Phenotypic expression of mtDNA heteroplasmy in the skeletal muscle of patients with oculomyopathy: defect in mitochondrial protein synthesis

Mitochondrial diseases: from molecular mechanisms to therapeutic advances

Mitochondria are essential for cellular function and viability, serving as central hubs of metabolism and signaling. They possess various metabolic and quality control mechanisms crucial for maintaining normal cellular activities. Mitochondrial genetic disorders can arise from a wide range of mutations in either mitochondrial or nuclear DNA, which encode mitochondrial proteins or other contents. These genetic defects can lead to a breakdown of mitochondrial function and metabolism, such as the collapse of oxidative phosphorylation, one of the mitochondria's most critical functions. Mitochondrial diseases, a common group of genetic disorders, are characterized by significant phenotypic and genetic heterogeneity. Clinical symptoms can manifest in various systems and organs throughout the body, with differing degrees and forms of severity. The complexity of the relationship between mitochondria and mitochondrial diseases results in an inadequate understanding of the genotype-phenotype correlation of these diseases, historically making diagnosis and treatment challenging and often leading to unsatisfactory clinical outcomes. However, recent advancements in research and technology have significantly improved our understanding and management of these conditions. Clinical translations of mitochondria-related therapies are actively progressing. This review focuses on the physiological mechanisms of mitochondria, the pathogenesis of mitochondrial diseases, and potential diagnostic and therapeutic applications. Additionally, this review discusses future perspectives on mitochondrial genetic diseases.

Mitochondrial DNA deletions inhibit proteasomal activity and stimulate an autophagic transcript

Deletions within the mitochondrial DNA (mtDNA) cause Kearns Sayre syndrome (KSS) and chronic progressive external opthalmoplegia (CPEO). The clinical signs of KSS include muscle weakness, heart block, pigmentary retinopathy, ataxia, deafness, short stature, and dementia. The identical deletions occur and rise exponentially as humans age, particularly in substantia nigra. Deletions at >30% concentration cause deficits in basic bioenergetic parameters, including membrane potential and ATP synthesis, but it is poorly understood how these alterations cause the pathologies observed in patients. To better understand the consequences of mtDNA deletions, we microarrayed six cell types containing mtDNA deletions from KSS and CPEO patients. There was a prominent inhibition of transcripts encoding ubiquitin-mediated proteasome activity, and a prominent induction of transcripts involved in the AMP kinase pathway, macroautophagy, and amino acid degradation. In mutant cells, we confirmed a decrease in proteasome biochemical activity, significantly lower concentration of several amino acids, and induction of an autophagic transcript. An interpretation consistent with the data is that mtDNA deletions increase protein damage, inhibit the ubiquitin-proteasome system, decrease amino acid salvage, and activate autophagy. This provides a novel pathophysiological mechanism for these diseases, and suggests potential therapeutic strategies.

The 'common deletion' is not increased in parkinsonian substantia nigra as shown by competitive polymerase chain reaction

Previous studies have estimated levels of mitochondrial DNA (mtDNA) carrying the 4,977-base-pair 'common deletion' in tissues from patients with Parkinson's disease (PD) by using semiquantitative techniques. The role of this deleted mtDNA species in the pathogenesis of PD has remained controversial. We have applied competitive polymerase chain reaction to achieve exact quantitation of deleted mtDNA in the substantia nigra and additional brain regions of cases with neuropathologically confirmed Lewy-body parkinsonism. In addition, genotyping was carried out for CYP2D6G1,934A and CYP2D6C2,938T alleles and the mitochondrial ND2 (nucleotide 5,460) and transfer RNA for glutamine (nucleotide 4,336) sequence variants. Parkinsonian brains showed 1-3% deleted mtDNA in the substantia nigra, that is, deletion levels were not higher than in age-matched controls. Our findings suggest that the defect in complex I of the respiratory chain observed in PD is not primarily due to the 'common deletion.'

Residual muscle cytochrome c oxidase activity accounts for submaximal exercise lactate threshold in chronic progressive external ophthalmoplegia

The data from histological, biochemical, and mitochondrial DNA (mtDNA ) studies of muscle biopsies from 10 patients affected with chronic progressive external ophthalmoplegia (CPEO) were related to dynamic and metabolic parameters of incremental submaximal exercise. Maximum power output was reduced in all patients as compared to controls. Analysis of the venous lactate curve during exercise revealed a lactate threshold at exercise levels ranging from 40 to 50% of the predicted maximal power output. An earlier significant increase in lactate could be detected by calculating the mean delta lactate. Lactate values were inversely correlated with the cytochrome c oxidase (COX) activity of isolated muscle mitochondria. No relationship was found between lactate values and the number of ragged red fibers, or cytochrome c oxidase-negative fibers of the proportion of deleted mtDNA measured in muscle biopsy specimens. The discussion underscores the value of lactate kinetics in assessing skeletal muscle function, as well as the use of muscle COX levels to predict the effectiveness of wild-type complementation of deleted skeletal muscle mtDNA in in vivo contractile performance of CPEO subjects.