Respiratory chain deficiencies exhibit a wide variety of clinical phenotypes resulting from defective mitochondrial energy production through oxidative phosphorylation. These defects can be caused by either mutations in the mtDNA or mutations in nuclear genes coding for mitochondrial proteins. The underlying pathomechanisms can affect numerous pathways involved in mitochondrial physiology. By whole-exome and candidate gene sequencing, we identified 11 individuals from 9 families carrying compound heterozygous or homozygous mutations in GTPBP3, encoding the mitochondrial GTP-binding protein 3. Affected individuals from eight out of nine families presented with combined respiratory chain complex deficiencies in skeletal muscle. Mutations in GTPBP3 are associated with a severe mitochondrial translation defect, consistent with the predicted function of the protein in catalyzing the formation of 5-taurinomethyluridine (τm(5)U) in the anticodon wobble position of five mitochondrial tRNAs. All case subjects presented with lactic acidosis and nine developed hypertrophic cardiomyopathy. In contrast to individuals with mutations in MTO1, the protein product of which is predicted to participate in the generation of the same modification, most individuals with GTPBP3 mutations developed neurological symptoms and MRI involvement of thalamus, putamen, and brainstem resembling Leigh syndrome. Our study of a mitochondrial translation disorder points toward the importance of posttranscriptional modification of mitochondrial tRNAs for proper mitochondrial function.

The human mitochondrial genome encodes RNA components of its own translational machinery to produce the 13 mitochondrial-encoded subunits of the respiratory chain. Nuclear-encoded gene products are essential for all processes within the organelle, including RNA processing. Transcription of the mitochondrial genome generates large polycistronic transcripts punctuated by the 22 mitochondrial (mt) tRNAs that are conventionally cleaved by the RNase P-complex and the RNase Z activity of ELAC2 at 5' and 3' ends, respectively. We report the identification of mutations in ELAC2 in five individuals with infantile hypertrophic cardiomyopathy and complex I deficiency. We observed accumulated mtRNA precursors in affected individuals muscle and fibroblasts. Although mature mt-tRNA, mt-mRNA, and mt-rRNA levels were not decreased in fibroblasts, the processing defect was associated with impaired mitochondrial translation. Complementation experiments in mutant cell lines restored RNA processing and a yeast model provided additional evidence for the disease-causal role of defective ELAC2, thereby linking mtRNA processing to human disease.

Polyadenylation of mRNA in human mitochondria is crucial for gene expression and perturbation of poly(A) tail length has been linked to a human neurodegenerative disease. Here we show that 2′-phosphodiesterase (2′-PDE), (hereafter PDE12), is a mitochondrial protein that specifically removes poly(A) extensions from mitochondrial mRNAs both in vitro and in mitochondria of cultured cells. In eukaryotes, poly(A) tails generally stabilize mature mRNAs, whereas in bacteria they increase mRNA turnover. In human mitochondria, the effects of increased PDE12 expression were transcript dependent. An excess of PDE12 led to an increase in the level of three mt-mRNAs (ND1, ND2 and CytB) and two (CO1 and CO2) were less abundant than in mitochondria of control cells and there was no appreciable effect on the steady-state level of the remainder of the mitochondrial transcripts. The alterations in poly(A) tail length accompanying elevated PDE12 expression were associated with severe inhibition of mitochondrial protein synthesis, and consequently respiratory incompetence. Therefore, we propose that mRNA poly(A) tails are important in regulating protein synthesis in human mitochondria, as it is the case for nuclear-encoded eukaryotic mRNA.