Mutant PIK3CA Is Detected in Both Pre-Invasive and Recurrent Breast Cancer.

BackgroundWe identified somatic PIK3CA mutations in 32.5% of 590 primary invasive breast cancers (BC) (manuscript in press: Clinical Cancer Research). Detected by massARRAY genotyping, PIK3CA mutations significantly associate with favorable clinicopathologic features and improved clinical outcome, including overall and breast cancer-specific survival. Given the strong association between PIK3CA mutations and hormone receptor (HR) positivity, one hypothesis is that PIK3CA mutations 9drive9 HR positive BC and will be detected in pre-invasive breast tumors. As PIK3CA mutations offer a protective effect in BC, it has not been determined whether PIK3CA mutations are selected for in disease progression or whether additional collaborating mutations are required.MethodsTo determine the concordance of PIK3CA mutations and assess the acquisition of additional oncogene mutations, available matched tissue samples, from the previous database, were procured and underwent massARRAY genotyping (n = 83). Two mm cores were macro-dissected from matched formalin-fixed, paraffin embedded tissue, including normal breast tissue, benign lymph nodes (LN), ductal carcinoma-in-situ (DCIS), regional LN metastases (mets), and distant mets. MassARRAY (Sequenom) genotyping was performed on native DNA to identify rare and hotspot PIK3CA mutations, as well as AKT1 (E17K), RAS, and RET mutations.ResultsConcordance of PIK3CA mutations is noted between primary BC and DCIS, except for one rare PIK3CA mutation (Q546R) not detected in DCIS (4/5). No PIK3CA mutations are detected in normal breast tissue (0/8) or benign LN (0/6). Rare and hotspot PIK3CA mutations in primary BC are detected in 87.5% (7/8) matched regional LN met and 80 % (4/5) distant mets. For the patient with the disconcordant regional LN met, the rare PIK3CA mutation (H1047L) identified in primary BC is also present in the distant met site. Notably, for the single disconcordant PIK3CA met site, a rare PIK3CA mutation (E545A) is detected in a primary tumor, regional LN met, and bone met and is absent in a second bone met, in which a KRAS (G12C) mutation has been identified. One concomitant hotspot PIK3CA (E542K)/KRAS (G12C) mutation is present in both primary BC and paired DCIS. Complete concordance of AKT1 (E17K) mutations has been identified between in DCIS, primary BC, regional LN met, and distant mets (n=4).ConclusionsWe recently defined the positive prognostic significance of PIK3CA mutations in breast cancer. PIK3CA and AKT1 (E17K) mutations are early events in breast cancer, occurring in pre-invasive tumors. Despite the protective effect of PIK3CA mutations on clinical outcome, they persist and are selected for in disease progression, as they are detected in regional LN and distant mets. Complete concordance is identified between hotspot PIK3CA mutated primary BC and matched tumors samples, suggesting that PIK3CA mutations with higher oncogenic potency are maintained in tumor progression. These findings support that targeting the PI3K pathway may assist in tailoring therapy to appropriate patient populations. We are expanding matched tissue collection from a separate patient cohort to further assess genomic and functional genomic change with disease progression. Citation Information: Cancer Res 2009;69(24 Suppl):Abstract nr 5164.