Joubert syndrome Cell Models for Research

Joubert syndrome (JBTS) is a rare autosomal recessive ciliopathy with an estimated prevalence of 1 in 80,000 to 1 in 100,000 live births (Orphanet). The condition is characterized by the molar tooth sign on brain imaging, hypotonia, ataxia, developmental delay, and variable multiorgan involvement including retinal dystrophy, renal cysts, and hepatic fibrosis. Life expectancy is reduced, with many patients succumbing to respiratory failure or renal disease in childhood or early adulthood. There is no cure, and management is supportive. The disease imposes a significant burden on patients and families, and research into pathogenic mechanisms and therapies is urgently needed.

Joubert syndrome is an ideal model for studying primary cilia function and ciliogenesis. Over 30 genes are associated with JBTS, many encoding proteins of the primary cilium or its basal body. The disorder exhibits phenotypic variability and genetic heterogeneity, providing a rich system to dissect genotype-phenotype correlations. Public datasets such as the International Rare Diseases Research Consortium (IRDiRC) and ClinVar provide extensive variant data. Open questions include the precise molecular pathways disrupted, the role of specific genes in different tissues, and the development of targeted therapies. Gene-edited cell models are essential to address these questions.

The primary cilium is a microtubule-based organelle that acts as a signaling hub. In Joubert syndrome, defects in ciliary structure or function disrupt multiple signaling pathways. Key pathways include:

• • Hedgehog (Hh) signaling: Ciliary localization of Smoothened (SMO) and Gli transcription factors is essential for Hh signal transduction. Mutations in JBTS genes impair Hh signaling, leading to developmental defects.
• • Wnt signaling: Both canonical and non-canonical Wnt pathways are modulated by ciliary proteins. Disruption can affect cell polarity and proliferation.
• • Planar cell polarity (PCP): Ciliary proteins are involved in PCP, which is critical for tissue organization.
• • G-protein-coupled receptor (GPCR) signaling: Primary cilia are enriched with GPCRs, and defects in ciliary transport can alter GPCR signaling.

Data compiled from NCBI Gene, ClinVar, and literature.

Joubert syndrome mutations disrupt ciliary function, leading to aberrant signaling. Key networks include:

• • Hedgehog signaling: Loss of ciliary function impairs Gli processing, leading to reduced Hh target gene expression.
• • Wnt signaling: Altered ciliary trafficking of Wnt receptors can cause both hyperactivation and suppression of canonical Wnt, depending on context.
• • mTOR signaling: Ciliary proteins interact with mTOR pathway components, and dysregulation may contribute to renal cysts.
• • DNA damage response: Some JBTS proteins are involved in DNA repair, and their loss may lead to genomic instability.

Organoids, such as kidney or brain organoids derived from patient iPSCs, recapitulate tissue-specific ciliary defects and are valuable for studying disease mechanisms and drug screening.

• • Mouse models: Knockout mice for Cep290, Tmem67, and other JBTS genes exhibit cerebellar and retinal defects, recapitulating human phenotypes.
• • Zebrafish models: Morpholino or CRISPR-generated mutants show ciliary defects and are useful for high-throughput screening.
• • Patient-derived xenografts (PDX): Not commonly used for JBTS, but organoid-based xenografts can be employed for drug testing.
• • Genetically engineered mouse models (GEMM): Conditional knockouts allow tissue-specific deletion to study organ-specific roles.

CRISPR-based gene editing enables the creation of isogenic cell lines with precise mutations in JBTS genes. For example:

• • CEP290 knockout cell lines: Generated in RPE1 or HEK293 cells to study ciliary trafficking defects.
• • TMEM67 knock-in lines: Introduction of patient-specific missense mutations to assess functional impact.
• • AHI1 knockout lines: Used to investigate ciliary signaling.

These models are commercially available as sequence-verified, clonal cell lines, ensuring reproducibility. They are essential for functional validation and drug screening. Isogenic pairs (wild-type vs. mutant) allow direct comparison, minimizing confounding factors.

Knockout and knock-in cell lines are used to validate the function of JBTS genes. For example, CEP290 knockout in RPE1 cells leads to reduced ciliation and impaired Hedgehog signaling, confirming its role. Similarly, TMEM67 knockout disrupts ciliary membrane composition. These models allow researchers to study gene function in a controlled environment and to identify genetic modifiers.

Isogenic cell line pairs are ideal for high-throughput screening. For instance, a CEP290 knockout line can be used to screen for compounds that rescue ciliation defects. Drug resistance studies can be performed by exposing mutant cells to therapeutic candidates and selecting for resistant clones, enabling identification of resistance mechanisms.

CRISPR-based synthetic lethality screens can identify genes that are essential in JBTS-mutant cells but not in wild-type cells. This approach can reveal novel therapeutic targets and biomarkers. For example, a screen in TMEM67 knockout cells might identify kinases that are selectively required for survival, providing potential drug targets.