Multicentric Osteolysis, Nodulosis, and Arthropathy (MONA) Cell Models for Research

Multicentric Osteolysis, Nodulosis, and Arthropathy (MONA) is an ultra-rare autosomal recessive disorder caused by mutations in the MMP2 gene. The exact prevalence is unknown; fewer than 100 cases have been reported worldwide. It is characterized by progressive osteolysis (bone resorption), nodulosis (subcutaneous nodules), and arthropathy (joint disease). Onset typically occurs in early childhood, leading to severe skeletal deformities, joint contractures, and chronic pain. There is no cure, and management is symptomatic. The disease significantly impacts quality of life and life expectancy, with respiratory complications and infections being common causes of mortality. Research is crucial to understand the molecular mechanisms and develop targeted therapies.

MONA is an ideal model for studying bone metabolism, extracellular matrix remodeling, and the role of matrix metalloproteinases (MMPs) in development and disease. The disease is monogenic, making it amenable to gene editing. Key research questions include: How do MMP2 mutations lead to osteolysis? What are the downstream signaling pathways? Can gene therapy or enzyme replacement be effective? Public datasets are limited, but patient-derived fibroblasts and induced pluripotent stem cells (iPSCs) offer valuable resources. Gene-edited cell models can help dissect genotype-phenotype correlations and test therapeutic interventions.

MONA is primarily caused by loss-of-function mutations in MMP2, which encodes gelatinase A (matrix metalloproteinase-2). MMP2 is involved in the degradation of extracellular matrix components, including type IV collagen, gelatin, and elastin. The pathogenic mechanisms include:

• • Impaired extracellular matrix remodeling: MMP2 deficiency leads to accumulation of collagen and other matrix proteins, disrupting tissue homeostasis.
• • Altered cell signaling: MMP2 cleaves various substrates, including growth factors and cytokines, affecting signaling pathways such as TGF-β and VEGF.
• • Increased osteoclast activity: MMP2 may regulate osteoclast function, and its loss leads to enhanced bone resorption.
• • Fibrosis: Accumulation of extracellular matrix contributes to nodule formation and joint stiffness.

Data from ClinVar and literature. Most mutations are homozygous or compound heterozygous.

MMP2 deficiency affects multiple signaling networks:

• • TGF-β signaling: MMP2 cleaves latent TGF-β binding protein, regulating TGF-β availability. Loss of MMP2 leads to increased TGF-β activity, promoting fibrosis.
• • VEGF signaling: MMP2 releases VEGF from extracellular matrix, influencing angiogenesis. Altered VEGF signaling may affect bone vascularization.
• • Integrin signaling: MMP2 interacts with integrins, affecting cell adhesion and migration.
• • RANKL/OPG pathway: MMP2 may modulate osteoclast differentiation through RANKL signaling, leading to increased bone resorption.

Organoids derived from patient iPSCs can recapitulate bone and joint tissue, providing a 3D model for studying MONA pathophysiology.

• • Mmp2 knockout mouse: Recapitulates some features of MONA, including osteolysis and joint abnormalities.
• • Mmp2 mutant mouse models: Generated via CRISPR to introduce specific patient mutations.
• • Zebrafish models: Used for rapid drug screening.
• • PDX models: Not applicable due to the genetic nature of MONA.

CRISPR-Cas9 technology enables the creation of isogenic cell lines with precise MMP2 mutations. For example:

• • MMP2 knockout cell lines: Complete loss of function, useful for studying null phenotypes.
• • MMP2 knock-in cell lines: Introduction of specific patient mutations (e.g., p.R101H) to model genotype-phenotype correlations.
• • Reporter cell lines: GFP-tagged MMP2 to track expression and localization.

These models are commercially available from various sources and are sequence-verified, providing reliable tools for drug discovery and functional studies.

Gene-edited cell lines allow researchers to validate the functional impact of MMP2 mutations. For example, comparing MMP2 knockout cells to wild-type controls can identify downstream gene expression changes and pathway alterations. Knock-in lines with specific mutations can be used to assess the effect of different mutations on protein function and cellular phenotype.

Isogenic cell lines provide a controlled system for high-throughput screening of potential therapeutic compounds. For instance, screening libraries of small molecules for their ability to rescue the osteolytic phenotype in MMP2-deficient cells. Additionally, these models can be used to test drug resistance mechanisms, such as the development of resistance to MMP inhibitors.

CRISPR-based synthetic lethality screens can identify genes that are essential for survival of MMP2-deficient cells. This can reveal novel therapeutic targets and biomarkers. For example, knocking out candidate genes in MMP2 knockout cells and assessing viability can identify vulnerabilities that can be exploited therapeutically.