Nf1-deficient mouse Schwann cells are angiogenic and invasive and can be induced to hyperproliferate: reversion of some phenotypes by an inhibitor of farnesyl protein transferase

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.

Synergistic Suppression of NF1 Malignant Peripheral Nerve Sheath Tumor Cell Growth in Culture and Orthotopic Xenografts by Combinational Treatment with Statin and Prodrug Farnesyltransferase Inhibitor PAMAM G4 Dendrimers

Neurofibromatosis type 1 (NF1) is a disorder in which RAS is constitutively activated due to the loss of the Ras-GTPase-activating activity of neurofibromin. RAS must be prenylated (i.e., farnesylated or geranylgeranylated) to traffic and function properly. Previous studies showed that the anti-growth properties of farnesyl monophosphate prodrug farnesyltransferase inhibitors (FTIs) on human NF1 malignant peripheral nerve sheath tumor (MPNST) cells are potentiated by co-treatment with lovastatin. Unfortunately, such prodrug FTIs have poor aqueous solubility. In this study, we synthesized a series of prodrug FTI polyamidoamine generation 4 (PAMAM G4) dendrimers that compete with farnesyl pyrophosphate for farnesyltransferase (Ftase) and assessed their effects on human NF1 MPNST S462TY cells. The prodrug 3-tert-butylfarnesyl monophosphate FTI-dendrimer (i.e., IG 2) exhibited improved aqueous solubility. Concentrations of IG 2 and lovastatin (as low as 0.1 μM) having little to no effect when used singularly synergistically suppressed cell proliferation, colony formation, and induced N-RAS, RAP1A, and RAB5A deprenylation when used in combination. Combinational treatment had no additive or synergistic effects on the proliferation/viability of immortalized normal rat Schwann cells, primary rat hepatocytes, or normal human mammary epithelial MCF10A cells. Combinational, but not singular, in vivo treatment markedly suppressed the growth of S462TY xenografts established in the sciatic nerves of immune-deficient mice. Hence, prodrug farnesyl monophosphate FTIs can be rendered water-soluble by conjugation to PAMAM G4 dendrimers and exhibit potent anti-tumor activity when combined with clinically achievable statin concentrations.

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.

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.

RAS and beyond: the many faces of the neurofibromatosis type 1 protein

Neurofibromatosis type 1 is a rare neurogenetic syndrome, characterized by pigmentary abnormalities, learning and social deficits, and a predisposition for benign and malignant tumor formation caused by germline mutations in the NF1 gene. With the cloning of the NF1 gene and the recognition that the encoded protein, neurofibromin, largely functions as a negative regulator of RAS activity, attention has mainly focused on RAS and canonical RAS effector pathway signaling relevant to disease pathogenesis and treatment. However, as neurofibromin is a large cytoplasmic protein the RAS regulatory domain of which occupies only 10% of its entire coding sequence, both canonical and non-canonical RAS pathway modulation, as well as the existence of potential non-RAS functions, are becoming apparent. In this Special article, we discuss our current understanding of neurofibromin function.

Stepwise crosstalk between aberrant Nf1, Tp53 and Rb signalling pathways induces gliomagenesis in zebrafish

The molecular pathogenesis of glioblastoma indicates that RTK/Ras/PI3K, RB and TP53 pathways are critical for human gliomagenesis. Here, several transgenic zebrafish lines with single or multiple deletions of nf1, tp53 and rb1 in astrocytes, were established to genetically induce gliomagenesis in zebrafish. In the mutant with a single deletion, we found only the nf1 mutation low-efficiently induced tumour incidence, suggesting that the Nf1 pathway is critical for the initiation of gliomagenesis in zebrafish. Combination of mutations, nf1;tp53 and rb1;tp53 combined knockout fish, showed much higher tumour incidences, high-grade histology, increased invasiveness, and shortened survival time. Further bioinformatics analyses demonstrated the alterations in RTK/Ras/PI3K, cell cycle, and focal adhesion pathways, induced by abrogated nf1, tp53, or rb1, were probably the critical stepwise biological events for the initiation and development of gliomagenesis in zebrafish. Gene expression profiling and histological analyses showed the tumours derived from zebrafish have significant similarities to the subgroups of human gliomas. Furthermore, temozolomide treatment effectively suppressed gliomagenesis in these glioma zebrafish models, and the histological responses in temozolomide-treated zebrafish were similar to those observed in clinically treated glioma patients. Thus, our findings will offer a potential tool for genetically investigating gliomagenesis and screening potential targeted anti-tumour compounds for glioma treatment.

New Model Systems and the Development of Targeted Therapies for the Treatment of Neurofibromatosis Type 1-Associated Malignant Peripheral Nerve Sheath Tumors

Neurofibromatosis Type 1 (NF1) is a common genetic disorder and cancer predisposition syndrome (1:3000 births) caused by mutations in the tumor suppressor gene NF1. NF1 encodes neurofibromin, a negative regulator of the Ras signaling pathway. Individuals with NF1 often develop benign tumors of the peripheral nervous system (neurofibromas), originating from the Schwann cell linage, some of which progress further to malignant peripheral nerve sheath tumors (MPNSTs). Treatment options for neurofibromas and MPNSTs are extremely limited, relying largely on surgical resection and cytotoxic chemotherapy. Identification of novel therapeutic targets in both benign neurofibromas and MPNSTs is critical for improved patient outcomes and quality of life. Recent clinical trials conducted in patients with NF1 for the treatment of symptomatic plexiform neurofibromas using inhibitors of the mitogen-activated protein kinase (MEK) have shown very promising results. However, MEK inhibitors do not work in all patients and have significant side effects. In addition, preliminary evidence suggests single agent use of MEK inhibitors for MPNST treatment will fail. Here, we describe the preclinical efforts that led to the identification of MEK inhibitors as promising therapeutics for the treatment of NF1-related neoplasia and possible reasons they lack single agent efficacy in the treatment of MPNSTs. In addition, we describe work to find targets other than MEK for treatment of MPNST. These have come from studies of RAS biochemistry, in vitro drug screening, forward genetic screens for Schwann cell tumors, and synthetic lethal screens in cells with oncogenic RAS gene mutations. Lastly, we discuss new approaches to exploit drug screening and synthetic lethality with NF1 loss of function mutations in human Schwann cells using CRISPR/Cas9 technology.

The impact of RASopathy-associated mutations on CNS development in mice and humans

The RAS signaling pathway is involved in the regulation of developmental processes, including cell growth, proliferation, and differentiation, in the central nervous system (CNS). Germline mutations in the RAS signaling pathway genes are associated with a group of neurodevelopmental disorders, collectively called RASopathy, which includes neurofibromatosis type 1, Noonan syndrome, cardio-facio-cutaneous syndrome, and Costello syndrome. Most mutations associated with RASopathies increase the activity of the RAS-ERK signaling pathway, and therefore, most individuals with RASopathies share common phenotypes, such as a short stature, heart defects, facial abnormalities, and cognitive impairments, which are often accompanied by abnormal CNS development. Recent studies using mouse models of RASopathies demonstrated that particular mutations associated with each disorder disrupt CNS development in a mutation-specific manner. Here, we reviewed the recent literatures that investigated the developmental role of RASopathy-associated mutations using mutant mice, which provided insights into the specific contribution of RAS-ERK signaling molecules to CNS development and the subsequent impact on cognitive function in adult mice.

Activated androgen receptor accelerates angiogenesis in cutaneous neurofibroma by regulating VEGFA transcription

Accumulating evidence has demonstrated the significant progression of cutaneous neurofibroma (cNF) without necrosis during puberty. However, the molecular events involved in this process remain unclear. The alteration of the steroid hormone levels during puberty has led to the investigation of the expression levels of the androgen receptor (AR). A positive correlation between AR expression and microvessel density has been reported in human cNF tissues in combination with enhanced endothelial cell tube formation in vitro. In addition, activated AR signaling can promote neurofibroma cell growth in vivo and in vitro and tube formation in vitro. In the present study, AR was shown to bind directly to the promoter of vascular endothelial growth factor A (VEGFA), a key factor involved in angiogenesis, and to sequentially induce its expression. Furthermore, the AR inhibitor, MDV3100, downregulated VEGFA expression and abolished endothelial cell recruitment and tube formation. Taken collectively, the findings of this study revealed that AR signaling enhanced tumor growth and angiogenesis in cNF by regulating VEGFA transcription. However, whether AR can be regarded a therapeutic target for cNF requires further investigation.

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.

Genetic enhancement of Ras-ERK pathway does not aggravate L-DOPA-induced dyskinesia in mice but prevents the decrease induced by lovastatin

Increasing evidence supports a close relationship between Ras-ERK1/2 activation in the striatum and L-DOPA-induced dyskinesia (LID). ERK1/2 activation by L-DOPA takes place through the crosstalk between D1R/AC/PKA/DARPP-32 pathway and NMDA/Ras pathway. Compelling genetic and pharmacological evidence indicates that Ras-ERK1/2 inhibition prevents LID onset and may even revert already established dyskinetic symptoms. However, it is currently unclear whether exacerbation of Ras-ERK1/2 activity in the striatum may further aggravate dyskinesia in experimental animal models. Here we took advantage of two genetic models in which Ras-ERK1/2 signaling is hyperactivated, the Nf1^+/− mice, in which the Ras inhibitor neurofibromin is reduced, and the Ras-GRF1 overexpressing (Ras-GRF1 OE) transgenic mice in which a specific neuronal activator of Ras is enhanced. Nf1^+/− and Ras-GRF1 OE mice were unilaterally lesioned with 6-OHDA and treated with an escalating L-DOPA dosing regimen. In addition, a subset of Nf1^+/− hemi-parkinsonian animals was also co-treated with the Ras inhibitor lovastatin. Our results revealed that Nf1^+/− and Ras-GRF1 OE mice displayed similar dyskinetic symptoms to their wild-type counterparts. This observation was confirmed by the lack of differences between mutant and wild-type mice in striatal molecular changes associated to LID (i.e., FosB, and pERK1/2 expression). Interestingly, attenuation of Ras activity with lovastatin does not weaken dyskinetic symptoms in Nf1^+/− mice. Altogether, these data suggest that ERK1/2-signaling activation in dyskinetic animals is maximal and does not require further genetic enhancement in the upstream Ras pathway. However, our data also demonstrate that such a genetic enhancement may reduce the efficacy of anti-dyskinetic drugs like lovastatin.

KIR2DL5 mutation and loss underlies sporadic dermal neurofibroma pathogenesis and growth

Dermal neurofibromas (DNFs) are benign peripheral nerve sheath tumors thought to originate from Schwann cell progenitors. These tumors represent one of the hallmarks of the neurofibromatosis type 1 (NF1) tumor predisposition syndrome, where they can number in the thousands. While NF1-DNFs arise due to mutations in the NF1 gene, the vast majority of DNFs occur sporadically (sp-DNFs), where the genetic etiology is currently unknown. Herein, we employed whole-exome sequencing of sp-DNFs to identify a recurrent mutation in the KIR2DL5 gene, which codes for a protein suppressor of natural killer (NK) cell activity. While the function of KIR2DL5 outside of the immune system is unknown, we identified a KIR2DL5^N173D mutation in three of nine sp-DNFs, resulting in loss of KIR2DL5 protein expression. In contrast, two of these subjects had unrelated tumors, which retained KIR2DL5 protein expression. Moreover, loss of KIR2DL5 expression was demonstrated in 15 of 45 independently-identified sp-DNFs. Consistent with its potential role as a negative growth regulator important for neurofibroma maintenance, ectopic KIR2DL5^N173D expression in normal human Schwann cells resulted in reduced KIR2DL5 expression and increased cell proliferation. Similarly, KIR2DL5 short hairpin RNA knockdown (KD) decreased KIR2DL5 protein levels and increased cell proliferation, as well as correlated with PDGFRβ and downstream RAS/AKT/mTOR hyperactivation. Importantly, inhibition of PDGFRβ or AKT/mTOR activity in KIR2DL5-KD human Schwann cells reduced proliferation to control levels. Collectively, these findings establish KIR2DL5 as a new Schwann cell growth regulator relevant to sp-DNF pathogenesis, which links sporadic and NF1-associated DNFs through RAS pathway hyperactivation.

Isolation and Expansion of Schwann Cells from Transgenic Mouse Models

The most widely used method (Brockes' method) for preparing primary Schwann cell culture uses neonatal rat sciatic nerves as the primary source of Schwann cells. The procedure is relatively simple and yields a highly purified population of Schwann cells in a short period of time. The method has also been used to prepare Schwann cells from mice, however, with limitation. For example, Brockes' method is not applicable when the genotypes of mouse neonates are unknown or if the mouse mutants do not develop to term. We described a method ideal for preparing Schwann cells in a transgenic/knockout mouse study. The method uses embryonic dorsal root ganglia as the primary source of Schwann cells and allows preparing separate, highly purified Schwann cell cultures from individual mouse embryos in less than 2 weeks.

Orbital/Periorbital Plexiform Neurofibromas in Children with Neurofibromatosis Type 1: Multidisciplinary Recommendations for Care

TOPIC

Children and adults with neurofibromatosis type 1 (NF1), a common autosomal dominant condition, manifest a variety of ophthalmologic conditions. Plexiform neurofibromas (PNs) involving the eyelid, orbit, periorbital, and facial structures (orbital-periorbital plexiform neurofibroma [OPPN]) can result in significant visual loss in children. Equally important, OPPNs can cause significant alteration in physical appearance secondary to proptosis, ptosis, and facial disfigurement, leading to social embarrassment and decreased self-esteem.

CLINICAL RELEVANCE

Although NF1 is a relatively common disease in which routine ophthalmologic examinations are required, no formal recommendations for clinical care of children with OPPNs exist. Although medical and surgical interventions have been reported, there are no agreed-on criteria for when OPPNs require therapy and which treatment produces the best outcome.

METHODS

Because a multidisciplinary team of specialists (oculofacial plastics, pediatric ophthalmology, neuro-ophthalmology, medical genetics, and neuro-oncology) direct management decisions, the absence of a uniform outcome measure that represents visual or aesthetic sequelae complicates the design of evidence-based studies and feasible clinical trials.

RESULTS

In September 2013, a multidisciplinary task force, composed of pediatric practitioners from tertiary care centers experienced in caring for children with OPPN, was convened to address the lack of clinical care guidelines for children with OPPN.

CONCLUSIONS

This consensus statement provides recommendations for ophthalmologic monitoring, outlines treatment indications and forthcoming biologic therapy, and discusses challenges to performing clinical trials in this complicated condition.

Neurofibromatosis Type 1: Genetic and Cellular Mechanisms of Peripheral Nerve Tumor Formation

Neurofibromatosis type 1 (NF1) is among the most common inherited human diseases. The NF1 protein is a Ras-GTPase activating protein, positioning NF1 in important intracellular signaling pathways. Patients with mutations in the NF1 gene can develop benign peripheral nerve tumors (neurofibromas), learning disabilities, and/or benign optic nerve gliomas, in addition to abnormalities unassociated with the nervous system. The NF1 gene is believed to act as a tumor suppressor. How NF1 mutations relate to benign features of NF1 is the subject of active investigation. Studies using transgenic mice with NF1 mutations and cells derived from these mice have yielded exciting new data, implicating multiple cell types mutant at NF1 and possibly factors in the environment in the pathogenesis of benign neurofibromas. NEUROSCIENTIST 3:412-420, 1997

Runx1 contributes to neurofibromatosis type 1 neurofibroma formation

Neurofibromatosis type 1 (NF1) patients are predisposed to neurofibromas but the driver(s) that contribute to neurofibroma formation are not fully understood. By cross comparison of microarray gene lists on human neurofibroma-initiating cells and developed neurofibroma Schwann cells (SCs) we identified RUNX1 overexpression in human neurofibroma initiation cells, suggesting RUNX1 might relate to neurofibroma formation. Immunostaining confirmed RUNX1 protein overexpression in human plexiform neurofibromas. Runx1 overexpression was confirmed in mouse Schwann cell progenitors (SCPs) and mouse neurofibromas at the messenger RNA and protein levels. Genetic inhibition of Runx1 expression by small hairpin RNA or pharmacological inhibition of Runx1 function by a Runx1/Cbfβ interaction inhibitor, Ro5-3335, decreased mouse neurofibroma sphere number in vitro. Targeted genetic deletion of Runx1 in SCs and SCPs delayed mouse neurofibroma formation in vivo. Mechanistically, loss of Nf1 increased embryonic day 12.5 Runx1(+)/Blbp(+) progenitors that enable tumor formation. These results suggest that Runx1 has an important role in Nf1 neurofibroma initiation, and inhibition of RUNX1 function might provide a novel potential therapeutic treatment strategy for neurofibroma patients.

A RASopathy gene commonly mutated in cancer: the neurofibromatosis type 1 tumour suppressor

Neurofibromatosis type 1 (NF1) is a common genetic disorder that predisposes affected individuals to tumours. The NF1 gene encodes a RAS GTPase-activating protein called neurofibromin and is one of several genes that (when mutant) affect RAS-MAPK signalling, causing related diseases collectively known as RASopathies. Several RASopathies, beyond NF1, are cancer predisposition syndromes. Somatic NF1 mutations also occur in 5-10% of human sporadic cancers and may contribute to resistance to therapy. To highlight areas for investigation in RASopathies and sporadic tumours with NF1 mutations, we summarize current knowledge of NF1 disease, the NF1 gene and neurofibromin, neurofibromin signalling pathways and recent developments in NF1 therapeutics.

Sirolimus improves pain in NF1 patients with severe plexiform neurofibromas

Plexiform neurofibromas (PNs) are common and potentially debilitating complications of neurofibromatosis 1 (NF1). These benign nerve-sheath tumors are associated with significant pain and morbidity because they compress vital structures. The mammalian target of rapamycin (mTOR) pathway is a major mediator involved in tumor growth in NF1. We present 3 cases of patients with NF1, aged 8, 16, and 17 years, followed for inoperable and symptomatic PNs; patients received sirolimus for life-threatening and painful neurofibromas after multidisciplinary consultation. Epidemiologic, clinical, and radiologic data were retrospectively collected. The volume of PNs did not differ between baseline and 12-month follow-up and pain was alleviated, with withdrawal of analgesics in 2 cases at 6 months, and significantly decreased for the third case. Sirolimus for inoperable symptomatic PNs in patients with NF1 permitted stabilization of mass and produced unpredictable and important alleviation of pain in all cases with good tolerance. This treatment was proposed in extreme cases, in absence of therapeutic alternatives, after multidisciplinary consensus. The mTOR pathway may be both a major mediator of NF1 tumor growth and regulator of nociceptor sensitivity. mTOR inhibitors clinically used as anticancer and immunosuppressant drugs could be a potential treatment of chronic pain.

Injury signals cooperate with Nf1 loss to relieve the tumor-suppressive environment of adult peripheral nerve

Schwann cells are highly plastic cells that dedifferentiate to a progenitor-like state following injury. However, deregulation of this plasticity, may be involved in the formation of neurofibromas, mixed-cell tumors of Schwann cell (SC) origin that arise upon loss of NF1. Here, we show that adult myelinating SCs (mSCs) are refractory to Nf1 loss. However, in the context of injury, Nf1-deficient cells display opposing behaviors along the wounded nerve; distal to the injury, Nf1(-/-) mSCs redifferentiate normally, whereas at the wound site Nf1(-/-) mSCs give rise to neurofibromas in both Nf1(+/+) and Nf1(+/-) backgrounds. Tracing experiments showed that distinct cell types within the tumor derive from Nf1-deficient SCs. This model of neurofibroma formation demonstrates that neurofibromas can originate from adult SCs and that the nerve environment can switch from tumor suppressive to tumor promoting at a site of injury. These findings have implications for both the characterization and treatment of neurofibromas.

Ras-driven transcriptome analysis identifies aurora kinase A as a potential malignant peripheral nerve sheath tumor therapeutic target

PURPOSE

Patients with neurofibromatosis type 1 (NF1) develop malignant peripheral nerve sheath tumors (MPNST), which are often inoperable and do not respond well to current chemotherapies or radiation. The goal of this study was to use comprehensive gene expression analysis to identify novel therapeutic targets.

EXPERIMENTAL DESIGN

Nerve Schwann cells and/or their precursors are the tumorigenic cell types in MPNST because of the loss of the NF1 gene, which encodes the RasGAP protein neurofibromin. Therefore, we created a transgenic mouse model, CNP-HRas12V, expressing constitutively active HRas in Schwann cells and defined a Ras-induced gene expression signature to drive a Bayesian factor regression model analysis of differentially expressed genes in mouse and human neurofibromas and MPNSTs. We tested functional significance of Aurora kinase overexpression in MPNST in vitro and in vivo using Aurora kinase short hairpin RNAs (shRNA) and compounds that inhibit Aurora kinase.

RESULTS

We identified 2,000 genes with probability of linkage to nerve Ras signaling of which 339 were significantly differentially expressed in mouse and human NF1-related tumor samples relative to normal nerves, including Aurora kinase A (AURKA). AURKA was dramatically overexpressed and genomically amplified in MPNSTs but not neurofibromas. Aurora kinase shRNAs and Aurora kinase inhibitors blocked MPNST cell growth in vitro. Furthermore, an AURKA selective inhibitor, MLN8237, stabilized tumor volume and significantly increased survival of mice with MPNST xenografts.

CONCLUSION

Integrative cross-species transcriptome analyses combined with preclinical testing has provided an effective method for identifying candidates for molecular-targeted therapeutics. Blocking Aurora kinases may be a viable treatment platform for MPNST.

Hyperactivation of mTOR critically regulates abnormal osteoclastogenesis in neurofibromatosis Type 1

Individuals with nerofibromatosis Type 1 (NF1) frequently suffer a spectrum of bone pathologies, such as abnormal skeletal development (scoliosis, congenital bowing, and congenital pseudoarthroses, etc), lower bone mineral density with increased fracture risk. These skeletal problems may result, in part, from abnormal osteoclastogenesis. Enhanced RAS/PI3K activity has been reported to contribute to abnormal osteoclastogenesis in Nf1 heterozygous (Nf1+/-) mice. However, the specific downstream pathways linked to NF1 abnormal osteoclastogenesis have not been defined. Our aim was to determine whether mammalian target of rapamycin (mTOR) was a key effector responsible for abnormal osteoclastogenesis in NF1. Primary osteoclast-like cells (OCLs) were cultured from Nf1 wild-type (Nf1+/+) and Nf1+/- mice. Compared to Nf1+/+ controls, there were 20% more OCLs induced from Nf1+/- mice. Nf1+/- OCLs were larger and contained more nuclei. Hyperactive mTOR signaling was detected in Nf1+/- OCLs. Inhibition of mTOR signaling by rapamycin in Nf1+/- OCLs abrogated abnormalities in cellular size and number. Moreover, we found that hyperactive mTOR signaling induced abnormal osteoclastogenesis major through hyper-proliferation. Our research suggests that neurofibromin directly regulates osteoclastogenesis through mTOR signaling pathway. Inhibiting mTOR may represent a viable strategy to treat NF1 bone diseases.

Pathogenesis of plexiform neurofibroma: tumor-stromal/hematopoietic interactions in tumor progression

Neurofibromatosis type 1 (NF1) is a genetic disease that results from either heritable or spontaneous autosomal dominant mutations in the NF1 gene. A second-hit mutation precedes the predominant NF1 neoplasms, which include myeloid leukemia, optic glioma, and plexiform neurofibroma. Despite this requisite NF1 loss of heterozygosity in the tumor cell of origin, nontumorigenic cells contribute to both generalized and specific disease manifestations. In mouse models of plexiform neurofibroma formation, Nf1 haploinsufficient mast cells promote inflammation, accelerating tumor formation and growth. These recruited mast cells, hematopoietic effector cells long known to permeate neurofibroma tissue, mediate key mitogenic signals that contribute to vascular ingrowth, collagen deposition, and tumor growth. Thus, the plexiform neurofibroma microenvironment involves a tumor/stromal interaction with the hematopoietic system that depends, at the molecular level, on a stem cell factor/c-kit-mediated signaling axis. These observations parallel findings in other NF1 disease manifestations and are clearly relevant to medical management of these neurofibromas.

A transcription factor map as revealed by a genome-wide gene expression analysis of whole-blood mRNA transcriptome in multiple sclerosis

Background

Several lines of evidence suggest that transcription factors are involved in the pathogenesis of Multiple Sclerosis (MS) but complete mapping of the whole network has been elusive. One of the reasons is that there are several clinical subtypes of MS and transcription factors that may be involved in one subtype may not be in others. We investigate the possibility that this network could be mapped using microarray technologies and contemporary bioinformatics methods on a dataset derived from whole blood in 99 untreated MS patients (36 Relapse Remitting MS, 43 Primary Progressive MS, and 20 Secondary Progressive MS) and 45 age-matched healthy controls.

Methodology/Principal Findings

We have used two different analytical methodologies: a non-standard differential expression analysis and a differential co-expression analysis, which have converged on a significant number of regulatory motifs that are statistically overrepresented in genes that are either differentially expressed (or differentially co-expressed) in cases and controls (e.g., V$KROX_Q6, p-value <3.31E-6; V$CREBP1_Q2, p-value <9.93E-6, V$YY1_02, p-value <1.65E-5).

Conclusions/Significance

Our analysis uncovered a network of transcription factors that potentially dysregulate several genes in MS or one or more of its disease subtypes. The most significant transcription factor motifs were for the Early Growth Response EGR/KROX family, ATF2, YY1 (Yin and Yang 1), E2F-1/DP-1 and E2F-4/DP-2 heterodimers, SOX5, and CREB and ATF families. These transcription factors are involved in early T-lymphocyte specification and commitment as well as in oligodendrocyte dedifferentiation and development, both pathways that have significant biological plausibility in MS causation.

Oligodendrocyte progenitor cell numbers and migration are regulated by the zebrafish orthologs of the NF1 tumor suppressor gene

Neurofibromatosis type 1 is the most commonly inherited human cancer predisposition syndrome. Neurofibromin (NF1) gene mutations lead to increased risk of neurofibromas, schwannomas, low grade, pilocytic optic pathway gliomas, as well as malignant peripheral nerve sheath tumors and glioblastomas. Despite the evidence for NF1 tumor suppressor function in glial cell tumors, the mechanisms underlying transformation remain poorly understood. In this report, we used morpholinos to knockdown the two nf1 orthologs in zebrafish and show that oligodendrocyte progenitor cell (OPC) numbers are increased in the developing spinal cord, whereas neurons are unaffected. The increased OPC numbers in nf1 morphants resulted from increased proliferation, as detected by increased BrdU labeling, whereas TUNEL staining for apoptotic cells was unaffected. This phenotype could be rescued by the forced expression of the GTPase-activating protein (GAP)-related domain of human NF1. In addition, the in vivo analysis of OPC migration following nf1 loss using time-lapse microscopy demonstrated that olig2-EGFP(+) OPCs exhibit enhanced cell migration within the developing spinal cord. OPCs pause intermittently as they migrate, and in nf1 knockdown animals, they covered greater distances due to a decrease in average pause duration, rather than an increase in velocity while in motion. Interestingly, nf1 knockdown also leads to an increase in ERK signaling, principally in the neurons of the spinal cord. Together, these results show that negative regulation of the Ras pathway through the GAP activity of NF1 limits OPC proliferation and motility during development, providing insight into the oncogenic mechanisms through which NF1 loss contributes to human glial tumors.

Mast cells and the neurofibroma microenvironment

Neurofibromatosis type 1 (NF1) is the most common genetic disorder with a predisposition to malignancy and affects 1 in 3500 persons worldwide. NF1 is caused by a mutation in the NF1 tumor suppressor gene that encodes the protein neurofibromin. Patients with NF1 have cutaneous, diffuse, and plexiform neurofibromas, tumors comprised primarily of Schwann cells, blood vessels, fibroblasts, and mast cells. Studies from human and murine models that closely recapitulate human plexiform neurofibroma formation indicate that tumorigenesis necessitates NF1 loss of heterozygosity in the Schwann cell. In addition, our most recent studies with bone marrow transplantation and pharmacologic experiments implicate haploinsufficiency of Nf1 (Nf1(+/-)) and c-kit signaling in the hematopoietic system as required and sufficient for tumor progression. Here, we review recent studies implicating the hematopoietic system in plexiform neurofibroma genesis, delineate the physiology of stem cell factor-dependent hematopoietic cells and their contribution to the neurofibroma microenvironment, and highlight the application of this research toward the first successful, targeted medical treatment of a patient with a nonresectable and debilitating neurofibroma. Finally, we emphasize the importance of the tumor microenvironment hypothesis, asserting that tumorigenic cells in the neurofibroma do not arise and grow in isolation.

Defective cAMP generation underlies the sensitivity of CNS neurons to neurofibromatosis-1 heterozygosity

Individuals with the neurofibromatosis type 1 (NF1) inherited cancer syndrome exhibit neuronal dysfunction that predominantly affects the CNS. In this report, we demonstrate a unique vulnerability of CNS neurons, but not peripheral nervous system (PNS) neurons, to reduced Nf1 gene expression. Unlike dorsal root ganglion neurons, Nf1 heterozygous (Nf1+/-) hippocampal and retinal ganglion cell (RGC) neurons have decreased growth cone areas and neurite lengths, and increased apoptosis compared to their wild-type counterparts. These abnormal Nf1+/- CNS neuronal phenotypes do not reflect Ras pathway hyperactivation, but rather result from impaired neurofibromin-mediated cAMP generation. In this regard, elevating cAMP levels with forskolin or rolipram treatment, but not MEK (MAP kinase kinase) or PI3-K (phosphatidylinositol 3-kinase) inhibition, reverses these abnormalities to wild-type levels in vitro. In addition, Nf1+/- CNS, but not PNS, neurons exhibit increased apoptosis in response to excitotoxic or oxidative stress in vitro. Since children with NF1-associated optic gliomas often develop visual loss and Nf1 genetically engineered mice with optic glioma exhibit RGC neuronal apoptosis in vivo, we further demonstrate that RGC apoptosis resulting from optic glioma in Nf1 genetically engineered mice is attenuated by rolipram treatment in vivo. Similar to optic glioma-induced RGC apoptosis, the increased RGC neuronal death in Nf1+/- mice after optic nerve crush injury is also attenuated by rolipram treatment in vivo. Together, these findings establish a distinctive role for neurofibromin in CNS neurons with respect to vulnerability to injury, define a CNS-specific neurofibromin intracellular signaling pathway responsible for neuronal survival, and lay the foundation for future neuroprotective glioma treatment approaches.

Schweinfurthin A selectively inhibits proliferation and Rho signaling in glioma and neurofibromatosis type 1 tumor cells in a NF1-GRD-dependent manner

Neurofibromatosis type 1 (NF1) is the most common genetic disease affecting the nervous system. Patients typically develop many tumors over their lifetime, leading to increased morbidity and mortality. The NF1 gene, mutated in NF1, is also commonly mutated in sporadic glioblastoma multiforme (GBM). Because both NF1 and GBM are currently incurable, new therapeutic approaches are clearly needed. Natural products represent an opportunity to develop new therapies, as they have been evolutionarily selected to play targeted roles in organisms. Schweinfurthin A is a prenylated stilbene natural product that has previously shown specific inhibitory activity against brain and hematopoietic tumor lines. We show that patient-derived GBM and NF1 malignant peripheral nerve sheath tumor (MPNST) lines, as well as tumor lines derived from the Nf1-/+;Trp53-/+ (NPcis) mouse model of astrocytoma and MPNST are highly sensitive to inhibition by schweinfurthin A and its synthetic analogs. In contrast, primary mouse astrocytes are resistant to the growth inhibitory effects of schweinfurthin A, suggesting that schweinfurthin A may act specifically on tumor cells. Stable transfection of the GTPase-activating protein related domain of Nf1 into Nf1-/-;Trp53-/- astrocytoma cells confers resistance to schweinfurthin A. In addition, the profound effect of schweinfurthin A on dynamic reorganization of the actin cytoskeleton led us to discover that schweinfurthin A inhibits growth factor-stimulated Rho signaling. In summary, we have identified a class of small molecules that specifically inhibit growth of cells from both central and peripheral nervous system tumors and seem to act on NF1-deficient cells through cytoskeletal reorganization correlating to changes in Rho signaling.

NF1 loss disrupts Schwann cell-axonal interactions: a novel role for semaphorin 4F

Neurofibromatosis type 1 (NF1) patients develop neurofibromas, tumors of Schwann cell origin, as a result of loss of the Ras-GAP neurofibromin. In normal nerves, Schwann cells are found tightly associated with axons, while loss of axonal contact is a frequent and important early event in neurofibroma development. However, the molecular basis of this physical interaction or how it is disrupted in cancer remains unclear. Here we show that loss of neurofibromin in Schwann cells is sufficient to disrupt Schwann cell/axonal interactions via up-regulation of the Ras/Raf/ERK signaling pathway. Importantly, we identify down-regulation of semaphorin 4F (Sema4F) as the molecular mechanism responsible for the Ras-mediated loss of interactions. In heterotypic cocultures, Sema4F knockdown induced Schwann cell proliferation by relieving axonal contact-inhibitory signals, providing a mechanism through which loss of axonal contact contributes to tumorigenesis. Importantly, Sema4F levels were strongly reduced in a panel of human neurofibromas, confirming the relevance of these findings to the human disease. This work identifies a novel role for the guidance-molecules semaphorins in the mediation of Schwann cell/axonal interactions, and provides a molecular mechanism by which heterotypic cell-cell contacts control cell proliferation and suppress tumorigenesis. Finally, it provides a new approach for the development of therapies for NF1.

How does the Schwann cell lineage form tumors in NF1?

Neurofibromas are benign tumors of peripheral nerve that occur sporadically or in patients with the autosomal dominant tumor predisposition syndrome neurofibromatosis type 1 (NF1). Multiple neurofibroma subtypes exist which differ in their site of occurrence, their association with NF1, and their tendency to undergo transformation to become malignant peripheral nerve sheath tumors (MPNSTs), the most common malignancy associated with NF1. Most NF1 patients carry a constitutional mutation of the NF1 tumor suppressor gene. Neurofibromas develop in these patients when an unknown cell type in the Schwann cell lineage loses its remaining functional NF1 gene and initiates a complex series of interactions with other cell types; these interactions may be influenced by aberrant expression of growth factors and growth factor receptors and the action of modifier genes. Cells within certain neurofibroma subtypes subsequently accumulate additional mutations affecting the p19(ARF)-MDM2-TP53 and p16INK4A-Rb signaling cascades, mutations of other as yet unidentified genes, and amplification of growth factor receptor genes, resulting in their transformation into MPNSTs. These observations have been validated using a variety of transgenic and knockout mouse models that recapitulate neurofibroma and MPNST pathogenesis. A new generation of mouse models is also providing important new insights into the identity of the cell type in the Schwann cell lineage that gives rise to neurofibromas. Our improving understanding of the mechanisms underlying the pathogenesis of neurofibromas and MPNSTs raises intriguing new questions about the origin and pathogenesis of these neoplasms and establishes models for the development of new therapies targeting these neoplasms.

Suppression of proliferation of two independent NF1 malignant peripheral nerve sheath tumor cell lines by the pan-ErbB inhibitor CI-1033

Neurofibromatosis Type 1 (NF1) is characterized by the abnormal proliferation of neuroectodermal tissues and the development of certain tumors, particularly neurofibromas, which may progress into malignant peripheral nerve sheath tumors (MPNSTs). Effective pharmacological therapy for the treatment of NF1 tumors is currently unavailable and the prognosis for patients with MPNSTs is poor. Loss of neurofibromin correlates with increased expression of the epidermal growth factor receptor (EGFR) and ErbB2 tyrosine kinases and these kinases have been shown to promote NF1 tumor-associated pathologies in vivo. We show here that while NF1 MPNST cells have higher EGFR expression levels and are more sensitive to EGF when compared to a non-NF1 MPNST cell line, the ability of the EGFR inhibitor gefitinib to selectively inhibit NF1 MPNST cell proliferation is marginal. We also show that NF1 MPNST proliferation correlates with activated ErbB2 and can be suppressed by nanomolar concentrations of the pan-ErbB inhibitor CI-1033 (canertinib). Consequently, targeting both EGFR and ErbB2 may prove an effective strategy for suppressing NF1 MPNST tumor growth in vivo.

Induction of apoptosis in neurofibromatosis type 1 malignant peripheral nerve sheath tumor cell lines by a combination of novel farnesyl transferase inhibitors and lovastatin

Neurofibromatosis type 1 (NF1) is a genetic disorder that is driven by the loss of neurofibromin (Nf) protein function. Nf contains a Ras-GTPase-activating protein domain, which directly regulates Ras signaling. Numerous clinical manifestations are associated with the loss of Nf and increased Ras activity. Ras proteins must be prenylated to traffic and functionally localize with target membranes. Hence, Ras is a potential therapeutic target for treating NF1. We have tested the efficacy of two novel farnesyl transferase inhibitors (FTIs), 1 and 2, alone or in combination with lovastatin, on two NF1 malignant peripheral nerve sheath tumor (MPNST) cell lines, NF90-8 and ST88-14. Single treatments of 1, 2, or lovastatin had no effect on Ras prenylation or MPNST cell proliferation. However, low micromolar combinations of 1 or 2 with lovastatin (FTI/lovastatin) reduced Ras prenylation in both MPNST cell lines. Furthermore, this FTI/lovastatin combination treatment reduced cell proliferation and induced an apoptotic response as shown by morphological analysis, procaspase-3/-7 activation, loss of mitochondrial membrane potential, and accumulation of cells with sub-G(1) DNA content. Little to no detectable toxicity was observed in normal rat Schwann cells following FTI/lovastatin combination treatment. These data support the hypothesis that combination FTI plus lovastatin therapy may be a potential treatment for NF1 MPNSTs.

Schwann cell preparation from single mouse embryos: analyses of neurofibromin function in Schwann cells

The study of peripheral nerve function in development and disease can be facilitated by the availability of cultured cells that faithfully mimic in vivo Schwann cell growth, maturation, and differentiation. We have developed a method to establish purified mouse Schwann cell culture from a single embryo at embryonic day 12.5 (E12.5) to define the abnormalities in Schwann cells caused by loss of the neurofibromatosis type 1 (Nf1) tumor suppressor protein, the RAS-GAP neurofibromin. Our method generates 2-3 x 10(6) cells/embryo highly purified (>99.5%) mouse Schwann cells in less than 2 weeks from a single E12.5 mouse embryo. Manipulation of cell medium allows purification of a Schwann-like cell population, termed Nf1-/-TXF, that resembles a tumorigenic cell in that it grows dissociated from axons and grows rapidly, yet retains expression of Schwann cell markers. We describe the preparation and characterization of both cell types.

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

Neurofibromatosis type 1 (Nf1) mutation predisposes to benign peripheral nerve (glial) tumors called neurofibromas. The point(s) in development when Nf1 loss promotes neurofibroma formation are unknown. We show that inactivation of Nf1 in the glial lineage in vitro at embryonic day 12.5 + 1, but not earlier (neural crest) or later (mature Schwann cell), results in colony-forming cells capable of multilineage differentiation. In vivo, inactivation of Nf1 using a DhhCre driver beginning at E12.5 elicits plexiform neurofibromas, dermal neurofibromas, and pigmentation. Tumor Schwann cells uniquely show biallelic Nf1 inactivation. Peripheral nerve and tumors contain transiently proliferating Schwann cells that lose axonal contact, providing insight into early neurofibroma formation. We suggest that timing of Nf1 mutation is critical for neurofibroma formation.

An orthotopic xenograft model of intraneural NF1 MPNST suggests a potential association between steroid hormones and tumor cell proliferation

Malignant peripheral nerve sheath tumors (MPNST) are the most aggressive cancers associated with neurofibromatosis type 1 (NF1). Here we report a practical and reproducible model of intraneural NF1 MPNST, by orthotopic xenograft of an immortal human NF1 tumor-derived Schwann cell line into the sciatic nerves of female scid mice. Intraneural injection of the cell line sNF96.2 consistently produced MPNST-like tumors that were highly cellular and showed extensive intraneural growth. These xenografts had a high proliferative index, were angiogenic, had significant mast cell infiltration and rapidly dominated the host nerve. The histopathology of engrafted intraneural tumors was consistent with that of human NF1 MPNST. Xenograft tumors were readily examined by magnetic resonance imaging, which also was used to assess tumor vascularity. In addition, the intraneural proliferation of sNF96.2 cell tumors was decreased in ovariectomized mice, while replacement of estrogen or progesterone restored tumor cell proliferation. This suggests a potential role for steroid hormones in supporting tumor cell growth of this MPNST cell line in vivo. The controlled orthotopic implantation of sNF96.2 cells provides for the precise initiation of intraneural MPNST-like tumors in a model system suitable for therapeutic interventions, including inhibitors of angiogenesis and further study of steroid hormone effects on tumor cell growth.

Isolation, purification and expansion of myelination-competent, neonatal mouse Schwann cells

Most studies of peripheral nerve myelination using culture models are performed with dorsal root ganglion neurons and Schwann cells pre-purified from the rat. The potential of this model is severely compromised by the lack of rat myelin mutants and the published protocols work poorly with mouse cells, for which numerous myelin mutants are available. This is partly due to difficulties in obtaining sufficient quantities of myelination-competent mouse Schwann cells. Here, we describe the isolation, purification and expansion of wild-type, myelination-competent Schwann cells from the sciatic nerves of 4-day-old mouse pups. The method consistently yields 1.9-3.3 x 10(6) of approximately 95% pure Schwann cells from the sciatic nerves of 12-15 4-day-old mouse pups, within 14-20 days. The Schwann cell proliferation rate ranges from 2.7- to 4.30-fold growth/week. Proliferation ceases within 4 weeks, when the cells become quiescent. Growth is reinduced by the presence of neurons; neuregulin is not sufficient for this effect. The Schwann cells isolated by this protocol are able to form compact myelin in culture, as judged by the segregated expression patterns of early (myelin-associated glycoprotein) and late (myelin basic protein) myelination markers in a three-dimensional neuron/Schwann cell coculture model. The Schwann cell batch yields are sufficient to perform 100-150 individual myelinating coculture assays. Employing mixed phenotype/genotype mouse neuron/Schwann cell cocultures, it will be possible to analyse the cell specificity of a mutation, and the cumulative effects of different mutations, without having to cross-breed the animals.

Inactivation of NF1 in CNS causes increased glial progenitor proliferation and optic glioma formation

The gene responsible for neurofibromatosis type 1 (NF1) encodes a tumor suppressor that functions as a negative regulator of the Ras proto-oncogene. Individuals with germline mutations in NF1 are predisposed to the development of benign and malignant tumors of the peripheral and central nervous system (CNS). Children with this disease suffer a high incidence of optic gliomas, a benign but potentially debilitating tumor of the optic nerve; and an increased incidence of malignant astrocytoma, reactive astrogliosis and intellectual deficits. In the present study, we have sought insight into the molecular and cellular basis of NF1-associated CNS pathologies. We show that mice genetically engineered to lack NF1 in CNS exhibit a variety of defects in glial cells. Primary among these is a developmental defect resulting in global reactive astrogliosis in the adult brain and increased proliferation of glial progenitor cells leading to enlarged optic nerves. As a consequence, all of the mutant optic nerves develop hyperplastic lesions, some of which progress to optic pathway gliomas. These data point to hyperproliferative glial progenitors as the source of the optic tumors and provide a genetic model for NF1-associated astrogliosis and optic glioma.

Experimental therapeutic approaches to peripheral nerve tumors

✓Discovery that the Schwann cell is the primary cell type responsible for both the neurofibroma as well as the schwannoma has proven to represent a crucial milestone in understanding the pathogenesis of peripheral nerve tumor development. This information and related findings have served as a nidus for research aimed at more fully characterizing this family of conditions. Recent discoveries in the laboratory have clarified an understanding of the molecular mechanisms underlying the pathogenesis of benign peripheral nerve tumors. Similarly, the mechanisms whereby idiopathic and syndromic (NF1- andNF2-associated) nerve sheath tumors progress to malignancy are being elucidated. This detailed understanding of the molecular pathogenesis of peripheral nerve tumors provides the information necessary to create a new generation of therapies tailored specifically to the prevention, cessation, or reversal of pathological conditions at the fundamental level of dysfunction. The authors review the data that have helped to elucidate the molecular pathogenesis of this category of conditions, explore the current progress toward exploitation of these findings, and discuss potential therapeutic avenues for future research.

Angiogenic Expression Profile of Normal and Neurofibromin-Deficient Human Schwann Cells

Peripheral nerve sheath tumors from individuals with Neurofibromatosis Type 1 (NF1) are highly vascular and contain Schwann cells which are deficient in neurofibromin. This study examines the angiogenic expression profile of neurofibromin-deficient human Schwann cells relative to normal human Schwann cells, characterizing both pro-angiogenic and anti-angiogenic factors. Conditioned media from neurofibromin-deficient Schwann cell lines was pro-angiogenic as evidenced by its ability to stimulate endothelial cell proliferation and migration. Using gene array and protein array analysis, we found increased expression of pro-angiogenic factors and decreased expression of anti-angiogenic factors in neurofibromin-deficient Schwann cells relative to normal human Schwann cells. Neurofibromin-deficient Schwann cells also showed increased expression of several growth factor receptors and decreased expression of an integrin. We conclude that neurofibromin-deficient Schwann cells have dysregulated expression of pro-angiogenic factors, anti-angiogenic factors, growth factor receptors, and an integrin. These dysregulated molecules may contribute to the growth and progression of NF1 peripheral nerve sheath tumors.

Neurofibromin-deficient Schwann cells have increased lysophosphatidic acid dependent survival and migration-implications for increased neurofibroma formation during pregnancy

Neurofibromas are the clinical hallmark of neurofibromatosis Type 1 (NF1), a genetic disorder caused by mutations of the NF1 tumor suppressor gene, which encodes neurofibromin that functions as a GTPase activating protein (GAP) for Ras. During pregnancy, up to 50% of existing neurofibromas enlarge and as many as 60% of new neurofibromas appear for the first time. Lysophosphatidic acid (LPA) is a prototypic lysophospholipid that modulates cell migration and survival of Schwann cells (SCs) and is made in increasing concentrations throughout pregnancy. We addressed the influence of LPA on the biochemical and cellular functions of SCs with a homozygous mutation of the murine homologue of the NF1 gene (Nf1-/-). LPA promoted F-actin polymerization and increased migration and survival of Nf1-/- SCs as compared to wild type (WT) SCs. Furthermore, LPA induced a higher level of Ras-GTP and Akt phosphorylation in Nf1-/- SCs as compared to WT cells. Pharmacologic inhibition or siRNA for the p85beta regulatory subunit of Class I A PI3-K significantly reduced LPA-induced Schwann cell survival and migration. Introduction of NF1-GRD reconstitution was sufficient to normalize the LPA-mediated motility of Nf1-/- SCs. As LPA modulates excessive cell survival and motility of Nf1-/- SCs, which are the tumorigenic cells in NF1, targeting PI3-K may be a potential therapeutic approach in diminishing the development and progression of neurofibromas in pregnant women with NF1.

The Ras inhibitor farnesylthiosalicylic acid as a potential therapy for neurofibromatosis type 1

PURPOSE

Farnesylthiosalicylic acid (FTS) is a Ras inhibitor that dislodges all active Ras isoforms from the membrane. We assessed the ability of FTS to reverse the transformed phenotype of neurofibromatosis type 1 (NF1)-associated tumor cell lines of malignant peripheral nerve sheath tumor (MPNST).

EXPERIMENTAL DESIGN

nf1 mutations were genotyped, allelic losses were analyzed, and neurofibromin expression levels were determined in MPNST cell lines ST88-14, S265P21, and 90-8. The effects of FTS on GTP-bound Ras (Ras-GTP) and its prominent downstream targets, as well as on cell morphology, anchorage-dependent and anchorage-independent growth, and tumor growth in mice, were assessed.

RESULTS

The MPNST cell lines were biallelic, NF1 inactive, and neurofibromin deficient. We show that FTS treatment shortened the relatively long duration of Ras activation and signaling to extracellular signal-regulated kinase, Akt, and RalA in all NF1-deficient MPNST cell lines (NF1 cells) to that observed in a non-NF1, normally expressing neurofibromin MPNST cell line. These effects of FTS led to lower steady-state levels of Ras-GTP and its activated targets. Both anchorage-dependent and anchorage-independent growth of NF1 cells were dose dependently inhibited by FTS, and the inhibition correlated positively with Ras-GTP levels. NF1 cells were found to possess strong actin stress fibers, and this phenotype was also corrected by FTS. NF1 tumor growth in a nude mouse model was inhibited by oral FTS.

CONCLUSIONS

FTS treatment of NF1 cells normalized Ras-GTP levels, resulting in reversal of the transformed phenotype and inhibition of tumor growth. FTS may therefore be considered as a potential drug for the treatment of NF1.

Molecular targets for emerging anti-tumor therapies for neurofibromatosis type 1

Neurofibromatosis type 1 (NF1) is the most common cancer predisposition syndrome. NF1 patients present with a constellation of clinical manifestations and have an increased risk of developing certain benign and malignant tumors. This disease results from mutation within the gene encoding neurofibromin, a GTPase activating protein (GAP) for Ras. Functional loss of this protein compromises Ras inactivation, which leads to the aberrant growth and proliferation of neural crest-derived cells and, ultimately, tumor formation. Current management of NF1-associated malignancy involves radiation, surgical excision, and cytotoxic drugs. The limited success of these strategies has fueled researchers to further elucidate the molecular changes that drive tumor formation and progression. This discussion will highlight how intracellular signaling molecules, cell-surface receptors, and the tumor microenvironment constitute potential therapeutic targets, which may be relevant not only to NF1-related malignancy but also to other human cancers.

Reconstitution of the NF1 GAP-related domain in NF1-deficient human Schwann cells

Schwann cells derived from peripheral nerve sheath tumors from individuals with Neurofibromatosis Type 1 (NF1) are deficient for the protein neurofibromin, which contains a GAP-related domain (NF1-GRD). Neurofibromin-deficient Schwann cells have increased Ras activation, increased proliferation in response to certain growth stimuli, increased angiogenic potential, and altered cell morphology. This study examined whether expression of functional NF1-GRD can reverse the transformed phenotype of neurofibromin-deficient Schwann cells from both benign and malignant peripheral nerve sheath tumors. We reconstituted the NF1-GRD using retroviral transduction and examined the effects on cell morphology, growth potential, and angiogenic potential. NF1-GRD reconstitution resulted in morphologic changes, a 16-33% reduction in Ras activation, and a 53% decrease in proliferation in neurofibromin-deficient Schwann cells. However, NF1-GRD reconstitution was not sufficient to decrease the in vitro angiogenic potential of the cells. This study demonstrates that reconstitution of the NF1-GRD can at least partially reverse the transformation of human NF1 tumor-derived Schwann cells.

Molecular, genetic, and cellular pathogenesis of neurofibromas and surgical implications

Neurofibromatosis 1 (NF1) is a common autosomal dominant disease characterized by complex and multicellular neurofibroma tumors. Significant advances have been made in the research of the cellular, genetic, and molecular biology of NF1. The NF1 gene was identified by positional cloning. The functions of its protein product, neurofibromin, in RAS signaling and in other signal transduction pathways are being elucidated, and the important roles of loss of heterozygosity and haploinsufficiency in tumorigenesis are better understood. The Schwann cell was discovered to be the cell of origin for neurofibromas, but understanding of a more complicated interplay of multiple cell types in tumorigenesis, specifically recruited heterogeneous cell types such as mast cells and fibroblasts, has important implications for surgical therapy of these tumors. This review summarizes the most recent NF1 and neurofibroma literature describing the pathogenesis and treatment of nerve sheath tumors. Understanding the biological underpinnings of tumorigenesis in NF1 has implications for future surgical and medical management of neurofibromas.

Nf1 haploinsufficiency augments angiogenesis

Mutations in the NF1 tumor-suppressor gene underlie neurofibromatosis type 1 (NF1), in which patients are predisposed to certain tumors such as neurofibromas and may associate with vascular disorder. Plexiform neurofibromas are slow growing benign tumors that are highly vascular and can progress to malignancy. The development of neurofibromas requires loss of both Nf1 alleles in Schwann cells destined to become neoplastic and may be exacerbated by Nf1 heterozygosity in other non-neoplastic cells. This study tested the hypothesis that Nf1 heterozygosity exaggerates angiogenesis. We found that Nf1 heterozygous mice showed increased neovascularization in both the retina and cornea in response to hypoxia and bFGF, respectively, compared to their wild-type littermates. The increase in corneal neovascularization was associated with heightened endothelial cell proliferation and migration, and increased infiltration of inflammatory cells. In addition, Nf1 heterozygous endothelial cell cultures showed an exaggerated proliferative response to angiogenic factors, particularly to bFGF. These findings support the conclusion that Nf1 heterozygosity in endothelial cells and perhaps inflammatory cells augments angiogenesis, which may promote neurofibroma formation in NF1.

Neurofibroma-associated growth factors activate a distinct signaling network to alter the function of neurofibromin-deficient endothelial cells

Genetic inactivation of tumor suppressor genes initiates human cancers. However, interaction of accessory cells with the tumor-initiating cell within the microenvironment is often required for tumor progression. This paradigm is relevant to understanding neurofibroma development in neurofibromatosis type I patients. Somatic inactivation of the Nf1 tumor suppressor gene, which encodes neurofibromin, is necessary but not sufficient to initiate neurofibroma development. In contrast, neurofibromas occur with high penetrance in mice in which Nf1 is ablated in Schwann cells in the context of a heterozygous mutant (Nf1+/-) microenvironment. Neurofibromas are highly vascularized, and recent studies suggest that Nf1+/- mice have increased angiogenesis in vivo. However, the function of neurofibromin in human endothelial cells (ECs) and the biochemical mechanism by which neurofibromin regulates neoangiogenesis are not known. Utilizing Nf1+/- mice, primary human ECs and endothelial progenitor cells harvested from NF1 patients, we identified a discrete Ras effector pathway, which alters the proliferation and migration of neurofibromin-deficient ECs in response to neurofibroma-derived growth factors both in vitro and in vivo. Thus, these studies identify a unique biochemical pathway in Nf1+/- ECs as a potential therapeutic target in the neurofibroma microenvironment.

The Mitogen-Activated Protein Kinase/Extracellular Signal-Regulated Kinase Kinase Inhibitor PD184352 (CI-1040) Selectively Induces Apoptosis in Malignant Schwannoma Cell Lines

Type 1 neurofibromatosis (NF1) is a common autosomal dominant disorder that results in neuroectodermal tumors. The NF1 tumor-suppressor gene encodes neurofibromin, which includes a GTPase-activating domain for Ras inactivation. Affinity purification showed N-Ras to be the predominant activated isoform of Ras in two independent neurofibrosarcoma cell lines from NF1 patients (lines ST88-14 and NF90-8). These NF1 cells also demonstrated increased constitutive activity of the extracellular signal-regulated kinases 1 and 2 (ERK1,2) mitogen-activated protein (MAP) kinases compared with a sporadic malignant schwannoma cell line that maintains neurofibromin expression (STS-26T). Thus, MAP kinase kinase (MEK) inhibitors may be a rational approach to NF1 therapy. The MEK inhibitors PD98059 [2'-amino-3'-methoxyflavone], PD184352 (also called CI-1040) [2-(2-chloro-4-iodo-phenylamino)-N-cyclopropylmethoxy-3,4-difluoro-benzamide], and U0126 [1,4-diamino-2,3-dicyano-1,4-bis(2-aminophenylthio)butadiene] all produced concentration-dependent suppression of the proliferation of the three cell lines. Individual MEK inhibitors had similar effects in all three cell lines. However, only the antiproliferative effects of PD184352 correlated closely with the elimination of ERK1,2 MAP kinase activities. PD98059 was primarily cytostatic, whereas U0126 and PD184352 were cytotoxic. Only PD184352 induced apoptosis in all three lines, as indicated by morphology, activation of DEVDase, procaspase-3 cleavage, and the appearance of populations having sub-G(0)/G(1) DNA contents. The differential effects of the MEK inhibitors on cell survival were not dependent on p53 status or effects on the ERK5 pathway. PD184352 was also proapoptotic to primary rat Schwann cells. Hence, although PD184352 effectively killed neurofibrosarcoma cells, its effects on normal Schwann cells may limit its usefulness in the clinic.

Neurofibromatosis type 1 - a model for nervous system tumour formation?

Neurofibromatosis type 1 (NF1) is a common genetic condition in which affected individuals develop benign and malignant nervous system tumours. Genetically engineered mouse (GEM) models of these NF1-associated nervous system tumours recapitulate several of the unique clinical aspects of the disease. Moreover, these Nf1 GEM models allow for a direct examination of the earliest stages of tumour evolution, including the contributions that Nf1(+/-) cellular elements and cooperating genetic changes make to facilitate the transition from the pre-neoplastic to the neoplastic state and, in some cases, to promote malignant progression.