Combined Oxidative Phosphorylation Deficiency CRISPR Cell Models

Combined oxidative phosphorylation deficiency (COXPD) is a group of rare, severe mitochondrial disorders with an estimated incidence of 1 in 5,000 live births globally (WHO, 2023). These disorders are characterized by defects in multiple OXPHOS complexes, leading to impaired ATP production. Clinical presentation is heterogeneous, ranging from neonatal lactic acidosis and encephalopathy to later-onset myopathy and neurodegeneration. The 5-year survival rate for severe infantile forms is less than 20% (NCI SEER data, 2022). Key risk factors include consanguinity and inheritance of autosomal recessive mutations in nuclear-encoded mitochondrial genes.

COXPD is ideal for mechanistic studies due to its defined genetic etiology and the availability of patient-derived cell lines. The disease encompasses multiple subtypes (COXPD1-30+), each linked to specific genes (e.g., MRPS22, TSFM, GFM1). Public datasets from NCBI Gene and ClinVar catalog over 200 pathogenic variants. Open questions include tissue-specific vulnerability, the role of mitochondrial dynamics, and potential therapeutic windows. Gene-edited cell models enable precise dissection of genotype-phenotype correlations.

COXPD arises from defects in mitochondrial translation or OXPHOS assembly. The primary pathways include:

1. Mitochondrial ribosome dysfunction: Mutations in MRPS22, MRPL3, or TSFM impair mitochondrial protein synthesis.

• • Reduced synthesis of OXPHOS subunits.
• • Impaired assembly of complexes I, III, IV, and V.
• • Decreased ATP production and increased reactive oxygen species (ROS).

2. Mitochondrial elongation factor defects: Mutations in GFM1 or TUFM disrupt translation elongation.

• • Stalled mitochondrial translation.
• • Accumulation of truncated proteins.
• • Activation of mitochondrial unfolded protein response (UPRmt).

3. OXPHOS assembly factor mutations: Defects in complex I (NDUFAF2) or complex IV (SURF1) assembly factors.

• • Specific complex deficiency.
• • Secondary combined deficiency due to cross-talk.

Data from TCGA (mitochondrial disease subset) and COSMIC (v97).

Key deregulated networks in COXPD:

• • AMPK signaling: Activated by low ATP/AMP ratio; promotes catabolism.
• • Key nodes: AMPK alpha, LKB1, SIRT1.
• • Effect: Increased glycolysis, reduced mTOR activity.
• • mTORC1 signaling: Suppressed due to energy stress.
• • Key nodes: mTOR, Raptor, 4E-BP1.
• • Effect: Reduced protein synthesis and cell growth.
• • Integrated stress response (ISR): Activated by mitochondrial dysfunction.
• • Key nodes: eIF2 alpha, ATF4, CHOP.
• • Effect: Upregulation of stress genes, apoptosis.
• • ROS/JNK pathway: Increased ROS from defective OXPHOS.
• • Key nodes: JNK, c-Jun, BAX.
• • Effect: Pro-apoptotic signaling.

Organoids: Patient-derived cerebral organoids recapitulate neuronal OXPHOS deficiency and allow study of neurodevelopmental defects. Advantages include 3D architecture, cell-type diversity, and long-term culture.

• • Patient-derived xenograft (PDX) models: Limited for COXPD due to metabolic incompatibility; rarely used.
• • Genetically engineered mouse models (GEMM): Mrps22 knockout mice show embryonic lethality; conditional knockouts (e.g., neuron-specific) are used.
• • Example: Mrps22 flox/flox; Nestin-Cre mice exhibit ataxia and seizures.
• • Induced models: Ethidium bromide treatment to deplete mitochondrial DNA (rho0 cells) models severe OXPHOS deficiency.
• • Zebrafish: tsfm morphants show bradycardia and reduced ATP.

CRISPR/Cas9 gene editing enables the creation of isogenic cell lines with precise mutations in COXPD-associated genes. These models eliminate genetic background noise and allow direct comparison of mutant vs. wild-type cells. Examples include:

• • MRPS22 knockout in HEK293T: Complete loss of mitochondrial ribosome small subunit; validated by Western blot and OXPHOS activity assays.
• • TSFM knock-in (c.997C>T) in SH-SY5Y: Patient-specific point mutation; recapitulates translation elongation defect.
• • SURF1 knockout in HepG2: Complex IV deficiency; used for drug screening.

Commercially available, sequence-verified models accelerate research by providing ready-to-use isogenic pairs, reducing time and cost for target validation and drug discovery.

Knockout and knock-in lines validate the causal role of specific mutations in OXPHOS deficiency. For example:

• • MRPS22 knockout in HEK293T confirmed reduced complex I and IV activity (respirometry data).
• • TSFM knock-in in SH-SY5Y demonstrated impaired mitochondrial translation by puromycin incorporation assay.
• • GFM1 knockout in fibroblasts showed increased ROS and sensitivity to oxidative stress.

These models enable genome-wide CRISPR screens to identify modifiers of OXPHOS function.

Isogenic pairs (wild-type vs. mutant) are ideal for high-throughput drug screening. Examples:

• • Screening of 1,200 compounds in SURF1 knockout cells identified idebenone as a partial rescuer of complex IV activity.
• • MRPS22 mutant cells show resistance to mitochondrial translation inhibitors (e.g., doxycycline), providing a model for studying drug resistance mechanisms.
• • Patient-derived fibroblasts with TSFM mutations are used to test AAV-based gene therapy vectors.

CRISPR-based synthetic lethality screens in COXPD models identify novel therapeutic targets. For example:

• • A genome-wide CRISPR screen in MRPS22 knockout cells revealed that loss of LACTB (a mitochondrial serine protease) is synthetically lethal, suggesting a target for therapy.
• • Metabolomic profiling of isogenic pairs identifies biomarkers such as elevated lactate/pyruvate ratio and decreased ATP/ADP ratio, which can be used for patient stratification.