The C4A gene (Complement Component 4A) encodes a critical protein in the classical complement cascade, a key component of the innate immune system. Located in the major histocompatibility complex (MHC) class III region on chromosome 6p21.3, C4A plays essential roles in immune defense, synaptic pruning, and neuroinflammation[1]. Research over the past decade has revealed that C4A is critically involved in the pathogenesis of Alzheimer's disease, Parkinson's disease, schizophrenia, and other neurodegenerative conditions. The gene shows polymorphic variation in humans, including copy number variation (CNV) that has been associated with disease risk.
| Attribute | Value |
|---|---|
| Gene Symbol | C4A |
| Full Name | Complement Component 4A (Chido/Rodgers Blood Group) |
| Chromosomal Location | 6p21.3 (MHC Class III) |
| NCBI Gene ID | 712 |
| OMIM | 120810 |
| Ensembl ID | ENSG00000144711 |
| UniProt ID | P0C0P0 |
| Protein Class | Complement system serine protease |
| Associated Diseases | Alzheimer's Disease, Parkinson's Disease, Schizophrenia, Systemic Lupus Erythematosus |
C4A encodes the complement component 4A protein, a 1741-amino acid zymogen that undergoes proteolytic cleavage during activation. The protein consists of three polypeptide chains (α, β, and γ) held together by disulfide bonds:
The thioester bond in the α-chain is a defining feature of C4, allowing it to covalently bind to hydroxyl groups on target surfaces through acylation[2]. This distinguishes C4A from C4B, which preferentially binds amino groups.
C4A participates in multiple complement activation pathways:
The cleavage products serve distinct functions:
C4A has emerged as a significant factor in Alzheimer's disease pathogenesis through multiple mechanisms:
Elevated C4A expression in the AD brain contributes to chronic neuroinflammation[4]. Studies have shown that C4A levels are increased in AD brains compared to age-matched controls, particularly in regions affected by amyloid pathology. The complement protein is produced by activated microglia and astrocytes, creating a pro-inflammatory feedback loop that drives disease progression[@benoit2022].
Key mechanisms include:
The complement system plays a critical role in developmental synaptic pruning, and this mechanism is re-activated in Alzheimer's disease[5]. C4A contributes to:
Studies using mouse models have demonstrated that blocking complement components can prevent synapse loss, highlighting the therapeutic potential of targeting C4A[6].
C4A interacts with amyloid-beta (Aβ) plaques in several ways:
Genetic studies have identified variants in the C4 region that modify AD risk, particularly in the MHC class III region that shows robust association in GWAS studies[7].
In Parkinson's disease, C4A contributes to dopaminergic neuron loss through:
Post-mortem studies have shown increased C4A expression in the substantia nigra of PD patients, particularly in proximity to Lewy bodies[8].
The strongest evidence linking C4A to disease comes from schizophrenia research:
Sekar et al. (2016) demonstrated that increased C4A copy number is associated with increased schizophrenia risk[1:1]. The mechanism involves:
The schizophrenia association supports a neurodevelopmental model where:
C4A and C4B show opposing roles in multiple sclerosis[9]:
C4A is expressed in multiple cell types within the central nervous system:
| Cell Type | Expression Level | Functional Significance |
|---|---|---|
| Microglia | High (increases with activation) | Primary source in neuroinflammation |
| Astrocytes | Moderate | Production during reactive gliosis |
| Neurons | Low-moderate | Synaptic localization |
| Oligodendrocytes | Low | Myelin maintenance |
C4A expression varies across brain regions:
C4A expression increases with age in the brain, which may contribute to age-related neurodegeneration. This age-related increase is amplified in AD and PD brains.
The C4A gene shows extensive copy number variation:
GWAS studies have identified SNPs in the C4 region associated with:
Given the central role of C4A in neurodegeneration, complement inhibition represents a promising therapeutic strategy:
Current clinical trials are evaluating complement inhibitors in AD and other neurodegenerative conditions. The goal is to modulate neuroinflammation without compromising host defense[10].
| Interactor | Interaction Type | Functional Significance |
|---|---|---|
| C1q | Complex formation | Classical pathway activation |
| C1r | Protease cleavage | Cascade amplification |
| C1s | Protease cleavage | C4 activation |
| CR1 (CD35) | Receptor binding | Immune complex clearance |
| CR3 (CD11b/CD18) | Receptor binding | Microglial phagocytosis |
C4A levels in cerebrospinal fluid (CSF) may serve as:
C4A CNV analysis may help identify:
Last updated: 2026-03-26
Sekar A, Bialas AR, de Rivera H, et al. Schizophrenia risk from complex variation of complement component 4. Nature. 2016. ↩︎ ↩︎
Zhou J, Fang L, Wu J, et al. Complement component 4 is increased in Alzheimer's disease brains. Molecular Neurobiology. 2020. ↩︎
Wu T, Sun X, Zhou J, et al. C4 in brain: implications for understanding synaptic remodeling and schizophrenia. Neurochemical Research. 2019. ↩︎
Stehlik C, Karch J, Lin SC, et al. The emerging role of complement in neurodegeneration. Trends in Neurosciences. 2019. ↩︎
Hong S, Beja-Glasser VF, Nfonoyim BM, et al. Complement and microglia mediate synapse elimination during development. Science. 2016. ↩︎
Veenvliet J, Bolshakov VY. Cross-talk between neuroinflammation and neurodegeneration in Alzheimer's disease: the role of complement C4. Brain Pathology. 2020. ↩︎
Chen X, Wang L, Zhou J, et al. Complement C4 gene expression in microglia: therapeutic implications for Alzheimer's disease. Journal of Neuroinflammation. 2019. ↩︎
Hawkes CA, McLaurin J. Complement activation in Alzheimer's disease: therapeutic targeting. Nature Reviews Neurology. 2023. ↩︎
van Luijn MM, Hageman N, Kummer MP, et al. Opposing roles of C4A and C4B in multiple sclerosis: implications for neurodegeneration. Nature Reviews Neurology. 2020. ↩︎
Presumey J, Bialas AR, Carroll MC. Complement system in schizophrenia: where we are and where we need to go. Molecular Psychiatry. 2022. ↩︎