A mitochondrial encephalo-myo-neuropathy with a nucleotide position 3271 (T-C) point mutation in the mitochondrial DNA

MERRF Classification: Implications for Diagnosis and Clinical Trials

BACKGROUND

Given the etiologic heterogeneity of disease classification using clinical phenomenology, we employed contemporary criteria to classify variants associated with myoclonic epilepsy with ragged-red fibers (MERRF) syndrome and to assess the strength of evidence of gene-disease associations. Standardized approaches are used to clarify the definition of MERRF, which is essential for patient diagnosis, patient classification, and clinical trial design.

METHODS

Systematic literature and database search with application of standardized assessment of gene-disease relationships using modified Smith criteria and of variants reported to be associated with MERRF using modified Yarham criteria.

RESULTS

Review of available evidence supports a gene-disease association for two MT-tRNAs and for POLG. Using modified Smith criteria, definitive evidence of a MERRF gene-disease association is identified for MT-TK. Strong gene-disease evidence is present for MT-TL1 and POLG. Functional assays that directly associate variants with oxidative phosphorylation impairment were critical to mtDNA variant classification. In silico analysis was of limited utility to the assessment of individual MT-tRNA variants. With the use of contemporary classification criteria, several mtDNA variants previously reported as pathogenic or possibly pathogenic are reclassified as neutral variants.

CONCLUSIONS

MERRF is primarily an MT-TK disease, with pathogenic variants in this gene accounting for ~90% of MERRF patients. Although MERRF is phenotypically and genotypically heterogeneous, myoclonic epilepsy is the clinical feature that distinguishes MERRF from other categories of mitochondrial disorders. Given its low frequency in mitochondrial disorders, myoclonic epilepsy is not explained simply by an impairment of cellular energetics. Although MERRF phenocopies can occur in other genes, additional data are needed to establish a MERRF disease-gene association. This approach to MERRF emphasizes standardized classification rather than clinical phenomenology, thus improving patient diagnosis and clinical trial design.

Mitochondria and peripheral neuropathies

There has been considerable progress during the past 24 years in the molecular genetics of mitochondrial DNA and related nuclear DNA mutations, and more than 100 nerve biopsies from hereditary neuropathies related to mitochondrial cytopathy have been accurately examined. Neuropathies were first reported in diseases related to point mutations of mitochondrial DNA, but they proved to be a prominent feature of the phenotype in mitochondrial disorders caused by defects in nuclear DNA, particularly in 3 genes: polymerase gamma 1 (POLG1), mitofusin 2 (MFN2), and ganglioside-induced differentiation-associated protein 1 (GDAP1). Most patients have sensory-motor neuropathy, sometimes associated with ophthalmoplegia, ataxia, seizures, parkinsonism, myopathy, or visceral disorders. Some cases are caused by consanguinity, but most are sporadic with various phenotypes mimicking a wide range of other etiologies. Histochemistry on muscle biopsy, as well as identification of crystalloid inclusions at electron microscopy, may provide a diagnostic clue to mitochondriopathy, but nerve biopsy is often less informative. Nevertheless, enlarged mitochondria containing distorted or amputated cristae are highly suggestive, particularly when located in the Schwann cell cytoplasm. Also noticeable are clusters of regenerating myelinated fibers surrounded by concentric Schwann cell processes, and such onion bulb-like formations are frequently observed in neuropathies caused by GDAP1 mutations.

Metabolic neuropathies and myopathies

Inborn errors of metabolism may impact on muscle and peripheral nerve. Abnormalities involve mitochondria and other subcellular organelles such as peroxisomes and lysosomes related to the turnover and recycling of cellular compartments. Treatable causes are β-oxidation defects producing progressive neuropathy; pyruvate dehydrogenase deficiency, porphyria, or vitamin B12 deficiency causing recurrent episodes of neuropathy or acute motor deficit mimicking Guillain-Barré syndrome. On the other hand, lysosomal (mucopolysaccharidosis, Gaucher and Fabry diseases), mitochondriopathic (mitochondrial or nuclear mutations or mDNA depletion), peroxisomal (adrenomyeloneuropathy, Refsum disease, sterol carrier protein-2 deficiency, cerebrotendinous xanthomatosis, α-methylacyl racemase deficiency) diseases are multisystemic disorders involving also the heart, liver, brain, retina, and kidney. Pathophysiology of most metabolic myopathies is related to the impairment of energy production or to abnormal production of reactive oxygen species (ROS). Main symptoms are exercise intolerance with myalgias, cramps and recurrent myoglobinuria or limb weakness associated with elevation of serum creatine kinase. Carnitine palmitoyl transferase deficiency, followed by acid maltase deficiency, and lipin deficiency, are the most common cause of isolated rhabdomyolysis. Metabolic myopathies are frequently associated to extra-neuromuscular disorders particularly involving the heart, liver, brain, retina, skin, and kidney.

Prevalence and severity of voice and swallowing difficulties in mitochondrial disease

BACKGROUND

Mutations of mitochondrial DNA (mtDNA) cause a broad spectrum of clinical phenotypes. Anecdotal evidence suggests that voice and swallow problems are a common feature of these diseases.

AIMS

To characterize accurately the prevalence and severity of voice and swallow problems in a large cohort of patients with mitochondrial disease.

METHODS & PROCEDURES

Patients with proven mitochondrial disease were sent validated questionnaires to assess both voice and swallow function. The presence of voice and swallow symptoms was correlated with other clinical features of mitochondrial disease in affected patients.

OUTCOMES & RESULTS

From the original 177 patients contacted, 98 swallowing status questionnaires and 96 Voice Handicap Index questionnaires were returned, response rates of 55% and 54%, respectively. Swallow: 48% of patients reported more difficulties with swallowing than control participants. Patients with single mtDNA deletions were most likely to report problems (65.2%), with patients with an m.8344A>G point mutation least likely (33.3%). All genotypes had a mean severity score in excess of the normal range, the highest mean score being found in the single large-scale mtDNA deletion group (10.12), the lowest in the m.3243A>G group (5.56) Voice; 48% of patients reported some difficulty with voice. Patients with single large-scale deletions showed the highest prevalence (65.2%), patients with the m.3243A>G mutation the lowest (33%). The most severe voice difficulties were reported by patients with an m.8344A>G point mutation. Patients with an m.3243A>G point mutation had the mildest and lowest incidence of voice problems. All genotypes scored outside of the normal range expected on the VHI overall (≥11.5 in control trials). Patients with an m.8344A>G point mutation reported a significantly higher degree of physical voice handicap than m.3243A>G patients (13.13 versus 4.40, p = 0.02). In patients with either single or multiple mtDNA deletions the likely pathophysiological mechanism is of proximal muscle weakness, whereas in patients with the m.8344A>G mutation cerebellar ataxia is the likely cause.

CONCLUSIONS & IMPLICATIONS

Dysphagia and dysarthria have been identified as symptoms in previous research, however the prevalence and pathophysiology of these symptoms have not been explored. This paper indicates that voice and swallow problems are a common, though predominantly mild feature of mitochondrial disease and that there is a core group of pathophysiological symptoms linked to the presence of voice and swallowing problems. This paper recommends early referral to speech and language therapists to identify emerging dysphonia and dysphagia and to provide appropriate intervention.

Inherited mitochondrial neuropathies

Mitochondrial disorders (MIDs) occasionally manifest as polyneuropathy either as the dominant feature or as one of many other manifestations (inherited mitochondrial neuropathy). MIDs in which polyneuropathy is the dominant feature, include NARP syndrome due to the transition m.8993T>, CMT2A due to MFN2 mutations, CMT2K and CMT4A due to GDAP1 mutations, and axonal/demyelinating neuropathy with external ophthalmoplegia due to POLG1 mutations. MIDs in which polyneuropathy is an inconstant feature among others is the MELAS syndrome, MERRF syndrome, LHON, Mendelian PEO, KSS, Leigh syndrome, MNGIE, SANDO; MIRAS, MEMSA, AHS, MDS (hepato-cerebral form), IOSCA, and ADOA syndrome. In the majority of the cases polyneuropathy presents in a multiplex neuropathy distribution. Nerve conduction studies may reveal either axonal or demyelinated or mixed types of neuropathies. If a hereditary neuropathy is due to mitochondrial dysfunction, the management of these patients is at variance from non-mitochondrial hereditary neuropathies. Patients with mitochondrial hereditary neuropathy need to be carefully investigated for clinical or subclinical involvement of other organs or systems. Supportive treatment with co-factors, antioxidants, alternative energy sources, or lactate lowering agents can be tried. Involvement of other organs may require specific treatment. Mitochondrial neuropathies should be included in the differential diagnosis of hereditary neuropathies.

Granular swollen epithelial cells: a histologic and diagnostic marker for mitochondrial nephropathy

Focal segmental glomerulosclerosis (FSGS) is a progressive kidney disease, and mitochondrial disease known to be a primary malady for secondary FSGS. Mitochondrial nephropathy with FSGS is diagnosed by genetic analysis or electron microscopy when it is suspected. As adequate morphologic features to diagnose mitochondrial nephropathy by light microscopy are lacking, this study used 10 cases with genetically proven mitochondrial disease and analyzed the kidney samples obtained by biopsy (n = 7) or autopsy (n = 3). We found granular swollen epithelial cells (GSECs) among the distal tubuli and collecting ducts in all patients, whereas such features were absent in IgA nephropathy, primary FSGS, and interstitial nephritis. Ultrastructural analysis of GSECs displayed accumulation of abnormal-shaped mitochondria in GSECs. To test whether GSECs were really associated with mitochondrial mutations, laser-captured single GSECs in 1 case with a position where 3,271 mutation were measured using a single-cell PCR analysis. This revealed that the mutant load of GSECs was significantly higher than normal-appearing epithelial cells within the same sample (63.4 + or - 17.8% vs. 32.5 + or - 4.6%; P <0.0001). This is direct evidence that GSEC is a characteristic cellular feature, indicating cells with mutant mitochondrial DNA accumulation. In addition, the incidence of GSECs did not correlate with serum creatinine levels, proteinuria, percent glomerulosclerosis, tubulointerstitial changes, or arteriolar hyalinosis, suggesting that GSECs per se may not cause tissue damage. In conclusion, GSEC is a distinct morphologic feature suggesting mitochondrial nephropathy and is a useful tool to identify secondary FSGS on the basis of mitochondrial abnormalities.

Ocular Motor Disorders in Mitochondrial Encephalopathy With Lactic Acid and Stroke-Like Episodes With the 3271 (T-C) Point Mutation in Mitochondrial DNA

BACKGROUND

Ocular motor function can provide insights into areas of dysfunction within the nervous system. There are no published eye movement recordings in patients with mitochondrial encephalopathy with lactic acid and stroke-like episodes (MELAS). Our purpose in this study was to analyze the ocular motor features of a family with MELAS with a (T-C) mutation at nucleotide position 3271 in the mitochondrial tRNA-Leu gene.

METHODS

The search coil method was used to record visually-guided saccades, antisaccades, and triangular pursuit tasks in the horizontal and vertical planes in three patients in a Japanese family with MELAS.

RESULTS

The patients showed saccadic dysmetria and prolonged saccadic reaction times, deficits in the ability to suppress reflex eye movements, and increased reaction time during antisaccades, downbeat nystagmus, square wave jerks, and impairment in pursuit.

CONCLUSIONS

On the basis of eye movement recordings, patients with MELAS have frontal cortex as well as cerebellar dysfunction.

Peripheral neuropathy in genetic mitochondrial diseases

Peripheral neuropathy is an underrecognized but common occurrence in genetic mitochondrial disorders. To gain insight into the frequency and clinical presentation of this complication, nerve conduction studies were performed on 43 subjects with congenital lactic acidosis enrolled in a controlled clinical trial of oral dichloroacetate. Median and peroneal motor conduction studies and median and sural sensory conduction studies were performed on each patient. The mean amplitude of the peroneal motor nerve (P < 0.001) and the conduction velocities of the median (P < 0.001) and peroneal (P < 0.001) motor nerves were uniformly lower in our subjects than in healthy literature control subjects. There were no significant differences in sensory nerve conduction studies. A generalized reduction in motor nerve conduction velocity was the dominant electrophysiological abnormality in the patients in this study and was independent of age, sex, or congenital mitochondrial disorder. We postulate that cellular energy failure is the most likely common cause of peripheral neuropathy in patients with genetic mitochondrial diseases, owing to the high demand for adenosine triphosphate via aerobic carbohydrate metabolism by nerve tissue.

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.

Clinical and genetic features in two families with MELAS and the T3271C mutation in mitochondrial DNA

The majority of patients with MELAS (mitochondrial encephalomyopathy, lactic acidosis, and strokelike episodes) have the A3243G point mutation. The much rarer T3271C mutation has been reported predominantly in Japanese subjects. Our objective was to better define the clinical phenotype and mutation load in patients with MELAS and the T3271C mutation in mitochondrial DNA. We present clinical and molecular genetic data in two pedigrees with the T3271C mutation. The age at onset was 8 years in one proband and 14 years in the other. Both patients had migrainelike headache, seizures, and strokelike episodes. Mutation loads were quantified in multiple tissues from the patients and from family members by polymerase chain reaction-restriction fragment length polymorphism analysis. The symptoms in both probands were typical of MELAS, and, contrary to previous reports, onset was early. Hearing loss was less common than in typical MELAS, and ragged red fibers were absent. The proportion of mutant genomes was consistently and markedly greater in DNA from urinary sediment than from blood. In the mother of one proband, mutant genomes were detected only in DNA from hair follicles and cheek mucosa The phenotype of patients with the T3271C mutation might not be as distinct as that of the A3243G mutation, as previously described. Our data also suggest that urine is a better source of DNA than blood for diagnosis and that multiple tissues should be studied in maternal relatives, especially when the mutation cannot be detected in blood.

Metabolic and drug-induced muscle disorders

PURPOSE OF REVIEW

The inherited disorders of muscle metabolism affect both substrate utilization and the final intramitochondrial oxidation through the Krebs cycle and the respiratory chain. Almost every step of these complex biochemical pathways can be affected by inborn errors, whose expression depends on peculiar tissue-specific or systemic gene expression. This review updates current knowledge in this broad field.

RECENT FINDINGS

New inherited defects are still being discovered, such as the beta-enolase deficiency in glycogenosis type XIII and mutations in the gene encoding an esterase/lipase/thioesterase protein in Chanarin-Dorfman syndrome, a multisystem triglyceride storage disease.

SUMMARY

Therapeutic approaches to the metabolic myopathies are still lagging behind, although remarkable observations have been made on the rare coenzyme Q10 deficiency syndrome. However, transgenic animal models may offer the opportunity both to investigate muscle pathogenesis and explore therapeutic targets. Finally, human myotoxicity may provide novel paradigms for naturally occurring muscle disorders.