SCF-β-TrCP E3 Ubiquitin Ligase-Mediated Proteolysis Of EZH2 Regulates H3K27 Trimethylation Activity In Germinal Center-Derived B-Cell Lymphomas

EZH2 is a critical enzymatic subunit of the PRC2 which trimethylates histone H3 (H3K27) to mediate gene repression. While overexpression of EZH2 and its increased H3K27 methylation have generally been associated with hematologic malignancies and solid tumors, recurrent gain of function somatic mutations in EZH2 have been recently reported in germinal center B-cell (GCB) derived lymphomas. These mutations, which replace a single tyrosine in the SET domain of the EZH2 protein (Y641), occur in 7.2% of FLs (follicular lymphoma) and 21.7% of GCB DLBCL (diffuse large B-cell lymphoma) and are absent from ABC DLBCLs. Consequently, B-cell lymphoma cell lines and lymphoma primary samples harboring heterozygous EZH2^Y641mutations exhibit increased levels of histone H3 Lys27 trimethylation (H3K27me3) and promote a lymphoproliferative phenotype with expansion of the GC B cell compartment.

Although extensive studies have been conducted to understand the role of polycomb group of proteins in cancer pathogenesis, few studies have been aimed at understanding the post-translational regulation of these proteins. Skp1/Cullin1/F- box protein (SCF) ubiquitin ligase complexes are the largest family of multicomponent ring E3 ubiquitin ligases that control the degradation of many important regulatory proteins and have been implicated in the regulation of epigenetic regulators. However, the mechanism of EZH2 degradation and its implication in PRC2 mediated methyl transferase activity in a cancer-specific context is not fully understood.

Cullin-ring ubiquitin ligases (CRLs) form the largest known class of multicomponent E3 ubiqutin ligase family in eukaryotes and target a wide array of substrates involved in several biological processes. To investigate a CRL-mediated basis for post-translational EZH2 regulation, we screened a panel of cullin family E3 ligases for interaction with EZH2 and discovered that EZH2 was immunoprecipitated by cullin1 exclusively among all other cullins (cul2,3,4a,4b,5 and 7). We engineered and expressed a cullin1 dominant negative protein which resulted in EZH2 accumulation in a dose dependent manner. Using co-immunoprecipitation approach, we screened 8 different F box proteins and observed that SCF E3 ubiquitin ligase β-TrCP (FBXW1) uniquely and specifically interacted with EZH2. Further, RNAi-mediated silencing of β-TrCP resulted in EZH2 stabilization with attendant increase in H3K27 trimethylation activity. Importantly, the Y641 mutants recurrently identified and relevant in lymphoma pathogenesis were unable to bind β-TrCP. Further, cycloheximide chase experiments showed that Y641 mutants endogenously expressed in lymphoma-derived cells exhibit increased EZH2 stability as well as enhanced H3K27 trimethylation activity.

Our studies demonstrate that β-TrCP plays an important role in controlling H3K27 trimethylation activity via an unusual recognition mechanism targeting EZH2 for proteasomal degradation. Our findings delineate a novel β-TrCP/EZH2 axis requiring an intact Y641 residue for EZH2 regulation by β-TrCP. Oncogenic mutations in Y641 in GCB-cell derived lymphomas confer increased stability to EZH2 and render it resistant to β-TrCP-mediated degradation. This newly identified mechanism might help in design potential novel therapeutic approaches for clinical management of cancer potentially driven by aberrant accumulation/function of EZH2.