The mitochondrial translation system is responsible for the synthesis of 13 proteins required for oxidative phosphorylation (OXPHOS), the major energy-generating process of our cells. Mitochondrial translation is controlled by various nuclear encoded proteins. In 27 patients with combined OXPHOS deficiencies, in whom complex II (the only complex that is entirely encoded by the nuclear DNA) showed normal activities, and mutations in the mitochondrial genome as well as polymerase gamma were excluded, we screened all mitochondrial translation factors for mutations. Here, we report a mutation in mitochondrial elongation factor G1 (GFM1) in a patient affected by severe, rapidly progressive mitochondrial encephalopathy. This mutation is predicted to result in an Arg250Trp substitution in subdomain G' of the elongation factor G1 protein and is presumed to hamper ribosome-dependent GTP hydrolysis. Strikingly, the decrease in enzyme activities of complex I, III and IV detected in patient fibroblasts was not found in muscle tissue. The OXPHOS system defects and the impairment in mitochondrial translation in fibroblasts were rescued by overexpressing wild-type GFM1, establishing the GFM1 defect as the cause of the fatal mitochondrial disease. Furthermore, this study evinces the importance of a thorough diagnostic biochemical analysis of both muscle tissue and fibroblasts in patients suspected to suffer from a mitochondrial disorder, as enzyme deficiencies can be selectively expressed.
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