Unraveling the Role of Topoisomerase 3β (TOP3B) in mRNA Translation and Human Disease

ABSTRACT

mRNA translation is a highly orchestrated process that requires spatiotemporal control to ensure each protein is synthesized at the correct abundance, time, and location during human development and physiology. Classically, trans‐acting RNA‐binding proteins (RBPs) recognize cis‐elements within mRNAs to provide one layer of gene‐specific translational control. The function and properties of RBPs are diverse, with some harboring enzymatic capabilities, and can be multifaceted if present in larger RBP complexes. In this review, we focus on the role of Topoisomerase 3β (TOP3B) as a non‐canonical RBP that is believed to influence the translation of select mRNAs and its connection with multiple human neurological disorders. Unlike any other encoded topoisomerase in the human genome, TOP3B is an mRNA‐binding protein, catalytically favors RNA over DNA, and primarily localizes to the cytoplasm. Here we highlight important aspects of TOP3B as an RBP and raise multiple key questions for the field as a roadmap to better define its function in translational control and neuropathology.

This article is categorized under:

• Translation > Regulation

• RNA in Disease and Development > RNA in Disease

• RNA Interactions with Proteins and Other Molecules > Protein‐RNA Interactions: Functional Implications

The human genome encodes six topoisomerases—TOP1, TOP1mt, TOP2A, TOP2B, TOP3A, and TOP3B. The subcellular localization of each is shown. TOP3B, which harbors a bona fide RNA‐binding domain, predominantly localizes to the cytosol and is reported to function in mRNA translation and stability. Mutation and deletion of TOP3B are linked to multiple neurological disorders, namely schizophrenia, autism, and intellectual disability.

1. Introduction

RNA‐binding proteins (RBPs) contribute to every level of gene expression and harbor a diverse set of functions (He et al. 2023; Musselman and Valkov 2022). In the context of mRNA translation, RBPs are both core components of the translation machinery that are required for nearly all transcripts and regulatory factors that contribute to gene‐specific regulation. One established role of a set of RBPs is to resolve and/or aid with RNA structure that could hinder translation. Highlighting the importance of RNA structure in gene expression, mammalian cells encode > 70 RNA helicases, ~40 of which have been shown to regulate and act on mRNAs (Bohnsack et al. 2023; Bourgeois et al. 2016). For example, eIF4A is a prototypical DEAD‐box helicase involved in translation initiation. Recent work has shown that two molecules of eIF4A contribute to efficient initiation (Brito Querido, Díaz‐López, and Ramakrishnan 2024; Waldron et al. 2018, 2019). One molecule is part of the eIF4F complex that recognizes the m⁷G cap and the Poly(A) Binding Protein (PABP)‐bound poly(A) tail. A second eIF4A molecule is at the mRNA entry channel of the ribosome and aids with RNA structure during 43S pre‐initiation complex scanning in the 5′ untranslated region (UTR) (Brito Querido, Sokabe, et al. 2024).

RNA structure within mature mRNAs is typically distinct between the UTRs and open reading frame (ORF) (Beaudoin et al. 2018; Byeon et al. 2021; Ganser et al. 2019). As the ribosome is generally able to resolve secondary structure, ORFs tend to be unstructured; whereas UTRs are known to harbor RNA structure that influences mRNA translation and stability. However, very thermostable synthetic RNA hairpins/stem‐loops in the ORF are able to sterically block the ribosome enough to stimulate no‐go mRNA decay. It is not inconceivable that RNA structures within ORFs have evolved to momentarily pause ribosomes, perhaps contributing to co‐translational folding or specific regulation. For example, more complex tertiary structures like pseudoknots that affect ribosome movement are key in programmed ribosomal frameshifting (Bhatt et al. 2021; Hsu et al. 2021; Namy et al. 2006).

Unlike DNA, RNA is typically not thought to be largely topologically constrained. This is primarily due to its more linear state; although mRNAs can form closed loop conformations with eIF4F and PABP (Alekhina et al. 2020; Kahvejian et al. 2001; Vicens et al. 2018), and some mRNAs are back‐spliced into circular RNAs (Wilusz 2018; Yang, Wilusz, and Chen 2022). However, contradicting this idea, mammalian Topoisomerase 3β (TOP3B) has been identified as a dual functional topoisomerase that acts on both DNA and RNA in cells and in vitro (Saha et al. 2020; Stoll et al. 2013; Xu et al. 2013), and most likely catalytically favors RNA over DNA (Yang, Saha, et al. 2022). Recent work has also linked TOP3B to multiple neurological disorders, namely schizophrenia, autism, and intellectual disability (Ahmad, Shen, et al. 2017; Stoll et al. 2013; Xu et al. 2013). TOP3B null mice also display impaired synapse formation, defects in adult hippocampal neurogenesis, and behavioral abnormalities tied to cognitive impairment and psychiatric disorders (Joo et al. 2020; Xu et al. 2013; Zhu et al. 2024).

The precise function of TOP3B in neurons remains unclear. However, unlike any other topoisomerase encoded in the human genome, TOP3B predominantly localizes to the cytoplasm, contains a bona fide RNA‐binding domain, and associates with the active translation machinery on polysomes (Ahmad et al. 2016; Stoll et al. 2013; Xu et al. 2013). Multiple CLIP‐based investigations have concluded that TOP3B binds to the open reading frame of mRNAs, and Ribo‐seq data suggest differential translation of certain mRNAs in the absence of TOP3B (Xu et al. 2013; Yang, Saha, et al. 2022). Together, these data support that TOP3B functions on translating mRNA. In this review, we focus on these pieces of evidence and raise important questions for the field to further define the role of TOP3B in neuronal protein synthesis and neurological disease.

2. Classification and Function of DNA Topoisomerases

Traditionally, topoisomerases are enzymes that resolve DNA topology that arises during replication and transcription. The human genome encodes at least six topoisomerases—namely TOP1, TOP1mt, TOP2A, TOP2B, TOP3A, and TOP3B (Figure 1). According to The Human Protein Atlas, only TOP2A and TOP3A display signs of tissue specificity (Sjöstedt et al. 2020; Uhlén et al. 2015). However, mechanistically, all six human topoisomerases can be classified as either type I or type II. Type I topoisomerases cleave only single‐stranded DNA without the need for ATP, and type II topoisomerases cleave double‐stranded DNA in an ATP‐dependent manner (Pommier et al. 2022; Wang 2002).

FIGURE 1.

Human topoisomerases have separate requirements and functions. Schematic of all six known human topoisomerases (left). Summary of each topoisomerase describing the type, mechanism, required cofactors, localization, and preferred substrates and functions (right). TOP3B is the only known human topoisomerase that contains a bona fide RNA‐binding domain (i.e., RGG box domain), which allows for dual functionality to resolve DNA topological strain and R‐loops in the nucleus, and uniquely function in mRNA metabolism in the cytoplasm. sc = supercoiled.

All topoisomerases have a similar mechanism in which they first bind DNA, then introduce a break via a transesterification reaction. This reaction uses a conserved catalytic tyrosine residue that forms a covalent link with the phosphodiester backbone of the substrate to allow a strand rearrangement, either through a strand passage or swivel reaction. The cleaved strand(s) are then rejoined by nucleophilic attack of the DNA hydroxyl end, which is mediated by the active site residues, and the topoisomerase dissociates from the substrate (Pommier et al. 2016; Vos et al. 2011; Zhang et al. 2011). However, the slight differences in topoisomerase mechanisms allow them to be further classified into subfamilies, including type IA, IB, IC, IIA, and IIB (Forterre and Gadelle 2009; Vos et al. 2011). It should be noted that eukaryotic topoisomerases have only been classified as types IA, IB, or IIA; thus, type IC and IIB will not be further discussed.

Type IB topoisomerases, including TOP1 and TOP1mt, cleave a single strand of a bound DNA duplex and form a covalent complex with the 3′ end of their substrate, can resolve both positive and negative DNA supercoils, are less dependent on divalent cations, and perform a “swivel” reaction rather than a strand passage reaction to resolve topological constraints. This occurs through a controlled rotation of the cleaved DNA strand around the intact DNA strand (Champoux and Dulbecco 1972; Pommier et al. 2022; Takahashi et al. 2022). Type IIA topoisomerases, including TOP2A and TOP2B, function as homodimers to bind and cleave double‐stranded DNA, form a covalent intermediate with the 5′ end of the cleaved substrate, and use an enzyme‐gated mechanism to allow strand passage of an intact double‐stranded DNA through the double‐stranded break. These enzymes require both Mg²⁺ and ATP, and are capable of resolving positive and negative DNA supercoils, as well as DNA knots and catenanes (Goto and Wang 1982; Ling et al. 2022; Nitiss 2009; Wendorff et al. 2012).

Type IA topoisomerases, which include TOP3A and TOP3B, cleave single‐stranded DNA and form a covalent intermediate complex with the 5′ end of the cleaved substrate. Following cleavage, type IA topoisomerases use an enzyme‐gated mechanism to perform a strand passage reaction by allowing an intact single‐stranded DNA to pass through the broken DNA that is bound to the topoisomerase (Ahmad, Xu, and Wang 2017; Bizard and Hickson 2020; Capranico et al. 2017). These enzymes require either Mg²⁺ or Mn²⁺ ions for catalytic activity (Cao et al. 2018; Schmidt et al. 2010; Wilson et al. 2000); however, Yang et al. reported that TOP3B can cleave, but not rejoin, DNA substrates in vitro without the addition of metal ions, suggesting different metal ion requirements at different stages of the catalytic cycle (Yang, Saha, et al. 2022). Due to the specificity for single‐stranded nucleic acids, TOP3A and TOP3B relieve negatively supercoiled DNA. As discussed in greater detail in the remainder of this review, TOP3B has been reported as the only mammalian topoisomerase capable of performing strand passage reactions on RNA in vitro (Stoll et al. 2013; Xu et al. 2013; Yang, Saha, et al. 2022) and is catalytically active on RNA in cells (Saha et al. 2020, 2022; Wang et al. 2023), and does so largely due to harboring a bona fide RNA‐binding domain (i.e., RGG box domain) (Figure 1). For more details, the roles of all six human topoisomerases have been recently expertly reviewed by Pommier et al. (2022).

3. Identification of RNA Topoisomerases

While DNA topoisomerases have been generally well characterized, RNA topoisomerases have not been extensively studied. Reports of E. coli topoisomerases being active on RNA substrates in vitro, at least being able to cleave RNA, date back to the early 1990s. In 1992, DiGate and Marians first reported that DNA cleavage by E. coli Topo III, a type IA topoisomerase, was inhibited by the presence of tRNA in reaction mixtures, leading to the hypothesis that Topo III was already covalently linked or bound to RNA. This was confirmed by incubating purified Topo III with radiolabeled RNA substrates and observing cleaved RNA products by denaturing gel electrophoresis and autoradiography (DiGate and Marians 1992). Whether such activity is robust and physiologically important in E. coli in vivo remains unknown. However, at least one type IA RNA topoisomerase has been identified in all domains of life (Ahmad et al. 2016), which may point toward the importance of having enzymes capable of resolving RNA topological strain and may suggest that this family of enzymes generally functions as dual topoisomerases on both DNA and RNA.

The above cleavage reactions rely on only the first half of the topoisomerase reaction—substrate binding and cleavage. Thus, the field should be skeptical of whether an enzyme truly has topoisomerase activity if only half of the reaction is assayed in vitro. However, in 1996, Wang et al. developed an RNA knotting assay, which has now become a standard experiment in the field, to confirm that E. coli Topo III is an RNA topoisomerase and able to complete the full topoisomerase reaction using an RNA substrate in vitro. In this assay, a synthetic RNA circle was designed with two complementary regions (e.g., A is complementary to A′, and B is complementary to B′), separated by linkers, that can be converted into a stable RNA knot after strand passage and substrate rejoining. The RNA knot and circle can be easily differentiated by denaturing gel electrophoresis (Wang et al. 1996).

However, whether E. coli Topo I is catalytically active on RNA remains unclear. Two reports have concluded that E. coli Topo I is not active on RNA in RNA knotting (Wang et al. 1996) and RNA cleavage assays (Yang, Saha, et al. 2022). However, using slightly different RNA knotting substrates, separate reports have provided evidence that E. coli Topo I is active on RNA (Ahmad et al. 2016; Liu et al. 2017). Ahmad et al. suggested that the difference in observed activities could be due to the longer complementary regions (12 bp vs. 10 bp) in their RNA circle substrate that could help with substrate stability and thermodynamic favorability of the reaction.

Identification of a eukaryotic RNA topoisomerase is relatively more recent. In mammals, only TOP3B has been shown to be active on RNA substrates in multiple in vitro (Ahmad, Shen, et al. 2017; Saha et al. 2023; Stoll et al. 2013; Xu et al. 2013) and cell‐based assays (Saha et al. 2020, 2022; Wang et al. 2023). However, it should be noted that TOP1, TOP2A, and TOP3B were identified as putative mRNA‐binding proteins in two independent studies in HeLa and HEK293 cells using UV‐crosslinking coupled with mass spectrometry (Baltz et al. 2012; Castello et al. 2012). While additional studies have since identified these enzymes as RNA‐binding proteins (Bhola et al. 2024; Park et al. 2008), at least to our knowledge, no experiments have been reported to determine if TOP1 or TOP2A are catalytically active on RNA substrates. Additionally, in a separate report, using UV‐crosslinking coupled with immunoblotting, only TOP3B was identified as an mRNA‐binding protein in HEK293 cells (Ahmad, Shen, et al. 2017).

It is important to recognize (and is discussed in more detail in the subsequent sections) that TOP3B is active on both DNA and RNA. Specifically, TOP3B is able to form covalent intermediates with DNA and RNA substrates in vitro (Saha et al. 2023; Stoll et al. 2013; Yang, Saha, et al. 2022) and when overexpressed in cultured cells (Saha et al. 2020; Wang et al. 2023). Using the RNA knotting assay, Xu et al. established that TOP3B, but not TOP3A, is active on RNA in vitro (Xu et al. 2013). At the same time, Stoll et al. confirmed these results by detecting radiolabeled RNA cleavage products generated after incubation with recombinant WT, but not a catalytically inactive mutant (Y336F), TOP3B (Stoll et al. 2013). Unlike other topoisomerases, TOP3B harbors an RNA‐binding domain (i.e., RGG box domain), which is generally conserved among orthologs (Ahmad et al. 2016), that greatly aids its activity on RNA (Xu et al. 2013; Yang, Saha, et al. 2022). At least in cultured mammalian cells, TOP3B is predominantly localized to the cytoplasm and CLIP‐seq based experiments suggest mRNAs are its primary target (Su et al. 2022; Xu et al. 2013). Interestingly, TOP3B appears to more frequently bind mRNAs with longer than average lengths (Su et al. 2022; Teimuri and Suter 2025; Xu et al. 2013). Consistent with a role in regulating translation, TOP3B has also been shown to associate with polyribosomes (Ahmad et al. 2016; Stoll et al. 2013; Xu et al. 2013). Together, these data support that mRNA is the primary substrate for TOP3B in mammalian cells, and TOP3B has been concluded to be the major topoisomerase for mRNAs.

4. TOP3B Is a Dual Activity Topoisomerase

4.1. Structure

Eukaryotic TOP3B consists of three major structural domains that are present within plants and metazoans—the N‐terminal catalytic core (human aa 1‐612), the C‐terminal zinc finger domain (human aa 613‐823), and the C‐terminal RGG box domain (human aa 824‐862) (Figure 1) (Goto‐Ito et al. 2017; Yang et al. 2025; Yang, Saha, et al. 2022). As with other type IA topoisomerases, the catalytic core of TOP3B is further separated into four different domains. Domains I, III, and IV form the base of the core domain and provide a single‐stranded nucleic acid binding groove. Domain I contains residues essential for divalent metal ion binding, and domain III contains the catalytic tyrosine residue. Domain II, often further broken down into IIa and IIb, consists of two folds that bridge domains III and IV and is likely responsible for the “hinge” activity of the enzyme that allows gate opening during strand passage reactions (Dasgupta et al. 2020; Goto‐Ito et al. 2017; Yang et al. 2025) (Figures 1 and 2A).

FIGURE 2.

AlphaFold predicted structure of TOP3B and catalytic cycle on DNA and RNA. (A) AlphaFold predicted structure of full‐length human TOP3B (UniProtKB O95985) with residues associated with disease‐linked mutations (P378, R472, and C666) and the catalytic active site tyrosine (Y336) labeled with black arrows, and core sub‐domains I‐IV labeled in green. (B) Schematic of TOP3B catalytic cycle on DNA and RNA. TOP3B first binds the substrate, followed by catalytic cleavage that generates a covalently‐linked intermediate to allow strand passage. The cleaved substrate is then rejoined and TOP3B dissociates.

While the core domain is conserved among Type IA topoisomerases, the length and sequence of the C‐terminal domains are more variable (Ahmad, Xu, and Wang 2017; Tan et al. 2015). TOP3B harbors a predicted zinc finger domain and an RGG box domain (Figure 2A). Based on the primary sequence, the zinc finger domain in human TOP3B was originally thought to have four C4‐type zinc finger motifs, each predicted to coordinate a single Zn²⁺ ion (Moreira et al. 2021). However, AlphaFold 3 predicts five zinc finger motifs: three C4‐type motifs (motifs 1, 4, and 5), a single D1C3‐type motif (motif 2), and a single C3‐type motif (motif 3) (Jumper et al. 2021; Varadi et al. 2024). Structural analysis of the C‐terminal domain has proven to be difficult due to its inherent flexibility; however, recent work has reported a cryo‐EM structure that was able to delineate the first three zinc finger motifs (residues 612–717) which consist of a C4‐type motif, a D1C3‐type motif, and a C3‐type motif, each coordinating separate Zn²⁺ ions, supporting the likelihood of five zinc finger motifs in the zinc finger domain of TOP3B (Yang et al. 2025). Recent work with recombinant human WT and mutant TOP3B suggests that the zinc finger domain contributes to substrate binding and enzyme activity on DNA and RNA, and promotes enzyme processivity on supercoiled DNA substrates (Yang, Saha, et al. 2022); however, the effect of this domain on processivity on RNA has not been reported (Table 1). The unique RNA processing capability of TOP3B is significantly due to the RGG box domain (Ahmad, Shen, et al. 2017; Ahmad et al. 2016; Huang et al. 2018; Yang, Saha, et al. 2022). This bona fide RNA‐binding domain is found in many types of RBPs and is not thought to have strict sequence or structural specificity (Ghisolfi et al. 1992; Thandapani et al. 2013). While the TOP3B core domain alone is still active on RNA (i.e., in RNA cleavage assays), deletion of the C‐terminal RGG box domain severely reduces TOP3B binding and catalytic activity on DNA and RNA substrates in vitro (Yang, Saha, et al. 2022), suggesting an important role for this domain in both the DNA and RNA topoisomerase activities of TOP3B (Table 1).

TABLE 1.

Key residues and requirements of TOP3B on DNA and RNA substrates in vitro.

	DNA	RNA
Effect of TDRD3 on catalytic activity	Increases	Increases
Effect of Mg2+/Mn2+ on cleavage	Not required	Not required
Effect of Mg2+/Mn2+ on rejoining	Required	Not required
Effect of K10M mutation on cleavage	Inhibits	No effect
Effect of K10M mutation on rejoining	Inhibits	Slows
Effect of C‐terminal domain deletion on binding	Decreases	Decreases
Effect of C‐terminal domain deletion on cleavage activity	Decreases	Decreases (more severe)
Effect of C‐terminal domain deletion on processivity	Decreases	Not reported

4.2. General Reaction Cycle and Catalytic Active Site

TOP3B follows the standard type IA topoisomerase reaction cycle—binding, cleavage, strand passage, and rejoining—and we are unaware of any evidence (at least to date) that argues that this general cycle is different between DNA and RNA. Following binding to single‐stranded DNA or RNA, TOP3B performs a transesterification reaction to cleave the substrate via general acid–base catalysis. The conserved catalytic tyrosine forms a covalent linkage with the 5′ phosphate of the nucleic acid phosphodiester backbone, forming a transient covalent intermediate. This allows strand passage of an intact DNA or RNA strand through the “open” conformation of the intermediate. TOP3B then facilitates a reverse phosphoryl transfer to rejoin the cleaved phosphodiester backbone and returns to its “closed” conformation (Pommier et al. 2022, 2016; Vos et al. 2011; Yang et al. 2025) (Figure 2B). This step distinguishes TOP3B from traditional ribonucleases or RNA helicases, as this enzyme is capable of rejoining the cleaved nucleic acid, leaving the sequence exactly the same as it was prior to cleavage. Following rejoining, TOP3B presumably can either dissociate from the nucleic acid substrate or repeat this cycle multiple times during one binding event to fully resolve a structure. It is not known what exactly dictates this choice or if there is an inherent preference for TOP3B to perform a single reaction cycle and then dissociate. However, recent work has shown that the TOP3B core alone (C‐terminal zinc finger and RGG box domains deleted) performed a single catalytic cycle during one DNA binding event, whereas full‐length TOP3B was highly processive and performed multiple cycles per binding event (Yang, Saha, et al. 2022). These data suggest that the zinc finger and RGG box domains stimulate TOP3B enzyme processivity by preventing detachment from DNA substrates. Whether this molecular phenotype is also seen on RNA substrates has not been reported yet.

Multiple reports have detailed that the biochemical requirements in vitro for the transesterification reaction and rejoining are probably distinct between DNA and RNA substrates, although both require the same catalytic tyrosine (human aa Y336) (Table 1). A divalent cation, either Mg²⁺ or Mn²⁺, is also coordinated in the active site by residues E9, D117, and D119 (Goto‐Ito et al. 2017; Yang et al. 2025; Yang, Saha, et al. 2022). In cleavage reactions (thus, only measuring the first half of the topoisomerase reaction), TOP3B was more active in the presence of Mn²⁺ than Mg²⁺ on both DNA and RNA in vitro (Wilson et al. 2000; Yang, Saha, et al. 2022). Yet, the actual requirement for divalent cations in vitro appears not to be absolute as recombinant TOP3B purified with buffers containing EDTA does not require additional divalent cations for DNA or RNA cleavage, or for rejoining of RNA substrates. However, divalent cation addition was essential for rejoining of DNA substrates (Table 1). Nevertheless, high concentrations of EDTA did completely inhibit cleavage and rejoining on both DNA and RNA substrates, suggesting that recombinant TOP3B is purified with stably bound metal ions or was active with trace amounts of metal ions that could be present in buffers (Yang, Saha, et al. 2022).

Active site residues other than Y336 also have important known roles. R338 is highly conserved in type IA topoisomerases and is proximal to the catalytic Y336. Mutation of the arginine proximal to the catalytic tyrosine of Y. pestis (R327) and E. coli (R321) Topoisomerase I to a hydrophobic amino acid severely reduced the enzyme's ability to rejoin DNA after cleavage and resulted in accumulation of covalent intermediates (Narula et al. 2011); these data led others to hypothesize, based on sequence alignment, that substitution of R338 in TOP3B with tryptophan (W) may similarly stabilize covalent intermediates (Saha et al. 2020). Plasmid overexpression in HCT116 and HEK293 cells determined that TOP3B‐R338W robustly stabilized covalent intermediates on both DNA and RNA and has since been termed a “self‐trapping mutation” (Saha et al. 2020). In WT TOP3B, the R338 residue coordinates the scissile phosphate with the catalytic tyrosine residue, which must be perfectly aligned for proper rejoining of the cleaved substrate (Changela et al. 2001; Goto‐Ito et al. 2017; Yang et al. 2025; Yang, Saha, et al. 2022). However, the substituted tryptophan (R338W) is not able to coordinate this alignment, leading to inhibition of rejoining and robust stabilization of unresolved covalent intermediates.

Mutational analysis of the conserved active site K10 has been informative and supports that RNA rather than DNA is most likely the preferential substrate for TOP3B (Yang, Saha, et al. 2022). The solved crystal structure of E. coli Topo III demonstrates this conserved residue coordinates the scissile phosphate group during rejoining (Changela et al. 2001), and a recent crystal structure (Goto‐Ito et al. 2017) and cryo‐EM structures of human TOP3B support this (Yang et al. 2025). From this, Yang et al. demonstrated with recombinant TOP3B that the introduction of a K10M substitution completely abolished DNA cleavage in the absence of metal ions and severely inhibited DNA cleavage in the presence of metal ions in vitro. DNA rejoining was abolished regardless of metal ion addition. Additionally, overexpression of the K10M mutant, compared to WT TOP3B, led to reduced amounts of covalent intermediates on DNA in HEK293 cells. Interestingly, using the same recombinant TOP3B K10M mutant, RNA cleavage was unaffected in the presence or absence of metal ions in vitro. However, the TOP3B K10M mutant did rejoin RNA substrates slower than WT in the presence of metal ions and was unable to rejoin RNA substrates in the absence of metal ions (Yang, Saha, et al. 2022) (Table 1). Due to the lower dependency on metal ions and other active site residues, Yang et al. concluded that RNA is the more favorable catalytic substrate for TOP3B and proposed that the additional 2′‐OH may make the RNA strand more stable than DNA, thus enhancing cleavage and could promote rejoining by polarizing the nucleophile or serving as a general base. Yang et al. provide further details regarding the biochemical requirements of TOP3B, as well as schematic diagrams of the catalytic active site bound to DNA and RNA.

4.3. Targeted Degradation of Unresolved Covalent Intermediates

In a reaction cycle, topoisomerases are transiently in the covalently linked intermediate form. However, topoisomerases can fail to complete the cycle, resulting in a more stable covalent complex with the DNA or RNA substrate (Meng et al. 2003; Pommier et al. 2006, 1998; Pourquier et al. 1999). In fact, this step is targeted by topoisomerase inhibitors that are clinically used as antibiotics for bacterial topoisomerases (e.g., quinolones) (Hooper and Jacoby 2016; Laponogov et al. 2009) and, more commonly, as chemotherapeutics to introduce deleterious DNA damage in cancer cells (e.g., camptothecins, anthracyclines, etoposide) (Marinello et al. 2018; Pommier 2013; Saha and Pommier 2023; Wang et al. 2023; Zhang et al. 2021). TOP3B·DNA and TOP3B·RNA covalent intermediates that fail to be resolved are targeted for degradation by the ubiquitin‐proteasome system in HEK293 and HCT116 cells, with the ubiquitin E3 ligase TRIM41 having a critical role (Saha et al. 2020). Upon proteasomal degradation, the remaining covalent tyrosine‐scissile phosphate bond is then excised by tyrosyl DNA phosphodiesterase 2 (TDP2) (Figure 3). In cells, this ladder step most likely only happens after proteasomal degradation of TOP3B; however, in vitro, TDP2 is able to remove the full‐length TOP3B protein if denatured, as TDP2 must gain access to the covalent tyrosine‐scissile phosphate bond. Failure to resolve or degrade stable TOP3B·DNA and TOP3B·RNA covalent intermediates formed by the overexpression of the R338W mutant results in multiple cellular phenotypes, including genomic DNA damage, higher incidence of R‐loops, and reduced cell proliferation in colony formation assays (Saha et al. 2020). As TOP3B is active on both RNA and DNA, and is primarily cytoplasmic, it remains important for the field to decipher if such phenotypes are more related to activity on DNA or RNA.

FIGURE 3.

Unresolved TOP3B·RNA covalent intermediates are targeted for proteasomal degradation. Unresolved covalent intermediates are ubiquitylated by the E3 ubiquitin ligase TRIM41 and subsequently degraded by the proteasome. TDP2 excises the remaining covalent tyrosine‐scissile phosphate bond, presumably releasing the substrate for degradation by the exoribonuclease Xrn1.

4.4. TOP3B‐TDRD3 Dimer

At least in commonly cultured cancer cell lines, TOP3B most likely forms an equimolar dimer with Tudor Domain Containing Protein 3 (TDRD3) independent of RNA. Initial immunopurifications of endogenous TOP3B from both nuclear and cytoplasmic extracts revealed co‐purified TDRD3. Subsequent experiments with recombinant TOP3B and TDRD3 mapped that domain II of TOP3B interacts with the OB‐fold of TDRD3 (Stoll et al. 2013; Xu et al. 2013). Both a solved crystal structure and cryo‐EM structure support this direct interaction (Goto‐Ito et al. 2017; Yang et al. 2025). Additionally, a recent cryo‐EM structure using full‐length TOP3B‐TDRD3 heterodimer revealed interactions between the C‐terminal domain of TOP3B and TDRD3 that may further stabilize the complex (Yang et al. 2025) and other data support that arginine methylation of the C‐terminal RGG box domain of TOP3B enhances interaction with TDRD3 (Huang et al. 2018).

Several effects of TDRD3 interacting with TOP3B are known. Most notably, TDRD3 binding increases TOP3B stability. Multiple reports have shown that knockout or knockdown of TDRD3 reduces steady‐state protein levels of TOP3B in cell lines, with no effect on RNA levels (Saha et al. 2023; Su et al. 2022; Yang et al. 2014). In alignment with this phenotype, ectopic expression of TDRD3 increases TOP3B protein levels (Saha et al. 2023; Yang et al. 2014). The reciprocal effect where TOP3B levels influence TDRD3 levels has also been noted (Saha et al. 2023; Su et al. 2022). TOP3B has been reported to be targeted for proteasomal degradation in the absence of TDRD3 in MCF7 breast cancer cells (Yang et al. 2014), and elevated levels of ubiquitylated TOP3B have been observed in TDRD3‐depleted HEK293 and HCT116 cells (Saha et al. 2023). Additionally, in HEK293 and HCT116 cells, depletion of TDRD3 reduced the half‐life of TOP3B, supporting the role of TDRD3 in stabilizing TOP3B (Saha et al. 2023).

Secondly, while TOP3B is catalytically active alone in vitro, multiple pieces of evidence support that TDRD3 increases TOP3B activity on both DNA and RNA substrates. On negatively supercoiled plasmid DNA in vitro, TDRD3 increases the rate of plasmid unwinding driven by TOP3B (Siaw et al. 2016; Yang, Saha, et al. 2022). More recent experiments using magnetic tweezers and recombinant TOP3B support this conclusion, as these data show that TDRD3 increases the processivity of TOP3B on supercoiled DNA (Yang, Saha, et al. 2022). However, the exact molecular mechanics of this are not entirely known. The positive effect of TDRD3 on TOP3B activity with RNA substrates is also evident with enhanced RNA cleavage in the presence of TDRD3 (Saha et al. 2023; Yang, Saha, et al. 2022). In cells, TDRD3 levels positively correlate with the ability of TOP3B to co‐sediment with polysome‐bound mRNAs (Ahmad et al. 2016; Stoll et al. 2013; Xu et al. 2013) and TDRD3 depletion causes accumulation of unresolved TOP3B·DNA and TOP3B·RNA covalent intermediates and induces R‐loops (Saha et al. 2023; Yuan et al. 2021). As TDRD3 has been reported to bind DNA and RNA in vitro (Siaw et al. 2016), TDRD3 may direct TOP3B to a subset of its target mRNAs. Most evidence and models suggest that TDRD3 most likely allosterically affects TOP3B structure and activity, possibly aiding TOP3B to stay bound to substrates, increasing the rate of the transesterification reaction and/or rejoining reaction, or stimulating additional rounds of catalysis.

Tudor domain containing proteins commonly function as scaffolding proteins. The most notable example here is TDRD3 serving as a scaffolding protein between TOP3B and FMRP to allow formation of the TOP3B‐TDRD3‐FMRP (TTF) complex (discussed in more detail below). Other examples include TDRD3 recruiting TOP3B to RNA polymerase II (Sims 3rd et al. 2011; Su et al. 2023) and binding methyl‐arginine motifs at its C‐terminal end and TOP3B at its N‐terminal end to recruit TOP3B to chromatin and regulate transcription by preventing R‐loop accumulation (Huang et al. 2018; Saha et al. 2023; Su et al. 2023; Yang et al. 2014). Drosophila TDRD3 has also been reported to act as a scaffold to link TOP3B to RNA silencing machinery (e.g., RISC and p68) (Lee et al. 2018). However, the interaction between TOP3B and DDX5 (p68 in Drosophila) in human cells occurs independent of TDRD3, and it should be noted that TOP3B has been reported to be recruited to and resolve R‐loops independent of TDRD3 (Saha et al. 2022).

5. Evidence for TOP3B Regulating Translation

Although it remains unclear why an mRNA would require a topoisomerase over a helicase, multiple lines of evidence support that TOP3B regulates mRNA translation of a subset of mRNAs. The early evidence demonstrating the primarily cytoplasmic localization of TOP3B pointed to a predominant role in RNA metabolism rather than DNA (Stoll et al. 2013; Xu et al. 2013). HITS‐CLIP and eCLIP data from HeLa and HCT116 cells, respectively, show that TOP3B primarily crosslinks to the open reading frame of mRNAs, consistent with a role in translation (Su et al. 2022; Xu et al. 2013). Also common for factors that regulate mRNA stability and translation, TOP3B localizes to stress granules (Huang et al. 2018; Stoll et al. 2013; Xu et al. 2013). TOP3B co‐sedimenting with actively translating polyribosomes is highly consistent with a role in mRNA metabolism and common for other RBPs that regulate translation (Ahmad et al. 2016; Stoll et al. 2013; Xu et al. 2013).

The initial TOP3B HITS‐CLIP experiments suggested that TOP3B regulates translation of a subset of mRNAs and not global translation. This has recently been further supported by new data from the same lab. Using TOP3B depletion and a combination of Ribo‐seq (ribosome occupancy/translation), RNA‐seq (RNA levels) and PRO‐seq (transcription levels), Su et al. identified differentially expressed genes (DEGs) that were regulated at the levels of transcription, translation, and mRNA turnover in HCT116 cells. The majority of DEGs, both increased and decreased, were regulated at the level of translation (~60%) or mRNA turnover (~20%), suggesting that TOP3B primarily functions post‐transcriptionally to regulate mRNA translation. Among the DEGs with altered translation in TOP3B KO cells, 42 autism risk and 33 schizophrenia risk genes were identified, further supporting that TOP3B regulates translation of mRNAs important in neurological disorders. The eCLIP data also revealed that many target mRNAs were reduced in the absence of TOP3B, suggesting that TOP3B also aids mRNA stability (Su et al. 2022).

Su et al. also identified that most translationally regulated DEGs in HCT116 cells did not require catalytically active TOP3B. Specifically, Ribo‐seq was used to assess translation in WT cells, TOP3B KO cells, and catalytically inactive TOP3B knock‐in (Y336F KI) cells. Only a small fraction (~13%) of DEGs identified by Ribo‐seq in TOP3B KO cells overlapped with DEGs in Y336F KI cells, suggesting that translational control by TOP3B does not absolutely require its topoisomerase activity. However, further analysis of DEGs that overlap in TOP3B KO and TDRD3 KO cells vs. Y336F KI DEGs showed a 2–5 fold higher dependence on catalytically active TOP3B. These data suggest that TOP3B may more likely regulate mRNA translation in a catalytic‐dependent manner when complexed with TDRD3, and in a catalytic‐independent manner without TDRD3. Furthermore, when evaluating representative TOP3B target mRNAs, Su et al. provide data supporting that TOP3B in a topoisomerase‐dependent manner promotes the translation and stability of CHD8 mRNA by preventing translation‐associated mRNA decay; it was postulated that unresolved topological strain in the absence of TOP3B may cause ribosome collisions that trigger subsequent quality control pathways, such as no‐go mRNA decay (Su et al. 2022).

While there is no clear data in the field indicating which mRNA feature(s) may be targeted by TOP3B, it is possible that highly structured or topologically constrained regions (e.g., strong pseudoknots) within mRNAs could hinder the ribosome unless resolved by TOP3B (Figure 4A). Others in the field have also speculated that mRNAs in the closed loop conformation could be tangled together and would require TOP3B to allow translation to optimally proceed (Figure 4B). As TOP3B has been shown to also negatively regulate translation, it is possible that TOP3B binding to the mRNA acts as a roadblock to elongating ribosomes (Figure 4C) (Ahmad, Xu, and Wang 2017; Pommier et al. 2022; Xu et al. 2013). As FMRP is a known translation repressor (Athar and Joseph 2020; Darnell et al. 2011; Scarpitti et al. 2022), it is possible that when functioning as part of the TOP3B‐TDRD3‐FMRP complex (described more below), TOP3B is more likely to repress translation. Future work in vivo or more physiologically relevant contexts may be required to pinpoint the role of TOP3B in mRNA metabolism.

FIGURE 4.

Proposed models for TOP3B‐regulated mRNA translation. (A) Model of TOP3B targeting an RNA pseudoknot to allow efficient translation. (B) Model of TOP3B targeting “tangled” closed‐looped mRNAs. (C) Model of TOP3B acting as an mRNA‐binding protein and sterically blocking elongating ribosomes, similar to FMRP‐mediated translational repression. (D) Schematic of the TOP3B‐TDRD3‐FMRP complex. Binding of TDRD3 to TOP3B is directed via the OB‐fold. The Tudor and C‐terminal domains (CTD) of TDRD3 interact with FMRP. Adapted from Xu et al. (2013).

6. Human Disease Mutations and Animal Models

6.1. Deletion and Mutation of TOP3B Linked to Human Disease

In 2013, Stoll et al. evaluated a Northern Finnish sub‐isolate population that has a prevalence of schizophrenia almost three times higher than that of Finland's overall population. In the same sub‐isolate population, a 240 kb deletion in chromosome region 22q11.22 was seen at significantly higher frequency. The identified 240 kb deletion is within the 22q11.22 microdeletion syndrome region that has been implicated in schizophrenia and developmental delay. Importantly, the entire TOP3B gene is within the sub‐isolate 240 kb deletion, and TOP3B mRNA levels were decreased among deletion carriers. It should be noted that IGLV2‐14, a predicted transcript of unknown function, is also within the sub‐isolate 240 kb deletion region; however, due to the lack of genetic evidence, Stoll et al. focused on TOP3B in subsequent studies (Stoll et al. 2013). Genomic deletion of the TOP3B locus has also been observed in patients with juvenile myoclonic epilepsy, autism, cognitive impairment, facial dysmorphism, and behavioral concerns (Daghsni et al. 2018; Kaufman et al. 2016; Riley et al. 2020). TOP3B duplication has also been reported in patients with cognitive disorder, dysmorphic features, developmental delay, autism, and intellectual disability (Ranieri et al. 2024; Wu et al. 2019).

In addition to genetic deletion, three de novo point mutations in TOP3B have been identified in patients with intellectual disability (P378Q), schizophrenia (R472Q), and autism (C666R) (Iossifov et al. 2012; Riazuddin et al. 2017; Xu et al. 2012). The P378Q and R472Q mutations are both located in the catalytic core domain outside of the active site, with R472Q located near the TDRD3 binding site (Figure 2A). The C666R mutation is located within the zinc finger domain (Figure 2A) and it has been recently shown by cryo‐EM that the C666 residue is within a D1C3‐type motif and coordinates a Zn²⁺ ion (Yang et al. 2025). Disease‐causing amino acid changes often occur in highly conserved residues (Miller and Kumar 2001)—however, only the P378 and C666 residues are conserved in TOP3B (Moreira et al. 2021). While the impact of the P378Q mutation on TOP3B has not been evaluated to our knowledge, Goto‐Ito et al. showed that the R472Q mutation did not affect the tertiary structure of TOP3B (Goto‐Ito et al. 2017) and Ahmed et al. assessed the impact of the R472Q and C666R mutations on TOP3B (Ahmad, Shen, et al. 2017). Both mutations showed normal localization and reduced association with TDRD3 and FMRP. Compared to WT TOP3B, the R472Q mutation displayed similar topoisomerase activity in vitro and similar mRNA binding capability in cells. However, the C666R mutation showed defective topoisomerase activity in vitro, diminished mRNA binding capability in cells, and expression of the corresponding mutation (C660R) in Drosophila only partially rescued the abnormal neuromuscular junction phenotype seen in a TOP3B null background that can be rescued by WT TOP3B; these data together suggest that C666R is a partial loss‐of‐function mutation (Ahmad, Shen, et al. 2017). Future studies are needed to further evaluate the effect of each mutation on TOP3B and determine if any observed effects are due to defects in TOP3B function on DNA or RNA. Although most of the literature has focused on the link of TOP3B with neurological disorders, increased TOP3B expression correlates with shorter overall survival in gastric cancer and better overall survival in epithelial ovarian carcinoma (Bai et al. 2016; Hou et al. 2020). TOP3B genetic deletion has also been linked to breast cancer and renal cancer (Oliveira‐Costa et al. 2010; Zhang et al. 2019). While TOP3B is not an essential gene, it is clear that disrupting TOP3B function or expression influences human biology and disease.

6.2. TOP3B in Complex With FMRP

In the pioneering studies of TOP3B, Xu et al. and Stoll et al. reported that FMRP co‐immunoprecipitates with TOP3B and TDRD3, suggesting that it associates to some degree with the TOP3B‐TDRD3 complex. Subsequent biochemical studies showed that TDRD3 binds TOP3B with its N‐terminal OB‐fold domain and FMRP with its Tudor and C‐terminal domains, with the Tudor domain likely interacting with the methyl‐arginine residues of the RGG box domain of FMRP (Figure 4D). These data, and that TOP3B‐FMRP co‐immunoprecipitation is eliminated in the absence or depletion of TDRD3, suggest that TDRD3 acts as a scaffold protein between TOP3B and FMRP to allow formation of the TOP3B‐TDRD3‐FMRP (TTF) complex (Stoll et al. 2013; Xu et al. 2013).

FMRP is known to regulate mRNA translation (Athar and Joseph 2020; Darnell et al. 2011; Scarpitti et al. 2022), and loss of FMRP causes Fragile X syndrome, the most common monogenic cause of autism and intellectual disability (Kremer et al. 1991; Niu et al. 2017; Verkerk et al. 1991). Its association with TOP3B provided one of the first indicators that TOP3B may play an important role in translational control and neurological disorders. Although TOP3B most likely forms an equimolar complex with TDRD3, only a fraction of FMRP is associated with the TTF complex (Stoll et al. 2013; Xu et al. 2013), suggesting that FMRP functions independently of the complex most of the time. Supporting this notion, Su et al. reported that TOP3B co‐regulates more mRNAs with TDRD3 than with FMRP at the post‐transcriptional level, and only 17% of TOP3B CLIP targets overlap with those of FMRP, although it is important to note that the CLIP datasets were obtained from different cell types (Darnell et al. 2011; Su et al. 2022). Despite the small amount of overlap, many of the shared CLIP targets are known autism and schizophrenia risk genes, suggesting that TOP3B may work with FMRP to regulate the translation of certain neuronal mRNAs important in neurological disorders. Interestingly, Xu et al. showed by evaluating neuromuscular junctions and expression of an mRNA targeted by both TOP3B and FMRP in single and double knockout Drosophila models that TOP3B and FMRP can function antagonistically or cooperatively and proposed a model in which the TOP3B‐TDRD3 complex can regulate some mRNAs independent of, and perhaps antagonistically with, FMRP, while other mRNAs are regulated cooperatively by the entire TOP3B‐TDRD3‐FMRP complex (Xu et al. 2013). However, to our knowledge, no direct data further supporting this model have been reported.

6.3. Animal Models

Multiple phenotypes are observed in TOP3B null animal models. TOP3B null flies demonstrate reduced neuromuscular performance, premature aging, and abnormal neuromuscular junctions (NMJs), a phenotype also seen in FMRP null flies, which can be rescued with ectopic TOP3B expression (Ahmad, Shen, et al. 2017; Teimuri and Suter 2025; Xu et al. 2013). TOP3B null mice also demonstrate reduced density of presynaptic vesicles and synapses, as well as abnormal hippocampal synaptic plasticity and neurogenesis (Joo et al. 2020; Xu et al. 2013; Zhu et al. 2024). Together, these data suggest that TOP3B is required for normal synapse formation in Drosophila and mice. TOP3B null mice also show reduced life span and autoimmunity (Cuarenta et al. 2023; Kwan et al. 2007; Kwan and Wang 2001). In addition to these phenotypes, Joo et al. reported that TOP3B null mice demonstrate many behavioral phenotypes related to psychiatric disorders and cognitive impairment. For example, TOP3B null mice demonstrated higher generalized anxiety levels and abnormal social interactions, which are phenotypes prevalent in patients with schizophrenia and autism, respectively, and also showed increased fear behaviors, which have been observed in some schizophrenia mouse models. TOP3B null mice also demonstrate impaired hippocampus‐dependent cognitive function and retention of spatial memory. Diminished learning and memory have been observed in schizophrenia and autism, supporting that loss of TOP3B may promote psychiatric and cognitive disorders. TOP3B null mice also have decreased transcription of neuronal‐early response genes in response to fear conditioning, suggesting that transcriptional regulation by TOP3B (directly or indirectly) may be important in preventing psychiatric and cognitive disorders (Joo et al. 2020). Zhu et al. later confirmed many of these results, and reported that TDRD3 null mice demonstrate overlapping but not identical phenotypes to TOP3B null mice, supporting independent functions outside of the TOP3B–TDRD3 complex (Zhu et al. 2024). Rahman et al. also reported that TOP3B null mice demonstrate decreased neuromuscular activities and altered connectivity in multiple brain regions, which have been previously implicated in neurodevelopmental, psychiatric, and behavioral disorders (Rahman et al. 2021).

It is also important to note that TOP3B null mice share some but not all phenotypes with other disease models, as TOP3B null mice perform normally in some behavior tests that schizophrenia and autism mouse models show abnormal behavior in (Brunskill et al. 2005; Joo et al. 2020; Schilit Nitenson et al. 2015). Further studies are needed to determine if the observed phenotypes arise from perturbations in TOP3B function on DNA or RNA; however, ectopic expression of an RGG box domain deletion mutant in TOP3B null flies was unable to rescue the abnormal NMJ phenotype as the WT version did (Ahmad, Shen, et al. 2017), suggesting that activity on RNA contributes to this phenotype.

7. Conclusion and Critical Questions Moving Forward

The apparent importance of an RNA topoisomerase in human neurophysiology represents a new facet to RNA biology that may have been previously underestimated. But it remains unclear why, in mammals at least, the brain appears to be more sensitive to TOP3B loss than other tissues. Presumably this is due to specific RNA substrates that are enriched in the brain, which is known to have a very high degree of alternative splicing and gene expression diversity compared to other tissues (Hawrylycz et al. 2015, 2012; Su et al. 2018). A key critical aspect of answering this question, albeit a big challenge, is defining TOP3B substrates in vivo or at least in neurons. Initial investigations have used CLIP‐based strategies in HeLa and HCT116 cells to start defining mRNAs targeted by TOP3B (Su et al. 2022; Xu et al. 2013); however, recent work using HCT116 cells as a model provided evidence that TOP3B may act to regulate translation in both a catalytically‐dependent and ‐independent manner (Su et al. 2022). The potential issue here is that the CLIP‐based methods are unable to distinguish between the two proposed classes of TOP3B mRNA targets. One possible approach that could be used to address this question is to enrich for the covalently linked TOP3B·RNA intermediate under strong denaturing conditions. This would begin to elucidate consensus sequences or structures targeted by TOP3B and potentially provide a clearer picture of TOP3B catalytically‐dependent mRNA translation.

Mechanistically, it is not reported yet how TOP3B selects or recognizes RNA targets. The RGG box domain, which severely aids binding and activity on RNA substrates in vitro, is certainly involved, but its exact influence is still unclear as RGG boxes are generally not thought to have specific consensus binding sites (Chowdhury and Jin 2023; Ghisolfi et al. 1992). For example, does the RGG box direct TOP3B targeting or does it stabilize TOP3B binding after the catalytic core and/or zinc finger domain binds a potential RNA substrate? Recent cryo‐EM structures using full‐length TOP3B‐TDRD3 complex with a DNA mismatch bubble and an R‐loop substrate show that upon DNA binding, the zinc finger domain moves to bind the DNA substrate upstream of the cleavage site, suggesting that this domain stabilizes TOP3B‐substrate complex formation (Yang et al. 2025). However, the RGG box domain was unable to be resolved in this structure, and structures with RNA‐bound substrates utilized truncated TOP3B (catalytic core only; deletion of the zinc finger and RGG box domains), so the precise role of the RGG box domain remains unclear. Recent advances in time‐resolved cryo‐EM may provide the field with an opportunity to capture and study the order of events during a complete reaction cycle (Kaledhonkar et al. 2019; Torino et al. 2023). To more fully understand why TOP3B is needed, it will be important for the field to decipher how different substrates are selected and whether they affect this cycle. Additionally, as TOP3B is catalytically active on both DNA and RNA substrates, an important outstanding question remains whether disease pathology is due to loss of TOP3B activity on DNA, RNA, or both. Generating animal models with RGG box domain deletions to drastically reduce TOP3B targeting to RNA may begin addressing this question.

Lastly, the RNA structures within mRNAs that are regulated by TOP3B remain enigmatic. This is, of course, closely tied to defining the RNA targets in vivo, but presumably TOP3B is altering topological strain within translating mRNAs. Given the advancements in RNA structure probing, which now includes reactivity measurements for all four nucleotides (Busan et al. 2019; Merino et al. 2005), the field may have an opportunity to more directly test how RNA structure changes in the presence and absence of TOP3B. In combination with inhibiting translation, a more complete picture of the extent to which TOP3B is affecting structure during active translation may be elucidated. Together, these key insights will push the field into better defining how TOP3B influences post‐transcriptional gene regulation and why its deletion or mutation contributes to neurological disorders.

Author Contributions

Julia E. Warrick: conceptualization (equal), writing – original draft (equal), writing – review and editing (equal). Michael G. Kearse: conceptualization (equal), writing – original draft (equal), writing – review and editing (equal).

Conflicts of Interest

The authors declare no conflicts of interest.

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Warrick, J. E. , and Kearse M. G.. 2025. “Unraveling the Role of Topoisomerase 3β (TOP3B) in mRNA Translation and Human Disease.” Wiley Interdisciplinary Reviews: RNA 16, no. 4: e70020. 10.1002/wrna.70020.

Data Availability Statement

Data sharing is not applicable to this article as no new data were created or analyzed in this study.