MYC expression promotes the proliferation of neural progenitor cells in culture and in vivo

Single-cell transcriptomic sequencing identifies subcutaneous patient-derived xenograft recapitulated medulloblastoma

BACKGROUND

Medulloblastoma (MB) is one of the most common malignant brain tumors that mainly affect children. Various approaches have been used to model MB to facilitate investigating tumorigenesis. This study aims to compare the recapitulation of MB between subcutaneous patient-derived xenograft (sPDX), intracranial patient-derived xenograft (iPDX), and genetically engineered mouse models (GEMM) at the single-cell level.

METHODS

We obtained primary human sonic hedgehog (SHH) and group 3 (G3) MB samples from six patients. For each patient specimen, we developed two sPDX and iPDX models, respectively. Three Patch+/- GEMM models were also included for sequencing. Single-cell RNA sequencing was performed to compare gene expression profiles, cellular composition, and functional pathway enrichment. Bulk RNA-seq deconvolution was performed to compare cellular composition across models and human samples.

RESULTS

Our results showed that the sPDX tumor model demonstrated the highest correlation to the overall transcriptomic profiles of primary human tumors at the single-cell level within the SHH and G3 subgroups, followed by the GEMM model and iPDX. The GEMM tumor model was able to recapitulate all subpopulations of tumor microenvironment (TME) cells that can be clustered in human SHH tumors, including a higher proportion of tumor-associated astrocytes and immune cells, and an additional cluster of vascular endothelia when compared to human SHH tumors.

CONCLUSIONS

This study was the first to compare experimental models for MB at the single-cell level, providing value insights into model selection for different research purposes. sPDX and iPDX are suitable for drug testing and personalized therapy screenings, whereas GEMM models are valuable for investigating the interaction between tumor and TME cells.

Role of Non-coding RNAs in the Response of Glioblastoma to Temozolomide

Chemotherapy and radiotherapy are widely used in clinical practice across the globe as cancer treatments. Intrinsic or acquired chemoresistance poses a significant problem for medical practitioners and researchers, causing tumor recurrence and metastasis. The most dangerous kind of malignant brain tumor is called glioblastoma multiforme (GBM) that often recurs following surgery. The most often used medication for treating GBM is temozolomide chemotherapy; however, most patients eventually become resistant. Researchers are studying preclinical models that accurately reflect human disease and can be used to speed up drug development to overcome chemoresistance in GBM. Non-coding RNAs (ncRNAs) have been shown to be substantial in regulating tumor development and facilitating treatment resistance in several cancers, such as GBM. In this work, we mentioned the mechanisms of how different ncRNAs (microRNAs, long non-coding RNAs, circular RNAs) can regulate temozolomide chemosensitivity in GBM. We also address the role of these ncRNAs encapsulated inside secreted exosomes.

Multifactoriality of Parkinson's Disease as Explored Through Human Neural Stem Cells and Their Transplantation in Middle-Aged Parkinsonian Mice

Parkinson's disease (PD) is an age-associated neurodegenerative disorder for which there is currently no cure. Cell replacement therapy is a potential treatment for PD; however, this therapy has more clinically beneficial outcomes in younger patients with less advanced PD. In this study, hVM1 clone 32 cells, a line of human neural stem cells, were characterized and subsequently transplanted in middle-aged Parkinsonian mice in order to examine cell replacement therapy as a treatment for PD. In vitro analyses revealed that these cells express standard dopamine-centered markers as well as others associated with mitochondrial and peroxisome function, as well as glucose and lipid metabolism. Four months after the transplantation of the hVM1 clone 32 cells, striatal expression of tyrosine hydroxylase was minimally reduced in all Parkinsonian mice but that of dopamine transporter was decreased to a greater extent in buffer compared to cell-treated mice. Behavioral tests showed marked differences between experimental groups, and cell transplant improved hyperactivity and gait alterations, while in the striatum, astroglial populations were increased in all groups due to age and a higher amount of microglia were found in Parkinsonian mice. In the motor cortex, nonphosphorylated neurofilament heavy was increased in all Parkinsonian mice. Overall, these findings demonstrate that hVM1 clone 32 cell transplant prevented motor and non-motor impairments and that PD is a complex disorder with many influencing factors, thus reinforcing the idea of novel targets for PD treatment that tend to be focused on dopamine and nigrostriatal damage.

The current landscape of immunotherapy for pediatric brain tumors

Pediatric central nervous system tumors are the most common solid malignancies in childhood, and aggressive therapy often leads to long-term sequelae in survivors, making these tumors challenging to treat. Immunotherapy has revolutionized prospects for many cancer types in adults, but the intrinsic complexity of treating pediatric patients and the scarcity of clinical studies of children to inform effective approaches have hampered the development of effective immunotherapies in pediatric settings. Here, we review recent advances and ongoing challenges in pediatric brain cancer immunotherapy, as well as considerations for efficient clinical translation of efficacious immunotherapies into pediatric settings.

The Role of E3s in Regulating Pluripotency of Embryonic Stem Cells and Induced Pluripotent Stem Cells

Pluripotent embryonic stem cells (ESCs) are derived from early embryos and can differentiate into any type of cells in living organisms. Induced pluripotent stem cells (iPSCs) resemble ESCs, both of which serve as excellent sources to study early embryonic development and realize cell replacement therapies for age-related degenerative diseases and other cell dysfunction-related illnesses. To achieve these valuable applications, comprehensively understanding of the mechanisms underlying pluripotency maintenance and acquisition is critical. Ubiquitination modifies proteins with Ubiquitin (Ub) at the post-translational level to monitor protein stability and activity. It is extensively involved in pluripotency-specific regulatory networks in ESCs and iPSCs. Ubiquitination is achieved by sequential actions of the Ub-activating enzyme E1, Ub-conjugating enzyme E2, and Ub ligase E3. Compared with E1s and E2s, E3s are most abundant, responsible for substrate selectivity and functional diversity. In this review, we focus on E3 ligases to discuss recent progresses in understanding how they regulate pluripotency and somatic cell reprogramming through ubiquitinating core ESC regulators.

TRIM proteins in neuroblastoma

Neuroblastoma (NB) is the most common extracranial solid tumor in childhood. Outcome for children with high-risk NB remains unsatisfactory. Accumulating evidence suggests that tripartite motif (TRIM) family proteins express diversely in various human cancers and act as regulators of oncoproteins or tumor suppressor proteins. This review summarizes the TRIM proteins involving in NB and the underlying molecular mechanisms. We expect these new insights will provide important implications for the treatment of NB by targeting TRIM proteins.

LINC00470 promotes tumour proliferation and invasion, and attenuates chemosensitivity through the LINC00470/miR-134/Myc/ABCC1 axis in glioma

Glioma is the most common primary malignant tumour in the brain; temozolomide (TMZ) is the most prevalent chemotherapeutic drug currently used to combat this cancer. We reported previously that the long intergenic non‐protein coding RNA 470 (LINC00470) is a novel prognostic biomarker for glioma and promotes the tumour growth in an intracranial transplantation mouse model. However, the effects of LINC00470 on glioma cell proliferation, invasion and TMZ chemosensitivity, as well as its molecular mechanism, remain unclear. In this study, we found elevated expression levels of LINC00470 and MYC in glioma tissues and cells and decreased expression of microRNA‐134 (miR‐134). Functional studies have shown that LINC00470 promotes proliferation and invasion, and attenuates chemosensitivity of glioma cells, while miR‐134 exerts the opposite effect. In the rescue experiments, the tumorigenic effect of LINC00470 was offset by miR‐134. In the mechanism study, we found that LINC00470 was a competitive endogenous RNA (ceRNA) of miR‐134 and that miR‐134 can directly target MYC and negatively regulate its expression. Furthermore, MYC was positively correlated with ATP‐binding cassette subfamily C member 1 (ABCC1) expression in glioma cells and MYC up‐regulated ABCC1 expression. Further studies found that LINC00470 regulated MYC by sponging miR‐134 to regulate the expression of ABCC1. We concluded that LINC00470 promoted the expression of MYC and ABCC1 by suppressing miR‐134, thus promoting glioma cell proliferation and invasion, and attenuating TMZ chemosensitivity. Moreover, the LINC00470/miR‐134/MYC/ABCC1 axis constitutes a potential therapeutic target.

Anti-PD-1 Induces M1 Polarization in the Glioma Microenvironment and Exerts Therapeutic Efficacy in the Absence of CD8 Cytotoxic T Cells

PURPOSE

Anti-programmed cell death protein 1 (PD-1) therapy has demonstrated inconsistent therapeutic results in patients with glioblastoma (GBM) including those with profound impairments in CD8 T-cell effector responses.

EXPERIMENTAL DESIGN

We ablated the CD8α gene in BL6 mice and intercrossed them with Ntv-a mice to determine how CD8 T cells affect malignant progression in forming endogenous gliomas. Tumor-bearing mice were treated with PD-1 to determine the efficacy of this treatment in the absence of T cells. The tumor microenvironment of treated and control mice was analyzed by IHC and FACS.

RESULTS

We observed a survival benefit in immunocompetent mice with endogenously arising intracranial glioblastomas after intravenous administration of anti-PD-1. The therapeutic effect of PD-1 administration persisted in mice even after genetic ablation of the CD8 gene (CD8^-/-). CD11b⁺ and Iba1⁺ monocytes and macrophages were enriched in the glioma microenvironment of the CD8^-/- mice. The macrophages and microglia assumed a proinflammatory M1 response signature in the setting of anti-PD-1 blockade through the elimination of PD-1-expressing macrophages and microglia in the tumor microenvironment. Anti-PD-1 can inhibit the proliferation of and induce apoptosis of microglia through antibody-dependent cellular cytotoxicity, as fluorescently labeled anti-PD-1 was shown to gain direct access to the glioma microenvironment.

CONCLUSIONS

Our results show that the therapeutic effect of anti-PD-1 blockade in GBM may be mediated by the innate immune system, rather than by CD8 T cells. Anti-PD-1 immunologically modulates innate immunity in the glioma microenvironment-likely a key mode of activity.

mTOR pathway activation in focal cortical dysplasia

BACKGROUND

Focal cortical dysplasia (FCD) is a localized cortical malformation and considerable morphological overlap exists between FCD IIB and neurological lesions associated with Tuberous sclerosis complex (TSC). Abnormal mTOR pathway secondary to somatic mTOR mutation and TSC gene mutation linked to PI3K/AKT/mTOR pathway have supported the hypothesis of common pathogenesis involved. Role of converging pathway, viz. Wnt/β-Catenin and mTOR is unknown in FCD. We aimed to analyse FCD IIB for TSC1/TSC2 mutations, immunoreactivity of hamartin, tuberin, mTOR and Wnt signalling cascades, and stem cell markers.

MATERIALS AND METHODS

Sixteen FCD IIB cases were retrieved along with 16 FCD IIA cases for comparison. Immunohistochemistry was performed for tuberin, hamartin, mTOR pathway markers, markers of stem cell phenotype, and Wnt pathway markers. Mutation analysis for TSC1 and TSC2 was performed by sequencing in 9 FCD cases.

RESULTS

All FCD cases showed preserved hamartin and tuberin immunoreactivity. Aberrant immunoreactivity of phospho-P70S6 kinase, S6 ribosomal, phospho-S6 ribosomal and Stat3 was noted in FCD IIB, with variable phospho-4E-BP1 (45%) and absent phospho-Stat3 expression. Immunoreactivity for phospho-P70S6 kinase (100%), S6 ribosomal protein (100%) and Stat3 (100%) was noted in FCD IIA, but not for phospho-S6 ribosomal, phospho-4E-BP1 and phospho-Stat3. c-Myc immunoreactivity was noted in all FCD cases. Nestin (81%) and Sox 2 (88%) stained balloon cells in FCD IIB (44%), while in FCD IIA cases were negative. All FCD cases were immunopositive for Wnt, but were negative for β-Catenin and cyclin-D1. TSC mutations were detected in two cases of FCD IIB.

CONCLUSION

Abnormal mTOR pathway activation exists in FCD IIB and IIA, however, shows differential immunoreactivity profile, indicating varying degrees of dysregulation. Labelling of neuronal stem cell markers in balloon cells suggests they are phenotypically immature. TSC1/2 mutation play role in the pathogenesis of FCD. Deep targeted sequencing is preferred diagnostic technique since conventional sanger sequencing often fails to detect low-allele frequency variants involved in mTOR/TSC pathway genes, commonly found in FCD.

Modeling pediatric medulloblastoma

Mouse models of medulloblastoma have proven to be instrumental in understanding disease mechanisms, particularly the role of epigenetic and molecular drivers, and establishing appropriate preclinical pipelines. To date, our research community has developed murine models for all four groups of medulloblastoma, each of which will be critical for the identification and development of new therapeutic approaches. Approaches to modeling medulloblastoma range from genetic engineering with CRISPR/Cas9 or in utero electroporation, to orthotopic and patient‐derived orthotopic xenograft systems. Each approach or model presents unique advantages that have ultimately contributed to an appreciation of medulloblastoma heterogeneity and the clinical obstacles that exist for this patient population.

Vrk1 partial Knockdown in Mice Results in Reduced Brain Weight and Mild Motor Dysfunction, and Indicates Neuronal VRK1 Target Pathways

Mutations in Vaccinia-related kinase 1 (VRK1) have emerged as a cause of severe neuronal phenotypes in human, including brain developmental defects and degeneration of spinal motor neurons, leading to Spinal Muscular Atrophy (SMA) or early onset Amyotrophic Lateral Sclerosis (ALS). Vrk1 gene-trap partial Knockout (KO) mice (Vrk1GT3/GT3), which express decreased levels of Vrk1, are sterile due to impaired gamete production. Here, we examined whether this mouse model also presents neuronal phenotypes. We found a 20-50% reduction in Vrk1 expression in neuronal tissues of the Vrk1GT3/GT3 mice, leading to mild neuronal phenotypes including significant but small reduction in brain mass and motor (rotarod) impairment. Analysis of gene expression in the Vrk1GT3/GT3 cortex predicts novel roles for VRK1 in neuronal pathways including neurotrophin signaling, axon guidance and pathways implicated in the pathogenesis of ALS. Together, our studies of the partial KO Vrk1 mice reveal that even moderately reduced levels of Vrk1 expression result in minor neurological impairment and indicate new neuronal pathways likely involving VRK1.

A complex of the ubiquitin ligase TRIM32 and the deubiquitinase USP7 balances the level of c-Myc ubiquitination and thereby determines neural stem cell fate specification

The balance between stem cell maintenance and differentiation has been proposed to depend on antagonizing ubiquitination and deubiquitination reactions of key stem cell transcription factors (SCTFs) mediated by pairs of E3 ubiquitin ligases and deubiquitinating enzymes. Accordingly, increased ubiquitination results in proteasomal degradation of the SCTF, thereby inducing cellular differentiation, whereas increased deubiquitination stabilizes the SCTF, leading to maintenance of the stem cell fate. In neural stem cells, one of the key SCTFs is c-Myc. Previously, it has been shown that c-Myc is ubiquitinated by the E3 ligase TRIM32, thereby targeting c-Myc for proteasomal degradation and inducing neuronal differentiation. Accordingly, TRIM32 becomes upregulated during adult neurogenesis. This upregulation is accompanied by subcellular translocation of TRIM32 from the cytoplasm of neuroblasts to the nucleus of neurons. However, we observed that a subpopulation of proliferative type C cells already contains nuclear TRIM32. As these cells do not undergo neuronal differentiation, despite containing TRIM32 in the nucleus, where it can ubiquitinate c-Myc, we hypothesize that antagonizing factors, specifically deubiquitinating enzymes, are present in these particular cells. Here we show that TRIM32 associates with the deubiquitination enzyme USP7, which previously has been implicated in neural stem cell maintenance. USP7 and TRIM32 were found to exhibit a dynamic and partially overlapping expression pattern during neuronal differentiation both in vitro and in vivo. Most importantly, we are able to demonstrate that USP7 deubiquitinates and thereby stabilizes c-Myc and that this function is required to maintain neural stem cell fate. Accordingly, we propose the balanced ubiquitination and deubiquitination of c-Myc by TRIM32 and USP7 as a novel mechanism for stem cell fate determination.

Failure of the PTEN/aPKC/Lgl Axis Primes Formation of Adult Brain Tumours in Drosophila

Different regions in the mammalian adult brain contain immature precursors, reinforcing the concept that brain cancers, such as glioblastoma multiforme (GBM), may originate from cells endowed with stem-like properties. Alterations of the tumour suppressor gene PTEN are very common in primary GBMs. Very recently, PTEN loss was shown to undermine a specific molecular axis, whose failure is associated with the maintenance of the GBM stem cells in mammals. This axis is composed of PTEN, aPKC, and the polarity determinant Lethal giant larvae (Lgl): PTEN loss promotes aPKC activation through the PI3K pathway, which in turn leads to Lgl inhibition, ultimately preventing stem cell differentiation. To find the neural precursors responding to perturbations of this molecular axis, we targeted different neurogenic regions of the Drosophila brain. Here we show that PTEN mutation impacts aPKC and Lgl protein levels also in Drosophila. Moreover, we demonstrate that PI3K activation is not sufficient to trigger tumourigenesis, while aPKC promotes hyperplastic growth of the neuroepithelium and a noticeable expansion of the type II neuroblasts. Finally, we show that these neuroblasts form invasive tumours that persist and keep growing in the adult, leading the affected animals to untimely death, thus displaying frankly malignant behaviours.

Modeling and Targeting MYC Genes in Childhood Brain Tumors

Brain tumors are the second most common group of childhood cancers, accounting for about 20%–25% of all pediatric tumors. Deregulated expression of the MYC family of transcription factors, particularly c-MYC and MYCN genes, has been found in many of these neoplasms, and their expression levels are often correlated with poor prognosis. Elevated c-MYC/MYCN initiates and drives tumorigenesis in many in vivo model systems of pediatric brain tumors. Therefore, inhibition of their oncogenic function is an attractive therapeutic target. In this review, we explore the roles of MYC oncoproteins and their molecular targets during the formation, maintenance, and recurrence of childhood brain tumors. We also briefly summarize recent progress in the development of therapeutic approaches for pharmacological inhibition of MYC activity in these tumors.

TRIM32 modulates pluripotency entry and exit by directly regulating Oct4 stability

Induced pluripotent stem cells (iPSCs) have revolutionized the world of regenerative medicine; nevertheless, the exact molecular mechanisms underlying their generation and differentiation remain elusive. Here, we investigated the role of the cell fate determinant TRIM32 in modulating such processes. TRIM32 is essential for the induction of neuronal differentiation of neural stem cells by poly-ubiquitinating cMyc to target it for degradation resulting in inhibition of cell proliferation. To elucidate the role of TRIM32 in regulating somatic cell reprogramming we analysed the capacity of TRIM32-knock-out mouse embryonic fibroblasts (MEFs) in generating iPSC colonies. TRIM32 knock-out MEFs produced a higher number of iPSC colonies indicating a role for TRIM32 in inhibiting this cellular transition. Further characterization of the generated iPSCs indicated that the TRIM32 knock-out iPSCs show perturbed differentiation kinetics. Additionally, mathematical modelling of global gene expression data revealed that during differentiation an Oct4 centred network in the wild-type cells is replaced by an E2F1 centred network in the TRIM32 deficient cells. We show here that this might be caused by a TRIM32-dependent downregulation of Oct4. In summary, the data presented here reveal that TRIM32 directly regulates at least two of the four Yamanaka Factors (cMyc and Oct4), to modulate cell fate transitions.

The role of Myc and let-7a in glioblastoma, glucose metabolism and response to therapy

Glioblastoma multiforme (GBM) is thought to result from an imbalance between glucose metabolism and tumor growth. The Myc oncogene and lethal-7a microRNA (let-7a miRNA) have been suggested to cooperatively regulate multiple downstream targets leading to changes in chromosome stability, gene mutations, and/or modulation of tumor growth. Here, we review the roles of Myc and let-7a in glucose metabolism and tumor growth and addresses their future potential as prognostic markers and therapeutic tools in GBM. We focus on the functions of Myc and let-7a in glucose uptake, tumor survival, proliferation, and mobility of glioma cells. In addition, we discuss how regulation of different pathways by Myc or let-7a may be useful for future GBM therapies. A large body of evidence suggests that targeting Myc and let-7a may provide a selective mechanism for the deregulation of glucose metabolic pathways in glioma cells. Indeed, Myc and let-7a are aberrantly expressed in GBM and have been linked to the regulation of cell growth and glucose metabolism in GBM. This article is part of a Special Issue entitled "Targeting alternative glucose metabolism and regulate pathways in GBM cells for future glioblastoma therapies".

Myc proteins in brain tumor development and maintenance

Myc proteins are often deregulated in human brain tumors, especially in embryonal tumors that affect children. Many observations have shown how alterations of these pleiotropic Myc transcription factors provide initiation, maintenance, or progression of tumors. This review will focus on the role of Myc family members (particularly c-myc and Mycn) in tumors like medulloblastoma and glioma and will further discuss how to target stabilization of these proteins for future brain tumor therapies.

Intratumoral mediated immunosuppression is prognostic in genetically engineered murine models of glioma and correlates to immunotherapeutic responses

PURPOSE

Preclinical murine model systems used for the assessment of therapeutics have not been predictive of human clinical responses, primarily because their clonotypic nature does not recapitulate the heterogeneous biology and immunosuppressive mechanisms of humans. Relevant model systems with mice that are immunologically competent are needed to evaluate the efficacy of therapeutic agents, especially immunotherapeutics.

EXPERIMENTAL DESIGN

Using the RCAS/Ntv-a system, mice were engineered to coexpress platelet-derived growth factor B (PDGF-B) receptor + B-cell lymphoma 2 (Bcl-2) under the control of the glioneuronal specific Nestin promoter. The degree and type of tumor-mediated immunosuppression were determined in these endogenously arising gliomas on the basis of the presence of macrophages and regulatory T cells. The immunotherapeutic agent WP1066 was tested in vivo to assess therapeutic efficacy and immunomodulation.

RESULTS

Ntv-a mice were injected with RCAS vectors to express PDGF-B + Bcl-2, resulting in both low- and high-grade gliomas. Consistent with observations in human high-grade gliomas, mice with high-grade gliomas also developed a marked intratumoral influx of macrophages that was influenced by tumor signal transducer and activator of transduction 3 (STAT3) expression. The presence of intratumoral F4/80 macrophages was a negative prognosticator for long-term survival. In mice coexpressing PDGF-B + Bcl-2that were treated with WP1066, there was 55.5% increase in median survival time (P < 0.01), with an associated inhibition of intratumoral STAT3 and macrophages.

CONCLUSIONS

Although randomization is necessary for including mice in a therapeutic trial, these murine model systems are more suitable for testing therapeutics, especially immunotherapeutics, in the context of translational studies.

Molecular diagnostics of CNS embryonal tumors

Tremendous progress has recently been made in both molecular subgrouping, and the establishment of prognostic biomarkers for embryonal brain tumors, particularly medulloblastoma. Several prognostic biomarkers that were initially identified in retrospective cohorts of medulloblastoma, including MYC and MYCN amplification, nuclear β-catenin accumulation, and chromosome 17 aberrations have now been validated in clinical trials. Moreover, molecular subgroups based on distinct transcriptome profiles have been consistently reported from various groups on different platforms demonstrating that the concept of distinct medulloblastoma subgroups is very robust. Well-described subgroups of medulloblastomas include tumors showing wingless signaling pathway (Wnt) activation, and another characterized by sonic hedgehog pathway activity. Two or more additional subgroups were consistently reported to contain the vast majority of high-risk tumors, including most tumors with metastatic disease at diagnosis and/or large cell/anaplastic histology. Several years ago, atypical teratoid rhabdoid tumor (AT/RT) was recognized as a separate entity based on its distinct biology and particularly aggressive clinical behavior. These tumors may occur supra or infratentorially and are usually found to have genetic alterations of SMARCB1 (INI1/hSNF5), a tumor suppressor gene located on chromosome 22q. Subsequent loss of SMARCB1 protein expression comprises a relatively specific and sensitive diagnostic marker for AT/RT. For CNS primitive neuroectodermal tumors (CNS PNETs), a consistent finding has been that they are molecularly distinct from medulloblastoma. Furthermore, a distinct fraction of CNS PNETs with particularly poor prognosis only occurring in young children was delineated, which was previously labeled ependymoblastoma or embryonal tumor with abundant neuropil and true rosettes (ETANTR) and which is morphologically characterized by the presence of multilayered "ependymoblastic" rosettes. This group of tumors shows a unique cytogenetic abnormality not seen in other brain tumors: focal amplification of a micro-RNA cluster at chromosome 19q13.42, which has never been found to be amplified in other CNS PNETs, medulloblastoma or AT/RT. In summary, these consistent findings have significantly contributed to our ability to sub-classify embryonal brain tumors into clinically and biologically meaningful strata and, for some of the subgroups, have led to the identification of specific targets for future development of molecularly targeted therapies.

Early progenitor cell marker expression distinguishes type II from type I focal cortical dysplasias

Type I and type II focal cortical dysplasias (FCDs) exhibit distinct histopathologic features that suggest different pathogenic mechanisms. Type I FCDs are characterized by mild laminar disorganization and hypertrophic neurons, whereas type II FCDs exhibit dramatic laminar disorganization and cytomegalic cells (balloon cells). Both FCD types are associated with intractable epilepsy; therefore, identifying cellular or molecular differences between these lesion types that explains the histologic differences could provide new diagnostic and therapeutic insights. Type II FCDs express nestin, a neuroglial progenitor protein that is modulated in vitro by the stem cell proteins c-Myc, sex-determining region Y-box 2 (SOX2), and Octamer-4 (Oct-4) after activation of mammalian target of rapamycin complex 1 (mTORC1). Because mTORC1 activation has been demonstrated in type II FCDs, we hypothesized that c-Myc, SOX2, and Oct-4 expression would distinguish type II from type I FCDs. In addition, we assayed the expression of progenitor cell proteins forkhead box G1 (FOXG1), Kruppel-like factor 4 (KLF4), Nanog, and SOX3. Differential expression of 7 stem cellproteins and aberrant phosphorylation of2mTORC1 substrates, S6 andS6 kinase 1 proteins, clearly distinguished type II from type I FCDs(n = 10 each). Our results demonstrate new potential pathogenic pathways in type II FCDs and suggest biomarkers for diagnostic pathology in resected epilepsy specimens.

Mouse models of CNS embryonal tumors

Central nervous system (CNS) embryonal tumors are devastating cancers in children, consisting of medulloblastomas, CNS primitive neuroectodermal tumors, and atypical teratoid/rhabdoid tumors. One of the reasons that CNS embryonal tumors remain difficult to treat is their rarity, which makes conducting clinical trials for these tumors difficult. Recent advances of molecular biology have led us to identify molecular and genetic causality of brain tumors. Based on the genetic alterations found in humans, multiple models of human CNS embryonal tumors have been generated in genetically engineered mice. These mouse models are valuable tools for understanding brain tumor biology and discovering novel therapeutic targets and drugs. In this article, we review molecular and cytogenetic characteristics of human CNS embryonal tumors and corresponding mouse models that have been developed. These findings indicate that common genetic abnormalities are seen in variants of human CNS embryonal tumors, and multiple histological variants of these tumors can be generated from a single set of genetic abnormalities in mice. These data provide insight into the biology and classification of CNS embryonal tumors.

Oligodendrocyte progenitor cells can act as cell of origin for experimental glioma

Gliomas are primary brain tumors mainly affecting adults. The cellular origin is unknown. The recent identification of tumor-initiating cells in glioma, which share many similarities with normal neural stem cells, has suggested the cell of origin to be a transformed neural stem cell. In previous studies, using the RCAS/tv-a mouse model, platelet-derived growth factor B (PDGF-B)-induced gliomas have been generated from nestin or glial fibrillary acidic protein-expressing cells, markers of neural stem cells. To investigate if committed glial progenitor cells could be the cell of origin for glioma, we generated the Ctv-a mouse where tumor induction would be restricted to myelinating oligodendrocyte progenitor cells (OPCs) expressing 2',3'-cyclic nucleotide 3'-phosphodiesterase. We showed that PDGF-B transfer to OPCs could induce gliomas with an incidence of 33%. The majority of tumors resembled human WHO grade II oligodendroglioma based on close similarities in histopathology and expression of cellular markers. Thus, with the Ctv-a mouse we have showed that the cell of origin for glioma may be a committed glial progenitor cell.

The TRIM-NHL protein TRIM32 activates microRNAs and prevents self-renewal in mouse neural progenitors

In the mouse neocortex, neural progenitor cells generate both differentiating neurons and daughter cells that maintain progenitor fate. Here, we show that the TRIM-NHL protein TRIM32 regulates protein degradation and microRNA activity to control the balance between those two daughter cell types. In both horizontally and vertically dividing progenitors, TRIM32 becomes polarized in mitosis and is concentrated in one of the two daughter cells. TRIM32 overexpression induces neuronal differentiation while inhibition of TRIM32 causes both daughter cells to retain progenitor cell fate. TRIM32 ubiquitinates and degrades the transcription factor c-Myc but also binds Argonaute-1 and thereby increases the activity of specific microRNAs. We show that Let-7 is one of the TRIM32 targets and is required and sufficient for neuronal differentiation. TRIM32 is the mouse ortholog of Drosophila Brat and Mei-P26 and might be part of a protein family that regulates the balance between differentiation and proliferation in stem cell lineages.

Modeling Brain Tumors Using Avian Retroviral Gene Transfer

RCAS/tv-a is a system for postnatal cell-type-specific gene transfer. It is used for the modeling of gliomas and medulloblastomas. This system provides a combination of lineage tracing from the cell origin with oncogenesis induced by mis-expression of specific genes. The genes that are most potent at inducing tumors are those that encode components of signal transduction and undifferentiated cells are most capable of serving as the cell of origin. The system effectively generates tumors with the histologic characteristics of human disease. Mice bearing RCAS/tv-a-induced brain tumors are currently being used for preclinical trials to understand the biology of therapeutic response in the various cell types that make up gliomas and medulloblastomas.

Myc increases self-renewal in neural progenitor cells through Miz-1

The mechanisms underlying the decision of a stem or progenitor cell to either self-renew or differentiate are incompletely understood. To address the role of Myc in this process, we expressed different forms of the proto-oncogene Myc in multipotent neural progenitor cells (NPCs) using retroviral transduction. Expression of Myc in neurospheres increased the proportion of self-renewing cells fivefold, and 1% of the Myc-overexpressing cells, but none of the control cells, retained self-renewal capacity even under differentiation-inducing conditions. A Myc mutant (MycV394D) deficient in binding to Miz-1, did not increase the percentage of self-renewing cells but was able to stimulate proliferation of NPCs as efficiently as wild-type Myc, indicating that these two cellular phenomena are regulated by at least partially different pathways. Our results suggest that Myc, through Miz-1, enhances self-renewal of NPCs and influences the way progenitor cells react to the environmental cues that normally dictate the cellular identity of tissues containing self-renewing cells.

c-Myc and beta-catenin cooperate with loss of p53 to generate multiple members of the primitive neuroectodermal tumor family in mice

Primitive neuroectodermal tumors (PNETs) are a family of primary malignant brain tumors that include medulloblastomas. Although genetic models of a subset of medulloblastomas are documented over the past decade, the molecular basis of other subclasses of PNET remains unclear. As elevated c-Myc expression, activation of Wnt/beta-catenin signaling and dysfunction of p53 are seen in human PNETs, we investigated what role these abnormalities have in the formation of PNETs. Incorporating these abnormalities, we generated supratentorial PNET (sPNET) in mice using somatic cell gene transfer. We show that sPNETs arise from GFAP-expressing cells by forced c-Myc expression combined with p53 inactivation. beta-catenin activation promotes tumor progression and induces divergent differentiation. These c-Myc+beta-catenin-induced PNETs are histologically similar to large cell/anaplastic medulloblastomas and can occur in both cerebrum and cerebellum. Furthermore, we have obtained one PNET with marked epithelial differentiation having histological resemblance to choroid plexus carcinoma in this series. Our results in mice suggest that sPNET with varied differentiation and large cell/anaplastic medulloblastomas may be two tumor groups with similar genetic foundations. These data provide insights into the biology and classification of human PNETs and suggest that multiple tumor types or variants can be generated from a fixed set of genetic abnormalities.

Etiology-dependent molecular mechanisms in human hepatocarcinogenesis

Hepatocellular carcinoma (HCC) is one of the most common cancers worldwide and is characterized by aggressive tumor behavior coupled with poor prognosis. Various etiologies have been linked to HCC development, most prominently chronic hepatitis B and C virus infections as well as chronic alcohol consumption. In approximately 10% of HCCs, the etiology remains cryptic; however, recent epidemiological data suggest that most of these cryptogenic HCCs develop due to nonalcoholic steatohepatitis. To identify etiology-dependent DNA copy number aberrations and genes relevant to hepatocarcinogenesis, we performed array-based comparative genomic hybridization of 63 HCCs of well-defined etiology and 4 HCC cell lines followed by gene expression profiling and functional analyses of candidate genes. For a 10-megabase chromosome region on 8q24, we observed etiology-dependent copy number gains and MYC overexpression in viral and alcohol-related HCCs, resulting in up-regulation of MYC target genes. Cryptogenic HCCs showed neither 8q24 gains, nor MYC overexpression, nor target gene activation, suggesting that tumors of this etiology develop by way of a distinct MYC-independent pathomechanism. Furthermore, we detected several etiology-independent small chromosome aberrations, including amplification of MDM4 on 1q32.1 and frequent gains of EEF1A2 on 20q13.33. Both genes were overexpressed in approximately half the HCCs examined, and gene silencing reduced cell viability as well as proliferation and increased apoptosis rates in HCC cell lines.

CONCLUSION

Our findings suggest that MDM4 and EEF1A2 act as etiology-independent oncogenes in a significant percentage of HCCs.

Neural stem cells and the future treatment of neurological diseases: raising the standard

Neural stem and progenitor cells offer great potential for treatment of neurological disorders.AQ[16]Both neurologic and neurological were used in text; the latter was more common. Note changes have been made to match in title and text body. The current strategies of isolation, expansion, and characterization of these cells require in vitro manipulations that can change their intrinsic properties, specifically with the acquisition of chromosomal abnormalities. We have analyzed the rationale of using neural stem cells in neurological disorders, the caveats of current isolation and in vitro culture protocols of neural precursors. Addressing these challenges is crucial for translation of neural stem cell therapy to the clinic.

Neural stem cells, inflammation and NF-kappaB: basic principle of maintenance and repair or origin of brain tumours?

Several recent reports suggest that inflammatory signals play a decisive role in the self-renewal, migration and differentiation of multipotent neural stem cells (NSCs). NSCs are believed to be able to ameliorate the symptoms of several brain pathologies through proliferation, migration into the area of the lesion and either differentiation into the appropriate cell type or secretion of anti-inflammatory cytokines. Although NSCs have beneficial roles, current evidence indicates that brain tumours, such as astrogliomas or ependymomas are also caused by tumour-initiating cells with stem-like properties. However, little is known about the cellular and molecular processes potentially generating tumours from NSCs. Most pro-inflammatory conditions are considered to activate the transcription factor NF-kappaB in various cell types. Strong inductive effects of NF-kappaB on proliferation and migration of NSCs have been described. Moreover, NF-kappaB is constitutively active in most tumour cells described so far. Chronic inflammation is also known to initiate cancer. Thus, NF-kappaB might provide a novel mechanistic link between chronic inflammation, stem cells and cancer. This review discusses the apparently ambivalent role of NF-kappaB: physiological maintenance and repair of the brain via NSCs, and a potential role in tumour initiation. Furthermore, it reveals a possible mechanism of brain tumour formation based on inflammation and NF-kappaB activity in NSCs.

N-myc is an essential downstream effector of Shh signaling during both normal and neoplastic cerebellar growth

We examined the genetic requirements for the Myc family of oncogenes in normal Sonic hedgehog (Shh)-mediated cerebellar granule neuronal precursor (GNP) expansion and in Shh pathway-induced medulloblastoma formation. In GNP-enriched cultures derived from N-myc(Fl/Fl) and c-myc(Fl/Fl) mice, disruption of N-myc, but not c-myc, inhibited the proliferative response to Shh. Conditional deletion of c-myc revealed that, although it is necessary for the general regulation of brain growth, it is less important for cerebellar development and GNP expansion than N-myc. In vivo analysis of compound mutants carrying the conditional N-myc null and the activated Smoothened (ND2:SmoA1) alleles showed, that although granule cells expressing the ND2:SmoA1 transgene are present in the N-myc null cerebellum, no hyperproliferation or tumor formation was detected. Taken together, these findings provide in vivo evidence that N-myc acts downstream of Shh/Smo signaling during GNP proliferation and that N-myc is required for medulloblastoma genesis even in the presence of constitutively active signaling from the Shh pathway.

Prospects for neural stem cell-based therapies for neurological diseases

Neural stem and progenitor cells have great potential for the treatment of neurological disorders. However, many obstacles remain to translate this field to the patient's bedside, including rationales for using neural stem cells in individual neurological disorders; the challenges of neural stem cell biology; and the caveats of current strategies of isolation and culturing neural precursors. Addressing these challenges is critical for the translation of neural stem cell biology to the clinic. Recent work using neural stem cells has yielded novel biologic concepts such as the importance of the reciprocal interaction between neural stem cells and the neurodegenerative environment. The prospect of using transplants of neural stem cells and progenitors to treat neurological diseases requires a better understanding of the molecular mechanisms of both neural stem cell behavior in experimental models and the intrinsic repair capacity of the injured brain.

Notch signaling enhances nestin expression in gliomas

Recent findings suggest that Notch signaling is active in brain tumors and stem cells, and that stem cells or cells with progenitor characteristics contribute to brain tumor formation. These stem cells are marked by expression of several markers, including nestin, an intermediate filament protein. We have studied how the Notch signaling pathway affects nestin expression in brain tumors. We find that Notch receptors and ligands are expressed in vitro and in human samples of glioblastomas, the highest grade of malignant gliomas. In culture, Notch activity activates the nestin promoter. Activation of the Notch pathway also occurs in a glioblastoma multiforme mouse model induced by Kras, with translational regulation playing a role in Notch expression. Combined activation of Notch and Kras in wild-type nestin-expressing cells leads to their expansion within the subventricular zone and retention of proliferation and nestin expression. However, activation of Notch alone is unable to induce this cellular expansion. These data suggest that Notch may have a contributing role in the stem-like character of glioma cells.

Sonic hedgehog acts at multiple stages during pancreatic tumorigenesis

Activation of sonic hedgehog (Shh) signaling occurs in the majority of pancreatic ductal adenocarcinomas. Here we investigate the mechanisms by which Shh contributes to pancreatic tumorigenesis. We find that Shh expression enhances proliferation of pancreatic duct epithelial cells, potentially through the transcriptional regulation of the cell cycle regulators cyclin D1 and p21. We further show that Shh protects pancreatic duct epithelial cells from apoptosis through the activation of phosphatidylinositol 3-kinase signaling and the stabilization of Bcl-2 and Bcl-X(L). Significantly, Shh also cooperates with activated K-Ras to promote pancreatic tumor development. Finally, Shh signaling enhances K-Ras-induced pancreatic tumorigenesis by reducing the dependence of tumor cells on the sustained activation of the MAPK and phosphatidylinositol 3-kinase/Akt/mTOR signaling pathways. Thus, our data suggest that Shh signaling contributes to tumor initiation in the pancreas through at least two mechanisms and additionally enhances tumor cell resistance to therapeutic intervention. Collectively, our findings demonstrate crucial roles for Shh signaling in multiple stages of pancreatic carcinogenesis.

Glioma oncogenesis and animal models of glioma formation

Recent advances in animal models have improved our understanding of the pathway abnormalities driving glioma growth. This article reviews key molecular abnormalities that have been modeled in mice, and describes major tumor modeling techniques along with examples of astrocytoma and oligodendroglioma models. Animal models are important not only for the testing of novel therapeutics but also as a means to understand the molecular explanations for treatment success and failure in humans.

N-Myc and the cyclin-dependent kinase inhibitors p18Ink4c and p27Kip1 coordinately regulate cerebellar development

Conditional N-Myc deletion limits the proliferation of granule neuron progenitors (GNPs), perturbs foliation, and leads to reduced cerebellar mass. We show that c-Myc mRNA levels increase in N-Myc-null GNPs and that simultaneous deletion of both c- and N-Myc exacerbates defective cerebellar development. Moreover, N-Myc loss has been shown to trigger the precocious expression of two cyclin-dependent kinase inhibitors, Kip1 and Ink4c, in the cerebellar primordium. We now further demonstrate that the engineered disruption of the Kip1 and Ink4c genes in N-Myc-null cerebella partially rescues GNP cell proliferation and cerebellar foliation. These results provide definitive genetic evidence that expression of N-Myc and concomitant down-regulation of Ink4c and Kip1 contribute to the proper development of the cerebellum.

The neurobiology of neurooncology

The histological classification of brain tumors currently is based on the morphological appearance and protein expression patterns that reflect specific cell types within the central nervous system. Recent studies have suggested that the cells of origin for brain tumors may persist in the fully formed tumors, and that these "cancer stem cells" might represent the relevant cellular targets for anticancer therapy. In this regard, insights into the developmental neurobiology of brain tumors has significant impact on our understanding of the molecular and cellular pathogenesis of these devastating cancers, as well as the development of new strategies for treating brain tumors.

Abnormal expression of REST/NRSF and Myc in neural stem/progenitor cells causes cerebellar tumors by blocking neuronal differentiation

Medulloblastoma, one of the most malignant brain tumors in children, is thought to arise from undifferentiated neural stem/progenitor cells (NSCs) present in the external granule layer of the cerebellum. However, the mechanism of tumorigenesis remains unknown for the majority of medulloblastomas. In this study, we found that many human medulloblastomas express significantly elevated levels of both myc oncogenes, regulators of neural progenitor proliferation, and REST/NRSF, a transcriptional repressor of neuronal differentiation genes. Previous studies have shown that neither c-Myc nor REST/NRSF alone could cause tumor formation. To determine whether c-Myc and REST/NRSF act together to cause medulloblastomas, we used a previously established cell line derived from external granule layer stem cells transduced with activated c-myc (NSC-M). These immortalized NSCs were able to differentiate into neurons in vitro. In contrast, when the cells were engineered to express a doxycycline-regulated REST/NRSF transgene (NSC-M-R), they no longer underwent terminal neuronal differentiation in vitro. When injected into intracranial locations in mice, the NSC-M cells did not form tumors either in the cerebellum or in the cerebral cortex. In contrast, the NSC-M-R cells did produce tumors in the cerebellum, the site of human medulloblastoma formation, but not when injected into the cerebral cortex. Furthermore, the NSC-M-R tumors were blocked from terminal neuronal differentiation. In addition, countering REST/NRSF function blocked the tumorigenic potential of NSC-M-R cells. To our knowledge, this is the first study in which abnormal expression of a sequence-specific DNA-binding transcriptional repressor has been shown to contribute directly to brain tumor formation. Our findings indicate that abnormal expression of REST/NRSF and Myc in NSCs causes cerebellum-specific tumors by blocking neuronal differentiation and thus maintaining the "stemness" of these cells. Furthermore, these results suggest that such a mechanism plays a role in the formation of human medulloblastoma.

In vitro identification and functional characterization of glial precursor cells in human gliomas

Human gliomas including astrocytomas and oligodendrogliomas are defined as being composed of neoplastic astrocytes and oligodendrocytes respectively. Here, on the basis of in vitro functional assays, we show that gliomas contain a mixture of glial progenitor cells and their progeny. We have set up explant cultures from pilocytic astrocytomas, glioblastomas and oligodendrogliomas and studied antigens that characterize glial lineage, from the precursor cells (glial restricted precursors and oligodendrocyte-type2-astrocyte/oligodendrocyte precursor cells expressing the A2B5 ganglioside) to the differentiated cells (oligodendrocyte and type-1 and type-2 astrocytes). All tumoral explants contain A2B5+ cells and can generate migrating cells with distinctive functional properties according to glioma subtypes. In pilocytic astrocytomas, very few migrating cells are dividing and can differentiate in type-2 astrocytes or towards the oligodendrocyte lineage. In glioblastomas, most migrating cells are dividing, express A2B5 or glial fibrillary acid protein (GFAP) and can generate oligodendrocytes and type-1 and type-2 astrocytes in appropriate medium. Oligodendroglioma explants are made by actively dividing glial precursor cells expressing A2B5 or PSA-NCAM. Only few cells can migrate and differentiation towards oligodendrocyte lineage does not occur. Isolated A2B5+ cells from both glioblastomas and oligodendrogliomas showed similar genetic alterations as the whole tumour. Therefore, pilocytic astrocytomas contain slowly dividing oligodendrocyte-type2-astrocyte/oligodendrocyte precursor cells in keeping with their benign behaviour whereas both glioblastomas and oligodendrogliomas contain neoplastic glial restricted precursor cells. In oligodendrogliomas, these cells are trapped in undifferentiated and proliferating state. The precursor cells properties present in gliomas give new insight into their histogenesis and open up new avenues for research in the field of gliomagenesis.

Morphing into cancer: the role of developmental signaling pathways in brain tumor formation

Morphogens play a critical role in most aspects of development, including expansion and patterning of the central nervous system. Activating germline mutations in components of the Hedgehog and Wnt pathways have provided evidence for the important roles morphogens play in the genesis of brain tumors such as cerebellar medulloblastoma. In addition, aberrant expression of transforming growth factor-beta (TGF-beta) superfamily members has been demonstrated to contribute to progression of malignant gliomas. This review summarizes our current knowledge about the roles of morphogens in central nervous system tumorigenesis.

Molecular mechanisms in gliomagenesis

Glioma, and in particular high-grade astrocytoma termed glioblastoma multiforme (GBM), is the most common primary tumor of the brain. Primarily because of its diffuse nature, there is no effective treatment for GBM, and relatively little is known about the processes by which it develops. Therefore, in order to design novel therapies and treatments for GBM, research has recently intensified to identify the cellular and molecular mechanisms leading to GBM formation. Modeling of astrocytomas by genetic manipulation of mice suggests that deregulation of the pathways that control gliogenesis during normal brain development, such as the differentiation of neural stem cells (NSCs) into astrocytes, might contribute to GBM formation. These pathways include growth factor-induced signal transduction routes and processes that control cell cycle progression, such as the p16-CDK4-RB and the ARF-MDM2-p53 pathways. The expression of several of the components of these signaling cascades has been found altered in GBM, and recent data indicate that combinations of mutations in these pathways may contribute to GBM formation, although the exact mechanisms are still to be uncovered. Use of novel techniques including large-scale genomics and proteomics in combination with relevant mouse models will most likely provide novel insights into the molecular mechanisms underlying glioma formation and will hopefully lead to development of treatment modalities for GBM.

Immunohistochemical analysis of SOX6 expression in human brain tumors

We previously demonstrated that the developmentally regulated gene, SOX6, is strongly expressed in glioma cells and in the fetal brain, but only faintly in the normal adult brain. Recent studies have indicated that brain tumor cells may share antigens, signaling systems, and behavior with neural stem/progenitor cells. To test the validity of this proposition, we analyzed the expression of SOX6 in various human central nervous system (CNS) tumors. Immunohistochemical analysis revealed that astrocytic and oligodendroglial tumors expressed SOX6; neuronal-glial cell tumors (central neurocytoma) and embryonal tumors (medulloblastoma), which arise from multipotential stem cell precursors, also showed a high intensity of SOX6 staining. In contrast, ependymal tumors (ependymoma and subependymoma), meningioma, and schwannoma, which are all well differentiated tumors, showed either no staining or only faint staining for SOX6. These results suggest that SOX6 may be expressed in bipotential or multipotential cells capable of neuronal and glial differentiation, but not in fully differentiated cells. SOX6 may be a useful marker for the diagnosis of tumors arising from immature bipotential cells that may differentiate into neuronal and glial cells.

The absence of p53 promotes metastasis in a novel somatic mouse model for hepatocellular carcinoma

We have generated a mouse model for hepatocellular carcinoma using somatic delivery of oncogene-bearing avian retroviral vectors to the liver cells of mice expressing the viral receptor TVA under the control of the albumin gene promoter (Alb-TVA mice). Viruses encoding mouse polyoma virus middle T antigen (PyMT) induced tumors, which can be visualized with magnetic resonance imaging, in 65% of TVA-positive animals. While these tumors can exceed 10 mm in diameter, they do not invade locally or metastasize to the lungs. Delivery of PyMT-expressing viruses to Alb-TVA mice lacking an intact p53 gene does not increase tumor incidence. However, the resulting tumors are poorly differentiated, invasive, and metastatic to the lungs. Gene expression microarrays identified over 100 genes that are differentially expressed between tumors found in p53 wild-type and p53 null mice. Some of these genes, such as cathepsin E and Igf2, have been previously implicated in tumor cell migration and invasion. Tumors induced in p53 null, TVA transgenic mice by PyMT mutants with changes in specific tyrosine residues fail to form metastases, indicating that metastasis is dependent on both the oncogene and the absence of p53.

The c-myc and PyMT oncogenes induce different tumor types in a somatic mouse model for pancreatic cancer

We have generated a mouse model for pancreatic cancer through the somatic delivery of oncogene-bearing avian retroviruses to mice that express TVA, the receptor for avian leukosis sarcoma virus subgroup A (ALSV-A), under the control of the elastase promoter. Delivery of ALSV-A-based RCAS vectors encoding either mouse polyoma virus middle T antigen (PyMT) or c-Myc to elastase-tv-a transgenic, Ink4a/Arf null mice induced the formation of pancreatic tumors. RCAS-PyMT induced pancreatic tumors with the histologic features of acinar or ductal carcinomas. The induced pancreatic lesions express Pdx1, a marker for pancreas progenitor cells, and many tumors express markers for both exocrine and endocrine cell lineages, suggesting that the tumors may be derived from progenitor cells. In contrast, RCAS-c-myc induced endocrine tumors exclusively, as determined by histology and detection of differentiation markers. Thus, specific oncogenes can induce the formation of different pancreatic tumor types in a single transgenic line, most likely from one or more types of multipotential progenitor cells. Our model appears to be useful for elucidating the genetic alterations, target cells, and signaling pathways that are important in the genesis of different types of pancreatic cancer.

c-Myc enhances sonic hedgehog-induced medulloblastoma formation from nestin-expressing neural progenitors in mice

Medulloblastomas are malignant brain tumors that arise in the cerebella of children. The presumed cells-of-origin are undifferentiated precursors of granule neurons that occupy the external granule layer (EGL) of the developing cerebellum. The overexpression of proteins that normally stimulate proliferation of neural progenitor cells may initiate medulloblastoma formation. Two known mitogens for neural progenitors are the c-Myc oncoprotein and Sonic hedgehog (Shh), a crucial determinant of embryonic pattern formation in the central nervous system. We modeled the ability of c-Myc and Shh to induce medulloblastoma in mice using the RCAS/tv-a system, which allows postnatal gene transfer and expression in a cell type-specific manner. We targeted the expression of Shh and c-Myc to nestin-expressing neural progenitor cells by injecting replication-competent ALV splice acceptor (RCAS) vectors into the cerebella of newborn mice. Following injection with RCAS-Shh alone, 3/32 (9%) mice developed medulloblastomas and 5/32 showed multifocal hyperproliferation of the EGL, possibly a precursor stage of medulloblastoma. Following injection with RCAS-Shh plus RCAS-Myc, 9/39 (23%) mice developed medulloblastomas. We conclude that nestin-expressing neural progenitors, present in the cerebellum at birth, can act as the cells-of-origin for medulloblastoma, and that c-Myc cooperates with Shh to enhance tumorigenicity.

Complementation of melanocyte development in SOX10 mutant neural crest using lineage-directed gene transfer

An in vitro gene complementation approach has been developed to dissect gene function and regulation in neural crest (NC) development and disease. The approach uses the avian RCAS virus to express genes in NC cells derived from transgenic mice expressing the RCAS receptor TVA, under the control of defined promoter elements. Constructs for creating TVA transgenic mice were developed using site-specific recombination GATEWAY (GW), compatible vectors that can also be used to facilitate analysis of genomic fragments for transcriptional regulatory elements. By using these GW vectors to facilitate cloning, transgenic mouse lines were generated that express TVA in SOX10-expressing NC stem cells under the control of the Pax3 promoter. The Pax3-tv-a transgene was bred onto a Sox10-deficient background, and the feasibility of complementing genetic NC defects was demonstrated by infecting the Pax3-tv-a cells with an RCAS-Sox10 expression virus, thereby rescuing melanocyte development of Sox10-deficient NC cells. This system will be useful for assessing genetic hierarchies in NC development. Developmental Dynamics 229:54-62, 2004.

Expression of stabilized beta-catenin in differentiated neurons of transgenic mice does not result in tumor formation

BACKGROUND

Medulloblastomas, embryonal tumors arising in the cerebellum, commonly contain mutations that activate Wnt signaling. To determine whether increased Wnt signaling in the adult CNS is sufficient to induce tumor formation, we created transgenic mice expressing either wild-type or activated beta-catenin in the brain.

METHODS

Wild-type and mutant human beta-catenin transgenes were expressed under the control of a murine PrP promoter fragment that drives high level postnatal expression in the CNS. Mutant beta-catenin was stabilized by a serine to phenylalanine alteration in codon 37.

RESULTS

Expression of the mutant transgene resulted in an approximately two-fold increase in beta-catenin protein levels in the cortex and cerebellum of adult animals. Immunohistochemical analysis revealed nuclear beta-catenin in hippocampal, cortical and cerebellar neurons of transgenic animals but not in non-transgenic controls. Tail kinking was observed in some transgenic animals, but no CNS malformations or tumors were detected.

CONCLUSIONS

No tumors or morphologic alterations were detected in the brains of transgenic mice expressing stabilized beta-catenin, suggesting that postnatal Wnt signaling in differentiated neurons may not be sufficient to induce CNS tumorigenesis.