FCN3

FCN3 encodes ficolin-3, a secreted pattern recognition molecule that initiates the lectin pathway of complement (PMID:19535802). Homozygosity for the FCN3 +1637delC frameshift mutation distorts the protein's C-terminal end, abolishing pattern recognition; affected individuals have undetectable serum ficolin-3 and are deficient in ficolin-3-dependent complement activation, defining a complete lectin pathway deficiency (PMID:19535802, PMID:18261799). Beyond its extracellular complement role, FCN3 has a context-dependent intracellular function in cancer cells: when retained in the ER, its fibrinogen domain is necessary and sufficient to trigger the unfolded protein response, cell cycle arrest, and apoptosis, whereas the secreted form is inert in this setting (PMID:33859174). In hepatocellular carcinoma, intracellular FCN3 acts as a tumor suppressor by binding the ribosome maturation factor SBDS to modulate EIF6 nuclear translocation, inducing ribosomal stress and activating p53-dependent apoptosis (PMID:36632465), and by upregulating APC to inhibit Wnt/β-catenin signaling and suppress Treg activation (PMID:41493695); STT3A-mediated N-glycosylation of FCN3 at Asn189 antagonizes these tumor-suppressive activities (PMID:41493695).

1. 2008 High

   Established that the FCN3 +1637delC frameshift produces a functionally defective protein, answering whether the mutation merely altered the sequence or abrogated the molecule's recognition activity.

   Evidence Recombinant expression of the mutant allele with ELISA, SDS-PAGE, and western blot, detecting a truncated protein in heterozygotes

   PMID:18261799

   Open questions at the time

   • Did not establish the clinical/complement-activation consequence in vivo
   • Structural basis of how C-terminal distortion abolishes ligand binding not resolved
2. 2009 High

   Defined ficolin-3 as an essential initiator of the lectin complement pathway by showing that its complete loss in a homozygous human ablates ficolin-3-dependent complement activation.

   Evidence Clinical characterization of a homozygous patient with undetectable serum ficolin-3 plus complement activation assays

   PMID:19535802

   Open questions at the time

   • Specific microbial/molecular ligands recognized by ficolin-3 not enumerated here
   • Downstream effector partners of the activated lectin pathway not detailed
3. 2021 Medium

   Revealed a non-complement, intracellular activity of FCN3 by demonstrating that its ER-localized fibrinogen domain drives ER stress and apoptosis, separating the secreted function from a cell-intrinsic pro-apoptotic one.

   Evidence Ectopic overexpression and domain-deletion analysis in LUAD cell lines with ER stress rescue and apoptosis/cell-cycle assays

   PMID:33859174

   Open questions at the time

   • Mechanism by which the fibrinogen domain triggers the UPR not defined
   • Whether endogenous FCN3 is ever ER-retained physiologically unknown
   • Limited to two LUAD lines in a single lab
4. 2023 Medium

   Identified a molecular partner and pathway for FCN3's tumor-suppressive effect, showing it binds SBDS to perturb EIF6 nuclear shuttling and activate p53, linking FCN3 to ribosomal stress signaling.

   Evidence Co-immunoprecipitation, overexpression/knockdown in HCC lines, EIF6 translocation and p53 reporter assays

   PMID:36632465

   Open questions at the time

   • Single Co-IP-based interaction without reciprocal structural validation
   • Direct binding versus complex-mediated association not distinguished
   • p53-YBX1-SBDS feedback loop mapped only in HCC cell lines
5. 2026 Medium

   Showed that a post-translational switch governs FCN3's anti-tumor activity, with STT3A-mediated N-glycosylation at Asn189 disabling its suppression of Wnt/β-catenin and Treg activation.

   Evidence Co-IP, site-specific N189 glycosylation validation, lentiviral knockdown/overexpression, xenograft models, and Treg flow cytometry

   PMID:41493695

   Open questions at the time

   • Mechanism connecting FCN3 to APC upregulation not resolved
   • How glycosylation at N189 alters FCN3 localization or interactions unclear
   • Single-lab findings without independent replication

• How FCN3's canonical extracellular complement-initiating role mechanistically relates to its intracellular tumor-suppressive functions remains unresolved.
• No unifying model linking secreted ficolin-3 and ER-retained pro-apoptotic FCN3
• Physiological relevance of intracellular FCN3 outside cancer cell lines untested