Combined Oxidative Phosphorylation Deficiency 45 (COXPD45) Cell Models for Research

Combined Oxidative Phosphorylation Deficiency 45 (COXPD45) is an ultra-rare autosomal recessive mitochondrial disorder caused by mutations in the MRPS23 gene. Due to its extreme rarity, precise incidence and prevalence are not well-established; fewer than 20 cases have been reported worldwide (as of 2025). The disease manifests in infancy or early childhood with severe neurological and muscular symptoms, including hypotonia, developmental delay, seizures, and lactic acidosis. Prognosis is poor, with many patients succumbing in early childhood. There is no cure, and management is supportive. The disease is a significant model for studying mitochondrial dysfunction and oxidative phosphorylation (OXPHOS) defects.

COXPD45 serves as a valuable model for investigating mitochondrial biology, OXPHOS complex assembly, and the pathophysiology of mitochondrial diseases. The MRPS23 gene encodes a component of the mitochondrial small ribosomal subunit, essential for mitochondrial protein synthesis. Mutations lead to impaired translation of OXPHOS subunits, causing combined deficiency of complexes I, III, IV, and V. Research on COXPD45 can provide insights into mitochondrial translation, ribosome assembly, and potential therapeutic targets. Public datasets, such as those in ClinVar and OMIM, list pathogenic variants, but functional studies are limited, making gene-edited cell models crucial for mechanistic research.

• • The primary mechanism is defective mitochondrial translation due to MRPS23 mutations. This leads to:
• • Impaired assembly of the mitochondrial small ribosomal subunit.
• • Reduced synthesis of mitochondrial DNA-encoded OXPHOS subunits.
• • Combined deficiency of OXPHOS complexes I, III, IV, and V.
• • Decreased ATP production and increased reactive oxygen species (ROS).
• • Cellular energy crisis, particularly in high-energy-demand tissues like brain and muscle.

Data from ClinVar and literature reports.

• • The energy deficit activates stress-responsive signaling pathways:
• • AMPK pathway: Activated due to increased AMP/ATP ratio, promoting catabolism.
• • mTORC1 signaling: Inhibited, reducing protein synthesis and cell growth.
• • Integrated stress response (ISR): Activation of eIF2α phosphorylation, leading to ATF4 expression.
• • Mitochondrial unfolded protein response (UPRmt): Upregulation of chaperones and proteases.
• • Apoptosis: Increased ROS and mitochondrial permeability transition can trigger cell death.

Organoids, such as cerebral organoids, can be generated from patient iPSCs to model neuronal defects. These 3D models recapitulate tissue architecture and are valuable for studying disease mechanisms and drug testing.

• • Animal models for COXPD45 are limited, but potential models include:
• • Mrps23 knockout mice: Not yet reported, but would be valuable.
• • Zebrafish with mrps23 knockdown: Used to study mitochondrial dysfunction.
• • Drosophila models: Can be used for genetic screens.
• • Patient-derived xenografts (PDX) are not applicable for this non-cancer disease.

• • CRISPR-Cas9 gene editing enables the creation of isogenic cell lines with precise MRPS23 mutations. For example:
• • MRPS23 knockout cell lines: Complete loss of function, mimicking severe null mutations.
• • MRPS23 point-mutation knock-in lines: Introduction of specific pathogenic variants (e.g., c.382G>A) to study genotype-phenotype correlations.
• • Reporter lines: Tagging MRPS23 with fluorescent proteins to study localization and dynamics.

These sequence-verified, commercially available models accelerate research by providing consistent, reproducible systems. They are essential for functional studies, drug screening, and mechanistic investigations.

• • Gene-edited cell lines are used to validate the function of MRPS23 and its variants. For example:
• • Knockout lines can be used to assess the impact on mitochondrial translation and OXPHOS activity.
• • Knock-in lines with specific mutations can determine the pathogenicity of variants of uncertain significance.
• • CRISPR screens can identify genetic modifiers that rescue the phenotype, revealing potential therapeutic targets.

Isogenic pairs (wild-type vs. mutant) are ideal for high-throughput drug screening. They allow the identification of compounds that selectively affect mutant cells, providing leads for therapy. Additionally, they can be used to study resistance mechanisms to mitochondrial-targeted drugs.

Gene-edited models can be used to discover biomarkers of mitochondrial dysfunction. For example, secretome analysis can identify proteins released by mutant cells that could serve as diagnostic markers. CRISPR synthetic lethality screens can identify genes that are essential only in the context of MRPS23 deficiency, which could be targeted therapeutically.