Claudin 7 suppresses invasion and metastasis through repression of a smooth muscle actin program

Metastasis initiates when cancer cells escape from the primary tumor, which requires changes to intercellular junctions. Claudins are transmembrane proteins that form the tight junction, and their expression is reduced in aggressive breast tumors. However, claudins’ roles during breast cancer metastasis remain unclear. We used gain- and loss-of-function genetics in organoids isolated from murine breast cancer models to establish that Cldn7 suppresses invasion and metastasis. Transcriptomic analysis revealed that Cldn7 knockdown induced smooth muscle actin (SMA)-related genes and a broader mesenchymal phenotype. We validated our results in human cell lines, fresh human tumor tissue, bulk RNA-seq, and public single cell RNA-seq data. We consistently observed an inverse relationship between Cldn7 expression and expression of SMA-related genes. Furthermore, knockdown and overexpression of SMA-related genes demonstrated that they promote breast cancer invasion. Our data reveal that Cldn7 suppresses breast cancer invasion and metastasis through negative regulation of SMA-related and mesenchymal gene expression. To investigate the role of Cldn7 on invasion and metastasis, we used organoids derived from genetically engineered mouse models (GEMMs) to knockdown Cldn7 by using shRNA lentiviral particles. To test the effect of Cldn7 depletion in invasion, we embedded Cldn7 knockdown and control organoids in 3D collagen matrices for 5 days. After invasion for 5 days, we then performed transcriptomics analysis upon Cldn7 knockdown using RNA-Seq.