Tumor suppressor mutations and growth factor signaling in the pathogenesis of NF1-associated peripheral nerve sheath tumors. I. The role of tumor suppressor mutations

Role of nerves in neurofibromatosis type 1-related nervous system tumors

BACKGROUND

Neurofibromatosis type 1 (NF1) is an autosomal dominant genetic disorder that affects nearly 1 in 3000 infants. Neurofibromin inactivation and NF1 gene mutations are involved in various aspects of neuronal function regulation, including neuronal development induction, electrophysiological activity elevation, growth factor expression, and neurotransmitter release. NF1 patients often exhibit a predisposition to tumor development, especially in the nervous system, resulting in the frequent occurrence of peripheral nerve sheath tumors and gliomas. Recent evidence suggests that nerves play a role in the development of multiple tumor types, prompting researchers to investigate the nerve as a vital component in and regulator of the initiation and progression of NF1-related nervous system tumors.

CONCLUSION

In this review, we summarize existing evidence about the specific effects of NF1 mutation on neurons and emerging research on the role of nerves in neurological tumor development, promising a new set of selective and targeted therapies for NF1-related tumors.

Co-culture of Schwann cells and endothelial cells for synergistically regulating dorsal root ganglion behavior on chitosan-based anisotropic topology for peripheral nerve regeneration

Background

Anisotropic topologies are known to regulate cell-oriented growth and induce cell differentiation, which is conducive to accelerating nerve regeneration, while co-culture of endothelial cells (ECs) and Schwann cells (SCs) can significantly promote the axon growth of dorsal root ganglion (DRG). However, the synergistic regulation of EC and SC co-culture of DRG behavior on anisotropic topologies is still rarely reported. The study aims to investigate the effect of anisotropic topology co-cultured with Schwann cells and endothelial cells on dorsal root ganglion behavior for promoting peripheral nerve regeneration.

Methods

Chitosan/artemisia sphaerocephala (CS/AS) scaffolds with anisotropic topology were first prepared using micro-molding technology, and then the surface was modified with dopamine to facilitate cell adhesion and growth. The physical and chemical properties of the scaffolds were characterized through morphology, wettability, surface roughness and component variation. SCs and ECs were co-cultured with DRG cells on anisotropic topology scaffolds to evaluate the axon growth behavior.

Results

Dopamine-modified topological CS/AS scaffolds had good hydrophilicity and provided an appropriate environment for cell growth. Cellular immunofluorescence showed that in contrast to DRG growth alone, co-culture of SCs and ECs could not only promote the growth of DRG axons, but also offered a stronger guidance for orientation growth of neurons, which could effectively prevent axons from tangling and knotting, and thus may significantly inhibit neurofibroma formation. Moreover, the co-culture of SCs and ECs could promote the release of nerve growth factor and vascular endothelial growth factor, and up-regulate genes relevant to cell proliferation, myelination and skeletal development via the PI3K-Akt, MAPK and cytokine and receptor chemokine pathways.

Conclusions

The co-culture of SCs and ECs significantly improved the growth behavior of DRG on anisotropic topological scaffolds, which may provide an important basis for the development of nerve grafts in peripheral nerve regeneration.

Transgenic mice overexpressing neuregulin-1 model neurofibroma-malignant peripheral nerve sheath tumor progression and implicate specific chromosomal copy number variations in tumorigenesis

Patients with neurofibromatosis type 1 (NF1) develop benign plexiform neurofibromas that frequently progress to become malignant peripheral nerve sheath tumors (MPNSTs). A genetically engineered mouse model that accurately models plexiform neurofibroma-MPNST progression in humans would facilitate identification of somatic mutations driving this process. We previously reported that transgenic mice overexpressing the growth factor neuregulin-1 in Schwann cells (P(0)-GGFβ3 mice) develop MPNSTs. To determine whether P(0)-GGFβ3 mice accurately model human neurofibroma-MPNST progression, cohorts of these animals were monitored through death and were necropsied; 94% developed multiple neurofibromas, with 70% carrying smaller numbers of MPNSTs. Nascent MPNSTs were identified within neurofibromas, suggesting that these sarcomas arise from neurofibromas. Although neurofibromin expression was maintained, P(0)-GGFβ3 MPNSTs exhibited Ras hyperactivation, as in human NF1-associated MPNSTs. P(0)-GGFβ3 MPNSTs also exhibited abnormalities in the p16(INK4A)-cyclin D/CDK4-Rb and p19(ARF)-Mdm-p53 pathways, analogous to their human counterparts. Array comparative genomic hybridization (CGH) demonstrated reproducible chromosomal alterations in P(0)-GGFβ3 MPNST cells (including universal chromosome 11 gains) and focal gains and losses affecting 39 neoplasia-associated genes (including Pten, Tpd52, Myc, Gli1, Xiap, and Bbc3/PUMA). Array comparative genomic hybridization also identified recurrent focal copy number variations affecting genes not previously linked to neurofibroma or MPNST pathogenesis. We conclude that P(0)-GGFβ3 mice represent a robust model of neurofibroma-MPNST progression useful for identifying novel genes driving neurofibroma and MPNST pathogenesis.

Neurogenic tumors of soft tissue

Neurogenic tumors are an uncommon yet important category of soft tissue tumors in children and adolescents because of their frequent association with various genetic syndromes. The heterogeneous cellular composition of the peripheral nerve and the wide metaplastic capacity of the neural crest and its derivatives generate a variety of neoplasms with neurogenic differentiation. This article reviews the clinicopathologic features and differential diagnosis of neurogenic tumors in the first two decades of life, and highlights use of selected ancillary methods for diagnosis.

Molecular mechanisms promoting the pathogenesis of Schwann cell neoplasms

Neurofibromas, schwannomas and malignant peripheral nerve sheath tumors (MPNSTs) all arise from the Schwann cell lineage. Despite their common origin, these tumor types have distinct pathologies and clinical behaviors; a growing body of evidence indicates that they also arise via distinct pathogenic mechanisms. Identification of the genes that are mutated in genetic diseases characterized by the development of either neurofibromas and MPNSTs [neurofibromatosis type 1 (NF1)] or schwannomas [neurofibromatosis type 2 (NF2), schwannomatosis and Carney complex type 1] has greatly advanced our understanding of these mechanisms. The development of genetically engineered mice with ablation of NF1, NF2, SMARCB1/INI1 or PRKAR1A has confirmed the key role these genes play in peripheral nerve sheath tumorigenesis. Establishing the functions of the NF1, NF2, SMARCB1/INI1 and PRKAR1A gene products has led to the identification of key cytoplasmic signaling pathways promoting Schwann cell neoplasia and identified new therapeutic targets. Analyses of human neoplasms and genetically engineered mouse models have established that interactions with other tumor suppressors such as TP53 and CDKN2A promote neurofibroma-MPNST progression and indicate that intratumoral interactions between neoplastic and non-neoplastic cell types play an essential role in peripheral nerve sheath tumorigenesis. Recent advances have also provided new insights into the identity of the neural crest-derived populations that give rise to different types of peripheral nerve sheath tumors. Based on these findings, we now have an initial outline of the molecular mechanisms driving the pathogenesis of neurofibromas, MPNSTs and schwannomas. However, this improved understanding in turn raises a host of intriguing new questions.

Morphological and Immunohistochemical Characterization of Sarcomatous Tumors in Wild-Type and Genetically Engineered Mice

Malignant soft tissue tumors are commonly observed in wild-type and gene-targeted mice. These tumors have different degrees of differentiation, cellularity, cellular atypia, nuclear pleomorphism, normal and abnormal mitosis, and giant tumor cells with enlarged polylobulated nuclei. They are often diagnosed as pleomorphic sarcoma, undifferentiated sarcoma, fibrosarcoma, malignant fibrous histiocytoma, sarcoma, or sarcoma, not otherwise specified. Pleomorphic sarcomas have no morphological differentiation toward a differentiated mesenchymal or other tumor type in hematoxylin and eosin–stained sections. With the use of immunohistochemistry, human and mouse, tumors associated with these broad nonspecific diagnoses can often be demonstrated to be of a specific cellular lineage. With mouse models being used to delineate the molecular mechanisms, pathogenesis, and cellular origin of human sarcomas, it will be necessary to correlate the morphological and cellular lineage and the molecular profiles of the pleomorphic tumors associated with these mouse models. The results presented here show that with the use of immunohistochemistry, the cellular lineage of many mouse tumors with pleomorphic features can be determined.

Succinate Dehydrogenase Subunit B (SDHB) Is Expressed in Neurofibromatosis 1-Associated Gastrointestinal Stromal Tumors (Gists): Implications for the SDHB Expression Based Classification of Gists

Gastrointestinal Stromal Tumor (GIST) is the most common mesenchymal tumor of the digestive tract. GISTs develop with relatively high incidence in patients with Neurofibromatosis-1 syndrome (NF1). Mutational activation of KIT or PDGFRA is believed to be a driving force in the pathogenesis of familial and sporadic GISTs. Unlike those tumors, NF1-associated GISTs do not have KIT or PGDFRA mutations. Similarly, no mutational activation of KIT or PDGFRA has been identified in pediatric GISTs and in GISTs associated with Carney Triad and Carney-Stratakis Syndrome. KIT and PDGFRA-wild type tumors are expected to have lesser response to imatinib treatment. Recently, Carney Triad and Carney-Stratakis Syndrome -associated GISTs and pediatric GISTs have been shown to have a loss of expression of succinate dehydrogenase subunit B (SDHB), a Krebs cycle/electron transport chain interface protein. It was proposed that GISTs can be divided into SDHB- positive (type 1), and SDHB-negative (type 2) tumors because of similarities in clinical features and response to imatinib treatment. In this study, SDHB expression was examined immunohistochemically in 22 well-characterized NF1-associated GISTs. All analyzed tumors expressed SDHB. Based on SDHB-expression status, NF1-associated GISTs belong to type 1 category; however, similarly to SDHB type 2 tumors, they do not respond well to imatinib treatment. Therefore, a simple categorization of GISTs into SDHB-positive and-negative seems to be incomplete. A classification based on both SDHB expression status and KIT and PDGFRA mutation status characterize GISTs more accurately and allow subdivision of SDHB-positive tumors into different clinico-genetic categories.

Tamoxifen inhibits malignant peripheral nerve sheath tumor growth in an estrogen receptor-independent manner

Few therapeutic options are available for malignant peripheral nerve sheath tumors (MPNSTs), the most common malignancy associated with neurofibromatosis type 1 (NF1). Guided by clinical observations suggesting that some NF1-associated nerve sheath tumors are hormonally responsive, we hypothesized that the selective estrogen receptor (ER) modulator tamoxifen would inhibit MPNST tumorigenesis in vitro and in vivo. To test this hypothesis, we examined tamoxifen effects on MPNST cell proliferation and survival, MPNST xenograft growth, and the mechanism by which tamoxifen impeded these processes. We found that 1-5 μM 4-hydroxy-tamoxifen induced MPNST cell death, whereas 0.01-0.1 μM 4-hydroxy-tamoxifen inhibited mitogenesis. Dermal and plexiform neurofibromas, MPNSTs, and MPNST cell lines expressed ERβ and G-protein-coupled ER-1 (GPER); MPNSTs also expressed estrogen biosynthetic enzymes. However, MPNST cells did not secrete 17β-estradiol, exogenous 17β-estradiol did not stimulate mitogenesis or rescue 4-hydroxy-tamoxifen effects on MPNST cells, and the steroidal antiestrogen ICI-182,780 did not mimic tamoxifen effects on MPNST cells. Further, ablation of ERβ and GPER had no effect on MPNST proliferation, survival, or tamoxifen sensitivity, indicating that tamoxifen acts via an ER-independent mechanism. Consistent with this hypothesis, inhibitors of calmodulin (trifluoperazine, W-7), another known tamoxifen target, recapitulated 4-hydroxy-tamoxifen effects on MPNST cells. Tamoxifen was also effective in vivo, demonstrating potent antitumor activity in mice orthotopically xenografted with human MPNST cells. We conclude that 4-hydroxy-tamoxifen inhibits MPNST cell proliferation and survival via an ER-independent mechanism. The in vivo effectiveness of tamoxifen provides a rationale for clinical trials in cases of MPNSTs.

A genetic screen for anchorage-independent proliferation in mammalian cells identifies a membrane-bound neuregulin

Anchorage-independent proliferation is a hallmark of oncogenic transformation and is thought to be conducive to proliferation of cancer cells away from their site of origin. We have previously reported that primary Schwann cells expressing the SV40 Large T antigen (LT) are not fully transformed in that they maintain a strict requirement for attachment, requiring a further genetic change, such as oncogenic Ras, to gain anchorage-independence. Using the LT-expressing cells, we performed a genetic screen for anchorage-independent proliferation and identified Sensory and Motor Neuron Derived Factor (SMDF), a transmembrane class III isoform of Neuregulin 1. In contrast to oncogenic Ras, SMDF induced enhanced proliferation in normal primary Schwann cells but did not trigger cellular senescence. In cooperation with LT, SMDF drove anchorage-independent proliferation, loss of contact inhibition and tumourigenicity. This transforming ability was shared with membrane-bound class III but not secreted class I isoforms of Neuregulin, indicating a distinct mechanism of action. Importantly, we show that despite being membrane-bound signalling molecules, class III neuregulins transform via a cell intrinsic mechanism, as a result of constitutive, elevated levels of ErbB signalling at high cell density and in anchorage-free conditions. This novel transforming mechanism may provide new targets for cancer therapy.

Soft tissue sarcomas with complex genomic profiles

Soft tissue sarcomas (STS) with complex genomic profiles (50% of all STS) are predominantly composed of spindle cell/pleomorphic sarcomas, including leiomyosarcoma, myxofibrosarcoma, pleomorphic liposarcoma, pleomorphic rhabdomyosarcoma, malignant peripheral nerve sheath tumor, angiosarcoma, extraskeletal osteosarcoma, and spindle cell/pleomorphic unclassified sarcoma (previously called spindle cell/pleomorphic malignant fibrous histiocytoma). These neoplasms show, characteristically, gains and losses of numerous chromosomes or chromosome regions, as well as amplifications. Many of them share recurrent aberrations (e.g., gain of 5p13-p15) that seem to play a significant role in tumor progression and/or metastatic dissemination. In this paper, we review the cytogenetic, molecular genetic, and clinicopathologic characteristics of the most common STS displaying complex genomic profiles. Features of diagnostic or prognostic relevance will be discussed when needed.

Establishment and characterization of a novel human malignant peripheral nerve sheath tumor cell line, FMS-1, that overexpresses epidermal growth factor receptor and cyclooxygenase-2

Malignant peripheral nerve sheath tumor (MPNST) is a rare soft tissue sarcoma. We established a new human MPNST cell line (designated FMS-1) from MPNST of the right brachial plexus of a 69-year-old woman with NF1. The cell line has been maintained for >24 months with >100 passages. FMS-1 cells showed a fibrosarcoma-like or epithelioid pattern in the heterotransplanted tumor, compared with a fascicular growth pattern of short-spindle tumor cells in the primary tumor. Immunophenotypically, FMS-1 cells showed almost the same characteristics as the primary tumor. Cytogenetic and molecular analyses revealed a deletion in exons 5-8 of the p53 gene. Epidermal growth factor receptor (EGFR) and cyclooxygenase (COX)-2 were expressed in FMS-1 cells. To improve the highly aggressive course and poor prognosis and establish new therapeutic methods, molecular genetic and biological characterizations of MPNST are required. Thus, FMS-1 cells might be useful for investigating biological behaviors and developing new molecular-targeting antitumor drugs for MPNST expressing EGFR or COX-2.

Neuregulin-1 beta and neuregulin-1 alpha differentially affect the migration and invasion of malignant peripheral nerve sheath tumor cells

Malignant peripheral nerve sheath tumors (MPNSTs) are the most common malignancy associated with neurofibromatosis Type 1 (NF1). These Schwann cell lineage-derived sarcomas aggressively invade adjacent nerve and soft tissue, frequently precluding surgical resection. Little is known regarding the mechanisms underlying this invasive behavior. We have shown that MPNSTs express neuregulin-1 (NRG-1) beta isoforms, which promote Schwann cell migration during development, and NRG-1 alpha isoforms, whose effects on Schwann cells are poorly understood. Hypothesizing that NRG-1 beta and/or NRG-1 alpha promote MPNST invasion, we found that NRG-1 beta promoted MPNST migration in a substrate-specific manner, markedly enhancing migration on laminin but not on collagen type I or fibronectin. The NRG-1 receptors erbB3 and erbB4 were present in MPNST invadopodia (processes mediating invasion), partially colocalized with focal adhesion kinase and the laminin receptor beta(1)-integrin and coimmunoprecipitated with beta(1)-integrin. NRG-1 beta stimulated human and murine MPNST cell migration and invasion in a concentration-dependent manner in three-dimensional migration assays, acting as a chemotactic factor. Both baseline and NRG-1 beta-induced migration were erbB-dependent and required the action of MEK 1/2, SAPK/JNK, PI-3 kinase, Src family kinases and ROCK-I/II. In contrast, NRG-1 alpha had no effect on the migration and invasion of some MPNST lines and inhibited the migration of others. While NRG-1 beta potently and persistently activated Erk 1/2, SAPK/JNK, Akt and Src family kinases, NRG-1 alpha did not activate Akt and activated these other kinases with kinetics distinct from those evident in NRG-1 beta-stimulated cells. These findings suggest that NRG-1 beta enhances MPNST migration and that NRG-1 beta and NRG-1 alpha differentially modulate this process.

Clinically aggressive central giant cell granulomas in two patients with neurofibromatosis 1

BACKGROUND

Neurofibromatosis 1 (NF1) is an autosomal dominantly inherited disorder caused by a spectrum of mutations affecting the Nf1 gene. Affected patients develop benign and malignant tumors at an increased frequency. Clinical findings include multiple cutaneous café-au-lait pigmentations, neurofibromas, axillary freckling, optic gliomas, benign iris hamartomas (Lisch nodules), scoliosis, and poorly defined soft tissue lesions of the skeleton. Kerl first reported an association of NF1 with multiple central giant cell granulomas (CGCGs) of the jaws. There have since been 4 additional published cases of NF1 patients with CGCGs of the jaws.

CLINICAL CASES

We report on 2 patients who presented with NF1 and aggressive CGCGs of the jaws. In both cases, the clinical course was characterized by numerous recurrences despite mechanical curettage and surgical resection.

CONCLUSIONS

We review proposed mechanisms to explain the apparent association between NF1 and an increased incidence of CGCGs of the jaws. While the presence of CGCGs of the jaws in patients with NF1 could represent either a coincidental association or a true genetic linkage, we propose that this phenomenon is most likely related to NF1-mediated osseous dysplasia. Compared to normal bone, the Nf1-haploinsufficient bone in a patient with NF1 may be less able to remodel in response to as of yet unidentified stimuli (e.g. excessive mechanical stress and/or vascular fragility), and consequently may be more susceptible to developing CGCG-like lesions. Alternatively, the CGCG in NF1 patients could represent a true neoplasm, resulting from additional, as of yet unidentified, genetic alterations to Nf1-haploinsufficient bone.

Single-sperm analysis for haplotype construction of de-novo paternal mutations: application to PGD for neurofibromatosis type 1

BACKGROUND

Neurofibromatosis type 1 (NF1) is an autosomal dominant disorder caused by mutations in the neurofibromin gene. Approximately, 50% of cases are caused by de-novo mutations. Even when the NF1 mutation is known, accuracy of PGD is highly enhanced by simultaneous analysis of linked markers. In a childless couple referred to PGD, the male carried a de-novo mutation, precluding the possibility of typing relatives to establish the mutation-associated haplotype. We developed a single-sperm haplotype analysis strategy to establish the haplotype linked to the NF1 mutation.

METHODS

Spermatozoa from freshly ejaculated semen were used as a substrate for multiplex PCR on single sperm.

RESULTS

In addition to the NF1 mutation, six informative polymorphic markers flanking the NF1 gene (D17S1294, D17S1849, D17S841, D17S975, NF1TG2 and NF1AC5) were linked to individual alleles in single sperm from the affected male.

CONCLUSIONS

Single-sperm analysis established the haplotypes of both mutant and wild-type NF1 alleles and enabled the implementation of a PGD protocol using polymorphic marker analysis. This method is generally applicable to PGD for any disease in which the haplotype of paternal mutations cannot be determined by typing relatives.

Neuregulin growth factors and their ErbB receptors form a potential signaling network for schwannoma tumorigenesis

Sporadic and neurofibromatosis type 2-associated schwannomas contain a glial growth factor (GGF)-like activity that has been hypothesized to promote neoplastic Schwann cell mitogenesis. It is not known whether this GGF-like activity is neuregulin-1 (NRG-1), an epidermal growth factor (EGF)-related molecule that regulates the proliferation, survival, and differentiation of developing Schwann cells, the related factor NRG-2, or another NRG/EGF ligand. We report that neoplastic Schwann cells within schwannomas overexpress multiple alpha and beta transmembrane precursors from the class II and class III NRG-1 subfamilies. NRG-2 alpha and beta transcripts are similarly overexpressed in some tumors. Of the other 8 known NRG/EGF ligands, only heparin-binding EGF, epiregulin, and TGFalpha are detectable in schwannomas. Neoplastic Schwann cells almost uniformly express erbB2 and erbB3, 2 membrane receptor tyrosine kinases mediating NRG-1 and NRG-2 action. Expression of the NRG receptor erbB4 and EGF receptor is also evident in schwannomas, but is more limited, occurring in only a subset of these tumors. ErbB2, the preferred dimerization partner for all erbB kinases, is constitutively phosphorylated in schwannomas. These observations suggest that autocrine, paracrine, and/or juxtacrine NRG-1/NRG-2 signaling promotes schwannoma pathogenesis and that this signaling pathway may be an important therapeutic target in schwannomas.

Activation of the neuregulin-1/ErbB signaling pathway promotes the proliferation of neoplastic Schwann cells in human malignant peripheral nerve sheath tumors

Patients with neurofibromatosis type 1 develop aggressive Schwann cell neoplasms known as malignant peripheral nerve sheath tumors (MPNSTs). Although tumor suppressor gene mutations play an important role in MPNST pathogenesis, it is likely that dysregulated signaling by as yet unidentified growth factors also contributes to the formation of these sarcomas. To test the hypothesis that neuregulin-1 (NRG-1) growth factors promote mitogenesis in MPNSTs, we examined the expression and action of NRG-1 in human MPNSTs and neurofibromas, the benign precursor lesions from which MPNSTs arise. Multiple alpha and beta transmembrane precursors from the class II and III NRG-1 subfamilies are present in both tumor types. Neoplastic Schwann cells within these neoplasms variably express the erbB kinases mediating NRG-1 responses (erbB2, erbB3 and/or erbB4). Human MPNST cell lines (Mash-1, YST-1, NMS-2 and NMS-2PC cells) similarly coexpress multiple NRG-1 isoforms and erbB receptors. These MPNST lines are NRG-1 responsive and demonstrate constitutive erbB phosphorylation. Treatment with PD168393 and PD158780, two structurally and mechanistically distinct erbB inhibitors, abolishes erbB phosphorylation and reduces DNA synthesis in these lines. These findings suggest that autocrine and/or paracrine NRG-1/erbB signaling promotes neoplastic Schwann cell proliferation and may be an important therapeutic target in neurofibromas and MPNSTs.

A molecular map of mesenchymal tumors

A comprehensive study of the gene expression profile of 96 mesenchymal tumors identifies molecular fingerprints for most tumors in this group.

Background

Bone and soft tissue tumors represent a diverse group of neoplasms thought to derive from cells of the mesenchyme or neural crest. Histological diagnosis is challenging due to the poor or heterogenous differentiation of many tumors, resulting in uncertainty over prognosis and appropriate therapy.

Results

We have undertaken a broad and comprehensive study of the gene expression profile of 96 tumors with representatives of all mesenchymal tissues, including several problem diagnostic groups. Using machine learning methods adapted to this problem we identify molecular fingerprints for most tumors, which are pathognomonic (decisive) and biologically revealing.

Conclusion

We demonstrate the utility of gene expression profiles and machine learning for a complex clinical problem, and identify putative origins for certain mesenchymal tumors.

Tumor suppressor mutations and growth factor signaling in the pathogenesis of NF1-associated peripheral nerve sheath tumors: II. The role of dysregulated growth factor signaling

Patients with neurofibromatosis type 1 (NF1), one of the most common genetic disease affecting the nervous system, develop multiple neurofibromas that can transform into aggressive sarcomas known as malignant peripheral nerve sheath tumors (MPNSTs). Studies of human tumors and newly developed transgenic mouse models indicate that Schwann cells are the primary neoplastic cell type in neurofibromas and MPNSTs and that development of these peripheral nerve sheath tumors involves mutations of multiple tumor suppressor genes. However, it is widely held that tumor suppressor mutations alone are not sufficient to induce peripheral nerve sheath tumor formation and that dysregulated growth factor signaling cooperates with these mutations to promote neurofibroma and MPNST tumorigenesis. In Part I of this review, we discussed findings demonstrating that a loss of NF1 tumor suppressor gene function in neoplastic Schwann cells is a key early step in neurofibroma formation and that progression from neurofibroma to MPNST is associated with abnormalities of additional tumor suppressor genes, including p53, INK4A, andp27(kip1). In Part II of this review, we consider evidence that dysregulated signaling by specific growth factors and growth factor receptors promotes the proliferation, migration, and survival of neoplastic Schwann cells in neurofibromas and MPNSTs.