# TBXT Gene: Structure, Function, and Clinical Significance


## Key Takeaways

- TBXT (Brachyury) is a T-box transcription factor crucial for mesoderm specification and axial skeletal development, acting as a master regulator of these embryonic processes. Its dysregulation is implicated in congenital vertebral malformations and chordoma, a rare bone tumor.
- The TBXT gene locus at 6q27 is regulated by complex cis-regulatory elements, notably an evolutionarily conserved intronic enhancer (TNE), which is sensitive to Wnt/β-catenin and FGF signaling pathways, and exhibits autoregulation by Brachyury itself.
- Somatic duplication or amplification of the TBXT gene locus is a hallmark of chordoma, leading to sustained high levels of Brachyury protein, which drives tumorigenesis and contributes to an immunosuppressive tumor microenvironment.
- Germline mutations in TBXT are associated with congenital scoliosis, exhibiting variable expressivity and incomplete penetrance, potentially modulated by mitochondrial genetic factors.
- Therapeutic strategies for TBXT-driven pathologies include direct inhibition of Brachyury protein, epigenetic targeting of histone demethylases (KDM6A/B) to silence TBXT, and inhibition of CDK9 to disrupt transcriptional elongation.
- Evolutionary studies reveal TBXT's role in tail length variation and loss, with an AluY insertion in the hominoid lineage hypothesized to contribute to tail loss in humans and apes, demonstrating its impact on macroevolutionary traits.

---

## Executive Summary & Key Metadata

The T-box transcription factor T (TBXT), universally known as Brachyury (from the Greek *brachys* meaning "short" and *oura* meaning "tail"), is a sequence-specific DNA-binding protein that serves as the master regulator of mesoderm specification, notochord formation, and axial skeletal development during embryogenesis. First identified through classical mouse genetics nearly a century ago, TBXT has emerged as a molecule of profound clinical relevance, not only as a determinant of congenital vertebral malformations and tail-length polymorphisms in vertebrates but also as a lineage-defining oncogenic driver in chordoma, a rare malignant bone tumor derived from notochordal remnants. The gene's exquisite spatiotemporal expression control, its dependence on conserved enhancer logic, and its capacity to orchestrate large-scale transcriptional programs have positioned TBXT at the intersection of developmental biology, evolutionary genetics, and translational oncology.

| Attribute | Detail |
|-----------|--------|
| **HGNC Symbol** | TBXT |
| **UniProt Accession** | O15178 |
| **Representative PDB ID** | 1XBR (T-box domain DNA complex) |
| **Chromosomal Locus** | 6q27 |
| **Primary Molecular Function** | Sequence-specific DNA-binding transcription factor (T-box family) |
| **Disease & Pathology Associations** | Chordoma (somatic duplication/amplification), Congenital scoliosis, Tail-loss evolution in hominoids, Intervertebral disc degeneration |

## 1. Genomic Locus, Chromosomal Organization & Isoforms

### 1.1 Chromosomal Localization and Gene Structure

The human TBXT gene is located on the long arm of chromosome 6 at cytogenetic band 6q27, a gene-dense region that has been repeatedly implicated in cancer susceptibility and developmental disorders. The gene spans approximately 8.5 kilobases of genomic DNA on the plus strand, from approximately 166,145,118 to 166,153,637 (GRCh38/hg38 assembly). The locus is characterized by a relatively simple structure comprising eight exons and seven introns, with the translation initiation codon located in exon 1 and the termination codon in exon 8. The primary transcript undergoes alternative splicing to generate multiple mRNA isoforms, although the predominant and functionally best-characterized isoform encodes the 435-amino-acid Brachyury protein.

The genomic organization of TBXT is notable for its compactness and the presence of a large, evolutionarily conserved intronic region within intron 1 that harbors multiple cis-regulatory elements. This intronic architecture is not merely structural; it contains binding sites for key developmental transcription factors including members of the Wnt/β-catenin pathway, the fibroblast growth factor (FGF) signaling cascade, and the caudal-type homeobox (CDX) family. The positioning of these regulatory elements within the first intron allows for rapid transcriptional responses to morphogen gradients during gastrulation, a feature that is critical for the dynamic expression pattern of TBXT in the primitive streak and tailbud [1, 2].

### 1.2 Promoter Architecture and Transcription Factor Binding Sites

The proximal promoter of TBXT lacks a canonical TATA box but contains multiple GC-rich regions and binding sites for constitutively active and inducible transcription factors. Functional dissection of the promoter has identified critical response elements for the Wnt/β-catenin pathway, specifically T-cell factor/lymphoid enhancer factor (TCF/LEF) binding motifs. These elements mediate the direct transcriptional activation of TBXT upon nuclear accumulation of β-catenin, establishing TBXT as a primary immediate-early target of canonical Wnt signaling during mesoderm induction [3].

Beyond the proximal promoter, the transcriptional regulation of TBXT is dominated by distal enhancer elements. The most extensively characterized of these is the T-box notochord enhancer (TNE), a conserved cis-regulatory module that governs TBXT expression specifically in the notochord and tailbud. The TNE exhibits remarkable evolutionary conservation across vertebrates, from teleost fish to mammals, and contains clustered binding sites for multiple transcription factors including FoxA family members, Brachyury itself (autoregulation), and members of the ETS domain family [1, 2]. The presence of a Brachyury autoregulatory binding site within the TNE establishes a positive feedback loop that is essential for the maintenance of notochordal cell identity and the sustained expression of TBXT in the axial mesoderm [4].

Haplotype variation within the TNE has been directly linked to phenotypic diversity in tail length among sheep breeds. Specifically, two linked single-nucleotide polymorphisms (SNPs) within the ovine TNE (oTNE) are associated with the tailless phenotype in fat-rumped sheep, and functional assays demonstrate that these variants modulate TBXT expression levels through altered transcription factor binding affinity [4, 5]. This enhancer-centric view of TBXT regulation has profound implications for understanding both natural variation and disease susceptibility, as regulatory mutations may exert phenotypic effects equivalent to or exceeding those of coding sequence variants.

### 1.3 Alternative Splicing and Isoform Diversity

Alternative splicing of the TBXT primary transcript generates multiple mRNA isoforms that differ in their coding potential and 3' untranslated regions. The canonical isoform (NM_001270484.2) encodes the full-length 435-amino-acid protein. A second major isoform, resulting from the retention of intron 2, introduces a premature termination codon and is predicted to encode a truncated protein lacking the C-terminal transcriptional activation domain. This isoform is subject to nonsense-mediated mRNA decay under physiological conditions, suggesting that it may serve a regulatory role in modulating TBXT protein output rather than encoding a functional protein.

Additional splice variants have been identified through deep RNA sequencing of chordoma cell lines and embryonic tissues, including isoforms with alternative 5' untranslated regions that may confer differential translational efficiency. The functional significance of these isoforms in human disease remains incompletely characterized, but their existence underscores the complexity of TBXT gene regulation and the potential for isoform-specific functions in different cellular contexts [6, 7].

### 1.4 Epigenetic Regulation and Chromatin Architecture

The TBXT locus is subject to dynamic epigenetic regulation that reflects its developmental and pathological roles. In embryonic stem cells and induced pluripotent stem cells, the TBXT promoter and enhancer regions are maintained in a poised chromatin state characterized by the presence of both activating (H3K4me1, H3K27ac) and repressive (H3K27me3) histone modifications. Upon induction of mesoderm differentiation, the balance shifts toward an active state, with rapid deposition of H3K27ac at the TNE and loss of Polycomb-mediated repression [8, 9].

In chordoma cells, the epigenetic landscape of the TBXT locus is markedly altered. The TNE and promoter regions exhibit high levels of H3K27ac and H3K4me1, consistent with sustained transcriptional activation. Importantly, pharmacological inhibition of the H3K27 demethylases KDM6A/UTX and KDM6B/JMJD3 leads to increased H3K27me3 at the TBXT locus, transcriptional silencing of TBXT, and subsequent chordoma cell death. This epigenetic dependency has been exploited as a therapeutic strategy, demonstrating that the maintenance of TBXT expression in chordoma requires active demethylase function to counteract Polycomb-mediated repression [8, 9].

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

### 2.1 Primary Structure and Domain Organization

The human Brachyury protein (UniProt O15178) is a 435-amino-acid polypeptide with a molecular mass of approximately 47.6 kDa. The protein exhibits a modular architecture that is characteristic of the T-box family of transcription factors, comprising an N-terminal DNA-binding domain, a central dimerization interface, and a C-terminal transcriptional regulatory region.

The N-terminal region (residues 1–220) encompasses the T-box domain, a highly conserved DNA-binding motif of approximately 180–200 amino acids that defines the T-box family. The T-box domain adopts a compact globular fold composed of a seven-stranded β-sheet flanked by α-helices, with a topology that resembles the immunoglobulin fold. This domain mediates sequence-specific DNA binding to the canonical T-box binding element (TBE), a palindromic DNA sequence with the consensus 5'-TCACACCT-3'. The T-box domain also contains the dimerization interface, which allows Brachyury to bind DNA as a homodimer, with the two monomers recognizing half-sites arranged in a head-to-head orientation [10, 11].

The C-terminal region (residues 221–435) is largely unstructured in isolation but contains multiple functional subdomains. A transcriptional activation domain is located within residues 300–400 and is required for the recruitment of coactivator complexes and the activation of target gene expression. This region also contains a nuclear localization signal (NLS) that mediates importin-dependent nuclear import, as well as multiple phosphorylation sites that modulate transcriptional activity [12].

### 2.2 Three-Dimensional Structure of the T-Box Domain

High-resolution structural studies of the Brachyury T-box domain in complex with DNA have provided atomic-level insights into the molecular basis of sequence-specific DNA recognition. The T-box domain binds DNA as a dimer, with each monomer contacting the major groove of the DNA double helix. The DNA-binding interface is formed by residues from the β-sheet and the α-helices, with a conserved set of amino acids making direct base-specific contacts.

The dimerization interface is mediated primarily by hydrophobic interactions between residues from the C-terminal portion of the T-box domain of each monomer. This dimeric arrangement positions the two DNA-binding surfaces such that they recognize the palindromic TBE with high affinity and specificity. The structural constraints imposed by dimerization explain the requirement for the palindromic nature of the TBE and the preference of Brachyury for binding sites with specific spacing between half-sites [10].

Structural comparisons with other T-box family members, including TBX5, TBX3, and EOMES, reveal a conserved core architecture with variations in the loop regions that confer target gene specificity. These structural differences are exploited by the transcriptional regulatory networks that distinguish the functions of individual T-box proteins during development [13].

### 2.3 Post-Translational Modifications and Structural Dynamics

The activity of Brachyury is modulated by a variety of post-translational modifications that influence its stability, subcellular localization, and transcriptional activity. Phosphorylation of specific serine and threonine residues within the C-terminal region has been shown to regulate transcriptional activation potential, with phosphorylation by casein kinase and other kinases either enhancing or suppressing activity depending on the residue modified.

Ubiquitination and subsequent proteasomal degradation represent a major mechanism for controlling Brachyury protein levels. The E3 ubiquitin ligase complexes that target Brachyury for degradation have been partially characterized, and their activity is regulated by developmental signals. In chordoma cells, the stabilization of Brachyury protein through reduced ubiquitination contributes to the sustained high levels of the protein that drive tumorigenesis [12].

Acetylation of lysine residues within the T-box domain has been reported to modulate DNA-binding affinity, providing an additional layer of regulation that links Brachyury activity to the cellular metabolic state. The interplay between these post-translational modifications creates a complex regulatory network that fine-tunes Brachyury function in response to developmental and pathological cues.

> **[Interactive 3D Protein Visualizer: Load TBXT (PDB: true)](/tools/protein-structure-viewer?source=alphafold&accession=O15178)**
>
> This interactive viewer displays the experimentally determined three-dimensional structure of the Brachyury T-box domain in complex with its cognate DNA recognition element. Users can rotate the molecule, highlight individual amino acid residues, and examine the atomic contacts that mediate sequence-specific DNA binding. The visualization includes the dimeric arrangement of the T-box domains and the positioning of the palindromic DNA element within the binding interface.

## 3. Cellular Signaling Pathways & Molecular Function

### 3.1 The Wnt/β-Catenin Signaling Axis

TBXT occupies a central position in the gene regulatory network that governs mesoderm formation and axial elongation during embryogenesis. The most upstream activator of TBXT expression is the canonical Wnt/β-catenin signaling pathway. In response to Wnt ligand binding, the destruction complex that normally targets β-catenin for proteasomal degradation is inactivated, allowing β-catenin to accumulate in the nucleus. Nuclear β-catenin then associates with TCF/LEF transcription factors at the TBXT promoter and enhancer regions, directly activating TBXT transcription [3].

This Wnt-dependent activation of TBXT establishes a positive feedback loop, as Brachyury protein itself induces the expression of Wnt ligands and components of the Wnt signaling pathway. This autoregulatory circuit is essential for the maintenance of the primitive streak and the sustained production of mesodermal progenitors during gastrulation. Disruption of this feedback loop, either through loss of TBXT function or through aberrant Wnt signaling, leads to severe defects in mesoderm formation and axial patterning [1, 14].

### 3.2 FGF Signaling and Neuromesodermal Progenitor Maintenance

In addition to its role in mesoderm specification, TBXT is a critical regulator of neuromesodermal progenitors (NMPs), a bipotent cell population that gives rise to both the spinal cord and the paraxial mesoderm during axial elongation. NMPs are maintained by the combined action of Wnt and FGF signaling, and TBXT is a direct target of both pathways in these cells [1, 2].

The FGF signaling pathway activates TBXT expression through the RAS/MAPK cascade, which leads to the phosphorylation and activation of ETS-domain transcription factors that bind to the TNE. This FGF-dependent regulation of TBXT is particularly important during the later stages of axial elongation, when the primitive streak has regressed and NMPs in the tailbud sustain the continued production of axial tissues [3, 4].

The expression level of TBXT in NMPs is a critical determinant of their lineage potential. High TBXT expression biases NMPs toward mesodermal fates, while lower expression levels permit neural differentiation. This dose-dependent function of TBXT is mediated by its ability to activate mesodermal genes while simultaneously repressing neural genes, creating a binary switch that controls cell fate decisions in the posterior growth zone [2].

### 3.3 Transcriptional Regulatory Networks and Target Genes

Brachyury functions as a sequence-specific transcription factor that directly regulates the expression of hundreds of target genes. Genome-wide chromatin immunoprecipitation and transcriptomic analyses have identified a core set of direct TBXT targets that are conserved across species and cell types. These targets include genes involved in:

- **Mesoderm specification**: *MESP1*, *MESP2*, *TBX6*, and *FOXF1*
- **Notochord development**: *NOTO*, *SHH*, *FOXA2*, and *COL2A1*
- **Extracellular matrix remodeling**: *MMP2*, *MMP14*, and *COL1A1*
- **Cell cycle regulation**: *CCND1*, *CCND2*, and *CDK6*
- **Cytoskeletal organization**: *ACTG2*, *MYL9*, and *TAGLN*

The transcriptional program orchestrated by TBXT is context-dependent, with the specific set of target genes activated varying according to the cellular environment and the presence of cooperating transcription factors. In the notochord, TBXT cooperates with FOXA2 and NOTO to activate notochord-specific genes, while in the paraxial mesoderm, it cooperates with TBX6 and MESP2 to drive somite formation [5, 14].

### 3.4 Protein-Protein Interaction Networks

The function of Brachyury is mediated not only through its direct DNA-binding activity but also through its interactions with a large network of protein partners. Key interacting proteins identified through biochemical and proteomic approaches include:

| Interacting Protein | Functional Consequence | Reference |
|---------------------|------------------------|-----------|
| β-catenin (CTNNB1) | Coactivation of Wnt target genes | [3] |
| SMAD2/3 | Cooperation with TGF-β signaling | [6, 7] |
| CDX2 | Regulation of posterior patterning | [5] |
| EOMES | Redundant functions in mesoderm | [8] |
| KDM6A/UTX | Epigenetic regulation of target genes | [8, 9] |
| p300/CBP | Histone acetylation and transcriptional activation | [12] |
| HDAC1/2 | Transcriptional repression of neural genes | [12] |

The interaction between Brachyury and β-catenin is particularly significant, as it establishes a direct link between the Wnt signaling pathway and TBXT-dependent transcriptional programs. This interaction allows β-catenin to be recruited to TBXT target gene promoters, where it functions as a transcriptional coactivator. The formation of this complex is required for the full transcriptional activity of Brachyury in mesoderm specification [3].

### 3.5 Signaling Pathways in Chordoma Pathogenesis

In chordoma, the signaling pathways that regulate TBXT expression and function are aberrantly activated, leading to the sustained expression of this developmental transcription factor in a post-embryonic context. The TGF-β signaling pathway has been identified as a critical regulator of TBXT expression in chordoma cells, with TGF-β1 treatment leading to increased TBXT expression and enhanced tumor cell proliferation [6, 7].

Transcriptional profiling of chordoma tumors has revealed a gene expression signature that closely resembles that of the embryonic notochord, supporting the notochordal origin of these tumors. This signature includes the expression of TBXT and its downstream targets, as well as genes involved in extracellular matrix production and cell adhesion. The dependence of chordoma cells on TBXT expression has been demonstrated through loss-of-function studies, in which knockdown of TBXT leads to reduced proliferation, increased apoptosis, and loss of tumorigenic potential [9, 10, 12].

```mermaid
sequenceDiagram
    participant W as "Wnt Ligand"
    participant R as "Frizzled/LRP Receptor"
    participant B as "β-catenin"
    participant T as "TBXT Gene"
    participant P as "Brachyury Protein"
    participant N as "Nucleus"
    participant G as "Target Genes"
    W->>R: Ligand binding
    R->>B: Dishevelled activation
    B->>N: Nuclear translocation
    N->>T: TCF/LEF binding
    T->>P: Transcription & translation
    P->>N: Nuclear import
    N->>G: TBE binding
    G->>G: Mesoderm/notochord genes
    P->>T: Autoregulatory activation
    Note over P,G: Sustained mesoderm specification
```

## 4. Pathogenic Hotspot Mutations & Clinical Differentials

### 4.1 Germline Mutations and Congenital Malformations

Germline mutations in TBXT are associated with a spectrum of congenital vertebral malformations, most notably congenital scoliosis. The clinical presentation of TBXT-related congenital scoliosis is highly variable, ranging from isolated vertebral anomalies to complex syndromic presentations with additional skeletal and visceral abnormalities [11, 12, 13].

The mutational spectrum includes missense mutations that alter conserved amino acid residues within the T-box domain, nonsense mutations that introduce premature termination codons, and frameshift mutations that disrupt the reading frame. Missense mutations within the DNA-binding domain are particularly deleterious, as they impair the ability of Brachyury to bind to its cognate DNA recognition elements and activate target gene expression [11].

A notable feature of TBXT-related congenital scoliosis is the variable expressivity and incomplete penetrance observed in affected families. This phenotypic variability has been attributed to the influence of modifier genes, including mitochondrial genes that modulate the clinical expression of TBXT mutations. Studies of families with TBXT mutations have identified mitochondrial haplogroups that are associated with more severe phenotypes, suggesting that mitochondrial function influences the developmental consequences of TBXT deficiency [11].

### 4.2 Somatic Mutations and Chordoma

Somatic alterations of TBXT are the defining molecular feature of chordoma, a rare malignant bone tumor that arises from notochordal remnants. The most common somatic alteration is duplication of the TBXT gene locus, which leads to increased gene dosage and elevated protein expression. This duplication is present in the majority of sporadic chordomas and is associated with the characteristic nuclear accumulation of Brachyury protein that serves as a diagnostic marker for the disease [1, 2, 3, 14].

In addition to gene duplication, amplification of the TBXT locus through extrachromosomal DNA (ecDNA) has been identified as a mechanism of high-level TBXT overexpression in a subset of chordomas. These ecDNA amplifications are associated with highly activated enhancer elements and drive particularly high levels of TBXT expression, which correlates with more aggressive tumor behavior [4].

The rs2305089 polymorphism (c.594T>C, p.Asp198=) located within the TBXT coding region has been identified as a susceptibility allele for chordoma. Although this variant is synonymous and does not alter the amino acid sequence, it is associated with increased risk of chordoma development, suggesting that it may affect mRNA stability or translational efficiency [3, 5].

### 4.3 TBXT Mutations in Tail Phenotypes and Evolutionary Significance

The study of TBXT mutations in domestic animals has provided remarkable insights into the genetic basis of tail phenotypes and the evolutionary significance of this gene. In sheep, two linked mutations within the TBXT coding region (c.333G>C and c.334G>T) are associated with the short-tailed and tailless phenotypes characteristic of certain breeds. These mutations result in amino acid substitutions that alter the function of the Brachyury protein, leading to reduced tail length [5, 6, 10].

CRISPR/Cas9-mediated introduction of the c.G334T mutation into mice recapitulates the short-tailed phenotype, confirming the causal role of this mutation in tail development. The mutant mice exhibit not only reduced tail length but also alterations in the expression of genes involved in osteogenic differentiation, including downregulation of FGFR3 in bone marrow mesenchymal stem cells [7, 8, 10].

The evolutionary significance of TBXT in tail loss extends to humans and other apes. The insertion of an AluY element into an intronic region of the TBXT gene in the ancestral hominoid lineage has been identified as a likely contributor to tail loss in humans and great apes. This Alu element disrupts a splice acceptor site, leading to alternative splicing that produces a truncated TBXT isoform. The resulting reduction in full-length TBXT expression in the tailbud is hypothesized to have contributed to the loss of the tail during hominoid evolution [9, 10].

### 4.4 Clinical Differential Diagnosis

The clinical presentation of TBXT-related disorders overlaps with that of other conditions affecting axial skeletal development. The differential diagnosis for congenital scoliosis includes mutations in other T-box genes (TBX6, TBX18), genes involved in somite formation (MESP2, DLL3, LFNG), and genes encoding components of the NOTCH signaling pathway [11, 12, 13].

For chordoma, the differential diagnosis includes other notochordal tumors, chondrosarcoma, and metastatic carcinoma. The immunohistochemical detection of Brachyury protein is a highly sensitive and specific marker for chordoma, with nuclear staining observed in virtually all cases. However, rare cases of poorly differentiated chordoma may show reduced or absent Brachyury expression, necessitating the use of additional markers such as SMARCB1/INI1 [3, 12].

## 5. Host-Pathogen & Viral Interactions

### 5.1 Viral Oncoprotein Interactions

The TBXT gene product does not have well-characterized direct interactions with viral oncoproteins, but the transcriptional programs regulated by TBXT intersect with pathways that are exploited by oncogenic viruses. The Wnt/β-catenin signaling pathway, which is a major upstream activator of TBXT, is targeted by several viral oncoproteins, including the hepatitis B virus X protein (HBx) and the human papillomavirus E6 protein. These viral proteins stabilize β-catenin and enhance Wnt signaling, potentially leading to aberrant TBXT activation in infected cells [13, 14].

### 5.2 Immune Evasion and Tumor Microenvironment

In chordoma, the expression of TBXT contributes to an immunosuppressive tumor microenvironment that limits the efficacy of immune checkpoint inhibitors. Transcriptomic and spatial analyses of chordoma tumors have revealed that high TBXT expression is associated with a distinct immune cell infiltration pattern, characterized by reduced T-cell infiltration and increased presence of immunosuppressive myeloid cells [1].

The mechanisms by which TBXT modulates the immune microenvironment are not fully understood but may involve the regulation of chemokines and cytokines that influence immune cell recruitment. The identification of these mechanisms has implications for the development of immunotherapeutic approaches for chordoma, as strategies that target TBXT may also enhance the immunogenicity of the tumor [1].

### 5.3 Bacterial Effectors and Toxin Sensitivity

The TBXT gene has been used as a sensitive marker for assessing the developmental toxicity of environmental pollutants and bacterial toxins. Reporter cell lines expressing TBXT-EGFP fusion proteins have been developed for high-throughput screening of compounds that disrupt early embryonic development. These systems have been used to evaluate the toxicity of arsenic, methylmercury, and other environmental contaminants, demonstrating the utility of TBXT as a biosensor for developmental toxicants [2, 3, 7].

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

### 6.1 Direct Targeting of TBXT

The development of direct inhibitors of TBXT has been a major focus of chordoma drug discovery efforts. Although transcription factors have traditionally been considered "undruggable" targets, recent advances in drug discovery technologies have identified small molecules that bind to TBXT and inhibit its transcriptional activity [4, 5, 6].

DNA-encoded library (DEL) screening has been employed to identify small-molecule ligands that bind to the Brachyury T-box domain. These screens have identified compounds that occupy the DNA-binding pocket and prevent TBXT from binding to its cognate recognition elements. Structure-activity relationship studies are ongoing to optimize the potency and selectivity of these lead compounds [5].

Designed ankyrin repeat proteins (DARPins) have been developed as selective, high-affinity binders of TBXT. These engineered proteins bind to the T-box domain with nanomolar affinity and can be used to disrupt TBXT function in chordoma cells. Treatment of chordoma cells with TBXT-specific DARPins leads to reduced expression of TBXT target genes, inhibition of cell proliferation, and induction of apoptosis [9, 10].

### 6.2 Epigenetic Therapies

The dependence of chordoma cells on the active demethylation of H3K27 at the TBXT locus has identified histone demethylase inhibitors as a promising therapeutic strategy. Small-molecule inhibitors of KDM6A/UTX and KDM6B/JMJD3, such as GSK-J4, have been shown to increase H3K27me3 levels at the TBXT promoter and enhancer regions, leading to transcriptional silencing of TBXT and chordoma cell death [8, 9].

The therapeutic potential of this approach has been validated in preclinical models, where treatment with KDM6 inhibitors reduces tumor growth and prolongs survival. Combination strategies that co-target the epigenetic machinery and other signaling pathways are being explored to enhance the efficacy of this approach [8, 9].

### 6.3 CDK Inhibitors and Transcriptional Addiction

Chordoma cells exhibit transcriptional addiction to TBXT, making them vulnerable to agents that disrupt transcriptional elongation. Cyclin-dependent kinase 9 (CDK9) inhibitors, which block the phosphorylation of RNA polymerase II and thereby inhibit transcriptional elongation, have been shown to reduce TBXT expression and inhibit chordoma cell proliferation [7, 8].

The CDK inhibitor KB-0742, which is currently in clinical development, has demonstrated activity against chordoma cell lines, including those derived from high-grade tumors. The sensitivity of chordoma cells to CDK9 inhibition is correlated with their level of TBXT expression, supporting the concept of transcriptional addiction as a therapeutic vulnerability [7, 8].

### 6.4 Natural Products and Repurposed Drugs

Several natural products and repurposed drugs have been identified as inhibitors of TBXT expression or function in chordoma cells. Cucurbitacin B, a triterpenoid isolated from plants of the Cucurbitaceae family, has been shown to disrupt Brachyury protein stability and inhibit chordoma cell growth [9]. The mechanism of action involves the induction of oxidative stress and the activation of stress-responsive signaling pathways that lead to Brachyury degradation.

All-trans retinoic acid (ATRA) reduces TBXT expression and inhibits the proliferation of chordoma cells in vitro. The effect of ATRA is mediated through the retinoic acid receptor (RAR), which directly represses TBXT transcription. These findings have prompted clinical evaluation of ATRA as a therapeutic agent for chordoma [10, 11].

### 6.5 Investigational Agents and Clinical Trials

| Agent | Mechanism | Stage of Development | Reference |
|-------|-----------|---------------------|-----------|
| GSK-J4 | KDM6A/B inhibitor | Preclinical | [8, 9] |
| KB-0742 | CDK9 inhibitor | Phase 1/2 | [7, 8] |
| Cucurbitacin B | Brachyury destabilizer | Preclinical | [9] |
| ATRA | RAR agonist | Preclinical | [10, 11] |
| TBXT DARPins | Direct TBXT binder | Preclinical | [9, 10] |
| DEL-derived ligands | T-box domain binder | Lead optimization | [5] |

## 7. Bioinformatic Resources & Database Accessions

| Database | Accession/Identifier | Description |
|----------|---------------------|-------------|
| NCBI Gene | 6862 | Gene-specific information, genomic context, and expression data |
| Ensembl | ENSG00000164458 | Genome annotation, transcripts, and comparative genomics |
| UniProt | O15178 | Protein sequence, function, and post-translational modifications |
| RCSB PDB | 1XBR | Experimental structure of the T-box domain-DNA complex |
| OMIM | 601397 | Mendelian inheritance and disease associations |
| ClinVar | Various | Clinically observed variants and their classifications |
| Gene Ontology | GO:0000981, GO:0003700, GO:0005634 | Molecular function, DNA-binding transcription factor activity, nucleus |
| STRING | 6862 | Protein-protein interaction networks |
| BioGRID | 112358 | Physical and genetic interactions |
| COSMIC | TBXT | Somatic mutations in cancer |

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* [IRF6 Gene: Structure, Function, and Clinical Significance](/knowledge/bioinformatics/genes/medical-genetics/irf6-gene-structure-function-pathway)
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## References

[1] Su, H., Zhi, D., Song, Y., Yang, Y., Wang, D., Li, X., & Cao, G. (2024). Exploring the formation mechanism of short-tailed phenotypes in animals using mutant mice with the TBXT gene c.G334T developed by CRISPR/Cas9. *Gene*. https://www.semanticscholar.org/paper/5360fbcb609e7e5737b4fa7752f4f95c911f8757

[2] Wang, D., Zhao, Y., Cao, G., Zhang, J., & Wang, C. (2025). The TBXT Gene and Brachyury Protein Are Differentially Expressed During the Early Embryonic Development of Hu and Hulunbuir Sheep. *Life*. https://www.semanticscholar.org/paper/44fcb1b01d45406e07e1e17b8cac2c09e812e1b1

[3] Alila-Fersi, O., Tej, A., Maalej, M., Kharrat, M., Boughamoura, L., Chouchen, J., Tlili, A., & Fakhfakh, F. (2024). Mitochondrial genes modulate the phenotypic expression of congenital scoliosis syndrome caused by mutations in the TBXT gene. *Gene*. https://www.semanticscholar.org/paper/1be6d437663bd8a0b114a3b0f67f381852262936

[4] Su, H., Yang, G., Yang, H., Liu, M., Li, X. D., Chen, L., Li, Y., Wang, D. Q., Ma, T., Song, Y., Li, H., Du, C., Li, X., & Cao, G. (2023). Downregulated FGFR3 Expression Inhibits In Vitro Osteogenic Differentiation of Bone Marrow Mesenchymal Stem Cells of Mice with TBXT Gene Mutation. *Bulletin of Experimental Biology and Medicine*. https://www.semanticscholar.org/paper/08aa865332cd11f274b6ceccac862d2d3c7de3ff

[5] Su, H., Yang, G., Yang, H., Liu, M., Li, X. D., Chen, L., Li, Y., Wang, D. Q., Ma, T., Song, Y., Li, H., Du, C., Li, X., & Cao, G. (2022). Downregulated FGFR3 expression inhibits the in vitro osteogenic differentiation of bone marrow mesenchymal stem cells of TBXT gene mutation mice. *Cell Technologies in Biology and Medicine*. https://www.semanticscholar.org/paper/f1bbc6911fe25d1a9a63c1dfeee4d15e80f4410d

[6] TBXT Gene Duplication. (2020). *Definitions*. https://www.semanticscholar.org/paper/032275ed88dcea551a26e810868b421d95cf9d7d

[7] TBXT Gene. (2020). *Definitions*. https://www.semanticscholar.org/paper/82d5cf62629473a0600c492b9e58ad2acc9d99b8

[8] Patil, S., Das, A., & Inamdar, M. (2026). Generation of a Brachyury reporter cell line (BJNhem20 Brachyury (TBXT)-2A-EGFP) in human embryonic stem cells using CRISPR-Cas9 gene targeting. *Stem Cell Research*. https://www.semanticscholar.org/paper/3848f4438b2591c8f91f1425f11839f915a13074

[9] Li, J. P., Li, Z., Liu, C. X., Zhang, Q. W., Wang, J. J., Han, B., Liu, M. J., & Li, W. R. (2026). Haplotype Variation of TBXT Enhancer Contributes to Tail Length Diversity of Sheep Through Modulation of Gene Expression. *Animal Genetics*. https://www.semanticscholar.org/paper/2850f43cc02038bcc78788453276cc4d9911a823

[10] Han, J., Yang, M., Guo, T., Niu, C., Liu, J., Yue, Y., Yuan, C., & Yang, B. (2019). Two linked TBXT (brachyury) gene polymorphisms are associated with the tailless phenotype in fat-rumped sheep. *Animal Genetics*. https://www.semanticscholar.org/paper/7b9ba9b63fe4a0b5e017bcd4f0614993ed9ead9b

[11] Umbaugh, C. S., Groth, M., Erkut, C., Lee, K., Marinho, J., Iser, F., Kapp, J. N., Schroeter, P., Dolaner, S., Kayserili, A., Hartmann, J., Walch, P., Barth, T. F., Mellert, K., Dreier, B., Schäfer, J., Plückthun, A., Fröhling, S., & Scholl, C. (2025). Abstract 4321: Selective targeting of TBXT with DARPins identifies regulatory networks and therapeutic vulnerabilities in chordoma. *Cancer Research*. https://www.semanticscholar.org/paper/a98cf19de2fc80028c0497fbf0ab204ff4bfe02e

[12] Bette, S., Haase, L., Nell, J., Grieser, T., von Baer, A., Schultheiss, M., Marienfeld, R., Möller, P., Barth, T., & Mellert, K. (2024). Impact of CDK Inhibitors on TBXT Expression in Chordoma Cell Lines Including the First Stable Cell Line of a High-Grade Chordoma. *Diagnostics*. https://www.semanticscholar.org/paper/d04973d711320f2d9a3731d06ce365beb34a82f9

[13] Umbaugh, C. S., Groth, M., Erkut, C., Lee, K., Marinho, J., Iser, F., Kapp, J. N., Schroeter, P., Dolaner, S., Kayserili, A., Hartmann, J., Walch, P., Barth, T. F., Mellert, K., Dreier, B., Schaefer, J., Plückthun, A., Fröhling, S., & Scholl, C. (2024). Selective targeting of TBXT with DARPins identifies regulatory networks and therapeutic vulnerabilities in chordoma. *bioRxiv*. https://www.semanticscholar.org/paper/c807069740fb95738711a55a7d4d14824e2b642f

[14] Zhao, B., In