Overexpression of c-myc by amplification of negative promoter domain.

Amplification of myc proto-oncogenes has been reported in many human malignancies, but whether the amplified genes are properly regulated has not been well studied. Transcription factors that control gene expression are of limited abundance, and we have previously shown (Johnston, R.N., and Kucey, B.L. (1988) Science 242, 1551-1554) that amplified heat shock promoter sequences can compete with natural heat shock genes for these factors, resulting in heat shock gene disregulation. To establish whether protooncogenes may also be disregulated in this manner, a previously identified negative regulatory domain from the human c-myc promoter was amplified by up to 800-fold in Chinese hamster ovary cells. The amplified cells showed up to a 14-fold increase in hamster c-myc transcript abundance when compared with controls and also displayed reduced doubling times, enhanced incorporation of tritiated thymidine, elevated growth in soft agar, and a fusiform morphology, consistent with an elevation in the degree of cellular transformation. We infer that overexpression of endogenous hamster c-myc genes in the experimental cells was elicited in trans by sequestration of an inhibitory factor or complex that bound to excess nonfunctional human c-myc promoter domains. The transcriptional inhibitory activity we detect is consistent with anti-oncogene function.