Heterogeneity of PIK3CA mutational status at the single cell level in circulating tumor cells from metastatic breast cancer patients

Circulating Tumor Cells (CTCs) represent a "liquid biopsy of the tumor" which might allow real-time monitoring of cancer biology and therapies in individual patients. CTCs are extremely rare in the blood stream and their analysis is technically challenging. The CellSearch(®) system provides the enumeration of CTCs with prognostic significance in patients with metastatic breast cancer (mBC), but it does not allow their molecular characterization, which might be useful to identify therapeutically relevant targets for individualized treatment. Combining the CellSearch(®) and DEPArray™ technologies allows the recovery of single CTCs as a pure sample for molecular analysis. The purpose of the study was to investigate the heterogeneity of PIK3CA mutational status within single CTCs isolated from individual mBC patients. CTCs were enriched and enumerated by CellSearch(®) in blood samples collected from 39 mBC patients. In 20 out of 39 patients enriched samples with ≥5 CTCs were sorted using DEParray™ to isolate single CTCs or pools of CTCs to be submitted to Whole Genome Amplification (WGA) before sequencing analysis. In 18 out of 20 patients, it was possible to perform PIK3CA sequencing on exons 9 and 20. Twelve subjects were wild type (wt) for the PIK3CA gene. PIK3CA status could also be assessed in pools of CTCs in seven of these patients, with consistent wt status found. Six patients (33%) had a PIK3CA mutation identified. In 2 of the six patients, molecular heterogeneity was detected when mutational analysis was performed on more than one single CTC, including one patient with loss of heterozygosity on both single and pooled CTCs, and one patient with three different PIK3CA variants on single CTCs but PIK3CA wt status on pooled CTC samples. In six out of the 18 cases PIK3CA status was also evaluable on a primary tumor sample. In one of the six cases a discordance in PIK3CA status between the primary (wild-type) and the matched CTC (exon 20 mutation) was observed. This study demonstrates the feasibility of a non-invasive approach based on the liquid biopsy in mBC patients. Moreover, our data suggest the importance of characterizing CTCs at the single cell level in order to investigate the molecular heterogeneity within cells from the same patient.