Somatic Cell Genetics最新文献

Fourteen independent somatic cell hybrid clones made between diphtheria toxin (DT)-resistant mouse Cl1D cells and DT-sensitive Chinese hamster ovary (CHO) cells slowly segregated CHO chromosome. Concordant segregation analysis of electrophoretically resolvable Chinese hamster chromosome 2 gene products and CHO chromosomes 2 and Z2 (having q1-q24 deletion) in DT-selected and control hybrid subclones was conducted. Analysis revealed that loci for DT sensitivity and galactose-1-phosphate uridyltransferase could be regionally assigned to Chinese hamster chromosome 2q1-24 and were physically hemizygous in CHO cells. Enolase (ENO1), 6 phosphogluconate dehydrogenase (PGD), and phosphoglucomutase 1 (PGM1) were located outside the q1-24 region on Chinese hamster chromosome 2 and were dizygous in CHO cells. Functional dizygosity of ENO1, PGD, and PGM1 in CHO cells, as determined by the isolation of diploid heterozygous electrophoretic shift mutants following UV and EMS exposure, confirmed their location outside the Z2 deletion and indicated that the deletion did not result in the inactivation of adjacent loci. These results are discussed in relation to current theories on the basis for high frequency of drug-resistant autosomal recessive mutants in CHO cells and conservation of mammalian autosomal linkage groups.

Thy- mutants, in addition to being resistant to arabinosyl cytosine (arcC), show cross-resistance to 5-fluorouracil (5FU). When Chinese hamster ovary (CHO) cells were exposed to a selection system using both araC and 5FU, the resistant clones isolated were identical to thy- mutants by the following criteria: (1) all were auxotrophic for thymidine with a high reversion frequency to thymidine prototrophy; (2) those tested had a high level of dCTP relative to wild-type cells, while dTTP and dATP levels were unaffected, and (3) all tested had a 7- to 50-fold higher rate of spontaneous mutation than the wild-type strain for at least one independent genetic marker. Although spontaneous thy- mutants were rare, the frequencies of thy- mutants in untreated and mutagenized cultures are consistent with the conclusion that the thy- phenotype is the consequence of a single mutation in CHO cells.

Clonal variants of mouse hepatoma cells that either fail to produce albumin (variant 19/2) or show significantly reduced levels (100-fold less) of albumin production (variant 1/c/1) were isolated from the parental line. Hepa la, after a single exposure to N-methyl-N'-nitrosoguanidine (MNNG). Intracellular levels of albumin in both variants were below detection by our assay. Analyses by cDNA-RNA reassociation kinetics indicate that there are approximately 3900 molecules of cytoplasmic albumin mRNA per cell in the parent and less than 10 molecules per cell in both variants. Southern blotting of the Eco RI restriction fragments of cellular DNA from the parent and variants did not indicate any major deletions in the albumin gene DNA sequences. We conclude that in the two variants studied, processes that regulate albumin production via alterations in the level of cytoplasmic albumin mRNA have been affected. Our analyses have also shown that alpha-fetoprotein (AFP) production is lacking in one variant (19/2) and is slightly reduced in the other (1/c/1). Transferrin secretion is lower than the parental line in both variants. Thus multiple nonlethal defects in hepatic gene expression can be obtained in Hepa la cells in culture that will be useful in determining the number and kinds of genes that control the expression of liver-specific loci.

Chinese hamster ovary cell lines which are resistant to an amino acid analog, beta-aspartyl hydroxamate, have been isolated and characterized. Mutants resistant to 100-150 microM beta-aspartyl hydroxamate arose from ethyl methane sulfonate-treated parental lines at frequencies of 3.4 x 10(-6) to 1.3 x 10(-7). The mutants fell into at least two genetic classes: 18% of the mutants behaved codominantly in hybrids, the others recessively. Complementation studies indicated that all the recessive mutants belonged to the same class. Mutants selected after one step of mutagenesis overproduce the enzyme asparagine synthetase constitutively with four- to sixfold increases in specific activities over the basal levels of the parental lines. beta-Aspartyl hydroxamate-resistant cell lines with up to 20-fold elevations in asparagine synthetase activity have been isolated after two steps of mutageneis. In addition, highly resistant lines have been selected by long-term growth of a dominant mutant in increasing concentrations of the drug. Resistance in the latter appears to be due not only to overproduction of asparagine synthetase but also to an alteration in the affinity of the enzyme for beta-aspartyl hydroxamate.

Expression of the gene locus which codes for a form of human intestinal alkaline phosphatase (ALP) has been analyzed in intraspecific somatic cell hybrids. Hybrids were constructed between D98/AH-2, a line of HeLa which ectopically synthesizes high levels of this ALP isozyme, and three different nonintestinal ALP-producing diploid lines. In chromosomally complete hybrids, expression of the ALP isozyme was initially suppressed, but on extended culture, reexpression occurred, as did limited chromosome loss. Results from extensive subcloning experiments showed that events leading to reexpression occurred at high frequency, and this ALP reexpression appeared to confer some selective advantage, direct or indirect, on the cells. In the fibroblast hybrids, reexpression of the intestinal-like ALP was always accompanied by new, high-level expression of liver/bone/kidney ALP, the product of a separate ALP gene locus. Thus expression of the one ALP locus is not excluded and, in fact, appears to be promoted by expression of the other in these cells.

We have conferred methotrexate resistance on mouse 3T6 fibroblasts by chromosome-mediated transfer of an altered dihydrofolate reductase gene encoding a highly methotrexate-insensitive enzyme. The methotrexate-resistant 3T6 cell line from which the chromosomes were prepared contains multiple copies of the altered dihydrofolate reductase gene, all of which appear to reside on double-minute chromosomes. Transformants selected at 0.2 microM methotrexate contain 10-20 times more of the transferred altered gene than of the resident normal gene. The altered genes are associated with double-minute chromosomes and are permanently lost following growth of the transformants in the absence of methotrexate. Growth of the transformants in increasing concentrations of methotrexate leads to the emergence of cells which have accumulated double-minute chromosomes and which have amplified only the transferred dihydrofolate reductase gene.

Alpha 2-macroglobulin (alpha 2M), a high-molecular-weight plasma protease inhibitor has been shown, by both immunological and functional methods, to be produced by cultured adult lung fibroblasts. Cultured skin fibroblasts synthesized approximately one tenth as much alpha 2M as lung fibroblasts. This quantitative difference in alpha 2M production was also demonstrated in fibroblasts of isogenic origin. There was no difference in the amount of alpha 2M produced between adult and fetal fibroblasts of the same tissue type (i.e., of lung or of skin origin). alpha 2M was produced in culture during log-phase growth as well as at confluence. Two other plasma protease inhibitors, C1-esterase inhibitor and a substance immunologically crossreacting with human inter-alpha-trypsin inhibitor, were also made by the cultured fibroblasts. Plasma protease inhibitors not detectable in culture supernatants were alpha 1-antitrypsin, alpha 1-antichymotrypsin, and antithrombin III.

Five different clones of somatic hybrid cell lines between mouse and rat cells were examined for the expression of species-specific histone H1 and H2B subtypes. It was found that one hybrid (140-3) contained only mouse specific histones and the other four clones had both mouse and rat specific histones in various ratios. In three cases (141-B, NBr10A, NBr20A), the relative amount of mouse and rat specific histones was correlated with the karyotype of the hybrids. However, this was not true in one case (NG108-15). The results indicate that a single mammalian hybrid clone can express both parental types of histones, and the histone expression in these hybrids is not regulated by an allelic exclusion but may be expressed codominantly by gene dosage.

When mammalian cells are cultured at low concentrations of toxic drugs, they often become phenotypically resistant. We studied whether this phenotypic resistance is due to selection of preexisting variants. The drugs 8-azaguaine (AG) and 6-thioguanine (TG) were used and, as a parameter for resistance, the incorporation of hypoxanthine was determined. Preexisting variation among clones in the uptake of hypoxanthine was found, and this variation has a hereditary component. This transmission of aberrant incorporation of hypoxanthine does not appear a stable trait, and the aberrant cell lines returned gradually to the original steady state. There are indications that within a cell population cells with altered levels of incorporation of hypoxanthine arise continuously and at a high frequency. Treatment with marginally toxic concentrations of AG or TG indicates that, at least for AG, survival is not related to the preexisting variation in hypoxanthine uptake. The observed phenomena could be of importance for the selection of drugs to be used in cancer chemotherapy.

We previously postulated that the structural gene for epidermal growth factor (EGF) receptor is located on human chromosome 7 (1,2). In this study, EGF receptor and certain postreceptor functions were further analyzed in a unique cell hybrid line, C2B5, that retains only one human chromosome of an X;7 translocation besides a nearly complete mouse parental genome. Kinetics and Scatchard analysis of [125I]EGF binding to the C2B5 hybrid cells indicated that they carry a single class of EGF receptors with a dissociation constant of 4 x 10(-10) M. The receptors expressed in the hybrids are proven to be immunologically of human nature. The human EGF receptors now embedded in essentially mouse plasma membrane are subject to "down regulation" mediated by the ligand EGF. Analysis of the cell-bound EGF indicated that internalization and processing take place in the human-mouse cell hybrids. The degradation of EGF appears to be through a lysosomal pathway since it was substantially delayed or inhibited by lysosomotropic agents.