PIK3CA-mutation profiling in patients with breast cancer, using a highly sensitive detection system.

Shimoi T1,2, Hamada A3, Yamagishi M4, Hirai M4, Yoshida M5, Nishikawa T1, Sudo K1, Shimomura A1, NoguchI E1, Yunokawa M1, Yonemori K1, Shimizu C1, Kinoshita T6, Fukuda T2,7, Fujiwara Y1, Tamura K1.

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Abstract

PIK3CA mutations are common activating mutations associated with breast cancer (occurring in 20-30% of all cases) and are potent predictive markers for responses to PI3K inhibitors. Thus, it is important to develop sensitive methods to detect these mutations. We established a novel detection method using a quenching probe (QP) system to identify PIK3CA mutations, using DNA from 309 breast cancer tissues. In a developmental cohort, we determined the optimal detection threshold of the QP system with human tumor DNA from 119 freshly frozen tumor samples. We found a 96% concordance rate with the QP system between DNA from 26 matching fresh-frozen specimens and FFPE specimens from the same patients, and known PIK3CA-mutation statuses in the developmental cohort. In a validation cohort, we evaluated whether the threshold for judging mutations via the QP system with frozen specimen-derived DNA was applicable with FFPE-derived DNA. In the validation cohort, 30 DNA samples from 190 FFPE-derived DNA samples with known PIK3CA-mutational statuses were analyzed by direct sequencing (DS) and droplet digital polymerase chain reaction (ddPCR), in a blinded manner. The sensitivity and specificity of the ddPCR results were 100% and 100% (QP system), and 60% and 100% (DS), respectively. We also analyzed the relationship between clinical outcomes and the PIK3CA mutational status of 309 breast cancer samples, including the developmental-cohort and validation-cohort samples. Multivariate analysis suggested that PIK3CA mutations, especially H1047R, were prognostic factors of relapse-free survival. Our novel detection system may be more useful than DS for detecting clinical PIK3CA mutations. This article is protected by copyright. All rights reserved.