Leigh syndrome: clinical features and biochemical and DNA abnormalities

Leigh syndrome associated with a mutation in the NDUFS7 (PSST) nuclear encoded subunit of complex I

Leigh syndrome is the phenotypical expression of a genetically heterogeneous cluster of disorders, with pyruvate dehydrogenase complex deficiency and respiratory chain disorders as the main biochemical causes. We report the first missense mutation within the nuclear encoded complex I subunit, NDUFS7, in 2 siblings with neuropathologically proven complex I-deficient Leigh syndrome.

Respiratory chain complex III [correction of complex] in deficiency with pruritus: a novel vitamin responsive clinical feature

We report a child with an isolated complex III respiratory chain deficiency and global developmental delay who had severe pruritus with elevated plasma bile acid levels. A liver biopsy showed micronodular cirrhosis, and enzymologic evaluation demonstrated an isolated complex III deficiency in both liver and muscle. His pruritus improved and serum bile acid levels decreased after treatment with menadione and vitamin C.

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.

D-2-Hydroxyglutaric aciduria: biochemical marker or clinical disease entity?

D-2-Hydroxyglutaric aciduria has been observed in patients with extremely variable clinical symptoms, creating doubt about the existence of a disease entity related to the biochemical finding. An international survey of patients with D-2-hydroxyglutaric aciduria was initiated to solve this issue. The clinical history, neuroimaging, and biochemical findings of 17 patients were studied. Ten of the patients had a severe early-infantile-onset encephalopathy characterized by epilepsy, hypotonia, cerebral visual failure, and little development. Five of these patients had a cardiomyopathy. In neuroimaging, all patients had a mild ventriculomegaly, often enlarged frontal subarachnoid spaces and subdural effusions, and always signs of delayed cerebral maturation. In all patients who underwent neuroimaging before 6 months, subependymal cysts over the head or corpus of the caudate nucleus were noted. Seven patients had a much milder and variable clinical picture, most often characterized by mental retardation, hypotonia, and macrocephaly, but sometimes no related clinical problems. Neuroimaging findings in 3 patients variably showed delayed cerebral maturation, ventriculomegaly, or subependymal cysts. Biochemical findings included elevations of D-2-hydroxyglutaric acid in urine, plasma, and cerebrospinal fluid in both groups. Cerebrospinal fluid gamma-aminobutyric acid was elevated in almost all patients investigated. Urinary citric acid cycle intermediates were variably elevated. The conclusion of the study is that D-2-hydroxyglutaric aciduria is a distinct neurometabolic disorder with at least two phenotypes.

SURF1, encoding a factor involved in the biogenesis of cytochrome c oxidase, is mutated in Leigh syndrome

Leigh Syndrome (LS) is a severe neurological disorder characterized by bilaterally symmetrical necrotic lesions in subcortical brain regions that is commonly associated with systemic cytochrome c oxidase (COX) deficiency. COX deficiency is an autosomal recessive trait and most patients belong to a single genetic complementation group. DNA sequence analysis of the genes encoding the structural subunits of the COX complex has failed to identify a pathogenic mutation. Using microcell-mediated chromosome transfer, we mapped the gene defect in this disorder to chromosome 9q34 by complementation of the respiratory chain deficiency in patient fibroblasts. Analysis of a candidate gene (SURF1) of unknown function revealed several mutations, all of which predict a truncated protein. These data suggest a role for SURF1 in the biogenesis of the COX complex and define a new class of gene defects causing human neurodegenerative disease.

The first nuclear-encoded complex I mutation in a patient with Leigh syndrome

Nicotinamide adenine dinucleotide (NADH):ubiquinone oxidoreductase (complex I) is the largest multiprotein enzyme complex of the respiratory chain. The nuclear-encoded NDUFS8 (TYKY) subunit of complex I is highly conserved among eukaryotes and prokaryotes and contains two 4Fe4S ferredoxin consensus patterns, which have long been thought to provide the binding site for the iron-sulfur cluster N-2. The NDUFS8 cDNA contains an open reading frame of 633 bp, coding for 210 amino acids. Cycle sequencing of amplified NDUFS8 cDNA of 20 patients with isolated enzymatic complex I deficiency revealed two compound heterozygous transitions in a patient with neuropathologically proven Leigh syndrome. The first mutation was a C236T (P79L), and the second mutation was a G305A (R102H). Both mutations were absent in 70 control alleles and cosegregated within the family. A progressive clinical phenotype proceeding to death in the first months of life was expressed in the patient. In the 19 other patients with enzymatic complex I deficiency, no mutations were found in the NDUFS8 cDNA. This article describes the first molecular genetic link between a nuclear-encoded subunit of complex I and Leigh syndrome.

Mutations of SURF-1 in Leigh disease associated with cytochrome c oxidase deficiency

Leigh disease associated with cytochrome c oxidase deficiency (LD[COX-]) is one of the most common disorders of the mitochondrial respiratory chain, in infancy and childhood. No mutations in any of the genes encoding the COX-protein subunits have been identified in LD(COX-) patients. Using complementation assays based on the fusion of LD(COX-) cell lines with several rodent/human rho0 hybrids, we demonstrated that the COX phenotype was rescued by the presence of a normal human chromosome 9. Linkage analysis restricted the disease locus to the subtelomeric region of chromosome 9q, within the 7-cM interval between markers D9S1847 and D9S1826. Candidate genes within this region include SURF-1, the yeast homologue (SHY-1) of which encodes a mitochondrial protein necessary for the maintenance of COX activity and respiration. Sequence analysis of SURF-1 revealed mutations in numerous DNA samples from LD(COX-) patients, indicating that this gene is responsible for the major complementation group in this important mitochondrial disorder.

A Chinese girl with Leigh syndrome: effect of botulinum toxin on dystonia

Leigh syndrome is a form of neurodegenerative disease which is associated with intracranial infarcts. The diagnosis is made by finding hyperlactacidaemia together with cerebral infarcts on neuroimaging. We report a 4-year-old Chinese girl with Leigh syndrome who had several atypical features. She presented with generalized dystonia and developmental regression. In addition, she suffered from an unusual feature of bladder dystonia. This patient appeared to be suffering from respiratory chain complex I deficiency from studies on cultured skin fibroblasts. Assays for respiratory chain enzymes as well as mitochondrial DNA point mutations and major deletions in muscle were normal. Dystonia persisted despite treatments with muscle relaxants and a ketogenic diet. Intramuscular botulinum toxin resulted in significant relief of dystonia.

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.

A high rate (20%-30%) of parental consanguinity in cytochrome-oxidase deficiency

By studying a large series of 157 patients, we found that complex I (33%), complex IV (28%), and complex I+IV (28%) deficiencies were the most common causes of respiratory chain (RC) defects in childhood. Truncal hypotonia (36%), antenatal (20%) and postnatal (31%) growth retardation, cardiomyopathy (24%), encephalopathy (20%), and liver failure (20%) were the main clinical features in our series. No correlation between the type of RC defect and the clinical presentation was noted, but complex I and complex I+IV deficiencies were significantly more frequent in cases of cardiomyopathy (P<.01) and hepatic failure (P<.05), respectively. The sex ratio (male/female) in our entire series was mostly balanced but was skewed toward males being affected with complex I deficiency (sex ratio R=1.68). Interestingly, a high rate of parental consanguinity was observed in complex IV (20%) and complex I+IV (28%) deficiencies. When parental consanguinity was related to geographic origin, an even higher rate of inbreeding was observed in North African families (76%, P<.01). This study gives strong support to the view that an autosomal recessive mode of inheritance is involved in most cases of mitochondrial disorders in childhood, a feature that is particularly relevant to genetic counseling for this devastating condition.

Juvenile Leigh syndrome with protracted course presenting as chronic sensory motor neuropathy, ataxia, deafness and retinitis pigmentosa: a clinicopathological report

We herein describe a male patient who died at 37 years of age, after having suffered from a slowly progressive syndrome of chronic sensory motor neuropathy, deafness, retinitis pigmentosa and ataxia. The neuropathological study showed symmetric areas of necrosis and demyelination affecting the cerebellum and brainstem. The type of lesion was consistent with the characteristics of Leigh Syndrome. On the basis of the histology of the lesions, we believe that they appeared only a few months before the death of the patient. We underline the atypical clinical picture and suggest that, in certain cases, brain MRI may not be a reliable diagnostic tool.

Phenotypic differences between T-->C and T-->G mutations at nt 8993 of mitochondrial DNA in Leigh syndrome

This study reports on a patient with Leigh syndrome with a T-to-C mutation at nucleotide 8993 of mitochondrial deoxyribonucleic acid (T8993C). The authors reviewed 10 Leigh syndrome patients, including ours, with T8993C. Compared with 18 reported patients with Leigh syndrome caused by a T-to-G mutation at nucleotide 8993 (T8993G), Leigh syndrome with T8993C was characterized by a significantly higher frequency of ataxia (P < 0.01). None of the reviewed T8993C-associated Leigh syndrome patients had retinitis pigmentosa, which is one of the characteristic findings in Leigh syndrome with T8993G. The milder symptoms of T8993C-Leigh syndrome can be explained by the milder complex V dysfunction; however, the higher frequency of ataxia in T8993C-Leigh syndrome requires more study.

Reduced Ca2+ uptake by mitochondria in pyruvate dehydrogenase-deficient human diploid fibroblasts

Physiological and pathological Ca2+ loads are thought to be taken up by mitochondria via a process dependent on aerobic metabolism. We sought to determine whether human diploid fibroblasts from a patient with an inherited defect in pyruvate dehydrogenase (PDH) exhibit a decreased ability to sequester cytosolic Ca2+ into mitochondria. Mobilization of Ca2+ stores with bradykinin (BK) increased the cytosolic Ca2+ concentration ([Ca2+]c) to comparable levels in control and PDH-deficient fibroblasts. In normal fibroblasts transfected with plasmid DNA encoding mitochondrion-targeted apoaequorin, BK elicited an increase in Ca2(+)-dependent aequorin luminescence corresponding to an increase in the mitochondrial Ca2+ concentration ([Ca2+]mt) of 2.0 +/- 0.2 microM. The mitochondrial uncoupling agent carbonyl cyanide p-(trifluoromethoxy)phenylhydrazone blocked the BK-induced [Ca2+]mt increase, although it did not affect the [Ca2+]c transient. Basal [Ca2+]c and [Ca2+]mt in control and PDH-deficient cells were similar. However, confocal imaging of the potential-sensitive dye JC-1 indicated that the percentage of highly polarized mitochondria was reduced from 30 +/- 1% in normal cells to 19 +/- 2% in the PDH-deficient fibroblasts. BK-elicited [Ca2+]mt transients in PDH-deficient cells were reduced to 4% of control, indicating that PDH-deficient mitochondria have a decreased ability to take up cytosolic Ca2+. Thus cells with compromised aerobic metabolism have a reduced capacity to sequester Ca2+.

Mitochondrial disorders

Mitochondrial respiration, the most efficient metabolic pathway devoted to energy production, is at the crosspoint of 2 quite different genetic systems, the nuclear genome and the mitochondrial genome (mitochondrial DNA, mtDNA). The latter encodes a few essential components of the mitochondrial respiratory chain and has unique molecular and genetic properties that account for some of the peculiar features of mitochondrial disorders. However, the perpetuation, propagation, and expression of mtDNA, the majority of the subunits of the respiratory complexes, as well as a number of genes involved in their assembly and turnover, are contained in the nuclear genome. Although mitochondrial disorders have been known for more than 30 years, a major breakthrough in their understanding has come much later, with the discovery of an impressive, ever-increasing number of mutations of mitochondrial DNA. Partial deletions or duplications of mtDNA, or maternally inherited point mutations, have been associated with well-defined clinical syndromes. However, phenotypes transmitted as mendelian traits have also been identified. These include clinical entities defined on the basis of specific biochemical defects, and also a few autosomal dominant or recessive syndromes associated with multiple deletions or tissue-specific depletion of mtDNA. Given the complexity of mitochondrial genetics and biochemistry, the clinical manifestations of mitochondrial disorders are extremely heterogenous. They range from lesions of single tissues or structures, such as the optic nerve in Leber hereditary optic neuropathy or the cochlea in maternally inherited nonsyndromic deafness, to more widespread lesions including myopathies, encephalomyopathies, cardiopathies, or complex multisystem syndromes. The recent advances in genetic studies provide both diagnostic tools and new pathogenetic insights in this rapidly expanding area of human pathology.

HIV-1 protein Vpr causes gross mitochondrial dysfunction in the yeast Saccharomyces cerevisiae

The biological effects of the HIV-1 accessory protein, Vpr, have been studied in yeast expression systems. In our previous study [1], employing the pCUP1-vpr copper-inducible expression cassette, Vpr was shown to cause growth arrest and structural defects. In this study yeast constitutively expressing vpr, through elevated copy number and/or elevated transcription levels, displayed no growth arrest in fermentative growth conditions while Vpr was produced at much lower levels than in the inducible expression system. However, such cells were respiratory deficient and unable to utilise ethanol or glycerol as the sole carbon source. They exhibited gross mitochondrial dysfunction displayed in the loss of respiratory chain complex I, II, III, IV and citrate synthase activities. The effects on mitochondria required a C-terminal domain of Vpr that contains a conserved amino acid sequence motif HFRIGCRHSRIG. These results suggest that the widely observed phenomenon of 'Vpr-induced growth arrest' in human cells could be due to mitochondrial dysfunction.

Serine protease inhibition and mitochondrial dysfunction associated with cisplatin resistance in human tumor cell lines: targets for therapy

Indicators of mitochondrial function were studied in two different cell culture models of cis-diamminedichloroplatinum-II (CDDP) resistance: the intrinsically resistant human ovarian cancer cell line CI-80-13S, and resistant clones (HeLa-S1a and HeLa-S1b) generated by stable expression of the serine protease inhibitor-plasminogen activator inhibitor type-2 (PAI-2), in the human cervical cancer cell line HeLa. In both models, CDDP resistance was associated with sensitivity to killing by adriamycin, etoposide, auranofin, bis[1,2-bis(diphenylphosphino)ethane]gold(I) chloride ([Au(DPPE)2]Cl), CdCl2 and the mitochondrial inhibitors rhodamine-123 (Rh123), dequalinium chloride (DeCH), tetraphenylphosphonium (TPP), and ethidium bromide (EtBr) and with lower constitutive levels of ATP. Unlike the HeLa clones, CI-80-13S cells were additionally sensitive to chloramphenicol, 1-methyl-4-phenylpyridinium ion (MPP+), rotenone, thenoyltrifluoroacetone (TTFA), and antimycin A, and showed poor reduction of 1-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide (MTT), suggesting a deficiency in NADH dehydrogenase and/or succinate dehydrogenase activities. Total platinum uptake and DNA-bound platinum were slightly lower in CI-80-13S than in sensitive cells. The HeLa-S1a and HeLa-S1b clones, on the other hand, showed poor reduction of triphenyltetrazolium chloride (TTC), indicative of low cytochrome c oxidase activity. Total platinum uptake by HeLa-Sla was similar to HeLa, but DNA-bound platinum was much lower than for the parent cell line. The mitochondria of CI-80-13S and HeLa-S1a showed altered morphology and were fewer in number than those of JAM and HeLa. In both models, CDDP resistance was associated with less platinum accumulation and with mitochondrial and membrane defects, brought about one case with expression of a protease inhibitor which is implicated in tumor progression. Such markers may identify tumors suitable for treatment with gold phosphine complexes or other mitochondrial inhibitors.

A maternally transmitted lethal neonatal progeroid syndrome with prominent genitourinary and gastrointestinal features

Twin brothers and their maternal uncle with a previously undescribed neonatal progeroid syndrome are presented. In addition to progeroid features, they had pseudo-obstruction of the urinary and gastrointestinal tracts, severe leucocytosis, liver dysfunction, and low complex III and IV in muscle but not in liver. Previously described neonatal progeroid syndromes and syndromes featuring pseudo-obstruction are discussed. The two most likely aetiological mechanisms are an X linked single gene disorder or a mitochondrial disorder. The evidence for these possibilities is presented.

Skewed segregation of the mtDNA nt 8993 (T-->G) mutation in human oocytes

Rapid changes in mtDNA variants between generations have led to the bottleneck theory, which proposes a dramatic reduction in mtDNA numbers during early oogenesis. We studied oocytes from a woman with heteroplasmic expression of the mtDNA nt 8993 (T-->G) mutation. Of seven oocytes analyzed, one showed no evidence of the mutation, and the remaining six had a mutant load > 95%. This skewed expression of the mutation in oocytes is not compatible with the conventional bottleneck theory. A possible explanation is that, during amplification of mtDNA in the developing oocyte, mtDNA from one mitochondrion is preferentially amplified. Thus, subsequent mature oocytes may contain predominantly wild-type or mutant mitochondrial genomes.

Human cytochrome c oxidase: structure, function, and deficiency

As the terminal component of the mitochondrial respiratory chain, cytochrome c oxidase plays a vital role in cellular energy transformation. Human cytochrome c oxidase is composed of 13 subunits. The three major subunits form the catalytic core and are encoded by mitochondrial DNA (mtDNA). The remaining subunits are nuclear-encoded. The primary sequence is known for all human subunits and the crystal structure of bovine heart cytochrome c oxidase has recently been reported. However, despite this wealth of structural information, the role of the nuclear encoded subunits is still poorly understood. Yeast cytochrome c oxidase is a close model of its human counterpart and provides a means of studying the effects of mutations on the assembly, structure, stability and function of the enzyme complex. Defects in cytochrome c oxidase function are found in a clinically heterogeneous group of disorders. The molecular defects that underlie these diseases may arise from mutations of either mitochondrial or the nuclear genomes or both. A significant number of cytochrome c oxidase deficiencies, often associated with other respiratory chain enzyme defects, are attributed to mutations of mtDNA. Mutations of mtDNA appear, nonetheless, uncommon in early childhood. Pedigree analysis and cell fusion experiments have demonstrated a nuclear involvement in some infantile cases but a specific genomic lesion has not yet been reported. Detailed analyses of the many steps involved in the biogenesis of cytochrome c oxidase, often pioneered in yeast, offer several starting points for further molecular characterizations of cytochrome c oxidase deficiencies observed in clinical practice.

Mitochondrial electron transport chain defect presenting as hypoglycemia

A profoundly deaf female infant was found to have hypoglycemia and lactic acidemia after an episode of decreased oral intake and vomiting. Electron transport chain (ETC) enzyme studies revealed a combination defect of complexes I, III, and IV in liver but not in skeletal muscle. This case highlights the fact that defects of the ETC are clinically highly heterogeneous and should be considered with hypoglycemia and lactic acidosis in the absence of a glycogen storage disorder. Moreover, ETC defects can occur with a biochemical profile suggestive of a fatty acid oxidation disorder.

Mitochondrial myopathy with tRNA(Leu(UUR)) mutation and complex I deficiency responsive to riboflavin

Deficiency of complex I (reduced nicotinamide adenine dinucleotide dehydrogenase-ubiquinone oxidoreductase) of the mitochondrial respiratory chain may be seen as a pure myopathy or as a neuromuscular disorder at presentation. Efficacy of long- term therapy for these disorders is yet to be established. We report the case of a female patient with complex I deficiency and skeletal myopathy, who has had a sustained clinical response to riboflavin during 3 years of therapy. Molecular studies found no mutations in the putative flavin mononucleotide binding site in the 51 kd subunit of complex I, but a T-to-C transition at nucleotide 3250 in the mitochondrial DNA tRNA(Leu(UUR)) gene was identified. This mutation has been reported in one other family in that five members had fatigue with or without muscle weakness. There were also five cases of unexplained infant deaths in that family and two cases in the family reported here. Riboflavin therapy should be attempted in all patients with complex I deficiency when the clinical presentation is one of isolated skeletal myopathy.

Prospects for DNA-based prenatal diagnosis of mitochondrial disorders

Mitochondria have their own DNA which is maternally inherited. Mitochondrial DNA (mtDNA) diseases are extremely variable because of the genetics of mtDNA and the unique pathogenesis of these disorders. This makes predicting the prognosis and the transmission of mtDNA disorders difficult. While mtDNA polymorphisms at a single base position are common, the overwhelming majority of the mitochondrial genomes within a single human individual are usually identical. When there is a point mutation difference between a mother and her offspring, there may be a complete switching of mtDNA type within a single generation. It is generally assumed that there is a genetic bottleneck whereby a single or small number of founder mtDNA(s) populate the organism, but it is not known at which stages the restriction/amplification of mtDNA subtype(s) occur, and this uncertainty impedes antenatal diagnosis for mtDNA disorders. Autosomally inherited disorders of mitochondrial function may be caused by mutations in genes for the components of the respiratory chain and for the machinery of mitochondrial biogenesis, which are nuclear-encoded. Accurate diagnosis of these disorders is important as prenatal diagnosis is available in a minority of cases.