title: Identification of potentially cytotoxic lesions induced by UVA photoactivation of DNA 4-thiothymidine in human cells.
abstract: 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.
authors: King's College London, School of Medicine, Division of Genetics and Molecular Medicine, St John's Institute of Dermatology, London, SE1 9RT, UK. olivier.reelfs@kcl.ac.uk
authors: Olivier
authors: O
authors: Reelfs
authors: Peter
authors: P
authors: Macpherson
authors: Xiaolin
authors: X
authors: Ren
authors: Yao-Zhong
authors: YZ
authors: Xu
authors: Peter
authors: P
authors: Karran
authors: Antony R
authors: AR
authors: Young
mesh_terms: D000818:Animals; D002460:Cell Line; D006224:Cricetinae; D004247:DNA / Q000737:chemistry / Q000378:metabolism; D004249:DNA Damage; D004260:DNA Repair; D006801:Humans; D016296:Mutagenesis; D009841:Oligonucleotides / Q000737:chemistry; D011740:Pyrimidine Dimers / Q000378:metabolism; D017382:Reactive Oxygen Species / Q000378:metabolism; D012867:Skin / Q000033:anatomy & histology / Q000528:radiation effects; D013936:Thymidine / Q000031:analogs & derivatives* / Q000528:radiation effects; D014466:Ultraviolet Rays
chemical_list: C414668:4-thiothymidine; D009841:Oligonucleotides; D011740:Pyrimidine Dimers; D017382:Reactive Oxygen Species; D004247:DNA; D013936:Thymidine