C9 (Complement C9) Gene

Key component of the membrane attack complex in the complement system

Gene Information Card

Symbol C9
Full Name Complement C9
Gene Type Protein coding
Chromosomal Location 5p13.1
NCBI Gene ID 715 ncbi.nlm.nih.gov/gene/715
Ensembl ID ENSG00000113600
UniProt ID P02748
OMIM ID 120940
HGNC ID 1322
Aliases C9D, complement component 9

Description

The C9 gene encodes complement component C9, a plasma glycoprotein that is the final component of the membrane attack complex (MAC). Upon activation of the complement cascade, C9 polymerizes with C5b-8 to form a pore in the lipid bilayer of target cells, leading to cell lysis. C9 is primarily synthesized in the liver and is essential for the lytic pathway of the complement system.

Disease Associations

Disease category Pathophysiological mechanism Genomic evidence
Complement component 9 deficiency Loss-of-function mutations in C9 prevent MAC formation, impairing immune defense against Neisseria infections. ClinVar, OMIM
Age-related macular degeneration (AMD) Rare variants in C9 may alter complement regulation, contributing to drusen formation and retinal damage. NCBI Gene, ClinVar
Hemolytic uremic syndrome (atypical) Dysregulation of complement activation involving C9 can lead to endothelial damage and microvascular thrombosis. ClinVar

Expression Profile

Tissue Expression
Tissue nTPM level
Liver 12.5 High
Plasma N/A High (secreted)
Kidney 3.2 Medium
Lung 1.8 Low
Brain 0.5 Not detected
Cell Line Expression
Cell Line nTPM Notes
HepG2 (hepatocellular carcinoma) 10.1 High expression
THP-1 (monocyte) 2.3 Moderate expression
A549 (lung carcinoma) 0.9 Low expression
K562 (leukemia) 0.2 Not detected
Data source：Human Protein Atlas（proteinatlas.org）

Mutations & Variants

Hotspot Mutations
Variant Type Frequency Functional Description
c.1138C>T (p.Arg380*) Nonsense Rare Loss of function; associated with C9 deficiency
c.1279G>A (p.Gly427Arg) Missense Rare Reduced C9 secretion and MAC activity
c.164delG (p.Gly55Valfs*2) Frameshift Rare Complete loss of protein function
Mutation functional classification

Loss of Function (LOF)

Nonsense, frameshift, and missense mutations that impair C9 synthesis, secretion, or polymerization lead to complement component 9 deficiency and increased susceptibility to Neisseria infections.

Gain of Function (GOF)

No gain-of-function mutations are currently documented in C9.

Dominant Negative (DN)

No dominant-negative mutations have been reported for C9.
View complete mutation data：
ClinVar：https://www.clinvar.com/gene/715
COSMIC：https://cancer.sanger.ac.uk/cosmic/gene/analysis?ln=C9

Gene Ontology (GO)

• extracellular region (GO:0005576) • complement activation (GO:0006956)
• membrane attack complex (GO:0005579) • serine-type endopeptidase activity (GO:0004252)
• regulation of complement activation (GO:0030449)

Pathways

• Complement cascade (Reactome: R-HSA-166658)
• Terminal pathway of complement (Reactome: R-HSA-166665)
• Membrane attack complex formation (KEGG: hsa04610)

Protein Summary

Complement component C9 is a 71 kDa glycoprotein that circulates in plasma as a monomer. Upon activation of the complement cascade, C9 binds to the C5b-8 complex and undergoes a conformational change, exposing hydrophobic domains that insert into the target cell membrane. Multiple C9 molecules polymerize to form a transmembrane pore (the membrane attack complex), leading to osmotic lysis of pathogens. C9 is primarily produced in the liver and is a key effector of the innate immune system.

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Reference Databases

NCBI Gene https://www.ncbi.nlm.nih.gov/gene/715
OMIM https://www.omim.org/entry/120940
HGNC https://www.genenames.org/data/gene-symbol-report/#!/hgnc_id/HGNC:1322
Ensembl https://www.ensembl.org/Homo_sapiens/Gene/Summary?g=ENSG00000113600
UniProt https://www.uniprot.org/uniprot/P02748
ClinVar https://www.ncbi.nlm.nih.gov/clinvar/?term=C9%5Bgene%5D
COSMIC https://cancer.sanger.ac.uk/cosmic/gene/analysis?ln=C9
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