Haploid loss of Ki-ras delays mammary tumor progression in C3 (1)/SV40 Tag transgenic mice.

We have previously demonstrated that amplification and overexpression of the Ki-ras gene is associated with mammary tumor progression in C3(1)/SV40Tag transgenic mice (Liu et al., 1998). To further evaluate the functional significance of the Ki-ras proto-oncogene in mammary cancer development, in vivo studies were conducted to examine the effect of Ki-ras gene dosage on tumor progression. The lack of one normal Ki-ras allele C3(1)/SV40Tag transgenic mice resulted in significantly delayed mammary intraepithelial neoplasia (MIN) formation as well as in a decreased number of mammary gland carcinomas. However, despite the retardation of tumor development by reduced Ki-ras gene dosage, overall survival was only modestly affected. This appears to be due to several factors including significant mammary tumor growth associated with Ki-ras gene amplification and over-expression that occurs during the advanced stage of oncogenesis in mice carrying either one or two normal Ki-ras alleles. The retardation of tumor progression due to the haploid loss of Ki-ras did not appear to be related to accelerated apoptosis, or a reduced rate of cell proliferation at the tumor stages examined. These data strongly suggest that the gene dosage of Ki-ras affects tumor promotion at an early stage of mammary tumor progression in this SV40 Tag-induced model of mammary oncogenesis.