Development of High Throughput Single-Cell Analysis System for Circulating Tumor Cells Based on Digital Micromirror Device
Authors: Negishi Ryo , Saito Hyuga , Tanaka Tsuyoshi , Yoshino Tomoko 
Published: December 2020

Circulating tumor cells (CTCs) are widely known as useful biomarkers in the liquid biopsies of cancer patients. Recently, in addition to counting the number of CTCs, genetic analysis of CTCs provides critical insights into cancer metastases and add detailed clinical information that enhances patient care. It is widely known that CTCs are genetically heterogeneous, the molecular characterization of CTCs should be performed at the single-cell level. In fact, several studies have identified inter- and intra-patient heterogeneity in the mutational status of CTCs in metastatic breast and lung cancers. These studies also reported that CTCs related to drug resistance are novel therapeutic targets. Thus, genetic analysis of single CTCs has the potential to be widely applicable in personalized medicine and drug discovery. However, CTCs are extremely rare cells that only 1-100 cells are contained in 1 ml of blood, so the recovery of such extremely rare CTCs from the peripheral blood is technically challenging.
So far, we have developed a CTC recovery system using a microcavity array (MCA), which demonstrates highly efficient recovery of cancer cells based on differences in cell size and deformability. Furthermore, we have proposed a novel cell manipulation method through the visualization of single cells through hydrogel encapsulation for subsequent genetic analysis of single CTCs. Subsequently, recovered CTCs can be subjected to a hydrogel-encapsulation for the following genetic analysis of single cells. Clinical studies indicated that the genetic analysis of CTCs from lung, pancreatic and gastric cancer patients was successfully achieved. However, this hydrogel encapsulation has limitations in throughput. In this study, we demonstrate a high-throughput single-cell analysis for CTCs using a multiple single-cell encapsulation system with a digital micromirror device (DMD).
The novel multiple single-cell encapsulation systemwas composed of two optical systems: the wide-field fluorescence imaging system and the photopolymerization system equipped with DMD. The wide-field fluorescence imaging system using a CMOS sensor was designed to visualize 2D fluorescence imaging of the entire MCA (6.0 × 6.0 mm2). We evaluated the sensitivity of fluorescence detection of the wide-field fluorescence imaging system using the calibration slide. The limit of detection of the wide-field fluorescence imaging system was 2.1 × 102 molecules-Cy3/μm2, which is comparable to conventional fluorescence microscopy (1.0 × 102 molecules-Cy3/μm2). Generally, the expression of cellular marker protein, such as cytokeratin, was estimated to be 1.3 × 103 molecules/μm2. Therefore, our proposed system had enough sensitivity for the detection of antibody-stained CTCs. Furthermore, this wide-field fluorescence imaging system for CTC detection enables the rapid visualization of all stained cells within the field of the MCA via one-shot imaging. This rapid detection method also allows for the rapid completion of single-CTC manipulation.
The photopolymerization system for single-cell encapsulation was controlled by the DMD. The curing light (λmax = 365 nm) modulated by the DMD was projected onto an MCA for hydrogel generation, and single-cell isolation by hydrogel-photopolymerization was examined at the optimized conditions. After recovery of cancer cells from whole blood onto MCA and staining by the fluorescent-labeled antibodies, cancer cells were identified by the wide-field imaging system. Then light irradiation (3 sec) was performed by the photopolymerization system. To minimize the area of the hydrogel reserved for a single cell, we newly designed the convex-type hydrogel. In the spike-in experiment, single-cell isolation rates were 97.6% (41/42) for NCI-H1975 cells (non-small cell lung cancer cell) and 94.4% (51/54) for NCI-N87 (gastric cancer cell) that were comparable with our previous study using fluorescence microscopy. With this proposed system, more than 50 single cells were encapsulated simultaneously within 1 min and isolated with an efficiency. Furthermore, single cells were successfully isolated from adjacent cells on the same microcavity without any contamination. Single cancer cell was partially encapsulated on the hydrogel, and the partial encapsulation allows us to subject them to whole genome amplification (WGA). Light irradiation and photopolymerizaion did not affect the quality of the amplified WGA products for single-cell genomic analyses. We are currently investigating single cell genetic analysis of metastatic cancer patients, and this developed multiple single-cell encapsulation system will improve the high throughput single-cell genetic analysis of CTCs in clinical samples. Our system provides an attractive application for other targets such as adherent cells, tissue samples, and microorganisms, in addition to widespread use in the isolation of CTCs for liquid biopsy.