Mechanism of action of a WWTR1(TAZ)-CAMTA1 fusion oncoprotein

Anoikis Resistance in Cancer: Mechanisms, Therapeutic Strategies, Potential Targets, and Models for Enhanced Understanding

Anoikis, defined as programmed cell death triggered by the loss of cell-extracellular matrix (ECM) and cell-cell interactions, is crucial for maintaining tissue homeostasis and preventing aberrant cell migration. Cancer cells, however, display anoikis resistance (AR) which in turn enables cancer metastasis. AR results from alterations in apoptotic signaling, metabolic reprogramming, autophagy modulation, and epigenetic changes, allowing cancer cells to survive in detached conditions. In this review we describe the mechanisms underlying both anoikis and AR, focusing on intrinsic and extrinsic pathways, disrupted cell-ECM interactions, and autophagy in cancer. Recent findings (i.e., between 2014 and 2024) on epigenetic regulation of AR and its role in metastasis are discussed. Therapeutic strategies targeting AR, including chemical inhibitors, are highlighted alongside a network analysis of 122 proteins reported to be associated with AR which identifies 53 hub proteins as potential targets. We also evaluate in vitro and in vivo models for studying AR, emphasizing their role in advancing metastasis research. Our overall goal is to guide future studies and therapeutic developments to counter cancer metastasis.

C-terminal fusion partner activity contributes to the oncogenic functions of YAP1::TFE3

YAP1 gene fusions are found in a multitude of human tumors, are potent oncogenic drivers, and are the likely initiating tumorigenic events in these tumors. We and others have previously shown that a YAP1 fusion proteins exert TEAD-dependent oncogenic YAP1 activity that is resistant to inhibitory Hippo pathway signaling. However, the contributions of the C-terminal fusion partners to the oncogenic functions of YAP1 fusion proteins are understudied. Here, we used the RCAS/tv-a system to express eight different YAP1 gene fusions in vivo and observed significant differences in the latencies of tumors induced by the various YAP1 fusions. We observed that tumors induced by YAP1::TFE3 displayed a significantly different histomorphology compared to tumors induced by other YAP1 fusions or activated non-fusion YAP1. To assess the extent to which the functional TFE3 domains (DNA binding: leucine zipper (LZ) and basic-helix-loop-helix (bHLH); activation domain (AD)) contribute to the oncogenic functions of YAP1::TFE3, we generated several mutant variants and performed functional in vitro and in vivo assays. In vitro, mutation or deletion of the TFE3 DNA binding domains (LZ, bHLH) resulted in reduced TFE3 activity but increased YAP1 activity of YAP1::TFE3. In vivo, deletion of the LZ and bHLH domains did not result in a decrease in tumor incidence but induced the formation of more YAP1-like tumors that lacked prominent features of YAP1::TFE3-driven tumors. By contrast, loss of the TFE3 AD almost completely abrogated tumor formation. Our results suggest that the TFE3 domains significantly contribute to the oncogenic activity of YAP1::TFE3.

EHE cell cultures: a platform for mechanistic and therapeutic investigation

Epithelioid hemangioendothelioma (EHE) is a difficult to treat vascular sarcoma defined by TAZ- CAMTA1 or YAP-TFE3 fusion proteins. Human cell lines needed to further understand the pathogenesis of EHE have been lacking. Herein, we describe a method to generate EHE extended primary cell cultures. An integrated multi -omic and functional approach was used to characterize these cultures. The cell cultures, relatively homogenous by single cell RNA-Seq, demonstrated established characteristics of EHE including increased proliferation, anchorage independent growth, as well as the overall gene expression profile and secondary genetic alterations seen in EHE. Whole genome sequencing (WGS) identified links to epigenetic modifying complexes, metabolic processes, and pointed to the importance of the extracellular matrix (ECM) in these tumors. Bulk RNA-Seq demonstrated upregulation of pathways including PI3K-Akt signaling, ECM/ECM receptor interaction, and the Hippo signaling pathway. Development of these extended primary cell cultures allowed for single-cell profiling which demonstrated different cell compartments within the cultures. Furthermore, the cultures served as a therapeutic platform to test the efficacy of TEAD inhibitors in vitro . Overall, the development of EHE primary cell cultures will aid in the mechanistic understanding of this sarcoma and serve as a model system to test new therapeutic approaches.

PI3K regulates TAZ/YAP and mTORC1 axes that can be synergistically targeted

Purpose

Sarcomas are a heterogeneous group of cancers with few shared therapeutic targets. PI3K signaling is activated in various subsets of sarcomas, representing a shared oncogenic signaling pathway. Oncogenic PI3K signaling has been challenging to target therapeutically. An integrated view of PI3K and Hippo pathway signaling is examined to determine if this could be leveraged therapeutically.

Experimental design

A tissue microarray containing sarcomas of various histological types was evaluated for PTEN loss and correlated with levels of activated TAZ and YAP. PI3K and Hippo pathways were dissected in sarcoma cell lines. The role of TAZ and YAP were evaluated in a PI3K-driven mouse model. The efficacy of mTORC1 inhibition and TEAD inhibition were evaluated in sarcoma cell lines and in vivo .

Results

PI3K signaling is frequently activated in sarcomas due to PTEN loss (in 30-60%), representing a common therapeutic target. TAZ and YAP are transcriptional co-activators regulated by PI3K and drive a transcriptome necessary for tumor growth in a PI3K-driven sarcoma mouse model. Combination therapy using IK-930 (TEAD inhibitor) and everolimus (mTORC1 inhibitor) synergistically diminished proliferation and anchorage independent growth of PI3K-activated sarcoma cell lines at low, physiologically achievable doses. Furthermore, this combination therapy showed a synergistic effect in vivo , reducing tumor proliferation and size.

Conclusions

TAZ and YAP are transcriptional co-activators downstream of PI3K signaling, a pathway that has lacked a well-defined oncogenic transcription factor. This PI3K-TAZ/YAP axis exists in parallel to the known PI3K-Akt-mTORC1 axis allowing for synergistic combination therapy targeting the TAZ/YAP-TEAD interaction and mTORC1 in sarcomas.

CDK9 Inhibition by Dinaciclib Is a Therapeutic Vulnerability in Epithelioid Hemangioendothelioma

PURPOSE

There are no effective treatment options for patients with aggressive epithelioid hemangioendothelioma (EHE) driven by the TAZ-CAMTA1 (TC) fusion gene. Here, we aimed to understand the regulation of TC using pharmacologic tools and identify vulnerabilities that can potentially be exploited for the treatment of EHE.

EXPERIMENTAL DESIGN

TC is a transcriptional coregulator; we hypothesized that compounds that reduce TC nuclear levels, either through translocation of TC to the cytoplasm, or through degradation, would render TC less oncogenic. TC localization was monitored using immunofluorescence in an EHE tumor cell line. Two target-selective libraries were used to identify small molecules that reduce TC localization in the nucleus. The ability of the shortlisted hits to affect cell viability, apoptosis, and tumorigenesis was also evaluated.

RESULTS

Basal TC remained "immobile" in the nucleus; administration of cyclin-dependent kinase (CDK) inhibitors such as CGP60474 and dinaciclib (Dina) mobilized TC. "Mobile" TC shuttled between the nucleus and cytoplasm; however, it was eventually degraded through proteasomes. This dramatically suppressed the levels of TC-regulated transcripts and cell viability, promoted apoptosis, and reduced the area of metastatic lesions in the allograft model of EHE. We specifically identified that the inhibition of CDK9, a transcriptional CDK, destabilizes TC.

CONCLUSIONS

The CDK inhibitor Dina exhibited antitumorigenic properties both in vitro and in vivo in EHE models. Dina has been rigorously tested in clinical trials and displayed an acceptable toxicity profile. Therefore, there is a potential therapeutic window for repurposing Dina for the treatment of EHE.

Epithelioid hemangioendothelioma-its history, clinical features, molecular biology and current therapy

Epithelioid hemangioendothelioma (EHE) is a remarkably rare tumor arising from endothelial cells that is classified as a vascular tumor in the WHO classification. The tumor is predominantly characterized by the presence of fusion genes, such as WWTR1-CAMTA1 or YAP1-TFE3, with a minority of cases exhibiting other rare fusion genes. EHE exhibits a broad age of onset, typically presenting at ~50 years, but it is not uncommon in pediatric populations. It manifests in a variety of organs, including the liver, lung, soft tissue and bone. Initial multiple-organ involvement is also observed. The tumor's biological behavior and prognosis vary substantially based on the primary site of manifestation. From a therapeutic perspective, initial active surveillance might be considered in selected cases, although surgical intervention remains the mainstay of treatment, especially for localized single-organ involvement. Chemotherapy is administered to patients with progressive unresectable tumors. Recent advances in the biological analysis of EHE fusion genes have elucidated their diverse functions. Additionally, next-generation sequencing has facilitated the identification of other mutations beyond the fusion genes. These continuous efforts to understand the biology of the fusion genes themselves and/or the dysregulated signaling by fusion genes are expected to lead to the development of novel therapeutic strategies for EHE. This article aims to provide a comprehensive review of EHE, encompassing its historical context, clinical manifestations, molecular biology and the current state of treatment.

Direct and selective pharmacological disruption of the YAP-TEAD interface by IAG933 inhibits Hippo-dependent and RAS-MAPK-altered cancers

The YAP-TEAD protein-protein interaction mediates YAP oncogenic functions downstream of the Hippo pathway. To date, available YAP-TEAD pharmacologic agents bind into the lipid pocket of TEAD, targeting the interaction indirectly via allosteric changes. However, the consequences of a direct pharmacological disruption of the interface between YAP and TEADs remain largely unexplored. Here, we present IAG933 and its analogs as potent first-in-class and selective disruptors of the YAP-TEAD protein-protein interaction with suitable properties to enter clinical trials. Pharmacologic abrogation of the interaction with all four TEAD paralogs resulted in YAP eviction from chromatin and reduced Hippo-mediated transcription and induction of cell death. In vivo, deep tumor regression was observed in Hippo-driven mesothelioma xenografts at tolerated doses in animal models as well as in Hippo-altered cancer models outside mesothelioma. Importantly this also extended to larger tumor indications, such as lung, pancreatic and colorectal cancer, in combination with RTK, KRAS-mutant selective and MAPK inhibitors, leading to more efficacious and durable responses. Clinical evaluation of IAG933 is underway.

Natural compounds targeting YAP/TAZ axis in cancer: Current state of art and challenges

Cancer has become a burgeoning global healthcare concern marked by its exponential growth and significant economic ramifications. Though advancements in the treatment modalities have increased the overall survival and quality of life, there are no definite treatments for the advanced stages of this malady. Hence, understanding the diseases etiologies and the underlying molecular complexities, will usher in the development of innovative therapeutics. Recently, YAP/TAZ transcriptional regulation has been of immense interest due to their role in development, tissue homeostasis and oncogenic transformations. YAP/TAZ axis functions as coactivators within the Hippo signaling cascade, exerting pivotal influence on processes such as proliferation, regeneration, development, and tissue renewal. In cancer, YAP is overexpressed in multiple tumor types and is associated with cancer stem cell attributes, chemoresistance, and metastasis. Activation of YAP/TAZ mirrors the cellular "social" behavior, encompassing factors such as cell adhesion and the mechanical signals transmitted to the cell from tissue structure and the surrounding extracellular matrix. Therefore, it presents a significant vulnerability in the clogs of tumors that could provide a wide window of therapeutic effectiveness. Natural compounds have been utilized extensively as successful interventions in the management of diverse chronic illnesses, including cancer. Owing to their capacity to influence multiple genes and pathways, natural compounds exhibit significant potential either as adjuvant therapy or in combination with conventional treatment options. In this review, we delineate the signaling nexus of YAP/TAZ axis, and present natural compounds as an alternate strategy to target cancer.

Functional Classification of Fusion Proteins in Sarcoma

Sarcomas comprise a heterogeneous group of malignant tumors of mesenchymal origin. More than 80 entities are associated with different mesenchymal lineages. Sarcomas with fibroblastic, muscle, bone, vascular, adipocytic, and other characteristics are distinguished. Nearly half of all entities contain specific chromosomal translocations that give rise to fusion proteins. These are mostly pathognomonic, and their detection by various molecular techniques supports histopathologic classification. Moreover, the fusion proteins act as oncogenic drivers, and their blockade represents a promising therapeutic approach. This review summarizes the current knowledge on fusion proteins in sarcoma. We categorize the different fusion proteins into functional classes, including kinases, epigenetic regulators, and transcription factors, and describe their mechanisms of action. Interestingly, while fusion proteins acting as transcription factors are found in all mesenchymal lineages, the others have a more restricted pattern. Most kinase-driven sarcomas belong to the fibroblastic/myofibroblastic lineage. Fusion proteins with an epigenetic function are mainly associated with sarcomas of unclear differentiation, suggesting that epigenetic dysregulation leads to a major change in cell identity. Comparison of mechanisms of action reveals recurrent functional modes, including antagonism of Polycomb activity by fusion proteins with epigenetic activity and recruitment of histone acetyltransferases by fusion transcription factors of the myogenic lineage. Finally, based on their biology, we describe potential approaches to block the activity of fusion proteins for therapeutic intervention. Overall, our work highlights differences as well as similarities in the biology of fusion proteins from different sarcomas and provides the basis for a functional classification.

Primary vascular tumors of bone: A comprehensive literature review on classification, diagnosis and treatment

Primary vascular tumors of bone are a heterogeneous group of neoplasms, ranging from benign hemangiomas to frankly malignant epithelioid hemangioendotheliomas and angiosarcomas. Over the years, their classification has been a matter of discussion, due to morphologic similarities and uncertainty regarding biologic behavior. Over the past decade, with the development of next-generation sequencing, there has been a significant improvement in the molecular characterization of these lesions. The integration of their morphologic, immunohistochemical and molecular features has led to a better stratification, with important prognostic and therapeutic implications. Nevertheless, primary vascular bone tumors still represent a challenge for medical oncologists. Given their rarity and heterogeneity, in the last few years, there has been no significant progress in medical treatment options, so further research is needed. Here we present a review of the current knowledge regarding primary vascular tumors of the bone, correlating clinicopathologic features with tumor behavior and therapeutic approaches.

Epithelioid hemangioendothelioma (EHE) with WWTR1::TFE3 gene fusion, a novel fusion variant

Epithelioid hemangioendothelioma (EHE) is a rare endothelial sarcoma associated with a high incidence of metastases and for which there are no standard treatment options. Based on disease-defining mutations, most EHEs are classified into two subtypes: WWTR1::CAMTA1-fused EHE or YAP1::TFE3-fused EHE. However, rare non-canonical fusions have been identified in clinical samples of EHE cases and are challenging to classify. In this study, we report the identification of a novel WWTR1::TFE3 fusion variant in an EHE patient using targeted RNA sequencing. Histologically, the tumor exhibited hybrid morphological characteristics between WWTR1::CAMTA1-fused EHE and YAP1::TFE3-fused EHE. In addition to the driver fusion, there were six additional secondary mutations identified, including a loss-of-function FANCA mutation. Furthermore, in vitro studies were conducted to investigate the tumorigenic function of the WWTR1::TFE3 fusion protein in NIH3T3 cells and demonstrated that WWTR1::TFE3 promotes colony formation in soft agar. Finally, as the wild-type WWTR1 protein relies on binding the TEAD family of transcription factors to affect gene transcription, mutation of the WWTR1 domain of the fusion protein to inhibit such binding abrogates the transformative effect of WWTR1::TFE3. Overall, we describe a novel gene fusion in EHE with a hybrid histological appearance between the two major genetic subtypes of EHE. Further cases of this very rare subtype of EHE will need to be identified to fully elucidate the clinical and pathological characteristics of this unusual subtype of EHE.

The oncogenic fusion protein TAZ::CAMTA1 promotes genomic instability and senescence through hypertranscription

TAZ::CAMTA1 is a fusion protein found in over 90% of Epithelioid Hemangioendothelioma (EHE), a rare vascular sarcoma with an unpredictable disease course. To date, how TAZ::CAMTA1 initiates tumour formation remains unexplained. To study the oncogenic mechanism leading to EHE initiation, we developed a model system whereby TAZ::CAMTA1 expression is induced by doxycycline in primary endothelial cells. Using this model, we establish that upon TAZ::CAMTA1 expression endothelial cells rapidly enter a hypertranscription state, triggering considerable DNA damage. As a result, TC-expressing cells become trapped in S phase. Additionally, TAZ::CAMTA1-expressing endothelial cells have impaired homologous recombination, as shown by reduced BRCA1 and RAD51 foci formation. Consequently, the DNA damage remains unrepaired and TAZ::CAMTA1-expressing cells enter senescence. Knockout of Cdkn2a, the most common secondary mutation found in EHE, allows senescence bypass and uncontrolled growth. Together, this provides a mechanistic explanation for the clinical course of EHE and offers novel insight into therapeutic options.

Targeting Hippo signaling in cancer: novel perspectives and therapeutic potential

As highly conserved among diverse species, Hippo signaling pathway regulates various biological processes, including development, cell proliferation, stem cell function, tissue regeneration, homeostasis, and organ size. Studies in the last two decades have provided a good framework for how these fundamental functions of Hippo signaling are tightly regulated by a network with numerous intracellular and extracellular factors. The Hippo signaling pathway, when dysregulated, may lead to a wide variety of diseases, especially cancer. There is growing evidence demonstrating that dysregulated Hippo signaling is closely associated with tumorigenesis, cancer cell invasion, and migration, as well as drug resistance. Therefore, the Hippo pathway is considered an appealing therapeutic target for the treatment of cancer. Promising novel agents targeting the Hippo signaling pathway for cancers have recently emerged. These novel agents have shown antitumor activity in multiple cancer models and demonstrated therapeutic potential for cancer treatment. However, the detailed molecular basis of the Hippo signaling-driven tumor biology remains undefined. Our review summarizes current advances in understanding the mechanisms by which Hippo signaling drives tumorigenesis and confers drug resistance. We also propose strategies for future preclinical and clinical development to target this pathway.

Mesenchymal Neoplasms of the Liver

Mesenchymal neoplasms of the liver can be diagnostically challenging, particularly on core needle biopsies. Here, I discuss recent updates in neoplasms that are specific to the liver (mesenchymal hamartoma, undifferentiated embryonal sarcoma, calcifying nested stromal-epithelial tumor), vascular tumors of the liver (anastomosing hemangioma, hepatic small vessel neoplasm, epithelioid hemangioendothelioma, angiosarcoma), and other tumor types that can occur primarily in the liver (PEComa/angiomyolipoma, inflammatory pseudotumor-like follicular dendritic cell sarcoma, EBV-associated smooth muscle tumor, inflammatory myofibroblastic tumor, malignant rhabdoid tumor). Lastly, I discuss metastatic sarcomas to the liver, as well as pitfalls presented by metastatic melanoma and sarcomatoid carcinoma.

The spectrum and significance of secondary (co-occurring) genetic alterations in sarcomas: the hallmarks of sarcomagenesis

Bone and soft tissue tumors are generally classified into complex karyotype sarcomas versus those with recurrent genetic alterations, often in the form of gene fusions. In this review, we provide an overview of important co-occurring genomic alterations, organized by biological mechanisms and covering a spectrum of genomic alteration types: mutations (single-nucleotide variations or indels) in oncogenes or tumor suppressor genes, copy number alterations, transcriptomic signatures, genomic complexity indices (e.g. CINSARC), and complex genomic structural variants. We discuss the biological and prognostic roles of these so-called secondary or co-occurring alterations, arguing that recognition and detection of these alterations may be significant for our understanding and management of mesenchymal tumors. On a related note, we also discuss major recurrent alterations in so-called complex karyotype sarcomas. These secondary alterations are essential to sarcomagenesis via a variety of mechanisms, such as inactivation of tumor suppressors, activation of proliferative signal transduction, telomere maintenance, and aberrant regulation of epigenomic/chromatin remodeling players. The use of comprehensive genomic profiling, including targeted next-generation sequencing panels or whole-exome sequencing, may be incorporated into clinical workflows to offer more comprehensive, potentially clinically actionable information. © 2023 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of The Pathological Society of Great Britain and Ireland.

Searching for Novel Biomarkers in Thymic Epithelial Tumors: Immunohistochemical Evaluation of Hippo Pathway Components in a Cohort of Thymic Epithelial Tumors

Given the pivotal role of the Hippo pathway in different facets of tumorigenesis, which has been vigorously established in multiple heterogenous malignancies, we attempted to evaluate its potential utility as a prognostic-predictive biomarker in thymic epithelial tumors (TETs). For this purpose, we performed a comprehensive immunohistochemical analysis of four Hippo cascade components (YAP, TAZ, TEAD4 and LATS1) in a sizeable cohort of TETs and attempted to identify possible correlations of their H-score with various clinicopathological parameters. TAZ and TEAD4 displayed both cytoplasmic and nuclear immunoreactivity in almost equal frequency, with their cytoplasmic H-score being strongly associated with more aggressive high-grade tumors (type B3, thymic carcinoma) and more advanced pathological stages. On the other hand, a primarily nuclear staining pattern was encountered in both YAP and LATS1, with the YAP nuclear H-score being higher in more indolent (type A) and earlier stage tumors. Interestingly, none of the four examined factors displayed any statistically significant correlation with patient overall (OS) or disease-free survival (DFS). In summary, our results provide some initial insight into the expression profile of these core Hippo pathway components in thymic neoplasms and point towards some clear associations with tumor characteristics, which are of paramount translational-clinical research with profound implications in therapeutic targeting of this pathway in the context of precision medicine.

Current Model Systems for Investigating Epithelioid Haemangioendothelioma

Epithelioid haemangioendothelioma (EHE) is a rare sarcoma of the vascular endothelium with an unpredictable disease course. EHE tumours can remain indolent for long period of time but may suddenly evolve into an aggressive disease with widespread metastases and a poor prognosis. Two mutually exclusive chromosomal translocations define EHE tumours, each involving one of the transcription co-factors TAZ and YAP. The TAZ-CAMTA1 fusion protein results from a t(1;3) translocation and is present in 90% of EHE tumours. The remaining 10% of EHE cases harbour a t(X;11) translocation, resulting in the YAP1-TFE3 (YT) fusion protein. Until recently, the lack of representative EHE models made it challenging to study the mechanisms by which these fusion proteins promote tumorigenesis. Here, we describe and compare the recently developed experimental approaches that are currently available for studying this cancer. After summarising the key findings obtained with each experimental approach, we discuss the advantages and limitations of these different model systems. Our survey of the current literature shows how each experimental approach can be utilised in different ways to improve our understanding of EHE initiation and progression. Ultimately, this should lead to better treatment options for patients.

Leveraging Hot Spots of TEAD-Coregulator Interactions in the Design of Direct Small Molecule Protein-Protein Interaction Disruptors Targeting Hippo Pathway Signaling

The Hippo signaling pathway is a highly conserved pathway that plays important roles in the regulation of cell proliferation and apoptosis. Transcription factors TEAD1-4 and transcriptional coregulators YAP/TAZ are the downstream effectors of the Hippo pathway and can modulate Hippo biology. Dysregulation of this pathway is implicated in tumorigenesis and acquired resistance to therapies. The emerging importance of YAP/TAZ-TEAD interaction in cancer development makes it a potential therapeutic target. In the past decade, disrupting YAP/TAZ-TEAD interaction as an effective approach for cancer treatment has achieved great progress. This approach followed a trajectory wherein peptidomimetic YAP-TEAD protein-protein interaction disruptors (PPIDs) were first designed, followed by the discovery of allosteric small molecule PPIDs, and currently, the development of direct small molecule PPIDs. YAP and TEAD form three interaction interfaces. Interfaces 2 and 3 are amenable for direct PPID design. One direct YAP-TEAD PPID (IAG933) that targets interface 3 has entered a clinical trial in 2021. However, in general, strategically designing effective small molecules PPIDs targeting TEAD interfaces 2 and 3 has been challenging compared with allosteric inhibitor development. This review focuses on the development of direct surface disruptors and discusses the challenges and opportunities for developing potent YAP/TAZ-TEAD inhibitors for the treatment of cancer.

The Recent Advances in Molecular Diagnosis of Soft Tissue Tumors

Soft tissue tumors are rare mesenchymal tumors with divergent differentiation. The diagnosis of soft tissue tumors is challenging for pathologists owing to the diversity of tumor types and histological overlap among the tumor entities. Present-day understanding of the molecular pathogenesis of soft tissue tumors has rapidly increased with the development of molecular genetic techniques (e.g., next-generation sequencing). Additionally, immunohistochemical markers that serve as surrogate markers for recurrent translocations in soft tissue tumors have been developed. This review aims to provide an update on recently described molecular findings and relevant novel immunohistochemical markers in selected soft tissue tumors.

Therapeutic targeting of TEAD transcription factors in cancer

The Hippo signaling pathway inhibits the activity of the oncogenic YAP (Yes-associated protein)/TAZ (transcriptional co-activator with PDZ-binding motif)-TEAD (TEA/ATTS domain) transcriptional complex. In cancers, inactivating mutations in upstream Hippo components and/or enhanced activity of YAP/TAZ and TEAD have been observed. The activity of this transcriptional complex can be effectively inhibited by targeting the TEAD family of transcription factors. The development of TEAD inhibitors has been driven by the discovery that TEAD has druggable hydrophobic pockets, and is currently at the clinical development stage. Three small molecule TEAD inhibitors are currently being tested in Phase I clinical trials. In this review, we highlight the role of TEADs in cancer, discuss various avenues through which TEAD activity can be inhibited, and outline the opportunities for the administration of TEAD inhibitors.

Targeting Breast Cancer Stem Cells

The potential roles of breast cancer stem cells (BCSCs) in tumor initiation and recurrence have been recognized for many decades. Due to their strong capacity for self-renewal and differentiation, BCSCs are the major reasons for poor clinical outcomes and low therapeutic response. Several hypotheses on the origin of cancer stem cells have been proposed, including critical gene mutations in stem cells, dedifferentiation of somatic cells, and cell plasticity remodeling by epithelial-mesenchymal transition (EMT) and the tumor microenvironment. Moreover, the tumor microenvironment, including cellular components and cytokines, modulates the self-renewal and therapeutic resistance of BCSCs. Small molecules, antibodies, and chimeric antigen receptor (CAR)-T cells targeting BCSCs have been developed, and their applications in combination with conventional therapies are undergoing clinical trials. In this review, we focus on the features of BCSCs, emphasize the major factors and tumor environment that regulate the stemness of BCSCs, and discuss potential BCSC-targeting therapies.

Genetic Alterations and Deregulation of Hippo Pathway as a Pathogenetic Mechanism in Bone and Soft Tissue Sarcoma

The Hippo pathway is an evolutionarily conserved modulator of developmental biology with a key role in tissue and organ size regulation under homeostatic conditions. Like other signaling pathways with a significant role in embryonic development, the deregulation of Hippo signaling contributes to oncogenesis. Central to the Hippo pathway is a conserved cascade of adaptor proteins and inhibitory kinases that converge and regulate the activity of the oncoproteins YAP and TAZ, the final transducers of the pathway. Elevated levels and aberrant activation of YAP and TAZ have been described in many cancers. Though most of the studies describe their pervasive activation in epithelial neoplasms, there is increasing evidence pointing out its relevance in mesenchymal malignancies as well. Interestingly, somatic or germline mutations in genes of the Hippo pathway are scarce compared to other signaling pathways that are frequently disrupted in cancer. However, in the case of sarcomas, several examples of genetic alteration of Hippo members, including gene fusions, have been described during the last few years. Here, we review the current knowledge of Hippo pathway implication in sarcoma, describing mechanistic hints recently reported in specific histological entities and how these alterations represent an opportunity for targeted therapy in this heterogeneous group of neoplasm.

TAZ/YAP fusion proteins: mechanistic insights and therapeutic opportunities

The Hippo pathway is dysregulated in many different cancers, but point mutations in the pathway are rare. Transcriptional co-activator with PDZ-binding motif (TAZ) and Yes-associated protein (YAP) fusion proteins have emerged in almost all major cancer types and represent the most common genetic mechanism by which the two transcriptional co-activators are activated. Given that the N termini of TAZ or YAP are fused to the C terminus of another transcriptional regulator, the resultant fusion proteins hyperactivate a TEAD transcription factor-based transcriptome. Recent advances show that the C-terminal fusion partners confer oncogenic properties to TAZ/YAP fusion proteins by recruiting epigenetic modifiers that promote a hybrid TEAD-based transcriptome. Elucidating these cooperating epigenetic complexes represents a strategy to identify new therapeutic approaches for a pathway that has been recalcitrant to medical therapy.

Structure-based discovery of a novel small-molecule inhibitor of TEAD palmitoylation with anticancer activity

The paralogous oncogenic transcriptional coactivators YAP and TAZ are the distal effectors of the Hippo signaling pathway, which plays a critical role in cell proliferation, survival and cell fate specification. They are frequently deregulated in most human cancers, where they contribute to multiple aspects of tumorigenesis including growth, metabolism, metastasis and chemo/immunotherapy resistance. Thus, they provide a critical point for therapeutic intervention. However, due to their intrinsically disordered structure, they are challenging to target directly. Since YAP/TAZ exerts oncogenic activity by associating with the TEAD1-4 transcription factors, to regulate target gene expression, YAP activity can be controlled indirectly by regulating TEAD1-4. Interestingly, TEADs undergo autopalmitoylation, which is essential for their stability and function, and small-molecule inhibitors that prevent this posttranslational modification can render them unstable. In this article we report discovery of a novel small molecule inhibitor of YAP activity. We combined structure-based virtual ligand screening with biochemical and cell biological studies and identified JM7, which inhibits YAP transcriptional reporter activity with an IC50 of 972 nMoles/Ltr. Further, it inhibits YAP target gene expression, without affecting YAP/TEAD localization. Mechanistically, JM7 inhibits TEAD palmitoylation and renders them unstable. Cellular thermal shift assay revealed that JM7 directly binds to TEAD1-4 in cells. Consistent with the inhibitory effect of JM7 on YAP activity, it significantly impairs proliferation, colony-formation and migration of mesothelioma (NCI-H226), breast (MDA-MB-231) and ovarian (OVCAR-8) cancer cells that exhibit increased YAP activity. Collectively, these results establish JM7 as a novel lead compound for development of more potent inhibitors of TEAD palmitoylation for treating cancer.

Updated information regarding management of hepatic epithelioid hemangioendothelioma

Hepatic epithelioid hemangioendothelioma (HEHE) is a rare hepatic vascular tumor with a borderline biological behavior between hemangioma and hemangiosarcoma. It tends to be multiple or diffuse subcapsular lesions across the liver but has no characteristic clinical manifestations or imaging findings. On computed tomography and magnetic resonance imaging, these lesions usually have a hypodense appearance with heterogeneous enhancement and a "halo sign" or "lollipop sign" may be evident in some cases. HEHE is diagnosed mainly based on a pathological examination along with differential immunohistochemical markers such as CAMTA1, CD31, CD34, CD10, vimentin, and factor VIII antigen. Currently, there are no standardized treatment guidelines for HEHE, and surgery (curative resection and liver transplantation) remains the mainstay of treatment. Studies have indicated that extra-hepatic metastasis might not be a contraindication for resection or transplantation. Systemic chemotherapeutic agents including doxorubicin, vincristine, interferon-a, 5-fluorouracil, and thalidomide, as well as VEGF-related agents are being investigated, but no agents have been approved for the treatment of HEHE.

The TAZ-CAMTA1 Fusion Protein Promotes Tumorigenesis via Connective Tissue Growth Factor and Ras-MAPK Signaling in Epithelioid Hemangioendothelioma

PURPOSE

A consistent genetic alteration in vascular cancer epithelioid hemangioendothelioma (EHE) is the t(1;3)(p36;q25) chromosomal translocation, which generates a WWTR1(TAZ)-CAMTA1 (TC) fusion gene. TC is a transcriptional coactivator that drives EHE. Here, we aimed to identify the TC transcriptional targets and signaling mechanisms that underlie EHE tumorigenesis.

EXPERIMENTAL DESIGN

We used NIH3T3 cells transformed with TC (NIH3T3/TC) as a model system to uncover TC-dependent oncogenic signaling. These cells proliferated in an anchorage-independent manner in suspension and soft agar. The findings of the cell-based studies were validated in a xenograft model.

RESULTS

We identified connective tissue growth factor (CTGF) as a tumorigenic transcriptional target of TC. We show that CTGF binds to integrin αIIbβ3, which is essential for sustaining the anchorage-independent proliferation of transformed NIH3T3/TC cells. NIH3T3/TC cells also have enhanced Ras and MAPK signaling, and the activity of these pathways is reduced upon CTGF knockdown, suggesting that CTGF signaling occurs via the Ras-MAPK cascade. Further, pharmacologic inhibition of MAPK signaling through PD 0325901 and trametinib abrogated TC-driven anchorage-independent growth. Likewise, for tumor growth in vivo, NIH3T3/TC cells require CTGF and MAPK signaling. NIH3T3/TC xenograft growth was profoundly reduced upon CTGF knockdown and after trametinib treatment.

CONCLUSIONS

Collectively, our results demonstrated that CTGF and the Ras-MAPK signaling cascade are essential for TC-mediated tumorigenesis. These studies provided the preclinical rationale for SARC033 (NCI 10015-NCT03148275), a nonrandomized, open-label, phase II study of trametinib in patients with unresectable or metastatic EHE.

Unraveling the Biology of Epithelioid Hemangioendothelioma, a TAZ-CAMTA1 Fusion Driven Sarcoma

Simple Summary

Epithelioid hemangioendothelioma (EHE) is a rare vascular cancer that involves a gain-of-function gene fusion involving TAZ, a transcriptional coactivator, and one of two end effectors of the Hippo pathway. Although the activity of TAZ and/or YAP, a paralog of TAZ, is consistently altered in many cancers, genetic alterations involving YAP/TAZ are rare, and the precise mechanisms by which YAP/TAZ are activated are not well understood in most cancers. Because WWTR1(TAZ)–CAMTA1 is the only genetic alteration in approximately half of EHE, EHE is a genetically clean and homogenous system for understanding how the dysregulation of TAZ promotes tumorigenesis. Therefore, by using EHE as a model system, we hope to elucidate the essential biological pathways mediated by TAZ and identify mechanisms to target them. The findings of EHE research can be applied to other cancers that are addicted to high YAP/TAZ activity.

Abstract

The activities of YAP and TAZ, the end effectors of the Hippo pathway, are consistently altered in cancer, and this dysregulation drives aggressive tumor phenotypes. While the actions of these two proteins aid in tumorigenesis in the majority of cancers, the dysregulation of these proteins is rarely sufficient for initial tumor development. Herein, we present a unique TAZ-driven cancer, epithelioid hemangioendothelioma (EHE), which harbors a WWTR1(TAZ)–CAMTA1 gene fusion in at least 90% of cases. Recent investigations have elucidated the mechanisms by which YAP/TAP-fusion oncoproteins function and drive tumorigenesis. This review presents a critical evaluation of this recent work, with a particular focus on how the oncoproteins alter the normal activity of TAZ and YAP, and, concurrently, we generate a framework for how we can target the gene fusions in patients. Since EHE represents a paradigm of YAP/TAZ dysregulation in cancer, targeted therapies for EHE may also be effective against other YAP/TAZ-dependent cancers.

Overview of the roles of calcium sensors in plants’ response to osmotic stress signalling

Calcium signals serve an important function as secondary messengers between cells in various biological processes due to their robust homeostatic mechanism, maintaining an intracellular free Ca2+ concentration. Plant growth, development, and biotic and abiotic stress are all regulated by Ca2+ signals. Ca2+ binding proteins decode and convey the messages encoded by Ca2+ ions. In the presence of high quantities of Mg2+ and monovalent cations, such sensors bind to Ca2+ ions and modify their conformation in a Ca2+ -dependent manner. Calcium-dependent protein kinases (CPKs), calmodulins (CaMs), and calcineurin B-like proteins are all calcium sensors (CBLs). To transmit Ca2+ signals, CPKs, CBLs, and CaMs interact with target proteins and regulate the expression of their genes. These target proteins may be protein kinases, metabolic enzymes, or cytoskeletal-associated proteins. Beyond its role in plant nutrition as a macroelement and its involvement in the plant cell wall structure, calcium modulates many aspects of development, growth and adaptation to environmental constraints such as drought, salinity and osmotic stresses. This review summarises current knowledge on calcium sensors in plant responses to osmotic stress signalling.

Aurintricarboxylic acid is a canonical disruptor of the TAZ-TEAD transcriptional complex

Disrupting the formation of the oncogenic YAP/TAZ-TEAD transcriptional complex holds substantial therapeutic potential. However, the three protein interaction interfaces of this complex cannot be easily disrupted using small molecules. Here, we report that the pharmacologically active small molecule aurintricarboxylic acid (ATA) acts as a disruptor of the TAZ-TEAD complex. ATA was identified in a high-throughput screen using a TAZ-TEAD AlphaLISA assay that was tailored to identify disruptors of this transcriptional complex. We further used fluorescence polarization assays both to confirm disruption of the TAZ-TEAD complex and to demonstrate that ATA binds to interface 3. We have previously shown that cell-based models that express the oncogenic TAZ-CAMTA1 (TC) fusion protein display enhanced TEAD transcriptional activity because TC functions as an activated form of TAZ. Utilizing cell-based studies and our TC model system, we performed TC/TEAD reporter, RNA-Seq, and qPCR assays and found that ATA inhibits TC/TEAD transcriptional activity. Further, disruption of TC/TEAD and TAZ/TEAD interaction by ATA abrogated anchorage-independent growth, the phenotype most closely linked to dysregulated TAZ/TEAD activity. Therefore, this study demonstrates that ATA is a novel small molecule that has the ability to disrupt the undruggable TAZ-TEAD interface.

Diagnosis of soft tissue tumors using immunohistochemistry as a surrogate for recurrent fusion oncoproteins

Soft tissue neoplasms encompass a broad spectrum of clinicopathologic manifestations. In a subset of soft tissue tumors, spanning a wide range of clinical behavior from indolent to highly aggressive, recurrent genetic translocations yield oncogenic fusion proteins that drive neoplastic growth. Beyond functioning as primary mechanisms of tumorigenesis, recurrent translocations represent key diagnostic features insofar as the presence of a particular oncogenic gene fusion generally points to specific tumor entities. In addition to more direct methods for identifying recurrent translocations, such as conventional cytogenetics or fluorescence in situ hybridization, immunohistochemistry for a component of the fusion oncoprotein increasingly is being used as a surrogate marker, exploiting the tendency of these fusion components to be distinctively overexpressed by translocation-bearing tumor cells. Diagnostic immunohistochemistry can also be used to identify the characteristic gene expression changes that occur downstream of oncogenic fusions. Here, we review the use of immunohistochemistry to detect surrogate markers of recurrent translocations in soft tissue tumors, focusing on the practical applications and limitations of this diagnostic approach.

The RNA binding protein QKI5 suppresses ovarian cancer via downregulating transcriptional coactivator TAZ

RNA-binding proteins (RBPs) are a set of proteins involved in many steps of post-transcriptional regulation to maintain cellular homeostasis. Ovarian cancer (OC) is the most deadly gynecological cancer, but the roles of RBPs in OC are not fully understood. Here, we reported that the RBP QKI5 was significantly negatively correlated with aggressive tumor stage and worse prognosis in serous OC patients. QKI5 could suppress the growth and metastasis of OC cells both in vitro and in vivo. Transcriptome analysis showed that QKI5 negatively regulated the expression of the transcriptional coactivator TAZ and its downstream targets (e.g., CTGF and CYR61). Mechanistically, QKI5 bound to TAZ mRNA and recruited EDC4, thus decreasing the stability of TAZ mRNA. Functionally, TAZ was involved in the QKI5-mediated tumor suppression of OC cells, and QKI5 expression was inversely correlated with TAZ, CTGF, and CYR61 expression in OC patients. Together, our study indicates that QKI5 plays a tumor-suppressive role and negatively regulates TAZ expression in OC.

YAP1-TFE3-fused hemangioendothelioma: a multi-institutional clinicopathologic study of 24 genetically-confirmed cases

YAP1-TFE3-fused hemangioendothelioma is an extremely rare malignant vascular tumor. We present the largest multi-institutional clinicopathologic study of YAP1-TFE3-fused hemangioendothelioma to date. The 24 cases of YAP1-TFE3-fused hemangioendothelioma showed a female predominance (17 female, 7 male) across a wide age range (20-78 years old, median 44). Tumors were most commonly located in soft tissue (50%), followed by bone (29%), lung (13%), and liver (8%), ranging from 3 to 115 mm in size (median 40 mm). About two-thirds presented with multifocal disease, including 7 cases with distant organ metastasis. Histopathologically, we describe three dominant architectural patterns: solid sheets of coalescing nests, pseudoalveolar and (pseudo)vasoformative pattern, and discohesive strands and clusters of cells set in a myxoid to myxohyaline stroma. These patterns were present in variable proportions across different tumors and often coexisted within the same tumor. The dominant cytomorphology (88%) was large epithelioid cells with abundant, glassy eosinophilic to vacuolated cytoplasm, prominent nucleoli and well-demarcated cell borders. Multinucleated or binucleated cells, prominent admixed erythrocytic and lymphocytic infiltrates, and intratumoral fat were frequently present. Immunohistochemically, ERG, CD31, and TFE3 were consistently expressed, while expression of CD34 (83%) and cytokeratin AE1/AE3 (20%) was variable. CAMTA1 was negative in all but one case. All cases were confirmed by molecular testing to harbor YAP1-TFE3 gene fusions: majority with YAP1 exon 1 fused to TFE3 exon 4 (88%), or less commonly, TFE3 exon 6 (12%). Most patients (88%) were treated with primary surgical resection. Over a follow-up period of 4-360 months (median 36 months) in 17 cases, 35% of patients remained alive without disease, and 47% survived many years with stable, albeit multifocal and/or metastatic disease. Five-year progression-free survival probability was 88%. We propose categorizing YAP1-TFE3-fused hemangioendothelioma as a distinct disease entity given its unique clinical and histopathologic characteristics in comparison to conventional epithelioid hemangioendothelioma.

Gene fusions in vascular tumors and their underlying molecular mechanisms

INTRODUCTION

The group of vascular tumors contains many different entities, and is considered difficult by pathologists, as they often have overlapping histological characteristics. Chromosomal translocations have been identified in ~20% of mesenchymal tumors and are considered the drivers of tumor formation. Many translocations have been discovered over the past decade through next-generation sequencing. This technological advancement has also revealed several recurrent gene fusions in vascular tumors.

AREAS COVERED

This review will discuss the various vascular tumors for which recurrent gene fusions have been identified. The gene fusions and the presumed molecular mechanisms underlying tumorigenesis are shown, and potential implications for targeted therapies discussed. The identification of these gene fusions in vascular tumors has improved diagnostic accuracy, especially since several of these fusions can be easily detected using surrogate immunohistochemical markers.

EXPERT OPINION

The identification of gene fusions in a subset of vascular tumors over the past decade has improved diagnostic accuracy, and has provided the pathologists with novel diagnostic tools to accurately diagnose these often difficult tumors. Moreover, the increased understanding of the underlying molecular mechanisms can guide the development of targeted therapeutic strategies.

YAP1-TFE3 gene fusion variant in clear cell stromal tumour of lung: report of two cases in support of a distinct entity

AIMS

Clear cell (haemangioblastoma-like) stromal tumour of the lung is a newly described, rare pulmonary neoplasm. Recurrent YAP1-TFE3 gene fusions have recently been reported in three cases. We describe two additional cases and confirm the characteristic YAP1-TFE3 gene fusion.

METHODS AND RESULTS

Two mesenchymal tumours of lung were identified from our soft tissue pathology consultation services and RNA sequencing was performed. Both cases were in male patients, aged 35 and 77 years. Both presented as solitary lung nodules measuring 3.9 and 7.5 cm in greatest dimension. Histopathologically, the tumours were composed of epithelioid to plump spindle cells arranged in packets and solid sheets. The cells showed fusiform to ovoid nuclei with open chromatin, variably prominent nucleoli and scant to moderate, clear to eosinophilic cytoplasm. Cytological atypia and significant mitotic activity were minimal. None of the tumours expressed lineage-specific immunophenotypical markers. Both cases were diffusely positive for nuclear TFE3. Unlike YAP1-TFE3-fused epithelioid haemangioendothelioma, for which the fusion breakpoint occurs in YAP1 exon 1 and TFE3 exons 4 or 6, the fusion breakpoints of these tumours were located in YAP1 exon 4 and TFE3 exon 7. Following complete surgical resection, neither of the tumours has recurred or metastasised (follow-up period 6-7 months).

CONCLUSIONS

We validate the presence of YAP1-TFE3 gene fusion in a unique primary mesenchymal tumour of lung, adding additional support for clear cell stromal tumour of the lung as a distinct entity.

Epithelioid hemangioendothelioma, an ultra-rare cancer: a consensus paper from the community of experts

Epithelioid hemangioendothelioma (EHE) is an ultra-rare, translocated, vascular sarcoma. EHE clinical behavior is variable, ranging from that of a low-grade malignancy to that of a high-grade sarcoma and it is marked by a high propensity for systemic involvement. No active systemic agents are currently approved specifically for EHE, which is typically refractory to the antitumor drugs used in sarcomas. The degree of uncertainty in selecting the most appropriate therapy for EHE patients and the lack of guidelines on the clinical management of the disease make the adoption of new treatments inconsistent across the world, resulting in suboptimal outcomes for many EHE patients. To address the shortcoming, a global consensus meeting was organized in December 2020 under the umbrella of the European Society for Medical Oncology (ESMO) involving >80 experts from several disciplines from Europe, North America and Asia, together with a patient representative from the EHE Group, a global, disease-specific patient advocacy group, and Sarcoma Patient EuroNet (SPAEN). The meeting was aimed at defining, by consensus, evidence-based best practices for the optimal approach to primary and metastatic EHE. The consensus achieved during that meeting is the subject of the present publication.

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This consensus paper provides key recommendations on the management of epithelioid hemangioendothelioma (EHE).

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Recommendations followed a consensus meeting between experts and a representative of the EHE advocacy group and SPAEN.

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Authorship includes a multidisciplinary group of experts from different institutions from Europe, North America and Asia.

Toward a Personalized Therapy in Soft-Tissue Sarcomas: State of the Art and Future Directions

Soft-tissue sarcomas are rare tumors characterized by pathogenetic, morphological, and clinical intrinsic variability. Median survival of patients with advanced tumors are usually chemo- and radio-resistant, and standard treatments yield low response rates and poor survival results. The identification of defined genomic alterations in sarcoma could represent the premise for targeted treatments. Summarizing, soft-tissue sarcomas can be differentiated into histotypes with reciprocal chromosomal translocations, with defined oncogenic mutations and complex karyotypes. If the latter are improbably approached with targeted treatments, many suggest that innovative therapies interfering with the identified fusion oncoproteins and altered pathways could be potentially resolutive. In most cases, the characteristic genetic signature is discouragingly defined as "undruggable", which poses a challenge for the development of novel pharmacological approaches. In this review, a summary of genomic alterations recognized in most common soft-tissue sarcoma is reported together with current and future therapeutic opportunities.

TAZ-CAMTA1 and YAP-TFE3 alter the TAZ/YAP transcriptome by recruiting the ATAC histone acetyltransferase complex

Epithelioid hemangioendothelioma (EHE) is a vascular sarcoma that metastasizes early in its clinical course and lacks an effective medical therapy. The TAZ-CAMTA1 and YAP-TFE3 fusion proteins are chimeric transcription factors and initiating oncogenic drivers of EHE. A combined proteomic/genetic screen in human cell lines identified YEATS2 and ZZZ3, components of the Ada2a-containing histone acetyltransferase (ATAC) complex, as key interactors of both fusion proteins despite the dissimilarity of the C terminal fusion partners CAMTA1 and TFE3. Integrative next-generation sequencing approaches in human and murine cell lines showed that the fusion proteins drive a unique transcriptome by simultaneously hyperactivating a TEAD-based transcriptional program and modulating the chromatin environment via interaction with the ATAC complex. Interaction of the ATAC complex with both fusion proteins indicates that it is a key oncogenic driver and unifying enzymatic therapeutic target for this sarcoma. This study presents an approach to mechanistically dissect how chimeric transcription factors drive the formation of human cancers.

The proliferation of human cells is tightly regulated to ensure that enough cells are made to build and repair organs and tissues, while at the same time stopping cells from dividing uncontrollably and damaging the body. To get the right balance, cells rely on physical and chemical cues from their environment that trigger the biochemical signals that regulate two proteins called TAZ and YAP. These proteins control gene activity by regulating the rate at which genes are copied to produce proteins. If this process becomes dysregulated, cells can grow uncontrollably, causing cancer.

In cancer cells, it is common to find TAZ and YAP fused to other proteins. In epithelioid hemangioendothelioma, a rare cancer that grows in the blood vessels, cancerous growth can be driven by a version of TAZ fused to the protein CAMTA1, or a version of YAP fused to the protein TFE3. While the role of TAZ and YAP in promoting gene activity is known, it is unclear how CAMTA1 and TFE3 contribute to cell growth becoming dysregulated.

Merritt, Garcia et al. studied sarcoma cell lines to show that these two fusion proteins, TAZ-CAMTA1 and YAP-TFE3, change the pattern of gene activity seen in the cells compared to TAZ or YAP alone. An analysis of molecules that interact with the two fusion proteins identified a complex called ATAC as the cause of these changes. This complex adds chemical markers to DNA-packaging proteins, which control which genes are available for activation. The fusion proteins combine the ability of TAZ and YAP to control gene activity and the ability of CAMTA1 and TFE3 to make DNA more accessible, allowing the fusion proteins to drive uncontrolled cancerous growth.

Similar TAZ and YAP fusion proteins have been found in other cancers, which can activate genes and potentially alter DNA packaging. Targeting drug development efforts at the proteins that complex with TAZ and YAP fusion proteins may lead to new therapies.

Prognostic and therapeutic value of the Hippo pathway, RABL6A, and p53-MDM2 axes in sarcomas

Additional prognostic and therapeutic biomarkers effective across different histological types of sarcoma are needed. Herein we evaluate expression of TAZ and YAP, the p53-MDM2 axis, and RABL6A, a novel oncoprotein with potential ties to both pathways, in sarcomas of different histological types. Immunohistochemical staining of a tissue microarray including 163 sarcomas and correlation with clinical data showed that elevated YAP and TAZ independently predict worse overall and progression-free survival, respectively. In the absence of p53 expression, combined TAZ and YAP expression adversely affect overall, progression free, and metastasis free survival more than TAZ or YAP activation alone. RABL6A independently predicted shorter time to metastasis and was positively correlated with p53, MDM2 and YAP expression, supporting a possible functional relationship between the biomarkers. Network analysis further showed that TAZ is positively correlated with MDM2 expression. The data implicate all five proteins as clinically relevant downstream players in the Hippo pathway. Finally, a novel inhibitor of MDM2 (MA242), effectively suppressed the survival of sarcoma cell lines from different histological types regardless of p53 status. These findings suggest both independent and cooperative roles for all five biomarkers across different histological types of sarcoma in predicting patient outcomes and potentially guiding future therapeutic approaches.

WWTR1(TAZ)-CAMTA1 gene fusion is sufficient to dysregulate YAP/TAZ signaling and drive epithelioid hemangioendothelioma tumorigenesis

Epithelioid hemangioendothelioma (EHE) is a genetically homogenous vascular sarcoma that is a paradigm for TAZ dysregulation in cancer. EHE harbors a WWTR1(TAZ)-CAMTA1 gene fusion in >90% of cases, 45% of which have no other genetic alterations. In this study, we used a first of its kind approach to target the Wwtr1-Camta1 gene fusion to the Wwtr1 locus, to develop a conditional EHE mouse model whereby Wwtr1-Camta1 is controlled by the endogenous transcriptional regulators upon Cre activation. These mice develop EHE tumors that are indistinguishable from human EHE clinically, histologically, immunohistochemically, and genetically. Overall, these results demonstrate unequivocally that TAZ-CAMTA1 is sufficient to drive EHE formation with exquisite specificity, as no other tumor types were observed. Furthermore, we fully credential this unique EHE mouse model as a valid preclinical model for understanding the role of TAZ dysregulation in cancer formation and for testing therapies directed at TAZ-CAMTA1, TAZ, and YAP/TAZ signaling.

WWTR1(TAZ)-CAMTA1 reprograms endothelial cells to drive epithelioid hemangioendothelioma

In this study, Driskill et al. studied whether the TAZ-CAMTA1 gene fusion is a driver of epithelioid hemangioendothelioma (EHE), a poorly understood and devastating vascular cancer. They show that TAZ-CAMTA1 expression in endothelial cells is sufficient to drive the formation of vascular tumors with the distinctive features of EHE, and inhibition of TAZ-CAMTA1 results in the regression of these vascular tumors, and their findings provide the first genetic model of a TAZ fusion oncoprotein driving its associated human cancer, pinpointing TAZ-CAMTA1 as the key driver and a valid therapeutic target of EHE.

Epithelioid hemangioendothelioma (EHE) is a poorly understood and devastating vascular cancer. Sequencing of EHE has revealed a unique gene fusion between the Hippo pathway nuclear effector TAZ (WWTR1) and the brain-enriched transcription factor CAMTA1 in ∼90% of cases. However, it remains unclear whether the TAZ-CAMTA1 gene fusion is a driver of EHE, and potential targeted therapies are unknown. Here, we show that TAZ-CAMTA1 expression in endothelial cells is sufficient to drive the formation of vascular tumors with the distinctive features of EHE, and inhibition of TAZ-CAMTA1 results in the regression of these vascular tumors. We further show that activated TAZ resembles TAZ-CAMTA1 in driving the formation of EHE-like vascular tumors, suggesting that constitutive activation of TAZ underlies the pathological features of EHE. We show that TAZ-CAMTA1 initiates an angiogenic and regenerative-like transcriptional program in endothelial cells, and disruption of the TAZ-CAMTA1-TEAD interaction or ectopic expression of a dominant negative TEAD in vivo inhibits TAZ-CAMTA1-mediated transformation. Our study provides the first genetic model of a TAZ fusion oncoprotein driving its associated human cancer, pinpointing TAZ-CAMTA1 as the key driver and a valid therapeutic target of EHE.

New insights into the pathophysiology and clinical care of rare primary liver cancers

Hepatocholangiocarcinoma, fibrolamellar carcinoma, hepatic haemangioendothelioma and hepatic angiosarcoma represent less than 5% of primary liver cancers. Fibrolamellar carcinoma and hepatic haemangioendothelioma are driven by unique somatic genetic alterations (DNAJB1-PRKCA and CAMTA1-WWTR1 fusions, respectively), while the pathogenesis of hepatocholangiocarcinoma remains more complex, as suggested by its histological diversity. Histology is the gold standard for diagnosis, which remains challenging even in an expert centre because of the low incidences of these liver cancers. Resection, when feasible, is the cornerstone of treatment, together with liver transplantation for hepatic haemangioendothelioma. The role of locoregional therapies and systemic treatments remains poorly studied. In this review, we aim to describe the recent advances in terms of diagnosis and clinical management of these rare primary liver cancers.

Recurrent YAP1 and MAML2 Gene Rearrangements in Retiform and Composite Hemangioendothelioma

Retiform and composite hemangioendotheliomas (CHEs) are both locally aggressive, rarely metastasizing vascular neoplasms characterized by arborizing vascular channels lined by endothelial cells with a hobnail morphology. CHE displays additional cytologic and architectural components, including often vacuolated epithelioid cells, solid areas, or features reminiscent of well-differentiated angiosarcoma. Triggered by an index case of a soft tissue retiform hemangioendothelioma (RHE) which revealed a YAP1-MAML2 gene fusion by targeted RNA sequencing, we sought to investigate additional cases in this morphologic spectrum for this genetic abnormality. A total of 24 cases, 13 RHE and 11 CHE involving skin and soft tissue were tested by fluorescence in situ hybridization using custom BAC probes for rearrangements involving these genes. An additional visceral CHE with neuroendocrine differentiation was tested by targeted RNA sequencing. Among the soft tissue cohort, 5/13 (38%) RHE and 3/11 (27%) CHE showed YAP1 gene rearrangements, with 5 cases showing a YAP1-MAML2 fusion, including all 3 CHE. The single neuroendocrine CHE showed the presence of a PTBP1-MAML2 fusion. All YAP1-positive CHE lesions occurred in female children at acral sites, compared with fusion-negative cases which occurred in adults, with a wide anatomic distribution. YAP1-positive RHE occurred preferentially in males and lower limb, compared with negative cases. These results suggest that RHE and CHE represent a morphologic continuum, sharing abnormalities in YAP1 and MAML2 genes. In contrast, the neuroendocrine CHE occurring in a 37-year-old male harbored a distinct PTBP1-MAML2 fusion and showed aggressive clinical behavior (pancreatic mass with multiple liver and lung metastases). These preliminary findings raise the possibility that neuroendocrine CHE may be genetically distinct from the conventional RHE/CHE spectrum. Further studies are needed to investigate the pathogenetic relationship of fusion-negative cases with this subset and, less likely, with other members of the HE family of tumors.

YAP1 and its fusion proteins in cancer initiation, progression and therapeutic resistance

YAP1 is a transcriptional co-activator whose activity is controlled by the Hippo signaling pathway. In addition to important functions in normal tissue homeostasis and regeneration, YAP1 has also prominent functions in cancer initiation, aggressiveness, metastasis, and therapy resistance. In this review we are discussing the molecular functions of YAP1 and its roles in cancer, with a focus on the different mechanisms of de-regulation of YAP1 activity in human cancers, including inactivation of upstream Hippo pathway tumor suppressors, regulation by intersecting pathways, miRNAs, and viral oncogenes. We are also discussing new findings on the function and biology of the recently identified family of YAP1 gene fusions, that constitute a new type of activating mutation of YAP1 and that are the likely oncogenic drivers in several subtypes of human cancers. Lastly, we also discuss different strategies of therapeutic inhibition of YAP1 functions.

Identification of a subset of immunosuppressive P2RX1-negative neutrophils in pancreatic cancer liver metastasis

The immunosuppressive microenvironment that is shaped by hepatic metastatic pancreatic ductal adenocarcinoma (PDAC) is essential for tumor cell evasion of immune destruction. Neutrophils are important components of the metastatic tumor microenvironment and exhibit heterogeneity. However, the specific phenotypes, functions and regulatory mechanisms of neutrophils in PDAC liver metastases remain unknown. Here, we show that a subset of P2RX1-negative neutrophils accumulate in clinical and murine PDAC liver metastases. RNA sequencing of murine PDAC liver metastasis-infiltrated neutrophils show that P2RX1-deficient neutrophils express increased levels of immunosuppressive molecules, including PD-L1, and have enhanced mitochondrial metabolism. Mechanistically, the transcription factor Nrf2 is upregulated in P2RX1-deficient neutrophils and associated with PD-L1 expression and metabolic reprogramming. An anti-PD-1 neutralizing antibody is sufficient to compromise the immunosuppressive effects of P2RX1-deficient neutrophils on OVA-activated OT1 CD8+ T cells. Therefore, our study uncovers a mechanism by which metastatic PDAC tumors evade antitumor immunity by accumulating a subset of immunosuppressive P2RX1-negative neutrophils.

Pancreatic ductal adenocarcinoma (PDAC) is a highly aggressive metastatic disease characterized by an immunosuppressive microenvironment. Here the authors show that a subset of P2RX1-negative neutrophils with immunosuppressive properties accumulate in PDAC metastatic liver tissues and promote tumor growth.

The Hippo Pathway in Liver Homeostasis and Pathophysiology

Studies of the regenerative capacity of the liver have converged on the Hippo pathway, a serine/threonine kinase cascade discovered in Drosophila and conserved from unicellular organisms to mammals. Genetic studies of mouse and rat livers have revealed that the Hippo pathway is a key regulator of liver size, regeneration, development, metabolism, and homeostasis and that perturbations in the Hippo pathway can lead to the development of common liver diseases, such as fatty liver disease and liver cancer. In turn, pharmacological targeting of the Hippo pathway may be utilized to boost regeneration and to prevent the development and progression of liver diseases. We review current insights provided by the Hippo pathway into liver pathophysiology. Furthermore, we present a path forward for future studies to understand how newly identified components of the Hippo pathway may control liver physiology and how the Hippo pathway is regulated in the liver.

Comparison of tumor-associated YAP1 fusions identifies a recurrent set of functions critical for oncogenesis

YAP1 is a transcriptional coactivator and the principal effector of the Hippo signaling pathway, which is causally implicated in human cancer. Several YAP1 gene fusions have been identified in various human cancers and identifying the essential components of this family of gene fusions has significant therapeutic value. Here, we show that the YAP1 gene fusions YAP1-MAMLD1, YAP1-FAM118B, YAP1-TFE3, and YAP1-SS18 are oncogenic in mice. Using reporter assays, RNA-seq, ChIP-seq, and loss-of-function mutations, we can show that all of these YAP1 fusion proteins exert TEAD-dependent YAP activity, while some also exert activity of the C'-terminal fusion partner. The YAP activity of the different YAP1 fusions is resistant to negative Hippo pathway regulation due to constitutive nuclear localization and resistance to degradation of the YAP1 fusion proteins. Genetic disruption of the TEAD-binding domain of these oncogenic YAP1 fusions is sufficient to inhibit tumor formation in vivo, while pharmacological inhibition of the YAP1-TEAD interaction inhibits the growth of YAP1 fusion-expressing cell lines in vitro. These results highlight TEAD-dependent YAP activity found in these gene fusions as critical for oncogenesis and implicate these YAP functions as potential therapeutic targets in YAP1 fusion-positive tumors.

Targeting the Hippo pathway in cancer, fibrosis, wound healing and regenerative medicine

The Hippo pathway is an evolutionarily conserved signalling pathway with key roles in organ development, epithelial homeostasis, tissue regeneration, wound healing and immune modulation. Many of these roles are mediated by the transcriptional effectors YAP and TAZ, which direct gene expression via control of the TEAD family of transcription factors. Dysregulated Hippo pathway and YAP/TAZ-TEAD activity is associated with various diseases, most notably cancer, making this pathway an attractive target for therapeutic intervention. This Review highlights the key findings from studies of Hippo pathway signalling across biological processes and diseases, and discusses new strategies and therapeutic implications of targeting this pathway.