title: The homologous recombination protein RAD51D mediates the processing of 6-thioguanine lesions downstream of mismatch repair.
abstract: Thiopurines are extensively used as immunosuppressants and in the treatment of childhood cancers, even though there is concern about therapy-induced leukemias and myelodysplastic syndromes resulting from thiopurine use. Following metabolic activation, thiopurines are incorporated into DNA and invoke mismatch repair (MMR). Recognition of 6-thioguanine (6-thioG) in DNA by key MMR proteins results in cell death rather than repair. There are suggestions that homologous recombination (HR) is involved downstream of MMR following thiopurine treatment, but the precise role of HR is poorly understood. In this study, we demonstrate that cells deficient in RAD51D (a RAD51 paralogue) are extremely sensitive to 6-thioG. This sensitivity is almost completely rescued by the deletion of Mlh1, which suggests that HR is involved in the repair of the 6-thioG-induced recombinogenic lesions generated by MMR. Furthermore, 6-thioG induces chromosome aberrations in the Rad51d-deficient cells. Interestingly, Rad51d-deficient cells show a striking increase in the frequency of triradial and quadriradial chromosomes in response to 6-thioG therapy. The presence of these chromatid exchange-type aberrations indicates that the deficiency in RAD51D-dependent HR results in profound chromosomal damage precipitated by the processing of 6-thioG by MMR. The radials are notable as an important source of chromosomal translocations, which are the most common class of mutations found in hematologic malignancies. This study thus suggests that HR insufficiency could be a potential risk factor for the development of secondary cancers that result from long-term use of thiopurines in patients.
authors.affiliation: Department of Pharmaceutical and Biomedical Sciences, South Carolina College of Pharmacy, University of South Carolina, 715 Sumter Street, Columbia SC 29208, USA.
authors.forename: Preeti
authors.initials: P
authors.lastname: Rajesh
authors.forename: Alexandra V
authors.initials: AV
authors.lastname: Litvinchuk
authors.forename: Douglas L
authors.initials: DL
authors.lastname: Pittman
authors.forename: Michael D
authors.initials: MD
authors.lastname: Wyatt
mesh_terms: D000818:Animals; D002460:Cell Line; D043171:Chromosomal Instability / Q000187:drug effects; D004247:DNA / Q000378:metabolism; D004249:DNA Damage; D053843:DNA Mismatch Repair / Q000187:drug effects*; D004268:DNA-Binding Proteins / Q000172:deficiency / Q000378:metabolism*; D016195:G2 Phase / Q000187:drug effects; D015726:Giant Cells / Q000187:drug effects / Q000473:pathology; D051379:Mice; D011995:Recombination, Genetic / Q000187:drug effects / Q000235:genetics*; D013866:Thioguanine / Q000494:pharmacology*
chemical_list: D004268:DNA-Binding Proteins; C491807:Rad51d protein, mouse; D004247:DNA; D013866:Thioguanine