Combined genome and transcriptome analysis of single disseminated cancer cells from bone marrow of prostate cancer patients reveals unexpected transcriptomes

Bone is the most frequent site of metastasis in prostate cancer (PCa) and patients with bone metastases are deemed incurable. Targeting prostate cancer cells that disseminated to the bone marrow (BM) prior to surgery and before metastatic outgrowth may therefore prevent lethal metastasis. This prompted us to directly analyse the transcriptome of disseminated cancer cells (DCC) isolated from non-metastatic (UICC stage M0) prostate cancer patients. We screened 105 BM samples of M0-stage prostate cancer patients and 18 BM samples of patients without malignancy for the presence of EpCAM+ single cells. In total we isolated 270 cells from both groups by micromanipulation and globally amplified their mRNA. We used targeted transcriptional profiling to unambiguously identify DCCs for subsequent in-depth analysis. Transcriptomes of all cells were examined for the expression of EPCAM, KRT8, KRT18, KRT19, KRT14, KRT6a, KRT5, KLK3 (PSA), MAGEA2, MAGEA4, PTPRC (CD45), CD33, CD34, CD19, GYPC, SCL4A1 (band 3), and HBA2. Using these transcripts we found it impossible to reliably identify true DCCs. We then applied combined genome and transcriptome analysis of single cells and found that EpCAM+ cells from controls expressed transcripts thought to be epithelial-specific, while true DCCs may express haematopoietic transcripts. These results point to an unexpected transcriptome plasticity of epithelial cancer cells in bone marrow and question common transcriptional criteria to identify DCCs.