The Human Splice Variant Δ16HER2 Induces Rapid Tumor Onset in a Reporter Transgenic Mouse

Abstract

Several transgenic mice models solidly support the hypothesis that HER2 (ERBB2) overexpression or mutation promotes tumorigenesis. Recently, a HER2 splice variant lacking exon-16 (Δ16HER2) has been detected in human breast carcinomas. This alternative protein, a normal byproduct of HER2, has an increased transforming potency compared to wild-type (wt) HER2 receptors. To examine the ability of Δ16HER2 to transform mammary epithelium in vivo and to monitor Δ16HER2-driven tumorigenesis in live mice, we generated and characterized a mouse line that transgenically expresses both human Δ16HER2 and firefly luciferase under the transcriptional control of the MMTV promoter. All the transgenic females developed multifocal mammary tumors with a rapid onset and an average latency of 15.11 weeks. Immunohistochemical analysis revealed the concurrent expression of luciferase and the human Δ16HER2 oncogene only in the mammary gland and in strict correlation with tumor development. Transgenic Δ16HER2 expressed on the tumor cell plasma membrane from spontaneous mammary adenocarcinomas formed constitutively active homodimers able to activate the oncogenic signal transduction pathway mediated through Src kinase. These new transgenic animals demonstrate the ability of the human Δ16HER2 isoform to transform “per se” mammary epithelium in vivo. The high tumor incidence as well as the short latency strongly suggests that the Δ16HER2 splice variant represents the transforming form of the HER2 oncoprotein.

Analysis of different HER‐2 mutations in breast cancer progression and drug resistance

“…Both of these alterations could explain the clinical failure of trastuzumab 55, 56, 57, 58, 59, 60. The Δ16HER‐2, a type of oncogenic variant caused by the in‐frame deletion of exon 16 in the extracellular domain of HER‐2, is reported to comprise 4–9% of total HER‐2 transcripts 56, 61. The Δ16HER‐2 variant, when expressed at high levels, harbours enhanced transforming activity compared with wild‐type HER‐2 54.…”

delta16HER2 splice variant and its role in HER2-overexpressing breast cancer

“…Interestingly, those studies also revealed a direct correlation between node status and delta16HER2 mRNA expression (Mitra et al, 2009), implicating delta16HER2 in human BC progression. Further confirmation of delta16HER2 intrinsic oncogenicity came from our recent generation a mouse line transgenically expressing the human delta16HER2 variant using a bicistronic vector encoding both delta16HER2 and luciferase reporter genes under the MMTV promoter (Marchini et al, 2011). Our transgenic line was characterized by a higher tumor incidence as well as a shorter latency (about 8 weeks of age) compared to the MMTVhuHER2 transgenic model, which developed human WT HER2-overexpressing spontaneous mammary tumors after 28 weeks of age (Finkle et al, 2004;Marchini et al, 2011).…”

“…The delta16HER2 variant was expressed on mammary tumor cells as cell surface-associated phosphorylated stable homodimers and monomers, and its downstream signals were significantly coupled to an activated Src kinase. In this transgenic model, about 5 copies of the delta16HER2 transgene were sufficient to drive early transformation in contrast to 30-50 copies of WT HER2 transgene needed to develop mammary tumors in the MMTVhuHER2 model (Finkle et al, 2004;Marchini et al, 2011). Very recently, Alajati et al (2013) provided further in vitro evidence that delta16HER2 is able to increase invasion and proliferation and also to decrease apoptosis either in two-or three-dimensional conditions.…”

“…This splice variant produces an aberrant receptor that lacks exon 16 (delta16HER2) ( Figure 1) and, in turn, 16 amino acids (634-649) in the HER2ECD (domain IV), including two relevant cysteine residues close to the T-binding epitope. The deletions result in stable constitutively active homodimer formation, enhanced multi-signaling activity and accelerated transformation (Kwong and Hung, 1998;Castiglioni et al, 2006;Mitra et al, 2009;Marchini et al, 2011;Alajati et al, 2013). -delta16HER2 splice variant and HER2-driven transformation In our previous analysis of 46 human HER2-positive BCs, we found that delta16HER2 was constitutively co-expressed with WT HER2 in ~90% of the BCs and its transcript represented 4-9% of total WT HER2 mRNA (Castiglioni et al, 2006), suggesting a possible role for this variant in WT HER2-driven tumorigenesis.…”

“…Further confirmation of delta16HER2 intrinsic oncogenicity came from our recent generation a mouse line transgenically expressing the human delta16HER2 variant using a bicistronic vector encoding both delta16HER2 and luciferase reporter genes under the MMTV promoter (Marchini et al, 2011). Our transgenic line was characterized by a higher tumor incidence as well as a shorter latency (about 8 weeks of age) compared to the MMTVhuHER2 transgenic model, which developed human WT HER2-overexpressing spontaneous mammary tumors after 28 weeks of age (Finkle et al, 2004;Marchini et al, 2011). The delta16HER2 variant was expressed on mammary tumor cells as cell surface-associated phosphorylated stable homodimers and monomers, and its downstream signals were significantly coupled to an activated Src kinase.…”