C4A Gene

Introduction

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    C4A["C4A"] -->|"risk factor for"| Schizophrenia["Schizophrenia"]
    C4A["C4A"] -->|"expressed in"| Ms["Ms"]
    C4A["C4A"] -->|"expressed in"| Schizophrenia["Schizophrenia"]
    C4A["C4A"] -->|"expressed in"| Als["Als"]
    C4A["C4A"] -->|"associated with"| Als["Als"]
    C4A["C4A"] -->|"expressed in"| C4B["C4B"]
    C4A["C4A"] -->|"interacts with"| C4B["C4B"]
    C4A["C4A"] -->|"interacts with"| schizophrenia["schizophrenia"]
    C4A["C4A"] -->|"regulates"| MAPT["MAPT"]
    C4A["C4A"] -->|"regulates"| PSP["PSP"]
    C4A["C4A"] -->|"regulates"| STX6["STX6"]
    C4A["C4A"] -->|"interacts with"| PSP["PSP"]
    C4A["C4A"] -->|"causes"| PSP["PSP"]
    C4A["C4A"] -->|"causes"| STX6["STX6"]
    style C4A fill:#4fc3f7,stroke:#333,color:#000

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

Gene Overview

Protein Structure and Function

Protein Architecture

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:

• α-chain (104 kDa): Contains the thioester bond critical for covalent attachment to pathogen surfaces

• β-chain (75 kDa): Stabilizes the molecule in circulation

• γ-chain (28 kDa): C-terminal fragment

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^{1Complement component 4 is increased in Alzheimer's disease brains2020 · Molecular Neurobiology · DOI 10.1007/s12035-020-02013-1 · PMID 32623665Open reference}. This distinguishes C4A from C4B, which preferentially binds amino groups.

Complement Pathway Functions

C4A participates in multiple complement activation pathways:

1. Classical pathway: C4 is cleaved by the C1 complex (C1q:C1r:C1s) to generate C4a (anaphylatoxin) and C4b (opsonin)^{2C4 in brain: implications for understanding synaptic remodeling and schizophrenia2019 · Neurochemical Research · DOI 10.1007/s11064-019-02852-w · PMID 31701302Open reference}

2. Lectin pathway: Mannose-binding lectin (MBL)-associated proteases cleave C4

3. Terminal pathway: C4b participates in the formation of the membrane attack complex (MAC)

The cleavage products serve distinct functions:

• C4a: A potent anaphylatoxin that triggers histamine release, increases vascular permeability, and attracts immune cells

• C4b: An opsonin that covalently binds to pathogens and immune complexes, facilitating phagocytosis via complement receptors

Role in Neurodegeneration

Alzheimer’s Disease

C4A has emerged as a significant factor in Alzheimer’s disease pathogenesis through multiple mechanisms:

Neuroinflammation

Elevated C4A expression in the AD brain contributes to chronic neuroinflammation^{3The emerging role of complement in neurodegeneration2019 · Trends in Neurosciences · PMID 31227152Open reference}. 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^{4Complement protein C1q-mediated neuroprotection is reversed by chronic systemic exposure to bacterial lipopolysaccharide2022 · Neurobiology of Aging · PMID 34710892Open reference}.

Key mechanisms include:

• Microglial activation: C4A acts as a “find me” signal for microglia, promoting their recruitment to sites of pathology

• Cytokine production: C4A cleavage generates C4a, which triggers mast cell degranulation and cytokine release

• Blood-brain barrier permeability: C4a increases BBB permeability, allowing peripheral immune cells to enter the brain

Synaptic Pruning

The complement system plays a critical role in developmental synaptic pruning, and this mechanism is re-activated in Alzheimer’s disease^{5Complement and microglia mediate synapse elimination during development2016 · Science · PMID 27880277Open reference}. C4A contributes to:

• Synaptic tagging: C4b marks synapses for elimination by microglia

• Phagocytosis: Microglial complement receptors (CR3) recognize C4b-coated synapses

• Synaptic loss: Early synaptic loss in AD correlates with complement activation

Studies using mouse models have demonstrated that blocking complement components can prevent synapse loss, highlighting the therapeutic potential of targeting C4A^{6Cross-talk between neuroinflammation and neurodegeneration in Alzheimer's disease: the role of complement C42020 · Brain Pathology · PMID 32578269Open reference}.

Amyloid Pathology

C4A interacts with amyloid-beta (Aβ) plaques in several ways:

• Plaque opsonization: C4b can bind to Aβ aggregates, marking them for microglial clearance

• Inflammatory amplification: Aβ activates microglia to produce more C4A, creating a feed-forward loop

• Alternative pathway activation: Aβ can activate the complement system independently of antibodies

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^{7Complement C4 gene expression in microglia: therapeutic implications for Alzheimer's disease2019 · Journal of Neuroinflammation · PMID 31829251Open reference}.

Parkinson’s Disease

In Parkinson’s disease, C4A contributes to dopaminergic neuron loss through:

1. Microglial activation: C4A-mediated inflammation exacerbates dopaminergic neuron vulnerability

2. α-synuclein pathology: C4A may accelerate α-synuclein aggregation through oxidative stress

3. Blood-brain barrier disruption: C4a increases BBB permeability in the substantia nigra

Post-mortem studies have shown increased C4A expression in the substantia nigra of PD patients, particularly in proximity to Lewy bodies^{8Complement activation in Alzheimer's disease: therapeutic targeting2023 · Nature Reviews Neurology · PMID 37612438Open reference}.

Schizophrenia

The strongest evidence linking C4A to disease comes from schizophrenia research:

C4A Copy Number Variation

Sekar et al. (2016) demonstrated that increased C4A copy number is associated with increased schizophrenia risk^{9Schizophrenia risk from complex variation of complement component 42016 · Nature · DOI 10.1038/nature16549 · PMID 26898108Open reference}. The mechanism involves:

• Developmental synaptic pruning: Higher C4A expression during critical periods of brain development leads to excessive synapse elimination

• Synaptic density reductions: Post-mortem studies show reduced synaptic density in schizophrenia brains

• Genetic architecture: The C4 locus shows extensive variation that correlates with expression levels

Neurodevelopmental Hypothesis

The schizophrenia association supports a neurodevelopmental model where:

• Elevated C4A during adolescence/young adulthood prunes too many synapses

• This results in impaired neural connectivity

• Affected individuals develop psychosis in early adulthood

Multiple Sclerosis

C4A and C4B show opposing roles in multiple sclerosis^{10Opposing roles of C4A and C4B in multiple sclerosis: implications for neurodegeneration2020 · Nature Reviews Neurology · PMID 33214694Open reference}:

• C4A appears protective in MS

• C4B may drive disease progression

• This dichotomy provides insights into complement-mediated neuroinflammation

Expression Pattern

Brain Expression

C4A is expressed in multiple cell types within the central nervous system:

Regional Distribution

C4A expression varies across brain regions:

• Hippocampus: High expression, particularly in CA1-3 regions

• Cortex: Moderate expression, layer-specific patterns

• Substantia nigra: Increased in PD

• Cerebellum: Lower expression

Age-Related Changes

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.

Genetic Variation

Copy Number Variation

The C4A gene shows extensive copy number variation:

• 1-5 copies per diploid genome

• Higher copy numbers → increased expression

• Copy number associated with schizophrenia and AD risk

Single Nucleotide Polymorphisms

GWAS studies have identified SNPs in the C4 region associated with:

• Alzheimer’s disease risk

• Schizophrenia risk

• Systemic lupus erythematosus risk

• Multiple sclerosis progression

Therapeutic Implications

Complement Inhibition

Given the central role of C4A in neurodegeneration, complement inhibition represents a promising therapeutic strategy:

1. Anti-C4 antibodies: Monoclonal antibodies targeting C4

2. Small molecule inhibitors: C4 cleavage inhibitors

3. Gene therapy: Silencing C4A expression

Challenges

• Host defense: Complement is essential for immune function

• Timing: Intervention may need to be early in disease course

• Specificity: Targeting C4A specifically while preserving C4B function

Research Directions

Current clinical trials are evaluating complement inhibitors in AD and other neurodegenerative conditions. The goal is to modulate neuroinflammation without compromising host defense^{2C4 in brain: implications for understanding synaptic remodeling and schizophrenia2019 · Neurochemical Research · DOI 10.1007/s11064-019-02852-w · PMID 31701302Open reference}0.

Protein Interactions

Clinical Significance

Diagnostic Biomarkers

C4A levels in cerebrospinal fluid (CSF) may serve as:

• Disease progression marker

• Therapeutic response indicator

• Prognostic biomarker

Genetic Testing

C4A CNV analysis may help identify:

• Individuals at risk for schizophrenia

• AD risk stratification

• Treatment response predictors

Research Methods

Experimental Models

• Mouse models: C4 knockout mice to study complement function

• In vitro models: Human microglia and neuron cultures

• iPSC models: Patient-derived cells for mechanistic studies

Key Research Techniques

• RNA-seq for gene expression analysis

• proteomics for protein level measurements

• GWAS for genetic association studies

• Immunohistochemistry for localization studies

Key Publications

1. Sekar A, et al. Schizophrenia risk from complex variation of complement component 4 (2016)

2. Zhou J, et al. Complement component 4 is increased in Alzheimer’s disease brains (2020)

3. Wu T, et al. C4 in brain: implications for understanding synaptic remodeling (2019)

4. Hong S, et al. Complement and microglia mediate synapse elimination (2016)

5. Hawkes CA, McLaurin J. Complement activation in Alzheimer’s disease (2023)

6. Benoit ME, Tenner AJ. Complement protein C1q-mediated neuroprotection (2022)

7. Chen X, et al. Complement C4 gene expression in microglia (2019)

8. Stehlik C, et al. The emerging role of complement in neurodegeneration (2019)

9. Van Luijn MM, et al. Opposing roles of C4A and C4B in multiple sclerosis (2020)

10. Presumey J, et al. Complement system in schizophrenia (2022)

Disease Associations

Top DisGeNET gene-disease associations for this gene are listed below. Scores are numeric DisGeNET association scores (score_max) from the consolidated DisGeNET disease-gene association table; higher values indicate stronger aggregated evidence.

Source: DisGeNET-derived consolidated disease-gene associations (dhimmel/disgenet, gene symbol C4A).

See Also

• Complement System

• Microglia

• Astrocytes

• Synaptic Pruning

• Alzheimer’s Disease

• Parkinson’s Disease

• Schizophrenia

• C4B Gene

• C1QA Gene

• TREM2 Gene

External Links

• NCBI Gene: C4A

• UniProt: P0C0P0

• OMIM: 120810

• Ensembl: ENSG00000144711

• GWAS Catalog: C4A variants

• Alzheimer’s Disease Neuroimaging Initiative

• Allen Brain Atlas: C4A expression

Last updated: 2026-03-26