TDP1 facilitates repair of ionizing radiation-induced DNA single-strand breaks

Tyrosyl DNA phosphodiesterase-1 (TDP1) is the gene product mutated in spinocerebellar ataxia with axonal neuropathy1 (SCAN1). SCAN1 is a hereditary ataxia that lacks extra-neurological phenotype, pointing to a critical role for TDP1 in the nervous system. Recently, we showed that TDP1 is associated with the DNA single-strand break (SSBR) repair machinery through an interaction with DNA ligase 3 alpha (Lig3 alpha) and that SCAN1 cells are defective in the repair of chromosomal DNA single-strand breaks (SSBs) arising from abortive Topoisomerase 1 (Top1)-DNA intermediates. Here we demonstrate that TDPI is also required for the repair of SSBs induced by ionizing radiation (IR), though not measurably for IR-induced DNA double-strand breaks (DSBs). In addition, we provide evidence that abortive Top1 cleavage complexes are processed by the proteasome prior to the action of TDPI in vivo, and we exploit this observation to show that the SSBR defect in SCAN1 following IR reflects, in part at least, the presence of IR-induced protein-DNA cross-links. Finally we show that TDPI activity at abortive Top1-SSBs is stimulated by XRCC1/Lig3 alpha in vitro. These data expand the type of SSBs processed by TDPI to include those induced by ionizing radiation, and raise the possibility that TDPI inhibitors may improve radiotherapy. (C) 2007 Elsevier B.V. All rights reserved.