TBL1XR1 Mutations Drive Extranodal Lymphoma by Inducing a Pro-tumorigenic Memory Fate.

The most aggressive B cell lymphomas frequently manifest extranodal distribution and carry somatic mutations in the poorly characterized gene TBL1XR1. Here, we show that TBL1XR1 mutations skew the humoral immune response toward generating abnormal immature memory B cells (MB), while impairing plasma cell differentiation. At the molecular level, TBL1XR1 mutants co-opt SMRT/HDAC3 repressor complexes toward binding the MB cell transcription factor (TF) BACH2 at the expense of the germinal center (GC) TF BCL6, leading to pre-memory transcriptional reprogramming and cell-fate bias. Upon antigen recall, TBL1XR1 mutant MB cells fail to differentiate into plasma cells and instead preferentially reenter new GC reactions, providing evidence for a cyclic reentry lymphomagenesis mechanism. Ultimately, TBL1XR1 alterations lead to a striking extranodal immunoblastic lymphoma phenotype that mimics the human disease. Both human and murine lymphomas feature expanded MB-like cell populations, consistent with a MB-cell origin and delineating an unforeseen pathway for malignant transformation of the immune system.

Rewiring cancer drivers to activate apoptosis.

Genes that drive the proliferation, survival, invasion and metastasis of malignant cells have been identified for many human cancers1-4. Independent studies have identified cell death pathways that eliminate cells for the good of the organism5,6. The coexistence of cell death pathways with driver mutations suggests that the cancer driver could be rewired to activate cell death using chemical inducers of proximity (CIPs). Here we describe a new class of molecules called transcriptional/epigenetic CIPs (TCIPs) that recruit the endogenous cancer driver, or a downstream transcription factor, to the promoters of cell death genes, thereby activating their expression. We focused on diffuse large B cell lymphoma, in which the transcription factor B cell lymphoma 6 (BCL6) is deregulated7. BCL6 binds to the promoters of cell death genes and epigenetically suppresses their expression8. We produced TCIPs by covalently linking small molecules that bind BCL6 to those that bind to transcriptional activators that contribute to the oncogenic program, such as BRD4. The most potent molecule, TCIP1, increases binding of BRD4 by 50% over genomic BCL6-binding sites to produce transcriptional elongation at pro-apoptotic target genes within 15 min, while reducing binding of BRD4 over enhancers by only 10%, reflecting a gain-of-function mechanism. TCIP1 kills diffuse large B cell lymphoma cell lines, including chemotherapy-resistant, TP53-mutant lines, at EC50 of 1-10 nM in 72 h and exhibits cell-specific and tissue-specific effects, capturing the combinatorial specificity inherent to transcription. The TCIP concept also has therapeutic applications in regulating the expression of genes for regenerative medicine and developmental disorders.

Regulation of DNA methylation dictates Cd4 expression during the development of helper and cytotoxic T cell lineages.

During development, progenitor cells with binary potential give rise to daughter cells that have distinct functions. Heritable epigenetic mechanisms then lock in gene-expression programs that define lineage identity. Regulation of the gene encoding the T cell-specific coreceptor CD4 in helper and cytotoxic T cells exemplifies this process, with enhancer- and silencer-regulated establishment of epigenetic memory for stable gene expression and repression, respectively. Using a genetic screen, we identified the DNA-methylation machinery as essential for maintaining silencing of Cd4 in the cytotoxic lineage. Furthermore, we found a requirement for the proximal enhancer in mediating the removal of DNA-methylation marks from Cd4, which allowed stable expression of Cd4 in helper T cells. Our findings suggest that stage-specific methylation and demethylation events in Cd4 regulate its heritable expression in response to the distinct signals that dictate lineage 'choice' during T cell development.

CBX5 loss drives EGFR inhibitor resistance and results in therapeutically actionable vulnerabilities in lung cancer.

Although epidermal growth factor receptor (EGFR) tyrosine kinase inhibitors (EGFRi) are approved for treating EGFR-mutant lung adenocarcinoma (LUAD), emergence of acquired resistance limits their clinical benefits. Several mechanisms for acquired resistance to EGFRi in LUAD have been identified; however, the molecular basis for this resistance remains unknown in ~30% of LUAD. Chromatin and DNA modifiers and their regulators play important roles in determining response to anticancer therapies. Therefore, to identify nongenetic mechanisms of EGFRi resistance in LUAD, we performed an epigenome-wide shRNA screen targeting 363 human epigenetic regulator genes. This screen identified loss of the transcriptional repressor chromobox homolog 5 (CBX5) as a driver of EGFRi resistance in EGFR-mutant LUAD. Loss of CBX5 confers resistance to multiple EGFRi in both cell culture and mice. We found that CBX5 loss in EGFR-mutant LUAD cells leads to increased expression of the transcription factor E2F1, which in turn stimulates expression of the antiapoptotic gene BIRC5 (survivin). This E2F1-mediated upregulation of BIRC5 in CBX5-knockdown LUAD cells attenuates apoptosis induction following EGFRi treatment. Consistent with these results, knockdown of E2F1 or BIRC5 partly rescues CBX5-knockdown-induced EGFRi resistance in cell culture and mice. EGFRi-resistant LUAD cell lines show reduced CBX5 expression compared to parental lines; however, bromo- and extra-terminal (BET)-domain inhibitors (BETi) restore CBX5 expression in these cells and sensitize them to EGFRi/BETi combination therapy. Similarly, treatment with a BIRC5 inhibitor suppresses growth of EGFRi-resistant LUAD cells. Collectively, these studies identify CBX5 loss as a driver of EGFRi resistance and reveal therapeutic opportunities for treating EGFRi-resistant LUAD.

A mammalian siderophore synthesized by an enzyme with a bacterial homolog involved in enterobactin production.

Intracellular iron homeostasis is critical for survival and proliferation. Lipocalin 24p3 is an iron-trafficking protein that binds iron through association with a bacterial siderophore, such as enterobactin, or a postulated mammalian siderophore. Here, we show that the iron-binding moiety of the 24p3-associated mammalian siderophore is 2,5-dihydroxybenzoic acid (2,5-DHBA), which is similar to 2,3-DHBA, the iron-binding component of enterobactin. We find that the murine enzyme responsible for 2,5-DHBA synthesis, BDH2, is the homolog of bacterial EntA, which catalyzes 2,3-DHBA production during enterobactin biosynthesis. RNA interference-mediated knockdown of BDH2 results in siderophore depletion. Mammalian cells lacking the siderophore accumulate abnormally high amounts of cytoplasmic iron, resulting in elevated levels of reactive oxygen species, whereas the mitochondria are iron deficient. Siderophore-depleted mammalian cells and zebrafish embryos fail to synthesize heme, an iron-dependent mitochondrial process. Our results reveal features of intracellular iron homeostasis that are conserved from bacteria through humans.

Safety and efficacy of ibrutinib in combination with rituximab and lenalidomide in previously untreated follicular and marginal zone lymphoma: An open label, phase 2 study.

BACKGROUND: Follicular lymphoma (FL) and marginal zone lymphoma (MZL) are indolent non-Hodgkin lymphomas (iNHL). Median survival for iNHL is approximately 20 years. Because standard treatments are not curative, patients often receive multiple lines of therapy with associated toxicity-rationally designed, combination therapies with curative potential are needed. The immunomodulatory drug lenalidomide was evaluated in combination with rituximab for the frontline treatment of FL in the phase 3 RELEVANCE study. Ibrutinib, an oral Bruton tyrosine kinase inhibitor, is active in NHL and was evaluated in combination with lenalidomide, rituximab, and ibrutinib (IRR) in a phase 1 study. METHODS: The authors conducted an open-label, phase 2 clinical trial of IRR for previously untreated FL and MZL. The primary end point was progression-free survival (PFS) at 24 months. RESULTS: This study included 48 participants with previously untreated FL grade 1-3a (N = 38), or MZL (N = 10). Participants received 12, 28-day cycles of lenalidomide (15 mg, days 1-21 cycle 1; 20 mg, cycles 2-12), rituximab (375 mg/m2 weekly in cycle 1; day 1 cycles 2-12), and ibrutinib 560 mg daily. With a median follow-up of 65.3 months, the estimated PFS at 24 months was 78.8% (95% confidence interval [CI], 68.0%-91.4%) and 60-month PFS was 59.7% (95% CI, 46.6%-76.4%). One death occurred unrelated to disease progression. Grade 3-4 adverse events were observed in 64.6%, including 50% with grade 3-4 rash. CONCLUSIONS: IRR is highly active as frontline therapy for FL and MZL. Compared to historical results with lenalidomide and rituximab, PFS is similar with higher grade 3-4 toxicity, particularly rash. The study was registered with ClinicalTrials.gov (NCT02532257).

Effective therapy for AML with RUNX1 mutation by cotreatment with inhibitors of protein translation and BCL2.

The majority of RUNX1 mutations in acute myeloid leukemia (AML) are missense or deletion-truncation and behave as loss-of-function mutations. Following standard therapy, AML patients expressing mtRUNX1 exhibit inferior clinical outcome than those without mutant RUNX1. Studies presented here demonstrate that as compared with AML cells lacking mtRUNX1, their isogenic counterparts harboring mtRUNX1 display impaired ribosomal biogenesis and differentiation, as well as exhibit reduced levels of wild-type RUNX1, PU.1, and c-Myc. Compared with AML cells with only wild-type RUNX1, AML cells expressing mtRUNX1 were also more sensitive to the protein translation inhibitor homoharringtonine (omacetaxine) and BCL2 inhibitor venetoclax. Homoharringtonine treatment repressed enhancers and their BRD4 occupancy and was associated with reduced levels of c-Myc, c-Myb, MCL1, and Bcl-xL. Consistent with this, cotreatment with omacetaxine and venetoclax or BET inhibitor induced synergistic in vitro lethality in AML expressing mtRUNX1. Compared with each agent alone, cotreatment with omacetaxine and venetoclax or BET inhibitor also displayed improved in vivo anti-AML efficacy, associated with improved survival of immune-depleted mice engrafted with AML cells harboring mtRUNX1. These findings highlight superior efficacy of omacetaxine-based combination therapies for AML harboring mtRUNX1.

Risk assessment with low-pass whole-genome sequencing of cell-free DNA before CD19 CAR T-cell therapy for large B-cell lymphoma.

Patients with relapsed or refractory large B-cell lymphomas (rrLBCL) can achieve long-term remission after CD19 chimeric antigen receptor T-cell therapy (CART19). However, more than half of recipients will experience treatment failure. Thus, approaches are needed to identify high-risk patients who may benefit from alternative or consolidative therapy. We evaluated low-pass whole-genome sequencing (lpWGS) of cell-free DNA (cfDNA) before CART19 as a new approach for risk stratification. We performed lpWGS on pretreatment plasma samples from 122 patients at time of leukapheresis who received standard-of-care CART19 for rrLBCL to define DNA copy number alterations (CNAs). In multivariable selection, high focal CNA score (FCS) denoting genomic instability was the most significant pretreatment variable associated with inferior 3-month complete response rates (28% vs 56%, P = .0029), progression-free survival (PFS; P = .0007; hazard ratio, 2.11), and overall survival (OS; P = .0026; hazard ratio, 2.10). We identified 34 unique focal CNAs in 108 (89%) patients; of these, deletion 10q23.3 leading to loss of FAS death receptor was the most highly associated with poor outcomes, leading to inferior PFS (P < .0001; hazard ratio, 3.49) and OS (P = .0027; hazard ratio, 2.68). By combining FCS with traditional markers of increased tumor bulk (elevated lactate dehydrogenase and >1 extranodal site), we built a simple risk model that could reliably risk stratify patients. Thus, lpWGS of cfDNA is a minimally invasive assay that could rapidly identify high-risk patients and may guide patient selection for and targeted therapies to evaluate in future clinical trials.

Intra-tumoral and peripheral blood TIGIT and PD-1 as immune biomarkers in nodular lymphocyte predominant Hodgkin lymphoma.

In classical Hodgkin lymphoma (cHL), responsiveness to immune-checkpoint blockade (ICB) is associated with specific tumor microenvironment (TME) and peripheral blood features. The role of ICB in nodular lymphocyte predominant Hodgkin lymphoma (NLPHL) is not established. To gain insights into its potential in NLPHL, we compared TME and peripheral blood signatures between HLs using an integrative multiomic analysis. A discovery/validation approach in 121 NLPHL and 114 cHL patients highlighted >2-fold enrichment in programmed cell death-1 (PD-1) and T-cell Ig and ITIM domain (TIGIT) gene expression for NLPHL versus cHL. Multiplex imaging showed marked increase in intra-tumoral protein expression of PD-1+ (and/or TIGIT+) CD4+ T-cells and PD-1+CD8+ T-cells in NLPHL compared to cHL. This included T-cells that rosetted with lymphocyte predominant (LP) and Hodgkin Reed-Sternberg (HRS) cells. In NLPHL, intra-tumoral PD-1+CD4+ T-cells frequently expressed TCF-1, a marker of heightened T-cell response to ICB. The peripheral blood signatures between HLs were also distinct, with higher levels of PD-1+TIGIT+ in TH1, TH2, and regulatory CD4+ T-cells in NLPHL versus cHL. Circulating PD-1+CD4+ had high levels of TCF-1. Notably, in both lymphomas, highly expanded populations of clonal TIGIT+PD-1+CD4+ and TIGIT+PD-1+CD8+ T-cells in the blood were also present in the TME, indicating that immune-checkpoint expressing T-cells circulated between intra-tumoral and blood compartments. In in vitro assays, ICB was capable of reducing rosette formation around LP and HRS cells, suggesting that disruption of rosetting may be a mechanism of action of ICB in HL. Overall, results indicate that further evaluation of ICB is warranted in NLPHL.

U2AF35(S34F) Promotes Transformation by Directing Aberrant ATG7 Pre-mRNA 3' End Formation.

Recurrent mutations in the splicing factor U2AF35 are found in several cancers and myelodysplastic syndrome (MDS). How oncogenic U2AF35 mutants promote transformation remains to be determined. Here we derive cell lines transformed by the oncogenic U2AF35(S34F) mutant and identify aberrantly processed pre-mRNAs by deep sequencing. We find that in U2AF35(S34F)-transformed cells the autophagy-related factor 7 (Atg7) pre-mRNA is abnormally processed, which unexpectedly is not due to altered splicing but rather selection of a distal cleavage and polyadenylation (CP) site. This longer Atg7 mRNA is translated inefficiently, leading to decreased ATG7 levels and an autophagy defect that predisposes cells to secondary mutations, resulting in transformation. MDS and acute myeloid leukemia patient samples harboring U2AF35(S34F) have a similar increased use of the ATG7 distal CP site, and previous studies have shown that mice with hematopoietic cells lacking Atg7 develop an MDS-like syndrome. Collectively, our results reveal a basis for U2AF35(S34F) oncogenic activity.

G-rich motifs within phosphorothioate-based antisense oligonucleotides (ASOs) drive activation of FXN expression through indirect effects.

Friedreich's ataxia is an incurable disease caused by frataxin (FXN) protein deficiency, which is mostly induced by GAA repeat expansion in intron 1 of the FXN gene. Here, we identified antisense oligonucleotides (ASOs), complementary to two regions within the first intron of FXN pre-mRNA, which could increase FXN mRNA by ∼2-fold in patient fibroblasts. The increase in FXN mRNA was confirmed by the identification of multiple overlapping FXN-activating ASOs at each region, two independent RNA quantification assays, and normalization by multiple housekeeping genes. Experiments on cells with the ASO-binding sites deleted indicate that the ASO-induced FXN activation was driven by indirect effects. RNA sequencing analyses showed that the two ASOs induced similar transcriptome-wide changes, which did not resemble the transcriptome of wild-type cells. This RNA-seq analysis did not identify directly base-paired off-target genes shared across ASOs. Mismatch studies identified two guanosine-rich motifs (CCGG and G4) within the ASOs that were required for FXN activation. The phosphorodiamidate morpholino oligomer analogs of our ASOs did not activate FXN, pointing to a PS-backbone-mediated effect. Our study demonstrates the importance of multiple, detailed control experiments and target validation in oligonucleotide studies employing novel mechanisms such as gene activation.

EZH2 inhibits NK cell-mediated antitumor immunity by suppressing CXCL10 expression in an HDAC10-dependent manner.

Enhancer of zeste homolog 2 (EZH2) is a histone H3 lysine 27 methyltransferase that has been shown to function as an oncogene in some cancers. Previous reports have largely focused on the ability of EZH2 to regulate cell-intrinsic tumor regulatory pathways as its mechanism-of-oncogenic action. However, the role that EZH2-mediated immune suppression plays in its oncogenic activity is not fully known. In particular, the role of natural killer (NK) cells in EZH2-driven tumor growth remains incompletely understood. Here, we demonstrate that genetic or pharmacological inhibition of EZH2 induces reexpression of the chemokine CXCL10 in hepatic tumor cells. We find that histone deacetylase 10 (HDAC10) is necessary for EZH2 recruitment to the CXCL10 promoter, leading to CXCL10 transcriptional repression. Critically, CXCL10 is necessary and sufficient for stimulating NK cell migration, and EZH2's ability to inhibit NK cell migration via CXCL10 suppression is conserved in other EZH2-dependent cancers. NK cell depletion in an immunocompetent syngeneic mouse model of hepatic tumorigenesis reverses the tumor inhibitory effects of an EZH2 inhibitor (GSK343), and inhibitor-mediated reexpression of CXCL10 is required for its tumor suppressive effects in the same mouse model. Collectively, these results reveal a decisive role for NK cells and CXCL10 in mediating the oncogenic function of EZH2.

Interacting effects of environmental enrichment across multiple generations on early life phenotypes in zebrafish.

The environment plays an important role in an individual's development during early life, however, parents may also influence offspring development through so called "parental effects." We examined the effects of environmental enrichment in zebrafish (Danio rerio) across two generations through the paternal lineage. Fathers and grandfathers were exposed to either standard or high levels of housing enrichment for 4-weeks during adulthood. First-generation (F1) and second-generation (F2) offspring were obtained from controlled breeding and tested as larvae for changes in morphology at hatching stage (72hpf), and in locomotor activity at larval stage (120hpf) in both generations. We found paternal experience of enrichment resulted in changes in trunk length of F1 offspring and changes in spine curvature and dorsal length of F2 offspring, while changes in snout morphology of F2 offspring seemed to be driven by whether grandpaternal and paternal experience of the environment was matched or not. We found that while paternal enrichment increased the frequency of spontaneous movement in F1 and F2 offspring, interacting effects of paternal and grandpaternal enrichment on movement distance were seen in F2 offspring, and that spontaneous movement and the distance that larvae swam are thus distinct phenotypes that were differentially affected by the experiences of previous paternal generations. Taken together, these findings suggest that the parental and grandparental environment influence zebrafish behavior and morphology. The nature of these effects and the design of this study mean that these phenotypes were likely the result of nongenetic transmission through the paternal germline.

Prostaglandin E2 stimulates cAMP signaling and resensitizes human leukemia cells to glucocorticoid-induced cell death.

Glucocorticoid (GC) resistance remains a clinical challenge in pediatric acute lymphoblastic leukemia where response to GC is a reliable prognostic indicator. To identify GC resistance pathways, we conducted a genome-wide, survival-based, short hairpin RNA screen in murine T-cell acute lymphoblastic leukemia (T-ALL) cells. Genes identified in the screen interfere with cyclic adenosine monophosphate (cAMP) signaling and are underexpressed in GC-resistant or relapsed ALL patients. Silencing of the cAMP-activating Gnas gene interfered with GC-induced gene expression, resulting in dexamethasone resistance in vitro and in vivo. We demonstrate that cAMP signaling synergizes with dexamethasone to enhance cell death in GC-resistant human T-ALL cells. We find the E prostanoid receptor 4 expressed in T-ALL samples and demonstrate that prostaglandin E2 (PGE2) increases intracellular cAMP, potentiates GC-induced gene expression, and sensitizes human T-ALL samples to dexamethasone in vitro and in vivo. These findings identify PGE2 as a target for GC resensitization in relapsed pediatric T-ALL.

Prognostic impact of corticosteroids on efficacy of chimeric antigen receptor T-cell therapy in large B-cell lymphoma.

Corticosteroids are commonly used for the management of severe toxicities associated with chimeric antigen receptor (CAR) T-cell therapy. However, it remains unclear whether their dose, duration, and timing may affect clinical efficacy. Here, we determined the impact of corticosteroids on clinical outcomes in patients with relapsed or refractory large B-cell lymphoma treated with standard of care anti-CD19 CAR T-cell therapy. Among 100 patients evaluated, 60 (60%) received corticosteroids for management of CAR T-cell therapy-associated toxicities. The median cumulative dexamethasone-equivalent dose was 186 mg (range, 8-1803) and the median duration of corticosteroid treatment was 9 days (range, 1-30). Corticosteroid treatment was started between days 0 and 7 in 45 (75%) patients and beyond day 7 in 15 (25%). After a median follow-up of 10 months (95% confidence interval, 8-12 months), use of higher cumulative dose of corticosteroids was associated with significantly shorter progression-free survival. More importantly, higher cumulative dose of corticosteroids, and prolonged and early use after CAR T-cell infusion were associated with significantly shorter overall survival. These results suggest that corticosteroids should be used at the lowest dose and for the shortest duration and their initiation should be delayed whenever clinically feasible while managing CAR T-cell therapy-associated toxicities.

Senescence: not just for tumor suppression.

Cellular senescence provides an intrinsic barrier to tumor development by preventing the proliferation of cells that are at risk for malignant transformation. In this issue, Krizhanovsky et al. (2008) report that senescence is an important player not only in tumor suppression but also in the response of liver tissue to injury.

Oncogenic BRAF induces senescence and apoptosis through pathways mediated by the secreted protein IGFBP7.

Expression of an oncogene in a primary cell can, paradoxically, block proliferation by inducing senescence or apoptosis through pathways that remain to be elucidated. Here we perform genome-wide RNA-interference screening to identify 17 genes required for an activated BRAF oncogene (BRAFV600E) to block proliferation of human primary fibroblasts and melanocytes. Surprisingly, we find a secreted protein, IGFBP7, has a central role in BRAFV600E-mediated senescence and apoptosis. Expression of BRAFV600E in primary cells leads to synthesis and secretion of IGFBP7, which acts through autocrine/paracrine pathways to inhibit BRAF-MEK-ERK signaling and induce senescence and apoptosis. Apoptosis results from IGFBP7-mediated upregulation of BNIP3L, a proapoptotic BCL2 family protein. Recombinant IGFBP7 (rIGFBP7) induces apoptosis in BRAFV600E-positive human melanoma cell lines, and systemically administered rIGFBP7 markedly suppresses growth of BRAFV600E-positive tumors in xenografted mice. Immunohistochemical analysis of human skin, nevi, and melanoma samples implicates loss of IGFBP7 expression as a critical step in melanoma genesis.

Antigen presentation profiling reveals recognition of lymphoma immunoglobulin neoantigens.

Cancer somatic mutations can generate neoantigens that distinguish malignant from normal cells. However, the personalized identification and validation of neoantigens remains a major challenge. Here we discover neoantigens in human mantle-cell lymphomas by using an integrated genomic and proteomic strategy that interrogates tumour antigen peptides presented by major histocompatibility complex (MHC) class I and class II molecules. We applied this approach to systematically characterize MHC ligands from 17 patients. Remarkably, all discovered neoantigenic peptides were exclusively derived from the lymphoma immunoglobulin heavy- or light-chain variable regions. Although we identified MHC presentation of private polymorphic germline alleles, no mutated peptides were recovered from non-immunoglobulin somatically mutated genes. Somatic mutations within the immunoglobulin variable region were almost exclusively presented by MHC class II. We isolated circulating CD4+ T cells specific for immunoglobulin-derived neoantigens and found these cells could mediate killing of autologous lymphoma cells. These results demonstrate that an integrative approach combining MHC isolation, peptide identification, and exome sequencing is an effective platform to uncover tumour neoantigens. Application of this strategy to human lymphoma implicates immunoglobulin neoantigens as targets for lymphoma immunotherapy.

Global Promotion of Alternative Internal Exon Usage by mRNA 3' End Formation Factors.

The mechanisms that regulate alternative precursor mRNA (pre-mRNA) splicing are largely unknown. Here, we perform an RNAi screen to identify factors required for alternative splicing regulation by RBFOX2, an RNA-binding protein that promotes either exon inclusion or exclusion. Unexpectedly, we find that two mRNA 3' end formation factors, cleavage and polyadenylation specificity factor (CPSF) and SYMPK, are RBFOX2 cofactors for both inclusion and exclusion of internal exons. RBFOX2 interacts with CPSF/SYMPK and recruits it to the pre-mRNA. RBFOX2 and CPSF/SYMPK then function together to regulate binding of the early intron recognition factors U2AF and U1 small nuclear ribonucleoprotein particle (snRNP). Genome-wide analysis reveals that CPSF also mediates alternative splicing of many internal exons in the absence of RBFOX2. Accordingly, we show that CPSF/SYMPK is also a cofactor of NOVA2 and heterologous nuclear ribonucleoprotein A1 (HNRNPA1), RNA-binding proteins that also regulate alternative splicing. Collectively, our results reveal an unanticipated role for mRNA 3' end formation factors in global promotion of alternative splicing.