Investigating the oncogenic properties of HER2 splice variants in breast cancer.


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Victoria Hart1
1Newcastle University

Abstract

Background

The Human Epidermal Growth Factor Receptor 2 (HER2) is a key driver of tumorigenesis and aggressive disease phenotype in ~15% of breast cancer cases. It is a mediator of cellular phenotypes such as proliferation, cell cycling and survival via modulation of gene expression through cell signalling. Despite the development of targeted treatments, resistance caused by the overexpression or amplification of HER2 reduces survival.

Splice variants are produced from alternative pre-mRNA splicing and several HER2 splice variants have been identified. Owing to structural differences these protein variants behave aberrantly to HER2 wild-type (WT), altering cell signaling and cellular phenotypes. Here, two novel HER2 splice variants have been identified.

Method

PCR-based analysis identified two alternative splicing events in the HER2 gene, ErBb2. Functional protein products and in vitro models were produced. Functional assays were used to characterize the downstream effects of splice variant expression on cell signaling and cellular phenotypes. Real-time qPCR analysis of invasive ductal carcinomas (IDC) (n = 40) and normal breast tissue (n = 20) was used to identify expression in vivo.

Results

Two novel HER2 splice variants were identified, termed HER2-PI9 and HER2-I12, generated by inclusion of intron 9 in part and intron 12, respectively. These exhibit altered behaviour to HER2-WT. HER2-PI9 is ineffective at triggering activation of the signaling pathways RAS/MAPK and PI3K/Akt. HER2-I12 enhances this signaling, proliferation, migration, invasion, and anchorage-independent growth.

HER2-PI9 and HER2-I12 are present alongside HER2-WT in IDC and normal breast. High variation is seen in expression levels between patients. HER2-I12 expression outflanked HER2-WT in some samples; this pattern was particularly striking in a HER2+ ER- patient group.

Conclusion

We have identified two novel HER2 splice variants which are biologically active in breast cancer. HER2-PI9, despite only minor deviations from HER2-WT structure, is unable to activate oncogenic signaling. Surprisingly, the truncated HER-I12 variant retains oncogenic abilities associated with the full-length receptor. Both are present in IDC alongside normal breast tissue. Impact upon targeted treatment efficacy will be investigated.

Impact statement

Determining the precise functionality of HER2 splice variants during oncogenic transformation and treatment will provide more detail as to how this receptor influences tumour behaviour and drug resistance.