Elevated expression of wildtype RhoC promotes ErbB2- and Pik3ca-induced mammary tumor formation

Copy number gains in genes coding for Rho activating exchange factors as well as losses affecting genes coding for RhoGAP proteins are common in breast cancer, suggesting that elevated Rho signaling may play an important role. Extra copies and overexpression of RhoC also occur, although a role for RhoC overexpression in driving tumor formation has not been assessed in vivo. To this end, we report on the development of a Rosa26 (R26)-targeted Cre-conditional RhoC overexpression mouse (R26-RhoC). This mouse was crossed to two models for ERBB2/NEU+ breast cancer: one based on expression of an oncogenic ErbB2/Neu cDNA downstream of the endogenous ErbB2 promoter (FloxNeoNeuNT), the other, a metastatic model that is based on high-level expression from MMTV regulatory elements (NIC). RhoC overexpression dramatically enhanced mammary tumor formation in FloxNeoNeuNT mice but showed a more subtle effect in the NIC line, which forms multiple mammary tumors after a very short latency. Many mammary tumors that form in FloxNeoNeuNT mice show selection for increased NeuNT expression linked to high-level amplification of the ErbB2/NeuNT locus. In contrast, NeuNT overexpression did not require high-level amplification of ErbB2/NeuNT in RhoC-FloxNeoNeuNT lesions. RhoC overexpression also enhanced mammary tumor formation in a model for breast cancer induced by Pik3ca(H1047R). The transforming effect of RhoC was associated with epithelial/mesenchymal transition (EMT) in ErbB2/NeuNT and Pik3ca-H1047R systems. Thus, RhoC copy number gains with resultant overexpression contribute to breast tumor formation and/or progression.