Use of multiplex mutation genotyping to identify novel and protective mutations in breast cancer

11004 Background: We recently analyzed 590 primary invasive breast tumors for PIK3CA mutations by multiplex massARRAY genotyping. We identified a 32.5% PIK3CA mutation rate (manuscript submitted). Associations between PIK3CA mutation status and favorable pathology, such as lower grade tumors, lymph node negativity and hormone receptor (HR) positivity, were observed. Multiplex analysis allows the rapid assessment of mutations not commonly described in breast cancer. To identify activated pathways that may serve as predictive biomarkers, tissue microarrays (TMA) underwent immunohistochemical (IHC) analysis. Methods: 590 archival formalin-fixed paraffin embedded (FFPE) tumors > 1 cm were used to ensure adequate tissue procurement for mutation detection and TMA construction. MassARRAY (Sequenom) genotyping was performed on native DNA to identify PIK3CA, RAS, RET, and other known mutations. IHC staining of the TMAs included assessment of steroid nuclear receptors, HER2 expression, Akt pathway activation (including pAKT, p70S6K, eIF4E), and PTEN. Stained TMAs underwent digital quantitative image analysis and scoring (Aperio, Vista, CA). The Kaplan-Meier method and Cox models were used in univariate and multivariate analysis for associations of mutation status and clinicopathologic features on overall survival (OS) and breast cancer-specific survival (BCSS). Results: Compared to wild-type, PIK3CA mutated tumors (median f/u: 12.8 years) demonstrate a significant improvement in OS (p=0.03) and BCSS (p=0.004). For patients with any PIK3CA mutation, the improvement in BCSS is maintained in HR positive and ER negative subgroups. In addition to PIK3CA mutations, RAS and RET mutations are identified in a small proportion of breast tumors. Additional analysis to evaluate the association of mutation status and biomarker data derived from the TMA analysis will be performed for meeting presentation. Conclusions: We have recently defined the positive prognostic significance of PIK3CA mutations in breast cancer. RAS mutations are confirmed to occur rarely in breast cancer. The finding of RET mutations in breast cancer is novel. Future tumor biomarker identification directed towards predictive measurement will assist in tailoring therapy to appropriate patient populations. No significant financial relationships to disclose.