Omodysplasia 1 (OMOD1) Cell Models for Research

Omodysplasia 1 (OMOD1) is an extremely rare autosomal recessive skeletal dysplasia characterized by shortening of the limbs (rhizomelic) and facial dysmorphism. Due to its rarity, global incidence is not well-defined, but it is estimated to affect fewer than 1 in 1,000,000 individuals. The condition is caused by mutations in the GPC6 gene, which encodes glypican 6, a heparan sulfate proteoglycan involved in skeletal development. Clinical impact includes severe short stature, joint dislocations, and distinctive facial features. There is no cure, and management is supportive. Research focuses on understanding the molecular mechanisms of GPC6 in chondrocyte differentiation and bone growth.

OMOD1 serves as an excellent model for studying skeletal development and the role of heparan sulfate proteoglycans in growth factor signaling. The disease is monogenic, making it amenable to gene editing to create isogenic cell lines that recapitulate the disease phenotype. Public datasets, such as those from the International Skeletal Dysplasia Registry, provide clinical and genetic data. Open questions include the precise signaling pathways affected by GPC6 deficiency and potential therapeutic targets. Gene-edited cell models enable functional studies to answer these questions.

GPC6 is a glycosylphosphatidylinositol (GPI)-anchored heparan sulfate proteoglycan that regulates signaling pathways critical for skeletal development. The major pathways affected include:

• • Fibroblast growth factor (FGF) signaling: GPC6 modulates FGF ligand-receptor interactions, influencing chondrocyte proliferation.
• • Bone morphogenetic protein (BMP) signaling: GPC6 interacts with BMP ligands, affecting osteoblast differentiation.
• • Wnt signaling: GPC6 may modulate Wnt ligand distribution and receptor activation, impacting limb patterning.

Mutations in GPC6 lead to loss of function, disrupting these pathways and resulting in impaired endochondral ossification.

Data from ClinVar and literature indicate that biallelic loss-of-function mutations in GPC6 are the sole cause of OMOD1. No other recurrent mutations have been reported.

Loss of GPC6 disrupts multiple signaling networks:

• • FGF signaling: Reduced FGF signaling leads to decreased chondrocyte proliferation and differentiation.
• • BMP signaling: Altered BMP signaling affects osteoblast maturation and bone formation.
• • Wnt signaling: Dysregulated Wnt signaling impairs limb bud development and joint formation.
• • Hedgehog signaling: GPC6 may also influence Hedgehog ligand distribution, affecting growth plate organization.

These networks are interconnected, and GPC6 deficiency leads to a global disruption of skeletal development.

Organoids derived from patient iPSCs or gene-edited iPSCs can recapitulate cartilage and bone development, providing a three-dimensional model for studying OMOD1.

• • Gpc6 knockout mouse: A constitutive knockout mouse model exhibits skeletal abnormalities resembling OMOD1, including shortened limbs and craniofacial defects.
• • Conditional knockout mouse: Tissue-specific knockout using Cre-lox systems allows study of GPC6 in chondrocytes or osteoblasts.
• • Patient-derived xenograft (PDX): Not applicable for OMOD1 as it is not a cancer; however, xenografts of human bone cells could be used for mechanistic studies.
• • Induced models: CRISPR-engineered mouse models with specific GPC6 mutations (e.g., point mutations) can be generated to study genotype-phenotype correlations.

CRISPR-Cas9 gene editing enables the creation of isogenic cell lines with precise GPC6 mutations. For example:

• • GPC6 knockout cell lines: Complete loss of function to model null mutations.
• • GPC6 knock-in cell lines: Introduction of specific patient mutations (e.g., p.Arg365Ter) to study their impact.

These isogenic lines, where the only difference is the introduced mutation, are invaluable for dissecting the molecular consequences of GPC6 loss. Commercially available, sequence-verified models accelerate research by providing consistent and validated tools. Such models are essential for drug screening and functional genomics.

Gene-edited cell models allow functional validation of GPC6 variants. For instance, CRISPR knockout of GPC6 in hMSCs can be used to study the effects on chondrogenic differentiation. Knock-in of patient mutations can confirm pathogenicity and assess the impact on protein expression and signaling. These models enable high-throughput screening to identify genetic modifiers or downstream effectors.

Isogenic cell pairs (wild-type vs. GPC6 knockout) can be used to screen for compounds that rescue the defective phenotype. For example, small molecules that activate FGF or BMP signaling could be tested for their ability to restore chondrogenesis. Additionally, gene-edited cells can be used to test drug toxicity and efficacy in a disease-relevant context.

CRISPR-based synthetic lethality screens can identify genes that are essential in GPC6-deficient cells but not in wild-type cells. This approach can uncover potential therapeutic targets. Furthermore, transcriptomic and proteomic profiling of gene-edited cells can reveal biomarkers for disease progression or response to therapy.