Selective inhibition of c-myc expression by the ribonucleic acid synthesis inhibitor mithramycin.

Expression of the c-myc proto-oncogene has been shown to correlate with the rate of cellular proliferation and malignant transformation in a number of cell types. JEG-3 choriocarcinoma cells demonstrate c-myc transcript levels that are greater than those of nonmalignant trophoblastic tissue at any stage of gestation. Southern blot analysis documents c-myc gene amplification in JEG-3 cells, with a gene copy number of approximately 20. The methylation pattern and genomic structure of the amplified c-myc oncogene in JEG-3 choriocarcinoma cells are identical to those of normal placenta. Treatment of JEG-3 cells with mithramycin, a ribonucleic acid synthesis inhibitor, results in a dramatic decrease in c-myc expression relative to that of the c-Ha-Ras gene. The apparent selectivity of mithramycin for c-myc expression represents the only example, to date, of the selective pharmacologic modulation of oncogene expression.