Isolated complex III deficiency

Isolated complex III deficiency, also known as mitochondrial complex III deficiency or ubiquinol-cytochrome c reductase deficiency, is a rare mitochondrial disorder affecting the third complex (complex III, or cytochrome bc1 complex) of the mitochondrial respiratory chain. Complex III plays a critical role in cellular energy production by transferring electrons from ubiquinol to cytochrome c. When this complex is deficient, cells cannot produce adequate energy (ATP), particularly affecting tissues with high energy demands such as the brain, skeletal muscles, heart, liver, and kidneys. Clinical presentation is highly variable, ranging from severe neonatal multisystem disease to milder forms presenting later in life. Key features may include lactic acidosis, exercise intolerance, muscle weakness (myopathy), encephalopathy, cardiomyopathy, hepatic dysfunction, renal tubulopathy, and failure to thrive. Some patients present with Leigh syndrome or GRACILE syndrome (growth restriction, aminoaciduria, cholestasis, iron overload, lactic acidosis, and early death). The severity and combination of symptoms depend on the specific genetic cause and the degree of residual complex III activity. Isolated complex III deficiency can be caused by mutations in several nuclear-encoded genes (including BCS1L, UQCRB, UQCRQ, UQCRC2, CYC1, TTC19, LYRM7, and UQCC2) as well as in the mitochondrial gene MT-CYB. Treatment is primarily supportive and symptomatic, as no curative therapy currently exists. Management may include supplementation with coenzyme Q10, riboflavin, and other cofactors, along with avoidance of metabolic stressors. Multidisciplinary care involving neurology, cardiology, hepatology, and metabolic specialists is typically required.

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