Amplification and rearrangement of the Kirsten ras oncogene in virus-transformed BALB/c 3T3 cells during malignant tumor progression.

Analyses of the cellular and viral Kirsten ras genes (c-Ki-ras and v-Ki-ras, respectively) during malignant tumor progression were performed by using Kirsten murine sarcoma virus-transformed BALB/c 3T3 cells that harbor a replication-defective provirus. After injection into athymic nude mice by four different routes, primary tumors and secondary lung metastases were isolated, adapted to in vitro growth, and analyzed for DNA levels and mRNA expression of both genes for comparison with the originally injected transformed cells and untransformed 3T3 cells. For all tumors (primary or secondary), the v-Ki-ras gene was amplified and v-Ki-ras mRNA expression was highly elevated above that observed in the original transformed cell population. In two of five lung metastases from the i.v. and footpad injection routes, rearranged Ki-ras DNA sequences were observed. Micrometastases from the s.c. route of injection did not display these alterations. Injection of footpad lung tumor cells with rearrangements into a second group of animals led to multiple lung metastases with even further rearrangements correlating with more effective lung colonization/growth ability (overt lung tumors in five of eight animals less than 20 days after injection). However, reinjection of an i.v. lung tumor with rearranged Ki-ras led to no further rearrangements in the lung microfoci tumors isolated greater than 40 days after injection. These data suggest (i) the significance of amplification and elevated expression of v-Ki-ras in tumor formation, (ii) correlation of this amplification with more effective tumor progression, and (iii) the selective advantage that cells with Ki-ras DNA sequence additions have in the formation of overt lung tumors.