Tooth Agenesis, Selective, 4 (STHAG4) Cell Models for Research

Tooth agenesis is the most common human congenital anomaly, with prevalence ranging from 1.6% to 9.6% depending on the population and excluding third molars. Selective tooth agenesis (STHAG) is a nonsyndromic form characterized by the congenital absence of one or more permanent teeth. STHAG4 is a specific subtype linked to mutations in the WNT10A gene. The condition can lead to functional and aesthetic problems, affecting mastication, speech, and quality of life. Early diagnosis and intervention are crucial for dental rehabilitation.

STHAG4 serves as an excellent model for studying tooth development and the molecular mechanisms underlying odontogenesis. The disease is relatively well-defined genetically, with WNT10A mutations accounting for a significant proportion of cases. Public datasets such as the Human Gene Mutation Database (HGMD) and ClinVar provide extensive variant information. Open questions remain regarding the genotype-phenotype correlation and the role of WNT10A in other tissues, making it a valuable area for functional studies.

While tooth agenesis is not a cancer, the underlying pathways are critical for development. The WNT/β-catenin signaling pathway is central to tooth development. Here are the key steps:

1. WNT ligand (e.g., WNT10A) binds to Frizzled receptors and LRP5/6 co-receptors.

2. This leads to stabilization of β-catenin, which translocates to the nucleus.

3. β-catenin activates transcription of target genes such as MSX1, PAX9, and AXIN2, which are essential for odontogenesis.

Dysregulation of this pathway, often due to mutations in WNT10A, disrupts the signaling cascade, leading to arrested tooth development.

Data from ClinVar and literature.

The WNT/β-catenin pathway is the primary network affected in STHAG4. Key nodes include:

• • WNT10A: ligand, crucial for activation.
• • LRP6: co-receptor, mutations can affect signaling.
• • β-catenin: central mediator, its stability is regulated by a destruction complex (APC, AXIN, GSK3β).
• • AXIN2: scaffold protein, also a target gene, creating a negative feedback loop.
• • MSX1 and PAX9: transcription factors downstream, essential for dental mesenchyme development.

Additionally, the BMP and FGF pathways interact with WNT signaling during tooth morphogenesis, and their crosstalk is often disrupted in agenesis.

Organoids derived from dental stem cells are emerging as more physiologically relevant models, recapitulating tooth development in vitro. They can be generated from induced pluripotent stem cells (iPSCs) and used to study the effects of gene mutations in a 3D context.

• • Genetically engineered mouse models (GEMMs): WNT10A knockout mice exhibit tooth agenesis, providing a valuable in vivo model.
• • Induced models: Chemical induction or surgical removal of tooth germs can mimic agenesis.
• • PDX models: Not commonly used for tooth agenesis, but xenografts of dental tissues can be used for mechanistic studies.

These models allow for the study of tooth development in a whole-organism context.

CRISPR-based gene editing enables the creation of isogenic cell lines with specific mutations in WNT10A, AXIN2, MSX1, or PAX9. For example, a WNT10A knockout cell line can be generated in a dental epithelial cell line, while a knock-in line can introduce a specific point mutation (e.g., p.Cys107*) found in patients. These models are sequence-verified and commercially available from various sources, accelerating research by providing consistent and reproducible tools. They are ideal for studying the molecular consequences of mutations and for drug screening.

Knockout and knock-in lines are used to validate the role of genes in tooth development. For instance, WNT10A knockout cells show reduced WNT signaling activity, confirming its role in the pathway. Similarly, introducing a patient-specific mutation into a wild-type line can demonstrate the pathogenic effect. These models are essential for functional genomics studies, including transcriptomic and proteomic analyses.

Isogenic pairs (wild-type vs. mutant) are powerful tools for drug screening. They can be used to identify compounds that rescue the mutant phenotype, such as WNT pathway agonists. Additionally, they can be used to study resistance mechanisms to drugs that target the WNT pathway, which is relevant in cancer contexts where WNT signaling is often hyperactivated.

CRISPR-based synthetic lethality screens can identify genes that, when silenced, are lethal only in the context of a specific mutation. For example, in WNT10A mutant cells, silencing a gene that compensates for WNT loss might be lethal, revealing potential therapeutic targets. Such screens can also identify biomarkers for early diagnosis or prognosis.