Spinocerebellar ataxia with axonal neuropathy-1 (SCAN1) is a neurodegenerative disease that results from mutation of tyrosyl phosphodiesterase 1 (TDP1)1. In lower eukaryotes, Tdp1 removes topoisomerase 1 (top1) peptide from DNA termini during the repair of double-strand breaks created by collision of replication forks with top1 cleavage complexes in proliferating cells2,3,4. Although TDP1 most probably fulfils a similar function in human cells, this role is unlikely to account for the clinical phenotype of SCAN1, which is associated with progressive degeneration of post-mitotic neurons. In addition, this role is redundant in lower eukaryotes, and Tdp1 mutations alone confer little phenotype4,5,6,7. Moreover, defects in processing or preventing double-strand breaks during DNA replication are most probably associated with increased genetic instability and cancer, phenotypes not observed in SCAN1 (ref. 8). Here we show that in human cells TDP1 is required for repair of chromosomal single-strand breaks arising independently of DNA replication from abortive top1 activity or oxidative stress. We report that TDP1 is sequestered into multi-protein single-strand break repair (SSBR) complexes by direct interaction with DNA ligase IIIα and that these complexes are catalytically inactive in SCAN1 cells. These data identify a defect in SSBR in a neurodegenerative disease, and implicate this process in the maintenance of genetic integrity in post-mitotic neurons.
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We thank M. Withers and L. Ju for technical assistance, H. Nash for tyrosine oligonucleotide and critical reading of the manuscript, F. Karimi-Busheri and A. Rasouli-Nia for C-ter3 cells. This work was supported by an MRC Programme Grant to K.W.C. and by an Overseas Research Scholarship and a Stapley Trust award to S.F.E-K.
The authors declare that they have no competing financial interests.
Depicts the use of alkaline unwinding to measure strand breaks in normal and SCAN1 cells. (PPT 26 kb)
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