White Sponge Nevus (WSN) Cell Models for Research

White Sponge Nevus (WSN) is a rare, benign autosomal dominant disorder affecting the oral mucosa, with an estimated prevalence of 1 in 100,000 to 1 in 200,000 individuals. It typically presents in childhood or adolescence as bilateral, asymptomatic, white, spongy plaques on the buccal mucosa, but can also affect other mucosal surfaces. Although benign, WSN can cause cosmetic concerns and, rarely, functional issues. There is no malignant transformation risk, but the condition can be misdiagnosed as leukoplakia or other white lesions, leading to unnecessary biopsies. The disease is caused by mutations in keratin genes, primarily KRT4 and KRT13, which are critical for the structural integrity of oral epithelial cells. Research on WSN provides insights into keratin function, epithelial differentiation, and cell adhesion, with broader implications for other keratinopathies and oral mucosal disorders.

WSN serves as an excellent model for studying keratin intermediate filament biology and epithelial tissue homeostasis. The disease is monogenic, making it amenable to gene editing and functional studies. Key research questions include how specific keratin mutations disrupt filament assembly, impact cell proliferation and differentiation, and alter cell-cell adhesion. Additionally, WSN offers a platform to explore genotype-phenotype correlations, as different mutations in KRT4 and KRT13 can lead to variable clinical presentations. Public datasets, such as those from NCBI ClinVar, provide mutation information, while cell lines derived from oral mucosa can be used for mechanistic studies. The availability of CRISPR-engineered isogenic cell lines with specific keratin mutations enables precise dissection of molecular pathways and potential therapeutic targets.

WSN is not a malignant condition, so the term 'carcinogenic pathways' is not applicable. Instead, we focus on the pathogenic mechanisms leading to the phenotype. The primary defect is in keratin intermediate filaments, which provide mechanical support to epithelial cells. Mutations in KRT4 or KRT13 disrupt filament assembly, leading to cytoplasmic aggregation and collapse of the cytoskeleton. This results in impaired cell-cell adhesion, altered cell signaling, and abnormal epithelial differentiation. The affected pathways include:

• • Keratin filament assembly: Mutations in the helix initiation and termination motifs of KRT4 or KRT13 prevent proper dimerization and filament formation.
• • Cell adhesion: Disrupted keratin network affects desmosome integrity, leading to weakened intercellular adhesion.
• • Cell signaling: Keratin mutations can sequester signaling molecules, affecting pathways such as MAPK and PI3K/AKT, which regulate proliferation and differentiation.
• • Epithelial differentiation: Abnormal keratin expression alters the differentiation program of oral keratinocytes, leading to the characteristic spongy appearance.

Data from NCBI ClinVar and literature. The mutations are typically heterozygous, autosomal dominant, and cluster in the highly conserved helix boundary motifs of the keratin rod domain.

Keratin mutations in WSN affect multiple signaling networks:

• • MAPK pathway: Keratin filaments interact with RAF1 and other MAPK components; disruption can lead to altered ERK signaling, affecting cell proliferation.
• • PI3K/AKT pathway: Keratins can sequester PI3K; mutations may release it, leading to hyperactivation of AKT and increased cell survival.
• • Wnt/β-catenin: Keratin filaments may modulate β-catenin localization; disruption could affect differentiation.
• • Notch signaling: Keratin expression is regulated by Notch; feedback loops may be disturbed.

Key nodes: KRT4, KRT13, desmoplakin, plakoglobin, and signaling intermediates like ERK, AKT, and β-catenin.

For WSN, patient-derived primary keratinocytes can be used, but they are not immortalized. Organoids from oral mucosa can be generated from patient biopsies and maintain the disease phenotype. These 3D models are valuable for studying epithelial differentiation and testing therapeutic interventions.

• • Genetically engineered mouse models (GEMM): Knock-in mice carrying Krt4 or Krt13 mutations (e.g., Krt13 R80H) recapitulate the oral mucosal phenotype. These are used to study disease mechanism and test gene therapies.
• • Induced models: CRISPR-engineered mice with conditional knockout of Krt13 in oral epithelium can be used to study the role of keratin loss.
• • Patient-derived xenografts (PDX): Not commonly used for benign conditions, but oral mucosal tissue from patients can be transplanted into immunodeficient mice for short-term studies.

These models are essential for in vivo validation of gene editing approaches and understanding systemic effects.

CRISPR-Cas9 gene editing enables the creation of isogenic cell lines with specific KRT4 or KRT13 mutations. For example:

• • KRT13 knockout cell line: Complete loss of KRT13 to study the effect of null mutations.
• • KRT13 R80H knock-in cell line: Introduction of a common pathogenic mutation to model the dominant-negative effect.
• • KRT4 knockout cell line: To study the contribution of KRT4 loss.

These models are sequence-verified and can be generated in a background of immortalized oral keratinocytes (e.g., OKF6/TERT-1) to ensure phenotypic relevance. They are commercially available from various sources, allowing researchers to accelerate their studies without the need for in-house gene editing. Such models are invaluable for functional genomics, drug screening, and mechanistic studies.

Gene-edited cell lines with KRT4 or KRT13 mutations allow researchers to validate the functional impact of specific genetic variants. For example:

• • Knockout lines can be used to assess the effect of complete loss of a keratin on cell morphology, proliferation, and adhesion.
• • Knock-in lines with patient-specific mutations can be used to study the dominant-negative mechanism and identify downstream pathways.

These models are essential for confirming pathogenicity of novel variants and for understanding genotype-phenotype correlations.

Isogenic pairs (wild-type vs. mutant) can be used in high-throughput screens to identify compounds that rescue the mutant phenotype. For WSN, potential therapeutic approaches include:

• • Small molecules that stabilize keratin filaments or promote proper folding.
• • Gene therapy approaches using antisense oligonucleotides or CRISPR base editing to correct the mutation.

Drug resistance is not a major concern for a benign condition, but these models can be used to test the efficacy of topical treatments.

CRISPR-engineered cells can be used to identify biomarkers of disease progression or response to therapy. For example:

• • Transcriptomic and proteomic profiling of mutant vs. wild-type cells can reveal differentially expressed genes that serve as potential biomarkers.
• • Synthetic lethality screens: Although WSN is not cancer, such screens could identify genes that, when silenced, are lethal only in the context of keratin mutations, providing potential therapeutic targets.

These approaches can be applied to other keratinopathies as well.