Identification and characterization of the protein encoded by the human N-myc oncogene

The effect of exogenous ciliary neurotrophic factor on cell cycle and neural differentiation markers of in vitro model cells: New insights for future therapeutic approaches

Retinoblastoma is known as childhood rare malignancy of the retina. Ciliary neurotrophic factor (CNTF) was previously found to reduce degeneration and promote retina survival. This work investigated the effects of CNTF supplementation on in-vitro model cells including retinoblastoma (Y79) and adipose-derived mesenchymal stem cells (AMSCs) viability, proliferation, gene expression and cell cycle. A drop of viability was detected in Y79 treated with CNTF in a dose-dependent manner (P < .05). However, the proliferation of AMSCs was increased at lower concentrations of CNTF (5 ng/mL), but declined in higher doses (50 and 100 ng/mL). The BrdU assay confirmed the MTT assay results. Cell cycle was arrested in both Y79 and AMSCs in the G0/G1 phase by CNTF treatment. A considerable down-regulation of Bcl2, CycD1 and N-Myc genes expression (P < .05) inversely, P15 and P21 genes up-regulation in treated Y79 cells was observed. Besides, stemness genes' transcription was reduced in AMSCs (P < .05), and levels of neuronal-specific markers such as neuron-specific enolase (NSE) and neuronal nuclei (NeuN) were increased (P < .05). The findings of this study suggest a promising potential of CNTF in terms of arresting Y79 retinoblastoma cells, and differentiation-inducing to AMSCs, which could be valuable for managing future innovative treatments targeting retinoblastoma. SIGNIFICANCE OF THE STUDY: We demonstrate that CNTF has the potential to reduce proliferation of Y79 cells and induce the cell cycle arrest of them. Also, down-regulation of oncogenes (such as N-Myc) while up-regulation of tumour suppressor genes (such as P21) was detected by exposure of Y79 cells to CNTF. Furthermore, we observed the cell cycle arrest, reduction of stemness gene and up-regulation of neural differentiation markers in AMSCs treated with CNTF. These results support the probable promising effects of CNTF for controlling retinoblastoma.

MYC transcription activation mediated by OCT4 as a mechanism of resistance to 13-cisRA-mediated differentiation in neuroblastoma

Despite the improvement in clinical outcome with 13-cis-retinoic acid (13-cisRA) + anti-GD2 antibody + cytokine immunotherapy given in first response ~40% of high-risk neuroblastoma patients die of recurrent disease. MYCN genomic amplification is a biomarker of aggressive tumors in the childhood cancer neuroblastoma. MYCN expression is downregulated by 13-cisRA, a differentiating agent that is a component of neuroblastoma therapy. Although MYC amplification is rare in neuroblastoma at diagnosis, we report transcriptional activation of MYC medicated by the transcription factor OCT4, functionally replacing MYCN in 13-cisRA-resistant progressive disease neuroblastoma in large panels of patient-derived cell lines and xenograft models. We identified novel OCT4-binding sites in the MYC promoter/enhancer region that regulated MYC expression via phosphorylation by MAPKAPK2 (MK2). OCT4 phosphorylation at the S111 residue by MK2 was upstream of MYC transcriptional activation. Expression of OCT4, MK2, and c-MYC was higher in progressive disease relative to pre-therapy neuroblastomas and was associated with inferior patient survival. OCT4 or MK2 knockdown decreased c-MYC expression and restored the sensitivity to 13-cisRA. In conclusion, we demonstrated that high c-MYC expression independent of genomic amplification is associated with disease progression in neuroblastoma. MK2-mediated OCT4 transcriptional activation is a novel mechanism for activating the MYC oncogene in progressive disease neuroblastoma that provides a therapeutic target.

Role of OCT4 in cancer stem-like cells and chemotherapy resistance

Cancer stem-like cells (CSCs) contribute to the tumorigenicity, progression, and chemoresistance of cancers. It is not known whether CSCs arise from normal stem cells or if they arise from differentiated cancer cells by acquiring self-renewal features. These CSCs share stem cell markers that normal stem cells express. There is a rising interest in octamer-binding transcription factor 4 (OCT4), one of the stem cell factors that are essential in embryogenesis and pluripotency. OCT4 is also overexpressed in CSCs of various cancers. Although the majority of the studies in CSCs reported a positive association between the expression of OCT4 and chemoresistance and an inverse correlation between OCT4 and clinical prognosis, there are studies rebuking these findings, possibly due to the sparsity of stem cells within tumors and the heterogeneity of tumors. In addition, post-translational modification of OCT4 affects its activity and warrants further investigation for its association with chemoresistance and prognosis.

Protein arginine methyltransferase 1 is a novel regulator of MYCN in neuroblastoma

Amplification or overexpression of MYCN is associated with poor prognosis of human neuroblastoma. We have recently defined a MYCN-dependent transcriptional signature, including protein arginine methyltransferase 1 (PRMT1), which identifies a subgroup of patients with high-risk disease. Here we provide several lines of evidence demonstrating PRMT1 as a novel regulator of MYCN and implicating PRMT1 as a potential therapeutic target in neuroblastoma pathogenesis. First, we observed a strong correlation between MYCN and PRMT1 protein levels in primary neuroblastoma tumors. Second, MYCN physically associates with PRMT1 by direct protein-protein interaction. Third, depletion of PRMT1 through siRNA knockdown reduced neuroblastoma cell viability and MYCN expression. Fourth, we showed that PRMT1 regulates MYCN stability and identified MYCN as a novel substrate of PRMT1. Finally, we demonstrated that mutation of putatively methylated arginine R65 to alanine decreased MYCN stability by altering phosphorylation at residues serine 62 and threonine 58. These results provide mechanistic insights into the modulation of MYCN oncoprotein by PRMT1, and suggest that targeting PRMT1 may have a therapeutic impact on MYCN-driven oncogenesis.

Eya1 interacts with Six2 and Myc to regulate expansion of the nephron progenitor pool during nephrogenesis

Self-renewal and proliferation of nephron progenitor cells and the decision to initiate nephrogenesis are crucial events directing kidney development. Despite recent advancements in defining lineage and regulators for the progenitors, fundamental questions about mechanisms driving expansion of the progenitors remain unanswered. Here we show that Eya1 interacts with Six2 and Myc to control self-renewing cell activity. Cell fate tracing reveals a developmental restriction of the Eya1(+) population within the intermediate mesoderm to nephron-forming cell fates and a common origin shared between caudal mesonephric and metanephric nephrons. Conditional inactivation of Eya1 leads to loss of Six2 expression and premature epithelialization of the progenitors. Six2 mediates translocation of Eya1 to the nucleus, where Eya1 uses its threonine phosphatase activity to control Myc phosphorylation/dephosphorylation and function in the progenitor cells. Our results reveal a functional link between Eya1, Six2, and Myc in driving the expansion and maintenance of the multipotent progenitors during nephrogenesis.

Ciliary neurotrophic factor reduces the proliferation and promotes the differentiation of TH- MYCN transformed sympathoadrenal progenitors

Neuroblastoma is a childhood cancer caused by the transformation of sympathoadrenal progenitors. By following the formation of tumors in homozygous TH-MYCN mice, an established mouse model of neuroblastoma, we were able to capture transformed cells prior to the formation of large, vascularized tumors in order to determine the responsiveness of cells to neurotrophic factors. We discovered that the ciliary neurotrophic factor (CNTF) receptor is abundantly expressed in tumor cells from these mice. Furthermore, CNTF - but not nerve growth factor, brain-derived nerve growth factor, neurotrophin 3, or glial cell line-derived neurotrophic factor - promoted neuronal differentiation and withdrawal from the cell cycle. Thus, the transformation of sympathoadrenal progenitors by MYCN overexpression differentially affects responsiveness to neurotrophic molecules.

Tumour immunogenicity, antigen presentation and immunological barriers in cancer immunotherapy

Since the beginning of the 20^th century, scientists have tried to stimulate the anti-tumour activities of the immune system to fight against cancer. However, the scientific effort devoted on the development of cancer immunotherapy has not been translated into the expected clinical success. On the contrary, classical anti-neoplastic treatments such as surgery, radiotherapy and chemotherapy are the first line of treatment. Nevertheless, there is compelling evidence on the immunogenicity of cancer cells, and the capacity of the immune system to expand cancer-specific effector cytotoxic T cells. However, the effective activation of anti-cancer T cell responses strongly depends on efficient tumour antigen presentation from professional antigen presenting cells such as dendritic cells (DCs). Several strategies have been used to boost DC antigen presenting functions, but at the end cancer immunotherapy is not as effective as would be expected according to preclinical models. In this review we comment on these discrepancies, focusing our attention on the contribution of regulatory T cells and myeloid-derived suppressor cells to the lack of therapeutic success of DC-based cancer immunotherapy.

MYC-y mice: from tumour initiation to therapeutic targeting of endogenous MYC

MYC is one of the best-studied oncogenes in terms of mouse models of malignancy. MYC overexpression has been targeted to several tissues using transgenic constructs, and more recently as mouse models have evolved, conditional systems have been developed to allow the regulation of MYC expression or activity in vivo. The ability to target MYC expression to specific tissues and cell lineages, as well as the ability to regulate that expression, has made genetically engineered mouse models (GEMM) a valuable resource for studying the importance of MYC in the process of tumourigenesis. Here we review how these models have been used to address the role of MYC in tumour initiation and maintenance, how subtle changes in levels of MYC can influence tumourigenesis, and finally the ongoing efforts to target endogenous MYC genetically and with novel therapies.

The Cdk1 Complex Plays a Prime Role in Regulating N-Myc Phosphorylation and Turnover in Neural Precursors

Myc family transcription factors are destabilized by phosphorylation of a conserved amino-terminal GSK-3beta motif. In proliferating cerebellar granule neuron precursors (CGNPs), Sonic hedgehog signaling induces N-myc expression, and N-myc protein is stabilized by insulin-like growth factor-mediated suppression of GSK-3beta. N-myc phosphorylation-mediated degradation is a prerequisite for CGNP growth arrest and differentiation. We investigated whether N-myc phosphorylation and turnover are thus linked to cell cycle exit in primary mouse CGNP cultures and the developing cerebellum. We report that phosphorylation-induced turnover of endogenous N-myc protein in CGNPs increases during mitosis, due to increased priming phosphorylation of N-myc for GSK-3beta. The priming phosphorylation requires the Cdk1 complex, whose cyclin subunits are indirect Sonic hedgehog targets. These findings provide a mechanism for promoting growth arrest in the final cycle of neural precursor proliferation competency, or for resetting the cell cycle in the G1 phase, by destabilizing N-myc in mitosis.

Prominent transcription of zebrafish N-myc (nmyc1) in tectal and retinal growth zones during embryonic and early larval development

Because of its oncogenic capacity and expression restricted to embryonic and newborn tissues, the N-myc proto-oncogene is suggested to play a key role in vertebrate organogenesis as well as in the control of cell proliferation and differentiation. To further approach the developmental function of N-myc, we cloned full-length zebrafish N-myc (nmyc1) and analyzed its expression in the embryo and early larva. nmyc1 transcription is initiated at the mid-blastula stage. At somitogenesis stages, its expression was detected in the retina, midbrain, posterior hindbrain and presumptive spinal cord. nmyc1 was also transcribed in the endoderm and its derivatives as well as in branchial arches. At later developmental stages, posterior neural expression of nmyc1 was switched off, but expression remained intense in the brain, mainly in the optic tectum, cerebellar plate and dorsal rhombomere 2. Comparison of nmyc1 transcription with proliferation zones using a M phase mitotic marker revealed that nmyc1 expression is specifically associated with mitosis in the optic tectum and the retina. This result contrasts with previous studies in other vertebrates where N-myc expression can persist in differentiating cells.

Natural killer T cells infiltrate neuroblastomas expressing the chemokine CCL2

CD1d-restricted Valpha24-Jalpha18-invariant natural killer T cells (iNKTs) are potentially important in tumor immunity. However, little is known about their localization to tumors. We analyzed 98 untreated primary neuroblastomas from patients with metastatic disease (stage 4) for tumor-infiltrating iNKTs using TaqMan((R)) reverse transcription polymerase chain reaction and immunofluorescent microscopy. 52 tumors (53%) contained iNKTs, and oligonucleotide microarray analysis of the iNKT(+) and iNKT(-) tumors revealed that the former expressed higher levels of CCL2/MCP-1, CXCL12/SDF-1, CCL5/RANTES, and CCL21/SLC. Eight tested neuroblastoma cell lines secreted a range of CCL2 (0-21.6 ng/ml), little CXCL12 (</=0.1 ng/ml), and no detectable CCL5 or CCL21. CCR2, the receptor for CCL2, was more frequently expressed by iNKT compared with natural killer and T cells from blood (P < 0.001). Supernatants of neuroblastoma cell lines that produced CCL2 induced in vitro migration of iNKTs from blood of patients and normal adults; this was abrogated by an anti-CCL2 monoclonal antibody. CCL2 expression by tumors was found to inversely correlate with MYCN proto-oncogene amplification and expression (r = 0.5, P < 0.001), and MYCN-high/CCL2-low expression accurately predicted the absence of iNKTs (P < 0.001). In summary, iNKTs migrate toward neuroblastoma cells in a CCL2-dependent manner, preferentially infiltrating MYCN nonamplified tumors that express CCL2.

The expression of the developmentally regulated proto-oncogene Pax-3 is modulated by N-Myc

N-Myc is a member of the Myc family of transcription factors that have been shown to play a pivotal role in cell proliferation and differentiation. In this report, we have investigated the relationship between N-Myc and the developmental control gene Pax-3. Using transient transfection assays, we show that the Pax-3 promoter is activated by both N-Myc-Max and c-Myc-Max. Moreover, we show that Myc regulation of Pax-3 promoter activity is dependent upon a noncanonical E box site in the 5' promoter region of Pax-3. In addition, we show that ectopic expression of both N-Myc and c-Myc leads to increased expression of Pax-3 mRNA. Furthermore, we show that Pax-3 mRNA expression is cell cycle-regulated and that the 5' promoter region of Pax-3 (bp -1578 to +56) can direct cell cycle-dependent gene expression with kinetics similar to that of the endogenous transcript. Site-directed mutagenesis of the E box site within the Pax-3 promoter significantly altered the pattern of expression through the cell cycle. These results suggest that the Myc family of transcription factors may modulate Pax-3 expression in vivo.

Molecular biology of neuroblastoma

PURPOSE AND RESULTS: Neuroblastoma, the most common solid extracranial neoplasm in children, is remarkable for its clinical heterogeneity. Complex patterns of genetic abnormalities interact to determine the clinical phenotype. The molecular biology of neuroblastoma is characterized by somatically acquired genetic events that lead to gene overexpression (oncogenes), gene inactivation (tumor suppressor genes), or alterations in gene expression. Amplification of the MYCN proto-oncogene occurs in 20% to 25% of neuroblastomas and is a reliable marker of aggressive clinical behavior. No other oncogene has been shown to be consistently mutated or overexpressed in neuroblastoma, although unbalanced translocations resulting in gain of genetic material from chromosome bands 17q23-qter have been identified in more than 50% of primary tumors. Some children have an inherited predisposition to develop neuroblastoma, but a familial neuroblastoma susceptibility gene has not yet been localized. Consistent areas of chromosomal loss, including chromosome band 1p36 in 30% to 35% of primary tumors, 11q23 in 44%, and 14q23-qter in 22%, may identify the location of neuroblastoma suppressor genes. Alterations in the expression of the neurotrophins and their receptors correlate with clinical behavior and may reflect the degree of neuroblastic differentiation before malignant transformation. Alterations in the expression of genes that regulate apoptosis also correlate with neuroblastoma behavior and may help to explain the phenomenon of spontaneous regression observed in a well-defined subset of patients.

CONCLUSION

The molecular biology of neuroblastoma has led to a combined clinical and biologic risk stratification. Future advances may lead to more specific treatment strategies for children with neuroblastoma.

Development of a polyclonal antibody with defined specificity against synthetic peptides from the N-myc oncoprotein using multiple antigen peptide and hemocyanin conjugation methods

BACKGROUND/PURPOSE

The importance of determining N-myc oncoprotein rather than genomic N-myc amplification has been emphasized in neuroblastoma, especially in an international project to register biological risk factors in all neuroblastomas. A method to raise a specific polyclonal antibody against the N-myc oncoprotein in large quantities was sought using the synthetic antigen peptide and the multiple antigen peptide (MAP) method.

METHODS

Two sets of peptides, HGRGPPTAGSTAQSPG and GVAPPRPGGRQTSGGDH, conserved in the N-myc oncoprotein were synthesized. The hemocyanin-conjugated peptides and the lysine core-conjugated (multiple antigen peptide method) peptides were injected into rabbits with adjuvant. IgG fractions precipitated from the sera were purified on an affinity column coupled with these peptides, and the potency and specificity of the purified IgGs were examined by immunoblotting and immunohistochemistry in small cell lung cancer cell lines with known positivity-negativity of amplification and expression of N-myc, c-myc, and L-myc.

RESULTS

Peptides conjugated to the lysine core raised more potent antibodies than those conjugated to hemocyanin. Purified IgG against GVAPPRPGGRQTSGGDH reacted positively with an N-myc-amplified lung cancer cell line, but not with N-myc-unamplified and c-myc/L-myc-amplified cell lines on either immunoblotting or immunostaining. This IgG strongly stained the nuclei of cells in a series of surgical specimens and cell lines of neuroblastoma with N-myc amplification.

CONCLUSION

A polyclonal antibody specific for a synthetic peptide from the N-myc oncoprotein was thus obtained and will find wide international use.

Localization of human BRCA1 and its loss in high-grade, non-inherited breast carcinomas

Although the link between the BRCA1 tumour-suppressor gene and hereditary breast and ovarian cancer is established, the role, if any, of BRCA1 in non-familial cancers is unclear. BRCA1 mutations are rare in sporadic cancers, but loss of BRCA1 resulting from reduced expression or incorrect subcellular localization is postulated to be important in non-familial breast and ovarian cancers. Epigenetic loss, however, has not received general acceptance due to controversy regarding the subcellular localization of BRCA1 proteins, reports of which have ranged from exclusively nuclear, to conditionally nuclear, to the ER/golgi, to cytoplasmic invaginations into the nucleus. In an attempt to resolve this issue, we have comprehensively characterized 19 anti-BRCA1 antibodies. These reagents detect a 220-kD protein localized in discrete nuclear foci in all epithelial cell lines, including those derived from breast malignancies. Immunohistochemical staining of human breast specimens also revealed BRCA1 nuclear foci in benign breast, invasive lobular cancers and low-grade ductal carcinomas. Conversely, BRCA1 expression was reduced or undetectable in the majority of high-grade, ductal carcinomas, suggesting that absence of BRCA1 may contribute to the pathogenesis of a significant percentage of sporadic breast cancers.

A novel intron element operates posttranscriptionally To regulate human N-myc expression

Precisely regulated expression of oncogenes and tumor suppressor genes is essential for normal development, and deregulated expression can lead to cancer. The human N-myc gene normally is expressed in only a subset of fetal epithelial tissues, and its expression is extinguished in all adult tissues except transiently in pre-B lymphocytes. The N-myc gene is overexpressed due to genomic amplification in the childhood tumor neuroblastoma. In previous work to investigate mechanisms of regulation of human N-myc gene expression, we observed that N-myc promoter-chloramphemicol acelyltransferase reporter constructs containing sequences 5' to exon 1 were active in all cell types examined, regardless of whether endogenous N-myc RNA was detected. In contrast, inclusion of the first exon and a portion of the first intron allowed expression only in those cell types with detectable endogenous N-myc transcripts. We investigated further the mechanisms by which this tissue-specific control of N-myc expression is achieved. Using nuclear run-on analyses, we determined that the N-myc gene is actively transcribed in all cell types examined, indicating a posttranscriptional mode of regulation. Using a series of N-myc intron 1 deletion constructs, we localized a 116-bp element (tissue-specific element [TSE]) within the first intron that directs tissue-specific N-myc expression. The TSE can function independently to regulate expression of a heterologous promoter-reporter minigene in a cell-specific pattern that mirrors the expression pattern of the endogenous N-myc gene. Surprisingly, the TSE can function in both sense and antisense orientations to regulate gene expression. Our data indicate that the human N-myc TSE functions through a posttranscriptional mechanism to regulate N-myc expression.

Growth arrest failure, G1 restriction point override, and S phase death of sensory precursor cells in the absence of neurotrophin-3

More than half of the dorsal root ganglion (DRG) neurons are lost by excessive cell death coinciding with precursor proliferation and cell cycle exit in neurotrophin-3 null mutant (NT-3-/-) mice. We find that in the absence of NT-3, sensory precursor cells fail to arrest the cell cycle, override the G1 phase restriction point, and die by apoptosis in S phase, which can be prevented in vivo by a cell cycle blocker. Uncoordinated cell cycle reentry is preceded by a failure of nuclear N-myc downregulation and is paralleled by the activation of the full repertoire of G1 and S phase cell cycle proteins required for cell cycle entry. Our results provide evidence for novel activity of neurotrophins in cell cycle control and point toward an N-myc sensitization to cell death in the nervous system that is under the control of NT-3.

Familial neuroblastoma: report of a kindred with later age at diagnosis

PURPOSE

To describe the clinical and biologic features of neuroblastoma (NB) in two siblings and their maternal second cousin.

PATIENTS AND METHODS

NB was diagnosed in the siblings at 2 1/2 (patient 2) and 5 (patient 3) years of age. NB was diagnosed in their maternal second cousin (patient 1) when she was 7 years old. Standard clinical and biological data, tumor karyotype, and tumor allelotype at select loci were obtained.

RESULTS

Patient 1 had International Neuroblastoma Staging System (INSS) stage 4 NB and unfavorable histology but no evidence of MYCN amplification; she died from complications of autologous bone marrow transplantation in second remission. Patient 2 had INSS stage 4 NB with unfavorable histology but no MYCN amplification; her disease recurred 39 months after completing therapy. Patient 3 had INSS stage 1 NB with favorable biologic features; he was treated with surgical excision and remains free of disease.

CONCLUSIONS

Familial NB may occur at a later age than predicted by the tumor suppressor gene model of inherited cancer. This report further emphasizes the clinical and biological heterogeneity of familial NB.

Transcription factors in normal and neoplastic pituitary tissues

Transcription factors are proteins that bind to regulatory elements in DNA and have critical roles in gene regulation during development, in cellular growth and differentiation. The four major groups of transcription factors have been classified according to the motif in the DNA-binding domains and include: (1) the helix-turn-helix group, which includes the Pit-1/GHF-1 (Pit-1) transcription factor; (2) the zing finger group, which includes estrogen and other steroid hormone receptors; (3) the leucine zipper group, which includes c-fos protooncogene, and (4) the helix-loop-helix group, which includes the c-myc oncogene. Members of all four groups have been described in normal and neoplastic anterior pituitary gland tissues. Pit-1 has been shown to regulate prolactin (PRL), growth hormone (GH), and thyroid-stimulating hormone (TSH) cells during development and differentiation. Genetic defects in this transcription factor have led to specific diseases in rodents and humans such as dwarfism and cretinism. Estrogen receptor (ER) protein plays a critical role in the regulation of gene expression in some anterior pituitary cells. There is a differential distribution of ER in anterior pituitary cells and tumors; PRL, gonadotroph, and null cell tumors are the principal adenomas expressing ER. The protooncogene c-fos is regulated by estrogen in various tissues, linking the regulation of one transcription factor by another transcription factor with a different motif. The c-myc oncogene has been detected in the pituitary gland and in some pituitary tumors, although the exact role of this oncogene in pituitary tumor development is uncertain. Because of the critical role that transcription factors play in pituitary cell development and differentiation, we can anticipate many more studies to elucidate their many functions in normal and neoplastic pituitary tissues.

c-myb intron I protein binding and association with transcriptional activity in leukemic cells

Specific binding of nuclear proteins to the region of transcriptional attenuation has been shown to modulate the expression of c-myb, a nuclear proto-oncogene preferentially expressed in lympho-hematopoietic cells. Here, it plays an important role in processes of differentiation and proliferation. The mechanism that regulates c-myb expression is not yet fully understood. The block of transcriptional elongation which has been mapped to a 1 kb region within murine intron 1 may represent one regulatory pathway. The DNA sequences containing the transcriptional pause site are well conserved between murine and human species, thus Implying similar transcription-control strategies. We compared the binding potential of nuclear extracts (from human fibroblasts and MOLT4 as well as murine NIH3T3- and 70Z/3B- cell lines) to oligonucleotide sequences previously shown to be target binding sites in the murine system. One complex containing a 70 D protein was found to be associated specifically with transcriptionally active leukemia cells. We performed transient expression studies with a CAT reporter construct containing this putative enhancer sequence and yielded significant CAT activity. We identified further a putative 20 kD repressor protein in transcriptionally silent cells and demonstrated that c-Jun is part of an ubiquitously present complex. Our results confirm the participation of intron 1 in transcriptional regulation of the c-myb gene (in mouse and human) and implicate multiple and complex regulatory mechanisms of activation during myelomonocytic differentiation and leukemic cell growth control.

N-myc expression in the embryonic cochlea of the mouse

N-myc expression in the mouse embryo was examined during the late cochlear organogenesis. Tissue distribution of N-myc expression was histologically analyzed by in situ hybridization of the transcript in the cochlea between 15 and 18 days of gestation. At 15 days of gestation, N-myc expression was found very conspicuous in nervous structure of the cochlea such as the auditory nerve and the spiral ganglion. Moreover, N-myc was also present in the Köllikers organ and in the epithelium surrounding the cochlear canal. A few days later, N-myc expression was still clearly present in the Köllikers organ but less so in nervous structures. This study shows that cochlear tissues derived from the otic placode present a significant level of N-myc transcript during late embryogenesis. N-myc expression seems to be related to cell differentiation in the inner ear.

Human neuroblastoma cell lines that express N-myc without gene amplification

BACKGROUND

About one half of aggressive neuroblastomas lack N-myc amplification. Cell lines from such tumors are needed to determine the biological basis of aggressive tumor behavior.

METHODS

Neuroblastoma cell lines were established from a primary tumor (SMS-LHN) and a bone marrow metastasis (LA-N-6) of two children with Stage IV neuroblastoma. Although both cell lines and their original tumors lacked N-myc genomic amplification, these patients died of progressive disease.

RESULTS

SMS-LHN and LA-N-6 can be distinguished from primitive neuroectodermal tumor (PNET) lines by cell surface antigen expression and catecholamine production. Cytogenetic analysis of each cell line revealed unique genetic rearrangements, whereas both lines showed abnormalities involving chromosome 2. Neither cell line contained double-minute chromosomes, homogeneously staining regions, a 1p chromosomal deletion, or t(11;22). The growth rates of these two new lines in vitro and in vivo (as xenografts in nude mice) are slower than N-myc amplified neuroblastoma lines. Both lines express greater amounts of N-myc RNA and protein relative to nonneuroblastoma cell lines (including PNET), although not to the extent of cell lines with N-myc genomic amplification.

CONCLUSIONS

The relatively large amount of N-myc expression in these two new cell lines suggests that N-myc expression without amplification could play a role in the pathogenesis of some neuroblastomas. These cell lines should be useful for investigating mechanisms and consequences of N-myc gene activation other than genomic amplification.

Transcriptional regulation of the N-myc gene: identification of positive regulatory element and its double- and single-stranded DNA binding proteins

The N-myc gene is amplified and overexpressed in neuroblastoma, retinoblastoma and small cell lung carcinoma, and is considered to be related to cell proliferation and/or differentiation. The transcriptional regulatory sequences of the c-myc gene have been already identified, while those of N-myc have remained obscure for a long time. In this report, we have identified several positive and negative transcriptional regulatory elements in the upstream region of the mouse N-myc gene. Among them, an activating sequence spanning -860 to -797 bp (63 bp) could be reduced to a functional core of 21 bp from -846 to -826. This sequence, termed N21 box, worked as a positive transcriptional element when linked directly upstream (but not downstream) of the putative N-myc promoter in HeLa, not in IMR32 cells. At least two proteins, of 42 kDa and 100 kDa, bound to the double-stranded N21 box, and were expressed in HeLa as well as in IMR32 cells. Moreover, the plus strand of N21 box could be specifically bound by a species of 42 kDa from either cell type and by a 37 kDa protein found only in HeLa cells. These proteins may be factors binding to positive transcriptional regulatory elements and may have a role in the regulation of N-myc expression.

Defects of embryonic organogenesis resulting from targeted disruption of the N-myc gene in the mouse

The highest expression of the N-myc gene occurs during embryonic organogenesis in the mouse ontogeny, with the peak of expression around embryonic day 9.5. Homozygous N-myc-deficient mice, produced by germline transmission of a disrupted allele in ES cells, developed normally to day 10.5, indicating dispensability of N-myc expression in the earlier period, but later accumulated organogenic abnormalities and died around day 11.5. The most notable abnormalities were found in the limb bud, visceral organs (lung, stomach, liver and heart) and the central/peripheral nervous systems, and were highly correlated with the site of N-myc expression. The limb buds and the lungs excised from N-myc-deficient mutant embryos were placed in culture to allow their development to stages beyond the point of death of the embryos. Analyses indicated that the mutant limbs failed to develop distal structures and the development of bronchi from the trachea was defective in the lungs. The latter defect was largely corrected by addition of fetal calf serum to the culture medium, suggesting that an activity missing in the mutant lung was replenished by a component of the serum. The phenotype of N-myc-deficient mutant embryos indicated requirement of the N-myc function in many instances of tissue interactions in organogenesis and also in cell-autonomous regulation of tissue maturation.

Transition of localization of the N-Myc protein from nucleus to cytoplasm in differentiating neurons

N-myc is a developmentally regulated proto-oncogene encoding a putative sequence-specific DNA-binding protein. Previous studies on tissue distribution of N-myc transcripts indicated that one of the major sites of N-myc expression is the CNS and neural crest derivatives in developing embryos. We investigated N-Myc protein expression in embryonic neural tissues and found that the protein was usually localized in the nucleus, but accumulated in the cytoplasm upon differentiation of specific classes of neurons, e.g., retinal ganglion cells, neurons of spinal ganglia, and Purkinje cells of the cerebellum. The change of localization of N-Myc from the nucleus to the cytoplasm indicates a novel feature of regulation of myc family proteins and suggests functions of N-myc in the cytoplasm of maturing neurons.

Specific phosphorylation of the acidic central region of the N-myc protein by casein kinase II

The central region of the N-myc protein has a characteristic amino acid sequence EDTLSDSDDEDD, which is very similar to those of particular domains of adenovirus E1A, human papilloma virus E7, Simian virus 40 large T, c-myc and L-myc proteins. Domains of these three viral oncoproteins have recently been shown to be specific binding sites for the tumor-suppressor gene retinoblastoma protein. We have noted that the sequence of serine followed by a cluster of acidic amino acids is exactly the same as that of a typical substrate of casein kinase II (CKII). Therefore, we investigated whether these nuclear oncoproteins are phosphorylated by CKII. For this purpose, we fused the beta-galactosidase and N-myc genes including this domain and expressed it in Escherichia coli cells. Several mutant N-myc genes, containing single amino acid substitutions in this domain, were also used to produce fused proteins. Strong phosphorylation by CKII was detected with the fused protein of wild-type N-myc. However, no phosphorylation of beta-galactosidase itself was observed and the phosphorylations of fused mutant proteins were low. Another fused N-myc protein containing most of the C-terminal region downstream of this acidic region was not phosphorylated by CKII. Analysis of phosphorylation sites in synthetic peptides of this acidic region identified the major sites phosphorylated by CKII as Ser261 and Ser263. On two-dimensional tryptic mapping of phosphorylated N-myc proteins, major spots of in vitro-labeled and in-vivo-labeled N-myc proteins were detected in the same positions. These results suggest that two serine residues of the acidic central region of the N-myc protein are phosphorylated by CKII in vivo as well as in vitro. The functional significance of this acidic domain is discussed.

Oncogenes: cause or consequence in the development of glial tumors

Recent developments in the field of oncogenes and growth stimulatory factors have provided limited but essential models in neuro-oncology. The observation in gliomas of platelet growth factor (PDGF)-like immunoreactivity fits with the autocrine secretion model, rising the possibility for the growth factor independence of the cancer cells. The discovery of the tumor suppressor genes, for which loss of function mutations are oncogenic as in the RB gene of the retinoblastoma and p53 gene, has introduced a new concept of oncogenesis which could be useful even in the cure of the neoplasms. Several oncogenes are amplified and/or expressed in brain tumors, some associated with polymorphism leading to abnormal protein products. Therefore, corresponding functions, such as production of deficient epidermal growth factor receptor (EGFR) encoded by erb-B, are impaired. Abnormal chromosomal patterns have been recognized in brain tumors and found mainly in chromosomes 7 and 22 on which oncogenes erb-B and sis are located, respectively. Location of proto-oncogenes, which are normally expressed in the brain, indicate that they share common distribution patterns mainly involving the cerebellum, hippocampus and olfactory bulbs. These proto-oncogenes may be regulated by physiological and pathological events. The concept of oncogene involvement in brain tumors must be extended to include the other factors such as G-proteins, growth factor receptors, membrane-associated and cytoplasmic protein kinases, which are all responsible for the control of the cell growth and their response to external signals including chemotherapeutic drigs.

Contrasting patterns of c-myc and N-myc expression in proliferating, quiescent, and differentiating cells of the embryonic chicken lens

The present study uses the polymerase chain reaction and in situ hybridization to examine c-myc and N-myc mRNA in the embryonic chicken lens at 6, 10, 14 and 19 days of development and compares the pattern of expression obtained with the developmental pattern of cell proliferation and differentiation. In the central epithelium, c-myc mRNA levels were proportional to the percentage of proliferating cells throughout development. N-myc mRNA expression in this region was relatively low and showed no correlation with cell proliferation. The ratio of N-myc to c-myc mRNA increased markedly with the onset of epithelial cell elongation and terminal fiber cell differentiation, although both c-myc and N-myc mRNAs continued to be expressed in postmitotic, elongating cells of the equatorial epithelium and in terminally differentiating lens fiber cells. Thus, increased expression of N-myc, a gene whose protein product may compete with c-myc protein for dimerization partners, accompanies the dissociation of c-myc expression and cell proliferation during terminal differentiation of lens fiber cells.

Determination of the DNA sequence recognized by the bHLH-zip domain of the N-Myc protein

The DNA-binding domain of the murine N-Myc protein, comprising the basic helix-loop-helix-zipper (bHLH-zip) region was expressed as a fusion protein in E. coli. The affinity purified glutathione-S-transferase-N-Myc fusion protein (GST-N-MYC) was used to select the N-Myc specific DNA-recognition motif from a pool of random-sequence oligonucleotides. After seven rounds of binding-site selection, specifically enriched oligonucleotides were cloned and sequenced. Of 31 individual oligonucleotides whose sequences were determined, 30 contained a common DNA-motif, defining the hexameric consensus sequence CACGTG. We confirm by mutational analysis that binding of the N-Myc derived bHLH-zip domain to this motif is sequence-specific.

Development of a two color immunofluorescence stain and immunolocalization method for N-myc and c-myc oncoproteins with a newly generated mouse IgM anti N-myc antibody

A new mouse monoclonal antibody specific for N-myc oncoprotein was generated and used in combination with an anti-c-myc antibody to develop two color immunofluorescence staining and ultrastructural immunolocalization of N-myc and c-myc in well established (SK-N-SH; CHP 126) and in newly established neuroblastoma (NB) cell lines. Analysis and quantitation of c-myc and N-myc in dually stained cells was done by flow cytometry. Immunolocalization was done by staining with immunogold secondary antibodies and transmission electron microscopy. The results obtained from analysis of 13 newly established NB cell lines revealed, great heterogeneity in the expression of N-myc oncoprotein with 10/13 cell lines over expressing the protein. C-myc oncoprotein was also expressed in all cell lines, however, the level of expression was 4-10-fold lower than the N-myc oncoprotein. Localization studies of c-myc and N-myc oncoproteins on the level of light microscopy and electron microscopy revealed exclusive nuclear localization of c-myc whereas N-myc was localized to the nucleus and to the cytoplasm.

Oncogenes: a review of their clinical application

One objective of this review is to sort through and collate the recent data that suggest that human cellular oncogenes, which have been implicated as the etiologic agents in both animal and human malignancies, have also the potential to be employed as clinical tools in the struggle against cancer. For nearly 10 years, reports have been suggesting that advantage can be taken of cellular oncogenes as to their use as diagnostic and prognostic indicators of cancer and eventually as therapeutic cancer agents. It is also the purpose of this review to give an objective evaluation of these predictions. Moreover, this review will try to highlight some of the significant advances in this most rapidly evolving field of biology. Although the enormity of what has been learned about cellular oncogenes is nothing less than impressive, it is the view here that the routine implementation of oncogenes into the clinical setting will not become evident as early as the many predictions had purported.

Immunohistochemical analysis of N-myc protein expression in neuroblastoma: correlation with prognosis of patients

We have analyzed N-myc gene amplification and N-myc protein expression in 41 primary neuroblastomas. In this series, 22 patients are currently alive and disease-free, whereas 19 patients have died or are alive with progressive disease. All tumor samples were obtained at operation. N-myc gene amplification was detected by Southern blot analysis, and N-myc protein expression was detected using Bouin-fixed, paraffin-embedded tissue sections and immunohistochemical staining with anti-N-myc gene products serum. N-myc protein expression was detected in all 9 tumors with N-myc gene amplification (greater than or equal to 10 N-myc gene copies). Among 19 patients with poor prognosis, N-myc gene amplification was detected in 8 (42%) and N-myc protein expression in 18 (95%); neither was detected in 20 of the 22 patients who survived free of disease. We conclude that the immunohistochemical detection of N-myc protein expression is one of the most unfavorable prognostic factors in neuroblastoma patients.

A transient decrease in N-myc expression and its biological role during differentiation of human embryonal carcinoma cells

The human embryonal carcinoma (EC) cell line, NEC14 can be induced to morphologically differentiate by the addition of 10(-2) M N,N'-hexamethylene-bis-acetamide (HMBA) in vitro. The expression of several cellular oncogenes (c-onc) in NEC14 cells was examined after induction of differentiation by HMBA. The level of N-myc expression was the highest in undifferentiated cells but decreased transiently to less than 1/10 of the original level shortly after the induction of differentiation. To investigate the role of the transient decrease in N-myc level on NEC14 cell differentiation, a chimeric human N-myc gene in which transcription is initiated at the human beta-actin gene promoter was constructed and introduced into NEC14 cells. Several transformants expressing the exogenous N-myc gene constitutively were established. These transformants showed 10- to 70-fold increases in plating efficiency and shorter population doubling times as compared with the parental NEC14 cells. The transformants were hard to induce, spontaneously differentiated cells on the periphery of cell clusters in culture, unlike parental NEC14 cells, and took longer for HMBA-induced morphological differentiation. The populations of the cells expressing HLA and SSEA-1 antigens increased from 10%-20% to nearly 100% in NEC14 cells after the induction of differentiation, while the populations expressing these antigens increased only to 50%-60% in one of the transformants, S11. The transformants gained an increased tumorigenic potential in nude mice, and the tumors produced consisted exclusively of EC stem cells. These results suggest that the additional expression of the exogenous N-myc gene (increased about two-fold) confers the more transformed state on the cells.

Effect of nerve growth factor on C-1300 murine neuroblastoma tumor growth and catecholamine content in neonatally sympathectomized mice

The in situ C-1300 murine neuroblastoma (MNB) tumor model was used to investigate the influence of exogenously administered nerve growth factor (NGF) on tumor growth and tissue catecholamine concentration in mice sympathectomized with 6-hydroxy-dopamine (6-OHDA) on postnatal days 4-10. Mice were implanted with 1 x 10(6) disaggregated MNB cells 3 days after termination of 6-OHDA administration. NGF (12-15 micrograms/mouse/day) treatment was initiated at the time of MNB cell implantation and continued until sacrifice of the animal. The time interval between tumor cell implantation and detection of palpable tumor (tumor onset time), transverse tumor diameter, tumor weight, tumor weight to body weight ratio, and tumor catecholamine concentration were determined. Neonatal sympathectomy caused a decrease in myocardial norepinephrine concentration of 88% compared with vehicle-treated animals as well as a significant reduction in total body and organ weight. Average body, brain, heart, and spleen weights were decreased 31%, 16%, 25%, and 42%, respectively, below control values. The daily injection of NGF, from the time of MNB tumor implantation to sacrifice, did not prevent these effects of chemical sympathectomy from being expressed. Tumor onset time following implantation of MNB cells was significantly increased in neonatally sympathectomized mice and was not altered by treatment with NGF. In contrast, the decrease in MNB tumor growth rate observed in sympathectomized mice was reversed by administration of NGF. Mean tumor weight and mean tumor to body weight ratio were 89% and 115% of comparable control values, respectively, in sympathectomized mice receiving exogenous NGF.(ABSTRACT TRUNCATED AT 250 WORDS)

Nuclear colocalization of cellular and viral myc proteins with HSP70 in myc-overexpressing cells

The c-myc oncogene and its viral counterpart v-myc encode phosphoproteins which have been located within cell nuclei, excluding nucleoli. We have expressed the c-myc gene under the simian virus 40 early promoter and studied the distribution of its protein product in transient expression assays in COS, HeLa, and 293 cells. We found three distinct patterns of c-myc immunofluorescence in the transfected cells: one-third of the c-myc-positive cells displayed a diffuse nuclear distribution, and in two-thirds of the cells the c-myc fluorescence was accumulated either in small amorphous or in large multilobed phase-dense nuclear structures. Unexpectedly, these structures also stained for the HSP70 heat shock protein in both heat-shocked and untreated cells. Our results indicate that both transient and stable overexpression of either the c-myc or v-myc protein induces translocation of the endogenous HSP70 protein from the cytoplasm to the nucleus, where it becomes sequestered in structures containing the myc protein. Interestingly, the closely related N-myc protein does not stimulate substantial nuclear expression of the HSP70 protein. Studies with chimeric myc proteins revealed that polypeptide sequences encoded by the second exon of c-myc are involved in colocalization with HSP70.

The role of myc oncogenes in cell growth and differentiation

The myc oncoproteins are expressed in a wide range of normal adult and embryonic tissues. They are also found to be over-expressed in a variety of tumor types. All myc proteins are short-lived nuclear phosphoproteins thought to act as regulatory components of cell proliferation. The rapid induction of c-myc mRNA and protein following the addition of growth factors to quiescent cells, together with the short half-life of these molecules, suggests that they are sensitive and continuous indicators of external stimuli, consistent with a role in signal transduction. Furthermore, in untransformed cells, c-myc protein expression is tightly regulated, at least in part, by a mechanism of autoregulation. Deregulated expression of myc genes is a frequent observation in tumors and may lead to a cell becoming independent of one or more growth factors, with the concomitant potential for uncontrolled proliferation. Although the precise functions of the myc proteins are unknown, they all bear the hallmarks of multimeric DNA-binding proteins probably involved in the regulation of expression of specific genes.

Organization and expression of the chicken N-myc gene

We cloned the chicken N-myc gene and analyzed its structure and expression. We found that it consisted of three exons with coding regions in exons 2 and 3. Comparison to mammalian N-myc genomic sequence indicated that nucleotide sequences of the 5'-flanking region, noncoding exon 1, and introns were not conserved, but coding and 3' noncoding sequences showed significant homology to mammalian N-myc. Alignment of deduced amino acid sequences of chicken and mammalian N-myc proteins revealed nine conserved domains interrupted by different lengths of nonhomologous sequences. Two of the domains were specific to N-myc proteins, and the other seven were common to c-myc proteins. Northern blot (immunoblot) and in situ hybridization analyses of 3.5-day-old chicken embryos revealed that high-level expression of the N-myc gene was confirmed to certain tissues, e.g., the central nervous system, neural crest derivatives, and mesenchyme of limb buds. In the beak and limb primordia, N-myc expression in the mesenchyme was higher toward the distal end, suggesting possible involvement in positional assignment of the tissue within the rudimentary structures.

High-frequency disruption of the N-myc gene in embryonic stem and pre-B cell lines by homologous recombination

Identification of gene function has often relied on isolation of mutant cells in which expression of the gene was inactivated. Gene targeting by homologous recombination in tissue culture now may provide a technology to rapidly and directly produce such mutant mammalian cells. We demonstrate that selection of embryonic stem and pre-B cell lines for expression of a promoterless construct containing murine N-myc genomic sequences fused to a gene encoding neomycin resistance allows highly efficient recovery of variants in which the endogenous N-myc gene is disrupted. The high frequency of N-myc gene disruption by this method should permit targeted disruption of both allelic N-myc copies in various cell lines to study N-myc function.

Organization and expression of the chicken N-myc gene

We cloned the chicken N-myc gene and analyzed its structure and expression. We found that it consisted of three exons with coding regions in exons 2 and 3. Comparison to mammalian N-myc genomic sequence indicated that nucleotide sequences of the 5'-flanking region, noncoding exon 1, and introns were not conserved, but coding and 3' noncoding sequences showed significant homology to mammalian N-myc. Alignment of deduced amino acid sequences of chicken and mammalian N-myc proteins revealed nine conserved domains interrupted by different lengths of nonhomologous sequences. Two of the domains were specific to N-myc proteins, and the other seven were common to c-myc proteins. Northern blot (immunoblot) and in situ hybridization analyses of 3.5-day-old chicken embryos revealed that high-level expression of the N-myc gene was confirmed to certain tissues, e.g., the central nervous system, neural crest derivatives, and mesenchyme of limb buds. In the beak and limb primordia, N-myc expression in the mesenchyme was higher toward the distal end, suggesting possible involvement in positional assignment of the tissue within the rudimentary structures.

Characterization of an embryonal rhabdomyosarcoma cell line showing amplification and over-expression of the N-myc oncogene

A parent rhabdomyosarcoma cell line designated SCMC-RM2 was established from bone-marrow tumor cells taken from an 11-year-old girl with an embryonal rhabdomyosarcoma. Subsequently a cloned SCMC-RM2-1 cell line was isolated from a parent line. These cell lines grew as adherent monolayers in liquid culture with a doubling time of 50 and 52 hr, respectively. In addition, colonies were established in soft agar, which grew in a dose-dependent fashion with a cloning efficiency of 0.7 and 0.8%, respectively. Chromosomal analysis showed these cell lines had neither double minutes nor homogeneously staining regions. Chromosome number ranged from 61 to 93, translocation; t(9;13)(p22;q14) was identified, and no alteration of chromosome 2 was observed. Surface membrane antigen profile of parent and cloned lines by using a panel of 24 monoclonal antibodies (MAbs) excluded the possibility of these being neuroblastoma cell lines. In addition, MAbs to the cytoplasmic protein desmin, myoglobin, muscle actin (alpha and gamma) and alpha-sarcomeric actin reacted with these cell lines, SCMC-RM2 and SCMC-RM2-1 being thus identified as rhabdomyosarcoma. Southern blot analyses revealed 8- and 7-fold amplification of the N-myc gene in SCMC-RM2 and SCMC-RM2-1 as compared with the promyelocytic cell line HL60. Over-expression of the N-myc mRNA was noted over control cell lines.

High-frequency disruption of the N-myc gene in embryonic stem and pre-B cell lines by homologous recombination

Identification of gene function has often relied on isolation of mutant cells in which expression of the gene was inactivated. Gene targeting by homologous recombination in tissue culture now may provide a technology to rapidly and directly produce such mutant mammalian cells. We demonstrate that selection of embryonic stem and pre-B cell lines for expression of a promoterless construct containing murine N-myc genomic sequences fused to a gene encoding neomycin resistance allows highly efficient recovery of variants in which the endogenous N-myc gene is disrupted. The high frequency of N-myc gene disruption by this method should permit targeted disruption of both allelic N-myc copies in various cell lines to study N-myc function.