TOP1
DNA topoisomerase 1 is an enzyme that in humans is encoded by the TOP1 gene.

# 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, thus altering the topology of DNA. This gene is localized to chromosome 20 and has pseudogenes which reside on chromosomes 1 and 22.

# Mechanism
As reviewed by Champoux, 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 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 214 amino acids are dispensable for relaxation of supercoiling activity in vitro and there are four nuclear localization signals and sites for interaction with other cellular proteins 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.  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 Tyr723 is found within the C-terminal domain.
As further summarized by Pommier and by Seol et al.,  TOP1 breaks the DNA by a transesterification reaction using the active site tyrosine as the nucleophile that attacks the DNA phosphodiester backbone.  After the TOP1 covalently attaches to the 3' end of the broken strand, supercoiling 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 of the DNA.  The nicking and closing reactions are fast, and about 100 cycles can occur per second.

# Inhibition
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.  One of the first inhibitors shown to target TOP1 is irinotecan.  Irinotecan is an analogue of the cytotoxic natural alkaloid camptothecin, obtained from the Chinese tree Camptotheca acuminata.  Irinotecan is especially effective through its metabolic product SN-38.  Irinotecan and SN-38 act by trapping a subset of TOP1-DNA cleavage complexes, those with a guanine +1 in the DNA sequence.  One irinotecan or SN-38 molecule stacks against the base pairs flanking the topoisomerase-induced cleavage site and poisons (inactivates) the TOP1 enzyme.  The article Camptothecin lists other analogues of camptothecin and the article Topoisomerase inhibitor lists other compounds which inhibit TOP1.

# Cancer
Since 1985, TOP1 has been known as a target for the treatment of human cancers. Camptothecin analogues irinotecan and 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.

## Synthetic lethality
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, epigenetic alteration 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 gene in patients with colon cancer.  In a 2006 study, 45 patients had colonic tumors with hypermethylated WRN gene promoters (silenced WRN expression), and 43 patients had tumors with unmethylated WRN gene promoters, so that WRN protein expression was high.  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.
Irinotecan inactivation of TOP1 may be synthetically lethal with deficient expression of DNA repair gene MRE11.  A recent study was carried out with 1,264 patients with stage III colon cancer.  The patients were treated with a postoperative weekly adjuvant bolus 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 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.
There are a number of pre-clinical studies indicating synthetic lethality of irinotecan with other genetic or epigenetic DNA repair deficiencies common in cancers.  For instance, the DNA repair gene ATM is frequently hypermethylated (silenced) in many cancers (see hypermethylation 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.  This indicates synthetic lethality of ATM deficiency with irinotecan-mediated TOP1 deficiency.
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) expression.  NDRG1 is a metastasis-suppressor gene in prostate cancer, and appears to have a role in DNA repair.  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.

# Interactions
TOP1 has been shown to interact with:
- ASF/SF2,
- BTBD1,
- BTBD2,
- Nucleolin,
- P53,  and
- UBE2I.