Final Report Summary - REVERSIBLECOX (Reversibility and tissue specificity of mitochondrial translation defects in early childhood)
Most mitochondrial diseases are disabling, progressive or fatal, affecting the brain, liver, skeletal muscle, heart and other organs. Currently there are no effective cures and treatment is at best symptomatic. Although defective oxidative phosphorylation is the common final pathway, it is unknown why different mtDNA or nuclear mutations result in largely heterogeneous clinical presentations. The diagnosis in patients with multiple respiratory chain complex defects (~30% of all mitochondrial disease) due to abnormal mitochondrial translation is particularly difficult because of the massive number of nuclear genes involved in intra-mitochondrial protein synthesis. Many of these genes are not yet linked to human disease. Whole exome sequencing (WES) rapidly changed the diagnostic pathway by identifying the primary genetic defect, thus making invasive and complex biochemical testing unnecessary. However, our understanding of the mitochondrial protein synthesis apparatus and expression of mitochondrial proteins in health and disease remains limited, slowing down the development of personalized therapies.
While most mitochondrial diseases are progressive conditions, two unique mitochondrial conditions termed reversible infantile respiratory chain deficiency (RIRCD, homoplasmic mt-tRNAGlu mutation) and reversible infantile hepatopathy (TRMU mutations) stand out by showing complete spontaneous recovery. This research project was looking for genetic modifiers in several patients and families with RIRCD collected from international centres. We combined whole exome sequencing (35 patients) and RNA sequencing (5 fibroblast cell lines, 6 muscle samples) with proteomics. The analysis of the data is currently finalized and the results are being written up for a nigh impact publication. We have identified a novel approach explaining disease manifestation by our combined omics technology, which has a broad impact on the understanding of mitochondrial diseases and tissue specific manifestations.
Previously, our group reported that low cysteine concentrations may play a role in triggering a reversible mitochondrial translation defect in patients with reversible infantile respiratory chain deficiency (RIRCD, Boczonadi et al., 2013). RIRCD is caused by a homoplasmic mutation in the mitochondrial tRNAGlu. We hypothesised that the mutation changes the structure of the tRNA and thereby impairs its subsequent posttranscriptional modification by thiouridylation. Increase of substrate availability for thiouridylation by addition of L-cysteine to the culture medium rescued respiratory chain enzyme activities in human cell lines of patients with RIRCD (Boczonadi et al., 2013). This observation led to the hypothesis that increasing cysteine availability might reverse a mitochondrial translation defect also in other mitochondrial conditions affecting posttranslational mt-tRNA modifications. We performed the supplementation with L-cysteine in patient cell lines with Mitochondrial Encephalomyopathy and Lactic Acidosis with Stroke-like episodes (MELAS), Myoclonic Epilepsy with Red Ragged Fibres (MERRF), nuclear defects of mitochondrial translation and controls. We observed an improvement in mitochondrial function (Seahorse). In contrast, in the control fibroblast lines we observed a slight decrease of oxygen consumption after L-cysteine supplementation, suggesting that the increase of mitochondrial respiration seen in the patient cells is specific. However, the increase is not statistically significant and not reflected by an increase of protein levels in most of the cell lines. Further studies will determine the usefulness of L-cysteine supplementation in mitochondrial translation deficiencies.
While most mitochondrial diseases are progressive conditions, two unique mitochondrial conditions termed reversible infantile respiratory chain deficiency (RIRCD, homoplasmic mt-tRNAGlu mutation) and reversible infantile hepatopathy (TRMU mutations) stand out by showing complete spontaneous recovery. This research project was looking for genetic modifiers in several patients and families with RIRCD collected from international centres. We combined whole exome sequencing (35 patients) and RNA sequencing (5 fibroblast cell lines, 6 muscle samples) with proteomics. The analysis of the data is currently finalized and the results are being written up for a nigh impact publication. We have identified a novel approach explaining disease manifestation by our combined omics technology, which has a broad impact on the understanding of mitochondrial diseases and tissue specific manifestations.
Previously, our group reported that low cysteine concentrations may play a role in triggering a reversible mitochondrial translation defect in patients with reversible infantile respiratory chain deficiency (RIRCD, Boczonadi et al., 2013). RIRCD is caused by a homoplasmic mutation in the mitochondrial tRNAGlu. We hypothesised that the mutation changes the structure of the tRNA and thereby impairs its subsequent posttranscriptional modification by thiouridylation. Increase of substrate availability for thiouridylation by addition of L-cysteine to the culture medium rescued respiratory chain enzyme activities in human cell lines of patients with RIRCD (Boczonadi et al., 2013). This observation led to the hypothesis that increasing cysteine availability might reverse a mitochondrial translation defect also in other mitochondrial conditions affecting posttranslational mt-tRNA modifications. We performed the supplementation with L-cysteine in patient cell lines with Mitochondrial Encephalomyopathy and Lactic Acidosis with Stroke-like episodes (MELAS), Myoclonic Epilepsy with Red Ragged Fibres (MERRF), nuclear defects of mitochondrial translation and controls. We observed an improvement in mitochondrial function (Seahorse). In contrast, in the control fibroblast lines we observed a slight decrease of oxygen consumption after L-cysteine supplementation, suggesting that the increase of mitochondrial respiration seen in the patient cells is specific. However, the increase is not statistically significant and not reflected by an increase of protein levels in most of the cell lines. Further studies will determine the usefulness of L-cysteine supplementation in mitochondrial translation deficiencies.