MUTYH mediates the toxicity of combined DNA 6-thioguanine and UVA radiation

// Francesca Grasso 1,2,* , Vitalba Ruggieri 3,* , Gabriele De Luca 1 , Paola Leopardi 1 , Maria Teresa Mancuso 4 , Ida Casorelli 5 , Pietro Pichierri 1 , Peter Karran 6 and Margherita Bignami 1 1 Department of Environment and Primary Prevention, Istituto Superiore di Sanita, Rome, Italy 2 Department of Science, University Roma Tre, Rome, Italy 3 Laboratory of Pre-Clinical and Translational Research, IRCCS, Referral Cancer Center of Basilicata, Rionero in Vulture, Italy 4 Laboratory of Radiation Biology and Biomedicine, Agenzia Nazionale per le Nuove Tecnologie, l’Energia e lo Sviluppo Economico Sostenibile (ENEA) CR-Casaccia, Rome, Italy 5 Department of Immunohematology and Transfusion Unit, Azienda Ospedaliera Sant’Andrea, Rome, Italy 6 Cancer Research UK London Research Institute, Clare Hall Laboratories, South Mimms, Herts, UK * These authors contributed equally to this work Correspondence: Margherita Bignami, email: // Keywords : MUTYH, 6-thioguanine, azathioprine, UVA Received : October 22, 2014 Accepted : December 01, 2014 Published : December 02, 2014 Abstract The therapeutic thiopurines, including the immunosuppressant azathioprine (Aza) cause the accumulation of the UVA photosensitizer 6-thioguanine (6-TG) in the DNA of the patients’ cells. DNA 6-TG and UVA are synergistically cytotoxic and their interaction causes oxidative damage. The MUTYH DNA glycosylase participates in the base excision repair of oxidized DNA bases. Using Mutyh -nullmouse fibroblasts (MEFs) we examined whether MUTYH provides protection against the lethal effects of combined DNA 6-TG/UVA. Surprisingly, Mutyh -null MEFs were more resistant than wild-type MEFs, despite accumulating higher levels of DNA 8-oxo-7,8-dihydroguanine (8-oxoG).Their enhanced 6-TG/UVA resistance reflected the absence of the MUTYH protein and MEFs expressing enzymatically-dead human variants were as sensitive as wild-type cells. Consistent with their enhanced resistance, Mutyh -null cells sustained fewer DNA strand breaks and lower levels of chromosomal damage after 6-TG/UVA. Although 6-TG/UVA treatment caused early checkpoint activation irrespective of the MUTYH status, M utyh -null cells failed to arrest in S-phase at late time points. MUTYH-dependent toxicity was also apparent in vivo . Mutyh -/- mice survived better than wild-type during a 12-month chronicexposure to Aza/UVA treatments that significantly increased levels of skin DNA 8-oxoG. Two squamous cell skin carcinomas arose in Aza/UVA treated Mutyh -/- mice whereas similarly treated wild-type animals remained tumor-free.