The action of Mendelian genes in human diploid cell strains.

Some of the cells of every human being will grow outside the body as microorganisms. It is possible to show, in a variety of ways, that these cells resemble genetically the individual from whom they were obtained. Over 35 inherited human diseases and anomalies can now be studied in such cell lines. Human diploid cell strains, biochemically marked by one or more mutant Mendelian genes, have proven particularly useful for the study of gene action in man and for the detection of genetic changes such as mutation and somatic cell hybridization. In addition, the strains have a number of clinical applications, including the antenatal diagnosis of inherited disease. The failure of cultured human cells to display their phenotype at most loci continues to restrict their use in both genetics and medicine. There are reasons for hoping that this difficulty will eventually be solved, and some experiments bearing on the problem are already feasible. To study genetics, one must start with hereditary variation. In cell culture, this means one must have, as a minimum, two different strains of cells which differ from one another in a t least one attribute. Moreover, the difference must be hereditary over somatic generations. The initial problem of human cell genetics, therefore, has been to identify or generate this kind of variation. Two principal methods have been used to obtain lines of human cells that have some distinctive and hereditary attribute. The first method is to select infrequent cells, from a population which differs in a known way from the majority of the cells. For example, nearly all cells exposed to the synthetic analogue 8-azaguanine are killed by this agent, but, over a wide range of doses, a few are not (Szybalski and Szybalska, '62; inter aliae). The surviving cells are resistant to the drug, even if they are grown in its absence for an extended period of time. In most cases, the resistant subline is deficient in an enzyme which apparently converts 8-azaguanine to the corresponding ribotide (Brockman et al., '61; inter aliae). It is likely to be this ribotide, rather than the free base, which is cytotoxic. In any case, such an experiment frequently leads J. CELL. PHYSIOL., 76: 311-330. to the isolation of a variant subline; in this case as well as in several others, an enzyme deficiency is associated with the ability of the cell to survive in the selective medium. Indeed, there are several general methods for preferentially favoring the growth of auxotrophic mammalian cells (Hooper and DeMars, '60; Kao and Puck, '68; Pollack, Green and Todaro, '68). Most such auxotrophs may be presumed to be deficient in one or more enzyme activities. The second method for obtaining variant lines of human cells is quite different. One begins by selecting donors (rather than cells) who are known to carry genes which affect molecules normally present in cultured cells (Krooth and Weinberg, '61). The Mendelian basis for the cellular phenotype is shown by developing strains from various persons in the same family. There are important differences between the first and second method. In the first method, the variant line and the line with which it is compared usually come from the same donor. In the second method, the donors are different. It can be argued that variants isolated by the first method are 1 The originaI investisations reported in this paper were supported by NIH Research grant 1-Pol-15419-03. 2 Supported by NIH Training grant 5-TOl-GM-0007113.