C1QC

C1QC encodes the C-chain of the C1q heterotrimer, which together with the A- and B-chains forms the recognition subunit of the C1 complex that initiates the classical complement cascade. The three C1q chain genes are arranged in tandem (A-C-B) on chromosome 1p; the C-chain contributes a collagen-like stalk with a characteristic Gly-X-Y interruption at position C-36 that introduces the bend seen by electron microscopy, and a globular head domain whose apex harbors charged residues critical for binding IgG, CRP, PTX3, and phosphatidylserine on apoptotic cells (PMID:486087, PMID:12960167, PMID:16566583, PMID:18250442). Upon engagement of IgG hexamers, the C1q arms condense and induce conformational rearrangement of the C1r₂s₂ proteases, activating the classical pathway (PMID:24626930, PMID:29449492). Beyond complement activation, C1q suppresses IFN-α production by plasmacytoid dendritic cells (PMID:19790049), and compound heterozygous loss-of-function mutations in C1QC abolish serum C1q, causing classical-pathway deficiency and systemic lupus erythematosus (PMID:31357913).

1. 1979 High

   Determining the primary sequence of the C1QC collagen-like region revealed a Gly→Ala substitution at position C-36 that breaks the Gly-X-Y repeat, providing a structural explanation for the bend observed in electron micrographs of C1q stalks.

   Evidence Direct amino acid sequencing of the purified C-chain collagen-like fragment

   PMID:486087

   Open questions at the time

   • Full-length C-chain sequence not yet available
   • Three-dimensional structure of the collagen stalk unresolved
2. 1991 High

   Cloning and sequencing the complete C1QC gene established the tandem A-C-B gene arrangement on chromosome 1p and defined the full-length C-chain amino acid sequence, completing the molecular characterization of all three C1q subunits.

   Evidence cDNA cloning, cosmid library screening, and genomic DNA sequencing

   PMID:1706597

   Open questions at the time

   • Promoter regulation of the C1QC locus undefined
   • No structure of the globular head domain
3. 1991 High

   Identification of direct, antibody-independent binding between HIV-1 gp41 and C1q demonstrated that C1q can be activated by non-immunoglobulin ligands, broadening its recognized recognition repertoire.

   Evidence Radiolabeled C1q binding, gel exclusion chromatography, and synthetic peptide competition using gp41 fragments

   PMID:1744579

   Open questions at the time

   • Structural basis for gp41-C1q interaction unknown
   • In vivo relevance during HIV infection not established
4. 2003 High

   The 1.9 Å crystal structure of the gC1q heterotrimeric head domain (A, B, C chains) resolved how non-polar interactions and a Ca²⁺ ion stabilize the trimer, and the concurrent demonstration that this domain binds pentraxin PTX3 to activate or inhibit complement depending on context answered how C1q engages pattern-recognition ligands beyond immunoglobulins.

   Evidence X-ray crystallography at 1.9 Å; recombinant globular head binding assays and C4 deposition complement activation assay

   PMID:12645945 PMID:12960167

   Open questions at the time

   • Atomic-level models of C1q bound to full-length IgG or CRP not yet available
   • Mechanism of PTX3 inhibition of C1q–IgG interaction unresolved
5. 2006 High

   Site-directed mutagenesis of the globular head C-chain module identified specific charged apex residues critical for binding IgG1, CRP, and PTX3, establishing that multiple ligands share an overlapping ionic interaction surface rather than discrete binding sites.

   Evidence Recombinant globular head modules with point mutations; ELISA and SPR binding assays

   PMID:16566583

   Open questions at the time

   • Relative contribution of each chain to binding affinity for each ligand not fully quantified
   • Intact C1q context may modify individual head module behavior
6. 2008 High

   Demonstration that C1q's globular domain binds phosphatidylserine on apoptotic cell membranes with nanomolar affinity, confirmed by crystallography and SPR, established C1q as a direct sensor of 'eat-me' signals linking complement to apoptotic clearance.

   Evidence SPR, cosedimentation, X-ray crystallography of gC1q–PS complex, confocal microscopy with annexin V competition

   PMID:18250442

   Open questions at the time

   • Relative importance of PS recognition versus opsonin bridging in vivo unclear
   • Whether individual chains contribute differently to PS binding undefined
7. 2009 High

   Binding of C1q to plasmacytoid dendritic cells and suppression of IFN-α production revealed a complement-independent immunoregulatory role for C1q, linking C1q deficiency to the type I IFN signature in SLE.

   Evidence SPR and flow cytometry for binding; cytokine immunoassay after CpG/immune complex stimulation of purified PDCs

   PMID:19790049

   Open questions at the time

   • Receptor on PDCs mediating C1q binding not identified
   • Whether this function requires heterotrimeric C1q or individual chains unknown
8. 2014 High

   Structural and functional evidence that IgG hexamers formed via Fc–Fc interactions are the physiological C1q ligand on target surfaces resolved a long-standing question about the stoichiometry of complement activation, showing that engineered Fc–Fc contacts modulate C1 engagement and cell killing.

   Evidence Cryo-EM, native mass spectrometry, engineered IgG mutants, cell-killing assays

   PMID:24626930

   Open questions at the time

   • How different IgG subclass hexamers influence C1q binding geometry unresolved
9. 2018 High

   Cryo-EM of the C1–IgG1 hexamer complex revealed how C1q arm condensation upon antibody binding rearranges the C1r₂s₂ proteases, providing the first mechanistic model for signal transduction from ligand recognition to serine protease activation within the C1 complex.

   Evidence Cryo-EM at sub-nanometer resolution with IgG1 mutant functional validation

   PMID:29449492

   Open questions at the time

   • Full atomic model of activated C1r₂s₂ in complex not yet achieved
   • Whether activation can occur within a single C1 complex or requires trans-activation between complexes debated
10. 2019 Medium

    Identification of compound heterozygous C1QC mutations (Gly34Arg/Arg69X) causing complete C1q deficiency and SLE with cerebral involvement provided definitive human genetic evidence that C1QC loss of function is sufficient for classical pathway failure and lupus susceptibility.

    Evidence ELISA (absent serum C1q), Western blot, DNA and RNA sequencing in a clinical case

    PMID:31357913

    Open questions at the time

    • Single family; broader genotype–phenotype spectrum for C1QC mutations underexplored
    • Cerebral involvement mechanism not molecularly dissected
11. 2024 Medium

    Discovery that C1QC binds DDR2 and activates MMP9 in cerebral microvascular endothelial cells under diabetic conditions, disrupting the blood–brain barrier, identified a complement-independent pathological axis for C1QC in diabetic microangiopathy.

    Evidence siRNA knockdown and overexpression in high-glucose endothelial cell model and diabetic mouse; co-immunoprecipitation for C1QC–DDR2 interaction

    PMID:39531193

    Open questions at the time

    • C1QC–DDR2 binding domain not mapped
    • Single lab; independent replication needed
    • Whether monomeric C1QC or trimeric C1q engages DDR2 unresolved
12. 2024 Medium

    Functional studies in DLBCL showed that C1QC sustains M2 macrophage identity (CD163 expression) and that C1QC-expressing macrophages promote tumor cell proliferation and chemoresistance, positioning C1QC as a marker and functional mediator of tumor-associated macrophage immunosuppression.

    Evidence siRNA knockdown of C1QC in M2 macrophages, co-culture proliferation and drug sensitivity assays, single-cell RNA-seq

    PMID:39388888

    Open questions at the time

    • Downstream signaling from C1QC in macrophages not defined
    • Whether C1QC acts as secreted ligand or intracellular factor in this context unclear
13. 2025 Medium

    Elucidation of the circDnajc1/miR-27a-5p/C1qc axis in ischemic stroke showed that C1qc upregulation promotes microglial activation and neuroinflammation via C3/C5aR, placing C1qc downstream of a specific non-coding RNA regulatory circuit in cerebral ischemia-reperfusion injury.

    Evidence MCAO/R rat model and OGD/R cell model; RNA immunoprecipitation and luciferase reporter assays for miR-27a-5p–C1qc targeting

    PMID:40483386

    Open questions at the time

    • Whether miR-27a-5p regulation of C1qc is conserved in human stroke unknown
    • Relative contribution of C1qc versus other complement components downstream of circDnajc1 not dissected
14. 2025 Medium

    In diabetic kidney disease, C1QC upregulation in proximal tubular cells promotes lipid accumulation and inflammation; empagliflozin's renoprotective effect is partly mediated through C1QC downregulation, establishing C1QC as a pharmacologically targetable effector in diabetic nephropathy.

    Evidence siRNA knockdown and overexpression with rescue experiments in vitro and in db/db mice; empagliflozin intervention

    PMID:41252098

    Open questions at the time

    • Downstream signaling pathway from C1QC in tubular cells not identified
    • Single lab; mechanism of empagliflozin-mediated C1QC suppression not defined

• Key unresolved questions include: (1) the structural basis for C1QC interactions with non-canonical partners such as DDR2; (2) whether monomeric C1QC or assembled C1q mediates its complement-independent functions in disease contexts; and (3) the transcriptional and post-transcriptional regulatory circuits controlling C1QC expression in macrophages and epithelial cells.
• No structural data for C1QC–DDR2 interaction
• Monomeric versus trimeric functionality in disease not resolved
• Transcriptional regulation of C1QC incompletely characterized