Mitochondrial oxidative phosphorylation disorder due to a large-scale single deletion of mitochondrial DNA

Mitochondrial oxidative phosphorylation disorder due to a large-scale single deletion of mitochondrial DNA (mtDNA) encompasses a group of clinical syndromes caused by the loss of a segment of the mitochondrial genome, typically ranging from 1.3 to 10 kilobases. The most common deletion is approximately 4,977 base pairs, often referred to as the 'common deletion.' These deletions remove genes encoding essential subunits of the mitochondrial respiratory chain complexes and transfer RNAs, leading to impaired oxidative phosphorylation — the primary energy-generating pathway in cells. Because mitochondria are critical for energy production, tissues with high metabolic demands such as the brain, skeletal muscle, heart, eyes, and endocrine organs are preferentially affected. This category of mitochondrial disease classically presents as one of three major clinical phenotypes: Kearns-Sayre syndrome (KSS), characterized by progressive external ophthalmoplegia, pigmentary retinopathy, and cardiac conduction defects with onset before age 20; Pearson syndrome, a severe infantile disorder featuring sideroblastic anemia and exocrine pancreatic dysfunction; and chronic progressive external ophthalmoplegia (CPEO), which primarily involves ptosis and limitation of eye movements. Patients may also present with overlapping features including myopathy, exercise intolerance, lactic acidosis, sensorineural hearing loss, short stature, diabetes mellitus, cerebellar ataxia, and renal tubular dysfunction. The clinical severity and age of onset depend on the size and location of the deletion, the proportion of deleted versus normal mtDNA molecules (heteroplasmy), and the tissue distribution of the abnormal mitochondria. There is currently no curative treatment for these disorders. Management is supportive and multidisciplinary, focusing on surveillance and treatment of cardiac conduction abnormalities (which may require pacemaker implantation), endocrine monitoring, nutritional support, physical therapy, and supplementation with cofactors such as coenzyme Q10 and L-carnitine, although evidence for their efficacy remains limited. Patients with Pearson syndrome who survive infancy may evolve into a Kearns-Sayre phenotype. Regular monitoring by a team including neurologists, cardiologists, ophthalmologists, and endocrinologists is essential for optimal care.

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