Identifying dormant cells in colorectal cancer spheroids

Cellular dormancy and heterogeneous cell cycle lengths provide important explanations for treatment failure following adjuvant therapy with S-phase cytotoxics in colorectal cancer (CRC) yet the molecular control of the dormant versus cycling state remains unknown. In CRCs dormant cells are found to be highly clonogenic and resistant to chemotherapies. We sought to understand the molecular features of dormant CRC cells to facilitate rationale identification of compounds to target both dormant and cycling tumour cells. Six colorectal cancer cell lines (DLD1, HCT15, HT55, SW948, RKO and SW48) were labelled with the cell permeable dye CFSE and then grown in non-adherent spheroid culture for 6 days to enable identification of dormant cells that retain CFSE (LRC) and cycling cells (BULK). LRCs and BULK populations were then FACS sorted from each cell line in quadruplicate. As a control experiment, to identify off-target effects of the CFSE dye and culture artefacts, BULK populations from DLD1 cells at d1 and d6 after seeding both with and without CFSE labelling were included in the RNAseq analysis. RNA was extracted using the RNAeasy Micro Plus kit (Qiagen) and quantified using the Qubit RNA Assay Kit (Thermo Fisher Scientific). RNA quality was assessed using the Agilent Bioanalyser system as per manufacturer’s instructions. Following normalisation and sample randomisation, Truseq library (Illumina) preparation was carried out at the CRUK CI genomics facility and subsequent single end, 50bp sequencing using the HiSeq system (Illumina). Following human genome alignment (hg19), read counts were normalised and differential expression tested using the DEseq protocol.