Deficiency of mitochondrial ATP synthase of nuclear genetic origin

We present clinical and laboratory data from 14 cases with an isolated deficiency of the mitochondrial ATP synthase (7-30% of control) caused by nuclear genetic defects. A quantitative decrease of the ATP synthase complex was documented by Blue-Native electrophoresis and Western blotting and was supported by the diminished activity of oligomycin/aurovertin-sensitive ATP hydrolysis in fibroblasts (10 cases), muscle (6 of 7 cases), and liver (one case). All patients had neonatal onset and elevated plasma lactate levels. In 12 patients investigated 3-methyl-glutaconic aciduria was detected. Seven patients died, mostly within the first weeks of life and surviving patients showed psychomotor and various degrees of mental retardation. Eleven patients had hypertrophic cardiomyopathy; other clinical signs included hypotonia, hepatomegaly, facial dysmorphism and microcephaly. This phenotype markedly differs from the severe central nervous system changes of ATP synthase disorders caused by mitochondrial DNA mutations of the ATP6 gene presenting mostly as NARP and MILS.

[A new missense mutation of 574C>T in the SURF1 gene--biochemical and molecular genetic study in seven children with Leigh syndrome]

BACKGROUND

Leigh disease, subacute necrotizing encephalopathy, is a serious mitochondrial disorder of energy-providing metabolism. Clinical presentation usually starts in infancy as a progressive neurodegenerative disorder with retardation and regression of psychomotor development. The most common form of the disease is associated with deficiency of the cytochrome c oxidase (COX) due to SURF1 gene mutations. SURF1 encodes an inner mitochondrial membrane protein involved in the biogenesis and assembly of COX complex.

METHODS AND RESULTS

The activities of mitochondrial respiratory chain complexes were determined spectrophotometrically in isolated lymphocytes, platelets, muscle mitochondria and cultured fibroblasts. Generalised decrease of COX activity was found in 7 children with typical symptoms of Leigh disease. Two-dimensional electrophoresis of mitochondrial proteins showed altered assembly pattern of COX. As demonstrated by Western blot analysis of mitochondria or mitoplasts with anti-hSurf1 antibodies (gift from Dr. E. A. Shoubridge), the Surf1 protein was absent in all 5 investigated patients. Molecular analyses in the 7 patients revealed the presence of mutations in the SURF1 gene--six patients harboured previously described SURF1 mutations, a new mutation 574C > T was found in one patient.

CONCLUSIONS

The co-operation among the patient's families, clinicians and specialised laboratories is essential for the diagnostic of mitochondrial disorders. The treatment of Leigh syndrome is only symptomatic and the prognosis of the disease is unfavourable. The diagnostics on biochemical and molecular level is necessary for genetic counselling and prenatal diagnosis in affected families.

[Variation in manifestations of heteroplasmic mtDNA mutation 8993 T>G in two families]

BACKGROUND

The most frequent manifestations of heteroplasmic mitochondrial DNA (mtDNA) mutation 8993 T > G are Leigh syndrome or NARP syndrome (Neurogenic Muscle Weakness, Ataxia, and Retinitis Pigmentosa). The authors describe heterogeneity of clinical symptoms and results of biochemical and molecular analyses in seven severely clinically affected children from two unrelated families with heteroplasmic mtDNA mutation 8993 T > G.

METHODS AND RESULTS

Seven clinically affected children from two unrelated families were born in term after an uneventful pregnancy. The failure to thrive, psychomotor retardation, hypotonic or spastic quadruparesis, hypertrophic cardiomyopathy, hepatopathy and hyperlactacidaemia developed after birth. Five children died in the first year of life during acute respiratory infection, one girl died at the age of 3 months with sudden death syndrome, only one boy with spastic quadruparesis and severe psychomotor retardation survived to the age of 8 years. Molecular analyses in all investigated children and their clinically non-affected mothers revealed the presence of heteroplasmic mtDNA mutation 8993 T > G. Mutated copies of mtDNA molecules in maternal tissues were in the range of 15-22%. The mutation load in all analysed children's tissues was higher than 90%.

CONCLUSIONS

A broad spectrum of clinical symptoms may be observed in families with heteroplasmic mtDNA mutations 8993 T > G. Affected children with a mutation load higher than 90% usually do not survive after infancy. In both investigated families, a profound increase in the levels of heteroplasmy of mtDNA mutation 8993 T > G was observed in two subsequent generations.

A novel mutation in SURF1 causes skipping of exon 8 in a patient with cytochrome c oxidase-deficient leigh syndrome and hypertrichosis

Leigh syndrome is a rare pediatric neurodegenerative disorder attributed to impaired mitochondrial energy metabolism. Mutations in SURF1 have been described in several patients with Leigh syndrome associated with cytochrome c oxidase deficiency. We report a new 18-bp deletion (821del18), spanning the splice donor junction of exon 8 of SURF1, in an infant presenting with cytochrome c oxidase-deficient Leigh syndrome and hypertrichosis. cDNA sequencing demonstrated that this deletion results in a messenger lacking exon 8. RT-PCR experiments suggested a rapid degradation of the aberrant mRNA species from the 5'-end.

[Prenatal diagnosis in families with cytochrome C oxidase disorder]

OBJECTIVE

Cytochrome c oxidase (COX) deficiency presents with severe impairment of brain, muscle or heart. Prenatal diagnosis in affected families is difficult because the disease may be caused by mutations in nuclear or mtDNA. This study shows the results of prenatal diagnosis in two families where the first child died because of a generalised COX defect. In both cases the low activity of COX was accompanied by a low content of the enzyme.

SUBJECTS

In the first family the amniocentesis was performed during the second pregnancy and cultured amniocytes showed a marked decrease of COX activity and ATP production. Based on decision of the parents the pregnancy was terminated. Analysis of the foetal tissues confirmed a generalised COX defect. In the second family the nuclear origin of the COX defect was found using transmitochondrial cybrids derived from COX-deficient fibroblasts of the affected child. In the successive pregnancy with dizygotic twins a combined amniocentesis and chorionic villi biopsy has been performed. Prenatal diagnosis was based in both foetuses on three independent approaches. COX activity, the ATP production and protein content of COX complex was measured in cultivated foetal cells. The results of all investigations excluded a putative COX defect and both children are healthy at the age of 2 and half years.

CONCLUSION

Prenatal diagnosis of COX disorders is available in families with the generalised form of the disease based on a nuclear origin of COX deficiency. Three independent approaches to characterise COX at a functional, enzymatic and protein level may be used.

[Various manifestations of the A8344G mtDNA heteroplasmic mutation in 4 families with the MERRF syndrome]

BACKGROUND

The most frequent manifestation of mitochondrial DNA (mtDNA) mutation 8344 A-->G is MERRF syndrome (Myoclonic Epilepsy and Myopathy with Ragged Red Fibres). Less frequent symptoms include ataxia, perceptive type of deafness, cardiomyopathy or external ophthalmoplegia and mental and motor retardation in children. We describe heterogeneity of clinical symptoms and results of biochemical and molecular investigations in four families with the heteroplasmic mutation 8344 A-->G in mtDNA.

METHODS AND RESULTS

In co-operation with paediatric, neurological and genetic specialists from the Czech and Slovak Republic we found in 1993-1998 at the enzymatic or molecular level more than 90 children and adults with impaired mitochondrial energy metabolism. Heteroplasmic mutation 8344 A-->G in mtDNA was found in four families. Ataxia and progressive muscle weakness appeared in the first proband with 50% of mutated copies of mtDNA in muscle at the age of 30 years. The second proband with 95% of mutated mtDNA had his first clinical symptoms--muscle hypotonia, cardiomyopathy and mental and motor retardation--in infancy while his four relatives with 25-50% mutated mtDNA lack so far clinical symptoms. In a female from the third family with 50% mutated mtDNA in muscle the disease manifested at the age of 42 years with progressive external ophthalmoplegia (PEO) and muscle weakness. In the fourth proband with 50% of mutated mtDNA in blood the disease started in infancy with spastic quadruparesis and arrested mental and motor development. Enzymatic and histochemical investigation in muscle biopsy in two probands revealed lower cytochrom c oxidase activity. Ragged-red fibres were found only in one adult patient.

CONCLUSIONS

MtDNA mutation 8344 A-->G can manifest by heterogeneous symptoms. A higher percentage of mutated mtDNA is usually associated with more serious forms of the disease, but there is not always a correlation between the degree of heteroplasmy and severity of the disease or the age of the first clinical symptoms.

[Lactate acidosis in childhood]

BACKGROUND

Secondary lactate acidosis is found in children with hypoxaemia, with impaired tissue perfusion, in hepatic and renal failure or in intoxications. Primary lactate acidosis is usually caused by hereditary metabolic disorders. The objective of the trial was to analyze the causes which lead in childhood to the development of primary hyperlactacidaemia.

METHODS AND RESULTS

The authors examined during 1995-1996 the lactate and pyruvate concentration in 479 children referred by paediatric and neurological departments with a suspect hereditary metabolic disturbances. A raised lactate in blood or cerebrospinal fluid > 2.3 mmol/l was found in 230 children incl. 49 where a metabolic disorder was detected. Ten children had impaired cytochrome c oxidase, two children had a combined deficience of NADH dehydrogenase and cytochrome c oxidase, three children had a deficience of the pyruvate dehydrogenase (PDH) complex, one child had a deficience of ATP synthase and seven children suffered from impaired beta-oxidation. Glycogenosis type I, III or IX was found in 13 children. In three children organic aciduria was found, two children had an impaired urea cycle and three children impaired fructose metabolism. In five children a low level of free and total carnitene was found as a result of valproate treatment. A significant increase of the lactate level by more than 1 mmol/l during an oral glucose load was found in 11 of 16 children with impairment of the respiratory chain or PDH complex. In 58 children concurrently lactate in blood and cerebrospinal fluid assessed but no correlation of lactate levels was found.

CONCLUSIONS

In patients with suspect hereditary metabolic disorders examination of lactate, pyruvate and alanine levels can be considered a screening test for detection of mitochondrial disorders. It remains difficult to reveal the cause of hyperlactacidaemia in a sick child even if a wide range of laboratory methods are used which contribute to the diagnosis of hereditary metabolic disorders.

[Metabolic study in a child with Leigh's syndrome and deficient activity in complex I of the respiratory chain]

Leigh's syndrome--subacute necrotizing encephalomyopathy--is a serious disease of child age manifested by severe psychomotor retardation, a varied neurological symptomatology, a typically symmetrical neuropathological affection of the central nervous system in the area of the basal ganglia and a metabolic disorder affecting the energy system of cells. The authors describe the clinical course of the disease and the results of metabolic and neuropathological investigations in an infant with Leigh's syndrome and severe lactate acidosis based on deficiency of complex I activity of the respiratory chain.