Brain Metastases (BM) are the most common type of cerebral tumor in adult, occurring at a rate 10 times greater than that of primary brain cancers. The inherent abilities of a primary tumor cell capable of initiating a BM resembles that of a cancer stem cell (CSCs). We hypothesize that a subgroup of CSCs, termed brain metastasis-initiating cells (BMICs), are responsible for the initiation of BM and are identifiable by an exclusive subset of genes that regulate self-renewal and metastasis. Despite the prevalence and lethality of BM, there is no clinically relevant model that fully reflects metastasis in patients. We recently generated a novel human-mouse xenotransplantation model of BM that allows for interrogation of each phase of the metastatic process from lung to brain, through injection of human patient-derived BMICs into immunocompromised mice. Comprehensive interrogation of human BM using RNA interference screens with subsequent validation in our model identified SPOCK1 and TWIST2 as novel regulators of brain metastasis-initiating cell (BMIC) self-renewal and migration to the brain, respectively. A prospective cohort of primary lung cancer specimens showed SPOCK1 and TWIST2 over-expressed only in patients who ultimately developed BM. Protein-protein interaction network mapping identified novel BMIC regulatory genes with significant prognostic value in lung cancer patients. Specifically, the most significant connector between SPOCK1 and TWIST2, INHBA, a TGF-β ligand found mutated in lung adenocarcinoma, showed reduced expression in BMICs with knockdown of SPOCK1, defining a novel pathway between INHBA, SPOCK1 and TWIST2 and implicating the involvement of the TGF-β signaling pathway in BM development. Our development of a novel preclinical model of BM, through which we have identified several novel BMIC regulators, present potential therapeutic targets that could aid in blockage of the metastatic process, and transform a uniformly fatal systemic disease into a locally controlled and eminently more treatable one.
