Circulating Tumor Cell Clustering Triggers DNA Methylation Changes that Enable Metastasis Seeding

The ability of circulating tumor cells (CTCs) to form clusters is key to accelerate the metastatic process. Yet, biological features and vulnerabilities of CTC-clusters remain largely unknown. Here, we profile the DNA methylation landscape of single CTCs and CTC-clusters, matched within individual breast cancer patients and mouse models. We find that clustering of CTC leads to a remarkable hypomethylation of binding sites for stemness-related transcription factors. Through a screen with 2'486 FDA-approved compounds, we identify drugs that are able to dissociate CTC-clusters into single cells, leading to DNA re-methylation at critical sites and to a reduction in metastasis-seeding ability. Thus, clustering of CTCs favors accessibility to binding sites of stemness-related transcription factors, and cluster-dissociating compounds enable to suppress the spread of cancer.