Tumour predisposition in mice heterozygous for a targeted mutation in Nf1

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.

ALTERED CALCIUM CURRENTS AND AXONAL GROWTH IN Nf1 HAPLOINSUFFICIENT MICE

Mutations of the neurofibromin gene (NF1) cause neurofibromatosis type 1 (NF1), a disease in which learning disabilities are common. Learning deficits also are observed in mice with a heterozygous mutation of Nf1 (Nf1(+/-)). Dysregulation of regulated neurotransmitter release has been observed in Nf1(+/-) mice. However, the role of presynaptic voltage-gated Ca(2+) channels mediating this release has not been investigated. We investigated whether Ca(2+) currents and transmitter release were affected by reduced neurofibromin in Nf1(+/-) mice. Hippocampal Ca(2+) current density was greater in neurons from Nf1(+/-) mice and a greater fraction of Ca(2+) currents was activated at less depolarized potentials. In addition, release of the excitatory neurotransmitter, glutamate, was increased in neuronal cortical cultures from Nf1(+/-) mice. Dendritic complexity and axonal length were also increased in neurons Nf1(+/-) mice compared to wild-type neurons, linking loss of neurofibromin to developmental changes in hippocampal axonal/cytoskeletal dynamics. Collectively, these results show that altered Ca(2+) channel density and transmitter release, along with increased axonal growth may account for the abnormal nervous system functioning in NF1.

Pheochromocytoma and paraganglioma

Pheochromocytoma is a very special kind of tumor full of duplicity. On the one hand it represents its own microworld with unique clinical, biochemical and pathological features, while on the other it constitutes a tremendously significant part of whole body system, playing a vital role for practically every organ system. It has a very special character - sometimes like a child it can be sweet and predictable, while at times it can behave like a deadly wild beast, crashing and tearing everything on its path in a fierce rage. It also consists of the amazingly intelligent neuroendocrine cells that possess a magical ability to make miraculous substances of many kinds. But most of all, it is a system that is able to drive our curiosity and the itch of "Cogito, ergo sum" to limitless depths and year by year it still amazes us with new and unexpected discoveries that move our understanding of multiple pathways and metabolic events closer to the ultimate truth. Recent discoveries of succinate dehydrogenase (SHD) and prolyl hydroxylase (PHD) mutations, for example, propelled our understanding of neuroendocrine tumorigenesis as a whole, as well as physiology of mitochondrial respiratory chain and phenomenon of pseudohypoxia in particular. Good old discoveries make their way from dusty repositories to shine with new meaning, appropriate for the current level of knowledge. This acquired wisdom makes us better physicians - knowing the specific expression makeup of catecholamine transporters, GLUTs and SRIFs allows for better tailored imaging and therapeutic manipulations. There are still long ways to go, keeping in mind that pheochromocytoma is but so very special, and we are optimistic and expect many great things to come.

Skeletal abnormalities in neurofibromatosis type 1: approaches to therapeutic options

The skeleton is frequently affected in individuals with neurofibromatosis type 1, and some of these bone manifestations can result in significant morbidity. The natural history and pathogenesis of the skeletal abnormalities of this disorder are poorly understood and consequently therapeutic options for these manifestations are currently limited. The Children's Tumor Foundation convened an International Neurofibromatosis Type 1 Bone Abnormalities Consortium to address future directions for clinical trials in skeletal abnormalities associated with this disorder. This report reviews the clinical skeletal manifestations and available preclinical mouse models and summarizes key issues that present barriers to optimal clinical management of skeletal abnormalities in neurofibromatosis type 1. These concepts should help advance optimal clinical management of the skeletal abnormalities in this disease and address major difficulties encountered for the design of clinical trials.

Retroviral insertional mutagenesis identifies Zeb2 activation as a novel leukemogenic collaborating event in CALM-AF10 transgenic mice

The t(10;11) translocation results in a CALM-AF10 fusion gene in a subset of leukemia patients. Expression of a CALM-AF10 transgene results in leukemia, with prolonged latency and incomplete penetrance, suggesting that additional events are necessary for leukemic transformation. CALM-AF10 mice infected with the MOL4070LTR retrovirus developed acute leukemia, and ligation-mediated polymerase chain reaction was used to identify retroviral insertions at 19 common insertion sites, including Zeb2, Nf1, Mn1, Evi1, Ift57, Mpl, Plag1, Kras, Erg, Vav1, and Gata1. A total of 26% (11 of 42) of the mice had retroviral integrations near Zeb2, a transcriptional corepressor leading to overexpression of the Zeb2-transcript. A total of 91% (10 of 11) of mice with Zeb2 insertions developed B-lineage acute lymphoblastic leukemia, suggesting that Zeb2 activation promotes the transformation of CALM-AF10 hematopoietic precursors toward B-lineage leukemias. More than half of the mice with Zeb2 integrations also had Nf1 integrations, suggesting cooperativity among CALM-AF10, Zeb2, and Ras pathway mutations. We searched for Nras, Kras, and Ptpn11 point mutations in the CALM-AF10 leukemic mice. Three mutations were identified, all of which occurred in mice with Zeb2 integrations, consistent with the hypothesis that Zeb2 and Ras pathway activation promotes B-lineage leukemic transformation in concert with CALM-AF10.

Cardiac and vascular functions of the zebrafish orthologues of the type I neurofibromatosis gene NFI

Von Recklinghausen neurofibromatosis is a common autosomal dominant genetic disorder characterized by benign and malignant tumors of neural crest origin. Significant progress in understanding the pathophysiology of this disease has occurred in recent years, largely aided by the development of relevant animal models. Von Recklinghausen neurofibromatosis is caused by mutations in the NF1 gene, which encodes neurofibromin, a large protein that modulates the activity of Ras. Here, we describe the identification and characterization of zebrafish nf1a and nf1b, orthologues of NF1, and show neural crest and cardiovascular defects resulting from morpholino knockdown, including vascular and cardiac valvular abnormalities. Development of a zebrafish model of von Recklinghausen neurofibromatosis will allow for structure-function analysis and genetic screens in this tractable vertebrate system.

The neurofibromatosis type I pre-mRNA is a novel target of CELF protein-mediated splicing regulation

The CUG-BP and ETR-3 like factors (CELF) are a family of six highly conserved RNA-binding proteins that preferentially bind to UG-rich sequences. One of the key functions of these proteins is to mediate alternative splicing in a number of tissues, including brain, heart and muscle. To fully understand the function of CELF proteins, it is important to identify downstream targets of CELF proteins. In this communication, we report that neurofibromatosis type I (NF1) exon 23a is a novel target of CELF protein-mediated splicing regulation in neuron-like cells. NF1 regulates Ras signaling, and the isoform that excludes exon 23a shows 10 times greater ability to down-regulate Ras signaling than the isoform that includes exon 23a. Five of the six CELF proteins strongly suppress the inclusion of NF1 exon 23a. Over-expression or siRNA knockdown of these proteins in cell transfection experiments altered the levels of NF1 exon 23a inclusion. In vitro binding and splicing analyses demonstrate that CELF proteins block splicing through interfering with binding of U2AF⁶⁵. These studies, combined with our previous investigations demonstrating a role for Hu proteins and TIA-1/TIAR in controlling NF1 exon 23a inclusion, highlight the complex nature of regulation of this important alternative splicing event.

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.

Mice lacking neurofibromin develop gastric hyperplasia

Gastrointestinal (GI) neoplasms are among many manifestations of the genetic disease neurofibromatosis type 1 (NF1). However, the physiological and pathological functions of the Nf1 gene in the GI system have not been fully studied, possibly because of a lack of mouse models. In this study, we generated conditional knockout mice with Nf1 deficiency in the GI tract. These mice develop gastric epithelial hyperplasia and inflammation together with increased cell proliferation and apoptosis. The gastric phenotypes observed in these mutant mice seem to be the consequence of loss of Nf1 in gastric fibroblasts, resulting in paracrine hyperactivation of the ERK pathway in the gastric epithelium. These mice provide a useful model to study the pathogenesis of GI lesions in a subset of patients with NF1 and to investigate the role of the Nf1 gene in the development of GI neoplasms.

The neurofibroma cell of origin: SKPs expand the playing field

Mounting evidence suggests that stem/progenitor cells may be the cells of origin for many tumor types. In this issue of Cell Stem Cell, Le et al. (2009) demonstrate that skin-derived precursors (SKPs) can initiate dermal neurofibromas and highlight the importance of the microenvironment in the formation of this complex tumor.

Cardiomyocyte-specific loss of neurofibromin promotes cardiac hypertrophy and dysfunction

RATIONALE

Neurofibromatosis type 1 (NF1) is a common autosomal dominant disorder with a broad array of clinical manifestations, including benign and malignant tumors, and characteristic cutaneous findings. NF1 patients also have an increased incidence of cardiovascular diseases, including obstructive vascular disorders and hypertension. The disease gene, NF1, encodes neurofibromin, a ubiquitously expressed protein that acts, in part, as a Ras-GAP (GTP-ase activating protein), downregulating the activity of activated Ras protooncogenes. In animal models, endothelial and smooth muscle expression of the disease gene is critical for normal heart development and the prevention of vascular disease, respectively.

OBJECTIVE

To determine the role of NF1 in the postnatal and adult heart.

METHODS AND RESULTS

We generated mice with homozygous loss of the murine homolog Nf1 in myocardium (Nf1mKO) and evaluated their hearts for biochemical, structural, and functional changes. Nf1mKO mice have normal embryonic cardiovascular development but have marked cardiac hypertrophy, progressive cardiomyopathy, and fibrosis in the adult. Hyperactivation of Ras and downstream pathways are seen in the heart with the loss of Nf1, along with activation of a fetal gene program.

CONCLUSIONS

This report describes a critical role of Nf1 in the regulation of cardiac growth and function. Activation of pathways known to be involved in cardiac hypertrophy and dysfunction are seen with the loss of myocardial neurofibromin.

Hereditary tumor syndromes and gliomas

Several congenital syndromes caused by germline mutations in tumor suppressor genes predispose to the development of glial tumors. In the last few decades our knowledge about the molecular functions of these genes and the pathogenesis of hereditary tumor syndromes has greatly increased. The most common syndromes are the neurofibromatoses (type 1 and type 2) and the tuberous scleroses complex. There are interesting overlaps in the molecular pathogen-esis. Deregulation of Ras or downstream Ras pathways including MEK/ERK and AKT/ mTOR plays an important role in these three syndromes. Other rare syndromes include Li-Fraumeni, melanoma-astrocytoma, and Turcot syndrome involving cell cycle regulators and DNA repair genes. The genes and pathways involved in the pathogenesis of these syndromes also play an important role in the development of sporadic tumors. Therefore research on hereditary syndromes contributes substantially to our understanding of tumor formation.

How does the Schwann cell lineage form tumors in NF1?

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

Disease models and mechanisms in the classroom

At the University of Pennsylvania (PENN), we devote an entire graduate-level course to the study of human disease models: Seminar on Current Genetic Research: Modeling Human Disease in Diverse Genetic Systems

Nf1 haploinsufficiency and Icsbp deficiency synergize in the development of leukemias

Loss of neurofibromin or interferon consensus sequence binding protein (Icsbp) leads to a myeloproliferative disorder. Transcription of NF1 is directly controlled by ICSBP. It has been postulated that loss of NF1 expression resulting from loss of transcriptional activation by ICSBP contributes to human hematologic malignancies. To investigate the functional cooperation of these 2 proteins, we have established Icsbp-deficient mice with Nf1 haploinsufficiency. We here demonstrate that loss of Icsbp and Nf1 haploinsufficiency synergize to induce a forced myeloproliferation in Icsbp-deficient mice because of an expansion of a mature myeloid progenitor cell. Furthermore, Nf1 haploinsufficiency and loss of Icsbp contribute synergistically to progression of the myeloproliferative disorder toward transplantable leukemias. Leukemias are characterized by distinct phenotypes, which correlate with progressive genetic abnormalities. Loss of Nf1 heterozygosity is not mandatory for disease progression, but its occurrence with other genetic abnormalities indicates progressive genetic alterations in a defined subset of leukemias. These data show that loss of the 2 tumor suppressor genes Nf1 and Icsbp synergize in the induction of leukemias.

Creation and use of a cre recombinase transgenic database

In many cases, a gene "knockout" results in early embryonic lethality, which obscures the study of potential later functions. In other cases, the "knockout" does not show any phenotype due to the compensation of the gene deficiency by other family members. These limitations have called for further development of the powerful gene-targeting technology. One of the critical tools now being efficiently combined with gene-targeting is site-specific recombination. As the site-specific recombinase technology developed further in the mouse system, it became evident that this tool was going to have a significant impact on the power of mammalian genetics. The number of transgenic mouse lines expressing Cre recombinase with different specificities has steadily increased in the past 15 years and has now surpassed 500. Efficient utilization of this community-generated resource calls for a user-friendly database with all necessary information available about the properties of the Cre transgenic lines. The "CreXmice" database was created to meet these needs and has evolved over the past 4 years from flat file listings of transgenic lines into its current form, a professionally hosted SQL-driven web application. With hundreds of transgenic mouse lines, CreXmice is enriched by its presence on the World Wide Web allowing visitors the opportunity to search or contribute to the global effort by submitting the specific lines being developed by their laboratories.

Review: the role of neural crest cells in the endocrine system

The neural crest is a pluripotent population of cells that arises at the junction of the neural tube and the dorsal ectoderm. These highly migratory cells form diverse derivatives including neurons and glia of the sensory, sympathetic, and enteric nervous systems, melanocytes, and the bones, cartilage, and connective tissues of the face. The neural crest has long been associated with the endocrine system, although not always correctly. According to current understanding, neural crest cells give rise to the chromaffin cells of the adrenal medulla, chief cells of the extra-adrenal paraganglia, and thyroid C cells. The endocrine tumors that correspond to these cell types are pheochromocytomas, extra-adrenal paragangliomas, and medullary thyroid carcinomas. Although controversies concerning embryological origin appear to have mostly been resolved, questions persist concerning the pathobiology of each tumor type and its basis in neural crest embryology. Here we present a brief history of the work on neural crest development, both in general and in application to the endocrine system. In particular, we present findings related to the plasticity and pluripotency of neural crest cells as well as a discussion of several different neural crest tumors in the endocrine system.

Brain tumor susceptibility: the role of genetic factors and uses of mouse models to unravel risk

Brain tumors are relatively rare but deadly cancers, and present challenges in the determination of risk factors in the population. These tumors are inherently difficult to cure because of their protected location in the brain, with surgery, radiation and chemotherapy options carrying potentially lasting morbidity for patients and incomplete cure of the tumor. The development of methods to prevent or detect brain tumors at an early stage is extremely important to reduce damage to the brain from the tumor and the therapy. Developing effective prevention or early detection methods requires a deep understanding of the risk factors for brain tumors. This review explores the difficulties in assessing risk factors in rare diseases such as brain tumors, and discusses how mouse models of cancer can aid in a better understanding of genetic risk factors for brain tumors.

Genetically modified mouse models in cancer studies

Genetically modified animals represent a resource of immense potential for cancer research. Classically, genetic modifications in mice were obtained through selected breeding experiments or treatments with powerful carcinogens capable of inducing random mutagenesis. A new era began in the early 1980s when genetic modifications by inserting foreign DNA genes into the cells of an animal allowed for the development of transgenic mice. Since that moment, genetic modifications have been able to be made in a predetermined way. Gene targeting emerged later as a method of in vivo mutagenesis whereby the sequence of a predetermined gene is selectively modified within an intact cell. In this review we focus on how genetically modified mice can be created to study tumour development, and how these models have contributed to an understanding of the genetic alterations involved in human cancer. We also discuss the strengths and weaknesses of the different mouse models for identifying cancer genes, and understanding the consequences of their alterations in order to obtain the maximum benefit for cancer patients.

Tnk1/Kos1 knockout mice develop spontaneous tumors

Tnk1/Kos1 is a non-receptor protein tyrosine kinase implicated in negatively regulating cell growth in a mechanism requiring its intrinsic catalytic activity. Tnk1/Kos1 null mice were created by homologous recombination by deleting the catalytic domain. Both Tnk1(+/-) and Tnk1(-/-) mice develop spontaneous tumors, including lymphomas and carcinomas, at high rates [27% (14 of 52) and 43% (12 of 28), respectively]. Tnk1/Kos1 expression is silenced in tumors that develop in Tnk1(+/-) mice but not in adjacent uninvolved tissue, and silencing occurs in association with Tnk1 promoter hypermethylation. Tissues and murine embryonic fibroblasts derived from Tnk1/Kos1-null mice exhibit proportionally higher levels of basal and epidermal growth factor-stimulated Ras activation that results from increased Ras-guanine exchange factor (GEF) activity. Mechanistically, Tnk1/Kos1 can directly tyrosine phosphorylate growth factor receptor binding protein 2 (Grb2), which promotes disruption of the Grb2-Sos1 complex that mediates growth factor-induced Ras activation, providing dynamic regulation of Ras GEF activity with suppression of Ras. Thus, Tnk1/Kos1 is a tumor suppressor that functions to down-regulate Ras activity.

Tissue-specific functions of the Caenorhabditis elegans p120 Ras GTPase activating protein GAP-3

All metazoan genomes encode multiple RAS GTPase activating proteins (RasGAPs) that negatively regulate the conserved RAS/MAPK signaling pathway. In mammals, several RasGAPs exhibit tumor suppressor activity by preventing excess RAS signal transduction. We have identified gap-3 as the to date missing Caenorhabditiselegans member of the p120 RasGAP family. By studying the genetic interaction of gap-3 with the two previously identified RasGAPs gap-1 and gap-2, we find that different combinations of RasGAPs are used to repress LET-60 RAS signaling depending on the cellular context. GAP-3 is the predominant negative regulator of RAS during meiotic progression of the germ cells, while GAP-1 is the key inhibitor of RAS during vulval induction. In other tissues such as the sex myoblasts or the chemosensory neurons, all three RasGAPs act in concert. The C. elegans RasGAPs have thus undergone partial specialization after gene duplication to allow the differential regulation of the RAS/MAPK signaling pathway in different cell types. A similar tissue specialization of the human tumor suppressor genes may explain the strong bias in the type of cancer they promote when mutated.

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

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

Influence of hormones and hormone metabolites on the growth of Schwann cells derived from embryonic stem cells and on tumor cell lines expressing variable levels of neurofibromin

Loss of neurofibromin, the protein product of the tumor suppressor gene neurofibromatosis type 1 (NF1), is associated with neurofibromas, composed largely of Schwann cells. The number and size of neurofibromas in NF1 patients have been shown to increase during pregnancy. A mouse embryonic stem cell (mESC) model was used, in which mESCs with varying levels of neurofibromin were differentiated into Schwann-like cells. NF1 cell lines derived from a malignant and a benign human tumor were used to study proliferation in response to hormones. Estrogen and androgen receptors were not expressed or expressed at very low levels in the NF1+/+ cells, at low levels in NF1+/-cells, and robust levels in NF1-/-cells. A 17beta-estradiol (E2) metabolite, 2-methoxy estradiol (2ME2) is cytotoxic to the NF1-/- malignant tumor cell line, and inhibits proliferation in the other cell lines. 2ME2 or its derivatives could provide new treatment avenues for NF1 hormone-sensitive tumors at times of greatest hormonal influence.

Survival and tumorigenesis in O6-methylguanine DNA methyltransferase-deficient mice following cyclophosphamide exposure

O(6)-methylguanine DNA methyltransferase (MGMT) deficiency is associated with an increased susceptibility to alkylating agent toxicity. To understand the contribution of MGMT in protecting against cyclophosphamide (CP)-induced toxicity, mutagenesis and tumorigenesis, we compared the biological effects of this agent in transgenic Mgmt knockout and wild-type mice. In addition, neurofibromin (Nf1)+/- background was used to increase the likelihood of CP-induced tumorigenesis. Cohorts of Mgmt-proficient or -deficient mice (either Nf1+/+ or Nf1+/-) were given 6 weekly injections of a maximally tolerated dose of CP (250 mg/kg) or vehicle and followed for 15 months. CP-treated mice had more deaths than control mice but there was no difference in the long-term survival between Mgmt+/+ and Mgmt-/- mice (12 of 83 Mgmt+/+ mice died compared to 12 of 80 Mgmt-/- mice, disregarding Nf1 status). Lymphomas and adrenal tumours were the most frequent malignancies. Interestingly, CP-treated, Mgmt-deficient mice developed fewer tumours than controls. Ten of 71 (14%) Mgmt-proficient mice developed tumours after CP treatment compared to only 2 of 68 (3%) Mgmt-deficient mice (P = 0.02). Mgmt-/-, Nf1+/- mice developed fewer tumours (1 of 35, 3%) following CP compared to Mgmt+/+, Nf1+/- mice (7 of 37, 19%) (P = 0.03). Hypoxanthine-guanine phosphoribosyltransferase mutation assays showed no significant increases in mutant frequencies in Mgmt-/- (18.1 x 10(6)) compared to Mgmt+/+ mice (12.9 x 10(6)). These data indicate that MGMT deficiency does not protect against long-term toxicity or mutagenicity from CP and appears to attenuate the occurrence of CP-induced tumours in an Nf1+/- background.

A high through-put reverse genetic screen identifies two genes involved in remote memory in mice

Previous studies have revealed that the initial stages of memory formation require several genes involved in synaptic, transcriptional and translational mechanisms. In contrast, very little is known about the molecular and cellular mechanisms underlying later stages of memory, including remote memory (i.e. 7-day memory). To identify genes required for remote memory, we screened randomly selected mouse strains harboring known mutations. In our primary reverse genetic screen, we identified 4 putative remote memory mutant strains out of a total of 54 lines analyzed. Additionally, we found 11 other mutant strains with other abnormal profiles. Secondary screens confirmed that mutations of integrin β2 (Itgβ2) and steryl-O-acyl transferase 1 (Soat1) specifically disrupted remote memory. This study identifies some of the first genes required for remote memory, and suggests that screens of targeted mutants may be an efficient strategy to identify molecular requirements for this process.

Recent insights into bone development, homeostasis, and repair in type 1 neurofibromatosis (NF1)

Neurofibromatosis type 1 (NF1) is one of the most common single gene syndromes and is typified by a range of characteristic but variably penetrant manifestations. The orthopaedic manifestations of congenital pseudarthrosis of the tibia (CPT) and scoliosis, along with other skeletal defects including sphenoid wing dysplasia, rib penciling, and gracile bones have been classically associated with NF1. Added to this, several recent studies have reported a high prevalence of osteoporosis or osteopenia in NF1 cohorts. Clues to the underlying molecular and cellular deficiencies that cause these bony defects can be gleaned from genetically modified mouse models of Nf1 gene deficiency. These studies suggest that a variety of different cell lineages may be adversely affected by Nf1 haploinsufficiency or by double inactivation of the Nf1 gene. Osteoblasts, osteoclasts, chondrocytes, fibroblasts, and vascular endothelial cells all express the Nf1 gene product, neurofibromin, and may be functionally compromised when levels are decreased or absent. This paper reviews the current literature on NF1 bone development, homeostatic regulation, and repair, and highlights some emerging themes that may have relevance for managing orthopaedic disorders that can arise in individuals with NF1.

All in the family: using inherited cancer syndromes to understand de-regulated cell signaling in brain tumors

The cell signaling pathways that are tightly regulated during development are often co-opted by cancer cells to allow them to escape from the constraints that normally limit cell growth and cell movement. In this regard, de-regulated signaling in cancer cells confers a number of key tumor-associated properties, including increased cell proliferation, decreased cell death, and increased cell motility. The identification of some of these critical signaling pathways in the nervous system has come from studies of inherited cancer syndromes in which affected individuals develop brain tumors. The study of brain tumors arising in patients with neurofibromatosis 1 (NF1), neurofibromatosis 2 (NF2), and tuberous sclerosis complex (TSC) has already uncovered several key intracellular signaling pathways important for modulating brain tumor growth. An in-depth analysis of these intracellular signaling pathways will not only lead to an improved understanding of the process of brain tumorigenesis, but may also provide important molecular targets for future therapeutic drug design.

Neurofibromin is required for barrel formation in the mouse somatosensory cortex

The rodent barrel cortex is a useful system to study the role of genes and neuronal activity in the patterning of the nervous system. Several genes encoding either intracellular signaling molecules or neurotransmitter receptors are required for barrel formation. Neurofibromin is a tumor suppressor protein that has Ras GTPase activity, thus attenuating the MAPK (mitogen-activated protein kinase) and and PI-3 kinase (phosphatidylinositol 3-kinase) pathways, and is mutated in humans with the condition neurofibromatosis type 1 (NF1). Neurofibromin is widely expressed in the developing and adult nervous system, and a common feature of NF1 is deficits in intellectual development. In addition, NF1 is an uncommonly high disorder among individuals with autism. Thus, NF1 may have important roles in normal CNS development and function. To explore roles for neurofibromin in the development of the CNS, we took advantage of a mouse conditional allele. We show that mice that lack neurofibromin in the majority of cortical neurons and astrocytes fail to form cortical barrels in the somatosensory cortex, whereas segregation of thalamic axons within the somatosensory cortex appears unaffected.

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

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

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

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

Neurofibromin as a regulator of melanocyte development and differentiation

Patients with the genetic disease type I neurofibromatosis (NF1) exhibit characteristic pigmentary lesions associated with loss of a single allele of NF1, encoding the 260 kDa protein neurofibromin. To understand the basis for these pigmentary problems, the properties of melanocytes haploinsufficient for the murine gene Nf1 were studied using Nf1+/– knockout mice. We demonstrate that neurofibromin regulates the Kit-Mitf signaling axis in vivo during melanocyte development. Primary Nf1+/– melanocytes were purified by FACS to measure melanogenic gene expression. We found that Nf1+/– melanocytes exhibit higher levels of melanogenic gene expression than their wild-type counterparts. Both prior to and following Kit stimulation, Nf1+/– melanocytes also exhibit increased activation of the MAP kinase pathway compared with primary cells. The melanogenic response of primary melanocytes to Mek inhibition is consistent with the changes observed with Nf1 haploinsufficiency; however, these changes differ from those observed with their immortalized counterparts. The observation that reduction of neurofibromin, either from haploinsufficiency in the case of primary melanocytes or from neurofibromin knockdown in the case of melan-a cells, enhances melanogenic gene expression suggests that neurofibromin plays a dominant role to MEK activity in controlling melanogenic gene expression in murine melanocytes.

Real-time PCR analysis of candidate imprinted genes on mouse chromosome 11 shows balanced expression from the maternal and paternal chromosomes and strain-specific variation in expression levels

Imprinted genes are monoallelically expressed from either the maternal or paternal genome. Because cancer develops through genetic and epigenetic alterations, imprinted genes affect tumorigenesis depending on which parental allele undergoes alteration. We have shown previously in a mouse model of neurofibromatosis type 1 (NF1) that inheriting mutant alleles of Nf1 and Trp53 on chromosome 11 from the mother or father dramatically changes the tumor spectrum of mutant progeny, likely due to alteration in an imprinted gene(s) linked to Nf1 and Trp53. In order to identify imprinted genes on chromosome 11 that are responsible for differences in susceptibility, we tested candidate imprinted genes predicted by a bioinformatics approach and an experimental approach. We have tested 30 candidate genes (Havcr2, Camk2b, Ccdc85a, Cntnap1, Ikzf1, 5730522E02Rik, Gria1, Zfp39, Sgcd, Jup, Nxph3, Spnb2, Asb3, Rasd1, Map2k3, Map2k4, Trp53, Serpinf1, Crk, Rasl10b, Itga3, Hoxb5, Cbx1, Pparbp, Igfbp4, Smarce1, Stat3, Atp6v0a1, Nbr1 and Meox1), two known imprinted genes (Grb10 and Impact) and Nf1, which has not been previously identified as an imprinted gene. Although we confirmed the imprinting of Grb10 and Impact, we found no other genes imprinted in the brain. We did, however, find strain-biased expression of Camk2b, 5730522E02Rik, Havcr2, Map2k3, Serpinf1, Rasl10b, Itga3, Asb3, Trp53, Nf1, Smarce1, Stat3, Cbx1, Pparbp and Cntnap1. These results suggest that the prediction of imprinted genes is complicated and must be individually validated. This manuscript includes supplementary data listing primer sequences for Taqman assays and Ct values for Taqman PCR.

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

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

An update on mouse brain tumor models in cancer drug discovery

Gliomas and medulloblastomas are the most common primary brain tumors in adults and children, respectively. Although the standard of care for gliomas may have evolved slightly over the last 50 years, the clinical outcome of this disease remains unchanged. Therefore, further research to improve the treatment modalities is urgently needed. An important step forward is the use of genetically and histologically accurate mouse glioma models that mimic the human tumors in their native microenvironment in order to fully understand the biology and mechanistic causes of this disease. Such strategy will help us to identify novel targets for therapies and use these models for preclinical testing.

Syndromic and sporadic pediatric optic pathway gliomas: review of clinical and histopathological differences and treatment implications

✓Optic pathway gliomas (OPGs) are the most common primary neoplasm of the optic pathway. These lesions usually present in childhood and can arise anywhere along the optic pathway; they occur more frequently in women; and they rarely undergo late progression. Management strategies after the initial diagnosis are controversial, compounded by the different behaviors exhibited by sporadic and syndromic tumors. Neurofibromatosis Type 1 (NF1), with aberrant oncogenic signaling and consequent predisposition to intracranial tumors, is the most common associated syndrome, with nearly 20% of NF1 patients developing OPGs. A comorbid NF1 diagnosis has implications for tumor location with greater predilection for optic nerve involvement, whereas chiasmal and postchiasmal lesions are more frequently seen in sporadic cases. Syndromic OPGs often exhibit more indolent behavior and lower rates of clinical progression, and the majority of these are diagnosed by routine neuroophthalmological screening. When treatment is indicated, however, the molecular abnormalities that constitute this syndrome can limit the available chemotherapy and radiotherapy options because clinicians fear secondary malignancy and cerebrovascular complications. Furthermore, radiotherapy early in life can impair an individual's intellectual development, endocrine function, and physical growth, thereby limiting the role of this modality in the treatment of this childhood lesion. Differential gene expression and histogenesis among sporadic and syndromic OPGs may account for the different tumor behaviors, but studies correlating specific genetic and proteomic changes with patient outcome are pending. Loss of heterozygosity at 10 and 17q are more common among patients with NF1, and Ki67 labeling intensity of 2–3% and low p53 labeling intensity seem prognostic of aggressive tumor behavior. Recent advances in the development of a preclinical mouse model of NF1-associated OPG will permit investigation into improved detection strategies and chemotherapeutic and radiotherapy treatment protocols.

NF1 regulates a Ras-dependent vascular smooth muscle proliferative injury response

BACKGROUND

Neurofibromatosis type I (NF1) is a common autosomal dominant disorder with a broad array of clinical manifestations, including benign and malignant tumors, osseous dysplasias, and characteristic cutaneous findings. In addition, NF1 patients have an increased incidence of cardiovascular diseases, including obstructive vascular disorders. In animal models, endothelial expression of the disease gene, NF1, is critical for normal heart development. However, the pathogeneses of the more common vascular disorders are not well characterized.

METHODS AND RESULTS

To examine the role of NF1 in vascular smooth muscle, we generated mice with homozygous loss of the murine homolog Nf1 in smooth muscle (Nf1smKO). These mice develop and breed normally. However, in response to vascular injury, they display a marked intimal hyperproliferation and abnormal activation of mitogen-activated protein kinase, a downstream effector of Ras. Vascular smooth muscle cells cultured from these mice also display enhanced proliferation and mitogen-activated protein kinase activity. Smooth muscle expression of the NF1 Ras-regulatory domain (GTPase activating protein-related domain) rescues intimal hyperplasia in Nf1smKO mice and normalizes vascular smooth muscle cell Ras effector activity and proliferation in vitro, similar to blockade of downstream effectors of Ras.

CONCLUSIONS

In this in vivo model of NF1 obstructive vascular disease, we have shown that Nf1 regulation of Ras plays a critical role in vascular smooth muscle proliferation after injury. These results suggest opportunities for targeted therapeutics in the prevention and treatment of NF1-related vascular disease and in the treatment of neointimal proliferation in other settings.

Bub1 mediates cell death in response to chromosome missegregation and acts to suppress spontaneous tumorigenesis

The physiological role of the mitotic checkpoint protein Bub1 is unknown. To study this role, we generated a series of mutant mice with a gradient of reduced Bub1 expression using wild-type, hypomorphic, and knockout alleles. Bub1 hypomorphic mice are viable, fertile, and overtly normal despite weakened mitotic checkpoint activity and high percentages of aneuploid cells. Bub1 haploinsufficient mice, which have a milder reduction in Bub1 protein than Bub1 hypomorphic mice, also exhibit reduced checkpoint activity and increased aneuploidy, but to a lesser extent. Although cells from Bub1 hypomorphic and haploinsufficient mice have similar rates of chromosome missegregation, cell death after an aberrant separation decreases dramatically with declining Bub1 levels. Importantly, Bub1 hypomorphic mice are highly susceptible to spontaneous tumors, whereas Bub1 haploinsufficient mice are not. These findings demonstrate that loss of Bub1 below a critical threshold drives spontaneous tumorigenesis and suggest that in addition to ensuring proper chromosome segregation, Bub1 is important for mediating cell death when chromosomes missegregate.

A mouse embryonic stem cell model of Schwann cell differentiation for studies of the role of neurofibromatosis type 1 in Schwann cell development and tumor formation

The neurofibromatosis Type 1 (NF1) gene functions as a tumor suppressor gene. One known function of neurofibromin, the NF1 protein product, is to accelerate the slow intrinsic GTPase activity of Ras to increase the production of inactive rasGDP, with wide-ranging effects on p21ras pathways. Loss of neurofibromin in the autosomal dominant disorder NF1 is associated with tumors of the peripheral nervous system, particularly neurofibromas, benign lesions in which the major affected cell type is the Schwann cell (SC). NF1 is the most common cancer predisposition syndrome affecting the nervous system. We have developed an in vitro system for differentiating mouse embryonic stem cells (mESC) that are NF1 wild type (+/+), heterozygous (+/-), or null (-/-) into SC-like cells to study the role of NF1 in SC development and tumor formation. These mES-generated SC-like cells, regardless of their NF1 status, express SC markers correlated with their stage of maturation, including myelin proteins. They also support and preferentially direct neurite outgrowth from primary neurons. NF1 null and heterozygous SC-like cells proliferate at an accelerated rate compared to NF1 wild type; this growth advantage can be reverted to wild type levels using an inhibitor of MAP kinase kinase (Mek). The mESC of all NF1 types can also be differentiated into neuron-like cells. This novel model system provides an ideal paradigm for studies of the role of NF1 in cell growth and differentiation of the different cell types affected by NF1 in cells with differing levels of neurofibromin that are neither transformed nor malignant.

Biochemical and functional characterization of germ line KRAS mutations

Germ line missense mutations in HRAS and KRAS and in genes encoding molecules that function up- or downstream of Ras in cellular signaling networks cause a group of related developmental disorders that includes Costello syndrome, Noonan syndrome, and cardiofaciocutaneous syndrome. We performed detailed biochemical and functional studies of three mutant K-Ras proteins (P34R, D153V, and F156L) found in individuals with Noonan syndrome and cardiofaciocutaneous syndrome. Mutant K-Ras proteins demonstrate a range of gain-of-function effects in different cell types, and biochemical analysis supports the idea that the intrinsic Ras guanosine nucleotide triphosphatase (GTPase) activity, the responsiveness of these proteins to GTPase-activating proteins, and guanine nucleotide dissociation all regulate developmental programs in vivo.

Neurofibromatosis type 1 in genetic counseling practice: recommendations of the National Society of Genetic Counselors

The objective of this document is to provide recommendations for the genetic counseling of patients and families undergoing evaluation for neurofibromatosis type 1 (NF1) or who have received a diagnosis of NF1. These recommendations are the opinions of a multi-center working group of genetic counselors with expertise in the care of individuals with NF1. These recommendations are based on the committee's clinical experiences, a review of pertinent English language medical articles, and reports of expert committees. These recommendations are not intended to dictate an exclusive course of management, nor does the use of such recommendations guarantee a particular outcome. These recommendations do not displace a health care provider's professional judgment based on the clinical circumstances of an individual patient.

Role of NF2 haploinsufficiency in NF2-associated polyneuropathy

Neurofibromatosis 2 (NF2) is a hereditary tumor disease characterized by bilateral vestibular schwannomas. Polyneuropathy seems to occur quite frequently in NF2 and in most cases, the etiology of this neuropathy is unclear, especially when the neuropathy is symmetric. NF2 is believed to follow the two-hit hypothesis. According to this, one allele is mutated in the germline, and the second hit is somatic and results in tumor formation. The second hit most frequently is a loss of the NF2 locus, often the entire chromosome 22. We set out to investigate the underlying genetics in peripheral nerve of NF2 patients with polyneuropathy. We identified NF2 patients with polyneuropathy in which we could detect the germline mutation and analyzed NF2 gene dosage in archived nerve biopsies from these patients using a newly developed method. We observed merlin haploinsufficiency in peripheral nerves of two different patients with NF2-related polyneuropathy. This finding was further supported by showing that approximately 50% merlin expression in a cell line using shRNA results in altered gene expression as previously shown in schwannomas. Thus, we suggest that reduced merlin gene dosage is relevant in NF2-associated polyneuropathy.

Tumor microenvironment and neurofibromatosis type I: connecting the GAPs

The human disease von Recklinghausen's neurofibromatosis (Nf1) is one of the most common genetic disorders. It is caused by mutations in the NF1 tumor suppressor gene, which encodes a GTPase activating protein (GAP) that negatively regulates p21-RAS signaling. Dermal and plexiform neurofibromas as well as malignant peripheral nerve sheath tumors and other malignant tumors, are significant complications in Nf1. Neurofibromas are complex tumors and composed mainly of abnormal local cells including Schwann cells, endothelial cells, fibroblasts and additionally a large number of infiltrating inflammatory mast cells. Recent work has indicated a role for the microenvironment in plexiform neurofibroma genesis. The emerging evidence points to mast cells as crucial contributors to neurofibroma tumorigenesis. Therefore, further understanding of the molecular interactions between Schwann cells and their environment will provide tools to develop new therapies aimed at delaying or preventing tumor formation in Nf1 patients.

Germline NF1 mutational spectra and loss-of-heterozygosity analyses in patients with pheochromocytoma and neurofibromatosis type 1

BACKGROUND

Neurofibromatosis type 1 (NF1) is a pheochromocytoma-associated syndrome. Because of the low prevalence of pheochromocytoma in NF1, we ascertained subjects by pheochromocytoma that also had NF1 in the hope of describing the germline NF1 mutational spectra of NF1-related pheochromocytoma.

MATERIALS AND METHODS

An international registry for NF1-pheochromocytomas was established. Mutation scanning was performed using denaturing HPLC for intragenic variation and quantitative PCR for large deletions. Loss-of-heterozygosity analysis using markers in and around NF1 was performed.

RESULTS

There were 37 eligible subjects (ages 14-70 yr). Of 21 patients with corresponding tumor available, 67% showed somatic loss of the nonmutated allele at the NF1 locus vs. 0 of 12 sporadic tumors (P = 0.0002). Overall, 86% of the 37 patients had exonic or splice site mutations, 14% large deletions or duplications; 79% of the mutations are novel. The cysteine-serine rich domain (CSR) was affected in 35% but the RAS GTPase activating protein domain (RGD) in only 13%. There did not appear to be an association between any clinical features, particularly pheochromocytoma presentation and severity, and NF1 mutation genotype.

CONCLUSIONS

The germline NF1 mutational spectra comprise intragenic mutations and deletions in individuals with pheochromocytoma and NF1. NF1 mutations tended to cluster in the CSR over the RAS-GAP domain, suggesting that CSR plays a more prominent role in individuals with NF1-pheochromocytoma than in NF1 individuals without this tumor. Loss-of-heterozygosity of NF1 markers in NF1-related pheochromocytoma was significantly more frequent than in sporadic pheochromocytoma, providing further molecular evidence that pheochromocytoma is a true component of NF1.

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

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

Optic pathway gliomas in neurofibromatosis-1: controversies and recommendations

Optic pathway glioma (OPG), seen in 15% to 20% of individuals with neurofibromatosis type 1 (NF1), account for significant morbidity in young children with NF1. Overwhelmingly a tumor of children younger than 7 years, OPG may present in individuals with NF1 at any age. Although many OPG may remain indolent and never cause signs or symptoms, others lead to vision loss, proptosis, or precocious puberty. Because the natural history and treatment of NF1-associated OPG is different from that of sporadic OPG in individuals without NF1, a task force composed of basic scientists and clinical researchers was assembled in 1997 to propose a set of guidelines for the diagnosis and management of NF1-associated OPG. This new review highlights advances in our understanding of the pathophysiology and clinical behavior of these tumors made over the last 10 years. Controversies in both the diagnosis and management of these tumors are examined. Finally, specific evidence-based recommendations are proposed for clinicians caring for children with NF1.

Experimental therapeutic approaches to peripheral nerve tumors

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

The genetic predisposition to and histogenesis of neurofibromas and neurofibrosarcoma in neurofibromatosis Type 1

The author addresses the issue of neurofibroma classification and implications for treatment. He emphasizes the importance of understanding that not all neurofibromas are the same and that the key differences between the types of neurofibromas involve which portions of the nerve sheath contribute to the distinctive behavior of the different types of lesions.

Endoneurial neurofibromas derive from cellular elements ordinarily restricted to the endoneurium. Perineurial neurofibromas arise within individual fascicles of a nerve and are largely confined thereby, precluding a breach of the epineurium. Epineurial neurofibromas are contained only by the epineurium, and ultimately that portion of the nerve sheath is breached by these lesions.

Whether the perineurium is present or breached becomes the key element for exploiting this approach to neurofibroma origins, behaviors, and treatment, surgical and medical. With respect to surgical treatment, perineurial neurofibromas will have clean planes of dissection about the involved nerve. In contrast, endoneurial and epineurial neurofibromas infiltrate adjacent tissues, leading to surgical challenges. With respect to pharmaceutical approaches, the integrity of the perineurium is likely to prove critical: a specific function of the perineurium is to serve as a barrier to various materials, microbiological or chemical. Thus, drugs that might be effective when the perineurium is absent or rent may be less effective (or not effective at all) if the perineurium is intact, as is expected in cases of perineurial neurofibromas.

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

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