Polycomb Repressive Complex 2 Confers BRG1 Dependency on the CIITA Locus

Area of Science:

• Immunology
• Molecular Biology
• Cancer Biology

Background:

• CIITA (MHC2TA) regulates MHC class II gene expression, crucial for immune response.
• Interferon-gamma (IFN-γ) responsiveness of CIITA depends on BRG1 (SMARCA4) within the SWI/SNF complex.
• SWI/SNF complex defects in cancer lead to IFN-γ resistance, mediated by remote regulatory elements.

Purpose of the Study:

• To identify the distal repressor responsible for BRG1 dependency in CIITA regulation.
• To investigate the role of Polycomb Repressive Complex 2 (PRC2) in mediating IFN-γ resistance in cancer.
• To explore PRC2 as a potential therapeutic target for restoring IFN-γ responsiveness.

Main Methods:

• Chromatin immunoprecipitation (ChIP) to detect PRC2 components (EZH2, SUZ12) and H3K27me3 at CIITA locus enhancers.
• Analysis of IFN-γ-induced changes in histone modifications and nucleosome positioning.
• Gene knockdown experiments (SUZ12) to assess functional restoration of IFN-γ responsiveness.
• Reporter assays to evaluate enhancer activity in BRG1-deficient cells.

Main Results:

• PRC2 components (EZH2, SUZ12) and H3K27me3 were detected at remote CIITA interenhancer regions.
• IFN-γ treatment reduced H3K27me3 in a BRG1-dependent manner, correlating with nucleosome displacement.
• SUZ12 knockdown rescued IFN-γ responsiveness in BRG1-null cells and promoted active histone marks.
• PRC2 was identified as the factor conferring BRG1 dependency on the CIITA locus.

Conclusions:

• PRC2 directly inhibits IFN-γ-induced CIITA expression, establishing BRG1 dependency.
• PRC2 may suppress immune surveillance by hindering CIITA expression in SWI/SNF-deficient cancers.
• Targeting PRC2 offers a potential strategy to re-sensitize tumors to IFN-γ therapy.