Detection of the <i>PIK3CA</i> Mutation in Circulating Tumor DNA as a Possible Predictive Indicator for Poor Prognosis of Early-Stage Breast Cancer

Objectives: Circulating tumor DNA (ctDNA) is shown to provide the real-time genomic information of metastatic breast cancer. This study elucidates the clinico-pathological significance of ctDNA in early-stage breast cancer using the PIK3CA mutation as an indicator. Materials and Methods: Twenty-seven primary breast cancers without metastasis were surgically resected and pathologically diagnosed at the University of Tokyo Hospital, Japan. Genomic DNA of primary tumor was extracted from formalin-fixed and paraffin-embedded specimens. ctDNA was extracted from fresh-frozen plasma from patients. The PIK3CA mutations at E542K, E545K and H1047R were examined by Sanger sequencing or droplet digital PCR in 27 tumors and pre- and post-surgery plasma. Results: The PIK3CA mutations were detected in 13 (48%) of 27 primary tumors. These mutations did not significantly correlate with specific clinico-pathological characteristics of tumors. When ctDNA was examined, 4 (33%) of 12 cases carrying the mutated PIK3CA showed the identical mutation in pre-surgery plasma and 2 (50%) of them showed the identical mutations in post-surgery plasma. Interestingly, in these 2 cases in pathological stages IIIA and IA, fractional abundance of the mutated PIK3CA alleles to the total alleles in pre-surgery ctDNA was around 1% or more and was higher than that of the other two cases without PIK3CA mutations in post-surgery ctDNA. Conclusions: The PIK3CA mutation in ctDNA is detectable even in a subset of early-stage breast cancer. Furthermore, fractional abundance of the mutated PIK3CA in pre-surgery ctDNA could provide a possible predictive indicator for tumor burden and for choosing the appropriate adjuvant treatment of breast cancer.