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.

p53 and NF 1 loss plays distinct but complementary roles in glioma initiation and progression

Malignant glioma is one of the deadliest types of cancer. Understanding how the cell of origin progressively evolves toward malignancy in greater detail could provide mechanistic insights and lead to novel concepts for tumor prevention and therapy. Previously we have identified oligodendrocyte precursor cell (OPC) as the cell of origin for glioma following the concurrent deletion of p53 and NF1 using a mouse genetic mosaic system that can reveal mutant cells prior to malignancy. In the current study, we set out to deconstruct the gliomagenic process in two aspects. First, we determined how the individual loss of p53 or NF1 contributes to aberrant behaviors of OPCs. Second, we determined how signaling aberrations in OPCs progressively change from pre-malignant to transformed stages. We found that while the deletion of NF1 leads to mutant OPC expansion through increased proliferation and decreased differentiation, the deletion of p53 impairs OPC senescence. Signaling analysis showed that, while PI3K and MEK pathways go through stepwise over-activation, mTOR signaling remains at the basal level in pre-transforming mutant OPCs but is abruptly up-regulated in tumor OPCs. Finally, inhibiting mTOR via pharmacological or genetic methods, led to a significant blockade of gliomagenesis but had little impact on pre-transforming mutant OPCs, suggesting that mTOR is necessary for final transformation but not early progression. In summary, our findings show that deconstructing the tumorigenic process reveals specific aberrations caused by individual gene mutations and altered signaling events at precise timing during tumor progression, which may shed light on tumor-prevention strategies.

Pharmacologically targeting beta-catenin for NF1 associated deficiencies in fracture repair

Patients with Neurofibromatosis type 1 display delayed fracture healing and the increased deposition of fibrous tissue at the fracture site. Severe cases can lead to non-union and even congenital pseudarthrosis. Neurofibromatosis type 1 is caused by a mutation in the NF1 gene and mice lacking the Nf1 gene show a fracture repair phenotype similar to that seen in patients. Tissue from the fracture site of patients with Neurofibromatosis type 1 and from mice deficient in the Nf1 gene both show elevated levels of β-catenin protein and activation of β-catenin mediated signaling. Constitutively elevated β-catenin leads to a delayed and fibrous fracture repair process, and (RS)-5-methyl-1-phenyl-1,3,4,6-tetrahydro-2,5-benzoxazocine (Nefopam, a centrally-acting, non-narcotic analgesic agent) inhibits β-catenin mediated signaling during skin wound repair. Here we investigate Nefopam's potential as a modulator of bone repair in mice deficient in Nf1. Mice were treated with Nefopam and investigated for bone fracture repair. Bone marrow stromal cells flushed from the long bones of unfractured mice were treated with Nefopam and investigated for osteogenic potential. Treatment with Nefopam was able to lower the β-catenin level and the Axin2 transcript level in the fracture calluses of Nf1 deficient mice. Cultures from the bone marrow of Nf1^-/- mice had significantly lower osteoblastic colonies and mineralized nodules, which was increased when cells were cultured in the presence of Nefopam. Fracture calluses were harvested and analyzed 14days and 21days after injury. Nf1^-/- calluses had less bone, less cartilage, and higher fibrous tissue content than control calluses. Treatment with Nefopam increased the bone and cartilage content and decreased the fibrous tissue content in Nf1^-/- calluses. These findings present a potential treatment for patients with Neurofibromatosis 1 in the context of bone repair. Since Nefopam is already in use in patient care, it could be rapidly translated to the clinical setting.

An inflammatory gene signature distinguishes neurofibroma Schwann cells and macrophages from cells in the normal peripheral nervous system

Neurofibromas are benign peripheral nerve tumors driven by NF1 loss in Schwann cells (SCs). Macrophages are abundant in neurofibromas, and macrophage targeted interventions may have therapeutic potential in these tumors. We generated gene expression data from fluorescence-activated cell sorted (FACS) SCs and macrophages from wild-type and mutant nerve and neurofibroma to identify candidate pathways involved in SC-macrophage cross-talk. While in 1-month-old Nf1 mutant nerve neither SCs nor macrophages significantly differed from their normal counterparts, both macrophages and SCs showed significantly altered cytokine gene expression in neurofibromas. Computationally reconstructed SC-macrophage molecular networks were enriched for inflammation-associated pathways. We verified that neurofibroma SC conditioned medium contains macrophage chemo-attractants including colony stimulation factor 1 (CSF1). Network analysis confirmed previously implicated pathways and predict novel paracrine and autocrine loops involving cytokines, chemokines, and growth factors. Network analysis also predicted a central role for decreased type-I interferon signaling. We validated type-I interferon expression in neurofibroma by protein profiling, and show that treatment of neurofibroma-bearing mice with polyethylene glycolyated (PEGylated) type-I interferon-α2b reduces the expression of many cytokines overexpressed in neurofibroma. These studies reveal numerous potential targetable interactions between Nf1 mutant SCs and macrophages for further analyses.

Combined MEK inhibition and BMP2 treatment promotes osteoblast differentiation and bone healing in Nf1Osx -/- mice

Neurofibromatosis type I (NF1) is an autosomal dominant disease with an incidence of 1/3000, caused by mutations in the NF1 gene, which encodes the RAS/GTPase-activating protein neurofibromin. Non-bone union after fracture (pseudarthrosis) in children with NF1 remains a challenging orthopedic condition to treat. Recent progress in understanding the biology of neurofibromin suggested that NF1 pseudarthrosis stems primarily from defects in the bone mesenchymal lineage and hypersensitivity of hematopoietic cells to TGFβ. However, clinically relevant pharmacological approaches to augment bone union in these patients remain limited. In this study, we report the generation of a novel conditional mutant mouse line used to model NF1 pseudoarthrosis, in which Nf1 can be ablated in an inducible fashion in osteoprogenitors of postnatal mice, thus circumventing the dwarfism associated with previous mouse models where Nf1 is ablated in embryonic mesenchymal cell lineages. An ex vivo-based cell culture approach based on the use of Nf1(flox/flox) bone marrow stromal cells showed that loss of Nf1 impairs osteoprogenitor cell differentiation in a cell-autonomous manner, independent of developmental growth plate-derived or paracrine/hormonal influences. In addition, in vitro gene expression and differentiation assays indicated that chronic ERK activation in Nf1-deficient osteoprogenitors blunts the pro-osteogenic property of BMP2, based on the observation that only combination treatment with BMP2 and MEK inhibition promoted the differentiation of Nf1-deficient osteoprogenitors. The in vivo preclinical relevance of these findings was confirmed by the improved bone healing and callus strength observed in Nf1osx (-/-) mice receiving Trametinib (a MEK inhibitor) and BMP2 released locally at the fracture site via a novel nanoparticle and polyglycidol-based delivery method. Collectively, these results provide novel evidence for a cell-autonomous role of neurofibromin in osteoprogenitor cells and insights about a novel targeted approach for the treatment of NF1 pseudoarthrosis.

Signal therapy of NF1-deficient tumor xenograft in mice by the anti-PAK1 drug FK228

PAK1, a Rac/CDC42-dependent Ser/Thr kinase, is required for the malignant growth of RAS transformants as well as both NF1-deficient and NF2-deficient cancer cells. FK228, a histone deacetylase (HDAC) inhibitor, suppresses the growth of more than 70% of human cancers in vivo including RAS transformants, breast cancers and prostate cancers by activating a set of genes including the tumor suppressors gelsolin and p21(WAF1), that block upstream and downstream of PAK1, respectively. Here we demonstrate that (1) the anti-PAK1 drug FK228 (0.1 nM) completely blocks the growth of both NF1-deficient and NF2-deficient cancer cells in vitro, and that (2) FK228 (2.5 mg/kg, i.p., twice a week) causes the complete regression of an NF1-deficient human malignant peripheral nerve sheath tumor (MPNST) xenograft in nude mice. This is the very first case where a chemical drug in clinical trials for cancers has ever worked so effectively on neurofibromatosis (experimental neurofibromas) in vivo.

PRC2 loss amplifies Ras-driven transcription and confers sensitivity to BRD4-based therapies

The polycomb repressive complex 2 (PRC2) exerts oncogenic effects in many tumour types. However, loss-of-function mutations in PRC2 components occur in a subset of haematopoietic malignancies, suggesting that this complex plays a dichotomous and poorly understood role in cancer. Here we provide genomic, cellular, and mouse modelling data demonstrating that the polycomb group gene SUZ12 functions as tumour suppressor in PNS tumours, high-grade gliomas and melanomas by cooperating with mutations in NF1. NF1 encodes a Ras GTPase-activating protein (RasGAP) and its loss drives cancer by activating Ras. We show that SUZ12 loss potentiates the effects of NF1 mutations by amplifying Ras-driven transcription through effects on chromatin. Importantly, however, SUZ12 inactivation also triggers an epigenetic switch that sensitizes these cancers to bromodomain inhibitors. Collectively, these studies not only reveal an unexpected connection between the PRC2 complex, NF1 and Ras, but also identify a promising epigenetic-based therapeutic strategy that may be exploited for a variety of cancers.

Neurofibromin inactivation impairs osteocyte development in Nf1Prx1 and Nf1Col1 mouse models

Neurofibromin has been identified as a critical regulator of osteoblast differentiation. Osteoblast specific inactivation of neurofibromin in mice results in a high bone mass phenotype and hyperosteoidosis. Here, we show that inactivation of the Nf1 gene also impairs osteocyte development. We analyzed cortical bone tissue in two conditional mouse models, Nf1Prx1 and Nf1Col1, for morphological and molecular effects. Backscattered electron microscopy revealed significantly enlarged osteocyte lacunae in Nf1Prx1 and Nf1Col1 mice (level E2: ctrl=1.90±0.52%, Nf1Prx1=3.40±0.95%; ctrl 1.60±0.47%, Nf1Col1 2.46±0.91%). Moreover, the osteocyte lacunae appeared misshaped in Nf1Prx1 and Nf1Col1 mice as indicated by increased Feret ratios. Strongest osteocyte and dendritic network disorganization was observed in proximity of muscle attachment sites in Nf1Prx1 humeri. In contrast to control cells, Nf1Prx1 osteocytes contained abundant cytosolic vacuoles and accumulated immature organic matrix within the perilacunar space, a phenotype reminiscent of the hyperosteoidosis shown Nf1 deficient mice. Cortical bone lysates further revealed approx. twofold upregulated MAPK signalling in osteocytes of Nf1Prx1 mice. This was associated with transcriptional downregulation of collagens and genes involved in mechanical sensing in Nf1Prx1 and Nf1Col1 bone tissue. In contrast, matrix gla protein (MGP), phosphate regulating endopeptidase homolog, X-linked (PHEX), and genes involved in lipid metabolism were upregulated. In line with previously described hyperactivation of Nf1 deficient osteoblasts, systemic plasma levels of the bone formation markers osteocalcin (OCN) and procollagen typ I N-propeptide (PINP) were approx. twofold increased in Nf1Prx1 mice. Histochemical and molecular analysis ascertained that osteocytes in Nf1Prx1 cortical bone were viable and did not undergo apoptosis or autophagy. We conclude that loss of neurofibromin is not only critical for osteoblasts but also hinders normal osteocyte development. These findings expand the effect of neurofibromin onto yet another cell type where it is likely involved in the regulation of mechanical sensing, bone matrix composition and mechanical resistance of bone tissue.

Loss of NF1 in cutaneous melanoma is associated with RAS activation and MEK dependence

Melanoma is a disease characterized by lesions that activate ERK. Although 70% of cutaneous melanomas harbor activating mutations in the BRAF and NRAS genes, the alterations that drive tumor progression in the remaining 30% are largely undefined. Vemurafenib, a selective inhibitor of RAF kinases, has clinical utility restricted to BRAF-mutant tumors. MEK inhibitors, which have shown clinical activity in NRAS-mutant melanoma, may be effective in other ERK pathway-dependent settings. Here, we investigated a panel of melanoma cell lines wild type for BRAF and NRAS to determine the genetic alteration driving their transformation and their dependence on ERK signaling in order to elucidate a candidate set for MEK inhibitor treatment. A cohort of the BRAF/RAS wild type cell lines with high levels of RAS-GTP had loss of NF1, a RAS GTPase activating protein. In these cell lines, the MEK inhibitor PD0325901 inhibited ERK phosphorylation, but also relieved feedback inhibition of RAS, resulting in induction of pMEK and a rapid rebound in ERK signaling. In contrast, the MEK inhibitor trametinib impaired the adaptive response of cells to ERK inhibition, leading to sustained suppression of ERK signaling and significant antitumor effects. Notably, alterations in NF1 frequently co-occurred with RAS and BRAF alterations in melanoma. In the setting of BRAF(V600E), NF1 loss abrogated negative feedback on RAS activation, resulting in elevated activation of RAS-GTP and resistance to RAF, but not MEK, inhibitors. We conclude that loss of NF1 is common in cutaneous melanoma and is associated with RAS activation, MEK-dependence, and resistance to RAF inhibition.

Multiscale, converging defects of macro-porosity, microstructure and matrix mineralization impact long bone fragility in NF1

Bone fragility due to osteopenia, osteoporosis or debilitating focal skeletal dysplasias is a frequent observation in the Mendelian disease Neurofibromatosis type 1 (NF1). To determine the mechanisms underlying bone fragility in NF1 we analyzed two conditional mouse models, Nf1Prx1 (limb knock-out) and Nf1Col1 (osteoblast specific knock-out), as well as cortical bone samples from individuals with NF1. We examined mouse bone tissue with micro-computed tomography, qualitative and quantitative histology, mechanical tensile analysis, small-angle X-ray scattering (SAXS), energy dispersive X-ray spectroscopy (EDX), and scanning acoustic microscopy (SAM). In cortical bone of Nf1Prx1 mice we detected ectopic blood vessels that were associated with diaphyseal mineralization defects. Defective mineral binding in the proximity of blood vessels was most likely due to impaired bone collagen formation, as these areas were completely devoid of acidic matrix proteins and contained thin collagen fibers. Additionally, we found significantly reduced mechanical strength of the bone material, which was partially caused by increased osteocyte volume. Consistent with these observations, bone samples from individuals with NF1 and tibial dysplasia showed increased osteocyte lacuna volume. Reduced mechanical properties were associated with diminished matrix stiffness, as determined by SAM. In line with these observations, bone tissue from individuals with NF1 and tibial dysplasia showed heterogeneous mineralization and reduced collagen fiber thickness and packaging. Collectively, the data indicate that bone fragility in NF1 tibial dysplasia is partly due to an increased osteocyte-related micro-porosity, hypomineralization, a generalized defect of organic matrix formation, exacerbated in the regions of tensional and bending force integration, and finally persistence of ectopic blood vessels associated with localized macro-porotic bone lesions.

The Ras GTPase-activating protein neurofibromin 1 promotes the positive selection of thymocytes

TCR-mediated activation of the Ras signaling pathway is critical for T cell development in the thymus and function in the periphery. However, which members of a family of Ras GTPase-activating proteins (RasGAPs) negatively regulate Ras activation in T cells is unknown. In this study we examined a potential function for the neurofibromin 1 (NF1) RasGAP in the T cell lineage with the use of T cell-specific NF1-deficient mice. Surprisingly, on an MHC class I-restricted TCR transgenic background, NF1 was found to promote thymocyte positive selection. By contrast, NF1 neither promoted nor inhibited the negative selection of thymocytes. In the periphery, NF1 was found to be necessary for the maintenance of normal numbers of naïve CD4⁺ and CD8⁺ T cells but was dispensable as a regulator of TCR-induced Ras activation, cytokine synthesis, proliferation and differentiation and death. These findings point to a novel unexpected role for NF1 in T cell development as well as a regulator of T cell homeostasis.

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.

Elucidating distinct roles for NF1 in melanomagenesis

BRAF mutations play a well-established role in melanomagenesis; however, without additional genetic alterations, tumor development is restricted by oncogene-induced senescence (OIS). Here, we show that mutations in the NF1 tumor suppressor gene cooperate with BRAF mutations in melanomagenesis by preventing OIS. In a genetically engineered mouse model, Nf1 mutations suppress Braf-induced senescence, promote melanocyte hyperproliferation, and enhance melanoma development. Nf1 mutations function by deregulating both phosphoinositide 3-kinase and extracellular signal-regulated kinase pathways. As such, Nf1/Braf-mutant tumors are resistant to BRAF inhibitors but are sensitive to combined inhibition of mitogen-activated protein/extracellular signal-regulated kinase kinase and mTOR. Importantly, NF1 is mutated or suppressed in human melanomas that harbor concurrent BRAF mutations, NF1 ablation decreases the sensitivity of melanoma cell lines to BRAF inhibitors, and NF1 is lost in tumors from patients following treatment with these agents. Collectively, these studies provide mechanistic insight into how NF1 cooperates with BRAF mutations in melanoma and show that NF1/neurofibromin inactivation may have an impact on responses to targeted therapies.

An unholy alliance: cooperation between BRAF and NF1 in melanoma development and BRAF inhibitor resistance

In this issue of Cancer Discovery, 2 studies provide new evidence implicating loss of the tumor suppressor neurofibromin (NF1) in the biologic behavior of cutaneous melanoma. The first study from Maertens and colleagues describes a new transgenic mouse model in which mutant BRAF cooperates with NF1 loss to drive melanoma development through the abrogation of oncogene-induced senescence. The second, from Whittaker and colleagues, used a high-throughput short hairpin RNA screening approach to identify NF1 loss as a key mediator of acquired and intrinsic BRAF inhibitor resistance. Together these studies provide new insights into the signaling that underlies melanoma initiation and progression and suggests novel therapeutic strategies for patients whose melanomas are BRAF-mutant/NF1-deficient.

Sensitivity of malignant peripheral nerve sheath tumor cells to TRAIL is augmented by loss of NF1 through modulation of MYC/MAD and is potentiated by curcumin through induction of ROS

Malignant peripheral nerve sheath tumor (MPNST) is a rare aggressive form of sarcoma often associated with the tumor syndrome neurofibromatosis type 1 (NF1). We investigated the effects of tumor necrosis factor-related apoptosis inducing ligand (TRAIL) on NF1 associated MPNST and determinants of TRAIL sensitivity. MPNST cell lines with complete neurofibromin deficiency were sensitive to apoptotic cell death induced by TRAIL whereas MPNST cells with retained neurofibromin expression or normal human Schwann cells were resistant. Increased sensitivity to TRAIL was associated with overexpression of death receptors, especially DR5. Re-expression of the GAP related domain of neurofibromin (NF1-GRD) suppressed DR5 expression and decreased sensitivity to TRAIL. We show that death receptor expression and TRAIL sensitivity critically depend on c-MYC and that c-MYC amounts are increased by MEK/ERK and PI3K/AKT signalling pathways which are suppressed by neurofibromin. Furthermore PI3K/AKT signalling strongly suppresses the MYC-antagonist MAD1 which significantly contributes to TRAIL sensitivity. Re-expression of the NF1-GRD decreased c-MYC and increased MAD1 amounts suggesting that neurofibromin influences TRAIL sensitivity at least in part by modulating the MYC/MAX/MAD network. The phytochemical curcumin further increased the sensitivity of neurofibromin deficient MPNST cells to TRAIL. This was presumably mediated by ROS, as it correlated with increased ROS production, was blocked by N-acetylcysteine and mimicked by exogenous ROS.

A murine model of neurofibromatosis type 1 tibial pseudarthrosis featuring proliferative fibrous tissue and osteoclast-like cells

Neurofibromatosis type 1 (NF1) is a common genetic condition caused by mutations in the NF1 gene. Patients often suffer from tissue-specific lesions associated with local double-inactivation of NF1. In this study, we generated a novel fracture model to investigate the mechanism underlying congenital pseudarthrosis of the tibia (CPT) associated with NF1. We used a Cre-expressing adenovirus (AdCre) to inactivate Nf1 in vitro in cultured osteoprogenitors and osteoblasts, and in vivo in the fracture callus of Nf1(flox/flox) and Nf1(flox/-) mice. The effects of the presence of Nf1(null) cells were extensively examined. Cultured Nf1(null)-committed osteoprogenitors from neonatal calvaria failed to differentiate and express mature osteoblastic markers, even with recombinant bone morphogenetic protein-2 (rhBMP-2) treatment. Similarly, Nf1(null)-inducible osteoprogenitors obtained from Nf1 MyoDnull mouse muscle were also unresponsive to rhBMP-2. In both closed and open fracture models in Nf1(flox/flox) and Nf1(flox/-) mice, local AdCre injection significantly impaired bone healing, with fracture union being <50% that of wild type controls. No significant difference was seen between Nf1(flox/flox) and Nf1(flox/-) mice. Histological analyses showed invasion of the Nf1(null) fractures by fibrous and highly proliferative tissue. Mean amounts of fibrous tissue were increased upward of 10-fold in Nf1(null) fractures and bromodeoxyuridine (BrdU) staining in closed fractures showed increased numbers of proliferating cells. In Nf1(null) fractures, tartrate-resistant acid phosphatase-positive (TRAP+) cells were frequently observed within the fibrous tissue, not lining a bone surface. In summary, we report that local Nf1 deletion in a fracture callus is sufficient to impair bony union and recapitulate histological features of clinical CPT. Cell culture findings support the concept that Nf1 double inactivation impairs early osteoblastic differentiation. This model provides valuable insight into the pathobiology of the disease, and will be helpful for trialing therapeutic compounds.

JNK inhibitors increase osteogenesis in Nf1-deficient cells

Neurofibromatosis type 1 (NF1) is an autosomal dominant disorder that is associated with a variety of manifestations, including orthopedic complications such as scoliosis and tibial pseudarthrosis. Orthopedic management of these skeletal complications is rendered more challenging due to a lack of standardized adjunctive pharmacotherapies. NF1 leads to disruption of the canonical Ras/Raf-1/MEK/ERK axis, and this has been associated with defects in bone anabolism. The roles of other non-canonical Ras effector pathways, such as the c-Jun N-terminal Kinase (JNK) pathway, are less well understood. In this study we examine the effects of an anthrapyrazolone inhibitor of JNK (SP600125) on inducible osteoprogenitors as well as Nf1-deficient and Nf1-null primary osteoblasts. C2C12 cells, which are highly responsive to rhBMP-2, were examined with exogenous rhBMP-2 and a range of SP600125 doses. Based on the expression of early and late bone markers and matrix mineralization, 10 μM SP600125 was found to be pro-osteogenic whether delivered concurrent with or following 2 days of rhBMP-2 treatment. Aberrant JNK activity was identified in Nf1-deficient osteoprogenitors (increased rhBMP-2 induced phospho-c-Jun) and in Nf1-null mature osteoblasts (increased total c-Jun). Next, SP600125 was used to treat these cells and was found to facilitate osteogenesis in Nf1-deficient osteoprogenitors, and in Nf1-null osteoblasts when given in conjunction with rhBMP-2. Outcome measures included alkaline phosphatase activity, matrix mineralization, and osteogenic gene expression. In summary, JNK inhibitors represent a class of potentially useful adjunctive agents for orthopedic medicine, particularly in the context of NF1.

Motor deficits and neurofibromatosis type 1 (NF1)-associated MRI impairments in a mouse model of NF1

Neurofibromatosis type 1 (NF1) is a genetic disorder characterized inter alia by cognitive and motor dysfunction and appearance of high-signal foci on T2-weighted images in the brain. Nf1(+/-) mice are useful models for studying aspects of NF1, including cognitive deficits. Here we assessed their motor performance and used quantitative transverse T2 relaxation MRI to identify structural abnormalities in their brains. Nf1(+/-) mice exhibited both enhanced and reduced T2 signals in distinct brain regions compared to wild-type mice, and their motor performance was impaired. As in NF1 patients, enhanced T2 signals in Nf1(+/-) mice were observed in the thalamus and basal ganglia. Reduced T2 signals were seen in motor-associated regions along the motor pathway, predominantly in the white matter of the cerebral peduncle and the optic tract. Correlation analysis between T2 signals and motor performance suggested that the motor deficits are associated with impairments in the cerebral peduncle and the amygdala.

The loss of Nf1 transiently promotes self-renewal but not tumorigenesis by neural crest stem cells

Neurofibromatosis is caused by the loss of neurofibromin (Nf1), leading to peripheral nervous system (PNS) tumors, including neurofibromas and malignant peripheral nerve sheath tumors (MPNSTs). A long-standing question has been whether these tumors arise from neural crest stem cells (NCSCs) or differentiated glia. Germline or conditional Nf1 deficiency caused a transient increase in NCSC frequency and self-renewal in most regions of the fetal PNS. However, Nf1-deficient NCSCs did not persist postnatally in regions of the PNS that developed tumors and could not form tumors upon transplantation into adult nerves. Adult P0a-Cre+Nf1(fl/-) mice developed neurofibromas, and Nf1(+/-)Ink4a/Arf(-/-) and Nf1/p53(+/-) mice developed MPNSTs, but NCSCs did not persist postnatally in affected locations in these mice. Tumors appeared to arise from differentiated glia, not NCSCs.

Type 2 NF1 deletions are highly unusual by virtue of the absence of nonallelic homologous recombination hotspots and an apparent preference for female mitotic recombination

Approximately 5% of patients with neurofibromatosis type 1 (NF1) exhibit gross deletions that encompass the NF1 gene and its flanking regions. The breakpoints of the common 1.4-Mb (type 1) deletions are located within low-copy repeats (NF1-REPs) and cluster within a 3.4-kb hotspot of nonallelic homologous recombination (NAHR). Here, we present the first comprehensive breakpoint analysis of type 2 deletions, which are a second type of recurring NF1 gene deletion. Type 2 deletions span 1.2 Mb and are characterized by breakpoints located within the SUZ12 gene and its pseudogene, which closely flank the NF1-REPs. Breakpoint analysis of 13 independent type 2 deletions did not reveal any obvious hotspots of NAHR. However, an overrepresentation of polypyrimidine/polypurine tracts and triplex-forming sequences was noted in the breakpoint regions that could have facilitated NAHR. Intriguingly, all 13 type 2 deletions identified so far are characterized by somatic mosaicism, which indicates a positional preference for mitotic NAHR within the NF1 gene region. Indeed, whereas interchromosomal meiotic NAHR occurs between the NF1-REPs giving rise to type 1 deletions, NAHR during mitosis appears to occur intrachromosomally between the SUZ12 gene and its pseudogene, thereby generating type 2 deletions. Such a clear distinction between the preferred sites of mitotic versus meiotic NAHR is unprecedented in any other genomic disorder induced by the local genomic architecture. Additionally, 12 of the 13 mosaic type 2 deletions were found in females. The marked female preponderance among mosaic type 2 deletions contrasts with the equal sex distribution noted for type 1 and/or atypical NF1 deletions. Although an influence of chromatin structure was strongly suspected, no sex-specific differences in the methylation pattern exhibited by the SUZ12 gene were apparent that could explain the higher rate of mitotic recombination in females.

Spatiotemporal differences in CXCL12 expression and cyclic AMP underlie the unique pattern of optic glioma growth in neurofibromatosis type 1

Astrocytoma (glioma) formation in neurofibromatosis type 1 (NF1) occurs preferentially along the optic pathway during the first decade of life. The molecular basis for this unique pattern of gliomagenesis is unknown. Previous studies in mouse Nf1 optic glioma models suggest that this patterning results from cooperative effects of Nf1 loss in glial cells and the action of factors derived from the surrounding Nf1+/- brain. Because CXCL12 is a stroma-derived growth factor for malignant brain tumors, we tested the hypothesis that CXCL12 functions in concert with Nf1 loss to facilitate NF1-associated glioma growth. Whereas CXCL12 promoted cell death in wild-type astrocytes, it increased Nf1-/- astrocyte survival. This increase in Nf1-/- astrocyte survival in response to CXCL12 was due to sustained suppression of intracellular cyclic AMP (cAMP) levels. Moreover, the ability of CXCL12 to suppress cAMP and increase Nf1-/- astrocyte survival was a consequence of mitogen-activated protein/extracellular signal-regulated kinase kinase-dependent inhibition of CXCL12 receptor (CXCR4) desensitization. In support of an instructive role for CXCL12 in facilitating optic glioma growth, we also show that CXCL12 expression along the optic pathway is higher in infant children and young mice and is associated with low levels of cAMP. CXCL12 expression declines in multiple brain regions with increasing age, correlating with the age-dependent decline in glioma growth in children with NF1. Collectively, these studies provide a mechanism for the unique pattern of NF1-associated glioma growth.

Pheochromocytomas in Nf1 knockout mice express a neural progenitor gene expression profile

Pheochromocytomas are adrenal medullary tumors that typically occur in adult patients, with increased frequency in multiple endocrine neoplasia type 2, von Hippel-Lindau disease, familial paraganglioma syndromes and neurofibromatosis type 1 (NF1). Pheochromocytomas arise in adult mice with a heterozygous knockout mutation of exon 31 of the murine Nf1 gene, providing a mouse model for pheochromocytoma development in NF1. We performed a microarray-based gene expression profiling study comparing mouse pheochromocytoma tissue to normal adult mouse adrenal medulla to develop a basis for studying the pathobiology of these tumors. The findings demonstrate that pheochromocytomas from adult neurofibromatosis knockout mice express multiple developmentally regulated genes involved in early development of both the CNS and peripheral nervous system. One of the most highly overexpressed genes is receptor tyrosine kinase Ret, which is known to be transiently expressed in the developing adrenal gland, down-regulated in adult adrenals and often overexpressed in human pheochromocytomas. Real-time polymerase chain reaction validated the microarray results and immunoblots confirmed the overexpression of Ret protein. Other highly expressed validated genes include Sox9, which is a neural crest determinant, and Hey 1, which helps to maintain the progenitor status of neural precursors. The findings are consistent with the recently proposed concept that persistent neural progenitors might give rise to pheochromocytomas in adult mouse adrenals and suggest that events predisposing to tumor development might occur before formation of the adrenal medulla or migration of cells from the neural crest. However, the competing possibility that developmentally regulated neural genes arise secondarily to neoplastic transformation cannot be ruled out. In either case, the unique profile of gene expression opens the mouse pheochromocytoma model to new applications pertinent to neural stem cells and suggests potential new targets for treatment of pheochromocytomas or eradication of their precursors.

Neurofibromatosis-1 (Nf1) heterozygous brain microglia elaborate paracrine factors that promote Nf1-deficient astrocyte and glioma growth

The tumor microenvironment is considered to play an important role in tumor formation and progression by providing both negative and positive signals that influence tumor cell growth. We and others have previously shown that brain tumor (glioma) formation in Nf1 genetically engineered mice requires a microenvironment composed of cells heterozygous for a targeted Nf1 mutation. Using NF1 as a model system to understand the contribution of the tumor microenvironment to glioma formation, we show that Nf1+/- brain microglia produce specific factors that promote Nf1-/- astrocyte growth in vitro and in vivo and identify hyaluronidase as one of these factors in both genetically engineered Nf1 mouse and human NF1-associated optic glioma. We further demonstrate that blocking hyaluronidase ameliorates the ability of Nf1+/- microglia to increase Nf1-/- astrocyte proliferation and that hyaluronidase increases Nf1-/- astrocyte proliferation in an MAPK-dependent fashion. Lastly, inhibiting microglia activation in genetically engineered Nf1 mice significantly reduces mouse optic glioma proliferation in vivo. Collectively, these studies identify Nf1+/- microglia as an important stromal cell type that promotes Nf1-/- astrocyte and optic glioma growth relevant to the pathogenesis of NF1-associated brain tumors and suggest that future brain therapies might be directed against paracrine factors produced by cells in the tumor microenvironment.

Deletion of Nf1 in neurons induces increased axon collateral branching after dorsal root injury

Ras-mediated signaling pathways participate in multiple aspects of neural development and function. For example, Ras signaling lies downstream of neurotrophic factors and Trk family receptor tyrosine kinases to regulate neuronal survival and morphological differentiation, including axon extension and target innervation. Neurofibromin, the protein encoded by the tumor suppressor gene Nf1, is a negative regulator of Ras [Ras-GAP (GTPase-activating protein)], and we previously demonstrated that Nf1 null embryonic sensory and sympathetic neurons can survive and differentiate independent of neurotrophin support. In this report, we demonstrate that Nf1 loss in adult sensory neurons enhances their intrinsic capacity for neurite outgrowth and collateral branching in vitro and in vivo after dorsal root injury. In contrast to the permanent sensory deficits observed in control mice after dorsal rhizotomy, neuron-specific Nf1 mutant mice spontaneously recover proprioceptive function. This phenomenon appears to be mediated both by a cell-autonomous capacity of spared Nf1-/- DRG neurons for increased axonal sprouting, and by non-cell-autonomous contribution from Nf1-/- neurons in the denervated spinal cord.

K-ras is critical for modulating multiple c-kit-mediated cellular functions in wild-type and Nf1+/- mast cells

p21(ras) (Ras) proteins and GTPase-activating proteins (GAPs) tightly modulate extracellular growth factor signals and control multiple cellular functions. The specific function of each Ras isoform (H, N, and K) in regulating distinct effector pathways, and the role of each GAP in negatively modulating the activity of each Ras isoform in myeloid cells and, particularly, mast cells is incompletely understood. In this study, we use murine models of K-ras- and Nf1-deficient mice to examine the role of K-ras in modulating mast cell functions and to identify the role of neurofibromin as a GAP for K-ras in this lineage. We find that K-ras is required for c-kit-mediated mast cell proliferation, survival, migration, and degranulation in vitro and in vivo. Furthermore, the hyperactivation of these cellular functions in Nf1(+/-) mast cells is decreased in a K-ras gene dose-dependent fashion in cells containing mutations in both loci. These findings identify K-ras as a key effector in multiple mast cell functions and identify neurofibromin as a GAP for K-ras in mast cells.

Protein nutrition as therapy for a genetic disorder of bone?

Bone formation is controlled by a network of transcription factors and signaling molecules. In this issue, , studying the role of the transcription factor ATF4 in a new mouse model of neurofibromatosis type I skeletal defects, demonstrate striking effects of changing dietary protein on bone formation abnormalities.

ATF4 mediation of NF1 functions in osteoblast reveals a nutritional basis for congenital skeletal dysplasiae

The transcription factor ATF4 enhances bone formation by favoring amino acid import and collagen synthesis in osteoblasts, a function requiring its phosphorylation by RSK2, the kinase inactivated in Coffin-Lowry Syndrome. Here, we show that in contrast, RSK2 activity, ATF4-dependent collagen synthesis, and bone formation are increased in mice lacking neurofibromin in osteoblasts (Nf1(ob)(-/-) mice). Independently of RSK2, ATF4 phosphorylation by PKA is enhanced in Nf1(ob)(-/-) mice, thereby increasing Rankl expression, osteoclast differentiation, and bone resorption. In agreement with ATF4 function in amino acid transport, a low-protein diet decreased bone protein synthesis and normalized bone formation and bone mass in Nf1(ob)(-/-) mice without affecting other organ weight, while a high-protein diet overcame Atf4(-/-) and Rsk2(-/-) mice developmental defects, perinatal lethality, and low bone mass. By showing that ATF4-dependent skeletal dysplasiae are treatable by dietary manipulations, this study reveals a molecular connection between nutrition and skeletal development.

The tumor suppressor neurofibromin confers sensitivity to apoptosis by Ras-dependent and Ras-independent pathways

Neurofibromatosis type 1 (NF1) is characterized by a high incidence of benign and malignant tumors attributed to loss of function of Nf1, which encodes neurofibromin, a tumor suppressor with Ras-GAP activity. Neurofibromin deficiency typically causes chronic activation of Ras, considered the major contributor to manifestation of NF1. Resistance to radio- and chemotherapy are typical of NF1-associated tumors, but the underlying mechanism is unknown. Here, we investigated interrelationships between neurofibromin expression, Ras activity, and sensitivity to apoptosis. Neurofibromin-deficient mouse embryonic fibroblasts (MEFs) and human NF1 tumor cells were more resistant than neurofibromin-expressing cells to apoptosis. Moreover, Nf1(-/-), Nf1(+/-), and Nf1(+/+) MEFs exhibited gene-dosage-related resistance to apoptosis. Resistance of the Nf1-deficient cells was mediated by two survival pathways: a Ras-dependent pathway, and a Ras-independent pathway promoted by the lack of an NF1-GRD-independent proapoptotic action of neurofibromin. Therefore, besides its Ras-dependent growth inhibition, neurofibromin can exert tumor suppression via a proapoptotic effect.

Hyperactivation of p21ras and PI3K cooperate to alter murine and human neurofibromatosis type 1-haploinsufficient osteoclast functions

Individuals with neurofibromatosis type 1 (NF1) have a high incidence of osteoporosis and osteopenia. However, understanding of the cellular and molecular basis of these sequelae is incomplete. Osteoclasts are specialized myeloid cells that are the principal bone-resorbing cells of the skeleton. We found that Nf1(+/-) mice contain elevated numbers of multinucleated osteoclasts. Both osteoclasts and osteoclast progenitors from Nf1(+/-) mice were hyperresponsive to limiting concentrations of M-CSF and receptor activator of NF-kappaB ligand (RANKL) levels. M-CSF-stimulated p21(ras)-GTP and Akt phosphorylation was elevated in Nf1(+/-) osteoclasts associated with gains of function in survival, proliferation, migration, adhesion, and lytic activity. These gains of function are associated with more severe bone loss following ovariectomy as compared with that in syngeneic WT mice. Intercrossing Nf1(+/-) mice and mice deficient in class 1(A) PI3K (p85alpha) restored elevated PI3K activity and Nf1(+/-) osteoclast functions to WT levels. Furthermore, in vitro-differentiated osteoclasts from NF1 patients also displayed elevated Ras/PI3K activity and increased lytic activity analogous to those in murine Nf1(+/-) osteoclasts. Collectively, our results identify a what we believe to be a novel cellular and biochemical NF1-haploinsufficient phenotype in osteoclasts that has potential implications for the pathogenesis of NF1 bone disease.

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.

The neurofibromin GAP-related domain rescues endothelial but not neural crest development in Nf1 mice

Neurofibromatosis type I (NF1; also known as von Recklinghausen's disease) is a common autosomal-dominant condition primarily affecting neural crest-derived tissues. The disease gene, NF1, encodes neurofibromin, a protein of over 2,800 amino acids that contains a 216-amino acid domain with Ras-GTPase-activating protein (Ras-GAP) activity. Potential therapies for NF1 currently in development and being tested in clinical trials are designed to modify NF1 Ras-GAP activity or target downstream effectors of Ras signaling. Mice lacking the murine homolog (Nf1) have mid-gestation lethal cardiovascular defects due to a requirement for neurofibromin in embryonic endothelium. We sought to determine whether the GAP activity of neurofibromin is sufficient to rescue complete loss of function or whether other as yet unidentified functions of neurofibromin might also exist. Using cre-inducible ubiquitous and tissue-specific expression, we demonstrate that the isolated GAP-related domain (GRD) rescued cardiovascular development in Nf1(-/-) embryos, but overgrowth of neural crest-derived tissues persisted, leading to perinatal lethality. These results suggest that neurofibromin may possess activities outside of the GRD that modulate neural crest homeostasis and that therapeutic approaches solely aimed at targeting Ras activity may not be sufficient to treat tumors of neural crest origin in NF1.

Psoriatic skin expresses the transcription factor Gli1: possible contribution of decreased neurofibromin expression

BACKGROUND

Psoriasis is a chronic inflammatory disorder of skin characterized by hyperproliferation of keratinocytes. Intracellular signalling pathways inducing the hyperproliferation of keratinocytes remain to be elucidated. An inhibitor of Hedgehog (Hh) signalling, cyclopamine, was recently reported to clear psoriatic skin lesions, suggesting involvement of the Hh signalling pathway in the hyperproliferation of lesional keratinocytes. We have previously observed activation of the Hh signalling pathway in Schwann cells of plexiform neurofibroma in neurofibromatosis type 1 (NF1), which results from functional loss of the NF1 encoding protein, neurofibromin. In psoriasis, deficiency of neurofibromin expression has been observed in lesional keratinocytes.

OBJECTIVES

To investigate whether the Hh signalling pathway would be activated in psoriasis and whether inhibition of neurofibromin expression would enhance the activation of the Hh signalling pathway.

METHODS

Activation of the Hh signalling pathway was examined by protein expression of one of the target genes, GLI1, coding for the transcription factor Gli1. Immunohistochemical studies were performed on seven psoriatic skin samples and seven control normal skin samples with a standard immunoperoxidase technique. mRNA expression of GLI1 was analysed by reverse transcriptase-polymerase chain reaction in HaCaT cells transfected with double-strand small interfering RNA for NF1.

RESULTS

Our results showed Gli1 expression in psoriatic skin but not in control normal skin. Inhibition of neurofibromin expression in HaCaT cells upregulated mRNA expression of GLI1.

CONCLUSIONS

Our findings indicate that the Hh signalling pathway is activated in psoriasis and that neurofibromin deficiency may upregulate the pathway.

Neurofibromin plays a critical role in modulating osteoblast differentiation of mesenchymal stem/progenitor cells

Mutations in the NF1 tumor suppressor gene cause neurofibromatosis type 1, a pandemic autosomal dominant genetic disorder with an incidence of 1:3000. Individuals with NF1 have a variety of malignant and non-malignant manifestations, including skeletal manifestations, such as osteoporosis, scoliosis and short statures. However, the mechanism(s) underlying the osseous manifestations in NF1 are poorly understood. In the present study, utilizing Nf1 haploinsufficient (+/-) mice, we demonstrate that Nf1+/- mesenchymal stem/progenitor cells (MSPC) have increased proliferation and colony forming unit-fibroblast (CFU-F) capacity compared with wild-type (WT) MSPC. Nf1+/- MSPC also have fewer senescent cells and have a significantly higher telomerase activity compared with WT MSPC. Nf1+/- MSPC have impaired osteoblast differentiation as determined by alkaline phosphatase staining, and confirmed by single CFU-F replating assays. The impaired osteoblast differentiation in Nf1+/- MSPC is consistent with the reduced expression of osteoblast markers at the mRNA level, including osteocalcin and osteonectin. Importantly, re-expression of the full-length NF1 GTPase activating related domain (NF1 GAP-related domain) is sufficient to restore the impaired osteoblast differentiation in Nf1+/- MSPC. Taken together, our results suggest that neurofibromin plays a crucial role in modulating MSPC differentiation into osteoblasts, and the defect in osteoblast differentiation may contribute at least in part to the osseous abnormalities seen in individuals with NF1.

Signal therapy of human pancreatic cancer and NF1-deficient breast cancer xenograft in mice by a combination of PP1 and GL-2003, anti-PAK1 drugs (Tyr-kinase inhibitors)

The majority of cancers are caused by mutations of a few signal transducers such as the GTPase RAS, the kinase Src and the tumor suppressor p53. Thus, a group of specific chemical compounds called 'signal therapeutics', that block or reverse selectively these abnormally activated signaling pathways would be very useful for the treatment of these signally disordered cancers. More than 90% of human pancreatic cancers are associated with oncogenic mutations of RAS, in particular K-RAS at codon 12. We have previously shown that, PAK1, the Rac/CDC42-dependent Ser/Thr kinase, is essential for RAS/estrogen-induced transformation and neurofibromatosis (NF). Furthermore, we and others have demonstrated that the growth of mouse RAS-induced sarcomas allografts in mice is almost completely suppressed by either FK228 or a combination of two complimentary Tyr-kinase inhibitors, PP1 and AG 879, all of which block the RAS-induced activation of PAK1. Since, so far no effective therapeutic is available for the treatment of pancreatic cancer patients, we have examined the therapeutic potential of either FK228, the combination of these two Tyr-kinase inhibitors or GL-2003, a water-soluble derivative of AG 879, on human pancreatic cancer (Capan-1) xenograft in mice. Among these PAK1-blocking approaches, the PP1/GL-2003 combination is the most effective in the therapy of this cancer xenograft model. Its therapeutic potential is equivalent to those of gemcitabine and kigamicin D which suppress by 70-80% the growth of a similar human pancreatic cancer xenograft model. Also, this PP1/GL-2003 combination therapy has been proven to be very effective to suppress the estrogen-independent growth of an NF1-deficient multidrug/FK228-resistant human breast cancer (MDA-MB-231) xenograft in mice.

Cerebrospinal fluid proteomic analysis reveals dysregulation of methionine aminopeptidase-2 expression in human and mouse neurofibromatosis 1-associated glioma

Individuals affected with the neurofibromatosis 1 (NF1) tumor predisposition syndrome are prone to the development of multiple nervous system tumors, including optic pathway gliomas (OPG). The NF1 tumor suppressor gene product, neurofibromin, functions as a Ras GTPase-activating protein, and has been proposed to regulate cell growth by inhibiting Ras activity. Recent studies from our laboratory have shown that neurofibromin also regulates the mammalian target of rapamycin activity in a Ras-dependent fashion, and that the rapamycin-mediated mammalian target of rapamycin inhibition ameliorates the Nf1-/- astrocyte growth advantage. Moreover, Nf1-deficient astrocytes exhibit increased protein translation. As part of a larger effort to identify protein markers for NF1-associated astrocytomas that could be exploited for therapeutic drug design, we did an objective proteomic analysis of the cerebrospinal fluid from genetically engineered Nf1 mice with optic glioma. One of the proteins found to be increased in the cerebrospinal fluid of OPG-bearing mice was the eukaryotic initiation factor-2alpha binding protein, methionine aminopeptidase 2 (MetAP2). In this study, we show that Nf1 mouse OPGs and NF1-associated human astrocytic tumors, but not sporadic pilocytic or other low-grade astrocytomas, specifically expressed high levels of MetAP2. In addition, we show that Nf1-deficient astrocytes overexpress MetAP2 in vitro and in vivo, and that treatment with the MetAP2 inhibitor fumagillin significantly reduces Nf1-/- astrocyte proliferation in vitro. These observations suggest that MetAP2 is regulated by neurofibromin, and that MetAP2 inhibitors could be potentially employed to treat NF1-associated tumor proliferation.

Proteomic analysis reveals hyperactivation of the mammalian target of rapamycin pathway in neurofibromatosis 1-associated human and mouse brain tumors

Individuals with the tumor predisposition syndrome, neurofibromatosis 1 (NF1), are prone to development of nervous system tumors, including neurofibromas and pilocytic astrocytomas. Based on the ability of the NF1 gene product (neurofibromin) to function as a GTPase activating protein for RAS, initial biologically based therapies for NF1-associated tumors focused on the use of RAS inhibitors, but with limited clinical success. In an effort to identify additional targets for therapeutic drug design in NF1, we used an unbiased proteomic approach to uncover unanticipated intracellular signaling pathways dysregulated in Nf1-deficient astrocytes. We found that the expression of proteins involved in promoting ribosome biogenesis was increased in the absence of neurofibromin. In addition, Nf1-deficient astrocytes exhibit high levels of mammalian target of rapamycin (mTOR) pathway activation, which was inhibited by blocking K-RAS or phosphatidylinositol 3-kinase activation. This mTOR pathway hyperactivation was reflected by high levels of ribosomal S6 activation in both Nf1 mutant mouse optic nerve gliomas and in human NF1-associated pilocytic astrocytoma tumors. Moreover, inhibition of mTOR signaling in Nf1-/- astrocytes abrogated their growth advantage in culture, restoring normal proliferative rates. These results suggest that mTOR pathway inhibition may represent a logical and tractable biologically based therapy for brain tumors in NF1.

Glioma formation in neurofibromatosis 1 reflects preferential activation of K-RAS in astrocytes

Children with the tumor predisposition syndrome, neurofibromatosis 1 (NF1), develop optic pathway gliomas. The NF1 gene product, neurofibromin, functions as a negative regulator of RAS, such that NF1 inactivation results in RAS hyperactivation. Recent studies have highlighted the divergent biological and biochemical properties of the various RAS isoforms, which prompted us to examine the consequence of Nf1 inactivation in astrocytes on RAS isoform activation in vitro and in vivo. In this report, we show that only K-RAS is activated in Nf1-/- astrocytes and that activation of K-RAS, but not H-RAS, accounts for the proliferative advantage and abnormal actin cytoskeleton-mediated processes observed in Nf1-/- astrocytes in vitro. Moreover, dominant inhibitory K-RAS corrects these abnormalities in Nf1-/- astrocytes invitro. Lastly, we show that Nf1+/- mice with astrocyte-specific activated K-RAS expression in vivo develop optic pathway gliomas, similar to our previously reported Nf1+/- mice with astrocyte Nf1 inactivation. Collectively, our results show that K-RAS is the primary target for neurofibromin GTPase-activating protein activity in vitro and in vivo and that K-RAS activation in astrocytes recapitulates the biochemical, biological, and tumorigenic properties of neurofibromin loss.

Role of TC21/R-Ras2 in enhanced migration of neurofibromin-deficient Schwann cells

The neurofibromatosis type 1 tumor suppressor protein neurofibromin, is a GTPase activating protein for H-, N-, K-, R-Ras and TC21/R-Ras2 proteins. We demonstrate that Schwann cells derived from Nf1-null mice have enhanced chemokinetic and chemotactic migration in comparison to wild-type controls. Surprisingly, this migratory phenotype is not inhibited by a farnesyltransferase inhibitor or dominant-negative (dn) (N17)H-Ras (which inhibits H-, N-, and K-Ras activation). We postulated that increased activity of R-Ras and/or TC21/R-Ras2, due to loss of Nf1, contributes to increased migration. Mouse Schwann cells (MSCs) express R-Ras and TC21/R-Ras2 and their specific guanine exchange factors, C3G and AND-34. Infection of Nf1-null MSCs with a dn(43N)R-Ras adenovirus (to inhibit both R-Ras and TC21/R-Ras2 activation) decreases migration by approximately 50%. Conversely, expression of activated (72L)TC21/R-Ras2, but not activated (38V)R-Ras, increases migration, suggesting a role of TC21/R-Ras2 activation in the migration of neurofibromin-deficient Schwann cells. TC21/R-Ras2 preferentially couples to the phosphatidylinositol 3-kinase (PI3-kinase) and MAP kinase pathways. Treatment with a PI3-kinase or MAP kinase inhibitor reduces Nf1-null Schwann cell migration, implicating these TC21 effectors in Schwann cell migration. These data reveal a key role for neurofibromin regulation of TC21/R-Ras2 in Schwann cells, a cell type critical to NF1 tumor pathogenesis.

Leukemic potential of doubly mutant Nf1 and Wv hematopoietic cells

The development of molecularly targeted treatments of adult leukemias warrants investigation of these targets in similar pediatric leukemias. The NF1 tumor suppressor gene, which encodes a GTPase activating protein for p21(ras), is frequently inactivated in juvenile myelomonocytic leukemia (JMML). Other patients with JMML acquire activating RAS gene mutations. Recipient mice reconstituted with Nf1-/- fetal hematopoietic cells develop a myeloproliferative disease (MPD) that models the human disease. JMML arises from clonal expansion of a hematopoietic stem cell, and JMML cells and murine Nf1-/- hematopoietic cells are hypersensitive to granulocyte macrophage-colony stimulating factor and KitL, the ligand for c-kit. We generated embryos doubly mutant for the Wv allele of c-kit and Nf1 to ask if reduction of c-kit activity would delay or prevent the development of MPD. Despite a reduction in c-kit activity to approximately 10% of wild-type levels, Nf1-/-;Wv/Wv cells induced MPD in recipient mice.

Selective changes in the neurofilament and microtubule cytoskeleton of NF-H/LacZ mice

This study focused mainly on changes in the microtubule cytoskeleton in a transgenic mouse where beta-galactosidase fused to a truncated neurofilament subunit led to a decrease in neurofilament triplet protein expression and a loss in neurofilament assembly and abolished transport into neuronal processes in spinal cord and brain. Although all neurofilament subunits accumulated in neuronal cell bodies, our data suggest an increased solubility of all three subunits, rather than increased precipitation, and point to a perturbed filament assembly. In addition, reduced neurofilament phosphorylation may favor an increased filament degradation. The function of microtubules seemed largely unaffected, in that tubulin and microtubule-associated proteins (MAP) expression and their distribution were largely unchanged in transgenic animals. MAP1A was the only MAP with a reduced signal in spinal cord tissue, and differences in immunostaining in various brain regions corroborate a relationship between MAP1A and neurofilaments.

Nf1 has an essential role in endothelial cells

Neurofibromatosis type 1 (NF1) or von Recklinghausen neurofibromatosis is a genetic disorder that occurs in 1 of 4000 births and is characterized by benign and malignant tumors. Cardiovascular defects also contribute to NF1, though the pathogenesis is still unclear. Deficiency in neurofibromin (encoded by Nf1) in mice results in mid-embryonic lethality owing to cardiac abnormalities previously thought to be secondary to cardiac neural-crest defects. Using tissue-specific gene inactivation, we show that endothelial-specific inactivation of Nf1 recapitulates key aspects of the complete null phenotype, including multiple cardiovascular abnormalities involving the endocardial cushions and myocardium. This phenotype is associated with an elevated level of ras signaling in Nf1(-/-) endothelial cells and greater nuclear localization of the transcription factor Nfatc1. Inactivation of Nf1 in the neural crest does not cause cardiac defects but results in tumors of neural-crest origin resembling those seen in humans with NF1. These results establish a new and essential role for Nf1 in endothelial cells and confirm the requirement for neurofibromin in the neural crest.

Hyperactivation of protein kinase B and ERK have discrete effects on survival, proliferation, and cytokine expression in Nf1-deficient myeloid cells

The Nf1 tumor suppressor encodes a GTPase-activating protein for Ras. Previous work has implicated hyperactive Ras in the aberrant growth of Nf1-deficient cells; however, there are limited data on which effectors modulate specific phenotypes. To address this, we generated myeloid cell lines by infecting fetal liver cells with a retrovirus encoding a truncated allele of c-Myb. Granulocyte-macrophage colony stimulating factor (GM-CSF) promoted the survival of wild-type Myb cells in a dose-dependent manner. By contrast, Nf1-deficient myeloid cells deprived of growth factors, were resistant to apoptosis due to hyperactivation of the phosphoinositide-3-OH kinase/protein kinase B cascade. Nf1(-/-) cells also demonstrated growth factor-independent proliferation and upregulation of GM-CSF mRNA production that were dependent upon Raf/MEK/ERK signaling. These data link specific Ras effectors with discrete cellular phenotypes in Nf1-deficient cells.

Lymphoproliferative defects in mice lacking the expression of neurofibromin: functional and biochemical consequences of Nf1 deficiency in T-cell development and function

Ras plays an essential role in lymphocyte development and function. However, in vivo consequence(s) of regulation of Ras activity by guanosine triphosphatase (GTPase)-activating proteins (GAPs) on lymphocyte development and function are not known. In this study we demonstrate that neurofibromin, the protein encoded by the NF1 tumor suppressor gene functions as a GAP for Ras in T cells. Loss of Nf1 in T cells results in enhanced Ras activation, which is associated with thymic and splenic hyperplasia, and an increase in the absolute number of immature and mature T-cell subsets compared with control mice. Interestingly, in spite of a profound T-cell expansion and higher thymidine incorporation in unstimulated Nf1-deficient T cells, T-cell receptor and interleukin-2 receptor-mediated proliferation of thymocytes and mature T cells was substantially reduced compared with control mice. Collectively, these results identify neurofibromin as a GAP for Ras in T cells for maintaining immune homeostasis in vivo.

A novel cytokine pathway suppresses glial cell melanogenesis after injury to adult nerve

The neural crest gives rise to numerous cell types, including Schwann cells, neurons, and melanocytes. The extent to which adult neural crest-derived cells retain plasticity has not been tested previously. We report that cutting adult mouse sciatic nerve induces pigmentation around nerve fascicles, among muscle bundles, and in the hypodermis. Pigmented cells are derived from adult nerve, because pigmentation occurs even when nerve fragments are grafted into tyrosinase null albino mice. Pigmentation defects are pervasive in patients with neurofibromatosis type 1 (NF1). Mice hemizygous for Nf1 mutations show enhanced pigmentation after nerve lesion and occasionally form pigmented and unpigmented tumors. The Nf1 nerve and the Nf1 host environment both contribute to enhanced pigmentation. Grafted purified Nf1 mutant glial cells [S100(+)-p75NGFR(+)-GFAP(+)-EGFR(+) or S100(+)-p75NGFR(+)-GFAP(+)-EGFR(-)] mimic nerve-derived pigmentation. The NF1 protein, neurofibromin, is a Ras-GAP that acts downstream of a few defined receptor tyrosine kinases, including [beta-common (beta(c))] the shared common receptor for granulocyte and monocyte colony-stimulating factor, interleukin-3 (IL3), and IL5. Cytokines in the environment have the potential to suppress pigmentation as shown by nerve injury experiments in null mice; when is beta(c) absent or Nf1 is mutant, melanogenesis is increased. Thus, the adult nerve glial cell phenotype is maintained after nerve injury by response to cytokines, through neurofibromin.

Gene-targeted deletion of neurofibromin enhances the expression of a transient outward K+ current in Schwann cells: a protein kinase A-mediated mechanism

Mutations in the neurofibromatosis type 1 gene predispose patients to develop benign peripheral nerve tumors (neurofibromas) containing Schwann cells (SCs). SCs from neurofibromatosis type-1 gene (Nf1) null mutant mice showed increased levels of Ras-GTP and cAMP. The proliferation and differentiation of SCs are regulated by Ras-GTP and cAMP-mediated signaling, which have been linked to expression of K+ channels. We investigated the differential expression of K+ currents in Nf1 null mutant SCs (Nf1-/-) and their wild-type (Nf1+/+) counterparts and determined the mechanisms underlying the differences. The current densities of the sustained component of K+ currents were similar in the two genotypes. However, Nf1-/- SCs showed a significant increase (approximately 1.5-fold) in a 4-aminopyridine-sensitive transient outward K+ current (I(A)). Nonstationary fluctuation analysis revealed a significant increase in the number of functional channels in the null mutant cells. When the involvement of the Ras pathway in the modulation of the K+ current was examined using adenoviral-mediated gene transfer of a dominant-negative H-Ras N17 or the known H-Ras inhibitor (L-739,749), an additional increase in I(A) was observed. In contrast, protein kinase A (PKA) inhibitors, H89 and [PKI(2-22)amide] attenuated the enhancement of the current in the Nf1-/- cells, suggesting that the increase in I(A) was mediated via activation of protein kinase A. The unitary conductance of the channel underlying I(A) was unaltered by inhibitors of PKA. Activation of I(A) is thus negatively regulated by Ras-GTP and positively regulated by PKA.

The role of neurofibromin and melatonin in pathogenesis of pseudarthrosis after spinal fusion for neurofibromatous scoliosis

We might hypothesize that the high rate of pseudarthrosis after spinal fusion for neurofibromatous scoliosis is related to two factors: the absence of neurofibromin and melatonin deficiency. Loss of the up-regulation of neurofibromin during the healing process might abolish the bone-forming effects mediated through platelet-derived growth factor (PDGF) and transforming growth factor (TGF) beta1. The absence of neurofibromin might cause an increase in the Ras activity that increases the mitogen-activated protein kinase (MAPK) with resultant disturbance of the regulatory mechanism of core binding transcription factor (Cbfa 1) and increase of osteocalcin. These effects might inhibit bone formation. Melatonin deficiency might cause defective bone formation and favour excess fibrous tissue formation.

Mechanism for the learning deficits in a mouse model of neurofibromatosis type 1

Neurofibromatosis type I (NF1) is one of the most common single-gene disorders that causes learning deficits in humans. Mice carrying a heterozygous null mutation of the Nfl gene (Nfl(+/-) show important features of the learning deficits associated with NF1 (ref. 2). Although neurofibromin has several known properties and functions, including Ras GTPase-activating protein activity, adenylyl cyclase modulation and microtubule binding, it is unclear which of these are essential for learning in mice and humans. Here we show that the learning deficits of Nf1(+/-) mice can be rescued by genetic and pharmacological manipulations that decrease Ras function. We also show that the Nf1(+/-) mice have increased GABA (gamma-amino butyric acid)-mediated inhibition and specific deficits in long-term potentiation, both of which can be reversed by decreasing Ras function. Our results indicate that the learning deficits associated with NF1 may be caused by excessive Ras activity, which leads to impairments in long-term potentiation caused by increased GABA-mediated inhibition. Our findings have implications for the development of treatments for learning deficits associated with NF1.