Gene expression profiling identifies potential therapeutic targets in estrogen receptor positive breast cancer.

Loi S, Haibe-Kains B, Sotiriou C, Lallemand F, Piccart MJ, Campbell I, Wayne P and McArthur GA

We recently reported that the level of expression of proliferation genes in estrogen receptor  positive (ER+) breast cancer (BC) is a powerful predictor of  prognosis and high levels of proliferation is associated with tamoxifen resistance (Loi et al, JCO 2007). This study aimed to explain the biological basis for these observations using global gene expression profiling.

Methods246 ER+ BC samples from women treated with adjuvant tamoxifen only were analyzed with gene expression arrays and evaluated using gene set enrichment analysis (GSEA). 173 samples were sequenced for PIK3CA mutations. Ingenuity Pathway Analysis was used to generate interaction networks. MCF-7 BC cells treated heregulin (HRG) was used as an in-vitro model of ERBB2 pathway activation.

We found that gene sets suggesting ERBB2 and PI3K pathway activation were significantly enriched in the tamoxifen resistant subtype.
HRG treated MCF7 cells displayed phosphorylation of HER2/3 without HER2 overexpression. Treatment with HRG overcame tamoxifen induced cell cycle arrest (p<0.01) and increased anchorage-independent colony formation (p<0.01), validating the hypothesis that ERBB2 signaling without overexpression is an important for tamoxifen resistance.
Samples were sequenced for PIK3CA mutations to investigate PI3K pathway activation. Mutations were found in 45 samples (26%). There was no significant association with mutation and lymph node status, tumor grade or age. Tumor size had a borderline association (p=0.056). Contrasting gene expression profiles of mutation positive samples and wild type revealed 81 significantly differentially expressed genes (p=0.03). A gene-expression predictor consisting of 35 genes based on gene interactions was then constructed. The PIK3CA associated gene set could predict clinical outcome in 1228 independent ER+ BC samples HR: 1.39(95%CI 1.23-1.58) p<0.001.

Using gene expression profiling, we have identified signaling pathways in ER+ BC that could be targeted in the clinical setting. Furthermore, we propose gene signatures of activated ERBB2 and PI3K pathways that may help stratify patients for therapy selection.