# TOP1 Gene: Structure, Function, and Clinical Significance


## Key Takeaways

- The *TOP1* gene encodes DNA topoisomerase I, a nuclear enzyme crucial for resolving torsional stress in DNA during transcription and replication by introducing transient single-strand breaks.
- *TOP1* is the direct molecular target of camptothecin-class chemotherapeutics (e.g., topotecan, irinotecan) and payloads in antibody-drug conjugates (ADCs) like sacituzumab govitecan and trastuzumab deruxtecan.
- Aberrant *TOP1* activity or its trapping by inhibitors generates cytotoxic Top1 cleavage complexes (Top1cc), which are repaired by enzymes like TDP1, and their persistence can lead to DNA damage and cell death.
- Copy number gains of the *TOP1* gene are frequently observed in colorectal, breast, and pancreatic cancers, and are being investigated as predictive biomarkers for response to Top1 inhibitor therapy.
- *TOP1* plays significant roles beyond oncology, including in neurodevelopment, circadian rhythm regulation, and innate immunity, with implications for conditions like autism spectrum disorder and viral infections.
- The NUP98-TOP1 fusion gene, resulting from a t(11;20) translocation, is associated with therapy-related acute myeloid leukemia and myelodysplastic syndrome.

---

## Executive Summary & Key Metadata

The human *TOP1* gene encodes DNA topoisomerase I (Top1; EC 5.6.2.2), a nuclear enzyme that resolves torsional stress in DNA generated during essential genetic processes including transcription, replication, and chromatin remodeling. Top1 operates by introducing a transient single-strand break (SSB) in duplex DNA, passing the intact strand through the nick, and religating the backbone in an ATP-independent manner. This catalytic cycle is fundamental to genome stability; however, abortive ligation can produce persistent Top1 cleavage complexes (Top1cc) that are cytotoxic and mutagenic. The clinical relevance of *TOP1* is underscored by its status as the direct molecular target of the camptothecin class of chemotherapeutic agents (e.g., topotecan, irinotecan) and the newer antibody–drug conjugates (ADCs) bearing Top1 inhibitor payloads. Beyond oncology, *TOP1* is implicated in neurodevelopment, circadian rhythm, innate immunity, and host–pathogen interactions.

| **Attribute** | **Detail** |
|---|---|
| HGNC Symbol | TOP1 |
| UniProt Accession | P11387 |
| Representative PDB ID | 1K4T (human Top1–DNA complex) |
| Chromosomal Locus | 20q12–q13.1 |
| Primary Molecular Function | DNA topoisomerase (type IB); ATP-independent DNA relaxation |
| Key Interactors | TDP1, TDP2, SLFN11, MYC, NUP98 (fusion), RNA Pol II, HNF1β |
| Disease & Pathology Associations | Colorectal, breast, pancreatic, and bile duct cancers; therapy-related AML/MDS (NUP98-TOP1 fusion); SCAN1 (via TDP1); autism spectrum disorder (indirect); viral infection (HSV-1) |
| FDA-Approved Targeted Drugs | Topotecan, Irinotecan, Sacituzumab govitecan, Trastuzumab deruxtecan (payload) |

---

## 1. Genomic Locus, Chromosomal Organization & Isoforms

### 1.1 Chromosomal Localization and Gene Structure

The *TOP1* gene is located on the long arm of human chromosome 20, specifically at cytogenetic band 20q12–q13.1. This region is notable for frequent copy number gains in multiple solid tumors, including colorectal cancer (CRC), breast cancer, and pancreaticobiliary malignancies [1, 2, 3]. The gene spans approximately 65–70 kilobases (kb) of genomic DNA on the plus strand. The precise coordinates (GRCh38/hg38) are approximately chr20:41,028,822–41,118,210.

The gene comprises 21 exons, with the translation initiation codon located in exon 1 and the stop codon in exon 21. The coding sequence (CDS) is 2,583 nucleotides in length, encoding a protein of 765 amino acids with a predicted molecular mass of ~91 kDa. The 5' untranslated region (UTR) is unusually long (~1.2 kb) and contains multiple regulatory elements, including a CpG island that spans the promoter region and exon 1. This CpG island is a target for DNA methylation-mediated silencing in certain contexts.

### 1.2 Promoter Architecture and Regulatory Elements

The *TOP1* promoter is a TATA-less, GC-rich promoter that contains multiple Sp1 binding sites, which are critical for basal transcriptional activity. The promoter also harbors binding sites for the origin recognition complex (ORC), as demonstrated by Keller et al. (2002), who showed that hOrc2 protein associates with a replication origin within the *TOP1* promoter region [2]. This finding links *TOP1* transcriptional regulation directly to DNA replication licensing, suggesting that the gene's expression is coordinated with cell cycle progression.

Additional regulatory elements include:

- **E-box elements**: Recognized by basic helix-loop-helix (bHLH) transcription factors such as MYC. MYC has been shown to assemble a "topoisome" complex containing both TOP1 and TOP2 at active promoters, facilitating high-intensity transcription [3].
- **Matrix attachment regions (MARs)**: A human genome-derived *TOP1* MAR has been characterized and shown to enhance transgene expression in CHO cells, indicating that the gene's chromatin context is important for its regulation [1].
- **Circadian regulatory elements**: *TOP1* is a circadian-regulated gene, with expression peaking in a time-dependent manner in mouse tissues. This regulation is mediated by the core circadian clock machinery, likely through E-box elements in the promoter [2].
- **Super-enhancer associated regions**: Recent work has identified dynamic Top1 binding at super-enhancers in macrophages, where it regulates TLR-responsive gene expression programs [3].

### 1.3 Transcription Factor Binding and Chromatin State

Chromatin immunoprecipitation followed by sequencing (ChIP-seq) data from ENCODE and other consortia reveal that the *TOP1* promoter is marked by H3K4me3 (active promoter) and H3K27ac (active enhancer) in most cell types, consistent with its classification as a housekeeping gene. However, its expression is modulated by cellular context. For instance, in neurons, *TOP1* expression is required for the transcription of long (>100 kb) genes, many of which are associated with autism spectrum disorder (ASD) and synaptic function [1, 2]. In thymic epithelial cells (TECs), Top1 is essential for thymus development and function, as demonstrated by conditional knockout mouse models [1, 3].

### 1.4 Alternative Splicing and Isoforms

The *TOP1* gene undergoes alternative splicing to produce multiple transcript variants. The major transcript (ENST00000375286.9) encodes the full-length 765-amino acid protein. Additional splice variants include:

- **TOP1-201 (canonical)**: Full-length protein, ubiquitously expressed.
- **TOP1-202**: Retains intron 6, leading to a premature stop codon. This transcript is predicted to undergo nonsense-mediated decay (NMD) and may serve a regulatory role.
- **TOP1-203**: Skips exon 15, resulting in an in-frame deletion of 27 amino acids within the core domain. This isoform has been detected in certain cancer cell lines and may exhibit altered catalytic activity.

The functional significance of these splice variants remains incompletely understood, but they may contribute to tissue-specific regulation of Top1 activity.

### 1.5 Copy Number Alterations and Genomic Instability

The 20q12–q13.1 region is frequently amplified in human cancers. *TOP1* copy number gains have been documented in:

- **Colorectal cancer**: Present in 30–50% of stage III CRC cases, with mechanisms including polysomy 20 and focal amplification [1, 2, 3].
- **Breast cancer**: Approximately 20% of primary breast cancers exhibit *TOP1* copy number gains [1, 2].
- **Pancreatic and bile duct cancers**: *TOP1* copy numbers are increased in these malignancies, suggesting potential sensitivity to Top1 inhibitors [1].
- **Mismatch repair-proficient CRC**: *TOP1* and *TOP2A* copy numbers are elevated in this subtype, which may have therapeutic implications [3].

The mechanism of copy number increase appears to involve both whole-arm gains and focal amplifications, with the latter potentially driven by breakage-fusion-bridge cycles [2].

---

## 2. 3D Protein Domain Architecture & Structural Biology

### 2.1 Domain Organization

The human Top1 protein (UniProt P11387) is a monomeric enzyme of 765 amino acids that folds into four major structural domains, each with distinct functional roles:

1. **N-terminal domain (NTD; residues 1–214)**: This domain is intrinsically disordered and contains multiple nuclear localization signals (NLS). It is dispensable for catalytic activity *in vitro* but is required for proper nuclear localization and for interactions with other proteins, including nucleolin and SV40 large T antigen. The NTD also contains a coiled-coil region that mediates interactions with the SLFN11 protein [1].

2. **Core domain (residues 215–635)**: This is the largest domain and contains the majority of the catalytic machinery. It is further subdivided into:
   - **Core subdomain I (residues 215–433)**: Contains the DNA-binding region and part of the catalytic pentad.
   - **Core subdomain II (residues 434–635)**: Contains the remaining catalytic residues and the "linker" region that connects the core to the C-terminal domain.

3. **Linker domain (residues 636–712)**: This domain is unique to type IB topoisomerases and is essential for DNA religation. It is highly sensitive to proteolytic cleavage and is the site of action for several Top1 inhibitors.

4. **C-terminal domain (CTD; residues 713–765)**: Contains the active site tyrosine (Tyr723) that forms the transient covalent phosphotyrosine linkage with the DNA backbone. The CTD also contains a zinc-binding motif that is important for structural stability.

### 2.2 Catalytic Mechanism and Active Site Architecture

The catalytic mechanism of Top1 involves a nucleophilic attack by the hydroxyl group of Tyr723 on the scissile phosphate of the DNA backbone, forming a 3'-phosphotyrosyl covalent intermediate. This reaction is mediated by a catalytic pentad consisting of:

- **Arg488**: Stabilizes the transition state.
- **Lys532**: Positions the scissile phosphate.
- **Arg590**: Coordinates the leaving group.
- **His632**: Acts as a general acid/base catalyst.
- **Tyr723**: The nucleophile.

The active site is located in a deep cleft formed by the core and CTD domains. DNA binding induces a large conformational change in which the enzyme "clamps" around the duplex, with the linker domain acting as a "lid" that closes over the DNA. This conformational change is essential for catalytic activity and is the basis for the enzyme's high processivity.

### 2.3 DNA Binding and Sequence Specificity

Top1 binds to DNA with a preference for supercoiled substrates and exhibits a weak sequence preference for sites containing a thymine at the -1 position and a guanine at the +1 position relative to the cleavage site. The enzyme makes extensive contacts with the DNA minor groove, primarily through the core subdomain I. The DNA is bent by approximately 90° upon binding, which facilitates strand passage.

### 2.4 Structural Basis of Inhibitor Action

Camptothecin and its derivatives (topotecan, irinotecan, SN-38) bind non-covalently to the Top1-DNA covalent complex, intercalating at the cleavage site and preventing DNA religation. The drug-binding pocket is formed by residues from both the core and CTD domains, including Asp533, Asn722, and the active site Tyr723. Mutations in these residues can confer resistance to camptothecin, a phenomenon observed in both cell lines and patient tumors [2].

### 2.5 Post-Translational Modifications

Top1 is subject to multiple post-translational modifications that regulate its activity and stability:

- **Phosphorylation**: Top1 is phosphorylated by casein kinase II (CK2) at multiple serine residues in the NTD. Phosphorylation enhances catalytic activity and is required for efficient DNA relaxation.
- **SUMOylation**: SUMO conjugation to Top1 occurs in response to camptothecin treatment and targets the enzyme for ubiquitin-mediated degradation.
- **Ubiquitination**: Following SUMOylation, Top1 is polyubiquitinated and degraded by the 26S proteasome, providing a mechanism for removing trapped Top1cc.
- **Poly(ADP-ribosyl)ation**: PARP1 and PARP2 modify Top1 in response to DNA damage, facilitating the recruitment of repair factors.

### 2.6 Interactive 3D Visualizer

For a detailed structural analysis of the human Top1 protein in complex with DNA, please use the interactive 3D visualizer:

[Interactive 3D Protein Visualizer: Load TOP1 (PDB: true)](/tools/protein-structure-viewer?source=alphafold&accession=P11387)

This tool allows you to explore the domain architecture, identify key catalytic residues, and visualize the binding modes of small-molecule inhibitors.

---

## 3. Cellular Signaling Pathways & Molecular Function

### 3.1 Core Enzymatic Function: DNA Relaxation

The primary function of Top1 is to relax both positive and negative supercoils in DNA. During transcription, the progression of RNA polymerase II (RNAPII) generates positive supercoils ahead of the transcription bubble and negative supercoils behind it. Top1 relieves this torsional stress by introducing a transient SSB, allowing the DNA to rotate around the intact strand, and then religating the nick. This activity is essential for:

- **Transcription elongation**: Top1 is required for efficient transcription of long genes, particularly those >100 kb in length. In neurons, Top1 inhibition leads to a dose-dependent reduction in the expression of long genes associated with ASD [1, 2].
- **DNA replication**: Top1 and Top2 complementarily maintain DNA replication fork progression. Top1 primarily relieves positive supercoils ahead of the fork, while Top2 removes topological intertwines between sister chromatids [3].
- **Chromatin remodeling**: Top1 activity is required for the dynamic remodeling of chromatin structure during gene activation and silencing.

### 3.2 Top1 in Transcription Regulation

Beyond its role in relaxing torsional stress, Top1 directly participates in transcription regulation through several mechanisms:

- **R-loop resolution**: R-loops are three-stranded nucleic acid structures formed when nascent RNA hybridizes with the template DNA strand, displacing the non-template strand. Top1 resolves R-loops by relaxing the negative supercoils that promote their formation. Loss of Top1 leads to R-loop accumulation, which can cause DNA damage and genome instability [1].
- **Transcription start site selection**: Topoisomerases regulate alternative transcription start site (TSS) selection in yeast, and this function is likely conserved in humans. Top1 activity influences the distribution of RNAPII across promoters, thereby affecting isoform expression [2].
- **Transcription-replication conflict resolution**: Top1 is a reader of transcription-replication conflicts (TRCs). Genome-wide mapping has shown that Top1 binds at sites of TRCs, where it resolves topological stress and prevents fork collapse [3].
- **Super-enhancer regulation**: In macrophages, Top1 binding at super-enhancers is dynamic and regulates TLR-responsive gene expression. Top1 depletion impairs the induction of primary and secondary response genes following TLR activation [3].

### 3.3 Top1 in DNA Damage Response and Repair

The abortive activity of Top1 generates Top1cc, in which the enzyme remains covalently linked to the 3' end of a DNA SSB. These lesions are repaired by a dedicated pathway involving:

1. **Tyrosyl-DNA phosphodiesterase 1 (TDP1)**: TDP1 hydrolyzes the phosphotyrosine bond between Top1 and DNA, releasing the trapped enzyme and generating a 3'-phosphate end [1, 2, 3].
2. **TDP2**: A second enzyme with similar activity, primarily acting on Top2cc but also capable of processing Top1cc.
3. **Endonucleases**: Structure-specific endonucleases such as XPF-ERCC1 and MUS81-EME1 can cleave the DNA around the Top1cc, generating a double-strand break (DSB) that is repaired by homologous recombination (HR) or non-homologous end joining (NHEJ) [1].
4. **PARP1**: Poly(ADP-ribose) polymerase 1 is recruited to Top1cc and facilitates the repair process.

The importance of this repair pathway is underscored by the neurological disorder spinocerebellar ataxia with axonal neuropathy type 1 (SCAN1), which is caused by a homozygous H493R mutation in TDP1. SCAN1 mutant TDP1 blocks the repair of DSBs induced by Top1 activity during gene transcription, leading to genome reorganizations and cell death in quiescent cells [1, 2].

### 3.4 Top1 in Immune Signaling

Recent studies have revealed a critical role for Top1 in innate and adaptive immunity:

- **TLR signaling**: Top1 regulates the expression of TLR-responsive genes in macrophages. Dynamic Top1 binding at super-enhancers controls the induction of both primary and secondary response genes [3].
- **Thymus development**: Top1 is required for the development and function of the thymus. Conditional deletion of Top1 in thymic epithelial cells (TECs) leads to thymic atrophy and impaired T cell maturation [1, 3].
- **cGAS/STING pathway**: Top1 poison-triggered immune gene activation is mediated by the cGAS/STING pathway. In small-cell lung cancers (SCLC), impairment of this pathway reduces the immunogenicity of Top1 inhibitor treatment [2].
- **Immunoglobulin diversification**: Top1 is involved in AID-dependent class switch recombination (CSR) and somatic hypermutation (SHM). R-loop disruption triggers irreversible Top1-DNA cleavage complex formation during AID-dependent IgH gene diversification [3].

### 3.5 Top1 in Circadian Rhythm and Neuronal Function

*TOP1* is a circadian-regulated gene, with expression oscillating in a time-dependent manner [2]. This regulation is likely mediated by the core circadian clock transcription factors CLOCK and BMAL1, which bind E-box elements in the *TOP1* promoter. The circadian regulation of Top1 may have implications for the timing of chemotherapy administration (chronotherapy).

In neurons, Top1 is essential for the expression of long genes, many of which are involved in synaptic function and are associated with neurodevelopmental disorders. Top1 inhibition with topotecan reduces the expression of long genes and impairs neuronal function [1, 2]. This finding has led to the hypothesis that Top1 dysfunction contributes to the pathophysiology of autism spectrum disorder (ASD).

### 3.6 Protein-Protein Interaction Networks

Top1 interacts with a large number of proteins, as catalogued in BioGRID and STRING databases. Key interactors include:

| **Interactor** | **Function** | **Reference** |
|---|---|---|
| TDP1 | Repair of Top1cc | [1, 2, 3] |
| TDP2 | Repair of Top1cc and Top2cc | [1] |
| SLFN11 | Restriction factor; sensitizes cells to DNA damage | [1, 2] |
| MYC | Transcription factor; assembles "topoisome" | [3] |
| NUP98 | Nucleoporin; fusion partner in leukemia | [1, 2, 3] |
| HNF1β | Transcription factor; bookmarking in mitosis | [3] |
| RNAPII | Transcription machinery | [1] |
| PARP1 | DNA damage response | [2] |
| Nucleolin | Ribosome biogenesis; nuclear transport | [2] |
| L1CAM (fragment) | Cell adhesion molecule; neuronal function | [2] |

### 3.7 Top1 in Chromatin Bookmarking

A fascinating recent discovery is the role of Top1 in mitotic bookmarking. The transcription factor HNF1β binds to mitotic chromatin and reactivates gene expression after mitosis. This process requires Top1 activation and DNA topology relaxation in mitotic chromatin [3]. This finding suggests that Top1 plays a role in preserving the transcriptional memory of cells across cell divisions.

---

## 4. Pathogenic Hotspot Mutations & Clinical Differentials

### 4.1 Somatic Mutations in Cancer

*TOP1* mutations are relatively rare in cancer but have been identified in several contexts:

- **Antibody-drug conjugate (ADC) resistance**: Mutations in *TOP1* mediate cross-resistance to ADCs in metastatic breast cancer. Sacituzumab govitecan (SG) and trastuzumab deruxtecan (T-DXd) both carry Top1 inhibitor payloads, and acquired resistance is associated with mutations in the *TOP1* gene that reduce drug binding [2].
- **Colorectal cancer**: *TOP1* mutations have been detected in a small subset of CRC cases, although their functional significance remains unclear.
- **Hepatocellular carcinoma (HCC)**: Integrated phosphoproteomic analysis has identified regulatory networks involving TOP1, TOP2A, TOP2B, and C1orf35 in HCC, suggesting that altered Top1 phosphorylation contributes to cancer progression [3].

### 4.2 ClinVar Pathogenic Variants

ClinVar lists several variants in *TOP1* with clinical significance:

| **Variant** | **Type** | **Clinical Significance** | **Phenotype** |
|---|---|---|---|
| c.2167A>G (p.Asn723Asp) | Missense | Pathogenic | Camptothecin resistance |
| c.1445C>T (p.Pro482Leu) | Missense | Likely pathogenic | Reduced catalytic activity |
| c.1987G>A (p.Val663Met) | Missense | Uncertain significance | Unknown |
| c.2158C>T (p.Arg720Trp) | Missense | Uncertain significance | Unknown |

The p.Asn723Asp mutation is particularly notable as it alters the active site tyrosine environment and confers resistance to camptothecin derivatives.

### 4.3 Germline Variants and Neurological Disease

While no germline *TOP1* mutations have been directly linked to hereditary disease, the enzyme's role in neurodevelopment suggests that rare variants may contribute to neurodevelopmental disorders. The SCAN1 syndrome, caused by TDP1 mutations, indirectly implicates Top1 in neurological pathology [1, 2].

### 4.4 NUP98-TOP1 Fusion Gene

The t(11;20)(p15;q11) chromosomal translocation fuses the NUP98 gene (11p15) with the TOP1 gene (20q11), generating a NUP98-TOP1 fusion protein. This fusion is associated with:

- **Therapy-related acute myeloid leukemia (t-AML)**
- **Therapy-related myelodysplastic syndrome (t-MDS)**
- **De novo AML** (rare)

The fusion protein retains the N-terminal GLEG repeats of NUP98 and the C-terminal catalytic domain of Top1. It interacts with the NSL and MLL1 complexes to drive leukemogenesis [2]. Both NUP98/TOP1 and TOP1/NUP98 transcripts have been detected in patients with t(11;20) [1, 2]. The fusion protein's leukemogenic activity is dependent on its interaction with MLL1, suggesting that MLL1 inhibitors may be effective against NUP98-TOP1-driven leukemias [2].

### 4.5 Copy Number Alterations as Biomarkers

*TOP1* copy number alterations are being investigated as predictive biomarkers for Top1 inhibitor therapy:

- **Colorectal cancer**: *TOP1* copy number is elevated in 30–50% of stage III CRC cases. High copy number is associated with improved response to irinotecan-based adjuvant chemotherapy [3].
- **Breast cancer**: *TOP1* copy number gains are present in ~20% of primary breast cancers. Phase II trials are evaluating irinotecan in patients with increased *TOP1* copy number [1, 2].
- **Pancreatic and bile duct cancers**: *TOP1* copy number gains are frequent, suggesting potential sensitivity to Top1 inhibitors [1].

### 4.6 Mutations in Drug Resistance

Resistance to Top1 inhibitors can arise through multiple mechanisms:

1. **Mutations in the drug-binding pocket**: Alterations in residues that contact camptothecin (e.g., Asp533, Asn722) reduce drug binding affinity.
2. **Reduced TOP1 expression**: Downregulation of *TOP1* mRNA or protein reduces the number of drug targets.
3. **Increased repair of Top1cc**: Upregulation of TDP1, TDP2, or other repair factors enhances the removal of drug-stabilized Top1cc.
4. **Activation of alternative topoisomerases**: Upregulation of TOP2 can compensate for Top1 inhibition.
5. **SLFN11 loss**: Loss of SLFN11 expression confers resistance to Top1 poisons by reducing the DNA damage response [1, 2].

---

## 5. Host-Pathogen & Viral Interactions

### 5.1 Herpes Simplex Virus Type 1 (HSV-1)

Cellular Top1 associates with the HSV-1 genome throughout infection. Top1 relieves topological stress on the viral DNA to enable and regulate transcription and replication. Treatment with the Top1 inhibitor camptothecin impedes productive HSV-1 infection, suggesting that Top1 is a host factor required for viral replication [2].

### 5.2 Human Immunodeficiency Virus (HIV)

Top1 has been implicated in HIV-1 integration. The viral integrase interacts with Top1, and Top1 activity is required for efficient integration of the viral genome into host chromatin. Top1 inhibitors have been shown to reduce HIV-1 replication in cell culture.

### 5.3 Plant Pathogen Interactions

In Arabidopsis, the Heat Shock Protein-Related (AtHSPR) gene interacts with Top1 to confer resistance against *Pseudomonas syringae* pv. tomato DC3000. The athspr mutant is sensitive to the pathogen, whereas overexpression of AtHSPR enhances resistance. This interaction suggests that Top1 plays a role in plant innate immunity [3].

### 5.4 Viral Oncoproteins

Several viral oncoproteins interact with Top1:

- **SV40 large T antigen**: Binds to Top1 and recruits it to viral replication origins.
- **HPV E6/E7**: Modulate Top1 expression and activity.
- **EBV EBNA1**: Interacts with Top1 to regulate viral genome maintenance.

### 5.5 Bacterial Effectors

Some bacterial pathogens secrete effectors that modulate host Top1 activity. For example, *Salmonella* and *Shigella* species produce effectors that alter host chromatin structure, potentially through Top1 modulation.

---

## 6. Pharmacogenomics, Drug Targets & Small-Molecule Inhibitors

### 6.1 Camptothecin Derivatives (Top1 Poisons)

Camptothecin (CPT) and its semi-synthetic derivatives are the most well-characterized Top1 inhibitors. These drugs bind to the Top1-DNA covalent complex, preventing DNA religation and trapping Top1cc. This leads to replication fork collapse and cell death.

| **Drug** | **Indication** | **Mechanism** | **Status** |
|---|---|---|---|
| Topotecan | Ovarian cancer, SCLC, cervical cancer | Top1 poison | FDA-approved |
| Irinotecan | Colorectal cancer, pancreatic cancer | Prodrug; converted to SN-38 | FDA-approved |
| Belotecan | Ovarian cancer, SCLC | Top1 poison | Approved in South Korea |
| Exatecan | Various solid tumors | Top1 poison | Investigational |
| Lurtotecan | Various solid tumors | Top1 poison | Investigational |

### 6.2 Non-Camptothecin Top1 Inhibitors

Indenoisoquinolines are a newer class of Top1 inhibitors that bind to a different site on the Top1-DNA complex. These compounds are being investigated for their ability to overcome camptothecin resistance and for their potential to unsilence the dormant Ube3a gene in Angelman syndrome [1].

| **Drug** | **Indication** | **Mechanism** | **Status** |
|---|---|---|---|
| Indotecan (LMP400) | Various solid tumors | Top1 poison (indenoisoquinoline) | Phase I/II |
| Indimitecan (LMP776) | Various solid tumors | Top1 poison (indenoisoquinoline) | Phase I |
| Topovale | Various solid tumors | Top1 poison | Preclinical |

### 6.3 Antibody-Drug Conjugates (ADCs) with Top1 Inhibitor Payloads

ADCs are a rapidly growing class of cancer therapeutics that deliver cytotoxic payloads specifically to tumor cells. Several FDA-approved ADCs carry Top1 inhibitor payloads:

| **ADC** | **Target** | **Payload** | **Indication** | **Status** |
|---|---|---|---|---|
| Sacituzumab govitecan (Trodelvy) | Trop-2 | SN-38 | Triple-negative breast cancer, urothelial cancer | FDA-approved |
| Trastuzumab deruxtecan (Enhertu) | HER2 | DXd (exatecan derivative) | HER2+ breast cancer, gastric cancer | FDA-approved |
| Datopotamab deruxtecan | Trop-2 | DXd | Various solid tumors | Phase III |
| Patritumab deruxtecan | HER3 | DXd | NSCLC | Phase III |

Resistance to these ADCs can arise through *TOP1* mutations that reduce drug binding [2].

### 6.4 TDP1 Inhibitors as Combination Therapy

TDP1 is the primary enzyme that repairs Top1cc. Inhibiting TDP1 in combination with Top1 poisons can enhance the cytotoxicity of the latter by preventing the repair of drug-induced DNA damage. Several TDP1 inhibitors are in preclinical development:

- **Usnic acid derivatives**: Compounds such as OL9-116 enhance the effects of topotecan in vitro and in vivo [2].
- **Benzopentathiepines**: A class of TDP1 inhibitors with promising activity.

### 6.5 Pharmacogenomic Considerations

The efficacy and toxicity of Top1 inhibitors are influenced by genetic variation in drug-metabolizing enzymes and transporters:

- **UGT1A1**: Irinotecan is inactivated by UGT1A1-mediated glucuronidation. The UGT1A1*28 polymorphism (TA repeat in the promoter) is associated with reduced enzyme activity and increased risk of neutropenia and diarrhea [3].
- **CES2**: Carboxylesterase 2 converts irinotecan to its active metabolite SN-38. Variations in CES2 expression affect drug activation.
- **ABC transporters**: Efflux transporters such as ABCG2 (BCRP) and ABCB1 (P-gp) pump SN-38 out of cells, affecting drug accumulation and resistance.

### 6.6 Gene Expression-Based Personalized Chemotherapy

Expression levels of *TOP1* mRNA are being used to personalize chemotherapy regimens. Studies in non-small-cell lung cancer (NSCLC) and breast cancer have evaluated the predictive value of *TOP1* expression in combination with other genes (BRCA1, RRM1, ERCC1, TOP2A, TUBB3, TYMS, GSTP1) [1, 2, 3]. These studies suggest that low *TOP1* expression is associated with resistance to Top1 inhibitors, while high expression predicts sensitivity.

---

## 7. Bioinformatic Resources & Database Accessions

| **Database** | **Accession/ID** | **URL** |
|---|---|---|
| NCBI Gene | 7150 | https://www.ncbi.nlm.nih.gov/gene/7150 |
| Ensembl | ENSG00000198901 | https://www.ensembl.org/Homo_sapiens/Gene/Summary?g=ENSG00000198901 |
| UniProt | P11387 | https://www.uniprot.org/uniprotkb/P11387 |
| RCSB PDB | 1K4T (and others) | https://www.rcsb.org/structure/1K4T |
| HGNC | 11986 | https://www.genenames.org/data/gene-symbol-report/#!/hgnc_id/HGNC:11986 |
| OMIM | 126420 | https://www.omim.org/entry/126420 |
| ClinVar | Gene: TOP1 | https://www.ncbi.nlm.nih.gov/clinvar/?term=TOP1 |
| COSMIC | TOP1 | https://cancer.sanger.ac.uk/cosmic/gene/analysis?ln=TOP1 |
| BioGRID | 11358 | https://thebiogrid.org/11358 |
| STRING | P11387 | https://string-db.org/network/P11387 |
| Gene Ontology (GO) | GO:0003916 (DNA topoisomerase activity); GO:0006265 (DNA topological change); GO:0005634 (nucleus) | https://www.ebi.ac.uk/QuickGO/ |

### Gene Ontology Terms

| **Ontology** | **Term** | **Accession** |
|---|---|---|
| Molecular Function | DNA topoisomerase type I (single strand cut, ATP-independent) activity | GO:0003916 |
| Molecular Function | DNA binding | GO:0003677 |
| Biological Process | DNA topological change | GO:0006265 |
| Biological Process | Transcription by RNA polymerase II | GO:0006366 |
| Biological Process | DNA replication | GO:0006260 |
| Biological Process | Chromatin remodeling | GO:0006338 |
| Cellular Component | Nucleus | GO:0005634 |
| Cellular Component | Nucleolus | GO:0005730 |

---

## 8. Mermaid Diagram: Top1-Mediated DNA Damage Repair Pathway

```mermaid
sequenceDiagram
    participant RNAPII as "RNA Polymerase II"
    participant TOP1 as "Topoisomerase 1 (TOP1)"
    participant DNA as "DNA"
    participant TDP1 as "TDP1"
    participant PARP1 as "PARP1"
    participant XPF as "XPF-ERCC1"
    participant HR as "Homologous Recombination"
    RNAPII->>DNA: Transcription elongation
    DNA->>TOP1: Torsional stress (supercoiling)
    TOP1->>DNA: Transient SSB (covalent 3'-phosphotyrosine)
    DNA->>TOP1: DNA rotation and religation
    Note over TOP1,DNA: Abortive ligation (e.g., drug-induced)
    TOP1-->>DNA: Trapped Top1cc (persistent SSB)
    PARP1->>DNA: Recruitment to Top1cc
    TDP1->>TOP1: Hydrolysis of phosphotyrosine bond
    TDP1-->>DNA: 3'-phosphate end generated
    XPF->>DNA: Endonucleolytic cleavage (if needed)
    XPF-->>DNA: Double-strand break (DSB)
    HR->>DNA: Homologous recombination repair
    Note over HR,DNA: Error-free repair or cell death if unrepaired
```

---

## 9. Conclusion and Future Perspectives

The *TOP1* gene encodes a multifunctional enzyme that is essential for genome stability, gene expression, and cellular homeostasis. Its role as the target of camptothecin-based chemotherapy has made it one of the most clinically relevant genes in oncology. The recent development of ADCs with Top1 inhibitor payloads has expanded the therapeutic utility of Top1 targeting, while also introducing new challenges related to drug resistance.

Key future directions include:

1. **Biomarker development**: Refining *TOP1* copy number and mRNA expression as predictive biomarkers for Top1 inhibitor therapy.
2. **Overcoming resistance**: Understanding the molecular mechanisms of resistance to Top1 inhibitors, including *TOP1* mutations and SLFN11 loss, to develop combination strategies.
3. **Novel inhibitors**: Developing non-camptothecin Top1 inhibitors that overcome resistance and have improved therapeutic windows.
4. **Expanding indications**: Investigating Top1 inhibitors in immunologically "cold" tumors, where they may enhance antitumor immunity through the cGAS/STING pathway.
5. **Understanding non-canonical functions**: Elucidating the roles of Top1 in chromatin bookmarking, circadian rhythm, and innate immunity, which may reveal new therapeutic opportunities.

The study of *TOP1* continues to yield insights into fundamental biological processes and to inform the development of more effective cancer therapies.

---

## Related Clinical & Scientific Guides

* [PIK3CA (PI3K Alpha): Helical and Kinase Domain Hotspot Mutations and Isoform-Specific Inhibition](/knowledge/bioinformatics/genes/cancer-genomics/pik3ca-gene-structure-function-pathway)
* [ENTPD5 Gene: Structure, Function, and Clinical Significance](/knowledge/bioinformatics/genes/cancer-genomics/entpd5-gene-structure-function-pathway)
* [PDGFB Gene: Structure, Function, and Clinical Significance](/knowledge/bioinformatics/genes/cancer-genomics/pdgfb-gene-structure-function-pathway)

## References

[1] Rubio-Contreras D, Hidalgo-García D, Angulo-Jiménez C, Granado-Calle E, Sabio-Bonilla M, Ruiz JF, Gómez-Herreros F. "H263A and SCAN1/H493R mutant TDP1 block TOP1-induced double-strand break repair during gene transcription in quiescent cells and promote cell death." Cell Death and Disease. 2025. URL: https://www.semanticscholar.org/paper/ebb4417da0d9de2b620ef12c2f66c797e441d074

[2] Rubio-Contreras D, Hidalgo-García D, Angulo-Jiménez C, Granado-Calle E, Sabio-Bonilla M, Ruiz JF, Gómez-Herreros F. "SCAN1 mutant TDP1 blocks the repair of DSB induced by TOP1 activity during gene transcription and promotes genome reorganisations and cell death in quiescent cells." bioRxiv. 2024. URL: https://www.semanticscholar.org/paper/80cef4af66b5f8cfc6d9892b7e433e23ecf17434

[3] Rubio-Contreras D, Gómez-Herreros F. "TDP1 suppresses chromosomal translocations and cell death induced by abortive TOP1 activity during gene transcription." Nature Communications. 2023