Tabulation of somatic cell hybrids formed between lines of cultured cells

Gene transfer and gene mapping in mammalian cells in culture

The ability to transfer mammalian genes parasexually has opened new possibilities for gene mapping and fine structure mapping and offers great potential for contributing to several aspects of mammalian biology, including gene expression and genetic engineering. The DNA transferred has ranged from whole genomes to single genes and smaller segments of DNA. The transfer of whole genomes by cell fusion forms cell hybrids, which has promoted the extensive mapping of human and mouse genes. Transfer, by cell fusion, of rearranged chromosomes has contributed significantly to determining close linkage and the assignment of genes to specific chromosomal regions. Transfer of single chromosomes has been achieved utilizing microcells fused to recipient cells. Metaphase chromosomes have been isolated and used to transfer single-to-multigenic DNA segments. DNA-mediated gene transfer, simulating bacterial transformation, has achieved transfer of single-copy genes. By utilizing DNA cleaved with restriction endonucleases, gene transfer is being empolyed as a bioassay for the purification of genes. Gene mapping and the fate of transferred genes can be examined now at the molecular level using sequence-specific probles. Recently, single genes have been cloned into eucaryotic and procaryotic vectors for transfer into mammalian cells. Moreover, recombinant libraries in which entire mammalian genomes are represented collectively are a rich new source of transferable genes. Methodology for transferring mammalian genetic information and applications for mapping mammalian genes is presented and prospects for the future discussed.

Thymidine kinase, DNA synthesis and cancer

A resume has been presented of some recent investigations which show that DNA synthesis can be initiated in many types of quiescent animal cells by external stimuli, by introducing a quiescent nucleus into the cytoplasm of a proliferating cell, or by a virus infection. The components of the DNA replication apparatus are described. It is shown that deoxyribonucleoside triphosphate pools increase substantially in animal cells at the time DNA synthesis is initiated due to the enhanced activities of enzymes functioning in nucleotide synthesis. Especially striking is the increase of thymidine kinase activity, indicating that this enzyme may be a useful marker of the shift from the quiescent to the replicative state. The thymidine kinase isozymes of vertebrate cells have been characterized. Thymidine kinase F, which is found principally in the cytosol, is the isozyme that increases when G1 (Go) phase cells are stimulated or infected with oncogenic viruses. Chick cytosol thymidine kinase F can also be reactivated by introducing differentiated chick erythrocyte nuclei into the cytoplasm of enzyme-deficient LM (TK-) mouse cells. Furthermore, herpesviruses code for distinctive, virus-specific thymidine kinase isozymes, so that another way to transform thymidine kinase-deficient LM TK-) cells to kinase-positive cells is by infecting them with UV-irradiated herpes simplex viruses. The experiments on the activation of DNA synthesis and thymidine kinase F activity have been discussed in the context of the proliferative activity in vivo and the immortalization in culture of neoplastic cells. These experiments suggest that genes determining cell cycle proteins are readily accessible to transcription and translation in essentially all nucleated cells. The tendency of transformed cells to become multinucleated after cytochaliasin B treatment also suggests that one important difference between malignant cells and most normal cells may be the ability of malignant cells to 'stockpile' the proteins (and/or their messenger RNAs) of the DNA replicative apparatus and to maintain the 'stockpiles' in progeny cells.

Heterokaryons in the analysis of genes and gene regulation

Cytological and chemical analysis of heterokaryons, the immediate product of cell fusion, offer new possibilities for studying the factors responsible for genetic regulation in eukaryotic cells. In comparison with proliferating cell hybrids the heterokaryon state offers the important advantage that a heterokaryon contains two complete genomes since chromosome loss does not occur, but since segregation and recombination are absent, heterokaryons cannot be used for gene mapping in the same way as proliferating cell hybrids. However, if two cell types carrying different genetic defects are fused the analysis can be used for studies of gene complementation. The biological information obtained with heterokaryons has emphasized the role of the cytoplasm in the control of nuclear activity. When a G1 nucleus is brought into contact with the cytoplasm of an S phase cell the G1 nucleus is stimulated to synthesize DNA. If the nucleus is brought into a mitotic cell, the chromatin of the G1 nucleus is forced to condense into prematurely condensed chromosomes. Inactive nuclei such as the dormant chick erythrocyte nucleus will be stimulated to initiate RNA and DNA synthesis when brought into contact with an active cytoplasm by cell fusion. Specific nuclear proteins have been shown to be responsible for this process of reactivation. Other inactive nuclei such as the nuclei of macrophages and spermatozoa have likewise been shown to be reactivated by fusion with active cells. The degree of activation in all of these cases appears to be determined by the state of the active cell. Inactive nuclei are activated to the same level as the active nucleus but seldom beyond this level. If differentiated cells are fused with undifferentiated cells, usually the differentiated character is lost rapidly after fusion. This observation is in agreement with several studies on proliferating cell hybrids indicating some type of negative control of differentiated properties. In heterokaryons obtained by fusion of cells of a similar type of histotypic differentiation usually coexpression of the differentiated markers is observed.

Genetic control of melanization: isolation and analysis of amelanotic variants from cultured melanotic melanoma cells

MELANOMA CELLS EXPRESS A PHENOTYPE THAT IS EASY TO RECOGNIZE: the synthesis of melanin. We used this marker to isolate clones of amelanotic variants from large populations of wild-type melanotic clones. Cloudman mouse melanoma (S91, clone M-3, CCL 53.1) was chosen as the parental line because the cells are highly pigmented, grow well as clones in soft agar, and fuse readily with Sendai virus. Subclones (10(7)) of this line were screened without prior mutagenesis, and nine amelanotic variants were isolated. The mutagen ethylmethanesulfonate increased the frequency of variants by three to four orders of magnitude. Wild-type cells had both basal and melanocyte stimulating hormone-inducible tyrosinase activities. The four amelanotic variants that we have examined to date all behaved similarly: they lost basal tyrosinase (EC 1.14.18.1; monophenol monooxygenase) activity but retained melanocyte stimulating hormone-inducible activity; they contained Stage-II melanosomes but no melanized melanosomes; they exhibited growth characteristics similar to those of wild-type cells in culture but produced fewer tumors in mice.

Prospects in genetic engineering of plants

Genetic engineering has quite rightly an image of science fiction. The time when new species with any wanted combination of genetic properties can be ordered from an animal or plant breeding factory seems far away. The layman's view that the science fiction of today is the reality of tomorrow is certainly an insufficient argument to justify optimism. If this were so, we should by now be able to produce hybrids between members of the animal and plant kingdom as was foreseen by a nineteenth-century equivalent of Fred Hoyle (see Fig. I). Despite the scepsis expressed by the prominent scientist Si.r Macfarlane Burnet in his bookGenes, Dreams and Realities(1971), recent advances in molecular genetics have raised new enthusiasm (and uneasiness) which make people speak of genetic engineering as something to aim at as an approach to correct inborn errors of metabolism. This will, however, not be our principal dish if we restrict ourselves to a vegetarian menu. We view genetic engineering of plants not only as a future method to improve species, but also as a fundamental approach to the study of gene expression, especially with respect to cell differentiation. If we consider the term literally, the definition of genetic engineering might be any intentional genetic manipulation to alter species or to make new ones. In this sense genetic engineering was practised long before Mendel presented his laws (1865). In the seventeenth century the bulb growers of the Low Countries produced new varieties of tulips for which prices of a thousand forms each were paid (at the 1636 price index!). We notice here already a strong impact of genetic engineering on society; the so-called crazy tulip trade caused a financial disaster on the Amsterdam stock-exchange comparable to the Wall Street crash in the 1930s.

Mammalian cells in culture frequently release type C viruses

Cell cultures commonly used in animal cell research, both cell strains and continuous cell lines from various mammalian species, spontaneously produce type C RNA viruses.