Joubert Syndrome 4 (JBTS4) Cell Models for Research

Joubert Syndrome 4 (JBTS4) is a rare autosomal recessive ciliopathy with an estimated incidence of 1 in 100,000 live births (WHO, 2023). It is characterized by cerebellar vermis hypoplasia, molar tooth sign on MRI, and intellectual disability. The condition is progressive, with many patients developing retinal dystrophy, renal cysts, and hepatic fibrosis. Life expectancy is reduced, with many patients surviving into adulthood but with significant morbidity. There is no cure, and management is supportive. The rarity and heterogeneity of JBTS4 make it a challenging but important disease for mechanistic research and therapeutic development.

JBTS4 is an ideal model for studying ciliary function and ciliopathies. The disease is primarily caused by mutations in the NPHP1 gene, which encodes a ciliary protein. Research models, including gene-edited cell lines, allow for the investigation of ciliary signaling pathways, the role of primary cilia in development, and the pathophysiology of ciliopathies. Public datasets, such as those from the International Rare Diseases Research Consortium (IRDiRC), provide valuable resources for studying genotype-phenotype correlations. Open questions include the molecular mechanisms linking NPHP1 mutations to ciliary dysfunction and the development of targeted therapies.

Although JBTS4 is not a cancer, its pathogenesis involves ciliary dysfunction that can affect cell signaling pathways relevant to cancer. The primary cilia act as sensory organelles that regulate key signaling pathways. In JBTS4, loss of NPHP1 leads to defective ciliary signaling, which can impact:

• • Hedgehog (Hh) signaling: Cilia are essential for Hh signal transduction. Disruption can lead to abnormal cell proliferation and differentiation.
• • Wnt signaling: Cilia modulate both canonical and non-canonical Wnt pathways. Altered Wnt signaling can affect cell polarity and proliferation.
• • Planar cell polarity (PCP): Ciliary dysfunction can disrupt PCP, leading to tissue organization defects.

These pathways are also implicated in various cancers, making JBTS4 models useful for studying ciliary contributions to tumorigenesis.

Data from ClinVar and COSMIC (2023).

JBTS4 is primarily a ciliopathy, and the deregulated signaling networks are those dependent on primary cilia. Key networks include:

• • Hedgehog signaling: NPHP1 interacts with ciliary proteins to regulate Gli transcription factors. Loss of NPHP1 leads to aberrant Hh signaling, affecting cell fate and proliferation.
• • Wnt signaling: Cilia modulate both canonical (β-catenin-dependent) and non-canonical (planar cell polarity) Wnt pathways. NPHP1 deficiency can lead to hyperactivation of canonical Wnt, promoting proliferation.
• • mTOR signaling: Cilia regulate mTOR activity; NPHP1 loss may cause mTOR hyperactivation, contributing to renal cyst formation.
• • Autophagy: Ciliary dysfunction can impair autophagy, leading to accumulation of damaged proteins and organelles.

These networks are interconnected and provide potential therapeutic targets.

Organoids, particularly kidney organoids derived from patient iPSCs, offer a more physiologically relevant model for studying ciliopathy phenotypes. They can recapitulate renal cyst formation and allow for drug screening.

• • Knockout mouse models: Nphp1 knockout mice exhibit cerebellar and renal defects, mimicking human JBTS4. These are used to study disease mechanisms and test therapies.
• • Conditional knockout mice: Tissue-specific Nphp1 deletion (e.g., in the kidney) helps study organ-specific effects.
• • Zebrafish models: nphp1 morpholino or CRISPR knockout in zebrafish show ciliary defects and are useful for high-throughput screening.
• • Patient-derived xenografts (PDX): Not applicable for JBTS4 as it is not a cancer, but patient-derived organoids can be transplanted into mice for in vivo studies.

CRISPR-based gene editing enables the creation of isogenic cell lines with precise mutations in NPHP1. For example:

• • NPHP1 knockout cell lines: Complete loss-of-function models to study the effects of NPHP1 absence on ciliary function and signaling.
• • NPHP1 point mutation knock-in lines: Introduction of specific patient mutations (e.g., R413X) to study genotype-phenotype correlations.
• • Reporter lines: Tagging NPHP1 with fluorescent proteins to track its localization and dynamics.

These models are commercially available from various sources and are sequence-verified, providing reliable tools for drug discovery and functional genomics. They allow for controlled experiments in a defined genetic background, accelerating research.

Gene-edited cell lines are essential for validating the function of NPHP1 and other ciliary genes. For example:

• • NPHP1 knockout lines can be used to identify downstream effectors via transcriptomics and proteomics.
• • Knock-in lines with specific mutations can reveal allele-specific effects on ciliary assembly and signaling.
• • CRISPR screens using these lines can identify genetic modifiers that rescue or exacerbate ciliary defects.

Isogenic pairs (wild-type vs. NPHP1 knockout) are powerful for drug screening. They allow for the identification of compounds that specifically target mutant cells while sparing normal cells. For example:

• • High-throughput screening of small molecules that restore ciliogenesis in NPHP1-deficient cells.
• • Testing drugs that modulate mTOR or Hedgehog pathways to reduce renal cyst formation.
• • Resistance modeling: Since JBTS4 is not a cancer, resistance is less relevant, but drug efficacy and toxicity can be assessed.

CRISPR synthetic lethality screens can identify genes that are essential only in NPHP1-deficient cells. This can reveal novel therapeutic targets and biomarkers. For example:

• • Screening for genes whose knockdown selectively kills NPHP1 knockout cells but not wild-type cells.
• • Identifying secreted proteins that are altered in NPHP1-deficient cells as potential biomarkers for disease progression.