EPOP

EPOP (C17orf96) is an accessory, PRC2-specific subunit that fine-tunes Polycomb repressive complex 2 activity at CpG island promoters, balancing repression against active transcription rather than acting as a simple silencer (PMID:24550272, PMID:27462409). It is a strong, mutually exclusive PRC2 subunit (exclusive of C10orf12) that co-localizes genome-wide with EZH2 and H3K27me3 (PMID:24550272). Mechanistically, EPOP physically bridges PRC2 to the transcription elongation factor Elongin BC and to the deubiquitinase USP7, recruiting these factors to bivalent and actively transcribed targets to keep target genes expressed at low but non-zero levels (PMID:27863225, PMID:27863226); the EPOP–Elongin BC interface is defined structurally, with the conserved BC-box leucine (Leu40) embedded in the hydrophobic pocket of Elongin C (PMID:40153999). EPOP exerts opposing activities on PRC2: in vitro it stimulates PRC2 histone methyltransferase activity and promotes chromatin binding in a GCN-rich, DNA-sequence-dependent manner, depositing H3K27me3 de novo in cooperation with MTF2 and JARID2 (PMID:41650228), while in cells it restrains PRC2.1 by disrupting complex dimerization and reducing avidity-driven chromatin association, an inhibitory function that is PRC2-dependent but largely independent of Elongin BC (PMID:39314288, PMID:41519789). This restraint of PRC2.1 over-targeting is required in vivo: Epop-knockout mice are viable but show highly penetrant posterior homeotic transformations arising from premature, anteriorly shifted Hox gene activation in the presomitic mesoderm (PMID:40834979).

1. 2014 High

   Establishing that C17orf96/EPOP is a bona fide PRC2 subunit answered whether this uncharacterized protein belongs to the Polycomb machinery and defined its mutual exclusivity with C10orf12.

   Evidence BioTAP-XL cross-linking affinity purification with mass spectrometry and ChIP-seq in cells

   PMID:24550272

   Open questions at the time

   • Did not resolve whether EPOP activates or restrains PRC2
   • No structural or domain-level interaction map
   • Functional consequence of incorporation untested
2. 2015 Medium

   Genome-wide depletion revealed that EPOP has opposite effects at PRC2-rich versus active CpG islands, establishing it as a context-dependent modulator rather than a uniform repressor or activator.

   Evidence ChIP-seq with depletion in mouse ES cells reading H3K27me3, Suz12, Pol II and H3K4me3

   PMID:27462409

   Open questions at the time

   • Molecular basis of the antagonism at PRC2-rich sites unresolved
   • Direct partners mediating active-transcription role not yet identified
   • Single lab
3. 2016 High

   Identifying Elongin BC and USP7 as EPOP partners showed how EPOP physically couples PRC2 to transcription elongation and deubiquitination machinery to maintain low expression at target genes.

   Evidence Reciprocal co-immunoprecipitation, genome-wide ChIP-seq, and knockdown expression/proliferation assays in mouse ESCs and human cancer lines (two concurrent papers)

   PMID:27863225 PMID:27863226

   Open questions at the time

   • Atomic basis of the EPOP–Elongin BC interface not defined
   • Functional role of USP7 recruitment not mechanistically dissected
   • Link to Myc module correlative
4. 2025 High

   Crystallography of the EPOP BC-box bound to Elongin BC pinpointed Leu40 in the Elongin C hydrophobic pocket, giving the structural basis for the bridging interaction.

   Evidence X-ray crystallography of the EPOP BC-box peptide–Elongin BC complex

   PMID:40153999

   Open questions at the time

   • Structure of full-length EPOP within PRC2 not determined
   • Does not address the Elongin-BC-independent inhibitory function
5. 2025 High

   Reconstituted enzymatic assays demonstrated that EPOP stimulates PRC2 methyltransferase activity in a GCN-rich DNA-sequence-dependent manner, defining a positive, cooperative role with MTF2 and JARID2.

   Evidence In vitro HMT and dinucleosome binding assays on defined substrates plus EED-rescue in vivo system

   PMID:41650228

   Open questions at the time

   • How sequence preference is read out mechanistically unclear
   • Reconciliation with in-cell inhibitory role not fully resolved
   • Single lab
6. 2026 High

   Separation-of-function mutants and dimerization assays established that EPOP restrains PRC2.1 by disrupting dimerization and lowering chromatin avidity, defining a distinct PRC2.1 subclass that prevents over-repression.

   Evidence Biochemical dimerization assays, PRC2-binding-deficient EPOP mutant, and genome-wide ChIP-seq in mouse epiblast-like cells (peer-reviewed publication of the 2024 preprint)

   PMID:39314288 PMID:41519789

   Open questions at the time

   • Structural detail of the dimerization-disrupting interaction not resolved
   • How activating and inhibitory modes are partitioned across loci unknown
7. 2025 High

   An Epop-knockout mouse linked EPOP's molecular restraint of PRC2 to organismal axial patterning, showing it prevents premature Hox activation in the presomitic mesoderm.

   Evidence Mouse knockout with skeletal phenotyping, tissue-specific RNA-seq, and Hox expression boundary analysis

   PMID:40834979

   Open questions at the time

   • Which EPOP biochemical activity (dimerization restraint vs Elongin BC bridging) drives the phenotype not dissected
   • Human disease relevance untested

• How EPOP's opposing activities — stimulating PRC2 methyltransferase activity yet restraining PRC2.1 chromatin over-targeting — are mechanistically partitioned at individual loci and contexts remains unresolved.
• No unified model integrating activation and inhibition at the same target
• Role of USP7/H2B deubiquitination in the switch undefined
• No structure of EPOP within an intact PRC2.1 dimer