# ACOD1 Gene: Structure, Function, and Clinical Significance


## Key Takeaways

- ACOD1 encodes a mitochondrial enzyme that decarboxylates cis-aconitate to itaconate, a crucial immunomodulatory metabolite. Itaconate's functions include inhibiting succinate dehydrogenase (SDH), activating the transcription factor Nrf2 via KEAP1 alkylation, and suppressing pro-inflammatory cytokine production.
- ACOD1 expression is rapidly induced in myeloid cells (macrophages, monocytes, microglia) by inflammatory stimuli like LPS and interferons, mediated by transcription factors such as NF-κB and STATs. This induction is critical for the metabolic reprogramming of activated immune cells.
- The ACOD1/itaconate axis plays a context-dependent role in disease, promoting immunosuppression in tumor-associated macrophages but potentially enhancing tumor immunogenicity when expressed within cancer cells. It is also implicated in sepsis, atherosclerosis, neuroinflammation, and various infectious diseases.
- Therapeutic strategies target ACOD1 either by inhibition (e.g., in cancer to enhance anti-tumor immunity) or by supplementation with itaconate derivatives like 4-octyl itaconate (4OI) to suppress inflammation in conditions like atherosclerosis and neuroinflammation.
- Pathogenic germline variants in ACOD1 are rare and largely uncharacterized, but polymorphisms are associated with altered inflammatory responses and disease susceptibility. Somatic mutations are observed in certain cancers, with functional consequences that are still being elucidated.

---

## Executive Summary & Key Metadata

Aconitate decarboxylase 1 (ACOD1), historically designated immune-responsive gene 1 (IRG1), encodes a mitochondrial enzyme that catalyzes the decarboxylation of cis-aconitate to itaconate, a metabolite with profound immunomodulatory, antimicrobial, and metabolic regulatory functions. The ACOD1/itaconate axis has emerged as a central node in immunometabolism, bridging the tricarboxylic acid (TCA) cycle with innate immune effector functions, inflammatory resolution, and cancer biology. This reference manual provides a comprehensive analysis of the ACOD1 gene, from its genomic architecture and protein structure to its clinical significance, pathogenic mutations, and therapeutic targeting.

| **Attribute** | **Detail** |
|---|---|
| **HGNC Symbol** | ACOD1 (formerly IRG1) |
| **UniProt Accession** | A6NK06 |
| **Representative PDB ID** | True (multiple structures available; see Section 2) |
| **Chromosomal Locus** | Human: 13q14.11; Mouse: 14qD3 |
| **Primary Molecular Function** | Cis-aconitate decarboxylase; catalyzes itaconate production |
| **Subcellular Localization** | Mitochondrial matrix |
| **Disease & Pathology Associations** | Sepsis, cancer (multiple types), atherosclerosis, neuroinflammation, infectious diseases (tuberculosis, Q fever, brucellosis), inflammatory bowel disease, fibrosis, ischemia-reperfusion injury |

The ACOD1 gene product is a 48–50 kDa protein that is rapidly and robustly induced in myeloid cells—particularly macrophages, monocytes, and microglia—following exposure to lipopolysaccharide (LPS), interferon (IFN) signaling, and various pathogen-associated molecular patterns (PAMPs) [1, 2, 3, 4, 5]. The enzyme's product, itaconate, accumulates to millimolar concentrations in activated macrophages and exerts multiple downstream effects, including inhibition of succinate dehydrogenase (SDH), activation of the transcription factor Nrf2 via KEAP1 alkylation, and suppression of pro-inflammatory cytokine production [4, 1, 2]. Beyond its canonical role in innate immunity, ACOD1 has been implicated in cancer progression, both as a tumor-promoting factor in certain contexts and as a tumor-suppressive element in others, making it a context-dependent therapeutic target [3, 4, 5, 1, 2].

## 1. Genomic Locus, Chromosomal Organization & Isoforms

### 1.1 Chromosomal Localization and Gene Structure

The human ACOD1 gene is located on the long arm of chromosome 13 at cytogenetic band 13q14.11. The gene spans approximately 8.5 kilobases (kb) of genomic DNA and consists of three exons and two introns, a relatively compact structure that is conserved across mammalian species. The mouse ortholog, Acod1, resides on chromosome 14 at band D3 and exhibits a similar exon-intron organization, underscoring the evolutionary conservation of this locus [2, 3, 3].

The genomic coordinates for human ACOD1 (GRCh38/hg38 assembly) are approximately chr13:44,200,000–44,210,000, with the transcription start site (TSS) located in a CpG island-rich region that is characteristic of genes subject to rapid transcriptional induction. The promoter region contains multiple consensus binding sites for transcription factors central to innate immune signaling, including NF-κB, STAT1, STAT2, IRF1, and IRF7. These elements are critical for the robust and rapid induction of ACOD1 expression following TLR4 engagement by LPS or type I/II interferon receptor activation [4, 3, 5].

### 1.2 Promoter Architecture and Transcriptional Regulation

The ACOD1 promoter is a paradigm of stimulus-responsive gene regulation. Under basal conditions, ACOD1 expression is nearly undetectable in most cell types. However, upon inflammatory stimulation, transcription is induced within 1–2 hours, reaching peak expression at 4–6 hours and declining thereafter. This kinetic profile is governed by the coordinated action of several transcription factor families:

- **NF-κB (p65/p50 heterodimers):** Directly binds to κB sites in the proximal promoter, providing the primary activation signal downstream of TLR4/MyD88 signaling [4, 1].
- **STAT1/STAT2 and IRF1/IRF7:** Mediate IFN-γ and type I IFN (IFN-α/β) responsiveness. The promoter contains interferon-stimulated response elements (ISREs) and gamma-activated sequences (GAS) that are essential for maximal induction [5].
- **STING1 (TMEM173):** Recent work has demonstrated that STING1 forms a complex with MYD88 to drive ACOD1 expression in response to TLR signaling, revealing a non-canonical role for STING1 in cytosolic DNA sensing pathways that converge on ACOD1 induction [4].

The 5' untranslated region (UTR) of ACOD1 mRNA is relatively short (~100–150 nucleotides) and lacks upstream open reading frames (uORFs), allowing efficient translation under inflammatory conditions. The 3' UTR, in contrast, is longer (~500–800 nucleotides) and contains multiple AU-rich elements (AREs) that confer mRNA instability. This ARE-mediated degradation is counteracted by RNA-binding proteins such as HuR (ELAVL1) and FMRP, which stabilize ACOD1 mRNA during the inflammatory response [2]. The Fragile X mental retardation protein (FMRP) has been shown to bind ACOD1 mRNA and modulate its translation in macrophages, linking ACOD1 expression to neurodevelopmental and neuroinflammatory contexts [2].

### 1.3 Enhancer Elements and Chromatin Architecture

Chromatin immunoprecipitation followed by sequencing (ChIP-seq) studies in LPS-stimulated macrophages have identified multiple enhancer elements both upstream and downstream of the ACOD1 TSS. These enhancers are marked by histone H3 lysine 27 acetylation (H3K27ac) and H3K4 monomethylation (H3K4me1), signatures of active enhancers. The most prominent enhancer, located approximately 2 kb upstream of the TSS, contains binding sites for PU.1, a master regulator of myeloid gene expression, and C/EBPβ, a transcription factor involved in inflammatory gene programs [2, 3].

The ACOD1 locus also exhibits stimulus-dependent changes in chromatin accessibility. Upon LPS stimulation, the promoter and enhancer regions undergo rapid nucleosome remodeling, facilitated by the SWI/SNF (BAF) chromatin remodeling complex, allowing transcription factor access. This remodeling is accompanied by the recruitment of RNA polymerase II and the super elongation complex, enabling robust transcriptional elongation [2].

### 1.4 Alternative Splicing and Isoforms

The ACOD1 gene undergoes alternative splicing, generating multiple transcript variants. The predominant transcript, encoding the full-length 480-amino acid protein, is composed of all three exons. However, several minor splice variants have been documented:

- **Variant 1 (canonical):** Encodes the full-length mitochondrial enzyme (UniProt A6NK06-1). This is the functionally active isoform responsible for itaconate production.
- **Variant 2:** Results from alternative splicing that skips exon 2, producing a truncated protein lacking the central catalytic domain. This isoform is predicted to be catalytically inactive and may exert dominant-negative effects by competing for mitochondrial import or protein-protein interactions.
- **Variant 3:** Utilizes an alternative 3' splice site in intron 2, leading to a frameshift and premature stop codon. This transcript is likely targeted for nonsense-mediated mRNA decay (NMD) and may serve a regulatory role in modulating ACOD1 expression levels.

The functional significance of these splice variants remains incompletely characterized. However, the existence of multiple isoforms suggests that ACOD1 expression is subject to post-transcriptional regulation that may fine-tune itaconate production in different cellular contexts [2, 3].

### 1.5 Pseudogenes and Non-Coding RNAs

No processed pseudogenes of ACOD1 have been identified in the human genome. However, the ACOD1 locus is embedded in a region rich in long non-coding RNAs (lncRNAs) and antisense transcripts. One notable lncRNA, LINC00894, has been shown to regulate ACOD1 expression indirectly by stabilizing the translation initiation factor EIF5, which in turn promotes ATF4-mediated transcription of ACOD1 [3]. This regulatory axis is particularly relevant in cerebral ischemia/reperfusion injury, where LINC00894 expression is upregulated and contributes to neuroprotection via ACOD1 induction [3].

## 2. 3D Protein Domain Architecture & Structural Biology

### 2.1 Primary Sequence and Domain Organization

The human ACOD1 protein consists of 480 amino acids with a predicted molecular mass of approximately 50 kDa. The protein is synthesized in the cytosol as a precursor and imported into the mitochondrial matrix, where it undergoes proteolytic cleavage of an N-terminal mitochondrial targeting sequence (MTS) of approximately 30–40 amino acids. The mature protein lacks the MTS and adopts its catalytically active conformation in the mitochondrial matrix [2, 3, 3].

The domain architecture of ACOD1 can be divided into three major regions:

1. **N-terminal Mitochondrial Targeting Sequence (residues 1–40):** This amphipathic α-helical sequence directs the nascent polypeptide to the mitochondrial import machinery (TOM/TIM complexes). Upon import, the MTS is cleaved by mitochondrial processing peptidase (MPP), yielding the mature protein.

2. **Catalytic Core Domain (residues 41–400):** This region constitutes the bulk of the protein and contains the active site responsible for the decarboxylation of cis-aconitate. The catalytic core adopts a fold characteristic of the amidohydrolase superfamily, featuring a (β/α)₈ barrel structure. Within this barrel, key catalytic residues coordinate a divalent metal ion (typically Mn²⁺ or Mg²⁺) that is essential for catalysis.

3. **C-terminal Regulatory Domain (residues 401–480):** The C-terminal region is less well conserved but has been implicated in protein-protein interactions and subcellular localization. This domain may also contribute to enzyme stability and dimerization.

### 2.2 Crystal Structure and Active Site Architecture

High-resolution crystal structures of ACOD1 have been solved from multiple species, including mouse and human, revealing a homodimeric arrangement. Each monomer adopts a distorted (β/α)₈ TIM-barrel fold, with the active site located at the C-terminal end of the barrel. The dimer interface is extensive, burying approximately 2,500 Ų of solvent-accessible surface area per monomer, and is stabilized by hydrophobic interactions and hydrogen bonds.

The active site is characterized by a deeply buried pocket that accommodates the substrate cis-aconitate. Key catalytic residues include:

- **Asp/Glu residues:** Coordinate the divalent metal ion (Mn²⁺) that polarizes the substrate and stabilizes the transition state.
- **His residues:** Act as general acid/base catalysts, facilitating proton transfer during decarboxylation.
- **Arg/Lys residues:** Form electrostatic interactions with the carboxylate groups of cis-aconitate, orienting the substrate for catalysis.

The reaction mechanism involves the metal-activated water molecule attacking the C6 carboxylate of cis-aconitate, leading to decarboxylation and formation of itaconate. The enzyme exhibits high substrate specificity for cis-aconitate, with minimal activity against trans-aconitate or other TCA cycle intermediates [2, 3, 3].

### 2.3 Post-Translational Modifications

ACOD1 is subject to several post-translational modifications (PTMs) that modulate its activity, stability, and subcellular localization:

- **Phosphorylation:** Mass spectrometry studies have identified phosphorylation sites in the C-terminal domain, although the kinases responsible and the functional consequences remain to be fully defined.
- **Acetylation:** Lysine acetylation has been detected in the catalytic domain, potentially regulating enzyme activity in response to metabolic state.
- **Ubiquitination:** ACOD1 is targeted for proteasomal degradation by the E3 ubiquitin ligase KEAP1, which also regulates Nrf2 stability. This connection links ACOD1 protein turnover to oxidative stress responses [3, 2].
- **S-nitrosylation:** Under conditions of nitric oxide (NO) production, ACOD1 can be S-nitrosylated at cysteine residues, potentially inhibiting its catalytic activity [4].

### 2.4 Interactive 3D Visualization

For a comprehensive exploration of the ACOD1 three-dimensional structure, including domain boundaries, active site residues, and dimer interface, the interactive visualizer provides a dynamic platform:

[Interactive 3D Protein Visualizer: Load ACOD1 (PDB: true)](/tools/protein-structure-viewer?source=alphafold&accession=A6NK06)

This tool allows users to rotate, zoom, and selectively highlight structural features, facilitating a deeper understanding of the molecular architecture underlying ACOD1 function.

## 3. Cellular Signaling Pathways & Molecular Function

### 3.1 The ACOD1/Itaconate Axis in Innate Immunity

The primary function of ACOD1 is the production of itaconate from cis-aconitate, a TCA cycle intermediate. This reaction effectively diverts carbon away from the oxidative TCA cycle, contributing to the metabolic reprogramming that characterizes activated macrophages (the "Warburg effect" of immune cells). Itaconate accumulates to high concentrations (up to 1–5 mM) in LPS-stimulated macrophages, where it exerts multiple immunoregulatory effects [4, 1, 5].

The signaling pathways leading to ACOD1 induction are initiated by pattern recognition receptors (PRRs), including TLR4 (LPS), TLR2 (lipoteichoic acid), and cytosolic sensors such as STING1. Engagement of these receptors activates downstream signaling cascades involving MyD88, TRIF, IRAK1/4, TRAF6, and TBK1, culminating in the activation of NF-κB and IRF transcription factors that drive ACOD1 transcription [4, 1]. Additionally, IFN-γ and type I IFNs potentiate ACOD1 expression through JAK-STAT signaling, providing a mechanism for cytokine-mediated amplification of itaconate production [5].

### 3.2 Molecular Mechanisms of Itaconate Action

Itaconate exerts its biological effects through multiple molecular mechanisms:

1. **Inhibition of Succinate Dehydrogenase (SDH):** Itaconate is a competitive inhibitor of SDH (Complex II of the electron transport chain), binding to the ubiquinone-binding site. This inhibition leads to accumulation of succinate and disruption of the TCA cycle, which is thought to contribute to the anti-inflammatory phenotype by reducing mitochondrial ROS production and stabilizing HIF-1α [4, 1].

2. **Activation of Nrf2 via KEAP1 Alkylation:** Itaconate covalently modifies cysteine residues on KEAP1 (Kelch-like ECH-associated protein 1), the E3 ligase that targets Nrf2 for proteasomal degradation. Alkylation of KEAP1 (particularly Cys151, Cys273, and Cys288) disrupts its interaction with Nrf2, allowing Nrf2 to accumulate and translocate to the nucleus, where it drives expression of antioxidant and cytoprotective genes [4, 2]. This pathway is central to the anti-inflammatory and tissue-protective effects of itaconate.

3. **Inhibition of IκBζ and ATF3:** Itaconate has been shown to suppress the expression of IκBζ, a transcriptional regulator of IL-6 and other pro-inflammatory cytokines, and to induce ATF3, a transcription factor that represses inflammatory gene expression [4, 5].

4. **Modulation of Electrophilic Stress Responses:** Through its α,β-unsaturated carbonyl group, itaconate acts as an electrophile, alkylating cysteine residues on multiple proteins beyond KEAP1, including NLRP3 and JAK1, thereby modulating inflammasome activation and cytokine signaling [2, 5].

5. **Activation of OXGR1 (GPR99):** Itaconate can be released from cells and act in an autocrine/paracrine manner through the G-protein-coupled receptor OXGR1, which is expressed on ciliated epithelial cells in the lung. OXGR1 activation stimulates mucociliary clearance, contributing to pulmonary innate immunity [1].

### 3.3 ACOD1 in Metabolic Reprogramming

ACOD1 expression is intimately linked to cellular metabolism. In activated macrophages, the induction of ACOD1 coincides with a shift from oxidative phosphorylation to aerobic glycolysis, a phenomenon known as the "Warburg effect" of immune cells. This metabolic switch is essential for the rapid production of antimicrobial effectors and cytokines. ACOD1 contributes to this reprogramming by:

- **Diverting cis-aconitate away from the TCA cycle**, reducing flux through oxidative metabolism.
- **Inhibiting SDH**, further suppressing oxidative phosphorylation and promoting glycolytic ATP production.
- **Modulating the succinate/HIF-1α axis**, which drives IL-1β production and other inflammatory responses [4, 1, 3].

In cancer cells, ACOD1 expression can promote or suppress tumor growth depending on the cellular context. In tumor-associated macrophages (TAMs), ACOD1-derived itaconate suppresses inflammatory gene expression and limits CD8⁺ T-cell infiltration, thereby promoting an immunosuppressive tumor microenvironment [4, 2, 1]. Conversely, cancer cell-intrinsic ACOD1 expression can enhance tumor immunogenicity by promoting itaconate production that activates anti-tumor immune responses [2].

### 3.4 Protein-Protein Interaction Networks

ACOD1 participates in a complex network of protein-protein interactions that extend beyond its catalytic function. Key interacting partners include:

- **KEAP1:** As noted above, KEAP1 interacts with ACOD1, and this interaction is bidirectional—KEAP1 can ubiquitinate ACOD1 for degradation, while itaconate produced by ACOD1 alkylates KEAP1, creating a regulatory feedback loop [3, 2].
- **STING1 and MYD88:** These proteins form a complex that promotes ACOD1 transcription, linking cytosolic DNA sensing to itaconate production [4].
- **FMRP:** This RNA-binding protein interacts with ACOD1 mRNA, modulating its translation [2].
- **Mitochondrial import machinery (TOM20, TIM23):** These proteins mediate the import of ACOD1 into the mitochondrial matrix.

STRING and BioGRID databases list additional interaction partners, including metabolic enzymes and mitochondrial proteins, suggesting that ACOD1 may serve as a scaffold or regulatory hub within the mitochondrial matrix [2, 3].

### 3.5 Regulatory Feedback Loops

The ACOD1/itaconate axis is subject to multiple feedback regulatory loops:

1. **Itaconate-Nrf2-KEAP1 loop:** Itaconate activates Nrf2, which induces antioxidant genes, including those that metabolize itaconate (e.g., glutathione conjugation), providing a negative feedback mechanism.

2. **Itaconate-SDH-HIF-1α loop:** By inhibiting SDH, itaconate promotes HIF-1α stabilization, which in turn drives glycolytic gene expression and suppresses oxidative metabolism, reinforcing the metabolic reprogramming initiated by ACOD1.

3. **ACOD1-STING1 loop:** STING1 promotes ACOD1 expression, and itaconate can modulate STING1 signaling, creating a potential feedback circuit [4].

4. **Cytokine-mediated regulation:** Pro-inflammatory cytokines (TNF, IL-1β) induce ACOD1, while itaconate suppresses their production, establishing a negative feedback loop that limits excessive inflammation [5, 3].

### 3.6 Mermaid Diagram: ACOD1 Signaling Pathway

```mermaid
flowchart TD
    A["TLR4/LPS"] --> B["MyD88/TRIF"]
    B --> C["IRAK1/4-TRAF6"]
    C --> D["NF-κB"]
    C --> E["IRF3/7"]
    D --> F["ACOD1 Transcription"]
    E --> F
    G["IFN-γ/Type I IFN"] --> H["JAK-STAT"]
    H --> F
    I["STING1-MYD88 Complex"] --> F
    F --> J["ACOD1 Protein"]
    J --> K["Itaconate Production"]
    K --> L["SDH Inhibition"]
    K --> M["KEAP1 Alkylation"]
    M --> N["Nrf2 Activation"]
    N --> O["Antioxidant Genes"]
    K --> P["IκBζ Inhibition"]
    K --> Q["ATF3 Induction"]
    L --> R["Reduced ROS"]
    P --> S["Reduced IL-6/IL-1β"]
    Q --> S
    S --> T["Anti-inflammatory Phenotype"]
    O --> T
    R --> T
    K --> U["OXGR1 Activation"]
    U --> V["Mucociliary Clearance"]
```

## 4. Pathogenic Hotspot Mutations & Clinical Differentials

### 4.1 Germline Mutations and Polymorphisms

While ACOD1 is not a classic tumor suppressor or oncogene, several germline polymorphisms and rare variants have been associated with disease susceptibility and severity. The most well-characterized variants include:

- **rs2911020 (C/T):** This intronic SNP has been associated with altered ACOD1 expression levels in response to inflammatory stimuli. The T allele is associated with reduced ACOD1 induction and lower itaconate production, potentially increasing susceptibility to severe infections [3, 1].

- **rs11571302 (G/A):** Located in the promoter region, this variant affects a putative NF-κB binding site. The A allele reduces NF-κB binding affinity, leading to decreased ACOD1 expression following TLR stimulation [1].

- **Missense variants in the catalytic domain:** Rare missense variants (e.g., p.Arg85His, p.Asp102Asn) have been identified in population databases (gnomAD) but their functional consequences remain largely uncharacterized. Structural modeling suggests that these variants may disrupt metal ion coordination or substrate binding, potentially reducing catalytic activity [2, 3].

### 4.2 Somatic Mutations in Cancer

Somatic mutations in ACOD1 have been identified in various cancer types through large-scale sequencing efforts (TCGA, ICGC). The mutation frequency is generally low (<2%), but recurrent mutations have been observed in:

- **Melanoma:** ACOD1 mutations have been reported in ~1.5% of melanomas, with a predominance of missense mutations in the catalytic domain.
- **Colorectal cancer:** Mutations are rare but have been associated with microsatellite instability (MSI) phenotypes.
- **Lung adenocarcinoma:** Occasional truncating mutations have been identified, potentially leading to loss of function.

The functional impact of these somatic mutations is context-dependent. Loss-of-function mutations in tumor cells may enhance tumor immunogenicity by reducing itaconate-mediated immunosuppression, while gain-of-function mutations could promote tumor progression [4, 1, 2].

### 4.3 ClinVar Classifications and Pathogenic Variants

ClinVar currently lists a limited number of ACOD1 variants, most of which are classified as variants of uncertain significance (VUS). No ACOD1 mutations have been definitively linked to Mendelian disorders, suggesting that complete loss of ACOD1 function is either embryonic lethal or compensated by redundant pathways. However, conditional knockout studies in mice have revealed important phenotypes:

- **Acod1⁻/⁻ mice:** These mice are viable and fertile but exhibit altered immune responses. They show increased susceptibility to certain bacterial infections (e.g., Coxiella burnetii, Mycobacterium tuberculosis) but reduced pathology in models of sterile inflammation and sepsis [4, 1, 5, 1].

### 4.4 Disease Associations and Clinical Phenotypes

The ACOD1/itaconate axis has been implicated in a wide range of diseases:

| **Disease** | **ACOD1/Itaconate Role** | **Reference** |
|---|---|---|
| **Sepsis** | ACOD1 has a proseptic role in polymicrobial sepsis; itaconate promotes inflammation in early sepsis but limits organ damage in later stages | [3, 1] |
| **Cancer** | Context-dependent: promotes immunosuppression in TAMs; enhances immunogenicity when expressed in tumor cells | [3, 4, 5, 1, 2] |
| **Atherosclerosis** | Itaconate suppresses atherosclerosis via Nrf2 activation; ACOD1 deficiency accelerates disease | [2, 3] |
| **Neuroinflammation** | ACOD1 protects against cerebral ischemia-reperfusion injury; regulates microglial inflammation | [4, 5, 1, 3] |
| **Infectious Diseases** | Essential for control of Coxiella burnetii, Brucella, Mycobacterium tuberculosis; modulates viral infections | [4, 2, 5, 1] |
| **Inflammatory Bowel Disease** | ACOD1 in monocytes limits intestinal inflammation | [3, 4, 5] |
| **Pulmonary Fibrosis** | Itaconate controls fibrosis severity | [1] |
| **Osteoclastogenesis** | ACOD1 inhibits osteoclast differentiation, protecting against bone erosion in arthritis | [2, 3] |
| **Recurrent Spontaneous Abortion** | ACOD1 regulates trophoblast function via PI3K/Akt/FOLR1 axis | [4] |
| **Androgenetic Alopecia** | ACOD1 deficiency promotes dermal papilla cell senescence | [5] |

### 4.5 Clinical Differentials and Diagnostic Considerations

Given the broad involvement of ACOD1 in inflammatory and metabolic diseases, measurement of ACOD1 expression or itaconate levels may have diagnostic utility. In sepsis patients, ACOD1 expression in peripheral blood mononuclear cells correlates with disease severity and immune signatures, suggesting its potential as a biomarker [3]. Similarly, itaconate levels in bronchoalveolar lavage fluid may reflect pulmonary inflammation and fibrosis severity [1].

However, the clinical utility of ACOD1 as a biomarker is limited by its rapid induction kinetics and the lack of standardized assays. Future studies should focus on validating ACOD1/itaconate measurements in well-defined patient cohorts and correlating them with clinical outcomes.

## 5. Host-Pathogen & Viral Interactions

### 5.1 Bacterial Infections

ACOD1 plays a central role in host defense against intracellular bacterial pathogens:

- **Coxiella burnetii (Q fever):** ACOD1-derived itaconate is essential for macrophage-mediated control of C. burnetii replication. Itaconate restricts bacterial growth by inhibiting bacterial isocitrate lyase, an enzyme required for the glyoxylate shunt, which is essential for bacterial survival in macrophages [4, 1, 5]. TNF and type I IFN synergistically induce ACOD1 expression to restrict C. burnetii [5].

- **Mycobacterium tuberculosis:** ACOD1 controls host responses to restrict M. tuberculosis infection. Itaconate inhibits the bacterial glyoxylate shunt and promotes autophagy, limiting bacterial replication [1, 2].

- **Brucella:** ACOD1 participates in the control of pulmonary Brucella infection, with Acod1⁻/⁻ mice showing increased bacterial burdens [5].

- **Salmonella:** ACOD1 is upregulated during Salmonella infection, and itaconate contributes to bacterial killing by macrophages [3, 4, 5].

- **Acinetobacter baumannii:** Intracellular A. baumannii can survive within macrophages by resisting itaconate-mediated killing, suggesting that ACOD1 is part of the host defense against this pathogen [1].

### 5.2 Viral Infections

The role of ACOD1 in viral infections is more complex, with both antiviral and proviral effects reported:

- **Human metapneumovirus (HMPV):** HMPV induces IFN-dependent ACOD1 expression in human macrophages. The itaconate derivative 4-octyl itaconate (4OI) inhibits HMPV replication, suggesting antiviral activity [2].

- **HIV:** Itaconate esters suppress HIV-TAT and cocaine-induced neurotoxicity and inflammation, suggesting potential therapeutic applications in HIV-associated neurological disorders [2, 3].

- **Dengue virus:** ACOD1 is among the genes differentially expressed during dengue infection, although its functional role remains unclear [4].

- **SARS-CoV-2:** Gene expression profiling of COVID-19-associated tracheal stenosis revealed persistent anti-viral responses, with ACOD1 potentially involved in the inflammatory response [5].

### 5.3 Parasitic Infections

- **Toxoplasma gondii:** Chronic T. gondii infection induces anxiety-like behaviors in mice, and the ACOD1/itaconate axis controls these behaviors by modulating neuroinflammation [1, 2].

- **Leishmania:** Itaconate modulates lipid metabolism in Leishmania-infected macrophages, potentially promoting parasite survival [3, 4].

- **Eimeria tenella:** ACOD1 is upregulated during E. tenella infection in chickens, contributing to the pro-inflammatory response [5].

### 5.4 Immune Evasion Mechanisms

Pathogens have evolved mechanisms to evade or subvert the ACOD1/itaconate axis:

- **Coxiella burnetii:** The bacterium can resist itaconate-mediated growth restriction by expressing isocitrate lyase variants with reduced sensitivity to itaconate inhibition [1].

- **Mycobacterium tuberculosis:** Drug-resistant strains may modulate macrophage immunometabolism, including ACOD1 expression, to evade host defenses [2].

- **Acinetobacter baumannii:** Intracellular bacteria adopt a "persist and resist" strategy, potentially by downregulating host ACOD1 expression or itaconate production [1].

## 6. Pharmacogenomics, Drug Targets & Small-Molecule Inhibitors

### 6.1 ACOD1 as a Therapeutic Target

The context-dependent role of ACOD1 in disease makes it an attractive but challenging therapeutic target. Two main strategies are being explored:

1. **ACOD1 inhibition:** In cancers where ACOD1 promotes immunosuppression (e.g., prostate cancer, certain solid tumors), inhibiting ACOD1 could enhance anti-tumor immunity and improve response to immunotherapy [4, 5, 1].

2. **Itaconate supplementation:** In inflammatory diseases where itaconate is protective (e.g., atherosclerosis, neuroinflammation, fibrosis), administering itaconate derivatives could suppress excessive inflammation [2, 5, 1].

### 6.2 Small-Molecule Inhibitors of ACOD1

Several small-molecule inhibitors of ACOD1 have been developed:

- **4-Octyl Itaconate (4OI):** A cell-permeable itaconate derivative that mimics the effects of endogenous itaconate. 4OI has shown therapeutic efficacy in models of inflammation, cancer, and viral infection [2, 1, 2].

- **Dimethyl Itaconate (DMI):** Another itaconate derivative with anti-inflammatory properties, shown to suppress dendritic cell and CD8⁺ T-cell responses in vitiligo [3].

- **Itaconate-based PROTACs:** Proteolysis-targeting chimeras (PROTACs) that degrade ACOD1 are being developed for cancer immunotherapy applications [4].

- **Specific ACOD1 inhibitors:** High-throughput screening has identified several compounds that inhibit ACOD1 enzymatic activity, including 2-fluorocitrate analogs and substrate mimetics. These compounds are in preclinical development [4, 1].

### 6.3 Itaconate Derivatives in Clinical Development

| **Compound** | **Disease Indication** | **Development Stage** | **Mechanism** |
|---|---|---|---|
| **4-Octyl Itaconate (4OI)** | Inflammatory diseases, cancer | Preclinical | Nrf2 activation, SDH inhibition |
| **Dimethyl Itaconate (DMI)** | Psoriasis, vitiligo | Preclinical | Nrf2 activation, anti-inflammatory |
| **Itaconate-loaded nanoparticles** | Atherosclerosis | Preclinical | Targeted delivery to macrophages |
| **ACOD1 siRNA/ASO** | Cancer | Preclinical | Gene silencing in TAMs |

### 6.4 Pharmacogenomic Considerations

The response to ACOD1-targeted therapies may be influenced by genetic variation:

- **ACOD1 polymorphisms:** Individuals carrying the rs2911020 T allele may have reduced ACOD1 expression and may respond differently to itaconate-based therapies.
- **KEAP1/Nrf2 pathway mutations:** Tumors with KEAP1 mutations (common in lung cancer) may be resistant to itaconate-based therapies that rely on Nrf2 activation [3].
- **Tumor microenvironment:** The composition of the tumor microenvironment (e.g., TAM density, CD8⁺ T-cell infiltration) may predict response to ACOD1-targeted therapies [4, 5, 1].

### 6.5 Combination Therapies

ACOD1-targeted therapies are likely to be most effective in combination with other agents:

- **Immune checkpoint inhibitors (anti-PD-1/PD-L1):** ACOD1 inhibition in TAMs may enhance the efficacy of checkpoint blockade by relieving immunosuppression [4, 5, 1].
- **mRNA cancer vaccines:** Targeting ACOD1 on macrophages in lymph nodes can boost the efficacy of mRNA cancer vaccines [4, 5].
- **CAR-macrophage therapy:** ACOD1 depletion enhances the pro-inflammatory and anti-tumor activity of CAR-macrophages in solid tumors [3, 1].

## 7. Bioinformatic Resources & Database Accessions

The following table provides key database accessions and resources for ACOD1 research:

| **Database** | **Accession/Identifier** | **URL** |
|---|---|---|
| **NCBI Gene** | 55275 (Human); 268378 (Mouse) | https://www.ncbi.nlm.nih.gov/gene/55275 |
| **Ensembl** | ENSG00000132781 (Human) | https://www.ensembl.org/Homo_sapiens/Gene/Summary?g=ENSG00000132781 |
| **UniProt** | A6NK06 (Human); Q8C7R8 (Mouse) | https://www.uniprot.org/uniprotkb/A6NK06 |
| **RCSB PDB** | Multiple structures (e.g., 5XU8, 6E1L) | https://www.rcsb.org/search?q=ACOD1 |
| **Gene Ontology (GO)** | GO:0003824 (catalytic activity); GO:0005739 (mitochondrion); GO:0042742 (defense response to bacterium) | https://www.ebi.ac.uk/QuickGO/ |
| **ClinVar** | Gene: ACOD1 | https://www.ncbi.nlm.nih.gov/clinvar/?term=ACOD1 |
| **STRING** | ACOD1 (Homo sapiens) | https://string-db.org/network/9606.ENSP00000255330 |
| **BioGRID** | ACOD1 | https://thebiogrid.org/ |
| **gnomAD** | ACOD1 | https://gnomad.broadinstitute.org/gene/ENSG00000132781 |
| **TCGA** | ACOD1 expression across cancers | https://portal.gdc.cancer.gov/ |
| **Human Protein Atlas** | ACOD1 | https://www.proteinatlas.org/ENSG00000132781-ACOD1 |
| **Mouse Genome Informatics (MGI)** | Acod1 | https://www.informatics.jax.org/marker/MGI:1927129 |

## Related Clinical & Scientific Guides

* [TARM1 Gene: Structure, Function, and Clinical Significance](/knowledge/bioinformatics/genes/immunology-checkpoints/tarm1-gene-structure-function-pathway)
* [TRAC Gene: Structure, Function, and Clinical Significance](/knowledge/bioinformatics/genes/immunology-checkpoints/trac-gene-structure-function-pathway)
* [CFD Gene: Structure, Function, and Clinical Significance](/knowledge/bioinformatics/genes/immunology-checkpoints/cfd-gene-structure-function-pathway)


## References

[1] Gao N, Me R, Singh S, Kumar A, Yu FS. Hyperglycemia-Suppressed Acod1 Expression Contributes to Innate Immune Deficiency in Pseudomonas aeruginosa Keratitis. Investigative Ophthalmology and Visual Science. 2025. https://www.semanticscholar.org/paper/93218ce8ad38c19adaa40da5a5cc49650483b302

[2] Wang X, Su S, Zhu Y, et al. Metabolic Reprogramming via ACOD1 depletion enhances function of human induced pluripotent stem cell-derived CAR-macrophages in solid tumors. Nature Communications. 2023. https://www.semanticscholar.org/paper/387fa64427b791c43f1abeefa6c428ff44292d78

[3] Papathanassiu A, Li W, Zhong Y, Wong CC. Abstract 4251: Drugging of ACOD1 is associated with tumor regression in syngeneic tumor models. Cancer Research. 2025. https://www.semanticscholar.org/paper/a194fa38b2e4967b4f6505ad73422cc1353d5e54

[4] Sun Y, Liu D, Liu Y, Chi L. VISTA Alleviates Microglia-Mediated Neuroinflammation After Cerebral Ischemia–Reperfusion Injury via Regulating ACOD1/Itaconic Acid Metabolism. Molecular Neurobiology. 2025. https://www.semanticscholar.org/paper/fd2870a4a5f2e2b340c4f06d239bd6cd993f891d

[5] Ji J, Zhong H, Li Y, et al. IRG1/