Chromosomes and cancer

Cytogenetics and molecular genetics of ovarian cancer

Genetic alterations identified in human ovarian tumors by conventional banding, fluorescence in situ hybridization, comparative genomic hybridization, chromosome microdissection, loss of heterozygosity, chromosome microcell-mediated chromosome transfer, and microarray gene expression analysis are summarized and correlated. The significance of these findings with respect to pathologic classification and clinical application are discussed.

Isolation of differentiated squamous and undifferentiated spindle carcinoma cell lines with differing metastatic potential from a 4-nitroquinoline N-Oxide-induced tongue carcinoma in a F344 rat

One differentiated squamous cell carcinoma (SCC) cell line (RSC3-E2) and two undifferentiated tumor cell lines (RSC3-LM and RSC3-E2R) with different metastatic potential were established from a 4-nitroquinoline N-oxide (4NQO)-induced differentiated SCC in F344 rat tongue. The RSC3-E2 subline was isolated from a parental cell line (RSC3-P) by single cell cloning in vitro, whereas the RSC3-LM subline was isolated from a lung metastatic focus after subcutaneous (s.c.) injection of RSC3-P cells. The RSC3-E2R cell line was isolated from a lung metastatic focus following s.c. injection of RSC3-E2 cells after X-irradiation in vitro. The RSC3-E2 cell line is keratin-positive and grows as a keratinizing tumor in nude mice, whereas RSC3-LM and RSC3-E2R cells are keratin-negative, vimentin-positive and form undifferentiated tumors. When s.c. injected into nude mice, the RSC3-E2 cell line proved to be non-metastatic, while the RSC3-LM cell line was metastatic by both hematogenous and lymphogenous routes, and the RSC3-E2R cell line was metastatic only hematogenously. In vitro relative growth rates and in vitro invasion activity of these cell lines were in the order RSC3-LM > RSC3-E2R > RSC3-E2. Chromosome analysis revealed two peaks with modal chromosome numbers of 83 and 78 for RSC3-P cells and single peaks at 83, 78 and 56 for RSC3-LM, RSC3-E2 and RSC3-E2R cell lines, respectively. Common structural abnormalities on chromosome 11 were shared by all cell lines. Mutation analysis of the p53 gene using a yeast functional assay demonstrated RSC3-LM cell line to have a point mutation at codon 269, whereas RSC3-E2 and RSC3-E2R had double mutations at codons 106 and 170 on each allele. These results suggest that the two undifferentiated RSC3-LM and RSC3-E2R tumor cell lines with different metastatic potential were generated from differentiated SCC cells via different genetic pathways as a consequence of tumor progression in vivo and in vitro, respectively. These cell lines should provide a useful model for understanding mechanisms of hematogenous and lymphogenous metastasis, as well as tumor progression of oral SCCs.

Anticlastogenic potential of 1alpha,25-dihydroxyvitamin D3 in murine lymphoma

Vitamin D3, having gained scientific interest for so long because of its role in mineral homeostasis, has now received great importance as a possible antitumor agent. This study was undertaken in an attempt to visualize the possible anticlastogenic potential of the vitamin in an ascitic mouse lymphoma model namely, Dalton's lymphoma. Frequencies of structural type chromosomal aberrations, sister chromatid exchanges and micronucleus assays have been chosen as the genotoxic endpoints in the proposed investigation. All these cytogenetic markers have been found to be markedly elevated during the progression of lymphoma in bone marrow cells. Vitamin D3 effectively suppressed the frequencies of chromosomal aberrations and sister chromatid exchanges in the lymphoma-bearing mice during the entire phase of tumor growth that significantly coupled with almost two-fold increase in survival time (37 +/- 2 and 68 +/- 2 days in lymphoma controls and vitamin D3-treated lymphoma-bearing mice, respectively), thus substantiating the antineoplastic efficacy of this secosteroid. The outcome of this study also is clearly reflected in the depletion of circulating (serum) vitamin D3 levels in the lymphoma control mice compared with normal (vehicle) controls while a still higher level was maintained in the VD3-treated lymphoma mice. This anticlastogenic property of the vitamin has so far been neglected and this is the first attempt to unravel the vitamin D3's effect in combating tumor development in vivo by limiting the frequencies of chromosomal aberrations, sister chromatid exchanges and micronuclei at least in transplantable murine model studied herein.

Amplification of C-MYC as the origin of the homogeneous staining region in ovarian carcinoma detected by micro-FISH

Homogeneous staining region (hsr), a cytogenetic indicator of gene amplification, has been frequently found in ovarian carcinoma (ovc). To identify the origin of the hsr, chromosome microdissection combined with polymerase chain reaction and fluorescence in situ hybridization (FISH) was applied to two human ovarian cancer cell lines, GR and MLS/P. The hsr probes were labeled with biotin or digoxigenin and hybridized to normal metaphase spreads to elucidate the chromosomal origin and regional localization of the amplified genes. FISH to normal metaphase spreads with the probe generated from the whole hsr-bearing chromosome from GR hybridized to 8q24, 2p13-->2q11.2, 10pter-->10p15, 10p12-->10q11.2, 5q23-->5q31, and 5q33-->5qter. For MLS/P, the hsr-bearing marker chromosome hybridized to 8q and 15q. In both cases, detailed FISH analysis revealed enhanced signal intensity at the 8q24 locus, which coincides with the chromosomal location of the C-MYC oncogene. To verify the involvement of C-MYC in hsr formation, in situ hybridization with a probe specific for the C-MYC oncogene was conducted and confirmed the amplification of C-MYC as the origin of the hsr. The whole hsr-bearing chromosome for GR is designated as rev ish der(10) (10pter-->10p15::8q24hsr:: 10p12-->10q11.2::8q24::2q11.2-->2p13::2p13 -->2q11.2::8q24::10q11-->10p11.2:: 5q23-->5q31::5q33-->5qter (wcp10+,D10Z1++,wcp2+,D2Z++,wcp5+,wcp8+ ,C-MYC++/hsr). The hsr-bearing marker for MLS/P is designated as rev ish der(8)(qter-->8q24::8q24::8q24-->8q10:: 8q10-->8q24::8q24::8q24-->8qter:: 15q11-->15qter)(wcp8+, D8Z1+,wcp15+,C-MYC++. FISH with the probe generated from the hsr of GR also painted the hsr in MLS/P, indicating that the two hsrs have shared homology, which indicates that the amplification of 8q24/C-MYC as the origin of hsr may be a nonrandom genomic alteration in ovc.

Hypomethylation of the rat aryl sulfotransferase IV gene and amplification of a DNA sequence during multistage 2-acetylaminofluorene hepatocarcinogenesis

Rat hepatic aryl sulfotransferase IV (AST IV), which catalyses sulfuric acid esterification of N-hydroxy-2-acetylaminofluorene to its ultimate carcinogenic form, is differentially expressed during multistep 2-acetylaminofluorene (AAF) hepatocarcinogenesis. Two molecular mechanisms associated with this effect involve modulation of mRNA translational capacity at the early stages, and gene transcription at the late stages of the carcinogenic process. To characterize further the molecular mechanisms that may be involved in the transient regulation of the enzyme expression, an AST IV cDNA was used to assess the change in methylation profile and restriction fragment length polymorphism (RFLP) in the gene domain of genomic DNA derived from rats at different stages of carcinogenesis. The onset of hypomethylation of the AST IV gene domain and amplification of a 5.3-kb DNA sequence was found to correlate with the stage in AAF hepatocarcinogenesis, where rats begin to exhibit irreversible loss in hepatic enzyme expression and the liver becomes committed to hepatoma formation. This represents the first observation of both altered methylation status of AST IV gene domain and amplification of a DNA sequence whose expression may play a role in the genesis and/or progression of neoplastic transformation of initiated cells during AAF hepatocarcinogenesis.

Comparative patterns of hepatic drug metabolizing enzymes and their possible correlation with chromosomal aberrations in transplantable murine lymphoma: a time course study

The differential levels of induction of hepatic microsomal cytochrome P-450 (cyt. P-450), UDP-glucuronyl transferase (UDPGT), cytosolic glutathione-S-transferase (GSHT) activities, and major chromosomal aberrations were evaluated over various periods of time, following tumor transplantation in male Swiss Albino mice. Changes in the above markers were studied (1) to monitor the entire carcinogenic process and (2) to test the suitability of chemopreventive exposures in terms of phase and duration of tumor growth. The microsomal cyt. P-450 content and the UDPGT activity were significantly elevated (p < 0.01-0.001) from the early stages of tumor growth while the cytosolic GSHT activity reached its highest level (p < 0.01-0.001) only 10 days after tumor transplantation. During the later stages of tumor growth all the biotransforming enzyme activities showed a downhill trend, which was significantly lower than that of their normal counterparts (p < 0.01-0.001). The frequency of different chromosomal aberrations, which were of major structural, numerical, and physiological types, increased steadily throughout the entire length of the carcinogenic process (30 +/- 2 days).

Gene amplification in a human osteosarcoma cell line results in the persistence of the original chromosome and the formation of translocation chromosomes

Although gene amplification, a process that is markedly enhanced in tumor cells, has been studied in many different cell systems, there is still controversy about the mechanism(s) involved in this process. It is still unclear what happens to the DNA sequences that become amplified, whether they remain present at their original location (conservative gene amplification) or whether gene amplification necessarily results in a deletion at the original location (non-conservative gene amplification). We have studied gene amplification in a human osteosarcoma cell line, starting from a cell clone which contains only one copy of a plasmid integrate. Independent amplificants, originating from this clone and containing elevated plasmid copy numbers, were isolated and analyzed. Based on previous observations, encompassing the persistence of single-copy DNA sequences besides amplified DNA sequences clustered at a different location in the independent amplificants, we proposed an amplification pathway including a local duplication step and transposition of the duplicated DNA to other chromosomal positions. Now we have extended our study to more independent amplificants. We prove that the single-copy plasmid-containing chromosomes in the different amplificants and the single-copy plasmid-containing chromosome in the original parental cell clone are indeed identical, namely a translocation chromosome composed of at least three parts of which two originate from chromosomes 14 and 17. We show that the unit of amplification and the unit of the proposed transposition event are at least 1.5 Mb. We also demonstrate that the amplified DNA sequences, present at genomic locations other than the original single-copy DNA sequences, are preferentially associated with chromosome 16. We find that the amplified DNA sequences are often located at or near a site of chromosome translocation involving chromosome 16. In one cell clone we detect the amplified DNA sequences in most of the cells to be located within a complete chromosome 16 while in a minority of cells the amplified sequences are located at or near a breakpoint on a translocation chromosome 16. This indicates that this amplification region is highly unstable and frequently gives rise to translocation events.

Double minute chromatin bodies in carcinoma of the human cervix uteri

In direct G-banded preparations of 150 cases of cervical cancer, double minutes (dmin) were observed in 43% (64 cancers) of the patients, including a case of carcinoma-in-situ (CIS). The incidence of dmin varied from 1 to over 20 per cell. The karyotypic findings in the tumors containing dmin did not reveal any similarities and the prognosis of the patients with dmin was significantly worse compared to those without dmin. The presence of dmin in cytogenetic preparations of primary tumors may indicate a role in the maintenance and/or progression of malignancy, particularly as dmin have been suggested to represent amplified genes.

Karyotypic change from heteroploidy to near diploidy associated with development of cisplatin resistance in a rat ovarian tumour cell line

In a rat ovarian tumour cell line a 33-fold resistance to cisplatin (O-342/DDP) was developed in vitro by continuous exposure of the parental cell line (O-342) to stepwise increase cisplatin concentration in the culture medium. Both cell lines had a similar growth rate in vitro. Development of resistance was accompanied by a change of the karyotype from heteroploidy in chemosensitive O-342 cells to near diploidy in resistant O-342/DDP cells as shown by chromosome number distribution. This finding was confirmed by measuring cellular DNA content using flow-cytometry analysis. Flow karyotyping showed significant differences in chromosomal DNA contents between both cell lines. Our results suggest that the parent line O-342 consists of at least two subpopulations, a cisplatin-sensitive and a cisplatin-resistant one, corresponding to hyperploidy and near diploidy, respectively. Continuous cisplatin exposure of O-342 cells selectively killed the sensitive fraction, resulting in the karyotypic change observed.

Clonotypic chromosomal aberrations in long-term lines of myelin-specific rat T lymphocytes

A panel of 16 long-term rat T lymphocyte lines and clones were screened for cytogenetical abnormalities using chromosomal banding techniques. All T lines were CD4+, recognizing the relevant antigen in the molecular context of major histocompatibility complex (MHC) class II determinants. With one exception (an ovalbumin-specific line), all lines were specific for myelin proteins, and apart of one BS rat-derived T line and its clones, all lines were selected from the Lewis strain of rat. After in vitro culture of more than 1 year, all lines and clones exhibited subtle but definite chromosomal aberrations, which included deletions, enlargement, translocations and formation of isochromosomes. All lines were near diploid, structural chromosomal changes being more frequent than numerical abnormalities. Each T line investigated had an individual pattern of chromosomal changes. In our analysis, 16 of the 22 different chromosomes had changes in at least one line. Chromosome 9 and the X chromosome appeared to have an enhanced susceptibility of alterations. In two cases, chromosomal markers could be traced through different stages of in vitro culture of the T lines.

Karyotypic analyses of metastatic and non-metastatic mouse tumor cells by G- and C-banding

Cytogenetic studies using G- and C-banding were performed in order to compare non-metastasizing BWO mouse thymoma cells with their metastatic BW-IS-Li and BW-O-Li1 variants. BW-IS-Li and BW-O-Li1 cells were obtained from liver metastases generated respectively after intra-splenic or intravenous inoculation of mice with BWO cells. BW-O-Li1 cells contained normal chromosomes 4 and 18 which were never found in BWO or BW-IS-Li cells. As they also contained several markers common to the BWO parental cell line, our data provide definite evidence that the BW-O-Li1 cell line results from spontaneous in vivo cell fusion between a BWO cell and a normal host cell. Chromosome 3 was also shown to be "rearrangement-prone" and was less represented among metastatic cell variants. These variants also lack a C-band-deficient marker chromosome which was found in most of the BWO cells. However, as many complex chromosome rearrangements were found in all 3 cell lines, at least some chromosomal material may be present in a hidden form. We were not able in this particular cell system to link the presence or absence of a single chromosome, chromosome fragment or rearrangement to the acquisition or expression of metastatic properties.

Double minutes and other chromosomal aberrations in malignant cell line of Shionogi carcinoma 115

G-banded karyotypes and structural abnormalities of chromosomes are described in two clones of the mouse mammary carcinoma SC 115 cell line. The cells showed multiple numerical and structural abnormalities. Structural anomalies, such as double minutes, acentric fragments, gaps or breaks, radials, ring chromosomes, endoreduplications, dicentrics, and elongated chromosomes were observed in varying proportions. Unstable chromosomal aberrations, such as breaks, gaps, fragments, and radials, frequently appeared without any particular pretreatment. Increased numbers of double minutes per cell correlated positively with the presence of elongated chromosomes, which did not contain homogeneously staining regions by the G-band technique.

Carcinogen-mediated methotrexate resistance and dihydrofolate reductase amplification in Chinese hamster cells

We have investigated different parameters characterizing carcinogen-mediated enhancement of methotrexate resistance in Chinese hamster ovary (CHO) cells and in simian virus 40-transformed Chinese hamster embryo (C060) cells. We show that this enhancement reflects dihydrofolate reductase (dhfr) gene amplification. The carcinogens used in this work are alkylating agents and UV irradiation. Both types of carcinogens induce a transient enhancement of methotrexate resistance which increases gradually from the time of treatment to 72 to 96 h later and decreases thereafter. Increasing doses of carcinogens decrease cell survival and increase the enhancement of methotrexate resistance. Enhancement was observed when cells were treated at different stages in the cell cycle, and it was maximal when cells were treated during the early S phase. These studies of carcinogen-mediated dhfr gene amplification coupled with our earlier studies on viral DNA amplification in simian virus 40-transformed cells demonstrate that the same parameters characterize the amplification of both genes. Possible cellular mechanisms responsible for the carcinogen-mediated gene amplification phenomenon are discussed.

Carcinogen-mediated methotrexate resistance and dihydrofolate reductase amplification in Chinese hamster cells

We have investigated different parameters characterizing carcinogen-mediated enhancement of methotrexate resistance in Chinese hamster ovary (CHO) cells and in simian virus 40-transformed Chinese hamster embryo (C060) cells. We show that this enhancement reflects dihydrofolate reductase (dhfr) gene amplification. The carcinogens used in this work are alkylating agents and UV irradiation. Both types of carcinogens induce a transient enhancement of methotrexate resistance which increases gradually from the time of treatment to 72 to 96 h later and decreases thereafter. Increasing doses of carcinogens decrease cell survival and increase the enhancement of methotrexate resistance. Enhancement was observed when cells were treated at different stages in the cell cycle, and it was maximal when cells were treated during the early S phase. These studies of carcinogen-mediated dhfr gene amplification coupled with our earlier studies on viral DNA amplification in simian virus 40-transformed cells demonstrate that the same parameters characterize the amplification of both genes. Possible cellular mechanisms responsible for the carcinogen-mediated gene amplification phenomenon are discussed.

Patterns of BrdU incorporation in homogeneously staining regions and double minutes

The replication chronology of two structural chromosome abnormalities linked to the amplification phenomenon of DNA sequences was investigated. Three cell lines containing homogeneously staining region (HSR) chromosomes (IMR-32, MK42, and COLO-320) and one line with double minutes (DM) (SW-613) were examined. Using a bromodeoxyuridine-Hoechst 33258-Giemsa method, the HSR in the three cell lines were shown to be composed of subunits that replicated their DNA throughout all portions of the S-phase of the cell cycle. The double minute chromosomes were observed to replicate randomly throughout the entire S-phase, with no pattern evident. These results are consistent with the suggestion that DNA from HSR and DM are structurally and functionally related. Moreover, this observation that these amplified regions replicate their DNA throughout the entire S-phase favors the idea that, during amplification processes, both early and late replicating sequences are included. The apparent discordance between staining characteristics and replication behavior exhibited by some HSR and DM are also discussed.

Chemically induced aneuploidy in mammalian cells: mechanisms and biological significance in cancer

A growing body of evidence from human and animal cancer cytogenetics indicates that aneuploidy is an important chromosome change in carcinogenesis. Aneuploidy may be associated with a primary event of carcinogenesis in some cancers and a later change in other tumors. Evidence from in vitro cell transformation studies supports the idea that aneuploidy has a direct effect on the conversion of a normal cell to a preneoplastic or malignant cell. Induction of an aneuploid state in a preneoplastic or neoplastic cell could have any of the following four biological effects: a change in gene dosage, a change in gene balance, expression of a recessive mutation, or a change in genetic instability (which could secondarily lead to neoplasia). To understand the role of aneuploidy in carcinogenesis, cellular and molecular studies coupled with the cytogenetic studies will be required. There are a number of possible mechanisms by which chemicals might induce aneuploidy, including effects on microtubules, damage to essential elements for chromosome function (ie, centromeres, origins of replication, and telomeres), reduction in chromosome condensation or pairing, induction of chromosome interchanges, unresolved recombination structures, increased chromosome stickiness, damage to centrioles, impairment of chromosome alignment, ionic alterations during mitosis, damage to the nuclear membrane, and a physical disruption of chromosome segregation. Therefore, a number of different targets exist for chemically induced aneuploidy. Because the ability of certain chemicals to induce aneuploidy differs between mammalian cells and lower eukaryotic cells, it is important to study the mechanisms of aneuploidy induction in mammalian cells and to use mammalian cells in assays for potential aneuploidogens (chemicals that induce aneuploidy). Despite the wide use of mammalian cells for studying chemically induced mutagenesis and chromosome breakage, aneuploidy studies with mammalian cells are limited. The lack of a genetic assay with mammalian cells for aneuploidy is a serious limitation in these studies.

Chromosome aberrations in peripheral lymphocytes of male homosexuals

Karyotypes of peripheral lymphocytes of 19 male homosexuals showed increased hypodiploidy. Chromosomes #19 and #20 were most frequently lost. Also, structural chromosome aberrations frequently occurred consisting chiefly of translocations and simple chromosome breaks. Terminal deletions, inversions, and isochromosomes occurred less commonly. In three of the cases, 100% of the cells were involved in a pericentric inversion of a chromosome #9. Chromosomes #3 in p21.1 and 1 in p32.3 were repeatedly affected. Structural aberrations were seen less frequently in men with acquired immunodeficiency syndrome(AIDS) and AIDS-related complex than in asymptomatic homosexuals. The hypodiploidy with preferential loss of chromosomes was constantly present. The marker chromosomes and simple breaks at repeated sites are another manifestation of damage to the immune system in these male homosexuals from Greenwich Village in New York City. The chromosomal damage was potentially the result of exposure to amyl and butyl nitrites, viral infections, or immunologic reactions to sperm, which crossreact with lymphocytes.

Amplification and altered expression of the c-myc oncogene in A-MuLV-transformed fibroblasts

Chromosomal rearrangements involving the c-myc oncogene are a prevalent feature of plasmacytomas that arise after inoculating BALB/c mice with pristane and Abelson murine leukaemia virus (A-MuLV). With this observation in mind, we decided to determine if any genetic alterations of the c-myc locus could be observed in cells of a different type, when transformed in vitro by A-MuLV. Here we have analysed three independent A-MuLV-transformed NIH 3T3 lines (ANN-I, 54c12 and N25), and found that the c-myc locus is amplified 8-19-fold in each transformant. Quantitative S1 nuclease mapping performed on ANN-I and 54c12 RNAs demonstrated that: (1) c-myc messenger RNAs accumulated to double the levels found in NIH 3T3 cells; and (2) a shift in the use of the two normal c-myc transcription initiation sites (P1 and P2) occurred in favour of the 3' site, P2. Analysis of c-myc chromatin by DNase I treatment of 54c12 nuclei revealed that most, if not all, of the c-myc gene copies were transcriptionally competent. We present alternative ideas to explain why amplification of the c-myc gene occurs repeatedly in A-MuLV-transformed fibroblasts. Finally, we discuss our results in relation to the hypothesis linking the phenomenon of tumour progression with the amplification of oncogenes.

Gene amplification: an example of accelerated evolution in tumorigenic cells

During selection for methotrexate resistance, tumorigenic CHEF/16 cells and derivatives from CHEF/16 tumors underwent amplification of the dihydrofolate reductase gene (DHFR) at accelerated rates compared with closely related nontumorigenic CHEF/18 cells. "Dot blot" analysis showed that the CHEF/16 cells contained many more copies of the DHFR gene than did the CHEF/18 cells, when assayed at similar elevated levels of methotrexate resistance. Chromosome analysis of cell samples taken at several time points during amplification revealed large differences between the nontumorigenic CHEF/18 cells and the two tumorigenic cell lines. The tumorigenic cells developed few chromosome rearrangements over a 4-log increase in methotrexate resistance, other than increased length of a single chromosome, which was shown by in situ hybridization to contain most or all of the amplified DHFR gene copies. In contrast, the CHEF/18 cells underwent complex, progressive changes in almost every chromosome, and in situ hybridization suggested a dispersed pattern of gene amplification. The data support the hypothesis that unregulated amplification is a pathological process, occurring readily in neoplastic but rarely in normal cells, that together with other chromosomal disturbances contributes to the rapid evolution and progression of cancer.

Double minutes in the HeLa cell line

Metaphase preparations of three sublines of the HeLa line showed the presence of double minutes (DM) in varying frequencies. In two sublines (S3 and TCH-3753), the size of the DM was variable, whereas in the Fe-1000 subline, they were uniform. Giemsa banding preparations revealed typical HeLa marker chromosomes in all sublines.

Tumorigenicity, major histocompatibility antigens, and karyotypes of interspecific hybrids between mouse neuroblastoma and rat glioma or liver cells

Five interspecific hybrids of mouse neuroblastoma with rat glioma (NG108-15, 140-3, and 141-B) or with nontransformed rat liver cells (NBr-10A and NBr-20A) were examined for major histocompatibility (MHC) antigens and tumorigenicity in comparison with their karyotypes. Both mouse and rat MHC antigens were present in each hybrid population, as determined by a simple cytotoxicity test. All five hybrid cell lines produced tumors in athymic nude mice with varied take incidences. Four hybrid cells, NG108-15, 140-3, NBr-10A, and NBr-20A, were highly tumorigenic. Their karyotypes were characterized by a higher modal chromosome numbers than would be expected from the fusion of parent cells in which at least one parent contained an increased number of chromosomes. In contrast, 141-B cells, with massive loss of chromosomes from both malignant parents, were weakly tumorigenic. The results suggest that the retention of marker chromosomes as well as double minutes (DMs) or microchromosomes of neuroblastoma origin may be required for expression of malignancy in these hybrid cells. The survival time of tumor-bearing mice also varied within the five cell lines, but it was significantly short in NG108-15, which yielded lung metastases in the host animals.

Sister chromatid exchange and proliferation pattern in stimulated lymphocytes of cutaneous malignant melanoma patients

Sister chromatid exchange (SCE) and the proliferative pattern of phytohemagglutinin-stimulated lymphocytes were examined in 36 nonfamilial cutaneous malignant melanoma (CMM) patients. One close relative of each of 27 CMM patients was also examined. All the patients had undergone surgical treatment for the neoplasm, but had received no chemotherapy or radiotherapy. The SCE rates were found to be higher and more variable in a significant fraction of CMM patients, and in relatively fewer unaffected relatives, which is in contrast to findings in unrelated subjects taken as controls. Also, variable and higher proportions of cells in metaphase of the first cell cycle (M1), after 72-hr culture in the presence of bromodeoxyuridine, were more often found among the CMM patients than in the controls; however, no effect of clinical progression of the neoplastic disease on SCE rates or on the lymphoproliferative pattern was observed. The present study indicates heterogeneity among subjects who develop CMM and suggests that the peculiarities of SCE rates and of the lymphoproliferative patterns observed in some of the CMM patients and in a few of their close relatives may be connected with the mechanism of onset of the neoplasm.

Replication pattern of double minutes derived from an insect cell line

The DNA replication pattern of double minutes derived from an established cell line of Aedes albopictus is described. Although the vast majority of double minutes replicate semiconservatively once during the S phase, some double minutes appear to exhibit different pattern(s). Two theories are suggested as possible explanations of our findings.

UV radiation facilitates methotrexate resistance and amplification of the dihydrofolate reductase gene in cultured 3T6 mouse cells

Pretreatment of 3T6 murine cells with the carcinogen UV radiation or N-acetoxy-N-acetylaminofluorene increased the number of methotrexate-resistant colonies. This carcinogen-induced enhancement was seen only at low toxicities. The enhancement was transient and was observed at its maximum when cells were subjected to methotrexate selection 12 to 24 h after treatment. The addition of a tumor-promoting agent, 12-O-tetradecanoylphorbol-13-acetate, during or after carcinogen treatment further enhanced this effect. A large proportion of the resistant colonies had an increase in the dihydrofolate reductase gene copy number and the relative proportions of colonies with amplified genes were similar, regardless of whether selected cells were untreated, treated with carcinogen, or treated with carcinogen plus promoter. We discuss some of the variables which both enhance the generation and improve the detection of methotrexate-resistant colonies, as well as certain implications of our results for the generation and mechanism of gene amplification.

UV radiation facilitates methotrexate resistance and amplification of the dihydrofolate reductase gene in cultured 3T6 mouse cells

Pretreatment of 3T6 murine cells with the carcinogen UV radiation or N-acetoxy-N-acetylaminofluorene increased the number of methotrexate-resistant colonies. This carcinogen-induced enhancement was seen only at low toxicities. The enhancement was transient and was observed at its maximum when cells were subjected to methotrexate selection 12 to 24 h after treatment. The addition of a tumor-promoting agent, 12-O-tetradecanoylphorbol-13-acetate, during or after carcinogen treatment further enhanced this effect. A large proportion of the resistant colonies had an increase in the dihydrofolate reductase gene copy number and the relative proportions of colonies with amplified genes were similar, regardless of whether selected cells were untreated, treated with carcinogen, or treated with carcinogen plus promoter. We discuss some of the variables which both enhance the generation and improve the detection of methotrexate-resistant colonies, as well as certain implications of our results for the generation and mechanism of gene amplification.

Karyotypic analysis of murine cytolytic T-cell lines

G-banding techniques have been used to analyze the karyotypes of four independently derived murine cytolytic T lymphocyte (CTL) lines established in culture 2-4 yr earlier. For two of them, several subclones were included in the analysis. The modal arm number of three of the lines was close to diploid, whereas in the fourth line, both near-diploid and near-tetraploid subclones were found. The most frequent type of rearrangement was the formation of metacentrics by centromeric fusion. A comparison of the abnormalities (markers) occurring in the different CTL lines showed the formation of similar chromosomal rearrangements in independent lines, thus suggesting nonrandom chromosomal changes in these cells. Several of the metacentrics are derived from the fusion of normal murine chromosomes that are apparently prone to form such "markers," as the occurrence of similar abnormalities has been reported in other established cell lines and tumors. On the other hand, some of the biarmed and telocentric marker chromosomes seem to characteristically occur in CTL but not in other types of cell lines.

Gene amplification in a mouse embryo? Double minutes in cell lines independently derived from a Mus musculus X M. caroli fetus

Double minutes (DM) were found to be present in six of seven clones derived from a 16-day female Mus musculus X M. caroli fetus. The DM-positive clones derived from three primary populations independently set up from the fetus, and included clones with an active M. caroli X chromosome as well as clones with an active M. musculus X. The simplest explanation of these findings is that DM were already present in cells of the M. musculus X M. caroli embryo at the time of X chromosome inactivation and persisted during in vivo development and in vitro culture. This suggests that gene amplification occurred in the early embryo, or even the fertilized egg, perhaps because of interactions between components of germ cells contributed by the M. musculus and M. caroli parents. Alternatively, induction may have occurred independently in these lines, requiring that amplification is an unusually common occurrence in cells from interspecific hybrids.

Genetic analysis of hyperproduction of epidermal growth factor receptors in human epidermoid carcinoma A431 cells

Human epidermoid carcinoma A431 cells, possessing an extraordinarily high number of epidermal growth factor (EGF) receptors (1), were found to be hypotetraploid in their chromosome constitution and to contain two copies of intact chromosome 7 and two types of the translocation chromosomes involving chromosome 7 (M4 and M14) as well as several other rearranged chromosomes. The A431 cells were fused with mouse A9 cells, which lack EGF receptors (2) and are deficient in hypoxanthine phosphoribosyltransferase (3), and the human-mouse cell hybrid (AA series) were selected in HAT/ouabain medium (3, 4). The expression of high EGF binding ability was correlated with the presence of human translocation chromosome M4. AA hybrid clones that contained intact human chromosome 7 but not the marker chromosome M4 expressed only ordinary levels of EGF receptors. The EGF receptors expressed in the AA hybrids were proven to be of human nature by immunoprecipitation of the receptors cross-linked with [125I]EGF. These observations and our previous gene assignment of the EGF receptor to human chromosome 7 (2, 5) suggest that the marker chromosome M4 may carry an alteration(s) in the gene(s) involved in EGF receptor biosynthesis.

Cytogenetic analysis of 62 transitional cell bladder carcinomas

Chromosome analysis of biopsy material obtained after vinblastine pretreatment was carried out in 108 specimens from 89 patients with transitional cell carcinoma of the urinary bladder. Analyzable metaphases were obtained in 62 tumors, but only in nine tumors could karyotypes be analyzed by banding; a conventional technique was used in all others. Ploidy and occurrence of markers corresponded with tumor morphology and invasion and sometimes aided in the clinical evaluation; chromosome anomalies specific for bladder cancer were not revealed. In noninvasive tumors of WHO grade 1 and 2, near-diploid karyotypes with occasional marker chromosomes dominated. Grade 3 tumors showed a variety of grossly aneuploid karyotypes, with an almost constant occurrence of different markers. Superficially invasive G2 tumors had moderately pronounced aberrations with more deviations than non-invasive tumors but without the great variety of G3 tumors.

Chromosome analysis of two rat tumor cell lines. Possible role of DMs and HSR in metastasis

The rat tumor cell lines BSp73AS (AS, non-metastasizing with pronounced adherent capacities) and BSp73ASML (ASML, highly metastasizing with reduced adherent capacities) were cytogenetically investigated. The ASML cell line is reportedly derived from the AS cell line. Both lines exhibited abnormal numerical and structural chromosomal characteristics. The metastasizing ASML cells showed a higher chromosome number (modal number: 62-63) than the nonmetastasizing AS cells (modal number:48). The AS karyotype was characterized by the presence of a large metacentric marker chromosome resulting from a Robertsonian translocation (Rb 6.7). This chromosome is as such absent in ASML cells but perhaps it may be present in these cells with a major part of chromosome 7 being deleted. The most interesting feature of the ASML karyotype was the presence of double minutes (DMs) and a homogeneously staining region (HSR) at the telomeric end of chromosome 6. These were peculiar to the ASML cells, being absent in the AS cells. DMs and HSR are reported to be correlated with the resistance to various drugs and with the acquired virulence of tumor cells through gene amplification. Therefore, we assume that in the metastasizing ASML cell line the DMs and HSR were established through genetic selection and that they are probably related to the acquired metastasizing capacity of these cells.

Analysis of CAD Gene Amplification Using a Combined Approach of Molecular Genetics and Cytogenetics

CAD is a multifunctional protein which catalyzes the first three steps of de novo uridine biosynthesis. Rodent cells resistant to PALA, a specific inhibitor of the ATCase activity of CAD, overproduce the CAD protein and CAD mRNA as a direct result of the amplification of the CAD gene. In order to study the mechanism of CAD gene amplification, a functional Syrian hamster CAD gene was inserted into a cosmid vector using molecular cloning techniques. The cloned genes were assayed for biological function by fusing CAD-deficient Chinese hamster ovary (CHO) cell mutants with protoplasts of E. coli containing the CAD cosmids. Two clones with functional CAD genes were isolated and shown to contain inserts 40 and 45 kb long. The cloned genes could also be introduced into wild type CHO cells by selecting for cells which became resistant to high PALA concentrations in a single step. Transformations of mutant and wild type CHO cells contained multiple active copies of the donated Syrian hamster CAD genes in addition to their endogenous CHO CAD genes. The cloned genes in all transformants analyzed are integrated into host cell chromosomes at single locations defined by in situ hybridization. Independently isolated transformants contain the donated genes in different chromosomes. Co-transformation of CHO cells with two different genes by protoplast fusion is also shown to be possible.

Double minutes and other chromosomal aberrations in two malignant cell lines of the German cockroach Blattella germanica

Structural anomalies of mitotic chromosomes from two tumorigenic cell lines of the German cockroach (Blattella germanica) is described. Aberrations, such as unpairable marker chromosomes, double minutes, di- and tricentrics, ring chromosomes, tri- and quadriradials, and chromosome and chromatid gaps and breaks, were observed in varying proportions. This study reports that the double minute chromosomes (DMs) are associated with insect tumor cells, similar to the findings in both murine and human tumor cells.

Cytogenetic changes during the early stages of liver carcinogenesis in Chinese hamster: an in vivo--in vitro comparison

Cytogenetic changes were investigated during the early stages of hepatic adenocarcinoma development in Chinese hamsters injected with a single dose of dimethylnitrosamine (DMN). An in vivo-in vitro comparison was made from 7 to 35 weeks after injection. A partial hepatectomy was used to stimulate mitosis for in vivo analysis, and the excised liver, grown to the primary culture stage, was used for chromosome analysis in vitro. Aneuploidy, tetraploidy, and chromosome aberrations increased significantly in the hepatic cells of DMN-treated animals in vivo, with no significant change over the 7- to 35-week period. No differences, however, could be detected between the primary cultures of control and DMN-treated animals because of an inherent tendency for all cultures to develop aneuploid stem lines at an early stage in culture. A preferential involvement of chromosome #6 in the single trisomic state was demonstrated in vitro and to a minor extent in vivo. The relevance of increased aneuploidy in early carcinogenesis and the differences between the in vivo and in vitro results are discussed.

Gene amplification in methotrexate-resistant mouse cells. IV. Different DNA sequences are amplified in different resistant lines

DNA was purified from double minutes isolated from MTX-resistant EL4/8 mouse lymphoma cells, digested to completion with Bam H1 restriction endonuclease and cloned in lambda-1059. The properties of the library suggest that the DNA from which it was made was not detectably contaminated with non-dm chromosome material, and that the library is essentially complete for sequences contained in Bam H1 restriction fragments between 9 and 19 kb. The inserts of some selected lambda-recombinants were subcloned in pBR328 or pAT153 to separate sequences of differing repetition frequency. Clones representative of different classes of sequences were used as probes to Southern transfers of Bam H1 digested total nuclear DNAs of various MTX-resistant cell lines. The results clearly show that the amplified unit of each cell line has a unique structure, and that different amplified units differ widely in their sequence composition.

Variable content of double minute chromosomes is not correlated with degree of phenotype instability in methotrexate-resistant human cell lines

Several variants resistant to 1.8 x 10(-4) M DL-methotrexate (MTX) have been isolated from the human cell lines HeLa BU25 and VA2-B by exposing them to progressively increasing concentrations of the drug. A striking variability of phenotype and chromosome constitution was observed among the different variants. All resistant cell lines exhibited a greatly increased dihydrofolic acid reductase (DHFR) activity and DHFR content; however, the DHFR activity levels varied considerably among the variants, ranging between about 35 and 275 times the parental level. In the absence of selective pressure, the increased DHFR activity was unstable, and in all cell lines but one was completely lost over a period ranging in different variants between 25 and 200 days. The MTX-resistant cells lines showed anomalies in their chromosome constitution, which involved the occurrence of a duplicated set of chromosomes in most cells of some of the variants and the presence of double minute chromosomes in all cell lines. An analysis of the correlation of loss of double minute chromosomes and loss of DHFR activity in the absence of MTX has given results consistent with the idea that the double-minute chromosomes contain amplified DHFR genes. However, the most significant finding is that, in contrast to what has been reported in the mouse system, the recognizable double-minute chromosomes varied greatly in number in different variants without any relationship to either the level of DHFR activity or the degree of instability of MTX resistance in the absence of selective pressure. These and other observations point to the occurrence in the human MTX-resistant variants of another set of DHFR genes, representing a varied proportion of the total, which is associated with the regular chromosomes, and which may be unstable in the absence of selective pressure.

Genetic analysis of tumorigenesis: X. Chromosome studies of transformed mutants and tumor-derived CHEF/18 cells

Chinese hamster embryo fibroblast cell line CHEF/18 is stably diploid, anchorage-dependent, has a high serum requirement, and a does not form tumors in nude mice. The chromosome constitutions of spontaneous and chemically induced anchorage-independent and/or low-serum CHEF/18 mutants and tumors produced in nude mice by some of these mutants are compared. We find a correlation between diploidy and nontumorigenicity among the anchorage-independent mutants but not in the low-serum mutants. One of the four spontaneous and six of the 15 chemically induced anchorage mutants have remained diploid. The remaining 12 mutants are pseudodiploid or aneuploid, and seven of them contain changes in chromosome 1, either a translocation or a deletion involving breakage at the same position (1q11-12). Each of the tumors induced by six mutants has a unique pattern of rearrangements; however five of the six have changes involving chromosome 3. This chromosome was also frequency rearranged in tumor-derived cells previously investigated.

Influence of strain difference on the karyotypic changes in N-nitroso-N-butylurea--induced mouse lymphomas

Karyotypes were analyzed in 44 cases of mouse lymphomas induced by continuous oral administration of N-nitroso-N-butylurea (NBU): 14 cases of C57BL/6JKok, 6 from LT/sv, 5 from MT/Hok, 2 from (AKR/MsHok x T1Ct)F1, 2 from (AKR/MsHok x CBA/H-T6Hok)F1, and 15 from the first backcross generation of (CBA/Hok x CBA/H-T6Hok)F1 to CBA/Hok. Thirty-six were thymic lymphomas (TLs) and the remaining 8 nonthymic lymphomas (NTLs). No chromosome abnormality was found in 8 NTLs, whereas modal cells were karyotypically changed in 13 TLs. Unlike the situation in NTLs, chromosomally abnormal cells were always present in the remaining 23 TL cases in which modal cells showed no apparent deviation from the constitutional host karyotype. A total of 31 independent karyotypic changes were identified in 26 cytogenetically abnormal major clones from 19 TLs. Although trisomy 15 was the most frequent karyotypic change, being present in 9 TLs, its incidence was substantially low compared with experimentally induced murine TLs so far reported. Furthermore, 6 of the 9 trisomy 15 clones clustered to (CBA x T6)F1 x CBA. Thus, it seems probable that the genetic background of the host animals has a profound influence upon the frequency of trisomy 15 in TLs.

Mutagens in human faeces. Are they relevant to cancer of the large bowel?

1. Mutagenic activity has been detected in faecal extracts, prepared by a number of methods, from donors living under widely differing geographical, cultural and dietary circumstances. Faecal extracts cause point mutations in bacteria and chromosomal damage in cultured mammalian cells. 2. The claims that nitroso compounds are present in human faeces have been retracted, and the chemical nature of faecal mutagens is still unknown. Indirect evidence suggests the presence of several classes of mutagen. 3. The use of different methods of mutation assay gives conflicting estimates of the proportion of people who excrete mutagenic faeces. There is wide variation in mutagenic activity between different stool samples from one person, and between different stool samples from different people. There is conflicting evidence for inhibition or enhancement of the mutagenicity of reference mutagens by faecal extracts. The effects of air oxidation on the mutagenicity of faecal extracts have not been investigated in detail. 4. It has been claimed that the proportion of people excreting mutagenic faeces is higher in groups representing populations at high risk of large-bowel cancer than in groups at low risk of large-bowel cancer. For the reasons given in paragraph 3, these claims must be regarded as premature. 5. The part played by faecal mutagens in the aetiology of large-bowel cancer has yet to be determined.

Variable content of double minute chromosomes is not correlated with degree of phenotype instability in methotrexate-resistant human cell lines

Several variants resistant to 1.8 x 10(-4) M DL-methotrexate (MTX) have been isolated from the human cell lines HeLa BU25 and VA2-B by exposing them to progressively increasing concentrations of the drug. A striking variability of phenotype and chromosome constitution was observed among the different variants. All resistant cell lines exhibited a greatly increased dihydrofolic acid reductase (DHFR) activity and DHFR content; however, the DHFR activity levels varied considerably among the variants, ranging between about 35 and 275 times the parental level. In the absence of selective pressure, the increased DHFR activity was unstable, and in all cell lines but one was completely lost over a period ranging in different variants between 25 and 200 days. The MTX-resistant cells lines showed anomalies in their chromosome constitution, which involved the occurrence of a duplicated set of chromosomes in most cells of some of the variants and the presence of double minute chromosomes in all cell lines. An analysis of the correlation of loss of double minute chromosomes and loss of DHFR activity in the absence of MTX has given results consistent with the idea that the double-minute chromosomes contain amplified DHFR genes. However, the most significant finding is that, in contrast to what has been reported in the mouse system, the recognizable double-minute chromosomes varied greatly in number in different variants without any relationship to either the level of DHFR activity or the degree of instability of MTX resistance in the absence of selective pressure. These and other observations point to the occurrence in the human MTX-resistant variants of another set of DHFR genes, representing a varied proportion of the total, which is associated with the regular chromosomes, and which may be unstable in the absence of selective pressure.