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
Background: 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
Methods: 246 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
Results: 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.
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.
Conclusions: 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.