# TOP1

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DNA topoisomerase enzyme

TOP1 Available structures PDB Ortholog search: PDBe RCSB List of PDB id codes 1A31, 1A35, 1A36, 1EJ9, 1K4S, 1K4T, 1LPQ, 1NH3, 1R49, 1RR8, 1RRJ, 1SC7, 1SEU, 1T8I, 1TL8 Identifiers Aliases TOP1, TOPI, topoisomerase (DNA) I, DNA topoisomerase I External IDs OMIM: 126420; MGI: 98788; HomoloGene: 2467; GeneCards: TOP1; OMA:TOP1 - orthologs Gene location (Human) Chr. Chromosome 20 (human)[1] Band 20q12 Start 41,028,822 bp[1] End 41,124,487 bp[1] Gene location (Mouse) Chr. Chromosome 2 (mouse)[2] Band 2 H2|2 80.96 cM Start 160,487,808 bp[2] End 160,564,684 bp[2] RNA expression pattern Bgee Human Mouse (ortholog) Top expressed in oocyte ventricular zone ganglionic eminence secondary oocyte pericardium pylorus vena cava pancreatic ductal cell superior surface of tongue monocyte Top expressed in Ileal epithelium zygote secondary oocyte maxillary prominence mandibular prominence tail of embryo human fetus genital tubercle ventricular zone ganglionic eminence More reference expression data BioGPS n/a Gene ontology Molecular function DNA topoisomerase type I (single strand cut, ATP-independent) activity chromatin binding isomerase activity DNA topoisomerase activity core promoter sequence-specific DNA binding protein binding RNA binding DNA binding double-stranded DNA binding single-stranded DNA binding protein serine/threonine kinase activity ATP binding protein domain specific binding supercoiled DNA binding RNA polymerase II cis-regulatory region sequence-specific DNA binding Cellular component cytoplasm perikaryon P-body replication fork protection complex chromosome nuclear chromosome nucleus fibrillar center nucleoplasm nucleolus DNA topoisomerase type II (double strand cut, ATP-hydrolyzing) complex protein-DNA complex Biological process chromatin remodeling embryonic cleavage programmed cell death phosphorylation rhythmic process chromosome segregation protein sumoylation circadian regulation of gene expression DNA replication circadian rhythm viral process DNA topological change peptidyl-serine phosphorylation Sources:Amigo / QuickGO Orthologs Species Human Mouse Entrez 7150 21969 Ensembl ENSG00000198900 ENSMUSG00000070544 UniProt P11387 Q04750 RefSeq (mRNA) NM_003286 NM_009408 RefSeq (protein) NP_003277 NP_033434 Location (UCSC) Chr 20: 41.03 – 41.12 Mb Chr 2: 160.49 – 160.56 Mb PubMed search [3] [4] Wikidata View/Edit Human View/Edit Mouse

**DNA topoisomerase 1** is an [enzyme](/source/Enzyme) that in humans is encoded by the *TOP1* [gene](/source/Gene). It is a [DNA topoisomerase](/source/DNA_topoisomerase), an enzyme that catalyzes the transient breaking and rejoining of a single strand of [DNA](/source/DNA).

## Function

This gene encodes a DNA topoisomerase, an enzyme that controls and alters the topologic states of DNA during transcription. This enzyme catalyzes the transient breaking and rejoining of a single strand of DNA which lets the broken strand rotate around the intact strand,[5] thus altering the topology of DNA. This gene is localized to chromosome 20 and has pseudogenes which reside on chromosomes 1 and 22.[6]

## Mechanism

As reviewed by Champoux,[7] the type IB topoisomerases, including TOP1, form a covalent intermediate in which the active site tyrosine becomes attached to the 3' phosphate end of the cleaved strand rather than the 5' phosphate end.

The eukaryotic [topoisomerases I](/source/Type_I_topoisomerase) were found to nick the DNA with a preference for a sequence of nucleotides that extends from positions -4 to -1 from the nick. The preferred nucleotides in the strand to be cut are 5'-(A/T)(G/C)(A/T)T-3' with the enzyme covalently attached to the -1 T residue, though sometimes a C residue is found at the -1 position.

The TOP1 protein of humans has been subdivided into four regions. The [N-terminal](/source/N-terminus) 214 amino acids are dispensable for relaxation of [supercoiling](/source/DNA_supercoil) activity *[in vitro](/source/In_vitro)* and there are four [nuclear localization signals](/source/Nuclear_localization_sequence) and sites for [interaction with other cellular proteins](/source/Protein%E2%80%93protein_interaction) within the N-terminal domain. The N-terminal domain is followed by a highly conserved, 421 amino acid core domain containing all of the catalytic residues except the active site [tyrosine](/source/Tyrosine). This is followed by a poorly conserved linker domain of 77 amino acids. Finally there is a 53 amino acid C-terminal domain. The [active site](/source/Active_site) [Tyr](/source/Tyrosine)723 is found within the C-terminal domain.

As further summarized by Pommier and by Seol et al.,[5][8] TOP1 breaks the DNA by a [transesterification](/source/Transesterification) reaction using the active site tyrosine as the [nucleophile](/source/Nucleophile) that attacks the DNA phosphodiester backbone. After the TOP1 covalently attaches to the 3' end of the broken strand, [supercoiling](/source/DNA_supercoil) of the DNA is relaxed by controlled rotation of DNA about the intact strand. Then the 5' hydroxyl end of the broken DNA strand can reverse the phosphotyrosyl bond, enabling the release of TOP1 and [religation](/source/Ligation_(molecular_biology)) of the DNA. The nicking and closing reactions are fast, and about 100 cycles can occur per second.

## Inhibition

Main article: [Topoisomerase inhibitor](/source/Topoisomerase_inhibitor)

The briefly attached, covalently bonded TOP1-DNA structure at the 3' end of a cleaved DNA single strand is called a TOP1-DNA cleavage complex, or TOP1cc. The TOP1cc is a specific target of TOP1 [inhibitors](/source/Topoisomerase_inhibitor). One of the first inhibitors shown to target TOP1 is [irinotecan](/source/Irinotecan). Irinotecan is an analogue of the cytotoxic natural alkaloid [camptothecin](/source/Camptothecin), obtained from the Chinese tree *Camptotheca acuminata*.[9] Irinotecan is especially effective through its [metabolic](/source/Active_metabolite) product [SN-38](/source/SN-38). Irinotecan and [SN-38](/source/SN-38) act by trapping a subset of TOP1-DNA cleavage complexes, those with a guanine +1 in the DNA sequence.[5] One irinotecan or SN-38 molecule stacks against the base pairs flanking the topoisomerase-induced cleavage site and poisons (inactivates) the TOP1 enzyme.[5] The article [Camptothecin](/source/Camptothecin) lists other analogues of camptothecin and the article [Topoisomerase inhibitor](/source/Topoisomerase_inhibitor) lists other compounds which inhibit TOP1.

## Cancer

Since 1985, TOP1 has been known as a target for the treatment of human cancers.[9] Camptothecin analogues irinotecan and [topotecan](/source/Topotecan), which inhibit TOP1, are among the most effective FDA-approved anticancer chemotherapeutic agents used in clinical practice. Higher expression of TOP1 in KRAS mutant non-small cell lung cancer and correlation to survival suggests that TOP1 inhibitors might have increased benefit when administered to treat patients with a KRAS mutant tumor.[10]

### Synthetic lethality

[Synthetic lethality](/source/Synthetic_lethality) arises when a combination of deficiencies in the expression of two or more genes leads to cell death, whereas a deficiency in only one of these genes does not. The deficiencies can arise through [mutation](/source/Mutation), [epigenetic alteration](/source/Cancer_epigenetics) or by inhibition of a gene's expression.

Irinotecan inactivation of TOP1 appears to be synthetically lethal in combination with deficiencies in expression of some specific DNA repair genes.

Irinotecan inactivation of TOP1 was synthetically lethal with deficient expression of the DNA repair [*WRN*](/source/Werner_syndrome_ATP-dependent_helicase) gene in patients with colon cancer.[11] In a 2006 study, 45 patients had colonic tumors with [hypermethylated](/source/DNA_methylation) *WRN* gene [promoters](/source/Promoter_(genetics)) (silenced *WRN* expression), and 43 patients had tumors with unmethylated *WRN* gene promoters, so that WRN protein expression was high.[11] Irinotecan was more strongly beneficial for patients with hypermethylated *WRN* promoters (39.4 months survival) than for those with unmethylated *WRN* promoters (20.7 months survival). The *WRN* gene promoter is hypermethylated in about 38% of [colorectal cancers](/source/Colorectal_cancer).[11]

Irinotecan inactivation of TOP1 may be synthetically lethal with deficient expression of DNA repair gene *[MRE11](/source/MRE11A)*. A recent study was carried out with 1,264 patients with stage III colon cancer.[12] The patients were treated with a postoperative weekly [adjuvant](/source/Adjuvant_therapy) [bolus](/source/Bolus_(medicine)) of 5-fluorouracil/leucovorin (FU/LV) or else with irinotecan+FU/LV and were followed up for 8 years. Eleven percent of the tumors were deficient for DNA repair enzyme [MRE11](/source/MRE11A) due to a deletion of a string of thymidines in the DNA sequence of the *MRE11* gene. The addition of irinotecan to FU/LV in the treatment protocol resulted in *MRE11*-deficient patients having better long-term disease free survival than patients with wild-type *MRE11* (though the effect was small), indicating some degree of synthetic lethality between irinotecan-induced TOP1 inactivation and *MRE11* deficiency.[12]

There are a number of [pre-clinical](/source/Phases_of_clinical_research#Phases) studies indicating synthetic lethality of irinotecan with other genetic or [epigenetic](/source/Cancer_epigenetics) DNA repair deficiencies common in cancers. For instance, the DNA repair gene [*ATM*](/source/ATM_serine%2Fthreonine_kinase) is frequently [hypermethylated](/source/DNA_methylation_in_cancer) (silenced) in many cancers (see [hypermethylation of ATM in cancers](/source/ATM_serine%2Fthreonine_kinase#Frequent_epigenetic_deficiencies_of_ATM_in_cancers)). A 2016 study showed that low expression of the ATM protein in gastric cancer cells *in vitro* and in a mouse model caused increased sensitivity to inactivation by irinotecan compared to cells with high expression of ATM.[13] This indicates synthetic lethality of ATM deficiency with irinotecan-mediated TOP1 deficiency.[13]

Another pre-clinical effort was a screening study to find a compound that would be synthetically lethal with a deficiency of N-myc downstream regulated gene 1 (*[NDRG1](/source/NDRG1)*) expression. *NDRG1* is a metastasis-suppressor gene in prostate cancer,[14] and appears to have a role in DNA repair.[15] Screening of 3360 compounds revealed that irinotecan-mediated TOP1 deficiency (and one other compound, cetrimonium bromide) exhibit synthetic lethality with NDRG1 deficiency in prostate cancer cells.[14]

## DNA repair

Exposure of human HeLA cells to [UVB](/source/Ultraviolet) irradiation specifically stimulates the formation of covalent complexes between topoisomerase I and [DNA](/source/DNA).[16] Topoisomerase I appears to have a direct role in [nucleotide excision repair](/source/Nucleotide_excision_repair), a process that removes UVB-induced, and other, DNA damages.[16]

## Interactions

TOP1 has been shown to [interact](/source/Protein-protein_interaction) with:

- [ASF/SF2](/source/ASF%2FSF2),[17][18]

- [BTBD1](/source/BTBD1),[19]

- [BTBD2](/source/BTBD2),[19]

- [Nucleolin](/source/Nucleolin),[20][21]

- [P53](/source/P53),[22][23] and

- [UBE2I](/source/UBE2I).[24]

## See also

- [Type I topoisomerase](/source/Type_I_topoisomerase)

- [Topoisomerase](/source/Topoisomerase)

## References

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1. **[^](#cite_ref-pmid10759568_24-0)** Mao Y, Sun M, Desai SD, [Liu LF](/source/Leroy_Liu) (Apr 2000). ["SUMO-1 conjugation to topoisomerase I: A possible repair response to topoisomerase-mediated DNA damage"](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC18143). *Proceedings of the National Academy of Sciences of the United States of America*. **97** (8): 4046–51. [Bibcode](/source/Bibcode_(identifier)):[2000PNAS...97.4046M](https://ui.adsabs.harvard.edu/abs/2000PNAS...97.4046M). [doi](/source/Doi_(identifier)):[10.1073/pnas.080536597](https://doi.org/10.1073%2Fpnas.080536597). [PMC](/source/PMC_(identifier)) [18143](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC18143). [PMID](/source/PMID_(identifier)) [10759568](https://pubmed.ncbi.nlm.nih.gov/10759568).

## External links

- Overview of all the structural information available in the [PDB](/source/Protein_Data_Bank) for [UniProt](/source/UniProt): *[P11387](https://www.ebi.ac.uk/pdbe/pdbe-kb/proteins/P11387)* (DNA topoisomerase 1) at the [PDBe-KB](/source/PDBe-KB).

v t e PDB gallery 1a31: HUMAN RECONSTITUTED DNA TOPOISOMERASE I IN COVALENT COMPLEX WITH A 22 BASE PAIR DNA DUPLEX 1a35: HUMAN TOPOISOMERASE I/DNA COMPLEX 1a36: TOPOISOMERASE I/DNA COMPLEX 1ej9: CRYSTAL STRUCTURE OF HUMAN TOPOISOMERASE I DNA COMPLEX 1k4s: HUMAN DNA TOPOISOMERASE I IN COVALENT COMPLEX WITH A 22 BASE PAIR DNA DUPLEX 1k4t: HUMAN DNA TOPOISOMERASE I (70 KDA) IN COMPLEX WITH THE POISON TOPOTECAN AND COVALENT COMPLEX WITH A 22 BASE PAIR DNA DUPLEX 1lpq: Human DNA Topoisomerase I (70 Kda) In Non-Covalent Complex With A 22 Base Pair DNA Duplex Containing an 8-oxoG Lesion 1nh3: Human Topoisomerase I Ara-C Complex 1r49: Human topoisomerase I (Topo70) double mutant K532R/Y723F 1rr8: Structural Mechanisms of Camptothecin Resistance by Mutations in Human Topoisomerase I 1rrj: Structural Mechanisms of Camptothecin Resistance by Mutations in Human Topoisomerase I 1sc7: Human DNA Topoisomerase I (70 Kda) In Complex With The Indenoisoquinoline MJ-II-38 and Covalent Complex With A 22 Base Pair DNA Duplex 1seu: Human DNA Topoisomerase I (70 Kda) In Complex With The Indolocarbazole SA315F and Covalent Complex With A 22 Base Pair DNA Duplex 1t8i: Human DNA Topoisomerase I (70 Kda) In Complex With The Poison Camptothecin and Covalent Complex With A 22 Base Pair DNA Duplex 1tl8: Human DNA topoisomerase I (70 kDa) in complex with the indenoisoquinoline AI-III-52 and covalent complex with a 22 base pair DNA duplex

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Adapted from the Wikipedia article [TOP1](https://en.wikipedia.org/wiki/TOP1) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/TOP1?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.