MELAS: clinical phenotype and morphological brain abnormalities

Mitochondrial encephalomyopathy with lactic acidosis and stroke-like episodes with an MT-TL1 m.3243A>G point mutation: Neuroradiological features and their implications for underlying pathogenesis

Objective

Mitochondrial encephalomyopathy with lactic acidosis and stroke-like episodes (MELAS) is one of the most common inherited mitochondrial disorders. Due to the high clinical and genetic heterogeneity of MELAS, it is still a major challenge for clinicians to accurately diagnose the disease at an early stage. Herein, we evaluated the neuroimaging findings of MELAS with an m.3243A>G mutation in MT-TL1 and analyzed the possible underlying pathogenesis of stroke-like episodes.

Materials and methods

Fifty-nine imaging studies in 24 patients who had a confirmed genetic diagnosis of m.3243A>G (MT-TL1; tRNA ^Leu) associated with MELAS were reviewed in our case series. The anatomic location, morphological features, signal/intensity characteristics and temporal evolution of lesions were analyzed on magnetic resonance imaging (MRI), and computed tomography (CT) images. The supplying vessels and metabolite content of the lesions were also evaluated by using MR angiography (MRA)/CT angiography (CTA), and MR spectroscopy (MRS), respectively.

Results

The lesions were most commonly located in the posterior brain, with 37 (37/59, 63%) in the occipital lobe, 32 (32/59, 54%) in the parietal lobe, and 30 (30/59, 51%) in the temporal lobe. The signal characteristics of the lesions varied and evolved over time. Bilateral basal ganglia calcifications were found in 6 of 9 (67%) patients who underwent CT. Cerebral and cerebellar atrophy were found in 38/59 (64%) and 40/59 (68%) patients, respectively. Lesion polymorphism was found in 37/59 (63%) studies. MRS showed elevated lactate doublet peaks in 9/10 (90%) cases. MRA or CTA revealed that the lesion-related arteries were slightly dilated compared with those of the contralateral side in 4 of 6 (67%) cases.

Conclusion

The imaging features of MELAS vary depending on the disease stage. Polymorphic lesions in a single imaging examination should be considered a diagnostic clue for MELAS. Stroke-like episodes may be involved in a complex pathogenetic process, including mitochondrial angiopathy, mitochondrial cytopathy, and neuronal excitotoxicity.

Classificação das doenças neurometabólicas hereditárias baseada em aspectos radiológicos: ensaio iconográfico

Inherited neurometabolic disorders represent a diagnostic challenge, and an efficient classification system is needed in order to improve the understanding of these diseases. Although they constitute a group of rare diseases, they have a collective incidence of at least one case per 1,000 live births. Some inherited neurometabolic disorders are treatable. The clinical and radiological presentations are variable and sometimes overlap, depending on the stage of the disease. Therefore, a number of classification systems have been devised, some of which are difficult to apply in practice. The aim of this study was to illustrate a classification system for inherited neurometabolic disorders, based exclusively on radiological findings. This was a retrospective study of imaging examinations of the central nervous system, particularly of children, performed in a network of hospitals. All of the cases were studied by multidetector computed tomography, magnetic resonance imaging, or both, the images having been obtained by two neuroradiologists. We included only cases in which a definitive diagnosis was made. The classification system separates the relevant radiological findings into 10 categories. All of the cases studied presented at least one of those findings. In most of the cases, more than one finding was observed, which increased specificity and narrowed the differential diagnosis. Data from the literature and from this study demonstrate that it is possible to classify inherited neurometabolic disorders by their radiological aspects, which favors a definitive diagnosis.

Molecular biomarkers correlate with brain grey and white matter changes in patients with mitochondrial m.3243A > G mutation

INTRODUCTION

The mitochondrial DNA (mtDNA) m.3243A > G mutation in the MT-TL1 gene results in a multi-systemic disease, that is commonly associated with neurodegenerative changes in the brain.

METHODS

Seventeen patients harboring the m3243A > G mutation were enrolled (age 43.1 ± 11.4 years, 10 M/7F). A panel of plasma biomarkers including lactate acid, alanine, L-arginine, fibroblast growth factor 21 (FGF-21), growth/differentiation factor 15 (GDF-15) and circulating cell free -mtDNA (ccf-mtDNA), as well as blood, urine and muscle mtDNA heteroplasmy were evaluated. Patients also underwent a brain standardized MR protocol that included volumetric T1-weighted images and diffusion-weighted MRI. Twenty sex- and age-matched healthy controls were included. Voxel-wise analysis was performed on T1-weighted and diffusion imaging, respectively with VBM (voxel-based morphometry) and TBSS (Tract-based Spatial Statistics). Ventricular lactate was also evaluated by ¹H-MR spectroscopy.

RESULTS

A widespread cortical gray matter (GM) loss was observed, more severe (p < 0.001) in the bilateral calcarine, insular, frontal and parietal cortex, along with infratentorial cerebellar cortex. High urine mtDNA mutation load, high levels of plasma lactate and alanine, low levels of plasma arginine, high levels of serum FGF-21 and ventricular lactate accumulation significantly (p < 0.05) correlated with the reduced brain GM density. Widespread microstructural alterations were highlighted in the white matter, significantly (p < 0.05) correlated with plasma alanine and arginine levels, with mtDNA mutation load in urine, with high level of serum GDF-15 and with high content of plasma ccf-mtDNA.

CONCLUSIONS

Our results suggest that the synergy of two pathogenic mechanisms, mtDNA-related mitochondrial respiratory deficiency and defective nitric oxide metabolism, contributes to the brain neurodegeneration in m.3243A > G patients.

A novel m.11406 T > A mutation in mitochondrial ND4 gene causes MELAS syndrome

Pathogenic point mutations of mitochondrial DNA (mtDNA) are associated with a large number of heterogeneous diseases involving multiple systems with which patients may present with a wide range of clinical phenotypes. In this study, we describe a novel heteroplasmic missense mutation, m.11406 T > A, of the ND4 gene encoding the subunit 4 of mitochondrial complex I in a 32-year-old woman with recurrent epileptic seizure, headache and bilateral hearing loss. Skeletal muscle histochemistry demonstrated that approximately 20% of fibers were cytochrome C oxidase (COX) deficient with increased activity of succinate dehydrogenase (SDH). Further investigations in muscle specimens showed significantly reduced level of ND4 protein. It is interesting that the subunits of complex I (ND1 and NDFUB8) and complex IV(CO1) were also remarkably decreased. These findings indicate that ND1, NDFUB8 and CO1 are more susceptible than other subunits to mutations in the mitochondrial ND4 gene.

Mitochondrial Migraine: Disentangling the angiopathy paradigm in m.3243A>G patients

Migraine, characterized by recurrent attacks of predominantly unilateral throbbing headache, affects approximately 15% of the adult population and is an important cause of disability worldwide. Knowledge required for the development of new classes of antimigraine drugs might come from studying rare metabolic diseases associated with migraine. An illustrative example of a monogenetic disorder associated with migraine is the spectrum of disorders caused by the m.3243A>G mutation in the mitochondrial transfer RNA Leucine. Reported migraine prevalence figures in patients with this particular mutation vary considerably, but compared to the general population, m.3243A>G patients have a higher migraine prevalence. This burdensome symptom might sometimes even be the only clinical feature in maternal relatives carrying the m.3243A>G mutation. Although the exact sequence of events and the relative importance of factors underlying migraine in m.3243A>G MELAS spectrum disorders are still enigmatic, substantial evidence in man exist that dysfunctional mitochondria in both the vascular, the smooth muscle cells and the neuronal system and the interaction between these are at the starting point of the migraine developing pathophysiological cascade. Exclusively based on results of studies performed in patients harboring the m.3243A>G mutation, either in vivo or ex vivo, we here summarize our current understanding of mitochondrial angiopathy associated migraine in m.3243A>G patients which knowledge might lead to potential new avenues for migraine drug development.

Anatomic & metabolic brain markers of the m.3243A>G mutation: A multi-parametric 7T MRI study

One of the most common mitochondrial DNA (mtDNA) mutations, the A to G transition at base pair 3243, has been linked to changes in the brain, in addition to commonly observed hearing problems, diabetes and myopathy. However, a detailed quantitative description of m.3243A>G patients' brains has not been provided so far. In this study, ultra-high field MRI at 7T and volume- and surface-based data analyses approaches were used to highlight morphology (i.e. atrophy)-, microstructure (i.e. myelin and iron concentration)- and metabolism (i.e. cerebral blood flow)-related differences between patients (N = 22) and healthy controls (N = 15). The use of quantitative MRI at 7T allowed us to detect subtle changes of biophysical processes in the brain with high accuracy and sensitivity, in addition to typically assessed lesions and atrophy. Furthermore, the effect of m.3243A>G mutation load in blood and urine epithelial cells on these MRI measures was assessed within the patient population and revealed that blood levels were most indicative of the brain's state and disease severity, based on MRI as well as on neuropsychological data. Morphometry MRI data showed a wide-spread reduction of cortical, subcortical and cerebellar gray matter volume, in addition to significantly enlarged ventricles. Moreover, surface-based analyses revealed brain area-specific changes in cortical thickness (e.g. of the auditory cortex), and in T₁, T₂* and cerebral blood flow as a function of mutation load, which can be linked to typically m.3243A>G-related clinical symptoms (e.g. hearing impairment). In addition, several regions linked to attentional control (e.g. middle frontal gyrus), the sensorimotor network (e.g. banks of central sulcus) and the default mode network (e.g. precuneus) were characterized by alterations in cortical thickness, T₁, T₂* and/or cerebral blood flow, which has not been described in previous MRI studies. Finally, several hypotheses, based either on vascular, metabolic or astroglial implications of the m.3243A>G mutation, are discussed that potentially explain the underlying pathobiology. To conclude, this is the first 7T and also the largest MRI study on this patient population that provides macroscopic brain correlates of the m.3243A>G mutation indicating potential MRI biomarkers of mitochondrial diseases and might guide future (longitudinal) studies to extensively track neuropathological and clinical changes.

Black Toenail Sign in MELAS Syndrome

BACKGROUND

Mitochondrial encephalopathy with lactic acidosis and stroke-like episodes (MELAS) syndrome is a mitochondrial disorder often causing progressive brain injury that is not confined to large arterial territories. Severe insults ultimately lead to gyral necrosis affecting the cortex and juxtacortical white matter; the neuroimaging correlate is partial gyral signal suppression on T2/FLAIR sequences that resemble black toenails. We aimed to characterize the imaging features and the natural history of MELAS-related gyral necrosis.

MATERIALS AND METHODS

Databases at two children's hospitals were searched for brain magnetic resonance imaging studies of individuals with MELAS. Examinations with motion artifact and those lacking T2/FLAIR sequences were excluded. The location, the cumulative number, and the maximum transverse diameter of necrotic gyral lesions were assessed using T2-weighted images and T2/FLAIR sequences. Wilcoxon signed-rank test was employed to evaluate the relationship between disease duration and the number of necrotic lesions.

RESULTS

One hundred twenty-four examinations from patients with 14 unique MELAS patients (16 ± 3 years) were evaluated. Six of the eight patients who developed brain lesions also developed gyral necroses (mean 13, range 0 to 44). Necrotic lesions varied in maximal diameter from 4 to 25 mm. Cumulative necrotic lesions correlated with disease duration (P < 0.001).

CONCLUSIONS

The black toenail sign signifying gyral necrosis is a common imaging feature in individuals with MELAS syndrome. The extent of gyral necrosis correlates with disease duration.

Cortical venous disease severity in MELAS syndrome correlates with brain lesion development

PURPOSE

MELAS syndrome is a mitochondrial disorder typified by recurrent stroke-like episodes, seizures, and progressive brain injury. Abnormal mitochondria have been found in arterial walls implicating a vasculogenic etiology. We have observed abnormal cortical vein T2/FLAIR signal in MELAS patients, potentially representing wall thickening and sluggish flow. We sought to examine the relationship of hyperintense veins and brain lesions in MELAS.

METHODS

Imaging databases at two children's hospitals were searched for brain MRIs from MELAS patients. Artifact, sedated exams, and lack of 2D-T2/FLAIR sequences were exclusion criteria. Each exam was assigned a venous score based on number of T2/FLAIR hyperintense veins: 1 = <10, 2 = 10 to 20, 3 = >20. Cumulative brain lesions and venous score in MELAS and aged-matched normal exams were compared by Mann-Whitney test.

RESULTS

A total of 106 exams from 14 unique MELAS patients (mean 16 ± 3 years) and 30 exams from normal aged-matched patients (mean 15 ± 3 years) were evaluated. Median venous score between MELAS and control patients significantly differed (3 versus 1; p < 0.001). In the MELAS group, venous score correlated with presence (median = 3) or absence (median = 1) of cumulative brain lesions. In all 8 MELAS patients who developed lesions, venous hyperintensity was present prior to, during, and after lesion onset. Venous score did not correlate with brain lesion acuity.

CONCLUSION

Abnormal venous signal correlates with cumulative brain lesion severity in MELAS syndrome. Cortical venous stenosis, congestion, and venous ischemia may be mechanisms of brain injury. Identification of cortical venous pathology may aid in diagnosis and could be predictive of lesion development.

Review: Central nervous system involvement in mitochondrial disease

Mitochondrial respiratory chain defects are an important cause of inherited disorders affecting approximately 1 in 5000 people in the UK population. Collectively these disorders are termed ‘mitochondrial diseases’ and they result from either mitochondrial DNA mutations or defects in nuclear DNA. Although they are frequently multisystem disorders, neurological deficits are particularly common, wide‐ranging and disabling for patients. This review details the manifold neurological impairments associated with mitochondrial disease, and describes the efforts to understand how they arise and progressively worsen in patients with mitochondrial disease. We describe advances in our understanding of disease pathogenesis through detailed neuropathological studies and how this has spurred the development of cellular and animal models of disease. We underscore the importance of continued clinical, molecular genetic, neuropathological and animal model studies to fully characterize mitochondrial diseases and understand mechanisms of neurodegeneration. These studies are instrumental for the next phase of mitochondrial research that has a particular emphasis on finding novel ways to treat mitochondrial disease to improve patient care and quality of life.

Mitochondrial Dysfunction and Migraine

The molecular basis of migraine is still not completely understood. An impairment of mitochondrial oxidative metabolism might play a role in the pathophysiology of this disease, by influencing neuronal information processing. Biochemical assays of platelets and muscle biopsies performed in migraine sufferers have shown a decreased activity of the respiratory chain enzymes. Studies with phosphorus magnetic resonance spectroscopy (31P-MRS) have demonstrated an impairment of the brain oxidative energy metabolism both during and between migraine attacks. However, molecular genetic studies have not detected specific mitochondrial DNA (mtDNA) mutations in patients with migraine, although other studies suggest that particular genetic markers (i.e. neutral polymorphisms or secondary mtDNA mutations) might be present in some migraine sufferers. Further studies are still needed to clarify if migraine is associated with unidentified mutations on the mtDNA or on nuclear genes that code mitochondrial proteins. In this paper, we review morphological, biochemical, imaging and genetic studies which bear on the hypothesis that migraine may be related to mitochondrial dysfunction at least in some individuals.

MELAS: A Multigenerational Impact of the MTTL1 A3243G MELAS Mutation

BACKGROUND

the maternally inherited MTTL1 A3243G mutation in the mitochondrial genome causes MelaS (Mitochondrial encephalopathy lactic acidosis with Stroke-like episodes), a condition that is multisystemic but affects primarily the nervous system. Significant intra-familial variation in phenotype and severity of disease is well recognized.

METHODS

retrospective and ongoing study of an extended family carrying the MTTL1 A3243G mutation with multiple symptomatic individuals. tissue heteroplasmy is reviewed based on the clinical presentations, imaging studies, laboratory findings in affected individuals and pathological material obtained at autopsy in two of the family members.

RESULTS

there were seven affected individuals out of thirteen members in this three generation family who each carried the MTTL1 A3243G mutation. the clinical presentations were varied with symptoms ranging from hearing loss, migraines, dementia, seizures, diabetes, visual manifestations, and stroke like episodes. three of the family members are deceased from MelaS or to complications related to MelaS.

CONCLUSIONS

the results of the clinical, pathological and radiological findings in this family provide strong support to the current concepts of maternal inheritance, tissue heteroplasmy and molecular pathogenesis in MelaS. neurologists (both adult and paediatric) are the most likely to encounter patients with MelaS in their practice. genetic counselling is complex in view of maternal inheritance and heteroplasmy. newer therapeutic options such as arginine are being used for acute and preventative management of stroke like episodes.

Investigating complex I deficiency in Purkinje cells and synapses in patients with mitochondrial disease

Aims

Cerebellar ataxia is common in patients with mitochondrial disease, and despite previous neuropathological investigations demonstrating vulnerability of the olivocerebellar pathway in patients with mitochondrial disease, the exact neurodegenerative mechanisms are still not clear. We use quantitative quadruple immunofluorescence to enable precise quantification of mitochondrial respiratory chain protein expression in Purkinje cell bodies and their synaptic terminals in the dentate nucleus.

Methods

We investigated NADH dehydrogenase [ubiquinone] 1 alpha subcomplex subunit 13 protein expression in 12 clinically and genetically defined patients with mitochondrial disease and ataxia and 10 age‐matched controls. Molecular genetic analysis was performed to determine heteroplasmy levels of mutated mitochondrial DNA in Purkinje cell bodies and inhibitory synapses.

Results

Our data reveal that complex I deficiency is present in both Purkinje cell bodies and their inhibitory synapses which surround dentate nucleus neurons. Inhibitory synapses are fewer and enlarged in patients which could represent a compensatory mechanism. Mitochondrial DNA heteroplasmy demonstrated similarly high levels of mutated mitochondrial DNA in cell bodies and synapses.

Conclusions

This is the first study to use a validated quantitative immunofluorescence technique to determine complex I expression in neurons and presynaptic terminals, evaluating the distribution of respiratory chain deficiencies and assessing the degree of morphological abnormalities affecting synapses. Respiratory chain deficiencies detected in Purkinje cell bodies and their synapses and structural synaptic changes are likely to contribute to altered cerebellar circuitry and progression of ataxia.

Cognitive dysfunction in mitochondrial disorders

Among the various central nervous system (CNS) manifestations of mitochondrial disorders (MIDs), cognitive impairment is increasingly recognized and diagnosed (mitochondrial cognitive dysfunction). Aim of the review was to summarize recent findings concerning the aetiology, pathogenesis, diagnosis and treatment of cognitive decline in MIDs. Among syndromic MIDs due to mitochondrial DNA (mtDNA) mutations, cognitive impairment occurs in patients with mitochondrial encephalopathy, lactic acidosis and stroke-like episodes syndrome, myoclonus epilepsy with ragged-red fibres syndrome, mitochondrial chronic progressive external ophthalmoplegia, Kearns-Sayre syndrome, neuropathy, ataxia and retinitis pigmentosa syndrome and maternally inherited diabetes and deafness. Among syndromic MIDs due to nuclear DNA (nDNA) mutations, cognitive decline has been reported in myo-neuro-gastro-intestinal encephalopathy, mitochondrial recessive ataxia syndrome, spinocerebellar ataxia with encephalopathy, Mohr-Tranebjaerg syndrome, leuko-encephalopathy; brain and spinal cord involvement and lactic acidosis, CMT2, Wolfram syndrome, Wolf-Hirschhorn syndrome and Leigh syndrome. In addition to syndromic MIDs, a large number of non-syndromic MIDs due to mtDNA as well as nDNA mutations have been reported, which present with cognitive impairment as the sole or one among several other CNS manifestations of a MID. Delineation of mitochondrial cognitive impairment from other types of cognitive impairment is essential to guide the optimal management of these patients. Treatment of mitochondrial cognitive impairment is largely limited to symptomatic and supportive measures. Cognitive impairment may be a CNS manifestation of syndromic as well as non-syndromic MIDs. Correct diagnosis of mitochondrial cognitive impairment is a prerequisite for the optimal management of these patients.

Beyond the serotonin hypothesis: mitochondria, inflammation and neurodegeneration in major depression and affective spectrum disorders

For many years, a deficiency of monoamines including serotonin has been the prevailing hypothesis on depression, yet research has failed to confirm consistent relations between brain serotonin and depression. High degrees of overlapping comorbidities and common drug efficacies suggest that depression is one of a family of related conditions sometimes referred to as the "affective spectrum disorders", and variably including migraine, irritable bowel syndrome, chronic fatigue syndrome, fibromyalgia and generalized anxiety disorder, among many others. Herein, we present data from many different experimental modalities that strongly suggest components of mitochondrial dysfunction and inflammation in the pathogenesis of depression and other affective spectrum disorders. The three concepts of monoamines, energy metabolism and inflammatory pathways are inter-related in many complex manners. For example, the major categories of drugs used to treat depression have been demonstrated to exert effects on mitochondria and inflammation, as well as on monoamines. Furthermore, commonly-used mitochondrial-targeted treatments exert effects on mitochondria and inflammation, and are increasingly being shown to demonstrate efficacy in the affective spectrum disorders. We propose that interactions among monoamines, mitochondrial dysfunction and inflammation can inspire explanatory, rather than mere descriptive, models of these disorders.

The appearance of ADCs in the non-affected areas of the patients with MELAS

INTRODUCTION

The exact mechanism of the mitochondrial myopathy, encephalopathy, lactic acidosis and stroke-like episodes (MELAS) remain unclear. Diffusion-weighted imaging (DWI) is a magnetic resonance (MR) imaging technique for studying the pathophysiologic change of the MELAS. The purpose of the study is to see whether the apparent diffusion coefficient (ADC) of MELAS in the non-affected areas is different from the ADC of the normal subjects and to speculate the pathophysiological mechanisms of the MELAS.

METHODS

Sixteen cases of MELAS were retrospectively analyzed. Thirty healthy subjects were chosen to constitute the control group. All of them were performed on the 3.0T whole-body MR scanner with axial view T2 fluid attenuated inversion recovery (flair), T2-weighted imaging, T1flair, and DWI. An ADC map was reconstructed in the workstation. Two to five regions of interest were put in the non-affected frontal lobe and basal ganglia. All data took statistical analysis.

RESULTS

There were significant differences between the ADC of the patients with MELAS and the controls in the non-affected areas, including the superior frontal gyrus, precentral gyrus, corpus striatum, thalamus, and white matter of the semi-oval centrum.

CONCLUSION

ADCs in the non-affected areas of the patients with MELAS are higher than those of the normal subjects. Pathological changes take place in the non-affected areas of the patients with MELAS.

Mechanism of neurodegeneration of neurons with mitochondrial DNA mutations

Mutations of mitochondrial DNA are associated with a wide spectrum of disorders, primarily affecting the central nervous system and muscle function. The specific consequences of mitochondrial DNA mutations for neuronal pathophysiology are not understood. In order to explore the impact of mitochondrial mutations on neuronal biochemistry and physiology, we have used fluorescence imaging techniques to examine changes in mitochondrial function in neurons differentiated from mouse embryonic stem-cell cybrids containing mitochondrial DNA polymorphic variants or mutations. Surprisingly, in neurons carrying a severe mutation in respiratory complex I (<10% residual complex I activity) the mitochondrial membrane potential was significantly increased, but collapsed in response to oligomycin, suggesting that the mitochondrial membrane potential was maintained by the F₁F_o ATPase operating in ‘reverse’ mode. In cells with a mutation in complex IV causing ∼40% residual complex IV activity, the mitochondrial membrane potential was not significantly different from controls. The rate of generation of mitochondrial reactive oxygen species, measured using hydroethidium and signals from the mitochondrially targeted hydroethidine, was increased in neurons with both the complex I and complex IV mutations. Glutathione was depleted, suggesting significant oxidative stress in neurons with a complex I deficiency, but not in those with a complex IV defect. In the neurons with complex I deficiency but not the complex IV defect, neuronal death was increased and was attenuated by reactive oxygen species scavengers. Thus, in neurons with a severe mutation of complex I, the maintenance of a high potential by F₁F_o ATPase activity combined with an impaired respiratory chain causes oxidative stress which promotes cell death.

Cerebral oxygen and glucose metabolism in patients with mitochondrial m.3243A>G mutation

The m.3243A>G mutation is the most common pathogenic mutation in mitochondrial DNA. It leads to defective oxidative phosphorylation, decreased oxygen consumption and increased glucose utilization and lactate production in vitro. However, oxygen and glucose metabolism has not been studied in the brain of patients harbouring the m.3243A>G mutation. Therefore, 14 patients with the m.3243A>G mutation, not experiencing acute stroke-like episodes and 14 age-matched controls underwent positron emission tomography using 2-[(18)F]fluoro-2-deoxyglucose, [(15)O]H(2)O and [(15)O]O(2) as the tracers during normoglycaemia. The metabolic rate of oxygen and glucose were determined using a quantitative region of interest analysis. Metabolites in unaffected periventricular tissue were measured using magnetic resonance spectroscopy. We found that the cerebral metabolic rate of oxygen was decreased by 26% (range 18%-29%) in the grey as well as the white matter of patients with the m.3243A>G mutation. A decrease in the metabolic rate of glucose was found with predilection to the posterior part of the brain. No major changes were detected in cerebral blood flow or the number of white matter lesions. Our results show that the m.3243A>G mutation leads to a global decrease in oxygen consumption in the grey matter including areas where no other signs of disease were present.

Pathophysiology of stroke-like episodes in MELAS: neuron–astrocyte uncoupling in neuronal hyperexcitability

Mitochondrial myopathy, encephalopathy, lactic acidosis and stroke-like episodes (MELAS) is a distinct clinical syndrome characterized by fluctuated encephalopathy, migraineous headache, seizure and stroke-like episodes. The molecular mechanism of MELAS mutations has been elucidated; however, the pathogenesis of stroke-like episodes remains largely unknown. Three main hypotheses include ischemic, metabolic and neuronal hyperexcitability hypotheses. Recently, emerging hypotheses include alterations in nitric oxide homeostasis and over-reduction/oxidative stress mechanisms. Although neuron–astrocyte communication is crucial in various physiological functions, it has not been seriously considered in the pathophysiology of stroke-like episodes. This review summarizes what is known about the molecular mechanisms of gene mutation, clinico-radiological, clinico-physiological and pathological features of stroke-like episodes, as well as its pathogenesis. We finally discuss potential mechanisms involved in the pathogenesis of stroke-like episodes based on currently available clinical data and the current understanding of the mechanisms of neuron–astrocyte communications. We propose that neuron–astrocyte uncoupling is a new target of research in mitochondrial disorders.

Friedreich's ataxia: oxidative stress and cytoskeletal abnormalities

Friedreich's ataxia (FRDA) is an autosomal recessive disorder caused by mutations in the gene encoding frataxin, a mitochondrial protein implicated in iron metabolism. Current evidence suggests that loss of frataxin causes iron overload in tissues, and increase in free-radical production leading to oxidation and inactivation of mitochondrial respiratory chain enzymes, particularly Complexes I, II, III and aconitase. Glutathione plays an important role in the detoxification of ROS in the Central Nervous System (CNS), where it also provides regulation of protein function by glutathionylation. The cytoskeletal proteins are particularly susceptible to oxidation and appear constitutively glutathionylated in the human CNS. Previously, we showed loss of cytoskeletal organization in fibroblasts of patients with FRDA found to be associated with increased levels of glutathione bound to cytoskeletal proteins. In this study, we analysed the glutathionylation of proteins in the spinal cord of patients with FRDA and the distribution of tubulin and neurofilaments in the same area. We found, for the first time, a significant rise of the dynamic pool of tubulin as well as abnormal distribution of the phosphorylated forms of human neurofilaments in FRDA motor neurons. In the same cells, the cytoskeletal abnormalities co-localized with an increase in protein glutathionylation and the mitochondrial proteins were normally expressed by immunocytochemistry. Our results suggest that in FRDA oxidative stress causes abnormally increased protein glutathionylation leading to prominent abnormalities of the neuronal cytoskeleton.

High prevalence of impaired glucose homeostasis and myopathy in asymptomatic and oligosymptomatic 3243A>G mitochondrial DNA mutation-positive subjects

INTRODUCTION

The point mutation of 3243A>G mtDNA is the most frequent cause of mitochondrial diabetes, often presenting as the syndrome maternally inherited diabetes and deafness (MIDD). The mutation may also cause myopathy, ataxia, strokes, ophthalmoplegia, epilepsy, and cardiomyopathy in various combinations. Consequently, it is difficult to predict the "phenotypic risk profile" of 3243A>G mutation-positive subjects. The 3243A>G mutation coexists in cells with wild-type mtDNA, a phenomenon called heteroplasmy. The marked variability in mutation loads in different tissues is the main explanation for the different phenotypes associated with this mutation.

AIM

The aim of the study was to screen asymptomatic and oligosymptomatic 3243A>G mtDNA carriers for diabetes and myopathy.

METHODS

The study is a case-control study. Nineteen adult 3243A>G carriers presumed to be normoglycemic and matched healthy controls were subjected to an oral glucose tolerance test. Twenty-six adult 3243A>G carriers with unknown myopathy status and 17 healthy controls had a maximal cycle test and a muscle biopsy performed. The mutation loads were quantified in blood and muscle biopsies and correlated to the clinical manifestations of the mutation.

RESULTS

In the presumed normoglycemic 3243A>G-positive subjects, one subject had overt diabetes, and 10 subjects had impaired glucose tolerance. Sixteen of the 26 subjects with unknown oxidative capacity fulfilled criteria for myopathy. The mutation load in blood and muscle correlated with the age for diagnosis of impaired glucose homeostasis and hearing impairment (rho = -0.71 to -0.78; P < 0.0001).

CONCLUSION

The findings suggest that 3243A>G mutation carriers should be screened for diabetes and myopathy.

MELAS with diffuse degeneration of the cerebral white matter: report of an autopsy case

Up to now diffuse white matter demyelination of the cerebrum has been reported in only a few cases of mitochondrial encephalopathy with lactic acidosis and stroke-like episodes (MELAS). Here we document an autopsy case with this rare neuropathology. Most MELAS cases are diagnosed antemortem by A3243G transition of mitochondrial DNA. While cerebral damage including necrotic foci in the cerebral cortex are common findings in MELAS, prominent white matter involvement best characterizes this MELAS case. There were numerous necrotic foci, varying in size and chronological stage, in the cerebral white matter. In the areas of the white matter without necrotic foci, there was diffuse fibrillary gliosis with the loss of axons and oligodendrocytes. The gliosis was dominant in the deep white matter, sparing the U-fiber. The cerebral cortex showed diffuse cortical atrophy with few scattered necrotic foci. Distribution of the cerebral lesions does not coincide with the territory of blood supply. The vascular wall presented only slight to mild hyalinosis. We assumed a common pathogenesis to the cortical lesions and the white matter change. The pathogenesis of the present diffuse cerebral lesions may not be just secondary to circulatory disturbance but partly due to metabolic abnormality.

Mitochondrial function is related to alterations at brain SPECT in depressed patients

INTRODUCTION

99mTc-d,l-hexamethylpropylene amine oxime (99mTc-HMPAO) retention in brain is proportional to cerebral blood flow and related to both the local hemodynamic state and to the cellular content of reduced glutathione. Alterations of the regional distribution of 99mTc-HMPAO retention, with discrepant results, have been reported at functional brain imaging of unipolar depression. Since mitochondrial involvement has been reported in depressed patients, the aim of the study was to explore whether the 99mTc-HMPAO retention at single-photon emission computed tomography in depressed patients may relate to different levels of mitochondrial function.

METHODS

All patients had audiological and muscular symptoms, somatic symptoms that are common in depression. Citrate synthase (CS) activity assessed in muscle mitochondria correlated strongly with the activities of three mitochondrial respiratory chain enzymes and was used as a marker of mitochondrial function. K-means clustering performed on CS grouped eight patients with low and 11 patients with normal CS. Voxel-based analysis was performed on the two groups by statistical parametric mapping.

RESULTS

Voxel-based analysis showed significantly higher 99mTc-HMPAO retention in the patients with low CS compared with the patients with normal CS in the posterior and inferior frontal cortex, the superior and posterior temporal cortex, the somato-sensory cortex, and the associative parietal cortex.

CONCLUSION

Low muscle CS in depressed patients is related to higher regional 99mTc-HMPAO retention that may reflect cerebrovascular adaptation to impaired intracellular metabolism and/or intracellular enzymatic changes, as previously reported in mitochondrial disorder. Mitochondrial dysfunction in varying proportions of the subjects may explain some of the discrepant results for 99mTc-HMPAO retention in depression.

Cerebrospinal fluid analysis in the diagnosis and treatment of arterial ischemic stroke

With the advent of magnetic resonance imaging as a rapid and accurate way to diagnose arterial ischemic stroke, cerebrospinal fluid assessment is rarely performed, unless infectious or inflammatory processes are obvious. Recent advances in the understanding of the pathophysiology of childhood stroke have implicated a growing list of discrete or occult infectious and inflammatory conditions which may involve intracranial arteries and neighboring structures. Cerebrospinal-fluid assessment may allow the detection of markers identifying processes (including infectious, inflammatory, metabolic, and traumatic) potentially involved in cerebral vasculopathy and stroke. The analysis of cerebrospinal fluid in arterial ischemic strokes, including apparently idiopathic strokes, may yield essential information on pathophysiology, allowing for optimal therapeutic decisions and prognostic considerations.

Genetic, pathogenetic, and phenotypic implications of the mitochondrial A3243G tRNALeu(UUR) mutation

Mitochondrial disorders are frequently caused by mutations in mitochondrial genes and usually present as multisystem disease. One of the most frequent mitochondrial mutations is the A3,243G transition in the tRNALeu(UUR) gene. The phenotypic expression of the mutation is variable and comprises syndromic or non-syndromic mitochondrial disorders. Among the syndromic manifestations the mitochondrial encephalopathy, lactacidosis, and stroke-like episode (MELAS) syndrome is the most frequent. In single cases the A3,243G mutation may be associated with maternally inherited diabetes and deafness syndrome, myoclonic epilepsy and ragged-red fibers (MERRF) syndrome, MELAS/MERRF overlap syndrome, maternally inherited Leigh syndrome, chronic external ophthalmoplegia, or Kearns-Sayre syndrome. The wide phenotypic variability of the mutation is explained by the peculiarities of the mitochondrial DNA, such as heteroplasmy and mitotic segregation, resulting in different mutation loads in different tissues and family members. Moreover, there is some evidence that additional mtDNA sequence variations (polymorphisms, haplotypes) influence the phenotype of the A3,243G mutation. This review aims to give an overview on the actual knowledge about the genetic, pathogenetic, and phenotypic implications of the A3,243G mtDNA mutation.

Central nervous system manifestations of mitochondrial disorders

The central nervous system (CNS) is, after the peripheral nervous system, the second most frequently affected organ in mitochondrial disorders (MCDs). CNS involvement in MCDs is clinically heterogeneous, manifesting as epilepsy, stroke-like episodes, migraine, ataxia, spasticity, extrapyramidal abnormalities, bulbar dysfunction, psychiatric abnormalities, neuropsychological deficits, or hypophysial abnormalities. CNS involvement is found in syndromic and non-syndromic MCDs. Syndromic MCDs with CNS involvement include mitochondrial encephalomyopathy, lactacidosis, stroke-like episodes syndrome, myoclonic epilepsy and ragged red fibers syndrome, mitochondrial neuro-gastrointestinal encephalomyopathy syndrome, neurogenic muscle weakness, ataxia, and retinitis pigmentosa syndrome, mitochondrial depletion syndrome, Kearns-Sayre syndrome, and Leigh syndrome, Leber's hereditary optic neuropathy, Friedreich's ataxia, and multiple systemic lipomatosis. As CNS involvement is often subclinical, the CNS including the spinal cord should be investigated even in the absence of overt clinical CNS manifestations. CNS investigations comprise the history, clinical neurological examination, neuropsychological tests, electroencephalogram, cerebral computed tomography scan, and magnetic resonance imaging. A spinal tap is indicated if there is episodic or permanent impaired consciousness or in case of cognitive decline. More sophisticated methods are required if the CNS is solely affected. Treatment of CNS manifestations in MCDs is symptomatic and focused on epilepsy, headache, lactacidosis, impaired consciousness, confusion, spasticity, extrapyramidal abnormalities, or depression. Valproate, carbamazepine, corticosteroids, acetyl salicylic acid, local and volatile anesthetics should be applied with caution. Avoiding certain drugs is often more beneficial than application of established, apparently indicated drugs.

Molecular neuropathology of MELAS: level of heteroplasmy in individual neurones and evidence of extensive vascular involvement

Mitochondrial DNA (mtDNA) disease is an important genetic cause of neurological disability. A variety of different clinical features are observed and one of the most common phenotypes is MELAS (Mitochondrial Myopathy, Encephalopathy, Lactic Acidosis and Stroke-like episodes). The majority of patients with MELAS have the 3243A>G mtDNA mutation. The neuropathology is dominated by multifocal infarct-like lesions in the posterior cortex, thought to underlie the stroke-like episodes seen in patients. To investigate the relationship between mtDNA mutation load, mitochondrial dysfunction and neuropathological features in MELAS, we studied individual neurones from several brain regions of two individuals with the 3243A>G mutation using dual cytochrome c oxidase (COX) and succinate dehydrogenase (SDH) histochemistry, and Polymerase Chain Reaction Restriction Fragment Lenght Polymorphism (PCR-RFLP) analysis. We found a low number of COX-deficient neurones in all brain regions. There appeared to be no correlation between the threshold level for the 3243A>G mutation to cause COX deficiency within single neurones and the degree of pathology in affected brain regions. The most severe COX deficiency associated with the highest proportion of mutated mtDNA was present in the walls of the leptomeningeal and cortical blood vessels in all brain regions. We conclude that vascular mitochondrial dysfunction is important in the pathogenesis of the stroke-like episodes in MELAS patients. As migraine is a commonly encountered feature in MELAS, we propose that coupling of the vascular mitochondrial dysfunction with cortical spreading depression (CSD) might underlie the selective distribution of ischaemic lesions in the posterior cortex in these patients.

The transmission of OXPHOS disease and methods to prevent this

Diseases owing to defects of oxidative phosphorylation (OXPHOS) affect approximately 1 in 8,000 individuals. Clinical manifestations can be extremely variable and range from single-affected tissues to multisystemic syndromes. In general, tissues with a high energy demand, like brain, heart and muscle, are affected. The OXPHOS system is under dual genetic control, and mutations in both nuclear and mitochondrial genes can cause OXPHOS diseases. The expression and segregation of mitochondrial DNA (mtDNA) mutations is different from nuclear gene defects. The mtDNA mutations can be either homoplasmic or heteroplasmic and in the latter case disease becomes manifest when the mutation exceeds a tissue-specific threshold. This mutation load can vary between tissues and often an exact correlation between mutation load and phenotypic expression is lacking. The transmission of mtDNA mutations is exclusively maternal, but the mutation load between embryos can vary tremendously because of a segregational bottleneck. Diseases by nuclear gene mutations show a normal Mendelian inheritance pattern and often have a more constant clinical manifestation. Given the prevalence and severity of OXPHOS disorders and the lack of adequate therapy, existing and new methods for the prevention of transmission of OXPHOS disorders, like prenatal diagnosis (PND), preimplantation genetic diagnosis (PGD), cytoplasmic transfer (CT) and nuclear transfer (NT), are technically and ethically evaluated.

Les encéphalomyopathies mitochondriales

Mitochondrial encephalomyopathies include various syndromes involving both muscles and the nervous system. They are characterized by morphological and/or functional mitochondrial abnormalities. Relevant histological modifications in muscle are ragged-red fibers with or without cytochrome C oxidase (COX) activity. Neuropathological alterations in the brain are not specific. They consist of spongiosis with or without preferential involvement of territories of "system degeneration", neuronal loss, focal necrosis, capillary proliferation and mineral deposits. Their topographic patterns are characteristic of each syndrome. Mitochondrial encephalomyopathies are due to defects in mitochondrial DNA, sporadic, with maternal inheritance or defects in nuclear DNA with mendelian inheritance. The first group is more frequent including MERRF, MELAS, KEARNS-SAYRE, and some LEIGH syndromes. LEIGH syndrome is also the most frequent in the second group. However, in accordance with the progress in molecular genetics, these syndromes might be reclassified.

Stroke-like episode involving a cerebral artery in a patient with MELAS

This report describes a patient with mitochondrial myopathy, encephalopathy, lactic acidosis, and stroke-like episodes who exhibited a segmental vascular narrowing in the crural segment of the right posterior cerebral artery by magnetic imaging angiography in the acute phase of the first stroke-like episode. The vascular stenosis almost improved on the subsequent neuroimaging study. This result suggested that major cerebral arteries might be occasionally involved in a stroke-like episode in MELAS (mitochondrial myopathy, encephalopathy, lactic acidosis, and stroke-like episodes).

Hypocitrullinemia in patients with MELAS: an insight into the “MELAS paradox”

L-citrulline, classified as a nonessential amino acid, is synthesized predominantly via Delta-1-pyrroline carboxylate synthase in the endothelial cells of the small intestine. In mammals, small quantities of citrulline are also produced in nitric oxide synthase-expressing cells. Considering the fact that the enzymes involved in the endogenous synthesis of L-citrulline are all located in the mitochondria and the fact that citrulline is a component of the citrulline-nitric oxide (NO) cycle, we hypothesized that the distinct clinical, biochemical, and morphological characteristics of MELAS, a maternally inherited mitochondrial disorder, might be due to alterations in nitric oxide homeostasis. Analysis of serum from MELAS patients showed that levels of plasma arginine were similar in both patients and in controls. However, levels of citrulline in MELAS patients were significantly lower than in controls, and there was a clear inverse correlation between arginine and citrulline levels in these patients. We found no correlation between the level of heteroplasmy and the plasma levels of either arginine or citrulline. We discuss the depressed citrulline levels in MELAS patients, who have an unusual and paradoxical pattern of vascular respiratory chain expression, in the context of NO homeostasis.

Toward the definition of cerebral arteriopathies of childhood

PURPOSE OF REVIEW

To facilitate and standardize the diagnosis of cerebrovascular conditions in childhood, particularly in the field of arterial ischemic diseases.

RECENT FINDINGS

Progress in diagnostic techniques in the past decade have led to newly established etiologies for childhood stroke, most of which represent some form of vascular pathology. These advances must be integrated into a modern nomenclature system with revised definitions of stroke and arterial wall diseases-arteriopathies-in childhood.

SUMMARY

This nomenclature system is intended to facilitate and enhance clinical research in childhood stroke, particularly multicenter collaborative studies.