Cohesin protein STAG2 regulates expression of IRF9 and interferon signaling in melanoma via enhancer loop reprogramming [HiChIP]

The cohesin complex participates in the organization of 3D genome through generating and maintaining DNA loops. Stromal antigen 2 (STAG2), a core subunit of the cohesin complex, is frequently mutated in various cancers. However, the impact of STAG2 inactivation on 3D genome organization and subsequent gene expression in cancer remain poorly understood. Here we show that depletion of STAG2 in melanoma cells leads to expansion of topologically associating domains (TADs) and enhances the formation of H3K27Ac-associated DNA loops at sites where binding of STAG2 is switched to its paralog STAG1. We further identify Interferon Regulatory Factor 9 (IRF9) as a major direct target of STAG2 in melanoma cells via integrated RNA-Seq, STAG2 ChIP-Seq and H3K27Ac HiChIP analyses. We demonstrate that loss of STAG2 activates IRF9 through modulating the 3D genome organization, which in turn enhances type I interferon signaling and increases the expression of PD-L1. Our findings not only establish a previously unknown role of the STAG2 to STAG1 switch in 3D genome organization, but also reveal a functional link between STAG2 and interferon signaling in cancer cells, which may contribute to malignant phenotype in STAG2-mutant cancer. We investigated how STAG2 KD affact 3D genome structure and its related functional consequnece in malignant phenotype in STAG2-mutant melanomas.