An oncogenic isoform of HER2 associated with locally disseminated breast cancer and trastuzumab resistance

Abstract

The HER2-targeted therapy trastuzumab is widely used for the treatment of patients with metastatic breast tumors overexpressing HER2. However, an objective response is observed in only 12% to 24% of patients treated with trastuzumab as a single agent and initial responders regress in <6 months (1-3). The reason for the clinical failure of trastuzumab in this setting remains unclear. Here we show that local lymph node-positive disease progression in 89% of breast cancer patients with HER2-positive tumors involves the HER2 oncogenic variant HER2Δ16. We further show that ectopic expression of HER2Δ16, but not wild-type HER2, promotes receptor dimerization, cell invasion, and trastuzumab resistance of NIH3T3 and MCF-7 tumor cell lines. The potentiated metastatic and oncogenic properties of HER2Δ16 were mediated through direct coupling of HER2Δ16 to Src kinase. Cotargeting of HER2Δ16 and Src kinase with the singleagent tyrosine kinase inhibitor dasatinib resulted in Src inactivation, destabilization of HER2Δ16, and suppressed tumorigenicity. Activated Src kinase was also observed in 44% of HER2Δ16-expressing breast carcinomas underscoring the potential clinical implications of coupled HER2Δ16 and Src signaling. Our results suggest that HER2Δ16 expression is an important genetic event driving trastuzumab-refractory breast cancer. We propose that successful targeted therapeutics for intervention of aggressive HER2-positive breast cancers will require a strategy to suppress HER2Δ16 oncogenic signaling. One possibility involves a therapeutic strategy employing single-agent tyrosine kinase inhibitors to disengage the functionally coupled oncogenic HER2Δ16 and Src tyrosine kinase pathways.

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

“…2) 53, 54. 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. The primary activating mechanism is that this conformation with removed relevant cysteine residues promotes intermolecular disulfide bonding and the generation of homodimers, thus transforming cells 62.…”

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

“…In addition, exogenous expression of delta16HER2 in comparison to WT HER2 resulted in an increased activation of multiple downstream signaling pathways involving FAK, PI3K/Akt, MAPK and Src kinase. 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).…”

“…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.…”

“…Thus, WT HER2 requires additional genetic alterations to induce neoplastic transformation. More recently, Mitra et al (2009) showed that delta16HER2-overexpressing MCF7 human BC transfectants expressed stable dimers and were more invasive than WT HER2-transfected MCF7 cells. In addition, exogenous expression of delta16HER2 in comparison to WT HER2 resulted in an increased activation of multiple downstream signaling pathways involving FAK, PI3K/Akt, MAPK and Src kinase.…”