Circulating tumor cells (CTCs) are shed from primary tumors into the bloodstream, mediating the spread of cancer to other organs. Little is known about these cells, partly because they are rare (∼1–10 tumor cells per 10 billion normal blood cells in 1 ml of blood) and difficult to isolate for molecular analysis.

David Ting (Massachusetts General Hospital (MGH) Cancer Center, Boston) and his team have previously attempted to study pooled CTCs enriched from the blood, but this approach only makes it possible to identify the most highly expressed genes in the pooled cells and cannot be used to distinguish genetic differences within the cell population.

But now Ting and his colleagues have developed a new technique for isolating CTCs for transcriptional analysis. They demonstrate this technique in a new study, published in Cell Reports (8, 1905–1918; 2014). The researchers isolated 168 single CTCs from the bloodstreams of five pancreatic tumor–bearing mice. Of these cells, 75 had RNA of sufficient quality to proceed with next-generation RNA sequencing. The researchers noted that the remaining cells with damaged RNA were likely nonviable cancer cells and that the damage was not the result of the CTC isolation process. Ting's team compared the genome-wide expression profiles of these viable CTCs with matched primary tumors from mouse models as well as human CTCs of pancreatic, breast and prostate cancers. Three different subsets of CTCs were identified, all of which were distinct from the sequences of the mouse primary tumors and human cancer cell lines. Extracellular matrix genes, which are important for the establishment of metastases, were highly expressed in the CTCs.

The device developed by Ting's team, called the CTC-iChip, enables the isolation of all CTCs in a blood sample using a process called epitope-independent microfluidic capture. The first step was hydrodynamic size-based separation of all nucleated cells away from red blood cells, platelets and plasma. The next step of the process excluded the vast majority of white blood cells from the sample so that only CTCs remained. The advantage of this approach is that it leaves the CTCs in solution, rendering them more suitable for advanced RNA sequencing techniques to reveal the gene expression patterns of each individual cell.

Said Ting in a press release, “Our ability to combine a novel microfluidic CTC isolation device, developed here at MGH, with single-cell RNA sequencing has given us new biological insights into these cells and revealed novel avenues to try and block the spread of cancer.”