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In this chapter we describe about 18 years of experience with prenatal diagnosis in oxidative phosphorylation (OXPHOS) diseases in our centre. We start diagnostics of OXPHOS disorders in patients with a mitochondrial (encephalo)myopathy by preference by measuring oxidation rates of pyruvate, malate and succinate and ATP production rates from oxidation of pyruvate in a “fresh” muscle biopsy. In the same biopsy activities of the mitochondrial respiratory chain enzymes complex-I, complex-II, complex-III and complex-IV are also measured. When decreased substrate oxidation rates and ATP production rates give indication for suspicion on a complex-V or a pyruvate dehydrogenase complex (PDHC) deficiency, activities of these enzymes are also measured. In frozen muscle biopsies we only can measure the respiratory chain enzymes. In which cases now can we offer prenatal diagnosis? In about 30% of the muscle biopsies with clearly decreased substrate oxidation rates and ATP production rates, all respiratory chain enzymes, complex-V and PDHC show normal activities. In these cases it is impossible at the moment to offer prenatal diagnosis. In the remainder of the biopsies with clearly reduced substrate oxidation- and ATP production rates, decreased activities are measured of one or more of the afore mentioned enzymes. The most frequendy occurring deficiencies in fresh as well as in frozen muscle biopsies are complex-I, complex-IV or combined deficiencies of these enzymes. The next step is to search if the deficiency is also expressed in cultured fibroblasts and to exclude a mtDNA mutation as a cause of the deficiency. If the deficiency is also expressed in cultured fibroblasts and mtDNA mutations have been excluded we are willing to offer prenatal diagnosis. This chapter is aggravated on prenatal diagnosis for complex-I, complex-IV or a combined deficiency of these enzymes because the majority of the total number of requests for prenatal diagnosis that reach us concerns pregnancies in families in which the index patient was suffering from a deficiency of one of these (or both) enzymes.

Laboratory diagnosis of OXPHOS disorders has evolved substantially since mitochondrial DNA mutations were first shown to cause human disease in 1988. Traditional approaches such as skeletal muscle OXPHOS enzyme analysis and histochemistry remain among the most important diagnostic tools. However, molecular diagnosis of mitochondrial DNA mutations and, more recently, nuclear gene mutations are responsible for an increasing number of diagnoses. This trend will continue over the next two decades with new genomic approaches such as mutation chips eventually likely to become front-line diagnostic tools. In order to completely replace traditional approaches though, much work needs to be done identifying novel OXPHOS disease-causing genes and distinguishing pathogenic mutations in such genes from polymorphisms. In the interim, a number of other approaches will improve the ease and certainty of OXPHOS diagnosis. These are likely to include increased use of spectroscopic and other methods for assessing in vivo OXPHOS function, use of minimally invasive tissue biopsies, improved assays of OXPHOS function using immunocapture antibodies or fluorescent probes, and methods for assessing expression of OXPHOS genes and proteins using antibody chips, proteomics and cDNA microarrays. All these methods will require extensive validation studies to distinguish primary OXPHOS defects from other disease states and development of improved algorithms for defining how much evidence is needed for a definite diagnosis of OXPHOS disease.