The extent of heterocellular communication mediated by gap junctions is predictive of bystander tumor cytotoxicity in vitro

Herpes simplex virus thymidine kinase (HSV-tk)/ganciclovir (GCV) viral-directed enzyme prodrug gene therapy causes potent, tumor-selective cytotoxicity in animal models in which HSV-tk gene transduction is limited to a minority of tumor cells. The passage of toxic molecules from HSV-tk+ cells to neighboring HSV-tk- cells during GCV therapy is one mechanism that may account for this "bystander" cytotoxicity. To investigate whether gap junction-mediated intercellular coupling could mediate this bystander effect, we used a flow cytometry assay to quantitate the extent of heterocellular coupling between HSV-tk+ murine fibroblasts and both rodent and human tumor cell lines. Bystander tumor cytotoxicity during GCV treatment in a coculture assay was highly correlated (P < 0.001) with the extent of gap junction-mediated coupling. These findings show that gap junction-mediated intercellular coupling contributes to the in vitro bystander effect during HSV-tk/GCV therapy and that retroviral transduction of tumor cells is not required for bystander cytotoxicity.

The molecular biology of brain tumors

Recent years have seen rapid progress in our understanding of the molecular basis of brain tumors, particularly malignant astrocytomas. Translation of these advances from basic molecular genetics into the clinical arena is now underway. Molecular markers that correlate with prognosis in individual patients are being sought, and work is beginning on the identification of individuals who are at increased risk for developing a malignant brain tumor. Goals for the future include the development of more effective treatment strategies based on a clearer understanding of the pathogenesis of these tumors.

Differentiation of neuroblastoma enhances Bcl-2 expression and induces alterations of apoptosis and drug resistance

Recent evidence suggests that resistance to antineoplastic therapy may result from mutations in genes mediating the apoptotic response to DNA damage. To determine the effects of epigenetic changes on tumor responsiveness to cytotoxic agents inducing DNA damage, we examined the chemosensitivity of neuroblastoma (NB) after differentiation by retinoic acid (RA). Differentiation of the cell lines SH-SY5Y and SMS-KCNR by RA abolished the cytotoxic effects of adriamycin (Adr) and cisplatin. Chemoresistance was not the result of decreased proliferation induced by RA because: (a) growth arrest by nutrient deprivation did not affect sensitivity; (b) growth arrested NB cell lines, which did not differentiate, remained chemosensitive; and (c) RA concentrations which promoted differentiation without affecting growth, induced resistance. Apoptosis characterized NB cells responding to Adr, although differentiated SH-SY5Y did not apoptose and were resistant to Adr and cisplatin. Marked induction of bcl-2 in NB cells followed RA-induced differentiation, whereas in cell lines failing to differentiate, bcl-2 was not detected. Our data indicate that NB differentiation induces drug resistance after a loss of the apoptotic response to antineoplastic drugs and suggest that bcl-2 overexpression is an important mechanism of resistance in differentiated tumor cells.

Inactivation of p53 is associated with decreased levels of radiation-induced apoptosis in medulloblastoma cell lines

Radiation is the primary therapeutic modality for children with medulloblastoma, a pediatric brain tumour. We examined the response of four medulloblastoma cell lines to ionising radiation. Our evaluation utilising flow cytometry, morphological analysis and terminal deoxynucleotidyl transferase assays demonstrated that medulloblastoma cells undergo radiation-induced apoptosis. p53 mediates radiation-induced apoptosis in many cell types, and p53 mutations have been associated with increased resistance to ionising radiation. p53 mutations are rare in medulloblastoma. We found that wildtype p53 is required for high levels of apoptosis in medulloblastoma, and cell lines in which p53 had been inactivated by mutation had very low levels of apoptosis. Inactivation of endogenous wildtype p53 in medulloblastoma cells by introduction of a dominant negative mutant of p53 decreased the level of radiation-induced apoptosis. Our results suggest that the sensitivity of medulloblastoma to irradiation involves p53-mediated apoptosis and that p53 gene status may be a predictor of response to radiation therapy.

Molecular analysis at the NF1 locus in astrocytic brain tumors

BACKGROUND

Patients with neurofibromatosis type 1 (NF1) are at increased risk for developing malignant neural crest tumors and juvenile myeloid leukemia. Although the normal allele of the NF1 tumor-suppressor gene is frequently deleted in some of the malignant tumors that arise in patients with NF1, the role of NF1 alterations in the sporadic forms of these cancers is unclear.

METHODS

A series of intragenic sequence polymorphisms was used to investigate lymphocyte and tumor DNA samples from 22 adults with high grade malignant gliomas for loss of heterozygosity (LOH) at NF1. In addition, an assay based on the polymerase chain reaction was used to screen these tumors for point mutations at codon 1423.

RESULTS

One recurrent anaplastic astrocytoma showed LOH within NF1 but not with a flanking marker located near the gene. Of 21 informative tumors, none showed point mutations affecting codon 1423 of NF1.

CONCLUSION

These data suggest that LOH at NF1 is uncommon in sporadic high grade astrocytoma, and codon 1423 is not a "hot spot" for activating point mutations in these tumors.

Constitutional p53 mutations associated with brain tumors in young adults

Identification of patients at risk for developing brain tumors is important for the development of preventative strategies. Because individuals with germline p53 mutations may be at increased risk, we examined DNA from brain tumor-derived cell lines and malignant and normal nervous system tissue for p53 gene mutations using the single strand conformation polymorphism assay and direct sequencing of polymerase chain reaction-amplified DNA. We found mutations in the p53 gene in eight of 22 adult glioma tissue specimens and germline mutations in two of these eight patients. In contrast, mutation of the p53 gene was not detectable in either 16 glial tumors occurring in children, glial tumor tissue from three unrelated glioblastoma multiforme patients with a familial history of cancer, or in benign meningiomas. One constitutional p53 mutation was a G to T transversion at codon 154, and the second was a C to T transition at codon 256. Both patients with germline mutations developed glioblastoma multiforme before the age of 31, although the median age for glioma patients is above 50. These findings suggest that p53 germline mutations may identify a subset of young adults predisposed to the development of high-grade astrocytic tumors.

Id gene expression during development and molecular cloning of the human Id-1 gene

Id genes encode helix-loop-helix proteins that inhibit transcription by forming inactive heterodimers with basic helix-loop-helix (bHLH) proteins. bHLH proteins normally form either homodimers or heterodimers with other bHLH proteins and bind to a DNA sequence element activating transcription. Id-containing heterodimers are inactive because Id proteins lack the basic amino acid region necessary to form a DNA-binding domain. We have examined the relative levels of Id-1 and Id-2 mRNA during normal development and in malignant tissues. In the course of these experiments we cloned and sequenced the human Id-1 cDNA. Two related cDNA molecules encoding human Id-1 mRNAs were identified. Id-1a is a cDNA of 958 nucleotides and can encode a protein of 135 amino acids. Id-1b cDNA is 1145 nucleotides, can encode a protein of 149 amino acids, and appears to be a splice variant of Id-1a. The amino acid sequence of human Id-1 is greater than 90% homologous to that of mouse Id-1. The patterns of Id-1 and Id-2 expression during mouse development vary widely, and we detected Id-1 expression in human fetal and adult tissues from lung, liver, and brain. High Id-1 mRNA expression was found in many human tumor cell lines, including those isolated from nervous system tumors. We mapped Id-2 to human chromosome 2p25.

Expression of nitric oxide synthase in human central nervous system tumors

The nitric oxide synthases (NOS) are a family of related enzymes which regulate the production of NO, a free radical gas implicated in a wide variety of biological processes. Vasodilation and increased tumor blood flow, increased vascular permeability, modulation of host tumoricidal activity, and free radical injury to tumor cells and adjacent normal tissues are pathophysiological features of malignant tumors that may be mediated by NO. We examined human brain tumors for three NOS isoforms and NADPH diaphrase, a histochemical marker of NOS activity in the brain. We detected increased expression of the brain and endothelial forms of NOS [NOS I and NOS II, respectively (C. Nathan and Q. Xie. Cell, 78: 915-919, 1994)] in astrocytic tumors, and the highest levels of expression was found in higher grade tumors. Each of these two isoforms was found in tumor cells and tumor endothelial cells. The macrophage isoform of NOS (NOS III) was less frequently detected and expressed at a lower level, predominantly in tumor endothelial cells. NADPH diaphorase staining for NOS activity paralleled this pattern of NOS expression. Western blot analysis of tumor tissues for these NOS isoforms confirmed these observations. Our data indicate that malignant central nervous system neoplasms express unexpectedly high levels of NOS and suggest that NO production may be associated with pathophysiological processes important to these tumors.

Inhibition of apoptosis by the retinoblastoma gene product

Tissue homeostasis and the prevention of neoplasia require regulatory co-ordination between cellular proliferation and apoptosis. Several cellular proteins, including c-myc and E2F, as well as viral proteins such as E1A, have dual functions as positive regulators of apoptosis and proliferation. The product of the retinoblastoma tumor suppressor gene, pRb, binds these proteins and is known to function in growth suppression. To examine whether pRb may function as a negative regulator of both proliferation and apoptosis, we analyzed apoptosis induced in transfected derivatives of the human osteosarcoma cell line SAOS-2. Ionizing radiation induced apoptosis in a time- and dose-dependent manner in SAOS-2 cells, which lack pRb expression. In both a transient and stable transfection assay, SAOS-2 derivatives expressing wild-type (wt) pRb exhibited increased viability and decreased apoptosis following treatment at a variety of radiation doses. Expression in SAOS-2 of a mutant pRb that fails to complex with several known binding partners of pRb, including E1A and E2F, did not protect SAOS-2 cells from apoptosis. Radiation exposure induced a G2 arrest in SAOS-2 and in derivatives expressing pRb. Inhibition of DNA synthesis and cell cycle progression by aphidicolin treatment failed to protect SAOS-2 cells or pRb-expressing isolates from undergoing apoptosis. Our data document a novel function for pRb in suppressing apoptosis and suggest that several proteins shown to induce apoptosis, including E1A, E2F and c-myc, may do so by interfering with the protective function of pRb.

Gene therapy for diseases of the nervous system

Advances in molecular biology and recombinant DNA technologies have contributed to our understanding of the molecular basis of many diseases. Now the possibility of gene transfer into normal cells to produce a gene product of therapeutic potential, or into diseased cells to correct the pathologic alteration, promises to revolutionize medical practice. In contemporary medicine, many therapeutic strategies focus on the link between a biochemical deficiency and the ensuing disorder. The treatment of noninfectious disease is often based on replacement therapy; medication is given to compensate for biochemical defects and to prevent or reverse the progression of disease. Although conventional therapies seldom alter the fundamental cause of a disease, gene therapy potentially could correct, at a molecular level, the genetic abnormalities contributing to its pathogenesis. Treatment directed at specific molecular alterations associated with the development of neurologic disease provides expectations of more effective and less toxic therapy. The development of gene therapy for nervous system tumors has progressed rapidly and may be prototypical in the development of therapies for inherited and acquired disorders of the nervous system. We describe possible strategies for using gene therapy to treat nervous system disorders, and we review recent advances in gene therapy for nervous system tumors.

The helix-loop-helix protein Id-2 enhances cell proliferation and binds to the retinoblastoma protein

Cell growth and differentiation are usually antagonistic. Proteins of the basic helix-loop-helix (bHLH) family bind DNA and play important roles in the differentiation of specific cell types. Id proteins heterodimerize with bHLH transcription factors, blocking their activation of lineage-specific gene expression and thereby inhibiting cellular differentiation. To examine the effect of Id-2 on cell proliferation, we overexpressed Id-2 in the human osteosarcoma cell line U2OS. Id-2 expression in U2OS reduced the serum requirement for growth and stimulated cellular proliferation by shortening the doubling time and increasing the percentage of cells in S phase. We demonstrated that Id-2 expression was able to reverse the inhibition of cellular proliferation and the block in cell cycle progression mediated by the product of the retinoblastoma tumor suppressor gene pRB. This effect was not associated with changes in the state of pRb phosphorylation in transfected cells. In vitro, unphosphorylated pRb from cell lysates specifically bound Id-2 but was not able to bind a mutated form of Id-2 lacking the HLH domain that also did not antagonize the growth arrest by pRb. In vitro-synthesized pRb containing mutations within the E1A/large T-binding pocket did not bind Id-2. However, wild-type pRb was able to bind to a region of Id-2 corresponding to only the HLH domain. In vivo, a physical association between Id-2 and pRb was seen in cross-linked extracts from SAOS-2 cells transfected with Id-2 and pRb. Our data identify a role for Id-2 in the regulation of cellular proliferation and suggest that the interaction between Id-2 and pRB is a molecular pathway over which synchronous changes in growth and differentiation are mediated in vivo.

Disordered differentiation as a target for novel approaches to the treatment of neuroblastoma

Some childhood malignant neoplasms are thought to arise in embryonic tissues. These tumors present unique opportunities for studying the maturation of specific cellular lineages and examining the possible role of alterations in the regulation of differentiation in tumor development. Several features of neuroblastoma, a tumor thought to arise in cells originating in the embryonic neural crest, suggest that it may be particularly useful in this regard. The identification of a series of markers that characterize the various cell types of the peripheral nervous system that are detectable in neuroblastoma tumor tissues has made it possible to recognize that the cells of neuroblastoma tumor cell lines and tissues correspond to specific stages of adrenal gland development. Experiments directed at understanding the cellular signals by which neural crest cell maturation is mediated may provide insights of therapeutic import because neuroblastoma tumors corresponding to some stages of differentiation respond very differently to nonspecific cytotoxic therapies than tumors corresponding to other stages.

Developmental regulation of annexin II (Lipocortin 2) in human brain and expression in high grade glioma

In experiments to identify molecules that might be important in the pathogenesis of glioblastoma multiforme, the most common malignant brain tumor, we found that annexin II (Lipocortin 2, p36), a likely second messenger in several different mitogenic pathways, was highly expressed in tumor tissue of glioblastoma multiforme (9 of 9) and highly anaplastic astrocytoma (2 of 6), but not in astrocytomas of lower pathological grade (0 of 6). We also detected high levels of annexin II expression in fetal brain during the period when radial glia proliferate, although annexin II expression was not detected in normal adult brain. These data demonstrate that annexin II expression is developmentally regulated in the human central nervous system and suggest that the early progenitor radial glia share important characteristics with highly malignant glial tumors.

Plasticity of neuroblastoma tumor cells to differentiate along a fetal adrenal ganglionic lineage predicts for improved patient survival

We have recently presented a model of human adrenal medullary histogenesis that incorporates all neural crest-derived lineages (chromaffin, sustentacular, and ganglionic) known to compose this tissue. To determine if neuroblastomas correspond to the arrested maturation of embryonal adrenal medullary cells, we evaluated the expression of adrenal medullary developmental markers in 81 neuroblastoma tumors. We found that patterns of chromaffin-related gene expression in these tumors correlated exactly with the patterns observed during maturation of adrenal medullary cells (P2 < 10(-5). In a multivariate Cox proportional hazards analysis of developmental marker expression and other well-recognized prognostic variables, evidence of maturation along a fetal ganglionic lineage, as monitored by HNK-1 immunoreactivity (relative risk of 6.42, P2 = 0.0001), and age at diagnosis (relative risk of 5.05, P2 = 0.0042) were independent and significant prognostic indicators of patient survival. These studies demonstrate that neuroblastomas correspond to embryonal adrenal medullary cells arrested at recognizable stages during development, and that evidence of maturation along a fetal ganglionic lineage appears to have major importance in predicting patient survival.

A novel POU homeodomain gene specifically expressed in cells of the developing mammalian nervous system

We report the isolation of a novel human POU domain encoding gene named RDC-1. The POU domain of the RDC-1 encoded protein is highly related to the POU domain potentially encoded by the rat brain-3 sequence and to that of the Drosophila I-POU protein; outside of the POU region, RDC-1 is unrelated to any previously characterized protein. The RDC-1 gene is expressed almost exclusively in normal tissues and transformed cells of neural origin. In the developing mouse and human fetus, RDC-1 is expressed in a spatially and temporally restricted pattern that suggests a critical role in the differentiation of neuronal tissues. In addition, RDC-1 is expressed in a unique subset of tumors of the peripheral nervous system including neuroepitheliomas and Ewing's sarcomas but not neuroblastomas. Based on its unique structural characteristics and expression pattern, we discuss potential functions for the RDC-1 protein.

Expression of the cholecystokinin gene in pediatric tumors

We have examined a wide range of cultured human tumor cell lines and found that a specific subset of tumors expresses the cholecystokinin (CCK) gene. All neuroepitheliomas (eight) and Ewing sarcoma (eight) cell lines that were tested express CCK RNA. In addition, two of six rhabdomyosarcoma cell lines also express the CCK gene, suggesting that rhabdomyosarcomas are probably heterogenous and that a subset may be similar to Ewing sarcoma and neuroepithelioma. Very few of the positive tumors express completely processed immunoreactive CCK. However, we have used a radioimmunoassay that detects the CCK precursor to demonstrate synthesis of CCK precursor-like peptides by all of the Ewing sarcoma and neuroepithelioma lines that were tested and by the rhabdomyosarcoma cell line that expresses CCK mRNA. These data demonstrate a consistent association of CCK gene expression with a specific group of human neoplasms. The data also add credence to the theory that Ewing sarcoma and neuroepithelioma are derived from the same transformed cell type. Finally, our results suggest that CCK gene expression may serve as a marker to distinguish these tumors, which are considered to be small-round-cell tumors of childhood, from other pediatric tumors.

Germ-line and somatic p53 gene mutations in multifocal osteogenic sarcoma

Multifocal osteogenic sarcoma patients without familial histories of increased tumor predisposition were examined for mutations in the highly conserved regions of the p53 gene. p53 point mutations were found in tumor DNA from each of the four patients we examined. A germ-line p53 mutation was detected in one of these patients, and a further rearrangement of the residual wild-type allele was detected in tumor tissue. p53 germ-line mutations can contribute to the enhanced predisposition to tumor development manifest in patients with multifocal osteosarcoma.

A human Id-like helix-loop-helix protein expressed during early development

The interaction of helix-loop-helix (HLH) proteins is known to regulate the differentiation of several different tissues, including mammalian muscle and the insect peripheral nervous system. In myoblasts, the products of myogenic HLH genes such as MyoD and ubiquitous HLH proteins such as E12 are present at constant levels throughout development. An E12 monomer and a MyoD monomer form a DNA binding heterodimer that activates muscle-specific genes. These two proteins are unable to dimerize in proliferating myoblasts because a negative regulator HLH protein, Id, is present. We now report the sequence and structure of a human HLH gene related to Id, which has been designated Id-2. Two prominent Id-2 RNA molecules of 2.5 and 1.3 kilobases were found in a number of different human normal and neoplastic tissues. We believe the larger RNA is a precursor of the 1.3-kilobase mRNA that encodes an Id-2 protein of 134 amino acids. The HLH region of the Id-2 protein is 90% homologous to that of myogenic Id, but the homology is much less extensive outside the HLH region. The Id-2 gene is highly expressed during early fetal development in several tissues, including those of the central nervous system, but is not expressed in the corresponding mature tissues. Id-2 expression is modulated in association with retinoic acid-induced ganglionic differentiation of the neuroblastoma cell line SMS-KCNR. These findings suggest that Id-2 is an inhibitor of tissue-specific gene expression, although its distinctive pattern of expression during development suggests a role different from that of Id.

Molecular origins of pediatric embryonal tumors

The different but distinctly related lines of investigation discussed during this workshop are likely to provide important insights into the molecular events that initiate and mediate the oncogenic process. Pediatric embryonal tumors were the first malignancies in which chemotherapeutic responses were recognized, and they have been singularly important clinical models for the development of many important principles of cancer treatment. Judging from the emerging insights into the pathogenesis of these tumors, it seems likely that they will also serve as important models for designing novel approaches to cancer treatment.

Insulin-like growth factor II-mediated proliferation of human neuroblastoma

Neuroblastoma is an embryonal tumor that typically arises in cells of the developing adrenal medulla. IGF-II mRNA is expressed at high levels in the adrenal cortex before birth but it is not detectable until after birth in the adrenal medulla. Neuroblastoma cell lines corresponding to early adrenal medullary precursors did not express IGF-II, although all three cell lines we tested were growth stimulated by IGF-II. Cell lines corresponding to more mature adrenal medullary cells expressed IGF-II, and one, SK-N-AS, grows by an IGF-II autocrine mechanism (J. Clin. Invest. 84:829-839) El-Badry, Romanus, Helman, Cooper, Rechler, and Israel. 1989. An examination of human neuroblastoma tumor tissues for IGF-II gene expression using in situ hybridization histochemistry revealed that IGF-II is expressed by tumor cells in only 5 of 21 neuroblastomas, but is detectable in cells of nonmalignant tissues including adrenal cortical cells, stromal fibroblasts, and eosinophils in all 21 tumors. These findings indicate that IGF-II may function as an autocrine growth factor for some neuroblastomas and as a paracrine growth factor for others. They suggest that the growth regulatory pathways utilized by neuroblastoma mimic those used in the precursor cell type from which individual tumors arise.