Neuromuscular Disorder Associated With a Defect in Mitochondrial Energy Supply

• A limb muscle biopsy specimen from a patient with a slowly progressive congenital neuromuscular disorder disclosed, by electron microscopy, widespread mitochondrial crystalline inclusions. Biochemical studies of isolated mitochondria showed decreased respiratory rate and respiratory control with both nicotine adenine dinucleotide and flavoprotein-linked substrates. Mitochondrial adenosine triphosphatase (ATPase) activity, both basal and magnesium (Mg++) or 2,4-dinitrophenol- (DNP) stimulated, was greatly reduced in contrast to normal. The rate and extent of mitochondrial calcium accumulation was normal. These findings are consistent with a defect of the respiratory chain-linked energy transfer at a level common to all three energy coupling sites of the respiratory chain. The defect in ATPase activity may be secondary to replacement of functional mitochondrial inner membrane by crystalline inclusions.