TY - JOUR T1 - Nijmegen breakage syndrome (NBN) protein causes resistance to methylating anticancer drugs like temozolomide JF - Molecular Pharmacology JO - Mol Pharmacol DO - 10.1124/mol.110.066076 SP - mol.110.066076 AU - Marcus Eich AU - Wynand Paul Roos AU - Grigory L Dianov AU - Martin Digweed AU - Bernd Kaina Y1 - 2010/08/20 UR - http://molpharm.aspetjournals.org/content/early/2010/08/20/mol.110.066076.abstract N2 - Methylating agents are first-line therapeutics for gliomas and malignant melanomas. They attack DNA at various sites, and both O6-methylguanine and N-methylated base adducts contribute to the killing response. The mechanism of cellular defense against these agents primarily involves O6-methylguanine-DNA methyltransferase (MGMT) and base excision repair (BER). Here, we determined whether a key protein involved in DNA double-strand break (DSB) recognition and signaling, nibrin (NBN alias NBS-1), plays a role in the cellular defense against methylating agents. Comparing NBN mutated fibroblasts and lymphoblastoid cells from patients suffering from Nijmegen breakage syndrome, we show that NBN mutants are clearly more sensitive to N-methyl-N'nitro-N-nitrosoguanidine (MNNG) and temozolomide than the corresponding wild-type cells. Hypersensitivity was due to the induction of both apoptosis and necrosis. The mismatch repair proteins MSH2, MSH6, MLH1 and PMS2 were expressed at a similar level in the cell lines and BER was not affected by NBN mutation. Since MGMT expression abrogated the hypersensitivity of NBN mutated cells, we conclude that O6-methylguanine derived lesions are responsible for triggering the response. Down-regulation of NBN in melanoma cells by siRNA rendered them more sensitive to temozolomide, suggesting that NBN is a novel modulator of temozolomide sensitivity. As NBN is part of the MRN complex that recognizes DSBs, the data strongly indicate that MRN is critically involved in DSB processing following O6-methylguanine induction. The data provide first evidence that NBN is involved in the cellular defense against O6-methylguanine inducing agents like temozolomide and identify NBN as a critical target of methylating anticancer drug resistance. ER -