Thinking beyond the tumor cell: Nf1 haploinsufficiency in the tumor environment

A Clinicopathologic Study on the Role of Estrogen, Progesterone, and Their Classical and Nonclassical Receptors in Cutaneous Neurofibromas of Individuals With Neurofibromatosis 1

OBJECTIVES

To evaluate the expression of progesterone receptor (PR), estrogen receptor (ER), and G protein-coupled estrogen receptor 1 (GPER-1) in cutaneous neurofibromas (cNFs) and their correlation with demographic, clinical, and laboratory data of individuals with neurofibromatosis 1 (NF1). The association of PROGINS polymorphism and PR expression in cNFs, as well as the serum steroidal hormones and the number of cNFs, was investigated.

METHODS

The sample comprised 80 large and 80 small cNFs from 80 individuals with NF1. PR, ER, GPER-1, and Ki-67 expression were investigated by immunohistochemistry in tissue micro- and macroarrays and quantified using a digital computer-assisted method. The number of cNFs, the levels of serum 17β estradiol and progesterone, and the PROGINS polymorphism were identified.

RESULTS

Twelve (8.5%) small cNFs were weakly positive for ER, 131 (92.3%) cNFs expressed PR, and all (100%) cNFs expressed GPER-1. Large cNFs showed a higher expression of PR (P < .0001) and GPER-1 (P = .019) and had a higher intensity of staining for these receptors (P < .0001). The cell proliferation index was positively correlated with PR (P = .001). Persons with more cNFs had higher serum levels of progesterone (P = .001).

CONCLUSIONS

These findings emphasize the role of estrogen and progesterone in cNF development and suggest that these hormones may act on cNF cells via a noncanonical pathway through GPER-1.

NF1 gene silencing induces upregulation of vascular endothelial growth factor expression in both Schwann and non-Schwann cells

Neurofibromatosis type I (NF1) is associated with typical hypervascular tumors, including neurofibroma, glioma, malignant peripheral nerve sheath tumors (MPNST) and glomus tumors. Previously, we and other groups reported that neurofibromas showed high-level expression of vascular endothelial growth factor (VEGF), a potent angiogenic factor involved in neovascularization. However, the molecular mechanism underlying the upregulation of VEGF in neurofibromas remains unclear. In this study, we examined the effects of Nf1 gene silencing on VEGF expression in Schwann cell and non-Schwann cell line and the upstream mTOR-HIF-1α - VEGF pathway in Schwann cell line. The results indicated that Nf1 gene silencing by lentiviral-mediated RNA interference resulted in elevated expression of VEGF, HIF-1α and phosphorylated mTOR at the protein level. The results obtained from Nf1 gene silencing in murine Schwann cell line analogously suggest that NF1 gene haploinsufficiency in human tumor Schwann cells may directly elicit upregulation of VEGF expression without the tumor microenvironment by activation of the mTOR-HIF-1α - VEGF pathway. We also showed that interleukin-6 is upregulated in Nf1 gene knock-down Schwann cells at the protein level.

Partial Blindness to Submicron Topography in NF1 Haploinsufficient Cultured Fibroblasts Indicates a New Function of Neurofibromin in Regulation of Mechanosensoric

Cells sense physical properties of their extracellular environment and translate them into biochemical signals. In this study, cell responses to surfaces with submicron topographies were investigated in cultured human NF1 haploinsufficient fibroblasts. Age-matched fibroblasts from 8 patients with neurofibromatosis type 1 (NF1(+/-)) and 9 controls (NF1(+/+)) were cultured on surfaces with grooves of 200 nm height and lateral distance of 2 μm. As cellular response indicator, the mean cell orientation along microstructured grooves was systematically examined. The tested NF1 haploinsufficient fibroblasts were significantly less affected by the topography than those from healthy donors. Incubation of the NF1(+/-) fibroblasts with the farnesyltransferase inhibitor FTI-277 and other inhibitors of the neurofibromin pathway ameliorates significantly the cell orientation. These data indicate that NF1 haploinsufficiency results in an altered response to specific surface topography in fibroblasts. We suggest a new function of neurofibromin in the sensoric mechanism to topographies and a partial mechanosensoric blindness by NF1 haploinsufficiency.

Is double inactivation of the Nf1 gene responsible for the development of congenital pseudarthrosis of the tibia associated with NF1?

The pathogenic mechanism responsible for congenital pseudarthrosis of the tibia (CPT) is not well understood although the possibility of double inactivation of the neurofibromatosis type 1 (Nf1) gene has been suggested. In the present study, loss of heterozygosity was investigated in fibrous hamartoma tissues harvested from 16 patients with CPT associated with NF1 using four genetic markers that span the Nf1 gene. Based on the assumption that a single cell with double inactivation of Nf1 would undergo clonal growth and cause fibrous hamartoma, we investigated clonality in fibrous hamartoma tissues by analyzing X-chromosome inactivation patterns in 11 female patients. Loss of Nf1 heterozygosity in fibrous hamartoma tissues was observed at one or two genetic markers in 4 out of the 16 patients tested. In clonality assays, 3 of 11 patients showed a clonal growth pattern, 5 a non-clonal pattern, and 3 were non-informative. These findings support that double inactivation of the Nf1 gene and subsequent clonal growth could be a pathogenic feature of the fibrous hamartoma tissue at least in some of the CPT but might not be essential requirements of CPT development.

Neurofibromatosis Type 1 and tumorigenesis: molecular mechanisms and therapeutic implications

Neurofibromatosis Type 1 (NF1) is a common autosomal dominant disease characterized by complex and multicellular neurofibroma tumors, and less frequently by malignant peripheral nerve sheath tumors (MPNSTs) and optic nerve gliomas. Significant advances have been made in elucidating the cellular, genetic, and molecular biology involved in tumor formation in NF1. Neurofibromatosis Type 1 is caused by germline mutations of the NF1 tumor suppressor gene, which generally result in decreased intracellular neurofibromin protein levels, leading to increased cascade Ras signaling to its downstream effectors. Multiple key pathways are involved with the development of tumors in NF1, including Ras/mitogen-activated protein kinase (MAPK) and Akt/mammalian target of rapamycin (mTOR). Interestingly, recent studies demonstrate that multiple other developmental syndromes (in addition to NF1) share phenotypic features resulting from germline mutations in genes responsible for components of the Ras/MAPK pathway. In general, a somatic loss of the second NF1 allele, also referred to as loss of heterozygosity, in the progenitor cell, either the Schwann cell or its precursor, combined with haploinsufficiency in multiple supporting cells is required for tumor formation. Importantly, a complex series of interactions with these other cell types in neurofibroma tumorigenesis is mediated by abnormal expression of growth factors and their receptors and modification of gene expression, a key example of which is the process of recruitment and involvement of the NF1+/– heterozygous mast cell. In general, for malignant transformation to occur, there must be accumulation of additional mutations of multiple genes including INK4A/ARF and P53, with resulting abnormalities of their respective signal cascades. Further, abnormalities of the NF1 gene and molecular cascade described above have been implicated in the tumorigenesis of NF1 and some sporadically occurring gliomas, and thus, these treatment options may have wider applicability. Finally, increased knowledge of molecular and cellular mechanisms involved with NF1 tumorigenesis has led to multiple preclinical and clinical studies of targeted therapy, including the mTOR inhibitor rapamycin, which is demonstrating promising preclinical results for treatment of MPNSTs and gliomas.

PTEN dosage is essential for neurofibroma development and malignant transformation

Patients with neurofibromatosis type 1 (NF1) carry approximately a 10% lifetime risk of developing a malignant peripheral nerve sheath tumor (MPNST). Although the molecular mechanisms underlying NF1 to MPNST malignant transformation remain unclear, alterations of both the RAS/RAF/MAPK and PI3K/AKT/mTOR signaling pathways have been implicated. In a series of genetically engineered murine models, we perturbed RAS/RAF/MAPK or/and PTEN/PI3K/AKT pathway, individually or simultaneously, via conditional activation of K-ras oncogene or deletion of Nf1 or Pten tumor suppressor genes. Only K-Ras activation in combination with a single Pten allele deletion led to 100% penetrable development of NF lesions and subsequent progression to MPNST. Importantly, loss or decrease in PTEN expression was found in all murine MPNSTs and a majority of human NF1-associated MPNST lesions, suggesting that PTEN dosage and its controlled signaling pathways are critical for transformation of NFs to MPNST. Using noninvasive in vivo PET-CT imaging, we demonstrated that FDG can be used to identify the malignant transformation in both murine and human MPNSTs. Our data suggest that combined inhibition of RAS/RAF/MAPK and PTEN/PI3K/AKT pathways may be beneficial for patients with MPNST.

Cloning and characterization of a novel intracellular protein p48.2 that negatively regulates cell cycle progression

Neurofibromatosis type 1 (NF1) microdeletion is a large genomic deletion that embraces at least 11 continuous genes at human chromosome 17q11.2. To date, most of these genes' functions still remain undefined. In this study, we report an unknown cytokine receptor like molecule (p48.2) that is frequently deleted in patients with type-1 and type-2 NF1 microdeletions in the neurofibromin locus. The cloned gene has 1317 base pair long that encodes a 438aa intracellular protein. The gene was subsequently named p48.2 based on its predicted molecular weight. A typical fibronectin type III (FNIII) domain was identified in p48.2 between Arg(176) and Pro(261) in which a palindromic Arg-Gly-Asp (RGD) repeat plus a putative Trp-Ser-X-Trp-Ser (WSXWS) motif were found at the domain's C-terminus. p48.2 mRNAs were abundant in many tumor cell lines and normal human tissues and up-regulated in some freshly isolated lung cancer and leukemia cells. Interestingly, over-expression of p48.2 in human embryo kidney 293T cells could significantly cause G0/G1 arrest and prevented S phase entry. In contrast, repressing endogenous p48.2 gene expression by specific siRNA markedly reduced G0/G1 population. Importantly, over-expression of p48.2 could significantly up-regulate rather than down-regulate cyclin D1 and cyclin D3 expressions. We further showed that the induction of cyclin D1 expression was directly due to the activation of signal transducers and activators of transcription 3 (STAT3), but was independent of RAS/mitogen-activated protein kinase (RAS/MAPK) signaling pathway. Thus, p48.2 may represent a novel type of intracellular protein functioning as a negative regulator at the G0/G1 phase.

A brief history of surgery for peripheral nerve sheath tumors

✓The authors present a brief and selective history of surgery for peripheral nerve tumors to illustrate how the current understanding of the nature of disease influences the choice of surgical intervention. There was very little understanding of the anatomy and function of peripheral nerves in ancient times; consequently, surgical treatments for peripheral nerve tumors were based on the writings of authorities. The confusion between traumatic neuromas and genuine nerve sheath tumors coupled with the belief that manipulation of a peripheral nerve might be lethal to the patient stifled the development of surgical techniques for the management of nerve tumors in the 18th and 19th centuries. It was not until the 20th century, with an increased understanding of the microscopic anatomy of nerve sheath tumors, that efficacious surgical treatments for these diseases were developed. Continued advances in the understanding of the biology of these tumors will continue to impact their surgical management.

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.

Nf1 expression is dependent on strain background: implications for tumor suppressor haploinsufficiency studies

Neurofibromatosis type 1 (NF1) is the most common cancer predisposition syndrome affecting the nervous system, with elevated risk for both astrocytoma and peripheral nerve sheath tumors. NF1 is caused by a germline mutation in the NF1 gene, with tumors showing loss of the wild type copy of NF1. In addition, NF1 heterozygosity in surrounding stroma is important for tumor formation, suggesting an additional role of haploinsufficiency for NF1. Studies in mouse models and NF1 families have implicated modifier genes unlinked to NF1 in the severity of the disease and in susceptibility to astrocytoma and peripheral nerve sheath tumors. To determine if differences in Nf1 expression may contribute to the strain-specific effects on tumor predisposition, we examined the levels of Nf1 gene expression in mouse strains with differences in tumor susceptibility using quantitative polymerase chain reaction. The data presented in this paper demonstrate that strain background has as much effect on Nf1 expression levels as mutation of one Nf1 allele, indicating that studies of haploinsufficiency must be carefully interpreted with respect to strain background. Because expression levels do not correlate entirely with the susceptibility or resistance to tumors observed in the strain, these data suggest that either variation in Nf1 levels is not responsible for the differences in astrocytoma and peripheral nerve sheath tumor susceptibility in Nf1-/+;Trp53-/+cis mice, or that certain mouse strains have evolved compensatory mechanisms for differences in Nf1 expression.

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

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

Clusterin is a secreted marker for a hypoxia-inducible factor-independent function of the von Hippel-Lindau tumor suppressor protein

Germline mutations in the von Hippel-Lindau (VHL) tumor suppressor gene predispose people to renal cancer, hemangioblastomas, and pheochromocytomas in an allele-specific manner. The best documented function of the VHL gene product (pVHL) relates to its ability to polyubiquitinate, and hence target for destruction, the alpha subunits of the heterodimeric transcription factor hypoxia-inducible factor (HIF). pVHL mutants linked to familial pheochromocyctoma (type 2C VHL disease), in contrast to classical VHL disease, appear to be normal with respect to HIF regulation. Using a simple method for identifying proteins that are differentially secreted by isogenic cell line pairs, we confirmed that the HIF targets IGBP3 and PAI-1 are overproduced by pVHL-defective renal carcinoma cells. In addition, cells lacking wild-type pVHL, including cells producing type 2C pVHL mutants, were defective with respect to expression and secretion of clusterin, which does not behave like a HIF target. Decreased clusterin secretion by pVHL-defective tumors was confirmed in vivo by immunohistochemistry. Therefore, clusterin is a secreted marker for a HIF-independent pVHL function that might be especially important in pheochromocytoma development.

Analysis of NF1 transcriptional regulatory elements

Mutations of the NF1 locus cause neurofibromatosis 1 (NF1), a clinically variable autosomal dominant disease. Expression of neurofibromin, the protein product of the NF1 gene, is regulated in a tissue- and developmentally-specific fashion, but the basis for this regulation is not understood. We used phylogenetic footprinting and other bioinformatic methods to identify potential transcriptional regulatory regions in the 5' upstream region and intron 1 of the NF1 gene from human, mouse, rat, and pufferfish. Three genomic segments that have equal or higher homology than the coding region were found in the NF1 5' upstream region, and four more very highly homologous regions were found in intron 1. Five of these highly homologous regions are confidently predicted to contain transcription factor binding sites. One highly homologous segment in the 5' upstream region spans the transcription start site and contains several potential transcription factor binding sites. This segment includes a novel 24-bp sequence (acttccggtggggtgtcatggcgg) that lies 310-333 bp upstream of the translation initiation site. This sequence, which is identical in human, mouse, and rat and differs by only 1-bp in Fugu, may contain the core promoter element for NF1 transcription.

The neurofibromatosis type 1 gene product neurofibromin enhances cell motility by regulating actin filament dynamics via the Rho-ROCK-LIMK2-cofilin pathway

Neurofibromin is a neurofibromatosis type 1 (NF1) tumor suppressor gene product with a domain that acts as a GTPase-activating protein and functions, in part, as a negative regulator of Ras. Loss of neurofibromin expression in NF1 patients is associated with elevated Ras activity and increased cell proliferation, predisposing to a variety of tumors of the peripheral and central nervous systems. We show here, using the small interfering RNA (siRNA) technique, that neurofibromin dynamically regulates actin cytoskeletal reorganization, followed by enhanced cell motility and gross cell aggregation in Matrigel matrix. NF1 siRNA induces characteristic morphological changes, such as excessive actin stress fiber formation, with elevated negative phosphorylation levels of cofilin, which regulates actin cytoskeletal reorganization by depolymerizing and severing actin filaments. We found that the elevated phosphorylation of cofilin in neurofibromin-depleted cells is promoted by activation of a Rho-ROCK-LIMK2 pathway, which requires Ras activation but is not transduced through three major Ras-mediated downstream pathways via Raf, phosphatidylinositol 3-kinase, and RalGEF. In addition, the exogenous expression of the NF1-GTPase-activating protein-related domain suppressed the NF1 siRNA-induced phenotypes. Neurofibromin was demonstrated to play a significant role in the machinery regulating cell proliferation and in actin cytoskeletal reorganization, which affects cell motility and adhesion. These findings may explain, in part, the mechanism of multiple neurofibroma formation in NF1 patients.

Hereditary hormone excess: genes, molecular pathways, and syndromes

Hereditary origin of a tumor helps toward early discovery of its mutated gene; for example, it supports the compilation of a DNA panel from index cases to identify that gene by finding mutations in it. The gene for a hereditary tumor may contribute also to common tumors. For some syndromes, such as hereditary paraganglioma, several genes can cause a similar syndrome. For other syndromes, such as multiple endocrine neoplasia 2, one gene supports variants of a syndrome. Onset usually begins earlier and in more locations with hereditary than sporadic tumors. Mono- or oligoclonal ("clonal") tumor usually implies a postnatal delay, albeit less delay than for sporadic tumor, to onset and potential for cancer. Hormone excess from a polyclonal tissue shows onset at birth and no benefit from subtotal ablation of the secreting organ. Genes can cause neoplasms through stepwise loss of function, gain of function, or combinations of these. Polyclonal hormonal excess reflects abnormal gene dosage or effect, such as activation or haploinsufficiency. Polyclonal hyperplasia can cause the main endpoint of clinical expression in some syndromes or can be a precursor to clonal progression in others. Gene discovery is usually the first step toward clarifying the molecule and pathway mutated in a syndrome. Most mutated pathways in hormone excess states are only partly understood. The bases for tissue specificity of hormone excess syndromes are usually uncertain. In a few syndromes, tissue selectivity arises from mutation in the open reading frame of a regulatory gene (CASR, TSHR) with selective expression driven by its promoter. Polyclonal excess of a hormone is usually from a defect in the sensor system for an extracellular ligand (e.g., calcium, glucose, TSH). The final connections of any of these polyclonal or clonal pathways to hormone secretion have not been identified. In many cases, monoclonal proliferation causes hormone excess, probably as a secondary consequence of accumulation of cells with coincidental hormone-secretory ability.

Tumour-stroma interactions in carcinogenesis: basic aspects and perspectives

In contrast to the conventional notion regarding tumour development as a cell autonomous process in which the major participants were the cancer cells, increasing evidence attributes important role in the stromal components, namely fibroblasts, and view the tumour as a heterogenous mixture of different cell types. These different types of cells, being cancer cells, fibroblasts, endothelial cells, and others, interact reciprocally and play an almost equally important role in the manifestation of certain aspects of the malignant phenotype. The elucidation of the mechanistic base of such interactions, besides the contribution to understand fundamental aspects of tumour cell biology, promises important applications in diagnosis, prognosis and therapy of the disease.

Epidemiology and molecular pathology at crossroads to establish causation: molecular mechanisms of malignant transformation

Epidemiology is a very reliable science for the identification of carcinogens. Epidemiological studies require that the effect, cancer in this case, has already occurred, when of course it would be more desirable to identify potential carcinogenic substances at an earlier stage before they have caused a large number of malignancies and thus become identifiable by epidemiological studies. In the past 30 years, molecular pathology (which includes chemistry, biochemistry, molecular biology, molecular virology, molecular genetics, epigenetics, genomics, proteomics, and other molecular-based approaches) has identified some key alterations that are required for cellular transformation and malignancy. Agents that specifically interfere with some of these mechanisms are suspected human carcinogens. It can be stated that tumor formation requires the following steps: (1) inactivation of Rb and p53 cellular pathways; (2) activation of Ras and/or other growth promoting pathways; (3) inactivation of phosphatase 2A that causes changes in the phosphorylation and activity of several cellular proteins; (4) evasion of apoptosis; (5) telomerase activation or alternative mechanisms of cellular immortalization; (6) angiogenic activity; and (7) the ability to invade surrounding tissues and to metastasize. Here, we review the molecular mechanisms of cellular transformation. The integration of this knowledge with classical epidemiology and animal studies should permit a more rapid and accurate identification of human carcinogens.

Haploinsufficiency for tumour suppressor genes: when you don't need to go all the way

Classical tumour suppressor genes are thought to require mutation or loss of both alleles to facilitate tumour progression. However, it has become clear over the last few years that for some genes, haploinsufficiency, which is loss of only one allele, may contribute to carcinogenesis. These effects can either be directly attributable to the reduction in gene dosage or may act in concert with other oncogenic or haploinsufficient events. Here we describe the genes that undergo this phenomenon and discuss possible mechanisms that allow haploinsufficiency to display a phenotype and facilitate the pathogenesis of cancer.

Myristoylation of the fus1 protein is required for tumor suppression in human lung cancer cells

FUS1 is a novel tumor suppressor gene identified in the human chromosome 3p21.3 region that is deleted in many cancers. Using surface-enhanced laser desorption/ionization mass spectrometric analysis on an anti-Fus1-antibody-capture ProteinChip array, we identified wild-type Fus1 as an N-myristoylated protein. N-myristoylation is a protein modification process in which a 14-carbon myristoyl group is cotranslationally and covalently added to the NH2-terminal glycine residue of the nascent polypeptide. Loss of expression or a defect of myristoylation of the Fus1 protein was observed in human primary lung cancer and cancer cell lines. A myristoylation-deficient mutant of the Fus1 protein abrogated its ability to inhibit tumor cell-induced clonogenicity in vitro, to induce apoptosis in lung tumor cells, and to suppress the growth of tumor xenografts and lung metastases in vivo and rendered it susceptible to rapid proteasome-dependent degradation. Our results show that myristoylation is required for Fus1-mediated tumor-suppressing activity and suggest a novel mechanism for the inactivation of tumor suppressors in lung cancer and a role for deficient posttranslational modification in tumor suppressor-gene-mediated carcinogenesis.

Loss of the nf1 tumor suppressor gene decreases fas antigen expression in myeloid cells

Genetic loss of surface Fas antigen expression leads to reduced apoptosis of myeloid and lymphoid progenitor cells, and a propensity to develop autoimmunity and myeloid leukemia in mouse models. Oncogenic p21(ras) decreases surface Fas antigen expression and renders fibroblasts resistant to Fas mediated apoptosis. Neurofibromin, which is encoded by NF1, is a GTPase activating protein that negatively regulates p21(ras) activity. NF1 loss leads to deregulation of p21(ras)-effector pathways, which control myeloid cell survival. Heterozygous inactivation of Nf1 increases mast cell numbers in Nf1 +/- mice, and enhances mast cell survival in response to c-kit ligand (kit-L). Here, we show that Nf1-deficient mast cells have reduced surface Fas antigen expression in response to kit-L and are resistant to Fas ligand-mediated apoptosis. Using genetic intercrosses between Nf1 +/- and class I (A)-PI-3K-deficient mice, we demonstrate that hyperactivation of the p21(ras)-class I(A) PI-3K pathway is the mechanism for this phenotype. Finally, we demonstrate that mast cells from both Fas antigen-deficient mice and Nf1 +/- mice are resistant to apoptosis following kit-L withdrawal in vivo. Thus, therapies designed to decrease p21(ras) activity and up-regulate Fas antigen expression may limit the pathological accumulation of myeloid cells in disease states where p21(ras) is hyperactivated.

Characterization of the somatic mutational spectrum of the neurofibromatosis type 1 (NF1) gene in neurofibromatosis patients with benign and malignant tumors

One of the main features of neurofibromatosis type 1 (NF1) is benign neurofibromas, 10-20% of which become transformed into malignant peripheral nerve sheath tumors (MPNSTs). The molecular basis of NF1 tumorigenesis is, however, still unclear. Ninety-one tumors from 31 NF1 patients were screened for gross changes in the NF1 gene using microsatellite/restriction fragment length polymorphism (RFLP) markers; loss of heterozygosity (LOH) was found in 17 out of 91 (19%) tumors (including two out of seven MPNSTs). Denaturing high performance liquid chromatography (DHPLC) was then used to screen 43 LOH-negative and 10 LOH-positive tumors for NF1 microlesions at both RNA and DNA levels. Thirteen germline and 12 somatic mutations were identified, of which three germline (IVS7-2A>G, 3731delT, 6117delG) and eight somatic (1888delG, 4374-4375delCC, R2129S, 2088delG, 2341del18, IVS27b-5C>T, 4083insT, Q519P) were novel. A mosaic mutation (R2429X) was also identified in a neurofibroma by DHPLC analysis and cloning/sequencing. The observed somatic and germline mutational spectra were similar in terms of mutation type, relative frequency of occurrence, and putative underlying mechanisms of mutagenesis. Tumors lacking mutations were screened for NF1 gene promoter hypermethylation but none were found. Microsatellite instability (MSI) analysis revealed MSI in five out of 11 MPNSTs as compared to none out of 70 neurofibromas (p=1.8 x 10(-5)). The screening of seven MPNSTs for subtle mutations in the CDKN2A and TP53 genes proved negative, although the screening of 11 MPNSTs detected LOH involving either the TP53 or the CDKN2A gene in a total of four tumors. These findings are consistent with the view that NF1 tumorigenesis is a complex multistep process involving a variety of different types of genetic defect at multiple loci.

Neurofibromin-deficient Schwann cells secrete a potent migratory stimulus for Nf1+/- mast cells

The NF1 tumor suppressor gene encodes a GTPase-activating protein called neurofibromin that negatively regulates Ras signaling. Mutations in NF1 cause neurofibromatosis type 1 (NF1). The development of neurofibromas, which are complex tumors composed of multiple cell types, is a hallmark of NF1. Somatic inactivation of murine Nf1 in Schwann cells is necessary, but not sufficient, to initiate neurofibroma formation. Neurofibromas occur with high penetrance in mice in which Nf1 is ablated in Schwann cells in the context of a heterozygous mutant (Nf1+/-) microenvironment. Mast cells infiltrate neurofibromas, where they secrete proteins that can remodel the ECM and initiate angiogenesis. Thus, identification of mechanisms responsible for mast cell migration to tumor microenvironments is important for understanding tumorigenesis and for designing potential therapies. Here, we show that homozygous Nf1 mutant (Nf1-/-) Schwann cells secrete Kit ligand (KitL), which stimulates mast cell migration, and that Nf1+/- mast cells are hypermotile in response to KitL. Furthermore, we link hyperactivation of the Ras-class IA-PI3K-Rac2 pathway to increased Nf1+/- mast cell migration. Thus, these studies identify a novel interaction between Nf1-/- Schwann cells and Nf1+/- mast cells that is likely to be important in neurofibroma formation.

p190RhoGAP can act to inhibit PDGF-induced gliomas in mice: a putative tumor suppressor encoded on human chromosome 19q13.3

p190RhoGAP and Rho are key regulators of oligodendrocyte differentiation. The gene encoding p190RhoGAP is located at 19q13.3 of the human chromosome, a locus that is deleted in 50%-80% of oligodendrogliomas. Here we provide evidence that p190RhoGAP may suppress gliomagenesis by inducing a differentiated glial phenotype. Using a cell culture model of autocrine loop PDGF stimulation, we show that reduced Rho activity via p190RhoGAP overexpression or Rho kinase inhibition induced cellular process extension, a block in proliferation, and reduced expression of the neural precursor marker nestin. In vivo infection of mice with retrovirus expressing PDGF and the p190 GAP domain caused a decreased incidence of oligodendrogliomas compared with that observed with PDGF alone. Independent experiments revealed that the retroviral vector insertion site in 3 of 50 PDGF-induced gliomas was within the p190RhoGAP gene. This evidence strongly suggests that p190 regulates critical components of PDGF oncogenesis and can act as a tumor suppressor in PDGF-induced gliomas by down-regulating Rho activity.