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 protoo ncogenes, such as point-mutated proto-ras, ''play a dominant part in c ancer,'' because they are sufficient to transform transfected mouse ce ll lines in vitro [Alberts, B., Bray, D., Lewis, J., Raff, M., Roberts , K. & Watson, J. D. (1994) Molecular Biology of the Cell (Garland, Ne w York)]. However, in cells transformed in vitro mutated human ms gene s 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 tran sformed kt vitro, we have investigated the minimal level of human ms e xpression for transformation in vitro. Using point-mutated human ms ge nes recombined with different promoters from either human metallothion ein-IIA or human fibronectin or from retroviruses we found dominant ir t vitro transformation of the mouse C3H cell line only with ms genes l inked 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, nond ominant transformation was observed in a small percentage (2-7%) of C3 H cells transfected with ms genes that are expressed less than 20 time s higher than native C3H ms genes. Because over 90% of cells expressin g ms at this moderately enhanced level were untransformed, transformat ion must follow either a nondominant ras mechanism or a non-ras mechan ism. We conclude that the mutated, but normally expressed, ms genes fo und in human and animal cancers are not likely to ''play a dominant pa rt in cancer.'' The conclusion that mutated ras genes are not sufficie nt or dominant for cancer is directly supported by recent discoveries of mutated ms in normal animals, and in benign human tissue, ''which h as little potential to progress'' [Jen, J., Powell, S. M., Papadopoulo s, N., Smith, K.J., Hamilton, S. R, Vogelstein, B. & Kinzler, K.W. (19 94) Cancer Res. 54, 5523-5526]. Even the view that mutated ms is neces sary for cancer is hard to reconcile with (i) otherwise indistinguisha ble cancers with and without ms mutations, (ii) metastases of the same human cancers with and without ras mutations, (iii) retroviral ras ge nes that are oncogenic without point mutations, and (iv) human tumor c ells having spontaneously lost ras mutation but not tumorigencity.