SCAN1 mutant Tdp1 accumulates the enzyme-DNA intermediate and causes camptothecin hypersensitivity

Tyrosyl-DNA phosphodiesterase (Tdp1) catalyzes the hydrolysis of the tyrosyl-3′ phosphate linkage found in topoisomerase I-DNA covalent complexes. The inherited disorder, spinocerebellar ataxia with axonal neuropathy (SCAN1), is caused by a H493R mutation in Tdp1. Contrary to earlier proposals that this disease results from a loss-of-function mutation, we show here that this mutation reduces enzyme activity ∼25-fold and importantly causes the accumulation of the Tdp1-DNA co valent reaction intermediate. Thus, the attempted repair of topoisomerase I-DNA complexes by Tdp1 unexpectedly generates a new protein-DNA complex with an apparent half-life of ∼13 min that, in addition to the unrepaired topoisomerase I-DNA complex, may interfere with transcription and replication in human cells and contribute to the SCAN1 phenotype. The analysis of Tdp1 mutant cell lines derived from SCAN1 patients reveals that they are hypersensitive to the topoisomerase I-specific anticancer drug camptothecin (CPT), implicating Tdp1 in the repair of CPT-induced topoisomerase I damage in human cells. This finding suggests that inhibitors of Tdp1 could act synergistically with CPT in anticancer therapy.