Cytochrome c oxidase-associated Leigh syndrome: phenotypic features and pathogenetic speculations

Immunohistochemical analysis of muscle cytochrome c oxidase deficiency in children

Despite the demonstration of a clear biochemical defect, the genetic alterations causing childhood forms of cytochrome c oxidase (COX) deficiency remain unknown. The double genetic origin (nuclear and mitochondrial DNA), and the complexity of COX enzyme structure and regulation, indicate the need for genetic investigations of the molecular structure of individual COX subunits. In the present study a new monoclonal antibody, which reacts exclusively with heart-type human COX subunit VIIa (VIIa-H), and other monoclonal antibodies against human COX subunits, were used in the immunohistochemical analysis of skeletal muscle from children with different forms of mitochondrial myopathy with COX deficiency. By immunohistochemical investigation a normal reaction was seen with antibodies to COX subunits IV, Va+Vb, and VIa+VIc in all four cases, and in two cases with antibodies to COX VIIa-H and VIIa+VIIb. In muscle from a fatal infantile case with cardiac and skeletal muscle involvement, no immunohistochemical reaction was seen with the monoclonal antibody against the tissue-specific subunit VIIa-H. In muscle from an 11-year-old boy with exclusive muscular symptoms and signs, immunohistological reactions were absent with COX subunit VIIa-H and COX subunits VIIa+VIIb, and slightly decreased with COX subunit II, thus demonstrating a different molecular mechanism in each case. It is concluded that the molecular basis of COX deficiency in childhood may vary greatly between patients.

CT and MR of the brain in the diagnosis of organic acidemias. Experiences from 107 patients

The results of CT and/or MRI of the brain in 107 patients with different types of organic acidemia are presented. The CSF spaces were wide in more than two-thirds of the patients, in 46 slightly-to-moderately and in 26 markedly-to-severely dilated. Marked widening of the operculae was found in all 5 patients with glutaric acidemia type 1, but open opercula was also found in other organic acidemias. White matter changes were found in about half the patients, in 28 mildly-to-moderately pronounced, in another 28 marked or severe. Basal ganglia or central pathway pathology was seen in a total of 34 patients, i.e. 32%. These changes in 25 patients involved the caudate and/or lentiform nuclei: in 14 cases the T2 signal was increased and volume loss was present, in 9 cases increased T2 signal with preserved volume was found (in one of these the changes were transient). In 2 patients, both with ethylmalonic aciduria (cause unknown), only small high T2 spots were seen in the caudate heads and the putamina. In 4 patients, all suffering from methylmalonic acidemia, only the globus pallidus was affected. In 3 patients, all with beta-ketothiolase deficiency, high T2 intensity changes were seen only in the postero-lateral putamina. The remaining 8 patients represent a variety of different locations of lesions. The CT or MRI findings in many patients with organic acidemias should alert the radiologist that a neurometabolic disorder may be present; in some cases the location and appearance of the lesions may even suggest the correct diagnosis.

Dystonia as the major manifestation of Leigh's syndrome

We report eight patients who had a progressive illness dominated by generalised dystonia and who had clinical and imaging features suggestive of Leigh's syndrome (LS). Six of the eight cases were male. Early development was usually normal but three cases exhibited impaired mental and/or motor development, and three others had a history of an earlier unexplained encephalopathy or febrile illness. The onset of the dystonia occurred at a mean age of 3 years (range 2 months-7 years). All had abnormalities in the basal ganglia on brain imaging; symmetrical bilateral lucencies or calcification were seen in the basal ganglia on computed tomography scan in five cases, and high signal lesions were evident in these regions on T2-weighted magnetic resonance imaging sequences in seven cases. Other causes of such changes in the basal ganglia were excluded by appropriate investigations. Raised blood lactate levels were found in four of the eight patients. Muscle biopsies were done in seven patients but histology and histochemistry were normal. The common mitochondrial DNA mutations associated with LS in mitochondrial encephalopathies were not found in the six cases examined. LS presenting as a pure dystonic syndrome is uncommon, but should be considered in the differential diagnosis of symptomatic dystonia presenting in childhood.

MtDNA and nuclear mutations affecting oxidative phosphorylation: correlating severity of clinical defect with extent of bioenergetic compromise

Rates of ATP synthesis were studied in cultured skin fibroblasts treated with digitonin. In fibroblasts from patients with complex I deficiency, complex IV and complex V deficiency rates of ATP synthesis were decreased below the levels found in controls. In mitochondria isolated from cultured lymphoblasts, ATP synthesis was also decreased by 35-50% in cases of Leigh's disease due to complex I, complex IV, or complex V deficiency. Calculating the effect of the mutations in the various complexes on the overall efficiency of oxidative phosphorylation, we show that the mtDNA 8993 mutation which affects the activity of the F1F0 ATPase (complex V) has the strongest effect.

Leigh syndrome and hypertrophic cardiomyopathy in an infant with a mitochondrial DNA point mutation (T8993G)

Mutation of mitochondrial (mt) DNA at nucleotide (nt) 8993 has been reported to cause neurogenic weakness, ataxia, retinitis pigmentosa (NARP), or Leigh syndrome (LS). We report a family in whom the mutation was expressed clinically as LS and hypertrophic cardiomyopathy (CMP) in a boy who presented with a history of developmental delay and hypotonia, and who had recurrent lactic acidosis. The mother's first pregnancy resulted in the birth of a stillborn female; an apparently healthy older brother had died suddenly (SIDS) at age 2 months. MtDNA analysis identified the presence of the T8993G point mutation, which was found to be heteroplasmic in the patient's skeletal muscle (90%) and fibroblasts (90%). The identical mutation was present in leukocytes (38%) isolated from the mother, but not from the father or maternal grandmother. Our findings expand the clinical phenotype of the nt 8993 mtDNA mutation to include hypertrophic cardiomyopathy and confirm its cause of LS.

Absence of cytochrome c oxidase activity in a boy with dysfunction of renal tubules, brain and muscle

We report on a boy who developed proximal renal tubular acidosis with loss of carnitine at the age of about 6 months. A few months later he began to suffer from progressive muscular weakness and neurological disturbances. Blood biochemistry showed elevated lactate and beta-hydroxybutyrate with increased lactate/pyruvate and beta-hydroxybutyrate/acetoacetate ratios. A high urinary excretion of lactate and citric acid cycle intermediates was found. These results indicated a defect of the mitochondrial respiratory chain. Analysis of biopsy material from skeletal muscle revealed low activities of all respiratory chain complexes. In muscle and fibroblasts cytochrome c-oxidase (complex IV) was absent. Despite high dose multi-vitamin therapy the boy died at the age of 30 months from central respiratory failure. At autopsy the neuropathological diagnosis of Leigh disease was made.

A new syndrome with ethylmalonic aciduria and normal fatty acid oxidation in fibroblasts

We describe four Italian male infants with a novel clinical phenotype characterized by orthostatic acrocyanosis, relapsing petechiae, chronic diarrhea, progressive pyramidal signs, mental retardation, and brain magnetic resonance imaging abnormalities. The first symptoms appeared after the termination of breast-feeding and introduction of formula feeding. Marked persistent 2-ethylmalonic aciduria was associated with abnormal excretion of C4-C5(n-butyryl-, isobutyryl-, isovaleryl-, and 2-methylbutyryl-)acylglycines and acylcarnitines and with intermittent lactic acidosis. Short- and branched-chain plasma acylcarnitine levels were also elevated. 2-Ethylmalonic aciduria is generally regarded as being indicative of a defect in fatty acid oxidation. Extensive studies of cultured fibroblasts failed to reveal such a defect. The observation of intermittent urinary excretion of 2-ethylhydracrylic acid pointed to involvement of the isoleucine R pathway in ethylmalonate biosynthesis. This hypothesis was tentatively corroborated by the biochemical responses to an oral isoleucine challenge in two patients. However, fibroblast studies showed normal oxidation rates of (14C)isoleucine (ul), indicating that this is not a defect of isoleucine oxidation expressed in skin fibroblasts. In one of two patients tested, cytochrome c oxidase activity was partially reduced (45%) in cultured fibroblasts. This unique clinical and biochemical phenotype identifies a new metabolic encephalopathy of yet undetermined cause.

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

An 18-year-old male patient presented with clinical and radiological evidence of Leigh's syndrome (LS), having developed progressive myoclonic epilepsy and ataxia 11 years previously. Muscle biopsy showed cytochrome oxidase deficiency but no ragged red fibres. Autopsy confirmed the diagnosis of LS; there was additional degenerative change in the cerebellum and dentate and olivary nuclei, and an axonal peripheral neuropathy. Biochemical studies showed reduced activity of complexes I and IV of the respiratory chain in mitochondria from heart, liver and kidney. The mutation of mitochondrial DNA (mtDNA) at position 8344, commonly associated with the syndrome of myoclonic epilepsy and ragged red fibres, was detected in the patient's blood and was present in muscle, brain, liver, heart, and kidney in uniformly high amounts. It is clear that LS is genetically heterogeneous and represents one of the most severe phenotypes of a number of different mtDNA defects.

The mutation at nt 8993 of mitochondrial DNA is a common cause of Leigh's syndrome

Twelve patients with Leigh's syndrome from 10 families harbored a T > G point mutation at nt 8993 of mtDNA. This mutation, initially associated with neurogenic weakness, ataxia, and retinitis pigmentosa, was later found to result in the Leigh phenotype when present in a high percentage. In our patients, the mutation was heteroplasmic, maternally inherited, and appeared to segregate rapidly within the pedigrees. Quantitative analysis revealed a good correlation between percentage of mutant mitochondrial genomes and severity of the clinical phenotype. The mutation was not found in > 200 patients with other mitochondrial encephalomyopathies or in controls. Mitochondrial enzyme activities were normal in all but 1 patient, and there were no ragged-red fibers in the muscle biopsy. Lactic acidosis was present in 92% of patients. Our findings suggest that the mtDNA nt 8993 mutation is a relatively common cause of Leigh's syndrome.

Defect in the lipoyl-bearing protein X subunit of the pyruvate dehydrogenase complex in two patients with encephalomyelopathy

Among the many metabolic encephalomyelopathies caused by deficiencies in the pyruvate dehydrogenase complex (PDHC), nearly all involve its E1 subunit. We describe two new familial cases of PDHC deficiency with encephalomyelopathy, chronic lactic acidemia, and a normal E1 subunit of PDHC but deficiency in another component. Activity of PDHC was measured in cultured skin fibroblasts and skeletal muscle, and immunoblot studies were performed on mitochondrial extracts from skin fibroblasts. Spectra of muscle tissue, obtained in vivo with phosphorus 31 nuclear magnetic resonance, were recorded both at rest and with exercise. The PDHC activity was markedly reduced to 10% to 20% of normal values in both cultured skin fibroblasts and skeletal muscle. Immunoblotting of skin fibroblast mitochondrial extracts showed a specific deficiency in the protein X component of PDHC but normal E1, E2, and E3 components. Spectra obtained with 31P nuclear magnetic resonance showed alterations compatible with those found in mitochondrial myopathies. This is the second description of an encephalomyelopathy associated with a specific absence of the lipoyl-containing protein X component, which has a structural role in the formation of a functional PDHC.

Proton spectroscopy in five patients with Leigh's disease and mitochondrial enzyme deficiency

Five children with Leigh's disease and progressive neurological symptoms were compared with 14 control children. In all patients, MRI showed bilateral lesions of the putamina and caudate heads. Serum lactate was normal for four of the children, and CSF lactate slightly elevated for three. Volume-selective proton MR spectroscopy (1H-MRS) of the basal ganglia in the Leigh patients revealed elevated lactate, giving further evidence for a defect of energy metabolism in the brain. 1H-MRS is an important tool for non-invasive brain tissue analysis in Leigh's disease, particularly in the absence of peripheral lactate elevation.

Molecular genetic characterization of an X-linked form of Leigh's syndrome

We report a patient with necrotizing encephalomyelopathy (Leigh's syndrome) associated with a deficiency of pyruvate dehydrogenase complex activity. The underlying mutation is an A to C transversion in the pyruvate dehydrogenase complex E1 alpha subunit gene. As the E1 alpha subunit is encoded on the X chromosome, this observation confirms that some patients with Leigh's syndrome may potentially exhibit X-linked inheritance.

A T-to-G mutation at nucleotide pair 8993 in mitochondrial DNA in a patient with Leigh's syndrome

We studied a patient with Leigh's syndrome using neurophysiologic, radiologic, enzymatic, biochemical, and molecular analysis. Her clinical course had started with acute encephalopathic symptoms at 7 months of age. With repeated remission and exacerbation, she developed hypotonia and symptoms of brainstem dysfunction, such as irregular respiration and swallowing difficulty. These symptoms were followed by epileptic seizures, including simple partial seizures and tonic spasms. Both serum lactate and serum pyruvate levels were elevated, and deficient activity was detected in cytochrome c oxidase in her quadriceps femoris muscle. From the early stages, we noted an abnormality in the auditory brainstem response and visual evoked potentials, and an abnormal symmetrical low-density area in the basal ganglia on the computed tomographic scan. We found a mitochondrial DNA point mutation at 8993 in blood samples from both the patient and her mother using a simple polymerase chain reaction method. The ratio of wild and mutant mitochondrial DNA calculated densitometrically on polymerase chain reaction products was 56.6% in the patient's blood cells and 8.4% in her mother's. This patient's disorder was thought to be maternally inherited Leigh's syndrome. Her brother had died of the identical clinical features at 1 year 9 months of age.

Approach to diagnosis of oxidative metabolism disorders

Mitochondrial oxidation of a variety of substrates produces the bulk of energy requirements for most cell types. Impairment of oxidative metabolism may result in a broad spectrum of clinical signs and symptoms. A disorder of oxidative metabolism should be suspected when an unexplained association of signs and symptoms occurs, particularly when it is progressive, involving organs with no common embryologic origin. Encephalopathy and myopathy are a particularly suspect combination. Numerous specific disorders affect oxidative metabolism. Lactate elevation frequently occurs and additional laboratory abnormalities often assist in focusing investigation. Diagnostic specificity may require, in addition to the blood and urine studies, tissue sampling, cerebral imaging, in vivo studies of tissue energetics, or molecular genetic analysis.

Maternally inherited Leigh syndrome

A 6 1/2-year-old girl had developmental regression, and Leigh syndrome was diagnosed. A second girl born to the same mother after heterologous artificial insemination also lost acquired skills and died at 2 1/2 years of age; neuropathologic examination confirmed the diagnosis of Leigh syndrome. Tissues from both children and from the mother had a point mutation at nucleotide 8993 in the adenosinetriphosphatase 6-gene of mitochondrial DNA. This family illustrates that Leigh syndrome can be transmitted by maternal inheritance.

The expanding clinical spectrum of mitochondrial diseases

The mitochondrion is the only extranuclear organelle containing DNA (mtDNA). As such, genetically determined mitochondrial diseases may result from a molecular defect involving the mitochondrial or the nuclear genome. The first is characterized by maternal inheritance and the second by Mendelian inheritance. Ragged-red fibers (RRF) are commonly seen with primary lesions of mtDNA, but this association is not invariant. Conversely, RRF are seldom associated with primary lesions of nuclear DNA. Large-scale rearrangements (deletions and insertions) and point mutations of mtDNA are commonly associated with RRF and lactic acidosis, e.g. Kearns-Sayre syndrome (KSS) (major large-scale rearrangements), Pearson syndrome (large-scale rearrangements), myoclonus epilepsy with RRF (MERRF) (point mutation affecting tRNA(lys) gene), mitochondrial myopathy, lactic acidosis, and stroke-like episodes (MELAS) (two point mutations affecting tRNA(leu)(UUR) gene) and a maternally-inherited myopathy with cardiac involvement (MIMyCa) (point mutation affecting tRNA(leu)(UUR) gene). However, RRF and lactic acidosis are absent in Leber hereditary optic neuropathy (LHON) (one point mutation affecting ND4 gene, two point mutations affecting ND1 gene, and one point mutation affecting the apocytochrome b subunit of complex III), and the condition associated with maternally inherited sensory neuropathy (N), ataxia (A), retinitis pigmentosa (RP), developmental delay, dementia, seizures, and limb weakness (NARP) (point mutation affecting ATPase subunit 6 gene). The point mutations in MELAS, MIMyCa, and MERRF, and the large-scale mtDNA rearrangements in KSS and Pearson syndrome have a broader biochemical impact since these molecular defects involve the translational sequence of mitochondrial protein synthesis. The nuclear defects involving mitochondrial function generally are not associated with RRF. The biochemical classification of mitochondrial diseases principally catalogues these nuclear defects. This classification divides mitochondrial diseases into five categories. Primary and secondary deficiencies of carnitine are examples of a substrate transport defect. A lipid storage myopathy is often present. Disturbances of pyruvate or fatty acid metabolism are examples of substrate utilization defects. Only four defects of the Krebs cycle are known: fumarase deficiency, dihydrolipoyl dehydrogenase deficiency, alpha-ketoglutarate dehydrogenase deficiency, and combined defects of muscle succinate dehydrogenase and aconitase. Luft disease is the singular example of a defect in oxidation-phosphorylation coupling. Defects of respiratory chain function are manifold. Two clinical syndromes predominate, one involving limb weakness, and the other primarily affecting brain function. Leigh syndrome may result from different enzyme defects, most notably pyruvate dehydrogenase complex deficiency, cytochrome c oxidase deficiency, complex I deficiency, and complex V deficiency associated with the recently described NARP point mutation. A new group of mitochondrial diseases has emerged.(ABSTRACT TRUNCATED AT 400 WORDS)

Modified structure and kinetics of cytochrome-c oxidase in fibroblasts from patients with Leigh syndrome

In this study we compared the properties of cytochrome-c oxidase (COX) in cultured fibroblasts from two patients with Leigh Syndrome with COX from control fibroblasts. The fibroblasts from patients showed decreased growth rates and elevated lactate production. COX activity of patients fibroblasts was about 25% of control. Kinetic studies with isolated mitochondria showed a higher Km for cytochrome c and a markedly reduced molecular turnover of COX from patients, indicating a different structure of the enzyme. A biphasic change of COX activity was obtained by titration of dodecylmaltoside solubilized mitochondria from control fibroblasts with increasing concentrations of anions. With patient mitochondria we found only the inhibiting phase of COX activity and, in contrast to control mitochondria, irreversible inhibition of COX activity by guanidinium chloride. ELISA titrations with monoclonal antibodies to subunit II, IV, Vab, Vlac and VIIab indicated a normal amount of mitochondrial coded subunit II, but a reduced amount of nuclear coded subunits. The data indicate incompletely assembled nuclear coded subunits of COX from patient fibroblasts.