Mitochondrial DNA mutation underlying Leigh's syndrome: clinical, pathological, biochemical, and genetic studies of a patient presenting with progressive myoclonic epilepsy

Primary cilia formation requires the Leigh syndrome-associated mitochondrial protein NDUFAF2

Mitochondria-related neurodegenerative diseases have been implicated in the disruption of primary cilia function. Mutation in an intrinsic mitochondrial complex I component NDUFAF2 has been identified in Leigh syndrome, a severe inherited mitochondriopathy. Mutations in ARMC9, which encodes a basal body protein, cause Joubert syndrome, a ciliopathy with defects in the brain, kidney, and eye. Here, we report a mechanistic link between mitochondria metabolism and primary cilia signaling. We discovered that loss of NDUFAF2 caused both mitochondrial and ciliary defects in vitro and in vivo and identified NDUFAF2 as a binding partner for ARMC9. We also found that NDUFAF2 was both necessary and sufficient for cilia formation and that exogenous expression of NDUFAF2 rescued the ciliary and mitochondrial defects observed in cells from patients with known ARMC9 deficiency. NAD+ supplementation restored mitochondrial and ciliary dysfunction in ARMC9-deficient cells and zebrafish and ameliorated the ocular motility and motor deficits of a patient with ARMC9 deficiency. The present results provide a compelling mechanistic link, supported by evidence from human studies, between primary cilia and mitochondrial signaling. Importantly, our findings have significant implications for the development of therapeutic approaches targeting ciliopathies.

Expert Panel Curation of 113 Primary Mitochondrial Disease Genes for the Leigh Syndrome Spectrum

OBJECTIVE

Primary mitochondrial diseases (PMDs) are heterogeneous disorders caused by inherited mitochondrial dysfunction. Classically defined neuropathologically as subacute necrotizing encephalomyelopathy, Leigh syndrome spectrum (LSS) is the most frequent manifestation of PMD in children, but may also present in adults. A major challenge for accurate diagnosis of LSS in the genomic medicine era is establishing gene-disease relationships (GDRs) for this syndrome with >100 monogenic causes across both nuclear and mitochondrial genomes.

METHODS

The Clinical Genome Resource (ClinGen) Mitochondrial Disease Gene Curation Expert Panel (GCEP), comprising 40 international PMD experts, met monthly for 4 years to review GDRs for LSS. The GCEP standardized gene curation for LSS by refining the phenotypic definition, modifying the ClinGen Gene-Disease Clinical Validity Curation Framework to improve interpretation for LSS, and establishing a scoring rubric for LSS.

RESULTS

The GDR with LSS across the nuclear and mitochondrial genomes was classified as definitive for 31 of 114 GDRs curated (27%), moderate for 38 (33%), limited for 43 (38%), and disputed for 2 (2%). Ninety genes were associated with autosomal recessive inheritance, 16 were maternally inherited, 5 were autosomal dominant, and 3 were X-linked.

INTERPRETATION

GDRs for LSS were established for genes across both nuclear and mitochondrial genomes. Establishing these GDRs will allow accurate variant interpretation, expedite genetic diagnosis of LSS, and facilitate precision medicine, multisystem organ surveillance, recurrence risk counseling, reproductive choice, natural history studies, and determination of eligibility for interventional clinical trials. ANN NEUROL 2023;94:696-712.

Mitochondrial Neurodegeneration

Mitochondria are cytoplasmic organelles, which generate energy as heat and ATP, the universal energy currency of the cell. This process is carried out by coupling electron stripping through oxidation of nutrient substrates with the formation of a proton-based electrochemical gradient across the inner mitochondrial membrane. Controlled dissipation of the gradient can lead to production of heat as well as ATP, via ADP phosphorylation. This process is known as oxidative phosphorylation, and is carried out by four multiheteromeric complexes (from I to IV) of the mitochondrial respiratory chain, carrying out the electron flow whose energy is stored as a proton-based electrochemical gradient. This gradient sustains a second reaction, operated by the mitochondrial ATP synthase, or complex V, which condensates ADP and Pi into ATP. Four complexes (CI, CIII, CIV, and CV) are composed of proteins encoded by genes present in two separate compartments: the nuclear genome and a small circular DNA found in mitochondria themselves, and are termed mitochondrial DNA (mtDNA). Mutations striking either genome can lead to mitochondrial impairment, determining infantile, childhood or adult neurodegeneration. Mitochondrial disorders are complex neurological syndromes, and are often part of a multisystem disorder. In this paper, we divide the diseases into those caused by mtDNA defects and those that are due to mutations involving nuclear genes; from a clinical point of view, we discuss pediatric disorders in comparison to juvenile or adult-onset conditions. The complementary genetic contributions controlling organellar function and the complexity of the biochemical pathways present in the mitochondria justify the extreme genetic and phenotypic heterogeneity of this new area of inborn errors of metabolism known as ‘mitochondrial medicine’.

Comprehensive summary of mitochondrial DNA alterations in the postmortem human brain: A systematic review

BACKGROUND

Mitochondrial DNA (mtDNA) encodes 37 genes necessary for synthesizing 13 essential subunits of the oxidative phosphorylation system. mtDNA alterations are known to cause mitochondrial disease (MitD), a clinically heterogeneous group of disorders that often present with neuropsychiatric symptoms. Understanding the nature and frequency of mtDNA alterations in health and disease could be a cornerstone in disentangling the relationship between biochemical findings and clinical symptoms of brain disorders. This systematic review aimed to summarize the mtDNA alterations in human brain tissue reported to date that have implications for further research on the pathophysiological significance of mtDNA alterations in brain functioning.

METHODS

We searched the PubMed and Embase databases using distinct terms related to postmortem human brain and mtDNA up to June 10, 2021. Reports were eligible if they were empirical studies analysing mtDNA in postmortem human brains.

FINDINGS

A total of 158 of 637 studies fulfilled the inclusion criteria and were clustered into the following groups: MitD (48 entries), neurological diseases (NeuD, 55 entries), psychiatric diseases (PsyD, 15 entries), a miscellaneous group with controls and other clinical diseases (5 entries), ageing (20 entries), and technical issues (5 entries). Ten entries were ascribed to more than one group. Pathogenic single nucleotide variants (pSNVs), both homo- or heteroplasmic variants, have been widely reported in MitD, with heteroplasmy levels varying among brain regions; however, pSNVs are rarer in NeuD, PsyD and ageing. A lower mtDNA copy number (CN) in disease was described in most, but not all, of the identified studies. mtDNA deletions were identified in individuals in the four clinical categories and ageing. Notably, brain samples showed significantly more mtDNA deletions and at higher heteroplasmy percentages than blood samples, and several of the deletions present in the brain were not detected in the blood. Finally, mtDNA heteroplasmy, mtDNA CN and the deletion levels varied depending on the brain region studied.

INTERPRETATION

mtDNA alterations are well known to affect human tissues, including the brain. In general, we found that studies of MitD, NeuD, PsyD, and ageing were highly variable in terms of the type of disease or ageing process investigated, number of screened individuals, studied brain regions and technology used. In NeuD and PsyD, no particular type of mtDNA alteration could be unequivocally assigned to any specific disease or diagnostic group. However, the presence of mtDNA deletions and mtDNA CN variation imply a role for mtDNA in NeuD and PsyD. Heteroplasmy levels and threshold effects, affected brain regions, and mitotic segregation patterns of mtDNA alterations may be involved in the complex inheritance of NeuD and PsyD and in the ageing process. Therefore, more information is needed regarding the type of mtDNA alteration, the affected brain regions, the heteroplasmy levels, and their relationship with clinical phenotypes and the ageing process.

FUNDING

Hospital Universitari Institut Pere Mata; Institut d'Investigació Sanitària Pere Virgili; Instituto de Salud Carlos III, Ministerio de Ciencia e Innovación (PI18/00514).

Leigh Syndrome: A Tale of Two Genomes

Leigh syndrome is a rare, complex, and incurable early onset (typically infant or early childhood) mitochondrial disorder with both phenotypic and genetic heterogeneity. The heterogeneous nature of this disorder, based in part on the complexity of mitochondrial genetics, and the significant interactions between the nuclear and mitochondrial genomes has made it particularly challenging to research and develop therapies. This review article discusses some of the advances that have been made in the field to date. While the prognosis is poor with no current substantial treatment options, multiple studies are underway to understand the etiology, pathogenesis, and pathophysiology of Leigh syndrome. With advances in available research tools leading to a better understanding of the mitochondria in health and disease, there is hope for novel treatment options in the future.

The molecular pathology of pathogenic mitochondrial tRNA variants

Mitochondrial diseases are clinically and genetically heterogeneous disorders, caused by pathogenic variants in either the nuclear or mitochondrial genome. This heterogeneity is particularly striking for disease caused by variants in mitochondrial DNA-encoded tRNA (mt-tRNA) genes, posing challenges for both the treatment of patients and understanding the molecular pathology. In this review, we consider disease caused by the two most common pathogenic mt-tRNA variants: m.3243A>G (within MT-TL1, encoding mt-tRNALeu(UUR) ) and m.8344A>G (within MT-TK, encoding mt-tRNALys ), which together account for the vast majority of all mt-tRNA-related disease. We compare and contrast the clinical disease they are associated with, as well as their molecular pathologies, and consider what is known about the likely molecular mechanisms of disease. Finally, we discuss the role of mitochondrial-nuclear crosstalk in the manifestation of mt-tRNA-associated disease and how research in this area not only has the potential to uncover molecular mechanisms responsible for the vast clinical heterogeneity associated with these variants but also pave the way to develop treatment options for these devastating diseases.

Leigh Syndrome as a Phenotype of Near-Homoplasmic m.8344 A>G Variant in Children

In the field of mitochondrial medicine, correlation of clinical phenotype with mutation heteroplasmy remains an outstanding question with few, if any, clear thresholds corresponding to a given phenotype. The m.8344A>G mutation is most commonly associated with myoclonus epilepsy and ragged red fiber syndrome (MERRF) at varying levels of heteroplasmy. However, a handful of cases been previously reported in which individuals homoplasmic or nearly homoplasmic for this mutation in the blood have presented with multiple bulbar palsies, respiratory failure, and progressive neurologic decline almost uniformly following a respiratory illness. MRI brain in all affected individuals revealed symmetric T2 hyperintense lesions of subcortical gray matter structures, consistent with Leigh syndrome. Here, we present 3 cases with clinical, biochemical, and neuro-imaging findings with the additional reporting of spinal lesions. This new phenotype supports a heteroplasmy-dependent phenotype model for this mutation and recognition of this can help clinicians with diagnosis and anticipatory clinical guidance.

Overlapping Leigh Syndrome/Myoclonic Epilepsy With Ragged Red Fibres in an Adolescent Patient With a Mitochondrial DNA A8344G Mutation

We present the case of a 16-year-old boy with a family history of epilepsy who presented with acute respiratory failure, limb weakness, diabetes mellitus, sinus tachycardia, lactic acidosis, and pneumonia. He went on to develop cranial nerve palsy, myoclonus, generalized seizures, ataxia, recurrent pneumonia, and hypotension. Biochemical investigation revealed elevated lactate, pyruvate, and glucose levels. Cerebral magnetic resonance imaging (MRI) revealed bilateral, symmetric, high-intensity T2-weighted signals in the thalamus, brainstem, and gray matter of the spinal cord. Histochemical analyses revealed ragged red fibers (RRF) and decreased cytochrome oxidase activity. Blood and muscle-derived DNA demonstrated a high level (95% and 96%, respectively) of the m.8344A>G mutation, while almost all of his maternal relatives (n = 17, including his mother) carried the same point mutation. The point mutation level of his mother (who had short stature, high blood lactate levels, and epilepsy) was 77% (blood-derived DNA). Although this mutation has been identified in approximately 30 individuals with these disorders, to our knowledge, this is the first reported case of overlapping Leigh syndrome/myoclonic epilepsy with RRF in an adolescent patient, and the largest reported pedigree of mitochondrial DNA A8344G mutation.

Mitochondrial ataxias

Ataxia is one of the most frequent symptoms of mitochondrial disease. In most cases it occurs as part of a syndromic disorder and the combination of ataxia with other neurologic involvement such as epilepsy is common. Mitochondrial ataxias can be caused by disturbance of the cerebellum and its connections, involvement of proprioception (i.e., sensory ataxia) or a combination of both (spinocerebellar). There are no specific features that define an ataxia as mitochondrial, except perhaps the tendency for it to occur together with involvement of multiple other sites, both in the nervous system and outside. In this review we will concentrate on the mitochondrial disorders in which ataxia is a prominent and consistent feature and focus on the clinical features and genetic causes.

Identifying and Analyzing Novel Epilepsy-Related Genes Using Random Walk with Restart Algorithm

As a pathological condition, epilepsy is caused by abnormal neuronal discharge in brain which will temporarily disrupt the cerebral functions. Epilepsy is a chronic disease which occurs in all ages and would seriously affect patients' personal lives. Thus, it is highly required to develop effective medicines or instruments to treat the disease. Identifying epilepsy-related genes is essential in order to understand and treat the disease because the corresponding proteins encoded by the epilepsy-related genes are candidates of the potential drug targets. In this study, a pioneering computational workflow was proposed to predict novel epilepsy-related genes using the random walk with restart (RWR) algorithm. As reported in the literature RWR algorithm often produces a number of false positive genes, and in this study a permutation test and functional association tests were implemented to filter the genes identified by RWR algorithm, which greatly reduce the number of suspected genes and result in only thirty-three novel epilepsy genes. Finally, these novel genes were analyzed based upon some recently published literatures. Our findings implicate that all novel genes were closely related to epilepsy. It is believed that the proposed workflow can also be applied to identify genes related to other diseases and deepen our understanding of the mechanisms of these diseases.

Leigh syndrome: Resolving the clinical and genetic heterogeneity paves the way for treatment options

Leigh syndrome is a progressive neurodegenerative disorder, affecting 1 in 40,000 live births. Most patients present with symptoms between the ages of three and twelve months, but adult onset Leigh syndrome has also been described. The disease course is characterized by a rapid deterioration of cognitive and motor functions, in most cases resulting in death due to respiratory failure. Despite the high genetic heterogeneity of Leigh syndrome, patients present with identical, symmetrical lesions in the basal ganglia or brainstem on MRI, while additional clinical manifestations and age of onset varies from case to case. To date, mutations in over 60 genes, both nuclear and mitochondrial DNA encoded, have been shown to cause Leigh syndrome, still explaining only half of all cases. In most patients, these mutations directly or indirectly affect the activity of the mitochondrial respiratory chain or pyruvate dehydrogenase complex. Exome sequencing has accelerated the discovery of new genes and pathways involved in Leigh syndrome, providing novel insights into the pathophysiological mechanisms. This is particularly important as no general curative treatment is available for this devastating disorder, although several recent studies imply that early treatment might be beneficial for some patients depending on the gene or process affected. Timely, gene-based personalized treatment may become an important strategy in rare, genetically heterogeneous disorders like Leigh syndrome, stressing the importance of early genetic diagnosis and identification of new genes/pathways. In this review, we provide a comprehensive overview of the most important clinical manifestations and genes/pathways involved in Leigh syndrome, and discuss the current state of therapeutic interventions in patients.

Late-onset Leigh syndrome with myoclonic epilepsy with ragged-red fibers

We report the case of a boy with myoclonic epilepsy with ragged-red fibers (MERRF) who had astatic seizures since 2 years of age and later developed ataxia, absence seizures, and myoclonus. Almost homoplasmic A8344G mutation of mitochondrial DNA (m.8344A>G mutation) was detected in lymphocytes. He developed late-onset Leigh syndrome (LS) when he contracted pneumonia at 6 years. He developed bulbar palsy and deep coma. MRI demonstrated lesions in the brainstem, basal ganglia, and cerebral cortex. Three similar cases have been reported; two carried the almost-homoplasmic m.8344A>G mutation in muscle tissue. These suggested that almost homoplastic m.8344A>G mutation developed clinical phenotype of MERRF in the early stage and late-onset Leigh syndrome in the late course of the disease.

Ataxia in mitochondrial disorders

Mitochondria are subcellular organelles whose major function is to generate energy by coupling through oxidation of nutrient substrates with ATP synthesis, via ADP phosphorylation. This process, known as oxidative phosphorylation, is carried out by the mitochondrial respiratory chain, a pathway consisting of five multi-subunit complexes, four of which take contribution from genes located in two separate compartments, the nuclear chromosomes, and a genome found in mitochondria themselves, mitochondrial DNA (mtDNA). Defects affecting either genome give rise to mitochondrial dysfunction, causing disease that often affects the brain and in particular the cerebellum. Mitochondrial disorders can give rise to pure cerebellar, spinocerebellar, or sensory ataxia, usually as part of a multisystem (and multisymptom) disorder. In this chapter we divide the diseases into those caused by mtDNA defects and those due to mutations involving nuclear genes. With more than 100 mutations in mtDNA and new nuclear genes being described all the time, we have focused on the commonest disorders and used these as examples of the different types of mitochondrial ataxia.

Early Developmental Pathology Due to Cytochrome c Oxidase Deficiency Is Revealed by a New Zebrafish Model

Deficiency of cytochrome c oxidase (COX) is associated with significant pathology in humans. However, the consequences for organogenesis and early development are not well understood. We have investigated these issues using a zebrafish model. COX deficiency was induced using morpholinos to reduce expression of CoxVa, a structural subunit, and Surf1, an assembly factor, both of which impaired COX assembly. Reduction of COX activity to 50% resulted in developmental defects in endodermal tissue, cardiac function, and swimming behavior. Cellular investigations revealed different underlying mechanisms. Apoptosis was dramatically increased in the hindbrain and neural tube, and secondary motor neurons were absent or abnormal, explaining the motility defect. In contrast, the heart lacked apoptotic cells but showed increasingly poor performance over time, consistent with energy deficiency. The zebrafish model has revealed tissue-specific responses to COX deficiency and holds promise for discovery of new therapies to treat mitochondrial diseases in humans.

Bilateral putaminal necrosis associated with the mitochondrial DNA A8344G myoclonus epilepsy with ragged red fibers (MERRF) mutation: an infantile case

Myoclonus epilepsy with ragged red fibers (MERRF) is one of the major mitochondrial encephalomyopathies. Its main clinical features are myoclonus epilepsy, ataxia, and myopathy with ragged red fibers. Whereas there is a close correlation between MERRF syndrome and the A8344G mutation of mitochondrial DNA, the reverse is not true. In fact, this mutation is also responsible for various other syndromes, such as Leigh syndrome, spinocerebellar degeneration, atypical Charcot-Marie-Tooth disease, and multiple truncal lipomas. We describe a child with the A8344G mutation of mitochondrial DNA and an unusual clinical, neuroradiologic, and biochemical phenotype, characterized by early-onset, nonprogressive cerebellar ataxia, and subclinical myoclonias in association with bilateral putaminal necrosis on magnetic resonance imaging and a reduction in complex V activity. Our case confirms the existence of a relationship between alteration in adenosine triphosphatase activity and basal ganglia involvement. We recommend that the possibility of a mitochondrial pathology should always be taken into consideration in the presence of bilateral symmetric lesions of the basal ganglia, even when the typical clinical picture is lacking. (J Child Neurol 2006;21:79-82).

New phenotypic diversity associated with the mitochondrial tRNA(SerUCN) gene mutation

We performed detailed clinical, histopathological, biochemical, in vitro translation and molecular genetic analysis in patients from two unrelated families harbouring the tRNA(SerUCN) 7472C-insertion mutation. Proband 1 developed a progressive neurodegenerative phenotype characterised by myoclonus, epilepsy, cerebellar ataxia and progressive hearing loss. Proband 2 had a comparatively benign phenotype characterised by isolated myopathy with exercise intolerance. Both patients had the 7472C-insertion mutation in identical proportions and they exhibited a similar muscle biochemical and histopathological phenotype. However, proband 2 also had a previously unreported homoplasmic A to C transition at nucleotide position 7472 in the tRNA(SerUCN) gene. This change lengthens further the homopolymeric C run already expanded by the 7472C-insertion. These data extend the phenotypic range associated with the 7472C-insertion to include isolated skeletal myopathy, as well as a MERRF-like phenotype.

A mutation in mitochondrial DNA-encoded cytochrome c oxidase II gene in a child with Alpers-Huttenlocher-like disease

OBJECTIVE

Cytochrome c oxidase (COX) deficiency has been demonstrated in some patients with Alpers-Huttenlocher disease, but no genetic background has been identified. Our objective was to determine the molecular defect underlying the mitochondrial respiratory chain deficiency in a child with Alpers-Huttenlocher-like progressive cerebrohepatic disease.

METHODS

The entire coding region of mitochondrial DNA was analyzed by conformation-sensitive gel electrophoresis and sequencing. Biochemical and morphologic investigations were performed on tissue biopsy material, including oximetric and spectrophotometric analyses of oxidative phosphorylation, histochemistry, and electron microscopy.

RESULTS

Postmortem histologic examination revealed a marked loss of neurons in the olivary nuclei and a spongy change in the calcarine cortex, fatty infiltration and micronodular cirrhosis of the liver, and atrophic ovaries. A novel heteroplasmic 7706G>A mutation was found in the COX II gene. The median degree of the mutant heteroplasmy was 90% in 5 tissues examined but was lower in the blood of asymptomatic maternal relatives. The distribution of the mutant heteroplasmy was skewed to the left in single muscle fibers of the proband and her mother. The 7706G>A mutation converts a hydrophobic alanine in a conserved transmembrane segment to hydrophilic threonine.

CONCLUSIONS

The 7706G>A mutation is pathogenic and may lead to impaired dioxygen transfer to the active site of COX. The clinical phenotype of this patient resembled that in Alpers-Huttenlocher disease, suggesting that analysis of mitochondrial DNA is worthwhile in patients with a progressive cerebrohepatic disease.

Mitochondrial DNA depletion in Leigh syndrome

Leigh syndrome is a heterogenous neurologic disease characterized by seizures, developmental delay, muscle weakness, respiratory abnormalities, optic abnormalities, including atrophy and ophthalmoplegia, and progressive cranial nerve degeneration with early onset in infants and children. Diagnosis can be confirmed by characteristic pathologic findings of necrosis in the basal ganglia, thalamus, and brainstem. Severe dysfunction of mitochondrial energy metabolism is generally present and involved in the etiology of this degenerative central nervous system disease. At the molecular level, a number of point mutations have been located in mitochondrial DNA genes, including ATPase6 and tRNA(Lys) genes, and in nuclear genes encoding subunits of oxidative enzymes, such as pyruvate dehydrogenase. Biochemically these mutations are responsible for enzymatic defects in either respiratory complexes (I, IV, or V) or pyruvate dehydrogenase. We describe here the first case of Leigh syndrome with marked depletion of mitochondrial DNA levels in skeletal muscle and abnormal activities in skeletal muscle of mitochondrial respiratory complexes I, III, IV, and V.

Clinical differences in patients with mitochondriocytopathies due to nuclear versus mitochondrial DNA mutations

Defects in oxidative phosphorylation (OXPHOS) are genetically unique because the different components involved in this process, respiratory chain enzyme complexes (I, III, and IV) and complex V, are encoded by nuclear and mitochondrial genome. The objective of the study was to assess whether there are clinical differences in patients suffering from OXPHOS defects caused by nuclear or mitochondrial DNA (mtDNA) mutations. We studied 16 families with > or = two siblings with a genetically established OXPHOS deficiency, four due to a nuclear gene mutation and 12 due to a mtDNA mutation. Siblings with a nuclear gene mutation showed very similar clinical pictures that became manifest in the first years (ranging from first months to early childhood). There was a severe progressive course. Seven of the eight children died in their first decade. Conversely, siblings with a mtDNA mutation had clinical pictures that varied from almost alike to very distinct. They became symptomatic at an older age (ranging from childhood to adulthood), with the exception of defects associated with Leigh or Leigh-like phenotype. The clinical course was more gradual and relatively less severe; four of the 26 patients died, one in his second year, another in her second decade and two in their sixth decade. There are differences in age at onset, severity of clinical course, outcome, and intrafamilial variability in patients affected of an OXPHOS defect due to nuclear or mtDNA mutations. Patients with nuclear mutations become symptomatic at a young age, and have a severe clinical course. Patients with mtDNA mutations show a wider clinical spectrum of age at onset and severity. These differences may be of importance regarding the choice of which genome to study in affected patients as well as with respect to genetic counseling.

Disconnection of cerebellar Purkinje cells in Kearns-Sayre syndrome

Kearns-Sayre syndrome (KSS) is a sporadic multisystem disorder due to rearrangements in mitochondrial DNA (mtDNA). To gain further insight into the pathogenesis of cerebellar dysfunction in KSS, antibodies against synaptophysin (SY) were used to identify presynaptic terminals and antibodies to calbindin D (CB) to identify Purkinje cells in the cerebellar cortex and in the dentate nucleus from two autopsied cases of KSS. By conventional neuropathology we found marked spongiform degeneration and by immunohistochemistry a disruption of presynaptic terminals and of the terminal arborizations of Purkinje cell axons on multipolar neurons of the dentate nucleus in the KSS patients. We suggest that a disconnection of Purkinje cells at the dentate nucleus may play a role in the pathogenesis of cerebellar ataxia in KSS.

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.

Phenotypic variability in a family with a mitochondrial DNA T8993C mutation

Two patients are described in a family with a mitochondrial DNA T8993C point mutation. Patient 1, the proband, was a 4-year-old male, and his clinical features were consistent with those of Leigh syndrome, including lactic acidosis, motor development delay, and symmetric basal ganglia lesions on magnetic resonance imaging (MRI). His mental development was delayed mildly, but he has not demonstrated neurologic deterioration. Patient 2 was his maternal aunt. She developed her first neurologic sign at 18 months of age, thereafter her development ceased and regressed. She had lost her head control and become bedridden by 4 years of age and died at 20 years of age, demonstrating a more severe clinical course than that of Patient 1. Analysis of mitochondrial DNA from peripheral leukocytes of Patient 1 revealed a T8993C mutation of 99%. Patient 2 was demonstrated to have the same mutation at high abundance (99%) in the frozen myocardium and in the formaldehyde preserved spinal cord, with only 18% mutant mitochondrial DNA present in the formaldehyde preserved sciatic nerve. The mother of Patient 1, who was phenotypically normal (sister of Patient 2), had 35% mutant mitochondrial DNA in peripheral leukocytes. The authors' findings suggest that T8993C phenotypes are highly variable and that the proportion of the mutant mitochondrial DNA may vary among tissues and not correlate well with clinical severity.

Clinical and laboratory findings in referrals for mitochondrial DNA analysis

BACKGROUND

Increasingly, mutations of mitochondrial DNA (mtDNA) are being considered when investigating the aetiology of neurological diseases in childhood. However, they are often difficult to predict clinically.

METHOD

Mitochondrial DNA analysis was carried out on 190 children from 1992 to 1996. Most patients were screened for large scale rearrangements and point mutations at nucleotide positions 3243, 3271, 8344, and 8993.

RESULTS

Mutations were found in only 15 patients (7.9%) and were either large scale rearrangements (seven patients) or point mutations at nucleotide position 3243 (eight patients). Other point mutations were screened for depending on the clinical picture. The age of symptom onset was significantly older in children with an mtDNA mutation (mean 7.0 years) compared with children without a mutation (mean 2.8 years). Neither Leigh's syndrome (28 cases) nor severe infantile lactic acidosis (12 cases) was associated with mtDNA mutation. Only three clinical features were significantly associated with an mtDNA mutation: progressive external ophthalmoplegia, myopathy, and pigmentary retinopathy. Family history was valuable: the point mutation at nucleotide 3243 (but not the large scale rearrangements) was associated with maternal inheritance; and consanguinity was not associated with mtDNA mutations. The only investigation that provided specific evidence of an underlying mtDNA mutation was histochemical staining of muscle biopsy specimens. The large scale mutations associated with Kearns-Sayre syndrome and progressive external ophthalmoplegia were found in DNA from muscle only, not leucocyte DNA; whereas point mutations were found in leucocyte DNA.

CONCLUSIONS

Even among children seen at a neurogenetic referral centre, mtDNA mutations were very uncommon. Muscle biopsy was the only investigation to provide evidence of mtDNA abnormality.

Gene dosage effect in one family with myoclonic epilepsy and ragged-red fibers (MERRF)

OBJECTIVES

We analyzed the percentage of mitochondrial DNA (mtDNA) heteroplasmy in blood samples of 13 individuals belonging to a three family generation of myoclonic epilepsy with ragged-red fibers (MERRF) and compared the 5 affected patients and the 8 unaffected relatives.

MATERIAL AND METHODS

DNA was extracted from blood and muscle of the proband and from blood of 12 maternal relatives. A PCR restriction analysis method was used to detect the mutation.

RESULTS

The proband had the complete MERRF phenotype. The phenotype in three other individuals in the maternal lineage was consistent with the MERRF syndrome. The remaining were asymptomatic. The np 8344 mutation was observed in muscle and blood of the proband, and in blood from every one of 12 maternal relatives, ranging from 44% to 83% of mutated genomes. Symptomatic individuals had higher levels (P < 0.001) of mutated mtDNA than asymptomatic maternal relatives. However, high proportions of mutant genomes (up to 63%) were found in asymptomatic relatives.

CONCLUSIONS

Although there seems to be a gene dosage effect in MERRF, we found no absolute relationship between the relative proportion of mutant genomes in blood and clinical severity. Factors other than gene dosage in blood may account for the differences in clinical phenotype.

Biochemical analysis of fibroblasts from patients with cytochrome c oxidase-associated Leigh syndrome

Cultured skin fibroblasts from four patients with Leigh syndrome and cytochrome c oxidase deficiency were studied. Mitochondrial DNA (mtDNA) analysis excluded large-scale deletions and known point mutations associated with Leigh syndrome. The COX activities were reduced to 18-44% of healthy probands, when measured in the presence of laurylmaltoside. COX activity from patients was shown to be more temperature sensitive than COX activity from control cells. In order to determine the subunit composition of COX immunoblotting studies were performed using mono- and polyclonal antibodies to distinct subunits. A monoclonal antibody to subunit IV crossreacted with two unknown proteins of higher apparent molecular weight in mitochondria from three patients, but not in mitochondria from control and the fourth patient. Quantification of immunoreactivity revealed a decrease of subunits II/III and IV parallel to the determined enzyme activity. In contrast, a variable amount of subunit VIIa (and/or VIIb) was found in mitochondria from different patients. The results indicate a defective COX holoenzyme complex in patients with Leigh syndrome and suggest different molecular origins of the defect.

Deficiency of respiratory chain complex I is a common cause of Leigh disease

We reviewed the clinical features and etiologies of Leigh disease in 66 patients from 60 pedigrees. Biochemical or molecular defects were identified in 50% of all pedigrees, and in 74% of the 19 pedigrees with pathologically proved Leigh disease. Isolated deficiency of respiratory chain complex I was found in 7 patients, though the complex was only assayed in 25 patients, making this the second most common biochemical abnormality after complex IV deficiency. Mutations at residue 8993 of mitochondrial DNA were found in only 2 patients. No correlation was found between the clinical features and etiologies. No defects were identified in the 8 patients with normal lactate concentrations in the cerebrospinal fluid.

Tissue distribution and disease manifestations of the tRNA(Lys) A-->G(8344) mitochondrial DNA mutation in a case of myoclonus epilepsy and ragged red fibres

This man with myoclonus epilepsy and ragged red fibres (MERRF) syndrome due to the tRNA(Lys) A-->G(8344) mutation of mitochondrial DNA (mtDNA) died of bronchopneumonia at 18 years of age. He had progressive clinical symptoms from 6 months of age manifesting as ataxia, myoclonic seizures, and muscle weakness. A post-mortem examination revealed 91-99% mutated mtDNA in all 32 examined tissue samples, including various organs and different brain regions. The brain appeared without macroscopic changes, but microscopic examination showed degeneration with loss of nerve cells and gliosis affecting the globus pallidus, substantia nigra, red nucleus, dentate nucleus, inferior olivary nucleus, cerebellar cortex, and the spinal cord. Skeletal muscle showed cytochrome c oxidase deficient muscle fibres with proliferation of mitochondria. In addition to pathological changes of muscle and brain there were few morphological changes that could be attributed to his mitochondrial disease. These data support the concept that in patients with the tRNA(Lys) A-->G(8344) mutation who are manifesting disease there are high levels of mutated mtDNA in all tissues, but only some tissues and brain regions are vulnerable.

A case of recurrent headache and neurological deficit

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Kearns-Sayre syndrome associated with mitochondrial DNA deletion or duplication: a molecular genetic and pathological study

The neuropathological findings in 2 patients with Kearns-Sayre syndrome and mitochondrial DNA (mtDNA) rearrangements, one a predominant deletion and the other a predominant duplication, were remarkably similar, showing diffuse vacuolation of white matter. There were some of the pathological features of Leigh's syndrome in the spinal cord of the patient with a duplication. In the patient with a predominant deletion, rearranged mtDNA was undetectable in blood, spleen, and testis, and present in highest amounts in muscle and the brain, but relatively low in cerebellum, reflecting the ratio seen, albeit in much smaller amounts, in normal aged brains. MtDNA rearrangements in this patient were largely deletions or deletion dimers; duplicated mtDNA was present in only trace amounts in some tissues and there was none in skeletal muscle. The patient with a predominant duplication of mtDNA had higher amounts of rearranged mtDNA in blood (mainly duplicated) than muscle (mainly deleted). Correlation of these data with tissue dysfunction is probably complicated by the replicative behaviour of deleted, duplicated and normal mtDNA.

Impaired mitochondrial translation in human myoblasts harbouring the mitochondrial DNA tRNA lysine 8344 A-->G (MERRF) mutation: relationship to proportion of mutant mitochondrial DNA

The mitochondrial DNA transfer RNA lysine A8344G mutation is commonly associated with the MERRF (myoclonus epilepsy with ragged red fibre) phenotype. The molecular pathogenesis of disease associated with this mutation is unclear. Theoretically, a mitochondrial tRNA mutation might affect transcription or translation, or both. We therefore studied these processes in cloned primary human myoblast cultures containing different proportions of mutant mtDNA. No abnormality of transcription was observed. However, there was a progressive decrease in mitochondrially encoded protein synthesis as the proportion of mutant mtDNA increased. Furthermore, there was evidence that subunits were differentially affected, based on selective reduction of cytochrome c oxidase subunits with relatively low proportions of mutant mtDNA.

Mitochondrial DNA (mtDNA) diseases: correlation of genotype to phenotype

This study examines the relationship of genotype to phenotype in 14 unselected patients who were found to harbour the A3243G transition in the mitochondrial transfer RNALeu(UUR) gene commonly associated with the syndrome of mitochondrial encephalopathy, lactic acidosis and strokes (MELAS). Only 6 of the 14 cases (43%) had seizures and recurrent strokes, the core clinical features of the MELAS phenotype. Of the remaining cases, four had an encephalomyopathy with deafness, ataxia and dementia, two had syndromes with progressive external ophthalmoplegia and two had limb weakness alone. Even within the MELAS subgroup, the majority of patients had one or more clinical manifestations considered to be atypical of the MELAS syndrome. They included developmental delay, ophthalmoparesis, pigmentary retinopathy and intestinal pseudo-obstruction. The proportion of mutant mitochondrial DNA (mtDNA) in muscle was generally higher in patients with recurrent strokes than in those without strokes, the highest levels being observed in MELAS cases with early onset disease. Studies of isolated muscle mitochondria identified a range of respiratory chain abnormalities mostly involving Complex I; immunoblots of Complex I in 3 of 10 cases showed selective loss of specific subunits encoded by nuclear genes. In the group as a whole, however, no clear correlations were observed between the severity or extent of the respiratory chain abnormality and clinical phenotype or the proportion of mutant mtDNA in biopsied skeletal muscle. These discrepancies suggest that, in patients harbouring the common MELAS3243 mutation, differences in heteroplasmy and the proportions of mutant mtDNA may not be the sole determinants of disease expression and that additional genetic mechanisms are involved in defining the range of clinical and biochemical phenotypes associated with this aberrant mitochondrial genome.

A new mitochondrial DNA mutation associated with progressive dementia and chorea: a clinical, pathological, and molecular genetic study

A novel mitochondrial DNA transfer RNA mutation at position 5549 was identified in a patient with dementia, chorea, cerebellar ataxia, deafness, and peripheral neuropathy in the absence of clinical myopathy. Muscle biopsy specimens showed ragged red and cytochrome oxidase-negative fibers, and reduced complex I activity on polarography. There was diffuse neuronal loss and gliosis throughout the brain on postmortem examination. The heteroplasmic mutation had a widespread distribution in autopsy tissues.