RNPC3

RNPC3 encodes the 65K protein of the minor (U12-dependent) spliceosome, functioning as a dual-RRM domain RNA-binding protein that bridges U11/U12 di-snRNP complex formation and is essential for removal of U12-type introns from a subset of mammalian genes (PMID:14974681, PMID:30254136). Conditional loss of RNPC3 in intestinal epithelium, germinal center B cells, and pituitary causes U12-type intron retention in proliferation- and survival-related transcripts, leading to impaired cell proliferation, increased apoptosis, and tissue degeneration, with homozygous germline deletion being pre-implantation lethal (PMID:30254136, PMID:40170400, PMID:34906446). Beyond splicing, RNPC3 interacts with ANKRD11, which bridges it to HDAC3 on chromatin; this tripartite association promotes H3K9 and H4K5 deacetylation, coupling minor spliceosome components to transcriptional regulation via histone modification (PMID:38837887). A pathogenic missense variant (p.Leu483Phe) in the RRM2 domain reduces pituitary growth hormone content in knock-in mice, establishing RNPC3 as a determinant of pituitary development (PMID:34906446).

1. 2003 Medium

   Initial cloning established that RNPC3 encodes a nuclear protein with two RRM domains and bipartite nuclear localization signals, providing the structural basis for its later assignment to RNA-containing spliceosomal complexes.

   Evidence cDNA cloning, sequence analysis, and GFP-fusion imaging in cultured cells

   PMID:14974681

   Open questions at the time

   • No functional assay performed; RRM domains not tested for RNA binding specificity
   • Nuclear localization shown only by overexpressed GFP fusion, not endogenous protein
2. 2018 High

   Conditional knockout in mice demonstrated that RNPC3 is essential for U12-type intron removal in vivo; its loss caused widespread intron retention, loss of phospho-ERK1/2 in intestinal crypt cells, epithelial degeneration, and pre-implantation embryonic lethality, establishing RNPC3 as indispensable for proliferating tissues.

   Evidence Cre-lox conditional and germline knockout mice; RT-PCR intron retention assays; immunofluorescence and western blot for phospho-ERK1/2

   PMID:30254136

   Open questions at the time

   • Mechanism by which U12-type intron retention leads to ERK1/2 signaling loss not resolved
   • Relative contribution of individual minor intron-containing genes to the phenotype unknown
3. 2021 Medium

   Expression mapping and CRISPR knock-in of a patient-derived RRM2 missense variant (p.Leu483Phe) showed RNPC3 is required for pituitary development and growth hormone production, extending its essential role to endocrine organogenesis.

   Evidence In situ hybridization on mouse and human embryos; CRISPR/Cas9 knock-in mice with hormone content measurement

   PMID:34906446

   Open questions at the time

   • Splicing targets affected by the Leu483Phe variant in pituitary cells not identified
   • Whether the GH deficit reflects a cell-autonomous spliceosome defect versus a developmental patterning defect is unresolved
4. 2024 High

   Discovery that RNPC3 associates with ANKRD11 and HDAC3 on chromatin to drive H3K9/H4K5 deacetylation revealed an unexpected non-splicing function, coupling the minor spliceosome to epigenetic transcriptional regulation.

   Evidence Affinity purification from Drosophila lysates, CUT&Tag chromatin profiling, RNAi knockdown, histone modification analysis; confirmed across Drosophila and mammalian systems

   PMID:38837887

   Open questions at the time

   • Whether RNPC3's chromatin role is independent of ongoing minor splicing or mechanistically linked remains unclear
   • Target genes regulated through the RNPC3–ANKRD11–HDAC3 axis not systematically defined
   • Structural basis of the RNPC3–ANKRD11 interaction not determined
5. 2025 High

   B cell-specific ablation showed RNPC3 is required for germinal center responses and antibody production by ensuring minor intron splicing in proliferation and apoptosis genes, generalizing its essential role to adaptive immunity.

   Evidence Conditional B cell-specific Cre knockout mice; flow cytometry for GC B cells; RT-PCR/sequencing for intron retention; serum antibody titers

   PMID:40170400

   Open questions at the time

   • Identity of the critical minor intron-containing transcripts whose mis-splicing drives GC B cell failure not fully delineated
   • Whether RNPC3's chromatin/HDAC3-associated function contributes to the B cell phenotype is untested

• It remains unknown how RNPC3's splicing and chromatin-regulatory functions are coordinated, which specific minor intron-containing transcripts are the critical effectors in each tissue, and whether the RNPC3–ANKRD11–HDAC3 axis operates independently of U12-type intron splicing.
• No structural model of RNPC3 in the U11/U12 di-snRNP context
• Genome-wide identification of direct RNPC3 chromatin targets in mammalian cells lacking
• Separation-of-function alleles distinguishing splicing from chromatin roles not generated