Activated neu oncogene sequences in primary tumors of the peripheral nervous system induced in rats by transplacental exposure to ethylnitrosourea

The inhibiting activity of meadowsweet extract on neurocarcinogenesis induced transplacentally in rats by ethylnitrosourea

Inhibitory activity of a decoction of meadowsweet, given postnatally, was studied in rats at risk for neurogenic and renal tumors initiated by transplacental exposure to ethylnitrosourea (ENU). Chemical analysis of ethanol and aqueous extracts of meadowsweet has shown high content of biologically active flavonoids and tannins. Pregnant rats of LIO strain were given a single i.v. injection of ENU, 75 mg/kg, оn the 21st day of gestation. After weaning at 3 weeks after birth, the offspring were divided into two groups: the first was a positive control (ENU), while rats in the second group (ENU + meadowsweet) were given daily a decoction of meadowsweet as drinking water throughout their lifetime. All rats of the first group (ENU) developed multiple malignant tumors, which occurred in brain (86%), spinal cord (43%), peripheral and cranial nerves (29%) and in kidney (31%). More than one-third of CNS tumors were oligodendrogliomas. Mixed gliomas (oligoastrocytomas) occurred less frequently. All other types including astrocytomas, glioblastomas, and ependymomas were rare. All PNS tumors were neurinomas (schwannomas). The spectrum of tumors was similar in rats of the second group. Postnatal consumption of meadowsweet significantly reduced number of tumor-bearing rats (by 1.2 times), the incidence and multiplicity of CNS tumors (brain-by 2.0 and 2.1 times, respectively; spinal cord-by 3.1 and 3.0 times, respectively) and significantly increased latency period, compared to rats of the first group. No significant reduction in PNS or renal tumors was seen in rats given meadowsweet. Meadowsweet extract can be considered an effective cancer preventive agent especially as a neurocarcinogenesis inhibitor.

The development of Wilms tumor: from WT1 and microRNA to animal models

Wilms tumor recapitulates the development of the kidney and represents a unique opportunity to understand the relationship between normal and tumor development. This has been illustrated by the findings that mutations of Wnt/β-catenin pathway-related WT1, β-catenin, and WTX together account for about one-third of Wilms tumor cases. While intense efforts are being made to explore the genetic basis of the other two-thirds of tumor cases, it is worth noting that, epigenetic changes, particularly the loss of imprinting of the DNA region encoding the major fetal growth factor IGF2, which results in its biallelic over-expression, are closely associated with the development of many Wilms tumors. Recent investigations also revealed that mutations of Drosha and Dicer, the RNases required for miRNA generation, and Dis3L2, the 3'-5' exonuclease that normally degrades miRNAs and mRNAs, could cause predisposition to Wilms tumors, demonstrating that miRNA can play a pivotal role in Wilms tumor development. Interestingly, Lin28, a direct target of miRNA let-7 and potent regulator of stem cell self-renewal and differentiation, is significantly elevated in some Wilms tumors, and enforced expression of Lin28 during kidney development could induce Wilms tumor. With the success in establishing mice nephroblastoma models through over-expressing IGF2 and deleting WT1, and advances in understanding the ENU-induced rat model, we are now able to explore the molecular and cellular mechanisms induced by these genetic, epigenetic, and miRNA alterations in animal models to understand the development of Wilms tumor. These animal models may also serve as valuable systems to assess new treatment targets and strategies for Wilms tumor.

Replication across regioisomeric ethylated thymidine lesions by purified DNA polymerases

Causal links exist between smoking cigarettes and cancer development. Some genotoxic agents in cigarette smoke are capable of alkylating nucleobases in DNA, and higher levels of ethylated DNA lesions were observed in smokers than in nonsmokers. In this study, we examined comprehensively how the regioisomeric O(2)-, N3-, and O(4)-ethylthymidine (O(2)-, N3-, and O(4)-EtdT, respectively) perturb DNA replication mediated by purified human DNA polymerases (hPols) η, κ, and ι, yeast DNA polymerase ζ (yPol ζ), and the exonuclease-free Klenow fragment (Kf(-)) of Escherichia coli DNA polymerase I. Our results showed that hPol η and Kf(-) could bypass all three lesions and generate full-length replication products, whereas hPol ι stalled after inserting a single nucleotide opposite the lesions. Bypass conducted by hPol κ and yPol ζ differed markedly among the three lesions. Consistent with its known ability to efficiently bypass the minor groove N(2)-substituted 2'-deoxyguanosine lesions, hPol κ was able to bypass O(2)-EtdT, though it experienced great difficulty in bypassing N3-EtdT and O(4)-EtdT. yPol ζ was only modestly blocked by O(4)-EtdT, but the polymerase was strongly hindered by O(2)-EtdT and N3-EtdT. LC-MS/MS analysis of the replication products revealed that DNA synthesis opposite O(4)-EtdT was highly error-prone, with dGMP being preferentially inserted, while the presence of O(2)-EtdT and N3-EtdT in template DNA directed substantial frequencies of misincorporation of dGMP and, for hPol ι and Kf(-), dTMP. Thus, our results suggested that O(2)-EtdT and N3-EtdT may also contribute to the AT → TA and AT → GC mutations observed in cells and tissues of animals exposed to ethylating agents.

Ablation of T cell immunity differentially influences tumor risk in inbred BD rat strains

Inbred rat strains BDIX and BDIV are constitutionally susceptible and resistant, respectively, to the development of malignant peripheral nerve sheath tumors (MPNST) induced by neonatal exposure to N-ethyl-N-nitrosourea (EtNU). They represent a model system for analysis of molecular and cellular processes underlying differential cancer susceptibility. A point mutation in the Neu/ErbB-2 gene is an early marker of Schwann precursor cells at high risk of malignant conversion and is diagnostic of the resulting MPNST predominantly developing in the trigeminal nerves. Initially considerable amounts of Neu/ErbB-2-mutant cells arise in nerve tissue of both rat strains subsequently disappearing in resistant BDIV rats, but persisting and giving rise to MPNST in susceptible BDIX animals. An almost identical cellular immune response-sequentially involving macrophages, T helper- and cytotoxic T lymphocytes-is mounted in the trigeminal nerves of EtNU-treated rats of both strains. In this study, T cell maturation was prevented by neonatal thymectomy following EtNU-exposure. While resistance against MPNST development significantly decreased in BDIV rats MPNST incidence and survival time remained unaltered in thymectomized BDIX rats. Contrary to euthymic animals a number of both thymectomized BDIV and BDIX rats developed MPNST lacking the Neu/ErbB-2-mutation. This suggests that Schwann cells initiated by other genetic alterations can progress to full malignancy in immune-compromised rats only. T cell-dependent resistance against tumorigenesis originating from non-Neu/ErbB-2-mutant Schwann precursors might thus be shared by both strains while BDIV T lymphocytes additionally prevent the development of Neu/ErbB-2-mutant MPNST. Rat strain-specific differences in the interaction of T lymphocytes with (pre)malignant Neu-mutant cells may thus critically contribute to susceptibility and resistance towards EtNU-induced MPNST development.

Inheritance of susceptibility to induction of nephroblastomas in the Noble rat

Noble (Nb) strain rats are susceptible to nephroblastoma induction with transplacental exposure to direct-acting alkylating agent N-nitrosoethylurea (ENU), while F344 strain rats are highly resistant. To study the inheritance of susceptibility to induction of these embryonal renal tumors, fetal Nb and F344 rats and F1, F2 and reciprocal backcross hybrids were exposed transplacentally to ENU once on day 18 of gestation. Nephroblastomas developed in 53% of Nb offspring with no apparent gender difference, while no nephroblastomas developed in inbred F344 offspring. F1 and F2 hybrid offspring had intermediate responses, 28% and 30%, respectively. Nephroblastoma incidence in the offspring of F1 hybrids backcrossed to the susceptible strain Nb was 46%, while that in F1 hybrids backcrossed to resistant strain F344 was much lower (16%). Carcinogenic susceptibility is therefore consistent with the involvement of one major autosomal locus; the operation of a gene dosage effect; and a lack of simple Mendelian dominance for either susceptibility or resistance. Since established Wilms tumor-associated suppressor genes, Wt1 and Wtx, were not mutated in normal or neoplastic tissues, genomic profiling was performed on isolated Nb and F344 metanephric progenitors to identify possible predisposing factors to nephroblastoma induction. Genes preferentially elevated in expression in Nb rat progenitors included Wnt target genes Epidermal growth factor receptor, Inhibitor of DNA binding 2, and Jagged1, which were further increased in nephroblastomas. These studies demonstrate the value of this model for genetic analysis of nephroblastoma development and implicate both the Wnt and Notch pathways in its pathogenesis.

Signal integration: a framework for understanding the efficacy of therapeutics targeting the human EGFR family

The human EGFR (HER) family is essential for communication between many epithelial cancer cell types and the tumor microenvironment. Therapeutics targeting the HER family have demonstrated clinical success in the treatment of diverse epithelial cancers. Here we propose that the success of HER family-targeted monoclonal antibodies in cancer results from their ability to interfere with HER family consolidation of signals initiated by a multitude of other receptor systems. Ligand/receptor systems that initiate these signals include cytokine receptors, chemokine receptors, TLRs, GPCRs, and integrins. We further extrapolate that improvements in cancer therapeutics targeting the HER family are likely to incorporate mechanisms that block or reverse stromal support of malignant progression by isolating the HER family from autocrine and stromal influences.

The Interaction Mode of Premalignant Schwann and Immune Effector Cells during Chemically Induced Carcinogenesis in the Rat Peripheral Nervous System Is Strongly Influenced by Genetic Background

Contrary to rats of the highly sensitive inbred strain BDIX, BDIV rats are resistant to the induction of malignant schwannomas by N-ethyl-N-nitrosourea, arising predominantly in the trigeminal nerves. A point mutation of the neu/erbB-2 gene diagnostic of N-ethyl-N-nitrosourea-induced rat schwannomas is an early marker of Schwann precursor cells at high risk of subsequent malignant transformation. Neu-mutant cells initially arise at a similar frequency in sensitive and resistant animals. However, these cells disappear from the trigeminal nerves of resistant rats while giving rise to highly malignant schwannomas in susceptible animals. The resistance of BDIV rats obviously includes mechanisms to recognize and eliminate premalignant cells. The involvement of a cellular immune response was investigated in trigeminal nerves of both strains at different times after neonatal carcinogen exposure. An inflammatory reaction involving sequentially CD4(+) macrophages and T helper cells, CD8(+) cytotoxic T cells, and ED1(+) and ED2(+) macrophages was detected as a consequence of N-ethyl-N-nitrosourea treatment as early as postnatal day 40, briefly after the emergence of premalignant neu-mutant Schwann cells. It persisted throughout the observation period (40-250 days). However, there were no gross differences in immune cell counts between tumor-susceptible and tumor-resistant rats, except for a moderate increase of ED2(+) macrophages in N-ethyl-N-nitrosourea-treated BDIX rats only. Differential interactions of immune effector cells with premalignant Schwann cells may thus be involved in genetically determined tumor susceptibility or resistance, which could include functional differences of immune effector cells and/or a differential capability of premalignant Schwann cells to escape or counteract the cellular immune response.

Inducible and transmissible genetic events and pediatric tumors of the nervous system

Tumors of the nervous system most often occur in both children and adults as sporadic events with no family history of the disease, but they are also among the clinical manifestations of a significant number of familial cancer syndromes, including familial retinoblastoma, neurofibromatosis 1 and 2, tuberous sclerosis, and Cowden, Turcot, Li-Fraumeni and nevoid basal cell carcinoma (Gorlin) syndromes. All of these syndromes involve transmissible genetic risk resulting from loss of a functional allele, or inheritance of a structurally defective allele, of a specific gene. These genes include RB1, NF1, NF2, TSC1, TSC2, TP53, PTEN, APC, hMLH1, hPSM2, and PTCH, most of which function as tumor suppressor genes. The same genes are also observed in mutated and inactive forms, or are deleted, in tumor cells in sporadic cases of the same tumors. The nature of the mutational events that give rise to these inactivated alleles suggests a possible role of environmental mutagens in their causation. However, only external ionizing radiation at high doses is clearly established as an environmental cause of brain, nerve and meningeal tumors in humans. Transplacental carcinogenesis studies in rodents and other species emphasize the extraordinary susceptibility of the developing mammalian nervous system to carcinogenesis, but the inverse relationship of latency to dose suggests that low transplacental exposures to genotoxicants are more likely to result in brain tumors late in life, rather than in childhood. While not all neurogenic tumor-related genes in humans have similar effects in experimental rodents, genetically engineered mice (GEM) increasingly provide useful insights into the combined effects of multiple tumor suppressor genes and of gene-environment interactions in the genesis of brain tumors, especially pediatric brain tumors such as medulloblastoma.

Isolation of immortalized, INK4a/ARF-deficient cells from the subventricular zone after in utero N-ethyl-N-nitrosourea exposure

OBJECT

Brain tumors, including gliomas, develop several months after rats are exposed in utero to N-ethyl-N-nitroso-urea (ENU). Although pathological changes cannot be detected until these animals are several weeks old, the process that eventually leads to glioma formation must begin soon after exposure given the rapid clearance of the carcinogen and the observation that transformation of brain cells isolated soon after exposure occasionally occurs. This model can therefore potentially provide useful insights about the early events that precede overt glioma formation. The authors hypothesized that future glioma cells arise from stem/progenitor cells residing in or near the subventricular zone (SVZ) of the brain.

METHODS

Cells obtained from the SVZ or corpus striatum in ENU-exposed and control rats were cultured in an epidermal growth factor (EGF)-containing, chemically defined medium. Usually, rat SVZ cells cultured in this manner (neurospheres) are nestin-positive, undifferentiated, and EGF-dependent and undergo cell senescence. Consistent with these prior observations, control SVZ cells undergo senescence by the 12th to 15th doubling (20 of 20 cultures). In contrast, three of 15 cultures of cells derived from the SVZs of individual ENU-treated rats continue to proliferate for more than 60 cell passages. Each of these nestin-expressing immortalized cell lines harbored a common homozygous deletion spanning the INK4a/ARF locus and was unable to differentiate into neural lineages after exposure to specific in vitro stimuli. Nevertheless, unlike the rat C6 glioma cell line, these immortalized cell lines demonstrate EGF dependence and low clonogenicity in soft agar and did not form tumors after intracranial transplantation.

CONCLUSIONS

Data in this study indicated that immortalized cells may represent glioma precursors that reside in the area of the SVZ after ENU exposure that may serve as a reservoir for further genetic and epigenetic hits that could eventually result in a full glioma phenotype.

Causation of nervous system tumors in children: insights from traditional and genetically engineered animal models

Pediatric neurogenic tumors include primitive neuroectodermal tumors (PNETs), especially medulloblastoma; ependymomas and choroid plexus papillomas; astrocytomas; retinoblastoma; and sympathetic neuroblastoma. Meningiomas and nerve sheath tumors, although uncommon in childhood, are also significant because they can result from exposures of children to ionizing radiation. Specific chromosomal loci and specific genes are related to each of these tumor types. Virtually all these genes appear to act as tumor suppressor genes, which are inactivated in tumor cells by mutations or by chromosomal loss. In genetically engineered mice, some genes that are clearly associated with specific human tumors (e.g., RB1 in retinoblastoma and NF2 in meningiomas and schwannomas) have no such effect. Other genetic constructs in mice involving the genes p53, ptc1, and Nf1 have produced tumors remarkably similar to some of the human pediatric neoplasms. Some of these tumors become clinically apparent after only a few weeks, while the mice are still juveniles, especially when two or more tumor suppressor genes are inactivated in the same genetic construct. Conversely, at least one genetic pathway in rodents involving point mutation in the coding region of a transforming gene (neu in malignant schwannomas) does not appear to operate in any human tumors. The nervous system is markedly susceptible to experimental carcinogenesis during early life in rodents, dogs, primates, and other nonhuman species, and there is no obvious reason why this generalization should not also apply to humans. However, except for therapeutic ionizing radiation, no physical, chemical, or biological cause of human pediatric nervous system tumors is known. The failure of experimental transplacental carcinogenesis to mirror human pediatric experience more closely may reflect the need for multiple mutational events in target cells, and for experimental carcinogens that are capable of causing the full spectrum of mutations that occur in cancer-related genes in pediatric neurogenic tumors.

Hypertrophic neuropathies and malignant peripheral nerve sheath tumors in transgenic mice overexpressing glial growth factor beta3 in myelinating Schwann cells

The neuregulin-1 (NRG-1) family of growth and differentiation factors exerts a variety of effects on Schwann cells and their precursors during nervous system development; however, NRG-1 effects on adult Schwann cells are poorly defined. Several lines of evidence suggest that NRG-1 actions on adult Schwann cells are distinct from those observed during development. To test this hypothesis, we generated transgenic mice overexpressing the NRG-1 isoform glial growth factor beta3 (GGFbeta3) in myelinating Schwann cells [protein zero (P0)GGFbeta3 mice]. P0-GGFbeta3 mice develop resting tremors, gait abnormalities, decreased hindlimb strength, and paralysis by approximately 7 months of age. Sciatic nerves from these animals show a hypertrophic neuropathy characterized by demyelination, remyelination, and "onion bulb" formation. Development of this hypertrophic neuropathy is preceded by Schwann cell hyperplasia that is prominent in 1-month-old mice and present but decreased in 2- and 4-month-old animals. P0-GGFbeta3 mice also develop peripheral ganglion-associated malignant peripheral nerve sheath tumors. Motor, sensory, and sympathetic ganglia from 1-, 2-, and 4-month-old P0-GGFbeta3 mice uniformly contain intraganglionic, likely preneoplastic, Schwann cell proliferations. Examination of bromodeoxyuridine incorporation and caspase-3 activation in sciatic nerves and trigeminal ganglia indicates that Schwann cell hyperplasia in P0-GGFbeta3 mice reflects increased proliferation rather than decreased apoptosis. These observations are consistent with the hypothesis that GGFbeta3 induces proliferation of adult Schwann cells and demyelination of peripheral nerve axons. Furthermore, overexpression of this NRG-1 isoform frequently induces neoplastic Schwann cell proliferation within PNS ganglia, suggesting that NRG-1 may contribute to human Schwann cell neoplasia.

Hypertrophic neuropathies and malignant peripheral nerve sheath tumors in transgenic mice overexpressing glial growth factor beta3 in myelinating Schwann cells

The neuregulin-1 (NRG-1) family of growth and differentiation factors exerts a variety of effects on Schwann cells and their precursors during nervous system development; however, NRG-1 effects on adult Schwann cells are poorly defined. Several lines of evidence suggest that NRG-1 actions on adult Schwann cells are distinct from those observed during development. To test this hypothesis, we generated transgenic mice overexpressing the NRG-1 isoform glial growth factor beta3 (GGFbeta3) in myelinating Schwann cells [protein zero (P0)GGFbeta3 mice]. P0-GGFbeta3 mice develop resting tremors, gait abnormalities, decreased hindlimb strength, and paralysis by approximately 7 months of age. Sciatic nerves from these animals show a hypertrophic neuropathy characterized by demyelination, remyelination, and "onion bulb" formation. Development of this hypertrophic neuropathy is preceded by Schwann cell hyperplasia that is prominent in 1-month-old mice and present but decreased in 2- and 4-month-old animals. P0-GGFbeta3 mice also develop peripheral ganglion-associated malignant peripheral nerve sheath tumors. Motor, sensory, and sympathetic ganglia from 1-, 2-, and 4-month-old P0-GGFbeta3 mice uniformly contain intraganglionic, likely preneoplastic, Schwann cell proliferations. Examination of bromodeoxyuridine incorporation and caspase-3 activation in sciatic nerves and trigeminal ganglia indicates that Schwann cell hyperplasia in P0-GGFbeta3 mice reflects increased proliferation rather than decreased apoptosis. These observations are consistent with the hypothesis that GGFbeta3 induces proliferation of adult Schwann cells and demyelination of peripheral nerve axons. Furthermore, overexpression of this NRG-1 isoform frequently induces neoplastic Schwann cell proliferation within PNS ganglia, suggesting that NRG-1 may contribute to human Schwann cell neoplasia.

Overexpression of Ogt reduces MNU and ENU induced transition, but not transversion, mutations in E. coli

Studies of alkylation-induced mutations in Escherichia coli FX-11 revealed that both N-ethyl-N-nitrosourea (ENU) and N-methyl-N-nitrosourea (MNU) produced tRNA suppressor mutations (G:C to A:T) but only ENU produced a significant number of backmutations (A:T to G:C, A:T to T:A and A:T to C:G). Further, the ENU-induced transversions were absent in a UmuC-defective strain. This suggested that transition mutations could result from alkylation of guanine or thymine at the O(6)- and O(4)-positions, respectively, but that transversions might result from alkylation of thymine at the O(2)-position. To test this idea, the gene encoding O(6)-alkylguanine-DNA methyltransferase (ogt) was recombined into a plasmid to overexpress the cellular levels of this enzyme. Ogt protein can de-alkylate O(6)-alkylguanine and O(4)-alkylthymine, but not O(2)-alkylthymine. Cells harboring the plasmid (or a control plasmid lacking the ogt gene) were exposed to different concentrations of MNU or ENU and the resulting mutations were analyzed. With either MNU or ENU, the frequency of GlnV(o) suppressors was reduced about 70-fold in the Ogt-overexpressing cells, suggesting that Ogt eliminated O(6)-alkylguanine. Similarly, GlnU(o) suppressor frequencies were substantially reduced. In contrast, the reduction in frequency for the backmutations was slight, only about 2.5-fold with MNU and less than two-fold for ENU. However, DNA sequence analysis of the backmutations showed that only A:T to G:C transitions were affected by overexpression of Ogt, suggesting repair of O(4)-alkylthymine. The frequency of transversions, in comparison, was essentially unaltered. These results implicate O(2)-alkylthymine as a likely candidate for transversion mutagenesis induced by ENU.

Role of DNA repair in carcinogen-induced ras mutation

In this contribution we discuss the gene- and cell type-specific repair of miscoding DNA alkylation products as a risk parameter in both mutation induction and malignant transformation by N-nitroso carcinogens. Upon exposure to N-nitroso compounds such as N-methyl-N-nitrosourea (MeNU) or N-ethyl-N-nitrosourea (EtNU), about a dozen different alkylation products are formed in cellular DNA. Among these are O(6)-methylguanine (O(6)-MeGua) and O(6)-ethylguanine (O(6)-EtGua), respectively, which differ only by one CH(2) group in their alkyl residue and, when unrepaired, cause G:C-->A:T transition mutations by anomalous base pairing during DNA replication. We have analyzed the global and gene-specific repair of O(6)-MeGua and O(6)-EtGua in target cell DNA, ras gene mutation frequencies, and tumor incidence, in the model of mammary carcinogenesis induced in 50-day-old female Sprague-Dawley rats by a single application of MeNU or EtNU. Both carcinogens induce histologically indistinguishable mammary adenocarcinomas at high yield. In the target mammary epithelia, O(6)-MeGua is repaired at similar slow rates in both transcriptionally active genes (Ha-ras, beta-actin), silent genes (lgE heavy chain), and in bulk DNA, by the one-step repair protein O(6)-alkylguanine-DNA alkyltransferase (MGMT; low level of expression in the target cells). The slow repair of O(6)-MeGua translates into a high frequency of mutations at the central position of Ha-ras codon 12 (GGA) in MeNU-induced tumors. O(6)-EtGua, however, is removed approximately 20 times faster than O(6)-MeGua selectively from transcribed genes via an MGMT independent, as yet uncharacterized excision mechanism. Accordingly, no Ha-ras codon 12 mutations are found in the EtNU-induced mammary tumors. Neither MeNU- nor EtNU-induced tumors exhibit mutations at codons 13 and 61 of Ha-ras or at codons 12, 13 and 61 of Ki-ras. While a moderate surplus MGMT activity of the target cells - contributed by a bacterial MGMT transgene (ada) - significantly counteracts mammary tumorigenesis in MeNU-exposed rats, this is not the case in the EtNU-treated animals. Differential repair of structurally distinct DNA lesions in transcribed or (temporarily) silent genes thus determines the probability of mutation and, together with cell type-specific and interindividual differences in DNA repair capacity, influences carcinogenic risk.

Ethylnitrosourea induces neoplasia in zebrafish (Danio rerio)

The zebrafish (Danio rerio) has been successfully used to discover hundreds of genes involved in development and organogenesis. To address the potential of zebrafish as a cancer model, it is important to determine the susceptibility of zebrafish to tumors. Germ line mutations are most commonly induced for zebrafish mutant screens by exposing adult male zebrafish to the alkylating agent, ethylnitrosourea (ENU). To determine whether ENU induces tumors, we compared the incidence of tumors in ENU-treated fish with untreated controls. Interestingly, 18 of 18 (100%) fish mutagenized with either 2.5 or 3.0 mM ENU developed epidermal papillomas, which numbered 1 to 22 per fish, within 1 year of treatment. The induced epidermal lesions included epidermal hyperplasia, flat papillomas (0.2 to 1.2 mm), and pedunculated papillomas (1.2 to 8 mm in greatest dimension), but no skin cancers. Angiogenesis was evident in papillomas larger than approximately 1 mm. All but two papillomas contained the three cell types (keratinocytes, club, and mucous cells) of normal zebrafish epidermis; histologic variants lacked either club cells or mucous cells. Two cavernous hemangiomas and a single malignant peripheral nerve sheath tumor were also found in the treated fish. None of five untreated controls developed tumors. These studies establish the feasibility of the zebrafish as an experimental model for the study of skin tumors.

Tumors of the nervous system in carcinogenic hazard identification

In the absence of adequate data on humans, it is biologically plausible and prudent to regard agents and mixtures for which there is sufficient evidence of carcinogenicity in experimental animals, usually rats and mice, as if they presented a carcinogenic risk to humans. Prediction of cancer sites in humans from bioassay data in rodents is much less certain, however, regardless of organ or tissue. For tumors of the nervous system, there is practically no basis for judging the validity of such predictions, as only ionizing radiation is known to cause tumors of the central nervous system (CNS) in humans. Brain tumors are relatively uncommon findings in bioassays and are rare in untreated rodents, even in rats, which appear to be the most susceptible species. However, CNS tumors have been readily induced in rodents by systemic exposures to some chemicals, notably N-nitrosoalkylureas and other alkylating agents and certain alkyl hydrazine derivatives. CNS tumors in rodents have played a significant role in carcinogenic hazard evaluations of several other chemicals, including acrylonitrile, ethylene oxide, and acrylamide, and have been implicated as part of the tumor spectrum induced by vinyl chloride and certain inorganic lead compounds. In some of these evaluations, it is not certain that all tumors diagnosed as primary brain tumors were correctly identified. Diagnostic difficulties have been presented by undifferentiated small-cell tumors that may invade the brain, including carcinomas of the nasal cavity and undifferentiated schwannomas arising in cranial nerve ganglia, and by the difficulty of reliably distinguishing between focal reactive gliosis and early glial neoplasms. The most striking experimental finding regarding the induction by chemicals of tumors of the nervous system is the dramatically greater susceptibility of the fetal and neonatal nervous system to some carcinogens, as compared with the susceptibility of the nervous system in adults of the same species.

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

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

The biology of human epidermal growth factor receptor 2

Our understanding of the normal signaling mechanisms and functions of human epidermal growth factor receptor 2 (HER2) and other members of the HER family, namely epidermal growth factor receptor, HER3, and HER4, is growing rapidly. Activation of these receptors results in a diverse array of signals through the formation of homodimeric and heterodimeric receptor complexes; HER2 is the preferred dimerization partner for the other HERs. These oligomeric receptor complexes activate distinct signaling pathways, such as the Ras-MAPK and PI3-kinase pathways. These, in turn, affect various cellular processes. Recent gene deletion experiments in mice point to an important role for HER2 in cardiac and neural development, and evidence from other studies indicates that HER2 is involved in normal breast growth and development. Thus, HER2 is a key component of a complex signaling network that plays a critical role in the regulation of tissue development, growth, and differentiation.

Fast repair of O6-ethylguanine, but not O6-methylguanine, in transcribed genes prevents mutation of H-ras in rat mammary tumorigenesis induced by ethylnitrosourea in place of methylnitrosourea

Differential repair of structurally distinct mutagenic lesions in critical genes may influence the cellular risk of malignant conversion. We have investigated rat mammary tumorigenesis induced by N-ethyl-N-nitrosourea (EtNU) versus N-methyl-N-nitrosourea (MeNU) with respect to tumor incidence, ras gene mutation, and gene-specific repair. Both carcinogens induced mammary adenocarcinomas at high yield. In mammary epithelia (very low expression of O6-alkylguanine-DNA alkyltransferase, MGMT), O6-methylguanine (O6-MeGua) was eliminated from transcribed (H-ras and beta-actin) and inactive genes (IgE heavy chain) at the same slow rate as determined for bulk genomic DNA. The persistence of O6-MeGua in DNA correlated with a high frequency of G:C --> A:T transition mutations at codon 12 of the H-ras gene in MeNU-induced tumors. Repair of O6-ethylguanine (O6-EtGua), too, was slow in the IgE heavy chain gene as in bulk DNA. Contrasting with O6-MeGua, however, O6-EtGua was removed approximately 20 times faster from the active H-ras and beta-actin genes via MGMT-independent mechanism(s). Accordingly, no H-ras codon 12 mutations were found in EtNU-induced tumors, and 5- to 8-fold surplus alkyltransferase activity of the mammary epithelia-via a bacterial ada transgene-did not significantly counteract tumorigenesis in EtNU-exposed contrary to MeNU-treated animals. Neither MeNU- nor EtNU-induced tumors exhibited mutations at codons 13 and 61 of H-ras or codons 12, 13, and 61 of K-ras. Fast repair of O6-EtGua, but not O6-MeGua, in transcribed genes thus prevents mutational activation of H-ras when rat mammary carcinogenesis is initiated by EtNU in place of MeNU.

Multiple regulatory elements control transcription of the peripheral myelin protein zero gene

The gene encoding protein zero (P0), the most abundant protein of peripheral nervous system myelin, is expressed uniquely in Schwann cells. Previous studies have demonstrated that much of the cell type specificity of this expression is due to transcriptional control elements in the 1.1-kilobase pair 5'-regulatory region of the gene. We have now analyzed this region and have identified a set of functional elements in the 500 base pairs proximal to the transcription start site. DNA sequence conservation within the 5' regions of the human, mouse, and rat P0 genes correlates closely with the results of promoter deletion analysis of the 1.1-kilobase pair region assayed in Schwann cell cultures and reveals a potent proximal region from position -350 to +45. Sites of protein/DNA interaction within the proximal 500 base pairs of the promoter were identified by footprinting assays. Functional transcriptional elements were identified within the protected regions in the proximal promoter by mutation and transient transfection analysis in P0-expressing cell lines. The core (or basal) P0 promoter is identified as two regulatory elements, a G/C-rich element that binds nuclear factor Sp1 and a CAAT box that binds NF-Y. These core elements are essential for the transcription observed from the transfected promoter in cultured Schwann cells.

Development and utilization of the rat lymphocyte hprt mutation assay

Much of the recent progress in the field of genetic toxicology has come from an increased understanding of the molecular and cellular biology of the mammalian organism. Most prominent has been the ability to detect and quantify somatic mutation and relate the nature of the mutation to the specific type of chemical damage. Building upon the foundation of the human lymphocyte hypoxanthine guanine phosphoribosyl transferase (hprt) system, and later, the mouse hprt system, methods for the detection and quantification of hprt mutations in rat lymphocytes were developed. These methods are described in this report as is the ongoing validation of the assay. Additionally, the characterization of the recovered mutants and a comparison of the mutation spectrum in the rat lymphocyte system to the spectrum in cancer genes, such as H-ras and p53, and the spectrum in transgenic systems, such as lacI, are included. The development of the rat lymphocyte hprt system and validation of the assay at the molecular level, provide an effective and reliable measure of genetic damage in an in vivo system which is readily comparable to measurement of genetic damage in the human.

Functional nucleotide excision repair is required for the preferential removal of N-ethylpurines from the transcribed strand of the dihydrofolate reductase gene of Chinese hamster ovary cells

Transcription-coupled repair of DNA adducts is an essential factor that must be considered when one is elucidating biological endpoints resulting from exposure to genotoxic agents. Alkylating agents comprise one group of chemical compounds which modify DNA by reacting with oxygen and nitrogen atoms in the bases of the double helix. To discern the role of transcription-coupled DNA repair of N-ethylpurines present in discrete genetic domains, Chinese hamster ovary cells were exposed to N-ethyl-N-nitrosourea, and the clearance of the damage from the dihydrofolate reductase gene was investigated. The results indicate that N-ethylpurines were removed from the dihydrofolate reductase gene of nucleotide excision repair-proficient Chinese hamster ovary cells; furthermore, when repair rates in the individual strands were determined, a statistically significant bias in the removal of ethyl-induced, alkali-labile sites was observed, with clearance occurring 30% faster from the transcribed strand than from its nontranscribed counterpart at early times after exposure. In contrast, removal of N-ethylpurines was observed in the dihydrofolate reductase locus in cells that lacked nucleotide excision repair, but both strands were repaired at the same rate, indicating that transcription-coupled clearance of these lesions requires the presence of active nucleotide excision repair.

Growth inhibition by dominant-negative mutations of the neu-encoded oncoprotein

In the present study, kinase-deficient mutants of the neu gene were constructed in order to generate dominant-negative receptor molecules, which should abolish phosphorylation of receptor complexes. One construct carried a mutation of the putative ATP-binding site (K758M), while the other mutant was generated by deletion of the kinase domain (ID400). Neither receptor showed phosphorylation by in vitro kinase assay. When NIH3T3 fibroblasts were co-transfected by the oncogenic neu gene and one of either construct, the transforming effect could be partially reversed. Therefore, kinase-negative mutations of the neu-encoded receptor seemed to have a dominant-negative effect on the action of the activated protein. To test this hypothesis, rat neurinoma cell lines containing oncogenic neu genes were transfected with the constructs. Expression of the kinase-defective mutants and reduced phosphorylation could be detected in different clones derived from single transfected cells. Striking growth inhibition and reduction of colony formation in soft agar were observed in these cell lines when compared with untransfected cells. Thus, kinase-deficient mutants exert a dominant-negative effect on phosphorylation of receptor complexes, resulting in a reversion of the transformed phenotype.

Age- and dose-dependent transplacental carcinogenesis by N-nitrosoethylurea in Syrian golden hamsters

Syrian golden hamsters have a very short period (15 days) of gestation. The implantation of the blastocyst occurs on day 5, embryogenesis proceeds very rapidly thereafter and neural tube closure is completed by day 9. In the present study the effects of two different doses of N-nitrosoethylurea (NEU) administered at various stages of gestation were quantitatively evaluated in Syrian golden hamsters. NEU at either 0.2 or 0.5 mmol/kg was administered transplacentally as a single i.p. injection to pregnant hamsters on gestation days 7, 8, 9, 10, 11, 12, 13, or 14. The incidence, latency period and multiplicity of tumors varied with the dose of NEU and the stage of development at the time of NEU administration. Although tumors of the peripheral nervous system predominated, a variety of other tumors, including melanomas and visceral tumors of epithelial and mesenchymal origin, were also observed in hamster offspring exposed transplacentally to NEU. Sensitivity to transplacental carcinogenesis was maximal during late gestation and very low before day 9.

Neuregulins with an Ig-like domain are essential for mouse myocardial and neuronal development

Neuregulins are ligands for the erbB family of receptor tyrosine kinases and mediate growth and differentiation of neural crest, muscle, breast cancer, and Schwann cells. Neuregulins contain an epidermal growth factor-like domain located C-terminally to either an Ig-like domain or a cysteine-rich domain specific to the sensory and motor neuron-derived isoform. Here it is shown that elimination of the Ig-like domain-containing neuregulins by homologous recombination results in embryonic lethality associated with a deficiency of ventricular myocardial trabeculation and impairment of cranial ganglion development. The erbB receptors are expressed in myocardial cells and presumably mediate the neuregulin signal originating from endocardial cells. The trigeminal ganglion is reduced in size and lacks projections toward the brain stem and mandible. We conclude that IgL-domain-containing neuregulins play a major role in cardiac and neuronal development.

Schwann cell tumors express characteristic patterns of CD44 splice variants

Members of the CD44 family of cell surface hyaluronate-binding proteins have been implicated in cell migration, cell-matrix interactions and tumor progression. To determine whether these proteins might play a role in the normal functions of Schwann cells and in their tumorigenesis, we examined the patterns of CD44 expression in Schwann cells from rat peripheral nerve, rat Schwann cell tumor lines, and human schwannomas. Normal rat spinal nerves and primary Schwann cell cultures expressed standard CD44 (CD44s) but not alternatively spliced variant isoforms. In contrast, rat Schwann cell tumor lines expressed both CD44s and a number of variants, including proteins containing sequences encoded by exon v6. Furthermore, we found that these cell lines bind hyaluronate, and that their cell surface hyaluronate binding correlates with CD44 expression. All of the human schwannomas also expressed CD44 variants, especially epitopes encoded by exon v5, the border between v7 and v8, and v9-10. These data indicate that Schwann cells normally express CD44s, that Schwann cell tumors express both CD44s and particular variants of CD44, and that CD44s and possibly variants of CD44 are involved in hyaluronate recognition by Schwann cell tumors.

Transformation of type 1 astrocytes with N-ethyl-N-nitrosourea: establishment of an in vitro system and the role of the p53 gene

N-ethyl-N-nitrosourea (ENU)-induced gliomas, animal models of human gliomas, are most frequently oligodendrocytic, while human gliomas tend to be astrocytic. To facilitate a detailed study of human glial carcinogenesis, we developed an in vitro system using type 1 astrocyte transformation with ENU. Type 1 astrocytes from fetal Wistar rat brain were treated by a single dose of ENU. Transformed colonies appeared 50 days after exposure to single doses of ENU greater than 150 micrograms/mL. Cloned cells from these colonies retained the immunohistochemical characteristics of type 1 astrocytes. They showed rapid growth and high saturation densities, colony formation in low (2%) serum medium and gave rise to tumors when injected into nude mice. When p53 expression was studied at each passage, a single cell positive for mutant p53 protein emerged 40 days after ENU treatment. In the next 1-3 passages, the mutant p53 positive cell formed piled-up colonies and exhibited dominant growth. Northern blot analysis showed markedly increased accumulations of p53 mRNA in transformed cells. This in vitro transformation system of type 1 astrocytes provides a valuable tool for further investigations of astrocyte carcinogenesis.

Analysis of in vivo mutation induced by N-ethyl-N-nitrosourea in the hprt gene of rat lymphocytes

The rat lymphocyte hprt assay measures in vivo mutagenicity by quantifying the frequency of 6-thioguanine-resistant (TGr) spleen lymphocytes cultured in vitro. In this study we have examined the types of mutations induced in the hprt gene of TGr lymphocyte clones from female Fischer 344 rats exposed to 100 mg/kg N-ethyl-N-nitrosourea (ENU). Hprt exons 3 and 8 were amplified from DNA extracted from each of 249 clones, and the resulting products were screened for mutant:wild-type heteroduplex formation by denaturing gradient gel electrophoresis. The analysis revealed 59 clones with mutations in exon 3, and 20 clones with mutations in exon 8. DNA sequence analysis of the heteroduplexes identified 84 mutations: all of the mutations were base pair substitutions, and 88% were mutations of A:T base pairs. At least 82% were induced independently. These results suggest that the mutations found in TGr rat lymphocytes from ENU-treated rats were due mainly to ethylthymidine adducts. In addition, a comparison of these results with previously reported in vivo ENU mutational profiles indicates that the types of mutation detected by heteroduplex screening of rat hprt exons 3 and 8 are representative of mutation in the entire protein coding sequence.

The role of mutagenic metal ions in mediating in vitro mispairing by alkylpyrimidines

A variety of alkylating mutagens and carcinogens produce pyrimidine adducts in DNA that block DNA synthesis in vitro. Since DNA synthesis past the lesion is a necessary step to produce mutations, we investigated the role of the mutagenic metal ion Mn++ in facilitating DNA synthesis past alkylpyrimidines. In the presence of the natural metal activator Mg++, N3-ethyldeoxythymidine (N3-Et-dT) and O2-ethyldeoxythymidine (O2-Et-dT), present at a single site in DNA, blocked in vitro DNA synthesis 3' to the lesion and after incorporating dA opposite each lesion. The presence of Mn++ permitted postlesion synthesis with dT misincorporated opposite N3-Et-dT and O2-Et-dT, implicating these lesions in A.T-->T.A transversion mutagenesis. The DNA synthesis block by O4-ethyldeoxythymidine (O4-Et-dT) in the presence of Mg++ was partial and was also removed by Mn++. Consistent with in vivo studies, dG was incorporated opposite O4-Et-dT during postlesion synthesis, leading to A.T-->G.C transition mutagenesis. We also have discovered a new class of DNA adducts, N3-hydroxyalkyldeoxyuridine (3-HA-dU) lesions, which are produced by mutagenic and carcinogenic aliphatic epoxides. 3-HA-dU is formed after initial alkylation at the N3 position of dC followed by a rapid hydrolytic deamination. As observed with the analogous mutagenic N3-Et-dT, the ethylene oxide-induced 3-hydroxyethyldeoxyuridine (3-HE-dU) blocked in vitro DNA synthesis, which could be by-passed in the presence of Mn++. The nucleotide incorporated opposite 3-HE-dU during postlesion synthesis is being identified. These studies suggest a role for Mn++ in mediating mutagenic and carcinogenic effects of environmentally important ethylating agents and aliphatic epoxides.

Pathophysiology of glial growth factor receptors

Activation and proliferation of glial cells are common events in the pathology of the nervous system. Although we are only beginning to understand the molecular signals leading to glial activation in vivo, there is increasing evidence that growth factors and their receptors may play an important part. In this paper we summarize the data on the pathophysiology of glial growth factor receptors and their ligands in the central and peripheral nervous systems.

Infrequent mutation of Ha-ras and p53 in rat mammary carcinomas induced by 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine

2-Amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) is the most abundant of the heterocyclic amines, a group of potent carcinogens contained in cooked meat and fish. Female F344 rats fed a diet containing 100 or 400 ppm PhIP developed mammary carcinomas within 104 or 52 wk, respectively, at the rate of 47% for each group; these carcinomas were examined for mutations in three members of the ras gene family and in the p53 gene. Single-strand conformation polymorphism (SSCP) analysis and direct sequencing demonstrated a G-->A transition at the second position of Ha-ras codon 12, with the resultant substitution of glutamic acid for glycine, in two of 10 carcinomas induced by 100 ppm PhIP and in one of seven induced by the 400 ppm dose. No mutations in Ki-ras or N-ras were detected. cDNA polymerase chain reaction-SSCP analysis and direct sequencing demonstrated a G-->T transversion at the third position of p53 codon 130, with the resultant substitution of asparagine for lysine, in one of the 10 carcinomas induced by 100 ppm of PhIP for which freshly frozen samples were available. PhIP-induced rat mammary carcinogenesis can be regarded as a unique system in that rat mammary carcinomas are negative for ras and p53 mutations.

Chemical carcinogenesis in the nervous system: past and future

The model of experimental tumors of the nervous system has greatly contributed to our understanding of growth and management of intracranial tumors, but has been somewhat neglected in the last years, because a wealth of new data concerning oncogenic action came from viral oncogenesis. These new issues led to a much better insight into human tumor induction and promotion. Yet one example of the impact of oncogenic transformation stems from the "neurooncogenic" model: the discovery of the neu oncogene and its product as a putative differentiation receptor in the cell membrane of experimental Schwann cell derived tumors. In the light of this unique finding the history of the "neurooncogenic" model and the morphological and "clinical" result of tumors produced within the model are reviewed. There is a large open field for future investigation both in basic and applied science.

Infrequent transforming mutations in the transmembrane domain of the neu oncogene in spontaneous rat schwannomas

Ethylnitrosourea (ENU) given transplacentally to rats induces schwannomas of the cranial, spinal, and peripheral nerves, with a high frequency of mutations in the neu proto-oncogene. To establish the requirement for such mutations in tumorigenesis of the Schwann cell, spontaneous schwannomas from BD-VI rats were evaluated for transforming mutations in the transmembrane domain of the protein encoded by the neu proto-oncogene. While all five schwannomas induced transplacentally with ENU were shown to contain T-->A transversions in base 2012 of neu by selective oligonucleotide hybridization and dideoxy sequencing of polymerase chain reaction-amplified products from paraffin sections, only one of nine spontaneous schwannomas from untreated rats had the same mutation. Examination of tumors for mutations in codon 12 of Ki-ras revealed normal alleles. Therefore, the high frequency of mutations in neu in ENU-induced tumors may be directly attributable to the carcinogen or to the period of development at which exposure occurred, and transforming mutations of the transmembrane domain of neu are not required for tumorigenesis of the Schwann cell.

Development of malignant fibrous histiocytoma induced by 7,12-dimethylbenz[a]anthracene in the rat: characterization of early atypical cells

Morphologically atypical cells were first detected in the adjacent connective tissue 98 days after implanting a paraffin pill containing 2 mg of 7,12-dimethylbenz[a]anthracene (DMBA) into the subcutaneous tissues of rats. These cells subsequently formed groups and finally produced gross malignant fibrous histiocytomas (MFH). Early atypical cells were located between proliferating fibroblasts and histiocytes in the center of a fibrous capsule surrounding the DMBA pill. They exhibited a smooth cell surface, dilated rough endoplasmic reticulum, multiple Golgi complexes, and were often associated with newly formed collagen. These cells incorporated [3H]thymidine and [3H]proline intensively, and showed weak acid phosphatase activity but no features diagnostic of macrophages (microvilli, numerous lysosomes, high acid phosphatase and non-specific esterase activities, antigens recognized by monoclonal antibodies ED1 and OX-42 and vital staining with trypan blue). There was no evidence that atypical cells differentiated into muscle cells (no expression of desmin or the alpha-sarcomeric form of actin) or Schwann cells (no expression of S-100 protein). No point mutation in the neu gene at nucleotide 2007, specific for N-ethyl-N-nitrosourea- and DMBA-induced malignant rat schwannomas, was detected by polymerase chain reaction (PCR)-restriction fragment length polymorphism (RFLP) analyses. These results support the view that malignant fibrous histiocytoma is derived from immature fibroblasts exhibiting pronounced phenotypic diversity during the later stages of carcinogenesis.

Migration of fresh human malignant astrocytoma cells into hydrated gel wafers in vitro

Individual astrocytoma cells expressing a cytoplasmic form of p185c-neu migrated along basement membrane lined surfaces after xenografing fresh low or high grade human malignant astrocytomas into host rat brain. We now study the migratory capacity of fresh human malignant astrocytoma cells seeded on hydrated gel wafers composed of artificial basement membrane or collagen I, a normal and lesion-related CNS extracellular matrix component. Approximately 10(7) mechanically disrupted cells (with small clumps) of 3 fresh low grade and 6 fresh high grade astrocytomas were seeded on the surface of artificial basement membrane and collagen I wafers (11 x 16 mm). The wafers were then prepared for scanning electron microscopy and immunohistochemistry at 1, 3, 5, and 7 days after seeding. Regardless of tumor grade, a morphologically similar class of cells was observed to migrate through collagen I gels in 24 hours and 0.5-1.5 mm into artificial basement membrane gels in 7 days. Immunohistochemistry revealed that the migrated cells from low and high grade astrocytomas were positive for glial fibrillary acidic protein (GFAP)) and expressed cytoplasmic human-specific p185c-neu. These data indicate that fresh human malignant astrocytoma cells that contain GFAP and express cytoplasmic p185c-neu have a high degree of migratory capacity and could be the cell in the tumor involved in intraparenchymal metastasis and poor patient survival in high grade astrocytomas of the human brain.

Glial growth factors are alternatively spliced erbB2 ligands expressed in the nervous system

Glial growth factors, proteins that are mitogenic for Schwann cells, and several ligands for the p185erbB2 receptor, are products of the same gene. Alternative splicing of the messenger RNA generates an array of putative membrane-attached, intracellular and secreted signalling proteins, at least some of which are expressed in the developing spinal cord and brain. These factors are probably important in the development and regeneration of the nervous system.

Neural expression and chromosomal mapping of Neu differentiation factor to 8p12-p21

Neu differentiation factor (NDF/heregulin) is a 44-kDa glycoprotein that interacts with the Neu/ErbB-2 receptor tyrosine kinase to increase its phosphorylation on tyrosine residues. In vitro NDF promotes differentiation of certain mammary tumor cell lines to milk-producing cells. As a first step toward understanding the physiological role of NDF, we performed in situ hybridization analyses to determine mRNA distribution in the mouse embryo and to map the gene to human karyotypes. In 14.5-day-postcoitum mouse embryos, NDF expression is confined predominantly to the central and peripheral nervous system, including the neuroepithelium that lines the lateral ventricles of the brain, the ventral horn of the spinal cord, and the intestinal as well as dorsal root ganglia. Other tissues that contain NDF transcripts are the adrenal gland, liver, and distinct cell layers of the dermis and germinal ridge. In situ hybridization of a 3H-labeled probe to human metaphase spreads localized the NDF gene to the short arm of chromosome 8 at bands p12-p21.

Perinatal and multigenerational effect of carcinogens: possible contribution to determination of cancer susceptibility

Perinatal exposure to carcinogens may contribute to the determination of susceptibility to cancer in two situations: a) exposure in utero of embryonal or fetal somatic cells to carcinogens, and b) prezygotic exposure of the germ cells of one or both parents to carcinogens. Epidemiological as well as experimental studies demonstrate that exposure to carcinogens in utero increases the occurrence of cancer postnatally. Studies with experimental animals suggest that prezygotic exposure of germ cells to carcinogens can result in an increased incidence of cancer not only in immediate but also in subsequent generations. Although several studies suggest a transgeneration effect of carcinogens in human populations, the evidence cannot yet be considered conclusive. In particular, while some hypotheses can be advanced, the mechanism(s) by which increased susceptibility or predisposition to cancer may be transmitted via the germ cells has not yet been clarified. In conjunction with exposure both in utero and prezygotically, it is important to consider postnatal exposure to possible tumor-promoting agents. Results from experimental animals suggest that oncogenes can be activated transplacentally, and human studies indicate that tumor-suppressor gene inactivation may be involved in the transgenerational effect of carcinogens.

A new approach to the molecular basis of neoplastic transformation in the brain

Gene transfer into living organisms has evolved as a powerful approach to study in vivo effects of specific genes and to devise animal models of hereditary disorders. We have been particularly interested in an approach to introducing transforming genes into the nervous system. Since specific promoter sequences for targeting the expression of a transgene to many cell types of the brain are not yet isolated, a suitable transgenic mouse model was not available for these experiments. This has prompted us to develop an alternative strategy for gene transfer into the brain. The rationale is to introduce foreign genes into fetal brain transplants using embryonic CNS as donor tissue and replication-defective retroviral vectors as genetic vehicles. This technique relies on the extraordinary organotypic differentiation capacity of neural grafts and the expression of retrovirally transmitted genes in different cell types of CNS transplants. In contrast to transgenic animals but analogous to sporadic tumour formation, target cells for the retroviral vector will develop in an environment of unmodified neural tissue. We have introduced a number of neurotropic oncogenes into fetal brain transplants to study potential effects of such genes on the brain. This review will summarize some of the findings which have emerged from this experimental study including the tropism of several genes for endothelial cells, attempts to identify cooperating combinations of transforming genes and an experimental model for primitive neuroectodermal tumours in neural grafts.

Mutational spectrum at the Hprt locus in splenic T cells of B6C3F1 mice exposed to N-ethyl-N-nitrosourea

We have determined the mutational spectrum of N-ethyl-N-nitrosourea (ENU) in exon 3 of the hypoxanthine (guanine) phosphoribosyltransferase gene (Hprt) in splenic T cells following in vivo exposure of male B6C3F1 mice (5-7 weeks old) to ENU. Hprt- mutants were isolated by culturing splenic T cells in microtiter dishes containing medium supplemented with interleukin 2, concanavalin A, and 6-thioguanine. DNA was extracted from 6-thioguanine-resistant colonies and amplified by the polymerase chain reaction (PCR) using primers flanking Hprt exon 3. Identification of mutant sequences and purification of mutant DNA from contaminating wild-type Hprt DNA was accomplished by denaturing-gradient gel electrophoresis. Purified mutant DNA was then sequenced. Treatment of mice with ENU at 40 mg/kg of body weight produced a Hprt- mutant frequency of 7.3 x 10(-5) in splenic T cells, approximately 35-fold above background levels. Sixty-nine of the 521 Hprt- mutants analyzed contained mutations in exon 3 (13%). Transversions and transitions at A.T base pairs dominated the spectrum; 62 of the 69 exon 3 mutations were at A.T base pairs (14 different sites). Thirteen of 14 thymine bases undergoing mutation (61 of 62 mutations at A.T bases) were located on the nontranscribed strand of exon 3. The majority of the remaining mutations (6 of 69) were transitions at a single G.C base pair. These results suggest the importance of thymidine alkylation in ENU-induced mutagenesis in vivo. The mouse Hprt- T-cell cloning/sequencing assay described here may represent a useful system for studying the molecular mechanism of chemically induced mutation occurring in vivo in an endogenous gene.

Genotypic analysis of N-ethyl-N-nitrosourea-induced mutations by Taq I restriction fragment length polymorphism/polymerase chain reaction in the c-H-ras1 gene

In genotypic mutation analysis DNA sequence changes are determined without the in vivo or in vitro selection of phenotypically altered cells. We have studied the induction of base-pair changes by N-ethyl-N-nitrosourea in Taq I endonuclease recognition site 2508-2511 (TCGA) of the c-H-ras1 gene in human fibroblasts by the restriction fragment length polymorphism/polymerase chain reaction (RFLP/PCR) method. This site contains the four bases, and all 12 possible single base-pair changes can be monitored. The transition of guanine to adenine at position 2510 was the major mutation detected by lambda plaque oligonucleotide hybridization and quantitative sequence analysis of the RFLP/PCR products. It involves the G residue of the CpG sequence of the coding strand. Data calibration with an internal mutant standard indicates that absolute frequencies for this transition lie in the range of 4-12 x 10(-7). The present study documents the capacity of the RFLP/PCR approach to measure mutagen-induced base-pair changes in a specific gene sequence without the selection of a phenotypically altered cell.

Expression of the neu proto-oncogene by Schwann cells during peripheral nerve development and Wallerian degeneration

The neu gene, which encodes a putative tyrosine kinase growth factor receptor termed p185neu, was originally identified as a dominant transforming gene in neurogliomas and schwannomas induced by transplacental treatment of rat embryos with ethylnitrosourea. The present studies were undertaken to determine the expression pattern of the neu gene in peripheral nerve. Northern blot analysis of total RNA isolated from rat sciatic nerves demonstrated prominent neu mRNA expression on postnatal days 1 and 7, with substantially lower expression up to adulthood. Immunohistochemical studies confirmed expression of p185neu by Schwann cells (SC) in developing sciatic nerve and minimal p185neu immunoreactivity in adult nerves. However, neu mRNA and p185neu protein progressively increased following sciatic nerve transection in adult animals. In addition, neu mRNA and p185neu were found in neonatal rat sciatic nerve SC and several SC-derived cell lines. In resting SC, neu mRNA was expressed at a low level, but was greatly increased by treatment with forskolin and glial growth factor. These studies demonstrate that the neu gene and its protein product, p185neu, are expressed by SC both in vivo and in vitro and suggest that p185neu plays a role in the regulation of SC proliferation or differentiation.

In vitro DNA replication implicates O2-ethyldeoxythymidine in transversion mutagenesis by ethylating agents

A 36-nucleotide oligomer containing a single O2-ethyldeoxythymidine (O2-Et-dT) adduct at a specific site was synthesized. The oligomer, which corresponds to a specific DNA sequence in gene G of bacteriophage phi X174, was used as a template by T7 DNA polymerase to investigate the in vitro mutagenic specificity of O2-Et-dT. At 10 microM dNTP and 5 mM Mg++, the progress of T7 DNA polymerase was interrupted by O2-Et-dT: 80% 3' to O2-Et-dT and 14% after incorporating a nucleotide opposite O2-Et-dT (incorporation-dependent blocked product). DNA synthesis past the lesion was low (6%). Incorporation of a nucleotide opposite O2-Et-dT and subsequent postlesion synthesis were enhanced by increasing the dNTP concentration, with postlesion synthesis reaching 30% at 200 microM. Postlesion synthesis was further increased to 45% by addition of 10 mM dAMP to the polymerization reactions. DNA sequencing revealed that both dA and dT were incorporated opposite O2-Et-dT with dA incorporation impeding the progress of DNA synthesis. dT incorporation was efficiently extended implicating O2-Et-dT in transversion mutagenesis in vivo. These studies provide a basis for understanding the molecular mechanisms by which ethylating agents contribute to cytotoxicity, A.T transversion mutagenesis and activation of the oncogene neu by an A.T----T.A transversion event in rat neuroblastomas.

Expression of the neu oncogene under the transcriptional control of the myelin basic protein gene in transgenic mice: generation of transformed glial cells

We have taken a transgenic approach in an effort to specifically transform oligodendrocytes, the myelinating glial cells of the central nervous system (CNS). Transgenic mice were generated with a DNA construct that contained the activated neu oncogene under the transcriptional control of the myelin basic protein (MBP) gene. The MBP/c-neu transgenic animals have experienced a low incidence of brain tumors that express molecular markers specific to oligodendrocytes, providing a mouse model to study the formation and progression of oligodendrocyte tumors. A tumor from a transgenic animal has been dispersed in culture, and transformed cells that express properties of oligodendrocytes and astrocytes have been maintained. The degree to which these cells express phenotypic characteristic of oligodendrocytes or astrocytes is influenced by culture conditions. These transformed cells should serve as a valuable resource with which to study various molecular and biochemical aspects of the myelination process, as well as the lineage interrelationship of CNS glial cells.

Early mutation of the neu (erbB-2) gene during ethylnitrosourea-induced oncogenesis in the rat Schwann cell lineage

The development of malignant tumors of the peripheral nervous system (schwannomas) within a defined intracranial section of the rat trigeminal nerve ("trigeminal box") was used as a model to identify genetic alterations typically associated with the process of cell-lineage-specific oncogenesis induced by exposure to N-ethyl-N-nitrosourea on postnatal day 1. All 47 trigeminal schwannomas (and 12 extracranial neurinomas) investigated carried a T.A----A.T transversion mutation at nucleotide 2012 of the neu (erbB-2) gene sequence encoding the transmembrane domain of pg185neu. This mutation was absent in all 18 tumors in the brain and spinal cord (central nervous system) isolated from the same animals. Identical observations were made in cell lines derived from N-ethyl-N-nitrosourea-induced rat schwannomas vs. brain tumors. By asymmetric PCR and mutant-specific Mnl I restriction fragment length analyses, cells carrying the mutant neu allele became detectable and could be localized within the trigeminal box as early as 7 days after the carcinogen pulse. The proliferation rate of the mutant cells strongly exceeded that of the wild-type cells up to the time of maturation of the trigeminal nerve around postnatal day 30 and thereafter to a lesser extent until the appearance of schwannomas. A specific mutation of the neu gene thus represents a very early, probably the first, step in the malignant conversion of immature rat Schwann cells exposed to N-ethyl-N-nitrosourea in vivo and is diagnostic for a subset of proliferative cells at high risk of progressing toward the expression of fully malignant phenotypes. Loss of heterozygosity for the mutant neu allele is a candidate event for a critical second step in the process.

Rat model of the human "Triton" tumor: direct genetic evidence for the myogenic differentiation capacity of schwannoma cells using the mutant neu gene as a cell lineage marker

Spontaneous myogenic differentiation was observed in 2 out of 15 cases when cells from schwannomas induced in the offspring of BDIX rats by transplacental exposure to N-ethyl-N-nitrosourea (EtNU) were grown in monolayer culture following fluorescence-activated cell sorting with monoclonal antibody (Mab) 217c. Myotubes and numerous mononucleated cells no longer expressed the Schwann cell antigens 217c and S-100 protein, but rather revealed the presence of desmin, the alpha-sarcomeric form (alpha-sr) of actin, and the cell surface antigen specified by Mab RB21-7, a 250 kD glycoprotein sharing an epitope with the neural cell adhesion molecule (N-CAM). Subcutaneous reimplantation of such cells into syngeneic animals led to the appearance of tumors composed of both S-100 positive Schwann cells and desmin and alpha-sr-actin positive rhabdomyoblasts, thus closely resembling the human "Triton" tumor. With the use of the polymerase chain reaction and allele-specific oligonucleotide hybridization, DNA isolated from individual myotubes was analyzed for the presence of a T----A transversion mutation at nucleotide 2012 of the neu gene, which is diagnostic of EtNU-induced rat schwannomas. All of the amplified DNA isolates contained the mutant neu allele, thus providing direct genetic proof for the capacity of mammalian neuroectodermal cells for myogenic differentiation.

The role of non-ras transforming genes in chemical carcinogenesis

DNA transfection experiments using the NIH 3T3 mouse fibroblast cell line have demonstrated that chemically induced tumors and chemically transformed cell lines frequently contain dominant transforming genes. Although many of the genes detected using the NIH 3T3 transfection-transformation assay are activated versions of H-ras, K-ras, and N-ras, in some experimental systems activated forms of genes such as met and neu that are unrelated to ras have been observed. The activated met gene was originally detected in a human cell line that had been transformed by exposure to N-methyl-N'-nitro-N-nitrosoguanidine. Subsequent studies demonstrated that the met proto-oncogene encodes a novel growth factor receptor and that gene activation involves the production of a chimeric gene in which the regions of met encoding the extracellular and transmembrane domains of the receptor are replaced by the 5'-region of an unrelated gene called trp. The activated neu gene was detected in tumors of the nervous system that arose in mice following transplacental exposure to N-ethyl-N-nitrosourea. The neu gene also encodes a novel growth factor receptor but, in contrast to met, its activation involves a single T:A----A:T point mutation in the region of the neu gene encoding the receptor transmembrane domain. The presence of genetic alterations in chemically induced malignancies has also been assessed in cytogenetic studies and by Southern analysis of DNA from neoplastic cells.(ABSTRACT TRUNCATED AT 250 WORDS)