Amplification and enhanced expression of the c-myc oncogene in mouse SEWA tumour cells

Analysis of ALK, MYCN, and the ALK ligand ALKAL2 (FAM150B/AUGα) in neuroblastoma patient samples with chromosome arm 2p rearrangements

Gain of chromosome arm 2p is a previously described entity in neuroblastoma (NB). This genomic address is home to two important oncogenes in NB-MYCN and anaplastic lymphoma kinase (ALK). MYCN amplification is a critical prognostic factor coupled with poor prognosis in NB. Mutation of the ALK receptor tyrosine kinase has been described in both somatic and familial NB. Here, ALK activation occurs in the context of the full-length receptor, exemplified by activating point mutations in NB. ALK overexpression and activation, in the absence of genetic mutation has also been described in NB. In addition, the recently identified ALK ligand ALKAL2 (previously described as FAM150B and AUGα) is also found on the distal portion of 2p, at 2p25. Here we analyze 356 NB tumor samples and discuss observations indicating that gain of 2p has implications for the development of NB. Finally, we put forward the hypothesis that the effect of 2p gain may result from a combination of MYCN, ALK, and the ALK ligand ALKAL2.

Genome instability mechanisms and the structure of cancer genomes

Genomic instability is a hallmark of cancer cells, and arises from the aberrations that these cells exhibit in the normal biological mechanisms that repair and replicate the genome, or ensure its accurate segregation during cell division. Increasingly detailed descriptions of cancer genomes have begun to emerge from next-generation sequencing (NGS), providing snapshots of their nature and heterogeneity in different cancers at different stages in their evolution. Here, we attempt to extract from these sequencing studies insights into the role of genome instability mechanisms in carcinogenesis, and to identify challenges impeding further progress.

Amplification of the MYC Gene in Osteosarcoma Secondary to Paget's Disease of Bone

Purpose. In a previous series of 25 human osteosarcoma samples studied for MYC gene amplification, we found amplification in two cases (8%), including one arising in association with Paget's disease (pagetic osteosarcoma). Based on this observation, we further investigated the prevalence of MYC gene amplification in pagetic osteosarcomas.Methods. MYC gene amplification was assessed by Southern blot analysis using frozen tissue samples in five cases of pagetic osteosarcoma and 53 cases of primary (non-pagetic) osteosarcoma. Amplification was considered present if the MYC copy number was six or greater.Results. Three out of five patients (60%) with pagetic osteosarcoma showed MYC gene amplification, whereas it was present in only 5/53 patients (9.4%) with primary osteosarcoma. The incidence of MYC amplification in pagetic osteosarcoma was thus significantly higher than that in primary osteosarcoma (p = 0.016).Discussion. The finding that MYC gene amplification may be more common in pagetic than primary osteosarcoma warrants further study and suggests pathogenetic differences between primary osteosarcomas and those arising in the setting of Paget's disease. Three of the four pagetic osteosarcomas from the present study were previously shown to be immunoreactive for p53, suggesting that p53 mutation may also be a frequent genetic lesion in these tumors.

Amplification of oncogenes revisited: from expression profiling to clinical application

Regulatory or structural alterations of cellular oncogenes have been implicated in the causation of cancers. Amplification represents one of the major molecular pathways by which gene expression is constitutively enhanced above the level of physiologically normal variation. Consequently, the significance of oncogene amplification in tumorigenesis originally had emerged from expression profiling of tumor cells by oncogene arrays. Amplified oncogenes have been found associated with more aggressive tumor variants and in selected settings are clinical markers to determine patient prognosis.

Amplification and overexpression of the hepatocyte growth factor receptor (HGFR/MET) in rat DMBA sarcomas

In the present study subcutaneous fibrosarcomas were induced by the carcinogen 7,12-dimethylbenz(a)anthracene (DMBA) in rats from F1 generation cross breedings of two different inbred strains. Comparative genomic hybridization (CGH) analysis, which allows detection of DNA sequence copy changes, was applied to one of the tumors and it was found that there were increased copy numbers of sequences at chromosome 4q12-q21 in this tumor. We have previously determined that the loci for the hepatocyte growth factor (Hgf) and hepatocyte growth factor receptor (Hgfr/Met), a protooncogene, are situated in this particular chromosome region. Using probes for the two genes in FISH (fluorescence in situ hybridization) and in Southern blots we found that the Hgfr/Met gene was amplified in five of the 19 sarcomas studied, and that the Hgf gene was coamplified in two of them. Northern and Western blots and tyrosine phosphorylation analysis showed that the HGF receptor was overexpressed and functional in all five tumors, as well as in two additional tumors. In summary, both amplification and overexpression of the Hgfr/Met gene was found in about 25% of DMBA-induced experimental rat sarcomas, and HGF receptor overexpression alone was seen in two additional tumors. Possibly this reflects an involvement in paracrine or autocrine stimulation of growth and invasiveness by HGF. Our finding could provide a rodent model system to increased knowledge about causality and therapy, which may be applicable to the sizeable fraction of human musculoskeletal tumors displaying MET overexpression.

Cloning and expression of a myc family member from the pituitary gland of the Rainbow trout, Oncorhynchus mykiss

A myc gene cloned from a Rainbow trout pituitary gland cDNA library is described. This clone (Tmyc2) shows extensive homology to Rainbow trout C-myc, which is expressed in the liver. Tmyc2 does not appear in the liver but is expressed in the pituitary gland (1.9 kb transcript), brain (2.0 kb transcript) and, at very low levels, in the heart (2.0 kb transcript). Tmyc2 contains three highly modified areas within the coding sequence, one of which shows an extensive loss of acidic residues that is uncommon in C-myc family members and may be important in determining the function of Tmyc2 in the pituitary gland and brain.

Integration of viral sequences into the c-myc gene in two mammary adenocarcinomas induced by polyomavirus in athymic nude mice

We report the analysis of polyomavirus (Py) DNA integration into chromosomal DNA of two Py-induced mammary adenocarcinomas of athymic nude mice. Prior observations had established that these tumors had high levels of episomal Py DNA, making analysis of integration sites difficult. Propagation of tumor cells in culture allows the isolation of lines which have lost episomal Py DNA but are still tumorigenic and thus can be used for in situ and Southern analysis of Py sequences. The data reported here support the conclusion that Py DNA integrated into and next to the c-myc gene, adding further importance to this tumor system which, in its modifications of c-myc expression, appears to be similar to some human mammary cancers. In situ hybridization experiments on metaphase chromosomes of tumor cells showed that (i) in both cases, there was a single integration site at the same position on the same chromosome in all cells of a given tumor, and (ii) integration sites were different in the two tumors; in one, it was located on chromosome 15, near the c-myc proto-oncogene, and in the other, it was situated in the distal part of chromosome 1. We have demonstrated a probable rearrangement between chromosome 1 and chromosome 15, in the region of Py insertion, thus suggesting that a specific site on chromosome 15 is involved in tumorigenesis. The discovery that Py DNA was integrated at specific sites in host chromosomes raised the questions of whether such integrations were correlated with the activation of specific oncogenes. The rearrangements of the c-myc proto-oncogene observed on Southern blot analysis for both tumors, along with similar integration patterns of Py sequences, the overexpression of the c-myc gene, and the synthesis of abnormal oversized hybrid transcripts between c-myc and Py genes, favor this hypothesis. Finally, the analysis of episomal Py DNA in various tumors shows viral populations presenting a specific deletion in a part of the Py late region. This deleted region in the episomal virus genome was systematically found integrated in chromosomal DNA, thus arguing for the importance of Py integration in the induction of mammary tumor.

Amplification of c-myc oncogene and absence of c-Ha-ras point mutation in human bone sarcoma

The genomic organization of four oncogenes, c-myc, c-myb, c-Ha-ras, and v-fms, was analyzed in 21 patients with malignant bone tumors. Amplification of the c-myc proto-oncogene without rearrangement was the sole abnormality detected in four tumors: two chondrosarcomas, one osteosarcoma, and one lymphoma of bone. DNA hybridizations with c-myb, c-Ha-ras, and v-fms probes disclosed no structural gene abnormalities. Point mutations at the 12th codon of the c-Ha-ras gene were investigated with the polymerase chain reaction technique; no alterations were detected. The observed amplification of the c-myc there was not related to histologic type, grade, surgical stage, or ploidy level of the tumors. The results indicated that c-myc amplification, presumed to be involved in the development of malignancy in a variety of solid tumors, is encountered sporadically in malignant bone tumors; however, this occurs without relation to common histopathologic features. The clinical significance of oncogene amplification in bone sarcoma remains to be established.

Amplification of the 11q13 region in human carcinoma cell lines: a mechanistic view

We previously proposed that a local duplication, not the loss of the subsequently amplified marker from its original site, might be the first step in gene amplification in human cells. It is important to investigate this issue in naturally occurring amplification and when copy numbers are relatively low. We have examined the location of single-copy and amplified 11q13 sequences in cell lines from human breast cancers and squamous cell carcinomas using fluorescence in situ hybridization both with a probe specific for the 11q13 amplifying region and with a chromosome 11-specific library. We show that in most cell lines the 11q13 amplicons are physically linked to chromosome 11 or to a chromosome derived from chromosome 11 by various rearrangements near the 11q13 region. In none of the cell lines were interstitial deletions of 11q13 detected. These results indicate that 11q13 amplification in human tumor cells generally does not involve deletion as the initial step. One cell line with chromosomally located amplified 11q13 sequences contained double minutes that harbored the MYC gene but no 11q13 sequences. This suggests that the genetic outcome and the mechanism of gene amplification are probably dependent on specific DNA sequences rather than on the origin of the cells.

Expression of the myc proto-oncogene in the rainbow trout Oncorhynchus mykiss

1. The myc proto-oncogene is shown to be expressed in the pituitary gland, brain and heart of the rainbow trout. 2. Based on transcript sizes the pituitary gland expressed B-myc and L-myc, the brain expresses L-myc and the heart expresses either C-myc or L-myc. 3. Pituitary B-myc is expressed at a level 3-fold greater than liver C-myc and 8-fold greater than the heart transcript. 4. The amounts of the pituitary B-myc and heart transcript nearly double as the fish approach sexual maturity.

Aldolase-DNA interactions in a SEWA cell system

In this report we demonstrate the novel finding that aldolase A interacts with DNA sequences in mouse SEWA sarcoma cells. This interaction was initially observed through the identification of a 40 kDa protein which was eluted from a DNA affinity chromatography column consisting of the long terminal repeat (LTR) of the endogenous intracisternal A-type particle (IAP). Microsequencing analysis identified this 40 kDa protein as the glycolytic enzyme, aldolase A. The use of specific anti-aldolase antibodies enabled the identification and subsequent purification of aldolase from the nuclear protein fraction of two SEWA sublines, one that is adherent and one that grows in suspension. In order to confirm our initial finding that aldolase is capable of interacting with DNA, proteins from each subline were immunopurified with anti-aldolase antibodies, eluted and then tested for their ability to interact with IAP-LTR DNA sequences. Interestingly, only aldolase derived from the anchorage dependent SEWA cells was capable of interacting with the IAP-LTR, however, several cell lines derived from human tumors also exhibited this activity. Subsequent studies revealed the ability of aldolase to interact with some but not every DNA sequence tested, implying that there may be a minimal DNA conformation and/or sequence requirement for this activity. The presence of aldolase A in the nuclei and its ability to interact with certain DNA sequences suggest a novel role for this metabolic enzyme.

Interaction of SEWA sarcoma cell proteins with the intracisternal A-type particle long terminal repeat DNA sequence

Intracisternal A-type particle (IAP) transcripts are endogenous retrovirus-like sequences expressed during specific stages of normal development and in a variety of murine tumors. In this study, we have analyzed two cell lines derived originally from the SEWA murine osteosarcoma and grown either as ascites or as solid tumors, for proteins that might regulate IAP expression. We found that subline AA7-NA, originally derived from the ascites tumor, expressed about five times more IAP RNA than the AS12-AD subline, which was derived from a solid tumor. In view of this finding, we examined the binding of cellular proteins from the two cell lines to the 5' end of an IAP long terminal repeat sequence. Gel retardation assays of DNA-protein complexes and DNase I footprinting assays identified several DNA sequences within the long terminal repeat fragment that were protected by protein extracts from both SEWA sublines. Gel retardation assays using specific synthetic oligonucleotide sequences that correspond to two of these protected regions revealed different patterns of DNA-protein complexes with extracts from the two SEWA sublines. These data suggest that expression of IAP sequences is regulated by complex mechanisms involving several proteins that appear to differ between the two sublines.

Cytogenetics and DNA amplification in colorectal cancers

In vitro cultivated cells of 28 colorectal cancers were analyzed for chromosomal abnormalities that might signal amplification of DNA, either as double minutes (DMs) or homogeneously staining chromosomal regions (HSRs). Cells derived from 18 tumors showed DMs in 10 to 100% of all metaphases examined. Surveys that employed a panel of available oncogene probes failed to detect amplification of a known cellular oncogene with the exception of three cases where the ERBB2 gene was amplified. In one of these three cases neither DMs nor HSRs were detectable. Our studies show that from 28 lines established in culture, 19 (68%) show amplification of DNA, and indicate that DNA amplification is a frequent genetic alteration in colorectal cancers in addition to other genetic changes. Amplification is correlated with high Dukes stage, but not with histological grade. The identity of the amplified DNA remains to be established for most cases.

The molecular biology of radiation-induced carcinogenesis: thymic lymphoma, myeloid leukaemia and osteosarcoma

In mice, external X- or gamma-irradiation may induce thymic lymphomas or myeloid leukaemias, while bone-seeking alpha-emitters may induce osteosarcomas and, to a lesser extent, acute myeloid leukaemia. The present paper aims to review briefly some of the experimental data with respect to the molecular mechanisms underlying these radiation-induced carcinogenic processes. Thymic lymphomagenesis proceeds through an indirect mechanism. Recombinant proviruses often occur in the tumour cell DNA, favouring the idea that they might be involved. However, there are indications that they might mediate tumour growth rather than induction. It is plausible that activation of ras oncogenes by somatic point mutations might play a role in the carcinogenic process, although at a yet undetermined stage. Myeloid leukaemogenesis is characterized by a very early, putative initiating event, consisting of non-random rearrangements and/or deletions of chromosome 2. These may be related to deletions in the developmentally important homeobox gene clusters and to rearrangements of the sequences flanking the IL-1 beta gene. Either a gene of the homeobox family or IL-1 beta might be considered as potentially involved in the induction process. Osteosarcomagenesis in mice is often associated with the expression of proviruses, and the tumours often contain somatically acquired proviruses. These viruses may contribute to tumour development by affecting various growth-suppressor genes. Viruses isolated from bone tumours, although non-sarcomagenic, induce osteopetrosis, osteomas and lymphomas upon infection of newborn mice. Osteogenic tumours frequently display amplification of a region on mouse chromosome 15, which encompasses c-myc and Mlvi-1 sequences. Enhanced transcription of various oncogenes is found in individual tumours, but no specificity for osteosarcomas has been identified. In vitro systems of skeletoblast differentiation are being developed to study tumour induction in vitro.

Differences in c-myc and pvt-1 amplification in SEWA sarcoma sublines selected for adherent or non-adherent growth

Conversion of solid sarcomas and carcinomas into ascites tumors depends on the in vivo selection of phenotypically altered tumor cell variants that can grow in the dissociated form. Once selected, they retain this property even after prolonged s.c. growth as solid tumors. From an s.c.-passaged subline of an ascites-converted murine sarcoma (SEWA-AS12), we were able to separate cells adapted to the ascites form of growth from cells that can only grow in the solid form on the basis of their differential adherence to plastic. Both c-myc and pvt-1 were amplified approximately 63- to 77-fold in the nonadherent subline (SEWA-AS12-NA), but only 5- to 8-fold in the adherent subline (SEWA-AS12-ADH). This suggests that c-myc and/or pvt-1 amplification may provide a selective advantage to cells that can grow in the dissociated form.

Complete transformation of embryonal rat fibroblasts by polyomavirus occurs during passage in vitro

The tumorigenicity of secondary rat embryo fibroblasts transfected with a plasmid harboring a replication origin-defective polyomavirus was found to increase during in vitro propagation. Thus, polyomavirus-transfected cells were found to be more than 10,000-fold more tumorigenic when injected into syngenic rats at 3 months after transfection compared to those injected at an earlier time point. Furthermore, most clones of polyomavirus-transfected cells did not grow in semisolid medium at 52 days after transfection but did grow at 95 days. Addition of glucocorticoid hormones, but not of 25% fetal calf serum, to the growth medium of the early passage cells resulted in limited anchorage-independent growth. An altered level of expression of a number of proteins was found in cells analyzed at different times after transfection. Notably, the expression of a component of the actin filament system, tropomyosin 2, was shown to decrease during growth in vitro. The development of a more fully transformed phenotype at late passages correlated with loss of the requirement for large T-antigen for growth. Thus, cells transfected with a polyomavirus mutant encoding a thermolabile large T-antigen did not grow at the restrictive temperature at 6 weeks after transfection, but grew well at 5 months after transfection. We suggest that these phenomena may be explained by assuming that establishment of rodent fibroblasts, and thereby sensitivity to transformation by middle T-antigen, is not an immediate consequence of expression of large T-antigen but occurs after a period of growth in vitro.

Excision of N-myc from chromosome 2 in human neuroblastoma cells containing amplified N-myc sequences

Amplification of one of three growth-stimulating myc genes is a common method by which many tumor types gain a proliferative advantage. In metastatic human neuroblastoma, the amplification of the N-myc locus, located on chromosome 2, is a dominant feature of this usually fatal pediatric cancer. Of the many models proposed to explain this amplification, all incorporate as the initial step either disproportionate overreplication of the chromosomal site or recombination across a loop structure. The original locus is retained within the chromosome in the overreplication models but is excised in the recombination models. To test these models, we have used somatic cell hybrids to separate and analyze the chromosomes 2 from a neuroblastoma cell line containing in vivo amplified N-myc. Our results demonstrate that N-myc is excised from one of the chromosomes, suggesting that deletion is a requisite part of gene amplification in a naturally occurring system.

Amplified N-myc in human neuroblastoma cells is often arranged as clustered tandem repeats of differently recombined DNA

Human neuroblastoma cells often carry amplified DNA encompassing the gene N-myc. Amplified N-myc has been found localized in "double minutes" in direct tumor cell preparations. In contrast, later passages carried amplified N-myc almost exclusively within a single homogeneously staining chromosomal region located at a chromosomal site different from the normal location of N-myc. We used pulsed field gel electrophoresis to define the structural arrangement of the amplified DNA. Long-range mapping was facilitated by the presence of several sites for rare cutting restriction endonucleases in the 5' region of N-myc. Amplified DNAs of different neuroblastoma cell lines were heterogeneous in size and had undergone recombination at various distances from N-myc. N-myc occupied a central position within the amplified DNA, and in no case was the coding region affected by recombination. Among neuroblastoma cells, varying proportions of amplified DNA (in some instances close to 100%) consisted of multiple tandem arrays of DNA segments ranging in size from 100 to 700 kilobase pairs. Tumor cells with low degrees of amplification revealed regions of amplified DNA in excess of 1,500 kilobase pairs without apparent rearrangement. Our observations, in concert with the cytogenetic findings, suggest a model of gene amplification which involves unscheduled DNA replication, recombination, and formation of extrachromosomal DNA followed by integration into a chromosome and subsequent in situ multiplication. The central position which N-myc occupies within the amplified sequences and the lack of recombination within the coding region of N-mc indicate that N-myc rather than other genetic information provides the selective advantage for retention of the amplified DNA.

Excision of N-myc from chromosome 2 in human neuroblastoma cells containing amplified N-myc sequences

Amplification of one of three growth-stimulating myc genes is a common method by which many tumor types gain a proliferative advantage. In metastatic human neuroblastoma, the amplification of the N-myc locus, located on chromosome 2, is a dominant feature of this usually fatal pediatric cancer. Of the many models proposed to explain this amplification, all incorporate as the initial step either disproportionate overreplication of the chromosomal site or recombination across a loop structure. The original locus is retained within the chromosome in the overreplication models but is excised in the recombination models. To test these models, we have used somatic cell hybrids to separate and analyze the chromosomes 2 from a neuroblastoma cell line containing in vivo amplified N-myc. Our results demonstrate that N-myc is excised from one of the chromosomes, suggesting that deletion is a requisite part of gene amplification in a naturally occurring system.

Amplification of cellular oncogenes: a predictor of clinical outcome in human cancer

Increased dosage of cellular oncogenes resulting from amplification of DNA is a frequent genetic abnormality of tumor cells and the study of oncogene amplification has been paradigmatic for the usefulness of molecular genetic research in clinical oncology. Certain types of human tumors carry an amplified cellular oncogene at frequencies of up to 50-60%. Human neuroblastoma has been prototypic for the importance of oncogene amplification in tumorigenesis, and evidence is emerging that amplification may be an early event involved in a more malignant form of this cancer. It is unclear at which stage amplification plays a role in other cancers. Amplification of cellular oncogenes is a good predictor of clinical outcome in some human malignancies.

Polyoma DNA replication dependent upon growth condition of SEWA sarcoma cells

Extrachromosomal replication of viral DNA sequences has been observed in transformed as well as in normal cells following "stress"-inducing treatments. To explore the effect of growth conditions on the ability to support such replication, we analyzed SEWA sarcoma cells that grew subcutaneously or as ascites tumors in vivo as well as cell lines that were established from each of these tumors. The replicative form of polyoma DNA sequences was observed in SEWA tumors grown in ascites fluids but not in cells maintained as solid tumors. Polyoma DNA replication was found in ascites-derived cells that were adapted to grow in culture, only when the cultured cells are stimulated with UV irradiation. Immunoprecipitation of T antigens enabled detection of large T antigen only in the ascites-derived cells. The mechanisms that may regulate this phenomenon and the possible role large T may play in different growth conditions of SEWA cells are discussed.

Amplified N-myc in human neuroblastoma cells is often arranged as clustered tandem repeats of differently recombined DNA

Human neuroblastoma cells often carry amplified DNA encompassing the gene N-myc. Amplified N-myc has been found localized in "double minutes" in direct tumor cell preparations. In contrast, later passages carried amplified N-myc almost exclusively within a single homogeneously staining chromosomal region located at a chromosomal site different from the normal location of N-myc. We used pulsed field gel electrophoresis to define the structural arrangement of the amplified DNA. Long-range mapping was facilitated by the presence of several sites for rare cutting restriction endonucleases in the 5' region of N-myc. Amplified DNAs of different neuroblastoma cell lines were heterogeneous in size and had undergone recombination at various distances from N-myc. N-myc occupied a central position within the amplified DNA, and in no case was the coding region affected by recombination. Among neuroblastoma cells, varying proportions of amplified DNA (in some instances close to 100%) consisted of multiple tandem arrays of DNA segments ranging in size from 100 to 700 kilobase pairs. Tumor cells with low degrees of amplification revealed regions of amplified DNA in excess of 1,500 kilobase pairs without apparent rearrangement. Our observations, in concert with the cytogenetic findings, suggest a model of gene amplification which involves unscheduled DNA replication, recombination, and formation of extrachromosomal DNA followed by integration into a chromosome and subsequent in situ multiplication. The central position which N-myc occupies within the amplified sequences and the lack of recombination within the coding region of N-mc indicate that N-myc rather than other genetic information provides the selective advantage for retention of the amplified DNA.

The PVT gene frequently amplifies with MYC in tumor cells

The line of human colon carcinoma cells known as COLO320-DM contains an amplified and abnormal allele of the proto-oncogene MYC (DMMYC). Exon 1 and most of intron 1 of MYC have been displaced from DMMYC by a rearrangement of DNA. The RNA transcribed from DMMYC is a chimera that begins with an ectopic sequence of 176 nucleotides and then continues with exons 2 and 3 of MYC. The template for the ectopic sequence represents exon 1 of a gene known as PVT, which lies 50 kilobase pairs downstream of MYC. We encountered three abnormal configurations of MYC and PVT in the cell lines analyzed here: (i) amplification of the genes, accompanied by insertion of exon 1 and an undetermined additional portion of PVT within intron 1 of MYC to create DMMYC; (ii) selective deletion of exon 1 of PVT from amplified DNA that contains downstream portions of PVT and an intact allele of MYC; and (iii) coamplification of MYC and exon 1 of PVT, but not of downstream portions of PVT. We conclude that part or all of PVT is frequently amplified with MYC and that intron 1 of PVT represents a preferred boundary for amplification affecting MYC.

Arrangement and expression of integrated adenovirus type 12 DNA in the transformed hamster cell line HA12/7: amplification of Ad12 and c-myc DNAs and evidence for hybrid viral-cellular transcripts

In the genome of the adenovirus type 12 (Ad12)-transformed hamster cell line HA12/7 about three copies of the viral DNA are fixed by integration. The results of blot-hybridization, molecular cloning, and nucleotide sequencing experiments suggest a model for the arrangement of Ad12 DNA molecules in which the left hand terminus of one of the Ad12 DNA copies is linked to unique hamster DNA. The right hand end of this DNA molecule is fused to an inverted copy of a left terminal approximately 4.3 kb fragment of Ad12 DNA. This ensemble is followed by the second Ad12 DNA copy whose right terminus is again joined to an inverted, supernumerary left terminal approximately 4.3 kb Ad12 DNA fragment. There is a third Ad12 DNA copy whose right terminus is linked to cellular DNA. In this sequence arrangement, the left terminus of Ad12 DNA is overrepresented, as had been shown earlier (S. Stabel, W. Doerfler and R.R. Friis (1980) J. Virol. 36, 22-40). In the presented model, cellular DNA sequences are interspersed in between the three copies of Ad12 DNA. In the left terminus of the integrated Ad12 DNA, transcription of RNA is initiated which extends out into cellular DNA. The interviral DNA junctions are also transcribed. The c-myc gene in cell line HA12/7 is amplified about 10-fold and considerably more c-myc RNA has been identified in the Ad12-transformed cells than in BHK21 or in LSH hamster cells. It has been shown previously that the E1 region of Ad12 DNA is transcribed into mRNA in HA12/7 cells (Ortin et al. (1976) J. Virol. 20, 355-372). It remains to be investigated whether c-myc amplification and expression are related to the transformed phenotype of HA12/7 cells.

The PVT gene frequently amplifies with MYC in tumor cells

The line of human colon carcinoma cells known as COLO320-DM contains an amplified and abnormal allele of the proto-oncogene MYC (DMMYC). Exon 1 and most of intron 1 of MYC have been displaced from DMMYC by a rearrangement of DNA. The RNA transcribed from DMMYC is a chimera that begins with an ectopic sequence of 176 nucleotides and then continues with exons 2 and 3 of MYC. The template for the ectopic sequence represents exon 1 of a gene known as PVT, which lies 50 kilobase pairs downstream of MYC. We encountered three abnormal configurations of MYC and PVT in the cell lines analyzed here: (i) amplification of the genes, accompanied by insertion of exon 1 and an undetermined additional portion of PVT within intron 1 of MYC to create DMMYC; (ii) selective deletion of exon 1 of PVT from amplified DNA that contains downstream portions of PVT and an intact allele of MYC; and (iii) coamplification of MYC and exon 1 of PVT, but not of downstream portions of PVT. We conclude that part or all of PVT is frequently amplified with MYC and that intron 1 of PVT represents a preferred boundary for amplification affecting MYC.

Structure and expression of B-myc, a new member of the myc gene family

The myc family of genes contains five functional members. We describe the cloning of a new member of the myc family from rat genomic and cDNA libraries, designated B-myc. A fragment of cloned B-myc was used to map the corresponding rat locus by Southern blotting of DNA prepared from rat X mouse somatic cell hybrids. B-myc mapped to rat chromosome 3. We have previously mapped the c-myc to rat chromosome 7 (J. Sümegi, J. Spira, H. Bazin, J. Szpirer, G. Levan, and G. Klein, Nature [London] 306:497-498, 1983) and N-myc and L-myc to rat chromosomes 6 and 5, respectively (S. Ingvarsson, C. Asker, Z. Wirschubsky, J. Szpirer, G. Levan, G. Klein, and J. Sümegi, Somat. Cell Mol. Genet. 13:335-339, 1987). A partial sequence of B-myc had extensive sequence homology to the c-myc protein-coding region, and the detection of intron homology further indicated that these two genes are closely related. The DNA regions conserved among the myc family members, designated myc boxes, were highly conserved between c-myc and B-myc. A lower degree of homology was detected in other parts of the coding region in c-myc and B-myc not present in N-myc and L-myc. A 1.3-kilobase B-myc-specific mRNA was detected in most rat tissues, with the highest expression in the brain. This resembled the expression pattern of c-myc, although at different relative levels, and was in contrast to the more tissue-specific expression of N-myc and L-myc. B-myc was expressed at uniformly high levels in all fetal tissues and during subsequent postnatal development, in contrast to the stage-specific expression of c-myc.

Selection of cells with different chromosomal localizations of the amplified c-myc gene during in vivo and in vitro growth of the breast carcinoma cell line SW 613-S

The c-myc gene is amplified in the human breast carcinoma cell line SW 613-S. At early in vitro passages, the extra copies of the gene were mainly localized in double minute chromosomes (DMs), as shown by in situ hybridization with a biotinylated c-myc probe. However, cells without DMs were also present in which the c-myc genes were found integrated into any of several distinct chromosomes (mainly 7q+, 4 and 4q+, and 1). When this cell line was propagated in vitro, the level of c-myc amplification decreased because cells with DMs and a high amplification level were lost and replaced by cells without DMs and having a low amplification level. On the contrary, when early passage SW 613-S cells were grown in vivo, as subcutaneous tumours in nude mice, cells with numerous DMs and a high level of c-myc amplification were selected for. In one cell line (SW 613-Tu1) established from such a tumour, the DM-containing cells were substituted at late passages for cells with a high number of c-myc copies integrated within an abnormally banded region, at band 17q24 of a 17q+ chromosome. When only cells with integrated genes were present, this cell line was still highly tumorigenic indicating that the localization of the c-myc genes in DMs was not required for these cells to be tumorigenic in nude mice. Furthermore, cells of the secondary tumours induced by SW 613-Tu1 did not contain any DMs showing that in vivo growth did not promote the release of integrated c-myc copies into DMs.

The myc family of nuclear proto-oncogenes

Several members of the myc family of proto-oncogenes have been described, and some (c-, N-, and L-myc) have been characterized in considerable detail. They are united by a common gene structure and nucleotide homologies that were used to identify some of them initially. Their protein products also have scattered regions of amino acid identity or homology. Although the cellular activities of the various proteins are unknown, some members may play a role in regulating cell growth and differentiation. They share the ability to cooperate with an activated ras gene and cotransform embryonic rodent cells. In naturally occurring tumors, the members of the myc family of oncogenes appear to be activated by genetic changes (proviral insertion, chromosomal translocation, and gene amplification) that augment or otherwise disrupt normally regulated expression. The members of this family of genes differ markedly in their tissue specificity and developmental regulation of expression. This may account in part for the frequent appearance of activated c-myc genes in a wide variety of neoplasms and the limited appearance of activated N- and L-myc genes in tumors of embryonic or neural origin. The c-myc gene may be activated in tumors by a variety of mechanisms, whereas N- and L-myc appear to be activated only by gene amplification. Regulation of expression of the different myc genes also appears to occur by different mechanisms. Finally, the products of the different genes differ in may regions of the protein, and this divergence probably reflects their specific and individual functions.

Analysis of gene amplification in human tumor cell lines

Oncogene amplification has been observed in various primary tumors and tumor-derived cell lines. In several types of cancer, amplification of specific oncogenes is correlated with the stage of tumor progression. To estimate the frequency of gene amplification in other tumor types and to determine whether the ability to grow in vivo is associated with gene amplification in tumor cell lines, we have developed a modified version of the in-gel renaturation assay that detects human DNA sequences of unknown nature amplified as little as 7- to 8-fold. This assay was used to screen 16 cell lines derived from various solid tumors and leukemias. Amplified DNA sequences were detected in only one cell line, Calu-3 lung adenocarcinoma. This cell line was found to contain coamplified NGL (formerly termed neu) and ERBA1 oncogenes. However, when one of the amplification-negative cell lines, PC-3 prostatic carcinoma, was selected for in vivo growth in nude mice, amplified DNA sequences became detectable in these cells. The amplified sequences included the MYC oncogene, which showed no amplification in the parental cell line but was amplified 10- to 12-fold in the in vivo-selected cells. MYC amplification may, therefore, provide tumor cells with a selective advantage specific for in vivo growth.

Structure and expression of B-myc, a new member of the myc gene family

The myc family of genes contains five functional members. We describe the cloning of a new member of the myc family from rat genomic and cDNA libraries, designated B-myc. A fragment of cloned B-myc was used to map the corresponding rat locus by Southern blotting of DNA prepared from rat X mouse somatic cell hybrids. B-myc mapped to rat chromosome 3. We have previously mapped the c-myc to rat chromosome 7 (J. Sümegi, J. Spira, H. Bazin, J. Szpirer, G. Levan, and G. Klein, Nature [London] 306:497-498, 1983) and N-myc and L-myc to rat chromosomes 6 and 5, respectively (S. Ingvarsson, C. Asker, Z. Wirschubsky, J. Szpirer, G. Levan, G. Klein, and J. Sümegi, Somat. Cell Mol. Genet. 13:335-339, 1987). A partial sequence of B-myc had extensive sequence homology to the c-myc protein-coding region, and the detection of intron homology further indicated that these two genes are closely related. The DNA regions conserved among the myc family members, designated myc boxes, were highly conserved between c-myc and B-myc. A lower degree of homology was detected in other parts of the coding region in c-myc and B-myc not present in N-myc and L-myc. A 1.3-kilobase B-myc-specific mRNA was detected in most rat tissues, with the highest expression in the brain. This resembled the expression pattern of c-myc, although at different relative levels, and was in contrast to the more tissue-specific expression of N-myc and L-myc. B-myc was expressed at uniformly high levels in all fetal tissues and during subsequent postnatal development, in contrast to the stage-specific expression of c-myc.

Control of myogenic differentiation by cellular oncogenes

The establishment of a differentiated phenotype in skeletal muscle cells requires withdrawal from the cell cycle and termination of DNA synthesis. Myogenesis can be inhibited by serum components, purified mitogens, and transforming growth factors, but the intracellular signaling pathways utilized by these molecules are unknown. Recent studies have confirmed a role for proteins encoded by cellular proto-oncogenes in transduction of growth factor effects that lead to cell proliferation. To test the contrasting hypothesis that cellular oncogenes might also regulate tissue-specific gene expression in developing muscle cells, myoblasts have been modified by incorporation of the cognate viral oncogenes, the corresponding normal or oncogenic cellular homologs, and chimeric oncogenes, whose expression can be induced reversibly. Regulation of the endogenous cellular oncogenes also has been examined in detail. Down-regulation of c-myc is not obligatory for myogenesis; rather, inhibitory effects of myc on muscle differentiation are contingent on sustained proliferation. In contrast, activated src and ras genes block myocyte differentiation directly, through a mechanism that is independent of DNA synthesis and is rapidly reversible, resembling the effects of inhibitory growth factors. The coordinate regulation of diverse tissue-specific gene products including muscle creatine kinase, nicotinic acetylcholine receptors, sarcomeric proteins, and voltage-gated ion channels, raises the hypothesis that inhibitors such as transforming growth factor-beta and ras proteins might exert their effects through a transacting transcriptional signal shared by multiple muscle-specific genes.

Oncogenes and tumor suppressor genes

The artificial selection of the directly acting or acute RNA tumor viruses for high transforming ability has led to the isolation of defective retroviral genomes that have picked up, by accidental recombination, some of the important genes that influence, trigger or regulate cell division. These genes belong to at least four functionally different groups. Each of them can contribute to tumor development and/or progression after activation by structural or regulatory changes. Growth factor genes may act as oncogenes following constitutive activation in a cell that normally responds to, but does not produce, the corresponding growth factor (the autocrine model, exemplified by sis). Growth factor receptors may be fixed in a state of continuous, faulty signalling by the truncation of their external, ligand binding portion (examples: erb-B, fms). Genes coding for proteins involved in signal transduction may be activated by point mutations in certain, important domains (example: the ras-family). DNA binding proteins, presumably involved in DNA replication may drive cell division after constitutive activation by retroviral insertion, chromosomal translocation or gene amplification (example: the myc-family).

Expression of c-myc proto-oncogene in normal human lymphocytes. Regulation by transcriptional and posttranscriptional mechanisms

Aberrant expression of the c-myc gene results from nonrandom chromosomal translocations involving the transcriptionally active antigen receptor gene loci, in particular lymphocytic leukemias and lymphomas, and is believed to contribute to the etiology of these neoplasms. In addition to its expression in abnormal lymphocytes, increased accumulation of c-myc mRNA occurs rapidly in normal B- and T-lymphocytes after stimulation with appropriate mitogens. The mechanisms that mediate these mitogen-induced elevations in c-myc mRNA levels, however, have not been determined for normal B and T cells. By using enriched populations of B- and T-lymphocytes obtained from freshly isolated human tonsils and stimulated with Staphylococcus-A or with phytohemagglutinin, respectively, we observed marked elevations (20-40-fold) in the steady state levels of accumulated c-myc messenger RNA (mRNA) within 1 h of exposure of cells to mitogens; modest increases (three- to fivefold) in the relative rate of transcription of the c-myc gene through protein synthesis-independent (cycloheximide-insensitive) mechanisms; and rapid rates of degradation of mature c-myc mRNAs through protein synthesis-dependent (cycloheximide-sensitive) mechanisms. These findings corroborate previous studies in other cell types and provide evidence for both transcriptional and posttranscriptional control of c-myc proto-oncogene expression in normal human lymphocytes.

Dose effects of transfected c-Ha-rasVal 12 oncogene in transformed cell clones

We have examined the expression of the transformed phenotype in a series of clonal lines of NIH/3T3 cells transfected with the human c-Ha-rasVal 12 oncogene and the neomycin phosphotransferase gene. Cells from individual transformed foci were cloned and subjected to detailed analyses of the ras sequences. Three clones were found that expressed approximately one, 2-4, or 4-8 copies of the human c-ras oncogene, respectively. A fourth clone had multiple copies of the transfected sequences, and expressed abundant c-Ha-ras RNA. Analysis of the transformed phenotype of various clones indicated that cells expressing low levels of mutant c-Ha-ras had lost some of their extracellular fibronectin network, and were barely altered in their cytoskeleton. In contrast, cells expressing abundant c-Ha-ras had lost both their actin and fibronectin networks and showed an increase in plasminogen activator activity. Cells with amplified c-Ha-rasVal 12 grew better in low serum, formed large colonies in soft agar and showed enhanced activity of ornithine decarboxylase, the rate-controlling enzyme in polyamine biosynthesis. These results show that the dosage level of the mutant oncogene makes a significant contribution to the transformed phenotype of c-Ha-ras oncogene-transformed cells.

The molecular genetics of cancer

The search for genetic damage in neoplastic cells now occupies a central place in cancer research. Diverse examples of such damage are in hand, and they in turn hint at biochemical explanations for neoplastic growth. The way may be open to solve the riddles of how normal cells govern their replication and why cancer cells do not.

Inducible overproduction of the mouse c-myc protein in mammalian cells

We have made Chinese hamster ovary (CHO) cell lines that contain up to 2000 copies of the coding region of the mouse c-myc gene fused to the promoter of the Drosophila gene (hsp70) encoding a Mr 70,000 heat shock protein. Incubation of these cells at 43 degrees C results in an estimated 100-fold induction of c-myc mRNA. Translation of this mRNA occurs when the cells are returned to 37 degrees C, and during the first 3 hr of recovery at 37 degrees C, the c-myc protein is one of the most abundantly synthesized proteins in the cells. The products of the induced c-myc gene are phosphoproteins of apparent Mr 64,000, 66,000, and 75,000. Induced cells die, suggesting that elevated levels of c-myc are cytotoxic. Amplification of genes placed under control of the Drosophila hsp70 promoter may provide a general method for inducibly over expressing proteins in mammalian cells.