Os06.2.a Fibroblasts Accelerate Cancer Growth in the Leptomeninges

BACKGROUND Solid tumor spread to the spinal-fluid-filled compartment surrounding the brain and spinal cord, or leptomeningeal metastasis (LM), represents a fatal complication of malignancy. How cancer within this unique microenvironment grows remains incompletely understood. MATERIAL AND METHODS To address this, we carried out proteomics of breast cancer patient spinal fluid in tandem with granular molecular dissection in immune-competent mouse models of LM. RESULTS Untargeted proteomics of human spinal fluid uncovered multiple inflammatory species present at significantly elevated concentrations in the setting of LM. Subsequent targeted proteomics of these specimens revealed a cluster of inflammatory mediators reminiscent of those generated by cancer-associated-fibroblasts, including IL-6 and IL-8. We hypothesized that leptomeningeal fibroblasts, while not canonical fibroblasts, might play the role of a CAF in LM. Indeed, we found that leptomeningeal fibroblasts support cancer cell growth in vitro, accelerate cancer cell migration, and promote cancer cell growth in vivo. To identify the key cancer cell-generated mediators that provoke this process, we carried out orthogonal proteomic and genomic screens, and identified the wnt-antagonist, DKK1. To understand how these leptomeningeal fibroblasts might respond similarly and differently from classical cancer-associated fibroblasts, we collected cancer cells and fibroblasts from models of breast and lung cancer LM as well as pancreatic cancer (a classical CAF-dependent malignancy) and subjected these to single cell RNAseq on 10X. In doing so, we found that the leptomeningeal fibroblasts and CNS fibroblasts more generally are transcriptionally distinct from classical pancreatic, lung, and mammary fat pad fibroblasts. Further, we uncovered transcriptionally distinct populations of leptomeningeal fibroblasts in the setting of LM: The first, a DKK1-responsive cell that provokes cancer cell proliferation; the second, a Wnt5a-responsive cell that promotes cancer cell migration. Genetic and antibody-based interruption of these pathways inhibited LM growth. CONCLUSION We have revealed novel pharmacologic targets for the treatment of LM. In doing so, we underline the distinct pathophysiology of cancer growth within the central nervous system.