Studies on 1-beta-D-arabinofuranosyl-cytosine (Ara-C) resistant mutants of Chinese hamster fibroblasts. II. High resistance to Ara-C as a genetic marker for cellular hybridization

Induction of resistance to 6-thioguanine and cytarabine by a range of anticancer drugs in Chinese hamster AA8 cells

A mutagenicity assay using AA8 Chinese hamster cells has been used to explore the potential of some currently used clinical anticancer drugs to induce cells resistant to 6-thioguanine and cytarabine. Preliminary experiments gave evidence of a "low dose" and "high dose" resistance to cytarabine, and subsequent work considered only the latter of these events. When ethyl methane sulphonate was used as a reference mutagen, induced resistance to cytarabine developed substantially later and at a lower frequency than resistance to 6-thioguanine. Of the clinical drugs tested, carmustine showed the highest ability to induce either 6-thioguanine or cytarabine resistant cells. Bleomycin, daunomycin and amsacrine showed moderate ability, while vincristine was essentially inactive in these assays. Such information could potentially be used in selecting new drug combinations or timing of drug administration in cancer chemotherapy.

DNA methylation and epigenetic inheritance

Classical genetics has revealed the mechanisms for the transmission of genes from generation to generation, but the strategy of the genes in unfolding the developmental programme remains obscure. Epigenetics comprises the study of the mechanisms that impart temporal and spatial control on the activities of all those genes required for the development of a complex organism from the zygote to the adult. Epigenetic changes in gene activity can be studied in relation to DNA methylation in cultured mammalian cells and it is also possible to isolate and characterize mutants with altered DNA methylase activity. Although this experimental system is quite far removed from the epigenetic controls acting during development it does provide the means to clarify the rules governing the silencing of genes by specific DNA methylation and their reactivation by demethylation. This in turn will facilitate studies on the control of gene expression in somatic cells of the developing organism or the adult. The general principles of epigenetic mechanisms can be defined. There are extreme contrasts between instability or switches in gene expression, such as those in stem-line cells, and the stable heritability of a specialized pattern of gene activities. In some situations cell lineages are known to be important, whereas in others coordinated changes in groups of cells have been demonstrated. Control of numbers of cell divisions and the size of organisms, or parts of organisms, is also essential. The epigenetic determination of gene expression can be reversed or reprogrammed in the germ line. The extent to which methylation or demethylation of specific DNA sequences can help explain these basic epigenetic mechanisms is briefly reviewed.

Construction of human T-cell hybrids with helper function

Human T-cell hybrids with helper activity were obtained after fusion of phytohemagglutinin-activated normal human T cells with a 6-thioguanine-resistant, aminopterin-sensitive human T-cell line. This mutant line, designated CEM-T15, was derived from the human T-cell line CEM after mutagenesis with ethyl methanesulfonate. The polyethylene glycol induced fusion and the selection in hypoxanthine- aminopeterin -thymidine medium were performed by modification of standard somatic cell hybridization techniques. After fusion, the strategy for selecting hybrids consisted in screening growing cultures for the presence of cells expressing the OKT3 cell surface differentiation antigen. OKT3 was chosen because it is present in 85-95% of normal human T cells but absent from CEM-T15 cells. Thus, OKT3+ cells growing 5-7 weeks after fusion most likely represented hybrids between normal T cells (OKT3+) and continuously growing CEM-T15 cells (OKT3-). Several of the hybrids were tested for their capacity to promote pokeweed mitogen-induced antibody production by B cells. These experiments demonstrated that many of the hybrids had helper activity. Periodical testing of these uncloned hybrids for helper activity revealed functional instability, with most of the hybrids losing helper activity after 20 weeks of continuous culture. However, early and repeated cloning of the same hybrids resulted in a series of hybrid clones with helper activity still present more than 8 months after fusion. In more recent fusions, we have demonstrated that human helper hybrids producing helper factor(s) can also be obtained. These and similar hybrids with different functions will be of considerable importance in further studies of the immunobiology of human T lymphocytes.

Stepwise isolation and properties of unstable Chinese hamster cell variants that overproduce adenylate deaminase

Addition of coformycin (0.5 microgram/ml) to a culture medium containing adenine causes in Chinese hamster fibroblasts a lethal depletion of IMP. Resistant variants have been recovered, some of which exhibit increased adenylate deaminase activity. (Debatisse et al., J. Cell. Physiol., 106:1-11, 1981). The selective medium was made more specific for the isolation of this class of variants by supplementation with azaserine. The hyperactive variants remained sensitive to coformycin concentrations above that used for their selection and were unstable. Their frequency was not increased by ethyl methane sulfonate mutagenesis. The resistant phenotype and the increased activity of adenylate deaminase behaved as semidominant traits in hybrids. No change was detected in the Km for AMP, the cofactor requirement, or the chromatographic properties of adenylate deaminase in the variants. Through stepwise selection in media supplemented with increasing coformycin concentrations, unstable clones with adenylate deaminase activity up to 150-fold the wild-type level were isolated; from an unstable clone, a stable subclone with reduced resistance and enzyme activity was recovered. Evidence that increased adenylate deaminase activity is the manifestation of overaccumulation of the enzyme protein was supplied by the correlation of enzyme activity with the intensity of a protein band comigrating with purified adenylate deaminase during sodium dodecyl sulfate-polyacrylamide gel electrophoresis of cell extracts. Several unidentified additional bands showed comparable quantitative changes. The striking similarity between the adenylate deaminase-overproducing lines and unstable dihydrofolate reductase-overproducing lines generated by gene amplification strongly suggests that the coformycin-resistant variants also resulted from amplification of an adenylate deaminase gene.

Stepwise isolation and properties of unstable Chinese hamster cell variants that overproduce adenylate deaminase

Addition of coformycin (0.5 microgram/ml) to a culture medium containing adenine causes in Chinese hamster fibroblasts a lethal depletion of IMP. Resistant variants have been recovered, some of which exhibit increased adenylate deaminase activity. (Debatisse et al., J. Cell. Physiol., 106:1-11, 1981). The selective medium was made more specific for the isolation of this class of variants by supplementation with azaserine. The hyperactive variants remained sensitive to coformycin concentrations above that used for their selection and were unstable. Their frequency was not increased by ethyl methane sulfonate mutagenesis. The resistant phenotype and the increased activity of adenylate deaminase behaved as semidominant traits in hybrids. No change was detected in the Km for AMP, the cofactor requirement, or the chromatographic properties of adenylate deaminase in the variants. Through stepwise selection in media supplemented with increasing coformycin concentrations, unstable clones with adenylate deaminase activity up to 150-fold the wild-type level were isolated; from an unstable clone, a stable subclone with reduced resistance and enzyme activity was recovered. Evidence that increased adenylate deaminase activity is the manifestation of overaccumulation of the enzyme protein was supplied by the correlation of enzyme activity with the intensity of a protein band comigrating with purified adenylate deaminase during sodium dodecyl sulfate-polyacrylamide gel electrophoresis of cell extracts. Several unidentified additional bands showed comparable quantitative changes. The striking similarity between the adenylate deaminase-overproducing lines and unstable dihydrofolate reductase-overproducing lines generated by gene amplification strongly suggests that the coformycin-resistant variants also resulted from amplification of an adenylate deaminase gene.

Studies on 1-beta-D-arabinofuranosyl cytosine-resistant mutants of Chinese hamster fibroblasts: III. Joint resistance to arabinofuranosyl cytosine and to excess thymidine--a semidominant manifestation of deoxycytidine triphosphate pool expansion

Variants isolated from mutagenized Chinese hamster fibroblasts by a single cycle of exposure to ara-C distributed into two classes: (1) deoxycytidine (dC) kinase deficient clones with a high level of resistance, this phenotype was recessive in hybrids; and (2) clones exhibiting joint resistance to thymidine (dT) and to "low" ara-C concentration, this phenotype was accounted for by an increased dCTP pool. The incorporation of exogenous dC into macromolecules was markedly altered in these variants. In hybrids, the phenotype of joint resistance to dT and ara-C was semidominant. Through a second selection step, variants cumulating recessive high resistance to ara-C and semidominant dT resistance were recovered. The identification of these two classes of ara-C-resistant variants suggests an interpretation of the known phenotypes of ara-C resistance as manifestations of chromosomal gene mutations. Dominant resistance mutations might contribute to the survival of cancer cells during prolonged ara-C chemotherapy.

Adenosine kinase as a new selective marker in somatic cell genetics: isolation of adenosine kinase--deficient mouse cell lines and human--mouse hybrid cell lines containing adenosine kinase

A new selective system for isolating somatic cell hybrids, using adenosine kinase as the selective marker, has been developed. The selective medium for forward selection (to select for cells containing adenosine kinase) contains alanosine, adenosine and uridine. To survive in the presence of alanosine, cells must have adenosine kinase in order to utilize exogenous adenosine as the sole source of AMP. Uridine is added to the selective medium to prevent the toxic effects of adenosine on cultured mammalian cells. The selective medium for reverse selection (to select for cells lacking adenosine kinase) contains 2-fluoroadenosine, an analogue of adenosine, which is converted to a toxic nucleotide by the action of adenosine kinase. Mouse mutant cell lines deficient in adenosine kinase have been derived. Human--mouse hybrid cells containing the kinase have been prepared from one of these mutant lines. Karyotype data of these hygrid lines and their adenosine kinase-minus sublines are consistent with assignment by others of the human gene for adenosine kinase on chromosome 10.

The control of cell proliferation by preformed purines: a genetic study. II. Pleiotropic manifestations and mechanism of a control exerted by adenylic purines on PRPP synthesis

When plated in medium containing 0.5 microgram/ml coformycin and adenosine (or adenine) fibroblasts were killed, even if pyrimidines were supplied. Measurements of N-formylglycine amide ribonucleotide synthesis showed that lethality is a manifestation of purine starvation. In the case of adenosine kinase deficient cells, growth was restored by hypoxanthine. The adenylic derivatives block only purine biosynthesis, presumably by inhibition of PRPP-amidotransferase. In this same medium, wild-type cells exhibited symptoms of PRPP deprivation: purine and pyrimidine syntheses were both shut off and HGPRT was simultaneously inactivated. The pleiotropic control by adenosine was abolished in adenosine-resistant mutants that behaved as PRPP "over-producers." These mutations conferred partial resistance to various toxic purine and pyrimidine analogs and preserved HGPRT activity in adenosine-containing medium. This permits selection against these mutants. Evidence suggesting that adenosine kinase products may fulfill a specific function in the regulation of PRPP synthesis is discussed.

Chinese hamster X mouse hybrid cells segregating mouse chromosomes and isozymes

Hybrid cells are readily formed by fusing clonal Chinese hamster cells to fresh, noncultured, adult mouse spleen cells followed by isolation in selective medium. The vast majority of such hybrids retain Chinese hamster chromosomes and isozymes while segregating mouse chromosomes and isozymes. The growth, plating efficiency, ease of karyology, and rapid segregation of mouse markers allows linkage tests in primary clones. Analysis of 13 isozymes showed 12 to be asyntenic and on epair (PGD-PGM2) to be syntenic This system will allow extensive somatic cell hybrid gene mapping in the mouse and permit a comparison of human and mouse linkage relationships.