Modulation of Erbb2 signaling during development: a threshold level of Erbb2 signaling is required for development.

We have generated a series of Erbb2 cDNA knock-in animals to explore the role of signaling pathways coupled to Erbb2 during development. Although this knock-in allele was hypomorphic, expressing tenfold less Erbb2 protein than wild type, the knock-in animals were healthy. However, a further twofold reduction in Erbb2 levels in hemizygous knock-in animals resulted in perinatal lethality with defects in the innervation of the diaphragm. Genetic rescue of this hypomorph was accomplished by expression of the Erbb2-Y1028F mutant in a comparable knock-in allele. Interestingly, hemizygous Y1028F animals were viable with normal innervation of the diaphragm. Molecular analyses revealed that the Y1028F allele expressed higher levels of Erbb2 and that Y1028 promoted the turnover of the receptor. In addition, ablation of the Shc-binding site in Erbb2 (Y1227) resulted in subtle defects in the sensory nerves not observed in the other mutant erbb2 strains. Thus, we have established how Erbb2 levels may be modulated through development and that a minimum threshold level of Erbb2 is required.

Germ-line expression of an oncogenic erbB2 allele confers resistance to erbB2-induced mammary tumorigenesis.

We have previously shown that mammary epithelial specific expression of the activated erbB2 allele under the control of the endogenous promoter in mice resulted in the formation of mammary adenocarcinomas. To assess whether mammary tumorigenesis in this model is influenced by the developmental window of expression, we generated mice that expressed the activated erbB2 allele in the germ line. Although we were able to derive viable transgenic mice that were heterozygous for the activated erbB2 allele, mice homozygous for the activated erbB2 allele died at 12.5 days of embryogenesis. These two separate lines of mice expressed activated erbB2 at equal levels in the mammary gland. Surprisingly, unlike the tumor-prone mice expressing activated ErbB2 in the mammary epithelium, mice with the germ-line erbB2 allele failed to develop tumors. Gene expression analysis of the preneoplastic mammary glands revealed that there were a number of luminal epithelial markers expressed at higher levels in the tumor-prone mice. These data suggest either an expansion of a susceptible population in the tumor-prone mice or the loss of this population in the tumor-resistant mice. Taken together, these observations suggest that the temporal pattern of expression of activated ErbB2 is a critical determinant in mammary tumorigenesis. These results strongly suggest that there are feedback mechanisms present that can compensate for the expression of a potent oncogene.

Gene expression profiling of neu-induced mammary tumors from transgenic mice reveals genetic and morphological similarities to ErbB2-expressing human breast cancers.

Numerous studies have shown that the overexpression and amplification of ErbB2/Neu are observed in 20-30% of patients afflicted with breast cancer. Furthermore, it has also been observed that the elevated expression of ErbB2/Neu also correlates with poor prognosis and clinical outcome. Given the prevalence of this disease, we sought to create mouse models that mimic the human condition. In this study, we compared two mouse models expressing activated neu under the control of the endogenous and mouse mammary tumor virus promoters. Although histologically similar, the latency and metastatic potential of these tumors are remarkably different. Gene expression profiling of tumor RNA from the two Neu mouse models revealed distinctive and nonoverlapping patterns of gene expression. Consistent with noninvasive nature of the mammary tumors induced by expression of neu under the endogenous promoter, these tumors expressed a number of markers characteristic of a highly differentiated state. In addition to these differences, these analyses revealed that in contrast to the mouse mammary tumor virus-based Neu model, the endogenous promoter tumors expressed elevated levels of two genes (Grb7 and Cab1) that are closely linked to ErbB2 and often coamplified in noninvasive ductal carcinoma in situ. Furthermore, this analysis has revealed several transcription factors that may be involved in ErbB2-mediated tumorigenesis. Taken together, these results illustrate the similarity of the endogenously regulated Neu tumor model to the human disease.

The catalytic activity of the ErbB-2 receptor tyrosine kinase is essential for embryonic development.

Activation of the epidermal growth factor receptor (EGFR) family is thought to play a critical role in both embryogenesis and oncogenesis. The diverse biological activities of the EGFR family are achieved through various ligand-receptor and receptor-receptor interactions. One receptor that has been found to play a central role in this signaling network is ErbB-2/Neu, and it is considered the preferred heterodimerization partner for other members of the EGFR family. To assess the importance of the catalytic activity of ErbB-2 in embryonic development, we have generated mice expressing a kinase-dead erbB-2 cDNA under the transcriptional control of the endogenous promoter. Here, we show that mice homozygous for the kinase-dead erbB-2 allele die at midgestation and display the same spectrum of embryonic defects seen in erbB-2 knockout mutants. These observations suggest that the catalytic activity of ErbB-2 is essential for normal embryonic development.