Mutational Analysis and mtDNA Haplogroup Characterization in Three Serbian Cases of Mitochondrial Encephalomyopathies and Literature Review

Mitochondrial encephalomyopathies (MEMP) are heterogeneous multisystem disorders frequently associated with mitochondrial DNA (mtDNA) mutations. Clinical presentation varies considerably in age of onset, course, and severity up to death in early childhood. In this study, we performed molecular genetic analysis for mtDNA pathogenic mutation detection in Serbian children, preliminary diagnosed clinically, biochemically and by brain imaging for mitochondrial encephalomyopathies disorders. Sanger sequencing analysis in three Serbian probands revealed two known pathogenic mutations. Two probands had a heteroplasmic point mutation m.3243A>G in the MT-TL1 gene, which confirmed mitochondrial myopathy, encephalopathy, lactic acidosis, and stroke-like episode syndrome (MELAS), while a single case clinically manifested for Leigh syndrome had an almost homoplasmic (close to 100%) m.8993T>G mutation in the MT-ATP6 gene. After full mtDNA MITOMASTER analysis and PhyloTree build 17, we report MELAS’ association with haplogroups U and H (U2e and H15 subclades); likewise, the mtDNA-associated Leigh syndrome proband shows a preference for haplogroup H (H34 subclade). Based on clinical–genetic correlation, we suggest that haplogroup H may contribute to the mitochondrial encephalomyopathies’ phenotypic variability of the patients in our study. We conclude that genetic studies for the distinctive mitochondrial encephalomyopathies should be well-considered for realizing clinical severity and possible outcomes.

Different Phenotypes Caused by the Unique Mutation in the Same Family with Mitochondrial Encephalomyopathy

Background and Objective: Mitochondrial encephalomyopathies represent a clinically heterogeneous group of disorders resulting from abnormal mitochondrial function. This study investigates the clinical and genetic characteristics of families with mitochondrial encephalomyopathy. Methods: The clinical manifestations, biopsy and gene detection were retrospectively analyzed for four probands with definitively diagnosed mitochondrial encephalomyopathy with lactic acidosis and stroke-like episodes (MELAS) from three families with MELAS and/or maternally inherited diabetes and deafness (MIDD). Results: The initial symptoms of probands were convulsive headache and/or epilepsy. The members of the three families also had diabetes, deafness, muscle weakness and short statures. Typical characteristics were indicated by muscle biopsy and gene detection in all. Conclusion: We reveal that the same family can have MIDD and MELAS cases, which clearly show that the unique mutation may cause different syndromes in one family. Neurologists should take into account more possibilities and phenotypes in screening and genetic counselling for the families of probands.

Progressive External Ophthalmoplegia in Polish Patients—From Clinical Evaluation to Genetic Confirmation

Mitochondrial encephalomyopathies comprise a group of heterogeneous disorders resulting from impaired oxidative phosphorylation (OxPhos). Among a variety of symptoms progressive external ophthalmoplegia (PEO) seems to be the most common. The aim of this study is to present clinical and genetic characteristics of Polish patients with PEO. Clinical, electrophysiological, neuroradiological, and morphological data of 84 patients were analyzed. Genetic studies of mitochondrial DNA (mtDNA) were performed in all patients. Among nuclear DNA (nDNA) genes POLG was sequenced in 41 patients, TWNK (C10orf2) in 13 patients, and RNASEH1 in 2 patients. Total of 27 patients were included in the chronic progressive external ophthalmoplegia (CPEO) group, 24 in the CPEO+ group. Twenty-six patients had mitochondrial encephalomyopathy (ME), six patients Kearns–Sayre syndrome (KSS), and one patient sensory ataxic neuropathy, dysarthria, ophthalmoparesis (SANDO) syndrome. Genetic analysis of nDNA genes revealed the presence of pathogenic or possibly pathogenic variants in the POLG gene in nine patients, the TWNK gene in five patients and the RNASEH1 gene in two patients. Detailed patients’ history and careful assessment of family history are essential in the diagnostic work-up. Genetic studies of both mtDNA and nDNA are necessary for the final diagnosis of progressive external ophthalmoplegia and for genetic counseling.

P0064RENAL PHENOTYPES IN A GROUP OF PATIENTS WITH MITOCHONDRIAL DISEASES

Background and Aims Mitochondrial diseases (MDs) are one of the most common inherited metabolic disorders caused by mutations of either mitochondrial or nuclear genes involved in the oxidative phosphorylation pathway. Tissues with high energy demand (i.e. brain and skeletal muscle) are preferentially affected, and MDs are not rarely referred to as mitochondrial encephalomyopathies. However, one of the hallmarks of MDs is multisystemic involvement, with large clinical variability and severity due to different mitochondrial DNA heteroplasmy levels. Given the possibility of multisystemic phenotype of MDs, renal involvement has been suggested, based on renal cells vulnerability to poor energy supply. A specific mitochondrial kidney disease is thought to underlie renal manifestations, therefore theoretically every district of the nephron might be affected by MDs. Regarding tubular involvement, we suggested a possible association between nephrolithiasis and MDs, based on data of 3 related stone formers with reduced urinary citrate excretion and MELAS syndrome. The aim of this study is to provide a clinical description of the renal phenotypes of patients with MDs. Method In 2019, 6 patients with MDs routinely followed by the Neurology outpatient clinic of our Institution were randomly asked to perform renal screening. Genetic tests, blood and 24-hour urine analyses were assessed in the whole population. The main comorbidities were recorded. Glomerular filtration rate (GFR) was estimated using creatinine clearance. Results Half of the patients were female, the average age at renal evaluation was 48±15 years; diabetes, hypertension and neurosensorial hypoacusia occurred in 4, 1 and 2 subjects, respectively. Five patients had m.3243A>G mutation, responsible for more than 85% of maternal inherited diabetes and deafness (MIDD) and syndrome of mitochondrial encephalopathy with lactic acidosis and stroke-like episodes (MELAS). The remaining patient carried the m.8363G>A variant, reported to be associated with a multisystem MDs known as Leigh syndrome (subacute necrotizing encephalomyelopathy). Median serum creatinine was 0.6 mg/dL (IQR: 0.5-1.1) with median GFR of 92 mL/min/1.73 m2 (IQR: 40-117). Three patients exhibited signs of kidney disease: one had both reduced GFR and proteinuria (eGFR 37 mL/min/1.73 m2 and 750 mg/day respectively), one had renal impairment (58 mL/min/1.7 3m2) and severe hypocitraturia (22 mg/day), one showed increased urinary calcium excretion (348 mg/day). In one patient, evaluation of the type of proteinuria has not been performed yet. Serum lactate was 3.1±1.9 mmol/L, urinary pH 5.8±0.3 and median urinary citrate excretion 593 mg/24h (IQR: 310-923). No relevant results were found on serum and/or urinary levels of sodium, potassium, magnesium, phosphorus, urate, chloride, oxalate, parathyroid hormone and 25-OH vitamin D. Conclusion These results confirmed the presence of renal disease and its heterogeneity in patients with MDs. A complete renal screening is necessary in these subjects in order to early recognize renal involvement and carefully select candidates for further diagnostic work-up including renal biopsy. A consensus on the optimal renal screening for these patients has yet to be established.

Risk of mitochondrial deletions is affected by the global secondary structure of the mitochondrial genome

Ageing is associated with accumulation of somatic mutations. This process is especially pronounced in mitochondrial genomes (mtDNA) of postmitotic cells, where the accumulation of somatic mitochondrial deletions is associated with healthy ageing and mitochondrial encephalomyopathies. Deletions are often flanked by direct nucleotide repeats, however, they do not provide an exhaustive explanation of deletion distribution. We hypothesized that in parallel with the role of direct repeats there is also a global secondary structure of mtDNA, shaping deletion formation. Analyzing the folding energies of the heavy chain, which stays single-stranded during mtDNA replication, we observed a potential contact zone between 6-9kb and 13-16kb of the major arc of mtDNA. Describing the distribution of deletions in the human mtDNA we demonstrated that the contact zone is 3-times more mutagenic under all else equal. The proposed topological model improves our understanding of the mechanisms of deletion formation in the human mitochondrial genome.

Migraine in mitochondrial disorders: Prevalence and characteristics

Background Migraine is a well-known feature of mitochondrial disorders (MDs). However, no systematic epidemiological data are available in large populations of patients. Aims The aim of this cross-sectional cohort study was to describe the prevalence and migraine characteristics in a large cohort of patients with mitochondrial encephalomyopathies. Methods We studied 93 consecutive patients with characterised MDs referred to our Neuromuscular Unit during a 12-month period. All patients (age range = 16–78 years; 31 men; 58 progressive external ophthalmoplegia [PEO], 12 myoclonic epilepsy with ragged red fibres [MERRF], eight mitochondrial encephalomyopathy, lactic acidosis and stroke-like episodes [MELAS], two mitochondrial neurogastrointestinal encephalomyopathy [MNGIE] and 13 other MDs) underwent a structured diagnostic headache interview using an operational diagnostic tool following the IHS criteria. If they met the criteria for migraine, they were included in the ‘Migraine Group’. The other patients were counted in the ‘No Migraine Group’. Patient demographic and migraine characteristics were examined. Clinical, neuroradiological and neurophysiological data were compared between groups. Results Migraine was reported in 35.5% of patients. Migraine without aura was the most common headache (81.8%). The migraine group showed younger age ( P < 0.01), increased prevalence of epilepsy ( P = 0.01), myoclonus ( P = 0.03), stroke-like episodes ( P = 0.03) and decreased prevalence of muscle weakness ( P < 0.01). Multivariate analysis showed that migraine was positively associated with absence of muscle weakness ( P = 0.04) and presence of EEG abnormalities ( P = 0.02). Conclusion Migraine has a higher prevalence in MDs compared with general population-based data, independently from genotype or phenotype. Migraine is not merely a phenotypic aspect of specific MDs but is rather the expression of vulnerability of the central nervous system, probably directly related with defects of the respiratory chain.

Diseases of Mitochondrial Energy Metabolism

Mitochondrial disorders are a group of inherited diseases of energy metabolism caused by impairment of mitochondrial function-primarily disorders of the oxidative phosphorylation system but also the more recently described disorders of mitochondrial transport and fission. This review will focus on primary disorders of mitochondrial oxidative phosphorylation. The neurologic system is one of the most profoundly affected by mitochondrial dysfunction and the effects can be varied and widespread. This has led to these diseases being commonly called mitochondrial encephalomyopathies. The heterogeneity of clinical presentation, laboratory findings, neuroimaging findings, pathologic findings, and genetic findings in these diseases make diagnosis extremely difficult. Treatment for mitochondrial disorders is currently lacking a solid evidence base but this is a rapidly expanding area of research.