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, 34 (2), 283-92

Biochemical Diagnosis of Mitochondrial Disorders


Biochemical Diagnosis of Mitochondrial Disorders

Richard J T Rodenburg. J Inherit Metab Dis.


Establishing a diagnosis in patients with a suspected mitochondrial disorder is often a challenge. Both knowledge of the clinical spectrum of mitochondrial disorders and the number of identified disease-causing molecular genetic defects are continuously expanding. The diagnostic examination of patients requires a multi-disciplinary clinical and laboratory evaluation in which the biochemical examination of the mitochondrial functional state often plays a central role. In most cases, a muscle biopsy provides the best opportunity to examine mitochondrial function. In addition to activity measurements of individual oxidative phosphorylation enzymes, analysis of mitochondrial respiration, substrate oxidation, and ATP production rates is performed to obtain a detailed picture of the mitochondrial energy-generating system. On the basis of the compilation of clinical, biochemical, and other laboratory test results, candidate genes are selected for molecular genetic testing. In patients in whom an unknown genetic variant is identified, a compatible biochemical phenotype is often required to firmly establish the diagnosis. In addition to the current role of the biochemical analysis in the diagnostic examination of patients with a suspected mitochondria disorder, this report gives a future perspective on the biochemical diagnosis in view of both the expanding genotypes of mitochondrial disorders and the possibilities for high throughput molecular genetic diagnosis.


Fig. 1
Fig. 1
Schematic representation of the mitochondrial energy generating system. Substrates are shuttled into mitochondria and are metabolized in the matrix in the TCA cycle, during which reduction equivalents NADH and FADH2 are formed. These are oxidized by complex I (CI) and II (CII) of the respiratory chain, and electrons coming from NADH and FADH2 are shuttled through complexes I or II, coenzyme Q, complex III (CIII), cytochrome c, and complex IV (CIV) to molecular oxygen. During the electron transport process, protons are pumped out of the mitochondrial matrix to the intermembrane space by complexes I, III, and IV. The electrochemical gradient thus formed is used by complex V (CV; F1/F0 ATPase) to convert ADP into ATP. The intramitochondrial ATP/ADP ratio is balanced by shuttling ADP and ATP in and out of the mitochondrial matrix by the adenosine nucleotide transporter ANT (not shown here). For diagnostic purposes, the mitochondrial energy generating system can be analyzed in several ways. By using 14C labeled pyruvate, malate, or succinate, the conversion rates of these substrates can be determined by measuring the amounts of released 14CO2 as parameters for the overall capacity of the mitochondrial energy-generating system (Janssen et al 2006). A similar parameter is the oxygen consumption rate in the presence of different substrates, e.g., pyruvate + malate, and which can be measured by respirometry or by fluorescent probes (Jonckheere et al ; Rustin et al 1994). The rate of synthesis of the end product ATP in the presence of different mitochondrial substrates is also representative for the capacity of the mitochondrial energy-generating system (Janssen et al 2006). In addition to these flux-parameters of the mitochondrial energy-generating system, individual enzymes can be determined by spectrophotometric and radiochemical assays

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