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

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.

Tumorigenic properties of neurofibromin-deficient Schwann cells in culture and as syngrafts inNf1 knockout mice

Neurofibromatosis type 1 (NF1) is one of the most common dominantly inherited genetic diseases associated with the nervous system. Functional loss of the NF1 tumor suppressor is frequently associated with the generation of benign neurofibromas that can progress to malignancy. Recent evidence in genetic mouse models indicates that the development of neurofibromas requires a loss of Nf1 in the cells destined to become neoplastic as well as heterozygosity in nonneoplastic cells. We tested this hypothesis in a newly developed syngraft mouse model in which Nf1-/- Schwann cells isolated from knockout embryos were grafted into the sciatic nerves of Nf1+/- mice, corresponding to the genetic background of NF1 patients. Furthermore, we also characterized in vitro growth of these cells. We found that embryonic mouse Nf1-/- Schwann cells exhibit increased proliferation and less growth factor-dependence in vitro compared with heterozygous and wild-type counterparts. Moreover, Nf1-/- Schwann cells showed tumorigenic growth when implanted into nerve of adult Nf1 heterozygous mice. These findings support the conclusion that loss of Nf1 in embryonic mouse Schwann cells is sufficient for tumor development in the heterozygous environment of adult mouse nerve. In addition, this syngraft model provides a practical means for the controlled induction of neurofibromas, greatly facilitating localized application of therapeutic agents and gene delivery.

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.

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

Patients with neurofibromatosis type 1 (NF1), a common autosomal dominant tumor predisposition syndrome, develop benign cutaneous, intraneural, and plexiform neurofibromas and malignant peripheral nerve sheath tumors (MPNSTs), an aggressive form of Schwann cell neoplasm that frequently arises from plexiform neurofibromas. Impressive advances have been made in defining the molecular mechanisms responsible for neurofibroma and MPNST tumorigenesis, including the identification of key tumor suppressor gene mutations, an improved understanding of the functions of these tumor suppressors, and the production of transgenic mouse models in which tumor suppressor gene mutations predispose animals to the development of neurofibromas and MPNSTs. It has also become apparent that dysregulated growth factor signaling cooperates with tumor suppressor mutations to promote neurofibroma and MPNST tumorigenesis. In Part I of this two-part review, we consider findings demonstrating that Schwann cells are the primary neoplastic cell type in neurofibromas and MPNSTs and that specific tumor suppressor gene mutations promote the development of these tumors. In Part II, which will be published in a later issue, we will review evidence indicating that inappropriate growth factor signaling contributes to this process by stimulating the proliferation, survival, and migration of Schwann cells whose regulatory mechanisms have been crippled by a loss of tumor suppressor function.

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.

Adenosine: an activity-dependent axonal signal regulating MAP kinase and proliferation in developing Schwann cells

Nonsynaptic release of ATP from electrically stimulated dorsal root gangion (DRG) axons inhibits Schwann cell (SC) proliferation and arrests SC development at the premyelinating stage, but the specific types of purinergic receptor(s) and intracellular signaling pathways involved in this form of neuron-glia communication are not known. Recent research shows that adenosine is a neuron-glial transmitter between axons and myelinating glia of the CNS. The present study investigates the possibility that adenosine might have a similar function in communicating between axons and premyelinating SCs. Using a combination of pharmacological and molecular approaches, we found that mouse SCs in culture express functional adenosine receptors and ATP receptors, a far more complex array of purinergic receptors than thought previously. Adenosine, but not ATP, activates ERK/MAPK through stimulation of cAMP-linked A2(A) adenosine receptors. Both ATP and adenosine inhibit proliferation of SCs induced by platelet-derived growth factor (PDGF), via mechanisms that are partly independent. In contrast to ATP, adenosine failed to inhibit the differentiation of SCs to the O4+ stage. This indicates that, in addition to ATP, adenosine is an activity-dependent signaling molecule between axons and premyelinating Schwann cells, but that electrical activity, acting through adenosine, has opposite effects on the differentiation of myelinating glia in the PNS and CNS.

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.

Aberrant growth and differentiation of oligodendrocyte progenitors in neurofibromatosis type 1 mutants

Neurofibromatosis type 1 (NF1) patients are predisposed to learning disabilities, macrocephaly, and brain tumors as well as abnormalities on magnetic resonance imaging that are postulated to result from abnormal myelination. Here we show that Nf1+/- spinal cords in adult mice have more than twofold-increased numbers of NG2+ progenitor cells. Nf1-/- embryonic spinal cords have increased numbers of Olig2+ progenitors. Also, cultures from Nf1 mutant embryos with hemizygous and biallelic Nf1 mutations have dramatically increased numbers of CNS oligodendrocyte progenitor cells. In medium that allows growth of neuroepithelial cells and glial progenitors, mutant cells hyper-respond to FGF2, have increased basal and FGF-stimulated Ras-GTP, and fail to accumulate when treated with a farnesyltransferase inhibitor. Cell accumulation results in part from increased proliferation and decreased cell death. In contrast to wild-type cells, Nf1-/- progenitors express the glial differentiation marker O4 while retaining expression of the progenitor marker nestin. Nf1 mutant progenitors also abnormally coexpress the glial differentiation markers O4 and GFAP. Importantly, Nf1-/- spinal cord-derived oligodendrocyte progenitors, which are amplified 12-fold, retain the ability to form oligodendrocytes after in vivo transplantation. The data reveal a key role for neurofibromin and Ras signaling in the maintenance of CNS progenitor cell pools and also suggest a potential role for progenitor cell defects in the CNS abnormalities of NF1 patients.

Aberrant growth and differentiation of oligodendrocyte progenitors in neurofibromatosis type 1 mutants

Neurofibromatosis type 1 (NF1) patients are predisposed to learning disabilities, macrocephaly, and brain tumors as well as abnormalities on magnetic resonance imaging that are postulated to result from abnormal myelination. Here we show that Nf1+/- spinal cords in adult mice have more than twofold-increased numbers of NG2+ progenitor cells. Nf1-/- embryonic spinal cords have increased numbers of Olig2+ progenitors. Also, cultures from Nf1 mutant embryos with hemizygous and biallelic Nf1 mutations have dramatically increased numbers of CNS oligodendrocyte progenitor cells. In medium that allows growth of neuroepithelial cells and glial progenitors, mutant cells hyper-respond to FGF2, have increased basal and FGF-stimulated Ras-GTP, and fail to accumulate when treated with a farnesyltransferase inhibitor. Cell accumulation results in part from increased proliferation and decreased cell death. In contrast to wild-type cells, Nf1-/- progenitors express the glial differentiation marker O4 while retaining expression of the progenitor marker nestin. Nf1 mutant progenitors also abnormally coexpress the glial differentiation markers O4 and GFAP. Importantly, Nf1-/- spinal cord-derived oligodendrocyte progenitors, which are amplified 12-fold, retain the ability to form oligodendrocytes after in vivo transplantation. The data reveal a key role for neurofibromin and Ras signaling in the maintenance of CNS progenitor cell pools and also suggest a potential role for progenitor cell defects in the CNS abnormalities of NF1 patients.

NF1 modulates the effects of ras oncogenes: Evidence of other NF1 function besides its GAP activity

Neurofibromin (NF1) (the product of Nf1 gene) is a large cytosolic protein known as a negative regulator of Ras. A fragment of some 400 residues located at the center of the NF1 GAP-Related Domain (NF1-GRD) has strong identity with other molecules of the GAP family, which comprises, among others, the mammalian proteins NF1 and p120GAP, and the yeast proteins IRA1 and IRA2. GAP family members are known by their ability to promote the GTPase activity of Ras proteins, facilitating the transit of those proteins to their inactive state. Recent findings (Tong et al., 2002, Nat Neurosci 5:95-96) indicate that NF1 may be involved in the regulation of adenyl cyclase activity. Our results show that NF1-GRD cooperates with Ras in the anchorage-independent growth capacity of Ras-expressing fibroblasts, without affecting: (i) their ability to grow in low serum, (ii) their cellular adhesion capability, or (iii) the expression of key proteins involved in cell-cell and cell-matrix interactions. On the other hand, NF1 overexpression induces an increase in the expression levels of the focal adhesion kinase (FAK), and specific changes in the activation status of the mitogen-activated protein kinases (MAPKs). These results suggest the existence of a Ras-independent NF1-dependent pathway able to modify the levels of expression of FAK and the levels of activation of MAPKs. Because FAK and many proteins recently found to bind NF1 have a role in the cytoskeleton, this pathway may involve rearrangement of cytoskeletal components that facilitate anchorage independence.

Delayed rectifier K currents in NF1 Schwann cells. Pharmacological block inhibits proliferation

K+(K) currents are related to the proliferation of many cell types and have a relationship to second messenger pathways implicated in regulation of the cell cycle in development and certain disease states. We examined the role of K currents in Schwann cells (SC) cultured from tumors that arise in the human disease neurofibromatosis type 1 (NF1). Comparisons were made between whole cell voltage clamp recordings from normal human SC cultures and from neurofibroma cultures and malignant peripheral nerve sheath tumor (MPNST) cell lines. The outward K currents of normal and tumor cells could be divided into three types based on pharmacology and macroscopic inactivation: (1) "A type" current blocked by 4-aminopyridine, (2) delayed rectifier (DR) current blocked by tetraethylammonium, and (3) biphasic current consisting of a combination of these two current types. The DR K current was present in MPNST- and neurofibroma-derived SC, but not in quiescent, nondividing, normal SC. DR currents were largest in MPNST-derived SC (50 pA/pF vs. 2.1-4.9 pA/pF in dividing and quiescent normal SC). Normal SC cultures had significantly more cells with A type current than cultures of MPNST and the plexiform neurofibroma. Conversely, MPNST and plexiform neurofibroma cultures had significantly more SC with DR current than did normal cultures, and these DR currents were significantly larger. In addition, the plexiform neurofibroma culture had significantly more cells with DR current than the dermal neurofibroma culture. K currents in SC from normal NF1 SC cultures had current abundances similar to GGF-exposed normal SC and the plexiform neurofibroma. We have established a link between DR K current blockade via TEA analogs and inhibition of proliferation of NF1 SC in vitro. In addition, a farnysyl transferase inhibitor (FTI), a blocker of Ras activation, blocked cell proliferation without blocking K currents in all cultures except a plexiform neurofibroma, suggesting that regulation of proliferation in neoplastic and normal SC in vitro is complex.

Malignant peripheral nerve sheath tumor cell invasion is facilitated by Src and aberrant CD44 expression

Malignant peripheral nerve sheath tumors (MPNSTs) are aggressive malignancies that arise within peripheral nerves. These tumors occur with increased incidence in patients with neurofibromatosis type 1 (NF1), exhibiting increased Ras activity due to loss of the NF1 gene product, neurofibromin, and abnormal expression of the epidermal growth factor receptor (EGFR). We previously found that MPNSTs express increased levels of the CD44 family of transmembrane glycoproteins that have been implicated in tumor cell invasion and metastasis. In two MPNST cell lines, we have found that elevated CD44 expression and cell invasion are dependent on Src kinase activity but are independent of mitogen-activated protein kinases (MAPK) kinase (MEK) activity. In contrast, inhibition of Src kinase activity has no influence on MPNST cell proliferation. Reduction of CD44 levels, using antisense oligonucleotides, results in reduced MPNST cell invasion in vitro, suggesting that Src contributes in part to MPNST cell invasion by increasing CD44 levels. At least some of this increased CD44 expression results from elevated EGFR levels through a Src-dependent mechanism, consistent with the notion that EGFR promotes constitutive Src activation in MPNSTs. These data indicate that Src and CD44 are putative targets for the treatment of MPNST invasion and metastasis.

Brain lipid binding protein in axon-Schwann cell interactions and peripheral nerve tumorigenesis

Loss of axonal contact characterizes Schwann cells in benign and malignant peripheral nerve sheath tumors (MPNST) from neurofibromatosis type 1 (NF1) patients. Tumor Schwann cells demonstrate NF1 mutations, elevated Ras activity, and aberrant epidermal growth factor receptor (EGFR) expression. Using cDNA microarrays, we found that brain lipid binding protein (BLBP) is elevated in an EGFR-positive subpopulation of Nf1 mutant mouse Schwann cells (Nf1(-/-) TXF) that grows away from axons; BLBP expression was not affected by farnesyltransferase inhibitor, an inhibitor of H-Ras. BLBP was also detected in EGFR-positive cell lines derived from Nf1:p53 double mutant mice and human MPNST. BLBP expression was induced in normal Schwann cells following transfection with EGFR but not H-Ras12V. Furthermore, EGFR-mediated BLBP expression was not inhibited by dominant-negative H-Ras, indicating that BLBP expression is downstream of Ras-independent EGFR signaling. BLBP-blocking antibodies enabled process outgrowth from Nf1(-/-) TXF cells and restored interaction with axons, without affecting cell proliferation or migration. Following injury, BLBP expression was induced in normal sciatic nerves when nonmyelinating Schwann cells remodeled their processes. These data suggest that BLBP, stimulated by Ras-independent pathways, regulates Schwann cell-axon interactions in normal peripheral nerve and peripheral nerve tumors.

Neurofibromatosis 1: closing the GAP between mice and men

Neurofibromatosis 1 (NF1) is a common genetic condition in which affected individuals are prone to the development of benign and malignant tumors. The NF1 tumor suppressor encodes a protein product, neurofibromin, which functions in part as a negative regulator of RAS. Loss of neurofibromin expression in NF1-associated tumors or Nf1-deficient mouse cells is associated with elevated RAS activity and increased cell proliferation. Despite this straightforward pathophysiologic association between neurofibromin, RAS, and tumorigenesis, recent insights from mouse and Drosophila modeling studies have suggested additional functions for neurofibromin and have implicated Nf1 heterozygosity in tumor formation. Lastly, Nf1 knockout mouse studies have also demonstrated important roles for cooperating genetic changes that accelerate tumorigenesis as well as modifier genes that impact on cancer susceptibility.

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.

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.

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.

Increased noise as an effect of haploinsufficiency of the tumor-suppressor gene neurofibromatosis type 1 in vitro

In human diseases related to tumor-suppressor genes, it is suggested that only the complete loss of the protein results in specific symptoms such as tumor formation, whereas simple reduction of protein quantity to 50%, called haploinsufficiency, essentially does not affect cellular behavior. Using a model of gene expression, it was presumed that haploinsufficiency is related to an increased noise in gene expression also in vivo [Cook, D. L., Gerber, A. N. & Tapscott, S. J. (1998) Proc. Natl. Acad. Sci. USA 95, 15641-15646]. Here, we demonstrate that haploinsufficiency of the tumor-suppressor gene neurofibromatosis type 1 (NF1) results in an increased variation of dendrite formation in cultured NF1 melanocytes. These morphological differences between NF1 and control melanocytes can be described by a mathematical model in which the cell is considered to be a self-organized automaton. The model describes the adjustment of the cells to a set point and includes a noise term that allows for stochastic processes. It describes the experimental data of control and NF1 melanocytes. In the cells haploinsufficient for NF1 we found an altered signal-to-noise ratio detectable as increased variation in dendrite formation in two of three investigated morphological parameters. We also suggest that in vivo NF1 haploinsufficiency results in an increased noise in cellular regulation and that this effect of haploinsufficiency may be found also in other tumor suppressors.

Inhibitors of the ras oncogene as therapeutic targets

Advances in our understanding of the molecular pathways and genetic mutations that control tumor cell proliferation and metastasis present an opportunity to develop novel, mechanism-based therapeutic strategies. Ras mutations are the most frequently activated oncogenes in human tumors, with over 30% expressing ras mutations. Molecular dissection of the signaling pathway and the mechanisms of ras anchorage, post-translational modification, and downstream effector signaling of ras now under intensive investigation will help us to design additional methods for ras-directed therapy in an effort to reach an optimal treatment for human tumors that will most likely comprise a combination of modalities targeted at the different underlying genetic defects. The successes and limitations of ras-targeted therapies must be viewed in light of the increasing understanding of the complexity of the ras-signaling pathway. Only now are we beginning to discover the many functions of this integrated pathway, such as the differences between the actions of various ras isoforms that may affect our choice of therapeutic approach. Many of these Ras therapeutic targets have shown success in preclinical studies, and some have shown efficacy in clinical trials with minimal toxicities. Compounds that block ras-transforming activity without affecting normal ras function seem more attractive for the future development of ras-targeted therapy. FTIs may partially fulfill such requirements. Based on their specific, novel, and mechanism-based action; minimal toxicity; and encouraging responses in clinical trials, the development of Ras therapeutic targets as single agents or in combination with conventional chemotherapy and radiotherapy should be pursued.

The molecular and genetic basis of neurological tumours

There are no effective therapies for many tumours of the nervous system. This is, in part, a consequence of their location within relatively inaccessible tissues. It is also likely, however, that the unique characteristics of the cells that give rise to these tumours create a set of conditions that facilitate tumour development. Here, we consider recent advances in molecular genetics, the development of mouse models and developmental neurobiology as they relate to tumours of neuroectodermal origin. It is likely that these advances will provide insight into underlying mechanisms and provide a rational framework for the development of effective interventions.

Antiangiogenesis in neurofibromatosis 1

Antiangiogenesis therapy has become a potentially promising tool to inhibit tumor growth by targeting an essential yet untransformed tissue component. Identifying the factors involved and understanding the mechanisms required for tumor angiogenesis will facilitate efficient and specific targeting. In neurofibromas, tumor growth is facilitated by a genetically and cytologically diverse mixture of cell types, including Schwann cells, fibroblast, mast cells, and neurons where nf-/- Schwann cells are most likely the tumorigenic cell type. The matrix forming nf+/- cells may provide a permissive environment, facilitating tumor development, perhaps by providing landscaping factors such as the angiogenic molecules fibroblast growth factor-2, platelet-derived growth factor, endothelial growth factor, vascular endothelial growth factor, and midkine, which have been detected in neurofibromas. Systemic overexpression of specific factors such as midkine owing to loss of one nf allele might further lower the overall threshold for tumorigenesis and development of a tumor vasculature. Targeting these heparin-binding growth factors might inhibit not only angiogenesis but also proliferation of tumor cells because most of these factors also stimulate proliferation of neurofibroma-derived Schwann cells. We discuss the role of specific secreted molecules for angiogenesis in tumors of neurofibromatosis 1 and possible Approaches for their targeting. Furthermore, results are discussed that demonstrate the efficacy of antiangiogenesis targeting to inhibit growth of neurofibrosarcomas in experimental animal models.

Mouse models of neurofibromatosis 1 and 2

The neurofibromatoses represent two of the most common inherited tumor predisposition syndromes affecting the nervous system. Individuals with neurofibromatosis 1 (NF1) are prone to the development of astrocytomas and peripheral nerve sheath tumors whereas those affected with neurofibromatosis 2 (NF2) develop schwannomas and meningiomas. The development of traditional homozygous knockout mice has provided insights into the roles of the NF1 and NF2 genes during development and in differentiation, but has been less instructive regarding the contribution of NF1 and NF2 dysfunction to the pathogenesis of specific benign and malignant tumors. Recent progress employing novel mouse targeting strategies has begun to illuminate the roles of the NF1 and NF2 gene products in the molecular pathogenesis of NF-associated tumors.

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

Deletion of both copies of the Nf1 gene in Schwann cells combined with Nf1 heterozygosity in the tumor environment promotes neurofibroma formation in mice.

Regulation of the cell cycle in normal and pathological glia

Precise regulation of the glial cell cycle is essential during nervous system development and in response to injury, whereas disruption of cell cycle control is associated with malignant glial tumors and other nervous system diseases. The Ras signaling pathway plays a central role in regulating the mammalian cell cycle, and uncontrolled Ras signaling has been implicated in a wide range of human cancers, including malignant gliomas. Recent studies in glia have demonstrated that activation of Ras can either induce or inhibit proliferation through complex interactions among downstream signaling pathways impinging on cell cycle regulatory proteins. Studies in Schwann cells have begun to delineate the pathways by which Ras regulates the cell cycle in normal and pathological glia, and have identified promising targets for therapeutic intervention in the treatment of PNS and CNS malignant glial tumors.

Neurofibromin regulates G protein-stimulated adenylyl cyclase activity

Neurofibromatosis type 1 (NF1) is a dominant genetic disorder characterized by multiple benign and malignant nervous system tumors, and by learning defects in 45% of children with NF1 mutations. Studies of neurofibromin, the protein encoded by NF1, have focused on its functions in tumorigenesis and regulation of Ras activity; however, Drosophila NF1 regulates both Ras and cyclic AMP (cAMP) pathways. Expression of a human NF1 transgene rescued cAMP-related phenotypes in NF1 mutant flies (small body size and G protein-stimulated adenylyl cyclase (AC) activity defects), and neuropeptide- and G protein-stimulated AC activity were lower in Nf1-/- as compared to Nf1+/- mouse brains, demonstrating that neurofibromin regulates AC activity in both mammals and flies.

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.

Hemarthrosis: an unusual complication of type 1 neurofibromatosis

Type 1 neurofibromatosis is one of the most common autosomal-dominant disorders and often includes orthopedic manifestations. We report the case of a 48-year-old woman with hemarthrosis caused by a popliteal cyst infiltrated by a diffuse neurofibroma associated with angiodysplasia. Surgical resection of this tumor was followed by postoperative hemorrhagic complications.

Schwann cell proliferative responses to cAMP and Nf1 are mediated by cyclin D1

In most mammalian cells, the cAMP-dependent protein kinase A pathway promotes growth arrest and cell differentiation. However in Schwann cells, the reverse is true. Elevated levels of cAMP function as the cofactor to a broad range of mitogenic cues in culture and in animals. Previous studies have suggested that cAMP acts at an early point in the Schwann cell mitogenic response, perhaps by stimulating the expression of growth factor receptors. We show here that cAMP acts downstream rather than upstream of growth factor receptor expression. The essential function(s) of cAMP is exerted as Schwann cells progress through the G(1) phase of the cell cycle. Ectopic expression studies using an inducible retroviral vector show that the G(1) phase requirement for cAMP can be alleviated by a single protein, cyclin D1. We show, in addition, that at least one function of the Nf1 tumor suppressor is to antagonize the accumulation of cAMP and the expression of cyclin D1 in Schwann cells. Thus a G(1) phase-specific protein, cyclin D1, accounts for two salient features of Schwann cell growth control: the promitotic response to cAMP and the antimitotic response to the Nf1 tumor suppressor.

The NF2 tumor suppressor gene product, merlin, mediates contact inhibition of growth through interactions with CD44

The neurofibromatosis-2 (NF2) gene encodes merlin, an ezrin-radixin-moesin-(ERM)-related protein that functions as a tumor suppressor. We found that merlin mediates contact inhibition of growth through signals from the extracellular matrix. At high cell density, merlin becomes hypo-phosphorylated and inhibits cell growth in response to hyaluronate (HA), a mucopolysaccharide that surrounds cells. Merlin's growth-inhibitory activity depends on specific interaction with the cytoplasmic tail of CD44, a transmembrane HA receptor. At low cell density, merlin is phosphorylated, growth permissive, and exists in a complex with ezrin, moesin, and CD44. These data indicate that merlin and CD44 form a molecular switch that specifies cell growth arrest or proliferation.

Neurofibromatosis 1 (NF1) heterozygosity results in a cell-autonomous growth advantage for astrocytes

Individuals with neurofibromatosis 1 (NF1) develop low-grade astrocytomas at an increased frequency. To gain insight into the function of the Nf1 gene product as a growth regulator for astrocytes, we examined mice heterozygous for a targeted Nf1 mutation. In our previous studies, we demonstrated increased numbers of proliferating astrocytes in Nf1 heterozygote (Nf1+/-) mice in vivo. We now show that cultured Nf1+/- astrocytes exhibit a cell-autonomous growth advantage in vitro associated with increased p21-ras pathway activation. Furthermore, we demonstrate that Nf1+/-;wild-type N-ras mice have a similar astrocyte growth advantage in vitro and in vivo as either oncogenic N-ras or Nf1+/-; oncogenic N-ras mice. Lastly, mice heterozygous for targeted defects in both Nf1 and p53 as well as Nf1 and Rb exhibit 3- and 2.5-fold increases in astrocyte proliferation in vivo, respectively, suggesting that abnormalities in Nf1- and p53/Rb-regulated pathways cooperate in the heterozygous state to confer a growth advantage for brain astrocytes. Collectively, these results provide evidence for a cell-autonomous growth advantage in Nf1+/- astrocytes and suggest that some of the brain pathology in individuals with NF1 might result from reduced, but not absent, NF1 gene function.

Malignancy in neurofibromatosis type 1

Neurofibromatosis type 1 (NF1) represents a major risk factor for development of malignancy, particularly malignant peripheral nerve sheath tumors (MPNST), optic gliomas, other gliomas, and leukemias. The oncologist will see NF1 patients referred for treatment of malignancy, and should be alert to the possibility of undiagnosed NF1 among patients with cancer. Brain tumors tend to have a more indolent course in NF1 than in the general population, and hence are best managed conservatively. MPNST, in contrast, do not respond to standard chemotherapy or radiation therapy. The most effective treatment of MPNST appears to be early diagnosis and surgery, but early diagnosis is hampered by frequent occurrence within preexisting large tumors, making new growth or change difficult to detect. New insights into pathogenesis now offer hope of development of specific methods of treatment with reduced toxicity and more precise molecular targeting. There is an urgent need, however, to develop methods to measure tumor growth and monitor outcomes, develop preclinical drug screening systems, and further explore the pathogenesis of the disorder to determine whether mechanisms other than Ras regulation may be important in pathogenesis.

Schwann cell proliferative responses to cAMP and Nf1 are mediated by cyclin D1

In most mammalian cells, the cAMP-dependent protein kinase A pathway promotes growth arrest and cell differentiation. However in Schwann cells, the reverse is true. Elevated levels of cAMP function as the cofactor to a broad range of mitogenic cues in culture and in animals. Previous studies have suggested that cAMP acts at an early point in the Schwann cell mitogenic response, perhaps by stimulating the expression of growth factor receptors. We show here that cAMP acts downstream rather than upstream of growth factor receptor expression. The essential function(s) of cAMP is exerted as Schwann cells progress through the G(1) phase of the cell cycle. Ectopic expression studies using an inducible retroviral vector show that the G(1) phase requirement for cAMP can be alleviated by a single protein, cyclin D1. We show, in addition, that at least one function of the Nf1 tumor suppressor is to antagonize the accumulation of cAMP and the expression of cyclin D1 in Schwann cells. Thus a G(1) phase-specific protein, cyclin D1, accounts for two salient features of Schwann cell growth control: the promitotic response to cAMP and the antimitotic response to the Nf1 tumor suppressor.

The angiogenic factor midkine is aberrantly expressed in NF1-deficient Schwann cells and is a mitogen for neurofibroma-derived cells

Loss of the tumor suppressor gene NF1 in neurofibromatosis type 1 (NF1) contributes to the development of a variety of tumors, including malignant peripheral nerve sheath tumors (MPNST) and benign neurofibromas. Of the different cell types found in neurofibromas, Schwann cells usually provide between 40 and 80%, and are thought to be critical for tumor growth. Here we describe the identification of growth factors that are upregulated in NF1-/- mouse Schwann cells and are potential regulators of angiogenesis and cell growth. Basic fibroblast growth factor (FGF-2), platelet-derived growth factor (PDGF) and midkine (MK) were found to be induced by loss of neurofibromin and MK was further characterized. MK was induced in human neurofibromas, schwannomas, and various nervous system tumors associated with NF1 or NF2; midkine showed an expression pattern overlapping but distinct from its homolog pleiotrophin (PTN). Immunohistochemistry revealed expression of MK in S-100 positive Schwann cells of dermal and plexiform neurofibromas, and in endothelial cells of tumor blood vessels, but not in normal blood vessels. Furthermore, MK demonstrated potent mitogenic activity for human systemic and brain endothelial cells in vitro and stimulated proliferation and soft agar colony formation of human MPNST derived S100 positive cells and fibroblastoid cells derived from an NF1 neurofibroma. The data support a possible central role for MK as a mediator of angiogenesis and neurofibroma growth in NF1. Oncogene (2001) 20, 97 - 105.

Single cell Ras-GTP analysis reveals altered Ras activity in a subpopulation of neurofibroma Schwann cells but not fibroblasts

Neurofibromatosis type 1 (NF1) is a common genetic disorder characterized by multiple neurofibromas, peripheral nerve tumors containing mainly Schwann cells and fibroblasts. The NF1 gene encodes neurofibromin, a tumor suppressor postulated to function in part as a Ras GTPase-activating protein. The roles of different cell types and of elevated Ras-GTP in neurofibroma formation are unclear. To determine which neurofibroma cell type has altered Ras-GTP regulation, we developed an immunocytochemical assay for active, GTP-bound Ras. In NIH 3T3 cells, the assay detected overexpressed, constitutively activated K-, N-, and Ha-Ras and insulin-induced endogenous Ras-GTP. In dissociated neurofibroma cells from NF1 patients, Ras-GTP was elevated in Schwann cells but not fibroblasts. Twelve to 62% of tumor Schwann cells showed elevated Ras-GTP, unexpectedly revealing neurofibroma Schwann cell heterogeneity. Increased basal Ras-GTP did not correlate with increased cell proliferation. Normal human Schwann cells, however, did not demonstrate elevated basal Ras activity. Furthermore, compared with cells from wild type littermates, Ras-GTP was elevated in all mouse Nf1(-/-) Schwann cells but never in Nf1(-/-) mouse fibroblasts. Our results indicate that Ras activity is detectably increased in only some neurofibroma Schwann cells and suggest that neurofibromin is not an essential regulator of Ras activity in fibroblasts.

CD44 enhances neuregulin signaling by Schwann cells

We describe a key role for the CD44 transmembrane glycoprotein in Schwann cell-neuron interactions. CD44 proteins have been implicated in cell adhesion and in the presentation of growth factors to high affinity receptors. We observed high CD44 expression in early rat neonatal nerves at times when Schwann cells proliferate but low expression in adult nerves, where CD44 was found in some nonmyelinating Schwann cells and to varying extents in some myelinating fibers. CD44 constitutively associated with erbB2 and erbB3, receptor tyrosine kinases that heterodimerize and signal in Schwann cells in response to neuregulins. Moreover, CD44 significantly enhanced neuregulin-induced erbB2 phosphorylation and erbB2-erbB3 heterodimerization. Reduction of CD44 expression in vitro resulted in loss of Schwann cell-neurite adhesion and Schwann cell apoptosis. CD44 is therefore crucial for maintaining neuron-Schwann cell interactions at least partly by facilitating neuregulin-induced erbB2-erbB3 activation.

Long-term culture and characterization of human neurofibroma-derived Schwann cells

Neurofibromas are benign tumors arising from the peripheral nerve sheath and are a typical finding in neurofibromatosis type 1 (NF1). Schwann cells are the predominant cell type in neurofibromas and thus are supposed to play a major role in the pathogenesis of these tumors. It is not known, however, if NF1 mutations in Schwann cells result in an altered phenotype that subsequently leads to tumor formation. To characterize the biological properties of neurofibroma-derived Schwann cells we developed cell culture techniques that enabled us to isolate Schwann cells from neurofibromas and grow them in vitro for several weeks without significant fibroblast contamination. Neurofibroma-derived Schwann cells were characterized by altered morphology, heterogeneous growth behavior, and increased expression of the P0 antigen while several other features of normal human Schwann cells were retained. We conclude that neurofibroma-derived Schwann cells exhibit a distinct phenotype in vitro but that the observed abnormalities by themselves are insufficient to explain neurofibroma formation. Application of our improved culture conditions makes neurofibroma-derived Schwann cells readily available for further studies to define their role in tumorigenesis in neurofibromatosis type 1.

The neurofibromatosis type 1 (Nf1) tumor suppressor is a modifier of carcinogen-induced pigmentation and papilloma formation in C57BL/6 mice

There is increasing evidence implicating the human NF1 gene in epithelial carcinogenesis. To test if NF1 can play a part in skin tumor formation, we analyzed effects of the skin cancer initiator dimethylbenz-anthracene and/or the tumor promoter 12-O-tetradecanoyl-13-acetylphorbol on mice heterozygous for null mutations in Nf1 (Nf1+/-). Mice were on the C57BL/6 background, noted for resistance to chemical carcinogens. Nf1+/- mice (18 of 24) developed papillomas after treatment with dimethylbenzanthracene and 12-O-tetradecanoyl-13-acetylphorbol; papillomas did not develop in wild-type C57BL/6 mice nor Nf1+/- mice treated with 12-O-tetradecanoyl-13-acetylphorbol alone. All papillomas analyzed (six of six) had mutations in codon 61 of H-ras, demonstrating strong cooperation between the Nf1 GTPase activating protein for Ras, neurofibromin, and Ras-GTP. After exposure to 12-O-tetradecanoyl-13-acetylphorbol, Nf1+/- keratinocytes showed significant, sustained, increases in proliferation, implicating Nf1 in phorbol ester responsive pathways. Thus, Nf1 levels regulate the response of keratinocytes to 12-O-tetradecanoyl-13-acetylphorbol. Nf1+/- mice also showed a 2-fold increase in the development of pigmented skin patches stimulated by dimethylbenzanthracene; patches were characterized by hair follicles in anagen phase, implicating keratinocytes in the aberrant hyperpigmentation. Our results show that mutation in the Nf1 gene causes abnormal keratinocyte proliferation that can be revealed by environmental assaults such as carcinogen exposure. The data support a plausible role for NF1 mutation in human epithelial carcinogenesis.

Epidermal growth factor receptor expression in neurofibromatosis type 1-related tumors and NF1 animal models

We have found that EGF-R expression is associated with the development of the Schwann cell-derived tumors characteristic of neurofibromatosis type 1 (NF1) and in animal models of this disease. This is surprising, because Schwann cells normally lack EGF-R and respond to ligands other than EGF. Nevertheless, immunoblotting, Northern analysis, and immunohistochemistry revealed that each of 3 malignant peripheral nerve sheath tumor (MPNST) cell lines from NF1 patients expressed the EGF-R, as did 7 of 7 other primary MPNSTs, a non-NF1 MPNST cell line, and the S100(+) cells from each of 9 benign neurofibromas. Furthermore, transformed derivatives of Schwann cells from NF1(-/-) mouse embryos also expressed the EGF-R. All of the cells or cell lines expressing EGF-R responded to EGF by activation of downstream signaling pathways. Thus, EGF-R expression may play an important role in NF1 tumorigenesis and Schwann cell transformation. Consistent with this hypothesis, growth of NF1 MPNST lines and the transformed NF1(-/-) mouse embryo Schwann cells was greatly stimulated by EGF in vitro and could be blocked by agents that antagonize EGF-R function.

A developmentally regulated switch directs regenerative growth of Schwann cells through cyclin D1

Sciatic nerve axons in cyclin D1 knockout mice develop normally, become properly ensheathed by Schwann cells, and appear to function normally. However, in the Wallerian degeneration model of nerve injury, the mitotic response of Schwann cells is completely inhibited. The mitotic block is Schwann cell autonomous and developmentally regulated. Rescue analysis (by "knockin" of cyclin E) indicates that D1 protein, rather than regulatory elements of the D1 gene, provides the essential Schwann cell function. Genetic inhibition of the Schwann cell cycle shows that neuronal responses to nerve injury are surprisingly independent of Schwann cell mitotic responses. Even axonal regrowth into the distal zone of a nerve crush injury is not markedly impaired in cyclin D1-/- mice.

Inhibition of juvenile myelomonocytic leukemia cell growth in vitro by farnesyltransferase inhibitors

Juvenile myelomonocytic leukemia (JMML) is an early childhood disease for which there is no effective therapy. Therapy with 13-cis retinoic acid or low-dose chemotherapy can induce some responses, but neither mode is curative. Stem cell transplantation can produce lasting remissions but is hampered by high rates of relapse. The pathogenesis of JMML involves deregulated cytokine signal transduction through the Ras signaling pathway, with resultant selective hypersensitivity of JMML cells to granulocyte-macrophage colony-stimulating factor (GM-CSF). A JMML mouse model, achieved through homozygous deletion of the neurofibromatosis gene, confirmed the involvement of deregulated Ras in JMML pathogenesis. With this pathogenetic knowledge, mechanism-based treatments are now being developed and tested. Ras is critically dependent on a prenylation reaction for its signal transduction abilities. Farnesyltransferase inhibitors are compounds that were developed specifically to block the prenylation of Ras. Two of these compounds, L-739,749 and L-744,832, were tested for their ability to inhibit spontaneous JMML granulocyte-macrophage colony growth. Within a dose range of 1 to 10 μmol/L, each compound demonstrated dose-dependent inhibition of JMML colony growth. An age-matched patient with a different disease and GM-CSF–stimulated normal adult marrow cells also demonstrated dose-dependent inhibitory effects on colony growth, but they were far less sensitive to these compounds than JMML hematopoietic progenitors. Even if the addition of L-739,749 were delayed for 5 days, significant inhibitory effects would still show in JMML cultures. These results demonstrate that a putative Ras-blocking compound can have significant growth inhibitory effects in vitro, perhaps indicating a potential treatment for JMML.

Overexpression of activated neu/erbB2 initiates immortalization and malignant transformation of immature Schwann cells in vitro

The neu/erbB2 protooncogene is overexpressed in numerous human cancers and is mutationally activated in N-ethyl-N-nitrosourea (ENU)-induced rodent tumors of the Schwann cell lineage. We investigated whether expression of activated neu in Schwann cells is sufficient to initiate their immortalization and transformation. Clones of embryonic dorsal root ganglia cells infected with a retrovirus bearing activated neu (NID cells) were selected based on their expression of Schwann cell-specific markers. Compared to embryonic Schwann cells infected with a virus encoding empty vector, we found that NID cells have altered shapes and disorganized cytoskeletons, grow in the absence of growth factors required for normal Schwann cell survival and proliferation, and can be repeatedly passaged. Furthermore, NID cells are invasive in an in vitro matrix invasion assay and form metastatic tumors when injected into syngeneic animals. The neu-induced growth and invasive phenotypes could be reversed by drugs that inhibit Ras and Src activity. Interestingly, later stage Schwann cells infected with activated neu failed to become immortalized. These findings indicate that constitutive activation of erbB2 is sufficient to initiate the immortalization and transformation of immature Schwann cells, and support the notion that Schwann cells have particular developmental stages during which they are susceptible to immortalizing and transforming events.