Phosphorylation of spliceosomal protein SAP 155 coupled with splicing catalysis

SAP30BP interacts with RBM17/SPF45 to promote splicing in a subset of human short introns

Human pre-mRNA splicing requires the removal of introns with highly variable lengths, from tens to over a million nucleotides. Therefore, mechanisms of intron recognition and splicing are likely not universal. Recently, we reported that splicing in a subset of human short introns with truncated polypyrimidine tracts depends on RBM17 (SPF45), instead of the canonical splicing factor U2 auxiliary factor (U2AF) heterodimer. Here, we demonstrate that SAP30BP, a factor previously implicated in transcriptional control, is an essential splicing cofactor for RBM17. In vitro binding and nuclear magnetic resonance analyses demonstrate that a U2AF-homology motif (UHM) in RBM17 binds directly to a newly identified UHM-ligand motif in SAP30BP. We show that this RBM17-SAP30BP interaction is required to specifically recruit RBM17 to phosphorylated SF3B1 (SF3b155), a U2 small nuclear ribonucleoprotein (U2 snRNP) component in active spliceosomes. We propose a mechanism for splicing in a subset of short introns, in which SAP30BP guides RBM17 in the assembly of active spliceosomes.

Systematic analysis of alternative exon-dependent interactome remodeling reveals multitasking functions of gene regulatory factors

Alternative splicing significantly expands biological complexity, particularly in the vertebrate nervous system. Increasing evidence indicates that developmental and tissue-dependent alternative exons often control protein-protein interactions; yet, only a minor fraction of these events have been characterized. Using affinity purification-mass spectrometry (AP-MS), we show that approximately 60% of analyzed neural-differential exons in proteins previously implicated in transcriptional regulation result in the gain or loss of interaction partners, which in some cases form unexpected links with coupled processes. Notably, a neural exon in Chtop regulates its interaction with the Prmt1 methyltransferase and DExD-Box helicases Ddx39b/a, affecting its methylation and activity in promoting RNA export. Additionally, a neural exon in Sap30bp affects interactions with RNA processing factors, modulating a critical function of Sap30bp in promoting the splicing of <100 nt "mini-introns" that control nuclear RNA levels. AP-MS is thus a powerful approach for elucidating the multifaceted functions of proteins imparted by context-dependent alternative exons.

Synthesis and Conformational Analysis of FR901464-Based RNA Splicing Modulators and Their Synergism in Drug-Resistant Cancers

FR901464 is a cytotoxic natural product that binds splicing factor 3B subunit 1 (SF3B1) and PHD finger protein 5A (PHF5A), the components of the human spliceosome. The amide-containing tetrahydropyran ring binds SF3B1, and it remains unclear how the substituents on the ring contribute to the binding. Here, we synthesized meayamycin D, an analogue of FR901464, and three additional analogues to probe the conformation through methyl scanning. We discovered that the amide-containing tetrahydropyran ring assumes only one of the two possible chair conformations and that methylation of the nitrogen distorts the chair form, dramatically reducing cytotoxicity. Meayamycin D induced alternative splicing of MCL-1, showed strong synergism with venetoclax in drug-resistant lung cancer cells, and was cancer-specific over normal cells. Meayamycin D incorporates an alkyl ether and shows a long half-life in mouse plasma. The characteristics of meayamycin D may provide an approach to designing other bioactive L-shaped molecules.

Studying the connection between SF3B1 and four types of cancer by analyzing networks constructed based on published research

Splicing factor 3B subunit 1 (SF3B1) is the largest component of SF3b protein complex which is involved in the pre-mRNA splicing mechanism. Somatic mutations of SF3B1 were shown to be associated with aberrant splicing, producing abnormal transcripts that drive cancer development and/or prognosis. In this study, we focus on the relationship between SF3B1 and four types of cancer, namely myelodysplastic syndrome (MDS), acute myeloid leukemia (AML), and chronic lymphocytic leukemia (CLL) and breast cancer (BC). For this purpose, we identified from the Pubmed library only articles which mentioned SF3B1 in connection with the investigated types of cancer for the period 2007 to 2018 to reveal how the connection has developed over time. We left out all published articles which mentioned SF3B1 in other contexts. We retrieved the target articles and investigated the association between SF3B1 and the mentioned four types of cancer. For this we utilized some of the publicly available databases to retrieve gene/variant/disease information related to SF3B1. We used the outcome to derive and analyze a variety of complex networks that reflect the correlation between the considered diseases and variants associated with SF3B1. The results achieved based on the analyzed articles and reported in this article illustrated that SF3B1 is associated with hematologic malignancies, such as MDS, AML, and CLL more than BC. We found that different gene networks may be required for investigating the impact of mutant splicing factors on cancer development based on the target cancer type. Additionally, based on the literature analyzed in this study, we highlighted and summarized what other researchers have reported as the set of genes and cellular pathways that are affected by aberrant splicing in cancerous cells.

A Truncated Form of the p27 Cyclin-Dependent Kinase Inhibitor Translated from Pre-mRNA Causes G2-Phase Arrest

Pre-mRNA splicing is an indispensable mechanism for eukaryotic gene expression. Splicing inhibition causes cell cycle arrest at the G₁ and G₂/M phases, and this is thought to be one of the reasons for the potent antitumor activity of splicing inhibitors. However, the molecular mechanisms underlying the cell cycle arrest have many unknown aspects. In particular, the mechanism of G₂/M-phase arrest caused by splicing inhibition is completely unknown. Here, we found that lower and higher concentrations of pladienolide B caused M-phase and G₂-phase arrest, respectively. We analyzed protein levels of cell cycle regulators and found that a truncated form of the p27 cyclin-dependent kinase inhibitor, named p27*, accumulated in G₂-arrested cells. Overexpression of p27* caused partial G₂-phase arrest. Conversely, knockdown of p27* accelerated exit from G₂/M phase after washout of splicing inhibitor. These results suggest that p27* contributes to G₂/M-phase arrest caused by splicing inhibition. We also found that p27* bound to and inhibited M-phase cyclins, although it is well known that p27 regulates the G₁/S transition. Intriguingly, p27*, but not full-length p27, was resistant to proteasomal degradation and remained in G₂/M phase. These results suggest that p27*, which is a very stable truncated protein in G₂/M phase, contributes to G₂-phase arrest caused by splicing inhibition.

CDK11 regulates pre-mRNA splicing by phosphorylation of SF3B1

RNA splicing, the process of intron removal from pre-mRNA, is essential for the regulation of gene expression. It is controlled by the spliceosome, a megadalton RNA-protein complex that assembles de novo on each pre-mRNA intron through an ordered assembly of intermediate complexes^1,2. Spliceosome activation is a major control step that requires substantial protein and RNA rearrangements leading to a catalytically active complex^1-5. Splicing factor 3B subunit 1 (SF3B1) protein-a subunit of the U2 small nuclear ribonucleoprotein⁶-is phosphorylated during spliceosome activation^7-10, but the kinase that is responsible has not been identified. Here we show that cyclin-dependent kinase 11 (CDK11) associates with SF3B1 and phosphorylates threonine residues at its N terminus during spliceosome activation. The phosphorylation is important for the association between SF3B1 and U5 and U6 snRNAs in the activated spliceosome, termed the B^act complex, and the phosphorylation can be blocked by OTS964, a potent and selective inhibitor of CDK11. Inhibition of CDK11 prevents spliceosomal transition from the precatalytic complex B to the activated complex B^act and leads to widespread intron retention and accumulation of non-functional spliceosomes on pre-mRNAs and chromatin. We demonstrate a central role of CDK11 in spliceosome assembly and splicing regulation and characterize OTS964 as a highly selective CDK11 inhibitor that suppresses spliceosome activation and splicing.

Exploring the mechanistic link between SF3B1 mutation and ring sideroblast formation in myelodysplastic syndrome

Acquired sideroblastic anemia, characterized by bone marrow ring sideroblasts (RS), is predominantly associated with myelodysplastic syndrome (MDS). Although somatic mutations in splicing factor 3b subunit 1 (SF3B1), which is involved in the RNA splicing machinery, are frequently found in MDS-RS, the detailed mechanism contributing to RS formation is unknown. To explore the mechanism, we established human umbilical cord blood-derived erythroid progenitor-2 (HUDEP-2) cells stably expressing SF3B1^K700E. SF3B1^K700E expressing cells showed higher proportion of RS than the control cells along with erythroid differentiation, indicating the direct contribution of mutant SF3B1 expression in erythroblasts to RS formation. In SF3B1^K700E expressing cells, ABCB7 and ALAS2, known causative genes for congenital sideroblastic anemia, were downregulated. Additionally, mis-splicing of ABCB7 was observed in SF3B1^K700E expressing cells. ABCB7-knockdown HUDEP-2 cells revealed an increased frequency of RS formation along with erythroid differentiation, demonstrating the direct molecular link between ABCB7 defects and RS formation. ALAS2 protein levels were obviously decreased in ABCB7-knockdown cells, indicating decreased ALAS2 translation owing to impaired Fe–S cluster export by ABCB7 defects. Finally, RNA-seq analysis of MDS clinical samples demonstrated decreased expression of ABCB7 by the SF3B1 mutation. Our findings contribute to the elucidation of the complex mechanisms of RS formation in MDS-RS.

A critical update on the strategies towards small molecule inhibitors targeting Serine/arginine-rich (SR) proteins and Serine/arginine-rich proteins related kinases in alternative splicing

>90% of genes in the human body undergo alternative splicing (AS) after transcription, which enriches protein species and regulates protein levels. However, there is growing evidence that various genetic isoforms resulting from dysregulated alternative splicing are prevalent in various types of cancers. Dysregulated alternative splicing leads to cancer generation and maintenance of cancer properties such as proliferation differentiation, apoptosis inhibition, invasion metastasis, and angiogenesis. Serine/arginine-rich proteins and SR protein-associated kinases mediate splice site recognition and splice complex assembly during variable splicing. Based on the impact of dysregulated alternative splicing on disease onset and progression, the search for small molecule inhibitors targeting alternative splicing is imminent. In this review, we discuss the structure and specific biological functions of SR proteins and describe the regulation of SR protein function by SR protein related kinases meticulously, which are closely related to the occurrence and development of various types of cancers. On this basis, we summarize the reported small molecule inhibitors targeting SR proteins and SR protein related kinases from the perspective of medicinal chemistry. We mainly categorize small molecule inhibitors from four aspects, including targeting SR proteins, targeting Serine/arginine-rich protein-specific kinases (SRPKs), targeting Cdc2-like kinases (CLKs) and targeting dual-specificity tyrosine-regulated kinases (DYRKs), in terms of structure, inhibition target, specific mechanism of action, biological activity, and applicable diseases. With this review, we are expected to provide a timely summary of recent advances in alternative splicing regulated by kinases and a preliminary introduction to relevant small molecule inhibitors.

Two oppositely-charged sf3b1 mutations cause defective development, impaired immune response, and aberrant selection of intronic branch sites in Drosophila

SF3B1 mutations occur in many cancers, and the highly conserved His662 residue is one of the hotspot mutation sites. To address effects on splicing and development, we constructed strains carrying point mutations at the corresponding residue His698 in Drosophila using the CRISPR-Cas9 technique. Two mutations, H698D and H698R, were selected due to their frequent presence in patients and notable opposite charges. Both the sf3b1-H698D and–H698R mutant flies exhibit developmental defects, including less egg-laying, decreased hatching rates, delayed morphogenesis and shorter lifespans. Interestingly, the H698D mutant has decreased resistance to fungal infection, while the H698R mutant shows impaired climbing ability. Consistent with these phenotypes, further analysis of RNA-seq data finds altered expression of immune response genes and changed alternative splicing of muscle and neural-related genes in the two mutants, respectively. Expression of Mef2-RB, an isoform of Mef2 gene that was downregulated due to splicing changes caused by H698R, partly rescues the climbing defects of the sf3b1-H698R mutant. Lariat sequencing reveals that the two sf3b1-H698 mutations cause aberrant selection of multiple intronic branch sites, with the H698R mutant using far upstream branch sites in the changed alternative splicing events. This study provides in vivo evidence from Drosophila that elucidates how these SF3B1 hotspot mutations alter splicing and their consequences in development and in the immune system.

In the past decade, one of the important findings in the RNA splicing field has been that somatic SF3B1 mutations widely occur in many cancers. Including R625, H662, K666, K700 and E902, there are five hotspot mutation sites in the highly conserved HEAT repeats of SF3B1. Several kinds of H662 mutations have been found widely in MDS, AML, CLL and breast cancers; however, it remains unclear how these H662 mutations alter splicing and whether they have in vivo effects on development. To address these questions, in this manuscript, we first summarized the H662 mutations in human diseases and constructed two corresponding Drosophila mutant strains, sf3b1-H698D and -H698R using CRISPR-Cas9. Analyses of these two fly strains find that the two oppositely charged Sf3b1-H698 mutants are defective in development. In addition, one mutant has decreased climbing ability, whereas the other mutant has impaired immune response. Further RNA-seq allows us to find responsible genes in each mutant strain, and lariat sequencing reveals that both mutations cause aberrant selection of the intronic branch sites. Our findings provide the first in vivo evidence that Sf3b1 mutations result in defective development, and also reveal a molecular mechanism of these hotspot histidine mutations that enhance the use of cryptic branch sites to alter splicing. Importantly, we demonstrate that the H698R mutant prefers to use far upstream branch sites.

The long non-coding RNA DKFZp434J0226 regulates the alternative splicing process through phosphorylation of SF3B6 in PDAC

BACKGROUND

Long noncoding RNAs (lncRNAs), a type of pervasive genes that regulates various biological processes, are differentially expressed in different types of malignant tumors. The role of lncRNAs in the carcinogenesis of pancreatic ductal adenocarcinoma (PDAC) remains unclear. Here, we investigated the role of the lncRNA DKFZp434J0226 in PDAC.

METHODS

Aberrantly expressed mRNAs and lncRNAs among six PDAC and paired non-tumorous tissues were profiled using microarray analysis. Quantitative real-time polymerase chain reaction was used to evaluate DKFZp434J0226 expression in PDAC tissues. CCK-8 assay, wound-healing assay, soft agar colony formation assay, and transwell assay were performed to assess the invasiveness and proliferation of PDAC cells. Furthermore, RNA pull-down, immunofluorescence, RNA immunoprecipitation, and western blotting assays were performed to investigate the association between DKFZp434J0226 and SF3B6. Tumor xenografts in mice were used to test for tumor formation in vivo.

RESULTS

In our study, 222 mRNAs and 128 lncRNAs were aberrantly expressed (≥ twofold change). Of these, 66 mRNAs and 53 lncRNAs were upregulated, while 75 lncRNAs and 156 mRNAs were downregulated. KEGG pathway analysis and the Gene ontology category indicated that these genes were associated with the regulation of mRNA alternative splicing and metabolic balance. Clinical analyses revealed that overexpression of DKFZp434J0226 was associated with worse tumor grading, frequent perineural invasion, advanced tumor-node-metastasis stage, and decreased overall survival and time to progression. Functional assays demonstrated that DKFZp434J0226 promoted PDAC cell migration, invasion, and growth in vitro and accelerated tumor proliferation in vivo. Mechanistically, DKFZp434J0226 interacted with the splicing factor SF3B6 and promoted its phosphorylation, which further regulated the alternative splicing of pre-mRNA.

CONCLUSIONS

This study indicates that DKFZp434J0226 regulates alternative splicing through phosphorylation of SF3B6 in PDAC and leads to an oncogenic phenotype in PDAC.

U2AF65-Dependent SF3B1 Function in SMN Alternative Splicing

Splicing factor 3b subunit 1 (SF3B1) is an essential protein in spliceosomes and mutated frequently in many cancers. While roles of SF3B1 in single intron splicing and roles of its cancer-linked mutant in aberrant splicing have been identified to some extent, regulatory functions of wild-type SF3B1 in alternative splicing (AS) are not well-understood yet. Here, we applied RNA sequencing (RNA-seq) to analyze genome-wide AS in SF3B1 knockdown (KD) cells and to identify a large number of skipped exons (SEs), with a considerable number of alternative 5′ splice-site selection, alternative 3′ splice-site selection, mutually exclusive exons (MXE), and retention of introns (RI). Among altered SEs by SF3B1 KD, survival motor neuron 2 (SMN2) pre-mRNA exon 7 splicing was a regulatory target of SF3B1. RT-PCR analysis of SMN exon 7 splicing in SF3B1 KD or overexpressed HCT116, SH-SY5Y, HEK293T, and spinal muscular atrophy (SMA) patient cells validated the results. A deletion mutation demonstrated that the U2 snRNP auxiliary factor 65 kDa (U2AF65) interaction domain of SF3B1 was required for its function in SMN exon 7 splicing. In addition, mutations to lower the score of the polypyrimidine tract (PPT) of exon 7, resulting in lower affinity for U2AF65, were not able to support SF3B1 function, suggesting the importance of U2AF65 in SF3B1 function. Furthermore, the PPT of exon 7 with higher affinity to U2AF65 than exon 8 showed significantly stronger interactions with SF3B1. Collectively, our results revealed SF3B1 function in SMN alternative splicing.

Splicing to Keep Cycling: The Importance of Pre-mRNA Splicing during the Cell Cycle

Pre-mRNA splicing is a fundamental process in mammalian gene expression, and alternative splicing plays an extensive role in generating protein diversity. Because the majority of genes undergo pre-mRNA splicing, most cellular processes depend on proper spliceosome function. We focus on the cell cycle and describe its dependence on pre-mRNA splicing and accurate alternative splicing. We outline the key cell-cycle factors and their known alternative splicing isoforms. We discuss different levels of pre-mRNA splicing regulation such as post-translational modifications and changes in the expression of splicing factors. We describe the effect of chromatin dynamics on pre-mRNA splicing during the cell cycle. In addition, we focus on spliceosome component SF3B1, which is mutated in many types of cancer, and describe the link between SF3B1 and its inhibitors and the cell cycle.

Complex landscape of alternative splicing in myeloid neoplasms

Myeloid neoplasms are characterized by frequent mutations in at least seven components of the spliceosome that have distinct roles in the process of pre-mRNA splicing. Hotspot mutations in SF3B1, SRSF2, U2AF1 and loss of function mutations in ZRSR2 have revealed widely different aberrant splicing signatures with little overlap. However, previous studies lacked the power necessary to identify commonly mis-spliced transcripts in heterogeneous patient cohorts. By performing RNA-Seq on bone marrow samples from 1,258 myeloid neoplasm patients and 63 healthy bone marrow donors, we identified transcripts frequently mis-spliced by mutated splicing factors (SF), rare SF mutations with common alternative splicing (AS) signatures, and SF-dependent neojunctions. We characterized 17,300 dysregulated AS events using a pipeline designed to predict the impact of mis-splicing on protein function. Meta-splicing analysis revealed a pattern of reduced levels of retained introns among disease samples that was exacerbated in patients with splicing factor mutations. These introns share characteristics with “detained introns,” a class of introns that have been shown to promote differentiation by detaining pro-proliferative transcripts in the nucleus. In this study, we have functionally characterized 17,300 targets of mis-splicing by the SF mutations, identifying a common pathway by which AS may promote maintenance of a proliferative state.

Spliceosomal factor mutations and mis-splicing in MDS

Somatic, heterozygous missense and nonsense mutations in at least seven proteins that function in the spliceosome are found at high frequency in MDS patients. These proteins act at various steps in the process of splicing by the spliceosome and lead to characteristic alterations in the alternative splicing of a subset of genes. Several studies have investigated the effects of these mutations and have attempted to identify a commonly affected gene or pathway. Here, we summarize what is known about the normal function of these proteins and how the mutations alter the splicing landscape of the genome. We also summarize the commonly mis-spliced gene targets and discuss the state of mechanistic unification that has been achieved. Finally, we discuss alternative mechanisms by which these mutations may lead to disease.

Hepatitis delta virus interacts with splicing factor SF3B155 and alters pre-mRNA splicing of cell cycle control genes

Hepatitis delta virus (HDV) is the agent responsible for the most severe form of human viral hepatitis. The HDV genome consists of a single-stranded circular RNA molecule that encodes for one single protein, the delta antigen. Given its simplicity, HDV must make use of several host cellular proteins to accomplish its life cycle processes, including transcription, replication, post-transcriptional, and post-translational modifications. Consequently, identification of the interactions established between HDV components and host proteins assumes a pivotal interest in the search of novel therapeutic targets. Here, we used the yeast three-hybrid system to screen a human liver cDNA library to identify host proteins that interact with the HDV genomic RNA. One of the identified proteins corresponded to the splicing factor SF3B155, a component of the U2snRNP complex that is essential for the early recognition of 3' splice sites in the pre-mRNAs of human genes. We show that the interaction between the HDV genomic RNA and SF3B155 occurs in vivo and that the expression of HDV promotes changes in splicing of human genes whose alternative splicing is SF3B155-dependent. We further show that expression of HDV triggers alterations in several constitutive and alternative splicing events in the tumor suppressor RBM5 transcript, with consequent reduction of its protein levels. This is the first description that HDV expression promotes changes in the splicing of human genes, and we suggest that the HDV-induced alternative splicing changes, through SF3B155 sequester, may contribute for the early progression to hepatocellular carcinoma characteristic of HDV-infected patients.

In Silico Analysis of a Highly Mutated Gene in Cancer Provides Insight into Abnormal mRNA Splicing: Splicing Factor 3B Subunit 1K700E Mutant

Cancer is the second leading cause of death worldwide. The etiology of the disease has remained elusive, but mutations causing aberrant RNA splicing have been considered one of the significant factors in various cancer types. The association of aberrant RNA splicing with drug/therapy resistance further increases the importance of these mutations. In this work, the impact of the splicing factor 3B subunit 1 (SF3B1) K700E mutation, a highly prevalent mutation in various cancer types, is investigated through molecular dynamics simulations. Based on our results, K700E mutation increases flexibility of the mutant SF3B1. Consequently, this mutation leads to i) disruption of interaction of pre-mRNA with SF3B1 and p14, thus preventing proper alignment of mRNA and causing usage of abnormal 3' splice site, and ii) disruption of communication in critical regions participating in interactions with other proteins in pre-mRNA splicing machinery. We anticipate that this study enhances our understanding of the mechanism of functional abnormalities associated with splicing machinery, thereby, increasing possibility for designing effective therapies to combat cancer at an earlier stage.

The SF3b complex: splicing and beyond

The SF3b complex is an intrinsic component of the functional U2 small nuclear ribonucleoprotein (snRNP). As U2 snRNP enters nuclear pre-mRNA splicing, SF3b plays key roles in recognizing the branch point sequence (BPS) and facilitating spliceosome assembly and activation. Since the discovery of SF3b, substantial progress has been made in elucidating its molecular mechanism during splicing. In addition, numerous recent studies indicate that SF3b and its components are engaged in various molecular and cellular events that are beyond the canonical role in splicing. This review summarizes the current knowledge on the SF3b complex and highlights its multiple roles in splicing and beyond.

Disease-Causing Mutations in SF3B1 Alter Splicing by Disrupting Interaction with SUGP1

SF3B1, which encodes an essential spliceosomal protein, is frequently mutated in myelodysplastic syndromes (MDS) and many cancers. However, the defect of mutant SF3B1 is unknown. Here, we analyzed RNA sequencing data from MDS patients and confirmed that SF3B1 mutants use aberrant 3' splice sites. To elucidate the underlying mechanism, we purified complexes containing either wild-type or the hotspot K700E mutant SF3B1 and found that levels of a poorly studied spliceosomal protein, SUGP1, were reduced in mutant spliceosomes. Strikingly, SUGP1 knockdown completely recapitulated the splicing errors, whereas SUGP1 overexpression drove the protein, which our data suggest plays an important role in branchsite recognition, into the mutant spliceosome and partially rescued splicing. Other hotspot SF3B1 mutants showed similar altered splicing and diminished interaction with SUGP1. Our study demonstrates that SUGP1 loss is a common defect of spliceosomes with disease-causing SF3B1 mutations and, because this defect can be rescued, suggests possibilities for therapeutic intervention.

Somatic SF3B1 mutations in myelodysplastic syndrome with ring sideroblasts and chronic lymphocytic leukaemia

SF3B1 is the largest subunit of the Spliceosome Factor 3b (SF3B) complex and part of the U2 small nuclear ribosomal protein. It functions as an important part of spliceosomal assembly, converting precursor messenger RNA (mRNA) to mRNA ready for ribosomal translation. Mutations of SF3B1 are commonly seen in myelodysplastic syndromes with ring sideroblasts (MDS-RS)and MDS/myeloproliferative neoplasm (MPN-RS-T). These mutations are typically heterozygous missense substitutions, of which, 55% involve K700E. MDS-RS and MDS/MPN-RS-T usually carry a more favourable prognosis than other subtypes of MDS. SF3B1 itself does not influence survival in these conditions, but does correlate with increased thrombotic risk. Mutated SF3B1 is present in 9%–15% of chronic lymphocytic leukaemia cases and on its own correlates with improved responsiveness to ibrutinib, but is associated with additional adverse genetic abnormalities including TP53 and ATM mutations, which traditionally confer adverse outcomes.

Degenerate minigene library analysis enables identification of altered branch point utilization by mutant splicing factor 3B1 (SF3B1)

Cancer-associated mutations of the core splicing factor 3 B1 (SF3B1) result in selection of novel 3′ splice sites (3′SS), but precise molecular mechanisms of oncogenesis remain unclear. SF3B1 stabilizes the interaction between U2 snRNP and branch point (BP) on the pre-mRNA. It has hence been speculated that a change in BP selection is the basis for novel 3′SS selection. Direct quantitative determination of BP utilization is however technically challenging. To define BP utilization by SF3B1-mutant spliceosomes, we used an overexpression approach in human cells as well as a complementary strategy using isogenic murine embryonic stem cells with monoallelic K700E mutations constructed via CRISPR/Cas9-based genome editing and a dual vector homology-directed repair methodology. A synthetic minigene library with degenerate regions in 3′ intronic regions (3.4 million individual minigenes) was used to compare BP usage of SF3B1^K700E and SF3B1^WT. Using this model, we show that SF3B1^K700E spliceosomes utilize non-canonical sequence variants (at position −1 relative to BP adenosine) more frequently than wild-type spliceosomes. These predictions were confirmed using minigene splicing assays. Our results suggest a model of BP utilization by mutant SF3B1 wherein it is able to utilize non-consensus alternative BP sequences by stabilizing weaker U2-BP interactions.

Cus2 enforces the first ATP-dependent step of splicing by binding to yeast SF3b1 through a UHM-ULM interaction

Stable recognition of the intron branchpoint (BP) by the U2 snRNP to form the pre-spliceosome is the first ATP-dependent step of splicing. Genetic and biochemical data from yeast indicate that Cus2 aids U2 snRNA folding into the stem IIa conformation prior to pre-spliceosome formation. Cus2 must then be removed by an ATP-dependent function of Prp5 before assembly can progress. However, the location from which Cus2 is displaced and the nature of its binding to the U2 snRNP are unknown. Here, we show that Cus2 contains a conserved UHM (U2AF homology motif) that binds Hsh155, the yeast homolog of human SF3b1, through a conserved ULM (U2AF ligand motif). Mutations in either motif block binding and allow pre-spliceosome formation without ATP. A 2.0 Å resolution structure of the Hsh155 ULM in complex with the UHM of Tat-SF1, the human homolog of Cus2, and complementary binding assays show that the interaction is highly similar between yeast and humans. Furthermore, we show that Tat-SF1 can replace Cus2 function by enforcing ATP dependence of pre-spliceosome formation in yeast extracts. Cus2 is removed before pre-spliceosome formation, and both Cus2 and its Hsh155 ULM binding site are absent from available cryo-EM structure models. However, our data are consistent with the apparent location of the disordered Hsh155 ULM between the U2 stem-loop IIa and the HEAT repeats of Hsh155 that interact with Prp5. We propose a model in which Prp5 uses ATP to remove Cus2 from Hsh155 such that extended base-pairing between U2 snRNA and the intron BP can occur.

Nucleolin Interacts and Co-Localizes with Components of Pre-Catalytic Spliceosome Complexes

Nucleolin is an RNA binding protein that is involved in many post-transcriptional regulation steps of messenger RNAs in addition to its nucleolar role in ribosomal RNA transcription and assembly in pre-ribosomes. Acetylated nucleolin was found to be associated with nuclear speckles and to co-localize with the splicing factor SC35. Previous nuclear pull down of nucleolin identified several splicing components and factors involved in RNA polymerase II transcription associated with nucleolin. In this report, we show that these splicing components are specifics of the pre-catalytic A and B spliceosomes, while proteins recruited in the Bact, C and P complexes are absent from the nucleolin interacting proteins. Furthermore, we show that acetylated nucleolin co-localized with P-SF3B1, a marker of co-transcriptional active spliceosomes. P-SF3B1 complexes can be pulled down with nucleolin specific antibodies. Interestingly, the alternative splicing of Fibronectin at the IIICS and EDB sites was affected by nucleolin depletion. These data are consistent with a model where nucleolin could be a factor bridging RNA polymerase II transcription and assembly of pre-catalytic spliceosome similarly to its function in the co-transcriptional maturation of pre-rRNA.

A U2-snRNP-independent role of SF3b in promoting mRNA export

To ensure efficient and accurate gene expression, pre-mRNA processing and mRNA export need to be balanced. However, how this balance is ensured remains largely unclear. Here, we found that SF3b, a component of U2 snRNP that participates in splicing and 3' processing of pre-mRNAs, interacts with the key mRNA export adaptor THO in vivo and in vitro. Depletion of SF3b reduces THO binding with the mRNA and causes nuclear mRNA retention. Consistently, introducing SF3b binding sites into the mRNA enhances THO recruitment and nuclear export in a dose-dependent manner. These data demonstrate a role of SF3b in promoting mRNA export. In support of this role, SF3b binds with mature mRNAs in the cells. Intriguingly, disruption of U2 snRNP by using a U2 antisense morpholino oligonucleotide does not inhibit, but promotes, the role of SF3b in mRNA export as a result of enhanced SF3b-THO interaction and THO recruitment to the mRNA. Together, our study uncovers a U2-snRNP-independent role of SF3b in mRNA export and suggests that SF3b contributes to balancing pre-mRNA processing and mRNA export.

The Development and Use of Scalable Systems for Studying Aberrant Splicing in SF3B1-Mutant CLL

Mutational landscape of CLL is now known to include recurrent non-synonymous mutations in SF3B1, a core splicing factor. About 5-10% of newly diagnosed CLL harbor these mutations which are typically limited to HEAT domains in the carboxyl-terminus of the protein. Importantly, the mutations are not specific to CLL but also present in several unrelated clonal disorders. Analysis of patient samples and cell lines has shown the primary splicing aberration in SF3B1-mutant cells to the use of novel or "cryptic" 3' splice sites (3SS). Advances in genome-editing and next-generation sequencing (NGS) have allowed development of isogenic models and detailed analysis of changes to the transcriptome with relative ease. In this manuscript, we focus on two relevant methods to study splicing factor mutations in CLL: development of isogenic scalable cell lines and informatics analysis of RNA-Seq datasets.

Aberrant splicing and defective mRNA production induced by somatic spliceosome mutations in myelodysplasia

Spliceosome mutations are frequently found in myelodysplasia. Splicing alterations induced by these mutations, their precise targets, and the effect at the transcript level have not been fully elucidated. Here we report transcriptomic analyses of 265 bone marrow samples from myelodysplasia patients, followed by a validation using CRISPR/Cas9-mediated gene editing and an assessment of nonsense-mediated decay susceptibility. Small but widespread reduction of intron-retaining isoforms is the most frequent splicing alteration in SF3B1-mutated samples. SF3B1 mutation is also associated with 3′ splice site alterations, leading to the most pronounced reduction of canonical transcripts. Target genes include tumor suppressors and genes of mitochondrial iron metabolism or heme biosynthesis. Alternative exon usage is predominant in SRSF2- and U2AF1-mutated samples. Usage of an EZH2 cryptic exon harboring a premature termination codon is increased in both SRSF2- and U2AF1-mutated samples. Our study reveals a landscape of splicing alterations and precise targets of various spliceosome mutations.

Mutations to the splicing machinery may have an important role in myelodysplasia. Here, the authors describe splicing factor gene mutations in myelodysplasia and report tumor suppressor, epigenetic, iron metabolism and heme biosynthesis genes as their targets.

Glycogen synthase kinase-3 and alternative splicing

Glycogen synthase kinase-3 (GSK-3) is a highly conserved negative regulator of receptor tyrosine kinase, cytokine, and Wnt signaling pathways. Stimulation of these pathways inhibits GSK-3 to modulate diverse downstream effectors that include transcription factors, nutrient sensors, glycogen synthesis, mitochondrial function, circadian rhythm, and cell fate. GSK-3 also regulates alternative splicing in response to T-cell receptor activation, and recent phosphoproteomic studies have revealed that multiple splicing factors and regulators of RNA biosynthesis are phosphorylated in a GSK-3-dependent manner. Furthermore, inhibition of GSK-3 alters the splicing of hundreds of mRNAs, indicating a broad role for GSK-3 in the regulation of RNA processing. GSK-3-regulated phosphoproteins include SF3B1, SRSF2, PSF, RBM8A, nucleophosmin 1 (NPM1), and PHF6, many of which are mutated in leukemia and myelodysplasia. As GSK-3 is inhibited by pathways that are pathologically activated in leukemia and loss of Gsk3 in hematopoietic cells causes a severe myelodysplastic neoplasm in mice, these findings strongly implicate GSK-3 as a critical regulator of mRNA processing in normal and malignant hematopoiesis. This article is categorized under: RNA Processing > Splicing Mechanisms RNA Processing > Splicing Regulation/Alternative Splicing RNA in Disease and Development > RNA in Disease RNA Interactions with Proteins and Other Molecules > Protein-RNA Interactions: Functional Implications.

Functional analysis of Hsh155/SF3b1 interactions with the U2 snRNA/branch site duplex

SF3b1 is an essential component of the U2 snRNP implicated in branch site (BS) recognition and found to be frequently mutated in several human cancers. While recent structures of yeast and human SF3b1 have revealed its molecular architecture, the importance of specific RNA:protein contacts and conformational changes remains largely uncharacterized. Here, we performed mutational analysis of yeast SF3b1, guided by recent structures of the spliceosome. We find that conserved amino acids contacting the U2 snRNA backbone of the U2/BS duplex are nonessential, and that yeast can tolerate truncation of the HEAT repeats containing these amino acids. The pocket housing the branchpoint adenosine (BP-A) is also amenable to mutation despite strong conservation. However, mutations that support viability can still lead to defects in splicing pre-mRNAs with nonconsensus BS substitutions found at -3, -2, -1, and +1 positions relative to the BP-A or at the branchpoint position. Through the generation of yeast and human chimeric proteins, we further defined the functionally conserved regions of Hsh155 as well as identify changes in BS usage resulting from inclusion of human SF3b1 HEAT repeats. Moreover, these chimeric proteins confer a sensitivity to small molecule inhibition by pladienolide B to yeast splicing. Together, these data reveal the importance of individual contacts of Hsh155/SF3b1 to the U2/BS duplex and define their contribution to BS usage by the spliceosome.

Post-transcriptional regulation of gene expression and human disease

A large number of mutations in genes that encode RNA binding proteins cause human disease. Many of these RNA binding proteins mediate key steps in post-transcriptional regulation of gene expression from mRNA processing to eventual decay in the cytoplasm. Surprisingly, these RNA binding proteins, which are ubiquitously expressed and play fundamental roles in gene expression, are often altered in tissue-specific disease. Mutations linked to disease impact nearly every post-transcriptional processing step and cause diverse disease phenotypes in a variety of specific tissues. This review summarizes steps in post-transcriptional regulation of gene expression that have been linked to disease providing specific examples of some of the many genes affected. Finally, recent advances that hold promise for treatment of some of these diseases are presented.

Cyclin-dependent kinase 1 (CDK1) and CDK2 have opposing roles in regulating interactions of splicing factor 3B1 with chromatin

Splicing factor 3B1 (SF3B1) is a core splicing protein that stabilizes the interaction between the U2 snRNA and the branch point in the mRNA target during splicing. SF3B1 is heavily phosphorylated at its N terminus and a substrate of cyclin-dependent kinases (CDKs). Although SF3B1 phosphorylation coincides with splicing catalysis, the functional significance of SF3B1 phosphorylation is largely undefined. Here, we show that SF3B1 phosphorylation follows a dynamic pattern during cell cycle progression that depends on CDK activity. SF3B1 is known to interact with chromatin, and we found that SF3B1 maximally interacts with nucleosomes during G₁/S and that this interaction requires CDK2 activity. In contrast, SF3B1 disassociates from nucleosomes at G₂/M, coinciding with a peak in CDK1-mediated SF3B1 phosphorylation. Thus, CDK1 and CDK2 appear to have opposing roles in regulating SF3B1-nucleosome interactions. Importantly, these interactions were modified by the presence and phosphorylation status of linker histone H1, particularly the H1.4 isoform. Performing genome-wide analysis of SF3B1-chromatin binding in synchronized cells, we observed that SF3B1 preferentially bound exons. Differences in SF3B1 chromatin binding to specific sites, however, did not correlate with changes in RNA splicing, suggesting that the SF3B1-nucleosome interaction does not determine cell cycle-dependent changes to mRNA splicing. Our results define a cell cycle stage-specific interaction between SF3B1 and nucleosomes that is mediated by histone H1 and depends on SF3B1 phosphorylation. Importantly, this interaction does not seem to be related to SF3B1's splicing function and, rather, points toward its potential role as a chromatin modifier.

Stoichiometries of U2AF35, U2AF65 and U2 snRNP reveal new early spliceosome assembly pathways

The selection of 3΄ splice sites (3΄ss) is an essential early step in mammalian RNA splicing reactions, but the processes involved are unknown. We have used single molecule methods to test whether the major components implicated in selection, the proteins U2AF35 and U2AF65 and the U2 snRNP, are able to recognize alternative candidate sites or are restricted to one pre-specified site. In the presence of adenosine triphosphate (ATP), all three components bind in a 1:1 stoichiometry with a 3΄ss. Pre-mRNA molecules with two alternative 3΄ss can be bound concurrently by two molecules of U2AF or two U2 snRNPs, so none of the components are restricted. However, concurrent occupancy inhibits splicing. Stoichiometric binding requires conditions consistent with coalescence of the 5΄ and 3΄ sites in a complex (I, initial), but if this cannot form the components show unrestricted and stochastic association. In the absence of ATP, when complex E forms, U2 snRNP association is unrestricted. However, if protein dephosphorylation is prevented, an I-like complex forms with stoichiometric association of U2 snRNPs and the U2 snRNA is base-paired to the pre-mRNA. Complex I differs from complex A in that the formation of complex A is associated with the loss of U2AF65 and 35.

Mutations of RNA splicing factors in hematological malignancies

Systematic large-scale cancer genomic studies have produced numerous significant findings. These studies have not only revealed new cancer-promoting genes, but they also have identified cancer-promoting functions of previously known "housekeeping" genes. These studies have identified numerous mutations in genes which play a fundamental role in nuclear precursor mRNA splicing. Somatic mutations and copy number variation in many of the splicing factors which participate in the formation of multiple spliceosomal complexes appear to play a role in many cancers and in particular in myelodysplastic syndromes (MDS). Mutated proteins seem to interfere with the recognition of the authentic splice sites (SS) leading to utilization of suboptimal alternative splicing sites generating aberrantly spliced mRNA isoforms. This short review is focusing on the function of the splice factors involved in the formation of splicing complexes and potential mechanisms which affect usage of the authentic splice site recognition.

DDX54 regulates transcriptome dynamics during DNA damage response

The cellular response to genotoxic stress is mediated by a well-characterized network of DNA surveillance pathways. The contribution of post-transcriptional gene regulatory networks to the DNA damage response (DDR) has not been extensively studied. Here, we systematically identified RNA-binding proteins differentially interacting with polyadenylated transcripts upon exposure of human breast carcinoma cells to ionizing radiation (IR). Interestingly, more than 260 proteins, including many nucleolar proteins, showed increased binding to poly(A)+ RNA in IR-exposed cells. The functional analysis of DDX54, a candidate genotoxic stress responsive RNA helicase, revealed that this protein is an immediate-to-early DDR regulator required for the splicing efficacy of its target IR-induced pre-mRNAs. Upon IR exposure, DDX54 acts by increased interaction with a well-defined class of pre-mRNAs that harbor introns with weak acceptor splice sites, as well as by protein-protein contacts within components of U2 snRNP and spliceosomal B complex, resulting in lower intron retention and higher processing rates of its target transcripts. Because DDX54 promotes survival after exposure to IR, its expression and/or mutation rate may impact DDR-related pathologies. Our work indicates the relevance of many uncharacterized RBPs potentially involved in the DDR.

SF1 Phosphorylation Enhances Specific Binding to U2AF65 and Reduces Binding to 3'-Splice-Site RNA

Splicing factor 1 (SF1) recognizes 3' splice sites of the major class of introns as a ternary complex with U2AF⁶⁵ and U2AF³⁵ splicing factors. A conserved SPSP motif in a coiled-coil domain of SF1 is highly phosphorylated in proliferating human cells and is required for cell proliferation. The UHM kinase 1 (UHMK1), also called KIS, double-phosphorylates both serines of this SF1 motif. Here, we use isothermal titration calorimetry to demonstrate that UHMK1 phosphorylation of the SF1 SPSP motif slightly enhances specific binding of phospho-SF1 to its cognate U2AF⁶⁵ protein partner. Conversely, quantitative fluorescence anisotropy RNA binding assays and isothermal titration calorimetry experiments establish that double-SPSP phosphorylation reduces phospho-SF1 and phospho-SF1-U2AF⁶⁵ binding affinities for either optimal or suboptimal splice-site RNAs. Domain-substitution and mutagenesis experiments further demonstrate that arginines surrounding the phosphorylated SF1 loop are required for cooperative 3' splice site recognition by the SF1-U2AF⁶⁵ complex (where cooperativity is defined as a nonadditive increase in RNA binding by the protein complex relative to the individual proteins). In the context of local, intracellular concentrations, the subtle effects of SF1 phosphorylation on its associations with U2AF⁶⁵ and splice-site RNAs are likely to influence pre-mRNA splicing. However, considering roles for SF1 in pre-mRNA retention and transcriptional repression, as well as in splicing, future comprehensive investigations are needed to fully explain the requirement for SF1 SPSP phosphorylation in proliferating human cells.

Pharmacology of Modulators of Alternative Splicing

More than 95% of genes in the human genome are alternatively spliced to form multiple transcripts, often encoding proteins with differing or opposing function. The control of alternative splicing is now being elucidated, and with this comes the opportunity to develop modulators of alternative splicing that can control cellular function. A number of approaches have been taken to develop compounds that can experimentally, and sometimes clinically, affect splicing control, resulting in potential novel therapeutics. Here we develop the concepts that targeting alternative splicing can result in relatively specific pathway inhibitors/activators that result in dampening down of physiologic or pathologic processes, from changes in muscle physiology to altering angiogenesis or pain. The targets and pharmacology of some of the current inhibitors/activators of alternative splicing are demonstrated and future directions discussed.

Splicing Factor Mutations in Myelodysplasias: Insights from Spliceosome Structures

Somatic mutations of pre-mRNA splicing factors recur among patients with myelodysplastic syndrome (MDS) and related malignancies. Although these MDS-relevant mutations alter the splicing of a subset of transcripts, the mechanisms by which these single amino acid substitutions change gene expression remain controversial. New structures of spliceosome intermediates and associated protein complexes shed light on the molecular interactions mediated by 'hotspots' of the SF3B1 and U2AF1 pre-mRNA splicing factors. The frequently mutated SF3B1 residues contact the pre-mRNA splice site. Based on structural homology with other spliceosome subunits, and recent findings of altered RNA binding by mutant U2AF1 proteins, we suggest that affected U2AF1 residues also contact pre-mRNA. Altered pre-mRNA recognition emerges as a molecular theme among MDS-relevant mutations of pre-mRNA splicing factors.

SF3B1 is a stress-sensitive splicing factor that regulates both HSF1 concentration and activity

The heat shock response (HSR) is a well-conserved, cytoprotective stress response that activates the HSF1 transcription factor. During severe stress, cells inhibit mRNA splicing which also serves a cytoprotective function via inhibition of gene expression. Despite their functional interconnectedness, there have not been any previous reports of crosstalk between these two pathways. In a genetic screen, we identified SF3B1, a core component of the U2 snRNP subunit of the spliceosome, as a regulator of the heat shock response in Caenorhabditis elegans. Here, we show that this regulatory connection is conserved in cultured human cells and that there are at least two distinct pathways by which SF3B1 can regulate the HSR. First, inhibition of SF3B1 with moderate levels of Pladienolide B, a previously established small molecule inhibitor of SF3B1, affects the transcriptional activation of HSF1, the transcription factor that mediates the HSR. However, both higher levels of Pladienolide B and SF3B1 siRNA knockdown also change the concentration of HSF1, a form of HSR regulation that has not been previously documented during normal physiology but is observed in some forms of cancer. Intriguingly, mutations in SF3B1 have also been associated with several distinct types of cancer. Finally, we show that regulation of alternative splicing by SF3B1 is sensitive to temperature, providing a new mechanism by which temperature stress can remodel the transcriptome.

Identification of a small molecule inhibitor that stalls splicing at an early step of spliceosome activation

Small molecule inhibitors of pre-mRNA splicing are important tools for identifying new spliceosome assembly intermediates, allowing a finer dissection of spliceosome dynamics and function. Here, we identified a small molecule that inhibits human pre-mRNA splicing at an intermediate stage during conversion of pre-catalytic spliceosomal B complexes into activated B^act complexes. Characterization of the stalled complexes (designated B⁰²⁸) revealed that U4/U6 snRNP proteins are released during activation before the U6 Lsm and B-specific proteins, and before recruitment and/or stable incorporation of Prp19/CDC5L complex and other B^act complex proteins. The U2/U6 RNA network in B⁰²⁸ complexes differs from that of the B^act complex, consistent with the idea that the catalytic RNA core forms stepwise during the B to B^act transition and is likely stabilized by the Prp19/CDC5L complex and related proteins. Taken together, our data provide new insights into the RNP rearrangements and extensive exchange of proteins that occurs during spliceosome activation.

DOI:http://dx.doi.org/10.7554/eLife.23533.001

SF3B1/Hsh155 HEAT motif mutations affect interaction with the spliceosomal ATPase Prp5, resulting in altered branch site selectivity in pre-mRNA splicing

Mutations in the U2 snRNP component SF3B1 are prominent in myelodysplastic syndromes (MDSs) and other cancers and have been shown recently to alter branch site (BS) or 3' splice site selection in splicing. However, the molecular mechanism of altered splicing is not known. We show here that hsh155 mutant alleles in Saccharomyces cerevisiae, counterparts of SF3B1 mutations frequently found in cancers, specifically change splicing of suboptimal BS pre-mRNA substrates. We found that Hsh155p interacts directly with Prp5p, the first ATPase that acts during spliceosome assembly, and localized the interacting regions to HEAT (Huntingtin, EF3, PP2A, and TOR1) motifs in SF3B1 associated with disease mutations. Furthermore, we show that mutations in these motifs from both human disease and yeast genetic screens alter the physical interaction with Prp5p, alter branch region specification, and phenocopy mutations in Prp5p. These and other data demonstrate that mutations in Hsh155p and Prp5p alter splicing because they change the direct physical interaction between Hsh155p and Prp5p. This altered physical interaction results in altered loading (i.e., "fidelity") of the BS-U2 duplex into the SF3B complex during prespliceosome formation. These results provide a mechanistic framework to explain the consequences of intron recognition and splicing of SF3B1 mutations found in disease.

Unmasking the U2AF homology motif family: a bona fide protein-protein interaction motif in disguise

U2AF homology motifs (UHM) that recognize U2AF ligand motifs (ULM) are an emerging family of protein-protein interaction modules. UHM-ULM interactions recur in pre-mRNA splicing factors including U2AF1 and SF3b1, which are frequently mutated in myelodysplastic syndromes. The core topology of the UHM resembles an RNA recognition motif and is often mistakenly classified within this large family. Here, we unmask the charade and review recent discoveries of UHM-ULM modules for protein-protein interactions. Diverse polypeptide extensions and selective phosphorylation of UHM and ULM family members offer new molecular mechanisms for the assembly of specific partners in the early-stage spliceosome.

Cryptic splicing events in the iron transporter ABCB7 and other key target genes in SF3B1-mutant myelodysplastic syndromes

The splicing factor SF3B1 is the most frequently mutated gene in myelodysplastic syndromes (MDS), and is strongly associated with the presence of ring sideroblasts (RS). We have performed a systematic analysis of cryptic splicing abnormalities from RNA sequencing data on hematopoietic stem cells (HSCs) of SF3B1-mutant MDS cases with RS. Aberrant splicing events in many downstream target genes were identified and cryptic 3' splice site usage was a frequent event in SF3B1-mutant MDS. The iron transporter ABCB7 is a well-recognized candidate gene showing marked downregulation in MDS with RS. Our analysis unveiled aberrant ABCB7 splicing, due to usage of an alternative 3' splice site in MDS patient samples, giving rise to a premature termination codon in the ABCB7 mRNA. Treatment of cultured SF3B1-mutant MDS erythroblasts and a CRISPR/Cas9-generated SF3B1-mutant cell line with the nonsense-mediated decay (NMD) inhibitor cycloheximide showed that the aberrantly spliced ABCB7 transcript is targeted by NMD. We describe cryptic splicing events in the HSCs of SF3B1-mutant MDS, and our data support a model in which NMD-induced downregulation of the iron exporter ABCB7 mRNA transcript resulting from aberrant splicing caused by mutant SF3B1 underlies the increased mitochondrial iron accumulation found in MDS patients with RS.

Transcriptomic Characterization of SF3B1 Mutation Reveals Its Pleiotropic Effects in Chronic Lymphocytic Leukemia

Mutations in SF3B1, which encodes a spliceosome component, are associated with poor outcome in chronic lymphocytic leukemia (CLL), but how these contribute to CLL progression remains poorly understood. We undertook a transcriptomic characterization of primary human CLL cells to identify transcripts and pathways affected by SF3B1 mutation. Splicing alterations, identified in the analysis of bulk cells, were confirmed in single SF3B1-mutated CLL cells and also found in cell lines ectopically expressing mutant SF3B1. SF3B1 mutation was found to dysregulate multiple cellular functions including DNA damage response, telomere maintenance, and Notch signaling (mediated through KLF8 upregulation, increased TERC and TERT expression, or altered splicing of DVL2 transcript, respectively). SF3B1 mutation leads to diverse changes in CLL-related pathways.

Molecular Architecture of SF3b and Structural Consequences of Its Cancer-Related Mutations

SF3b is a heptameric protein complex of the U2 small nuclear ribonucleoprotein (snRNP) that is essential for pre-mRNA splicing. Mutations in the largest SF3b subunit, SF3B1/SF3b155, are linked to cancer and lead to alternative branch site (BS) selection. Here we report the crystal structure of a human SF3b core complex, revealing how the distinctive conformation of SF3b155's HEAT domain is maintained by multiple contacts with SF3b130, SF3b10, and SF3b14b. Protein-protein crosslinking enabled the localization of the BS-binding proteins p14 and U2AF65 within SF3b155's HEAT-repeat superhelix, which together with SF3b14b forms a composite RNA-binding platform. SF3b155 residues, the mutation of which leads to cancer, contribute to the tertiary structure of the HEAT superhelix and its surface properties in the proximity of p14 and U2AF65. The molecular architecture of SF3b reveals the spatial organization of cancer-related SF3b155 mutations and advances our understanding of their effects on SF3b structure and function.

Myelodysplasia-associated mutations in serine/arginine-rich splicing factor SRSF2 lead to alternative splicing of CDC25C

BACKGROUND

Serine-arginine rich splicing factor 2 (SRSF2) is a protein known for its role in RNA splicing and genome stability. It has been recently discovered that SRSF2, along with other splicing regulators, is frequently mutated in patients with myelodysplastic syndrome (MDS). The most common MDS mutations in SRSF2 occur at proline 95; the mutant proteins are shown to have different RNA binding preferences, which may contribute to splicing changes detected in mutant cells. However, the influence of these SRSF2 MDS-associated mutations on specific splicing events remains poorly understood.

RESULTS

A tetracycline-inducible TF-1 erythroleukemia cell line was transduced with retroviruses to create cell lines expressing HA-tagged wildtype SRSF2, SRSF2 with proline 95 point mutations found in MDS, or SRSF2 with a deletion of one of the four major domains of the protein. Effects of these mutants on apoptosis and specific alternative splicing events were evaluated. Cells were also treated with DNA damaging drugs for comparison. MDS-related P95 point mutants of SRSF2 were expressed and phosphorylated at similar levels as wildtype SRSF2. However, cells expressing mutant SRSF2 exhibited higher levels of apoptosis than cells expressing wildtype SRSF2. Regarding alternative splicing events, in nearly all examined cases, SRSF2 P95 mutants acted in a similar fashion as the wildtype SRSF2. However, cells expressing SRSF2 P95 mutants had a percent increase in the C5 spliced isoform of cell division cycle 25C (CDC25C). The same alternative splicing of CDC25C was detected by treating cells with DNA damaging drugs, such as cisplatin, camptothecin, and trichostatin A at appropriate dosage. However, unlike DNA damaging drugs, SRSF2 P95 mutants did not activate the Ataxia telangiectasia mutated (ATM) pathway.

CONCLUSION

SRSF2 P95 mutants lead to alternative splicing of CDC25C in a manner that is not dependent on the DNA damage response.

Cancer-associated SF3B1 mutants recognize otherwise inaccessible cryptic 3' splice sites within RNA secondary structures

Recurrent mutations in core splicing factors have been reported in several clonal disorders, including cancers. Mutations in SF3B1, a component of the U2 splicing complex, are the most common. SF3B1 mutations are associated with aberrant pre-mRNA splicing using cryptic 3’ splice sites (3’SS) but the mechanism of their selection is not clear. To understand how cryptic 3’SS are selected, we performed comprehensive analysis of transcriptome-wide changes to splicing and gene expression associated with SF3B1 mutations in patient samples as well as an experimental model of inducible expression. Hundreds of cryptic 3’SS were detectable across the genome in cells expressing mutant SF3B1. These 3’SS are typically sequestered within RNA secondary structures and poorly accessible compared to their corresponding canonical 3’SS. We hypothesized that these cryptic 3’SS are inaccessible during normal splicing catalysis and that this constraint is overcome in spliceosomes containing mutant SF3B1. This model of secondary structure-dependent selection of cryptic 3’SS was found across multiple clonal processes associated with SF3B1 mutations (myelodysplastic syndrome and chronic lymphocytic leukemia). We validated our model predictions in mini-gene splicing assays. Additionally, we found deregulated expression of proteins with relevant functions in splicing factor-related diseases both in association with aberrant splicing and without corresponding splicing changes. Our results show that SF3B1 mutations are associated with a distinct splicing program shared across multiple clonal processes and define a biochemical mechanism for altered 3’SS choice.

Structural and mechanistic insights into human splicing factor SF3b complex derived using an integrated approach guided by the cryo-EM density maps

Pre-mRNA splicing in eukaryotes is performed by the spliceosome, a highly complex macromolecular machine. SF3b is a multi-protein complex which recognizes the branch point adenosine of pre-mRNA as part of a larger U2 snRNP or U11/U12 di-snRNP in the dynamic spliceosome machinery. Although a cryo-EM map is available for human SF3b complex, the structure and relative spatial arrangement of all components in the complex are not yet known. We have recognized folds of domains in various proteins in the assembly and generated comparative models. Using an integrative approach involving structural and other experimental data, guided by the available cryo-EM density map, we deciphered a pseudo-atomic model of the closed form of SF3b which is found to be a "fuzzy complex" with highly flexible components and multiplicity of folds. Further, the model provides structural information for 5 proteins (SF3b10, SF3b155, SF3b145, SF3b130 and SF3b14b) and localization information for 4 proteins (SF3b10, SF3b145, SF3b130 and SF3b14b) in the assembly for the first time. Integration of this model with the available U11/U12 di-snRNP cryo-EM map enabled elucidation of an open form. This now provides new insights on the mechanistic features involved in the transition between closed and open forms pivoted by a hinge region in the SF3b155 protein that also harbors cancer causing mutations. Moreover, the open form guided model of the 5' end of U12 snRNA, which includes the branch point duplex, shows that the architecture of SF3b acts as a scaffold for U12 snRNA: pre-mRNA branch point duplex formation with potential implications for branch point adenosine recognition fidelity.

How Are Short Exons Flanked by Long Introns Defined and Committed to Splicing?

The splice sites (SSs) delimiting an intron are brought together in the earliest step of spliceosome assembly yet it remains obscure how SS pairing occurs, especially when introns are thousands of nucleotides long. Splicing occurs in vivo in mammals within minutes regardless of intron length, implying that SS pairing can instantly follow transcription. Also, factors required for SS pairing, such as the U1 small nuclear ribonucleoprotein (snRNP) and U2AF65, associate with RNA polymerase II (RNAPII), while nucleosomes preferentially bind exonic sequences and associate with U2 snRNP. Based on recent publications, we assume that the 5' SS-bound U1 snRNP can remain tethered to RNAPII until complete synthesis of the downstream intron and exon. An additional U1 snRNP then binds the downstream 5' SS, whereas the RNAPII-associated U2AF65 binds the upstream 3' SS to facilitate SS pairing along with exon definition. Next, the nucleosome-associated U2 snRNP binds the branch site to advance splicing complex assembly. This may explain how RNAPII and chromatin are involved in spliceosome assembly and how introns lengthened during evolution with a relatively minimal compromise in splicing.

Modulating splicing with small molecular inhibitors of the spliceosome

Small molecule inhibitors that target components of the spliceosome have great potential as tools to probe splicing mechanism and dissect splicing regulatory networks in cells. These compounds also hold promise as drug leads for diseases in which splicing regulation plays a critical role, including many cancers. Because the spliceosome is a complicated and dynamic macromolecular machine comprised of many RNA and protein components, a variety of compounds that interfere with different aspects of spliceosome assembly is needed to probe its function. By screening chemical libraries with high-throughput splicing assays, several labs have added to the collection of splicing inhibitors, although the mechanistic insight into splicing yielded from the initial compound hits is somewhat limited so far. In contrast, SF3B1 inhibitors stand out as a great example of what can be accomplished with small molecule tools. This group of compounds were first discovered as natural products that are cytotoxic to cancer cells, and then later shown to target the core spliceosome protein SF3B1. The inhibitors have since been used to uncover details of SF3B1 mechanism in the spliceosome and its impact on gene expression in cells. Continuing structure activity relationship analysis of the compounds is also making progress in identifying chemical features key to their function, which is critical in understanding the mechanism of SF3B1 inhibition. The knowledge is also important for the design of analogs with new and useful features for both splicing researchers and clinicians hoping to exploit splicing as pressure point to target in cancer therapy. WIREs RNA 2017, 8:e1381. doi: 10.1002/wrna.1381 For further resources related to this article, please visit the WIREs website.

The pathogenicity of splicing defects: mechanistic insights into pre-mRNA processing inform novel therapeutic approaches

Removal of introns from pre-mRNA precursors (pre-mRNA splicing) is a necessary step for the expression of most genes in multicellular organisms, and alternative patterns of intron removal diversify and regulate the output of genomic information. Mutation or natural variation in pre-mRNA sequences, as well as in spliceosomal components and regulatory factors, has been implicated in the etiology and progression of numerous pathologies. These range from monogenic to multifactorial genetic diseases, including metabolic syndromes, muscular dystrophies, neurodegenerative and cardiovascular diseases, and cancer. Understanding the molecular mechanisms associated with splicing-related pathologies can provide key insights into the normal function and physiological context of the complex splicing machinery and establish sound basis for novel therapeutic approaches.