Gene SymbolC7
Full NameComplement Component 7
Chromosomal Location5p13.1
Ensembl IDENSG00000124429
Associated DiseasesComplement Deficiency, Systemic Lupus Erythematosus
The C7 gene encodes complement component 7, a terminal complement protein essential for membrane attack complex (MAC) formation in the innate immune system. C7 is a secreted glycoprotein that plays a critical role in the complement cascade, a series of proteins that enhance opsonization, inflammation, and cell lysis. The gene is located on chromosome 5p13.1 and encodes a protein of approximately 809 amino acids [1].
C7 is synthesized primarily in the liver as a single polypeptide chain that undergoes post-translational modifications to become a functional secreted protein. It circulates in plasma at concentrations of approximately 40-80 μg/mL in healthy individuals. The protein contains multiple thrombospondin type I repeats (TSRs) and a hydrophobic region that facilitates membrane insertion [2].
The primary function of C7 is in the terminal pathway of the complement system. C7 binds to the C5b-6 complex to form C5b-6-7, which has the unique ability to insert into lipid bilayers of target cell membranes. This insertion is a critical step in the formation of the membrane attack complex (MAC), which creates a transmembrane pore leading to cell lysis.
The biological functions of C7 include:
- Membrane Insertion: The C5b-6-7 complex can insert into cell membranes, initiating the formation of the MAC pore
- Pore Formation: C7 provides the structural foundation for C8 and C9 addition to complete the MAC
- Immune Defense: The MAC is crucial for defense against Neisseria infections (N. meningitidis, N. gonorrhoeae)
- Immunomodulation: Sublytic MAC deposition can trigger inflammatory responses without cell death
The complement system, including terminal components, has been increasingly recognized in Alzheimer's disease pathogenesis. The MAC (C5b-9) is detected in AD brain tissue, particularly in association with amyloid plaques and in the cerebral vasculature. Research suggests that:
- MAC deposition on neurons may contribute to synaptic loss and neuronal damage [3]
- Complement activation products colocalize with neurofibrillary tangles
- The membrane attack complex may be involved in microvascular injury in AD
- Genetic variants in complement genes may modify AD risk [4]
Complement activation has been implicated in Parkinson's disease pathogenesis:
- MAC deposition has been observed in the substantia nigra of PD patients
- Microglial activation in PD is associated with complement upregulation
- C7 and other complement proteins are elevated in the CSF of PD patients
- The complement system may mediate neuroinflammation in PD [5]
¶ Multiple Sclerosis and Demyelination
In multiple sclerosis and related demyelinating diseases:
- MAC-mediated demyelination is a key pathological mechanism
- Oligodendrocyte death can be complement-dependent
- Demyelinating lesions show complement deposition
- Therapeutic strategies targeting complement are being explored [6]
- Amyotrophic Lateral Sclerosis (ALS): Complement activation is observed in ALS motor cortex and spinal cord
- Frontotemporal Dementia: MAC deposition found in FTD brain tissue
- Huntington's Disease: Complement proteins upregulated in HD brain
- Liver: Primary site of complement protein synthesis
- Brain: Expressed by astrocytes and microglia, particularly under inflammatory conditions
- Kidney: Local synthesis in glomerular cells
- Lung: Expression in alveolar epithelial cells
- Immune cells: Monocytes and macrophages can produce C7
C7 expression is regulated by:
- Inflammatory cytokines (IL-6, TNF-α, IFN-γ) upregulate hepatic C7 synthesis
- Acute phase response increases C7 levels 2-3 fold
- Glucocorticoids can suppress complement expression
- Aging is associated with dysregulated complement activity
Complete C7 deficiency is rare but leads to:
- Severe susceptibility to Neisseria infections
- Increased risk of meningitis and sepsis
- Generally normal immune function otherwise
Given the role of complement in neurodegeneration, complement inhibitors are being investigated:
- Eculizumab and ravulizumab (anti-C5) are approved for other conditions
- Anti-C7 therapeutics are in development
- Gene therapy approaches for complement deficiency are experimental
- Various single nucleotide polymorphisms (SNPs) in the C7 gene have been identified
- Some variants may affect complement activity levels
- Associations with autoimmune diseases (SLE, rheumatoid arthritis) reported
- C7 shows moderate polymorphism across populations
- Deficiency alleles are rare (1:1,000,000)
- NCBI Gene: C7 (719)
- UniProt: Complement component C7 (P10643)
- Staiko et al., Complement activation in Alzheimer's disease (2019)
- Riddell et al., Complement in Alzheimer's disease (2020)
- Gebbink et al., Complement activation in Parkinson's disease (2021)
- Rus et al., Complement in demyelinating disease (2006)
- Stahel et al., Complement membrane attack complex (2000)
- Michailidou et al., Complement C7 and Alzheimer disease risk (2018)
- Watowich et al., Structure and function of complement component C7 (1995)