Oncogenic dysregulation of pre-mRNA processing by protein kinases: challenges and therapeutic opportunities

SR proteins in cancer: function, regulation, and small inhibitor

Alternative splicing of pre-mRNAs is a fundamental step in RNA processing required for gene expression in most metazoans. Serine and arginine-rich proteins (SR proteins) comprise a family of multifunctional proteins that contain an RNA recognition motif (RRM) and the ultra-conserved arginine/serine-rich (RS) domain, and play an important role in precise alternative splicing. Increasing research supports SR proteins as also functioning in other RNA-processing-related mechanisms, such as polyadenylation, degradation, and translation. In addition, SR proteins interact with N6-methyladenosine (m6A) regulators to modulate the methylation of ncRNA and mRNA. Dysregulation of SR proteins causes the disruption of cell differentiation and contributes to cancer progression. Here, we review the distinct biological characteristics of SR proteins and their known functional mechanisms during carcinogenesis. We also summarize the current inhibitors that directly target SR proteins and could ultimately turn SR proteins into actionable therapeutic targets in cancer therapy.

Kinase activities in pancreatic ductal adenocarcinoma with prognostic and therapeutic avenues

Pancreatic ductal adenocarcinoma (PDAC) is a devastating disease with a limited number of known driver mutations but considerable cancer cell heterogeneity. Phosphoproteomics provides a direct read-out of aberrant signaling and the resultant clinically relevant phenotype. Mass spectrometry (MS)-based proteomics and phosphoproteomics were applied to 42 PDAC tumors. Data encompassed over 19 936 phosphoserine or phosphothreonine (pS/T; in 5412 phosphoproteins) and 1208 phosphotyrosine (pY; in 501 phosphoproteins) sites and a total of 3756 proteins. Proteome data identified three distinct subtypes with tumor intrinsic and stromal features. Subsequently, three phospho-subtypes were apparent: two tumor intrinsic (Phos1/2) and one stromal (Phos3), resembling known PDAC molecular subtypes. Kinase activity was analyzed by the Integrative iNferred Kinase Activity (INKA) scoring. Phospho-subtypes displayed differential phosphorylation signals and kinase activity, such as FGR and GSK3 activation in Phos1, SRC kinase family and EPHA2 in Phos2, and EGFR, INSR, MET, ABL1, HIPK1, JAK, and PRKCD in Phos3. Kinase activity analysis of an external PDAC cohort supported our findings and underscored the importance of PI3K/AKT and ERK pathways, among others. Interestingly, unfavorable patient prognosis correlated with higher RTK, PAK2, STK10, and CDK7 activity and high proliferation, whereas long survival was associated with MYLK and PTK6 activity, which was previously unknown. Subtype-associated activity profiles can guide therapeutic combination approaches in tumor and stroma-enriched tissues, and emphasize the critical role of parallel signaling pathways. In addition, kinase activity profiling identifies potential disease markers with prognostic significance.

[Targeting the spliceosome: A new therapeutic strategy to counteract chemotherapy resistance in lung cancer?]

Lung cancer is the first cancer-related cause of death worldwide. This is in partially due to therapeutic resistance, which occurs in around 70% of patients, especially those receiving platinum salts, the gold-standard chemotherapy. The massive deregulation of alternative transcript splicing processes observed in many cancers has led to the development of a new class of pharmacological agents aimed at inhibiting the activity of the splicing machinery (spliceosome). The molecular mechanisms by which these inhibitors act remain largely unknown, as do the benefits of using them in combination with other therapies. In this context, our work is focused on an inhibitor of the SRPK1 kinase, a major regulator of the spliceosome.

Splicing targeting drugs highlight intron retention as an actionable vulnerability in advanced prostate cancer

BACKGROUND

Advanced prostate cancer (PC) is characterized by insensitivity to androgen deprivation therapy and chemotherapy, resulting in poor outcome for most patients. Thus, advanced PC urgently needs novel therapeutic strategies. Mounting evidence points to splicing dysregulation as a hallmark of advanced PC. Moreover, pharmacologic inhibition of the splicing process is emerging as a promising option for this disease.

METHOD

By using a representative androgen-insensitive PC cell line (22Rv1), we have investigated the genome-wide transcriptomic effects underlying the cytotoxic effects exerted by three splicing-targeting drugs: Pladienolide B, indisulam and THZ531. Bioinformatic analyses were performed to uncover the gene structural features underlying sensitivity to transcriptional and splicing regulation by these treatments. Biological pathways altered by these treatments were annotated by gene ontology analyses and validated by functional experiments in cell models.

RESULTS

Although eliciting similar cytotoxic effects on advanced PC cells, Pladienolide B, indisulam and THZ531 modulate specific transcriptional and splicing signatures. Drug sensitivity is associated with distinct gene structural features, expression levels and cis-acting sequence elements in the regulated exons and introns. Importantly, we identified PC-relevant genes (i.e. EZH2, MDM4) whose drug-induced splicing alteration exerts an impact on cell survival. Moreover, computational analyses uncovered a widespread impact of splicing-targeting drugs on intron retention, with enrichment in genes implicated in pre-mRNA 3'-end processing (i.e. CSTF3, PCF11). Coherently, advanced PC cells displayed high sensitivity to a specific inhibitor of the cleavage and polyadenylation complex, which enhances the effects of chemotherapeutic drugs that are already in use for this cancer.

CONCLUSIONS

Our study uncovers intron retention as an actionable vulnerability for advanced PC, which may be exploited to improve therapeutic management of this currently incurable disease.

SRPKIN-1 as an inhibitor against hepatitis B virus blocking the viral particle formation and the early step of the viral infection

New antiviral agents are needed for the treatment of hepatitis B virus (HBV) infection because currently available drugs do not completely eradicate chronic HBV in patients. Phosphorylation dynamics of the HBV core protein (HBc) regulate several processes in the HBV life cycle, including nucleocapsid formation, cell trafficking, and virus uncoating after entry. In this study, the SRPK inhibitors SPHINX31, SRPIN340, and SRPKIN-1 showed concentration-dependent anti-HBV activity. Detailed analysis of the effects of SRPKIN-1, which exhibited the strongest inhibitory activity, on the HBV replication process showed that it inhibits the formation of infectious particles by inhibiting pregenomic RNA packaging into capsids and nucleocapsid envelopment. Mass spectrometry analysis combined with cell-free translation system experiments revealed that hyperphosphorylation of the C-terminal domain of HBc is inhibited by SRPKIN-1. Further, SRPKIN-1 exhibited concentration-dependent inhibition of HBV infection not only in HepG2-hNTCP-C4 cells but also in fresh human hepatocytes (PXB cells) and in the single-round infection system. Treatment with SRPKIN-1 at the time of infection reduced the nuclease sensitivity of HBV DNA in the nuclear fraction. These results suggest that SRPKIN-1 has the potential to not only inhibit the HBV particle formation process but also impair the early stages of viral infection.

Recent Advances in Drug Discovery for Triple-Negative Breast Cancer Treatment

Triple-negative breast cancer (TNBC) is one of the most heterogeneous and aggressive breast cancer subtypes with a high risk of death on recurrence. To date, TNBC is very difficult to treat due to the lack of an effective targeted therapy. However, recent advances in the molecular characterization of TNBC are encouraging the development of novel drugs and therapeutic combinations for its therapeutic management. In the present review, we will provide an overview of the currently available standard therapies and new emerging therapeutic strategies against TNBC, highlighting the promises that newly developed small molecules, repositioned drugs, and combination therapies have of improving treatment efficacy against these tumors.

Functions of SRPK, CLK and DYRK kinases in stem cells, development, and human developmental disorders

Human developmental disorders encompass a wide range of debilitating physical conditions and intellectual disabilities. Perturbation of protein kinase signalling underlies the development of some of these disorders. For example, disrupted SRPK signalling is associated with intellectual disabilities, and the gene dosage of DYRKs can dictate the pathology of disorders including Down's syndrome. Here, we review the emerging roles of the CMGC kinase families SRPK, CLK, DYRK, and sub-family HIPK during embryonic development and in developmental disorders. In particular, SRPK, CLK, and DYRK kinase families have key roles in developmental signalling and stem cell regulation, and can co-ordinate neuronal development and function. Genetic studies in model organisms reveal critical phenotypes including embryonic lethality, sterility, musculoskeletal errors, and most notably, altered neurological behaviours arising from defects of the neuroectoderm and altered neuronal signalling. Further unpicking the mechanisms of specific kinases using human stem cell models of neuronal differentiation and function will improve our understanding of human developmental disorders and may provide avenues for therapeutic strategies.

Targeting serine- and arginine-rich splicing factors to rectify aberrant alternative splicing

Serine- and arginine-rich splicing factors are pivotal modulators of constitutive splicing and alternative splicing that bind to the cis-acting elements in precursor mRNAs and facilitate the recruitment and assembly of the spliceosome. Meanwhile, SR proteins shuttle between the nucleus and cytoplasm with a broad implication in multiple RNA-metabolizing events. Recent studies have demonstrated the positive correlation of overexpression and/or hyperactivation of SR proteins and development of the tumorous phenotype, indicating the therapeutic potentials of targeting SR proteins. In this review, we highlight key findings concerning the physiological and pathological roles of SR proteins. We have also investigated small molecules and oligonucleotides that effectively modulate the functions of SR proteins, which could benefit future studies of SR proteins.

MYC up-regulation confers vulnerability to dual inhibition of CDK12 and CDK13 in high-risk Group 3 medulloblastoma

BACKGROUND

Medulloblastoma (MB) is the most common cerebellar malignancy during childhood. Among MB, MYC-amplified Group 3 tumors display the worst prognosis. MYC is an oncogenic transcription factor currently thought to be undruggable. Nevertheless, targeting MYC-dependent processes (i.e. transcription and RNA processing regulation) represents a promising approach.

METHODS

We have tested the sensitivity of MYC-driven Group 3 MB cells to a pool of transcription and splicing inhibitors that display a wide spectrum of targets. Among them, we focus on THZ531, an inhibitor of the transcriptional cyclin-dependent kinases (CDK) 12 and 13. High-throughput RNA-sequencing analyses followed by bioinformatics and functional analyses were carried out to elucidate the molecular mechanism(s) underlying the susceptibility of Group 3 MB to CDK12/13 chemical inhibition. Data from International Cancer Genome Consortium (ICGC) and other public databases were mined to evaluate the functional relevance of the cellular pathway/s affected by the treatment with THZ531 in Group 3 MB patients.

RESULTS

We found that pharmacological inhibition of CDK12/13 is highly selective for MYC-high Group 3 MB cells with respect to MYC-low MB cells. We identified a subset of genes enriched in functional terms related to the DNA damage response (DDR) that are up-regulated in Group 3 MB and repressed by CDK12/13 inhibition. Accordingly, MYC- and CDK12/13-dependent higher expression of DDR genes in Group 3 MB cells limits the toxic effects of endogenous DNA lesions in these cells. More importantly, chemical inhibition of CDK12/13 impaired the DDR and induced irreparable DNA damage exclusively in MYC-high Group 3 MB cells. The augmented sensitivity of MYC-high MB cells to CDK12/13 inhibition relies on the higher elongation rate of the RNA polymerase II in DDR genes. Lastly, combined treatments with THZ531 and DNA damage-inducing agents synergically suppressed viability of MYC-high Group 3 MB cells.

CONCLUSIONS

Our study demonstrates that CDK12/13 activity represents an exploitable vulnerability in MYC-high Group 3 MB and may pave the ground for new therapeutic approaches for this high-risk brain tumor.

CDK12/13 promote splicing of proximal introns by enhancing the interaction between RNA polymerase II and the splicing factor SF3B1

Transcription-associated cyclin-dependent kinases (CDKs) regulate the transcription cycle through sequential phosphorylation of RNA polymerase II (RNAPII). Herein, we report that dual inhibition of the highly homologous CDK12 and CDK13 impairs splicing of a subset of promoter-proximal introns characterized by weak 3' splice sites located at larger distance from the branchpoint. Nascent transcript analysis indicated that these introns are selectively retained upon pharmacological inhibition of CDK12/13 with respect to downstream introns of the same pre-mRNAs. Retention of these introns was also triggered by pladienolide B (PdB), an inhibitor of the U2 small nucelar ribonucleoprotein (snRNP) factor SF3B1 that recognizes the branchpoint. CDK12/13 activity promotes the interaction of SF3B1 with RNAPII phosphorylated on Ser2, and disruption of this interaction by treatment with the CDK12/13 inhibitor THZ531 impairs the association of SF3B1 with chromatin and its recruitment to the 3' splice site of these introns. Furthermore, by using suboptimal doses of THZ531 and PdB, we describe a synergic effect of these inhibitors on intron retention, cell cycle progression and cancer cell survival. These findings uncover a mechanism by which CDK12/13 couple RNA transcription and processing, and suggest that combined inhibition of these kinases and the spliceosome represents an exploitable anticancer approach.

Targeting pre-mRNA splicing in cancers: roles, inhibitors, and therapeutic opportunities

Accumulating evidence has indicated that pre-mRNA splicing plays critical roles in a variety of physiological processes, including development of multiple diseases. In particular, alternative splicing is profoundly involved in cancer progression through abnormal expression or mutation of splicing factors. Small-molecule splicing modulators have recently attracted considerable attention as a novel class of cancer therapeutics, and several splicing modulators are currently being developed for the treatment of patients with various cancers and are in the clinical trial stage. Novel molecular mechanisms modulating alternative splicing have proven to be effective for treating cancer cells resistant to conventional anticancer drugs. Furthermore, molecular mechanism-based combination strategies and patient stratification strategies for cancer treatment targeting pre-mRNA splicing must be considered for cancer therapy in the future. This review summarizes recent progress in the relationship between druggable splicing-related molecules and cancer, highlights small-molecule splicing modulators, and discusses future perspectives of splicing modulation for personalized and combination therapies in cancer treatment.

Dual inhibition of CDK12 and CDK13 uncovers actionable vulnerabilities in patient-derived ovarian cancer organoids

BACKGROUND

High grade serous ovarian cancer (HGSOC) is highly lethal, partly due to chemotherapy resistance and limited availability of targeted approaches. Cyclin dependent kinases 12 and 13 (CDK12/13) are promising therapeutic targets in human cancers, including HGSOC. Nevertheless, the effects of their inhibition in HGSOC and the potential synergy with other drugs are poorly known.

METHODS

We analyzed the effects of the CDK12/13 inhibitor THZ531 in HGSOC cells and patient-derived organoids (PDOs). RNA sequencing and quantitative PCR analyses were performed to identify the genome-wide effects of short-term CDK12/13 inhibition on the transcriptome of HGSOC cells. Viability assays with HGSOC cells and PDOs were performed to assess the efficacy of THZ531 as single agent or in combination with clinically relevant drugs.

RESULTS

The CDK12 and CDK13 genes are deregulated in HGSOC and their concomitant up-regulation with the oncogene MYC predicts poor prognosis. HGSOC cells and PDOs display high sensitivity to CDK12/13 inhibition, which synergizes with drugs in clinical use for HGSOC. Transcriptome analyses revealed cancer-relevant genes whose expression is repressed by dual CDK12/13 inhibition through impaired splicing. Combined treatment with THZ531 and inhibitors of pathways regulated by these cancer relevant genes (EGFR, RPTOR, ATRIP) exerted synergic effects on HGSOC PDO viability.

CONCLUSIONS

CDK12 and CDK13 represent valuable therapeutic targets for HGSOC. We uncovered a wide spectrum of CDK12/13 targets as potential therapeutic vulnerabilities for HGSOC. Moreover, our study indicates that CDK12/13 inhibition enhances the efficacy of approved drugs that are already in use for HGSOC or other human cancers.

Cdc2-like kinases: structure, biological function, and therapeutic targets for diseases

The CLKs (Cdc2-like kinases) belong to the dual-specificity protein kinase family and play crucial roles in regulating transcript splicing via the phosphorylation of SR proteins (SRSF1-12), catalyzing spliceosome molecular machinery, and modulating the activities or expression of non-splicing proteins. The dysregulation of these processes is linked with various diseases, including neurodegenerative diseases, Duchenne muscular dystrophy, inflammatory diseases, viral replication, and cancer. Thus, CLKs have been considered as potential therapeutic targets, and significant efforts have been exerted to discover potent CLKs inhibitors. In particular, clinical trials aiming to assess the activities of the small molecules Lorecivivint on knee Osteoarthritis patients, and Cirtuvivint and Silmitasertib in different advanced tumors have been investigated for therapeutic usage. In this review, we comprehensively documented the structure and biological functions of CLKs in various human diseases and summarized the significance of related inhibitors in therapeutics. Our discussion highlights the most recent CLKs research, paving the way for the clinical treatment of various human diseases.

Targeting splicing factors for cancer therapy

Alternative splicing (AS) of mRNAs is an essential regulatory mechanism in eukaryotic gene expression. AS misregulation, caused by either dysregulation or mutation of splicing factors, has been shown to be involved in cancer development and progression, making splicing factors suitable targets for cancer therapy. In recent years, various types of pharmacological modulators, such as small molecules and oligonucleotides, targeting distinct components of the splicing machinery, have been under development to treat multiple disorders. Although these approaches have promise, targeting the core spliceosome components disrupts the early stages of spliceosome assembly and can lead to nonspecific and toxic effects. New research directions have been focused on targeting specific splicing factors for a more precise effect. In this Perspective, we will highlight several approaches for targeting splicing factors and their functions and suggest ways to improve their specificity.

The role of SR protein kinases in regulating lipid storage in the Drosophila fat body

The survival of animals during periods of limited nutrients is dependent on the organism's ability to store lipids during times of nutrient abundance. However, the increased availability of food in modern western society has led to an excess storage of lipids resulting in metabolic diseases. To better understand the genes involved in regulating lipid storage, genome-wide RNAi screens were performed in cultured Drosophila cells and one group of genes identified includes mRNA splicing factor genes. Our lab has previously shown that a group of splicing factors important for intron/exon border recognition known as SR proteins are involved in controlling lipid storage in Drosophila; however, how these SR proteins are regulated to control lipid storage is not fully understood. Here, we focus on two SR protein kinases (SRPKs) in Drosophila: SRPK and SRPK79D. Decreasing the expression of these genes specifically in the adult fat body using RNAi resulted in lower levels of triglycerides and this is due to a decrease in the amount of fat stored per cell, despite having more fat cells, when compared to control flies. Decreasing SRPK and SRPK79D levels in the fat body leads to altered splicing of the β-oxidation gene, carnitine palmitoyltransferase 1 (CPT1), resulting in increased production of a more active enzyme, which would increase lipid breakdown and be consistent with the lean phenotype observed in these flies. In addition, flies with decreased SRPK and SRPK79D levels in their fat bodies eat less, which may also contribute to the decreased triglyceride phenotype. Together, these findings provide evidence to support that lipid storage is controlled by the phosphorylation of factors involved in mRNA splicing.

MSC-1186, a Highly Selective Pan-SRPK Inhibitor Based on an Exceptionally Decorated Benzimidazole-Pyrimidine Core

The highly conserved catalytic sites in protein kinases make it difficult to identify ATP competitive inhibitors with kinome-wide selectivity. Serendipitously, during a dedicated fragment campaign for the focal adhesion kinase (FAK), a scaffold that had lost its initial FAK affinity showed remarkable potency and selectivity for serine-arginine-protein kinases 1-3 (SRPK1-3). Non-conserved interactions with the uniquely structured hinge region of the SRPK family were the key drivers of the exclusive selectivity of the discovered fragment hit. Structure-guided medicinal chemistry efforts led to the SRPK inhibitor MSC-1186, which fulfills all hallmarks of a reversible chemical probe, including nanomolar cellular potency and excellent kinome-wide selectivity. The combination of MSC-1186 with CDC2-like kinase (CLK) inhibitors showed additive attenuation of SR-protein phosphorylation compared to the single agents. MSC-1186 and negative control (MSC-5360) are chemical probes available via the Structural Genomics Consortium chemical probe program (https://www.sgc-ffm.uni-frankfurt.de/).

HiFENS: high-throughput FISH detection of endogenous pre-mRNA splicing isoforms

Splicing factors play an essential role in regulation of alternative pre-mRNA splicing. While much progress has been made in delineating the mechanisms of the splicing machinery, the identity of signal transduction pathways and upstream factors that regulate splicing factor activity is largely unknown. A major challenge in the discovery of upstream regulatory factors of pre-mRNA splicing is the scarcity of functional genomics screening methods to monitor splicing outcomes of endogenous genes. Here, we have developed HiFENS (high throughput FISH detection of endogenous splicing isoforms), a high-throughput imaging assay based on hybridization chain reaction (HCR) and used HiFENS to screen for cellular factors that regulate alternative splicing of endogenous genes. We demonstrate optimized detection with high specificity of endogenous splicing isoforms and multiplexing of probes for accurate detection of splicing outcomes with single cell resolution. As proof-of-principle, we perform an RNAi screen of 702 human kinases and identify potential candidate upstream splicing regulators of the FGFR2 gene. HiFENS should be a useful tool for the unbiased delineation of cellular pathways involved in alternative splicing regulation.

Targeting the mRNA endonuclease CPSF73 inhibits breast cancer cell migration, invasion, and self-renewal

Cleavage by the endonuclease CPSF73 and polyadenylation of nascent RNA is an essential step in co-transcriptional mRNA maturation. Recent work has surprisingly identified CPSF73 as a promising drug target for inhibiting the growth of specific cancers, triggering further studies on understanding CPSF73 regulation and functions in cells. Here, we report that a HECT-like E3 ligase, UBE3D, participates in stabilizing CPFS73 protein by preventing its ubiquitin-mediated degradation by the proteasome. Depletion of UBE3D leads to CPSF73 downregulation, a pre-mRNA cleavage defect, and dysregulated gene expression in cells. UBE3D dysfunction or chemical inactivation of CPSF73 inhibited migration and invasion as well as stem cell renewal phenotypes in vitro in triple-negative breast cancer cells. In addition, genetic overexpression of CPSF73 promoted breast cancer stemness and knocking down CPSF73 inhibited stem cell renewal properties. Together, our findings indicate that targeting the pre-mRNA processing nuclease CPSF73 has potential for breast cancer therapy.

DNA Damage Regulates the Functions of the RNA Binding Protein Sam68 through ATM-Dependent Phosphorylation

Simple Summary

Alterations of the complex network of interactions between the DNA damage response pathway and RNA metabolism have been described in several tumors, and increasing efforts are devoted to the elucidation of the molecular mechanisms involved in this network. Previous large-scale proteomic studies identified the RNA binding protein Sam68 as a putative target of the ATM kinase. Herein, we demonstrate that ATM phosphorylates Sam68 upon DNA damage induction, and this post-translational modification regulates both the signaling function of Sam68 in the initial phase of the DNA damage response and its RNA processing activity. Thus, our study uncovers anew crosstalk between ATM and Sam68, which may represent a paradigm for the functional interaction between the DDR pathway and RNA binding proteins, and a possible actionabletarget in human cancers.

Abstract

Cancer cells frequently exhibit dysregulation of the DNA damage response (DDR), genomic instability, and altered RNA metabolism. Recent genome-wide studies have strongly suggested an interaction between the pathways involved in the cellular response to DDR and in the regulation of RNA metabolism, but the molecular mechanism(s) involved in this crosstalk are largely unknown. Herein, we found that activation of the DDR kinase ATM promotes its interaction with Sam68, leading to phosphorylation of this multifunctional RNA binding protein (RBP) on three residues: threonine 61, serine 388 and serine 390. Moreover, we demonstrate that ATM-dependent phosphorylation of threonine 61 promotes the function of Sam68 in the DDR pathway and enhances its RNA processing activity. Importantly, ATM-mediated phosphorylation of Sam68 in prostate cancer cells modulates alternative polyadenylation of transcripts that are targets of Sam68, supporting the notion that the ATM–Sam68 axis exerts a multifaceted role in the response to DNA damage. Thus, our work validates Sam68 as an ATM kinase substrate and uncovers an unexpected bidirectional interplay between ATM and Sam68, which couples the DDR pathway to modulation of RNA metabolism in response to genotoxic stress.

Targeting CLK4 inhibits the metastasis and progression of breast cancer by inactivating TGF-β pathway

Triple-negative breast cancer (TNBC) represents the most aggressive subtype of breast cancer that is highly resistant to current therapeutic options. According to the public databases Oncomine and KM plotter, the CLK4 expression is correlated with poor patient survival in TNBC, especially in mesenchymal-like TNBC (MES-TNBC) that has strong metastatic potential. Therefore, we investigated the potential involvement of CLK4 in the metastasis and progression of MES-TNBC. In the MES-TNBC cell lines, the CLK4 expression was elevated. Notably, the RNAi-mediated silencing of CLK4 reduced the expression of multiple epithelial-mesenchymal transition (EMT) genes that mediate metastasis. Furthermore, CLK4 silencing reduced both the invasive behaviors of the cultured cells and tumor metastasis in the mouse xenograft model. It is also noteworthy that CLK4 silencing repressed the invasive and cancer stem cell (CSC) properties that are induced by the TGF-β signaling. Importantly, the pharmacological inhibition of CLK4 potently repressed the invasion and proliferation of MES-TNBC cell lines and patient-derived cells, which demonstrates its clinical applicability. Collectively, our results suggest that CLK4 plays a crucial role in invasion and proliferation of MES-TNBC, especially in the processes that are induced by TGF-β. Also, this study characterizes CLK4 as a novel therapeutic target in breast cancer.

Good Cop, Bad Cop: The Different Roles of SRPKs

SR Protein Kinases (SRPKs), discovered approximately 30 years ago, are widely known as splice factor kinases due to their decisive involvement in the regulation of various steps of mRNA splicing. However, they were also shown to regulate diverse cellular activities by phosphorylation of serine residues residing in serine-arginine/arginine-serine dipeptide motifs. Over the last decade, SRPK1 has been reported as both tumor suppressor and promoter, depending on the cellular context and has been implicated in both chemotherapy sensitivity and resistance. Moreover, SRPK2 has been reported to exhibit contradictory functions in different cell contexts promoting either apoptosis or tumor growth. The aim of the current review is to broaden and deepen our understanding of the SRPK function focusing on the subcellular localization of the kinases. There is ample evidence that the balance between cytoplasmic and nuclear SRPK levels is tightly regulated and determines cell response to external signals. Specific cell states coupled to kinase levels, spatial specific interactions with substrates but also changes in the extent of phosphorylation that allow SRPKs to exhibit a rheostat-like control on their substrates, could decide the proliferative or antiproliferative role of SRPKs.

Functional Interaction Between the Oncogenic Kinase NEK2 and Sam68 Promotes a Splicing Program Involved in Migration and Invasion in Triple-Negative Breast Cancer

Triple-negative breast cancer (TNBC) represents the most aggressive breast cancer subtype. Poor prognosis in TNBC is partly due to lack of efficacious targeted therapy and high propensity to metastasize. Dysregulation of alternative splicing has recently emerged as a trait of TNBC, suggesting that unveiling the molecular mechanisms underlying its regulation could uncover new druggable cancer vulnerabilities. The oncogenic kinase NEK2 is significantly upregulated in TNBC and contributes to shaping their unique splicing profile. Herein, we found that NEK2 interacts with the RNA binding protein Sam68 in TNBC cells and that NEK2-mediated phosphorylation of Sam68 enhances its splicing activity. Genome-wide transcriptome analyses identified the splicing targets of Sam68 in TNBC cells and revealed a common set of exons that are co-regulated by NEK2. Functional annotation of splicing-regulated genes highlighted cell migration and spreading as biological processes regulated by Sam68. Accordingly, Sam68 depletion reduces TNBC cell migration and invasion, and these effects are potentiated by the concomitant inhibition of NEK2 activity. Our findings indicate that Sam68 and NEK2 functionally cooperate in the regulation of a splicing program that sustains the pro-metastatic features of TNBC cells.

Somatic Mutations in Core Spliceosome Components Promote Tumorigenesis and Generate an Exploitable Vulnerability in Human Cancer

Simple Summary

High throughput exome sequencing approaches have disclosed recurrent cancer-associated mutations in spliceosomal components, which drive aberrant pre-mRNA processing events and support the tumor phenotype. At the same time, mutations in spliceosome genes and aberrant splicing regulation establish a selective vulnerability of cancer cells to splicing-targeting approaches, which could be exploited therapeutically. It is conceivable that a better understanding of the mechanisms and roles of abnormal splicing in tumor metabolism will facilitate the development of a novel generation of tumor-targeting drugs. In this review, we describe recent advances in the elucidation of the biological impact and biochemical effects of somatic mutations in core spliceosome components on splicing choices and their associated targetable vulnerabilities.

Abstract

Alternative pre-mRNA processing enables the production of distinct mRNA and protein isoforms from a single gene, thus greatly expanding the coding potential of eukaryotic genomes and fine-tuning gene expression programs. Splicing is carried out by the spliceosome, a complex molecular machinery which assembles step-wise on mRNA precursors in the nucleus of eukaryotic cells. In the last decade, exome sequencing technologies have allowed the identification of point mutations in genes encoding splicing factors as a recurrent hallmark of human cancers, with higher incidence in hematological malignancies. These mutations lead to production of splicing factors that reduce the fidelity of the splicing process and yield splicing variants that are often advantageous for cancer cells. However, at the same time, these mutations increase the sensitivity of transformed cells to splicing inhibitors, thus offering a therapeutic opportunity for novel targeted strategies. Herein, we review the recent literature documenting cancer-associated mutations in components of the early spliceosome complex and discuss novel therapeutic strategies based on small-molecule spliceosome inhibitors that exhibit strong anti-tumor effects, particularly against cancer cells harboring mutations in spliceosomal components.

Reprogramming RNA processing: an emerging therapeutic landscape

The production of a mature mRNA requires coordination of multiple processing steps, which ultimately control its content, localization, and stability. These steps include some of the largest macromolecular machines in the cell, which were, until recently, considered undruggable due to their biological complexity. Building from an expanded understanding of the underlying mechanisms that drive these processes, a new wave of therapeutics is seeking to target RNA processing. With a focus on impacting gene regulation at the RNA level, such modalities offer potential for sequence-specific resolution in drug design. Here, we review our current understanding of RNA-processing events and their role in gene regulation, with a focus on the therapeutic opportunities that have emerged within this landscape.

The oncogenic kinase NEK2 regulates an RBFOX2-dependent pro-mesenchymal splicing program in triple-negative breast cancer cells

BACKGROUND

Triple-negative breast cancer (TNBC) is the most heterogeneous and malignant subtype of breast cancer (BC). TNBC is defined by the absence of expression of estrogen, progesterone and HER2 receptors and lacks efficacious targeted therapies. NEK2 is an oncogenic kinase that is significantly upregulated in TNBC, thereby representing a promising therapeutic target. NEK2 localizes in the nucleus and promotes oncogenic splice variants in different cancer cells. Notably, alternative splicing (AS) dysregulation has recently emerged as a featuring trait of TNBC that contributes to its aggressive phenotype.

METHODS

To investigate whether NEK2 modulates TNBC transcriptome we performed RNA-sequencing analyses in a representative TNBC cell line (MDA-MB-231) and results were validated in multiple TNBC cell lines. Bioinformatics and functional analyses were carried out to elucidate the mechanism of splicing regulation by NEK2. Data from The Cancer Genome Atlas were mined to evaluate the potential of NEK2-sensitive exons as markers to identify the TNBC subtype and to assess their prognostic value.

RESULTS

Transcriptome analysis revealed a widespread impact of NEK2 on the transcriptome of TNBC cells, with 1830 AS events that are susceptible to its expression. NEK2 regulates the inclusion of cassette exons in splice variants that discriminate TNBC from other BC and that correlate with poor prognosis, suggesting that this kinase contributes to the TNBC-specific splicing program. NEK2 elicits its effects by modulating the expression of the splicing factor RBFOX2, a well-known regulator of epithelial to mesenchymal transition (EMT). Accordingly, NEK2 splicing-regulated genes are enriched in functional terms related to cell adhesion and contractile cytoskeleton and NEK2 depletion in mesenchymal TNBC cells induces phenotypic and molecular traits typical of epithelial cells. Remarkably, depletion of select NEK2-sensitive splice-variants that are prognostic in TNBC patients is sufficient to interfere with TNBC cell morphology and motility, suggesting that NEK2 orchestrates a pro-mesenchymal splicing program that modulates migratory and invasive properties of TNBC cells.

CONCLUSIONS

Our study uncovers an extensive splicing program modulated by NEK2 involving splice variants that confer an invasive phenotype to TNBCs and that might represent, together with NEK2 itself, valuable therapeutic targets for this disease.

The ever-expanding landscape of cancer therapeutic approaches

Cancer is a leading cause of death and a major health problem worldwide, particularly in more developed countries. There is, therefore, an urgent clinical need to develop more effective therapies to treat cancer and metastatic disease. In this Editorial, the content of The FEBS Journal's Special Issue on Cancer Therapeutics is outlined. The interesting collection of recent articles in this issue covers a wide repertoire of cancer therapeutic approaches. While some of the articles discuss broad-spectrum applications such as immunotherapy and oncolytic virus therapy, others focus on a particular type of cancer or a signalling pathway that has gone awry such as aberrant Ca²⁺ signalling, glycosylation or pre-mRNA processing. Finally, an article featured in this issue reviews our current understanding of how cancer cells can become dormant, often for decades, and which pathways reactivate these cells to cause relapse. I am sure there is something for everyone in this issue.

Coordination of RNA Processing Regulation by Signal Transduction Pathways

Signal transduction pathways transmit the information received from external and internal cues and generate a response that allows the cell to adapt to changes in the surrounding environment. Signaling pathways trigger rapid responses by changing the activity or localization of existing molecules, as well as long-term responses that require the activation of gene expression programs. All steps involved in the regulation of gene expression, from transcription to processing and utilization of new transcripts, are modulated by multiple signal transduction pathways. This review provides a broad overview of the post-translational regulation of factors involved in RNA processing events by signal transduction pathways, with particular focus on the regulation of pre-mRNA splicing, cleavage and polyadenylation. The effects of several post-translational modifications (i.e., sumoylation, ubiquitination, methylation, acetylation and phosphorylation) on the expression, subcellular localization, stability and affinity for RNA and protein partners of many RNA-binding proteins are highlighted. Moreover, examples of how some of the most common signal transduction pathways can modulate biological processes through changes in RNA processing regulation are illustrated. Lastly, we discuss challenges and opportunities of therapeutic approaches that correct RNA processing defects and target signaling molecules.