Selective gene mutation in MEL cells.

MEL cells, undergoing erythroid differentiation and parasynchronized by dimethyl sulfoxide (DMSO) induction, were irradiated with a 3-s pulse of UV light at sublethal dose. A large number of clones deficient in different gene functions are found in the progeny of the treated cells, if the pulse irradiation is performed 18-24 h from the start of DMSO induction. Kinetics of thymidine incorporation into DNA show that the period of sensitivity corresponds to the S phase. The results show that the activities of the tested genes are differently affected depending on the exact time of cell irradiation. Maximum percent inhibition of cells not expressing glucose-6-phosphate dehydrogenase (G-6-PD) (70%) is produced by irradiating at 20 h from the start of DMSO induction; 6-phosphogluconate dehydrogenase (6-PGD) (55%), and hypoxanthine (guanine) phosphoribosyltransferase (HPRT) (33%), at 21 h; hemoglobin (50%), at 22 h. The time difference in the sensitivity to UV light is highly reproducible and has been exploited to isolate, with high efficiency, cellular clones deficient in any one of the tested functions. Determinations of enzymatic activities on cell lysates show that the expression of tested genes is actually altered in cells that, on the basis of cytochemical tests, appear unaffected by UV irradiation. While the production of mutant clones is observed only during the S phase of the cell cycle, immediate statistical damage of the cellular DNA is produced at all times of irradiation. This finding excludes that the two types of phenotypic alterations, blocked or altered gene expression, both propagated in the progeny of the cells as clonal properties, may derive from a preferential alteration of those functions during the S phase.