Dominant transformation by mutated human ras genes in vitro requires more than 100 times higher expression than is observed in cancers

The gene-mutation-cancer hypothesis holds that mutated cellular protooncogenes, such as point-mutated proto-ras, ''play a dominant part in cancer,'' because they are sufficient to transform transfected mouse cell lines in vitro [Alberts, B., Bray, D., Lewis, J., Raff, M., Roberts, K. & Watson, J. D. (1994) Molecular Biology of the Cell (Garland, New York)]. However, in cells transformed in vitro mutated human ms genes are expressed more than 100-fold than in the cancers from which they are isolated. In view of the discrepancy between the very low levels of ras transcription in cancers and the very high levels in cells transformed kt vitro, we have investigated the minimal level of human ms expression for transformation in vitro. Using point-mutated human ms genes recombined with different promoters from either human metallothionein-IIA or human fibronectin or from retroviruses we found dominant irt vitro transformation of the mouse C3H cell line only with ms genes linked to viral promoters. These ras genes were expressed more than 120-fold higher than are native ms genes of C3H cells. The copy number of transfected ms genes ranged from 2-6 in our system. In addition, nondominant transformation was observed in a small percentage (2-7%) of C3H cells transfected with ms genes that are expressed less than 20 times higher than native C3H ms genes. Because over 90% of cells expressing ms at this moderately enhanced level were untransformed, transformation must follow either a nondominant ras mechanism or a non-ras mechanism. We conclude that the mutated, but normally expressed, ms genes found in human and animal cancers are not likely to ''play a dominant part in cancer.'' The conclusion that mutated ras genes are not sufficient or dominant for cancer is directly supported by recent discoveries of mutated ms in normal animals, and in benign human tissue, ''which has little potential to progress'' [Jen, J., Powell, S. M., Papadopoulos, N., Smith, K.J., Hamilton, S. R, Vogelstein, B. & Kinzler, K.W. (1994) Cancer Res. 54, 5523-5526]. Even the view that mutated ms is necessary for cancer is hard to reconcile with (i) otherwise indistinguishable cancers with and without ms mutations, (ii) metastases of the same human cancers with and without ras mutations, (iii) retroviral ras genes that are oncogenic without point mutations, and (iv) human tumor cells having spontaneously lost ras mutation but not tumorigencity.