Myopathy Cell Models for Research

Myopathies are a heterogeneous group of disorders characterized by muscle weakness and wasting. The global prevalence of inherited myopathies is estimated at 1 in 3,000, with muscular dystrophies being the most common. Duchenne muscular dystrophy (DMD) affects approximately 1 in 3,500 male births worldwide (WHO). The clinical impact is profound, with many patients losing ambulation by adolescence and requiring respiratory support. Acquired myopathies, such as inflammatory myopathies, have an incidence of 1-2 per 100,000. The 5-year survival for DMD has improved to around 30% with multidisciplinary care, but remains poor (NCI).

Myopathies are ideal for mechanistic studies due to their well-defined genetic causes and the availability of patient-derived cell lines. Subtypes include DMD, Becker muscular dystrophy, limb-girdle muscular dystrophies, and congenital myopathies. Public datasets such as the NCBI GEO and the Muscular Dystrophy Association's patient registry provide extensive transcriptomic and clinical data. Open questions include the role of muscle stem cells in regeneration and the molecular pathways leading to muscle degeneration.

• • While myopathies are not cancers, they share pathways with cancer biology, such as dysregulation of cell survival and apoptosis. Key pathways include:
• • Dystrophin-glycoprotein complex (DGC) disruption
• • Calcium homeostasis dysregulation
• • Mitochondrial dysfunction
• • Inflammatory signaling (NF-κB)
• • Autophagy impairment

Data from TCGA and COSMIC (for cancer-related myopathies) and ClinVar.

• • Key signaling networks in myopathy include:
• • NF-κB pathway: activated in inflammatory myopathies, leading to muscle wasting.
• • PI3K/AKT pathway: dysregulated in insulin-resistant myopathies.
• • MAPK/ERK pathway: involved in muscle regeneration and hypertrophy.
• • TGF-β pathway: implicated in fibrosis in DMD.
• • Calcium signaling: disrupted in RYR1-related myopathies.

Organoids derived from patient iPSCs can recapitulate muscle architecture and are useful for drug testing.

• • mdx mouse: natural DMD model with a point mutation in the DMD gene.
• • GRMD dog: golden retriever muscular dystrophy model.
• • LMNA knockout mouse: models Emery-Dreifuss muscular dystrophy.
• • PDX models: patient-derived xenografts for cancer-related myopathies.

CRISPR-based gene editing enables the creation of isogenic cell lines with precise mutations. For example, a DMD knockout line can be generated by introducing a frameshift in exon 50, mimicking common patient mutations. Similarly, a LMNA knock-in line with a specific missense mutation (e.g., R453W) can be created. These models are sequence-verified and commercially available, providing reproducible tools for drug discovery and functional genomics.

Knockout and knock-in lines are used to validate the function of genes implicated in myopathy. For example, knocking out DMD in C2C12 cells leads to increased membrane fragility, confirming its role. Similarly, introducing a RYR1 mutation into a wild-type line can recapitulate calcium handling defects.

Isogenic pairs (wild-type vs. mutant) are used in high-throughput screens to identify compounds that rescue the mutant phenotype. For instance, screening for drugs that restore dystrophin expression in DMD knockout lines. Resistance models can be generated by chronic exposure to drugs, revealing mechanisms of drug resistance.

CRISPR synthetic lethality screens can identify genes that are essential only in the context of a specific mutation, providing potential therapeutic targets. For example, in DMD knockout cells, genes involved in membrane repair may be synthetic lethal, offering new drug targets.