Abstract P3-06-01: Hotspot mutations in PIK3CA are predictive for treatment outcome on aromatase inhibitors but not for tamoxifen
Authors: Ramirez Ardila DE , Helmijr JC , Lurkin I , Look M , Ruigrok-Ritstier K , Simon I , Van Laere S , Sweep F , Span P , Linn S , Foekens J , Sleijfer S , Berns EMJJ , Jansen MPHM 
Published: August 2014

Introduction: PIK3CA is the most frequent (30%) mutated oncogene in breast cancer and may lead to an activation of the PI3K/AKT/mTOR-pathway. Cell lines resistant to tamoxifen have an activated PI3K-pathway. Phase III clinical trials show substantial benefit when mTOR-inhibitors are added to aromatase inhibitor treatment. On the other hand, patients with PIK3CA exon 20 mutation expression signature show better treatment outcome after adjuvant tamoxifen therapy. To address this controversy, we evaluated the PIK3CA mutation status in 1423 primary breast cancer specimens for its relationship with prognosis and treatment outcome after first-line endocrine treatment.

Methods: Hotspot mutations in exon 9 and 20 of PIK3CA were detected by multiplex snapshot analyses. Mutation status in ER-positive tumors was related to metastasis free survival (MFS) in 292 untreated lymph node negative (LNN) patients and time to progression (TTP) in patients with metastatic disease treated with first-line tamoxifen (N = 482) or aromatase inhibitors (AIs; N=103). Whole genome mRNA and miRs expression profiling was performed in the latter patient subset to develop a PIK3CA gene signature in 64 specimens. This was validated in 28 independent ER-positive specimens.

Results: We could evaluate 1371 specimens and detected 437 hotspot mutations for PIK3CA (32%). Mutations in exon 20 were detected in 256 patients (59%), of which 40 cases with a H1047L (16%) and 216 with a H1047R (84%) mutation. Mutations in PIK3CA exon 9 were discovered in 174 patients (41%), with E542K and E545K mutations in 59 (34%) and 105 (60%) cases, respectively, as the most prevalent ones. Finally, 6 patients had PIK3CA double mutations for both exon 9 and 20 and one patient had a E542K and E545K mutation.

Evaluation of the untreated LNN patients for prognosis showed no relationship between MFS and PIK3CA mutations (HR = 1.07 [95% CI: 0.73–1.56]; p = 0.73), neither for exon 9 nor exon 20 compared to wild-type. In the multicenter cohort of 482 patients with advanced disease treated with first-line tamoxifen no link with treatment outcome and PIK3CA mutation status was observed (HR = 1.13 [95% CI: 0.92–1.38]; p = 0.24). However, patients with advanced disease treated with first-line AIs (N = 64) showed a significant longer TTP for patients with a PIK3CA mutation compared to wild-types (HR = 0.46 [95% CI: 0.25–0.87]; p = 0.017).

Expression profiles of mRNAs and miRs were integrated and resulted in signatures for PIK3CA status discovered by pathway (17 genes) and expression analyses (10 genes and 9 miRs). Validation and comparison with published signatures in the 28 independent tumors showed that our 10-genes signature had the highest PIK3CA prediction accuracy (75%), with 60% sensitivity and 78% specificity. Moreover, this signature better associates with TTP (HR = 0.38 [95% CI: 0.20–0.72]; P = 0.003).

Conclusion: Mutations in PIK3CA are not prognostic value in untreated ER-positive LNN patients, not predictive in ER-positive patients with advanced disease treated with first-line tamoxifen therapy, however, are predictive for favourable outcome after first-line AIs. Moreover, we propose an expression signature of 10 genes as a putative biomarker to predict the PI3K status and response to AIs.

Citation Information: Cancer Res 2012;72(24 Suppl):Abstract nr P3-06-01.