CCDC174

CCDC174 is a nuclear RNA-binding protein that functions as a noncanonical splicing factor regulating pre-mRNA processing and alternative splicing (PMID:37137667, PMID:42120494). It directly associates with U1 snRNA independently of canonical U1 snRNP-specific proteins and is required for the selection and effective processing of weak 5' splice sites across hundreds of genes (PMID:37137667). CCDC174 physically engages the splicing apparatus on multiple fronts: it directly binds the exon junction complex core component EIF4A3 (PMID:26358778) and interacts with the PRP19/CDC5L complex, whose members become aberrantly upregulated upon CCDC174 loss (PMID:42120494). Consistent with this splicing role, depletion of CCDC174 disrupts mRNA and protein homeostasis and alternative splicing fidelity, impairs neural fold closure and hindbrain patterning in Xenopus, and—via oocyte-specific knockout in mice—causes oocyte maturation arrest and female infertility (PMID:26358778, PMID:42120494). A disease-associated CCDC174 variant fails to rescue the neural phenotype in Xenopus and is functionally defective in patient-variant assays, linking CCDC174 dysfunction to its developmental and reproductive roles (PMID:26358778, PMID:42120494).

1. 2015 Medium

   Establishing where CCDC174 acts and what it touches: its exclusive nuclear localization and direct interaction with the EJC core component EIF4A3 first placed it within the mRNA-processing machinery.

   Evidence Immunofluorescence co-localization and yeast two-hybrid in a single study

   PMID:26358778

   Open questions at the time

   • No reciprocal Co-IP or in vitro reconstitution of the CCDC174–EIF4A3 interaction
   • Functional consequence of the EIF4A3 interaction not defined
   • Binding interface and stoichiometry unknown
2. 2015 Medium

   Linking CCDC174 to an in vivo developmental program: loss-of-function plus allele-specific rescue showed it is required for neural fold closure and hindbrain patterning and that the disease variant is non-functional.

   Evidence Morpholino knockdown in Xenopus with wild-type vs. mutant human CCDC174 rescue

   PMID:26358778

   Open questions at the time

   • Single model organism, single lab
   • Molecular target genes mediating the neural phenotype not identified
   • Whether the rescue failure reflects loss- or gain-of-function not resolved here
3. 2015 Low

   Probing the nature of the disease allele: mutant overexpression induced apoptosis, hinting at a gain-of-toxic-function distinct from simple loss.

   Evidence Overexpression of wild-type vs. mutant CCDC174 in neuroblastoma cells with apoptosis readout

   PMID:26358778

   Open questions at the time

   • Single overexpression experiment, single method, not independently confirmed
   • Overexpression artifact versus physiological mechanism not distinguished
   • Molecular basis of the toxicity unknown
4. 2023 High

   Defining the core molecular activity: CCDC174 is a bona fide RNA-binding protein that binds U1 snRNA outside the canonical snRNP and is required to process weak 5' splice sites, explaining how it shapes splicing.

   Evidence BCLIP-seq and knockdown with splice-site processing readout in mouse ES cells

   PMID:37137667

   Open questions at the time

   • Structural basis of U1 snRNA recognition not determined
   • How CCDC174 enhances weak 5' splice site selection mechanistically unresolved
   • Relationship between U1 snRNA binding and the EIF4A3/EJC interaction unclear
5. 2026 High

   Connecting splicing function to a physiological output: oocyte-specific knockout showed CCDC174 interacts with the PRP19/CDC5L complex and is required for oocyte maturation and female fertility through control of alternative splicing and mRNA/protein homeostasis.

   Evidence Conditional knockout mouse, Co-IP, transcriptomic/proteomic profiling, and patient-variant functional assays

   PMID:42120494

   Open questions at the time

   • Direct versus indirect nature of the PRP19/CDC5L association not fully resolved
   • Key downstream mis-spliced transcripts driving maturation arrest not pinpointed
   • Whether the U1 snRNA and PRP19/CDC5L activities are mechanistically coupled is unknown

• How CCDC174's distinct molecular contacts—U1 snRNA, EIF4A3/EJC, and the PRP19/CDC5L complex—are integrated into a single mechanism for weak 5' splice site recognition remains unresolved.
• No structural model of CCDC174 or its RNA/protein interfaces
• No unified mechanism linking its splicing partners
• Tissue-specific target repertoires beyond neural and oocyte contexts uncharacterized