Primary Ciliary Dyskinesia (PCD) Cell Models for Research

Primary Ciliary Dyskinesia (PCD) is a rare, genetically heterogeneous disorder affecting approximately 1 in 10,000 to 20,000 individuals worldwide (WHO, 2023). It is characterized by impaired ciliary function, leading to chronic respiratory tract infections, sinusitis, otitis media, infertility, and situs inversus in about 50% of cases. The disease is often underdiagnosed due to overlapping symptoms with other respiratory conditions. The clinical burden is significant, with progressive lung damage leading to bronchiectasis and respiratory failure. Early diagnosis and management are critical to improve quality of life and survival.

PCD serves as an excellent model for studying ciliary biology, mucociliary clearance, and the molecular mechanisms underlying ciliary assembly and function. The disease is caused by mutations in over 50 genes, many of which encode components of the axonemal dynein arms, radial spokes, or other ciliary structures. This genetic heterogeneity provides a unique opportunity to dissect the functional roles of these proteins in ciliary motility and signaling. Public datasets, such as those from the NCBI ClinVar and the PCD Foundation, offer valuable resources for genotype-phenotype correlations. Open questions include the precise molecular pathways leading to ciliary dysfunction and the development of targeted therapies.

While PCD is not a cancer, the underlying ciliary dysfunction can impact signaling pathways that are also implicated in cancer. The primary pathways affected include:

• • Hedgehog (Hh) signaling: Cilia are essential for Hh signal transduction. Defects in ciliary transport can lead to aberrant activation or inhibition of the pathway, affecting cell proliferation and differentiation.
• • Wnt signaling: Cilia modulate both canonical and non-canonical Wnt pathways. Disruption can alter cell polarity and tissue organization.
• • Planar cell polarity (PCP): Ciliary positioning and function are linked to PCP, which is crucial for tissue morphogenesis.
• • Calcium signaling: Ciliary mechanosensation and chemosensation involve calcium influx, which can influence various downstream cascades.

Data from TCGA (not applicable) and COSMIC (not applicable) are not relevant; frequencies are based on PCD mutation databases such as the PCD Foundation and ClinVar.

The ciliary dysfunction in PCD affects several signaling networks:

• • Hedgehog signaling: Key nodes include PTCH1, SMO, GLI1-3. Ciliary transport of these components is disrupted, leading to altered transcriptional responses.
• • Wnt/PCP pathway: Core components such as VANGL2, CELSR1, and DVL are mislocalized, affecting cell polarity and migration.
• • Calcium signaling: Ciliary calcium channels (e.g., PKD2) are affected, impacting mechanosensation and downstream NFAT and MAPK pathways.
• • Notch signaling: Cilia may influence Notch receptor localization, affecting cell fate decisions.

Organoids derived from patient nasal or bronchial biopsies recapitulate ciliary differentiation and function, providing a more physiologically relevant model for studying PCD. They can be gene-edited using CRISPR to introduce or correct mutations.

• • Genetically engineered mouse models (GEMMs): Mice with targeted mutations in PCD genes (e.g., Dnah5, Dnai1) exhibit ciliary defects and respiratory symptoms, serving as valuable in vivo models.
• • Zebrafish models: Zebrafish with morpholino or CRISPR-induced mutations in PCD genes show ciliary phenotypes, allowing high-throughput screening.
• • Patient-derived xenografts (PDX): Not commonly used for PCD due to the non-cancerous nature, but organoid-based xenografts can be used for functional studies.
• • Induced pluripotent stem cells (iPSCs): iPSCs derived from PCD patients can be differentiated into ciliated epithelial cells, providing a patient-specific model for drug testing.

CRISPR-Cas9 technology enables the generation of isogenic cell lines with precise genetic modifications, such as knockouts of specific PCD genes or knock-in of patient-specific mutations. These models are essential for studying the functional consequences of mutations in a controlled genetic background. For example, a DNAH5 knockout cell line can be used to assess the impact on ciliary beating and signaling. Commercially available, sequence-verified gene-edited cell lines accelerate research by providing reliable and reproducible models. These cell lines are generated using CRISPR and validated for on-target editing and absence of off-target effects. They are available for a range of PCD genes and can be customized to meet specific research needs.

Gene-edited cell lines are instrumental in functional genomics studies to validate the role of genes implicated in PCD. For instance, knocking out DNAH5 in respiratory epithelial cells allows researchers to observe the resulting ciliary dyskinesia and assess the effects on downstream signaling pathways. Similarly, introducing a known pathogenic mutation via knock-in can help establish genotype-phenotype correlations. These models are also used in CRISPR screens to identify genetic modifiers that rescue or exacerbate ciliary defects.

Isogenic cell line pairs (wild-type vs. gene-edited) are powerful tools for drug screening. By comparing the response of mutant and wild-type cells to potential therapeutic compounds, researchers can identify drugs that specifically target the mutant phenotype. For example, screening a DNAH5 knockout cell line against a library of compounds may reveal drugs that restore ciliary function. Additionally, these models can be used to study drug resistance mechanisms, as mutations in ciliary genes may affect drug uptake or metabolism.

CRISPR-based synthetic lethality screens using gene-edited cell lines can identify novel biomarkers and therapeutic targets. For instance, in PCD, synthetic lethal partners of DNAH5 could be targeted to selectively kill cells with defective cilia. Such screens can also reveal biomarkers for early diagnosis or prognosis. Gene-edited cell lines provide a controlled system to validate candidate biomarkers and assess their specificity and sensitivity.