Expression of the c-Myc protein in childhood medulloblastoma

Dependence of human cell survival and proliferation on the CASP3 prodomain

Mechanisms that regulate cell survival and proliferation are important for both the development and homeostasis of normal tissue, and as well as for the emergence and expansion of malignant cell populations. Caspase-3 (CASP3) has long been recognized for its proteolytic role in orchestrating cell death-initiated pathways and related processes; however, whether CASP3 has other functions in mammalian cells that do not depend on its known catalytic activity have remained unknown. To investigate this possibility, we examined the biological and molecular consequences of reducing CASP3 levels in normal and transformed human cells using lentiviral-mediated short hairpin-based knockdown experiments in combination with approaches designed to test the potential rescue capability of different components of the CASP3 protein. The results showed that a ≥50% reduction in CASP3 levels rapidly and consistently arrested cell cycle progression and survival in all cell types tested. Mass spectrometry-based proteomic analyses and more specific flow cytometric measurements strongly implicated CASP3 as playing an essential role in regulating intracellular protein aggregate clearance. Intriguingly, the rescue experiments utilizing different forms of the CASP3 protein showed its prosurvival function and effective removal of protein aggregates did not require its well-known catalytic capability, and pinpointed the N-terminal prodomain of CASP3 as the exclusive component needed in a diversity of human cell types. These findings identify a new mechanism that regulates human cell survival and proliferation and thus expands the complexity of how these processes can be controlled. The graphical abstract illustrates the critical role of CASP3 for sustained proliferation and survival of human cells through the clearance of protein aggregates.

Metabolism-based targeting of MYC via MPC-SOD2 axis-mediated oxidation promotes cellular differentiation in group 3 medulloblastoma

Group 3 medulloblastoma (G3 MB) carries the worst prognosis of all MB subgroups. MYC oncoprotein is elevated in G3 MB tumors; however, the mechanisms that support MYC abundance remain unclear. Using metabolic and mechanistic profiling, we pinpoint a role for mitochondrial metabolism in regulating MYC. Complex-I inhibition decreases MYC abundance in G3 MB, attenuates the expression of MYC-downstream targets, induces differentiation, and prolongs male animal survival. Mechanistically, complex-I inhibition increases inactivating acetylation of antioxidant enzyme SOD2 at K68 and K122, triggering the accumulation of mitochondrial reactive oxygen species that promotes MYC oxidation and degradation in a mitochondrial pyruvate carrier (MPC)-dependent manner. MPC inhibition blocks the acetylation of SOD2 and oxidation of MYC, restoring MYC abundance and self-renewal capacity in G3 MB cells following complex-I inhibition. Identification of this MPC-SOD2 signaling axis reveals a role for metabolism in regulating MYC protein abundance that has clinical implications for treating G3 MB.

Reciprocal Induction of MDM2 and MYCN in Neural and Neuroendocrine Cancers

MYC family oncoproteins MYC, MYCN, and MYCL are deregulated in diverse cancers and via diverse mechanisms. Recent studies established a novel form of MYCN regulation in MYCN-overexpressing retinoblastoma and neuroblastoma cells in which the MDM2 oncoprotein promotes MYCN translation and MYCN-dependent proliferation via a p53-independent mechanism. However, it is unclear if MDM2 also promotes expression of other MYC family members and has similar effects in other cancers. Conversely, MYCN has been shown to induce MDM2 expression in neuroblastoma cells, yet it is unclear if MYC shares this ability, if MYC family proteins upregulate MDM2 in other malignancies, and if this regulation occurs during tumorigenesis as well as in cancer cell lines. Here, we report that intrinsically high MDM2 expression is required for high-level expression of MYCN, but not for expression of MYC, in retinoblastoma, neuroblastoma, small cell lung cancer, and medulloblastoma cells. Conversely, ectopic overexpression of MYC as well as MYCN induced high-level MDM2 expression and gave rise to rapidly proliferating and MDM2-dependent cone-precursor-derived masses in a cultured retinoblastoma genesis model. These findings reveal a highly specific collaboration between the MDM2 and MYCN oncoproteins and demonstrate the origin of their oncogenic positive feedback circuit within a normal neuronal tissue.

Successful Use of Dose Dense Neoadjuvant Chemotherapy and Sodium Valproate with Minimal Toxicity in an Infant with Medulloblastoma in Extremely Poor General Condition

BACKGROUND

Medulloblastoma is the most common malignant brain tumor in children. Infants are in the high-risk category. Complete surgical resection is the single most important determinant of prognosis and survival in nonmetastatic disease. Infants with large primaries after incomplete resection/biopsy and poor general condition have bad prognosis. They are considered poor candidates for intensive chemotherapy involving high dose methotrexate/autologous stem cell transplantation as they are often unable to tolerate these aggressive regimens.

CASE DESCRIPTION

The patient, withinfantile medulloblastoma, was supposed to have complete resection but only a biopsy could be attempted because of increased tumor vascularity. He was in very poor general condition after surgery and his parents declined aggressive chemotherapy and shunt surgery. He was given dose dense neo-adjuvant chemotherapy along with the histone deactylase inhibitor valproate for 5 cycles, with minimal toxicity, after which the tumor was resected. The examination of the resected specimen revealed a complete pathologic response. He then received a total of 18 cycles of chemotherapy and valproate to complete 1 year of systemic treatment. The child is now 6.5 years of age, disease-free, without evidence of any neurocognitive or developmental abnormalities.

CONCLUSIONS

We suggest that the role of neoadjuvant chemotherapy should be explored in patients with infantile medulloblastoma in whom upfront complete resection is not possible, considering the gratifying results obtained in our case.

Neuroblastoma of undifferentiated subtype, prognostic significance of prominent nucleolar formation, and MYC/MYCN protein expression: a report from the Children's Oncology Group

BACKGROUND

This study sought to investigate biological/clinicopathological characteristics of neuroblastoma, undifferentiated subtype (NBUD).

METHODS

This study examined 157 NBUD cases filed at the Children's Oncology Group Neuroblastoma Pathology Reference Laboratory, and survival rates of the patients were analyzed with known prognostic factors. Immunostainings for MYCN and MYC protein were performed on 68 tumors.

RESULTS

NBUD cases had a poor prognosis (48.4% ± 5.0% 3-year event-free survival [EFS]; 56.5% ± 5.0% overall survival [OS]), and were often associated with high mitosis-karyorrhexis index (MKI, 65%), prominent nucleoli (PN, 83%), ≥ 18 months of age (75%), MYCN amplification (MYCN-A, 83%), diploid pattern (63%), and 1pLOH (loss of heterozygosity (72%). However, these prognostic indicators, except for MYCN status, had no significant impact on survival. Surprisingly, EFS for patients with MYCN-A tumors (53.4% ± 5.6%) was significantly better (P=.0248) than for patients with MYCN-nonamplified (MYCN-NA) tumors (31.7% ± 11.7%), with MYCN-NA and PN (+) tumors having the worst prognosis (9.3%  ± 8.8%, P=.0045). Immunohistochemically, MYCN expression was found in 42 of 48 MYCN-A tumors. In contrast, MYC expression was almost exclusively found in the MYCN-NA tumors (9 of 20) especially when they had PN (8 of 11). Those patients with only MYC-positive tumors had the worst EFS (N=8, 12.5% ± 11.7%) compared with only MYCN-positive (N=39, 49.9% ± 17.7%) and both negative tumors (N=15, 70.0% ± 17.1%) (P= .0029). High MKI was often found in only MYCN-positive (30 of 38) but rarely in only MYC-positive (2 of 8) tumors.

CONCLUSIONS

NBUD represents a unique subtype of neuroblastoma associated with a poor prognosis. In this subtype, MYC protein expression may be a new prognostic factor indicating more aggressive clinical behavior than MYCN amplification and subsequent MYCN protein expression.

The molecular classification of medulloblastoma: driving the next generation clinical trials

PURPOSE OF REVIEW

Most children diagnosed with cancer today are expected to be cured. Medulloblastoma, the most common pediatric malignant brain tumor, is an example of a disease that has benefitted from advances in diagnostic imaging, surgical techniques, radiation therapy and combination chemotherapy over the past decades. It was an incurable disease 50 years ago, but approximately 70% of children with medulloblastoma are now cured of their disease. However, the pace of increasing the cure rate has slowed over the past 2 decades, and we have likely reached the maximal benefit that can be achieved with cytotoxic therapy and clinical risk stratification. Long-term toxicity of therapy also remains significant. To increase cure rates and decrease long-term toxicity, there is great interest in incorporating biologic 'targeted' therapy into treatment of medulloblastoma, but this will require a paradigm shift in how we classify and study disease.

RECENT FINDINGS

Using genome-based high-throughput analytic techniques, several groups have independently reported methods of molecular classification of medulloblastoma within the past year. This has resulted in a working consensus to view medulloblastoma as four molecular subtypes, including wingless-type murine mammary tumor virus integration site (WNT) pathway subtype, Sonic Hedgehog pathway subtype and two less well defined subtypes (groups C and D).

SUMMARY

Novel classification and risk stratification based on biologic subtypes of disease will form the basis of further study in medulloblastoma and identify specific subtypes that warrant greater research focus.

c-myc and N-myc promote active stem cell metabolism and cycling as architects of the developing brain

myc genes are associated with a wide variety of human cancers including most types of nervous system tumors. While the mechanisms by which myc overexpression causes tumorigenesis are multifaceted and have yet to be clearly elucidated, they are at least in part related to endogenous myc function in normal cells. Knockout (KO) of either c-myc or N-myc genes in neural stem and precursor cells (NSC) driven by nestin-cre impairs mouse brain growth and mutation of N-myc also causes microcephaly in humans in Feingold Syndrome. To further define myc function in NSC and nervous system development, we created a double KO (DKO) for c- and N-myc using nestin-cre. The DKO mice display profoundly impaired overall brain growth associated with decreased cell cycling and migration of NSC, which are strikingly decreased in number. The DKO brain also exhibits specific changes in gene expression including downregulation of genes involved in protein and nucleotide metabolism, mitosis, and chromatin structure as well as upregulation of genes associated with differentiation. Together these data support a model of nervous system tumorigenesis in which excess myc aberrantly locks in a developmentally active chromatin state characterized by overactive cell cycling, and metabolism as well as blocked differentiation.

c- and N-myc regulate neural precursor cell fate, cell cycle, and metabolism to direct cerebellar development

Separate murine knockout (KO) of either c- or N-myc genes in neural stem and precursor cells (NSC) driven by nestin-cre causes microcephaly. The cerebellum is particularly affected in the N-myc KO, leading to a strong reduction in cerebellar granule neural progenitors (CGNP) and mature granule neurons. In humans, mutation of N-myc also causes microcephaly in Feingold Syndrome. We created a double KO (DKO) of c- and N-myc using nestin-cre, which strongly impairs brain growth, particularly that of the cerebellum. Granule neurons were almost absent from the Myc DKO cerebellum, and other cell types were relatively overrepresented, including astroglia, oligodendrocytes, and Purkinje neurons. These findings are indicative of a profound disruption of cell fate of cerebellar stem and precursors. DKO Purkinje neurons were strikingly lacking in normal arborization. Inhibitory neurons were ectopic and exhibited very abnormal GAD67 staining patterns. Also consistent with altered cell fate, the adult DKO cerebellum still retained a residual external germinal layer (EGL). CGNP in the DKO EGL were almost uniformly NeuN and p27KIP1 positive as well as negative for Math1 and BrdU at the peak of normal cerebellar proliferation at P6. The presence of some mitotic CGNP in the absence of S phase cells suggests a possible arrest in M phase. CGNP and NSC metabolism also was affected by loss of Myc as DKO cells exhibited weak nucleolin staining. Together these findings indicate that c- and N-Myc direct cerebellar development by maintaining CGNP and NSC populations through inhibiting differentiation as well as directing rapid cell cycling and active cellular metabolism.

Low-level copy number changes of MYC genes have a prognostic impact in medulloblastoma

High-level amplifications of MYC genes are associated with poor outcomes in childhood medulloblastoma (MB). However, the occurrence of MYCN and MYCC copy number increases below the intense amplification pattern is rarely reported, and its clinical impact has not yet been determined. Here, we describe this phenomenon and its prognostic significance in a cohort of 29 MB patients. Using interphase fluorescence in situ hybridization (I-FISH), low-level copy number alterations, i.e. gain of MYCN, were shown in 5/27 (19%) samples, whereas amplification was revealed in only 1/27 (4%) samples. MYCC gain was revealed in 6/29 (21%) MB, while amplification was disclosed in only 2/29 (7%). Hyperploidy and co-incidence of gains in both MYC loci were frequently observed in samples with copy number aberrations. Survival analysis has clearly shown that MYC copy number increases are associated with lowered event-free survival and overall survival in MB. In the case of MYCN, this negative correlation was statistically significant. We conclude that limited numerical alterations in loci 2p24 (MYCN) and 8q24 (MYCC), as assessed by I-FISH, are present in MB with a higher frequency than high-level amplifications. Poor prognoses were observed in patients with copy number increases in MYC genes. Our data illustrate the importance of further investigations in multicenter trials to better refine the emerging genomic-based prognostic stratification in MB.

Disabling c-Myc in childhood medulloblastoma and atypical teratoid/rhabdoid tumor cells by the potent G-quadruplex interactive agent S2T1-6OTD

We investigated here the effects of S2T1-6OTD, a novel telomestatin derivative that is synthesized to target G-quadruplex-forming DNA sequences, on a representative panel of human medulloblastoma (MB) and atypical teratoid/rhabdoid (AT/RT) childhood brain cancer cell lines. S2T1-6OTD proved to be a potent c-Myc inhibitor through its high-affinity physical interaction with the G-quadruplex structure in the c-Myc promoter. Treatment with S2T1-6OTD reduced the mRNA and protein expressions of c-Myc and hTERT, which is transcriptionally regulated by c-Myc, and decreased the activities of both genes. In remarkable contrast to control cells, short-term (72-hour) treatment with S2T1-6OTD resulted in a dose- and time-dependent antiproliferative effect in all MB and AT/RT brain tumor cell lines tested (IC(50), 0.25-0.39 micromol/L). Under conditions where inhibition of both proliferation and c-Myc activity was observed, S2T1-6OTD treatment decreased the protein expression of the cell cycle activator cyclin-dependent kinase 2 and induced cell cycle arrest. Long-term treatment (5 weeks) with nontoxic concentrations of S2T1-6OTD resulted in a time-dependent (mainly c-Myc-dependent) telomere shortening. This was accompanied by cell growth arrest starting on day 28 followed by cell senescence and induction of apoptosis on day 35 in all of the five cell lines investigated. On in vivo animal testing, S2T1-6OTD may well represent a novel therapeutic strategy for childhood brain tumors.

Medulloblastomas: a correlative study of MIB-1 proliferation index along with expression of c-Myc, ERBB2, and anti-apoptotic proteins along with histological typing and clinical outcome

BACKGROUND

Medulloblastoma (MB) is the most common pediatric brain tumor. It is however rare in adults. The genetic and protein expression profile of medulloblastoma is complex, which is worthwhile in terms of prognostication and development or selection of targeted therapy.

AIMS AND OBJECTIVES

The aims and objectives to correlate the MIB-1 proliferation index and protein expression profiles of c-Myc, ERBB2, and anti-apoptotic proteins (Bcl2 and Bcl-xL) in tumor cells with histological subtypes and clinical outcome.

METHODS AND MATERIAL

In 50 cases, histopathological subtyping was done, and protein expression profiling by immunohistochemical technique was performed by stains for MIB-1, Bcl2, Bcl-xL, c-Myc, and ERBB2 in 30 cases. The findings were correlated with histological types and patient's average follow-up data.

RESULTS

Histological subtypes were similar to that described in literatures. The average expression of Bcl2, Bcl-xL, MIB-1, c-Myc, and ERBB2 were as follows: 50.38%, 38.18%, 59.03%, 46.16%, and 59.62%, respectively. Bcl2 expression showed statistically significant correlation with progress-free survival (PFS) [p = 0.046], while ERBB2 and MIB-1 showed a trend of higher expression in progressive disease. The protein expression pattern did not correlate with histological subtypes.

CONCLUSION

Though Bcl-2, ERBB2, and MIB-1 LI came out to be potential markers of aggressive behavior, c-Myc did not correlate with PFS in MB.

A DNA transposon-based approach to validate oncogenic mutations in the mouse

Large-scale cancer genome projects will soon be able to sequence many cancer genomes to comprehensively identify genetic changes in human cancer. Genome-wide association studies have also identified putative cancer associated loci. Functional validation of these genetic mutations in vivo is becoming a challenge. We describe here a DNA transposon-based platform that permits us to explore the oncogenic potential of genetic mutations in the mouse. Briefly, promoter-less human cancer gene cDNAs were first cloned into Sleeping Beauty (SB) transposons. DNA transposition in the mouse that carried both the transposons and the SB transposase made it possible for the cDNAs to be expressed from an appropriate endogenous genomic locus and in the relevant cell types for tumor development. Consequently, these mice developed a broad spectrum of tumors at very early postnatal stages. This technology thus complements the large-scale cancer genome projects.

Medulloblastoma: what is the role of molecular genetics?

Among pediatric malignancies, medulloblastoma (MB) is one of the most common malignant tumors of the CNS. In the past few years, thanks to a multidisciplinary approach including surgery, chemo- and radiation therapy, survival has significantly improved. Despite that, a third of patients still have a low chance of being cured and long-term survivors experience severe treatment-related sequelae. MBs are usually classified according to a clinical risk stratification, based on histological features, age at diagnosis, extent of tumor resection and presence or absence of metastases. However, these clinical variables have recently been reported to be poor for defining risk-related disease. Retrospective studies have identified histological or biological factors that have distinct roles in prognosis. As several pathways have been discovered to be involved in MB pathogenesis, they should be taken into account to more accurately stratify patients and their treatment and to develop innovative therapies.

Biological background of pediatric medulloblastoma and ependymoma: a review from a translational research perspective

Survival rates of pediatric brain tumor patients have significantly improved over the years due to developments in diagnostic techniques, neurosurgery, chemotherapy, radiotherapy, and supportive care. However, brain tumors are still an important cause of cancer-related deaths in children. Prognosis is still highly dependent on clinical characteristics, such as the age of the patient, tumor type, stage, and localization, but increased knowledge about the genetic and biological features of these tumors is being obtained and might be useful to further improve outcome for these patients. It has become clear that the deregulation of signaling pathways essential in brain development, for example, sonic hedgehog (SHH), Wnt, and Notch pathways, plays an important role in pathogenesis and biological behavior, especially for medulloblastomas. More recently, data have become available about the cells of origin of brain tumors and the possible existence of brain tumor stem cells. Newly developed array-based techniques for studying gene expression, protein expression, copy number aberrations, and epigenetic events have led to the identification of other potentially important biological abnormalities in pediatric medulloblastomas and ependymomas.

Prognostic relevance of clinical and biological risk factors in childhood medulloblastoma: results of patients treated in the prospective multicenter trial HIT'91

PURPOSE

To identify better risk stratification systems in childhood medulloblastoma based on clinical factors and analysis of routinely processed formalin-fixed tumor material.

EXPERIMENTAL DESIGN

Formalin-fixed paraffin-embedded tumor samples from well-documented patients treated within the prospective randomized multicenter trial HIT'91 were analyzed for DNA amplification of c-myc and N-myc (n=133) and mRNA expression of c-myc and trkC (n=104; compared with human cerebellum) using validated methods of quantitative PCR and reverse transcription-PCR. Results were related to clinical data and outcome.

RESULTS

TrkC and c-myc mRNA expression were identified as independent prognostic factors by multivariate analysis. Three risk groups were identified. (a) Favorable risk group: all 8 patients (2 metastatic) with high trkC (>1x human cerebellum) and low c-myc mRNA expression (<or=1x human cerebellum) remained relapse-free [7-year event-free survival (EFS), 100%]. (b) Poor risk group: 10 of 15 patients with metastatic disease and high c-myc and low trkC mRNA expression relapsed (7-year EFS, 33%). (c) Intermediate risk group: the 7-year EFS of the remaining 78 patients was 65%. Among 47 M(0) stage patients, all 10 patients with high trkC mRNA expression remained relapse-free compared with 15 events in 37 patients with low trkC mRNA expression levels (7-year EFS, 100% versus 62%; P=0.056).

CONCLUSIONS

Whereas the collection of fresh-frozen tumor samples remains a major challenge in large clinical trials, routinely processed paraffin-embedded tissue samples can be used to quantitate the prognostic biological markers trkC and c-myc. On prospective validation of cutoff levels, this may lead to improved stratification of treatment for children with medulloblastoma.

Anti-proliferative activity of the quassinoid NBT-272 in childhood medulloblastoma cells

BACKGROUND

With current treatment strategies, nearly half of all medulloblastoma (MB) patients die from progressive tumors. Accordingly, the identification of novel therapeutic strategies remains a major goal. Deregulation of c-MYC is evident in numerous human cancers. In MB, over-expression of c-MYC has been shown to correlate with anaplasia and unfavorable prognosis. In neuroblastoma--an embryonal tumor with biological similarities to MB--the quassinoid NBT-272 has been demonstrated to inhibit cellular proliferation and to down-regulate c-MYC protein expression.

METHODS

To study MB cell responses to NBT-272 and their dependence on the level of c-MYC expression, DAOY (wild-type, empty vector transfected or c-MYC transfected), D341 (c-MYC amplification) and D425 (c-MYC amplification) human MB cells were used. The cells were treated with different concentrations of NBT-272 and the impact on cell proliferation, apoptosis and c-MYC expression was analyzed.

RESULTS

NBT-272 treatment resulted in a dose-dependent inhibition of cellular proliferation (IC50 in the range of 1.7-9.6 ng/ml) and in a dose-dependent increase in apoptotic cell death in all human MB cell lines tested. Treatment with NBT-272 resulted in up to 90% down-regulation of c-MYC protein, as demonstrated by Western blot analysis, and in a significant inhibition of c-MYC binding activity. Anti-proliferative effects were slightly more prominent in D341 and D425 human MB cells with c-MYC amplification and slightly more pronounced in c-MYC over-expressing DAOY cells compared to DAOY wild-type cells. Moreover, treatment of synchronized cells by NBT-272 induced a marked cell arrest at the G1/S boundary.

CONCLUSION

In human MB cells, NBT-272 treatment inhibits cellular proliferation at nanomolar concentrations, blocks cell cycle progression, induces apoptosis, and down-regulates the expression of the oncogene c-MYC. Thus, NBT-272 may represent a novel drug candidate to inhibit proliferation of human MB cells in vivo.

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.

Valproic acid induces growth arrest, apoptosis, and senescence in medulloblastomas by increasing histone hyperacetylation and regulating expression of p21Cip1, CDK4, and CMYC

Valproic acid is a well-tolerated anticonvulsant that has been identified recently as a histone deacetylase inhibitor. To evaluate the antitumor efficacy and mechanisms of action of valproic acid in medulloblastoma and supratentorial primitive neuroectodermal tumor (sPNET), which are among the most common malignant brain tumors in children with poor prognosis, two medulloblastoma (DAOY and D283-MED) and one sPNET (PFSK) cell lines were treated with valproic acid and evaluated with a panel of in vitro and in vivo assays. Our results showed that valproic acid, at clinically safe concentrations (0.6 and 1 mmol/L), induced potent growth inhibition, cell cycle arrest, apoptosis, senescence, and differentiation and suppressed colony-forming efficiency and tumorigenicity in a time- and dose-dependent manner. The medulloblastoma cell lines were more responsive than the sPNET cell line and can be induced to irreversible suppression of proliferation and significantly reduced tumorigenicity by 0.6 and 1 mmol/L valproic acid. Daily i.p. injection of valproic acid (400 mg/kg) for 28 days significantly inhibited the in vivo growth of DAOY and D283-MED s.c. xenografts in severe combined immunodeficient mice. With Western hybridization and real-time reverse transcription-PCR, we further showed that the antitumor activities of valproic acid correlated with induction of histone (H3 and H4) hyperacetylation, activation of p21, and suppression of TP53, CDK4, and CMYC expression. In conclusion, valproic acid possesses potent in vitro and in vivo antimedulloblastoma activities that correlated with induction of histone hyperacetylation and regulation of pathways critical for maintaining growth inhibition and cell cycle arrest. Therefore, valproic acid may represent a novel therapeutic option in medulloblastoma treatment.

Modeling medulloblastoma with genetically engineered mice

Medulloblastoma is a malignant tumor that arises in the cerebellum in children, presumably by transformation of granule neuron precursor cells. In vivo models of medulloblastoma in genetically engineered mice have shown that activation of signal transduction pathways that stimulate proliferation and inhibit differentiation of neural progenitor cells during cerebellar development initiate medulloblastoma formation. Activation of the Sonic hedgehog (Shh)/Patched signaling pathway in the postnatal cerebellum is sufficient to induce medulloblastoma in mice. Activation of the phosphatidylinositol 3-kinase (PI3K) signaling pathway by insulin-like growth factor-II, inactivation of the p53 tumor suppressor protein, loss of DNA damage repair mechanisms, and ectopic expression of Myc oncoproteins cooperate with Shh/Patched signaling to enhance tumor formation in mice. Ectopic expression of alpha and beta interferons in the developing brain also induces Shh-mediated medulloblastoma formation, suggesting a possible role for antiviral response in the genesis of medulloblastoma. By revealing which cell signaling proteins can initiate medulloblastoma formation, mouse models have enabled investigators to identify molecular targets for designing new therapies. Small-molecule inhibitors of the Shh/Patched and PI3K pathways are potential chemotherapeutic agents for patients with medulloblastoma.

Recent advances in embryonal tumours of the central nervous system

INTRODUCTION

Embryonal tumours of the central nervous system (CNS) are the commonest malignant paediatric brain tumours. This group includes medulloblastomas, supratentorial primitive neuroectodermal tumours, atypical teratoid/rhabdoid tumours, ependymoblastomas, and medulloepitheliomas. Earlier, all these tumours were grouped under a broad category of primitive neuroectodermal tumours (PNETs). However, the current WHO classification (2000) separates them into individual types based on significant progress in the understanding of their distinctive clinical, pathological, molecular genetic, histogenetic, and behavioural characteristics. Furthermore, advances in histopathology and molecular genetics have shown great promise for refining risk assessment in these tumours, especially medulloblastomas, thus providing a more accurate basis for tailoring therapies to individual patients. Correlation of histological changes with genetic events has also led to a new model of medulloblastoma tumorigenesis.

REVIEW

This review presents an updated comparative profile of these tumours, highlighting the clinical and biological relevance of the recent advances.

A Clinicobiological Model Predicting Survival in Medulloblastoma

PURPOSE

The purpose of this study was to determine the relative contributions of biological and clinical predictors of survival in patients with medulloblastoma (MB).

EXPERIMENTAL DESIGN

Clinical presentation and survival information were obtained for 119 patients who had undergone surgery for MB at the Hospital for Sick Children (Toronto, Ontario, Canada) between 1985 and 2001. A tissue microarray was constructed from the tumor samples. The arrays were assayed for immunohistochemical expression of MYC, p53, platelet-derived growth factor receptor-alpha, ErbB2, MIB-1, and TrkC and for apoptosis (terminal deoxynucleotidyl transferase-mediated nick end labeling). Both univariable and multivariable analyses were conducted to characterize the association between survival and both clinical and biological markers. For the strongest predictors of survival, a weighted predictive score was calculated based on their hazard ratios (HRs). The sum of these scores was then used to give an overall prediction of survival using a nomogram.

RESULTS

The four strongest predictors of survival in the final multivariable model were the presence of metastatic disease at presentation (HR, 2.02; P=0.01) and p53 (HR, 2.29; P=0.02), TrkC (HR, 0.65; P=0.14), and ErbB2 (HR, 1.51; P=0.21) immunopositivity. A linear prognostic index was derived, with coefficients equal to the logarithm of these HRs. The 5-year survival rate for patients at the 10th, 50th, and 90th percentiles of the score distribution was 80.0%, 71.0%, and 35.7%, respectively, with radiation therapy and 70.5%, 58.5%, and 20.0%, respectively, without radiation therapy.

CONCLUSIONS

In this study, we demonstrate an approach to combining both clinical and biological markers to quantify risk in MB patients. This provides further prognostic information than can be obtained when either clinical factors or biological markers are studied separately and establishes a framework for comparing prognostic markers in future clinical studies.

Identification of genes that are regulated transcriptionally by Myc in childhood tumors

BACKGROUND

Amplification of the N-myc oncogene is associated with adverse outcomes in the common childhood tumor, neuroblastoma. Because the transforming properties of Myc are related to its ability to modulate gene expression, the authors used cDNA microarrays to identify potential Myc target genes.

METHODS

Expression levels of 4608 genes were analyzed in a series of neuroblastoma cell lines. Identical analyses were performed in a panel of medulloblastoma cell lines to identify c-Myc targets and to determine the extent to which N-Myc targets and c-Myc targets were shared. Comparisons were made between cell lines with high levels versus low levels of Myc protein expression.

RESULTS

Array analyses yielded 121 genes with increased expression levels (>or= 1.65-fold) and 9 genes with decreased expression levels in N-Myc-expressing versus nonexpressing cell lines. Many of these were newly identified targets of biologic interest. Fifty percent of the N-Myc targets (60 of 121) were mutual c-Myc targets. A significant correlation between the level of N-myc and selected target gene expression was demonstrated independently in 27 neuroblastoma tumor samples and in an N-myc-inducible cell line system.

CONCLUSIONS

A number of diverse pathways are modulated by N-Myc in neuroblastoma. Although, overall, there was significant correlation between myc and target transcript expression among cohorts of tumors, great variability in levels of target expression was seen among individual tumor samples, and this biologic heterogeneity in the levels of target gene expression may offer insight into differences in the clinical behavior of neuroblastoma and may prove to be of prognostic significance in the future.

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.

Differential somatostatin receptor subtype expression in human normal pineal gland and pineal parenchymal tumors

Somatostatin is a potent antiproliferative signal in both tumoral and normal mammalian cells, and altered somatostatin receptor (sst) expression is associated with carcinogenesis in human tissues. In this study, two normal and three tumoral human pineal glands were analyzed using the reverse transcriptase-polymerase chain reaction (RT-PCR) for the presence of mRNA coding for the five different somatostatin receptors (sst1-sst5). Pineal parenchymal tumor (PPT) differentiation was confirmed by immunohistochemical detection of neuroendocrine markers (synaptophysin, neurofilaments, and chromogranin A). The presence of mRNA coding for c-myc, a proto-oncogene, and for tryptophan hydroxylase (TPOH), serotonin N-acetyltransferase (NAT), and hydroxyindole-O-methyltransferase (HIOMT), enzymes of the melatonin pathway, was also analyzed by RT-PCR. Only the tumoral tissues contained c-myc mRNA. All five tissues contained TPOH, NAT, and HIOMT mRNA, the levels of HIOMT mRNA being lower in PPT than in the normal pineal gland, suggesting that PPT retain the ability to synthesize melatonin. All tissues contained sst1, sst2, and sst3 transcripts, but not sst4, while small amounts of sst5 mRNA were only found in normal pineal glands. Real-time PCR, performed only with the most abundant subtpe sst2, evidenced an about sixfold higher level in in normal pineal glands. These results demonstrate the presence of somatostatin receptors in the human pineal gland, as described in other species, and point to a differential expression of the sst2 and sst5 subtypes associated with carcinogenesis.

I. Perinatal brain tumors: a review of 250 cases

Central nervous system tumors occur considerably less often in the fetus and neonate than in the older child. These tumors are not entirely the same as those found later in life. Their location, biologic behavior, response to therapy, and histologic types are different. Reports of 250 fetal and neonatal brain tumors were collected from the literature and studied for this review. The overall survival rate was 28%. The entire cranial cavity may be filled with tumor, and stillbirth is not uncommon. Macrocephaly was the most frequent presentation regardless of histology. Outcome is related to the size and location of the tumor, the histologic type, surgical resectability, and the condition of the infant at the time of diagnosis. Neonates with choroid plexus papillomas, gangliogliomas, and low-grade astrocytomas have the best prognosis, whereas those with teratomas and primitive neuroectodermal tumors have the worst prognosis.

The nucleophosmin-anaplastic lymphoma kinase fusion protein induces c-Myc expression in pediatric anaplastic large cell lymphomas

The majority of pediatric anaplastic large cell lymphomas (ALCLs) carry the t(2;5)(p23;q35) chromosomal translocation that juxtaposes the dimerization domain of nucleophosmin with anaplastic lymphoma kinase (ALK). The nucleophosmin-ALK fusion induces constitutive, ligand-independent activation of the ALK tyrosine kinase leading to aberrant activation of cellular signaling pathways. To study the early consequences of ectopic ALK activation, a GyrB-ALK fusion was constructed that allowed regulated dimerization with the addition of coumermycin. Expression of the fusion protein caused a coumermycin-dependent increase in cellular tyrosine phosphorylation and c-Myc immunoreactivity, which was paralleled by a rise in c-myc RNA. To assess the clinical relevance of this observation, c-Myc expression was determined in pediatric ALK-positive and -negative lymphomas. Co-expression of c-Myc and ALK was seen in tumor cells in 15 of 15 (100%) ALK-positive ALCL samples, whereas no expression of either ALK or c-Myc was seen in six of six cases of ALK-negative T-cell lymphoma. C-Myc may be a downstream target of ALK signaling and its expression a defining characteristic of ALK-positive ALCLs.

Platelet-derived growth factor-B and -C and active alpha-receptors in medulloblastoma cells

The malignant childhood brain tumor medulloblastoma belongs to the group of primitive neuroectodermal tumours (PNETs). Medulloblastomas are thought to arise from remnants of the transient external germinal layer in the cerebellum. Proliferation, differentiation, and motility of cells in the central nervous system are regulated by growth factors, e.g., platelet-derived growth factor (PDGF). Recently, it was shown that higher level of PDGF alpha-receptor expression is characteristic of metastatic medulloblastomas. We have investigated five medulloblastoma/PNET cell lines and found that the PDGF alpha-receptor is actively signalling in most of them, an activity most likely driven by endogenously produced PDGF-C. PDGF-C is normally present in cells of the developing external germinal layer and our results are consistent with the idea that medulloblastomas are derived from such cells undergoing early neuronal differentiation. Moreover, the expression of PDGF and its receptors was associated with neuronal characteristics, but not with high levels of c-myc expression in the medullablastoma cells.

Classifying the medulloblastoma: insights from morphology and molecular genetics

Significant advances in the treatment of the medulloblastoma (MB) have been made in the last 30 years, reducing mortality by 2-fold. Further improvements in the cure rate require an increased understanding of the biology of MBs, and this will translate into refinements in their classification. Scrutiny of the cytological variation found among MBs has recently led to the concept of the anaplastic MB, which overlaps the large-cell variant and appears to share its poor prognosis. In contrast, the MB with extensive nodularity, a distinctive nodular/desmoplastic variant occurring in infants, has a better outcome than most MBs in these young patients. Building on cytogenetic studies that have drawn attention to abnormalities on chromosome 17 in over a third of MBs, research shows non-random losses on chromosomes 8, 9, 10, 11 and 16, and gains on chromosomes 1, 7 and 9. Overexpression of ErbB2 receptors and losses on chromosome 17p have been proposed as independent indicators of aggressive behaviour, while high TrkC receptor expression indicates a favourable outcome. There is a strong association between anaplastic/large-cell tumours and MYC amplification, which has previously been linked with aggressive disease, but associations between abnormalities on chromosome 17 and anaplastic/large-cell MBs and between abnormalities in the shh/PTCH pathway and the desmoplastic variant are more controversial. Classification of the MB histopathologically and according to profiles of molecular abnormalities will help both to rationalize approaches to therapy, increasing the cure rate and reducing long-term side-effects, and to suggest novel treatments.

Immunohistochemical markers for prognosis of average-risk pediatric medulloblastomas. The effect of apoptotic index, TrkC, and c-myc expression

Medulloblastomas (MB) are the most common central nervous system malignancies in children. Numerous publications describe certain efforts to identify predictive value of various patterns of MB pathology and immunohistochemistry but received data appear to be controversial. In the present study, the apoptotic index (AI) and immunoexpression of TrkC, and c-myc proteins were investigated in biopsy samples from 68 MB with an average clinical risk to determine their prognostic utility in this tumor category. The number of cases with AI > 1.5% was significantly greater in the group of tumors in patients with recurrent MB and the mean AI was significantly higher in this group -4.7% vs. 1.1%. Furthermore, the number of tumors with AI > 1.5% was greater in the group of tumors in deceased patients and the mean Al was also higher in this group -4.6% vs. 1.2%. Immunoreactivity of the c-myc and TrkC did not show any differences between groups of patients with various clinical outcomes. A close association between Al as a continuous variable and the progression-free and overall survival was found. We found no any differences in survival times for c-myc and TrkC immunoreactivity. Multivariate revealed analysis that AI is a single significant prognostic factor for MB survival. Perhaps, investigations of c-myc and TrkC mRNA levels should be useful for clinical purposes, but in order to introduce these biomolecular markers in clinical protocols its distinct prognostic significance needs to be proved by prospective studies.

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

Primitive neuroectodermal tumors (PNETs) are pediatric brain tumors that result from defects in signaling molecules governing the growth and differentiation of neural progenitor cells. We used the RCAS-TVA system to study the growth effects of three genetic alterations implicated in human PNETs on a subset of neural progenitor cells that express the intermediate filament protein, nestin. The genetic alterations tested were: 1) overexpression of the cellular oncoprotein, MYC; 2) activation of transcription factor, beta-catenin; and 3) haploinsufficiency of Ptc, the hedgehog receptor gene. The RCAS-TVA system uses an avian retroviral vector, RCAS, to target gene expression to specific cell types in transgenic mice. To express exogenous genes in neural progenitor cells, we used Ntv-a mice. In these mice, the Nestin gene promoter drives expression of TVA, the cell surface receptor for the virus. Ectopic expression of MYC, but not activated beta-catenin, promoted the proliferation of neural progenitor cells in culture and in the cerebral leptomeninges in vivo. These effects were equally penetrant in mice with Ptc+/- and Ptc+/+ genetic backgrounds. Although overexpression of MYC is not sufficient to cause intraparenchymal tumors, it may facilitate PNET formation by sustaining the growth of undifferentiated progenitor cells.

Review of the molecular genetics and chemotherapeutic treatment of adult and paediatric medulloblastoma

Medulloblastoma is the most common primary brain tumour in children and accounts for 25% of newly diagnosed cases. Recent advances in treatment have extended 5-year survival rates from 3 - > 70% during the past 50 years. These improvements in survival have resulted from a multi-modality approach that includes surgical resection, posterior fossa and craniospinal irradiation and chemotherapy for selected, high-risk patients. The literature regarding chemotherapy of adult and paediatric patients is reviewed in-depth. The most active agents include cisplatin, CCNU, cyclophosphamide, vincristine and carboplatin. Although patients are living longer with their disease, neurocognitive function and quality of life are often impaired following radiation therapy (RT) to the developing brain. To safely allow reductions in the dose of RT, the specificity and efficacy of chemotherapy must be improved. Recent advances in the molecular genetics of medulloblastoma transformation (e.g., myc, PTCH ) are reviewed and discussed. A thorough understanding of these pathways will be critical for the development of more specific, novel drugs. Further clinical trials will be needed to evaluate the activity of these new drugs and determine their role in the treatment plan of patients with medulloblastoma.

Amplification of c-myc gene and overexpression of c-Myc protein in breast cancer and adjacent non-neoplastic tissue

BACKGROUND

Deregulated c-Myc expression and alterations of c-myc oncogene have been reported to play an important role in breast cancer tumorigenesis. We examined the relationship between c-Myc protein level, amplification of c-myc oncogene and commonly used clinical and pathologic factors.

METHODS

The studies were conducted on 94 ductal and lobular cancers. Amplification of c-Myc was assessed by the semiquantitative multiplex PCR assay. The amount of c-Myc protein was estimated by the densitometry analysis of Western blots.

RESULTS

Amplification of c-Myc was found in 21% of examined cancers. There was no association of c-myc amplification with established risk factors. Overexpression of c-Myc protein without c-myc amplification was associated with negative status of axillary lymph node. The size of lobular carcinoma displaying overexpression of c-Myc and the normal copy number of c-myc gene was significantly smaller than the size of tumor with elevated c-Myc and amplification of c-myc gene (p < 0.01). Within tumors displaying overexpression of c-Myc protein and c-myc gene amplification the size of ductal carcinoma was smaller than the size of lobular carcinoma (p < 0.007).

CONCLUSION

Data presented in this study suggest that alterations of c-myc gene and c-Myc protein level might be related to breast cancer progression. The prognostic utility of elevated level of c-Myc protein associated with normal status of c-myc gene for patients with lobular carcinoma requires further studies.

"Large cell/anaplastic" medulloblastomas: a Pediatric Oncology Group Study

495 medulloblastomas (MBs) from 6 Pediatric Oncology Group (POG) protocols were reviewed to assess the incidence and prognostic significance of "large cell" and "anaplastic" variants. "Large cell" medulloblastomas (LC MBs) were those with focal or diffuse, large, round neoplastic cells with prominent nucleoli. "Anaplastic" MBs (A MBs) were those with nuclei that were also large but markedly atypical with coarse chromatin and irregular shapes. Twenty-one cases were identified in the combined LC/A MB group, comprising about 4% of all MBs. Survival curves and Kaplan-Meier estimates of survival probabilities were examined separately for the LC/A MB and control groups. The logrank test for detecting poorer survival in the 21 cases was significant (p < 0.0001). Fluorescence in situ hybridization for c-myc showed amplification in 4 of 11 cases of the LC/A phenotype and 1 additional case of high level gain at 8q24 was disclosed by comparative genomic hybridization. Comparative genomic hybridization confirmed c-myc amplification and found evidence for isochromosome 17q in 3 of 4 LC/A cases studied successfully. One additional tumor showed high level gain restricted to 2p13 consistent with n-myc amplification. Monosomy 22, common in atypical teratoid/rhabdoid tumors, was not found. These results suggest that LC/A MB phenotype could be, at least in part, a correlate of c-myc, and possibly n-myc, amplification. The study thus confirms original observations about the LC MB in regard to histological features, immunohistochemical findings, c-myc amplification, cytogenetic findings, and poor prognosis.

Molecular insight into medulloblastoma and central nervous system primitive neuroectodermal tumor biology from hereditary syndromes: a review

Through the study of uncommon familial syndromes, physicians and scientists have been able to illuminate the underlying mechanisms of some of the more common sporadic diseases; this is illustrated best by studies of familial retinoblastoma. A number of rare familial syndromes have been described in which affected individuals are at increased risk of developing medulloblastoma and/or supratentorial primitive neuroectodermal tumors. The descriptions of many of these syndromes are based on patients observed by clinicians in their clinical practice. Determination of the underlying genetic defects in these patients with uncommon syndromes has led to identification of a number of genes subsequently found to be mutated in sporadic medulloblastomas (tumor suppressor genes). Associated genes in the same signaling pathways have also been found to be abnormal in sporadic medulloblastoma. Identification of patients with these rare syndromes is important, as they are often at increased risk for additional neoplasms, as are family members and future children. We review the published literature describing hereditary syndromes that have been associated with an increased incidence of medulloblastoma and/or central nervous system primitive neuroectodermal tumor. Review of the underlying molecular abnormalities in comparison to changes found in sporadic neoplasms suggests pathways important for tumorigenesis.

Gene amplification in PNETs/medulloblastomas: mapping of a novel amplified gene within the MYCN amplicon

OBJECTIVES

The pathological entity of primitive neuroectodermal tumour/medulloblastoma (PNET/MB) comprises a very heterogeneous group of neoplasms on a clinical as well as on a molecular level. We evaluated the importance of DNA amplification in medulloblastomas and other primitive neuroectodermal tumours (PNETs) of the CNS.

METHOD

Restriction landmark genomic scanning (RLGS), a method that allows the detection of low level amplification, was used. RLGS provides direct access to DNA sequences circumventing positional cloning efforts. Furthermore, we analysed several samples by CGH.

DESIGN

Twenty primary medulloblastomas, five supratentorial PNETs, and five medulloblastoma cell lines were studied.

RESULTS

Although our analysis confirms that gene amplification is generally a rare event in childhood PNET/MB, we found a total of 17 DNA fragments that were amplified in seven different tumours. Cloning and sequencing of several of these fragments confirmed the previous finding of MYC amplification in the cell line D341 Med and identified novel DNA sequences amplified in PNET/MB. We describe for the first time amplification of the novel gene, NAG, in a subset of PNET/MB. Despite genomic amplification, NAG was not overexpressed in the tumours studied. We have determined that NAG maps less than 50 kb 5' of DDX1 and approximately 400 kb telomeric of MYCN on chromosome 2p24.

CONCLUSION

We found a similar but slightly higher frequency of amplification than previously reported. We present several DNA fragments that may belong to the CpG islands of novel genes amplified in a small subset of PNET/MB. As an example we describe for the first time the amplification of NAG in the MYCN amplicon in PNET/MB.

A novel apoptotic pathway induced by nerve growth factor-mediated TrkA activation in medulloblastoma

Nerve growth factor (NGF) induces apoptosis in a human medulloblastoma cell line (MED283) engineered to express TrkA (MED283-TrkA) (Muragaki, Y., Chou, T. T., Kaplan, D. R., Trojanowski, J. Q., and Lee, V. M. (1997) J. Neurosci. 17, 530-542). To dissect the molecular signaling pathway that mediates this novel effect, specific receptor mutations in Trk have been employed. We showed that phosphorylation of tyrosine 490 is required for activation of phosphoinositide 3-OH kinase, whereas phosphorylation of tyrosine 785 is required for activation of phospholipase C-gamma. TrkA-mediated apoptosis was abolished when either the ATP-binding site or both tyrosines 490 and 785 were mutated. Because tyrosines 490 and 785 mediate redundant signaling through the Ras-extracellular signal-regulated kinase (Ras-ERK) pathway, we examined the role of Ras-ERK signaling in NGF-induced apoptosis. We found that MED283-TrkA cells expressing a dominant negative Ras inhibitor (N17Ras) failed to undergo ERK activation and apoptosis following NGF treatment, whereas the ERK kinase (mitogen-activated protein kinase kinase) inhibitors PD98059 and U0126 eliminated ERK activation but had no effect on apoptosis. We infer from these data that NGF-induced apoptosis is mediated by a novel Ras and/or Raf signaling pathway.