USP51

USP51 is a broadly acting deubiquitylating enzyme that counteracts ubiquitin-dependent signaling at chromatin during the DNA damage response and stabilizes multiple oncoproteins to promote epithelial–mesenchymal transition, hypoxia adaptation, and chemoresistance. USP51 directly deubiquitylates histone H2A at Lys13/Lys15 downstream of RNF168, dynamically regulating 53BP1 and BRCA1 focus assembly at DNA double-strand breaks (PMID:27083998, PMID:30192049). CDK4/6-mediated phosphorylation activates USP51 to deubiquitinate and stabilize ZEB1, linking cell-cycle kinase signaling to EMT and metastasis in breast and lung cancer (PMID:32296027, PMID:35058607). Beyond ZEB1, USP51 deubiquitinates and stabilizes HIF1α (in complex with the VHL/CUL2/ELOB/ELOC machinery, regulated by ELOC SUMOylation), PD-L1, TWIST1, GPX4, GRP78, and NEK8, thereby modulating hypoxia signaling, immune evasion, ferroptosis suppression, and drug efflux across diverse cancer contexts (PMID:37816999, PMID:37386737, PMID:37422632, PMID:39151312, PMID:40487653, PMID:41475333).

1. 2016 High

   Identification of USP51 as the DUB that removes H2AK13,15ub downstream of RNF168 established the first catalytic function for USP51 and placed it within the DNA damage response signaling cascade governing 53BP1/BRCA1 recruitment.

   Evidence In vitro deubiquitylation of H2A-H2B, chromatin fractionation, and epistasis analysis of IR-induced foci in human cells

   PMID:27083998

   Open questions at the time

   • Structural basis for H2A recognition by USP51 is unknown
   • Factors conferring in-cell site selectivity (K13/15 over K119) were not identified
   • Regulation of USP51 recruitment to damage sites was not characterized
2. 2017 High

   Discovery that USP51 deubiquitinates and stabilizes ZEB1 revealed a second, non-chromatin substrate and linked USP51 to EMT and cancer invasion, broadening its functional scope beyond DNA repair.

   Evidence DUB library screen, reciprocal Co-IP, in-cell ubiquitination assays, and invasion phenotyping in breast cancer cells

   PMID:29119051

   Open questions at the time

   • The domains mediating USP51-ZEB1 interaction were not mapped
   • Whether USP51 DUB activity on ZEB1 is regulated by post-translational modification was unknown
3. 2018 High

   Using chemically defined ubiquitylated nucleosomes, the intrinsic site selectivity of USP51 was resolved: it efficiently cleaves H2AK13Ub, H2AK15Ub, H2AK119Ub, and H2BK120Ub in vitro, indicating that in-cell specificity for K13/15 requires additional cellular factors.

   Evidence Total chemical synthesis of site-specifically ubiquitylated histones, in vitro nucleosome deubiquitylation assay, molecular dynamics simulation

   PMID:30192049

   Open questions at the time

   • Cellular factors that restrict USP51 selectivity to K13/15 in vivo remain unidentified
   • No high-resolution structure of USP51 bound to nucleosome substrate exists
4. 2020 High

   Demonstration that CDK4/6 phosphorylates and activates USP51, which then stabilizes ZEB1, established a kinase–DUB regulatory axis linking cell-cycle control to EMT-driven metastasis and provided a mechanistic rationale for CDK4/6 inhibitor efficacy.

   Evidence Phosphorylation assays, in vitro and in vivo deubiquitination, CDK4/6 inhibitor treatment, and xenograft metastasis models in breast cancer

   PMID:32296027

   Open questions at the time

   • The specific phosphorylation site(s) on USP51 were not fully mapped
   • Whether CDK4/6-dependent activation of USP51 also regulates its chromatin substrates is untested
5. 2019 Medium

   Identification of FAT4 as a USP51 substrate linked USP51 to Hippo pathway regulation, while its role in Bim stabilization connected it to TRAIL-induced apoptosis, expanding the substrate repertoire beyond chromatin and EMT factors.

   Evidence Co-IP for FAT4 interaction and shRNA knockdown in endometrial cancer; siRNA-based epistasis and Bim protein-level measurement with hispidulin/TRAIL treatment

   PMID:31217854 PMID:31817696

   Open questions at the time

   • Direct deubiquitination of Bim by USP51 was not biochemically demonstrated
   • FAT4-USP51 interaction lacks reciprocal validation or domain mapping
   • Neither finding has been independently replicated
6. 2022 Medium

   Replication of the CDK4/6–USP51–ZEB1 axis in lung adenocarcinoma confirmed the generality of this regulatory mechanism across epithelial cancer types.

   Evidence Phosphorylation and deubiquitination assays, CDK4/6 inhibitor treatment, and in vivo metastasis models in lung adenocarcinoma

   PMID:35058607

   Open questions at the time

   • Single-lab replication; independent lab confirmation still lacking
   • Whether additional kinases can phosphorylate and activate USP51 is unexplored
7. 2023 High

   Three new high-impact substrates—HIF1α, PD-L1, and TWIST1—were identified, revealing that USP51 operates at the nexus of hypoxia signaling, immune checkpoint regulation, and cancer stemness; the HIF1α study further showed USP51 is recruited via ELOC binding and regulated by ELOC SUMOylation.

   Evidence Co-IP of USP51/VHL/CUL2/ELOB/ELOC complex with ELOC K32 SUMOylation mapping; Ub-AMC activity assay and SPR for PD-L1; CHX chase and polyubiquitination assay for TWIST1; xenograft models for all three

   PMID:37386737 PMID:37422632 PMID:37816999

   Open questions at the time

   • The structural basis for USP51-ELOC interaction is unresolved
   • Whether USP51 deubiquitinates HIF1α directly or only in the context of the VHL complex is not fully dissected
   • TWIST1 and PD-L1 findings are each from single laboratories
8. 2024 Medium

   USP51 was shown to deubiquitinate GPX4 and suppress ferroptosis, connecting USP51 to lipid peroxide defense and identifying it as a target for ferroptosis-inducing anti-cancer strategies.

   Evidence USP51 overexpression/knockdown with GPX4 ubiquitination assay, ferroptosis markers, and ferrostatin rescue in esophageal squamous cell carcinoma xenografts

   PMID:39151312

   Open questions at the time

   • Direct in vitro reconstitution of USP51-mediated GPX4 deubiquitination not performed
   • The ubiquitin chain type cleaved from GPX4 was not determined
   • Single-lab finding
9. 2025 Medium

   Discovery of GRP78 and NEK8 as USP51 substrates extended USP51's role to drug efflux–mediated chemoresistance (via surface GRP78/ABCB1) and WNT/β-catenin pathway activation, and a third independent study confirmed the CDK4/6–USP51–ZEB1 axis.

   Evidence Deubiquitination and cell-surface localization assays for GRP78 in TNBC; Co-IP and ubiquitination assay for NEK8 in CRC; CDK4/6 inhibitor and USP51 knockdown with migration assays in breast cancer

   PMID:40487653 PMID:40877392 PMID:41475333

   Open questions at the time

   • GRP78 and NEK8 findings are each single-lab studies
   • No global substrate profiling (e.g., di-Gly proteomics) has been performed for USP51
   • How USP51 selects among its many substrates in different cellular contexts is unknown

• A unified model explaining how USP51 substrate selectivity is achieved across diverse cellular contexts—DNA repair, EMT, hypoxia, ferroptosis, and immune evasion—remains unresolved, and no structural information for USP51 exists.
• No crystal or cryo-EM structure of USP51 alone or in complex with any substrate
• No unbiased proteomics-based substrate identification (di-Gly or TUBE approaches)
• The full complement of CDK4/6 phosphorylation sites and their individual contributions to DUB activation are unmapped
• Physiological function in normal (non-cancer) tissues is essentially unstudied