DCAF4

DCAF4 functions as a substrate receptor within the CRL4 E3 ubiquitin ligase complex (CUL4A/CUL4B–DDB1–DCAF4), directing substrate-specific ubiquitination in both proteasomal and autophagic degradation pathways. In colitis-associated colorectal cancer, c-Myc transcriptionally activates CUL4A/CUL4B to drive CRL4DCAF4 assembly, which ubiquitinates and degrades the tumor suppressor ST7, promoting tumor cell proliferation; this axis is restrained by miR-34b-5p, whose silencing by inflammation-driven DNA hypermethylation derepresses c-Myc (PMID:30945288, PMID:31972160). Pharmacological disruption of the CUL4–DDB1 interface by the small molecule NSC1892 blocks CRL4DCAF4 assembly, stabilizes ST7, and inhibits colorectal cancer growth (PMID:32140073). DCAF4 also ubiquitinates the autophagy receptor optineurin (OPTN), facilitating autophagic clearance of misfolded SOD1 and thereby linking the CRL4DCAF4 complex to protein quality control (PMID:32014991).

1. 2019 High

   Establishing DCAF4 as a bona fide substrate receptor: it was unknown whether DCAF4 assembles a functional CRL4 ligase and what substrate it targets; this study demonstrated that DCAF4 bridges DDB1–CUL4A/CUL4B to ubiquitinate and degrade the tumor suppressor ST7, with c-Myc driving CUL4A/CUL4B expression to potentiate this activity in colitis-associated cancer.

   Evidence In vitro and in vivo ubiquitination assays, reciprocal co-immunoprecipitation, promoter-binding assays, and knockdown/overexpression with proliferation and xenograft tumor growth readouts in colorectal cancer cells

   PMID:30945288

   Open questions at the time

   • Structural basis of DCAF4 recognition of ST7 is unknown
   • Whether DCAF4 targets additional substrates beyond ST7 in cancer contexts was not addressed
   • Relative contributions of CUL4A versus CUL4B scaffolds within the complex are unclear
2. 2020 Medium

   Extending the upstream regulatory axis: it was unclear how CRL4DCAF4 activity is deregulated during inflammation; work showed that inflammation-induced DNA hypermethylation silences miR-34b-5p, derepressing c-MYC, which in turn transcriptionally induces CUL4A/CUL4B and thus CRL4DCAF4-mediated ST7 degradation.

   Evidence miRNA–target luciferase reporter assays, overexpression/knockdown with in vitro oncogenic phenotype assays, in vivo tumor formation, and DNA methylation analysis in colitis-associated cancer models

   PMID:31972160

   Open questions at the time

   • Validated in a single laboratory replicating the same disease model; independent confirmation in other inflammatory cancer settings is lacking
   • Whether miR-34b-5p also regulates DCAF4 expression directly was not tested
3. 2020 Medium

   Demonstrating druggability of the CRL4DCAF4 complex: it was unknown whether pharmacological disruption of CRL4 assembly could stabilize ST7 and suppress tumor growth; the small molecule NSC1892 was shown to block CUL4–DDB1 interaction, prevent CRL4DCAF4 complex formation, and inhibit colorectal cancer growth in vitro and in vivo.

   Evidence AlphaScreen binding assay for CUL4–DDB1 interaction, biochemical analyses of complex assembly, and in vitro/in vivo tumor growth assays

   PMID:32140073

   Open questions at the time

   • NSC1892 disrupts CUL4–DDB1 generally, so selectivity for CRL4DCAF4 versus other CRL4 complexes is unresolved
   • No structural data for the inhibitor-binding mode are available
4. 2020 Medium

   Revealing a second substrate and a link to autophagy: it was unknown whether DCAF4 functions beyond proteasomal degradation; a genome-wide screen identified DCAF4 as a ubiquitin ligase for the autophagy receptor optineurin, with DCAF4-mediated OPTN ubiquitination facilitating autophagic clearance of misfolded SOD1.

   Evidence Genome-wide siRNA screen with time-resolved FRET readout, co-immunoprecipitation, ubiquitination assays, and autophagy flux assays

   PMID:32014991

   Open questions at the time

   • The ubiquitin chain type deposited on OPTN by DCAF4 was not defined
   • Physiological relevance in neurodegenerative disease models (e.g., ALS-linked SOD1 mutants) was not tested in vivo
   • Whether DCAF4-mediated OPTN ubiquitination requires the full CRL4 complex or can occur independently was not resolved

• Key open questions include the full substrate repertoire of CRL4DCAF4, the structural basis by which DCAF4 recognizes its substrates, and whether its dual role in proteasomal (ST7) and autophagic (OPTN/SOD1) degradation reflects distinct regulatory contexts or a general protein quality-control function.
• No crystal or cryo-EM structure of the DCAF4–DDB1 interface or DCAF4–substrate complexes exists
• Comprehensive substrate profiling (e.g., ubiquitin remnant proteomics upon DCAF4 depletion) has not been performed
• The role of DCAF4 in tissues outside the colon and in non-cancer physiology is unexplored