Mitochondrial complex III deficiency is one of several conditions caused by dysfunction of mitochondria, which are specialized compartments in cells that generate more than 90% of the energy required by the body. It is a severe, multisystem disorder that includes features such as lactic acidosis, hypotonia, hypoglycemia, failure to thrive, encephalopathy, and delayed psychomotor development. Involvement of internal organs, including liver disease and renal tubulopathy, may also occur. Symptoms typically begin at birth. Many affected individuals die in early childhood, but some have survived longer. It is generally caused by mutations in nuclear DNA in the BCS1L, UQCRB and UQCRQ genes and inherited in an autosomal recessive manner. However, it may also be caused by mutations in mitochondrial DNA in the MTCYB gene, which is passed down maternally or occurs sporadically and may result in a milder form of the condition. Treatment generally focuses on alleviating symptoms and/or slowing down the progression of the disease, and effectiveness can vary among individuals.
The signs and symptoms of mitochondrial complex III deficiency are not the same for each affected individual, because a person with mitochondrial disease can have a unique mixture of healthy and defective mitochondria, with a unique distribution in the body. Several forms of mitochondrial complex III deficiency have been identified, and they are generally grouped based on the age of onset, nature and severity of symptoms.
The most severe form begins in infancy and causes life-threatening muscle and nervous system dysfunction (encephalomyopathy), lactic acidosis at birth, hypotonia (poor muscle tone), dystrophic posturing, seizures, and coma. Ragged-red fibers, a characteristic microscopic abnormality observed in muscle biopsy, are commonly present with this form.
In some individuals, encephalomyopathy does not begin until childhood or adulthood. For these individuals, symptoms may include various combinations of weakness, short stature, ataxia (inability to coordinate muscle movements), dementia, hearing loss, sensory neuropathy, pigmentary retinopathy (a disorder of the retina characterized by deposits of pigment and vision loss), and possible lactic acidosis or other features. Ragged-red fibers are common in these individuals as well.
Individuals with a less severe type may have myopathy with exercise intolerance that progresses to general weakness. Ragged-red fibers and lactic acidosis may be present.
A fourth described form is characterized by infantile histiocytoid cardiomyopathy. This is a condition characterized by cardiomegaly (enlarged heart), severe cardiac arrhythmias or sudden death, and the presence of histiocyte-like cells within the heart muscle.