Plexiform and dermal neurofibromas and pigmentation are caused by Nf1 loss in desert hedgehog-expressing cells

EZH2-dependent myelination following sciatic nerve injury

JOURNAL/nrgr/04.03/01300535-202508000-00028/figure1/v/2024-09-30T120553Z/r/image-tiff Demyelination and remyelination have been major focal points in the study of peripheral nerve regeneration following peripheral nerve injury. Notably, the gene regulatory network of regenerated myelin differs from that of native myelin. Silencing of enhancer of zeste homolog 2 (EZH2) hinders the differentiation, maturation, and myelination of Schwann cells in vitro. To further determine the role of EZH2 in myelination and recovery post-peripheral nerve injury, conditional knockout mice lacking Ezh2 in Schwann cells (Ezh2fl/fl;Dhh-Cre and Ezh2fl/fl;Mpz-Cre) were generated. Our results show that a significant proportion of axons in the sciatic nerve of Ezh2-depleted mice remain unmyelinated. This highlights the crucial role of Ezh2 in initiating Schwann cell myelination. Furthermore, we observed that 21 days after inducing a sciatic nerve crush injury in these mice, most axons had remyelinated at the injury site in the control nerve, while Ezh2fl/fl;Mpz-Cre mice had significantly fewer remyelinated axons compared with their wild-type littermates. This suggests that the absence of Ezh2 in Schwann cells impairs myelin formation and remyelination. In conclusion, EZH2 has emerged as a pivotal regulatory factor in the process of demyelination and myelin regeneration following peripheral nerve injury. Modulating EZH2 activity during these processes may offer a promising therapeutic target for the treatment of peripheral nerve injuries.

Stimulator of interferon gene facilitates recruitment of effector CD8 T cells that drive neurofibromatosis type 1 nerve tumor initiation and maintenance

Plexiform neurofibromas (PNFs) are benign nerve tumors driven by loss of the NF1 tumor suppressor in Schwann cells. PNFs are rich in immune cells, but whether immune cells are necessary for tumorigenesis is unknown. We show that inhibition of stimulator of interferon gene (STING) reduces plasma CXCL10, tumor T cell and dendritic cell (DC) recruitment, and tumor formation. Further, mice lacking XCR-1+ DCs showed reduced tumor-infiltrating T cells and PNF tumors. Antigen-presenting cells from tumor-bearing mice promoted CD8+ T cell proliferation in vitro, and PNF T cells expressed high levels of CCL5, implicating T cell activation. Notably, tumors and nerve-associated macrophages were absent in Rag1-/-; Nf1f/f; DhhCre mice and adoptive transfer of CD8+ T cells from tumor-bearing mice restored PNF initiation. In this setting, PNF shrunk upon subsequent T cell removal. Thus, STING pathway activation contributes to CD8+ T cell-dependent inflammatory responses required for PNF initiation and maintenance.

Investigating therapeutic nonsense suppression in a neurofibromatosis mouse model

Neurofibromatosis type 1 (NF1) is a human genetic disorder caused by variants in the NF1 gene. Plexiform neurofibromas, one of many NF1 manifestations, are benign peripheral nerve sheath tumors occurring in up to 50% of NF1 patients. A substantial fraction of NF1 pathogenetic variants are nonsense mutations, which result in the synthesis of truncated non-functional NF1 protein (neurofibromin). To date, no therapeutics have restored neurofibromin expression or addressed the consequences of this protein's absence in NF1 nonsense mutation patients, but nonsense suppression is a potential approach to the problem. Ataluren is a small molecule drug that has been shown to stimulate functional nonsense codon readthrough in several models of nonsense mutation diseases, as well as in Duchenne muscular dystrophy patients. To test ataluren's potential applicability in nonsense mutation NF1 patients, we evaluated its therapeutic effects using three treatment regimens in a previously established NF1 patient-derived (c.2041C > T; p.Arg681X) nonsense mutation mouse model. Collectively, our experiments indicate that: i) ataluren appeared to slow the growth of neurofibromas and alleviate some paralysis phenotypes, ii) female Nf1-nonsense mutation mice manifested more severe paralysis and neurofibroma phenotypes than male mice, iii) ataluren doses with apparent effectiveness were lower in female mice than in male mice, and iv) age factors also influenced ataluren's effectiveness.

Cold Atmospheric Plasma Induces Growth Arrest and Apoptosis in Neurofibromatosis Type 1-Associated Peripheral Nerve Sheath Tumor Cells

Neurofibromatosis type 1 (NF1) is an autosomal dominant disorder resulting from mutations in the NF1 gene. Patients harboring these mutations are predisposed to a spectrum of peripheral nerve sheath tumors (PNSTs) originating from Schwann cells, of which malignant peripheral nerve sheath tumors (MPNSTs) are the deadliest, with limited treatment options. Therefore, an unmet need still exists for more effective therapies directed at these aggressive malignancies. Cold atmospheric plasma (CAP) is a reactive oxygen species (ROS) and reactive nitrogen species (RNS) generating ionized gas that has been proposed to be a potential therapeutic modality for cancer. In this study, we sought to determine the effects of CAP on NF1-associated PNSTs. Utilizing established mouse and human cell lines to interrogate the effects of CAP in both in vitro and in vivo settings, we found that NF1-associated PNSTs were highly sensitive to CAP exposure, resulting in cell death. To our knowledge, this is the first application of CAP to NF1-associated PNSTs and provides a unique opportunity to study the complex biology of NF1-associated tumors.

Ex Vivo Patient-Derived Explant Model for Neurofibromatosis Type 1-Related Cutaneous Neurofibromas

Cutaneous neurofibromas (CNFs) are benign tumors that occur in the dermis of individuals with the inherited tumor predisposition disorder, neurofibromatosis type 1. CNFs cause disfigurement, pain, burning, and itching, resulting in substantially reduced QOL in patients with neurofibromatosis type 1. CNFs are benign tumors that exhibit cellular and molecular heterogeneity, making it difficult to develop tractable in vitro or in vivo models. As a result, CNF research and drug discovery efforts have been limited. To address this need, we developed a reproducible patient-derived explant (PDE) ex vivo culture model using CNF tumors from patients with neurofibromatosis type 1. CNF PDEs remain viable in culture for over 9 days and recapitulate the cellular composition and molecular signaling of CNFs. Using CNF PDEs as a model system, we found that proliferation was associated with increased T-cell infiltration. Furthermore, we identified a pattern of reciprocal inflammatory signaling in CNF PDEs in which tumors rely on prostaglandin or leukotriene-mediated signaling pathways. As proof of principle, we show that ex vivo glucocorticoid treatment reduced the expression of proinflammatory genes, confirming that CNF PDEs are a useful model for both mechanistic studies and preclinical drug testing.

Schwann cell-secreted PGE2 promotes sensory neuron excitability during development

Electrical excitability-the ability to fire and propagate action potentials-is a signature feature of neurons. How neurons become excitable during development and whether excitability is an intrinsic property of neurons remain unclear. Here, we demonstrate that Schwann cells, the most abundant glia in the peripheral nervous system, promote somatosensory neuron excitability during development. We find that Schwann cells secrete prostaglandin E2, which is necessary and sufficient to induce developing somatosensory neurons to express normal levels of genes required for neuronal function, including voltage-gated sodium channels, and to fire action potential trains. Inactivating this signaling pathway in Schwann cells impairs somatosensory neuron maturation, causing multimodal sensory defects that persist into adulthood. Collectively, our studies uncover a neurodevelopmental role for prostaglandin E2 distinct from its established role in inflammation, revealing a cell non-autonomous mechanism by which glia regulate neuronal excitability to enable the development of normal sensory functions.

NF1-dependent disruption of the blood-nerve-barrier is improved by blockade of P2RY14

The blood-nerve-barrier (BNB) that regulates peripheral nerve homeostasis is formed by endoneurial capillaries and perineurial cells surrounding the Schwann cell (SC)-rich endoneurium. Barrier dysfunction is common in human tumorigenesis, including in some nerve tumors. We identify barrier disruption in human NF1 deficient neurofibromas, which were characterized by reduced perineurial cell glucose transporter 1 (GLUT1) expression and increased endoneurial fibrin(ogen) deposition. Conditional Nf1 loss in murine SCs recapitulated these alterations and revealed decreased tight junctions and decreased caveolin-1 (Cav1) expression in mutant nerves and in tumors, implicating reduced Cav1-mediated transcytosis in barrier disruption and tumorigenesis. Additionally, elevated receptor tyrosine kinase activity and genetic deletion of Cav1 increased endoneurial fibrin(ogen), and promoted SC tumor formation. Finally, when SC lacked Nf1, genetic loss or pharmacological inhibition of P2RY14 rescued Cav1 expression and barrier function. Thus, loss of Nf1 in SC causes dysfunction of the BNB via P2RY14-mediated G-protein coupled receptor (GPCR) signaling.

MEK-SHP2 inhibition prevents tibial pseudarthrosis caused by NF1 loss in Schwann cells and skeletal stem/progenitor cells

Congenital pseudarthrosis of the tibia (CPT) is a severe pathology marked by spontaneous bone fractures that fail to heal, leading to fibrous nonunion. Half of patients with CPT are affected by the multisystemic genetic disorder neurofibromatosis type 1 (NF1) caused by mutations in the NF1 tumor suppressor gene, a negative regulator of RAS-mitogen-activated protein kinase (MAPK) signaling pathway. Here, we analyzed patients with CPT and Prss56-Nf1 knockout mice to elucidate the pathogenic mechanisms of CPT-related fibrous nonunion and explored a pharmacological approach to treat CPT. We identified NF1-deficient Schwann cells and skeletal stem/progenitor cells (SSPCs) in pathological periosteum as affected cell types driving fibrosis. Whereas NF1-deficient SSPCs adopted a fibrotic fate, NF1-deficient Schwann cells produced critical paracrine factors including transforming growth factor-β and induced fibrotic differentiation of wild-type SSPCs. To counteract the elevated RAS-MAPK signaling in both NF1-deficient Schwann cells and SSPCs, we used MAPK kinase (MEK) and Src homology 2 containing protein tyrosine phosphatase 2 (SHP2) inhibitors. Combined MEK-SHP2 inhibition in vivo prevented fibrous nonunion in the Prss56-Nf1 knockout mouse model, providing a promising therapeutic strategy for the treatment of fibrous nonunion in CPT.

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.

C5aR plus MEK inhibition durably targets the tumor milieu and reveals tumor cell phagocytosis

Plexiform neurofibromas (PNFs) are nerve tumors caused by loss of NF1 and dysregulation of RAS-MAPK signaling in Schwann cells. Most PNFs shrink in response to MEK inhibition, but targets with increased and durable effects are needed. We identified the anaphylatoxin C5a as increased in PNFs and expressed largely by PNF m acrophages. We defined pharmacokinetic and immunomodulatory properties of a C5aR1/2 antagonist and tested if peptide antagonists augment the effects of MEK inhibition. MEK inhibition recruited C5AR1 to the macrophage surface; short-term inhibition of C5aR elevated macrophage apoptosis and Schwann cell death, without affecting MEK-induced tumor shrinkage. PNF macrophages lacking C5aR1 increased the engulfment of dying Schwann cells, allowing their visualization. Halting combination therapy resulted in altered T-cell distribution, elevated Iba1+ and CD169+ immunoreactivity, and profoundly altered cytokine expression, but not sustained trumor shrinkage. Thus, C5aRA inhibition independently induces macrophage cell death and causes sustained and durable effects on the PNF microenvironment.

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.

Gene-targeted therapy for neurofibromatosis and schwannomatosis: The path to clinical trials

Numerous successful gene-targeted therapies are arising for the treatment of a variety of rare diseases. At the same time, current treatment options for neurofibromatosis 1 and schwannomatosis are limited and do not directly address loss of gene/protein function. In addition, treatments have mostly focused on symptomatic tumors, but have failed to address multisystem involvement in these conditions. Gene-targeted therapies hold promise to address these limitations. However, despite intense interest over decades, multiple preclinical and clinical issues need to be resolved before they become a reality. The optimal approaches to gene-, mRNA-, or protein restoration and to delivery to the appropriate cell types remain elusive. Preclinical models that recapitulate manifestations of neurofibromatosis 1 and schwannomatosis need to be refined. The development of validated assays for measuring neurofibromin and merlin activity in animal and human tissues will be critical for early-stage trials, as will the selection of appropriate patients, based on their individual genotypes and risk/benefit balance. Once the safety of gene-targeted therapy for symptomatic tumors has been established, the possibility of addressing a wide range of symptoms, including non-tumor manifestations, should be explored. As preclinical efforts are underway, it will be essential to educate both clinicians and those affected by neurofibromatosis 1/schwannomatosis about the risks and benefits of gene-targeted therapy for these conditions.

Stem cell modeling of nervous system tumors

Nervous system tumors, particularly brain tumors, represent the most common tumors in children and one of the most lethal tumors in adults. Despite decades of research, there are few effective therapies for these cancers. Although human nervous system tumor cells and genetically engineered mouse models have served as excellent platforms for drug discovery and preclinical testing, they have limitations with respect to accurately recapitulating important aspects of the pathobiology of spontaneously arising human tumors. For this reason, attention has turned to the deployment of human stem cell engineering involving human embryonic or induced pluripotent stem cells, in which genetic alterations associated with nervous system cancers can be introduced. These stem cells can be used to create self-assembling three-dimensional cerebral organoids that preserve key features of the developing human brain. Moreover, stem cell-engineered lines are amenable to xenotransplantation into mice as a platform to investigate the tumor cell of origin, discover cancer evolutionary trajectories and identify therapeutic vulnerabilities. In this article, we review the current state of human stem cell models of nervous system tumors, discuss their advantages and disadvantages, and provide consensus recommendations for future research.

Schwann cells modulate nociception in neurofibromatosis 1

Pain of unknown etiology is frequent in individuals with the tumor predisposition syndrome neurofibromatosis 1 (NF1), even when tumors are absent. Nerve Schwann cells (SCs) were recently shown to play roles in nociceptive processing, and we find that chemogenetic activation of SCs is sufficient to induce afferent and behavioral mechanical hypersensitivity in wild-type mice. In mouse models, animals showed afferent and behavioral hypersensitivity when SCs, but not neurons, lacked Nf1. Importantly, hypersensitivity corresponded with SC-specific upregulation of mRNA encoding glial cell line-derived neurotrophic factor (GDNF), independently of the presence of tumors. Neuropathic pain-like behaviors in the NF1 mice were inhibited by either chemogenetic silencing of SC calcium or by systemic delivery of GDNF-targeting antibodies. Together, these findings suggest that alterations in SCs directly modulate mechanical pain and suggest cell-specific treatment strategies to ameliorate pain in individuals with NF1.

Shedding New Light: Novel Therapies for Common Disorders in Children with Neurofibromatosis Type I

Neurofibromatosis type I (NF1) is a common dominantly inherited disorder, and one of the most common of the RASopathies. Most individuals with NF1 develop plexiform neurofibromas and cutaneous neurofibromas, nerve tumors caused by NF1 loss of function in Schwann cells. Cell culture models and mouse models of NF1 are being used to test drug efficacy in preclinical trials, which led to Food and Drug Administration approval for use of MEK inhibitors to shrink most inoperable plexiform neurofibromas. This article details methods used for testing in preclinical models, and outlines newer models that may identify additional, curative, strategies.

Single-cell RNA sequencing of neurofibromas reveals a tumor microenvironment favorable for neural regeneration and immune suppression in a neurofibromatosis type 1 porcine model

Neurofibromatosis Type 1 (NF1) is one of the most common genetically inherited disorders that affects 1 in 3000 children annually. Clinical manifestations vary widely but nearly always include the development of cutaneous, plexiform and diffuse neurofibromas that are managed over many years. Recent single-cell transcriptomics profiling efforts of neurofibromas have begun to reveal cell signaling processes. However, the cell signaling networks in mature, non-cutaneous neurofibromas remain unexplored. Here, we present insights into the cellular composition and signaling within mature neurofibromas, contrasting with normal adjacent tissue, in a porcine model of NF1 using single-cell RNA sequencing (scRNA-seq) analysis and histopathological characterization. These neurofibromas exhibited classic diffuse-type histologic morphology and expected patterns of S100, SOX10, GFAP, and CD34 immunohistochemistry. The porcine mature neurofibromas closely resemble human neurofibromas histologically and contain all known cellular components of their human counterparts. The scRNA-seq confirmed the presence of all expected cell types within these neurofibromas and identified novel populations of fibroblasts and immune cells, which may contribute to the tumor microenvironment by suppressing inflammation, promoting M2 macrophage polarization, increasing fibrosis, and driving the proliferation of Schwann cells. Notably, we identified tumor-associated IDO1 +/CD274+ (PD-L1) + dendritic cells, which represent the first such observation in any NF1 animal model and suggest the role of the upregulation of immune checkpoints in mature neurofibromas. Finally, we observed that cell types in the tumor microenvironment are poised to promote immune evasion, extracellular matrix reconstruction, and nerve regeneration.

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.

Existing and Developing Preclinical Models for Neurofibromatosis Type 1-Related Cutaneous Neurofibromas

Neurofibromatosis type 1 (NF1) is caused by a nonfunctional copy of the NF1 tumor suppressor gene that predisposes patients to the development of cutaneous neurofibromas (cNFs), the skin tumor that is the hallmark of this condition. Innumerable benign cNFs, each appearing by an independent somatic inactivation of the remaining functional NF1 allele, form in nearly all patients with NF1. One of the limitations in developing a treatment for cNFs is an incomplete understanding of the underlying pathophysiology and limitations in experimental modeling. Recent advances in preclinical in vitro and in vivo modeling have substantially enhanced our understanding of cNF biology and created unprecedented opportunities for therapeutic discovery. We discuss the current state of cNF preclinical in vitro and in vivo model systems, including two- and three-dimensional cell cultures, organoids, genetically engineered mice, patient-derived xenografts, and porcine models. We highlight the models' relationship to human cNFs and how they can be used to gain insight into cNF development and therapeutic discovery.

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.

Combining SOS1 and MEK Inhibitors in a Murine Model of Plexiform Neurofibroma Results in Tumor Shrinkage

Individuals with neurofibromatosis type 1 develop rat sarcoma virus (RAS)-mitogen-activated protein kinase-mitogen-activated and extracellular signal-regulated kinase (RAS-MAPK-MEK)-driven nerve tumors called neurofibromas. Although MEK inhibitors transiently reduce volumes of most plexiform neurofibromas in mouse models and in neurofibromatosis type 1 (NF1) patients, therapies that increase the efficacy of MEK inhibitors are needed. BI-3406 is a small molecule that prevents Son of Sevenless (SOS)1 interaction with Kirsten rat sarcoma viral oncoprotein (KRAS)-GDP, interfering with the RAS-MAPK cascade upstream of MEK. Single agent SOS1 inhibition had no significant effect in the DhhCre;Nf1 fl/fl mouse model of plexiform neurofibroma, but pharmacokinetics (PK)-driven combination of selumetinib with BI-3406 significantly improved tumor parameters. Tumor volumes and neurofibroma cell proliferation, reduced by MEK inhibition, were further reduced by the combination. Neurofibromas are rich in ionized calcium binding adaptor molecule 1 (Iba1)+ macrophages; combination treatment resulted in small and round macrophages, with altered cytokine expression indicative of altered activation. The significant effects of MEK inhibitor plus SOS1 inhibition in this preclinical study suggest potential clinical benefit of dual targeting of the RAS-MAPK pathway in neurofibromas. SIGNIFICANCE STATEMENT: Interfering with the RAS-mitogen-activated protein kinase (RAS-MAPK) cascade upstream of mitogen activated protein kinase kinase (MEK), together with MEK inhibition, augment effects of MEK inhibition on neurofibroma volume and tumor macrophages in a preclinical model system. This study emphasizes the critical role of the RAS-MAPK pathway in controlling tumor cell proliferation and the tumor microenvironment in benign neurofibromas.

Runx1/3-driven adaptive endoplasmic reticulum stress pathways contribute to neurofibromagenesis

Neurofibromatosis type 1 (NF1) patients are predisposed to develop plexiform neurofibromas (PNFs). Three endoplasmic reticulum (ER) stress response pathways restore cellular homeostasis. The unfolded protein response (UPR) sensors contribute to tumor initiation in many cancers. We found that all three UPR pathways were activated in mouse and human PNFs, with protein kinase RNA [PKR]-like ER kinase (PERK) the most highly expressed. We tested if neurofibroma cells adapt to ER stress, leading to their growth. Pharmacological or genetic inhibition of PERK reduced mouse neurofibroma-sphere number, and genetic inhibition in PERK in Schwann cell precursors (SCPs) decreased tumor-like lesion numbers in a cell transplantation model. Further, in a PNF mouse model, deletion of PERK in Schwann cells (SCs) and SCPs reduced tumor size, number, and increased survival. Mechanistically, loss of Nf1 activated PERK-eIF2α-ATF4 signaling and increased ATF4 downstream target gene p21 translocation from nucleus to cytoplasm. This nucleus-cytoplasm translocation was mediated by exportin-1. Runx transcriptionally activated ribosome gene expression and increased protein synthesis to allow SCs to adapt to ER stress and tumor formation. We propose that targeting proteostasis might provide cytotoxic and/or potentially durable novel therapy for PNFs.

Malignant Peripheral Nerve Sheath Tumors: Latest Concepts in Disease Pathogenesis and Clinical Management

Malignant peripheral nerve sheath tumor (MPNST) is an aggressive soft tissue sarcoma with limited therapeutic options and a poor prognosis. Although neurofibromatosis type 1 (NF1) and radiation exposure have been identified as risk factors for MPNST, the genetic and molecular mechanisms underlying MPNST pathogenesis have only lately been roughly elucidated. Plexiform neurofibroma (PN) and atypical neurofibromatous neoplasm of unknown biological potential (ANNUBP) are novel concepts of MPNST precancerous lesions, which revealed sequential mutations in MPNST development. This review summarized the current understanding of MPNST and the latest consensus from its diagnosis to treatment, with highlights on molecular biomarkers and targeted therapies. Additionally, we discussed the current challenges and prospects for MPNST management.

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.

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.

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.

Concomitant variants in NF1, LZTR1 and GNAZ genes probably contribute to the aggressiveness of plexiform neurofibroma and warrant treatment with MEK inhibitor

Neurofibromatosis 1 (NF1) is caused by germline mutations in the NF1 gene and manifests as proliferation of various tissues, including plexiform neurofibromas. The plexiform neurofibroma phenotype varies from indolent to locally aggressive, suggesting contributions of other modifiers in addition to somatic loss of NF1. In this study, we investigated a life-threatening plexiform neurofibroma in a 9-month-old female infant with NF1. Germline mutations in two RASopathy-associated genes were identified using whole-exome sequencing-a de novo pathogenic variant in the NF1 gene, and a known pathogenic variant in the LZTR1 gene. Somatic analysis of the plexiform neurofibroma revealed NF1 loss of heterozygosity and a variant in GNAZ, a gene encoding a G protein-coupled receptor. Cells expressing mutant GNAZ exhibited increased ERK 1/2 activation compared to those expressing wild-type GNAZ. Taken together, we suggest the variants in NF1, LZRT1 and GNAZ act synergistically in our patient, leading to MAPK pathway activation and contributing to the severity of the patient's plexiform neurofibromatosis. After treatment with the MEK inhibitor, trametinib, a prominent clinical improvement was observed in this patient. This case study contributes to the knowledge of germline and somatic non-NF1 variants affecting the NF1 clinical phenotype and supports use of personalized, targeted therapy.

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.

Toward Understanding the Mechanisms of Malignant Peripheral Nerve Sheath Tumor Development

Malignant peripheral nerve sheath tumors (MPNSTs) originate from the neural crest lineage and are associated with the neurofibromatosis type I syndrome. MPNST is an unmet clinical need. In this review article, we summarize the knowledge and discuss research perspectives related to (1) the natural history of MPNST development; (2) the mouse models recapitulating the progression from precursor lesions to MPNST; (3) the role of the tumor microenvironment in MPNST development, and (4) the signaling pathways linked to MPNST development.

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.

Enhanced enteric neurogenesis by Schwann cell precursors in mouse models of Hirschsprung disease

Hirschsprung disease (HSCR) is characterized by congenital absence of enteric neurons in distal portions of the gut. Although recent studies identified Schwann cell precursors (SCPs) as a novel cellular source of enteric neurons, it is unknown how SCPs contribute to the disease phenotype of HSCR. Using Schwann cell-specific genetic labeling, we investigated SCP-derived neurogenesis in two mouse models of HSCR; Sox10 haploinsufficient mice exhibiting distal colonic aganglionosis and Ednrb knockout mice showing small intestinal aganglionosis. We also examined Ret dependency in SCP-derived neurogenesis using mice displaying intestinal aganglionosis in which Ret expression was conditionally removed in the Schwann cell lineage. SCP-derived neurons were abundant in the transition zone lying between the ganglionated and aganglionic segments, although SCP-derived neurogenesis was scarce in the aganglionic region. In the transition zone, SCPs mainly gave rise to nitrergic neurons that are rarely observed in the SCP-derived neurons under the normal condition. Enhanced SCP-derived neurogenesis was also detected in the transition zone of mice lacking RET expression in the Schwann cell lineage. Increased SCP-derived neurogenesis in the transition zone suggests that reduction in the vagal neural crest-derived enteric neurons promotes SCP-derived neurogenesis. SCPs may adopt a neuronal subtype by responding to changes in the gut environment. Robust SCP-derived neurogenesis can occur in a Ret-independent manner, which suggests that SCPs are a cellular source to compensate for missing enteric neurons in HSCR.

Mek/ERK1/2-MAPK and PI3K/Akt/mTOR signaling plays both independent and cooperative roles in Schwann cell differentiation, myelination and dysmyelination

Multiple signals are involved in the regulation of developmental myelination by Schwann cells and in the maintenance of a normal myelin homeostasis throughout adult life, preserving the integrity of the axons in the PNS. Recent studies suggest that Mek/ERK1/2-MAPK and PI3K/Akt/mTOR intracellular signaling pathways play important, often overlapping roles in the regulation of myelination in the PNS. In addition, hyperactivation of these signaling pathways in Schwann cells leads to a late onset of various pathological changes in the sciatic nerves. However, it remains poorly understood whether these pathways function independently or sequentially or converge using a common mechanism to facilitate Schwann cell differentiation and myelin growth during development and in causing pathological changes in the adult animals. To address these questions, we analyzed multiple genetically modified mice using simultaneous loss- and constitutive gain-of-function approaches. We found that during development, the Mek/ERK1/2-MAPK pathway plays a primary role in Schwann cell differentiation, distinct from mTOR. However, during active myelination, ERK1/2 is dependent on mTOR signaling to drive the growth of the myelin sheath and regulate its thickness. Finally, our data suggest that peripheral nerve pathology during adulthood caused by hyperactivation of Mek/ERK1/2-MAPK or PI3K is likely to be independent or dependent on mTOR-signaling in different contexts. Thus, this study highlights the complexities in the roles played by two major intracellular signaling pathways in Schwann cells that affect their differentiation, myelination, and later PNS pathology and predicts that potential therapeutic modulation of these pathways in PNS neuropathies could be a complex process.

Genetic Events and Signaling Mechanisms Underlying Schwann Cell Fate in Development and Cancer

In this review, we describe Schwann cell development from embryonic neural crest cells to terminally differentiated myelinated and nonmyelinated mature Schwann cells. We focus on the genetic drivers and signaling mechanisms mediating decisions to proliferate versus differentiate during Schwann cell development, highlighting pathways that overlap with Schwann cell development and are dysregulated in tumorigenesis. We conclude by considering how our knowledge of the events underlying Schwann cell development and mouse models of schwannoma, neurofibroma, and malignant peripheral nerve sheath tumor can inform novel therapeutic strategies for patients with cancers derived from Schwann cell lineages.

Neurofibromatosis in the Era of Precision Medicine: Development of MEK Inhibitors and Recent Successes with Selumetinib

PURPOSE OF REVIEW

Patients with neurofibromatosis type 1 (NF1) are at increased risk for benign and malignant neoplasms. Recently, targeted therapy with the MEK inhibitor class has helped address these needs. We highlight recent successes with selumetinib while acknowledging ongoing challenges for NF1 patients and future directions.

RECENT FINDINGS

MEK inhibitors have demonstrated efficacy for NF1-related conditions, including plexiform neurofibromas and low-grade gliomas, two common causes of NF1-related morbidity. Active investigations for NF1-related neoplasms have benefited from advanced understanding of the genomic and cell signaling alterations in these conditions and development of sound preclinical animal models. Selumetinib has become the first FDA-approved targeted therapy for NF1 following its demonstrated efficacy for inoperable plexiform neurofibroma. Investigations of combination therapy and the development of a representative NF1 swine model hold promise for translating therapies for other NF1-associated pathology.

Spontaneous and Engineered Large Animal Models of Neurofibromatosis Type 1

Animal models are crucial to understanding human disease biology and developing new therapies. By far the most common animal used to investigate prevailing questions about human disease is the mouse. Mouse models are powerful tools for research as their small size, limited lifespan, and defined genetic background allow researchers to easily manipulate their genome and maintain large numbers of animals in general laboratory spaces. However, it is precisely these attributes that make them so different from humans and explains, in part, why these models do not accurately predict drug responses in human patients. This is particularly true of the neurofibromatoses (NFs), a group of genetic diseases that predispose individuals to tumors of the nervous system, the most common of which is Neurofibromatosis type 1 (NF1). Despite years of research, there are still many unanswered questions and few effective treatments for NF1. Genetically engineered mice have drastically improved our understanding of many aspects of NF1, but they do not exemplify the overall complexity of the disease and some findings do not translate well to humans due to differences in body size and physiology. Moreover, NF1 mouse models are heavily reliant on the Cre-Lox system, which does not accurately reflect the molecular mechanism of spontaneous loss of heterozygosity that accompanies human tumor development. Spontaneous and genetically engineered large animal models may provide a valuable supplement to rodent studies for NF1. Naturally occurring comparative models of disease are an attractive prospect because they occur on heterogeneous genetic backgrounds and are due to spontaneous rather than engineered mutations. The use of animals with naturally occurring disease has been effective for studying osteosarcoma, lymphoma, and diabetes. Spontaneous NF-like symptoms including neurofibromas and malignant peripheral nerve sheath tumors (MPNST) have been documented in several large animal species and share biological and clinical similarities with human NF1. These animals could provide additional insight into the complex biology of NF1 and potentially provide a platform for pre-clinical trials. Additionally, genetically engineered porcine models of NF1 have recently been developed and display a variety of clinical features similar to those seen in NF1 patients. Their large size and relatively long lifespan allow for longitudinal imaging studies and evaluation of innovative surgical techniques using human equipment. Greater genetic, anatomic, and physiologic similarities to humans enable the engineering of precise disease alleles found in human patients and make them ideal for preclinical pharmacokinetic and pharmacodynamic studies of small molecule, cellular, and gene therapies prior to clinical trials in patients. Comparative genomic studies between humans and animals with naturally occurring disease, as well as preclinical studies in large animal disease models, may help identify new targets for therapeutic intervention and expedite the translation of new therapies. In this review, we discuss new genetically engineered large animal models of NF1 and cases of spontaneous NF-like manifestations in large animals, with a special emphasis on how these comparative models could act as a crucial translational intermediary between specialized murine models and NF1 patients.

Neuronal lineages derived from the nerve-associated Schwann cell precursors

For a long time, neurogenic placodes and migratory neural crest cells were considered the immediate sources building neurons of peripheral nervous system. Recently, a number of discoveries revealed the existence of another progenitor type-a nerve-associated multipotent Schwann cell precursors (SCPs) building enteric and parasympathetic neurons as well as neuroendocrine chromaffin cells. SCPs are neural crest-derived and are similar to the crest cells by their markers and differentiation potential. Such similarities, but also considerable differences, raise many questions pertaining to the medical side, fundamental developmental biology and evolution. Here, we discuss the genesis of Schwann cell precursors, their role in building peripheral neural structures and ponder on their role in the origin in congenial diseases associated with peripheral nervous systems.

MicroRNA-155 contributes to plexiform neurofibroma growth downstream of MEK

MicroRNAs (miRs) are small non-coding RNAs that can have large impacts on oncogenic pathways. Possible functions of dysregulated miRs have not been studied in neurofibromatosis type 1 (NF1) plexiform neurofibromas (PNFs). In PNFs, Schwann cells (SCs) have biallelic NF1 mutations necessary for tumorigenesis. We analyzed a miR-microarray comparing to normal and PNF SCs and identified differences in miR expression, and we validated in mouse PNFs versus normal mouse SCs by qRT-PCR. Among these, miR-155 was a top overexpressed miR, and its expression was regulated by RAS/MAPK signaling. Overexpression of miR-155 increased mature Nf1^−/− mouse SC proliferation. In SC precursors, which model tumor initiating cells, pharmacological and genetic inhibition of miR-155 decreased PNF-derived sphere numbers in vitro and we identified Maf as a miR-155 target. In vivo, global deletion of miR-155 significantly decreased tumor number and volume, increasing mouse survival. Fluorescent nanoparticles entered PNFs, suggesting that an anti-miR might have therapeutic potential. However, treatment of established PNFs using anti-miR-155 peptide nucleic acid-loaded nanoparticles marginally decreased tumor numbers and did not reduce tumor growth. These results suggest that miR-155 plays a functional role in PNF growth and/or SC proliferation, and that targeting neurofibroma miRs is feasible, and might provide novel therapeutic opportunities.

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.

Peripheral nerve tumors of the hand: Clinical features, diagnosis, and treatment

The majority of the tumors arising from the peripheral nerves of the hand are relatively benign. However, a tumor diagnosed as malignant peripheral nerve sheath tumor (MPNST) has destructive consequences. Clinical signs and symptoms are usually caused by direct and indirect effects of the tumor, such as nerve invasion or compression and infiltration of surrounding tissues. Definitive diagnosis is made by tumor biopsy. Complete surgical removal with maximum reservation of residual neurologic function is the most appropriate intervention for most symptomatic benign peripheral nerve tumors (PNTs) of the hand; however, MPNSTs require surgical resection with a sufficiently wide margin or even amputation to improve prognosis. In this article, we review the clinical presentation and radiographic features, summarize the evidence for an accurate diagnosis, and discuss the available treatment options for PNTs of the hand.

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.

Current status of MEK inhibitors in the treatment of plexiform neurofibromas

BACKGROUND

Neurofibromatosis type 1 (NF1)-related plexiform neurofibromas (pNF) can be debilitating and until recently, surgery was the only potentially effective therapy for these tumors.

METHODS

We review critical steps in the path towards the FDA approval of the first medical therapy for NF1 pNF and the current status of MEK inhbitor therapy.

RESULTS

Sustained efforts by the NF community have resulted in a detailed understanding of the natural history and biology of NF1-related peripheral nerve sheath tumors. This work provided the basis for the development of meaningful clinical trials targeting pNF. Inhibition of the RAS/MAPK signaling pathway with MEK inhibitors identified the first medical therapy which resulted in shrinkage in the majority of children with NF1 and large inoperable pNF. Based on this finding and subsequent demonstration of clinical benefit, the MEK inhibitor selumetinib recently received approval by the United States Food and Drug Administration (FDA) for children with symptomatic pNF.

CONCLUSIONS

Sustained efforts and collaborations have resulted in identification of MEK inhibitors as effective therapy for NF1 pNF. Future work work will be directed at prevention of pNF morbidity and deepening the reponse in symptomatic pNF.

Assessment of nociception and related quality-of-life measures in a porcine model of neurofibromatosis type 1

Neurofibromatosis type 1 (NF1) is an autosomal dominant genetic disorder resulting from germline mutations in the NF1 gene, which encodes neurofibromin. Patients experience a variety of symptoms, but pain in the context of NF1 remains largely underrecognized. Here, we characterize nociceptive signaling and pain behaviors in a miniswine harboring a disruptive NF1 mutation (exon 42 deletion). We present the first characterization of pain-related behaviors in a pig model of NF1, identifying unchanged agitation scores, lower tactile thresholds (allodynia), and decreased response latencies to thermal laser stimulation (hyperalgesia) in NF1 (females only) pigs. Male NF1 pigs with tumors showed reduced sleep quality and increased resting, 2 health-related quality-of-life symptoms found to be comorbid in people with NF1 pain. We explore these phenotypes in relationship to suppression of the increased activity of the N-type voltage-gated calcium (CaV2.2) channel by pharmacological antagonism of phosphorylation of a regulatory protein-the collapsin response mediator protein 2 (CRMP2), a known interactor of neurofibromin, and by targeting the interface between the α subunit of CaV2.2 and the accessory β-subunits with small molecules. Our data support the use of NF1 pigs as a large animal model for studying NF1-associated pain and for understanding the pathophysiology of NF1. Our findings demonstrate the translational potential of 2 small molecules in reversing ion channel remodeling seen in NF1. Interfering with CaV2.2, a clinically validated target for pain management, might also be a promising therapeutic strategy for NF1-related pain management.

Cdkn2a Loss in a Model of Neurofibroma Demonstrates Stepwise Tumor Progression to Atypical Neurofibroma and MPNST

Plexiform neurofibromas are benign nerve sheath Schwann cell tumors characterized by biallelic mutations in the neurofibromatosis type 1 (NF1) tumor suppressor gene. Atypical neurofibromas show additional frequent loss of CDKN2A/Ink4a/Arf and may be precursor lesions of aggressive malignant peripheral nerve sheath tumors (MPNST). Here we combined loss of Nf1 in developing Schwann cells with global Ink4a/Arf loss and identified paraspinal plexiform neurofibromas and atypical neurofibromas. Upon transplantation, atypical neurofibromas generated genetically engineered mice (GEM)-PNST similar to human MPNST, and tumors showed reduced p16INK4a protein and reduced senescence markers, confirming susceptibility to transformation. Superficial GEM-PNST contained regions of nerve-associated plexiform neurofibromas or atypical neurofibromas and grew rapidly on transplantation. Transcriptome analyses showed similarities to corresponding human tumors. Thus, we recapitulated nerve tumor progression in NF1 and provided preclinical platforms for testing therapies at each tumor grade. These results support a tumor progression model in which loss of NF1 in Schwann cells drives plexiform neurofibromas formation, additional loss of Ink4a/Arf contributes to atypical neurofibromas formation, and further changes underlie transformation to MPNST. SIGNIFICANCE: New mouse models recapitulate the stepwise progression of NF1 tumors and will be useful to define effective treatments that halt tumor growth and tumor progression in NF1.

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.