MERRF syndrome with overwhelming lactic acidosis

Comprehensive summary of mitochondrial DNA alterations in the postmortem human brain: A systematic review

BACKGROUND

Mitochondrial DNA (mtDNA) encodes 37 genes necessary for synthesizing 13 essential subunits of the oxidative phosphorylation system. mtDNA alterations are known to cause mitochondrial disease (MitD), a clinically heterogeneous group of disorders that often present with neuropsychiatric symptoms. Understanding the nature and frequency of mtDNA alterations in health and disease could be a cornerstone in disentangling the relationship between biochemical findings and clinical symptoms of brain disorders. This systematic review aimed to summarize the mtDNA alterations in human brain tissue reported to date that have implications for further research on the pathophysiological significance of mtDNA alterations in brain functioning.

METHODS

We searched the PubMed and Embase databases using distinct terms related to postmortem human brain and mtDNA up to June 10, 2021. Reports were eligible if they were empirical studies analysing mtDNA in postmortem human brains.

FINDINGS

A total of 158 of 637 studies fulfilled the inclusion criteria and were clustered into the following groups: MitD (48 entries), neurological diseases (NeuD, 55 entries), psychiatric diseases (PsyD, 15 entries), a miscellaneous group with controls and other clinical diseases (5 entries), ageing (20 entries), and technical issues (5 entries). Ten entries were ascribed to more than one group. Pathogenic single nucleotide variants (pSNVs), both homo- or heteroplasmic variants, have been widely reported in MitD, with heteroplasmy levels varying among brain regions; however, pSNVs are rarer in NeuD, PsyD and ageing. A lower mtDNA copy number (CN) in disease was described in most, but not all, of the identified studies. mtDNA deletions were identified in individuals in the four clinical categories and ageing. Notably, brain samples showed significantly more mtDNA deletions and at higher heteroplasmy percentages than blood samples, and several of the deletions present in the brain were not detected in the blood. Finally, mtDNA heteroplasmy, mtDNA CN and the deletion levels varied depending on the brain region studied.

INTERPRETATION

mtDNA alterations are well known to affect human tissues, including the brain. In general, we found that studies of MitD, NeuD, PsyD, and ageing were highly variable in terms of the type of disease or ageing process investigated, number of screened individuals, studied brain regions and technology used. In NeuD and PsyD, no particular type of mtDNA alteration could be unequivocally assigned to any specific disease or diagnostic group. However, the presence of mtDNA deletions and mtDNA CN variation imply a role for mtDNA in NeuD and PsyD. Heteroplasmy levels and threshold effects, affected brain regions, and mitotic segregation patterns of mtDNA alterations may be involved in the complex inheritance of NeuD and PsyD and in the ageing process. Therefore, more information is needed regarding the type of mtDNA alteration, the affected brain regions, the heteroplasmy levels, and their relationship with clinical phenotypes and the ageing process.

FUNDING

Hospital Universitari Institut Pere Mata; Institut d'Investigació Sanitària Pere Virgili; Instituto de Salud Carlos III, Ministerio de Ciencia e Innovación (PI18/00514).

Involvement of the Spinal Cord in Mitochondrial Disorders

This review aims at summarising and discussing the current status concerning the clinical presentation, pathogenesis, diagnosis, and treatment of spinal cord affection in mitochondrial disorders (MIDs). A literature search using the database Pubmed was carried out by application of appropriate search terms and their combinations. Involvement of the spinal cord in MIDs is more frequent than anticipated. It occurs in specific and non-specific MIDs. Among the specific MIDs it has been most frequently described in LBSL, LS, MERRF, KSS, IOSCA, MIRAS, and PCH and only rarely in MELAS, CPEO, and LHON. Clinically, spinal cord involvement manifests as monoparesis, paraparesis, quadruparesis, sensory disturbances, hypotonia, spasticity, urinary or defecation dysfunction, spinal column deformities, or as transverse syndrome. Diagnosing spinal cord involvement in MIDs requires a thoroughly taken history, clinical exam, and imaging studies. Additionally, transcranial magnetic stimulation, somato-sensory-evoked potentials, and cerebro-spinal fluid can be supportive. Treatment is generally not at variance compared to the underlying MID but occasionally surgical stabilisation of the spinal column may be necessary. It is concluded that spinal cord involvement in MIDs is more frequent than anticipated but may be missed if cerebral manifestations prevail. Spinal cord involvement in MIDs may strongly determine the mobility of these patients.

Causes of Death in Adults with Mitochondrial Disease

INTRODUCTION

Mitochondrial diseases are a clinically, biochemically and genetically heterogeneous group of disorders with a variable age of onset and rate of disease progression. It might therefore be expected that this variation be reflected in the age and cause of death. However, to date, little has been reported regarding the 'end-of-life' period and causes of death in mitochondrial disease patients. For some specific syndromes, the associated clinical problems might predict the cause of death, but for many patients, it remains difficult to provide an accurate prognosis.

AIMS

To describe a retrospective cohort of adult mitochondrial disease patients who had attended the NHS Highly Specialised Services for Rare Mitochondrial Diseases in Newcastle upon Tyne (UK), evaluate life expectancy and causes of death and assess the consequences for daily patient care.

METHODS

All deceased adult patients cared for at this centre over a period of 10 years were included in the study. Patient history, data on laboratory findings, biochemical investigations and genetic studies were analysed retrospectively.

RESULTS

A total of 30 adult mitochondrial patients died within the time period of the study. The main mitochondrial disease-related causes of death in this patient cohort were respiratory failure, cardiac failure and acute cerebral incidents such as seizures and strokes. In almost half of the patients, the cause of death remained unknown. Based on our study, we present recommendations regarding the care of patients with mitochondrial disease.

"Myo-cardiomyopathy" is commonly associated with the A8344G "MERRF" mutation

The objective of the study was to better characterize the clinical phenotype associated with the A8344G "MERRF" mutation of mitochondrial DNA. Fifteen mutated patients were extensively investigated. The frequency of main clinical features was: exercise intolerance and/or muscle weakness 67 %, respiratory involvement 67 %, lactic acidosis 67 %, cardiac abnormalities 53 %, peripheral neuropathy 47 %, myoclonus 40 %, epilepsy 40 %, ataxia 13 %. A restrictive respiratory insufficiency requiring ventilatory support was observed in about half of our patients. One patient developed a severe and rapidly progressive cardiomyopathy requiring cardioverter-defibrillator implantation. Five patients died of overwhelming, intractable lactic acidosis. Serial muscle MRIs identified a consistent pattern of muscle involvement and progression. Cardiac MRI showed non-ischemic late gadolinium enhancement in the left ventricle inferolateral part as early sign of myocardial involvement. Brain spectroscopy demonstrated increased peak of choline and reduction of N-acetylaspartate. Lactate was never detected in brain areas, while it could be documented in ventricles. We confirm that muscle involvement is the most frequent clinical feature associated with A8443G mutation. In contrast with previous reports, however, about half of our patients did not develop signs of CNS involvement even in later stages of the disease. The difference may be related to the infrequent investigation of A8344G mutation in 'pure' mitochondrial myo-cardiomyopathy, representing a bias and a possible cause of syndrome's underestimation. Our study highlights the importance of lactic acidosis and respiratory muscle insufficiency as critical prognostic factors. Muscle and cardiac MRI and brain spectroscopy may be useful tools in diagnosis and follow-up of MERRF.

Role of AMPK-mediated adaptive responses in human cells with mitochondrial dysfunction to oxidative stress

BACKGROUND

Mitochondrial DNA (mtDNA) mutations are an important cause of mitochondrial diseases, for which there is no effective treatment due to complex pathophysiology. It has been suggested that mitochondrial dysfunction-elicited reactive oxygen species (ROS) plays a vital role in the pathogenesis of mitochondrial diseases, and the expression levels of several clusters of genes are altered in response to the elevated oxidative stress. Recently, we reported that glycolysis in affected cells with mitochondrial dysfunction is upregulated by AMP-activated protein kinase (AMPK), and such an adaptive response of metabolic reprogramming plays an important role in the pathophysiology of mitochondrial diseases.

SCOPE OF REVIEW

We summarize recent findings regarding the role of AMPK-mediated signaling pathways that are involved in: (1) metabolic reprogramming, (2) alteration of cellular redox status and antioxidant enzyme expression, (3) mitochondrial biogenesis, and (4) autophagy, a master regulator of mitochondrial quality control in skin fibroblasts from patients with mitochondrial diseases.

MAJOR CONCLUSION

Induction of adaptive responses via AMPK-PFK2, AMPK-FOXO3a, AMPK-PGC-1α, and AMPK-mTOR signaling pathways, respectively is modulated for the survival of human cells under oxidative stress induced by mitochondrial dysfunction. We suggest that AMPK may be a potential target for the development of therapeutic agents for the treatment of mitochondrial diseases.

GENERAL SIGNIFICANCE

Elucidation of the adaptive mechanism involved in AMPK activation cascades would lead us to gain a deeper insight into the crosstalk between mitochondria and the nucleus in affected tissue cells from patients with mitochondrial diseases. This article is part of a Special Issue entitled Frontiers of Mitochondrial Research.

Myoclonic epilepsy and ragged red fibers (MERRF) syndrome: selective vulnerability of CNS neurons does not correlate with the level of mitochondrial tRNAlys mutation in individual neuronal isolates

Selective vulnerability of subpopulations of neurons is a striking feature of neurodegeneration. Mitochondrially transmitted diseases are no exception. In this study CNS tissues from a patient with myoclonus epilepsy and ragged red fibers (MERRF) syndrome, which results from an A to G transition of nucleotide (nt) 8344 in the mitochondrial tRNALys gene, were examined for the proportion of mutant mtDNA. Either individual neuronal somas or the adjacent neuropil and glia were microdissected from cryostat tissue sections of histologically severely affected brain regions, including dentate nuclei, Purkinje cells, and inferior olivary nuclei, and from a presumably less affected neuronal subpopulation, the anterior horn cells of the spinal cord. Mutant and normal mtDNA were quantified after PCR amplification with a mismatched primer and restriction enzyme digestion. Neurons and the surrounding neuropil and glia from all CNS regions that were analyzed exhibited high proportions of mutant mtDNA, ranging from 97.6 +/- 0.7% in Purkinje cells to 80.6 +/- 2.8% in the anterior horn cells. Within each neuronal group that was analyzed, neuronal soma values were similar to those in the surrounding neuropil and glia or in the regional tissue homogenate. Surprisingly, as compared with controls, neuronal loss ranged from 7% of the Purkinje cells to 46% of the neurons of the dentate nucleus in MERRF cerebellum. Thus, factors other than the high proportion of mutant mtDNA, in particular nuclear-controlled neuronal differences among various regions of the CNS, seem to contribute to the mitochondrial dysfunction and ultimate cell death.