EZH2 enables germinal center formation through epigenetic silencing of CDKN1A and an Rb-E2F1 positive feedback loop

The EZH2 histone methyltransferase is required for B cells to form germinal centers (GCs). Here we show that EZH2 mediates GC formation through repression of cyclin-dependent kinase inhibitor CDKN1A (p21Cip1). Deletion of Cdkn1a rescued the GC reaction in Ezh2 knockout mice. To study the effects of EZH2 in primary GC B cells we generated and validated a 3D B cell follicular organoid system that mimics the endogenous GC reaction. Using this system we found that depletion of EZH2 suppressed G1 to S phase transition of GC B cells in a Cdkn1a dependent manner. GC B cells of Cdkn1a;Ezh2 double knockout mice exhibited high levels of phospho Rb, indicating that loss of Cdkn1a allows progression of cell cycle. Moreover, we show that the transcription factor E2F1 plays a major role in inducing EZH2 upregulation during the GC reaction. E2F1 deficient mice manifest impaired GC responses, which was rescued by restoring EZH2 expression, thus defining a positive feedback loop whereby EZH2 controls GC B cell proliferation by suppressing CDKN1A, allowing cell cycle progression with a concomitant phosphorylation of Rb and release of E2F1. Overall design: gene expression profiles of murine B cells

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Béguelin W, Rivas MA, Calvo Fernández MT, Teater M, Purwada A, Redmond D, Shen H, Challman MF, Elemento O, Singh A, Melnick AM