Epithelial cell polarity defects support cancer progression. It is thus crucial to decipher the functional interactions within the polarity protein network. Here we show that Drosophila Girdin and its human ortholog (GIRDIN) sustain the function of crucial lateral polarity proteins by inhibiting the apical kinase aPKC. Loss of GIRDIN expression is also associated with overgrowth of disorganized cell cysts. Moreover, we observed cell dissemination from GIR-DIN knockdown cysts and tumorspheres, thereby showing that GIRDIN supports the cohesion of multicellular epithelial structures. Consistent with these observations, alteration of GIRDIN expression is associated with poor overall survival in subtypes of breast and lung cancers. Overall, we discovered a core mechanism contributing to epithelial cell polarization from flies to humans. Our data also indicate that GIRDIN has the potential to impair the progression of epithelial cancers by preserving cell polarity and restricting cell dissemination.

27Epithelial cell polarity defects support cancer progression. It is thus crucial to decipher the 28 functional interactions within the polarity protein network. Here we show that Drosophila Girdin 29 and its human ortholog (GIRDIN) sustain the function of crucial lateral polarity proteins by 30 inhibiting the apical kinase aPKC. Loss of GIRDIN expression is also associated with 31 overgrowth of disorganized cell cysts. Moreover, we observed cell dissemination 32 from GIRDIN knockdown cysts and tumorspheres, thereby showing that GIRDIN supports the 33 cohesion of multicellular epithelial structures. Consistent with these observations, alteration 34 of GIRDIN expression is associated with a poor overall survival in subtypes of breast and lung 35 cancers. Overall, we discovered a core mechanism contributing to epithelial cell polarization 36 from flies to humans. Our data also indicate that GIRDIN has the potential to impair the 37 progression of epithelial cancers by preserving cell polarity and restricting cell dissemination. 38 39

The contribution of deregulated chromatin architecture, including topologically associated domains (TADs), to cancer progression remains ambiguous. CCCTC-binding factor (CTCF) is a central regulator of higher-order chromatin structure that undergoes copy number loss in over half of all breast cancers, but the impact of this defect on epigenetic programming and chromatin architecture remains unclear. We find that under physiological conditions, CTCF organizes subTADs to limit the expression of oncogenic pathways, including phosphatidylinositol 3-kinase (PI3K) and cell adhesion networks. Loss of a single CTCF allele potentiates cell invasion through compromised chromatin insulation and a reorganization of chromatin architecture and histone programming that facilitates de novo promoter-enhancer contacts. However, this change in the higher-order chromatin landscape leads to a vulnerability to inhibitors of mTOR. These data support a model whereby subTAD reorganization drives both modification of histones at de novo enhancer-promoter contacts and transcriptional up-regulation of oncogenic transcriptional networks.

Breast cancer remains a leading cause of cancer‐related death for women. The stepwise development of breast cancer through preinvasive to invasive disease is associated with progressive disruption of cellular and tissue organization. Apical–basal polarity is thought to be a barrier to breast cancer development, but the extent and potential mechanisms that contribute to disrupted polarity are incompletely understood. To investigate the cell polarity status of invasive breast cancers, we performed multiplex imaging of polarity markers on tissue cores from 432 patients from a spectrum of grades, stages and molecular subtypes. Apical–basal cell polarity was lost in 100% of cells in all cases studied, indicating that loss of epithelial polarity may be a universal feature of invasive breast cancer. We then analyzed genomic events from the TCGA dataset for an 18‐gene set of core polarity genes. Coamplification of polarity genes with established breast oncogenes was found, which is consistent with functional cooperation within signaling amplicons. Gene‐expression levels of several polarity genes were significantly associated with survival, and protein localization of Par6 correlated with higher grade, nodal metastasis and molecular subtype. Finally, multiple hotspot mutations in protein–protein interaction domains critical for cell polarity were identified. Our data indicate that genomic events likely contribute to pervasive disruption of epithelial polarity observed in invasive breast cancer.