Cfi is an important component in the neurobiology of neurodegenerative diseases. This page provides detailed information about its structure, function, and role in disease processes.
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Gene SymbolCFI
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Full NameComplement Factor I
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Chromosomal Location4p25.3
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NCBI Gene ID[3426](https://www.ncbi.nlm.nih.gov/gene/3426)
OMIM[217030](https://omim.org/entry/217030)
Ensembl IDENSG00000167644
UniProt ID[P05156](https://www.uniprot.org/uniprot/P05156)
Associated DiseasesAge-Related Macular Degeneration, Complement Deficiency
The CFI gene encodes complement factor I, a serine protease that plays a critical role in regulating all three complement pathways. Factor I cleaves and inactivates C3b and C4b, requiring cofactors for its activity. The gene is located on chromosome 4p25.3 and encodes a protein of 406 amino acids [1].
Factor I is a member of the serine protease family and circulates in plasma at low concentrations (~35 μg/mL). It is synthesized primarily in the liver but is also expressed in various tissues including the brain. The protein consists of a heavy chain (containing the protease domain) and a light chain (containing the cofactor domains) held together by a disulfide bond [2].
Factor I is the only known protease that can cleave C3b and C4b, but requires cofactor proteins for its activity:
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C3b Cleavage: Factor I cleaves C3b at specific sites, generating inactive C3b (iC3b)
- Requires cofactors: CFH, MCP (CD46), C4BP, CR1
- Cleavage sites: Arg1281, Arg1298, Arg1299
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C4b Cleavage: Factor I inactivates C4b
- Requires cofactors: C4BP, CR1
- Generates C4c and C4d fragments
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Limited C5 Cleavage: Factor I can also cleave C5 (limited activity)
- Does not generate functional C5a
- May regulate terminal pathway activation
- Controls amplification of all complement pathways
- Prevents immune complex deposition
- Protects host tissues from complement-mediated damage
- Essential for immune homeostasis
- CFI variants have been implicated in AD risk in genome-wide studies
- Altered CFI levels observed in AD cerebrospinal fluid
- CFI may interact with complement activation in AD neuroinflammation
- The role of CFI in amyloid clearance is under investigation [3]
- CFI variants are associated with AMD risk, particularly in European populations [4]
- Loss-of-function CFI variants increase AMD susceptibility
- CFI polymorphisms interact with CFH variants in AMD risk
- Therapeutic targeting of CFI is being explored for AMD treatment
- CFI deficiency is a rare autosomal recessive disorder
- Leads to recurrent bacterial infections
- Increased risk of autoimmune disease
- Presents with angioedema and lupus-like symptoms
- Glomerulonephritis: CFI mutations associated with renal disease
- Infection Susceptibility: Particularly with encapsulated bacteria
- Liver: Primary site of synthesis (hepatocytes)
- Brain: Low baseline expression in astrocytes and microglia
- Kidney: Expression in tubular epithelial cells
- Immune cells: Monocytes and macrophages
CFI expression is regulated by:
- Inflammatory cytokines (IL-6, IFN-γ) upregulate CFI
- Acute phase response increases CFI levels
- Glucocorticoids suppress CFI expression
- TGF-β can modulate CFI expression
- G119R: Common variant affecting enzyme activity
- A252G: Polymorphism in the signal peptide region
- R399Q: Variant associated with AMD risk
- CFI polymorphisms vary across populations
- Certain variants enriched in European populations
- Serum CFI levels can be measured
- Genetic testing available for CFI variants
- Functional assays for CFI activity
The study of Cfi has evolved significantly over the past decades. Research in this area has revealed important insights into the underlying mechanisms of neurodegeneration and continues to drive therapeutic development.
Historical context and key discoveries in this field have shaped our current understanding and will continue to guide future research directions.