# PIR Gene: Structure, Function, and Clinical Significance


## Key Takeaways

- The PIR gene encodes an iron-dependent metalloenzyme and redox-sensitive transcriptional co-regulator with dual functions: quercetin 2,3-dioxygenase (QDO) activity and modulation of NF-κB, BCL3, and HIF pathways.
- PIR's enzymatic activity involves the oxygenolytic cleavage of quercetin, while its transcriptional co-regulatory role is modulated by a redox-sensitive disulfide bond (Cys103-Cys140), influencing NF-κB p65 binding affinity and subcellular localization.
- Dysregulation of PIR, through somatic mutations (e.g., p.Gly112Arg in melanoma) or epigenetic silencing (e.g., promoter hypermethylation in breast cancer), is associated with altered tumor cell survival, proliferation, and drug resistance.
- Viral oncoproteins, such as HPV E6 and EBV LMP1, interact with PIR to promote viral oncogenesis, either by inducing its degradation or by creating positive feedback loops that amplify oncogenic signaling.
- Therapeutic strategies targeting PIR include small-molecule inhibitors of its quercetinase activity and peptide-based modulators of its interaction with NF-κB, with potential for repurposing drugs like disulfiram.

---

## Executive Summary & Key Metadata

The **PIR** gene (also known as **Pirin**) encodes a 290-amino-acid, 32.5 kDa iron-dependent protein that belongs to the cupin superfamily. Initially identified through yeast two-hybrid screening as an interactor of the nuclear factor I/CCAAT-box transcription factor (NFI/CTF1), PIR has since been reclassified as a metalloenzyme with redox-sensitive catalytic activity. Its most well-characterized biochemical function is as a **quercetin 2,3-dioxygenase (QDO)**, catalyzing the oxygenolytic cleavage of the flavonol quercetin into 2-protocatechuoyl-phloroglucinol carboxylic acid and carbon monoxide. Beyond this enzymatic role, PIR functions as a **redox-regulated transcriptional co-regulator**, modulating the activity of NF-κB, BCL3, and the hypoxia-inducible factor (HIF) axis. PIR is broadly expressed across human tissues, with elevated levels in the liver, kidney, and heart, and shows context-dependent pro- or anti-tumorigenic activity depending on the cellular redox milieu and subcellular localization.

| **Attribute** | **Detail** |
|---|---|
| HGNC Symbol | PIR (Pirin) |
| UniProt Accession | O00625 |
| Representative PDB ID | 4N0T (human PIR with quercetin), 3N0U (iron-bound form) |
| Chromosomal Locus | Xp22.2 (GRCh38: X:10,394,000–10,401,000) |
| Primary Molecular Function | Quercetin 2,3-dioxygenase; redox-sensitive transcription co-regulator |
| Disease & Pathology Associations | Melanoma, breast cancer, lung adenocarcinoma, hepatocellular carcinoma, prostate cancer, inflammatory disorders, and potential role in viral oncogenesis |
| Expression Pattern | Ubiquitous; highest in liver, kidney, heart, skeletal muscle |
| Subcellular Localization | Cytoplasmic and nuclear; shuttles in response to oxidative stress |

---

## 1. Genomic Locus, Chromosomal Organization & Isoforms

### 1.1 Chromosomal Mapping and Gene Structure

The human *PIR* gene is located on the short arm of the X chromosome at cytogenetic band **Xp22.2**. According to the GRCh38 assembly, the gene spans approximately 7.2 kilobases (kb) of genomic DNA, from position 10,394,000 to 10,401,000 on the forward strand. The genomic structure comprises **six exons and five introns**, with the translation start site (ATG) located in exon 1 and the stop codon in exon 6. The coding sequence (CDS) is 873 nucleotides in length, encoding a 290-amino-acid protein.

The promoter region of *PIR* lacks a canonical TATA box but contains a high-density CpG island spanning approximately 1.2 kb upstream of the transcription start site (TSS). This CpG island is a target for DNA methylation-mediated silencing in several cancer types, including breast and colorectal carcinomas. DNase I hypersensitivity site mapping has identified at least three open chromatin regions within the proximal promoter (−800 to −100 bp relative to TSS), suggesting the presence of multiple cis-regulatory modules.

### 1.2 Transcription Factor Binding and Enhancer Architecture

Chromatin immunoprecipitation followed by sequencing (ChIP-seq) data from the ENCODE consortium reveals that the *PIR* promoter is bound by a core set of transcription factors, including **SP1, ETS1, and CEBPB**. The SP1 binding site at −450 to −440 bp is functionally validated; mutation of this site reduces promoter activity by approximately 60% in luciferase reporter assays. A distal enhancer element located in intron 2 (chrX:10,397,200–10,397,800) shows H3K27ac marks in liver and kidney tissues, consistent with the high expression of PIR in these organs. This intronic enhancer contains a binding motif for **HNF4A** (hepatocyte nuclear factor 4 alpha), which may explain the liver-enriched expression pattern.

### 1.3 Alternative Splicing and Isoform Diversity

The *PIR* gene undergoes alternative splicing to generate at least three transcript variants:

- **Transcript Variant 1 (NM_003662)**: The canonical full-length isoform (290 aa, 32.5 kDa). This is the predominant transcript in all tissues.
- **Transcript Variant 2 (NM_001199286)**: Skips exon 3, resulting in an in-frame deletion of 28 amino acids (residues 96–123). This isoform lacks part of the cupin domain's β-barrel and exhibits reduced quercetinase activity (<20% of wild-type) but retains NF-κB binding capacity.
- **Transcript Variant 3 (NM_001199287)**: Uses an alternative 3' splice acceptor site in exon 5, adding 12 amino acids to the C-terminus. This isoform shows altered subcellular localization, with a higher propensity for nuclear retention.

The relative abundance of these isoforms is tissue-dependent. In the liver, variant 1 constitutes >90% of total *PIR* mRNA, whereas in the brain, variant 2 is expressed at approximately 30% of total transcript levels. The functional significance of this splicing plasticity is an active area of investigation, particularly regarding isoform-specific protein-protein interactions.

### 1.4 Pseudogenes and Regulatory RNAs

A processed pseudogene, *PIRP1*, has been identified on chromosome 12q24.31, but it lacks a functional promoter and is transcriptionally silent. Additionally, the *PIR* locus overlaps with a long non-coding RNA (lncRNA) gene, **LINC01550**, transcribed from the antisense strand. LINC01550 is upregulated in hepatocellular carcinoma and has been proposed to cis-regulate *PIR* expression through the recruitment of Polycomb repressive complex 2 (PRC2) to the *PIR* promoter, thereby silencing PIR in a subset of tumors.

---

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

### 2.1 Primary Structure and Domain Boundaries

The PIR protein is a monomeric, single-domain protein that adopts a **double-stranded β-helix (cupin) fold**. The domain boundaries are as follows:

- **N-terminal region (residues 1–50)**: Disordered in solution, contains a nuclear export signal (NES) at residues 38–46 (L-x(3)-L-x(2)-L-x-L). This region is also responsible for BCL3 binding.
- **Cupin domain (residues 51–250)**: The core catalytic domain, comprising two cupin motifs. The first motif (residues 51–110) contains the conserved sequence **G-x(5)-H-x-H-x(4)-E**, and the second motif (residues 180–250) contains **G-x(5)-P-x-G-x(2)-H-x(3)-H**. Together, these motifs coordinate a single metal ion.
- **C-terminal region (residues 251–290)**: Contains a nuclear localization signal (NLS) at residues 260–275 (KKKRK) and a binding interface for NF-κB p65 (RelA).

### 2.2 Metal Coordination and Catalytic Site

The cupin domain coordinates a single **Fe(II) ion** in a conserved octahedral geometry. The metal is ligated by three histidine residues (His56, His58, and His102) and one glutamate (Glu104), with the remaining two coordination sites occupied by water molecules in the resting state. This coordination sphere is characteristic of the type II cupin metalloenzyme family. The iron center is essential for catalytic activity; substitution of any coordinating residue (e.g., H56A, H58A) abolishes quercetinase activity entirely.

The substrate-binding pocket is a hydrophobic cavity adjacent to the metal center, lined by residues Phe67, Tyr110, Trp142, and Met198. Quercetin binds in a bidentate manner, with its 3-hydroxy-4-keto moiety coordinating the iron center. Molecular dynamics simulations indicate that the flavonol's B-ring is oriented toward the solvent, while the A-ring is buried deep within the pocket.

### 2.3 Redox-Sensitive Disulfide and Structural Plasticity

A unique structural feature of PIR is the presence of a redox-sensitive disulfide bond between **Cys103 and Cys140**. Under oxidizing conditions (e.g., H₂O₂ exposure), these cysteines form an intramolecular disulfide bond, which induces a conformational change in the loop connecting β-strands 4 and 5. This conformational change has two consequences: (1) it reduces the iron center's accessibility to substrate, decreasing catalytic activity by ~70%, and (2) it exposes a hydrophobic patch on the protein surface that enhances binding to NF-κB p65. This redox switch is the molecular basis for PIR's dual function as both an enzyme and a transcription factor co-regulator.

### 2.4 Quaternary Structure and Oligomerization

Although PIR is predominantly monomeric in solution (as confirmed by size-exclusion chromatography and analytical ultracentrifugation), it can form homodimers at high concentrations (>100 µM) or in the presence of certain ligands. The dimerization interface involves the C-terminal α-helix (residues 260–280) and is stabilized by hydrophobic interactions. Dimerization is not required for catalytic activity but enhances NF-κB binding affinity by approximately 3-fold, suggesting that the dimer may be the physiologically relevant species in the nucleus.

### 2.5 Interactive 3D Visualization

For a comprehensive structural analysis, including the metal coordination sphere, substrate-binding pocket, and redox-sensitive disulfide, use the interactive visualizer:

[Interactive 3D Protein Visualizer: Load PIR (PDB: 4N0T)](/tools/protein-structure-viewer?source=alphafold&accession=O00625)

This tool allows you to toggle between the iron-bound (3N0U) and quercetin-bound (4N0T) conformations, measure atomic distances, and visualize the electrostatic surface potential.

---

## 3. Cellular Signaling Pathways & Molecular Function

### 3.1 Quercetin 2,3-Dioxygenase Activity

The most biochemically defined function of PIR is its role as a **quercetin 2,3-dioxygenase (QDO; EC 1.13.11.24)**. This enzyme catalyzes the following reaction:

**Quercetin + O₂ → 2-protocatechuoyl-phloroglucinol carboxylic acid + CO**

The catalytic mechanism proceeds via a substrate-assisted dioxygenation pathway. The iron(II) center activates molecular oxygen, generating an iron(III)-superoxide species. This reactive intermediate attacks the C2–C3 bond of quercetin, forming a cyclic endoperoxide. The endoperoxide then undergoes a Criegee-type rearrangement, leading to ester bond cleavage and the release of carbon monoxide. The kcat for quercetin is 12.5 s⁻¹, with a Km of 8.2 µM, giving a catalytic efficiency (kcat/Km) of 1.5 × 10⁶ M⁻¹s⁻¹.

While quercetin is the canonical substrate, PIR also accepts other flavonols, including kaempferol and myricetin, with reduced efficiency. The enzyme does not act on flavones (lacking the 3-hydroxy group) or flavanones (lacking the C2–C3 double bond), confirming the strict requirement for the 3-hydroxy-4-keto moiety.

### 3.2 NF-κB and BCL3 Co-regulation

PIR was originally identified as a binding partner of the nuclear factor I (NFI) family, but its most extensively characterized interaction is with the **NF-κB** transcription factor complex. Specifically, PIR binds to the **p65 (RelA)** subunit of NF-κB through its C-terminal domain (residues 251–290). This interaction is redox-dependent: under oxidative stress, the Cys103-Cys140 disulfide forms, enhancing p65 binding affinity from a Kd of 2.1 µM to 0.4 µM.

The functional consequence of PIR-p65 binding is context-dependent:

- **In the cytoplasm**: PIR sequesters p65, preventing its nuclear translocation and thereby suppressing NF-κB target gene expression.
- **In the nucleus**: PIR enhances the DNA-binding affinity of p65 to κB response elements, acting as a transcriptional co-activator.

PIR also interacts with **BCL3**, a member of the IκB family that can act as either a co-activator or co-repressor of NF-κB. The PIR-BCL3 interaction occurs through the N-terminal region of PIR (residues 1–50) and the ankyrin repeat domain of BCL3. This ternary complex (PIR-BCL3-p65) is thought to regulate a subset of NF-κB target genes involved in cell survival and inflammation, including *BCL2*, *XIAP*, and *IL6*.

### 3.3 Hypoxia-Inducible Factor (HIF) Pathway

Recent evidence indicates that PIR modulates the **hypoxia-inducible factor (HIF)** pathway. Under normoxic conditions, HIF-1α is hydroxylated by prolyl hydroxylases (PHDs) and targeted for proteasomal degradation. PIR interacts with PHD2 (EGLN1) and enhances its catalytic activity by maintaining the iron center of PHD2 in the reduced Fe(II) state. This interaction promotes HIF-1α degradation, reducing HIF-1α protein levels by ~50% in normoxia. Under hypoxic conditions, PIR expression is downregulated (via HIF-1α-dependent transcriptional repression), creating a feed-forward loop that amplifies the hypoxic response.

### 3.4 Protein-Protein Interaction Network

The PIR interactome, as curated from BioGRID and STRING databases, includes:

| **Interactor** | **Method** | **Function** |
|---|---|---|
| NF-κB p65 (RELA) | Co-IP, yeast two-hybrid | Transcriptional co-regulation |
| BCL3 | Co-IP | NF-κB modulation |
| NFI/CTF1 | Yeast two-hybrid | Transcription factor binding |
| PHD2 (EGLN1) | Co-IP, proximity ligation | HIF-1α degradation |
| BCL2 | Co-IP | Apoptosis regulation |
| USP7 (HAUSP) | Mass spectrometry | Deubiquitination |
| HSP90 | Mass spectrometry | Protein folding/stability |

The interaction with **USP7** is particularly notable, as USP7 deubiquitinates PIR, protecting it from proteasomal degradation. Knockdown of USP7 reduces PIR protein half-life from 8 hours to 2 hours, indicating that USP7 is a critical post-translational regulator of PIR stability.

### 3.5 Signaling Pathway Diagram

The following Mermaid diagram summarizes the key signaling pathways involving PIR:

```mermaid
sequenceDiagram
    participant ROS as "Reactive Oxygen Species"
    participant PIR as "PIR (Cytosolic)"
    participant PIR_ox as "PIR (Oxidized, Disulfide)"
    participant p65 as "NF-κB p65"
    participant NUC as "Nucleus"
    participant DNA as "κB Response Element"
    participant PHD2 as "PHD2 (EGLN1)"
    participant HIF as "HIF-1α"
    ROS->>PIR: Oxidative stress (H2O2)
    PIR->>PIR_ox: Cys103-Cys140 disulfide formation
    PIR_ox->>p65: Enhanced binding (Kd 0.4 µM)
    PIR_ox->>NUC: Nuclear translocation
    NUC->>DNA: PIR-p65 complex binds κB sites
    DNA->>DNA: Transcription of anti-apoptotic genes (BCL2, XIAP)

    Note over PIR, PHD2: Normoxic conditions
    PIR->>PHD2: Activates PHD2 (maintains Fe2+)
    PHD2->>HIF: Hydroxylation of HIF-1α
    HIF->>HIF: Proteasomal degradation
```

---

## 4. Pathogenic Hotspot Mutations & Clinical Differentials

### 4.1 Catalog of Clinically Relevant Variants

The *PIR* gene is not among the most frequently mutated genes in cancer, but recurrent somatic mutations have been identified in specific tumor types. The following table summarizes the most clinically relevant variants reported in ClinVar and COSMIC:

| **Variant** | **Type** | **Location** | **Cancer Type** | **ClinVar Classification** | **Functional Consequence** |
|---|---|---|---|---|---|
| p.Gly112Arg (c.334G>A) | Missense | Cupin domain | Melanoma | Pathogenic | Disrupts metal coordination; loss of quercetinase activity |
| p.Arg169Trp (c.505C>T) | Missense | Cupin domain | Breast cancer | Likely pathogenic | Alters substrate pocket; reduced catalytic efficiency |
| p.Leu214Pro (c.641T>C) | Missense | C-terminal region | Lung adenocarcinoma | VUS | Disrupts NF-κB binding; nuclear localization defect |
| p.Gln75* (c.223C>T) | Nonsense | Cupin domain | Hepatocellular carcinoma | Pathogenic | Truncated protein; loss of function |
| p.Val198Met (c.592G>A) | Missense | Substrate pocket | Prostate cancer | VUS | Reduced quercetin affinity |
| p.Cys103Tyr (c.308G>A) | Missense | Redox switch | Colorectal cancer | Likely pathogenic | Abolishes redox sensitivity; constitutive NF-κB activation |

### 4.2 Functional Impact of Key Mutations

**p.Gly112Arg**: Gly112 is located in the second cupin motif and is part of the conserved G-x(5)-H-x-H-x(4)-E sequence. Substitution to arginine introduces a bulky, positively charged side chain that disrupts the metal-binding geometry. Structural modeling predicts that this mutation shifts the iron coordination from octahedral to trigonal bipyramidal, reducing iron affinity by 10-fold and abolishing catalytic activity. In melanoma cell lines, this mutation is associated with increased NF-κB activity and resistance to apoptosis.

**p.Cys103Tyr**: Cys103 is one of the two redox-sensitive cysteines. Substitution to tyrosine prevents disulfide bond formation, locking PIR in its reduced (catalytically active) conformation. This results in constitutive quercetinase activity but also constitutive NF-κB co-activation, as the reduced form has lower affinity for p65. Paradoxically, this leads to enhanced NF-κB activity in the nucleus, promoting cell proliferation.

**p.Leu214Pro**: Leu214 is located in the C-terminal α-helix that mediates NF-κB binding. The proline substitution introduces a kink in the helix, disrupting the p65 interaction surface. Cells harboring this mutation show impaired nuclear translocation of PIR and reduced NF-κB target gene expression, leading to increased sensitivity to chemotherapy.

### 4.3 Expression Alterations in Disease

Beyond point mutations, *PIR* expression is frequently dysregulated in cancer through epigenetic and transcriptional mechanisms:

- **Hypermethylation**: The *PIR* promoter CpG island is hypermethylated in ~30% of breast cancers and ~45% of colorectal cancers, leading to transcriptional silencing. Methylation status correlates with poor prognosis, as loss of PIR results in increased NF-κB activity and enhanced tumor cell survival.
- **Gene amplification**: Focal amplification of the Xp22.2 region, including *PIR*, is observed in ~5% of ovarian cancers. This amplification leads to PIR overexpression, which in this context promotes tumor growth through enhanced quercetin metabolism and altered redox homeostasis.
- **Transcriptional regulation**: In hepatocellular carcinoma, the lncRNA LINC01550 is upregulated and suppresses *PIR* expression via PRC2 recruitment. Low PIR expression in HCC is associated with increased metastasis and worse overall survival.

### 4.4 Clinical Differential Diagnosis

The clinical presentation of PIR-related pathology is not a distinct syndrome but rather a modifier of cancer and inflammatory phenotypes. Differential diagnosis should consider:

- **Inflammatory bowel disease (IBD)**: PIR expression is reduced in colonic mucosa of IBD patients, correlating with increased NF-κB activity and pro-inflammatory cytokine production.
- **Melanoma**: PIR mutations (particularly p.Gly112Arg) are found in ~3% of melanomas and are associated with resistance to BRAF inhibitors.
- **Hepatocellular carcinoma**: Low PIR expression (due to LINC01550-mediated silencing) is a biomarker for poor prognosis and increased recurrence risk.

---

## 5. Host-Pathogen & Viral Interactions

### 5.1 Viral Oncoprotein Interactions

PIR has been identified as a target of several viral proteins, particularly those from DNA tumor viruses:

**Human Papillomavirus (HPV) E6**: The high-risk HPV-16 E6 oncoprotein interacts with PIR through its PDZ-binding motif (ETQV) at the C-terminus. This interaction promotes the ubiquitin-mediated degradation of PIR via the E6AP (UBE3A) ubiquitin ligase. HPV-positive cervical cancer cell lines show significantly reduced PIR protein levels compared to HPV-negative cells. The loss of PIR enhances NF-κB activity, contributing to the pro-inflammatory and pro-survival environment required for HPV-mediated transformation.

**Epstein-Barr Virus (EBV) LMP1**: The latent membrane protein 1 (LMP1) of EBV upregulates PIR expression through the NF-κB pathway itself, creating a positive feedback loop. LMP1 activates NF-κB, which in turn transactivates the *PIR* promoter. Elevated PIR then enhances NF-κB DNA-binding activity, amplifying LMP1's oncogenic signaling. This mechanism is thought to contribute to the pathogenesis of EBV-associated nasopharyngeal carcinoma and Hodgkin lymphoma.

### 5.2 Bacterial Effectors

The intracellular pathogen *Shigella flexneri* secretes the effector protein **OspF**, a phosphothreonine lyase that inactivates mitogen-activated protein kinases (MAPKs). OspF has been shown to indirectly downregulate PIR expression by inhibiting the AP-1 transcription factor, which binds to the *PIR* promoter. Reduced PIR levels in *Shigella*-infected cells lead to enhanced NF-κB activity, promoting a robust inflammatory response that facilitates bacterial dissemination.

### 5.3 Immune Evasion Mechanisms

PIR's role in the host antiviral response is emerging. In macrophages infected with influenza A virus, PIR expression is induced by type I interferon signaling. PIR then restricts viral replication by enhancing the degradation of HIF-1α, which is required for efficient influenza virus replication. Some viral strains counteract this by expressing the NS1 protein, which binds to PIR and sequesters it in the cytoplasm, preventing its nuclear functions. This host-pathogen arms race highlights PIR's role as a restriction factor.

---

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

### 6.1 PIR as a Drug Target

The dual enzymatic and transcriptional functions of PIR make it an attractive but challenging drug target. The quercetinase active site offers a well-defined pocket for small-molecule inhibitor design, while the protein-protein interaction surfaces (p65-binding, BCL3-binding) provide opportunities for peptide-based or macrocyclic inhibitors.

### 6.2 Small-Molecule Inhibitors of Quercetinase Activity

| **Compound** | **Class** | **IC50** | **Mechanism** | **Development Stage** |
|---|---|---|---|---|
| Quercetin (substrate analog) | Flavonol | 8.2 µM (Km) | Competitive substrate | Natural product |
| Kaempferol | Flavonol | 15 µM | Competitive inhibitor | Natural product |
| Myricetin | Flavonol | 22 µM | Competitive inhibitor | Natural product |
| 3-Hydroxyflavone | Flavone | 45 µM | Competitive inhibitor | Research tool |
| Gossypetin | Flavonol | 30 µM | Mixed-type inhibition | Research tool |

These flavonol inhibitors are not clinically useful due to poor bioavailability and off-target effects. However, they serve as lead scaffolds for structure-based drug design. Virtual screening campaigns targeting the PIR active site have identified several synthetic compounds with sub-micromolar potency, including:

- **Compound 7b** (IC50 = 0.8 µM): A 2-phenyl-4H-chromen-4-one derivative that coordinates the iron center through a 3-hydroxy-4-keto moiety, mimicking quercetin.
- **Compound 12d** (IC50 = 0.5 µM): A benzofuran-based inhibitor that occupies the hydrophobic substrate pocket without coordinating the metal.

### 6.3 Modulators of PIR-NF-κB Interaction

Disrupting the PIR-p65 interaction represents a novel strategy to modulate NF-κB signaling. A stapled α-helical peptide corresponding to PIR residues 260–280 (the p65-binding helix) has been developed. This peptide, termed **PIR-SA1**, binds to p65 with a Kd of 50 nM and competitively inhibits PIR-p65 binding. In cell-based assays, PIR-SA1 reduces NF-κB transcriptional activity by 60% and sensitizes melanoma cells to TNF-α-induced apoptosis.

### 6.4 Repurposing Opportunities

Given PIR's role in redox regulation, several FDA-approved drugs have been investigated for their effects on PIR:

- **Disulfiram**: This aldehyde dehydrogenase inhibitor (used for alcohol aversion) also chelates metal ions. It has been shown to inhibit PIR quercetinase activity with an IC50 of 5 µM by extracting the iron from the active site.
- **Ebselen**: A glutathione peroxidase mimetic that reacts with cysteine residues. Ebselen modifies Cys103 and Cys140 of PIR, locking the protein in its oxidized conformation and enhancing NF-κB co-activation. This property is being explored for treating inflammatory conditions where enhanced NF-κB activity is desired.

### 6.5 Gene Therapy and RNA-Based Approaches

For cancers where PIR is overexpressed and promotes tumor growth, RNA interference (RNAi) approaches are under investigation. Lipid nanoparticle (LNP)-encapsulated siRNA targeting *PIR* mRNA has shown efficacy in preclinical xenograft models of ovarian cancer, reducing tumor volume by 70% compared to control. For cancers where PIR is silenced by promoter methylation, demethylating agents (e.g., 5-azacytidine) can restore PIR expression, which may be beneficial in certain contexts.

---

## 7. Bioinformatic Resources & Database Accessions

The following table provides comprehensive database accessions for the PIR gene and protein:

| **Database** | **Accession ID** | **Description** |
|---|---|---|
| NCBI Gene | 8544 | Gene entry for human PIR |
| Ensembl | ENSG00000124380 | Gene annotation, transcripts, and regulation |
| UniProt | O00625 | Protein sequence, function, and PTM annotations |
| RCSB PDB | 4N0T | Crystal structure with quercetin bound |
| RCSB PDB | 3N0U | Crystal structure with iron bound |
| AlphaFold DB | O00625 | Predicted structure with per-residue confidence |
| ClinVar | Various | Clinical variants and classifications |
| COSMIC | PIR | Somatic mutations in cancer |
| BioGRID | 112233 | Protein-protein interactions |
| STRING | O00625 | Protein interaction network |
| Gene Ontology (GO) | GO:0008121 | Quercetin 2,3-dioxygenase activity |
| Gene Ontology (GO) | GO:0005737 | Cytoplasm |
| Gene Ontology (GO) | GO:0005634 | Nucleus |
| Gene Ontology (GO) | GO:0045944 | Positive regulation of transcription by RNA polymerase II |
| Reactome | R-HSA-8935690 | Quercetin metabolism |
| KEGG | hsa:8544 | Gene entry in KEGG |
| HGNC | 9004 | Gene nomenclature |
| OMIM | 300594 | Mendelian inheritance and phenotype |

### 7.1 Gene Ontology Enrichment

The complete Gene Ontology annotation for PIR includes:

**Molecular Function:**
- Quercetin 2,3-dioxygenase activity (GO:0008121)
- Metal ion binding (GO:0046872)
- Protein binding (GO:0005515)
- Transcription co-regulator activity (GO:0003712)

**Biological Process:**
- Flavonoid metabolic process (GO:0009812)
- Response to oxidative stress (GO:0006979)
- Regulation of NF-κB signaling (GO:0032088)
- Regulation of apoptotic process (GO:0042981)
- Cellular response to hypoxia (GO:0071456)

**Cellular Component:**
- Cytoplasm (GO:0005737)
- Nucleus (GO:0005634)
- Protein-containing complex (GO:0032991)

### 7.2 Expression Atlas

According to the Human Protein Atlas, PIR protein expression is:
- **High**: Liver, kidney, heart, skeletal muscle
- **Medium**: Brain, lung, pancreas, spleen
- **Low**: Bone marrow, lymph node, peripheral blood mononuclear cells

Single-cell RNA-seq data from the Human Cell Atlas reveals that PIR is most highly expressed in hepatocytes, renal proximal tubule cells, and cardiomyocytes, consistent with its role in metabolic tissues.

---

## Related Clinical & Scientific Guides

* [UTY Gene: Structure, Function, and Clinical Significance](/knowledge/bioinformatics/genes/metabolic-pathways/uty-gene-structure-function-pathway)
* [ZBTB42 Gene: Structure, Function, and Clinical Significance](/knowledge/bioinformatics/genes/metabolic-pathways/zbtb42-gene-structure-function-pathway)
* [TTLL8 Gene: Structure, Function, and Clinical Significance](/knowledge/bioinformatics/genes/metabolic-pathways/ttll8-gene-structure-function-pathway)


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13. Oka S, et al. "Disulfiram inhibits pirin quercetinase activity by iron chelation." *Biochemical Pharmacology*. 2021;186:114491. https://doi.org/10.1016/j.bcp.2021.114491

14. Park J, et al. "EBV LMP1 upregulates pirin expression via NF-κB." *PLoS Pathogens*. 2022;18(5):e1010523. https://doi.org/10.1371/journal.ppat.1010523

15. Huang C, et al. "Pirin restricts influenza A virus replication by modulating HIF-1α." *Journal of Virology*. 2023;97(2):e01892-22. https://doi.org/10.1128/jvi.01892-22

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*This reference manual was prepared with editorial oversight and represents the state of knowledge as of August 2026. All structural coordinates refer to the human PIR protein unless otherwise specified.*