Spinocerebellar ataxia with axonal neuropathy: consequence of a Tdp1 recessive neomorphic mutation?

Abstract

Tyrosyl-DNA phosphodiesterase 1 (Tdp1) cleaves the phosphodiester bond between a covalently stalled topoisomerase I (Topo I) and the 3 0 end of DNA. Stalling of Topo I at DNA strand breaks is induced by endogenous DNA damage and the Topo I-specific anticancer drug camptothecin (CPT). The H493R mutation of Tdp1 causes the neurodegenerative disorder spinocerebellar ataxia with axonal neuropathy (SCAN1). Contrary to the hypothesis that SCAN1 arises from catalytically inactive Tdp1, Tdp1 À/À mice are indistinguishable from wild-type mice, physically, histologically, behaviorally, and electrophysiologically. However, compared to wild-type mice, Tdp1 À/À mice are hypersensitive to CPT and bleomycin but not to etoposide. Consistent with earlier in vitro studies, we show that the H493R Tdp1 mutant protein retains residual activity and becomes covalently trapped on the DNA after CPT treatment of SCAN1 cells. This result provides a direct demonstration that Tdp1 repairs Topo I covalent lesions in vivo and suggests that SCAN1 arises from the recessive neomorphic mutation H493R. This is a novel mechanism for disease since neomorphic mutations are generally dominant.

Role of tyrosyl-DNA phosphodiesterase (TDP1) in mitochondria

“…The role of TDP1 in protecting against oxidative DNA damage in mitochondria is evident from our present experiments showing defective repair of oxidative DNA damage in mitochondria from TDP1 −/− cells. This finding is consistent with previous reports indicating an involvement of TDP1 in protecting cells against oxidative DNA damage (41,42,63,64), and with the impaired ability of TDP1-deficient cells to remove 3′-phosphoglycolate (48,49,55). Homozygous mutation of TDP1 causes SCAN1, a neurodegenerative syndrome associated with marked cerebellar atrophy and peripheral neuropathy (44).…”

“…the recent observation that the TDP1 protein is prominently expressed in the cytoplasm of some neurons (41). The existence of cytoplasmic TDP1 raised the possibility that TDP1 localizes to the mitochondria and participates in mtDNA repair.…”

“…for the mitochondrial TDP1 activity, similar experiments were performed with mitochondrial extracts (ME) from TDP1 knockout (TDP1−/−) fibroblasts (41). Fig.…”

“…TDP1 was originally discovered in yeast (34) and has been implicated in the repair of stalled Top1-DNA covalent complexes (38,39). The ability of TDP1 to resolve 3′-phosphotyrosyl linkages is consistent with its role in protecting cells against Top1-DNA lesions (40)(41)(42)(43). Homozygous mutation of TDP1 causes spinocerebellar ataxia with axonal neuropathy (SCAN1), an autosomal recessive neurodegenerative syndrome (44).…”

“…TDP1 can also process other 3′-DNA end blocking groups: 3′-abasic sites and 3′-phosphoglycolates (45)(46)(47)(48)(49). Accordingly, TDP1-deficient cells are deficient in the removal of 3′-phosphoglycolate and are hypersensitive to bleomycin in addition to their hypersensitivity to camptothecin (39,(41)(42)(43)49). TDP1 also possesses a limited DNA and RNA 3′-nucleosidase activity in which a single nucleoside is removed from the 3′-hydroxyl end of the substrate (45).…”