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

Area of Science:

• DNA repair mechanisms
• Neurodegenerative disease genetics

Background:

• Tyrosyl-DNA phosphodiesterase 1 (Tdp1) resolves topoisomerase I (Topo I) covalent complexes stalled at DNA strand breaks.
• Stalled Topo I-DNA complexes are induced by DNA damaging agents like camptothecin (CPT) and endogenous DNA damage.
• The H493R mutation in Tdp1 is linked to spinocerebellar ataxia with axonal neuropathy (SCAN1).

Purpose of the Study:

• To investigate the in vivo function of Tdp1 and the pathogenic mechanism of the H493R mutation in SCAN1.
• To determine if Tdp1 deficiency causes a SCAN1-like phenotype in mice.
• To elucidate the role of Tdp1 activity in response to CPT and other DNA damaging agents.

Main Methods:

• Generation and characterization of Tdp1 knockout (Tdp1-/-) mice.
• Phenotypic analysis of Tdp1-/- mice (physical, histological, behavioral, electrophysiological).
• Assessment of CPT, bleomycin, and etoposide sensitivity in Tdp1-/- mice.
• Analysis of H493R Tdp1 mutant protein activity and DNA trapping in SCAN1 patient cells.

Main Results:

• Tdp1-/- mice exhibit normal physical, histological, behavioral, and electrophysiological characteristics compared to wild-type mice.
• Tdp1-/- mice show hypersensitivity to CPT and bleomycin, but not etoposide.
• The H493R Tdp1 mutant protein retains residual catalytic activity and becomes covalently trapped on DNA after CPT treatment in SCAN1 cells.

Conclusions:

• Tdp1 deficiency alone does not recapitulate the SCAN1 phenotype, suggesting a more complex mechanism.
• Tdp1 plays a crucial role in repairing Topo I-DNA covalent lesions in vivo.
• The SCAN1 disease likely arises from a recessive neomorphic mutation (H493R) in Tdp1, a novel disease mechanism.
• Neomorphic mutations, typically dominant, may act recessively in the context of Tdp1 and SCAN1.