Transcriptome-wide analyses of CstF64-RNA interactions in global regulation of mRNA alternative polyadenylation

CSTF2-impeded innate αβ T cell infiltration and activation exacerbate immune evasion of pancreatic cancer

Alternative cleavage and polyadenylation (APA) have gained increasing attention in cancer biology, yet its role in modulating anti-tumor immune response remains largely unexplored. Here, we identify the cleavage stimulation factor 2 (CSTF2), an APA-related gene, as a pivotal suppressor of anti-tumor immunity in pancreatic ductal adenocarcinoma (PDAC). CSTF2 promotes tumor development by inhibiting the infiltration and cytotoxic immune cell recruitment function of TCRαβ+CD4-CD8-NK1.1- innate αβ T (iαβT) cells. Mechanistically, CSTF2 diminishes CXCL10 expression by promoting PolyA polymerase alpha (PAPα) binding to the 3' untranslated regions of CXCL10 RNA, resulting in shortened PolyA tails and compromised RNA stability. Furthermore, we identify Forsythoside B, a selective inhibitor targeting the RNA recognition motif of CSTF2, can effectively activate anti-tumor immunity and overcome resistance to immune checkpoint blockade (ICB) therapy. Collectively, our findings unveil CSTF2 as a promising therapeutic target for sensitizing PDAC to ICB therapy.

RBBP6 anchors pre-mRNA 3' end processing to nuclear speckles for efficient gene expression

Pre-mRNA 3' processing is an integral step in mRNA biogenesis. However, where this process occurs in the nucleus remains unknown. Here, we demonstrate that nuclear speckles (NSs), membraneless organelles enriched with splicing factors, are major sites for pre-mRNA 3' processing in human cells. We show that the essential pre-mRNA 3' processing factor retinoblastoma-binding protein 6 (RBBP6) associates strongly with NSs via its C-terminal intrinsically disordered region (IDR). Importantly, although the conserved N-terminal domain (NTD) of RBBP6 is sufficient for pre-mRNA 3' processing in vitro, its IDR-mediated association with NSs is required for efficient pre-mRNA 3' processing in cells. Through proximity labeling analyses, we provide evidence that pre-mRNA 3' processing for over 50% of genes occurs near NSs. We propose that NSs serve as hubs for RNA polymerase II transcription, pre-mRNA splicing, and 3' processing, thereby enhancing the efficiency and coordination of different gene expression steps.

CSTF2 Supports Hypoxia Tolerance in Hepatocellular Carcinoma by Enabling m6A Modification Evasion of PGK1 to Enhance Glycolysis

Cleavage stimulation factor subunit 2 (CSTF2) is a fundamental factor in the regulation of 3'-end cleavage and alternative polyadenylation of pre-mRNAs. Previous work has identified a tumor-promoting role of CSTF2, suggesting that it may represent a potential therapeutic target. In this study, we aimed to elucidate the mechanistic function of CSTF2 in hepatocellular carcinoma (HCC). CSTF2 upregulation was frequent in HCC, and elevated levels of CSTF2 correlated with poor patient prognosis. Although CSTF2 inhibition did not suppress HCC growth under nonstress conditions, it supported tolerance and survival of HCC cells under hypoxic conditions. Mechanistically, CSTF2 increased phosphoglycerate kinase 1 (PGK1) protein production to enhance glycolysis, thereby sustaining the energy supply under hypoxic conditions. CSTF2 shortened the 3' untranslated region of PGK1 pre-mRNA by binding near the proximal polyadenylation site. This shortening led to a loss of N6-methyladenosine (m6A) modification sites that are bound by YTH m6A RNA-binding protein F2 and increase degradation of PGK1 mRNA. Concurrently, hypoxia increased m6A modification of PGK1 mRNA near the proximal polyadenylation site that was recognized by the YTH m6A RNA-binding protein C1, which recruited CSTF2 to enhance the shortening of the PGK1 3' untranslated region. A small-molecule screen identified masitinib as an inhibitor of CSTF2. Masitinib counteracted PGK1 upregulation by CSTF2 and suppressed the growth of HCC xenograft and patient-derived organoid models. In conclusion, this study revealed a function of CSTF2 in supporting HCC survival under hypoxia conditions through m6A modification evasion and metabolic reprogramming, indicating that inhibiting CSTF2 may overcome hypoxia tolerance in HCC. Significance: Targeting CSTF2 inhibits hepatocellular carcinoma survival in hypoxic microenvironments, which may be a promising therapeutic strategy for treating liver cancer.

A special focus on polyadenylation and alternative polyadenylation in neurodegenerative diseases: A systematic review

Neurodegenerative diseases (NDDs) are one of the prevailing conditions characterized by progressive neuronal loss. Polyadenylation (PA) and alternative polyadenylation (APA) are the two main post-transcriptional events that regulate neuronal gene expression and protein production. This systematic review analyzed the available literature on the role of PA and APA in NDDs, with an emphasis on their contributions to disease development. A comprehensive literature search was performed using the PubMed, Scopus, Cochrane, Google Scholar, Embase, Web of Science, and ProQuest databases. The search strategy was developed based on the framework introduced by Arksey and O'Malley and supplemented by the inclusion and exclusion criteria. The study selection was performed by two independent reviewers. Extraction and data organization were performed in accordance with the predefined variables. Subsequently, quantitative and qualitative analyses were performed. Forty-seven studies were included, related to a variety of NDDs, namely Alzheimer's disease, Parkinson's disease, Huntington's disease, and amyotrophic lateral sclerosis. Disease induction was performed using different models, including human tissues, animal models, and cultured cells. Most investigations were related to PA, although some were related to APA or both. Amyloid precursor protein (APP), Tau, SNCA, and STMN2 were the major genes identified; most of the altered PA patterns were related to mRNA stability and translation efficiency. This review particularly underscores the key roles of PA and APA in the pathogenesis of NDDs through their mechanisms that contribute to gene expression dysregulation, protein aggregation, and neuronal dysfunction. Insights into these mechanisms may lead to new therapeutic strategies focused on the modulation of PA and APA activities. Further research is required to investigate the translational potential of targeting these pathways for NDD treatment.

Characterization and molecular targeting of CFIm25 (NUDT21/CPSF5) mRNA using miRNAs

Changes in protein levels of the mammalian cleavage factor, CFIm25, play a role in regulating pathological processes including neural dysfunction, fibrosis, and tumorigenesis. However, despite these effects, little is known about how CFIm25 (NUDT21) expression is regulated at the RNA level. A potential regulator of NUDT21 mRNA are small non-coding microRNAs (miRNAs). In general, miRNAs bind to the 3'untranslated regions (3'UTRs) and can target the bound mRNA for degradation or inhibit translation thus affecting the levels of protein in cells. Interestingly, a mechanism known as alternative polyadenylation (APA) enables mRNAs to escape miRNA regulation by generating mRNAs with 3'UTRs of different sizes. As many miRNA target sites are located within the 3'UTR, shortening the 3'UTR allows mRNAs to evade miRNAs targeting this region. The differences in the lengths and the sequence composition of the 3'UTRs may also impact the mRNA's translatability and subcellular localization. APA has been reported to regulate over 70% of protein coding genes, thus increasing the transcript repertoire. Several proteins, including mammalian cleavage factor, CFIm25 (NUDT21), have been shown to regulate APA. In this study we wanted to determine whether CFIm25 (NUDT21), itself a regulator of APA, undergoes APA to evade miRNA regulation. We used the blood cancer mantle cell lymphoma (MCL) cells as a model and showed that in these cells, NUDT21 is relatively stable with a long half-life. In addition, the NUDT21 pre-mRNA undergoes alternative APA within the same terminal exon. The three different sized NUDT21 mRNAs have different 3'UTR lengths and they each use a different canonical polyadenylation signal, AAUAAA, for 3'end cleavage and polyadenylation. Use of miRNA mimics and inhibitors showed that miR-23a, miR-222, and miR-323a play a significant role in regulating NUDT21 expression. Hence, these results suggest that NUDT21 mRNA is stable and the different 3'UTRs generated through APA of NUDT21 play an important role in evading miRNA regulation and offers insights into how levels of CFIm25 (NUDT21) may be fine-tuned as needed under different physiological and pathological conditions.

Fateful Decisions of Where to Cut the Line: Pathology Associated with Aberrant 3' End Processing and Transcription Termination

Aberrant gene expression lies at the heart of many pathologies. This review will point out how 3' end processing, the final mRNA-maturation step in the transcription cycle, is surprisingly prone to regulated as well as stochastic variations with a wide range of consequences. Whereas smaller variations contribute to the plasticity of gene expression, larger alternations to 3' end processing and coupled transcription termination can lead to pathological consequences. These can be caused by the local mutation of one gene or affect larger numbers of genes systematically, if aspects of the mechanisms of 3' end processing and transcription termination are altered.

Decoding protein-RNA interactions using CLIP-based methodologies

Protein-RNA interactions are central to all RNA processing events, with pivotal roles in the regulation of gene expression and cellular functions. Dysregulation of these interactions has been increasingly linked to the pathogenesis of human diseases. High-throughput approaches to identify RNA-binding proteins and their binding sites on RNA - in particular, ultraviolet crosslinking followed by immunoprecipitation (CLIP) - have helped to map the RNA interactome, yielding transcriptome-wide protein-RNA atlases that have contributed to key mechanistic insights into gene expression and gene-regulatory networks. Here, we review these recent advances, explore the effects of cellular context on RNA binding, and discuss how these insights are shaping our understanding of cellular biology. We also review the potential therapeutic applications arising from new knowledge of protein-RNA interactions.

Light regulates widespread plant alternative polyadenylation through the chloroplast

Transcription of eukaryotic protein-coding genes generates immature mRNAs that are subjected to a series of processing events, including capping, splicing, cleavage, and polyadenylation (CPA), and chemical modifications of bases. Alternative polyadenylation (APA) greatly contributes to mRNA diversity in the cell. By determining the length of the 3' untranslated region, APA generates transcripts with different regulatory elements, such as miRNA and RBP binding sites, which can influence mRNA stability, turnover, and translation. In the model plant Arabidopsis thaliana, APA is involved in the control of seed dormancy and flowering. In view of the physiological importance of APA in plants, we decided to investigate the effects of light/dark conditions and compare the underlying mechanisms to those elucidated for alternative splicing (AS). We found that light controls APA in approximately 30% of Arabidopsis genes. Similar to AS, the effect of light on APA requires functional chloroplasts, is not affected in mutants of the phytochrome and cryptochrome photoreceptor pathways, and is observed in roots only when the communication with the photosynthetic tissues is not interrupted. Furthermore, mitochondrial and TOR kinase activities are necessary for the effect of light. However, unlike AS, coupling with transcriptional elongation does not seem to be involved since light-dependent APA regulation is neither abolished in mutants of the TFIIS transcript elongation factor nor universally affected by chromatin relaxation caused by histone deacetylase inhibition. Instead, regulation seems to correlate with changes in the abundance of constitutive CPA factors, also mediated by the chloroplast.

Alternative polyadenylation determines the functional landscape of inverted Alu repeats

Inverted Alu repeats (IRAlus) are abundantly found in the transcriptome, especially in introns and 3' untranslated regions (UTRs). Yet, the biological significance of IRAlus embedded in 3' UTRs remains largely unknown. Here, we find that 3' UTR IRAlus silences genes involved in essential signaling pathways. We utilize J2 antibody to directly capture and map the double-stranded RNA structure of 3' UTR IRAlus in the transcriptome. Bioinformatic analysis reveals alternative polyadenylation as a major axis of IRAlus-mediated gene regulation. Notably, the expression of mouse double minute 2 (MDM2), an inhibitor of p53, is upregulated by the exclusion of IRAlus during UTR shortening, which is exploited to silence p53 during tumorigenesis. Moreover, the transcriptome-wide UTR lengthening in neural progenitor cells results in the global downregulation of genes associated with neurodegenerative diseases, including amyotrophic lateral sclerosis, via IRAlus inclusion. Our study establishes the functional landscape of 3' UTR IRAlus and its role in human pathophysiology.

A distinct class of pan-cancer susceptibility genes revealed by an alternative polyadenylation transcriptome-wide association study

Alternative polyadenylation plays an important role in cancer initiation and progression; however, current transcriptome-wide association studies mostly ignore alternative polyadenylation when identifying putative cancer susceptibility genes. Here, we perform a pan-cancer 3' untranslated region alternative polyadenylation transcriptome-wide association analysis by integrating 55 well-powered (n > 50,000) genome-wide association studies datasets across 22 major cancer types with alternative polyadenylation quantification from 23,955 RNA sequencing samples across 7,574 individuals. We find that genetic variants associated with alternative polyadenylation are co-localized with 28.57% of cancer loci and contribute a significant portion of cancer heritability. We further identify 642 significant cancer susceptibility genes predicted to modulate cancer risk via alternative polyadenylation, 62.46% of which have been overlooked by traditional expression- and splicing- studies. As proof of principle validation, we show that alternative alleles facilitate 3' untranslated region lengthening of CRLS1 gene leading to increased protein abundance and promoted proliferation of breast cancer cells. Together, our study highlights the significant role of alternative polyadenylation in discovering new cancer susceptibility genes and provides a strong foundational framework for enhancing our understanding of the etiology underlying human cancers.

Multiplexed screening reveals how cancer-specific alternative polyadenylation shapes tumor growth in vivo

Alternative polyadenylation (APA) is strikingly dysregulated in many cancers. Although global APA dysregulation is frequently associated with poor prognosis, the importance of most individual APA events is controversial simply because few have been functionally studied. Here, we address this gap by developing a CRISPR-Cas9-based screen to manipulate endogenous polyadenylation and systematically quantify how APA events contribute to tumor growth in vivo. Our screen reveals individual APA events that control mouse melanoma growth in an immunocompetent host, with concordant associations in clinical human cancer. For example, forced Atg7 3' UTR lengthening in mouse melanoma suppresses ATG7 protein levels, slows tumor growth, and improves host survival; similarly, in clinical human melanoma, a long ATG7 3' UTR is associated with significantly prolonged patient survival. Overall, our study provides an easily adaptable means to functionally dissect APA in physiological systems and directly quantifies the contributions of recurrent APA events to tumorigenic phenotypes.

WDR33 alternative polyadenylation is dependent on stochastic poly(a) site usage and splicing efficiencies

Transcripts from the human WDR33 gene, which encodes a central component of the mRNA polyadenylation (PA) machinery, are subject to alternative polyadenylation (APA) within promoter-proximal introns/exons. This APA, which itself involves usage of multiple PA sites, results in the production of two non-canonical protein isoforms, V2 and V3, that are functionally completely unrelated to the full-length protein, with roles in innate immunity. The mechanism and regulation of WDR33 APA are unclear. Here, we report that levels of the PA factor CFIm25 modulate V2 and V3 expression, and that PA site usage of both V2 and V3 varies in distinct immune responses. Using newly developed assays to measure splicing and PA site strength, we show that splicing of V2-associated intron 6 is inefficient, allowing V2 to be produced using weak PA sites. Usage of V3's strong PA sites, on the other hand, is relatively low, reflecting the high efficiency of intron 7 splicing coupled with dependency on usage of an alternative 3' splice site within the intron. Overall, our findings demonstrate that usage of WDR33 alternative PA sites is stochastic, dependent on a complex interplay between splicing and PA, and thus provide new insights into mechanisms underlying APA.

Restrictor synergizes with Symplekin and PNUTS to terminate extragenic transcription

Transcription termination pathways mitigate the detrimental consequences of unscheduled promiscuous initiation occurring at hundreds of thousands of genomic cis-regulatory elements. The Restrictor complex, composed of the Pol II-interacting protein WDR82 and the RNA-binding protein ZC3H4, suppresses processive transcription at thousands of extragenic sites in mammalian genomes. Restrictor-driven termination does not involve nascent RNA cleavage, and its interplay with other termination machineries is unclear. Here we show that efficient termination at Restrictor-controlled extragenic transcription units involves the recruitment of the protein phosphatase 1 (PP1) regulatory subunit PNUTS, a negative regulator of the SPT5 elongation factor, and Symplekin, a protein associated with RNA cleavage complexes but also involved in cleavage-independent and phosphatase-dependent termination of noncoding RNAs in yeast. PNUTS and Symplekin act synergistically with, but independently from, Restrictor to dampen processive extragenic transcription. Moreover, the presence of limiting nuclear levels of Symplekin imposes a competition for its recruitment among multiple transcription termination machineries, resulting in mutual regulatory interactions. Hence, by synergizing with Restrictor, Symplekin and PNUTS enable efficient termination of processive, long-range extragenic transcription.

Genetic Variants That Impact Alternative Polyadenylation in Cancer Represent Candidate Causal Risk Loci

Alternative polyadenylation (APA) is emerging as a major mechanism of posttranscriptional regulation. APA can impact the development and progression of cancer, suggesting that the genetic determinants of APA might play an important role in regulating cancer risk. Here, we depicted a pan-cancer atlas of human APA quantitative trait loci (apaQTL), containing approximately 0.7 million apaQTLs across 32 cancer types. Systematic multiomics analyses indicated that cancer apaQTLs could contribute to APA regulation by altering poly(A) motifs, RNA-binding proteins (RBP), and chromatin regulatory elements and were preferentially enriched in genome-wide association studies (GWAS)-identified cancer susceptibility loci. Moreover, apaQTL-related genes (aGene) were broadly related to cancer signaling pathways, high mutational burden, immune infiltration, and drug response, implicating their potential as therapeutic targets. Furthermore, apaQTLs were mapped in Chinese colorectal cancer tumor tissues and then screened for functional apaQTLs associated with colorectal cancer risk in 17,789 cases and 19,951 controls using GWAS-ChIP data, with independent validation in a large-scale population consisting of 6,024 cases and 10,022 controls. A multi-ancestry-associated apaQTL variant rs1020670 with a C>G change in DNM1L was identified, and the G allele contributed to an increased risk of colorectal cancer. Mechanistically, the risk variant promoted aberrant APA and facilitated higher usage of DNM1L proximal poly(A) sites mediated by the RBP CSTF2T, which led to higher expression of DNM1L with a short 3'UTR. This stabilized DNM1L to upregulate its expression, provoking colorectal cancer cell proliferation. Collectively, these findings generate a resource for understanding APA regulation and the genetic basis of human cancers, providing insights into cancer etiology.

SIGNIFICANCE

Cancer risk is mediated by alternative polyadenylation quantitative trait loci, including the rs1020670-G variant that promotes alternative polyadenylation of DNM1L and increases colorectal cancer risk.

CPSF6 regulates alternative polyadenylation and proliferation of cancer cells through phase separation

Cancer cells usually exhibit shortened 3' untranslated regions (UTRs) due to alternative polyadenylation (APA) to promote cell proliferation and migration. Upregulated CPSF6 leads to a systematic prolongation of 3' UTRs, but CPSF6 expression in tumors is typically higher than that in healthy tissues. This contradictory observation suggests that it is necessary to investigate the underlying mechanism by which CPSF6 regulates APA switching in cancer. Here, we find that CPSF6 can undergo liquid-liquid phase separation (LLPS), and elevated LLPS is associated with the preferential usage of the distal poly(A) sites. CLK2, a kinase upregulated in cancer cells, destructs CPSF6 LLPS by phosphorylating its arginine/serine-like domain. The reduction of CPSF6 LLPS can lead to a shortened 3' UTR of cell-cycle-related genes and accelerate cell proliferation. These results suggest that CPSF6 LLPS, rather than its expression level, may be responsible for APA regulation in cancer cells.

CSTF2 mediated mRNA N6-methyladenosine modification drives pancreatic ductal adenocarcinoma m6A subtypes

N6-methyladenosine (m6A) modification of gene transcripts plays critical roles in cancer. Here we report transcriptomic m6A profiling in 98 tissue samples from 65 individuals with pancreatic ductal adenocarcinoma (PDAC). We identify 17,996 m6A peaks with 195 hyper-methylated and 93 hypo-methylated in PDAC compared with adjacent normal tissues. The differential m6A modifications distinguish two PDAC subtypes with different prognosis outcomes. The formation of the two subtypes is driven by a newly identified m6A regulator CSTF2 that co-transcriptionally regulates m6A installation through slowing the RNA Pol II elongation rate during gene transcription. We find that most of the CSTF2-regulated m6As have positive effects on the RNA level of host genes, and CSTF2-regulated m6As are mainly recognized by IGF2BP2, an m6A reader that stabilizes mRNAs. These results provide a promising PDAC subtyping strategy and potential therapeutic targets for precision medicine of PDAC.

A Novel Gene Pair CSTF2/DPE2A Impacts Prognosis and Cell Cycle of Hepatocellular Carcinoma

Background

Hepatocellular carcinoma (HCC), one of the commonest cancers at present, possesses elevated mortality. This study explored the predictive value of CSTF2/PDE2A for HCC prognosis.

Methods

In this study, clinical information and RNA sequencing expression profiles of HCC patients were acquired from common databases. Kaplan-Meier curve compound with time-dependent ROC curve, nomogram model, and univariate/multivariate Cox analysis were carried out to access the prediction capacity of CSTF2/PDE2A. The immune status, tumor microenvironment, drug sensitivity, biological function and pathway between HCC and adjacent non-tumorous tissue were analyzed and compared. Finally, RT-qPCR, Western blot, and apoptosis assays were performed to verify the effect on HCC cells of CSTF2/PDE2A.

Results

The optimal cut-off value of CSTF2, PDE2A and CSTF2/PDE2A was 6.95, 0.95 and 3.63, respectively. In TCGA and ICGC cohorts, the high group of CSTF2/PDE2A presented higher OS compared to low group. The area under the curve (AUC) for OS at 1-, 2-, and 3-years predicted by CSTF2/PDE2A were 0.731/0.695, 0.713/0.732 and 0.689/0.755, higher than the counterparts of the single gene CSTF2 and PDE2A. Multivariate Cox analysis revealed that CSTF2/PDE2A (HR = 1.860/3.236, 95% CI = 1.265-2.733/1.575-6.645) was an independent prognostic factor for HCC. The OS nomogram model created according to five independent factors including CSTF2/PDE2A showed excellent capacity for HCC prognosis. Furthermore, the immune status of the CSTF2/PDE2A high group was deleted, cell cycle-related genes and chemotherapy resistance were increased. Finally, cell experiments revealed distinct differences in the proliferation, apoptosis, protein and mRNA expression of HCC cells after si-CSTF2 transfection compared with the negative control.

Conclusion

Taken together, the gene pair CSTF2/PDE2A is able to forecast the prognosis of HCC and regulates cell cycle, which is promising as a novel prognostic predictor of HCC.

3'-end mRNA processing within apicomplexan parasites, a patchwork of classic, and unexpected players

The 3'-end processing of mRNA is a co-transcriptional process that leads to the formation of a poly-adenosine tail on the mRNA and directly controls termination of the RNA polymerase II juggernaut. This process involves a megadalton complex composed of cleavage and polyadenylation specificity factors (CPSFs) that are able to recognize cis-sequence elements on nascent mRNA to then carry out cleavage and polyadenylation reactions. Recent structural and biochemical studies have defined the roles played by different subunits of the complex and provided a comprehensive mechanistic understanding of this machinery in yeast or metazoans. More recently, the discovery of small molecule inhibitors of CPSF function in Apicomplexa has stimulated interest in studying the specificities of this ancient eukaryotic machinery in these organisms. Although its function is conserved in Apicomplexa, the CPSF complex integrates a novel reader of the N6-methyladenosine (m6A). This feature, inherited from the plant kingdom, bridges m6A metabolism directly to 3'-end processing and by extension, to transcription termination. In this review, we will examine convergence and divergence of CPSF within the apicomplexan parasites and explore the potential of small molecule inhibition of this machinery within these organisms. This article is categorized under: RNA Processing > 3' End Processing RNA Processing > RNA Editing and Modification.

The U1 antisense morpholino oligonucleotide (AMO) disrupts U1 snRNP structure to promote intronic PCPA modification of pre-mRNAs

Functional depletion of the U1 small nuclear ribonucleoprotein (snRNP) with a 25 nt U1 AMO (antisense morpholino oligonucleotide) may lead to intronic premature cleavage and polyadenylation of thousands of genes, a phenomenon known as U1 snRNP telescripting; however, the underlying mechanism remains elusive. In this study, we demonstrated that U1 AMO could disrupt U1 snRNP structure both in vitro and in vivo, thereby affecting the U1 snRNP-RNAP polymerase II interaction. By performing chromatin immunoprecipitation sequencing for phosphorylation of Ser2 and Ser5 of the C-terminal domain of RPB1, the largest subunit of RNAP polymerase II, we showed that transcription elongation was disturbed upon U1 AMO treatment, with a particular high phosphorylation of Ser2 signal at intronic cryptic polyadenylation sites (PASs). In addition, we showed that core 3'processing factors CPSF/CstF are involved in the processing of intronic cryptic PAS. Their recruitment accumulated toward cryptic PASs upon U1 AMO treatment, as indicated by chromatin immunoprecipitation sequencing and individual-nucleotide resolution CrossLinking and ImmunoPrecipitation sequencing analysis. Conclusively, our data suggest that disruption of U1 snRNP structure mediated by U1 AMO provides a key for understanding the U1 telescripting mechanism.

Deciphering the RNA-binding protein interaction with the mRNAs encoded from human chromosome 15q11.2 BP1-BP2 microdeletion region

Microdeletion of the 15q11.2 BP1-BP2 region, also known as Burnside-Butler susceptibility region, is associated with phenotypes like delayed developmental language abilities along with motor skill disabilities, combined with behavioral and emotional problems. The 15q11.2 microdeletion region harbors four evolutionarily conserved and non-imprinted protein-coding genes: NIPA1, NIPA2, CYFIP1, and TUBGCP5. This microdeletion is a rare copy number variation frequently associated with several pathogenic conditions in humans. The aim of this study is to investigate the RNA-binding proteins binding with the four genes present in 15q11.2 BP1-BP2 microdeletion region. The results of this study will help to better understand the molecular intricacies of the Burnside-Butler Syndrome and also the possible involvement of these interactions in the disease aetiology. Our results of enhanced crosslinking and immunoprecipitation data analysis indicate that most of the RBPs interacting with the 15q11.2 region are involved in the post-transcriptional regulation of the concerned genes. The RBPs binding to this region are found from the in silico analysis, and the interaction of RBPs like FASTKD2 and EFTUD2 with exon-intron junction sequence of CYFIP1 and TUBGCP5 has also been validated by combined EMSA and western blotting experiment. The exon-intron junction binding nature of these proteins suggests their potential involvement in splicing process. This study may help to understand the intricate relationship of RBPs with mRNAs within this region, along with their functional significance in normal development, and lack thereof, in neurodevelopmental disorders. This understanding will help in the formulation of better therapeutic approaches.

The RNA-binding protein CSTF2 regulates BAD to inhibit apoptosis in glioblastoma

RNA-binding proteins (RBP) regulate several aspects of co- and post-transcriptional gene expression in cancer cells. CSTF2 is involved in the expression of many cellular mRNAs and involved in the 3'-end cleavage and polyadenylation of pre-mRNAs to terminate transcription. However, the role of CSTF2 in human glioblastoma (GBM) and the underlying mechanisms remain unclear. In the present study, CSTF2 was found to be upregulated in GBM, and its high expression predicted poor prognosis. Knockdown CSTF2 induced GBM cell apoptosis both in vitro and in vivo. Specific mechanism studies showed that CSTF2 unstabilized the mRNA of the BAD protein by shortening its 3' UTR. Additionally, an increase in the expression level of CSTF2 decreased the expression level of BAD. In conclusion, CSTF2 binds to the mRNA of the BAD protein to shorten its 3'UTR, which negatively affects the BAD mediated apoptosis and promotes GBM cell survival.

Macrophage differentiation is marked by increased abundance of the mRNA 3' end processing machinery, altered poly(A) site usage, and sensitivity to the level of CstF64

Regulation of mRNA polyadenylation is important for response to external signals and differentiation in several cell types, and results in mRNA isoforms that vary in the amount of coding sequence or 3' UTR regulatory elements. However, its role in differentiation of monocytes to macrophages has not been investigated. Macrophages are key effectors of the innate immune system that help control infection and promote tissue-repair. However, overactivity of macrophages contributes to pathogenesis of many diseases. In this study, we show that macrophage differentiation is characterized by shortening and lengthening of mRNAs in relevant cellular pathways. The cleavage/polyadenylation (C/P) proteins increase during differentiation, suggesting a possible mechanism for the observed changes in poly(A) site usage. This was surprising since higher C/P protein levels correlate with higher proliferation rates in other systems, but monocytes stop dividing after induction of differentiation. Depletion of CstF64, a C/P protein and known regulator of polyadenylation efficiency, delayed macrophage marker expression, cell cycle exit, attachment, and acquisition of structural complexity, and impeded shortening of mRNAs with functions relevant to macrophage biology. Conversely, CstF64 overexpression increased use of promoter-proximal poly(A) sites and caused the appearance of differentiated phenotypes in the absence of induction. Our findings indicate that regulation of polyadenylation plays an important role in macrophage differentiation.

U1 snRNP proteins promote proximal alternative polyadenylation sites by directly interacting with 3' end processing core factors

In eukaryotic cells, both alternative splicing and alternative polyadenylation (APA) play essential roles in the gene regulation network. U1 small ribonucleoprotein particle (U1 snRNP) is a major component of spliceosome, and U1 snRNP complex can suppress proximal APA sites through crosstalking with 3' end processing factors. However, here we show that both knockdown and overexpression of SNRPA, SNRPC, SNRNP70, and SNRPD2, the U1 snRNP proteins, promote the usage of proximal APA sites at the transcriptome level. SNRNP70 can drive the phase transition of PABPN1 from droplet to aggregate, which may reduce the repressive effects of PABPN1 on the proximal APA sites. Additionally, SNRNP70 can also promote the proximal APA sites by recruiting CPSF6, suggesting that the function of CPSF6 on APA is related with other RNA-binding proteins and cell context-dependent. Consequently, these results reveal that, on the contrary to U1 snRNP complex, the free proteins of U1 snRNP complex can promote proximal APA sites through the interaction with 3' end processing machinery.

The transcriptional terminator XRN2 and the RNA-binding protein Sam68 link alternative polyadenylation to cell cycle progression in prostate cancer

Alternative polyadenylation (APA) yields transcripts differing in their 3'-end, and its regulation is altered in cancer, including prostate cancer. Here we have uncovered a mechanism of APA regulation impinging on the interaction between the exonuclease XRN2 and the RNA-binding protein Sam68, whose increased expression in prostate cancer is promoted by the transcription factor MYC. Genome-wide transcriptome profiling revealed a widespread impact of the Sam68/XRN2 complex on APA. XRN2 promotes recruitment of Sam68 to its target transcripts, where it competes with the cleavage and polyadenylation specificity factor for binding to strong polyadenylation signals at distal ends of genes, thus promoting usage of suboptimal proximal polyadenylation signals. This mechanism leads to 3' untranslated region shortening and translation of transcripts encoding proteins involved in G1/S progression and proliferation. Thus, our findings indicate that the APA program driven by Sam68/XRN2 promotes cell cycle progression and may represent an actionable target for therapeutic intervention.

Diverse cell-specific patterns of alternative polyadenylation in Drosophila

Most genes in higher eukaryotes express isoforms with distinct 3' untranslated regions (3' UTRs), generated by alternative polyadenylation (APA). Since 3' UTRs are predominant locations of post-transcriptional regulation, APA can render such programs conditional, and can also alter protein sequences via alternative last exon (ALE) isoforms. We previously used 3'-sequencing from diverse Drosophila samples to define multiple tissue-specific APA landscapes. Here, we exploit comprehensive single nucleus RNA-sequencing data (Fly Cell Atlas) to elucidate cell-type expression of 3' UTRs across >250 adult Drosophila cell types. We reveal the cellular bases of multiple tissue-specific APA/ALE programs, such as 3' UTR lengthening in differentiated neurons and 3' UTR shortening in spermatocytes and spermatids. We trace dynamic 3' UTR patterns across cell lineages, including in the male germline, and discover new APA patterns in the intestinal stem cell lineage. Finally, we correlate expression of RNA binding proteins (RBPs), miRNAs and global levels of cleavage and polyadenylation (CPA) factors in several cell types that exhibit characteristic APA landscapes, yielding candidate regulators of transcriptome complexity. These analyses provide a comprehensive foundation for future investigations of mechanisms and biological impacts of alternative 3' isoforms across the major cell types of this widely-studied model organism.

The androgen receptor couples promoter recruitment of RNA processing factors to regulation of alternative polyadenylation at the 3' end of transcripts

Prostate cancer (PC) relies on androgen receptor (AR) signaling. While hormonal therapy (HT) is efficacious, most patients evolve to an incurable castration-resistant stage (CRPC). To date, most proposed mechanisms of acquired resistance to HT have focused on AR transcriptional activity. Herein, we uncover a new role for the AR in alternative cleavage and polyadenylation (APA). Inhibition of the AR by Enzalutamide globally regulates APA in PC cells, with specific enrichment in genes related to transcription and DNA topology, suggesting their involvement in transcriptome reprogramming. AR inhibition selects promoter-distal polyadenylation sites (pAs) enriched in cis-elements recognized by the cleavage and polyadenylation specificity factor (CPSF) complex. Conversely, promoter-proximal intronic pAs relying on the cleavage stimulation factor (CSTF) complex are repressed. Mechanistically, Enzalutamide induces rearrangement of APA subcomplexes and impairs the interaction between CPSF and CSTF. AR inhibition also induces co-transcriptional CPSF recruitment to gene promoters, predisposing the selection of pAs depending on this complex. Importantly, the scaffold CPSF160 protein is up-regulated in CRPC cells and its depletion represses HT-induced APA patterns. These findings uncover an unexpected role for the AR in APA regulation and suggest that APA-mediated transcriptome reprogramming represents an adaptive response of PC cells to HT.

Alternative polyadenylation regulation: insights from sequential polyadenylation

The processing of the proximal and distal poly(A) sites in alternative polyadenylation (APA) has long been thought to independently occur on pre-mRNAs during transcription. However, a recent study by our groups demonstrated that the proximal sites for many genes could be activated sequentially following the distal ones, suggesting a multi-cleavage-same-transcript mode beyond the canonical one-cleavage-per-transcript view. Here, we review the established mechanisms for APA regulation and then discuss the additional insights into APA regulation from the perspective of sequential polyadenylation, resulting in a unified leverage model for understanding the mechanisms of regulated APA.

CSTF2 Promotes Hepatocarcinogenesis and Hepatocellular Carcinoma Progression via Aerobic Glycolysis

Background

The shortening of 3’ untranslated regions (3’UTRs) of messenger RNAs(mRNAs) by alternative polyadenylation (APA) is an important mechanism for oncogene activation. Cleavage stimulation factor 2 (CSTF2), an important regulator of APA, has been reported to have a tumorigenic function in urothelial carcinoma of the bladder and lung cancers. However, the tumor-promoting role of CSTF2 in hepatocellular carcinoma (HCC) and its underlying molecular mechanism remains unclear.

Methods

Multiple databases were used to analyze the expression level and prognostic value of CSTF2 in HCC. Function enrichment analysis was used to investigate the molecular mechanism of CSTF2 for the occurrence and development of HCC. The biological function in HCC cell lines in vitro was determined by CCK8, colony formation, Transwell migration, and invasion assay. Moreover, the tumorigenic function of CSTF2 in vivo was measured by a subcutaneous tumor formation or injecting four plasmids into a mouse tail vein within 5–7 s in an immunocompetent HCC mouse model. In addition, aerobic glycolysis in HCC cells was determined by measuring the extracellular acid rate (ECAR) and extracellular glucose and lactate levels.

Results

Bioinformatics analysis revealed that CSTF2 was overexpressed in HCC tissues. The high expression of CSTF2 was correlated with a poor prognosis and high histological grades. CSTF2 knockout inhibited the proliferation, migration, and invasion of HCC cells. In addition, CSTF2 knockout HCC cells failed to form tumors by a subcutaneous graft experiment. Furthermore, endogenous CSTF2 knockout attenuated hepatocarcinogenesis in an immunocompetent HCC mouse model. Function enrichment analysis suggested that the high expression of CSTF2 was associated with enhanced glycolysis. Moreover, we found that CSTF2 knockout reduced the level of the short 3’ UTR isoform of hexokinase 2 and increased its level of long 3’UTR. Furthermore, CSTF2 knockout inhibited ECAR levels, glucose uptake, and lactate production.

Conclusion

Our results indicated that CSTF2 is highly expressed in HCC and is correlated with a poor prognosis and high histological grade. The knockout of CSTF2 inhibits the tumorigenesis and procession of HCC both in vitro and in vivo. Moreover, CSTF2 is associated with enhanced glycolysis. Therefore, this study suggests that CSTF2 might be a new prognostic biomarker and therapeutic target for HCC.

CFIm25 regulates human stem cell function independently of its role in mRNA alternative polyadenylation

It has recently been shown that CFIm25, a canonical mRNA 3’ processing factor, could play a variety of physiological roles through its molecular function in the regulation of mRNA alternative polyadenylation (APA). Here, we used CRISPR/Cas9-mediated gene editing approach in human embryonic stem cells (hESCs) for CFIm25, and obtained three gene knockdown/mutant cell lines. CFIm25 gene editing resulted in higher proliferation rate and impaired differentiation potential for hESCs, with these effects likely to be directly regulated by the target genes, including the pluripotency factor rex1. Mechanistically, we unexpected found that perturbation in CFIm25 gene expression did not significantly affect cellular mRNA 3’ processing efficiency and APA profile. Rather, we provided evidences that CFIm25 may impact RNA polymerase II (RNAPII) occupancy at the body of transcribed genes, and promote the expression level of a group of transcripts associated with cellular proliferation and/or differentiation. Taken together, these results reveal novel mechanisms underlying CFIm25ʹs modulation in determination of cell fate, and provide evidence that the process of mammalian gene transcription may be regulated by an mRNA 3’ processing factor.

Regulation of the Alternative Neural Transcriptome by ELAV/Hu RNA Binding Proteins

The process of alternative polyadenylation (APA) generates multiple 3' UTR isoforms for a given locus, which can alter regulatory capacity and on occasion change coding potential. APA was initially characterized for a few genes, but in the past decade, has been found to be the rule for metazoan genes. While numerous differences in APA profiles have been catalogued across genetic conditions, perturbations, and diseases, our knowledge of APA mechanisms and biology is far from complete. In this review, we highlight recent findings regarding the role of the conserved ELAV/Hu family of RNA binding proteins (RBPs) in generating the broad landscape of lengthened 3' UTRs that is characteristic of neurons. We relate this to their established roles in alternative splicing, and summarize ongoing directions that will further elucidate the molecular strategies for neural APA, the in vivo functions of ELAV/Hu RBPs, and the phenotypic consequences of these regulatory paradigms in neurons.

Alternative polyadenylation by sequential activation of distal and proximal PolyA sites

Analogous to alternative splicing, alternative polyadenylation (APA) has long been thought to occur independently at proximal and distal polyA sites. Using fractionation-seq, we unexpectedly identified several hundred APA genes in human cells whose distal polyA isoforms are retained in chromatin/nuclear matrix and whose proximal polyA isoforms are released into the cytoplasm. Global metabolic PAS-seq and Nanopore long-read RNA-sequencing provide further evidence that the strong distal polyA sites are processed first and the resulting transcripts are subsequently anchored in chromatin/nuclear matrix to serve as precursors for further processing at proximal polyA sites. Inserting an autocleavable ribozyme between the proximal and distal polyA sites, coupled with a Cleave-seq approach that we describe here, confirms that the distal polyA isoform is indeed the precursor to the proximal polyA isoform. Therefore, unlike alternative splicing, APA sites are recognized independently, and in many cases, in a sequential manner. This provides a versatile strategy to regulate gene expression in mammalian cells.

CstF64-Induced Shortening of the BID 3'UTR Promotes Esophageal Squamous Cell Carcinoma Progression by Disrupting ceRNA Cross-talk with ZFP36L2

The majority of human genes have multiple polyadenylation sites, which are differentially used through the process of alternative polyadenylation (APA). Dysregulation of APA contributes to numerous diseases, including cancer. However, specific genes subject to APA that impact oncogenesis have not been well characterized, and many cancer APA landscapes remain underexplored. Here, we used dynamic analyses of APA from RNA-seq (DaPars) to define both the 3'UTR APA profile in esophageal squamous cell carcinoma (ESCC) and to identify 3'UTR shortening events that may drive tumor progression. In four distinct squamous cell carcinoma datasets, BID 3'UTRs were recurrently shortened and BID mRNA levels were significantly upregulated. Moreover, system correlation analysis revealed that CstF64 is a candidate upstream regulator of BID 3'UTR length. Mechanistically, a shortened BID 3'UTR promoted proliferation of ESCC cells by disrupting competing endogenous RNA (ceRNA) cross-talk, resulting in downregulation of the tumor suppressor gene ZFP36L2. These in vitro and in vivo results were supported by human patient data whereby 3'UTR shortening of BID and low expression of ZFP36L2 are prognostic factors of survival in ESCC. Collectively, these findings demonstrate that a key ceRNA network is disrupted through APA and promotes ESCC tumor progression.Significance: High-throughput analysis of alternative polyadenylation in esophageal squamous cell carcinoma identifies recurrent shortening of the BID 3'UTR as a driver of disease progression.

Multi-resBind: a residual network-based multi-label classifier for in vivo RNA binding prediction and preference visualization

BACKGROUND

Protein-RNA interactions play key roles in many processes regulating gene expression. To understand the underlying binding preference, ultraviolet cross-linking and immunoprecipitation (CLIP)-based methods have been used to identify the binding sites for hundreds of RNA-binding proteins (RBPs) in vivo. Using these large-scale experimental data to infer RNA binding preference and predict missing binding sites has become a great challenge. Some existing deep-learning models have demonstrated high prediction accuracy for individual RBPs. However, it remains difficult to avoid significant bias due to the experimental protocol. The DeepRiPe method was recently developed to solve this problem via introducing multi-task or multi-label learning into this field. However, this method has not reached an ideal level of prediction power due to the weak neural network architecture.

RESULTS

Compared to the DeepRiPe approach, our Multi-resBind method demonstrated substantial improvements using the same large-scale PAR-CLIP dataset with respect to an increase in the area under the receiver operating characteristic curve and average precision. We conducted extensive experiments to evaluate the impact of various types of input data on the final prediction accuracy. The same approach was used to evaluate the effect of loss functions. Finally, a modified integrated gradient was employed to generate attribution maps. The patterns disentangled from relative contributions according to context offer biological insights into the underlying mechanism of protein-RNA interactions.

CONCLUSIONS

Here, we propose Multi-resBind as a new multi-label deep-learning approach to infer protein-RNA binding preferences and predict novel interactions. The results clearly demonstrate that Multi-resBind is a promising tool to predict unknown binding sites in vivo and gain biology insights into why the neural network makes a given prediction.

mRNA alternative polyadenylation (APA) in regulation of gene expression and diseases

The mRNA polyadenylation plays essential function in regulation of mRNA metabolism. Mis-regulations of mRNA polyadenylation are frequently linked with aberrant gene expression and disease progression. Under the action of polyadenylate polymerase, poly(A) tail is synthesized after the polyadenylation signal (PAS) sites on the mRNAs. Alternative polyadenylation (APA) often occurs in mRNAs with multiple poly(A) sites, producing different 3' ends for transcript variants, and therefore plays important functions in gene expression regulation. In this review, we first summarize the classical process of mRNA 3'-terminal formation and discuss the length control mechanisms of poly(A) in nucleus and cytoplasm. Then we review the research progress on alternative polyadenylation regulation and the APA site selection mechanism. Finally, we summarize the functional roles of APA in the regulation of gene expression and diseases including cancers.

Alternative polyadenylation: An enigma of transcript length variation in health and disease

Alternative polyadenylation (APA) is a molecular mechanism during a pre-mRNA processing that involves usage of more than one polyadenylation site (PA-site) generating transcripts of varying length from a single gene. The location of a PA-site affects transcript length and coding potential of an mRNA contributing to both mRNA and protein diversification. This variation in the transcript length affects mRNA stability and translation, mRNA subcellular and tissue localization, and protein function. APA is now considered as an important regulatory mechanism in the pathophysiology of human diseases. An important consequence of the changes in the length of 3'-untranslated region (UTR) from disease-induced APA is altered protein expression. Yet, the relationship between 3'-UTR length and protein expression remains a paradox in a majority of diseases. Here, we review occurrence of APA, mechanism of PA-site selection, and consequences of transcript length variation in different diseases. Emerging evidence reveals coordinated involvement of core RNA processing factors including poly(A) polymerases in the PA-site selection in diseases-associated APAs. Targeting such APA regulators will be therapeutically significant in combating drug resistance in cancer and other complex diseases. This article is categorized under: RNA Processing > 3' End Processing RNA in Disease and Development > RNA in Disease Translation > Regulation.

The three as: Alternative splicing, alternative polyadenylation and their impact on apoptosis in immune function

The latest advances in next-generation sequencing studies and transcriptomic profiling over the past decade have highlighted a surprising frequency of genes regulated by RNA processing mechanisms in the immune system. In particular, two control steps in mRNA maturation, namely alternative splicing and alternative polyadenylation, are now recognized to occur in the vast majority of human genes. Both have the potential to alter the identity of the encoded protein, as well as control protein abundance or even protein localization or association with other factors. In this review, we will provide a summary of the general mechanisms by which alternative splicing (AS) and alternative polyadenylation (APA) occur, their regulation within cells of the immune system, and their impact on immunobiology. In particular, we will focus on how control of apoptosis by AS and APA is used to tune cell fate during an immune response.

Conserved long-range base pairings are associated with pre-mRNA processing of human genes

The ability of nucleic acids to form double-stranded structures is essential for all living systems on Earth. Current knowledge on functional RNA structures is focused on locally-occurring base pairs. However, crosslinking and proximity ligation experiments demonstrated that long-range RNA structures are highly abundant. Here, we present the most complete to-date catalog of conserved complementary regions (PCCRs) in human protein-coding genes. PCCRs tend to occur within introns, suppress intervening exons, and obstruct cryptic and inactive splice sites. Double-stranded structure of PCCRs is supported by decreased icSHAPE nucleotide accessibility, high abundance of RNA editing sites, and frequent occurrence of forked eCLIP peaks. Introns with PCCRs show a distinct splicing pattern in response to RNAPII slowdown suggesting that splicing is widely affected by co-transcriptional RNA folding. The enrichment of 3'-ends within PCCRs raises the intriguing hypothesis that coupling between RNA folding and splicing could mediate co-transcriptional suppression of premature pre-mRNA cleavage and polyadenylation.

On the function and relevance of alternative 3'-UTRs in gene expression regulation

Messanger RNA (mRNA) isoforms with alternative 3'-untranslated regions (3'-UTRs) are produced by alternative polyadenylation (APA), which occurs during transcription in most eukaryotic genes. APA fine-tunes gene expression in a cell-type- and cellular state-dependent manner. Selection of an APA site entails the binding of core cleavage and polyadenylation factors to a particular polyadenylation site localized in the pre-mRNA and is controlled by multiple regulatory determinants, including transcription, pre-mRNA cis-regulatory sequences, and protein factors. Alternative 3'-UTRs serve as platforms for specific RNA binding proteins and microRNAs, which regulate gene expression in a coordinated manner by controlling mRNA fate and function in the cell. Genome-wide studies illustrated the full extent of APA prevalence and revealed that specific 3'-UTR profiles are associated with particular cellular states and diseases. Generally, short 3'-UTRs are associated with proliferative and cancer cells, and long 3'-UTRs are mostly found in polarized and differentiated cells. Fundamental new insights on the physiological consequences of this widespread event and the molecular mechanisms involved have been revealed through single-cell studies. Publicly available comprehensive databases that cover all APA mRNA isoforms identified in many cellular states and diseases reveal specific APA signatures. Therapies tackling APA mRNA isoforms or APA regulators may be regarded as innovative and attractive tools for diagnostics or treatment of several pathologies. We highlight the function of APA and alternative 3'-UTRs in gene expression regulation, the control of these mechanisms, their physiological consequences, and their potential use as new biomarkers and therapeutic tools. This article is categorized under: RNA Processing > 3' End Processing RNA Interactions with Proteins and Other Molecules > Protein-RNA Interactions: Functional Implications RNA in Disease and Development > RNA in Disease.

Targeting Polyadenylation for Retention of RNA at Chromatin

The various steps of RNA polymerase II transcription, including transcription initiation, splicing, and termination, are interlinked and tightly coordinated. Efficient 3'end processing is defined by sequence motifs emerging in the nascent-transcribed RNA strand and the cotranscriptional binding of regulatory proteins. The processing of a mature 3'end consists of cleavage and polyadenylation and is coupled with RNA polymerase II transcription termination and the dissociation of the nascent RNA transcript from the chromatin-associated transcriptional template. The subcellular and subnuclear topological specificity of the various RNA species is important for their functions. For instance, the formation of RNA-binding protein interactions, critical for the final outcome of gene expression, may require the nucleoplasmic fully spliced and polyadenylated form of an RNA transcript. Thus, interfering with the critical step of transcription termination and 3'end formation provides a means for assaying the functional potential of a given RNA of interest.In this protocol, we describe a method for blocking 3'end processing of the nascent RNA transcript, by using RNase H-inactive antisense oligonucleotides targeting cleavage and polyadenylation, delivered via transient transfection in cell culture.

SRSF3 and SRSF7 modulate 3'UTR length through suppression or activation of proximal polyadenylation sites and regulation of CFIm levels

Background

Alternative polyadenylation (APA) refers to the regulated selection of polyadenylation sites (PASs) in transcripts, which determines the length of their 3′ untranslated regions (3′UTRs). We have recently shown that SRSF3 and SRSF7, two closely related SR proteins, connect APA with mRNA export. The mechanism underlying APA regulation by SRSF3 and SRSF7 remained unknown.

Results

Here we combine iCLIP and 3′-end sequencing and find that SRSF3 and SRSF7 bind upstream of proximal PASs (pPASs), but they exert opposite effects on 3′UTR length. SRSF7 enhances pPAS usage in a concentration-dependent but splicing-independent manner by recruiting the cleavage factor FIP1, generating short 3′UTRs. Protein domains unique to SRSF7, which are absent from SRSF3, contribute to FIP1 recruitment. In contrast, SRSF3 promotes distal PAS (dPAS) usage and hence long 3′UTRs directly by counteracting SRSF7, but also indirectly by maintaining high levels of cleavage factor Im (CFIm) via alternative splicing. Upon SRSF3 depletion, CFIm levels decrease and 3′UTRs are shortened. The indirect SRSF3 targets are particularly sensitive to low CFIm levels, because here CFIm serves a dual function; it enhances dPAS and inhibits pPAS usage by binding immediately downstream and assembling unproductive cleavage complexes, which together promotes long 3′UTRs.

Conclusions

We demonstrate that SRSF3 and SRSF7 are direct modulators of pPAS usage and show how small differences in the domain architecture of SR proteins can confer opposite effects on pPAS regulation.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13059-021-02298-y.

Alternative polyadenylation: methods, mechanism, function, and role in cancer

Occurring in over 60% of human genes, alternative polyadenylation (APA) results in numerous transcripts with differing 3'ends, thus greatly expanding the diversity of mRNAs and of proteins derived from a single gene. As a key molecular mechanism, APA is involved in various gene regulation steps including mRNA maturation, mRNA stability, cellular RNA decay, and protein diversification. APA is frequently dysregulated in cancers leading to changes in oncogenes and tumor suppressor gene expressions. Recent studies have revealed various APA regulatory mechanisms that promote the development and progression of a number of human diseases, including cancer. Here, we provide an overview of four types of APA and their impacts on gene regulation. We focus particularly on the interaction of APA with microRNAs, RNA binding proteins and other related factors, the core pre-mRNA 3'end processing complex, and 3'UTR length change. We also describe next-generation sequencing methods and computational tools for use in poly(A) signal detection and APA repositories and databases. Finally, we summarize the current understanding of APA in cancer and provide our vision for future APA related research.

U1 snRNP telescripting: molecular mechanisms and beyond

U1 snRNP is one of the most abundant ribonucleoprotein (RNP) complexes in eukaryotic cells and is estimated to be approximately 1 million copies per cell. Apart from its canonical role in mRNA splicing, this complex has emerged as a key regulator of eukaryotic mRNA length via inhibition of mRNA 3'-end processing at numerous intronic polyadenylation sites, in a process that is also termed 'U1 snRNP telescripting'. Several reviews have extensively described the concept of U1 telescripting and subsequently highlighted its potential impacts in mRNA metabolism. Here, we review what is currently known regarding the underlying mechanisms of this important phenomenon and discuss open questions and future challenges.

A missense mutation in the CSTF2 gene that impairs the function of the RNA recognition motif and causes defects in 3' end processing is associated with intellectual disability in humans

CSTF2 encodes an RNA-binding protein that is essential for mRNA cleavage and polyadenylation (C/P). No disease-associated mutations have been described for this gene. Here, we report a mutation in the RNA recognition motif (RRM) of CSTF2 that changes an aspartic acid at position 50 to alanine (p.D50A), resulting in intellectual disability in male patients. In mice, this mutation was sufficient to alter polyadenylation sites in over 1300 genes critical for brain development. Using a reporter gene assay, we demonstrated that C/P efficiency of CSTF2^D50A was lower than wild type. To account for this, we determined that p.D50A changed locations of amino acid side chains altering RNA binding sites in the RRM. The changes modified the electrostatic potential of the RRM leading to a greater affinity for RNA. These results highlight the significance of 3′ end mRNA processing in expression of genes important for brain plasticity and neuronal development.

Overlapping Activities of ELAV/Hu Family RNA Binding Proteins Specify the Extended Neuronal 3' UTR Landscape in Drosophila

The tissue-specific deployment of highly extended neural 3' UTR isoforms, generated by alternative polyadenylation (APA), is a broad and conserved feature of metazoan genomes. However, the factors and mechanisms that control neural APA isoforms are not well understood. Here, we show that three ELAV/Hu RNA binding proteins (Elav, Rbp9, and Fne) have similar capacities to induce a lengthened 3' UTR landscape in an ectopic setting. These factors promote accumulation of chromatin-associated, 3' UTR-extended, nascent transcripts, through inhibition of proximal polyadenylation site (PAS) usage. Notably, Elav represses an unannotated splice isoform of fne, switching the normally cytoplasmic Fne toward the nucleus in elav mutants. We use genomic profiling to reveal strong and broad loss of neural APA in elav/fne double mutant CNS, the first genetic background to largely abrogate this distinct APA signature. Overall, we demonstrate how regulatory interplay and functionally overlapping activities of neural ELAV/Hu RBPs drives the neural APA landscape.

Activation and inhibition of nonsense-mediated mRNA decay control the abundance of alternative polyadenylation products

Alternative polyadenylation (APA) produces transcript 3' untranslated regions (3'UTRs) with distinct sequences, lengths, stabilities and functions. We show here that APA products include a class of cryptic nonsense-mediated mRNA decay (NMD) substrates with extended 3'UTRs that gene- or transcript-level analyses of NMD often fail to detect. Transcriptome-wide, the core NMD factor UPF1 preferentially recognizes long 3'UTR products of APA, leading to their systematic downregulation. Counteracting this mechanism, the multifunctional RNA-binding protein PTBP1 regulates the balance of short and long 3'UTR isoforms by inhibiting NMD, in addition to its previously described modulation of co-transcriptional polyadenylation (polyA) site choice. Further, we find that many transcripts with altered APA isoform abundance across multiple tumor types are controlled by NMD. Together, our findings reveal a widespread role for NMD in shaping the outcomes of APA.

Dynamic landscape and evolution of m6A methylation in human

m⁶A is a prevalent internal modification in mRNAs and has been linked to the diverse effects on mRNA fate. To explore the landscape and evolution of human m⁶A, we generated 27 m⁶A methylomes across major adult tissues. These data reveal dynamic m⁶A methylation across tissue types, uncover both broadly or tissue-specifically methylated sites, and identify an unexpected enrichment of m⁶A methylation at non-canonical cleavage sites. A comparison of fetal and adult m⁶A methylomes reveals that m⁶A preferentially occupies CDS regions in fetal tissues. Moreover, the m⁶A sub-motifs vary between fetal and adult tissues or across tissue types. From the evolutionary perspective, we uncover that the selection pressure on m⁶A sites varies and depends on their genic locations. Unexpectedly, we found that ∼40% of the 3′UTR m⁶A sites are under negative selection, which is higher than the evolutionary constraint on miRNA binding sites, and much higher than that on A-to-I RNA modification. Moreover, the recently gained m⁶A sites in human populations are clearly under positive selection and associated with traits or diseases. Our work provides a resource of human m⁶A profile for future studies of m⁶A functions, and suggests a role of m⁶A modification in human evolutionary adaptation and disease susceptibility.

Gene expression dynamics are a proxy for selective pressures on alternatively polyadenylated isoforms

Alternative polyadenylation (APA) produces isoforms with distinct 3′-ends, yet their functional differences remain largely unknown. Here, we introduce the APA-seq method to detect the expression levels of APA isoforms from 3′-end RNA-Seq data by exploiting both paired-end reads for gene isoform identification and quantification. We detected the expression levels of APA isoforms in individual Caenorhabditis elegans embryos at different stages throughout embryogenesis. Examining the correlation between the temporal profiles of isoforms led us to distinguish two classes of genes: those with highly correlated isoforms (HCI) and those with lowly correlated isoforms (LCI) across time. We hypothesized that variants with similar expression profiles may be the product of biological noise, while the LCI variants may be under tighter selection and consequently their distinct 3′ UTR isoforms are more likely to have functional consequences. Supporting this notion, we found that LCI genes have significantly more miRNA binding sites, more correlated expression profiles with those of their targeting miRNAs and a relative lack of correspondence between their transcription and protein abundances. Collectively, our results suggest that a lack of coherence among the regulation of 3′ UTR isoforms is a proxy for selective pressures acting upon APA usage and consequently for their functional relevance.

Nucleocytoplasmic shuttling of Gle1 impacts DDX1 at transcription termination sites

Gle1 is a nucleocytoplasmic shuttling protein with well-documented cytoplasmic roles as a modulator of ATP-dependent DEAD-box RNA helicases involved in messenger (m)RNA export, translation initiation and termination, and stress granule dynamics. Here, we identify a novel nuclear role for Gle1 during transcription termination. In HeLa cells treated with a peptide that disrupts Gle1 nucleocytoplasmic shuttling, we detected nuclear accumulation of specific mRNAs with elongated 3′-UTR (untranslated region). Enriched mRNAs were nascently transcribed and accumulated in the nucleus due to a change in transcription state and not due to altered nuclear export. Whereas Gle1 shuttling inhibition did not appear to perturb nuclear DDX19 functions, it did result in increased DDX1 nucleoplasmic localization and decreased DDX1 interactions with Gle1 and the pre-mRNA cleavage stimulation factor CstF-64. An increase in nuclear R-loop signal intensity was also observed with diminished Gle1 shuttling, as well as colocalization of Gle1 at R-loops. Taken together, these studies reveal a nuclear role for Gle1 in coordinating DDX1 function in transcription termination complexes.

A potential mechanism underlying U1 snRNP inhibition of the cleavage step of mRNA 3' processing

It is well established that U1 snRNP inhibits the cleavage of cryptic polyadenylation site (PAS) within introns, thereby facilitating full-length mRNA transcription for numerous genes in vertebrate cells, yet the underlying mechanism remains poorly understood. Here, by using a model PAS of wdr26 mRNA, we show that U1 snRNP predominantly interferes with the association of PAS with a core 3' processing factor CstF64, which can promote the cleavage step of mRNA 3' processing. Furthermore, we provide evidence that U1A, a component of U1 snRNP, might directly interfere with CstF64 binding on PAS through its RNA binding capacity. Consistently, U1A could potentially associate with U1-suppressed intronic PASs at the transcriptome level in human cells, showing a binding peak ∼50 nt downstream of the cleavage site, as revealed by U1A iCLIP-seq (individual-nucleotide resolution UV crosslinking and immunoprecipitation coupled with RNA sequencing) analysis. Together, our data suggest a molecular mechanism underlying U1 snRNP inhibition of the cleavage step of mRNA 3' processing. More generally, we argue that U1 snRNP might inhibit the usage of cryptic PASs through disturbing the recruitment of core 3' processing factors.

Emerging Roles of RNA 3'-end Cleavage and Polyadenylation in Pathogenesis, Diagnosis and Therapy of Human Disorders

A crucial feature of gene expression involves RNA processing to produce 3′ ends through a process termed 3′ end cleavage and polyadenylation (CPA). This ensures the nascent RNA molecule can exit the nucleus and be translated to ultimately give rise to a protein which can execute a function. Further, alternative polyadenylation (APA) can produce distinct transcript isoforms, profoundly expanding the complexity of the transcriptome. CPA is carried out by multi-component protein complexes interacting with multiple RNA motifs and is tightly coupled to transcription, other steps of RNA processing, and even epigenetic modifications. CPA and APA contribute to the maintenance of a multitude of diverse physiological processes. It is therefore not surprising that disruptions of CPA and APA can lead to devastating disorders. Here, we review potential CPA and APA mechanisms involving both loss and gain of function that can have tremendous impacts on health and disease. Ultimately we highlight the emerging diagnostic and therapeutic potential CPA and APA offer.