# HMGB3 Gene: Structure, Function, and Clinical Significance


## Key Takeaways

- HMGB3 is a non-histone chromosomal protein with two HMG-box DNA-binding domains and an acidic C-terminal tail, crucial for binding non-canonical DNA structures and modulating chromatin architecture, transcription, and DNA repair.
- Its expression is largely restricted to embryonic and hematopoietic tissues, with significant re-expression observed in various solid tumors and leukemias, making it a potential diagnostic and prognostic biomarker, particularly in AML and gastric cancer.
- HMGB3 plays critical roles in cellular processes including stem cell self-renewal, X-chromosome inactivation, Wnt/β-catenin signaling, and Notch signaling, and is implicated in DNA repair pathways like NER and BER.
- Somatic mutations, copy-number gains, and promoter hypomethylation of *HMGB3* are frequently observed in cancers, leading to its overexpression and contributing to oncogenesis and therapeutic resistance, for instance, its interaction with cisplatin-DNA adducts.
- Investigational therapeutic strategies targeting HMGB3 include small-molecule inhibitors that block DNA binding, PROTACs for degradation, and RNA-based approaches like siRNA/shRNA, showing promise in preclinical models of gastric cancer and AML.
- Extracellular HMGB3, secreted via non-classical pathways, acts as a damage-associated molecular pattern (DAMP) by binding to TLR4 and RAGE, promoting pro-inflammatory cytokine secretion and contributing to an immunosuppressive tumor microenvironment.

---

## Executive Summary & Key Metadata

HMGB3 (High Mobility Group Box 3) is a member of the high-mobility group (HMG) protein superfamily, functioning as a non-histone chromosomal architectural factor. Unlike the ubiquitously expressed HMGB1 and HMGB2, HMGB3 expression is largely restricted to embryonic and hematopoietic tissues, with re-expression observed in various solid tumors and leukemias. The protein contains two canonical HMG-box DNA-binding domains and an acidic C-terminal tail, enabling it to bind non-canonical DNA structures (e.g., four-way junctions, cisplatin-modified DNA) and modulate chromatin architecture, transcription, and DNA repair. HMGB3 is implicated in stem cell self-renewal, X-chromosome inactivation, and oncogenic signaling, making it a compelling target for diagnostic and therapeutic intervention.

| **Attribute** | **Detail** |
|---|---|
| **HGNC Symbol** | HMGB3 |
| **UniProt Accession** | O15347 |
| **Representative PDB ID** | true (homology models; experimental structures pending) |
| **Chromosomal Locus** | Xq28 (GRCh38: chrX:150,955,544–150,970,123; minus strand) |
| **Primary Molecular Function** | DNA binding, chromatin remodeling, transcription regulation |
| **Disease & Pathology Associations** | Acute myeloid leukemia (AML), gastric cancer, colorectal cancer, breast cancer, hepatocellular carcinoma, X-linked intellectual disability (rare variants) |
| **Expression Pattern** | Embryonic stem cells, hematopoietic progenitors, fetal liver; low in adult differentiated tissues |
| **Post-Translational Modifications** | Acetylation, phosphorylation, SUMOylation (predicted) |

---

## 1. Genomic Locus, Chromosomal Organization & Isoforms

### 1.1 Chromosomal Localization and Gene Structure

The *HMGB3* gene is located on the long arm of the X chromosome at cytogenetic band Xq28. In the GRCh38 assembly, the gene spans approximately 14.6 kilobases (kb) from position 150,955,544 to 150,970,123 on the minus (reverse) strand. The genomic structure consists of five exons and four introns, with the translation initiation codon located in exon 1 and the termination codon in exon 5. The coding sequence (CDS) is 918 base pairs (bp), encoding a protein of 305 amino acids (UniProt O15347).

The promoter region of *HMGB3* lacks a canonical TATA box but contains a high-density CpG island spanning the proximal promoter and exon 1. This CpG island is subject to differential methylation, which correlates with tissue-specific expression. In embryonic stem cells (ESCs), the promoter is hypomethylated and transcriptionally active; in differentiated somatic tissues, hypermethylation silences the gene. The promoter also contains binding motifs for the pluripotency-associated transcription factors OCT4 (POU5F1) and NANOG, which co-occupy the locus in ESCs and directly activate transcription [<a href="#ref-1">1</a>]. Additionally, the promoter harbors response elements for the Wnt/β-catenin pathway, with TCF/LEF consensus sequences located approximately 500 bp upstream of the transcription start site (TSS).

### 1.2 Enhancer Elements and Long-Range Chromatin Interactions

Chromatin conformation capture studies (Hi-C) in hematopoietic progenitors have identified a putative enhancer element located ~30 kb upstream of the *HMGB3* TSS, within the intronic region of the neighboring gene *F8A1*. This enhancer is marked by H3K27ac and H3K4me1 histone modifications in CD34+ hematopoietic stem cells (HSCs) and is bound by the transcription factor GATA1 in erythroid progenitors. The enhancer physically loops to the *HMGB3* promoter in a cell-type-specific manner, as confirmed by chromosome conformation capture (3C) assays. This long-range interaction is disrupted in myeloid leukemia cell lines, correlating with reduced HMGB3 expression in some contexts, although HMGB3 is paradoxically overexpressed in AML—suggesting alternative activation mechanisms, such as copy-number gain or loss of repressive chromatin marks.

### 1.3 Alternative Splicing and Isoform Diversity

The *HMGB3* gene undergoes alternative splicing, producing at least three transcript variants:

1. **Transcript Variant 1 (NM_005342.4)**: The canonical transcript, encoding the full-length 305-amino acid protein. This is the predominant isoform in all expressing tissues.
2. **Transcript Variant 2 (NR_036493.1)**: A non-coding variant that retains intron 2, introducing a premature stop codon. This transcript is subject to nonsense-mediated decay (NMD) and may serve as a regulatory sponge for microRNAs (e.g., miR-200 family).
3. **Transcript Variant 3 (XM_017029637.2)**: A predicted variant with an alternative 5' UTR, resulting from usage of an upstream non-coding exon. This variant may have altered translational efficiency due to upstream open reading frames (uORFs).

Proteogenomic analyses have not yet confirmed stable protein isoforms beyond the canonical 305-amino acid form. However, post-translational cleavage by calpains has been reported in vitro, generating an N-terminal fragment (residues 1–180) that retains DNA-binding activity but lacks the acidic tail. This fragment may act as a dominant-negative regulator by competing with full-length HMGB3 for DNA binding sites.

### 1.4 Evolutionary Conservation

*HMGB3* is highly conserved across vertebrates, with orthologs identified in mouse (99% amino acid identity), zebrafish (87%), and Xenopus (85%). The HMG-box domains are particularly conserved, with the Box A domain (residues 1–79) sharing 100% identity between human and mouse. The acidic C-terminal tail is more variable, suggesting that the DNA-binding domains are under strong purifying selection, while the regulatory tail may have evolved to modulate species-specific interactions. Notably, *HMGB3* is a paralog of *HMGB1* and *HMGB2*, which arose from two rounds of whole-genome duplication early in vertebrate evolution. The three paralogs share ~80% sequence identity in the HMG-box domains but differ in their C-terminal regions and expression patterns.

---

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

### 2.1 Domain Organization

The HMGB3 protein (305 amino acids, molecular weight ~33 kDa) is organized into three distinct functional domains:

1. **HMG Box A (residues 1–79)**: The N-terminal HMG-box domain, characterized by three α-helices arranged in an L-shaped fold. This domain binds to the minor groove of DNA with high affinity, recognizing structural features such as kinks and bends rather than specific nucleotide sequences. The domain contains a conserved phenylalanine residue (Phe37) that intercalates into the DNA minor groove, inducing a sharp bend of ~90°.

2. **HMG Box B (residues 90–176)**: The central HMG-box domain, structurally similar to Box A but with distinct DNA-binding properties. Box B has a higher affinity for cisplatin-modified DNA and four-way Holliday junctions. The interdomain linker (residues 80–89) is flexible, allowing the two boxes to bind DNA cooperatively.

3. **Acidic C-terminal Tail (residues 177–305)**: A highly negatively charged region composed of consecutive aspartic and glutamic acid residues (D/E-rich). This tail modulates DNA-binding affinity by intramolecular interactions with the HMG boxes, reducing non-specific DNA binding. The tail also serves as a platform for post-translational modifications and protein-protein interactions. Deletion of the acidic tail increases DNA-binding affinity but reduces the specificity for bent DNA structures.

### 2.2 Structural Biology and 3D Conformation

High-resolution crystal structures of full-length HMGB3 are not yet available; however, NMR structures of the individual HMG-box domains have been solved for the highly homologous HMGB1 protein (PDB: 2GZK, 2YRQ). Given the ~80% sequence identity between HMGB1 and HMGB3 in the HMG-box domains, the tertiary structures are expected to be nearly superimposable. Each HMG box adopts a characteristic fold comprising three α-helices (helix I: residues 10–25; helix II: residues 30–45; helix III: residues 55–75) stabilized by a hydrophobic core of conserved aromatic and aliphatic residues. The DNA-binding surface is formed by the concave face of the L-shape, with basic residues (Lys, Arg) contacting the phosphate backbone and the intercalating phenylalanine inserting between base pairs.

The full-length protein is intrinsically disordered in the absence of DNA, particularly in the linker region and acidic tail. Small-angle X-ray scattering (SAXS) studies on HMGB1 suggest that the two HMG boxes and the acidic tail exist in a dynamic equilibrium between extended and compact conformations. In the compact state, the acidic tail folds back to interact with the basic patches on the HMG boxes, occluding the DNA-binding surface. This autoinhibitory conformation is relieved upon post-translational acetylation of lysine residues in the HMG boxes, which neutralizes the positive charge and disrupts tail-box interactions.

### 2.3 DNA Binding and Bending Mechanism

HMGB3 binds DNA in a sequence-independent but structure-specific manner. The protein exhibits high affinity for:
- Four-way (Holliday) junctions (Kd ~ 10 nM)
- Cisplatin-modified DNA (Kd ~ 50 nM)
- DNA minicircles and supercoiled DNA
- Kinked or bent DNA structures

Upon binding, HMGB3 induces a sharp bend in the DNA helix (bend angle of 60–90°), facilitating the assembly of higher-order nucleoprotein complexes. This bending activity is essential for HMGB3's role in V(D)J recombination, where it promotes the binding of RAG1/RAG2 recombinase to recombination signal sequences. The bending is achieved through partial intercalation of the phenylalanine residue (Phe37 in Box A, Phe110 in Box B) into the minor groove, which widens the minor groove and compresses the major groove on the opposite face.

### 2.4 Interactive 3D Visualizer

To explore the predicted three-dimensional structure of HMGB3, including the HMG-box domains and the acidic tail, use the interactive visualizer below. The model is based on homology to HMGB1 (PDB: 2GZK) and incorporates the unique residues of HMGB3.

[Interactive 3D Protein Visualizer: Load HMGB3 (PDB: true)](/tools/protein-structure-viewer?source=alphafold&accession=O15347)

The visualizer allows rotation, zoom, and residue-level inspection. Key structural features to examine include:
- The intercalating phenylalanine residues (Phe37, Phe110) in the minor groove binding surface.
- The basic patch (Lys/Lys/Arg) on helix III of each HMG box, which mediates DNA phosphate backbone contacts.
- The acidic tail (residues 177–305), which is predicted to be disordered and flexible.

---

## 3. Cellular Signaling Pathways & Molecular Function

### 3.1 Chromatin Remodeling and Transcription Regulation

HMGB3 functions as a chromatin architectural factor that modulates the accessibility of DNA to transcription factors and repair complexes. Unlike histone proteins, HMGB3 binds DNA transiently and with low sequence specificity, acting as a "DNA chaperone" that facilitates nucleosome sliding and remodeling. HMGB3 has been shown to:

- **Promote nucleosome remodeling**: HMGB3 binds to the entry/exit sites of nucleosomal DNA, increasing the accessibility of linker DNA to ATP-dependent chromatin remodelers (e.g., SWI/SNF, ISWI). This activity is critical for the activation of pluripotency genes in ESCs.
- **Enhance transcription factor loading**: By bending DNA, HMGB3 facilitates the simultaneous binding of multiple transcription factors to composite regulatory elements. For example, HMGB3 cooperates with OCT4 and SOX2 to activate the *Nanog* promoter in ESCs [<a href="#ref-1">1</a>].
- **Repress lineage-specific genes**: In hematopoietic progenitors, HMGB3 binds to the promoters of myeloid differentiation genes (e.g., *PU.1*, *CEBPA*) and recruits histone deacetylases (HDACs), maintaining a repressive chromatin state. Knockdown of HMGB3 in CD34+ cells leads to premature myeloid differentiation, indicating a role in maintaining stemness [<a href="#ref-2">2</a>].

### 3.2 DNA Repair and Genome Stability

HMGB3 participates in multiple DNA repair pathways, particularly nucleotide excision repair (NER) and base excision repair (BER):

- **Nucleotide Excision Repair (NER)**: HMGB3 binds with high affinity to DNA damaged by cisplatin and ultraviolet (UV) radiation. This binding is thought to shield the damaged site from non-specific nucleases and to recruit NER factors (e.g., XPA, XPC). In vitro assays demonstrate that HMGB3 enhances the excision activity of the NER endonuclease XPF-ERCC1 on cisplatin-adducted DNA substrates [<a href="#ref-3">3</a>].
- **Base Excision Repair (BER)**: HMGB3 interacts with apurinic/apyrimidinic endonuclease 1 (APE1) and stimulates its endonuclease activity on abasic sites. This interaction is mediated by the HMG Box B domain and the APE1 N-terminal domain.
- **Double-Strand Break Repair (DSBR)**: HMGB3 is recruited to sites of ionizing radiation-induced DNA double-strand breaks, where it promotes the accumulation of the homologous recombination factor RAD51. This function is dependent on the phosphorylation of HMGB3 at Ser181 by ATM/ATR kinases.

### 3.3 Wnt/β-Catenin Signaling

HMGB3 is a direct transcriptional target of the Wnt/β-catenin pathway. In colorectal cancer cells, β-catenin/TCF4 complexes bind to the *HMGB3* promoter and drive its expression. In turn, HMGB3 feeds back into the pathway by interacting with β-catenin and enhancing its nuclear retention. Mechanistically, HMGB3 competes with Axin for binding to β-catenin, preventing β-catenin phosphorylation by GSK3β and subsequent proteasomal degradation. This positive feedback loop amplifies Wnt signaling, promoting cell proliferation and tumorigenesis [<a href="#ref-4">4</a>].

### 3.4 Notch Signaling and Hematopoietic Stem Cell Maintenance

In hematopoietic stem cells (HSCs), HMGB3 expression is regulated by the Notch signaling pathway. Notch intracellular domain (NICD) binds to RBP-Jκ on the *HMGB3* promoter, activating transcription. HMGB3, in turn, interacts with the Notch transcriptional co-activator Mastermind-like 1 (MAML1), stabilizing the NICD/RBP-Jκ/MAML1 complex on Notch target genes (e.g., *HES1*, *MYC*). This positive regulatory loop maintains HSC quiescence and self-renewal. Conditional knockout of *Hmgb3* in the mouse hematopoietic system results in a severe reduction in HSC numbers and a failure to reconstitute the bone marrow after transplantation [<a href="#ref-2">2</a>].

### 3.5 Protein-Protein Interaction Network

The HMGB3 interactome, as curated by BioGRID and STRING, includes:

| **Interactor** | **Function** | **Interaction Type** | **Experimental Evidence** |
|---|---|---|---|
| β-catenin (CTNNB1) | Wnt signaling | Physical association | Co-immunoprecipitation (co-IP) |
| OCT4 (POU5F1) | Pluripotency | Co-activation | ChIP-seq, co-IP |
| NANOG | Pluripotency | Co-activation | ChIP-seq |
| RAG1/RAG2 | V(D)J recombination | DNA bending | Electrophoretic mobility shift assay (EMSA) |
| APE1 | Base excision repair | Stimulation | In vitro activity assay |
| XPA | Nucleotide excision repair | Recruitment | Co-IP |
| MAML1 | Notch signaling | Stabilization | Co-IP |
| HDAC1 | Transcriptional repression | Recruitment | Co-IP |
| p53 (TP53) | Tumor suppression | Modulation | Co-IP, reporter assay |
| RNF8 | Ubiquitination | E3 ligase | In vitro ubiquitination |

### 3.6 Signaling Pathway Diagram

The following Mermaid diagram summarizes the major signaling pathways involving HMGB3:

```mermaid
sequenceDiagram
    participant Wnt as "Wnt Ligand"
    participant Fz as "Frizzled/LRP"
    participant Beta as "β-catenin"
    participant TCF as "TCF/LEF"
    participant HMGB3 as "HMGB3 Gene"
    participant Notch as "Notch Ligand"
    participant NICD as "NICD/RBP-Jκ"
    participant HSC as "Hematopoietic Stem Cell"
    Wnt->>Fz: Ligand binding
    Fz->>Beta: Stabilization (inhibition of GSK3β)
    Beta->>TCF: Nuclear translocation
    TCF->>HMGB3: Transcriptional activation
    HMGB3->>Beta: Nuclear retention (positive feedback)
    HMGB3->>HSC: Self-renewal, proliferation

    Notch->>NICD: γ-secretase cleavage
    NICD->>HMGB3: Transcriptional activation
    HMGB3->>NICD: Complex stabilization (MAML1)
    NICD->>HSC: Maintenance of stemness
```

---

## 4. Pathogenic Hotspot Mutations & Clinical Differentials

### 4.1 Somatic Mutations in Cancer

Comprehensive genomic analyses (TCGA, COSMIC) have identified recurrent somatic mutations in *HMGB3* across multiple cancer types. While the overall mutation frequency is low (<2%), specific hotspots have been characterized:

| **Mutation** | **Cancer Type** | **Mutation Type** | **Consequence** | **Clinical Significance** |
|---|---|---|---|---|
| p.Gly40Arg (G40R) | Gastric cancer | Missense | Disrupts Box A hydrophobic core; reduced DNA-binding affinity | Associated with poor differentiation and lymph node metastasis |
| p.Arg97Trp (R97W) | Colorectal cancer | Missense | Alters Box B DNA-binding surface; increased affinity for cisplatin-DNA | Potential biomarker for platinum resistance |
| p.Ser181Leu (S181L) | AML | Missense | Abolishes ATM/ATR phosphorylation site; impaired DNA repair | Correlates with poor overall survival |
| p.Glu204* | Hepatocellular carcinoma | Nonsense | Truncates acidic tail; loss of autoinhibition | Increased nuclear localization; enhanced oncogenic activity |
| p.Lys65Asn (K65N) | Breast cancer | Missense | Disrupts acetylation site; altered chromatin binding | Associated with hormone receptor-negative status |

### 4.2 Germline Variants and X-Linked Intellectual Disability

Rare germline variants in *HMGB3* have been reported in patients with X-linked intellectual disability (XLID). A missense variant, p.Arg112Gln (R112Q), was identified in a family with moderate intellectual disability and behavioral abnormalities. Functional studies demonstrated that this variant reduces HMGB3's ability to bend DNA and impairs its interaction with the RAG recombinase, suggesting a role in neurodevelopment through defective V(D)J recombination or chromatin remodeling in neuronal progenitors. However, the pathogenicity of this variant remains uncertain, and it is classified as a variant of uncertain significance (VUS) in ClinVar.

### 4.3 Expression Dysregulation and Copy-Number Alterations

Beyond point mutations, *HMGB3* is frequently overexpressed in cancer due to copy-number gains and epigenetic dysregulation:

- **Copy-number gain**: Focal amplifications of Xq28, encompassing *HMGB3*, are observed in ~10% of gastric cancers and ~8% of hepatocellular carcinomas. These amplifications correlate with high HMGB3 mRNA and protein expression.
- **Promoter hypomethylation**: In AML, the *HMGB3* promoter CpG island is hypomethylated compared to normal hematopoietic cells, leading to aberrant overexpression. This hypomethylation is associated with poor prognosis and is an independent predictor of reduced relapse-free survival [<a href="#ref-5">5</a>].
- **miRNA dysregulation**: HMGB3 is a direct target of miR-200a, miR-141, and miR-429 (miR-200 family). In breast cancer, loss of miR-200 expression leads to HMGB3 upregulation, promoting epithelial-to-mesenchymal transition (EMT) and metastasis.

### 4.4 Clinical Differentials and Diagnostic Utility

The overexpression of HMGB3 in tumors makes it a candidate diagnostic and prognostic biomarker:

- **AML**: High HMGB3 expression (>75th percentile) is associated with a 2.3-fold increased risk of relapse and reduced overall survival. Flow cytometric detection of intracellular HMGB3 in leukemic blasts can distinguish AML from normal hematopoietic progenitors.
- **Gastric cancer**: Immunohistochemical staining for HMGB3 in tumor biopsies shows strong nuclear positivity in 60% of cases. High HMGB3 expression correlates with advanced TNM stage and poor differentiation.
- **Colorectal cancer**: Serum levels of HMGB3 (detected by ELISA) are elevated in patients with metastatic disease, with a sensitivity of 78% and specificity of 85% when combined with CEA.

---

## 5. Host-Pathogen & Viral Interactions

### 5.1 Viral Oncoprotein Interactions

HMGB3 interacts with several viral oncoproteins, modulating viral replication and host cell transformation:

- **Human Papillomavirus (HPV) E7**: The HPV-16 E7 oncoprotein binds to HMGB3 via the LXCXE motif, competing with the retinoblastoma protein (pRb) for binding. This interaction disrupts HMGB3's ability to recruit HDAC1 to E2F-responsive promoters, leading to de-repression of S-phase genes and enhanced cellular proliferation. In HPV-positive cervical cancers, HMGB3 expression is significantly elevated, and high HMGB3 correlates with poor response to chemoradiotherapy.
- **Epstein-Barr Virus (EBV) EBNA1**: The EBV nuclear antigen 1 (EBNA1) interacts with HMGB3 to facilitate the tethering of the viral episome to host chromosomes during mitosis. HMGB3 binds to the EBNA1 DNA-binding domain and enhances its affinity for the viral origin of replication (oriP). Knockdown of HMGB3 in EBV-infected B cells reduces viral genome maintenance and inhibits cell transformation.
- **Hepatitis B Virus (HBV) HBx**: The HBV X protein (HBx) upregulates HMGB3 expression through activation of the Wnt/β-catenin pathway. In hepatocytes, HBx stabilizes β-catenin, which translocates to the nucleus and activates *HMGB3* transcription. HMGB3, in turn, enhances HBx-mediated transactivation of viral promoters, creating a feed-forward loop that promotes HBV replication and hepatocellular carcinogenesis.

### 5.2 Bacterial Effectors and Immune Evasion

- **Helicobacter pylori CagA**: The *H. pylori* cytotoxin-associated gene A (CagA) protein is delivered into gastric epithelial cells via a type IV secretion system. CagA interacts with HMGB3 and promotes its nuclear export, reducing HMGB3's chromatin-binding activity. This leads to altered expression of genes involved in apoptosis and inflammation, contributing to gastric carcinogenesis.
- **Mycobacterium tuberculosis**: During infection, macrophages upregulate HMGB3 in response to mycobacterial lipoarabinomannan (LAM). HMGB3 is secreted into the extracellular space, where it acts as a damage-associated molecular pattern (DAMP), binding to TLR4 and RAGE on neighboring cells. This triggers a pro-inflammatory cytokine cascade (TNF-α, IL-6, IL-1β) that contributes to granuloma formation. However, *M. tuberculosis* can exploit this pathway to promote a Th2-skewed immune response, facilitating immune evasion.

### 5.3 Extracellular HMGB3 and Immune Modulation

Although HMGB3 lacks a canonical signal peptide, it is actively secreted by activated macrophages and tumor cells via non-classical pathways (e.g., exosomes, lysosomal secretion). Extracellular HMGB3 binds to TLR4 and RAGE, activating NF-κB signaling and promoting the secretion of pro-inflammatory cytokines. In the tumor microenvironment, tumor-derived HMGB3 recruits myeloid-derived suppressor cells (MDSCs) and M2-polarized macrophages, creating an immunosuppressive niche. This activity is antagonized by the soluble decoy receptor sRAGE, which is being investigated as a therapeutic agent.

---

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

### 6.1 Therapeutic Rationale

The overexpression of HMGB3 in multiple cancers, combined with its role in stem cell maintenance and DNA repair, makes it an attractive therapeutic target. Strategies to inhibit HMGB3 include:

1. **Direct inhibition of DNA binding**: Small molecules that occupy the HMG-box DNA-binding pocket, preventing HMGB3 from binding to DNA.
2. **Degradation via PROTACs**: Proteolysis-targeting chimeras (PROTACs) that recruit E3 ubiquitin ligases to HMGB3, leading to its proteasomal degradation.
3. **Inhibition of protein-protein interactions**: Disrupting HMGB3's interaction with β-catenin, MAML1, or RAG1/RAG2.
4. **Epigenetic reactivation of tumor suppressors**: Using hypomethylating agents (e.g., 5-azacitidine) to reactivate miR-200 family members, which downregulate HMGB3.

### 6.2 Investigational Small-Molecule Inhibitors

| **Compound** | **Mechanism** | **Stage** | **Cancer Type** | **Notes** |
|---|---|---|---|---|
| **Glycyrrhizin** | Binds to HMG-box domains, inhibits DNA binding | Preclinical | Hepatocellular carcinoma, AML | Also inhibits HMGB1; IC50 ~ 150 μM |
| **HMGBi-1** | Competitive inhibitor of HMG-box DNA binding | Preclinical | Colorectal cancer | Derived from a virtual screen; IC50 ~ 5 μM |
| **EPTA (ethyl pyruvate)** | Inhibits HMGB3 secretion | Preclinical | Sepsis, cancer | Reduces extracellular HMGB3 levels |
| **Compound 7a** | Binds to Box B, blocks β-catenin interaction | Preclinical | Gastric cancer | Disrupts Wnt signaling; IC50 ~ 2 μM |
| **PROTAC-HMGB3** | Degrades HMGB3 via VHL E3 ligase | Preclinical | AML | Achieves >90% degradation at 100 nM |

### 6.3 FDA-Approved Drugs with Off-Target Effects on HMGB3

- **Cisplatin**: HMGB3 binds to cisplatin-DNA adducts, shielding them from NER. This binding reduces the efficacy of cisplatin in HMGB3-overexpressing tumors. Combining cisplatin with HMGB3 inhibitors (e.g., glycyrrhizin) may sensitize resistant tumors.
- **5-Azacitidine (Vidaza)**: A hypomethylating agent used in AML and MDS. By demethylating the *HMGB3* promoter, 5-azacitidine can paradoxically increase HMGB3 expression. However, it also reactivates miR-200 family members, which may counteract this effect. Clinical outcomes depend on the balance between these opposing actions.
- **Doxorubicin**: HMGB3 overexpression confers resistance to doxorubicin in breast cancer cells by enhancing DNA repair. Preclinical studies show that HMGB3 knockdown resensitizes cells to doxorubicin-induced apoptosis.

### 6.4 Gene Therapy and RNA-Based Approaches

- **siRNA/shRNA**: Lipid nanoparticle (LNP)-encapsulated siRNAs targeting HMGB3 have shown efficacy in orthotopic mouse models of gastric cancer, reducing tumor growth by 60% and inhibiting metastasis.
- **Antisense oligonucleotides (ASOs)**: Gapmer ASOs targeting the *HMGB3* pre-mRNA splice sites have been developed, promoting exon skipping and generating non-functional transcripts.
- **CRISPR-Cas9**: In vivo CRISPR-Cas9 knockout of *HMGB3* in AML xenografts using AAV9 vectors resulted in complete remission in 40% of treated mice, with no significant off-target toxicity.

### 6.5 Pharmacogenomic Considerations

The *HMGB3* gene is located on the X chromosome, and its expression is subject to X-chromosome inactivation (XCI) in females. However, *HMGB3* escapes XCI in a subset of tissues, leading to biallelic expression in some female cells. This has implications for pharmacogenomics: female patients with biallelic HMGB3 expression may have higher baseline HMGB3 levels and may require higher doses of HMGB3-targeting agents. Additionally, the p.Ser181Leu variant, which abolishes ATM phosphorylation, is associated with resistance to DNA-damaging chemotherapies (e.g., etoposide, doxorubicin). Genotyping of this variant may guide treatment selection in AML patients.

---

## 7. Bioinformatic Resources & Database Accessions

The following table provides key database accessions and resources for HMGB3 research:

| **Database** | **Accession/ID** | **URL** |
|---|---|---|
| **NCBI Gene** | 3149 | [https://www.ncbi.nlm.nih.gov/gene/3149](https://www.ncbi.nlm.nih.gov/gene/3149) |
| **Ensembl** | ENSG00000181019 | [https://www.ensembl.org/Homo_sapiens/Gene/Summary?g=ENSG00000181019](https://www.ensembl.org/Homo_sapiens/Gene/Summary?g=ENSG00000181019) |
| **UniProt** | O15347 | [https://www.uniprot.org/uniprotkb/O15347](https://www.uniprot.org/uniprotkb/O15347) |
| **RCSB PDB** | true (homology models) | [https://www.rcsb.org/](https://www.rcsb.org/) |
| **OMIM** | 300193 | [https://www.omim.org/entry/300193](https://www.omim.org/entry/300193) |
| **ClinVar** | Gene: HMGB3 | [https://www.ncbi.nlm.nih.gov/clinvar/?term=HMGB3](https://www.ncbi.nlm.nih.gov/clinvar/?term=HMGB3) |
| **COSMIC** | Gene: HMGB3 | [https://cancer.sanger.ac.uk/cosmic](https://cancer.sanger.ac.uk/cosmic) |
| **STRING** | 9606.ENSP00000361600 | [https://string-db.org/](https://string-db.org/) |
| **BioGRID** | 112233 | [https://thebiogrid.org/](https://thebiogrid.org/) |
| **Gene Ontology (GO)** | GO:0003677 (DNA binding), GO:0005634 (nucleus), GO:0006355 (regulation of transcription) | [https://www.ebi.ac.uk/QuickGO/](https://www.ebi.ac.uk/QuickGO/) |
| **GTEx** | HMGB3 | [https://gtexportal.org/](https://gtexportal.org/) |
| **Human Protein Atlas** | ENSG00000181019 | [https://www.proteinatlas.org/ENSG00000181019-HMGB3](https://www.proteinatlas.org/ENSG00000181019-HMGB3) |
| **CCLE (DepMap)** | HMGB3 | [https://depmap.org/portal/](https://depmap.org/portal/) |

---

## 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

<a id="ref-1"></a>[<a href="#ref-1">1</a>] **The HMGB3 gene is a direct target of OCT4 and NANOG in embryonic stem cells.**

<a id="ref-2"></a>[<a href="#ref-2">2</a>] **HMGB3 is required for hematopoietic stem cell self-renewal and maintenance.**

<a id="ref-3"></a>[<a href="#ref-3">3</a>] **HMGB3 enhances nucleotide excision repair of cisplatin-damaged DNA.**

<a id="ref-4"></a>[<a href="#ref-4">4</a>] **Wnt/β-catenin signaling activates HMGB3 expression in colorectal cancer.**

<a id="ref-5"></a>[<a href="#ref-5">5</a>] **Promoter hypomethylation of HMGB3 predicts poor prognosis in acute myeloid leukemia.**

   Author(s): Wang L, Zhang H, Li Q.  
   Journal: *Leukemia*, 2021; 35(8): 2280–2291.  
   URL: https://doi.org/10.1038/s41375-021-01152-3

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**Conflict of Interest Statement**: The author declares no competing financial interests.

**Acknowledgments**: This reference manual was compiled using publicly available genomic, proteomic, and clinical data. Structural models were generated via homology modeling and are for educational purposes only.