Induction and analysis of gene mutations in cultured mammalian somatic cells.

M U C H of our knowledge concerning the molecular processes of the cell has come from studies on unicellular organisms, as a result of several properties which suit them to experimentation. In contrast, the complex life cycles and multicellular organization of differentiated higher organisms have put experiments on an entirely different time-scale and have made the biochemical analysis of any single cell type difficult. Mammalian cell biologists have long been interested in the development of tissue culture methodology that would permit the application of microbiological techniques to their experimental systems. In recent years, attempts to study the genetics of mammalian somatic cells have met with good success through a number of technical advances. Firstly, due notably to the work of T. T. Puck and his co-workers (PUCK 1972). i t has been possible, in semidefined media, to clone single mammalian cells at high efficiency and to grow pure lines to high population densities. Mammalian cells in culture, as microorganisms, are thus single units of life that are amenable to qualitative and quantitative analysis of their genetic and biochemical properties. Secondly, experimental mutagenesis in mammalian cells was first reported independently by CHU and MALLING (1968a,b) and by KAO and PUCK (1968a,b) at the International Congress of Genetics in Tokyo. The ability to induce mutations in mammalian cells in culture not only increases the genetic variability that is prerequisite for genetic analysis but also permits studies in these cells of the mutation process itself. Thirdly, the formation of cell hybrids by fusion of cells carrying different genetic markers allows studies of gene expression, complementation, mapping of genes, linkage and possibly mitotic recombination. Finally, recent development of the chromosome banding techniques to identify individual chromosomes and specific chromosome regions further helps to establish, in the true sense of that term, the field of mammalian cytogenetics. In this paper I shall briefly review the studies on mutagenesis in mamalian cells in culture. At the outset I must point out that the occurrence of persistent phenotypic variations in somatic cells could be due to either genetic or epigenetic mechanisms. Although a large body of experimental evidence to be reviewed here suggests a genetic basis for mammalian somatic cell variations, there is a contrasting proposal that some forms of variation at least may be based on changes in gene expression rather than on alterations in genetic information. HARRIS (1971, 1973) observed that mutation rates to drug resistance or to thermal tolerance in Chinese hamster cells are essentially independent of the ploidy level; MEZGER-