SUMMARY Clusters of circulating tumor cells (CTC-clusters) are present in the blood of patients with cancer but their contribution to metastasis is not well defined. Using mouse models with tagged mammary tumors, we demonstrate that CTC-clusters arise from oligoclonal tumor cell groupings and not from intravascular aggregation events. Although rare in the circulation compared with single CTCs, CTC-clusters have 23-50-fold increased metastatic potential. In patients with breast cancer, single-cell resolution RNA sequencing of CTC-clusters and single CTCs, matched within individual blood samples, identifies the cell junction component plakoglobin as highly differentially expressed. In mouse models, knockdown of plakoglobin abrogates CTC-cluster formation and suppresses lung metastases. In breast cancer patients, both abundance of CTC-clusters and high tumor plakoglobin levels denote adverse outcomes. Thus, CTC-clusters are derived from multicellular groupings of primary tumor cells held together through plakoglobin-dependent intercellular adhesion, and while rare, they greatly contribute to the metastatic spread of cancer.

Cancer cells metastasize through the bloodstream either as single migratory circulating tumor cells (CTCs) or as multicellular groupings (CTC-clusters). Existing technologies for CTC enrichment are designed primarily to isolate single CTCs, and while CTC-clusters are detectable in some cases, their true prevalence and significance remain to be determined. Here, we developed a microchip technology (Cluster-Chip) specifically designed to capture CTC-clusters independent of tumor-specific markers from unprocessed blood. CTC-clusters are isolated through specialized bifurcating traps under low shear-stress conditions that preserve their integrity and even two-cell clusters are captured efficiently. Using the Cluster-Chip, we identify CTC-clusters in 30–40% of patients with metastatic cancers of the breast, prostate and melanoma. RNA sequencing of CTC-clusters confirms their tumor origin and identifies leukocytes within the clusters as tissue-derived macrophages. Together, the development of a device for efficient capture of CTC-clusters will enable detailed characterization of their biological properties and role in cancer metastasis.

The KDM5/JARID1 family of Fe(II)-and ␣-ketoglutarate-dependent demethylases remove methyl groups from tri-and dimethylated lysine 4 of histone H3. Accumulating evidence from primary tumors and model systems supports a role for KDM5A (JARID1A/RBP2) and KDM5B (JARID1B/PLU1) as oncogenic drivers. The KDM5 family is unique among the Jumonji domain-containing histone demethylases in that there is an atypical insertion of a DNA-binding ARID domain and a histone-binding PHD domain into the Jumonji domain, which separates the catalytic domain into two fragments (JmjN and JmjC). Here we demonstrate that internal deletion of the ARID and PHD1 domains has a negligible effect on in vitro enzymatic kinetics of the KDM5 family of enzymes. We present a crystal structure of the linked JmjN-JmjC domain from KDM5A, which reveals that the linked domain fully reconstitutes the cofactor (metal ion and ␣-ketoglutarate) binding characteristics of other structurally characterized Jumonji domain demethylases. Docking studies with GSK-J1, a selective inhibitor of the KDM6/ KDM5 subfamilies, identify critical residues for binding of the inhibitor to the reconstituted KDM5 Jumonji domain. Further, we found that GSK-J1 inhibited the demethylase activity of KDM5C with 8.5-fold increased potency compared with that of KDM5B at 1 mM ␣-ketoglutarate. In contrast, JIB-04 (a paninhibitor of the Jumonji demethylase superfamily) had the opposite effect and was ϳ8-fold more potent against KDM5B than against KDM5C. Interestingly, the relative selectivity of JIB-04 toward KDM5B over KDM5C in vitro translates to a ϳ10 -50-fold greater growth-inhibitory activity against breast cancer cell lines. These data define the minimal requirements for enzymatic activity of the KDM5 family to be the linked JmjNJmjC domain coupled with the immediate C-terminal helical zinc-binding domain and provide structural characterization of the linked JmjN-JmjC domain for the KDM5 family, which should prove useful in the design of KDM5 demethylase inhibitors with improved potency and selectivity.