Alternative cleavage and polyadenylation in spermatogenesis connects chromatin regulation with post-transcriptional control

The clinical impact of oligozoospermia in oocyte donation ICSI cycles using preimplantation genetic test for aneuploidy

STUDY QUESTION

Do severely impaired sperm concentrations (oligozoospemia) in male factor infertility affect embryo aneuploidy rates and clinical results after IVF oocyte donation cycles?

SUMMARY ANSWER

Severe oligozoospermia (SO) in IVF cycles utilizing donor oocytes does not significantly affect embryo euploidy rates or IVF outcomes or cumulative live birth rates (CLBRs).

WHAT IS KNOWN ALREADY

SO has previously been linked to elevated rates of chromosomal abnormalities in spermatozoa and altered rates of embryo development with poorer reproductive outcomes. Nonetheless, the precise impacts of severe male factor infertility on embryonic aneuploidy rates and the success of IVF, in the context of controlled female ages in oocyte donation cycles with preimplantation genetic testing for aneuploidy (PGT-A), are still not fully understood.

STUDY DESIGN, SIZE, DURATION

This retrospective observational cohort study involved 690 IVF oocyte donation cycles undergoing PGT-A from multiple clinics across Europe between January 2017 and December 2023. The study population was divided into three groups based on sperm concentration: SO (<5 million sperm/ml), moderate oligozoospermia (MO, 5-16 million sperm/ml), and a normozoospermia control group (≥16 million sperm/ml) for outcome analysis.

PARTICIPANTS/MATERIALS, SETTING, METHODS

The IVF outcomes and variables for couples undergoing oocyte donor cycles were investigated. ICSI was performed in all cycles using fresh sperm samples that were assessed for standard semen parameters with fresh or frozen donated oocytes. All of the resulting embryos were biopsied at the blastocyst stage for PGT-A, and all transfers were of single embryos performed in subsequent cycles after vitrification and warming. Statistical analysis was performed using multivariate regression models to identify correlations between rates of oligozoospermia and clinical, embryological and genetic outcomes.

MAIN RESULTS AND THE ROLE OF CHANCE

The study included 690 couples, in the SO (N = 202), MO (N = 102), and the control normozoospermia (N = 386) groups. The SO group had significantly lower sperm motility (P < 0.0001) and significantly reduced fertilization rates compared to the MO and control groups (P < 0.01). However, no statistical difference was observed for the blastocyst formation rate (per fertilized oocyte). Mostly due to the reduced fertilization rates, the number of euploid blastocysts obtained was significantly lower in the SO group (P = 0.007), however, this did not affect the pregnancy, biochemical or clinical miscarriage (P = 0.37, P = 0.22, and P = 0.86), or CLBRs (P = 0.26) after single blastocyst transfers. The multivariate analysis showed no effect of SO or MO on aneuploidy, miscarriage, or live birth rates. Interestingly, the PGT laboratory strategy (<0.001) was associated with the rate of euploid blastocysts but had no significant effect on pregnancy outcomes.

LIMITATIONS, REASONS FOR CAUTION

This is a retrospective observational study focusing on outcomes associated with reduced sperm concentration. Other elements of severe male factor infertility may have different associations with outcomes. Also, potential confounding factors including male lifestyle factors that can influence sperm quality were not considered.

WIDER IMPLICATIONS OF THE FINDINGS

In this oocyte donation setting, we demonstrated that SO is unlikely to significantly affect IVF outcomes following blastocyst development, which contradicts previous findings in less controlled settings. This suggests that male factor infertility in the setting of oocyte donation is not an indication per se to perform PGT-A. These results provide reassurance for couples when undergoing treatment with oocyte donation that any related male factor infertility will not significantly impact treatment outcomes. Further research should use similar controlled oocyte donation settings to investigate other severely impaired sperm parameters to better understand associations with paternal embryonic aneuploidy and IVF outcomes, guiding a more informed path for male factor infertility treatment.

STUDY FUNDING/COMPETING INTEREST(S)

No external funding was used and the authors have no conflicting interests.

TRIAL REGISTRATION NUMBER

N/A.

The Dynamic Landscape of 3'-UTR Alternative Polyadenylation Across Mouse Fetal Development and Anatomy

Alternative cleavage and polyadenylation (APA) in the 3'-untranslated region (3'-UTR) of mRNA produces transcripts with varied 3'-UTR and plays a key role in development and organogenesis. This work characterizes 3'-UTR APA using 85 high-quality RNA-seq datasets encompassing 12 tissue types and eight developmental stages of mouse fetuses. Results show that 46.4% of expressed genes undergo APA in a tissue-specific manner. Changes in polyadenylation site (pAS) usage often operate beyond transcriptional control, revealing APA as an additional layer of gene regulation. Sequence analysis demonstrates that pAS selection, governed by polyadenylation signal strength and adenine preferences, is evolutionarily conserved between mice and humans. Intriguingly, brain tissues display complex 3'-UTR APA dynamics during development, potentially regulated by RNA-binding proteins such as Rbm38, potentially impacting 3' UTR extension by restricting distal pAS usage. These APA events are associated with a depletion of conserved miRNA binding sites and an enrichment of transposable elements within alternative 3' UTRs. To facilitate further research, this work develops APApedia (http://xozhanglab.com/apapedia/), a comprehensive database cataloging identified 3'-UTR APA events, which serves as a valuable resource for the community to study APA in development and tissue-specific regulation. This comprehensive resource aids in deciphering the functional implications of APA in mouse fetal development.

The nuclear exosome co-factor MTR4 shapes the transcriptome for meiotic initiation

Nuclear RNA decay has emerged as a mechanism for post-transcriptional gene regulation in cultured cells. However, whether this process occurs in animals and holds biological relevance remains largely unexplored. Here, we demonstrate that MTR4, the central cofactor of the nuclear RNA exosome, is essential for embryogenesis and spermatogenesis. Embryonic development of Mtr4 knockout mice arrests at 6.5 day. Germ cell-specific knockout of Mtr4 results in male infertility with a specific and severe defect in meiotic initiation. During the pre-meiotic stage, MTR4/exosome represses meiotic genes, which are typically shorter in size and possess fewer introns, through RNA degradation. Concurrently, it ensures the expression of mitotic genes generally exhibiting the opposite features. Consistent with these regulation rules, mature replication-dependent histone mRNAs and polyadenylated retrotransposon RNAs were identified as MTR4/exosome targets in germ cells. In addition, MTR4 regulates alternative splicing of many meiotic genes. Together, our work underscores the importance of nuclear RNA degradation in regulating germline transcriptome, ensuring the appropriate gene expression program for the transition from mitosis to meiosis during spermatogenesis.

Supervised analysis of alternative polyadenylation from single-cell and spatial transcriptomics data with spvAPA

Alternative polyadenylation (APA) is an important driver of transcriptome diversity that generates messenger RNA isoforms with distinct 3' ends. The rapid development of single-cell and spatial transcriptomic technologies opened up new opportunities for exploring APA data to discover hidden cell subpopulations invisible in conventional gene expression analysis. However, conventional gene-level analysis tools are not fully applicable to APA data, and commonly used unsupervised dimensionality reduction methods often disregard experimentally derived annotations such as cell type identities. Here, we proposed a supervised analytical framework termed spvAPA, specifically used for APA analysis from both single-cell and spatial transcriptomics data. First, an iterative imputation method based on weighted nearest neighbor was designed to recover missing APA signatures, by integrating both gene expression and APA modalities. Second, a supervised feature selection method based on sparse partial least squares discriminant analysis was devised to identify APA features distinguishing cell types or spatial morphologies. Additionally, spvAPA improves the visualization of high-dimensional data for discovering novel cell subtypes, which considers APA features and dual modalities of gene expression and APA. Evaluations across nine single-cell and spatial transcriptomics datasets demonstrate the effectiveness and applicability of spvAPA. spvAPA is available at https://github.com/BMILAB/spvAPA.

Comprehensive analysis of single-nucleotide variants and alternative polyadenylation between inbred and outbred pigs

Inbreeding can lead to the accumulation of homozygous single nucleotide polymorphisms (SNPs) in the genome, which can significantly affect gene expression and phenotype. In this study, we examined the impact of homozygous SNPs resulting from inbreeding on alternative polyadenylation (APA) site selection and the underlying genetic mechanisms using inbred Luchuan pigs. Genome resequencing revealed that inbreeding results in a high accumulation of homozygous SNPs within the pig genome. 3' mRNA-seq on leg muscle, submandibular lymph node, and liver tissues was performed to identify differences in APA events between inbred and outbred Luchuan pigs. We revealed different tissue-specific APA usage caused by inbreeding, which were associated with different biological processes. Furthermore, we explored the role of polyadenylation signal (PAS) SNPs in APA regulation under inbreeding and identified key genes such as PUM1, SCARF1, RIPOR2, C1D, and LRRK2 that are involved in biological processes regulation. This study provides resources and sheds light on the impact of genomic homozygosity on APA regulation, offering insights into genetic characteristics and biological processes associated with inbreeding.

Natural antisense transcripts as versatile regulators of gene expression

Long non-coding RNAs (lncRNAs) are emerging as a major class of gene products that have central roles in cell and developmental biology. Natural antisense transcripts (NATs) are an important subset of lncRNAs that are expressed from the opposite strand of protein-coding and non-coding genes and are a genome-wide phenomenon in both eukaryotes and prokaryotes. In eukaryotes, a myriad of NATs participate in regulatory pathways that affect expression of their cognate sense genes. Recent developments in the study of NATs and lncRNAs and large-scale sequencing and bioinformatics projects suggest that whether NATs regulate expression, splicing, stability or translation of the sense transcript is influenced by the pattern and degrees of overlap between the sense-antisense pair. Moreover, epigenetic gene regulatory mechanisms prevail in somatic cells whereas mechanisms dependent on the formation of double-stranded RNA intermediates are prevalent in germ cells. The modulating effects of NATs on sense transcript expression make NATs rational targets for therapeutic interventions.

A developmental mechanism to regulate alternative polyadenylation in an adult stem cell lineage

Alternative cleavage and polyadenylation (APA) often results in production of mRNA isoforms with either longer or shorter 3' UTRs from the same genetic locus, potentially impacting mRNA translation, localization, and stability. Developmentally regulated APA can thus make major contributions to cell type-specific gene expression programs as cells differentiate. During Drosophila spermatogenesis, ∼500 genes undergo APA when proliferating spermatogonia differentiate into spermatocytes, producing transcripts with shortened 3' UTRs, leading to profound stage-specific changes in the proteins expressed. The molecular mechanisms that specify usage of upstream polyadenylation sites in spermatocytes are thus key to understanding the changes in cell state. Here, we show that upregulation of PCF11 and Cbc, the two components of cleavage factor II (CFII), orchestrates APA during Drosophila spermatogenesis. Knockdown of PCF11 or cbc in spermatocytes caused dysregulation of APA, with many transcripts normally cleaved at a proximal site in spermatocytes now cleaved at their distal site, as in spermatogonia. Forced overexpression of CFII components in spermatogonia switched cleavage of some transcripts to the proximal site normally used in spermatocytes. Our findings reveal a developmental mechanism where changes in expression of specific cleavage factors can direct cell type-specific APA at selected genes.

A Developmental Mechanism to Regulate Alternative Polyadenylation in an Adult Stem Cell Lineage

Alternative Cleavage and Polyadenylation (APA) often results in production of mRNA isoforms with either longer or shorter 3'UTRs from the same genetic locus, potentially impacting mRNA translation, localization and stability. Developmentally regulated APA can thus make major contributions to cell-type-specific gene expression programs as cells differentiate. During Drosophila spermatogenesis, approximately 500 genes undergo APA when proliferating spermatogonia differentiate into spermatocytes, producing transcripts with shortened 3' UTRs, leading to profound stage-specific changes in the proteins expressed. The molecular mechanisms that specify usage of upstream polyadenylation sites in spermatocytes are thus key to understanding the changes in cell state. Here, we show that upregulation of PCF11 and Cbc, the two components of Cleavage Factor II (CFII), orchestrates APA during Drosophila spermatogenesis. Knock down of PCF11 or cbc in spermatocytes caused dysregulation of APA, with many transcripts normally cleaved at a proximal site in spermatocytes now cleaved at their distal site, as in spermatogonia. Forced overexpression of CFII components in spermatogonia switched cleavage of some transcripts to the proximal site normally used in spermatocytes. Our findings reveal a developmental mechanism where changes in expression of specific cleavage factors can direct cell-type-specific APA at selected genes.

Integrated RNA expression and alternative polyadenylation analysis identified CPSF1-CCDC137 oncogenic axis in lung adenocarcinoma

This study aims to analyze the RNA expression and alternative polyadenylation (APA) events and identify APA tuned genes with prognostic significance in lung adenocarcinoma (LUAD). Genome-wide RNA expression profile and APA events were acquired in LUAD cancer and normal samples in GSE197346. Comparative analysis screened common deregulated genes and transcripts. All 11 and 19 transcripts were up and down expressed and polyadenylated in cancer samples, respectively. Clinical analysis found eight genes with prognostic significance, such as coiled-coil domain containing 137 (CCDC137). Role of CCDC137 in LUAD was first reported in this study. The cellular and animal experiments indicated that downregulated CCDC137 suppressed the malignant tumor phenotype and tumor growth in LUAD. Then, to identify APA regulators for elevated CCDC137, we analyzed the expression of 26 APA regulators in GSE197346 and The Cancer Genome Atlas (TCGA), and found 4 differential regulators: CPSF1, CELF2, NUDT21, and ELAVL1. At last, the correlation of eight genes with four differential APA regulators was analyzed, and CPSF1 showed a strong positive correlation with CCDC137. Based on the above results, we propose an oncogenic axis of CPSF1-CCDC137 in LUAD. This study first constructed a polyadenylation tuned RNA expression map in LUAD, and the proposed oncogenic axis of CPSF1-CCDC137 would shed light on the pathogenesis of LUAD.

Transcriptome Responses to Different Salinity Conditions in Litoditis marina, Revealed by Long-Read Sequencing

The marine nematode Litoditis marina is widely distributed in intertidal zones around the globe, yet the mechanisms underlying its broad adaptation to salinity remain elusive. In this study, we applied ONT long-read sequencing technology to unravel the transcriptome responses to different salinity conditions in L. marina. Through ONT sequencing under 3‱, 30‱ and 60‱ salinity environments, we obtained 131.78 G clean data and 26,647 non-redundant long-read transcripts, including 6464 novel transcripts. The DEGs obtained from the current ONT lrRNA-seq were highly correlated with those identified in our previously reported Illumina short-read RNA sequencing data. When we compared the 30‱ to the 3‱ salinity condition, we found that GO terms such as oxidoreductase activity, cation transmembrane transport and ion transmembrane transport were shared between the ONT lrRNA-seq and Illumina data. Similarly, GO terms including extracellular space, structural constituents of cuticle, substrate-specific channel activity, ion transport and substrate-specific transmembrane transporter activity were shared between the ONT and Illumina data under 60‱ compared to 30‱ salinity. In addition, we found that 79 genes significantly increased, while 119 genes significantly decreased, as the salinity increased. Furthermore, through the GO enrichment analysis of 214 genes containing DAS, in 30‱ compared to 3‱ salinity, we found that GO terms such as cellular component assembly and coenzyme biosynthetic process were enriched. Additionally, we observed that GO terms such as cellular component assembly and coenzyme biosynthetic process were also enriched in 60‱ compared to 30‱ salinity. Moreover, we found that 86, 125, and 81 genes that contained DAS were also DEGs, in comparisons between 30‱ and 3‱, 60‱ and 30‱, and 60‱ and 3‱ salinity, respectively. In addition, we demonstrated the landscape of alternative polyadenylation in marine nematode under different salinity conditions This report provides several novel insights for the further study of the mechanisms by which euryhalinity formed and evolved, and it might also contribute to the investigation of salinity dynamics induced by global climate change.

Comparative alternative polyadenylation profiles in differentiated adipocytes of subcutaneous and intramuscular fat tissue in cattle

Alternative polyadenylation (APA) is a key molecular mechanism involved in the post-transcriptional regulation of gene expression, which has been proven to play a critical role in cell differentiation. In the present study, we performed IVT-SAPAS sequencing to profile the dynamic changes of APA sites in bovine subcutaneous preadipocytes and intramuscular preadipocytes during adipogenesis. A total of 52621 high quality APA sites were identified in preadipocytes and adipocytes. Compared with preadipocytes, the increased usage of canonical AATAAA was observed in the cell-biased APA sites of adipocytes. Furthermore, 1933 and 2140 differentially expressed APA (DE-APA) sites, as well as 341 and 337 untranslated region-APA (UTR-APA) switching genes were identified in subcutaneous preadipocytes and intramuscular preadipocytes during adipogenesis, respectively. The UTR-APA switching genes showed divergent trends in preadipocytes, among which UTR-APA switching genes in intramuscular preadipocytes tended to use shorter 3'UTR for differentiation into mature adipocytes. APA events mediated by UTR-APA switching in intramuscular adipocytes were enriched in lipid synthesis and adipocyte differentiation. TRIB3, WWTR1, and INSIG1 played important roles in the differentiation of intramuscular preadipocytes. Briefly, our results provided new insights into understanding the mechanisms of bovine adipocyte differentiation.

Downregulation of CPSF6 leads to global mRNA 3' UTR shortening and enhanced antiviral immune responses

Alternative polyadenylation (APA) is a widespread mechanism of gene regulation that generates mRNA isoforms with alternative 3' untranslated regions (3' UTRs). Our previous study has revealed the global 3' UTR shortening of host mRNAs through APA upon viral infection. However, how the dynamic changes in the APA landscape occur upon viral infection remains largely unknown. Here we further found that, the reduced protein abundance of CPSF6, one of the core 3' processing factors, promotes the usage of proximal poly(A) sites (pPASs) of many immune related genes in macrophages and fibroblasts upon viral infection. Shortening of the 3' UTR of these transcripts may improve their mRNA stability and translation efficiency, leading to the promotion of type I IFN (IFN-I) signalling-based antiviral immune responses. In addition, dysregulated expression of CPSF6 is also observed in many immune related physiological and pathological conditions, especially in various infections and cancers. Thus, the global APA dynamics of immune genes regulated by CPSF6, can fine-tune the antiviral response as well as the responses to other cellular stresses to maintain the tissue homeostasis, which may represent a novel regulatory mechanism for antiviral immunity.

RNA polymerase II pausing is essential during spermatogenesis for appropriate gene expression and completion of meiosis

Male germ cell development requires precise regulation of gene activity in a cell-type and stage-specific manner, with perturbations in gene expression during spermatogenesis associated with infertility. Here, we use steady-state, nascent and single-cell RNA sequencing strategies to comprehensively characterize gene expression across male germ cell populations, to dissect the mechanisms of gene control and provide new insights towards therapy. We discover a requirement for pausing of RNA Polymerase II (Pol II) at the earliest stages of sperm differentiation to establish the landscape of gene activity across development. Accordingly, genetic knockout of the Pol II pause-inducing factor NELF in immature germ cells blocks differentiation to spermatids. Further, we uncover unanticipated roles for Pol II pausing in the regulation of meiosis during spermatogenesis, with the presence of paused Pol II associated with double-strand break (DSB) formation, and disruption of meiotic gene expression and DSB repair in germ cells lacking NELF.

Identification of experimentally-supported poly(A) sites in single-cell RNA-seq data with SCINPAS

Alternative polyadenylation is a main driver of transcriptome diversity in mammals, generating transcript isoforms with different 3' ends via cleavage and polyadenylation at distinct polyadenylation (poly(A)) sites. The regulation of cell type-specific poly(A) site choice is not completely resolved, and requires quantitative poly(A) site usage data across cell types. 3' end-based single-cell RNA-seq can now be broadly used to obtain such data, enabling the identification and quantification of poly(A) sites with direct experimental support. We propose SCINPAS, a computational method to identify poly(A) sites from scRNA-seq datasets. SCINPAS modifies the read deduplication step to favor the selection of distal reads and extract those with non-templated poly(A) tails. This approach improves the resolution of poly(A) site recovery relative to standard software. SCINPAS identifies poly(A) sites in genic and non-genic regions, providing complementary information relative to other tools. The workflow is modular, and the key read deduplication step is general, enabling the use of SCINPAS in other typical analyses of single cell gene expression. Taken together, we show that SCINPAS is able to identify experimentally-supported, known and novel poly(A) sites from 3' end-based single-cell RNA sequencing data.

mRNA isoform balance in neuronal development and disease

Balanced mRNA isoform diversity and abundance are spatially and temporally regulated throughout cellular differentiation. The proportion of expressed isoforms contributes to cell type specification and determines key properties of the differentiated cells. Neurons are unique cell types with intricate developmental programs, characteristic cellular morphologies, and electrophysiological potential. Neuron-specific gene expression programs establish these distinctive cellular characteristics and drive diversity among neuronal subtypes. Genes with neuron-specific alternative processing are enriched in key neuronal functions, including synaptic proteins, adhesion molecules, and scaffold proteins. Despite the similarity of neuronal gene expression programs, each neuronal subclass can be distinguished by unique alternative mRNA processing events. Alternative processing of developmentally important transcripts alters coding and regulatory information, including interaction domains, transcript stability, subcellular localization, and targeting by RNA binding proteins. Fine-tuning of mRNA processing is essential for neuronal activity and maintenance. Thus, the focus of neuronal RNA biology research is to dissect the transcriptomic mechanisms that underlie neuronal homeostasis, and consequently, predispose neuronal subtypes to disease. This article is categorized under: RNA in Disease and Development > RNA in Disease RNA in Disease and Development > RNA in Development.

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.

Specific expression of alternatively spliced genes in the turkey (Meleagris gallopavo) reproductive tract revealed their function in spermatogenesis and post-testicular sperm maturation

The tissue-specific profile of alternatively spliced genes (ASGs) and their involvement in reproduction processes characteristic of turkey testis, epididymis, and ductus deferens were investigated for the first time in birds. Deep sequencing of male turkey reproductive tissue RNA samples (n = 6) was performed using Illumina RNA-Seq with 2 independent methods, rMATs and SUPPA2, for differential alternative splicing (DAS) event prediction. The expression of selected ASGs was validated using quantitative real-time reverse transcriptase-polymerase chain reaction. The testis was found to be the site of the highest number of posttranscriptional splicing events within the reproductive tract, and skipping exons were the most frequently occurring class of alternative splicing (AS) among the reproductive tract. Statistical analysis revealed 86, 229, and 6 DAS events in the testis/epididymis, testis/ductus deferens, and epididymis/ductus deferens comparison, respectively. Alternative splicing was found to be a mechanism of gene expression regulation within the turkey reproduction tract. In testis, modification was observed for spermatogenesis specific genes; the changes in 5' UTR could act as regulator of MEIG1 expression (a player during spermatocytes meiosis), and modification of 3' UTR led to diversification of CREM mRNA (modulator of gene expression related to the structuring of mature spermatozoa). Sperm tail formation can be regulated by changes in the 5' UTR of testicular SLC9A3R1 and gene silencing by producing dysfunctional variants of ODF2 in the testis and ATP1B3 in the epididymis. Predicted differentially ASGs in the turkey reproductive tract seem to be involved in the regulation of spermatogenesis, including acrosome formation and sperm tail formation and binding of sperm to the zona pellucida. Several ASGs were classified as cilia by actin and microtubule cytoskeleton organization. Such genes may play a role in the organization of sperm flagellum and post-testicular motility development. To our knowledge, this is the first functional investigation of alternatively spliced genes associated with tissue-specific processes in the turkey reproductive tract.

Regulation and function of alternative polyadenylation in development and differentiation

Alternative processing of nascent mRNAs is widespread in eukaryotic organisms and greatly impacts the output of gene expression. Specifically, alternative cleavage and polyadenylation (APA) is a co-transcriptional molecular process that switches the polyadenylation site (PAS) at which a nascent mRNA is cleaved, resulting in mRNA isoforms with different 3'UTR length and content. APA can potentially affect mRNA translation efficiency, localization, stability, and mRNA seeded protein-protein interactions. APA naturally occurs during development and cellular differentiation, with around 70% of human genes displaying APA in particular tissues and cell types. For example, neurons tend to express mRNAs with long 3'UTRs due to preferential processing at PASs more distal than other PASs used in other cell types. In addition, changes in APA mark a variety of pathological states, including many types of cancer, in which mRNAs are preferentially cleaved at more proximal PASs, causing expression of mRNA isoforms with short 3'UTRs. Although APA has been widely reported, both the function of APA in development and the mechanisms that regulate the choice of 3'end cut sites in normal and pathogenic conditions are still poorly understood. In this review, we summarize current understanding of how APA is regulated during development and cellular differentiation and how the resulting change in 3'UTR content affects multiple aspects of gene expression. With APA being a widespread phenomenon, the advent of cutting-edge scientific techniques and the pressing need for in-vivo studies, there has never been a better time to delve into the intricate mechanisms of alternative cleavage and polyadenylation.

CFIm-mediated alternative polyadenylation safeguards the development of mammalian pre-implantation embryos

Alternative polyadenylation (APA) gives rise to transcripts with distinct 3' untranslated regions (3' UTRs), thereby affecting the fate of mRNAs. APA is strongly associated with cell proliferation and differentiation status, and thus likely plays a critical role in the embryo development. However, the pattern of APA in mammalian early embryos is still unknown. Here, we analyzed the 3' UTR lengths in human and mouse pre-implantation embryos using available single cell RNA-seq datasets and explored the underlying mechanism driving the changes. Although human and mouse early embryos displayed distinct patterns of 3' UTR changing, RNA metabolism pathways were involved in both species. The 3' UTR lengths are likely determined by the abundance of the cleavage factor I complex (CFIm) components NUDT21 and CPSF6 in the nucleus. Importantly, depletion of either component resulted in early embryo development arrest and 3' UTR shortening. Collectively, these data highlight an essential role for APA in the development of mammalian early embryos.

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.

ReadZS detects cell type-specific and developmentally regulated RNA processing programs in single-cell RNA-seq

RNA processing, including splicing and alternative polyadenylation, is crucial to gene function and regulation, but methods to detect RNA processing from single-cell RNA sequencing data are limited by reliance on pre-existing annotations, peak calling heuristics, and collapsing measurements by cell type. We introduce ReadZS, an annotation-free statistical approach to identify regulated RNA processing in single cells. ReadZS discovers cell type-specific RNA processing in human lung and conserved, developmentally regulated RNA processing in mammalian spermatogenesis-including global 3' UTR shortening in human spermatogenesis. ReadZS also discovers global 3' UTR lengthening in Arabidopsis development, highlighting the usefulness of this method in under-annotated transcriptomes.

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.

Developmentally regulated alternate 3' end cleavage of nascent transcripts controls dynamic changes in protein expression in an adult stem cell lineage

Alternative polyadenylation (APA) generates transcript isoforms that differ in the position of the 3' cleavage site, resulting in the production of mRNA isoforms with different length 3' UTRs. Although widespread, the role of APA in the biology of cells, tissues, and organisms has been controversial. We identified >500 Drosophila genes that express mRNA isoforms with a long 3' UTR in proliferating spermatogonia but a short 3' UTR in differentiating spermatocytes due to APA. We show that the stage-specific choice of the 3' end cleavage site can be regulated by the arrangement of a canonical polyadenylation signal (PAS) near the distal cleavage site but a variant or no recognizable PAS near the proximal cleavage site. The emergence of transcripts with shorter 3' UTRs in differentiating cells correlated with changes in expression of the encoded proteins, either from off in spermatogonia to on in spermatocytes or vice versa. Polysome gradient fractionation revealed >250 genes where the long 3' UTR versus short 3' UTR mRNA isoforms migrated differently, consistent with dramatic stage-specific changes in translation state. Thus, the developmentally regulated choice of an alternative site at which to make the 3' end cut that terminates nascent transcripts can profoundly affect the suite of proteins expressed as cells advance through sequential steps in a differentiation lineage.

scAPAmod: Profiling Alternative Polyadenylation Modalities in Single Cells from Single-Cell RNA-Seq Data

Alternative polyadenylation (APA) is a key layer of gene expression regulation, and APA choice is finely modulated in cells. Advances in single-cell RNA-seq (scRNA-seq) have provided unprecedented opportunities to study APA in cell populations. However, existing studies that investigated APA in single cells were either confined to a few cells or focused on profiling APA dynamics between cell types or identifying APA sites. The diversity and pattern of APA usages on a genomic scale in single cells remains unappreciated. Here, we proposed an analysis framework based on a Gaussian mixture model, scAPAmod, to identify patterns of APA usage from homogeneous or heterogeneous cell populations at the single-cell level. We systematically evaluated the performance of scAPAmod using simulated data and scRNA-seq data. The results show that scAPAmod can accurately identify different patterns of APA usages at the single-cell level. We analyzed the dynamic changes in the pattern of APA usage using scAPAmod in different cell differentiation and developmental stages during mouse spermatogenesis and found that even the same gene has different patterns of APA usages in different differentiation stages. The preference of patterns of usages of APA sites in different genomic regions was also analyzed. We found that patterns of APA usages of the same gene in 3′ UTRs (3′ untranslated region) and non-3′ UTRs are different. Moreover, we analyzed cell-type-specific APA usage patterns and changes in patterns of APA usages across cell types. Different from the conventional analysis of single-cell heterogeneity based on gene expression profiling, this study profiled the heterogeneous pattern of APA isoforms, which contributes to revealing the heterogeneity of single-cell gene expression with higher resolution.

A joint NCBI and EMBL-EBI transcript set for clinical genomics and research

Comprehensive genome annotation is essential to understand the impact of clinically relevant variants. However, the absence of a standard for clinical reporting and browser display complicates the process of consistent interpretation and reporting. To address these challenges, Ensembl/GENCODE1 and RefSeq2 launched a joint initiative, the Matched Annotation from NCBI and EMBL-EBI (MANE) collaboration, to converge on human gene and transcript annotation and to jointly define a high-value set of transcripts and corresponding proteins. Here, we describe the MANE transcript sets for use as universal standards for variant reporting and browser display. The MANE Select set identifies a representative transcript for each human protein-coding gene, whereas the MANE Plus Clinical set provides additional transcripts at loci where the Select transcripts alone are not sufficient to report all currently known clinical variants. Each MANE transcript represents an exact match between the exonic sequences of an Ensembl/GENCODE transcript and its counterpart in RefSeq such that the identifiers can be used synonymously. We have now released MANE Select transcripts for 97% of human protein-coding genes, including all American College of Medical Genetics and Genomics Secondary Findings list v3.0 (ref. 3) genes. MANE transcripts are accessible from major genome browsers and key resources. Widespread adoption of these transcript sets will increase the consistency of reporting, facilitate the exchange of data regardless of the annotation source and help to streamline clinical interpretation.

Tex13a Optimizes Sperm Motility via Its Potential Roles in mRNA Turnover

mRNAs have been found to undergo substantial selective degradation during the late stages of spermiogenesis. However, the mechanisms regulating this biological process are unknown. In this report, we have identified Tex13a, a spermatid-specific gene that interacts with the CCR4–NOT complex and is implicated in the targeted degradation of mRNAs encoding particular structural components of sperm. Deletion of Tex13a led to a delayed decay of these mRNAs, lowered the levels of house-keeping genes, and ultimately lowered several key parameters associated with the control of sperm motility, such as the path velocity (VAP, average path velocity), track speed (VCL, velocity curvilinear), and rapid progression.

Coupled protein synthesis and ribosome-guided piRNA processing on mRNAs

PIWI-interacting small RNAs (piRNAs) protect the germline genome and are essential for fertility. piRNAs originate from transposable element (TE) RNAs, long non-coding RNAs, or 3´ untranslated regions (3´UTRs) of protein-coding messenger genes, with the last being the least characterized of the three piRNA classes. Here, we demonstrate that the precursors of 3´UTR piRNAs are full-length mRNAs and that post-termination 80S ribosomes guide piRNA production on 3´UTRs in mice and chickens. At the pachytene stage, when other co-translational RNA surveillance pathways are sequestered, piRNA biogenesis degrades mRNAs right after pioneer rounds of translation and fine-tunes protein production from mRNAs. Although 3´UTR piRNA precursor mRNAs code for distinct proteins in mice and chickens, they all harbor embedded TEs and produce piRNAs that cleave TEs. Altogether, we discover a function of the piRNA pathway in fine-tuning protein production and reveal a conserved piRNA biogenesis mechanism that recognizes translating RNAs in amniotes.

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.

Non-Coding RNAs and Splicing Activity in Testicular Germ Cell Tumors

Testicular germ cell tumors (TGCTs) are the most common tumors in adolescent and young men. Recently, genome-wide studies have made it possible to progress in understanding the molecular mechanisms underlying the development of tumors. It is becoming increasingly clear that aberrant regulation of RNA metabolism can drive tumorigenesis and influence chemotherapeutic response. Notably, the expression of non-coding RNAs as well as specific splice variants is deeply deregulated in human cancers. Since these cancer-related RNA species are considered promising diagnostic, prognostic and therapeutic targets, understanding their function in cancer development is becoming a major challenge. Here, we summarize how the different expression of RNA species repertoire, including non-coding RNAs and protein-coding splicing variants, impacts on TGCTs’ onset and progression and sustains therapeutic resistance. Finally, the role of transcription-associated R-loop misregulation in the maintenance of genomic stability in TGCTs is also discussed.

QuantifyPoly(A): reshaping alternative polyadenylation landscapes of eukaryotes with weighted density peak clustering

The dynamic choice of different polyadenylation sites in a gene is referred to as alternative polyadenylation, which functions in many important biological processes. Large-scale messenger RNA 3' end sequencing has revealed that cleavage sites for polyadenylation are presented with microheterogeneity. To date, the conventional determination of polyadenylation site clusters is subjective and arbitrary, leading to inaccurate annotations. Here, we present a weighted density peak clustering method, QuantifyPoly(A), to accurately quantify genome-wide polyadenylation choices. Applying QuantifyPoly(A) on published 3' end sequencing datasets from both animals and plants, their polyadenylation profiles are reshaped into myriads of novel polyadenylation site clusters. Most of these novel polyadenylation site clusters show significantly dynamic usage across different biological samples or associate with binding sites of trans-acting factors. Upstream sequences of these clusters are enriched with polyadenylation signals UGUA, UAAA and/or AAUAAA in a species-dependent manner. Polyadenylation site clusters also exhibit species specificity, while plants ones generally show higher microheterogeneity than that of animals. QuantifyPoly(A) is broadly applicable to any types of 3' end sequencing data and species for accurate quantification and construction of the complex and dynamic polyadenylation landscape and enables us to decode alternative polyadenylation events invisible to conventional methods at a much higher resolution.

LABRAT reveals association of alternative polyadenylation with transcript localization, RNA binding protein expression, transcription speed, and cancer survival

BACKGROUND

The sequence content of the 3' UTRs of many mRNA transcripts is regulated through alternative polyadenylation (APA). The study of this process using RNAseq data, though, has been historically challenging.

RESULTS

To combat this problem, we developed LABRAT, an APA isoform quantification method. LABRAT takes advantage of newly developed transcriptome quantification techniques to accurately determine relative APA site usage and how it varies across conditions. Using LABRAT, we found consistent relationships between gene-distal APA and subcellular RNA localization in multiple cell types. We also observed connections between transcription speed and APA site choice as well as tumor-specific transcriptome-wide shifts in APA isoform abundance in hundreds of patient-derived tumor samples that were associated with patient prognosis. We investigated the effects of APA on transcript expression and found a weak overall relationship, although many individual genes showed strong correlations between relative APA isoform abundance and overall gene expression. We interrogated the roles of 191 RNA-binding proteins in the regulation of APA isoforms, finding that dozens promote broad, directional shifts in relative APA isoform abundance both in vitro and in patient-derived samples. Finally, we find that APA site shifts in the two classes of APA, tandem UTRs and alternative last exons, are strongly correlated across many contexts, suggesting that they are coregulated.

CONCLUSIONS

We conclude that LABRAT has the ability to accurately quantify APA isoform ratios from RNAseq data across a variety of sample types. Further, LABRAT is able to derive biologically meaningful insights that connect APA isoform regulation to cellular and molecular phenotypes.

Highly conserved sperm function-related transcripts across three species: human, rat and mouse

Assessing male reproductive toxicity of environmental and therapeutic agents relies on the histopathology of the testis and epididymis in a pre-clinical setting. Animal histopathology poorly correlates with human sperm parameters, and none of these current methods are strong indicators of sperm health or reproductive potential. Therefore, there is an urgent need to identify a translatable, non-invasive and reliable approach to monitor environmental and therapeutic agents' effects on male reproductive health. mRNA sequences were analyzed in mouse, rat and human sperm samples to identify sperm transcriptomic similarities across species that could be used as biomarkers to predict male reproductive toxicity in animal models. Semen specimens were collected from men aged 18 to 55 years with proven fertility. Rat and mouse semen specimens were collected via needle punctures of the cauda epididymides. Sperm RNAs were extracted using an optimized sperm RNA isolation protocol and subjected to polyA-purified mRNA-sequencing. Bioinformatics analyses, including differential abundance and gene set enrichment analysis, were used to investigate the biological and molecular functions of all shared and differentially abundant transcripts across species. Transcriptome profiling identified 6,684 similarly expressed transcripts within the three species of which 1,579 transcripts were found to be involved in spermatogenic functions. Our findings have shown that sperm transcriptome is highly species dependent, however, there are some key similarities among transcripts that are required for fertility. Based on these similarities, sperm mRNA biomarker may be developed to monitor male reproductive toxicity where rodent models would make suitable laboratory substitutes for human.

TRENDseq-A highly multiplexed high throughput RNA 3' end sequencing for mapping alternative polyadenylation

Alternative polyadenylation (APA) is a widespread and highly dynamic mechanism of gene regulation. It affects more than 70% of all genes, resulting in transcript isoforms with distinct 3' end termini. APA thereby considerably expands the diversity of the transcriptome 3' end (TREND). This leads to mRNA isoforms with profoundly different physiological effects, by affecting protein output, production of distinct protein isoforms, or modulating protein localization. APA is globally regulated in various conditions, including developmental and adaptive programs. Since perturbations of APA can disrupt biological processes, ultimately resulting in most devastating disorders, querying the APA landscape is crucial to decipher underlying mechanisms, resulting consequences and potential diagnostic and therapeutic implications. Here we provide a detailed step-by-step protocol for TRENDseq, a method for transcriptome-wide high-throughput sequencing of polyadenylated RNA 3' ends in a highly multiplexed fashion. TRENDseq exploits linear amplification of the starting material to improve sensitivity while significantly reducing the amount of input material. It thereby represents a powerful tool to study APA in numerous experimental set-ups and/or limited human samples in a highly multiplexed and reproducible manner.

Alternative Polyadenylation in Stem Cell Self-Renewal and Differentiation

Cellular function is shaped by transcriptional and post-transcriptional mechanisms, including alternative polyadenylation (APA). By directly controlling 3'- untranslated region (UTR) length and the selection of the last exon, APA regulates up to 70% of all cellular transcripts influencing RNA stability, output, and protein isoform expression. Cell-state-dependent 3'-UTR shortening has been identified as a hallmark of cellular proliferation. Hence, quiescent/dormant stem cells are characterized by long 3'-UTRs, whereas proliferative stem/progenitor cells exhibit 3'-UTR shortening. Here, the latest studies analyzing the role of APA in regulating stem cell state, self-renewal, differentiation, and metabolism are reviewed. The new role of APA in controlling stem cell fate opens novel potential therapeutic avenues in the field of regenerative medicine.

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.

Transcriptomics of Meiosis in the Male Mouse

Molecular studies of meiosis in mammals have been long relegated due to some intrinsic obstacles, namely the impossibility to reproduce the process in vitro, and the difficulty to obtain highly pure isolated cells of the different meiotic stages. In the recent years, some technical advances, from the improvement of flow cytometry sorting protocols to single-cell RNAseq, are enabling to profile the transcriptome and its fluctuations along the meiotic process. In this mini-review we will outline the diverse methodological approaches that have been employed, and some of the main findings that have started to arise from these studies. As for practical reasons most studies have been carried out in males, and mostly using mouse as a model, our focus will be on murine male meiosis, although also including specific comments about humans. Particularly, we will center on the controversy about gene expression during early meiotic prophase; the widespread existing gap between transcription and translation in meiotic cells; the expression patterns and potential roles of meiotic long non-coding RNAs; and the visualization of meiotic sex chromosome inactivation from the RNAseq perspective.

TREND-DB-a transcriptome-wide atlas of the dynamic landscape of alternative polyadenylation

Alternative polyadenylation (APA) profoundly expands the transcriptome complexity. Perturbations of APA can disrupt biological processes, ultimately resulting in devastating disorders. A major challenge in identifying mechanisms and consequences of APA (and its perturbations) lies in the complexity of RNA 3′ end processing, involving poorly conserved RNA motifs and multi-component complexes consisting of far more than 50 proteins. This is further complicated in that RNA 3′ end maturation is closely linked to transcription, RNA processing and even epigenetic (histone/DNA/RNA) modifications. Here, we present TREND-DB (http://shiny.imbei.uni-mainz.de:3838/trend-db), a resource cataloging the dynamic landscape of APA after depletion of >170 proteins involved in various facets of transcriptional, co- and post-transcriptional gene regulation, epigenetic modifications and further processes. TREND-DB visualizes the dynamics of transcriptome 3′ end diversification (TREND) in a highly interactive manner; it provides a global APA network map and allows interrogating genes affected by specific APA-regulators and vice versa. It also permits condition-specific functional enrichment analyses of APA-affected genes, which suggest wide biological and clinical relevance across all RNAi conditions. The implementation of the UCSC Genome Browser provides additional customizable layers of gene regulation accounting for individual transcript isoforms (e.g. epigenetics, miRNA-binding sites and RNA-binding proteins). TREND-DB thereby fosters disentangling the role of APA for various biological programs, including potential disease mechanisms, and helps identify their diagnostic and therapeutic potential.

Epidermal progenitors suppress GRHL3-mediated differentiation through intronic polyadenylation promoted by CPSF-HNRNPA3 collaboration

In self-renewing somatic tissue such as skin epidermis, terminal differentiation genes must be suppressed in progenitors to sustain regenerative capacity. Here we show that hundreds of intronic polyadenylation (IpA) sites are differentially used during keratinocyte differentiation, which is accompanied by downregulation of the Cleavage and Polyadenylation Specificity Factor (CPSF) complex. Sustained CPSF expression in undifferentiated keratinocytes requires the contribution from the transcription factor MYC. In keratinocytes cultured in undifferentiation condition, CSPF knockdown induces premature differentiation and partially affects dynamically used IpA sites. These sites include an IpA site located in the first intron of the differentiation activator GRHL3. CRISPR knockout of GRHL3 IpA increased full-length GRHL3 mRNA expression. Using a targeted genetic screen, we identify that HNRNPA3 interacts with CPSF and enhances GRHL3 IpA. Our data suggest a model where the interaction between CPSF and RNA-binding proteins, such as HNRNPA3, promotes site-specific IpA and suppresses premature differentiation in progenitors.

Emerging roles of the MAGE protein family in stress response pathways

The melanoma antigen (MAGE) proteins all contain a MAGE homology domain. MAGE genes are conserved in all eukaryotes and have expanded from a single gene in lower eukaryotes to ∼40 genes in humans and mice. Whereas some MAGEs are ubiquitously expressed in tissues, others are expressed in only germ cells with aberrant reactivation in multiple cancers. Much of the initial research on MAGEs focused on exploiting their antigenicity and restricted expression pattern to target them with cancer immunotherapy. Beyond their potential clinical application and role in tumorigenesis, recent studies have shown that MAGE proteins regulate diverse cellular and developmental pathways, implicating them in many diseases besides cancer, including lung, renal, and neurodevelopmental disorders. At the molecular level, many MAGEs bind to E3 RING ubiquitin ligases and, thus, regulate their substrate specificity, ligase activity, and subcellular localization. On a broader scale, the MAGE genes likely expanded in eutherian mammals to protect the germline from environmental stress and aid in stress adaptation, and this stress tolerance may explain why many cancers aberrantly express MAGEs Here, we present an updated, comprehensive review on the MAGE family that highlights general characteristics, emphasizes recent comparative studies in mice, and describes the diverse functions exerted by individual MAGEs.

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.

scAPAtrap: identification and quantification of alternative polyadenylation sites from single-cell RNA-seq data

Alternative polyadenylation (APA) generates diverse mRNA isoforms, which contributes to transcriptome diversity and gene expression regulation by affecting mRNA stability, translation and localization in cells. The rapid development of 3' tag-based single-cell RNA-sequencing (scRNA-seq) technologies, such as CEL-seq and 10x Genomics, has led to the emergence of computational methods for identifying APA sites and profiling APA dynamics at single-cell resolution. However, existing methods fail to detect the precise location of poly(A) sites or sites with low read coverage. Moreover, they rely on priori genome annotation and can only detect poly(A) sites located within or near annotated genes. Here we proposed a tool called scAPAtrap for detecting poly(A) sites at the whole genome level in individual cells from 3' tag-based scRNA-seq data. scAPAtrap incorporates peak identification and poly(A) read anchoring, enabling the identification of the precise location of poly(A) sites, even for sites with low read coverage. Moreover, scAPAtrap can identify poly(A) sites without using priori genome annotation, which helps locate novel poly(A) sites in previously overlooked regions and improve genome annotation. We compared scAPAtrap with two latest methods, scAPA and Sierra, using scRNA-seq data from different experimental technologies and species. Results show that scAPAtrap identified poly(A) sites with higher accuracy and sensitivity than competing methods and could be used to explore APA dynamics among cell types or the heterogeneous APA isoform expression in individual cells. scAPAtrap is available at https://github.com/BMILAB/scAPAtrap.

Systematic identification of functional SNPs interrupting 3'UTR polyadenylation signals

Alternative polyadenylation (APA) is emerging as a widespread regulatory layer since the majority of human protein-coding genes contain several polyadenylation (p(A)) sites in their 3’UTRs. By generating isoforms with different 3’UTR length, APA potentially affects mRNA stability, translation efficiency, nuclear export, and cellular localization. Polyadenylation sites are regulated by adjacent RNA cis-regulatory elements, the principals among them are the polyadenylation signal (PAS) AAUAAA and its main variant AUUAAA, typically located ~20-nt upstream of the p(A) site. Mutations in PAS and other auxiliary poly(A) cis-elements in the 3’UTR of several genes have been shown to cause human Mendelian diseases, and to date, only a few common SNPs that regulate APA were associated with complex diseases. Here, we systematically searched for SNPs that affect gene expression and human traits by modulation of 3’UTR APA. First, focusing on the variants most likely to exert the strongest effect, we identified 2,305 SNPs that interrupt the canonical PAS or its main variant. Implementing pA-QTL tests using GTEx RNA-seq data, we identified 330 PAS SNPs (called PAS pA-QTLs) that were significantly associated with the usage of their p(A) site. As expected, PAS-interrupting alleles were mostly linked with decreased cleavage at their p(A) site and the consequential 3’UTR lengthening. However, interestingly, in ~10% of the cases, the PAS-interrupting allele was associated with increased usage of an upstream p(A) site and 3’UTR shortening. As an indication of the functional effects of these PAS pA-QTLs on gene expression and complex human traits, we observed for few dozens of them marked colocalization with eQTL and/or GWAS signals. The PAS-interrupting alleles linked with 3’UTR lengthening were also strongly associated with decreased gene expression, indicating that shorter isoforms generated by APA are generally more stable than longer ones. Last, we carried out an extended, genome-wide analysis of 3’UTR variants and detected thousands of additional pA-QTLs having weaker effects compared to the PAS pA-QTLs.

mRNA molecules that encode for proteins end with a long stretch of adenosines, called poly(A) tail. The poly(A) tail contributes to the stability of the mRNA molecules, their translation to proteins and their import from the nucleus to the cytoplasm. The process of adding this tail to the mRNAs is called polyadenylation, and the termination site on the mRNAs at which the poly(A) tail is added is called the poly(A) site. In recent years it became evident that the vast majority of mRNAs of human genes contain several alternative poly(A) sites and their usage generates different mRNA isoforms that differ in their stability and translation efficiency. Therefore, alternative polyadenylation (APA) is emerging as a novel and important, yet underexplored, mechanism that regulate gene expression. The choice between alternative p(A) sites in an mRNA molecule is regulated by regulatory sequences located within a region in the mRNA called the 3’ untranslated region (3’UTR). A major challenge in present human genetics research is to understand how common genetic variants affect individuals’ health. In our study, we systematically identified dozens of genetic variants that affect the choice between alternative p(A) sites and demonstrated that by that, these variants influence the expression level of the target genes. Our results help to illuminate a novel mechanism by which genetic variants that are common in the population affect different traits including our risk for developing diseases.

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.

Functional Interaction between U1snRNP and Sam68 Insures Proper 3' End Pre-mRNA Processing during Germ Cell Differentiation

Male germ cells express the widest repertoire of transcript variants in mammalian tissues. Nevertheless, factors and mechanisms underlying such pronounced diversity are largely unknown. The splicing regulator Sam68 is highly expressed in meiotic cells, and its ablation results in defective spermatogenesis. Herein, we uncover an extensive splicing program operated by Sam68 across meiosis, primarily characterized by alternative last exon (ALE) regulation in genes of functional relevance for spermatogenesis. Lack of Sam68 preferentially causes premature transcript termination at internal polyadenylation sites, a feature observed also upon depletion of the spliceosomal U1snRNP in somatic cells. Notably, Sam68-regulated ALEs are characterized by proximity between U1snRNP and Sam68 binding motifs. We demonstrate a physical association between Sam68 and U1snRNP and show that U1snRNP recruitment to Sam68-regulated ALEs is impaired in Sam68^-/- germ cells. Thus, our study reveals an unexpected cooperation between Sam68 and U1snRNP that insures proper processing of transcripts essential for male fertility.

m6A-dependent biogenesis of circular RNAs in male germ cells

The majority of circular RNAs (circRNAs) spliced from coding genes contain open reading frames (ORFs) and thus, have protein coding potential. However, it remains unknown what regulates the biogenesis of these ORF-containing circRNAs, whether they are actually translated into proteins and what functions they play in specific physiological contexts. Here, we report that a large number of circRNAs are synthesized with increasing abundance when late pachytene spermatocytes develop into round and then elongating spermatids during murine spermatogenesis. For a subset of circRNAs, the back splicing appears to occur mostly at m6A-enriched sites, which are usually located around the start and stop codons in linear mRNAs. Consequently, approximately a half of these male germ cell circRNAs contain large ORFs with m6A-modified start codons in their junctions, features that have been recently shown to be associated with protein-coding potential. Hundreds of peptides encoded by the junction sequences of these circRNAs were detected using liquid chromatography coupled with mass spectrometry, suggesting that these circRNAs can indeed be translated into proteins in both developing (spermatocytes and spermatids) and mature (spermatozoa) male germ cells. The present study discovered not only a novel role of m6A in the biogenesis of coding circRNAs, but also a potential mechanism to ensure stable and long-lasting protein production in the absence of linear mRNAs, i.e., through production of circRNAs containing large ORFs and m6A-modified start codons in junction sequences.

A Cancer-Specific Ubiquitin Ligase Drives mRNA Alternative Polyadenylation by Ubiquitinating the mRNA 3' End Processing Complex

Alternative polyadenylation (APA) contributes to transcriptome complexity by generating mRNA isoforms with varying 3' UTR lengths. APA leading to 3' UTR shortening (3' US) is a common feature of most cancer cells; however, the molecular mechanisms are not understood. Here, we describe a widespread mechanism promoting 3' US in cancer through ubiquitination of the mRNA 3' end processing complex protein, PCF11, by the cancer-specific MAGE-A11-HUWE1 ubiquitin ligase. MAGE-A11 is normally expressed only in the male germline but is frequently re-activated in cancers. MAGE-A11 is necessary for cancer cell viability and is sufficient to drive tumorigenesis. Screening for targets of MAGE-A11 revealed that it ubiquitinates PCF11, resulting in loss of CFIm25 from the mRNA 3' end processing complex. This leads to APA of many transcripts affecting core oncogenic and tumor suppressors, including cyclin D2 and PTEN. These findings provide insights into the molecular mechanisms driving APA in cancer and suggest therapeutic strategies.

DAZL Regulates Germ Cell Survival through a Network of PolyA-Proximal mRNA Interactions

The RNA binding protein DAZL is essential for gametogenesis, but its direct in vivo functions, RNA targets, and the molecular basis for germ cell loss in Dazl-null mice are unknown. Here, we mapped transcriptome-wide DAZL-RNA interactions in vivo, revealing DAZL binding to thousands of mRNAs via polyA-proximal 3′ UTR interactions. In parallel, fluorescence-activated cell sorting and RNA-seq identified mRNAs sensitive to DAZL deletion in male germ cells. Despite binding a broad set of mRNAs, integrative analyses indicate that DAZL post-transcriptionally controls only a subset of its mRNA targets, namely those corresponding to a network of genes that are critical for germ cell proliferation and survival. In addition, we provide evidence that polyA sequences have key roles in specifying DAZL-RNA interactions across the transcriptome. Our results reveal a mechanism for DAZL-RNA binding and illustrate that DAZL functions as a master regulator of a post-transcriptional mRNA program essential for germ cell survival.

Combining transgenic mice, FACS, and multiple RNA-profiling methods, Zagore et al. show that DAZL binds thousands of mRNAs via GUU sites upstream of polyA tails. Loss of DAZL results in decreased mRNA levels for a network of genes that are essential for germ cell proliferation and differentiation.

Extensive transcriptional responses are co-ordinated by microRNAs as revealed by Exon-Intron Split Analysis (EISA)

Epithelial-mesenchymal transition (EMT) has been a subject of intense scrutiny as it facilitates metastasis and alters drug sensitivity. Although EMT-regulatory roles for numerous miRNAs and transcription factors are known, their functions can be difficult to disentangle, in part due to the difficulty in identifying direct miRNA targets from complex datasets and in deciding how to incorporate 'indirect' miRNA effects that may, or may not, represent biologically relevant information. To better understand how miRNAs exert effects throughout the transcriptome during EMT, we employed Exon-Intron Split Analysis (EISA), a bioinformatic technique that separates transcriptional and post-transcriptional effects through the separate analysis of RNA-Seq reads mapping to exons and introns. We find that in response to the manipulation of miRNAs, a major effect on gene expression is transcriptional. We also find extensive co-ordination of transcriptional and post-transcriptional regulatory mechanisms during both EMT and mesenchymal to epithelial transition (MET) in response to TGF-β or miR-200c respectively. The prominent transcriptional influence of miRNAs was also observed in other datasets where miRNA levels were perturbed. This work cautions against a narrow approach that is limited to the analysis of direct targets, and demonstrates the utility of EISA to examine complex regulatory networks involving both transcriptional and post-transcriptional mechanisms.

Cell-type-specific analysis of alternative polyadenylation using single-cell transcriptomics data

Alternative polyadenylation (APA) is emerging as an important layer of gene regulation because the majority of mammalian protein-coding genes contain multiple polyadenylation (pA) sites in their 3' UTR. By alteration of 3' UTR length, APA can considerably affect post-transcriptional gene regulation. Yet, our understanding of APA remains rudimentary. Novel single-cell RNA sequencing (scRNA-seq) techniques allow molecular characterization of different cell types to an unprecedented degree. Notably, the most popular scRNA-seq protocols specifically sequence the 3' end of transcripts. Building on this property, we implemented a method for analysing patterns of APA regulation from such data. Analyzing multiple datasets from diverse tissues, we identified widespread modulation of APA in different cell types resulting in global 3' UTR shortening/lengthening and enhanced cleavage at intronic pA sites. Our results provide a proof-of-concept demonstration that the huge volume of scRNA-seq data that accumulates in the public domain offers a unique resource for the exploration of APA based on a very broad collection of cell types and biological conditions.

Multi-strategic RNA-seq analysis reveals a high-resolution transcriptional landscape in cotton

Cotton is an important natural fiber crop, however, its comprehensive and high-resolution gene map is lacking. Here we integrate four complementary high-throughput techniques, including Pacbio long read Iso-seq, strand-specific RNA-seq, CAGE-seq, and PolyA-seq, to systematically explore the transcription landscape across 16 tissues or different organ types in Gossypium arboreum. We devise a computational pipeline, named IGIA, to reconstruct accurate gene structures from the integrated data. Our results reveal a dynamic and diverse transcriptional map in cotton: tissue-specific gene expression, alternative usage of TSSs and polyadenylation sites, hotspot of alternative splicing, and transcriptional read-through. These regulated events affect many genes in various aspects such as gain or loss of functional RNA motifs and protein domains, fine-tuning of DNA binding activity, and co-regulation for genes in the same complex or pathway. The methods and findings provide valuable resources for further functional genomic studies such as understanding natural SNP variations for plant community.