Upregulation of the MBM CTC Signature and CTC:B Cell Clusters Associate with Secondary Liver Metastasis: a Melanoma Brain-Liver Metastasis Axis [RNA-Seq]

Melanoma brain metastasis (MBM) is linked to dismal prognosis, low overall survival, and is detected in up to 80% of patients at autopsy. Circulating tumor cells (CTCs) are the smallest functional units of cancer and precursors of fatal metastasis. We previously employed an unbiased multilevel approach to discover a unique ribosomal protein large/small subunits (RPL/RPS) CTC gene signature associated with MBM. Here, we hypothesized that CTC-driven MBM secondary metastasis (?metastasis of metastasis? per clinical scenarios), has targeted organ specificity for liver. We injected parallel cohorts of immunodeficient and newly-developed humanized NBSGW (HuNBSGW) mice with cells from CTC-derived MBM to identify secondary metastatic patterns. We found the presence of a melanoma brain-liver metastasis axis in humanized NBSGW mice. Further, RNA-Seq analyses of tissues showed a significant upregulation of the RPL/RPS CTC gene signature linked to metastatic spread to liver. Additional RNA-Seq of CTCs from HuNBSGW blood revealed extensive CTC clustering with human B cells in these mice. CTC:B cell clusters were also upregulated in blood of primary melanoma patients, and maintained either in CTC-driven MBM or MBM CTC-derived cells promoting liver metastasis. CTC-generated tumor tissues were interrogated at single-cell gene and protein expression levels (10x Genomics Xenium and HALO spatial biology platforms, respectively). Collectively, our findings suggest that heterotypic CTC:B cell interactions can be critical at multiple stages of metastasis. This study provides important insights for relevance of pro-metastatic CTC:B cell clusters extending from primary metastatic disease, and identifies new targets for therapies of clinical metastasis to improve patient care. Mouse tissues were isolated during necropsies and frozen. They were thawed, homogenized, and RNA was extracted using Qiagen RNA kit. RNA was sent for RNA seq to the ATG core.