Gene expression study of MCF7/tet-off clones expressing different HER2 isoforms

HER2 is a tyrosine kinase receptor causally involved in cancer. A subgroup of breast cancer patients with particularly poor clinical outcome expresses a heterogeneous collection of HER2 carboxy-terminal fragments (CTFs). However, since the CTFs lack the extracellular domain that drives dimerization and subsequent activation of full-length HER2, they are in principle expected to be inactive. Here we present evidence that at low expression levels one of these fragments, 611-CTF, activated multiple signaling pathways because of its unanticipated ability to constitutively homodimerize. A transcriptomic analysis revealed that 611-CTF specifically controlled the expression of genes that we found correlated with poor prognosis in breast cancer. Among the 611-CTF-regulated genes were several that previously have been linked to metastasis, including MET, EPHA2, MMP1, IL11, ANGPTL4 and different Integrins. Transgenic mice overexpressing HER2 in the mammary gland develop tumors only after acquisition of activating mutations in the transgene. In contrast, we show that expression of 611-CTF led to development of aggressive and invasive mammary tumors without the need for mutations. These results demonstrate that 611-CTF is a potent oncogene capable of promoting mammary tumor progression and metastasis. Affymetrix Gene Array expression study, using MCF7/tet-off clones stably transfected with vectors encoding HER2 and different truncated forms of the receptor, were used to elucidate the activity of the various protein isoforms. MCF7 clones were selected, maintain and expanded in the presence of doxycycline to avoid expression of the HER2 receptor isoforms. The experiments were started by seeding equal numbers of cells in two dishes, one with and one without doxycycline. 15 or 60 hours later total RNA was isolated in parallel from the two dishes. In the dishes without doxycycline the cloned HER2 isoforms were expressed from a CMV promoter. In total 50 arrays were included in the analysis. For key clones and time points biological replicates were included.