Brain imaging in mitochondrial respiratory chain deficiency: combination of brain MRI features as a useful tool for genotype/phenotype correlations

Primary mitochondrial diseases

Primary mitochondrial diseases (PMDs) are a heterogeneous group of hereditary disorders characterized by an impairment of the mitochondrial respiratory chain. They are the most common group of genetic metabolic disorders, with a prevalence of 1 in 4,300 people. The presence of leukoencephalopathy is recognized as an important feature in many PMDs and can be a manifestation of mutations in both mitochondrial DNA (classic syndromes such as mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes; myoclonic epilepsy with ragged-red fibers [RRFs]; Leigh syndrome; and Kearns-Sayre syndrome) and nuclear DNA (mutations in maintenance genes such as POLG, MPV17, and TYMP; Leigh syndrome; and mitochondrial aminoacyl-tRNA synthetase disorders). In this chapter, PMDs associated with white matter involvement are outlined, including details of clinical presentations, brain MRI features, and elements of differential diagnoses. The current approach to the diagnosis of PMDs and management strategies are also discussed. A PMD diagnosis in a subject with leukoencephalopathy should be considered in the presence of specific brain MRI features (for example, cyst-like lesions, bilateral basal ganglia lesions, and involvement of both cerebral hemispheres and cerebellum), in addition to a complex neurologic or multisystem disorder. Establishing a genetic diagnosis is crucial to ensure appropriate genetic counseling, multidisciplinary team input, and eligibility for clinical trials.

Increased cerebral lactate levels in adults with autism spectrum disorders compared to non-autistic controls: a magnetic resonance spectroscopy study

INTRODUCTION

Autism spectrum disorder (ASD) encompasses a heterogeneous group with varied phenotypes and etiologies. Identifying pathogenic subgroups could facilitate targeted treatments. One promising avenue is investigating energy metabolism, as mitochondrial dysfunction has been implicated in a subgroup of ASD. Lactate, an indicator of energy metabolic anomalies, may serve as a potential biomarker for this subgroup. This study aimed to examine cerebral lactate (Lac+) levels in high-functioning adults with ASD, hypothesizing elevated mean Lac+ concentrations in contrast to neurotypical controls (NTCs).

MATERIALS AND METHODS

Magnetic resonance spectroscopy (MRS) was used to study cerebral Lac+ in 71 adults with ASD and NTC, focusing on the posterior cingulate cortex (PCC). After quality control, 64 ASD and 58 NTC participants remained. Lac+ levels two standard deviations above the mean of the control group were considered elevated.

RESULTS

Mean PCC Lac+ levels were significantly higher in the ASD group than in the NTC group (p = 0.028; Cohen's d = 0.404), and 9.4% of the ASD group had elevated levels as compared to 0% of the NTCs (p = 0.029). No significant correlation was found between blood serum lactate levels and MRS-derived Lac+ levels.

LIMITATIONS

A cautious interpretation of our results is warranted due to a p value of 0.028. In addition, a higher than anticipated proportion of data sets had to be excluded due to poor spectral quality.

CONCLUSION

This study confirms the presence of elevated cerebral Lac+ levels in a subgroup of adults with ASD, suggesting the potential of lactate as a biomarker for mitochondrial dysfunction in a subgroup of ASD. The lower-than-expected prevalence (20% was expected) and moderate increase require further investigation to elucidate the underlying mechanisms and relationships with mitochondrial function.

Mitochondrial encephalomyopathy

Mitochondrial dysfunction, especially perturbation of oxidative phosphorylation and adenosine triphosphate (ATP) generation, disrupts cellular homeostasis and is a surprisingly frequent cause of central and peripheral nervous system pathology. Mitochondrial disease is an umbrella term that encompasses a host of clinical syndromes and features caused by in excess of 300 different genetic defects affecting the mitochondrial and nuclear genomes. Patients with mitochondrial disease can present at any age, ranging from neonatal onset to late adult life, with variable organ involvement and neurological manifestations including neurodevelopmental delay, seizures, stroke-like episodes, movement disorders, optic neuropathy, myopathy, and neuropathy. Until relatively recently, analysis of skeletal muscle biopsy was the focus of diagnostic algorithms, but step-changes in the scope and availability of next-generation sequencing technology and multiomics analysis have revolutionized mitochondrial disease diagnosis. Currently, there is no specific therapy for most types of mitochondrial disease, although clinical trials research in the field is gathering momentum. In that context, active management of epilepsy, stroke-like episodes, dystonia, brainstem dysfunction, and Parkinsonism are all the more important in improving patient quality of life and reducing mortality.

Genetic and phenotypic spectrum of Chinese patients with epilepsy and photosensitivity

Objective

To determine the contribution of genetic etiologies in epilepsy with photosensitivity.

Methods

A total of 35 epileptic patients with genetic photosensitivity from January 2019 to May 2021 were analyzed.

Results

Pathogenic variants were identified in 35 patients, including SCN1A(7) CHD2(6), TPP1(3), SYNGAP1(3), GABRA1(2), GABRG2(1), KCTD7(1), MFSD8(1), KCNC1(1) GBA(1), CACNA1A(1), KCNMA1(1), FLNA(1), SZT2(1), SLC2A1(1), 5q33.2-34del(1), and mitochondrial variants(3). The predominant epileptic syndrome was progressive myoclonus epilepsy (PME) and Dravet syndrome, while the most common seizure type in both spontaneous seizures and photoconvulsive response (PCR) was myoclonic seizures. The abnormal EEG background and brain MRI were mainly seen in the PME patients. In PME, initial low-frequencies (1–6 Hz) photosensitivity was observed in 70% (7/10) of patients. Among the other patients, 12 patients (48.0%, 12/25) had photosensitivity at initial low -frequencies and 12 patients (48.0%, 12/25) had photosensitivity at initial middle frequencies (6–20 Hz). At the 1-year follow-up, 77.7% (21/27) still remained photosensitive.

Conclusion

The most common genes for epilepsy with genetic photosensitivity are SCN1A and CHD2, and the most common syndromes are PME and Dravet syndrome. MFSD8, KCNMA1, SZT2, FLNA, and SLC2A1 variants might be candidate genes for photosensitivity. PPRs at initial low-frequencies may be a marker of PME, and the most typical feature of genetic photosensitivity may be low- or middle- frequencies induced PPRs. Photosensitivity in epilepsy with genetic photosensitivity may be difficult to disappear in a short period of time.

Clinical Experience of Neurological Mitochondrial Diseases in Children and Adults: A Single-Center Study

The goal of the study was to retrospectively evaluate a cohort of children and adults with mitochondrial diseases (MDs) in a single-center experience. Neurological clinical examination, brain magnetic resonance imaging (MRI) and spectroscopy, muscle biopsy, metabolic and molecular-genetic analysis were evaluated in 26 children and 36 adult patients with MD in Slovenia from 2004 to 2018. Nijmegen MD criteria (MDC) were applied to all patients and the need for a muscle biopsy was estimated. Exome-sequencing was used in half of the patients. Twenty children (77.0%) and 12 adults (35.0%) scored a total of ≥8 on MDC, a result that is compatible with the diagnosis of definite MD. Yield of exome-sequencing was 7/22 (31.0%), but the method was not applied systematically in all patients from the beginning of diagnostics. Brain MRI morphological changes, which can be an imaging clue for the diagnosis of MD, were found in 17/24 children (71.0%). In 7/26 (29.0%) children, and in 20/30 (67.0%) adults, abnormal mitochondria were found on electron microscopy (EM) and ragged-red fibers were found in 16/30 (53.0%) adults. Respiratory chain enzymes (RCEs) and/or pyruvate dehydrogenase complex (PDHc) activities were abnormal in all the children and six adult cases. First, our data revealed that MDC was useful in the clinical diagnosis of MD, and second, until the use of NGS methods, extensive, laborious and invasive diagnostic procedures were performed to reach a final diagnosis. In patients with suspected MD, there is a need to prioritize molecular diagnosis with the more modern next-generation sequencing (NGS) method.

The Diagnostic Approach to Mitochondrial Disorders in Children in the Era of Next-Generation Sequencing: A 4-Year Cohort Study

Mitochondrial diseases (MDs) are a large group of genetically determined multisystem disorders, characterized by extreme phenotypic heterogeneity, attributable in part to the dual genomic control (nuclear and mitochondrial DNA) of the mitochondrial proteome. Advances in next-generation sequencing technologies over the past two decades have presented clinicians with a challenge: to select the candidate disease-causing variants among the huge number of data provided. Unfortunately, the clinical tools available to support genetic interpretations still lack specificity and sensitivity. For this reason, the diagnosis of MDs continues to be difficult, with the new “genotype first” approach still failing to diagnose a large group of patients. With the aim of investigating possible relationships between clinical and/or biochemical phenotypes and definitive molecular diagnoses, we performed a retrospective multicenter study of 111 pediatric patients with clinical suspicion of MD. In this cohort, the strongest predictor of a molecular (in particular an mtDNA-related) diagnosis of MD was neuroimaging evidence of basal ganglia (BG) involvement. Regression analysis confirmed that normal BG imaging predicted negative genetic studies for MD. Psychomotor regression was confirmed as an independent predictor of a definitive diagnosis of MD. The findings of this study corroborate previous data supporting a role for neuroimaging in the diagnostic approach to MDs and reinforce the idea that mtDNA sequencing should be considered for first-line testing, at least in specific groups of children.

Neuromitochondrial Disorders : Genomic Basis and an Algorithmic Approach to Imaging Diagnostics

Mitochondrial disorders have been an enigma for a long time due to the varied clinical presentations. Although a genetic confirmation will be mandatory most of the time, half the number of Leigh syndrome would be negative for genetic mutations. There are a growing number of mutations in clinical practice, which escape detection on routine clinical exome sequencing. Imaging would render help in pointing towards a mitochondrial disorder. There are a few case reports which brief about specific mitochondrial mutations and their specific imaging appearance. This article tries to provide a comprehensive review on the imaging-genomic correlation of mitochondrial disorders with an objective of performing a specific genetic testing to arrive at an accurate diagnosis.

Cerebral blood flow and acute episodes of Leigh syndrome in neurometabolic disorders

AIM

To investigate cerebral blood flow (CBF) in acute episodes of Leigh syndrome compared with basal state in patients carrying pathogenic mitochondrial disease gene variants responsible for neurometabolic disorders.

METHOD

Arterial spin labelling (ASL) magnetic resonance imaging (MRI) sequences were used to measure CBF in 27 patients with mitochondrial respiratory chain enzyme deficiencies, ascribed to pathogenic variants of reported disease genes who were undergoing either urgent neuroimaging for acute episodes of Leigh syndrome (Group I: 15 MRI, seven females, eight males; mean age 7y; range 7mo-14y) or routine brain MRI (Group II: 15 MRI, eight females, seven males; mean age 5y 2mo; range 2mo-12y).

RESULTS

Patients displayed markedly increased CBF in the striatum (2.8-fold greater, p<0.001 [1.05-2.53]) during acute episodes of Leigh syndrome compared to basal conditions. Detection of elevated CBF preceded identification of structural MRI lesions in four out of 15 cases.

INTERPRETATION

Our results suggest that increased CBF is an overt hallmark of Leigh syndrome episodes and ASL MRI sequences should facilitate early diagnosis of acute episodes of Leigh syndrome, especially during the first attack in young children, when structural MRI is insufficiently informative.

A homozygous variant in NDUFA8 is associated with developmental delay, microcephaly, and epilepsy due to mitochondrial complex I deficiency

Mitochondrial complex I deficiency is caused by pathogenic variants in mitochondrial and nuclear genes associated with complex I structure and assembly. We report the case of a patient with　NDUFA8-related mitochondrial disease. The patient presented with developmental delay, microcephaly, and epilepsy. His fibroblasts showed apparent biochemical defects in mitochondrial complex I. Whole-exome sequencing revealed that the patient carried a homozygous variant in NDUFA8. His fibroblasts showed a reduction in the protein expression level of not only NDUFA8, but also the other complex I subunits, consistent with assembly defects. The enzyme activity of complex I and oxygen consumption rate were restored by reintroducing wild-typeNDUFA8 cDNA into patient fibroblasts. The functional properties of the variant in NDUFA8 were also investigated using NDUFA8 knockout cells expressing wild-type or mutated NDUFA8 cDNA. These experiments further supported the pathogenicity of the variant in complex I assembly. This is the first report describing that the loss of NDUFA8, which has not previously been associated with mitochondrial disease, causes severe defect in the assembly of mitochondrial complex I, leading to progressive neurological and developmental abnormalities.

Stress and Psychiatric Disorders: The Role of Mitochondria

In seeking to understand mental health and disease, it is fundamental to identify the biological substrates that draw together the experiences and physiological processes that underlie observed psychological changes. Mitochondria are subcellular organelles best known for their central role in energetics, producing adenosine triphosphate to power most cellular processes. Converging lines of evidence indicate that mitochondria play a key role in the biological embedding of adversity. Preclinical research documents the effects of stress exposure on mitochondrial structure and function, and recent human research suggests alterations constituting recalibrations, both adaptive and nonadaptive. Current research suggests dynamic relationships among stress exposure, neuroendocrine signaling, inflammation, and mitochondrial function. These complex relationships are implicated in disease risk, and their elucidation may inform prevention and treatment of stress- and trauma-related disorders. We review and evaluate the evidence for mitochondrial dysfunction as a consequence of stress exposure and as a contributing factor to psychiatric disease.

Molecular basis of Leigh syndrome: a current look

Leigh Syndrome (OMIM 256000) is a heterogeneous neurologic disorder due to damage in mitochondrial energy production that usually starts in early childhood. The first description given by Leigh pointed out neurological symptoms in children under 2 years and premature death. Following cases brought some hypothesis to explain the cause due to similarity to other neurological diseases and led to further investigation for metabolic diseases. Biochemical evaluation and specific metabolic profile suggested impairment in energy production (OXPHOS) in mitochondria. As direct approach to involved tissues is not always possible or safe, molecular analysis is a great cost-effective option and, besides biochemical results, is required to confirm the underlying cause of this syndrome face to clinical suspicion. The Next Generation Sequencing (NGS) advance represented a breakthrough in molecular biology allowing simultaneous gene analysis giving short-time results and increasing the variants underlying this syndrome, counting over 75 monogenic causes related so far. NGS provided confirmation of emerging cases and brought up diagnosis in atypical presentations as late-onset cases, which turned Leigh into a heterogeneous syndrome with variable outcomes. This review highlights clinical presentation in both classic and atypical phenotypes, the investigation pathway throughout confirmation emphasizing the underlying genetic heterogeneity and increasing number of genes assigned to this syndrome as well as available treatment.

Genomics and Radiogenomics in Inherited Neurometabolic Disorders - A Practical Primer for Pediatricians

Advances in genetics has revolutionised the way we understand, diagnose and manage neurological disorders. Notwithstanding the fact that genetic confirmation has already become standard of care in routine clinical practice, radiological and clinical phenotyping has not diminished in value; in fact it has found an enhanced role in guiding and interpreting genetic test results. Inherited neurometabolic disorders are a prominent group of disorders which are seen commonly in clinical practice and many are potentially treatable. The concept of Radiogenomics is the bridge from phenotype to genotype and the strength of association varies widely across different inherited metabolic diseases. Understanding the strengths and limitations of these correlations forms the basis of success of multidisciplinary approach to diagnose these disorders. In this article authors give a brief overview of the genetic basis of a disease, available genetic tests and the prominent role of radiology in contemplating a diagnostic suspicion and guiding further confirmatory tests.

Epilepsy in Leigh Syndrome With Mitochondrial DNA Mutations

Background: Leigh syndrome is a mitochondrial cytopathy that presents as a neurodegenerative disease with apparent manifestation in the central nervous system. The aim of the present study was to describe its dominant neurological clinical features and analyze data related to epilepsy in Leigh syndrome accompanied by a mitochondrial DNA mutation.

Methods: Whole mitochondrial sequencing was performed on 125 patients clinically suspected of Leigh syndrome. Among them, 25 patients were identified to have mitochondrial DNA associated Leigh syndrome. Electroencephalography (EEG) findings, semiology, brain imaging findings, and biochemical results, were evaluated. We also compared brain magnetic resonance imaging findings and biochemical features in patients with Leigh syndrome based on the presence of epilepsy.

Results: Clinical seizures were observed in 14 out of 25 enrolled patients (56%), with focal seizures being the most common type (6/14, 42.8%). All patients were found to have slow and disorganized background neural activity while eight exhibited epileptic discharges on EEG. Mutations at base pairs 10,191 and 8,993 were revealed in a relatively larger number of patients of Leigh syndrome with epilepsy. The presence of gastrointestinal symptoms was significantly more frequent in the epilepsy group (P = 0.042). Diffuse cerebral atrophy was significantly increased (P = 0.042) and cortex signal abnormalities were also increased (P = 0.033) in the epilepsy group.

Conclusions: Patients with Leigh syndrome and mitochondrial DNA mutations had a high proportion of central nervous system comorbidities, though the prevalence of epilepsy in this population was not particularly high. Various types of seizure and EEG findings are common in those with Leigh syndrome. Future imaging studies involving more patients and proper mitochondrial DNA mutation analyses are needed to further evaluate the natural course of Leigh syndrome with epilepsy.

Neuroimaging of Mitochondrial Cytopathies

Mitochondrial diseases are a complex and heterogeneous group of genetic disorders that occur as a result of either nuclear DNA or mitochondrial DNA pathogenic variants, leading to a decrease in oxidative phosphorylation and cellular energy (ATP) production. Increasing knowledge about molecular, biochemical, and genetic abnormalities related to mitochondrial dysfunction has expanded the neuroimaging phenotypes of mitochondrial disorders. As a consequence of this growing field, the imaging recognition patterns of mitochondrial cytopathies are continually evolving. In this review, we describe the main neuroimaging characteristics of pediatric mitochondrial diseases, ranging from classical to more recent and challenging features. Due to the increased knowledge about the imaging findings of mitochondrial cytopathies, the pediatric neuroradiologist plays a crucial role in the diagnosis and evaluation of these patients.

NDUFB8 Mutations Cause Mitochondrial Complex I Deficiency in Individuals with Leigh-like Encephalomyopathy

Respiratory chain complex I deficiency is the most frequently identified biochemical defect in childhood mitochondrial diseases. Clinical symptoms range from fatal infantile lactic acidosis to Leigh syndrome and other encephalomyopathies or cardiomyopathies. To date, disease-causing variants in genes coding for 27 complex I subunits, including 7 mitochondrial DNA genes, and in 11 genes encoding complex I assembly factors have been reported. Here, we describe rare biallelic variants in NDUFB8 encoding a complex I accessory subunit revealed by whole-exome sequencing in two individuals from two families. Both presented with a progressive course of disease with encephalo(cardio)myopathic features including muscular hypotonia, cardiac hypertrophy, respiratory failure, failure to thrive, and developmental delay. Blood lactate was elevated. Neuroimaging disclosed progressive changes in the basal ganglia and either brain stem or internal capsule. Biochemical analyses showed an isolated decrease in complex I enzymatic activity in muscle and fibroblasts. Complementation studies by expression of wild-type NDUFB8 in cells from affected individuals restored mitochondrial function, confirming NDUFB8 variants as the cause of complex I deficiency. Hereby we establish NDUFB8 as a relevant gene in childhood-onset mitochondrial disease.

High predictive value of brain MRI imaging in primary mitochondrial respiratory chain deficiency

Background

Because the mitochondrial respiratory chain (RC) is ubiquitous, its deficiency can theoretically give rise to any symptom in any organ or tissue at any age with any mode of inheritance, owing to the twofold genetic origin of respiratory enzyme machinery, that is, nuclear and mitochondrial. Not all respiratory enzyme deficiencies are primary and secondary or artefactual deficiency is frequently observed, leading to a number of misleading conclusions and inappropriate investigations in clinical practice. This study is aimed at investigating the potential role of brain MRI in distinguishing primary RC deficiency from phenocopies and other aetiologies.

Methods

Starting from a large series of 189 patients (median age: 3.5 years (8 days–56 years), 58% males) showing signs of RC enzyme deficiency, for whom both brain MRIs and disease-causing mutations were available, we retrospectively studied the positive predictive value (PPV) and the positive likelihood ratio (LR+) of brain MRI imaging and its ability to discriminate between two groups: primary deficiency of the mitochondrial RC machinery and phenocopies.

Results

Detection of (1) brainstem hyperintensity with basal ganglia involvement (P≤0.001) and (2) lactate peak with either brainstem or basal ganglia hyperintensity was highly suggestive of primary RC deficiency (P≤0.01). Fourteen items had a PPV>95% and LR+ was greater than 9 for seven signs. Biallelic SLC19A3 mutations represented the main differential diagnosis. Non-significant differences between the two groups were found for cortical/subcortical atrophy, leucoencephalopathy and involvement of caudate nuclei, spinothalamic tract and corpus callosum.

Conclusion

Based on these results and owing to invasiveness of skeletal muscle biopsies and cost of high-throughput DNA sequencing, we suggest giving consideration to brain MRI imaging as a diagnostic marker and an informative investigation to be performed in patients showing signs of RC enzyme deficiency.

Neurological Disorders Associated with Striatal Lesions: Classification and Diagnostic Approach

Neostriatal abnormalities can be observed in a very large number of neurological conditions clinically dominated by the presence of movement disorders. The neuroradiological picture in some cases has been described as "bilateral striatal necrosis" (BSN). BSN represents a condition histo-pathologically defined by the involvement of the neostriata and characterized by initial swelling of putamina and caudates followed by degeneration and cellular necrosis. After the first description in 1975, numerous acquired and hereditary conditions have been associated with the presence of BSN. At the same time, a large number of disorders involving neostriata have been described as BSN, in some cases irrespective of the presence of signs of cavitation on MRI. As a consequence, the etiological spectrum and the nosographic boundaries of the syndrome have progressively become less clear. In this study, we review the clinical and radiological features of the conditions associated with MRI evidence of bilateral striatal lesions. Based on MRI findings, we have distinguished two groups of disorders: BSN and other neostriatal lesions (SL). This distinction is extremely helpful in narrowing the differential diagnosis to a small group of known conditions. The clinical picture and complementary exams will finally lead to the diagnosis. We provide an update on the etiological spectrum of BSN and propose a diagnostic flowchart for clinicians.

Radboud Centre for Mitochondrial Medicine Pediatric MRI score

We developed the first user-friendly, semi-quantitative, and quick-to-perform Radboud Centre for Mitochondrial Medicine Pediatric MRI score (RCMM-PMRIS), focusing on the six most commonly described neuroimaging abnormalities in the literature. The RCMM-PMRIS was validated through individual review of 30 sets of brain MRI studies in 24 patients with genetically confirmed mitochondrial disorders by six raters. The application of RCMM-PMRIS can help to define the extent of the brain involvement and therefore to assess the radiological mitochondrial disease severity, to monitor disease progression and consequently to act as an outcome measure for treatment effects in patients with mitochondrial disease.

The many faces of paediatric mitochondrial disease on neuroimaging

The knowledge about the genetic spectrum underlying paediatric mitochondrial diseases is rapidly growing. As a consequence, the range of neuroimaging findings associated with mitochondrial diseases became extremely broad. This has important implications for radiologists and clinicians involved in the care of these patients. Here, we provide a condensed overview of brain magnetic resonance imaging (MRI) findings in children with genetically confirmed mitochondrial diseases. The neuroimaging spectrum ranges from classical Leigh syndrome with symmetrical lesions in basal ganglia and/or brain stem to structural abnormalities including cerebellar hypoplasia and corpus callosum dysgenesis. We highlight that, although some imaging patterns can be suggestive of a genetically defined mitochondrial syndrome, brain MRI-based candidate gene prioritization is only successful in a subset of patients.

Leukoencephalopathy due to Complex II Deficiency and Bi-Allelic SDHB Mutations: Further Cases and Implications for Genetic Counselling

Isolated complex II deficiency is a rare cause of mitochondrial disease and bi-allelic mutations in SDHB have been identified in only a few patients with complex II deficiency and a progressive neurological phenotype with onset in infancy. On the other hand, heterozygous SDHB mutations are a well-known cause of familial paraganglioma/pheochromocytoma and renal cell cancer. Here, we describe two additional patients with respiratory chain deficiency due to bi-allelic SDHB mutations. The patients' clinical, neuroradiological, and biochemical phenotype is discussed according to current knowledge on complex II and SDHB deficiency and is well in line with previously described cases, thus confirming the specific neuroradiological presentation of complex II deficiency that recently has emerged. The patients' genotype revealed one novel SDHB mutation, and one SDHB mutation, which previously has been described in heterozygous form in patients with familial paraganglioma/pheochromocytoma and/or renal cell cancer. This is only the second example in the literature where one specific SDHx mutation is associated with both recessive mitochondrial disease in one patient and familial paraganglioma/pheochromocytoma in others. Due to uncertainties regarding penetrance of different heterozygous SDHB mutations, we argue that all heterozygous SDHB mutation carriers identified in relation to SDHB-related leukoencephalopathy should be referred to relevant surveillance programs for paraganglioma/pheochromocytoma and renal cell cancer. The diagnosis of complex II deficiency due to SDHB mutations therefore raises implications for genetic counselling that go beyond the recurrence risk in the family according to an autosomal recessive inheritance.

Mitochondrial dysfunction in inherited renal disease and acute kidney injury

Mitochondria are increasingly recognized as key players in genetic and acquired renal diseases. Most mitochondrial cytopathies that cause renal symptoms are characterized by tubular defects, but glomerular, tubulointerstitial and cystic diseases have also been described. For example, defects in coenzyme Q10 (CoQ10) biosynthesis and the mitochondrial DNA 3243 A>G mutation are important causes of focal segmental glomerulosclerosis in children and in adults, respectively. Although they sometimes present with isolated renal findings, mitochondrial diseases are frequently associated with symptoms related to central nervous system and neuromuscular involvement. They can result from mutations in nuclear genes that are inherited according to classic Mendelian rules or from mutations in mitochondrial DNA, which are transmitted according to more complex rules of mitochondrial genetics. Diagnosis of mitochondrial disorders involves clinical characterization of patients in combination with biochemical and genetic analyses. In particular, prompt diagnosis of CoQ10 biosynthesis defects is imperative because of their potentially reversible nature. In acute kidney injury (AKI), mitochondrial dysfunction contributes to the physiopathology of tissue injury, whereas mitochondrial biogenesis has an important role in the recovery of renal function. Potential therapies that target mitochondrial dysfunction or promote mitochondrial regeneration are being developed to limit renal damage during AKI and promote repair of injured tissue.

Consensus paper: radiological biomarkers of cerebellar diseases

Hereditary and sporadic cerebellar ataxias represent a vast and still growing group of diseases whose diagnosis and differentiation cannot only rely on clinical evaluation. Brain imaging including magnetic resonance (MR) and nuclear medicine techniques allows for characterization of structural and functional abnormalities underlying symptomatic ataxias. These methods thus constitute a potential source of radiological biomarkers, which could be used to identify these diseases and differentiate subgroups of them, and to assess their severity and their evolution. Such biomarkers mainly comprise qualitative and quantitative data obtained from MR including proton spectroscopy, diffusion imaging, tractography, voxel-based morphometry, functional imaging during task execution or in a resting state, and from SPETC and PET with several radiotracers. In the current article, we aim to illustrate briefly some applications of these neuroimaging tools to evaluation of cerebellar disorders such as inherited cerebellar ataxia, fetal developmental malformations, and immune-mediated cerebellar diseases and of neurodegenerative or early-developing diseases, such as dementia and autism in which cerebellar involvement is an emerging feature. Although these radiological biomarkers appear promising and helpful to better understand ataxia-related anatomical and physiological impairments, to date, very few of them have turned out to be specific for a given ataxia with atrophy of the cerebellar system being the main and the most usual alteration being observed. Consequently, much remains to be done to establish sensitivity, specificity, and reproducibility of available MR and nuclear medicine features as diagnostic, progression and surrogate biomarkers in clinical routine.

Magnetic resonance imaging correlates of genetically characterized patients with mitochondrial disorders: A study from south India

BACKGROUND

Large studies analyzing magnetic resonance imaging correlates in different genotypes of mitochondrial disorders are far and few. This study sought to analyze the pattern of magnetic resonance imaging findings in a cohort of genetically characterized patients with mitochondrial disorders.

METHODS

The study cohort included 33 patients (age range 18 months-50 years, M:F - 0.9:1) with definite mitochondrial disorders seen over a period of 8 yrs. (2006-2013). Their MR imaging findings were analyzed retrospectively.

RESULTS

The patients were classified into three groups according to the genotype, Mitochondrial point mutations and deletions (n=21), SURF1 mutations (n=7) and POLG1 (n=5). The major findings included cerebellar atrophy (51.4%), cerebral atrophy (24.2%), signal changes in basal ganglia (45.7%), brainstem (34.2%) & white matter (18.1%) and stroke like lesions (25.7%). Spinal cord imaging showed signal changes in 4/6 patients. Analysis of the special sequences revealed, basal ganglia mineralization (7/22), lactate peak on magnetic resonance spectrometry (10/15), and diffusion restriction (6/22). Follow-up images in six patients showed that the findings are dynamic. Comparison of the magnetic resonance imaging findings in the three groups showed that cerebral atrophy and cerebellar atrophy, cortical signal changes and basal ganglia mineralization were seen mostly in patients with mitochondrial mutation. Brainstem signal changes with or without striatal lesions were characteristically noted in SURF1 group. There was no consistent imaging pattern in POLG1 group.

CONCLUSION

Magnetic resonance imaging findings in mitochondrial disorders are heterogeneous. Definite differences were noted in the frequency of anatomical involvement in the three groups. Familiarity with the imaging findings in different genotypes of mitochondrial disorders along with careful analysis of the family history, clinical presentation, biochemical findings, histochemical and structural analysis will help the physician for targeted metabolic and genetic testing.

Novel, compound heterozygous, single-nucleotide variants in MARS2 associated with developmental delay, poor growth, and sensorineural hearing loss

Novel, single-nucleotide mutations were identified in the mitochondrial methionyl amino-acyl tRNA synthetase gene (MARS2) via whole exome sequencing in two affected siblings with developmental delay, poor growth, and sensorineural hearing loss.We show that compound heterozygous mutations c.550C>T:p.Gln 184* and c.424C>T:p.Arg142Trp in MARS2 lead to decreased MARS2 protein levels in patient lymphoblasts. Analysis of respiratory complex enzyme activities in patient fibroblasts revealed decreased complex I and IV activities. Immunoblotting of patient fibroblast and lymphoblast samples revealed reduced protein levels of NDUFB8 and COXII, representing complex I and IV, respectively. Additionally, overexpression of wild-type MARS2 in patient fibroblasts increased NDUFB8 and COXII protein levels. These findings suggest that recessive single-nucleotide mutations in MARS2 are causative for a new mitochondrial translation deficiency disorder with a primary phenotype including developmental delay and hypotonia. Identification of additional patients with single-nucleotide mutations in MARS2 is necessary to determine if pectus carinatum is also a consistent feature of this syndrome.

Neuroradiological findings expand the phenotype of OPA1-related mitochondrial dysfunction

OBJECTIVE

OPA1 mutations are responsible for more than half of autosomal dominant optic atrophy (ADOA), a blinding disease affecting the retinal ganglion neurons. In most patients the clinical presentation is restricted to the optic nerve degeneration, albeit in 20% of them, additional neuro-sensorial symptoms might be associated to the loss of vision, as frequently encountered in mitochondrial diseases. This study describes clinical and neuroradiological features of OPA1 patients.

METHODS

Twenty two patients from 17 families with decreased visual acuity related to optic atrophy and carrying an OPA1 mutation were enrolled. Patients underwent neuro-ophthalmological examinations. Brain magnetic resonance imaging (T1, T2 and flair sequences) was performed on a 1.5-Tesla MR Unit. Twenty patients underwent 2-D proton spectroscopic imaging.

RESULTS

Brain imaging disclosed abnormalities in 12 patients. Cerebellar atrophy mainly involving the vermis was observed in almost a quarter of the patients; other abnormalities included unspecific white matter hypersignal, hemispheric cortical atrophy, and lactate peak. Neurological examination disclosed one patient with a transient right hand motor deficit and ENT examination revealed hearing impairment in 6 patients. Patients with abnormal MRI were characterized by: (i) an older age (ii) more severe visual impairment with chronic visual acuity deterioration, and (iii) more frequent associated deafness.

CONCLUSIONS

Our results demonstrate that brain imaging abnormalities are common in OPA1 patients, even in those with normal neurological examination. Lactate peak, cerebellar and cortical atrophies are consistent with the mitochondrial dysfunction related to OPA1 mutations and might result from widespread neuronal degeneration.