Topoisomerase 3β interacts with RNAi machinery to promote heterochromatin formation and transcriptional silencing in Drosophila

Insights into the DNA and RNA Interactions of Human Topoisomerase III Beta Using Molecular Dynamics Simulations

Human topoisomerase III beta (hTOP3B) is the only topoisomerase in the human cell that can act on both DNA and RNA substrates. Recent findings have emphasized the physiological importance of hTOP3B and consolidated it as a valuable drug target for antiviral and anticancer therapeutics. Although type IA topoisomerases of different organisms have been studied over the years, the step-by-step interaction of hTOP3B and nucleic acid substrates is still not well understood. Due to the lack of hTOP3B-RNA structures as well as DNA/RNA covalent complexes, computational investigations have been limited. In our study, we utilized molecular dynamics (MD) simulations to study the interactions between hTOP3B and nucleic acids to get a closer look into the residues that play a role in binding DNA or RNA and facilitate catalysis, along with the differences and similarities when hTOP3B interacts with DNA compared to RNA. For this, we generated multiple models of hTOP3B complexed with DNA and RNA sequences using the hTOP3B crystal structure and 8-mer single-stranded DNA and RNA sequences. These models include both covalent and noncovalent complexes, which are then subjected to MD simulations and analyzed. Our findings highlight the complexes' stability, sequence preference, and interactions of the binding pocket residues with different nucleotides. Our work demonstrates that hTOP3B forms stable complexes with both DNA and RNA and provides a better understanding of the enzyme's interaction with different nucleic acid substrate sequences.

Variation of Structure and Cellular Functions of Type IA Topoisomerases across the Tree of Life

Topoisomerases regulate the topological state of cellular genomes to prevent impediments to vital cellular processes, including replication and transcription from suboptimal supercoiling of double-stranded DNA, and to untangle topological barriers generated as replication or recombination intermediates. The subfamily of type IA topoisomerases are the only topoisomerases that can alter the interlinking of both DNA and RNA. In this article, we provide a review of the mechanisms by which four highly conserved N-terminal protein domains fold into a toroidal structure, enabling cleavage and religation of a single strand of DNA or RNA. We also explore how these conserved domains can be combined with numerous non-conserved protein sequences located in the C-terminal domains to form a diverse range of type IA topoisomerases in Archaea, Bacteria, and Eukarya. There is at least one type IA topoisomerase present in nearly every free-living organism. The variation in C-terminal domain sequences and interacting partners such as helicases enable type IA topoisomerases to conduct important cellular functions that require the passage of nucleic acids through the break of a single-strand DNA or RNA that is held by the conserved N-terminal toroidal domains. In addition, this review will exam a range of human genetic disorders that have been linked to the malfunction of type IA topoisomerase.

Tdrd3-null mice show post-transcriptional and behavioral impairments associated with neurogenesis and synaptic plasticity

The Topoisomerase 3B (Top3b) - Tudor domain containing 3 (Tdrd3) protein complex is the only dual-activity topoisomerase complex that can alter both DNA and RNA topology in animals. TOP3B mutations in humans are associated with schizophrenia, autism and cognitive disorders; and Top3b-null mice exhibit several phenotypes observed in animal models of psychiatric and cognitive disorders, including impaired cognitive and emotional behaviors, aberrant neurogenesis and synaptic plasticity, and transcriptional defects. Similarly, human TDRD3 genomic variants have been associated with schizophrenia, verbal short-term memory and educational attainment. However, the importance of Tdrd3 in normal brain function has not been examined in animal models. Here we generated a Tdrd3-null mouse strain and demonstrate that these mice display both shared and unique defects when compared to Top3b-null mice. Shared defects were observed in cognitive behaviors, synaptic plasticity, adult neurogenesis, newborn neuron morphology, and neuronal activity-dependent transcription; whereas defects unique to Tdrd3-deficient mice include hyperactivity, changes in anxiety-like behaviors, olfaction, increased new neuron complexity, and reduced myelination. Interestingly, multiple genes critical for neurodevelopment and cognitive function exhibit reduced levels in mature but not nascent transcripts. We infer that the entire Top3b-Tdrd3 complex is essential for normal brain function, and that defective post-transcriptional regulation could contribute to cognitive and psychiatric disorders.

RNA-binding protein Nocte regulates Drosophila development by promoting translation reinitiation on mRNAs with long upstream open reading frames

RNA-binding proteins (RBPs) with intrinsically disordered regions (IDRs) are linked to multiple human disorders, but their mechanisms of action remain unclear. Here, we report that one such protein, Nocte, is essential for Drosophila eye development by regulating a critical gene expression cascade at translational level. Knockout of nocte in flies leads to lethality, and its eye-specific depletion impairs eye size and morphology. Nocte preferentially enhances translation of mRNAs with long upstream open reading frames (uORFs). One of the key Nocte targets, glass mRNA, encodes a transcription factor critical for differentiation of photoreceptor neurons and accessory cells, and re-expression of Glass largely rescued the eye defects caused by Nocte depletion. Mechanistically, Nocte counteracts long uORF-mediated translational suppression by promoting translation reinitiation downstream of the uORF. Nocte interacts with translation factors eIF3 and Rack1 through its BAT2 domain, and a Nocte mutant lacking this domain fails to promote translation of glass mRNA. Notably, de novo mutations of human orthologs of Nocte have been detected in schizophrenia patients. Our data suggest that Nocte family of proteins can promote translation reinitiation to overcome long uORFs-mediated translational suppression, and disruption of this function can lead to developmental defects and neurological disorders.

The TDRD3-USP9X complex and MIB1 regulate TOP3B homeostasis and prevent deleterious TOP3B cleavage complexes

TOP3B is stabilized by TDRD3. Hypothesizing that TDRD3 recruits a deubiquitinase, we find that TOP3B interacts with USP9X via TDRD3. Inactivation of USP9X destabilizes TOP3B, and depletion of both TDRD3 and USP9X does not promote further TOP3B ubiquitylation. Additionally, we observe that MIB1 mediates the ubiquitylation and proteasomal degradation of TOP3B by directly interacting with TOP3B independently of TDRD3. Combined depletion of USP9X, TDRD3 and MIB1 causes no additional increase in TOP3B levels compared to MIB1 knockdown alone indicating that the TDRD3-USP9X complex works downstream of MIB1. To comprehend why cells degrade TOP3B in the absence of TDRD3, we measured TOP3Bccs. Lack of TDRD3 increases TOP3Bccs in DNA and RNA, and induced R-loops, γH2AX and growth defect. Biochemical experiments confirm that TDRD3 increases the turnover of TOP3B. Our work provides molecular insights into the mechanisms by which TDRD3 protect cells from deleterious TOP3Bccs which are otherwise removed by TRIM41.

A dual-activity topoisomerase complex promotes both transcriptional activation and repression in response to starvation

Topoisomerases are required to release topological stress generated by RNA polymerase II (RNAPII) during transcription. Here, we show that in response to starvation, the complex of topoisomerase 3b (TOP3B) and TDRD3 can enhance not only transcriptional activation, but also repression, which mimics other topoisomerases that can also alter transcription in both directions. The genes enhanced by TOP3B-TDRD3 are enriched with long and highly-expressed ones, which are also preferentially stimulated by other topoisomerases, suggesting that different topoisomerases may recognize their targets through a similar mechanism. Specifically, human HCT116 cells individually inactivated for TOP3B, TDRD3 or TOP3B topoisomerase activity, exhibit similarly disrupted transcription for both starvation-activated genes (SAGs) and starvation-repressed genes (SRGs). Responding to starvation, both TOP3B-TDRD3 and the elongating form of RNAPII exhibit concomitantly increased binding to TOP3B-dependent SAGs, at binding sites that overlap. Notably, TOP3B inactivation decreases the binding of elongating RNAPII to TOP3B-dependent SAGs while increased it to SRGs. Furthermore, TOP3B-ablated cells display reduced transcription of several autophagy-associated genes and autophagy per se. Our data suggest that TOP3B-TDRD3 can promote both transcriptional activation and repression by regulating RNAPII distribution. In addition, the findings that it can facilitate autophagy may account for the shortened lifespan of Top3b-KO mice.

Tdrd3-null mice show post-transcriptional and behavioral impairments associated with neurogenesis and synaptic plasticity

The Topoisomerase 3B (Top3b) - Tudor domain containing 3 (Tdrd3) protein complex is the only dual-activity topoisomerase complex in animals that can alter the topology of both DNA and RNA. TOP3B mutations in humans are associated with schizophrenia, autism and cognitive disorders; and Top3b-null mice exhibit several phenotypes observed in animal models of psychiatric and cognitive disorders, including impairments in cognitive and emotional behaviors, aberrant neurogenesis and synaptic plasticity, and transcriptional defects. Similarly, human TDRD3 genomic variants have been associated with schizophrenia, verbal shorten-memory and learning, and educational attainment. However, the importance of Tdrd3 in normal brain function has not been examined in animal models. Here we built a Tdrd3-null mouse strain and demonstrate that these mice display both shared and unique defects when compared to Top3b-null mice. Shared defects were observed in cognitive behaviors, synaptic plasticity, adult neurogenesis, newborn neuron morphology, and neuronal activity-dependent transcription; whereas defects unique to Tdrd3-deficient mice include hyperactivity, changes in anxiety-like behaviors, increased new neuron complexity, and reduced myelination. Interestingly, multiple genes critical for neurodevelopment and cognitive function exhibit reduced levels in mature but not nascent transcripts. We infer that the entire Top3b-Tdrd3 complex is essential for normal brain function, and that defective post-transcriptional regulation could contribute to cognitive impairment and psychiatric disorders.

Nuclear architecture and the structural basis of mitotic memory

The nucleus is a complex organelle that hosts the genome and is essential for vital processes like DNA replication, DNA repair, transcription, and splicing. The genome is non-randomly organized in the three-dimensional space of the nucleus. This functional sub-compartmentalization was thought to be organized on the framework of nuclear matrix (NuMat), a non-chromatin scaffold that functions as a substratum for various molecular processes of the nucleus. More recently, nuclear bodies or membrane-less subcompartments of the nucleus are thought to arise due to phase separation of chromatin, RNA, and proteins. The nuclear architecture is an amalgamation of the relative organization of chromatin, epigenetic landscape, the nuclear bodies, and the nucleoskeleton in the three-dimensional space of the nucleus. During mitosis, the nucleus undergoes drastic changes in morphology to the degree that it ceases to exist as such; various nuclear components, including the envelope that defines the nucleus, disintegrate, and the chromatin acquires mitosis-specific epigenetic marks and condenses to form chromosome. Upon mitotic exit, chromosomes are decondensed, re-establish hierarchical genome organization, and regain epigenetic and transcriptional status similar to that of the mother cell. How this mitotic memory is inherited during cell division remains a puzzle. NuMat components that are a part of the mitotic chromosome in the form of mitotic chromosome scaffold (MiCS) could potentially be the seeds that guide the relative re-establishment of the epigenome, chromosome territories, and the nuclear bodies. Here, we synthesize the advances towards understanding cellular memory of nuclear architecture across mitosis and propose a hypothesis that a subset of NuMat proteome essential for nucleation of various nuclear bodies are retained in MiCS to serve as seeds of mitotic memory, thus ensuring the daughter cells re-establish the complex status of nuclear architecture similar to that of the mother cells, thereby maintaining the pre-mitotic transcriptional status.

Resolution of R-loops by topoisomerase III-β (TOP3B) in coordination with the DEAD-box helicase DDX5

The present study demonstrates how TOP3B is involved in resolving R-loops. We observed elevated R-loops in TOP3B knockout cells (TOP3BKO), which are suppressed by TOP3B transfection. R-loop-inducing agents, the topoisomerase I inhibitor camptothecin, and the splicing inhibitor pladienolide-B also induce higher R-loops in TOP3BKO cells. Camptothecin- and pladienolide-B-induced R-loops are concurrent with the induction of TOP3B cleavage complexes (TOP3Bccs). RNA/DNA hybrid IP-western blotting show that TOP3B is physically associated with R-loops. Biochemical assays using recombinant TOP3B and oligonucleotides mimicking R-loops show that TOP3B cleaves the single-stranded DNA displaced by the R-loop RNA-DNA duplex. IP-mass spectrometry and IP-western experiments reveal that TOP3B interacts with the R-loop helicase DDX5 independently of TDRD3. Finally, we demonstrate that DDX5 and TOP3B are epistatic in resolving R-loops in a pathway parallel with senataxin. We propose a decatenation model for R-loop resolution by TOP3B-DDX5 protecting cells from R-loop-induced damage.

A dual-activity topoisomerase complex regulates mRNA translation and turnover

Topoisomerase 3β (TOP3B) and TDRD3 form a dual-activity topoisomerase complex that interacts with FMRP and can change the topology of both DNA and RNA. Here, we investigated the post-transcriptional influence of TOP3B and associated proteins on mRNA translation and turnover. First, we discovered that in human HCT116 colon cancer cells, knock-out (KO) of TOP3B had similar effects on mRNA turnover and translation as did TDRD3-KO, while FMRP-KO resulted in rather distinct effects, indicating that TOP3B had stronger coordination with TDRD3 than FMRP in mRNA regulation. Second, we identified TOP3B-bound mRNAs in HCT116 cells; we found that while TOP3B did not directly influence the stability or translation of most TOP3B target mRNAs, it stabilized a subset of target mRNAs but had a more complex effect on translation-enhancing for some mRNAs whereas reducing for others. Interestingly, a point mutation that specifically disrupted TOP3B catalytic activity only partially recapitulated the effects of TOP3B-KO on mRNA stability and translation, suggesting that the impact of TOP3B on target mRNAs is partly linked to its ability to change topology of mRNAs. Collectively, our data suggest that TOP3B-TDRD3 can regulate mRNA translation and turnover by mechanisms that are dependent and independent of topoisomerase activity.

Transposable element landscapes in aging Drosophila

Genetic mechanisms that repress transposable elements (TEs) in young animals decline during aging, as reflected by increased TE expression in aged animals. Does increased TE expression during aging lead to more genomic TE copies in older animals? To address this question, we quantified TE Landscapes (TLs) via whole genome sequencing of young and aged Drosophila strains of wild-type and mutant backgrounds. We quantified TLs in whole flies and dissected brains and validated the feasibility of our approach in detecting new TE insertions in aging Drosophila genomes when small RNA and RNA interference (RNAi) pathways are compromised. We also describe improved sequencing methods to quantify extra-chromosomal DNA circles (eccDNAs) in Drosophila as an additional source of TE copies that accumulate during aging. Lastly, to combat the natural progression of aging-associated TE expression, we show that knocking down PAF1, a conserved transcription elongation factor that antagonizes RNAi pathways, may bolster suppression of TEs during aging and extend lifespan. Our study suggests that in addition to a possible influence by different genetic backgrounds, small RNA and RNAi mechanisms may mitigate genomic TL expansion despite the increase in TE transcripts during aging.

Transposable elements, also called transposons, are genetic parasites found in all animal genomes. Normally, transposons are compacted away in silent chromatin in young animals. But, as animals age and transposon-silencing defense mechanisms break down, transposon RNAs accumulate to significant levels in old animals like fruit flies. An open question is whether the increased levels of transposon RNAs in older animals also correspond to increased genomic copies of transposons. This study approached this question by sequencing the whole genomes of young and old wild-type and mutant flies lacking a functional RNA interference (RNAi) pathway, which naturally silences transposon RNAs. Although the wild-type flies with intact RNAi activity had little new accumulation of transposon copies, the sequencing approach was able to detect several transposon accumulation occurrences in some RNAi mutants. In addition, we found that some fly transposon families can also accumulate as extra-chromosomal circular DNA copies. Lastly, we showed that genetically augmenting the expression of RNAi factors can counteract the rising transposon RNA levels in aging and promote longevity. This study improves our understanding of the animal host genome relationship with transposons during natural aging processes.

Transcription-associated DNA breaks and cancer: A matter of DNA topology

Transcription is an essential cellular process but also a major threat to genome integrity. Transcription-associated DNA breaks are particularly detrimental as their defective repair can induce gene mutations and oncogenic chromosomal translocations, which are hallmarks of cancer. The past few years have revealed that transcriptional breaks mainly originate from DNA topological problems generated by the transcribing RNA polymerases. Defective removal of transcription-induced DNA torsional stress impacts on transcription itself and promotes secondary DNA structures, such as R-loops, which can induce DNA breaks and genome instability. Paradoxically, as they relax DNA during transcription, topoisomerase enzymes introduce DNA breaks that can also endanger genome integrity. Stabilization of topoisomerases on chromatin by various anticancer drugs or by DNA alterations, can interfere with transcription machinery and cause permanent DNA breaks and R-loops. Here, we review the role of transcription in mediating DNA breaks, and discuss how deregulation of topoisomerase activity can impact on transcription and DNA break formation, and its connection with cancer.

The conserved alternative splicing factor caper regulates neuromuscular phenotypes during development and aging

RNA-binding proteins play an important role in the regulation of post-transcriptional gene expression throughout the nervous system. This is underscored by the prevalence of mutations in genes encoding RNA splicing factors and other RNA-binding proteins in a number of neurodegenerative and neurodevelopmental disorders. The highly conserved alternative splicing factor Caper is widely expressed throughout the developing embryo and functions in the development of various sensory neural subtypes in the Drosophila peripheral nervous system. Here we find that caper dysfunction leads to aberrant neuromuscular junction morphogenesis, as well as aberrant locomotor behavior during larval and adult stages. Despite its widespread expression, our results indicate that caper function is required to a greater extent within the nervous system, as opposed to muscle, for neuromuscular junction development and for the regulation of adult locomotor behavior. Moreover, we find that Caper interacts with the RNA-binding protein Fmrp to regulate adult locomotor behavior. Finally, we show that caper dysfunction leads to various phenotypes that have both a sex and age bias, both of which are commonly seen in neurodegenerative disorders in humans.

The molecular biology of FMRP: new insights into fragile X syndrome

Fragile X mental retardation protein (FMRP) is the product of the fragile X mental retardation 1 gene (FMR1), a gene that - when epigenetically inactivated by a triplet nucleotide repeat expansion - causes the neurodevelopmental disorder fragile X syndrome (FXS). FMRP is a widely expressed RNA-binding protein with activity that is essential for proper synaptic plasticity and architecture, aspects of neural function that are known to go awry in FXS. Although the neurophysiology of FXS has been described in remarkable detail, research focusing on the molecular biology of FMRP has only scratched the surface. For more than two decades, FMRP has been well established as a translational repressor; however, recent whole transcriptome and translatome analyses in mouse and human models of FXS have shown that FMRP is involved in the regulation of nearly all aspects of gene expression. The emerging mechanistic details of the mechanisms by which FMRP regulates gene expression may offer ways to design new therapies for FXS.

DNA and RNA Cleavage Complexes and Repair Pathway for TOP3B RNA- and DNA-Protein Crosslinks

The present study demonstrates that topoisomerase 3B (TOP3B) forms both RNA and DNA cleavage complexes (TOP3Bccs) in vivo and reveals a pathway for repairing TOP3Bccs. For inducing and detecting cellular TOP3Bccs, we engineer a “self-trapping” mutant of TOP3B (R338W-TOP3B). Transfection with R338W-TOP3B induces R-loops, genomic damage, and growth defect, which highlights the importance of TOP3Bcc repair mechanisms. To determine how cells repair TOP3Bccs, we deplete tyrosyl-DNA phosphodiesterases (TDP1 and TDP2). TDP2-deficient cells show elevated TOP3Bccs both in DNA and RNA. Conversely, overexpression of TDP2 lowers cellular TOP3Bccs. Using recombinant human TDP2, we demonstrate that TDP2 can process both denatured and proteolyzed TOP3Bccs. We also show that cellular TOP3Bccs are ubiquitinated by the E3 ligase TRIM41 before undergoing proteasomal processing and excision by TDP2.

Saha et al. introduce an approach to generate and detect the catalytic intermediates of TOP3B in DNA and RNA by engineering a self-poisoning enzyme, R338W-TOP3B. They reveal the cellular consequences of abortive TOP3Bcc formation and a repair pathway involving TRIM41, the proteasome, and TDP2 for processing of TOP3Bcc.

Mechanism of Type IA Topoisomerases

Topoisomerases in the type IA subfamily can catalyze change in topology for both DNA and RNA substrates. A type IA topoisomerase may have been present in a last universal common ancestor (LUCA) with an RNA genome. Type IA topoisomerases have since evolved to catalyze the resolution of topological barriers encountered by genomes that require the passing of nucleic acid strand(s) through a break on a single DNA or RNA strand. Here, based on available structural and biochemical data, we discuss how a type IA topoisomerase may recognize and bind single-stranded DNA or RNA to initiate its required catalytic function. Active site residues assist in the nucleophilic attack of a phosphodiester bond between two nucleotides to form a covalent intermediate with a 5'-phosphotyrosine linkage to the cleaved nucleic acid. A divalent ion interaction helps to position the 3'-hydroxyl group at the precise location required for the cleaved phosphodiester bond to be rejoined following the passage of another nucleic acid strand through the break. In addition to type IA topoisomerase structures observed by X-ray crystallography, we now have evidence from biophysical studies for the dynamic conformations that are required for type IA topoisomerases to catalyze the change in the topology of the nucleic acid substrates.

Topoisomerase 3β knockout mice show transcriptional and behavioural impairments associated with neurogenesis and synaptic plasticity

Topoisomerase 3β (Top3β) is the only dual-activity topoisomerase in animals that can change topology for both DNA and RNA, and facilitate transcription on DNA and translation on mRNAs. Top3β mutations have been linked to schizophrenia, autism, epilepsy, and cognitive impairment. Here we show that Top3β knockout mice exhibit behavioural phenotypes related to psychiatric disorders and cognitive impairment. The mice also display impairments in hippocampal neurogenesis and synaptic plasticity. Notably, the brains of the mutant mice exhibit impaired global neuronal activity-dependent transcription in response to fear conditioning stress, and the affected genes include many with known neuronal functions. Our data suggest that Top3β is essential for normal brain function, and that defective neuronal activity-dependent transcription may be a mechanism by which Top3β deletion causes cognitive impairment and psychiatric disorders.

Mechanistic insights from structure of Mycobacterium smegmatis topoisomerase I with ssDNA bound to both N- and C-terminal domains

Type IA topoisomerases interact with G-strand and T-strand ssDNA to regulate DNA topology. However, simultaneous binding of two ssDNA segments to a type IA topoisomerase has not been observed previously. We report here the crystal structure of a type IA topoisomerase with ssDNA segments bound in opposite polarity to the N- and C-terminal domains. Titration of small ssDNA oligonucleotides to Mycobacterium smegmatis topoisomerase I with progressive C-terminal deletions showed that the C-terminal region has higher affinity for ssDNA than the N-terminal active site. This allows the C-terminal domains to capture one strand of underwound negatively supercoiled DNA substrate first and position the N-terminal domains to bind and cleave the opposite strand in the relaxation reaction. Efficiency of negative supercoiling relaxation increases with the number of domains that bind ssDNA primarily with conserved aromatic residues and possibly with assistance from polar/basic residues. A comparison of bacterial topoisomerase I structures showed that a conserved transesterification unit (N-terminal toroid structure) for cutting and rejoining of a ssDNA strand can be combined with two different types of C-terminal ssDNA binding domains to form diverse bacterial topoisomerase I enzymes that are highly efficient in their physiological role of preventing excess negative supercoiling in the genome.

The many lives of type IA topoisomerases

The double-helical structure of genomic DNA is both elegant and functional in that it serves both to protect vulnerable DNA bases and to facilitate DNA replication and compaction. However, these design advantages come at the cost of having to evolve and maintain a cellular machinery that can manipulate a long polymeric molecule that readily becomes topologically entangled whenever it has to be opened for translation, replication, or repair. If such a machinery fails to eliminate detrimental topological entanglements, utilization of the information stored in the DNA double helix is compromised. As a consequence, the use of B-form DNA as the carrier of genetic information must have co-evolved with a means to manipulate its complex topology. This duty is performed by DNA topoisomerases, which therefore are, unsurprisingly, ubiquitous in all kingdoms of life. In this review, we focus on how DNA topoisomerases catalyze their impressive range of DNA-conjuring tricks, with a particular emphasis on DNA topoisomerase III (TOP3). Once thought to be the most unremarkable of topoisomerases, the many lives of these type IA topoisomerases are now being progressively revealed. This research interest is driven by a realization that their substrate versatility and their ability to engage in intimate collaborations with translocases and other DNA-processing enzymes are far more extensive and impressive than was thought hitherto. This, coupled with the recent associations of TOP3s with developmental and neurological pathologies in humans, is clearly making us reconsider their undeserved reputation as being unexceptional enzymes.

Loss of TOP3B leads to increased R-loop formation and genome instability

Topoisomerase III beta (TOP3B) is one of the least understood members of the topoisomerase family of proteins and remains enigmatic. Our recent data shed light on the function and relevance of TOP3B to disease. A homozygous deletion for the TOP3B gene was identified in a patient with bilateral renal cancer. Analyses in both patient and modelled human cells show the disruption of TOP3B causes genome instability with a rise in DNA damage and chromosome bridging (mis-segregation). The primary molecular defect underlying this pathology is a significant increase in R-loop formation. Our data show that TOP3B is necessary to prevent the accumulation of excessive R-loops and identify TOP3B as a putative cancer gene, and support recent data showing that R-loops are involved in cancer aetiology.

Roles of Topoisomerases in Heterochromatin, Aging, and Diseases

Heterochromatin is a transcriptionally repressive chromatin architecture that has a low abundance of genes but an enrichment of transposons. Defects in heterochromatin can cause the de-repression of genes and transposons, leading to deleterious physiological changes such as aging, cancer, and neurological disorders. While the roles of topoisomerases in many DNA-based processes have been investigated and reviewed, their roles in heterochromatin formation and function are only beginning to be understood. In this review, we discuss recent findings on how topoisomerases can promote heterochromatin organization and impact the transcription of genes and transposons. We will focus on two topoisomerases: Top2α, which catenates and decatenates double-stranded DNA, and Top3β, which can change the topology of not only DNA, but also RNA. Both enzymes are required for normal heterochromatin formation and function, as the inactivation of either protein by genetic mutations or chemical inhibitors can result in defective heterochromatin formation and the de-silencing of transposons. These defects may contribute to the shortened lifespan and neurological disorders observed in individuals carrying mutations of Top3β. We propose that topological stress may be generated in both DNA and RNA during heterochromatin formation and function, which depend on multiple topoisomerases to resolve.