the amount of photoproducts. Moreover, the yields of formation of the DNA lesions were similar in cells and isolated DNA. In addition, the effect of the wavelength of the incident photons was found to be the same in isolated DNA and cells. This set of data shows that UVA-induced cyclobutane pyrimidine dimers are formed via a direct photochemical mechanism, without mediation of a cellular photosensitiser. This is possible because the double-stranded structure increases the capacity of DNA bases to absorb UVA photons, as evidenced in the case of the oligomer dA 20 :dT 20 . These results emphasize the need to consider UVA in the carcinogenic effects of sunlight. An efficient photoprotection is needed that can only be complete by completely blocking incident photons, rather than by systemic approaches such as antioxidant supplementation.
4-thiothymidine sensitization of DNA to UVA offers potential for a novel photochemotherapy
Photochemical & Photobiological Sciences, 2012
Photochemotherapy, in which ultraviolet radiation (UVR: 280-400 nm) or visible light is combined with a photosensitizing drug to produce a therapeutic effect that neither drug or radiation can achieve alone, is a proven therapeutic strategy for a number of non-malignant hyperproliferative skin conditions and various cancers. Examples are psoralen plus UVA (320-400 nm) radiation (PUVA) and photodynamic therapy (PDT). All existing photochemotherapies have drawbacks-for example the association of PUVA with the development of skin cancer, and pain that is often associated with PDT treatment of skin lesions. There is a clear need to develop alternative approaches that involve lower radiation doses and/or improved selectivity for target cells. In this review, we explore the possibility to address this need by exploiting thionucleoside-mediated DNA photosensitisation to low, non toxic doses of UVA radiation.
Thymine Dimerization in DNA Is an Ultrafast Photoreaction
Science, 2007
Femtosecond time-resolved infrared spectroscopy is used to study the formation of cyclobutane dimers in the all-thymine oligonucleotide (dT) 18 by ultraviolet light at 272 nanometers. The appearance of marker bands in time-resolved spectra indicate that dimers are fully formed ∼1 picosecond after ultraviolet excitation. The ultrafast appearance of this mutagenic photolesion points to an approximately barrierless excited-state reaction for bases that are properly oriented at the instant of light absorption. The low quantum yield of this photoreaction is proposed to result from infrequent conformational states in the unexcited polymer, revealing a strong link between conformation prior to light absorption and photodamage.
The Journal of Physical Chemistry Letters, 2022
Oxo and amino substituted purines and pyrimidines have been suggested as protonucleobases participating in ancient pre-RNA forms. Considering electromagnetic radiation as a key environmental selection pressure on early Earth, the investigation of the photophysics of modified nucleobases is crucial to determine their viability as nucleobases' ancestors and to understand the factors that rule the photostability of natural nucleobases. In this Letter, we combine femtosecond transient absorption spectroscopy and quantum mechanical simulations to reveal the photochemistry of 4-pyrimidinone, a close relative of uracil. Irradiation of 4pyrimidinone with ultraviolet radiation populates the S 1 (ππ*) state, which decays to the vibrationally excited ground state in a few hundred femtoseconds. Analysis of the postirradiated sample in water reveals the formation of a 6-hydroxy-5H-photohydrate and 3-(N-(iminomethyl)imino)propanoic acid as the primary photoproducts. 3-(N-(Iminomethyl)imino)propanoic acid originates from the hydrolysis of an unstable ketene species generated from the C4−N3 photofragmentation of the pyrimidine core.
DNA photodamage: Study of cyclobutane pyrimidine dimer formation in a locked thymine dinucleotide
Spectroscopy, 2010
The cyclobutane pyrimidine dimer (CPD) formed between two adjacent thymine bases is the most abundant DNA photolesion induced by UV radiation. The quantum yield of this reaction is on the order of ∼1% in DNA. This small quantum yield hampers the study of damage formation in naturally occurring DNA. Investigations with increased accuracy become possible for a locked nucleotide model compound T L pT L which exhibits a quantum yield of about 10% for CPD formation. Time resolved IR spectroscopy on T L pT L and two other DNA model compounds (TpT and (dT) 18 ) reveals that: (i) The absorption changes after ∼1 ps are due to CPD photodamage. (ii) The quantum efficiency of CPD formation on the few picosecond time scale equals the quantum efficiency reported in stationary experiments. CPD photodamage formation in the investigated DNA constructs is thus predominantly formed from the primarily photoexcited singlet ππ * state, whereas the triplet channel does not play an essential role.
UVA photoactivation of DNA containing halogenated thiopyrimidines induces cytotoxic DNA lesions
Journal of photochemistry and photobiology. B, Biology, 2015
Photochemotherapy, the combination of a photosensitiser and ultraviolet (UV) or visible light, is an effective treatment for skin conditions including cancer. The high mutagenicity and non-selectivity of photochemotherapy regimes warrants the development of alternative approaches. We demonstrate that the thiopyrimidine nucleosides 5-bromo-4-thiodeoxyuridine (SBrdU) and 5-iodo-4-thiodeoxyuridine (SIdU) are incorporated into the DNA of cultured human and mouse cells where they synergistically sensitise killing by low doses of UVA radiation. The DNA halothiopyrimidine/UVA combinations induce DNA interstrand crosslinks, DNA-protein crosslinks, DNA strand breaks, nucleobase damage and lesions that resemble UV-induced pyrimidine(6-4)pyrimidone photoproducts. These are potentially lethal DNA lesions and cells defective in their repair are hypersensitive to killing by SBrdU/UVA and SIdU/UVA. DNA SIdU and SBrdU generate lethal DNA photodamage by partially distinct mechanisms that reflect the...
Efficient DNA interstrand crosslinking by 6-thioguanine and UVA radiation
DNA Repair, 2011
Patients taking the immunosuppressant and anticancer thiopurines 6-mercaptopurine, azathioprine or 6-thioguanine (6-TG), develop skin cancer at a very high frequency. Their DNA contains 6-TG which absorbs ultraviolet A (UVA) radiation, and their skin is UVA hypersensitive, consistent with the formation of DNA photodamage. Here we demonstrate that UVA irradiation of 6-TG-containing DNA causes DNA interstrand crosslinking. In synthetic duplex oligodeoxynucleotides, the interstrand crosslinks (ICLs) can form between closely opposed 6-TG bases and, in a less favoured reaction, between 6-TG and normal bases on the opposite strand. In vivo, UVA irradiation of cultured cells containing 6-TG-substituted DNA also causes ICL formation and induces the chromosome aberrations that are characteristically associated with this type of DNA lesion. 6-TG/UVA activates the Fanconi anemia (FA) pathway via monoubiquitination of the FANCD2 protein. Cells defective in the FA pathway or other factors involved in ICL processing, such as XPF and DNA Pol, are all hypersensitive to killing by 6-TG/UVA-consistent with a significant contribution of photochemical ICLs to the cytotoxicity of this treatment. Our findings suggest that sunlight-exposed skin of thiopurine treated patients may experience chronic photochemical DNA damage that requires constant intervention of the FA pathway.
Nucleic Acids Research, 2011
Photochemotherapy-in which a photosensitizing drug is combined with ultraviolet or visible radiation-has proven therapeutic effectiveness. Existing approaches have drawbacks, however, and there is a clinical need to develop alternatives offering improved target cell selectivity. DNA substitution by 4-thiothymidine (S 4 TdR) sensitizes cells to killing by ultraviolet A (UVA) radiation. Here, we demonstrate that UVA photoactivation of DNA S 4 TdR does not generate reactive oxygen or cause direct DNA breakage and is only minimally mutagenic. In an organotypic human skin model, UVA penetration is sufficiently robust to kill S 4 TdR-photosensitized epidermal cells. We have investigated the DNA lesions responsible for toxicity. Although thymidine is the predominant UVA photoproduct of S 4 TdR in dilute solution, more complex lesions are formed when S 4 TdR-containing oligonucleotides are irradiated. One of these, a thietane/S 5-(6-4)T:T, is structurally related to the (6-4) pyrimidine:pyrimidone [(6-4) Py:Py] photoproducts induced by UVB/C radiation. These lesions are detectable in DNA from S 4 TdR/UVA-treated cells and are excised from DNA more efficiently by keratinocytes than by leukaemia cells. UVA irradiation also induces DNA interstrand crosslinking of S 4 TdR-containing duplex oligonucleotides. Cells defective in repairing (6-4) Py:Py DNA adducts or processing DNA crosslinks are extremely sensitive to S 4 TdR/UVA indicating that these lesions contribute significantly to S 4 TdR/UVA cytotoxicity.