Mitochondrial oxidative phosphorylation disorder due to nuclear DNA anomalies

Mitochondrial oxidative phosphorylation (OXPHOS) disorder due to nuclear DNA anomalies encompasses a large and heterogeneous group of genetic conditions in which defects in nuclear-encoded genes impair the mitochondrial respiratory chain (complexes I–V) and energy production. Although the mitochondrial respiratory chain resides within mitochondria, the vast majority of its protein subunits, assembly factors, and maintenance proteins are encoded by nuclear DNA. Mutations in these nuclear genes lead to deficient cellular energy production, predominantly affecting tissues with high energy demands such as the brain, skeletal muscle, heart, liver, and kidneys. Common synonyms include nuclear-encoded mitochondrial respiratory chain disorders and nuclear DNA-related OXPHOS deficiency. Clinical presentations are extremely variable and can include progressive encephalopathy, Leigh syndrome, lactic acidosis, hypotonia, seizures, developmental delay or regression, cardiomyopathy, hepatic failure, renal tubulopathy, sensorineural hearing loss, optic atrophy, and exercise intolerance with myopathy. Onset ranges from the neonatal period to adulthood, though many cases present in infancy or early childhood. Specific phenotypes depend on which complex or mitochondrial function is disrupted; for example, isolated complex I deficiency is the most common single-enzyme defect and frequently presents as Leigh syndrome or leukoencephalopathy. There is currently no curative treatment for these disorders. Management is largely supportive and symptomatic, including nutritional optimization, cofactor supplementation (such as coenzyme Q10, riboflavin, thiamine, and carnitine), management of seizures, cardiac surveillance, and physical therapy. Avoidance of metabolic stressors such as fasting, illness, and certain medications (e.g., valproic acid) is important. Prognosis varies widely depending on the specific genetic defect, the severity of respiratory chain dysfunction, and the organs involved, with severe neonatal-onset forms often carrying a poor prognosis.

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