Leigh and Leigh-like syndrome in children and adults

Leigh syndrome (also termed subacute, necrotizing encephalopathy) is a devastating neurodegenerative disorder, characterized by almost identical brain changes, e.g., focal, bilaterally symmetric lesions, particularly in the basal ganglia, thalamus, and brainstem, but with considerable clinical and genetic heterogeneity. Clinically, Leigh syndrome is characterized by a wide variety of abnormalities, from severe neurologic problems to a near absence of abnormalities. Most frequently the central nervous system is affected, with psychomotor retardation, seizures, nystagmus, ophthalmoparesis, optic atrophy, ataxia, dystonia, or respiratory failure. Some patients also present with peripheral nervous system involvement, including polyneuropathy or myopathy, or non-neurologic abnormalities, e.g., diabetes, short stature, hypertrichosis, cardiomyopathy, anemia, renal failure, vomiting, or diarrhea (Leigh-like syndrome). In the majority of cases, onset is in early childhood, but in a small number of cases, adults are affected. In the majority of cases, dysfunction of the respiratory chain (particularly complexes I, II, IV, or V), of coenzyme Q, or of the pyruvate dehydrogenase complex are responsible for the disease. Associated mutations affect genes of the mitochondrial or nuclear genome. Leigh syndrome and Leigh-like syndrome are the mitochondrial disorders with the largest genetic heterogeneity.     

The molecular background of Leigh syndrome

Leigh syndrome (subacute necrotizing encephalomyopathy, MIM 256,000) is a progressive neurodegenerative disorder of infancy and childhood, with characteristic pathological hallmarks including symmetric necrotizing lesions in the brainstem, basal ganglia, thalamus and spinal cord. It may result from several defects of mitochondrial enzyme complexes, including pyruvate dehydrogenase complex, and respiratory chain complexes I, II, III, IV, V. Clinical presentation mostly includes failure to thrive, developmental delay, muscle weakness, hypotonia, disorders of ocular movements, abnormal respiratory rate and bulbar dysfunction. Symptoms usually start after a few months of normal development and the course is typically rapid and relentless. Affected patients usually die before 5 years of life due to central ventilation failure. Leigh syndrome occurs with an estimated frequency of 1:77,000-1:34,000 live births. The disease demonstrates maternal, X-linked, and autosomal recessive inheritance.     

Mitochondrial cytopathies and neuromuscular disorders

In the last decade, mitochondrial diseases were shown not to be rare but to represent an important group of metabolic disorders. Defects are caused by mutations either located in nuclear genes or in mitochondrial genes. Nuclear gene defects are found in complex IV deficient and complex I deficient patients. Deficiencies of complex II are extremely rare. Different phenotypes are associated with complex IV deficiency, including a neonatal form, cardio-encephalomyopathy in young infants, Leigh syndrome, and pure myopathy. Mutations can be found in the complex IV assembly genes, such as the SURF-1 gene and the SCO2 gene. Different phenotypes are also found in complex I deficient patients and include a neonatal form, Leigh syndrome, pure myopathy, pure cardiomyopathy or multiple-system involvement. In some disorders, the mitochondrial DNA abnormalities are caused by a nuclear gene defect (Alpers-Huttenlocher syndrome, autosomal dominant multiple mitochondrial DNA deletion syndrome, and MNGIE syndrome). Since 1988, more then 70 different mutations were reported in the mitochondrial DNA. Some point mutations are associated with a specific phenotype, others have a wide range of clinical symptoms. We expect that many more mitochondrial DNA mutations will be identified in the future. The number of mutations in nuclear genes will also increase, especially since progress has been made in techniques used for identification of nuclear genes (microcell transfer).     

Juvenile Leigh syndrome, optic atrophy, ataxia, dystonia, and epilepsy due to T14487C mutation in the mtDNA-ND6 gene: a mitochondrial syndrome presenting from birth to adolescence

An increasing number of reports describe mutations in mitochondrial DNA coding regions, especially in mitochondrial DNA- encoded nicotinamide adenine dinucleotide dehydrogenase subunit genes of the respiratory chain complex I, as causing early-onset Leigh syndrome. The authors report the molecular findings in a 24-year-old patient with juvenile-onset Leigh syndrome presenting with optic atrophy, ataxia dystonia, and epilepsy. A brain magnetic resonance imaging revealed bilateral basal ganglia and thalamic hypointensities, and a magnetic resonance spectroscopy revealed an increased lactate peak. The authors identified a T14487C change causing M63V substitution in the mitochondrial ND6 gene. The mutation was heteroplasmic in muscle and blood samples, with different mutation loads, and was absent in the patient's mother's urine and blood samples. They suggest that the T14487C mtDNA mutation should be analyzed in Leigh syndrome, presenting with optic atrophy, ataxia, dystonia, and epilepsy, regardless of age.     

Multiple defects of the mitochondrial respiratory chain in a mitochondrial encephalopathy (MERRF): a clinical, biochemical and molecular study.

We describe a young man with a progressive neurological disorder including myoclonus, mental retardation, muscle weakness and a mitochondrial myopathy (myoclonus epilepsy and ragged red fibres--MERRF). Multiple abnormalities of the mitochondrial respiratory chain in skeletal muscle are shown by direct measurement of the flux through the individual complexes, low-temperature redox spectroscopy and decreased immunodetectable subunits of complexes I and IV by immunoblotting. No abnormality of mitochondrial DNA was found. This is the first report of combined defects of complexes I, III and IV as a cause of this clinical syndrome. However, we propose that the occurrence of multiple respiratory chain defects may be more common than previously recognised and that this particular combination of defects, involving complexes I, III and IV, may be the predominant biochemical abnormality in MERRF.     

Mitochondrial function and mitochondrial DNA in a series of 64 patients suspected of having mitochondrial myopathy

Biochemical results concerning 64 patients suspected of mitochondrial myopathies are presented. Four clinical groups were studied including 21 encephalomyopathies, 42 ocular myopathies, 8 isolated myopathies and 3 cardiomyopathies. In 26 cases, the coexistence of a normal mitochondrial DNA and a mutated mitochondrial DNA (heteroplasmy) was found (19 simple deletions, 4 multiple deletions and 3 punctual mutations) and all cases presented with ocular disorders (excepted 2 cases with MERRF). Furthermore, 1 complex I deficiency (1 ocular myopathy), 1 complex IV deficiency (1 adult encephalomyopathy type Leigh), 3 complexes I + IV deficiencies (2 cases with a cardiomyopathy and 1 familial MELAS) and 2 pyruvate (1 adult from of Leigh's encephalomyopathy) dehydrogenase deficiencies (clinically and genetically different) did not show evidence of mitochondrial DNA mutation.     

Cardiomyopathy associated with neurologic disorders and mitochondrial phenotype

Cardiomyopathy and neuromuscular abnormalities may simultaneously coexist and present with defects in mitochondrial DNA and bioenergetic function. We sought to evaluate the relationship between clinical and mitochondrial phenotypes in 28 young patients with both cardiomyopathy and neurologic disorders including seizures, dystonia, ophthalmoplegia, Kearns-Sayre syndrome, Leigh disease, and Friedreich's ataxia. All tissues examined displayed marked defects in respiratory complex activities. Five patients had abundant large-scale mitochondrial DNA deletions and one patient displayed a pathogenic point mutation previously reported with mitochondrial cytopathy. In this cohort, patients with hypertrophic cardiomyopathy displayed a higher incidence of complex I defects, fewer DNA deletions and mitochondrial structural abnormalities and were less often associated with developmental delay phenotype compared with patients with dilated cardiomyopathy. Although structural abnormalities are present in a subset of patients, evaluation of respiratory enzyme activity appears to be most informative whether tissues examined were derived from heart or skeletal muscle. Defects in mitochondrial DNA and bioenergetics are frequently present in children with cardiomyopathy presenting with a variety of neurologic abnormalities and are amenable to biochemical and molecular analysis.     

Immunocytological and histochemical correlation in Kearns-Sayre syndrome with mtDNA deletion and partial cytochrome c oxidase deficiency in skeletal muscle

We report histochemical, immunocytochemical, biochemical and molecular studies of skeletal muscle from a 23-year-old man with Kearns-Sayre syndrome. Southern blot analysis revealed a 4.7 kb heteroplasmic deletion of the mitochondrial DNA mapping within genes coding for subunits of complexes I, IV and V of the respiratory chain and for tRNA. Cytochrome c oxidase activity was decreased by 30% in isolated muscle mitochondria, without alteration of the Km. Histochemical and immunocytochemical correlation studies for cytochrome c oxidase revealed a lack of activity in 34% of individual muscle fibers including all the typical ragged-red fibers and a low percentage of immunodeficient fibers.     

Mitochondrial disorders

Mitochondrial disorders associated with defects in the respiratory chain can be attributable to mutations in the mitochondrial genome (mitochondrial DNA) or the nuclear genome (nuclear DNA). Because the brain is highly dependent on oxidative metabolism, encephalopathy is a common presentation, and epilepsy is a clinical hallmark of many of these conditions. Although most mutations in mitochondrial DNA do not present in infancy, a few mutations in the adenosine triphosphatase gene cause maternally inherited Leigh disease and infantile epilepsy. Early-onset epilepsy is more commonly associated with defects of nuclear genes encoding subunits of respiratory chain complexes or proteins needed for the correct assembly and functioning of the complexes. These defects generally cause autosomal recessive Leigh disease. In this review, the frequency and types of epilepsy (particularly early-onset seizures) are compared according to a genetic classification of the mitochondrial disorders.     

Biochemical and genetic analysis of Leigh syndrome patients in Korea

Sixteen Korean patients with Leigh syndrome were identified at the Seoul National University Children’s Hospital in 2001–2006. Biochemical or molecular defects were identified in 14 patients (87.5%). Thirteen patients had respiratory chain enzyme defects; 9 had complex I deficiency, and 4 had combined defects of complex I + III + IV. Based on the biochemical defects, targeted genetic studies in 4 patients with complex I deficiency revealed two heteroplasmic mitochondrial DNA mutations in ND genes. One patient had the mitochondrial DNA T8993G point mutation. No mitochondrial DNA defects were identified in 11 (68.7%) of our LS patients, who probably have mutations in nuclear DNA. Although a limited study based in a single tertiary medical center, our findings suggest that isolated complex I deficiency may be the most common cause of Leigh syndrome in Korea.     

Progressive myopathy with a combined respiratory chain defect including Complex II

Biochemical defects in the respiratory chain are mostly associated with deficiencies in Complexes I, III and IV, caused by nuclear or mitochondrial DNA mutations. Combined defects including Complex II have been reported very rarely and have muscular symptoms as the main manifestation, including muscle weakness, exercise intolerance and myoglobinuria. We report a patient with a fatal progressive myopathy and muscle biopsy showing diffuse reduction in succinate dehydrogenase activity, ragged red fibers and intense lipid accumulation. Cytochrome c oxidase (COX) histochemistry demonstrated 30% of fibers with increased subsarcolemmal staining while 27% were COX negative. Western blotting analysis showed reduction in the expression of the 39 kDa subunit of Complex I, subunit II of Complex IV and the 70 kDa subunit of Complex II. Our findings suggest that the patient had a complex pattern of mitochondrial dysfunction affecting multiple respiratory chain complexes (I, II and IV) and fatty acid metabolism. This report adds a new histological pattern associated to combined deficiencies of respiratory chain with involvement of Complex II and shows that this disease may be fatal with a rapid progression.     

Nuclear gene defects in mitochondrial disorders

An increasing number of nuclear genes have been associated with abnormalities of oxidative phosphorylation and mitochondrial disorders. The protein products of these genes can be grouped into three categories: structural components of the respiratory chain, factors influencing the structural integrity or the copy number of mitochondrial DNA, and proteins which control the formation, assembly and turnover of the respiratory complexes. Loss-of-function mutations in SURF-1, a gene belonging to the third category, have been associated with Leigh syndrome with cytochrome c oxidase deficiency. Mature Surf-1 protein (Surf-1p) is a 30 kDa hydrophobic polypeptide whose function is still unknown. Using antibodies against human Surf-1p, we demonstrated that this protein is imported into mitochondria as a larger precursor. The same analysis revealed that no protein is present in cell lines harboring loss-of-function mutations of SURF-1, regardless of their type and position. We also generated several constructs with truncated or partially deleted SURF-1 cDNAs. None of these constructs, expressed into SURF-1 null mutant cells, were able to rescue the COX phenotype, suggesting that different regions of the protein are all essential for function. Finally, experiments based on 2D gel electrophoresis indicated that assembly of COX in SURF-1 null mutants is blocked at an early step, most likely before the incorporation of subunit II in the nascent intermediates composed of subunit I alone or subunit I plus subunit IV.     

Mitochondrial dysfunction in myofibrillar myopathy

'Myofibrillar myopathy' defines a myopathic condition with focal myofibrillar destruction and accumulation of degraded myofibrillar elements. Despite the fact that a number of mutations in different genes as well as cytotoxic agents lead to the disease, abnormal accumulation of desmin is a typical, common feature. Pathological changes of mitochondrial morphology and function have been observed in animal models with intermediate filament pathology. Therefore, in the present study we tested for mitochondrial pathology in skeletal muscle of five patients with the pathohistological diagnosis of myofibrillar myopathy. Screening for large-scale mtDNA deletions and the frequent MERRF (myoclonic epilepsy; ragged red fibres) and MELAS (mitochondrial encephalomyopathy; lactic acidosis; stroke) point mutations was negative in all patients. Histologically, all muscle biopsies showed nonspecific abnormalities of the oxidative/mitochondrial enzyme stainings (histochemistry for reduced nicotinamide adenine dinucleotide, succinic dehydrogenase, cytochrome c oxidase), only one of them had ragged red fibres and a significant number of cytochrome c oxidase-negative fibres. Upon biochemical investigation, four of our patients showed pathologically low respiratory chain complex I activities. Only one of our patients had a pathologically low complex IV activity, while the measurements of the others were within low normal range. The single patient with pathological values for both complex I and IV was the one with the clear histological hallmarks (ragged red and cytochrome c oxidase-negative fibres) of mitochondrial pathology. She also was the only patient with clinical signs hinting at a mitochondrial disorder. Together with data from observations in desmin- and plectin-deficient mice, our results support the view that desmin intermediate filament pathology in these cases is closely linked to mitochondrial dysfunction in skeletal muscle.     

The expanding phenotype of mitochondrial myopathy

PURPOSE OF REVIEW: Our understanding of mitochondrial diseases (defined restrictively as defects in the mitochondrial respiratory chain) continues to progress apace. In this review we provide an update of information regarding disorders that predominantly or exclusively affect skeletal muscle. RECENT FINDINGS: Most recently described mitochondrial myopathies are due to defects in nuclear DNA, including coenzyme Q10 deficiency, and mutations in genes that control mitochondrial DNA (mtDNA) abundance and structure such as POLG and TK2. Barth syndrome, an X-linked recessive mitochondrial myopathy/cardiopathy, is associated with altered lipid composition of the inner mitochondrial membrane, but a putative secondary impairment of the respiratory chain remains to be documented. Concerning the 'other genome', the role played by mutations in protein encoding genes of mtDNA in causing isolated myopathies has been confirmed. It has also been confirmed that mutations in tRNA genes of mtDNA can cause predominantly myopathic syndromes and - contrary to conventional wisdom - these mutations can be homoplasmic. SUMMARY: Defects in the mitochondrial respiratory chain impair energy production and almost invariably involve skeletal muscle, causing exercise intolerance, myalgia, cramps, or fixed weakness, which often affects extraocular muscles and results in droopy eyelids (ptosis) and progressive external ophthalmoplegia.     

Re-evaluation of the dysfunction of mitochondrial respiratory chain in skeletal muscle of patients with Parkinson’s disease

Summary. The origin and tissue distribution of the mitochondrial dysfunction in Parkinsons disease (PD) remains still a matter of controversy. To re-evaluate a probably free radical-born, generalized mitochondrial impairment in PD, we applied optimized enzymatic assays, high resolution oxygraphic measurements of permeabilized muscle fibers, and application of metabolic control analysis to skeletal muscle samples of 19 PD patients and 36 age-matched controls. We detected decreased activities of respiratory chain complexes I and IV being accompanied by increased flux control coefficients of complexes I and IV on oxygen consumption of muscle fibers. We further investigated if randomly distributed point mutations in two discrete regions of the mitochondrial DNA are increased in PD muscle, and if they could contribute to the mitochondrial impairment. Our data confirm the previously debated presence of a mild mitochondrial defect in skeletal muscle of patients with PD which is accompanied with an about 1.5 to 2-fold increase of point mutated mtDNA.     

Cerebral white matter involvement in children with mitochondrial encephalopathies

In childhood mitochondrial encephalopathies the common MRI features are bilateral symmetric abnormalities in basal nuclei and brainstem. The presence of diffuse white matter abnormality has been described only in a few cases. Among a series of 110 children with mitochondrial encephalopathies, 8 patients with MR imaging consistent with a leukoencephalopathy were retrospectively evaluated. Diagnosis was based on the recognition of the biochemical defect in muscle homogenate. H-MR spectroscopic imaging was performed in six of them. Biochemical analysis demonstrated a defect of respiratory chain complexes in six patients: complex I in two cases, complex II in two, complex IV in one, multiple complexes defect in one. Pyruvate dehydrogenase deficiency was demonstrated in two patients. MRI showed severe involvement of the brain white matter without significant basal nuclei or brainstem abnormalities. Two patients developed large cystic areas since onset; in two others progressive vacuolisation of affected white matter was seen later in the course of the disease. One patient with pyruvate dehydrogenase deficiency also presented with a diffuse cortical polymicrogyria. H-MR spectroscopic imaging showed a decrease of N-acetylaspartate, choline and creatine with lactate accumulation in five patients, and was normal in one. These findings suggest that mitochondrial disorders should be included in the differential diagnosis of white matter disorders.     

Leigh encephalopathy: histologic and biochemical analyses of muscle biopsies.

To elucidate the pathogenesis of Leigh encephalopathy, histologic, biochemical, and mitochondrial DNA analyses were performed on biopsied muscles from 33 patients with the clinical characteristics of this disorder. On muscle histochemistry, cytochrome c oxidase activity was decreased or absent in 7 patients (21%), although none had ragged-red fibers. In 2 patients with cytochrome c oxidase deficiency, staining for this enzyme was poor in the muscle fibers and fibroblasts but was normal in the arterial wall, indicating tissue-specific involvement. Ten patients (30%) had biochemical defects, including 2 with pyruvate dehydrogenase complex, 4 with cytochrome c oxidase, 1 with NADH-cytochrome c reductase (complex I), and 3 with multiple complex deficiencies. None of the 28 patients in whom muscle mitochondrial (mt)DNA was analyzed had DNA deletions or point mutation at nucleotide positions 3,243 or 8,344. These results indicate that the underlying defect in Leigh encephalopathy is heterogeneous because only 30% of patients had enzyme defects demonstrable in muscle biopsy material.     

Antimitochondrial autoantibodies of primary biliary cirrhosis as a novel probe in the study of 2-oxo acid dehydrogenases in patients with mitochondrial myopathies

Autoantibodies present in the autoimmune disease primary biliary cirrhosis react by immunoblotting with four human skeletal muscle mitochondrial antigens of 70 kDa, 52 kDa, 50 kDa and 45 kDa, identified as the lipoate acetyl transferases (E2) of the pyruvate dehydrogenase, component X of E2 pyruvate dehydrogenase, E2 of 2-oxo glutarate dehydrogenase and E2 of branched-chain 2-oxo acid dehydrogenase complexes respectively. These autoantibodies have been employed as a novel probe to study whether there is a defect in the synthesis of the 2-oxo acid dehydrogenase complexes in patients with mitochondrial respiratory chain disorders. The reactive antigens are present normally in four patients with oculomyopathy in whom partial deletions of the mtDNA have been detected, and in two patients with MERRF and MELAS encephalomyopathy. Thus, unlike in the yeast Saccharomyces cerevisiae, there appear to be no regulatory interactions which coordinate the assembly of the mitochondrial respiratory chain with the development of the pyruvate dehydrogenase complex, which plays an important role in regulating the flow of metabolic intermediates to oxidative energy metabolism.     

Leigh syndrome associated with mitochondrial complex I deficiency due to a novel mutation in the NDUFS1 gene

BACKGROUND: Mutations in the nuclear-encoded subunits of complex I of the mitochondrial respiratory chain are a recognized cause of Leigh syndrome (LS). Recently, 6 mutations in the NDUFS1 gene were identified in 3 families. OBJECTIVE: To describe a Spanish family with LS, complex I deficiency in muscle, and a novel mutation in the NDUFS1 gene. DESIGN: Using molecular genetic approaches, we identified the underlying molecular defect in a patient with LS with a complex I defect. PATIENT: The proband was a child who displayed the clinical features of LS. RESULTS: Muscle biochemistry results showed a complex I defect of the mitochondrial respiratory chain. Sequencing analysis of the mitochondrial DNA-encoded ND genes, the nuclear DNA-encoded NDUFV1, NDUFS1, NDUFS2, NDUFS4, NDUFS6, NDUFS7, NDUFS8, and NDUFAB1 genes, and the complex I assembly factor CIA30 gene revealed a novel homozygous L231V mutation (c.691C-->G) in the NDUFS1 gene. The parents were heterozygous carriers of the L231V mutation. CONCLUSIONS: Identifying nuclear mutations as a cause of respiratory chain disorders will enhance the possibility of prenatal diagnosis and help us understand how molecular defects can lead to complex I deficiency.