Importance of muscle light microscopic mitochondrial subsarcolemmal aggregates in the diagnosis of respiratory chain deficiency.

The purpose of this study was to evaluate relationships between subsarcolemmal mitochondrial aggregates and electron transport chain deficiencies in skeletal muscle with the objective of establishing an association between mitochondrial accumulation and electron transport chain complex deficiency. We conducted a large-scale, retrospective study to evaluate factors associated with subsarcolemmal mitochondrial aggregates (percent) in pediatric patients who received muscle biopsies for suspected respiratory chain disorders. Patients were included if they had histochemical stains for assessment of mitochondrial pathology and had biochemical testing for muscle electron transport chain complex activities. Significant positive bivariate correlations (n = 337) were found between subsarcolemmal mitochondrial aggregate percentage and electron transport chain complexes II, IV, I + III, and II + III activities. Evaluation showed that a cutoff value of > 2% subsarcolemmal mitochondrial aggregates had poor overall diagnostic accuracy (mean, 32%), compared with a < 5% cutoff (mean, 60%). To better evaluate the effects of subsarcolemmal mitochondrial aggregates percentages, patients were stratified according to lower one-third (group 1, n = 120 plus ties) and upper one-third (group 2, n = 115 plus ties) of subsarcolemmal mitochondrial aggregates values. Although only minor clinical and pathologic differences were observed, group 1 participants had significantly lower electron transport chain complex activities than group 2 for all enzymes except complex III. Logistic regression showed over 2-fold greater odds of deficiency for electron transport chain complexes I + III (P = .01) and II + III (P = .03) for group 1 participants compared with group 2. We conclude that, contrary to the previous > 2.0% subsarcolemmal mitochondrial aggregates cutoff for respiratory chain disorder, patients with a low subsarcolemmal mitochondrial aggregates percentage (≤4%) are significantly more likely to have electron transport chain complex deficiency than patients with increased subsarcolemmal mitochondrial aggregates percentage (≥10%). This morphological approach for assessment of mitochondrial proliferation may assist clinicians to select further testing to rule out an electron transport chain complex deficiency in children by other methods, including direct biochemical testing of electron transport chain complex activities, measurement of muscle coenzyme Q10 content, or evaluation for a mitochondrial DNA depletion syndrome.