Combined Oxidative Phosphorylation Deficiency 50 (COXPD50) Cell Models for Research

Combined Oxidative Phosphorylation Deficiency 50 (COXPD50) is an ultra-rare autosomal recessive mitochondrial disorder caused by mutations in the MRPS23 gene. Due to its rarity, precise epidemiological data are limited; however, mitochondrial diseases collectively affect approximately 1 in 5,000 live births (WHO). COXPD50 presents in infancy with severe lactic acidosis, hypotonia, and failure to thrive, often leading to early mortality. The condition is characterized by combined deficiencies of mitochondrial respiratory chain complexes I, III, and IV. There is no cure, and management is supportive. The NCI does not track this disease as it is not a cancer; however, the underlying mitochondrial dysfunction has implications for cancer research, as mitochondrial metabolism is a key area of investigation.

COXPD50 serves as an excellent model for studying mitochondrial biology, OXPHOS function, and the role of mitochondrial ribosomal proteins. The disease is caused by mutations in MRPS23, which encodes a component of the mitochondrial small ribosomal subunit. Research using patient-derived fibroblasts or gene-edited cell models can elucidate the molecular mechanisms of mitochondrial translation defects and their downstream effects on cellular metabolism. Open questions include the tissue-specific vulnerability to OXPHOS defects and potential therapeutic targets. Public datasets, such as those in GEO, provide transcriptomic and proteomic data from patient samples, enabling further analysis.

Although COXPD50 is not a cancer, the underlying mitochondrial dysfunction can influence cancer-related pathways. The following pathways are relevant:

• • OXPHOS pathway: Defective mitochondrial translation leads to impaired assembly of respiratory chain complexes, reducing ATP production and increasing reactive oxygen species (ROS).
• • Apoptosis pathway: Mitochondrial dysfunction can trigger intrinsic apoptosis via cytochrome c release.
• • Metabolic reprogramming: Cells may shift to glycolysis (Warburg effect) to compensate for ATP deficit, a hallmark of cancer metabolism.
• • mTOR signaling: Mitochondrial stress can modulate mTOR activity, affecting cell growth and proliferation.

Data from ClinVar and literature reports (e.g., Kohda et al., 2016).

Key deregulated networks in COXPD50 include:

• • AMPK signaling: Activated in response to low ATP/AMP ratio, promoting catabolic pathways.
• • PI3K/AKT/mTOR pathway: May be downregulated due to energy stress, affecting cell growth.
• • HIF1α signaling: Hypoxia-inducible factor may be stabilized under mitochondrial dysfunction, promoting glycolysis.
• • NF-κB pathway: ROS can activate NF-κB, leading to inflammatory responses.

Organoids derived from patient cells can recapitulate tissue-specific mitochondrial defects and are useful for drug testing.

• • Knockout mouse models: Mrps23 knockout mice are embryonic lethal, highlighting the essential role of MRPS23.
• • Zebrafish models: Morpholino-based knockdown of mrps23 can recapitulate OXPHOS deficiency.
• • Induced pluripotent stem cells (iPSCs): Patient-derived iPSCs can be differentiated into various cell types to study tissue-specific effects.

CRISPR-Cas9 technology enables the creation of isogenic cell lines with specific MRPS23 mutations. For example, a knockout cell line lacking MRPS23 can be generated in HEK293T or HepG2 cells. Alternatively, knock-in lines can introduce patient-specific point mutations. These isogenic pairs (wild-type vs. mutant) are invaluable for studying the molecular consequences of MRPS23 mutations in a controlled genetic background. Commercially available, sequence-verified models accelerate research by providing ready-to-use tools for drug screening and functional studies.

Knockout and knock-in lines of MRPS23 allow researchers to validate the gene's role in mitochondrial translation and OXPHOS. For example, MRPS23 knockout cells exhibit reduced oxygen consumption rate (OCR) and decreased activity of complex I and IV. These models help identify downstream effectors and potential compensatory mechanisms.

Isogenic pairs can be used to screen for compounds that rescue mitochondrial function. For instance, screening libraries for molecules that increase OCR in mutant cells may identify therapeutic candidates. Additionally, these models can be used to study resistance to drugs that target mitochondrial metabolism, such as metformin or rotenone.

CRISPR-based synthetic lethality screens can identify genes that, when silenced, selectively kill MRPS23-mutant cells. This approach can reveal novel therapeutic targets and biomarkers for patient stratification.