Cells of origin in the embryonic nerve roots for NF1-associated plexiform neurofibroma

Granulocyte-Macrophage Colony Stimulating Factor Receptor Contributes to Plexiform Neurofibroma Initiation

Plexiform neurofibroma (PNF) is an immune cell-rich peripheral nerve sheath tumor that develops primarily in individuals with Neurofibromatosis Type 1 (NF1). Granulocyte-macrophage colony stimulating factor receptor-β (GM-CSFR-βc) is a shared component of receptors for the cytokines GM-CSF, IL-3, and IL-5, ligands with immunomodulatory and tumor promoting roles. In the present study, we use genetically engineered mouse model of neurofibroma. We identified the expression of GM-CSFR-βc and GM-CSFR-α on PNF cells and on macrophages and dendritic cells in the PNF, using the Nf1f/f; DhhCre mouse model of neurofibroma formation. Genetic deletion of GM-CSFR-βc in this model reduced the number of PNFs, which was associated with decreased numbers of tumor-associated Iba1+ macrophages and CD11c+ dendritic cells (DC), while loss of GM-CSFR-α had no effect. Deletion of GM-CSFR-α or GM-CSFR-βc did not improve mouse survival or the structure of Remak bundles in peripheral nerves. Proteome analysis of tumor lysates showed altered levels of numerous cytokines after receptor loss, suggesting that the compensatory effects of other cyto/chemokines maintain a proinflammatory environment promoting neurofibroma. Thus, GM-CSFR-βc signaling contributes modestly to neurofibroma formation, apparently independently of its ligand GM-CSF.

Schwann cells in regeneration and cancer: an epithelial-mesenchymal transition perspective

In the peripheral nervous system, glial cells, known as Schwann cells (SCs), are responsible for supporting and maintaining nerves. One of the most important characteristics of SCs is their remarkable plasticity. In various injury contexts, SCs undergo a reprogramming process that generates specialized cells to promote tissue regeneration and repair. However, in pathological conditions, this same plasticity and regenerative potential can be hijacked. Different studies highlight the activation of the epithelial-mesenchymal transition (EMT) as a driver of SC phenotypic plasticity. Although SCs are not epithelial, their neural crest origin makes EMT activation crucial for their ability to adopt repair phenotypes, mirroring the plasticity observed during development. These adaptive processes are essential for regeneration. However, EMT activation in SCs-derived tumours enhances cancer progression and aggressiveness. Furthermore, in the tumour microenvironment (TME), SCs also acquire activated phenotypes that contribute to tumour migration and invasion by activating EMT in cancer cells. In this review, we will discuss how EMT impacts SC plasticity and function from development and tissue regeneration to pathological conditions, such as cancer.

Socci N, Bale TA, Davis C, Dixon SAH, Zhang C, Wade Clapp D, Neel BG, Parada LF. Hydroxychloroquine prevents resistance and potentiates the antitumor effect of SHP2 inhibition in NF1-associated malignant peripheral nerve sheath tumors

Malignant peripheral nerve sheath tumors (MPNSTs) are aggressive sarcomas and the primary cause of mortality in patients with neurofibromatosis type 1 (NF1). These malignancies develop within preexisting benign lesions called plexiform neurofibromas (PNs). PNs are solely driven by biallelic NF1 loss eliciting RAS pathway activation, and they respond favorably to MEK inhibitor therapy. MPNSTs harbor additional mutations and respond poorly to MEK inhibition. Our analysis of genetically engineered and orthotopic patient-derived xenograft MPNST models indicates that MEK inhibition has poor antitumor efficacy. By contrast, upstream inhibition of RAS through the protein-tyrosine phosphatase SHP2 reduced downstream signaling and suppressed NF1 MPNST growth, although resistance eventually emerged. To investigate possible mechanisms of acquired resistance, kinomic analyses of resistant tumors were performed, and data analysis identified enrichment of activated autophagy pathway protein kinases. Combining SHP2 inhibition with hydroxychloroquine (HQ) resulted in durable responses in NF1 MPNSTs in both genetic and orthotopic xenograft mouse models. Our studies could be rapidly translated into a clinical trial to evaluate SHP2 inhibition in conjunction with HQ as a unique treatment approach for NF1 MPNSTs.

Emerging mechanism and therapeutic potential of neurofibromatosis type 1-related nerve system tumor: Advancing insights into tumor development

Neurofibromatosis Type 1 (NF1) is a genetic disorder resulting from mutations in the NF1 gene, which increases susceptibility to various nervous system tumors, including plexiform neurofibromas, malignant peripheral nerve sheath tumors, and optic pathway gliomas. Recent research has shown that these tumors are intricately connected to the complex, dynamic interactions within neurons, culminating in neuronal signaling that fosters tumor growth. These interactions offer crucial insights into the molecular mechanisms underpinning tumor development, as well as broader implications for therapeutic strategies. This review summarizes the mechanisms through which mutations in the NF1 gene within neural tissues trigger tumorigenesis, while examining the role of the neuron-via factors such as visual experience, neurotransmitter, tumor microenvironment, and psychological influences-in both promoting tumor progression and being affected by the tumors themselves. By investigating the dynamic relationship between NF1-associated nervous system tumor cells and neurons, we aim to shed light on novel biological pathways and disease processes, emphasizing the potential of interdisciplinary approaches that combine neurobiology, oncology, and pharmacology to enhance treatment strategies and even inhibit the tumorigenesis.

NSD3::NUTM1 Fusion Sarcoma Mimicking Malignant Peripheral Nerve Sheath Tumor with Prolonged Survival

Nuclear Protein in Testis (NUT)-rearranged tumors comprise predominantly NUT carcinoma but also include certain lymphomas, leukemias, skin appendage tumors, and sarcomas. Although histologically diverse, all are genetically identified by oncogenic rearrangement in the NUTM1 gene. Many fusion partners occur, and NSD3 is NUT carcinoma's third most common partner. Herein, we present a case of a 26-year-old man with an NSD3::NUTM1 fusion sarcoma. The patient presented at the age of 13 months with a scalp nodule. Over the next 24 years, he experienced five local recurrences and ultimately expired of a rapidly progressive recurrence. His treatment included surgical resections, radiation, and various chemotherapies. Deceptively, the clinical presentation and histopathology aligned with a malignant peripheral nerve sheath tumor, a diagnosis rendered at initial resection with concurrence by a national soft tissue tumor expert. The patient's exceptionally long survival could be due to NSD3 as the fusion partner, aided by the initial small tumor size and young patient age. Thus, this case expands NUT fusion sarcomas' histologic and immunohistochemical profile to include mimicking a malignant peripheral nerve sheath tumor (MPNST). Additionally, it indicates that the NSD3::NUTM1 fusion can drive sarcoma genesis.

Revisiting the NPcis mouse model: A new tool to model plexiform neurofibroma

Neurofibromatosis Type I (NF1) is a rare genetic disorder. NF1 patients frequently develop a benign tumor in peripheral nerve plexuses called plexiform neurofibroma. In the past two decades, tissue-specific Nf1 knockout mouse models were developed using commercially available tissue-specific Cre recombinase and the Nf1 flox mice to mimic neurofibroma development. However, these models develop para-spinal neurofibroma, recapitulating a rare type of neurofibroma found in NF1 patients. The NPcis mouse model developed a malignant version of neurofibroma called malignant peripheral nerve sheath tumor (MPNST) within 3 to 6 months but intriguingly without apparent benign precursor lesion. Here, we revisited the NPcis model and discovered that about 20% display clinical signs similar to Nf1 tissue-specific knockout mice models. However, a systematic histological analysis could not explain the clinical signs we observed although we noticed lesions reminiscent of a neurofibroma in a peripheral nerve, a cutaneous neurofibroma, and para-spinal neurofibroma on rare occasions in NPcis mice. We also observed that 10% of the mice developed a malignant peripheral nerve sheath tumor (MPNST) spontaneously, coinciding with their earring tag identification. Strikingly, half of the sciatic nerves from NPcis mice developed plexiform neurofibroma within 1-6 months when intentionally injured. Thus, we provided a procedure to turn the widely used NPcis sarcoma model into a model recapitulating plexiform neurofibroma.

In vitro competition between two transmissible cancers and potential implications for their host, the Tasmanian devil

Since the emergence of a transmissible cancer, devil facial tumour disease (DFT1), in the 1980s, wild Tasmanian devil populations have been in decline. In 2016, a second, independently evolved transmissible cancer (DFT2) was discovered raising concerns for survival of the host species. Here, we applied experimental and modelling frameworks to examine competition dynamics between the two transmissible cancers in vitro. Using representative cell lines for DFT1 and DFT2, we have found that in monoculture, DFT2 grows twice as fast as DFT1 but reaches lower maximum cell densities. Using co-cultures, we demonstrate that DFT2 outcompetes DFT1: the number of DFT1 cells decreasing over time, never reaching exponential growth. This phenomenon could not be replicated when cells were grown separated by a semi-permeable membrane, consistent with exertion of mechanical stress on DFT1 cells by DFT2. A logistic model and a Lotka-Volterra competition model were used to interrogate monoculture and co-culture growth curves, respectively, suggesting DFT2 is a better competitor than DFT1, but also showing that competition outcomes might depend on the initial number of cells, at least in the laboratory. We provide theories how the in vitro results could be translated to observations in the wild and propose that these results may indicate that although DFT2 is currently in a smaller geographic area than DFT1, it could have the potential to outcompete DFT1. Furthermore, we provide a framework for improving the parameterization of epidemiological models applied to these cancer lineages, which will inform future disease management.

The NF1+/- Immune Microenvironment: Dueling Roles in Neurofibroma Development and Malignant Transformation

Neurofibromatosis type 1 (NF1) is a common genetic disorder resulting in the development of both benign and malignant tumors of the peripheral nervous system. NF1 is caused by germline pathogenic variants or deletions of the NF1 tumor suppressor gene, which encodes the protein neurofibromin that functions as negative regulator of p21 RAS. Loss of NF1 heterozygosity in Schwann cells (SCs), the cells of origin for these nerve sheath-derived tumors, leads to the formation of plexiform neurofibromas (PNF)-benign yet complex neoplasms involving multiple nerve fascicles and comprised of a myriad of infiltrating stromal and immune cells. PNF development and progression are shaped by dynamic interactions between SCs and immune cells, including mast cells, macrophages, and T cells. In this review, we explore the current state of the field and critical knowledge gaps regarding the role of NF1(Nf1) haploinsufficiency on immune cell function, as well as the putative impact of Schwann cell lineage states on immune cell recruitment and function within the tumor field. Furthermore, we review emerging evidence suggesting a dueling role of Nf1+/- immune cells along the neurofibroma to MPNST continuum, on one hand propitiating PNF initiation, while on the other, potentially impeding the malignant transformation of plexiform and atypical neurofibroma precursor lesions. Finally, we underscore the potential implications of these discoveries and advocate for further research directed at illuminating the contributions of various immune cells subsets in discrete stages of tumor initiation, progression, and malignant transformation to facilitate the discovery and translation of innovative diagnostic and therapeutic approaches to transform risk-adapted care.

Econazole selectively induces cell death in NF1-homozygous mutant tumor cells

Cutaneous neurofibromas (cNFs) are tumors that develop in more than 99% of individuals with neurofibromatosis type 1 (NF1). They develop in the dermis and can number in the thousands. cNFs can be itchy and painful and negatively impact self-esteem. There is no US Food and Drug Administration (FDA)-approved drug for their treatment. Here, we screen a library of FDA-approved drugs using a cNF cell model derived from human induced pluripotent stem cells (hiPSCs) generated from an NF1 patient. We engineer an NF1 mutation in the second allele to mimic loss of heterozygosity, differentiate the NF1+/- and NF1-/- hiPSCs into Schwann cell precursors (SCPs), and use them to screen a drug library to assess for inhibition of NF1-/- but not NF1+/- cell proliferation. We identify econazole nitrate as being effective against NF1-/- hiPSC-SCPs. Econazole cream selectively induces apoptosis in Nf1-/- murine nerve root neurosphere cells and human cNF xenografts. This study supports further testing of econazole for cNF treatment.

Combined CDK4/6 and ERK1/2 Inhibition Enhances Antitumor Activity in NF1-Associated Plexiform Neurofibroma

PURPOSE

Plexiform neurofibromas (PNF) are peripheral nerve sheath tumors that cause significant morbidity in persons with neurofibromatosis type 1 (NF1), yet treatment options remain limited. To identify novel therapeutic targets for PNF, we applied an integrated multi-omic approach to quantitatively profile kinome enrichment in a mouse model that has predicted therapeutic responses in clinical trials for NF1-associated PNF with high fidelity.

EXPERIMENTAL DESIGN

Utilizing RNA sequencing combined with chemical proteomic profiling of the functionally enriched kinome using multiplexed inhibitor beads coupled with mass spectrometry, we identified molecular signatures predictive of response to CDK4/6 and RAS/MAPK pathway inhibition in PNF. Informed by these results, we evaluated the efficacy of the CDK4/6 inhibitor, abemaciclib, and the ERK1/2 inhibitor, LY3214996, alone and in combination in reducing PNF tumor burden in Nf1flox/flox;PostnCre mice.

RESULTS

Converging signatures of CDK4/6 and RAS/MAPK pathway activation were identified within the transcriptome and kinome that were conserved in both murine and human PNF. We observed robust additivity of the CDK4/6 inhibitor, abemaciclib, in combination with the ERK1/2 inhibitor, LY3214996, in murine and human NF1(Nf1) mutant Schwann cells. Consistent with these findings, the combination of abemaciclib (CDK4/6i) and LY3214996 (ERK1/2i) synergized to suppress molecular signatures of MAPK activation and exhibited enhanced antitumor activity in Nf1flox/flox;PostnCre mice in vivo.

CONCLUSIONS

These findings provide rationale for the clinical translation of CDK4/6 inhibitors alone and in combination with therapies targeting the RAS/MAPK pathway for the treatment of PNF and other peripheral nerve sheath tumors in persons with NF1.

RAS Signaling Gone Awry in the Skin: The Complex Role of RAS in Cutaneous Neurofibroma Pathogenesis, Emerging Biological Insights

Cutaneous neurofibromas (cNFs) are the most common tumor in people with the rasopathy neurofibromatosis type 1. They number in hundreds or even thousands throughout the body, and currently, there are no effective interventions to prevent or treat these skin tumors. To facilitate the identification of novel and effective therapies, essential studies including a more refined understanding of cNF biology and the role of RAS signaling and downstream effector pathways responsible for cNF initiation, growth, and maintenance are needed. This review highlights the current state of knowledge of RAS signaling in cNF pathogenesis and therapeutic development for cNF treatment.

Cutaneous Neurofibroma Heterogeneity: Factors that Influence Tumor Burden in Neurofibromatosis Type 1

Neurofibromatosis type 1 is one of the most common genetic disorders of the nervous system and predisposes patients to develop benign and malignant tumors. Cutaneous neurofibromas (cNFs) are NF1-associated benign tumors that affect nearly 100% of patients with NF1. cNFs dramatically reduce patients' QOL owing to their unaesthetic appearance, physical discomfort, and corresponding psychological burden. There is currently no effective drug therapy option, and treatment is restricted to surgical removal. One of the greatest hurdles for cNF management is the variability of clinical expressivity in NF1, resulting in intrapatient and interpatient cNF tumor burden heterogeneity, that is, the variability in the presentation and evolution of these tumors. There is growing evidence that a wide array of factors are involved in the regulation of cNF heterogeneity. Understanding the mechanisms underlying this heterogeneity of cNF at the molecular, cellular, and environmental levels can facilitate the development of innovative and personalized treatment regimens.

Basement membrane proteins in extracellular matrix characterize NF1 neurofibroma development and response to MEK inhibitor

Neurofibromatosis type 1 (NF1) is one of the most common tumor-predisposing genetic disorders. Neurofibromas are NF1-associated benign tumors. A hallmark feature of neurofibromas is an abundant collagen-rich extracellular matrix (ECM) that constitutes more than 50% of the tumor dry weight. However, little is known about the mechanism underlying ECM deposition during neurofibroma development and treatment response. We performed a systematic investigation of ECM enrichment during plexiform neurofibroma (pNF) development and identified basement membrane (BM) proteins, rather than major collagen isoforms, as the most upregulated ECM component. Following MEK inhibitor treatment, the ECM profile displayed an overall downregulation signature, suggesting ECM reduction as a therapeutic benefit of MEK inhibition. Through these proteomic studies, TGF-β1 signaling was identified as playing a role in ECM dynamics. Indeed, TGF-β1 overexpression promoted pNF progression in vivo. Furthermore, by integrating single-cell RNA sequencing, we found that immune cells including macrophages and T cells produce TGF-β1 to induce Schwann cells to produce and deposit BM proteins for ECM remodeling. Following Nf1 loss, neoplastic Schwann cells further increased BM protein deposition in response to TGF-β1. Our data delineate the regulation governing ECM dynamics in pNF and suggest that BM proteins could serve as biomarkers for disease diagnosis and treatment response.

Modeling nervous system tumors with human stem cells and organoids

Nervous system cancers are the 10th leading cause of death worldwide, many of which are difficult to diagnose and exhibit varying degrees of treatment resistance. The limitations of existing cancer models, such as patient-derived xenograft (PDX) models and genetically engineered mouse (GEM) models, call for the development of novel preclinical cancer models to more faithfully mimic the patient's cancer and offer additional insights. Recent advances in human stem cell biology, organoid, and genome-editing techniques allow us to model nervous system tumors in three types of next-generation tumor models: cell-of-origin models, tumor organoids, and 3D multicellular coculture models. In this review, we introduced and compared different human stem cell/organoid-derived models, and comprehensively summarized and discussed the recently developed models for various primary tumors in the central and peripheral nervous systems, including glioblastoma (GBM), H3K27M-mutant Diffuse Midline Glioma (DMG) and H3G34R-mutant High-grade Glioma (HGG), Low-grade Glioma (LGG), Neurofibromatosis Type 1 (NF1), Neurofibromatosis Type 2 (NF2), Medulloblastoma (MB), Atypical Teratoid/rhabdoid Tumor (AT/RT), and meningioma. We further compared these models with PDX and GEM models, and discussed the opportunities and challenges of precision nervous cancer modeling with human stem cells and organoids.

Single-cell multiomics identifies clinically relevant mesenchymal stem-like cells and key regulators for MPNST malignancy

Malignant peripheral nerve sheath tumor (MPNST), a highly aggressive Schwann cell (SC)-derived soft tissue sarcoma, arises from benign neurofibroma (NF); however, the identity, heterogeneity and origins of tumor populations remain elusive. Nestin+ cells have been implicated as tumor stem cells in MPNST; unexpectedly, single-cell profiling of human NF and MPNST and their animal models reveal a broad range of nestin-expressing SC lineage cells and dynamic acquisition of discrete cancer states during malignant transformation. We uncover a nestin-negative mesenchymal neural crest-like subpopulation as a previously unknown malignant stem-like state common to murine and human MPNSTs, which correlates with clinical severity. Integrative multiomics profiling further identifies unique regulatory networks and druggable targets against the malignant subpopulations in MPNST. Targeting key epithelial-mesenchymal transition and stemness regulators including ZEB1 and ALDH1A1 impedes MPNST growth. Together, our studies reveal the underlying principles of tumor cell-state evolution and their regulatory circuitries during NF-to-MPNST transformation, highlighting a hitherto unrecognized mesenchymal stem-like subpopulation in MPNST disease progression.

Multiple Nf1 Schwann cell populations reprogram the plexiform neurofibroma tumor microenvironment

To define alterations early in tumor formation, we studied nerve tumors in neurofibromatosis 1 (NF1), a tumor predisposition syndrome. Affected individuals develop neurofibromas, benign tumors driven by NF1 loss in Schwann cells (SCs). By comparing normal nerve cells to plexiform neurofibroma (PN) cells using single-cell and bulk RNA sequencing, we identified changes in 5 SC populations, including a de novo SC progenitor-like (SCP-like) population. Long after Nf1 loss, SC populations developed PN-specific expression of Dcn, Postn, and Cd74, with sustained expression of the injury response gene Postn and showed dramatic expansion of immune and stromal cell populations; in corresponding human PNs, the immune and stromal cells comprised 90% of cells. Comparisons between injury-related and tumor monocytes/macrophages support early monocyte recruitment and aberrant macrophage differentiation. Cross-species analysis verified each SC population and unique conserved patterns of predicted cell-cell communication in each SC population. This analysis identified PROS1-AXL, FGF-FGFR, and MIF-CD74 and its effector pathway NF-κB as deregulated in NF1 SC populations, including SCP-like cells predicted to influence other types of SCs, stromal cells, and/or immune cells in mouse and human. These findings highlight remarkable changes in multiple types of SCs and identify therapeutic targets for PN.

Single-cell sequencing reveals activation of core transcription factors in PRC2-deficient malignant peripheral nerve sheath tumor

Loss-of-function mutations in the polycomb repressive complex 2 (PRC2) occur frequently in malignant peripheral nerve sheath tumor, an aggressive sarcoma that arises from NF1-deficient Schwann cells. To define the oncogenic mechanisms underlying PRC2 loss, we use engineered cells that dynamically reassemble a competent PRC2 coupled with single-cell sequencing from clinical samples. We discover a two-pronged oncogenic process: first, PRC2 loss leads to remodeling of the bivalent chromatin and enhancer landscape, causing the upregulation of developmentally regulated transcription factors that enforce a transcriptional circuit serving as the cell's core vulnerability. Second, PRC2 loss reduces type I interferon signaling and antigen presentation as downstream consequences of hyperactivated Ras and its cross talk with STAT/IRF transcription factors. Mapping of the transcriptional program of these PRC2-deficient tumor cells onto a constructed developmental trajectory of normal Schwann cells reveals that changes induced by PRC2 loss enforce a cellular profile characteristic of a primitive mesenchymal neural crest stem cell.

Neurofibroma Development in Neurofibromatosis Type 1: Insights from Cellular Origin and Schwann Cell Lineage Development

BACKGROUND

Neurofibromatosis type 1 (NF1), a genetic tumor predisposition syndrome that affects about 1 in 3000 newborns, is caused by mutations in the NF1 gene and subsequent inactivation of its encoded neurofibromin. Neurofibromin is a tumor suppressor protein involved in the downregulation of Ras signaling. Despite a diverse clinical spectrum, one of several hallmarks of NF1 is a peripheral nerve sheath tumor (PNST), which comprises mixed nervous and fibrous components. The distinct spatiotemporal characteristics of plexiform and cutaneous neurofibromas have prompted hypotheses about the origin and developmental features of these tumors, involving various cellular transition processes.

METHODS

We retrieved published literature from PubMed, EMBASE, and Web of Science up to 21 June 2022 and searched references cited in the selected studies to identify other relevant papers. Original articles reporting the pathogenesis of PNSTs during development were included in this review. We highlighted the Schwann cell (SC) lineage shift to better present the evolution of its corresponding cellular origin hypothesis and its important effects on the progression and malignant transformation of neurofibromas.

CONCLUSIONS

In this review, we summarized the vast array of evidence obtained on the full range of neurofibroma development based on cellular and molecular pathogenesis. By integrating findings relating to tumor formation, growth, and malignancy, we hope to reveal the role of SC lineage shift as well as the combined impact of additional determinants in the natural history of PNSTs.

[Progress in diagnosis and treatment of neurofibromatosis in children]

Neurofibromatosis type 1（NF1） is an autosomal dominant genetic disease in which a mutation in the NF1 gene on chromosome 17q11.2 results in inactivation or down-regulation of neurofibromin. This results in a series of neurocutaneous lesions characterized by neurofibromatosis. Patients with plexiform neurofibromas（PN）, as one of the main manifestations of NF1, often experience pain, dysfunction, skeletal deformities, changes in appearance and other symptoms. In severe cases, compression of the airways and vital organs occurs, and the PN is at risk of malignancy progression. At present, its treatment is still challenging. Surgery is the primary treatment for PN, but complete resection is often difficult. In recent years, chemotherapy for PN has become a hot topic. This article reviews the research progress in the pathogenesis, diagnosis and treatment of PN in recent years.

Integrated Drug Mining Reveals Actionable Strategies Inhibiting Plexiform Neurofibromas

Neurofibromatosis Type 1 (NF1) is one of the most common genetic tumor predisposition syndromes, affecting up to 1 in 2500 individuals. Up to half of patients with NF1 develop benign nerve sheath tumors called plexiform neurofibromas (PNs), characterized by biallelic NF1 loss. PNs can grow to immense sizes, cause extensive morbidity, and harbor a 15% lifetime risk of malignant transformation. Increasingly, molecular sequencing and drug screening data from various preclinical murine and human PN cell lines, murine models, and human PN tissues are available to help identify salient treatments for PNs. Despite this, Selumetinib, a MEK inhibitor, is the only currently FDA-approved pharmacotherapy for symptomatic and inoperable PNs in pediatric NF1 patients. The discovery of alternative and additional treatments has been hampered by the rarity of the disease, which makes prioritizing drugs to be tested in future clinical trials immensely important. Here, we propose a gene regulatory network-based integrated analysis to mine high-throughput cell line-based drug data combined with transcriptomes from resected human PN tumors. Conserved network modules were characterized and served as drug fingerprints reflecting the biological connections among drug effects and the inherent properties of PN cell lines and tissue. Drug candidates were ranked, and the therapeutic potential of drug combinations was evaluated via computational predication. Auspicious therapeutic agents and drug combinations were proposed for further investigation in preclinical and clinical trials.

Neuronal hyperexcitability drives central and peripheral nervous system tumor progression in models of neurofibromatosis-1

Neuronal activity is emerging as a driver of central and peripheral nervous system cancers. Here, we examined neuronal physiology in mouse models of the tumor predisposition syndrome Neurofibromatosis-1 (NF1), with different propensities to develop nervous system cancers. We show that central and peripheral nervous system neurons from mice with tumor-causing Nf1 gene mutations exhibit hyperexcitability and increased secretion of activity-dependent tumor-promoting paracrine factors. We discovered a neurofibroma mitogen (COL1A2) produced by peripheral neurons in an activity-regulated manner, which increases NF1-deficient Schwann cell proliferation, establishing that neurofibromas are regulated by neuronal activity. In contrast, mice with the Arg1809Cys Nf1 mutation, found in NF1 patients lacking neurofibromas or optic gliomas, do not exhibit neuronal hyperexcitability or develop these NF1-associated tumors. The hyperexcitability of tumor-prone Nf1-mutant neurons results from reduced NF1-regulated hyperpolarization-activated cyclic nucleotide-gated (HCN) channel function, such that neuronal excitability, activity-regulated paracrine factor production, and tumor progression are attenuated by HCN channel activation. Collectively, these findings reveal that NF1 mutations act at the level of neurons to modify tumor predisposition by increasing neuronal excitability and activity-regulated paracrine factor production.

Neuronal activity is emerging as a driver of nervous system tumors. Here, the authors show in mouse models of Neurofibromatosis-1 (NF1) that Nf1 mutations differentially drive both central and peripheral nervous system tumor growth in mice through reduced hyperpolarization-activated cyclic nucleotide-gated (HCN) channel function.

Malignant peripheral nerve sheath tumor: models, biology, and translation

Malignant peripheral nerve sheath tumors (MPNSTs) are aggressive, invasive cancer that comprise around 10% of all soft tissue sarcomas and develop in about 8-13% of patients with Neurofibromatosis Type 1. They are associated with poor prognosis and are the leading cause of mortality in NF1 patients. MPNSTs can also develop sporadically or following exposure to radiation. There is currently no effective targeted therapy to treat MPNSTs and surgical removal remains the mainstay treatment. Unfortunately, surgery is not always possible due to the size and location of the tumor, thus, a better understanding of MPNST initiation and development is required to design novel therapeutics. Here, we provide an overview of MPNST biology and genetics, discuss findings regarding the developmental origin of MPNST, and summarize the various model systems employed to study MPNST. Finally, we discuss current management strategies for MPNST, as well as recent developments in translating basic research findings into potential therapies.

P2RY14 cAMP signaling regulates Schwann cell precursor self-renewal, proliferation, and nerve tumor initiation in a mouse model of neurofibromatosis

Neurofibromatosis type 1 (NF1) is characterized by nerve tumors called neurofibromas, in which Schwann cells (SCs) show deregulated RAS signaling. NF1 is also implicated in regulation of cAMP. We identified the G-protein-coupled receptor (GPCR) P2ry14 in human neurofibromas, neurofibroma-derived SC precursors (SCPs), mature SCs, and mouse SCPs. Mouse Nf1-/- SCP self-renewal was reduced by genetic or pharmacological inhibition of P2ry14. In a mouse model of NF1, genetic deletion of P2ry14 rescued low cAMP signaling, increased mouse survival, delayed neurofibroma initiation, and improved SC Remak bundles. P2ry14 signals via G_i to increase intracellular cAMP, implicating P2ry14 as a key upstream regulator of cAMP. We found that elevation of cAMP by either blocking the degradation of cAMP or by using a P2ry14 inhibitor diminished NF1-/- SCP self-renewal in vitro and neurofibroma SC proliferation in in vivo. These studies identify P2ry14 as a critical regulator of SCP self-renewal, SC proliferation, and neurofibroma initiation.

Modeling iPSC-derived human neurofibroma-like tumors in mice uncovers the heterogeneity of Schwann cells within plexiform neurofibromas

Plexiform neurofibromas (pNFs) are developmental tumors that appear in neurofibromatosis type 1 individuals, constituting a major source of morbidity and potentially transforming into a highly metastatic sarcoma (MPNST). pNFs arise after NF1 inactivation in a cell of the neural crest (NC)-Schwann cell (SC) lineage. Here, we develop an iPSC-based NC-SC in vitro differentiation system and construct a lineage expression roadmap for the analysis of different 2D and 3D NF models. The best model consists of generating heterotypic spheroids (neurofibromaspheres) composed of iPSC-derived differentiating NF1(-/-) SCs and NF1(+/-) pNF-derived fibroblasts (Fbs). Neurofibromaspheres form by maintaining highly proliferative NF1(-/-) cells committed to the NC-SC axis due to SC-SC and SC-Fb interactions, resulting in SC linage cells at different maturation points. Upon engraftment on the mouse sciatic nerve, neurofibromaspheres consistently generate human NF-like tumors. Analysis of expression roadmap genes in human pNF single-cell RNA-seq data uncovers the presence of SC subpopulations at distinct differentiation states.

Neurofibromin and suppression of tumorigenesis: beyond the GAP

Neurofibromatosis type 1 (NF1) is an autosomal dominant genetic disease and one of the most common inherited tumor predisposition syndromes, affecting 1 in 3000 individuals worldwide. The NF1 gene encodes neurofibromin, a large protein with RAS GTP-ase activating (RAS-GAP) activity, and loss of NF1 results in increased RAS signaling. Neurofibromin contains many other domains, and there is considerable evidence that these domains play a role in some manifestations of NF1. Investigating the role of these domains as well as the various signaling pathways that neurofibromin regulates and interacts with will provide a better understanding of how neurofibromin acts to suppress tumor development and potentially open new therapeutic avenues. In this review, we discuss what is known about the structure of neurofibromin, its interactions with other proteins and signaling pathways, its role in development and differentiation, and its function as a tumor suppressor. Finally, we discuss the latest research on potential therapeutics for neurofibromin-deficient neoplasms.

The differentiation state of the Schwann cell progenitor drives phenotypic variation between two contagious cancers

Contagious cancers are a rare pathogenic phenomenon in which cancer cells gain the ability to spread between genetically distinct hosts. Nine examples have been identified across marine bivalves, dogs and Tasmanian devils, but the Tasmanian devil is the only mammalian species known to have given rise to two distinct lineages of contagious cancer, termed Devil Facial Tumour 1 (DFT1) and 2 (DFT2). Remarkably, DFT1 and DFT2 arose independently from the same cell type, a Schwann cell, and while their ultra-structural features are highly similar they exhibit variation in their mutational signatures and infection dynamics. As such, DFT1 and DFT2 provide a unique framework for investigating how a common progenitor cell can give rise to distinct contagious cancers. Using a proteomics approach, we show that DFT1 and DFT2 are derived from Schwann cells in different differentiation states, with DFT2 carrying a molecular signature of a less well differentiated Schwann cell. Under inflammatory signals DFT1 and DFT2 have different gene expression profiles, most notably involving Schwann cell markers of differentiation, reflecting the influence of their distinct origins. Further, DFT2 cells express immune cell markers typically expressed during nerve repair, consistent with an ability to manipulate their extracellular environment, facilitating the cell’s ability to transmit between individuals. The emergence of two contagious cancers in the Tasmanian devil suggests that the inherent plasticity of Schwann cells confers a vulnerability to the formation of contagious cancers.

Cancer can be an infectious pathogen, with nine known cases, infecting bivalves, dogs and two independent tumours circulating in the endangered Tasmanian devil. These cancers, known as Devil Facial Tumour 1 (DFT1) and Devil Facial Tumour 2 (DFT2), spread through the wild population much like parasites, moving between genetically distinct hosts during social biting behaviours and persisting in the population. As DFT1 and DFT2 are independent contagious cancers that arose from the same cell type, a Schwann cell, they provide a unique model system for studying the emergence of phenotypic variation in cancers derived from a single progenitor cell. In this study, we have shown that these two remarkably similar tumours have emerged from Schwann cells in different differentiation states. The differentiation state of the progenitor has altered the characteristics of each tumour, resulting in different responses to external signals. This work demonstrates that the cellular origin of infection can direct the phenotype of a contagious cancer and how it responds to signals from the host environment. Further, the plasticity of Schwann cells may make these cells more prone to forming contagious cancers, raising the possibility that further parasitic cancers could emerge from this cell type.

Diffusion-Weighted Magnetic Resonance Imaging Improves the Accuracy of Differentiation of Benign from Malignant Peripheral Nerve Sheath Tumors

OBJECTIVE

In patients with neurofibromatosis type 1 (NF1), it is important to accurately determine when plexiform neurofibroma (pNF) transforms to a malignant peripheral nerve sheath tumor (MPNST). The purpose of this study is to investigate the usefulness of diffusion-weighted imaging (DWI) in differentiating pNF and MPNST in NF1 patients.

METHODS

Among the NF1 patients who were referred to our hospital between 1985 and 2015, 10 cases of MPNST and 19 cases of pNF were included. We evaluated features of standard magnetic resonance imaging according to the differentiation criteria of malignancy from benignancy as previously reported, apparent diffusion coefficient (ADC) value based on the DWI and the correlation between ADC value and benignancy/malignancy. ROC analysis was performed to determine the appropriate cutoff value of ADC.

RESULTS

There were significant differences between MPNST and pNF in the size of the tumor (P = 0.009), peripheral enhancement pattern (P = 0.002), perilesional edema-like zone (P = 0.0008), and intratumoral cystic change (P = 0.02). The mean and minimum values of ADC were significantly lower in MPNST than those in pNF (P = 0.03 and P = 0.003, respectively). When we set a cutoff value of mean ADC as 1.85 × 10^-3 mm²/s, the sensitivity and specificity were 80% and 74%, respectively. The area under the curve value improved by adding the Wasa score to the mean ADC evaluation.

CONCLUSIONS

ADC values determined by DWI are useful in differentiating MPNST from pNF and adding ADC evaluation to standard MRI evaluation improved the diagnostic accuracy.

Insights into the Pathogenesis of NF1-Associated Neoplasms

Neurofibromatosis type 1 (NF1) is one of the most common neurocutaneous genetic disorders, presenting with different cutaneous features such as café-au-lait macules, intertriginous skin freckling, and neurofibromas. Although most of the disease manifestations are benign, patients are at risk for a variety of malignancies, including malignant transformation of plexiform neurofibromas. Numerous studies have investigated the mechanisms by which these characteristic neurofibromas develop, with progress made toward unraveling the various players involved in their complex pathogenesis. In this review, we summarize the current understanding of the cells that give rise to NF1 neoplasms as well as the molecular mechanisms and cellular changes that confer tumorigenic potential. We also discuss the role of the tumor microenvironment and the key aspects of its various cell types that contribute to NF1-associated tumorigenesis. An increased understanding of these intrinsic and extrinsic components is critical for developing novel therapeutic approaches for affected patients.

Humanized neurofibroma model from induced pluripotent stem cells delineates tumor pathogenesis and developmental origins

Neurofibromatosis type 1 (NF1) is a common tumor predisposition syndrome caused by NF1 gene mutation, in which affected patients develop Schwann cell lineage peripheral nerve sheath tumors (neurofibromas). To investigate human neurofibroma pathogenesis, we differentiated a series of isogenic, patient-specific NF1-mutant human induced pluripotent stem cells (hiPSCs) into Schwannian lineage cells (SLCs). We found that, although WT and heterozygous NF1-mutant hiPSCs-SLCs did not form tumors following mouse sciatic nerve implantation, NF1-null SLCs formed bona fide neurofibromas with high levels of SOX10 expression. To confirm that SOX10+ SLCs contained the cells of origin for neurofibromas, both Nf1 alleles were inactivated in mouse Sox10+ cells, leading to classic nodular cutaneous and plexiform neurofibroma formation that completely recapitulated their human counterparts. Moreover, we discovered that NF1 loss impaired Schwann cell differentiation by inducing a persistent stem-like state to expand the pool of progenitors required to initiate tumor formation, indicating that, in addition to regulating MAPK-mediated cell growth, NF1 loss also altered Schwann cell differentiation to promote neurofibroma development. Taken together, we established a complementary humanized neurofibroma explant and, to our knowledge, first-in-kind genetically engineered nodular cutaneous neurofibroma mouse models that delineate neurofibroma pathogenesis amenable to future therapeutic target discovery and evaluation.

Enhancer reprogramming in PRC2-deficient malignant peripheral nerve sheath tumors induces a targetable de-differentiated state

Malignant peripheral nerve sheath tumors (MPNSTs) are soft tissue sarcomas that frequently harbor genetic alterations in polycomb repressor complex 2 (PRC2) components-SUZ12 and EED. Here, we show that PRC2 loss confers a dedifferentiated early neural-crest phenotype which is exclusive to PRC2-mutant MPNSTs and not a feature of neurofibromas. Neural crest phenotype in PRC2 mutant MPNSTs was validated via cross-species comparative analysis using spontaneous and transgenic MPNST models. Systematic chromatin state profiling of the MPNST cells showed extensive epigenomic reprogramming or chromatin states associated with PRC2 loss and identified gains of active enhancer states/super-enhancers on early neural crest regulators in PRC2-mutant conditions around genomic loci that harbored repressed/poised states in PRC2-WT MPNST cells. Consistently, inverse correlation between H3K27me3 loss and H3K27Ac gain was noted in MPNSTs. Epigenetic editing experiments established functional roles for enhancer gains on DLX5-a key regulator of neural crest phenotype. Consistently, blockade of enhancer activity by bromodomain inhibitors specifically suppressed this neural crest phenotype and tumor burden in PRC2-mutant PDXs. Together, these findings reveal accumulation of dedifferentiated neural crest like state in PRC2-mutant MPNSTs that can be targeted by enhancer blockade.

Tumorigenesis in neurofibromatosis type 1: role of the microenvironment

Neurofibromatosis Type 1 (NF1) is one of the most common inherited neurological disorders and predisposes patients to develop benign and malignant tumors. Neurofibromas are NF1-associated benign tumors but can cause substantial discomfort and disfigurement. Numerous studies have shown that neurofibromas arise from the Schwann cell lineage but both preclinical mouse models and clinical trials have demonstrated that the neurofibroma tumor microenvironment contributes significantly to tumorigenesis. This offers the opportunity for targeting new therapeutic vulnerabilities to treat neurofibromas. However, a translational gap exists between deciphering the contribution of the neurofibroma tumor microenvironment and clinically applying this knowledge to treat neurofibromas. Here, we discuss the key cellular and molecular components in the neurofibroma tumor microenvironment that can potentially be targeted therapeutically to advance neurofibroma treatment.

Defining Gene Functions in Tumorigenesis by Ex vivo Ablation of Floxed Alleles in Malignant Peripheral Nerve Sheath Tumor Cells

The development of new drugs that precisely target key proteins in human cancers is fundamentally altering cancer therapeutics. However, before these drugs can be used, their target proteins must be validated as therapeutic targets in specific cancer types. This validation is often performed by knocking out the gene encoding the candidate therapeutic target in a genetically engineered mouse (GEM) model of cancer and determining what effect this has on tumor growth. Unfortunately, technical issues such as embryonic lethality in conventional knockouts and mosaicism in conditional knockouts often limit this approach. To overcome these limitations, an approach to ablating a floxed embryonic lethal gene of interest in short-term cultures of malignant peripheral nerve sheath tumors (MPNSTs) generated in a GEM model was developed. This paper describes how to establish a mouse model with the appropriate genotype, derive short-term tumor cultures from these animals, and then ablate the floxed embryonic lethal gene using an adenoviral vector that expresses Cre recombinase and enhanced green fluorescent protein (eGFP). Purification of cells transduced with adenovirus using fluorescence-activated cell sorting (FACS) and the quantification of the effects that gene ablation exerts on cellular proliferation, viability, the transcriptome, and orthotopic allograft growth is then detailed. These methodologies provide an effective and generalizable approach to identifying and validating therapeutic targets in vitro and in vivo. These approaches also provide a renewable source of low-passage tumor-derived cells with reduced in vitro growth artifacts. This allows the biological role of the targeted gene to be studied in diverse biologic processes such as migration, invasion, metastasis, and intercellular communication mediated by the secretome.

Stem-like cells drive NF1-associated MPNST functional heterogeneity and tumor progression

NF1-associated malignant peripheral nerve sheath tumors (MPNSTs) are the major cause of mortality in neurofibromatosis. MPNSTs arise from benign peripheral nerve plexiform neurofibromas that originate in the embryonic neural crest cell lineage. Using reporter transgenes that label early neural crest lineage cells in multiple NF1 MPNST mouse models, we discover and characterize a rare MPNST cell population with stem-cell-like properties, including quiescence, that is essential for tumor initiation and relapse. Following isolation of these cells, we derive a cancer-stem-cell-specific gene expression signature that includes consensus embryonic neural crest genes and identify Nestin as a marker for the MPNST cell of origin. Combined targeting of cancer stem cells along with antimitotic chemotherapy yields effective tumor inhibition and prolongs survival. Enrichment of the cancer stem cell signature in cognate human tumors supports the generality and relevance of cancer stem cells to MPNST therapy development.

Distinctive epigenomic alterations in NF1-deficient cutaneous and plexiform neurofibromas drive differential MKK/p38 signaling

Benign peripheral nerve sheath tumors are the clinical hallmark of Neurofibromatosis Type 1. They account for substantial morbidity and mortality in NF1. Cutaneous (CNF) and plexiform neurofibromas (PNF) share nearly identical histology, but maintain different growth rates and risk of malignant conversion. The reasons for this disparate clinical behavior are not well explained by recent genome or transcriptome profiling studies. We hypothesized that CNFs and PNFs are epigenetically distinct tumor types that exhibit differential signaling due to genome-wide and site-specific methylation events. We interrogated the methylation profiles of 45 CNFs and 17 PNFs from NF1 subjects with the Illumina EPIC 850K methylation array. Based on these profiles, we confirm that CNFs and PNFs are epigenetically distinct tumors with broad differences in higher-order chromatin states and specific methylation events altering genes involved in key biological and cellular processes, such as inflammation, RAS/MAPK signaling, actin cytoskeleton rearrangement, and oxytocin signaling. Based on our identification of two separate DMRs associated with alternative leading exons in MAP2K3, we demonstrate differential RAS/MKK3/p38 signaling between CNFs and PNFs. Epigenetic reinforcement of RAS/MKK/p38 was a defining characteristic of CNFs leading to pro-inflammatory signaling and chromatin conformational changes, whereas PNFs signaled predominantly through RAS/MEK. Tumor size also correlated with specific CpG methylation events. Taken together, these findings confirm that NF1 deficiency influences the epigenetic regulation of RAS signaling fates, accounting for observed differences in CNF and PNF clinical behavior. The extension of these findings is that CNFs may respond differently than PNFs to RAS-targeted therapeutics raising the possibility of targeting p38-mediated inflammation for CNF treatment.

Modeling tumors of the peripheral nervous system associated with Neurofibromatosis type 1: Reprogramming plexiform neurofibroma cells

Plexiform neurofibromas (pNFs) are benign tumors of the peripheral nervous system (PNS) that can progress towards a deadly soft tissue sarcoma termed malignant peripheral nerve sheath tumor (MPNST). pNFs appear during development in the context of the genetic disease Neurofibromatosis type 1 (NF1) due to the complete loss of the NF1 tumor suppressor gene in a cell of the neural crest (NC) - Schwann cell (SC) axis of differentiation. NF1(-/-) cells from pNFs can be reprogrammed into induced pluripotent stem cells (iPSCs) that exhibit an increased proliferation rate and maintain full iPSC properties. Efficient protocols for iPSC differentiation towards NC and SC exist and thus NC cells can be efficiently obtained from NF1(-/-) iPSCs and further differentiated towards SCs. In this review, we will focus on the iPSC modeling of pNFs, including the reprogramming of primary pNF-derived cells, the properties of pNF-derived iPSCs, the capacity to differentiate towards the NC-SC lineage, and how well iPSC-derived NF1(-/-) SC spheroids recapitulate pNF-derived primary SCs. The potential uses of NF1(-/-) iPSCs in pNF modeling and a future outlook are discussed.

Neural crest lineage analysis: from past to future trajectory

Since its discovery 150 years ago, the neural crest has intrigued investigators owing to its remarkable developmental potential and extensive migratory ability. Cell lineage analysis has been an essential tool for exploring neural crest cell fate and migration routes. By marking progenitor cells, one can observe their subsequent locations and the cell types into which they differentiate. Here, we review major discoveries in neural crest lineage tracing from a historical perspective. We discuss how advancing technologies have refined lineage-tracing studies, and how clonal analysis can be applied to questions regarding multipotency. We also highlight how effective progenitor cell tracing, when combined with recently developed molecular and imaging tools, such as single-cell transcriptomics, single-molecule fluorescence in situ hybridization and high-resolution imaging, can extend the scope of neural crest lineage studies beyond development to regeneration and cancer initiation.

Schwannoma development is mediated by Hippo pathway dysregulation and modified by RAS/MAPK signaling

Schwannomas are tumors of the Schwann cells that cause chronic pain, numbness, and potentially life-threatening impairment of vital organs. Despite the identification of causative genes, including NF2 (Merlin), INI1/SMARCB1, and LZTR1, the exact molecular mechanism of schwannoma development is still poorly understood. Several studies have identified Merlin as a key regulator of the Hippo, MAPK, and PI3K signaling pathways; however, definitive evidence demonstrating the importance of these pathways in schwannoma pathogenesis is absent. Here, we provide direct genetic evidence that dysregulation of the Hippo pathway in the Schwann cell lineage causes development of multiple schwannomas in mice. We found that canonical Hippo signaling through the effectors YAP/TAZ is required for schwannomagenesis and that MAPK signaling modifies schwannoma formation. Furthermore, cotargeting YAP/TAZ transcriptional activity and MAPK signaling demonstrated a synergistic therapeutic effect on schwannomas. Our new model provides a tractable platform to dissect the molecular mechanisms underpinning schwannoma formation and the role of combinatorial targeted therapy in schwannoma treatment.

Canonical Hippo signaling through the effectors YAP/TAZ is required for the development of peripheral nervous system tumors of Schwann cells, and MAPK signaling modifies schwannoma formation.

Early administration of imatinib mesylate reduces plexiform neurofibroma tumor burden with durable results after drug discontinuation in a mouse model of neurofibromatosis type 1

BACKGROUND

Neurofibromatosis type 1 (NF1) is a common genetic disorder characterized by plexiform neurofibromas (pNF), which are thought to be congenital tumors that arise in utero and enlarge throughout life. Genetic studies in murine models delineated an indispensable role for the stem cell factor (SCF)/c-kit pathway in pNF initiation and progression. A subsequent phase 2 clinical trial using imatinib mesylate to inhibit SCF/c-kit demonstrated tumor shrinkage in a subset of preexisting pNF; however, imatinib's role on preventing pNF development has yet to be explored.

PROCEDURE

We evaluated the effect of imatinib dosed at 10-100 mg/kg/day for 12 weeks to one-month-old Nf1^flox/flox ;PostnCre(+) mice, prior to onset of pNF formation. To determine durability of response, we then monitored for pNF growth at later time points, comparing imatinib- with vehicle-treated mice. We assessed gross and histopathological analysis of tumor burden.

RESULTS

Imatinib administered preventatively led to a significant decrease in pNF number, even at doses as low as 10 mg/kg/day. Tumor development continued to be significantly inhibited after cessation of imatinib dosed at 50 and 100 mg/kg/day. In the cohort of treated mice that underwent prolonged follow-up, the size of residual tumors was significantly reduced as compared with age-matched littermates that received vehicle control.

CONCLUSIONS

Early administration of imatinib inhibits pNF genesis in vivo, and effects are sustained after discontinuation of therapy. These findings may guide clinical use of imatinib in young NF1 patients prior to the substantial development of pNF.

Genetic disruption of the small GTPase RAC1 prevents plexiform neurofibroma formation in mice with neurofibromatosis type 1

Neurofibromatosis type 1 (NF1) is a common cancer predisposition syndrome caused by mutations in the NF1 tumor suppressor gene. NF1 encodes neurofibromin, a GTPase-activating protein for RAS proto-oncogene GTPase (RAS). Plexiform neurofibromas are a hallmark of NF1 and result from loss of heterozygosity of NF1 in Schwann cells, leading to constitutively activated p21RAS. Given the inability to target p21RAS directly, here we performed an shRNA library screen of all human kinases and Rho-GTPases in a patient-derived NF1-/- Schwann cell line to identify novel therapeutic targets to disrupt PN formation and progression. Rho family members, including Rac family small GTPase 1 (RAC1), were identified as candidates. Corroborating these findings, we observed that shRNA-mediated knockdown of RAC1 reduces cell proliferation and phosphorylation of extracellular signal-regulated kinase (ERK) in NF1-/- Schwann cells. Genetically engineered Nf1flox/flox;PostnCre+ mice, which develop multiple PNs, also exhibited increased RAC1-GTP and phospho-ERK levels compared with Nf1flox/flox;PostnCre- littermates. Notably, mice in which both Nf1 and Rac1 loci were disrupted (Nf1flox/floxRac1flox/flox;PostnCre+) were completely free of tumors and had normal phospho-ERK activity compared with Nf1flox/flox ;PostnCre+ mice. We conclude that the RAC1-GTPase is a key downstream node of RAS and that genetic disruption of the Rac1 allele completely prevents PN tumor formation in vivo in mice.

New insights into the neurofibroma tumor cells of origin

Neurofibromatosis type I (NF1) is a debilitating inherited tumor syndrome affecting around 1 in 3000 people. Patients present with a variety of tumors caused by biallelic loss of the tumor suppressor neurofibromin (NF1), a negative regulator of Ras signaling. While the mechanism of tumor formation is similar in the majority of NF1 cases, the clinical spectrum of tumors can vary depending on spatiotemporal loss of heterozygosity of NF1 in cells derived from the neural crest during development. The hallmark lesions that give NF1 its namesake are neurofibromas, which are benign Schwann cell tumors composed of nervous and fibrous tissue. Neurofibromas can be found in the skin (cutaneous neurofibroma) or deeper in body near nerve plexuses (plexiform neurofibroma). While neurofibromas have been known to be Schwann cell tumors for many years, the exact timing and initiating cell has remained elusive. This has led to difficulties in developing animal models and successful therapies for NF1. A culmination of recent genetic studies has finally begun to shed light on the detailed cellular origins of neurofibromatosis. In this review, we will examine the hunt for neurofibroma tumor cells of origin through a historical lens, detailing the genetic systems used to delineate the source of plexiform and cutaneous neurofibromas. Through these novel findings, we can better understand the cellular, temporal, and developmental context during tumor initiation. By leveraging this data, we hope to uncover new therapeutic targets and mechanisms to treat NF1 patients.

Translating current basic research into future therapies for neurofibromatosis type 1

Neurofibromatosis type 1 (NF1) is a hereditary tumour syndrome that predisposes to benign and malignant tumours originating from neural crest cells. Biallelic inactivation of the tumour-suppressor gene NF1 in glial cells in the skin, along a nerve plexus or in the brain results in the development of benign tumours: cutaneous neurofibroma, plexiform neurofibroma and glioma, respectively. Despite more than 40 years of research, only one medication was recently approved for treatment of plexiform neurofibroma and no drugs have been specifically approved for the management of other tumours. Work carried out over the past several years indicates that inhibiting different cellular signalling pathways (such as Hippo, Janus kinase/signal transducer and activator of transcription, mitogen-activated protein kinase and those mediated by sex hormones) in tumour cells or targeting cells in the microenvironment (nerve cells, macrophages, mast cells and T cells) might benefit NF1 patients. In this review, we outline previous strategies aimed at targeting these signalling pathways or cells in the microenvironment, agents that are currently in clinical trials, and the latest advances in basic research that could culminate in the development of novel therapeutics for patients with NF1.

The Role of Polycomb Repressive Complex in Malignant Peripheral Nerve Sheath Tumor

Malignant peripheral nerve sheath tumors (MPNSTs) are aggressive soft tissue sarcomas that can arise most frequently in patients with neurofibromatosis type 1 (NF1). Despite an increasing understanding of the molecular mechanisms that underlie these tumors, there remains limited therapeutic options for this aggressive disease. One potentially critical finding is that a significant proportion of MPNSTs exhibit recurrent mutations in the genes EED or SUZ12, which are key components of the polycomb repressive complex 2 (PRC2). Tumors harboring these genetic lesions lose the marker of transcriptional repression, trimethylation of lysine residue 27 on histone H3 (H3K27me3) and have dysregulated oncogenic signaling. Given the recurrence of PRC2 alterations, intensive research efforts are now underway with a focus on detailing the epigenetic and transcriptomic consequences of PRC2 loss as well as development of novel therapeutic strategies for targeting these lesions. In this review article, we will summarize the recent findings of PRC2 in MPNST tumorigenesis, including highlighting the functions of PRC2 in normal Schwann cell development and nerve injury repair, as well as provide commentary on the potential therapeutic vulnerabilities of a PRC2 deficient tumor cell.

Cdkn2a (Arf) loss drives NF1-associated atypical neurofibroma and malignant transformation

Plexiform neurofibroma (PN) tumors are a hallmark manifestation of neurofibromatosis type 1 (NF1) that arise in the Schwann cell (SC) lineage. NF1 is a common heritable cancer predisposition syndrome caused by germline mutations in the NF1 tumor suppressor, which encodes a GTPase-activating protein called neurofibromin that negatively regulates Ras proteins. Whereas most PN are clinically indolent, a subset progress to atypical neurofibromatous neoplasms of uncertain biologic potential (ANNUBP) and/or to malignant peripheral nerve sheath tumors (MPNSTs). In small clinical series, loss of 9p21.3, which includes the CDKN2A locus, has been associated with the genesis of ANNUBP. Here we show that the Cdkn2a alternate reading frame (Arf) serves as a gatekeeper tumor suppressor in mice that prevents PN progression by inducing senescence-mediated growth arrest in aberrantly proliferating Nf1-/- SC. Conditional ablation of Nf1 and Arf in the neural crest-derived SC lineage allows escape from senescence, resulting in tumors that accurately phenocopy human ANNUBP and progress to MPNST with high penetrance. This animal model will serve as a platform to study the clonal development of ANNUBP and MPNST and to identify new therapies to treat existing tumors and to prevent disease progression.

The repair and autophagy mechanisms of hypoxia-regulated bFGF-modified primary embryonic neural stem cells in spinal cord injury

There is no effective strategy for the treatment of spinal cord injury (SCI), a devastating condition characterized by severe hypoxia and ischemic insults. In this study, we investigated the histology and pathophysiology of the SCI milieu in a rat model and found that areas of hypoxia were unevenly interspersed in compressed SCI. With this new knowledge, we generated embryonic neural stem cells (NSCs) expressing basic fibroblast growth factor (bFGF) under the regulation of five hypoxia-responsive elements (5HRE) using a lentiviral vector (LV-5HRE-bFGF-NSCs) to specifically target these hypoxic loci. SCI models treated with bFGF expressed by the LV-5HRE-bFGF-NSCs viral vector demonstrated improved recovery, increased neuronal survival, and inhibited autophagy in spinal cord lesions in the rat model due to the reversal of hypoxic conditions at day 42 after injury. Furthermore, improved functional restoration of SCI with neuron regeneration was achieved in vivo, accompanied by glial scar inhibition and the evidence of axon regeneration across the scar boundary. This is the first study to illustrate the presence of hypoxic clusters throughout the injury site of compressed SCI and the first to show that the transplantation of LV-5HRE-bFGF-NSCs to target this hypoxic microenvironment enhanced the recovery of neurological function after SCI in rats; LV-5HRE-bFGF-NSCs may therefore be a good candidate to evaluate cellular SCI therapy in humans.

Heterozygous Tumor Suppressor Microenvironment in Cancer Development

Hereditary cancer syndromes are typically caused by mutations of a tumor suppressor gene that lead to the early development of multifocal benign neoplasms followed by their malignant progression. However, the term 'hereditary cancer syndrome' may be misleading, as a large subgroup of syndromes are characterized by highly penetrant benign tumors. The reason why these cardinal tumors rarely progress to malignancy has been an elusive question in cancer biology. In this opinion article, we propose a framework where a heterozygous tumor suppressor gene microenvironment has antagonistic roles in tumorigenesis, by accelerating development of benign tumors while restraining further progression to malignant cancers.

Malignant Peripheral Nerve Sheath Tumors: From Epigenome to Bedside

Malignant peripheral nerve sheath tumors (MPNST) are aggressive sarcomas typically developing in the context of neurofibromatosis type 1 (NF-1). With the exception of surgical resection, these tumors are resistant to all current therapies, and unresectable, recurrent, or metastatic tumors are considered incurable. Preclinical studies have identified several novel candidate molecular targets for therapeutic intervention, but, to date, targeted therapies have proven ineffective. Recent studies have identified recurrent mutations in polycomb repressive complex 2 (PRC2) core components, embryonic ectoderm development protein (EED) and suppressor of zeste 12 homolog (SUZ12), in MPNST. These mutations result in global loss of the histone H3 lysine 27 trimethylation epigenetic mark, normally deposited by PRC2, and subsequent gain in acetylation at this residue. This altered chromatin state has been shown to promote MPNST malignancy; however, acetylation at this residue sensitizes MPNSTs to BRD4 and bromodomain and extra-terminal domain inhibition. Interestingly, the catalytic component of PRC2, enhancer of zeste homolog 2 (EZH2), is not mutated in MPNST, hinting that a noncanonical, PRC2-independent function of EZH2 may play a role in this cancer. This review examines the pathobiology of MPNST, the contribution of PRC2 subunits to this process, and the prospects for PRC2-related therapies for this cancer. IMPLICATIONS: Identification of mutations in the PRC2 components EED and SUZ12 in the majority of MPNSTs may imply noncanonical oncogenic activities of the intact component, EZH2, and provide new opportunities for therapeutic intervention.

Reprogramming Captures the Genetic and Tumorigenic Properties of Neurofibromatosis Type 1 Plexiform Neurofibromas

Neurofibromatosis type 1 (NF1) is a tumor predisposition genetic disease caused by mutations in the NF1 tumor suppressor gene. Plexiform neurofibromas (PNFs) are benign Schwann cell (SC) tumors of the peripheral nerve sheath that develop through NF1 inactivation and can progress toward a malignant soft tissue sarcoma. There is a lack of non-perishable model systems to investigate PNF development. We reprogrammed PNF-derived NF1(-/-) cells, descendants from the tumor originating cell. These NF1(-/-)-induced pluripotent stem cells (iPSCs) captured the genomic status of PNFs and were able to differentiate toward neural crest stem cells and further to SCs. iPSC-derived NF1(-/-) SCs exhibited a continuous high proliferation rate, poor myelination ability, and a tendency to form 3D spheres that expressed the same markers as their PNF-derived primary SC counterparts. They represent a valuable model to study and treat PNFs. PNF-derived iPSC lines were banked for making them available.

NF1 heterozygosity fosters de novo tumorigenesis but impairs malignant transformation

Neurofibromatosis type 1 (NF1) is an autosomal genetic disorder. Patients with NF1 are associated with mono-allelic loss of the tumor suppressor gene NF1 in their germline, which predisposes them to develop a wide array of benign lesions. Intriguingly, recent sequencing efforts revealed that the NF1 gene is frequently mutated in multiple malignant tumors not typically associated with NF1 patients, suggesting that NF1 heterozygosity is refractory to at least some cancer types. In two orthogonal mouse models representing NF1- and non-NF1-related tumors, we discover that an Nf1^+/− microenvironment accelerates the formation of benign tumors but impairs further progression to malignancy. Analysis of benign and malignant tumors commonly associated with NF1 patients, as well as those with high NF1 gene mutation frequency, reveals an antagonistic role for NF1 heterozygosity in tumor initiation and malignant transformation and helps to reconciliate the role of the NF1 gene in both NF1 and non-NF1 patient contexts.

Germline mono-allelic loss of the tumour suppressor NF1 predisposes patients to the development of benign lesions but rarely further progression into cancer development. Here the authors use mouse models to show that an NF1 heterozygous microenvironment accelerates the formation of benign tumours but impairs progression to malignancy.

Spatiotemporal Loss of NF1 in Schwann Cell Lineage Leads to Different Types of Cutaneous Neurofibroma Susceptible to Modification by the Hippo Pathway

Neurofibromatosis type 1 (NF1) is a cancer predisposition disorder that results from inactivation of the tumor suppressor neurofibromin, a negative regulator of RAS signaling. Patients with NF1 present with a wide range of clinical manifestations, and the tumor with highest prevalence is cutaneous neurofibroma (cNF). Most patients harboring cNF suffer greatly from the burden of those tumors, which have no effective medical treatment. Ironically, none of the numerous NF1 mouse models developed so far recapitulate cNF. Here, we discovered that HOXB7 serves as a lineage marker to trace the developmental origin of cNF neoplastic cells. Ablating Nf1 in the HOXB7 lineage faithfully recapitulates both human cutaneous and plexiform neurofibroma. In addition, we discovered that modulation of the Hippo pathway acts as a "modifier" for neurofibroma tumorigenesis. This mouse model opens the doors for deciphering the evolution of cNF to identify effective therapies, where none exist today. SIGNIFICANCE: This study provides insights into the developmental origin of cNF, the most common tumor in NF1, and generates the first mouse model that faithfully recapitulates both human cutaneous and plexiform neurofibroma. The study also demonstrates that the Hippo pathway can modify neurofibromagenesis, suggesting that dampening the Hippo pathway could be an attractive therapeutic target.This article is highlighted in the In This Issue feature, p. 1.