Can breast cancer cells be distinguished from blood cells by mechanical parameters? A label-free CTC detection approach

Background:

Even years after successful treatment of the primary tumor about one third of breast cancer patients are suffering from metastatic relapse. One reason might be hematogenous spread during early disease stages when isolated tumor cells change their physical properties from epithelial to mesenchymal features (EMT) and become able to disseminate from the primary tumor site. After entering the lymphatic system and the blood circulation they can travel to distant organs and seed metastases. Therefore, circulating tumor cells (CTCs) might be interesting and easy accessible surrogate markers to monitor disease progression and treatment response in the clinical setting.

Methodology:

One approach might be the detection of CTCs in the blood based on mechanical properties such as deformability using an optical stretcher device. In an optical rheometer, cells are deformed noninvasively by a dual beam trap. Thus, the softness of a single cell can be measured. PBMC were isolated from the peripheral blood of a healthy donor using density gradient centrifugation. Breast cancer cells MDA-MB 231 were cultured under standard conditions and harvested prior to analysis using trypsine. Both cell suspensions were applied separately to the optical stretcher (RS Zelltechnik, Leipzig) and rheological parameters such as deformability and stiffness were measured.

Results:

Distinct cellular profiles could be obtained from hematopoetic cells and breast cancer cells, respectively. Relative deformation was significantly different between both cell types. Furthermore, 10:1 mixture of PBMC and MDA-MB 231 cells were analyzed with the optical stretcher. Results indicated that the mixed samples could be sorted into subpopulations of significantly different cellular stiffness based on mechanical parameters such as cell size.

Conclusion & Outlook:

Further experiments to test the usability of the optical stretcher for possible CTC detection in peripheral blood of breast cancer patients are currently ongoing. Since CTCs are very rare events in patient's blood samples, mixed cell suspensions consisting of PBMC with different numbers of spiked tumor cells will be prepared to mimic clinical samples and to detect sensitivity and specificity of this method. Once significant differences between cellular profiles of the mixed samples with low numbers of spiked tumor cells can be measured, the methodology might be transferred to clinical samples in order to develop a liquid biopsy test.