ATM Deficiency Results in Accumulation of DNA-Topoisomerase I Covalent Intermediates in Neural Cells

Abstract

Accumulation of peptide-linked DNA breaks contributes to neurodegeration in humans. This is typified by defects in tyrosyl DNA phosphodiesterase 1 (TDP1) and human hereditary ataxia. TDP1 primarily operates at single-strand breaks (SSBs) created by oxidative stress or by collision of transcription machinery with topoisomerase I intermediates (Top1-CCs). Cellular and cell-free studies have shown that Top1 at stalled Top1-CCs is first degraded to a small peptide resulting in Top1-SSBs, which are the primary substrates for TDP1. Here we established an assay to directly compare Top1-SSBs and Top1-CCs. We subsequently employed this assay to reveal an increased steady state level of Top1-CCs in neural cells lacking Atm; the protein mutated in ataxia telangiectasia. Our data suggest that the accumulation of endogenous Top1-CCs in Atm-/- neural cells is primarily due to elevated levels of reactive oxygen species. Biochemical purification of Top1-CCs from neural cell extract and the use of Top1 poisons further confirmed a role for Atm during the formation/resolution of Top1-CCs. Finally, we report that global transcription is reduced in Atm-/- neural cells and fails to recover to normal levels following Top1-mediated DNA damage. Together, these data identify a distinct role for ATM during the formation/resolution of neural Top1-CCs and suggest that their accumulation contributes to the neuropathology of ataxia telangiectasia.

Catalytic and Noncatalytic Roles of the CtIP Endonuclease in Double-Strand Break End Resection

“…The importance of the ATM phosphorylation events for CtIP may explain a recent report showing that ATM activity is essential for repair involving topoisomerase adducts in human cells (Alagoz et al, 2013). …”

Genome integrity and disease prevention in the nervous system

“…ATM prevents the accumulation of endogenous Top1 cleavage complexes (Top1ccs) in mammalian brains and cultured cells by promoting the degradation of Top1 via ubiquitin/sumoylation, thereby minimizing the DNA damage generated by the Top1cc (Alagoz et al 2013;Carlessi et al 2014;Katyal et al 2014). Thus, this Top1 complex represents the first identified endogenous pathogenic lesion in the mammalian brain attenuated by ATM (Katyal et al 2014).…”

Topoisomerase-mediated chromosomal break repair: an emerging player in many games

“…Targeting TDP1 for cancer therapy is clinically attractive for many reasons. First, TDP1 is a broad-spectrum 3′ DNA end-processing enzyme: therefore, suppressing its activity is predicted to potentiate a number of cancer therapy protocols that induce PDBs either directly or indirectly, such as TOP1 poisons, alkylating agents, radiotherapy and chain terminators 22,29,53,61,63,[69][70][71] . Second, TDP1 deficiency is well tolerated in vertebrates and thus targeting TDP1 is likely to result in minimal toxicity.…”