Topoisomerases form transient covalent DNA cleavage complexes to perform their reactions. Topoisomerase I cleavage complexes (TOP1ccs) are trapped by camptothecin and TOP2ccs by etoposide. Proteolysis of the trapped topoisomerase DNA-protein cross-links (TOP-DPCs) is a key step for some pathways to repair these lesions. We describe a pathway that features a prominent role of the small ubiquitin-like modifier (SUMO) modification for both TOP1- and TOP2-DPC repair. Both undergo rapid and sequential SUMO-2/3 and SUMO-1 modifications in human cells. The SUMO ligase PIAS4 is required for these modifications. RNF4, a SUMO-targeted ubiquitin ligase (STUbL), then ubiquitylates the TOP-DPCs for their subsequent degradation by the proteasome. This pathway is conserved in yeast with Siz1 and Slx5-Slx8, the orthologs of human PIAS4 and RNF4.

DNA damage is considered a prime factor in multiple spinocerebellar neurodegenerative diseases; however, the DNA lesions underpinning disease etiology are unknown. Here we identify the endogenous accumulation of pathogenic topoisomerase-1-DNA cleavage complexes (Top1cc) in murine models of ataxia telangiectasia and spinocerebellar ataxia with axonal neuropathy 1. We also show that the defective DNA damage response factors in these two diseases cooperatively modulate Top1cc turnover in a non-epistatic and ATM kinase-independent manner. Furthermore, coincident neural inactivation of ATM and DNA single strand break repair factors including tyrosyl-DNA phosphodiesterase-1 or XRCC1 result in increased Top1cc formation and excessive DNA damage and neurodevelopmental defects. Importantly, direct topoisomerase-1 poisoning to elevate Top1cc levels phenocopies the neuropathology of the mouse models above. Our study identifies a critical endogenous pathogenic lesion associated with neurodegenerative syndromes arising from DNA repair deficiency, indicating the essential role that genome integrity plays in preventing disease in the nervous system.

Agents targeting topoisomerases are active against a wide range of human tumors. Stabilization of covalent complexes, converting topoisomerases into DNA-damaging agents, is an essential aspect of cell killing by these drugs. A unique aspect of the repair of topoisomerase-mediated DNA damage is the requirement for pathways that can remove protein covalently bound to DNA. Tyrosyl-DNA phosphodiesterase (Tdp1) is an enzyme that removes phosphotyrosyl moieties bound to the 3 end of DNA. Cells lacking Tdp1 are hypersensitive to camptothecin, consistent with a role for Tdp1 in processing 3 phosphotyrosyl protein-DNA covalent complexes. Because Top2p forms a 5 phosphotyrosyl linkage with DNA, previous work predicted that Tdp1p would not be active against lesions involving Top2p. We found that deletion of the TDP1 gene in yeast confers hypersensitivity to Top2 targeting agents. Combining tdp1 mutations with deletions of genes involved in nonhomologous end joining, excision repair, or postreplication repair enhanced sensitivity to Top2 targeting drugs over the level seen with single mutants, suggesting that Tdp1 may function in collaboration with multiple pathways involved in strand break repair. tdp1 mutations can sensitize yeast cells to drugs targeting Top2 even when TOP1 is deleted. Finally, bacterially expressed yeast Tdp1p is able to remove a peptide derived from yTop2 that is covalently bound to DNA by a 5 phosphotyrosyl linkage. Our results show that Tdp1 plays more general roles in DNA repair than repair of Top1 mediated DNA damage, and may participate in repairing many types of base damage to DNA.D NA topoisomerases (topos) carry out alterations in DNA structure by transiently introducing breaks in DNA. Topos use tyrosine residues to form a transient covalent intermediate between the enzyme and DNA, resulting in single or double strand DNA breaks (1). This mechanism of DNA breakage can create difficulties for a cell under conditions that interfere with the normal religation reaction of the enzyme, such as the presence of specific inhibitors. Then, the enzyme becomes trapped on DNA as a stable covalent adduct (2). Topos can become trapped by small molecule inhibitors such as camptothecin or etoposide, which are clinically used anticancer agents targeting topo I and topo II, respectively. The complete repair of this type of lesion requires DNA strand break repair pathways, and also requires activities that can remove covalent protein DNA adducts (3).An enzyme described by Nash and colleagues (4), tyrosyl-DNA phosphodiesterase (Tdp1), can remove peptides covalently bound to DNA by a 3Ј phosphotyrosine linkage. This enzyme participates in the repair of Top1 covalently trapped on DNA in the presence of camptothecin in both yeast (5, 6) and mammalian cells (7). Tdp1 is a phospholipase D superfamily enzyme (8), and the three-dimensional structure of the protein (9) as well as transition state intermediates have been reported (10). Tdp1 does not define the only pathway for removing Top1 that is trapped by camptothecin, ...

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Background: TDP2 is critical for repairing Top2 cleavage complexes (Top2cc) and as the VPg unlinkase for picornavirus replication. Results: Top2 proteolysis or denaturation is required for TDP2 activity. TDP2 also hydrolyzes Top2cc at ribonucleotides. Conclusion: TDP2 efficiently disjoints relatively large Top2 polypeptide-DNA and -RNA complexes. Significance: Top2 processing is critical prior to its unlinking from DNA or RNA by TDP2.