ABSTRACT
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.
Subject(s)
Cardiomyopathy, Hypertrophic/genetics , Mitochondria/genetics , Mutation , Neoplasm Proteins/genetics , RNA Processing, Post-Transcriptional , RNA, Messenger/genetics , Amino Acid Sequence , Cardiomyopathy, Hypertrophic/metabolism , Cardiomyopathy, Hypertrophic/pathology , Cell Nucleus/genetics , Cell Nucleus/metabolism , Electron Transport/genetics , Endoribonucleases/genetics , Endoribonucleases/metabolism , Female , Fibroblasts/metabolism , Fibroblasts/pathology , Genetic Complementation Test , Humans , Infant , Male , Mitochondria/metabolism , Molecular Sequence Data , Muscles/metabolism , Muscles/pathology , Neoplasm Proteins/metabolism , Pedigree , RNA, Messenger/metabolism , RNA, Mitochondrial , RNA, Ribosomal/genetics , RNA, Ribosomal/metabolism , RNA, Transfer/genetics , RNA, Transfer/metabolism , Saccharomyces cerevisiae/genetics , Saccharomyces cerevisiae/metabolismABSTRACT
Defects of mitochondrial oxidative phosphorylation constitute a clinical and genetic heterogeneous group of disorders affecting multiple organ systems at varying age. Biochemical analysis of biopsy material demonstrates isolated or combined deficiency of mitochondrial respiratory chain enzyme complexes. Co-occurrence of impaired activity of the pyruvate dehydrogenase complex has been rarely reported so far and is not yet fully understood. We investigated two siblings presenting with severe neonatal lactic acidosis, hypotonia, and intractable cardiomyopathy; both died within the first months of life. Muscle biopsy revealed a peculiar biochemical defect consisting of a combined deficiency of respiratory chain complexes I, II, and II+III accompanied by a defect of the pyruvate dehydrogenase complex. Joint exome analysis of both affected siblings uncovered a homozygous missense mutation in BOLA3. The causal role of the mutation was validated by lentiviral-mediated expression of the mitochondrial isoform of wildtype BOLA3 in patient fibroblasts, which lead to an increase of both residual enzyme activities and lipoic acid levels. Our results suggest that BOLA3 plays a crucial role in the biogenesis of iron-sulfur clusters necessary for proper function of respiratory chain and 2-oxoacid dehydrogenase complexes. We conclude that broad sequencing approaches combined with appropriate prioritization filters and experimental validation enable efficient molecular diagnosis and have the potential to discover new disease loci.
