DHX33

DHX33 is a DEAH-box RNA helicase that couples ATPase-driven nucleic acid unwinding to ribosome biogenesis, mRNA translation, transcriptional regulation, and innate immune sensing. Its ATPase activity, stimulated by DNA or RNA duplexes, is essential for promoting RNA Pol I-dependent 47S rRNA synthesis at rDNA loci (via interaction with UBF) and for facilitating elongation-competent 80S ribosome assembly required for global mRNA translation initiation (PMID:21930779, PMID:26100019, PMID:29870660). DHX33 also functions as a transcriptional regulator by binding CG-rich promoters and recruiting Gadd45a/Tet1 to promote DNA demethylation and RNA Pol II loading at target genes including cell cycle regulators and metabolic enzymes (PMID:27601587, PMID:32312884). In innate immunity, DHX33 senses cytosolic dsRNA through its HELICc domain and activates both the NLRP3 inflammasome and the IPS-1/MAVS-dependent type I interferon pathway, while its protein stability is controlled by opposing USP36-mediated deubiquitination and GSK-3β phosphorylation-triggered ubiquitin-dependent degradation (PMID:23871209, PMID:24037184, PMID:29273634, PMID:36403931).

1. 2011 High

   Establishing that DHX33 is required for rDNA transcription resolved how this DEAH-box helicase participates in ribosome biogenesis: it associates with rDNA loci and UBF to promote RNA Pol I occupancy and 47S rRNA synthesis, with its NTPase activity being essential.

   Evidence RNAi screen, ChIP at rDNA, Co-IP with UBF, dominant-negative K94R mutant, rRNA synthesis assays in human cells

   PMID:21930779

   Open questions at the time

   • Structural basis of DHX33–UBF interaction unknown
   • Direct versus indirect association with rDNA not resolved
   • Whether DHX33 unwinds R-loops or other structures at rDNA not tested
2. 2013 High

   Identification of DHX33 as a cytosolic dsRNA sensor that activates both the NLRP3 inflammasome and the MAVS-dependent type I IFN pathway revealed a dual innate immune function mediated through its HELICc domain.

   Evidence Domain-mapping Co-IP (HELICc with NLRP3 and IPS-1/MAVS), shRNA knockdown in macrophages and myeloid DCs, caspase-1 and cytokine secretion assays, poly I:C binding assays

   PMID:23871209 PMID:24037184

   Open questions at the time

   • Relative contribution of DHX33 versus RIG-I/MDA5 during physiological viral infection not determined
   • In vivo immune phenotype of DHX33 deletion in myeloid cells not reported
3. 2013 High

   Demonstrating that oncogenic Ras upregulates DHX33 translation via PI3K/mTOR/MAPK and that DHX33 is required for Ras-driven rRNA transcription and transformation placed DHX33 as a critical downstream effector linking growth factor signaling to ribosome biogenesis in cancer.

   Evidence Polysome fractionation, kinase inhibitor panels, DHX33 knockdown rescue of RasV12-driven transformation, in vivo tumor model

   PMID:23401854

   Open questions at the time

   • Whether DHX33 translation is directly controlled by mTORC1 phosphorylation of specific factors not shown
   • ARF-mediated translational repression mechanism not molecularly defined
4. 2015 High

   Showing that DHX33 promotes conversion of 80S monosomes to elongation-competent polysomes established a second, translation-level function distinct from its rRNA synthesis role, explaining how DHX33 loss causes global translational inhibition.

   Evidence Polyribosome profiling, RNA immunoprecipitation, Co-IP with ribosomal proteins and translation factors, helicase-dead mutant rescue failure

   PMID:26100019

   Open questions at the time

   • Specific RNA substrates unwound by DHX33 during 80S-to-polysome transition not identified
   • Whether DHX33 acts on mRNA secondary structures or ribosomal RNA rearrangements unclear
5. 2016 High

   Discovery that DHX33 occupies promoters of cell cycle genes and is required for RNA Pol II loading at these loci expanded its role from a ribosome biogenesis factor to a direct transcriptional regulator of Pol II-dependent genes.

   Evidence ChIP for DHX33 and RNA Pol II at cell cycle gene promoters, siRNA knockdown, CRISPR knockout in zebrafish, xenograft tumor model

   PMID:27601587

   Open questions at the time

   • Mechanism by which a helicase facilitates Pol II loading not determined
   • Whether DHX33 opens chromatin or resolves R-loops at Pol II promoters not tested
6. 2017 High

   Identification of USP36 as the deubiquitinase that stabilizes DHX33 revealed the first post-translational regulatory mechanism controlling DHX33 abundance, with loss of USP36 phenocopying DHX33 loss in rRNA synthesis and causing preimplantation lethality in mice.

   Evidence Ubiquitination and protein stability assays, USP36 knockout mouse, Northern blot for rRNA, puromycin incorporation for translation

   PMID:29273634

   Open questions at the time

   • E3 ligase responsible for DHX33 ubiquitination not identified at this point
   • Specific ubiquitin chain types on DHX33 not characterized
7. 2018 High

   Biochemical reconstitution with purified recombinant DHX33 demonstrated that it possesses bona fide ATPase-coupled unwinding activity on both DNA and RNA duplexes, confirming the enzymatic basis of its diverse cellular functions.

   Evidence In vitro ATPase assay with purified recombinant protein, helicase unwinding assay on DNA/RNA substrates, ATP-binding site mutagenesis

   PMID:29870660

   Open questions at the time

   • Substrate specificity (preferred duplex length, sequence, or structure) not systematically characterized
   • No structural model of DHX33 available
8. 2020 High

   Discovery that DHX33 recruits Gadd45a and Tet1 to CG-rich promoters to drive DNA demethylation and histone H4 acetylation provided a mechanistic explanation for how a helicase regulates Pol II transcription—through epigenetic remodeling.

   Evidence ChIP, 5hmC quantification, Co-IP of DHX33–AP-2β–Gadd45a complex, RNA-seq, promoter methylation analysis

   PMID:32312884 PMID:32617965

   Open questions at the time

   • Whether R-loop formation by DHX33 is required for Tet1 recruitment not directly shown
   • Genome-wide map of DHX33 binding sites (ChIP-seq) not reported
9. 2022 High

   Identification of GSK-3β-mediated phosphorylation at T482 as a trigger for DHX33 ubiquitination and degradation completed the regulatory circuit, explaining how growth-promoting signals (which inactivate GSK-3β) elevate DHX33 protein in cancer cells; K94 was identified as a major ubiquitination site that also overlaps the ATP-binding region.

   Evidence In vitro kinase assay, T482 mutagenesis, ubiquitination assay, GSK-3β inhibitor/activator treatments, protein stability measurements

   PMID:36403931

   Open questions at the time

   • Identity of the E3 ubiquitin ligase acting downstream of GSK-3β phosphorylation not determined
   • Whether T482 phosphorylation directly creates a phosphodegron not resolved
10. 2023 High

    B-cell-specific Dhx33 deletion in mice demonstrated that DHX33 is dispensable for steady-state B cells but essential for activation-induced rDNA transcription, proliferation, and germinal center responses, providing the first in vivo immune cell-autonomous requirement for DHX33 in ribosome biogenesis.

    Evidence Conditional CRISPR/Cas9 B-cell knockout mouse, 47S rRNA measurement, p53 accumulation assay, germinal center analysis

    PMID:36631557

    Open questions at the time

    • Whether DHX33 loss triggers nucleolar stress in other rapidly proliferating immune cells not examined
    • Extent to which p53-dependent apoptosis accounts for the full B-cell phenotype not dissected

• Major unresolved questions include the structural basis of DHX33 substrate recognition, the identity of the E3 ligase mediating its phosphodegron-triggered degradation, genome-wide mapping of DHX33 chromatin occupancy, and the physiological relevance of DHX33 in antiviral innate immunity in vivo.
• No structural model of DHX33
• E3 ubiquitin ligase downstream of GSK-3β phosphorylation unknown
• No genome-wide ChIP-seq for DHX33 reported
• In vivo role in antiviral immunity not tested with conditional knockout models