Export and stability of naturally intronless mRNAs require specific coding region sequences and the TREX mRNA export complex

Single-molecule live-cell RNA imaging with CRISPR-Csm

Understanding the diverse dynamic behaviors of individual RNA molecules in single cells requires visualizing them at high resolution in real time. However, single-molecule live-cell imaging of unmodified endogenous RNA has not yet been achieved in a generalizable manner. Here, we present single-molecule live-cell fluorescence in situ hybridization (smLiveFISH), a robust approach that combines the programmable RNA-guided, RNA-targeting CRISPR-Csm complex with multiplexed guide RNAs for direct and efficient visualization of single RNA molecules in a range of cell types, including primary cells. Using smLiveFISH, we track individual native NOTCH2 and MAP1B transcripts in living cells and identify two distinct localization mechanisms including the cotranslational translocation of NOTCH2 mRNA at the endoplasmic reticulum and directional transport of MAP1B mRNA toward the cell periphery. This method has the potential to unlock principles governing the spatiotemporal organization of native transcripts in health and disease.

The SARS-CoV-2 ORF6 protein inhibits nuclear export of mRNA and spliceosomal U snRNA

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is the causative agent of coronavirus disease 19 (COVID-19). SARS-CoV-2 infection suppresses host innate immunity and impairs cell viability. Among the viral proteins, ORF6 exhibits potent interferon (IFN) antagonistic activity and cellular toxicity. It also interacts with the RNA export factor RAE1, which bridges the nuclear pore complex and nuclear export receptors, suggesting an effect on RNA export. Using the Xenopus oocyte microinjection system, I found that ORF6 blocked the export of not only mRNA but also spliceosomal U snRNA. I further demonstrated that ORF6 affects the interaction between RAE1 and nuclear export receptors and inhibits the RNA binding of RAE1. These effects of ORF6 may cumulatively block the export of several classes of RNA. I also found that ORF6 binds RNA and forms oligomers. These findings provide insights into the suppression of innate immune responses and the reduction in cell viability caused by SARS-CoV-2 infection, contributing to the development of antiviral drugs targeting ORF6.

Exportin-5 binding precedes 5'- and 3'-end processing of tRNA precursors in Drosophila

Exportin5 (Exp5) is the major miRNA nuclear export factor and recognizes structural features of pre-miRNA hairpins, while it also exports other minihelix-containing RNAs. In Drosophila, Exp5 is suggested to play a major role in tRNA export because the gene encoding the canonical tRNA export factor Exportin-t is missing in its genome. To understand molecular functions of fly Exp5, we studied the Exp5/RNA interactome in the cell line S2R + using the crosslinking and immunoprecipitation (CLIP) technology. The CLIP experiment captured known substrates such as tRNAs and miRNAs and detected candidates of novel Exp5 substrates including various mRNAs and long non-coding RNAs (lncRNAs). Some mRNAs and lncRNAs enriched PAR-CLIP tags compared to their expression levels, suggesting selective binding of Exp5 to them. Intronless mRNAs tended to enrich PAR-CLIP tags; therefore, we proposed that Exp5 might play a role in the export of specific classes of mRNAs/lncRNAs. This result suggested that Drosophila Exp5 might have a wider variety of substrates than initially thought. Surprisingly, Exp5 CLIP reads often contained sequences corresponding to the flanking 5'-leaders and 3'-trailers of tRNAs, which were thought to be removed prior to nuclear export. In fact, we found pre-tRNAs before end-processing were present in the cytoplasm, supporting the idea that tRNA end-processing is a cytoplasmic event. In summary, our results provide a genome-wide list of Exp5 substrate candidates and suggest that flies may lack a mechanism to distinguish pre-tRNAs with or without the flanking sequences.

The GC-content at the 5' ends of human protein-coding genes is undergoing mutational decay

BACKGROUND

In vertebrates, most protein-coding genes have a peak of GC-content near their 5' transcriptional start site (TSS). This feature promotes both the efficient nuclear export and translation of mRNAs. Despite the importance of GC-content for RNA metabolism, its general features, origin, and maintenance remain mysterious. We investigate the evolutionary forces shaping GC-content at the transcriptional start site (TSS) of genes through both comparative genomic analysis of nucleotide substitution rates between different species and by examining human de novo mutations.

RESULTS

Our data suggests that GC-peaks at TSSs were present in the last common ancestor of amniotes, and likely that of vertebrates. We observe that in apes and rodents, where recombination is directed away from TSSs by PRDM9, GC-content at the 5' end of protein-coding gene is currently undergoing mutational decay. In canids, which lack PRDM9 and perform recombination at TSSs, GC-content at the 5' end of protein-coding is increasing. We show that these patterns extend into the 5' end of the open reading frame, thus impacting synonymous codon position choices.

CONCLUSIONS

Our results indicate that the dynamics of this GC-peak in amniotes is largely shaped by historic patterns of recombination. Since decay of GC-content towards the mutation rate equilibrium is the default state for non-functional DNA, the observed decrease in GC-content at TSSs in apes and rodents indicates that the GC-peak is not being maintained by selection on most protein-coding genes in those species.

Single-molecule live-cell RNA imaging with CRISPR-Csm

High-resolution, real-time imaging of RNA is essential for understanding the diverse, dynamic behaviors of individual RNA molecules in single cells. However, single-molecule live-cell imaging of unmodified endogenous RNA has not yet been achieved. Here, we present single-molecule live-cell fluorescence in situ hybridization (smLiveFISH), a robust approach that combines the programmable RNA-guided, RNA-targeting CRISPR-Csm complex with multiplexed guide RNAs for efficient, direct visualization of single RNA molecules in a range of cell types, including primary cells. Using smLiveFISH, we tracked individual endogenous NOTCH2 and MAP1B mRNA transcripts in living cells and identified two distinct localization mechanisms: co-translational translocation of NOTCH2 mRNA at the endoplasmic reticulum, and directional transport of MAP1B mRNA toward the cell periphery. This method has the potential to unlock principles governing the spatiotemporal organization of native transcripts in health and disease.

Archaeological approaches to RNA virus evolution

Studies with RNA enzymes (ribozymes) and protein enzymes have identified certain structural elements that are present in some cellular mRNAs and viral RNAs. These elements do not share a primary structure and, thus, are not phylogenetically related. However, they have common (secondary/tertiary) structural folds that, according to some lines of evidence, may have an ancient and common origin. The term 'mRNA archaeology' has been coined to refer to the search for such structural/functional relics that may be informative of early evolutionary developments in the cellular and viral worlds and have lasted to the present day. Such identified RNA elements may have developed as biological signals with structural and functional relevance (as if they were buried objects with archaeological value), and coexist with the standard linear information of nucleic acid molecules that is translated into proteins. However, there is a key difference between the methods that extract information from either the primary structure of mRNA or the signals provided by secondary and tertiary structures. The former (sequence comparison and phylogenetic analysis) requires strict continuity of the material vehicle of information during evolution, whereas the archaeological method does not require such continuity. The tools of RNA archaeology (including the use of ribozymes and enzymes to investigate the reactivity of the RNA elements) establish links between the concepts of communication and language theories that have not been incorporated into knowledge of virology, as well as experimental studies on the search for functionally relevant RNA structures.

mRNA nuclear export: how mRNA identity features distinguish functional RNAs from junk transcripts

The division of the cellular space into nucleoplasm and cytoplasm promotes quality control mechanisms that prevent misprocessed mRNAs and junk RNAs from gaining access to the translational machinery. Here, we explore how properly processed mRNAs are distinguished from both misprocessed mRNAs and junk RNAs by the presence or absence of various 'identity features'.

Viruses as archaeological tools for uncovering ancient molecular relationships

The entry of a virus into the host cell always implies the alteration of certain intracellular molecular relationships, some of which may involve the recovery of ancient cellular activities. In this sense, viruses are archaeological tools for identifying unexpressed activities in noninfected cells. Among these, activities that hinder virus propagation may represent cellular defense mechanisms, for example, activities that mutagenize the viral genome such as ADAR-1 or APOBEC activities. Instead, those that facilitate virus propagation can be interpreted as the result of viral adaptation to-or mimicking-cellular structures, enabling the virus to perform anthropomorphic activities, including hijacking, manipulating, and reorganizing cellular factors for their own benefit. The alternative we consider here is that some of these second set of cellular activities were already in the uninfected cell but silenced, under the negative control of the cell or lineage, and that they represent a necessary precondition for viral infection. For example, specifically loading an amino acid at the 3'-end of the mRNA of some plant viruses by aminoacyl-tRNA synthetases has proved essential for virus infection despite this reaction not occurring with cellular mRNAs. Other activities of this type are discussed here, together with the biological context in which they acquire a coherent meaning, that is, genetic latency and molecular conflict.

Herpes Simplex Virus-1 ICP27 Nuclear Export Signal Mutants Exhibit Cell Type-Dependent Deficits in Replication and ICP4 Expression

Herpes simplex virus type-1 (HSV-1) protein ICP27 is an essential immediate early (IE) protein that promotes the expression of viral early (E) and late (L) genes via multiple mechanisms. Our understanding of this complex regulatory protein has been greatly enhanced by the characterization of HSV-1 mutants bearing engineered alterations in the ICP27 gene. However, much of this analysis has been performed in interferon-deficient Vero monkey cells. Here, we assessed the replication of a panel of ICP27 mutants in several other cell types. Our analysis shows that mutants lacking ICP27's amino (N)-terminal nuclear export signal (NES) display a striking cell type-dependent growth phenotype, i.e., they grow semi-permissively in Vero and some other cells but are tightly blocked for replication in primary human fibroblasts and multiple human cell lines. This tight growth defect correlates with a failure of these mutants to replicate viral DNA. We also report that HSV-1 NES mutants are deficient in expressing the IE protein ICP4 at early times postinfection. Analysis of viral RNA levels suggests that this phenotype is due, at least in part, to a defect in the export of ICP4 mRNA to the cytoplasm. In combination, our results (i) show that ICP27's NES is critically important for HSV-1 replication in many human cells, and (ii) suggest that ICP27 plays a heretofore unappreciated role in the expression of ICP4. IMPORTANCE HSV-1 IE proteins drive productive HSV-1 replication. The major paradigm of IE gene induction, developed over many years, involves the parallel activation of the five IE genes by the viral tegument protein VP16, which recruits the host RNA polymerase II (RNAP II) to the IE gene promoters. Here, we provide evidence that ICP27 can enhance ICP4 expression early in infection. Because ICP4 is required for transcription of viral E and L genes, this finding may be relevant to understanding how HSV-1 enters and exits the latent state in neurons.

Mechanisms of RNA export and nuclear retention

With the identification of huge amount of noncoding RNAs in recent years, the concept of RNA localization has extended from traditional mRNA export to RNA export of mRNA and ncRNA as well as nuclear retention of ncRNA. This review aims to summarize the recent findings from studies on the mechanisms of export of different RNAs and nuclear retention of some lncRNAs in higher eukaryotes, with a focus on splicing-dependent TREX recruitment for the export of spliced mRNA and the sequence-dependent mechanism of mRNA export in the absence of splicing. In addition, evidence to support the involvement of m⁶ A modification in RNA export with the coordination between the methylase complex and TREX complex as well as sequence-dependent nuclear retention of lncRNA is recapitulated. Finally, a model of sequence-dependent RNA localization is proposed along with the many questions that remain to be answered. This article is categorized under: RNA Export and Localization > RNA Localization RNA Export and Localization > Nuclear Export/Import.

The hnRNP C tetramer binds to CBC on mRNA and impedes PHAX recruitment for the classification of RNA polymerase II transcripts

In eukaryotic cells, various classes of RNAs are exported to the cytoplasm by class-specific factors. Accumulating evidence has shown that export factors affect the fate of RNA, demonstrating the importance of proper RNA classification upon export. We previously reported that RNA polymerase II transcripts were classified after synthesis depending on their length, and identified heterogeneous nuclear ribonucleoprotein (hnRNP) C as the key classification factor. HnRNP C inhibits the recruitment of PHAX, an adapter protein for spliceosomal U snRNA export, to long transcripts, navigating these RNAs to the mRNA export pathway. However, the mechanisms by which hnRNP C inhibits PHAX recruitment to mRNA remain unknown. We showed that the cap-binding complex, a bridging factor between m7G-capped RNA and PHAX, directly interacted with hnRNP C on mRNA. Additionally, we revealed that the tetramer-forming activity of hnRNP C and its strong RNA-binding activity were crucial for the inhibition of PHAX binding to longer RNAs. These results suggest that mRNA is wrapped around the hnRNP C tetramer without a gap from the cap, thereby impeding the recruitment of PHAX. The results obtained on the mode of length-specific RNA classification by the hnRNP C tetramer will provide mechanistic insights into hnRNP C-mediated RNA biogenesis.

Viral-host interactions during splicing and nuclear export of influenza virus mRNAs

As influenza-A viruses (IAV) replicate in the host cell nucleus, intranuclear pathways are usurped for viral gene expression. The eight genomic viral ribonucleoproteins (vRNPs) segments of IAV are transcribed and two generate viral mRNAs (M and NS) that undergo alternative splicing followed by export from the nucleus. The focus of this review is on viral RNA splicing and nuclear export. Recent mechanistic advances on M and NS splicing show differential regulation by RNA-binding proteins as well as distinct intranuclear localization. After a review of IAV splicing, we will discuss the nuclear export of viral mRNAs, which occur by interacting with specific constituents of the host mRNA export machinery that translocate viral mRNAs through the nuclear pore complex for translation in the cytoplasm.

RBM33 directs the nuclear export of transcripts containing GC-rich elements

Although splicing is a major driver of RNA nuclear export, many intronless RNAs are efficiently exported to the cytoplasm through poorly characterized mechanisms. For example, GC-rich sequences promote nuclear export in a splicing-independent manner, but how GC content is recognized and coupled to nuclear export is unknown. Here, we developed a genome-wide screening strategy to investigate the mechanism of export of NORAD, an intronless cytoplasmic long noncoding RNA (lncRNA). This screen revealed an RNA binding protein, RBM33, that directs the nuclear export of NORAD and numerous other transcripts. RBM33 directly binds substrate transcripts and recruits components of the TREX-NXF1/NXT1 RNA export pathway. Interestingly, high GC content emerged as the feature that specifies RBM33-dependent nuclear export. Accordingly, RBM33 directly binds GC-rich elements in target transcripts. These results provide a broadly applicable strategy for the genetic dissection of nuclear export mechanisms and reveal a long-sought nuclear export pathway for transcripts with GC-rich sequences.

Phosphor-IWS1-dependent U2AF2 splicing regulates trafficking of CAR-E-positive intronless gene mRNAs and sensitivity to viral infection

AKT-phosphorylated IWS1 promotes Histone H3K36 trimethylation and alternative RNA splicing of target genes, including the U2AF65 splicing factor-encoding U2AF2. The predominant U2AF2 transcript, upon IWS1 phosphorylation block, lacks the RS-domain-encoding exon 2, and encodes a protein which fails to bind Prp19. Here we show that although both U2AF65 isoforms bind intronless mRNAs containing cytoplasmic accumulation region elements (CAR-E), only the RS domain-containing U2AF65 recruits Prp19 and promotes their nuclear export. The loading of U2AF65 to CAR-Elements was RS domain-independent, but RNA PolII-dependent. Virus- or poly(I:C)-induced type I IFNs are encoded by genes targeted by the pathway. IWS1 phosphorylation-deficient cells therefore, express reduced levels of IFNα1/IFNβ1 proteins, and exhibit enhanced sensitivity to infection by multiple cytolytic viruses. Enhanced sensitivity of IWS1-deficient cells to Vesicular Stomatitis Virus and Reovirus resulted in enhanced apoptotic cell death via caspase activation. Inhibition of this pathway may therefore sensitize cancer cells to oncolytic viruses.

Nuclear export of plant pararetrovirus mRNAs involves the TREX complex, two viral proteins and the highly structured 5' leader region

In eukaryotes, the major nuclear export pathway for mature mRNAs uses the dimeric receptor TAP/p15, which is recruited to mRNAs via the multisubunit TREX complex, comprising the THO core and different export adaptors. Viruses that replicate in the nucleus adopt different strategies to hijack cellular export factors and achieve cytoplasmic translation of their mRNAs. No export receptors are known in plants, but Arabidopsis TREX resembles the mammalian complex, with a conserved hexameric THO core associated with ALY and UIEF proteins, as well as UAP56 and MOS11. The latter protein is an orthologue of mammalian CIP29. The nuclear export mechanism for viral mRNAs has not been described in plants. To understand this process, we investigated the export of mRNAs of the pararetrovirus CaMV in Arabidopsis and demonstrated that it is inhibited in plants deficient in ALY, MOS11 and/or TEX1. Deficiency for these factors renders plants partially resistant to CaMV infection. Two CaMV proteins, the coat protein P4 and reverse transcriptase P5, are important for nuclear export. P4 and P5 interact and co-localise in the nucleus with the cellular export factor MOS11. The highly structured 5′ leader region of 35S RNAs was identified as an export enhancing element that interacts with ALY1, ALY3 and MOS11 in vitro.

Sequence-dependent recruitment of SRSF1 and SRSF7 to intronless lncRNA NKILA promotes nuclear export via the TREX/TAP pathway

The TREX-TAP pathway is vital for mRNA export. For spliced mRNA, the TREX complex is recruited during splicing; however, for intronless mRNA, recruitment is sequence dependent. However, the export of cytoplasmic long noncoding RNA (lncRNA) is poorly characterized. We report the identification of a cytoplasmic accumulation region (CAR-N) in the intronless lncRNA, NKILA. CAR-N removal led to strong nuclear retention of NKILA, and CAR-N insertion promoted the export of cDNA transcripts. In vitro RNP purification via CAR-N, mass spectrometry, and siRNA screening revealed that SRSF1 and SRSF7 were vital to NKILA export, and identified a cluster of SRSF1/7 binding sites within a 55 nucleotide sequence in CAR-N. Significant nuclear enrichment of NKILA was observed for NKILA lacking CAR-N or the cluster of binding sites in knock-in models. Depletion of TREX-TAP pathway components resulted in strong nuclear retention of NKILA. RNA and protein immunoprecipitation verified that SRSF1/7 were bound to NKILA and interacted with UAP56 and ALYREF. Moreover, NKILA lacking CAR-N was unable to inhibit breast cancer cell migration. We concluded that the binding of SRSF1/7 to clustered motifs in CAR-N facilitated TREX recruitment, promoting the export of NKILA, and confirmed the importance of NKILA localization to its function.

THOC4 regulates energy homeostasis by stabilizing TFEB mRNA during prolonged starvation

TFEB, a basic helix-loop-helix transcription factor, is a master regulator of autophagy, lysosome biogenesis and lipid catabolism. Compared to posttranslational regulation of TFEB, the regulation of TFEB mRNA stability remains relatively uncharacterized. In this study, we identified the mRNA-binding protein THOC4 as a novel regulator of TFEB. In mammalian cells, siRNA-mediated knockdown of THOC4 decreased the level of TFEB protein to a greater extent than other bHLH transcription factors. THOC4 bound to TFEB mRNA and stabilized it after transcription by maintaining poly(A) tail length. We further found that this mode of regulation was conserved in Caenorhabditis elegans and was essential for TFEB-mediated lipid breakdown, which becomes over-represented during prolonged starvation. Taken together, our findings reveal the presence of an additional layer of TFEB regulation by THOC4 and provide novel insights into the function of TFEB in mediating autophagy and lipid metabolism.

TPR is required for the efficient nuclear export of mRNAs and lncRNAs from short and intron-poor genes

While splicing has been shown to enhance nuclear export, it has remained unclear whether mRNAs generated from intronless genes use specific machinery to promote their export. Here, we investigate the role of the major nuclear pore basket protein, TPR, in regulating mRNA and lncRNA nuclear export in human cells. By sequencing mRNA from the nucleus and cytosol of control and TPR-depleted cells, we provide evidence that TPR is required for the efficient nuclear export of mRNAs and lncRNAs that are generated from short transcripts that tend to have few introns, and we validate this with reporter constructs. Moreover, in TPR-depleted cells reporter mRNAs generated from short transcripts accumulate in nuclear speckles and are bound to Nxf1. These observations suggest that TPR acts downstream of Nxf1 recruitment and may allow mRNAs to leave nuclear speckles and properly dock with the nuclear pore. In summary, our study provides one of the first examples of a factor that is specifically required for the nuclear export of intronless and intron-poor mRNAs and lncRNAs.

Out or decay: fate determination of nuclear RNAs

In eukaryotes, RNAs newly synthesized by RNA polymerase II (RNAPII) undergo several processing steps prior to transport to the cytoplasm. It has long been known that RNAs with defects in processing or export are removed in the nucleus. Recent studies revealed that RNAs without apparent defects are also subjected to nuclear degradation, indicating that nuclear RNA fate is determined in a more complex and dynamic way than previously thought. Nuclear RNA sorting directly determines the quality and quantity of RNA pools for future translation and thus is of significant importance. In this essay, we will summarize recent studies on this topic, mainly focusing on findings in mammalian system, and discuss about important remaining questions and possible biological relevance for nuclear RNA fate determination.

GC-content biases in protein-coding genes act as an "mRNA identity" feature for nuclear export

It has long been observed that human protein-coding genes have a particular distribution of GC-content: the 5' end of these genes has high GC-content while the 3' end has low GC-content. In 2012, it was proposed that this pattern of GC-content could act as an mRNA identity feature that would lead to it being better recognized by the cellular machinery to promote its nuclear export. In contrast, junk RNA, which largely lacks this feature, would be retained in the nucleus and targeted for decay. Now two recent papers have provided evidence that GC-content does promote the nuclear export of many mRNAs in human cells.

Strength in Diversity: Nuclear Export of Viral RNAs

The nuclear export of cellular mRNAs is a complex process that requires the orchestrated participation of many proteins that are recruited during the early steps of mRNA synthesis and processing. This strategy allows the cell to guarantee the conformity of the messengers accessing the cytoplasm and the translation machinery. Most transcripts are exported by the exportin dimer Nuclear RNA export factor 1 (NXF1)-NTF2-related export protein 1 (NXT1) and the transcription-export complex 1 (TREX1). Some mRNAs that do not possess all the common messenger characteristics use either variants of the NXF1-NXT1 pathway or CRM1, a different exportin. Viruses whose mRNAs are synthesized in the nucleus (retroviruses, the vast majority of DNA viruses, and influenza viruses) exploit both these cellular export pathways. Viral mRNAs hijack the cellular export machinery via complex secondary structures recognized by cellular export factors and/or viral adapter proteins. This way, the viral transcripts succeed in escaping the host surveillance system and are efficiently exported for translation, allowing the infectious cycle to proceed. This review gives an overview of the cellular mRNA nuclear export mechanisms and presents detailed insights into the most important strategies that viruses use to export the different forms of their RNAs from the nucleus to the cytoplasm.

Gene Architecture and Sequence Composition Underpin Selective Dependency of Nuclear Export of Long RNAs on NXF1 and the TREX Complex

The core components of the nuclear RNA export pathway are thought to be required for export of virtually all polyadenylated RNAs. Here, we depleted different proteins that act in nuclear export in human cells and quantified the transcriptome-wide consequences on RNA localization. Different genes exhibited substantially variable sensitivities, with depletion of NXF1 and TREX components causing some transcripts to become strongly retained in the nucleus while others were not affected. Specifically, NXF1 is preferentially required for export of single- or few-exon transcripts with long exons or high A/U content, whereas depletion of TREX complex components preferentially affects spliced and G/C-rich transcripts. Using massively parallel reporter assays, we identified short sequence elements that render transcripts dependent on NXF1 for their export and identified synergistic effects of splicing and NXF1. These results revise the current model of how nuclear export shapes the distribution of RNA within human cells.

Distinct effects on mRNA export factor GANP underlie neurological disease phenotypes and alter gene expression depending on intron content

Defects in the mRNA export scaffold protein GANP, encoded by the MCM3AP gene, cause autosomal recessive early-onset peripheral neuropathy with or without intellectual disability. We extend here the phenotypic range associated with MCM3AP variants, by describing a severely hypotonic child and a sibling pair with a progressive encephalopathic syndrome. In addition, our analysis of skin fibroblasts from affected individuals from seven unrelated families indicates that disease variants result in depletion of GANP except when they alter critical residues in the Sac3 mRNA binding domain. GANP depletion was associated with more severe phenotypes compared with the Sac3 variants. Patient fibroblasts showed transcriptome alterations that suggested intron content-dependent regulation of gene expression. For example, all differentially expressed intronless genes were downregulated, including ATXN7L3B, which couples mRNA export to transcription activation by association with the TREX-2 and SAGA complexes. Our results provide insight into the molecular basis behind genotype-phenotype correlations in MCM3AP-associated disease and suggest mechanisms by which GANP defects might alter RNA metabolism.

It's Not the Destination, It's the Journey: Heterogeneity in mRNA Export Mechanisms

The process of creating a translation-competent mRNA is highly complex and involves numerous steps including transcription, splicing, addition of modifications, and, finally, export to the cytoplasm. Historically, much of the research on regulation of gene expression at the level of the mRNA has been focused on either the regulation of mRNA synthesis (transcription and splicing) or metabolism (translation and degradation). However, in recent years, the advent of new experimental techniques has revealed the export of mRNA to be a major node in the regulation of gene expression, and numerous large-scale and specific mRNA export pathways have been defined. In this chapter, we will begin by outlining the mechanism by which most mRNAs are homeostatically exported ("bulk mRNA export"), involving the recruitment of the NXF1/TAP export receptor by the Aly/REF and THOC5 components of the TREX complex. We will then examine various mechanisms by which this pathway may be controlled, modified, or bypassed in order to promote the export of subset(s) of cellular mRNAs, which include the use of metazoan-specific orthologs of bulk mRNA export factors, specific cis RNA motifs which recruit mRNA export machinery via specific trans-acting-binding factors, posttranscriptional mRNA modifications that act as "inducible" export cis elements, the use of the atypical mRNA export receptor, CRM1, and the manipulation or bypass of the nuclear pore itself. Finally, we will discuss major outstanding questions in the field of mRNA export heterogeneity and outline how cutting-edge experimental techniques are providing new insights into and tools for investigating the intriguing field of mRNA export heterogeneity.

Exon size and sequence conservation improves identification of splice-altering nucleotides

Pre-mRNA splicing is regulated through multiple trans-acting splicing factors. These regulators interact with the pre-mRNA at intronic and exonic positions. Given that most exons are protein coding, the evolution of exons must be modulated by a combination of selective coding and splicing pressures. It has previously been demonstrated that selective splicing pressures are more easily deconvoluted when phylogenetic comparisons are made for exons of identical size, suggesting that exon size-filtered sequence alignments may improve identification of nucleotides evolved to mediate efficient exon ligation. To test this hypothesis, an exon size database was created, filtering 76 vertebrate sequence alignments based on exon size conservation. In addition to other genomic parameters, such as splice-site strength, gene position, or flanking intron length, this database permits the identification of exons that are size- and/or sequence-conserved. Highly size-conserved exons are always sequence-conserved. However, sequence conservation does not necessitate exon size conservation. Our analysis identified evolutionarily young exons and demonstrated that length conservation is a strong predictor of alternative splicing. A published data set of approximately 5000 exonic SNPs associated with disease was analyzed to test the hypothesis that exon size-filtered sequence comparisons increase detection of splice-altering nucleotides. Improved splice predictions could be achieved when mutations occur at the third codon position, especially when a mutation decreases exon inclusion efficiency. The results demonstrate that coding pressures dominate nucleotide composition at invariable codon positions and that exon size-filtered sequence alignments permit identification of splice-altering nucleotides at wobble positions.

Control of human testis-specific gene expression

Background

As a result of decades of effort by many investigators we now have an advanced level of understanding about several molecular systems involved in the control of gene expression. Examples include CpG islands, promoters, mRNA splicing and epigenetic signals. It is less clear, however, how such systems work together to integrate the functions of a living organism. Here I describe the results of a study to test the idea that a contribution might be made by focusing on genes specifically expressed in a particular tissue, the human testis.

Experimental design

A database of 239 testis-specific genes was accumulated and each was examined for the presence of features relevant to control of gene expression. These include: (1) the presence of a promoter, (2) the presence of a CpG island (CGI) within the promoter, (3) the presence in the promoter of a transcription factor binding site near the transcription start site, (4) the level of gene expression, and (5) the above features in genes of testis-specific cell types such as spermatocyte and spermatid that differ in their extent of differentiation.

Results

Of the 107 database genes with an annotated promoter, 56 were found to have one or more transcription factor binding sites near the transcription start site. Three of the binding sites observed, Pax-5, AP-2αA and GRα, stand out in abundance suggesting they may be involved in testis-specific gene expression. Compared to less differentiated testis-specific cells, genes of more differentiated cells were found to be (1) more likely to lack a CGI, (2) more likely to lack introns and (3) higher in expression level. The results suggest genes of more differentiated cells have a reduced need for CGI-based regulatory repression, reduced usage of gene splicing and a smaller set of expressed proteins.

The archaeology of coding RNA

Different theories concerning the origin of RNA (and, in particular, mRNA) point to the concatenation and expansion of proto-tRNA-like structures. Different biochemical and biophysical tools have been used to search for ancient-like RNA elements with a specific structure in genomic viral RNAs, including that of the hepatitis C virus, as well as in cellular mRNA populations, in particular those of human hepatocytes. We define this method as "archaeological," and it has been designed to discover evolutionary patterns through a nonphylogenetic and nonrepresentational strategy. tRNA-like elements were found in structurally or functionally relevant positions both in viral RNA and in one of the liver mRNAs examined, the antagonist interferon-alpha subtype 5 (IFNA5) mRNA. Additionally, tRNA-like elements are highly represented within the hepatic mRNA population, which suggests that they could have participated in the formation of coding RNAs in the distant past. Expanding on this finding, we have observed a recurring dsRNA-like motif next to the tRNA-like elements in both viral RNAs and IFNA5 mRNA. This suggested that the concatenation of these RNA motifs was an activity present in the RNA pools that might have been relevant in the RNA world. The extensive alteration of sequences that likely triggered the transition from the predecessors of coding RNAs to the first fully functional mRNAs (which was not the case in the stepwise construction of noncoding rRNAs) hinders the phylogeny-based identification of RNA elements (both sequences and structures) that might have been active before the advent of protein synthesis. Therefore, our RNA archaeological method is presented as a way to better understand the structural/functional versatility of a variety of RNA elements, which might represent "the losers" in the process of RNA evolution as they had to adapt to the selective pressures favoring the coding capacity of the progressively longer mRNAs.

Nuclear retention element recruits U1 snRNP components to restrain spliced lncRNAs in the nucleus

In contrast to cytoplasmic localization of spliced mRNAs, many spliced lncRNAs are localized in the nucleus. To investigate the mechanism, we used lncRNA MEG3 as a reporter and mapped a potent nuclear retention element (NRE), deletion of this element led to striking export of MEG3 from the nucleus to the cytoplasm. Insertion of the NRE resulted in nuclear retention of spliced lncRNA as well as spliced mRNA. We further purified RNP assembled on the NRE in vitro and identified the proteins by mass spectrometry. Screen using siRNA revealed depletion of U1 snRNP components SNRPA, SNRNP70 or SNRPD2 caused significant cytoplasmic localization of MEG3 reporter transcripts. Co-knockdown these factors in HFF1 cells resulted in an increased cytoplasmic distribution of endogenous lncRNAs. Together, these data support a model that U1 snRNP components restrain spliced lncRNAs in the nucleus via the interaction with nuclear retention element.

Sequences encoding C2H2 zinc fingers inhibit polyadenylation and mRNA export in human cells

The large C2H2-Zinc Finger (C2H2-ZNF) gene family has rapidly expanded in primates through gene duplication. There is consequently considerable sequence homology between family members at both the nucleotide and amino acid level, allowing for coordinated regulation and shared functions. Here we show that multiple C2H2-ZNF mRNAs experience differential polyadenylation resulting in populations with short and long poly(A) tails. Furthermore, a significant proportion of C2H2-ZNF mRNAs are retained in the nucleus. Intriguingly, both short poly(A) tails and nuclear retention can be specified by the repeated elements that encode zinc finger motifs. These Zinc finger Coding Regions (ZCRs) appear to restrict polyadenylation of nascent RNAs and at the same time impede their export. However, the polyadenylation process is not necessary for nuclear retention of ZNF mRNAs. We propose that inefficient polyadenylation and export may allow C2H2-ZNF mRNAs to moonlight as non-coding RNAs or to be stored for later use.

Sequence Determinants for Nuclear Retention and Cytoplasmic Export of mRNAs and lncRNAs

Eukaryotes are divided into two major compartments: the nucleus where RNA is synthesized and processed, and the cytoplasm, where mRNA is translated into proteins. Although many different RNAs are made, only a subset is allowed access to the cytoplasm, primarily RNAs involved in protein synthesis (mRNA, tRNA, and rRNA). In contrast, nuclear retained transcripts are mostly long non-coding RNAs (lncRNAs) whose role in cell physiology has been a source of much investigation in the past few years. In addition, it is likely that many non-functional RNAs, which arise by spurious transcription and misprocessing of functional RNAs, are also retained in the nucleus and degraded. In this review, the main sequence features that dictate whether any particular mRNA or lncRNA is a substrate for retention in the nucleus, or export to the cytoplasm, are discussed. Although nuclear export is promoted by RNA-splicing due to the fact that the spliceosome can help recruit export factors to the mature RNA, nuclear export does not require splicing. Indeed, most stable unspliced transcripts are well exported and associate with these same export factors in a splicing-independent manner. In contrast, nuclear retention is promoted by specialized cis-elements found in certain RNAs. This new understanding of the determinants of nuclear retention and cytoplasmic export provides a deeper understanding of how information flow is regulated in eukaryotic cells. Ultimately these processes promote the evolution of complexity in eukaryotes by shaping the genomic content through constructive neutral evolution.

Intronless mRNAs transit through nuclear speckles to gain export competence

Nuclear speckles (NSs) store splicing factors. Wang et al. show that many naturally intronless mRNAs associate with NSs and that speckle association enhances their export by facilitating TREX recruitment, suggesting that trafficking to NSs could be an important quality control step in intronless mRNA export.

Nuclear speckles (NSs) serve as splicing factor storage sites. In this study, we unexpectedly found that many endogenous intronless mRNAs, which do not undergo splicing, associate with NSs. These associations do not require transcription, polyadenylation, or the polyA tail. Rather, exonic splicing enhancers present in intronless mRNAs and their binding partners, SR proteins, promote intronless mRNA localization to NSs. Significantly, speckle targeting of mRNAs promotes the recruitment of the TREX export complex and their TREX-dependent nuclear export. Furthermore, TREX, which accumulates in NSs, is required for releasing intronless mRNAs from NSs, whereas NXF1, which is mainly detected at nuclear pores, is not. Upon NXF1 depletion, the TREX protein UAP56 loses speckle concentration but coaccumulates with intronless mRNAs and polyA RNAs in the nucleoplasm, and these RNAs are trapped in NSs upon UAP56 codepletion. We propose that the export-competent messenger RNP assembly mainly occurs in NSs for intronless mRNAs and that entering NSs serves as a quality control step in mRNA export.

Nucleosome Positioning of Intronless Genes in the Human Genome

Nucleosomes, the basic units of chromatin, are involved in transcription regulation and DNA replication. Intronless genes, which constitute 3 percent of the human genome, differ from intron-containing genes in evolution and function. Our analysis reveals that nucleosome positioning shows a distinct pattern in intronless and intron-containing genes. The nucleosome occupancy upstream of transcription start sites of intronless genes is lower than that of intron-containing genes. In contrast, high occupancy and well positioned nucleosomes are observed along the gene body of intronless genes, which is perfectly consistent with the barrier nucleosome model. Intronless genes have a significantly lower expression level than intron-containing genes and most of them are not expressed in CD4+ T cell lines and GM12878 cell lines, which results from their tissue specificity. However, the highly expressed genes are at the same expression level between the two types of genes. The highly expressed intronless genes require a higher density of RNA Pol II in an elongating state to compensate for the lack of introns. Additionally, 5' and 3' nucleosome depleted regions of highly expressed intronless genes are deeper than those of highly expressed intron-containing genes.

Viral tRNA Mimicry from a Biocommunicative Perspective

RNA viruses have very small genomes which limits the functions they can encode. One of the strategies employed by these viruses is to mimic key factors of the host cell so they can take advantage of the interactions and activities these factors typically participate in. The viral RNA genome itself was first observed to mimic cellular tRNA over 40 years ago. Since then researchers have confirmed that distinct families of RNA viruses are accessible to a battery of cellular factors involved in tRNA-related activities. Recently, potential tRNA-like structures have been detected within the sequences of a 100 mRNAs taken from human cells, one of these being the host defense interferon-alpha mRNA; these are then additional to the examples found in bacterial and yeast mRNAs. The mimetic relationship between tRNA, cellular mRNA, and viral RNA is the central focus of two considerations described below. These are subsequently used as a preface for a final hypothesis drawing on concepts relating to mimicry from the social sciences and humanities, such as power relations and creativity. Firstly, the presence of tRNA-like structures in mRNAs indicates that the viral tRNA-like signal could be mimicking tRNA-like elements that are contextualized by the specific carrier mRNAs, rather than, or in addition to, the tRNA itself, which would significantly increase the number of potential semiotic relations mediated by the viral signals. Secondly, and in particular, mimicking a host defense mRNA could be considered a potential new viral strategy for survival. Finally, we propose that mRNA's mimicry of tRNA could be indicative of an ancestral intracellular conflict in which species of mRNAs invaded the cell, but from within. As the meaning of the mimetic signal depends on the context, in this case, the conflict that arises when the viral signal enters the cell can change the meaning of the mRNAs' internal tRNA-like signals, from their current significance to that they had in the distant past.

ALYREF mainly binds to the 5' and the 3' regions of the mRNA in vivo

The TREX complex (TREX) plays key roles in nuclear export of mRNAs. However, little is known about its transcriptome-wide binding targets. We used individual cross-linking and immunoprecipitation (iCLIP) to identify the binding sites of ALYREF, an mRNA export adaptor in TREX, in human cells. Consistent with previous in vitro studies, ALYREF binds to a region near the 5′ end of the mRNA in a CBP80-dependent manner. Unexpectedly, we identified PABPN1-dependent ALYREF binding near the 3′ end of the mRNA. Furthermore, the 3′ processing factor CstF64 directly interacts with ALYREF and is required for the overall binding of ALYREF on the mRNA. In addition, we found that numerous middle exons harbor ALYREF binding sites and identified ALYREF-binding motifs that promote nuclear export of intronless mRNAs. Together, our study defines enrichment of ALYREF binding sites at the 5′ and the 3′ regions of the mRNA in vivo, identifies export-promoting ALYREF-binding motifs, and reveals CstF64- and PABPN1-mediated coupling of mRNA nuclear export to 3′ processing.

Exosome cofactor hMTR4 competes with export adaptor ALYREF to ensure balanced nuclear RNA pools for degradation and export

The exosome is a key RNA machine that functions in the degradation of unwanted RNAs. Here, we found that significant fractions of precursors and mature forms of mRNAs and long noncoding RNAs are degraded by the nuclear exosome in normal human cells. Exosome-mediated degradation of these RNAs requires its cofactor hMTR4. Significantly, hMTR4 plays a key role in specifically recruiting the exosome to its targets. Furthermore, we provide several lines of evidence indicating that hMTR4 executes this role by directly competing with the mRNA export adaptor ALYREF for associating with ARS2, a component of the cap-binding complex (CBC), and this competition is critical for determining whether an RNA is degraded or exported to the cytoplasm. Together, our results indicate that the competition between hMTR4 and ALYREF determines exosome recruitment and functions in creating balanced nuclear RNA pools for degradation and export.

Microinjection and Fluorescence In Situ Hybridization Assay for Studying mRNA Export in Mammalian Cells

Microinjection and Fluorescence in situ Hybridization (FISH) assay is a useful method for mRNA export studies, which can overcome the problems of traditional transfection in cells. Here, we describe the method of microinjection and FISH assay applied in investigation of mRNA export. By this method we can estimate the mRNA export kinetics, examining mRNA export in cells with low transfection efficiencies, and observing nuclear export of aberrant RNAs.

The role of TREX in gene expression and disease

TRanscription and EXport (TREX) is a conserved multisubunit complex essential for embryogenesis, organogenesis and cellular differentiation throughout life. By linking transcription, mRNA processing and export together, it exerts a physiologically vital role in the gene expression pathway. In addition, this complex prevents DNA damage and regulates the cell cycle by ensuring optimal gene expression. As the extent of TREX activity in viral infections, amyotrophic lateral sclerosis and cancer emerges, the need for a greater understanding of TREX function becomes evident. A complete elucidation of the composition, function and interactions of the complex will provide the framework for understanding the molecular basis for a variety of diseases. This review details the known composition of TREX, how it is regulated and its cellular functions with an emphasis on mammalian systems.

A matter of maturity: The impact of pre-mRNA processing in gene expression and antigen presentation

RNA processing plays a pivotal role in the diversification of high eukaryotes transcriptome and proteome. The expression of gene products controlling a variety of cellular and physiological processes depends largely on a complex maturation process undergone by pre-mRNAs to become translation-competent mRNAs. Here we review the different mechanisms involved in the pre-mRNA processing and disclose their impact in the gene regulation process in eukaryotic cells. We describe some viral strategies targeting pre-mRNA processing to control gene expression and host immune response and discuss their relevance as tools for a better understanding of cell biology. Finally, we highlight accumulating evidences toward the occurrence of a translation event coupled to mRNA biogenesis in the nuclear compartment and argue how this is relevant for the production of antigenic peptide substrates for the major histocompatibility complex class I pathway.

Pharmacological inhibition of the spliceosome subunit SF3b triggers exon junction complex-independent nonsense-mediated decay

Spliceostatin A, meayamycin, and pladienolide B are small molecules that target the SF3b subunit of the spliceosomal U2 small nuclear ribonucleoprotein (snRNP). These compounds are attracting much attention as tools to manipulate splicing and for use as potential anti-cancer drugs. We investigated the effects of these inhibitors on mRNA transport and stability in human cells. Upon splicing inhibition, unspliced pre-mRNAs accumulated in the nucleus, particularly within enlarged nuclear speckles. However, a small fraction of the pre-mRNA molecules were exported to the cytoplasm. We identified the export adaptor ALYREF as being associated with intron-containing transcripts and show its requirement for the nucleo-cytoplasmic transport of unspliced pre-mRNA. In contrast, the exon junction complex (EJC) core protein eIF4AIII failed to form a stable complex with intron-containing transcripts. Despite the absence of EJC, unspliced transcripts in the cytoplasm were degraded by nonsense-mediated decay (NMD), suggesting that unspliced transcripts are degraded by an EJC-independent NMD pathway. Collectively, our results indicate that although blocking the function of SF3b elicits a massive accumulation of unspliced pre-mRNAs in the nucleus, intron-containing transcripts can still bind the ALYREF export factor and be transported to the cytoplasm, where they trigger an alternative NMD pathway.

Structural and biochemical analyses of the DEAD-box ATPase Sub2 in association with THO or Yra1

mRNA is cotranscrptionally processed and packaged into messenger ribonucleoprotein particles (mRNPs) in the nucleus. Prior to export through the nuclear pore, mRNPs undergo several obligatory remodeling reactions. In yeast, one of these reactions involves loading of the mRNA-binding protein Yra1 by the DEAD-box ATPase Sub2 as assisted by the hetero-pentameric THO complex. To obtain molecular insights into reaction mechanisms, we determined crystal structures of two relevant complexes: a THO hetero-pentamer bound to Sub2 at 6.0 Å resolution; and Sub2 associated with an ATP analogue, RNA, and a C-terminal fragment of Yra1 (Yra1-C) at 2.6 Å resolution. We found that the 25 nm long THO clamps Sub2 in a half-open configuration; in contrast, when bound to the ATP analogue, RNA and Yra1-C, Sub2 assumes a closed conformation. Both THO and Yra1-C stimulated Sub2’s intrinsic ATPase activity. We propose that THO surveys common landmarks in each nuclear mRNP to localize Sub2 for targeted loading of Yra1.

DOI:http://dx.doi.org/10.7554/eLife.20070.001

Viral Evasion and Manipulation of Host RNA Quality Control Pathways

Viruses have evolved diverse strategies to maximize the functional and coding capacities of their genetic material. Individual viral RNAs are often used as substrates for both replication and translation and can contain multiple, sometimes overlapping open reading frames. Further, viral RNAs engage in a wide variety of interactions with both host and viral proteins to modify the activities of important cellular factors and direct their own trafficking, packaging, localization, stability, and translation. However, adaptations increasing the information density of small viral genomes can have unintended consequences. In particular, viral RNAs have developed features that mark them as potential targets of host RNA quality control pathways. This minireview focuses on ways in which viral RNAs run afoul of the cellular mRNA quality control and decay machinery, as well as on strategies developed by viruses to circumvent or exploit cellular mRNA surveillance.

Splicing promotes the nuclear export of β-globin mRNA by overcoming nuclear retention elements

Most current models of mRNA nuclear export in vertebrate cells assume that an mRNA must have specialized signals in order to be exported from the nucleus. Under such a scenario, mRNAs that lack these specialized signals would be shunted into a default pathway where they are retained in the nucleus and eventually degraded. These ideas were based on the selective use of model mRNA reporters. For example, it has been shown that splicing promotes the nuclear export of certain model mRNAs, such as human β-globin, and that in the absence of splicing, the cDNA-derived mRNA is retained in the nucleus and degraded. Here we provide evidence that β-globin mRNA contains an element that actively retains it in the nucleus and degrades it. Interestingly, this nuclear retention activity can be overcome by increasing the length of the mRNA or by splicing. Our results suggest that contrary to many current models, the default pathway for most intronless RNAs is to be exported from the nucleus, unless the RNA contains elements that actively promote its nuclear retention.

HuR antagonizes the effect of an intronic pyrimidine-rich sequence in regulating WT1 +/-KTS isoforms

WT1 + KTS and -KTS isoforms only differ in 3 amino acids in protein sequence but show significant functional difference. The +/-KTS isoforms were generated by alternative usage of 2 adjacent 5' splice sites at RNA level, however, how these 2 isoforms are regulated is still elusive. Here we report the identification of an intronic pyrimidine-rich sequence that is critical for the ratio of +/-KTS isoforms, deletion or partial replacement of the sequence led to full/significant shift to -KTS isoform. To identify trans-factors that can regulate +/-KTS isoforms via the binding to the element, we performed RNP assembly using in vitro transcribed RNA with or without the pyrimidine-rich sequence. Mass spectrometry analysis of purified RNPs showed that the element associated with many splicing factors. Co-transfection of these factors with WT1 reporter revealed that HuR promoted the production of -KTS isoform at the reporter level. RNA immuno-precipitation experiment indicated that HuR interacted with the pyrimidine-rich element in WT1 intron 9. We further presented evidence that transient or stable over-expression of HuR led to enhanced expression of endogenous -KTS isoform. Moreover, knockdown of HuR resulted in decreased expression of endogenous -KTS isoform in 293T, SW620, SNU-387 and AGS cell lines. Together, these data indicate that HuR binds to the pyrimidine-rich sequence and antagonize its effect in regulating WT1 +/-KTS isoforms.

Multiple Export Mechanisms for mRNAs

Nuclear mRNA export plays an important role in gene expression. We describe the mechanisms of mRNA export including the importance of mRNP assembly, docking with the nuclear basket of the nuclear pore complex (NPC), transit through the central channel of the NPC and cytoplasmic release. We describe multiple mechanisms of mRNA export including NXF1 and CRM1 mediated pathways. Selective groups of mRNAs can be preferentially transported in order to respond to cellular stimuli. RNAs can be selected based on the presence of specific cis-acting RNA elements and binding of specific adaptor proteins. The role that dysregulation of this process plays in human disease is also discussed.

Evolution of the unspliced transcriptome

BACKGROUND

Despite their abundance, unspliced EST data have received little attention as a source of information on non-coding RNAs. Very little is know, therefore, about the genomic distribution of unspliced non-coding transcripts and their relationship with the much better studied regularly spliced products. In particular, their evolution has remained virtually unstudied.

RESULTS

We systematically study the evidence on unspliced transcripts available in EST annotation tracks for human and mouse, comprising 104,980 and 66,109 unspliced EST clusters, respectively. Roughly one third of these are located totally inside introns of known genes (TINs) and another third overlaps exonic regions (PINs). Eleven percent are "intergenic", far away from any annotated gene. Direct evidence for the independent transcription of many PINs and TINs is obtained from CAGE tag and chromatin data. We predict more than 2000 3'UTR-associated RNA candidates for each human and mouse. Fifteen to twenty percent of the unspliced EST cluster are conserved between human and mouse. With the exception of TINs, the sequences of unspliced EST clusters evolve significantly slower than genomic background. Furthermore, like spliced lincRNAs, they show highly tissue-specific expression patterns.

CONCLUSIONS

Unspliced long non-coding RNAs are an important, rapidly evolving, component of mammalian transcriptomes. Their analysis is complicated by their preferential association with complex transcribed loci that usually also harbor a plethora of spliced transcripts. Unspliced EST data, although typically disregarded in transcriptome analysis, can be used to gain insights into this rarely investigated transcriptome component. The frequently postulated connection between lack of splicing and nuclear retention and the surprising overlap of chromatin-associated transcripts suggests that this class of transcripts might be involved in chromatin organization and possibly other mechanisms of epigenetic control.

THOC2 Mutations Implicate mRNA-Export Pathway in X-Linked Intellectual Disability

Export of mRNA from the cell nucleus to the cytoplasm is essential for protein synthesis, a process vital to all living eukaryotic cells. mRNA export is highly conserved and ubiquitous. Mutations affecting mRNA and mRNA processing or export factors, which cause aberrant retention of mRNAs in the nucleus, are thus emerging as contributors to an important class of human genetic disorders. Here, we report that variants in THOC2, which encodes a subunit of the highly conserved TREX mRNA-export complex, cause syndromic intellectual disability (ID). Affected individuals presented with variable degrees of ID and commonly observed features included speech delay, elevated BMI, short stature, seizure disorders, gait disturbance, and tremors. X chromosome exome sequencing revealed four missense variants in THOC2 in four families, including family MRX12, first ascertained in 1971. We show that two variants lead to decreased stability of THOC2 and its TREX-complex partners in cells derived from the affected individuals. Protein structural modeling showed that the altered amino acids are located in the RNA-binding domains of two complex THOC2 structures, potentially representing two different intermediate RNA-binding states of THOC2 during RNA transport. Our results show that disturbance of the canonical molecular pathway of mRNA export is compatible with life but results in altered neuronal development with other comorbidities.