# TOP3A

Source: https://onco.cc/targets/top3a/  
OnCo record `top3a` (Target). Data CC BY-NC 4.0, attribute "Data from OnCo (onco.cc)"; commercial use needs a licence.

## TL;DR

TOP3A (DNA topoisomerase 3-alpha) is an enzyme. In the public catalogues the evidence so far is association rather than a proven role.

## Summary

Releases the supercoiling and torsional tension of DNA introduced during the DNA replication and transcription by transiently cleaving and rejoining one strand of the DNA duplex. Introduces a single-strand break via transesterification at a target site in duplex DNA. The scissile phosphodiester is attacked by the catalytic tyrosine of the enzyme, resulting in the formation of a DNA-(5'-phosphotyrosyl)-enzyme intermediate and the expulsion of a 3'-OH DNA strand.

Open Targets scores its association with cancer at 0.55 (direct and indirect evidence; datatypes literature 0.83, affected pathway 0.87, genetic association 0.00).

## Fields

- Kind: Target
- Last checked: 2026-09-23
- Also known as: DNA topoisomerase III alpha; DNA topoisomerase 3-alpha; ZGRF7; TOP3
- Tags: cancer-genes-wave
- Symbol: TOP3A
- Class: enzyme
- Biology: Releases the supercoiling and torsional tension of DNA introduced during the DNA replication and transcription by transiently cleaving and rejoining one strand of the DNA duplex. Introduces a single-strand break via transesterification at a target site in duplex DNA. The scissile phosphodiester is attacked by the catalytic tyrosine of the enzyme, resulting in the formation of a DNA-(5'-phosphotyrosyl)-enzyme intermediate and the expulsion of a 3'-OH DNA strand. The free DNA strand then undergoes passage around the unbroken strand thus removing DNA supercoils. Finally, in the religation step, the DNA 3'-OH attacks the covalent intermediate to expel the active-site tyrosine and restore the DNA phosphodiester backbone. As an essential component of the RMI complex it is involved in chromosome separation and the processing of homologous recombination intermediates to limit DNA crossover formation in cells. Location: Mitochondrion matrix (UniProt). Locus 17p11.2 (HGNC).

## Notes

- Written by scripts/fetch-cancer-genes.ts from CIViC, Open Targets, IntOGen, HGNC and UniProt; the function text is UniProt's, condensed and in UK spelling. Roles: none stated by the sources. Evidence tier "association-only" is the strongest of those signals.
- Prevalence not recorded: none of the sources gives a positivity rate.

## Sources

- HGNC HGNC:11992: https://www.genenames.org/data/gene-symbol-report/#!/hgnc_id/HGNC:11992
- UniProt Q13472: https://www.uniprot.org/uniprotkb/Q13472/entry
- NCBI Gene 7156: https://www.ncbi.nlm.nih.gov/gene/7156
- Ensembl ENSG00000177302: https://www.ensembl.org/Homo_sapiens/Gene/Summary?g=ENSG00000177302

## Connected records

- collections: [Open Targets Platform](https://onco.cc/collections/open-targets/)

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JSON: https://onco.cc/api/v1/entities/top3a.json