Myoclonus epilepsy with ragged red fibers and multiple mtDNA deletions

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.

Epilepsy in adults with mitochondrial disease: A cohort study

OBJECTIVE

The aim of this work was to determine the prevalence and progression of epilepsy in adult patients with mitochondrial disease.

METHODS

We prospectively recruited a cohort of 182 consecutive adult patients attending a specialized mitochondrial disease clinic in Newcastle upon Tyne between January 1, 2005 and January 1, 2008. We then followed this cohort over a 7-year period, recording primary outcome measures of occurrence of first seizure, status epilepticus, stroke-like episode, and death.

RESULTS

Overall prevalence of epilepsy in the cohort was 23.1%. Mean age of epilepsy onset was 29.4 years. Prevalence varied widely between genotypes, with several genotypes having no cases of epilepsy, a prevalence of 34.9% in the most common genotype (m.3243A>G mutation), and 92.3% in the m.8344A>G mutation. Among the cohort as a whole, focal seizures, with or without progression to bilateral convulsive seizures, was the most common seizure type. Conversely, all of the patients with the m.8344A>G mutation and epilepsy experienced myoclonic seizures. Patients with the m.3243A>G mutation remain at high risk of developing stroke-like episodes (1.16% per year). However, although the standardized mortality ratio for the entire cohort was high (2.86), this ratio did not differ significantly between patients with epilepsy (2.96) and those without (2.83).

INTERPRETATION

Epilepsy is a common manifestation of mitochondrial disease. It develops early in the disease and, in the case of the m.3243A>G mutation, often presents in the context of a stroke-like episode or status epilepticus. However, epilepsy does not itself appear to contribute to the increased mortality in mitochondrial disease.

Buccal swab analysis of mitochondrial enzyme deficiency and DNA defects in a child with suspected myoclonic epilepsy and ragged red fibers (MERRF)

The authors describe mitochondrial studies in a 6-year-old patient with a seizure disorder that can be seen in myoclonic epilepsy and ragged red fibers. Using a recently developed noninvasive approach, analysis of buccal mitochondrial enzyme function revealed severe respiratory complex I and IV deficiencies in the patient. In addition, analysis of buccal mitochondrial DNA showed significant amounts of the common 5 kb and 7.4 kb mitochondrial DNA deletions, also detectable in blood. This study suggests that a buccal swab approach can be used to informatively examine mitochondrial dysfunction in children with seizures and may be applicable to screening mitochondrial disease with other clinical presentations.

Diagnostic screening of mitochondrial DNA mutations in Australian adults 1990-2001

BACKGROUND

Many diverse pathogenic mitochondrial DNA (mtDNA) mutations have been described since 1988. The Melbourne Neuromuscular Research Institute (MNRI) has undertaken diagnostic detection of selected mtDNA mutations since 1990. MtDNA mutations screened have included point mutations associated with Leber's hereditary optic neuropathy (LHON; G3460A, G11778A and T14484C), mitochondrial encephalopathy lactic acidosis and stroke-like episodes (MELAS; A3243G), myoclonus epilepsy and ragged red fibres (MERRF; A8344G) and Leigh's syndrome/neuropathy ataxia retinitis pigmentosa (LS/NARP; T8993C/G). Samples have also been screened for deletions/ rearrangements associated with Kearns-Sayre syndrome (KSS) and chronic progressive external ophthalmoplegia (CPEO).

AIMS

To present an audit of the MNRI mtDNA diagnostic service between 1990 and 2001, encompassing 1725 referred patients.

METHODS

The detection techniques carried out included polymerase chain reaction amplification of mtDNA combined with restriction fragment length polymorphism analysis for mtDNA point mutation detection, supplemented with selected sequence analysis and Southern blots for the detection of deletions/ rearrangements. Tissues tested included blood, hair and skeletal muscle.

RESULTS

Of the 1184 patients screened for MELAS A3243G, 6.17% were positive for the mutation, whereas for MERRF A8344G, 2.21% carried the mutation and for LS/NARP T8993C/G, 0.32% carried the mutation. The outcomes for the LHON mutations were G11778A, 6.60%, T14484C, 5.76% and G3460A, 0.29%. Of the patients referred for KSS and CPEO, 17.72% had deletions/rearrangements.

CONCLUSIONS

Overall, the detection rate of mtDNA point mutations was low. The protean clinical features of mitochondrial disorders and the frequency of partial phenotypes lead to requests for tests in many patients with a relatively low likelihood of mtDNA mutations. An improved algorithm could involve mutation screening appropriate to the phenotype using sequencing of selected mtDNA regions in patients with a high likelihood of mtDNA disease. Features increasing the likelihood of mtDNA mutations include the following: (i) a typical phenotype, (ii) a maternal inheritance pattern and (iii) histochemical evidence of mitochondrial abnormality in the muscle biopsy. Efficient laboratory diagnosis of mtDNA disease involves good communication between the physician and laboratory scientists, coupled with screening of the appropriate tissue.

Mitochondriopathies

Mitochondriopathies (MCPs) are either due to sporadic or inherited mutations in nuclear or mitochondrial DNA located genes (primary MCPs), or due to exogenous factors (secondary MCPs). MCPs usually show a chronic, slowly progressive course and present with multiorgan involvement with varying onset between birth and late adulthood. Although several proteins with signalling, assembling, transport, enzymatic function can be impaired in MCP, most frequently the activity of the respiratory chain (RC) protein complexes is primarily or secondarily affected, leading to impaired oxygen utilization and reduced energy production. MCPs represent a diagnostic challenge because of their wide variation in presentation and course. Systems frequently affected in MCP are the peripheral nervous system (myopathy, polyneuropathy, lactacidosis), brain (leucencephalopathy, calcifications, stroke-like episodes, atrophy with dementia, epilepsy, upper motor neuron signs, ataxia, extrapyramidal manifestations, fatigue), endocrinium (short stature, hyperhidrosis, diabetes, hyperlipidaemia, hypogonadism, amenorrhoea, delayed puberty), heart (impulse generation or conduction defects, cardiomyopathy, left ventricular non-compaction heart failure), eyes (cataract, glaucoma, pigmentary retinopathy, optic atrophy), ears (deafness, tinnitus, peripheral vertigo), guts (dysphagia, vomiting, diarrhoea, hepatopathy, pseudo-obstruction, pancreatitis, pancreas insufficiency), kidney (renal failure, cysts) and bone marrow (sideroblastic anaemia). Apart from well-recognized syndromes, MCP should be considered in any patient with unexplained progressive multisystem disorder. Although there is actually no specific therapy and cure for MCP, many secondary problems require specific treatment. The rapidly increasing understanding of the pathophysiological background of MCPs may further facilitate the diagnostic approach and open perspectives to future, possibly causative therapies.

Progressive external ophthalmoplegia characterized by multiple deletions of mitochondrial DNA: unraveling the pathogenesis of human mitochondrial DNA instability and the initiation of a genetic classification

Over the last decade, many sporadic and familial cases have been reported with multiple deletions of mitochondrial DNA (mtDNA) in postmitotic tissues. Most patients suffer from progressive external ophthalmoplegia (PEO) and may have a nuclear gene defect that predisposes to the accumulation of mtDNA deletions. Recently, positional cloning has led to the discovery of mutations in four such nuclear genes. Some mutations are dominant and others recessive. In all autosomal mutations, defective mtDNA replication and/or repair are probably responsible for the generation of secondary mtDNA deletions. There are also data suggestive of a prominent pathogenic role for disturbed nucleotide metabolism. We here present a tentative genotype-phenotype correlation. Since clinical presentations are heterogeneous and overlap with different previously described clinical syndromes, we advocate the use of a genetic, instead of a clinical, classification of disorders with multiple mtDNA deletions.

Diseases caused by nuclear genes affecting mtDNA stability

Diseases caused by nuclear genes that affect mitochondrial DNA (mtDNA) stability are an interesting group of mitochondrial disorders, involving both cellular genomes. In these disorders, a primary nuclear gene defect causes secondary mtDNA loss or deletion formation, which leads to tissue dysfunction. Therefore, the diseases clinically resemble those caused by mtDNA mutations, but follow a Mendelian inheritance pattern. Several clinical entities associated with multiple mtDNA deletions have been characterized, the most frequently described being autosomal dominant progressive external ophthalmoplegia (adPEO). MtDNA depletion syndrome (MDS) is a severe disease of childhood, in which tissue-specific loss of mtDNA is seen. Mitochondrial neurogastrointestinal encephalomyopathy (MNGIE) patients may have multiple mtDNA deletions and/or mtDNA depletion. Recent reports of thymidine phosphorylase mutations in MNGIE and adenine nucleotide translocator mutations in adPEO have given new insights into the mechanisms of mtDNA maintenance in mammals. The common mechanism underlying both of these gene defects could be disturbed mitochondrial nucleoside pools, the building blocks of mtDNA. Future studies on MNGIE and adPEO pathogenesis, and identification of additional gene defects in adPEO and MDS will provide further understanding about the mammalian mtDNA maintenance and the crosstalk between the nuclear and mitochondrial genomes.

Mitochondrial dysfunction and neuromuscular disease

Mitochondrial diseases are a heterogeneous group of disorders with widely varying clinical features, due to defects in mitochondrial function. Involvement of both muscle and nerve is common in mitochondrial disease. In some cases, this involvement is subclinical or a minor part of a multisystem disorder, but myopathy and neuropathy are a major, often presenting, feature of a number of mitochondrial syndromes. In addition, mitochondrial dysfunction may play a role in a number of classic neuromuscular diseases. This article reviews the role of mitochondrial dysfunction in neuromuscular disease and discusses a rational approach to diagnosis and treatment of patients presenting with a neuromuscular syndrome due to mitochondrial disease.

A unique junctional palindromic sequence in mitochondrial DNA from a patient with progressive external ophthalmoplegia

A polymerase chain reaction (PCR) based procedure was modified to determine the deletion of mitochondrial DNA (mtDNA). The protocol consists of coamplification both of deleted and wild-type segments of mtDNA using a long PCR technique; evaluation of the deleted portion within the amplified DNA segments by restriction enzyme digestion followed by densitometrical analysis; and direct subcloning into a plasmid vector for DNA sequencing. The procedure revealed a 5.3 kb deletion of mtDNA in the biopsied muscle tissue obtained from a patient clinically diagnosed with progressive external ophthalmoplegia. The 5' and 3' sequences at both sides of the breakpoint comprise a 17 bp palindrome and 5 bp tandem repeats, suggesting that the deletion might occur through slipped mispairing and other novel mechanisms. This improved procedure has the potential to detect deletions occurring in the entire length of mtDNA, and mighty be useful for clinical screening of progressive external ophthalmoplegia.

Mitochondrial encephalomyopathies: the enigma of genotype versus phenotype

Over the past decade a large body of evidence has accumulated implicating defects of human mitochondrial DNA in the pathogenesis of a group of disorders known collectively as the mitochondrial encephalomyopathies. Although impaired oxidative phosphorylation is likely to represent the final common pathway leading to cellular dysfunction in these diseases, fundamental issues still remain elusive. Perhaps the most challenging of these is to understand the mechanisms which underlie the complex relationship between genotype and phenotype. Here we examine this relationship and discuss some of the factors which are likely to be involved.