The development of resistance to methotrexate in a mouse melanoma cell line. I. Characterisation of the dihydrofolate reductases and chromosomes in sensitive and resistant cells

Chromatin organization in the homogeneously staining regions of a methotrexate-resistant mouse cell line: interspersion of inactive and active chromatin domains distinguished by acetylation of histone H4

We have analyzed the organization of the homogeneously staining regions (HSRs) in chromosomes from a methotrexate-resistant mouse melanoma cell line. Fluorescence in situ hybridization techniques were used to localize satellite DNA sequences and the amplified copies of the dihydrofolate reductase (DHFR) gene that confer drug-resistance, in combination with immunofluorescence using antibody probes to differentiate chromatin structure. We show that the major DNA species contained in the HSRs is mouse major satellite, confirming previous reports, and that this is interspersed with DHFR DNA in an alternating tandem array that can be resolved at the cytological level. Mouse minor satellite DNA, which is normally located at centromeres, is also distributed along the HSRs, but does not appear to interfere with centromere function. The blocks of major satellite DNA are coincident with chromatin domains that are labelled by an autoantibody that recognizes a mammalian homologue of Drosophila heterochromatin-associated protein 1, shown previously to be confined to centric heterochromatin in mouse. An antiserum that specifically recognizes acetylated histone H4, a marker for active chromatin, fails to bind to the satellite DNA domains, but labels the intervening segments containing DHFR DNA. We can find no evidence for the spreading of the inactive chromatin domains into adjacent active chromatin, even after extended passaging of cells in the absence of methotrexate selection.

Dihydrofolate reductase gene amplification-associated shift of differentiation in methotrexate-adapted HT-29 cells

Postconfluent cultures of HT-29 cells form a heterogeneous multilayer of which greater than 95% of the cells are undifferentiated. In contrast, when stably adapted to normally lethal concentrations of methotrexate (10(-6)-10(-5) M), they form a monolayer of gobletlike cells (Lesuffleur et al., 1990) which secrete large quantities of mucins and display a discrete brush border with the presence of villin, dipeptidylpeptidase-IV, and carcinoembryonic antigen. When adapted to even higher concentrations of methotrexate (10(-4) and 10(-3) M) there is a shift in the pattern of differentiation from gobletlike to dome-forming absorptive-like cells. These cells still display an apical brush border which expresses villin and dipeptidylpeptidase-IV, but no longer express significant levels of mucins and carcinoembryonic antigen. This shift of differentiation coincides with a sudden amplification of the gene coding for dihydrofolate reductase and an increased activity of the enzyme.

Mithramycin inhibits SP1 binding and selectively inhibits transcriptional activity of the dihydrofolate reductase gene in vitro and in vivo

The promoter of the human dihydrofolate reductase (DHFR) gene contains two consensus binding sites for the DNA binding protein Sp1. DNAse protection and gel mobility shift assays demonstrate binding of recombinant Sp1 to both decanucleotide Sp1 binding sequences which are located 49 and 14 base pairs upstream of the transcription start site. The more distal of the two binding sites exhibits a somewhat higher affinity for Sp1. The G-C specific DNA binding drug, mithramycin, binds to both consensus sequences and prevents subsequent Sp1 binding. Promoter-dependent in vitro transcription of a DHFR template is selectively inhibited by mithramycin when compared to the human H2b histone gene. A similar effect is also noted in vivo. Mithramycin treatment of MCF-7 human breast carcinoma cells containing an amplified DHFR gene induces selective inhibition of DHFR transcription initiation, resulting in a decline in DHFR mRNA level and enzyme activity. This selective inhibition of DHFR expression suggests that it is possible to modulate the overexpression of the DHFR gene in methotrexate resistant cells.

Replication of the dihydrofolate reductase genes on double minute chromosomes in a murine cell line

The purpose of this study is to determine the kinetics of the replication of intrachromosomal versus extrachromosomal amplified dihydrofolate reductase (DHFR) genes. Previous studies reported that the DHFR gene, when carried intrachromosomally on a homogeneously staining region, replicates (as a unit) within the first 2 h of the S phase of the cell cycle. We wished to determine if the extrachromosomal location of the amplified genes carried on double minute chromosomes effects the timing of their replication. Equilibrium cesium chloride ultracentrifugation was used to separate newly replicated (BUdR-labeled) DNA from bulk DNA in a synchronized cell population. Hybridization with the cDNA for the DHFR gene allowed us to determine the period of time within the cell cycle in which the DHFR DNA sequences were replicated. We found that, in contrast to intrachromosomal dihydrofolate reductase genes that uniformly replicate as a unit at the beginning of the S phase of the cell cycle, dihydrofolate reductase genes carried on double minute chromosomes (DMs) replicate throughout the S phase of the cell cycle. These results suggest that control of replication of extrachromosomal DNA sequences may differ from intrachromosomal sequences.

DNA content and structure of (double) minutes of a methotrexate-resistant cell line

We have determined the DNA content of intact double minutes (DMs) and of single minutes (SMs) by fluorometry of the individual chromatin bodies in metaphase spreads after staining with Feulgen-Schiff pararosaniline. We find that the intact DMs and SMs of the methotrexate-resistant mouse cell line 3T6R50 contain 4.4 megabase pairs (Mb) and 2.6 Mb DNA respectively, using the DNA content of E. coli (4.7 Mb) as a reference. As the pulsed field gradient gel electrophoresis experiments by van der Bliek et al. (1988) have indicated that the minutes of 3T6R50 cells contain a homogeneous population of 2.5 Mb DNA circles, we conclude that a SM contains one circular double strand DNA molecule of approximately 2.5 Mb, whereas DMs contain two.

Tetrahydrofolate dehydrogenase cytochemistry in acute lymphoblastic leukemia

We studied the cytochemical distribution of tetrahydrofolate dehydrogenase (FH4D), an enzyme involved in nucleic acid metabolism and thus in cell proliferation and differentiation processes, in bone marrow blasts from 37 cases of acute lymphoblastic leukemia (ALL), of whom 23 were pediatric patients. 26 cases were analyzed at onset, 11 in relapse. The ALL cases were immunologically classified as T (10), common (20), B (3) and null (4). In each subgroup the majority of lymphoblasts were positive, with heterogeneous positivity patterns and variable degrees of enzyme activity. Most T lymphoblasts were characterized by focal localization of FH4D, whereas in common blasts reactivity - usually less strong - was either focally localized or scattered with several fine granules. Finally, many B and null blasts showed diffuse positivity. A quantitative evaluation of FH4D activity using cytophotometric technique (Vickers M86) demonstrated higher degrees of reactivity in leukemic blasts than in normal lymphocytes. Moreover, slightly different levels of reactivity were observed in relation to immunological phenotype, age and stage of the disease. Therefore we think that FH4D is a useful additional marker for ALL characterization.

A clonal derivative of tunicamycin-resistant Chinese hamster ovary cells with increased N-acetylglucosamine-phosphate transferase activity has altered asparagine-linked glycosylation

A population of Chinese hamster ovary (CHO) cells resistant to the antibiotic tunicamycin (TM) had previously been isolated (Criscuolo, B.A., and Krag, S.S. (1982) J. Cell Biol. 94:586-591) by a stepwise selection procedure using progressive increments of TM added to the medium. TM inhibits asparagine-linked glycoprotein biosynthesis by blocking the transfer of N-acetylglucosamine-1-phosphate from the sugar nucleotide UDP-N-acetylglucosamine to the isoprenoid lipid carrier, dolichyl phosphate. Four clonal derivatives were isolated from the TM-resistant population in the presence of 27 micrograms TM/ml and were found to overproduce the N-acetylglucosamine-phosphate transferase activity to the same extent (approximately 15-fold compared to wild-type cells). One of these clones, 3E11, was greater than 550-fold more resistant to TM than wild-type cells. The resistance phenotype remained during at least 2.5 months of growth in the absence of TM. 3E11 cells exhibited chromosomal translocations, but no homogeneously staining regions (HSR) or double minute chromosomes. The N-acetylglucosamine-phosphate transferase activity in 3E11 cells was membrane-associated and was inhibited by TM. A 140,000-dalton membrane protein and at least four other membrane proteins were enriched in 3E11 cells. Mannosylphosphoryldolichol synthase and glucosylphosphoryldolichol synthase activities were not elevated in membranes prepared from 3E11 cells. Asparagine-linked glycosylation was altered such that 3E11 cells synthesized primarily a truncated oligosaccharide, Man5GlcNAc2, perhaps due to the reduced amount of mannosylphosphoryldolichol relative to wild-type cells.

A cytogenetic characterization comparing a rat 6TG-resistant strain and 6TG-sensitive clones

A 8.3 μ/ml 6-thioguanine (6TG)-resistant strain was isolated from a rat tetraploid cell line by step-by-step selection in 6TG-medium. In the 6TG-resistant cell population 51% of the cells were tetraploid and 35% of the cells were hypertetraploid, i.e., one chromosome more than a tetraploid. The 6TG-resistant strain grew very well in RPMI 1640 medium with intervals of three days between subcultures. The 6TG-resistant cells all have a homogeneously staining region (HSRs) in one of the X chromosomes which do not stain after chromosome C-banding. They also possess a higher NORs activity and much lower frequency of sister chromatid exchanges (SCE). When the 6TG-resistant RCT cells were subcultured in 6TG-free medium for three days, their SCE frequency did not change. 5'-bromodeoxyuridine (BrdU) significantly suppressed the NORs activity for both 6TG-resistant cells and 6TG-sensitive cells (P<0.001).

Assignment of genes encoding dihydrofolate reductase and hexosaminidase B to Mus musculus chromosome 13

The murine gene encoding dihydrofolate reductase (DHFR) has been localized to a particular mouse chromosome by complementation mapping. Microcells prepared from diploid mouse fibroblasts were fused with mutant hamster cells lacking the dihydrofolate reductase gene (Dhfr), and DHFR+ microcell hybrids were selected in medium lacking purines and pyrimidines. The complemented hybrids expressed wild-type levels of DHFR enzyme activity, and selectively retained a single mouse chromosome--chromosome 13. Genomic Southern blots of DNAs prepared from these hybrids and from an independently derived collection of clones isolated without selection on the DHFR phenotype confirmed the assignment of murine Dhfr gene sequences to chromosome 13. This assignment provides further evidence for the existence of genetic homology between regions of mouse chromosome 13 and human chromosome 5. In support of this view, we show that the gene encoding hexosamindase B, a chromosome 5 marker in man, also maps to mouse chromosome 13.

Regular pattern of karyotypic alterations accompanying gene amplification in Djungarian hamster cells: study of colchicine, adriablastin, and methotrexate resistance

Chromosomal analysis of 26 Djungarian hamster cell lines obtained from 11 independent clones and possessing different levels of resistance to colchicine or adriablastin as a consequence of gene amplification revealed regular patterns in the karyotypic changes that accompanied the development of drug resistance. Usually the sequence of karyotypic changes was as follows: first an additional chromosome 4 appeared: then single unpaired small chromatin bodies (SCBs) arose; later in the middle part of the long arm of one of three chromosomes 4 long homogeneously staining regions (HSRs) and double minute chromosomes (DMs) were formed; and finally in the most resistant variants large clusters of SCBs appeared. The emergence of the clusters of the SCBs correlated well with the occurrence of autonomously replicating, amplified DNA sequences. In contrast to DNA of the HSRs the DNA of the SCBs could replicate outside the S-phase of the cell cycle. When kept in a non-selective medium, the cells gradually lost their resistance to colchicine: 1%-4% of the cells lost the capacity to form colonies in the selective medium independently of the pattern of location in them of amplified genes (in chromosomal HSRs. SCBs, or DMs). Loss of drug resistance was accompanied by disappearance of the chromosomal HSRs, SCBs, and DMs. Chromosomal analysis of the set of methotrexate-resistant Djungarian hamster cell lines indicated the following karyotypic evolution: first the additional material on the distal part of one of two chromosomes 3 appeared; then the light HSRs were formed on the distal part of one of two chromosomes 4; later clusters of SCBs and HSRs arose on the distal part of the short arm of chromosome 3. Probably the amplification of different genes is characterized by specific patterns of karyotypic alterations.

Cytogenetical changes during early stages of development of methotrexate resistance in HeLa cells

HeLa cells cultured in ever-increasing doses of methotrexate (MTX) acquired resistance to this drug. Chromosomal changes occurring at early stages during the development of resistance to various doses (0.1, 0.3, 0.5, 1.0, 2.1, 3.0, 10.0 and 100.0 micrograms/ml) of MTX included: (a) an increase in the percentage of hypertetraploid cells (greater than 92 chromosomes) at all doses, and most profoundly at 1.0, 2.1 and 3.0 micrograms/ml; (b) an increase in the percentage of cells with structural abnormalities; (c) a remarkable increase in the percentage of hypertetraploid cells containing dicentrics, particularly at MTX dose levels 0.5-3.0 micrograms/ml and (d) emergence of increasing numbers of double minutes per cell with increasing MTX doses. At dose levels 2.1 and 3.0 micrograms/ml the modal chromosome number increased to 82, while at 0.1-1.0 microgram/ml it was similar to the mode of sensitive HeLa (64, 66) and at 10.0 and 100.0 micrograms/ml dropped to 62, 63. The results obtained suggest that polyploidization and formation of dicentrics are associated with the earlier stages of development of resistance to methotrexate.

Measurement of gene number for seed storage proteins in Pisum

We have measured the numbers of genes coding for the three seed storage proteins, vicilin, convicilin and legumin, in a number of Pisum genotypes of variant protein composition. No difference in gene number existed among P. sativum genotypes for any of the proteins. There were differences in the number of genes coding for individual proteins with approximately 11 genes (per haploid genome) for vicilin, 8 genes for legumin and 1 gene for convicilin.

Gene amplification-associated cytogenetic aberrations and protein changes in vincristine-resistant Chinese hamster, mouse, and human cells

We carried out cytogenetic studies of four Chinese hamster, mouse, and human cell lines selected for high levels of resistance (500- to 4,000-fold) to vincristine (VCR) by a multistep selection procedure. All cells examined contained gene amplification-associated metaphase chromosome abnormalities, either homogeneously staining regions (HSRs), abnormally banding regions (ABRs), or double-minute chromosomes (DMs); control actinomycin D- and daunorubicin-resistant hamster lines did not exhibit this type of chromosomal abnormality. VCR-resistant Chinese hamster sublines exhibited both increased synthesis of the protein V19 (Mr 19,000; pl = 5.7) and increased concentrations of V19 polysomal mRNA. When VCR-resistant cells were grown in drug-free medium, level of resistance, synthesis of V19, and amount of V19 mRNA declined in parallel with mean length of the HSR or mean number of DMs per cell. Cross-resistance studies indicate that VCR-resistant cells have increased resistance both to antimitotic agents and to a wide variety of agents unrelated to VCR in chemical structure and/or mechanism of action. Our studies of tubulin synthesis in Chinese hamster cells indicate no overproduction of tubulin or presence of a mutant tubulin species. Comparison with antifolate-resistant Chinese hamster cells known to contain amplified dihydrofolate reductase genes localized to HSRs or ABRs strongly suggests that the HSRs, ABRs, or DMs of the Vinca alkaloid-resistant sublines likewise represent cytological manifestations of specifically amplified genes, possibly encoding V19, involved in development of resistance to VCR.

Expression of the amplified domain in human neuroblastoma cells

Screening of a partial cDNA library prepared from the human neuroblastoma cell line BE(2)-C with genomic DNA probes containing sequences representative of the amplified domain of that cell line allowed us to identify cloned transcripts from an active gene within the domain. The gene BE(2)-C-59 is amplified ca. 150-fold and encodes a 3.0- and a 1.5-kilobase RNA transcript, both of which are overproduced in BE(2)-C cells. A survey of a large variety of human tumor cell types indicated that this gene is amplified to varying degrees in all neuroblastoma cell lines and a retinoblastoma cell line that exhibit obvious cytological manifestations of DNA sequence amplification, i.e., homogeneously staining regions and double-minute chromosomes. The BE(2)-C-59 gene is not amplified, however, in other nonrelated tumor types, even those containing amplified DNA. Although the functional significance of this specific gene amplification in neuroblastoma cells remains unknown, an indication that it may relate to the malignant phenotype of these cells follows from the remainder of our data which show that the amplified BE(2)-C-59 gene shares partial homology with both the second and third exons, but not the first exon, of the human c-myc oncogene.

Expression of the amplified domain in human neuroblastoma cells

Screening of a partial cDNA library prepared from the human neuroblastoma cell line BE(2)-C with genomic DNA probes containing sequences representative of the amplified domain of that cell line allowed us to identify cloned transcripts from an active gene within the domain. The gene BE(2)-C-59 is amplified ca. 150-fold and encodes a 3.0- and a 1.5-kilobase RNA transcript, both of which are overproduced in BE(2)-C cells. A survey of a large variety of human tumor cell types indicated that this gene is amplified to varying degrees in all neuroblastoma cell lines and a retinoblastoma cell line that exhibit obvious cytological manifestations of DNA sequence amplification, i.e., homogeneously staining regions and double-minute chromosomes. The BE(2)-C-59 gene is not amplified, however, in other nonrelated tumor types, even those containing amplified DNA. Although the functional significance of this specific gene amplification in neuroblastoma cells remains unknown, an indication that it may relate to the malignant phenotype of these cells follows from the remainder of our data which show that the amplified BE(2)-C-59 gene shares partial homology with both the second and third exons, but not the first exon, of the human c-myc oncogene.

Homogeneously staining regions (HSR) in a human malignant melanoma

A case of nodular malignant melanoma (level V of Clark's classification) with homogeneously staining regions (HSR) on the long arm of one chromosome #2 is described. Ultrastructural observation of melanosomic and promelanosomic granules near Golgi's vesicles confirmed the histologic diagnosis. Chromosome analysis was performed on nine metaphases from a bone marrow sample and 76 metaphases from culture of the malignant skin tumor. G-banding revealed the presence of a clone with trisomy #8 and another cell line with the HSR marker. This is the first report of HSR in human melanoma cells. As HSR has been found only in malignant cells, we believe that among the many factors that influence the patients' clinical evolution and poor response to treatment, the genic imbalance is of the utmost importance.

A preliminary model of double-minute-mediated gene amplification

A mathematical model of double minute (dm) population dynamics has been developed based upon current concepts of the saltatory replication, random partitioning, nuclear exclusion and loss, and cellular growth inhibition of these extrachromosomal elements. A highly accurate approximate analytical solution has been obtained for the dm frequency distribution at steady state and preliminary analysis of transient states has been performed. The steady state solution has been fit to experimental frequency data of the SW527N carcinoma line, the excellent goodness of fit (X2 = 2.6, d.f. = 29) providing preliminary evidence for the consistency of this set of mechanisms. Two special cases are examined in which extrareplicative dms are produced on both the chromosome and existing dms at equal rates or on the chromosome alone. The model predicts that the population--average rate of extrareplicative dm production is 0.039 +/- S.E. 0.009 dms/hr/cell in the first case and is tenfold higher than when such replication occurs on the chromosome alone (0.0043 +/- S.E. 0.0004 dms/hr/cell). Allowable ranges of the extent of dm-related growth inhibition and dm loss are determined for the SW527N cell line. It is found that dm-related growth inhibition can be nearly as high as that observed for the S180 sarcoma lines (on the order of 0.5% per dm lengthening of the doubling time) or as low as zero.

Specific DNA sequence amplification in human neuroblastoma cells

Southern blot analysis of a number of EcoRI-digested human neuroblastoma DNAs has revealed the presence of a family of discrete restriction fragments, the majority of which are amplified in most, but not all, of the neuroblastoma cell lines tested. None of these sequences is abundantly present in DNA from other human tumors of different tissue origins, including several either known or presumed to contain amplified DNA. Hence, these sequences appear to be specifically amplified by neuroblastoma cells. Hybridization with metaphase chromosomes in situ has localized these sequences to either the homogeneously staining regions or double-minute chromosomes of different neuroblastoma cell lines, indicating that these chromosomal structures, although present in cell lines established from different patients, share many sequences and may have a common, but as yet unknown, function.

Occurrence and evolution of homogeneously staining regions may be due to breakage-fusion-bridge cycles following telomere loss

Chromosomes with homogeneously staining regions (HSR) were analysed in a subclone of the H4 rat hepatoma cell line, where they represent amplification of the ribosomal RNA (rRNA) genes. Detailed G-band analysis of the subclone revealed that an HSR on the short arm of chromosome 3 became unstable and changed its position within the chromosome. The evolution of this marker chromosome was associated with the terminal deletion of the normal long arm of the HSR-bearing chromosome 3 and may have involved ring formation as a result of fusion between the HSR on the short arm and the broken end of the long arm. Evidence was obtained for breakage at different sites within the ring, producing chromosomes with HSRs located terminally on either the long arms or both arms. The terminally located HSR underwent elongation in some cells presumably as a result of a breakage-fusion-bridge cycle characteristic of instability due to telomeric loss. It is suggested that terminally located HSRs may generally occur this way.

Gene amplification in methotrexate-resistant mouse cells. V. Intact amplified units can be transferred to and amplified in methotrexate-sensitive mouse L cells

Wild-type mouse LtAp20 cells were treated with calcium phosphate-precipitated DNA or chromosomes from two highly Methotrexate (MTX)-resistant mouse lymphoma cell lines--EL4/8 and EL4/11. Transfections with purified MTX-resistant DNA produced colonies of LtAp20 cells resistant to 3 X 10(-8) M MTX, at about eight times the frequency with which resistant colonies arose in control transfections. DNA transfectants contained multiple copies of the dihydrofolate reductase (dhfr) gene, but other sequences characteristic of the donor DNA could not be detected. Transfections using isolated chromosomes were twice as efficient as those using purified DNA. Unlike DNA transfectants, over 90% of all chromosome transfectants took up large stretches of donor DNA intact and contained DNA sequences characteristic of donor DNA. Of chromosome transfectants selected for resistance to high levels of MTX (1 mM), 70% amplified a unit of DNA which was indistinguishable from that present in the donor cell. The results showed that large fragments of chromosomes (as opposed to purified DNA) can be taken up to recipient cells without detectable alteration to the fine structure of the DNA they contain. The results also support the notion that all amplified units within a MTX-resistant cell have the same overall complex DNA structure.

Paradoxical response of malignant melanoma to methotrexate in vivo and in vitro

Methotrexate (MTX) shows consistent cytotoxicity for melanoma cells in vitro but it is ineffective in clinical use at equivalent concentrations in vivo. This apparent paradox has been investigated by cell culture techniques and results quantified by cell number. In an in vitro model of high dose MTX therapy followed by leucovorin rescue (HD-MTX-LCR) there was survival of both melanoma and choriocarcinoma cell lines but not of an acute lymphocytic leukaemia cell line. The 70H metabolite of MTX was identified by HPLC in plasma samples of melanoma patients treated by HD-MTX-LCR, in which MTX concentrations approximately 10(-5) M were maintained for 24 h. However, metabolism per se is unlikely to account for the lack of response to MTX clinically. In vitro 70H MTX (10(-7) - 10(-6) M) was two orders of magnitude less cytotoxic for melanoma than MTX (10(-9) - 10(-8) M). The cellular accumulation of [3H]-MTX, using a rapid gradient centrifuge technique for separation of melanoma cells from medium, was reduced in the presence of 70H-MTX. The results suggest that reduced cellular uptake of MTX combined with biochemical rescue of tumour cells may partially explain the paradoxical lack of clinical response of melanoma to the drug.

Human chromosome analysis in 24 cases of primary carcinoma of the large intestine: contribution of the G-banding technique

As in the haemopathies, the application of cytogenetics to epithelial cancers could aid in the study of their pathogenesis evaluation. In this context we performed chromosome analyses on a series of human colo-rectal cancers. The technique was consistently reliable since the modal number of chromosomes could be determined in all 24 cases. In 22, karyotypes could also be established. Each tumour was characterized by a single cell clone in 21 cases and by a mosaic of 2 populations in 3 cases. Numerical anomalies were not due to chance: they enabled near-diploid (11 cases), near-triploid (9 cases), mosaic (3 cases) and highly polyploid (1 case) cancers to be distinguished. Supernumerary chromosomes were primarily in groups C and F. The most frequent markers before denaturation techniques were No 2q +, No F and minutes. Each time double-minutes were observed (5 cases), they were in invasive cancers (B and C Dukes classification). Cells were generally diploid in non-invasive cancers with fewer quantitative and structural anomalies. Tumour cytogenetics were related to the histological type and localization in the colon, as well as to the local and metastatic spread.

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.

Selective amplification of polymorphic dihydrofolate reductase gene loci in Chinese hamster lung cells

A series of antifolate-resistant Chinese hamster lung sublines that overproduce either a Mr 20,000 or a Mr 21,000 class of dihydrofolate reductase (DHFR; tetrahydrofolate dehydrogenase; 5,6,7,8-tetrahydrofolate:NADP+ oxidoreductase, EC 1.5.1.3), known to contain amplified DHFR genes, has been analyzed by DNA and RNA transfer techniques. The results suggest that the Mr 20,000 and Mr 21,000 DHFRs are encoded by at least two polymorphic DHFR genes, both of which are expressed in drug-sensitive parental cells. In drug-resistant sublines only one of the two DHFR gene types is amplified, thus accounting for the overproduction of one or the other molecular weight class of DHFR. In addition to the known differences between the DHFRs whose overproduction they direct, these allelic genes differ in restriction endonuclease profiles and in the relative abundances of their multiple mRNA transcripts.

Overproduction of dihydrofolate reductase and gene amplification in methotrexate-resistant Chinese hamster ovary cells

Stable isolates of Chinese hamster ovary cells that are highly resistant to methotrexate have been selected in a multistep selection process. Quantitative immunoprecipitations have indicated that these isolates synthesize dihydrofolate reductase at an elevated rate over its synthesis in sensitive cells. Restriction enzyme and Southern blot analyses with a murine reductase cDNA probe indicate that the highly resistant isolates contain amplifications of the dihydrofolate reductase gene number. Depending upon the parenteral line used to select these resistant cells, they overproduce either a wild-type enzyme or a structurally altered enzyme. Karyotype analysis shows that some of these isolates contain chromosomes with homogeneously staining regions whereas others do not contain such chromosomes.

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.

Loss and stabilization of amplified dihydrofolate reductase genes in mouse sarcoma S-180 cell lines

We studied the loss and stabilization of dihydrofolate reductase genes in clones of a methotrexate-resistant murine S-180 cell line. These cells contained multiple copies of the dihydrofolate reductase gene which were associated with double minute chromosomes. The growth rate of these cells in the absence of methotrexate was inversely related to the degree of gene amplification (number of double minute chromosomes). Cells could both gain and lose genes as a result of an unequal distribution of double minute chromosomes into daughter cells at mitosis. The loss of amplified dihydrofolate reductase genes during growth in the absence of methotrexate resulted from the continual generation of cells containing lower numbers of double minute chromosomes. Because of the growth advantage of these cells, they became dominant in the population. We also studied an unstably resistant S-180 cell line (clone) that, after 3 years of continuous growth in methotrexate, generated cells containing stably amplified dihydrofolate reductase genes. These genes were present on one or more chromosomes, and they were retained in a stable state.

Gene amplification accompanies low level increases in the activity of dihydrofolate reductase in antifolate-resistant Chinese hamster lung cells containing abnormally banding chromosomes

Three independently-derived, antifolate-resistant Chinese hamster lung cell lines that exhibit low level increases in dihydrofolate reductase (DHFR) activity, i.e., three- to fivefold vs. controls, have been compared with drug-sensitive cells to determine relative DHFR gene content. With a solution hybridization technique that makes use of genomic DNA and a cloned double-stranded Chinese hamster DHFR cDNA probe, it has been found that the enzyme activity increases are associated with an approximately proportionate amplification of DHFR genes. Trypsin-Giemsa staining techniques and hybridizations in situ further show that the amplified DHFR genes are located within abnormally banding regions along chromosome 2q and also suggest that, in each subline, only one chromosome 2 homolog is initially involved in the amplification process.

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.

Overproduction of dihydrofolate reductase and gene amplification in methotrexate-resistant Chinese hamster ovary cells

Stable isolates of Chinese hamster ovary cells that are highly resistant to methotrexate have been selected in a multistep selection process. Quantitative immunoprecipitations have indicated that these isolates synthesize dihydrofolate reductase at an elevated rate over its synthesis in sensitive cells. Restriction enzyme and Southern blot analyses with a murine reductase cDNA probe indicate that the highly resistant isolates contain amplifications of the dihydrofolate reductase gene number. Depending upon the parenteral line used to select these resistant cells, they overproduce either a wild-type enzyme or a structurally altered enzyme. Karyotype analysis shows that some of these isolates contain chromosomes with homogeneously staining regions whereas others do not contain such chromosomes.

Molecular biology and the clinician

Molecular biology has uncovered informational processes which broadly apply in medicine. The points of attack range from diagnosis at the most fundamental level of information flow to the production of therapeutic agents. This is promoting movement from serendipitous strategies of advance to those of predictive rational clinical design. Specific examples show how three principal types of molecular probe are benefiting a wide range of clinical disciplines and thereby blurring interdisciplinary boundaries. There are signs that current medical training is not making the best use of contemporary molecular biology. The identification of areas in which molecular biology can be usefully applied to medicine requires medical practitioners to be aware of its diagnostic and prognostic potential. Equally, advances are delayed by gaps between laboratory workers and their clinical colleagues. The dilemma is that by creating a specialist discipline of clinical molecular biology we risk it becoming isolated and delay the impact of molecular biology in medicine as a whole. An alternative is to find a means for raising molecular consciousness throughout all disciplines in medicine.

Loss and stabilization of amplified dihydrofolate reductase genes in mouse sarcoma S-180 cell lines

We studied the loss and stabilization of dihydrofolate reductase genes in clones of a methotrexate-resistant murine S-180 cell line. These cells contained multiple copies of the dihydrofolate reductase gene which were associated with double minute chromosomes. The growth rate of these cells in the absence of methotrexate was inversely related to the degree of gene amplification (number of double minute chromosomes). Cells could both gain and lose genes as a result of an unequal distribution of double minute chromosomes into daughter cells at mitosis. The loss of amplified dihydrofolate reductase genes during growth in the absence of methotrexate resulted from the continual generation of cells containing lower numbers of double minute chromosomes. Because of the growth advantage of these cells, they became dominant in the population. We also studied an unstably resistant S-180 cell line (clone) that, after 3 years of continuous growth in methotrexate, generated cells containing stably amplified dihydrofolate reductase genes. These genes were present on one or more chromosomes, and they were retained in a stable state.

Phorbol ester dramatically increases incidence of methotrexate-resistant mouse cells: possible mechanisms and relevance to tumor promotion

I have tested the hypothesis that at least some of the known tumor promoters may act by facilitating gene amplification. A series of single-step selections for resistance to methotrexate, a specific inhibitor of dihydrofolate reductase (DHFR), was carried out with 3T6 cells in the presence and in the absence of a potent tumor promoter, 12-O-tetradecanoyl-phorbol-13-acetate (TPA). Incidence of methotrexate-resistant, colony-forming 3T6 cells is increased up to 100 fold if selection is carried out in the presence of TPA. For a major portion of this new TPA effect to be observed, it is sufficient to add TPA simultaneously with methotrexate. The effect of TPA on the incidence of methotrexate resistance is detectable at less than 20 nM TPA and is maximal at about 200 nM TPA. Phorbol (a nonpromoting analog of TPA), thymidine and dimethylsulfoxide each fail to produce any TPA-like effect in this system. DHFR gene copy numbers per cell in clones resistant to 100, 200 and 300 nM methotrexate are approximately 3, 10 and 16 times higher, respectively, than the DHFR gene copy number in the parental 3T6 cells. These numbers do not depend on the presence or absence of TPA during methotrexate selection.

On the possibility of metabolic control of replicon "misfiring": relationship to emergence of malignant phenotypes in mammalian cell lineages

Constraints of a multireplicon chromosomal organization and of the necessity to maintain constant gene dosages demand that each origin of replication in a eukaryotic cell "fire" (initiate replication) only once per cell cycle. The central idea of this work is that a low probability of an extra ("illegitimate") round of DNA replication (called below "replicon misfiring") within any given chromosomal domain could be increased by certain substances of either intra- or extracellular origin. The term " "firone" is proposed for such a substance. It is shown that existence of firones could greatly speed up evolution of cellular systems under selection pressure, a developing tumor being one example of such a system. Experimentally testable predictions of the firone hypothesis are discussed.