Diagnoses and molecular bases of mitochondrial respiratory chain disorders

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BACKGROUND : Congenital and primary lactic acidosis is one of the most frequent inborn errors of metabolism, of whom only 30% have had its precise cause identified. Our aim is to make a prompt and correct diagnosis of mitochondrial respiratory chain disorders (MRCD), using Blue Native Polyacrylamide Gel Electrophoresis (BN-PAGE) combined with conventional enzyme assay. METHODS : Activities of the individual respiratory chain complexes and the mitochondrial matrix marker enzyme citrate synthase were measured in liver, heart and muscle homogenates and mitochondrial fractions isolated from cultured fibroblasts. Tissue homogenates or mitochondri isolated from skin fibroblasts were solubilised in n-dodecyl-maltoside and subjected to 4-13% BN-PAGE and western blotting using monoclonal antibodies specific for Complex I to IV subunits. Mitochondrial DNA and nuclear DNA copy numbers within tissues were determined by quantitative polymerase chain reaction. RESULTS : One hundred and ten patients were diagnosed to have MRCD out of 267 candidate patients. Most frequent was complex I deficiency, of whom many patients had tissue-specific type deficiency. Twenty patients out of 110 had mitochondrial DNA pathogenic mutations, which meant the majority of childhood-onset MRCD was nuclear origin. Patients with mtDNA mutations had milder symptoms than those suspected to have nuclear mutation. MtDNA depletion syndrome (MDS) was a prevalent cause of multiple MRCD. Twelve patients in 10 families were diagnosed to have hepatic MDS, and 5 patients were diagnosed to have myopathic MDS. Out of 12 hepatic MDS, we discovered nuclear genes mutations of DGUOK, POLG and MPV17 in 6 atients from 4 families. CONCLUSION : We must have a suspicion that almost every disease may be a MRCD. BN-PAGE is a useful guide to prompt and correct diagnosis, and future molecular analysis for categorizing respiratory chain disorders.