Use of Liquid Biopsy to Detect PIK3CA Mutation in Metastatic Breast Cancer

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Abstract
Background: PIK3CA is associated with tumor progression, and the prevalence of its mutation is high in breast cancer. Liquid biopsy offers convenient, non-invasive, and real-time insight into genetic alternation. In this study, we attempted to detect PIK3CA mutations in breast cancer patients through liquid biopsy.Methods: We recruited patients with histologically confirmed breast cancer with distant metastases between April 2020 and September 2020. Circulating DNA was extracted from plasma (ctDNA) and exosomes (exoDNA). PIK3CA mutations (exons 9 and 20) were analyzed by droplet digital PCR.Results: Of a total of 52 patients recruited, 16 had PIK3CA mutations in their tumor tissue or blood, which comprised 9 with exon 9 mutations (E542K and E545K) and 8 with exon 20 mutations (H1047L and H1047R). In 8 (15%) of the 52 patients, PIK3CA mutations were detected by liquid biopsies using ctDNA in 5 (9%), exoDNA in 6 (11%), and both ctDNA and exoDNA in 3 (6%). Of the 8 patients with PIK3CA mutations detected by liquid biopsies, 3 had no PIK3CA mutations in the primary tumors.Conclusions: PIK3CA mutations can be detected using liquid biopsy even in patients with no PIK3CA mutations in their primary tumors; thus, combination analysis using tissue and liquid biopsies can provide clinically useful information for patients with breast cancer.

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