Homozygous W748S mutation in the POLG1 gene in patients with juvenile-onset Alpers syndrome and status epilepticus

PURPOSE

Polymerase gamma (POLG) is the sole enzyme in the replication of mitochondrial DNA (mtDNA). Numerous mutations in the POLG1 gene have been detected recently in patients with various phenotypes including a classic infantile-onset Alpers-Huttenlocher syndrome (AHS). Here we studied the molecular etiology of juvenile-onset AHS manifesting with status epilepticus and liver disease in three teenagers.

PATIENTS AND METHODS

We examined 14- and 17-year-old female siblings (patients 1 and 2) and an unrelated 15-year-old girl (patient 3) with juvenile-onset AHS, sequenced POLG1, and the entire mtDNA, examined mtDNA deletions by amplification of the full-length mtDNA with the long PCR method and used real-time PCR to quantify mtDNA in the tissue samples.

RESULTS

The initial manifestations were migraine-like headache and epilepsy, and the terminal manifestations status epilepticus and hepatic failure. A homozygous W748S mutation in POLG1 was detected in the three patients. No deletions or pathogenic point mutations were found in mtDNA, but all three patients had mtDNA depletion.

CONCLUSIONS

POLG mutations should be considered in cases of teenagers and young adults with a sudden onset of intractable seizures or status epilepticus, and acute liver failure. The W748S POLG1 mutation seems to lead to tissue-specific, partial mtDNA depletion in patients with juvenile-onset Alpers syndrome. Valproic acid should be avoided in the treatment of epileptic seizures in these patients.

Inhibition of hypoxia-inducible factor (HIF) hydroxylases by citric acid cycle intermediates: possible links between cell metabolism and stabilization of HIF

The stability and transcriptional activity of the hypoxia-inducible factors (HIFs) are regulated by two oxygen-dependent events that are catalyzed by three HIF prolyl 4-hydroxylases (HIF-P4Hs) and one HIF asparaginyl hydroxylase (FIH). We have studied possible links between metabolic pathways and HIF hydroxylases by analyzing the abilities of citric acid cycle intermediates to inhibit purified human HIF-P4Hs and FIH. Fumarate and succinate were identified as in vitro inhibitors of all three HIF-P4Hs, fumarate having K(i) values of 50-80 microM and succinate 350-460 microM, whereas neither inhibited FIH. Oxaloacetate was an additional inhibitor of all three HIF-P4Hs with K(i) values of 400-1000 microM and citrate of HIF-P4H-3, citrate being the most effective inhibitor of FIH with a K(i) of 110 microM. Culturing of cells with fumarate diethyl or dimethyl ester, or a high concentration of monoethyl ester, stabilized HIF-1alpha and increased production of vascular endothelial growth factor and erythropoietin. Similar, although much smaller, changes were found in cultured fibroblasts from a patient with fumarate hydratase (FH) deficiency and upon silencing FH using small interfering RNA. No such effects were seen upon culturing of cells with succinate diethyl or dimethyl ester. As FIH was not inhibited by fumarate, our data indicate that the transcriptional activity of HIF is quite high even when binding of the coactivator p300 is prevented. Our data also support recent suggestions that the increased fumarate and succinate levels present in the FH and succinate dehydrogenase-deficient tumors, respectively, can inhibit the HIF-P4Hs with consequent stabilization of HIF-alphas and effects on tumor pathology.

A 3-year clinical follow-up of adult patients with 3243A>G in mitochondrial DNA

OBJECTIVE

To follow the clinical course of patients with the mitochondrial DNA mutation 3243A>G for 3 years.

METHODS

Thirty-three adult patients with the 3243A>G mutation entered a 3-year follow-up study. They were clinically evaluated annually, audiometry was performed, and samples were drawn for the analysis of blood chemistry and mutation heteroplasmy in leukocytes. Holter recording was performed three times during the follow-up and echocardiography, neuropsychological assessment, and quantitative EEG and brain imaging conducted at entry and after 3 years.

RESULTS

The incidence of new neurologic events was low during the 3-year follow-up. Sensorineural hearing impairment (SNHI) progressed, left ventricular wall thickness increased, mean alpha frequency in the occipital and parietal regions decreased, and the severity of disease index (modified Rankin score) progressed significantly. The rate of SNHI progression correlated with mutation heteroplasmy in muscle. The increase in left ventricular wall thickness was seen almost exclusively in diabetic patients. Seven patients died during the follow-up, and they were generally more severely affected than those who survived.

CONCLUSIONS

Significant changes in the severity of disease, sensorineural hearing impairment, left ventricular hypertrophy, and quantitative EEG were seen in adult patients with 3243A>G during the 3-year follow-up.

Sequence analysis of nuclear genes encoding functionally important complex I subunits in children with encephalomyopathy

Complex I has a vital role in the energy production of the cell, and the clinical spectrum of complex I deficiency varies from severe lactic acidosis in infants to muscle weakness in adults. It has been estimated that the cause of complex I deficiency, especially in children, is often a mutation in the nuclear-encoded genes and, more rarely, in the genes encoded by mitochondrial DNA. We sequenced nine complex I subunit coding genes, NDUFAB1, NDUFS1, NDUFS2, NDUFS3, NDUFS4, NDUFS7, NDUFS8, NDUFV1 and NDUFV2, in 13 children with defined complex I deficiency. Two novel substitutions were found: a synonymous replacement 201A>T in NDUFV2 and a non-synonymous base exchange 52C>T in NDUFS8. The 52C>T substitution produced the replacement Arg18Cys in the leading peptide of the TYKY subunit. This novel missense mutation was found as a heterozygote in one patient and her mother, but not among 202 healthy controls nor among 107 children with undefined encephalomyopathy. Bioinformatic analyses suggested that Arg18Cys could lead to marked changes in the physicochemical properties of the mitochondrial-targeting peptide of TYKY, but we could not see changes in the assembly or activity of complex I or in the transcription of NDUFS8 in the fibroblasts of our patient. We suggest that Arg18Cys in the leading peptide of the TYKY subunit is not solely pathogenic, and that other genetic factors contribute to the disease-causing potential of this mutation.

Comprehension of before and after in frontotemporal degeneration

Frontotemporal lobar degeneration (FTLD) is a dementia syndrome with behavioural problems and deterioration of language functions. The other cognitive domains are relatively preserved for at least the first two years of the disease. We studied the ability of patients with FTLD to understand before and after constructions expressed in Finnish. Ten patients and eight controls were presented eight sentences with different relations and asked questions to test their comprehension. The results showed that the patients generally found after constructions more difficult. The difficulty was not, however, related to normal ageing, as was shown by the performance of the control group.

Prevalence of large-scale mitochondrial DNA deletions in an adult Finnish population

BACKGROUND

Large-scale mitochondrial DNA (mtDNA) deletions are associated with clinical conditions such as Kearns-Sayre syndrome and chronic progressive external ophthalmoplegia in adults and Pearson syndrome in children. Reported case series have suggested that deletions are not uncommon in the population, but their prevalence has not been documented.

METHODS

The authors ascertained patients with clinical features associated with mtDNA deletions in a defined adult population in northern Finland. Buccal epithelial samples were requested from each patient fulfilling the selection criteria, and full-length mtDNA was amplified using the long PCR method. Deletion breakpoints were identified using sequencing. Patients with deletions were examined clinically.

RESULTS

The authors identified four patients with single large-scale mtDNA deletions. The prevalence of deletions was calculated to be 1.6/100,000 in the adult population in the province of Northern Ostrobothnia (0.0 to 3.2; 95% CI). Analysis of incident cases from a neighboring province revealed two patients with deletions and yielded a similar population frequency.

CONCLUSIONS

The frequency of large-scale mitochondrial DNA deletions is similar among populations, suggesting that there is a constant rate of new deletions.

Hereditary dementia with intracerebral hemorrhages and cerebral amyloid angiopathy

BACKGROUND

Deposition of the beta-amyloid peptide (Abeta) in neuritic plaques is a hallmark of Alzheimer disease (AD). Mutations in genes encoding amyloid precursor protein (APP) and presenilin 1 and 2 (PSEN1, PSEN2) are associated with increased accumulation of Abeta in neuritic plaques or in the walls of cerebral vessels. Intracerebral hemorrhage occasionally affects patients with AD.

METHODS

A Finnish family with dementia in four generations and with frequent co-occurrence of dementia and intracerebral hemorrhage was identified. Clinical features of 14 family members with a cognitive decline were evaluated. All exons in genes encoding APP, PSEN1, PSEN2, cystatin C, transthyretin, gelsolin, and ITM2B were sequenced, and an association study of APP was conducted by identification of single-nucleotide polymorphisms.

RESULTS

Neuropathologic examination revealed Alzheimer-type changes with Abeta in neuritic plaques and vessel walls, but the cognitive profile of the patients differed from that in AD, as the visuoconstructive functions and verbal fluency were well preserved even in the moderate stage of the disease. In addition to cognitive decline, five patients had had lobar intracerebral hemorrhages and one was diagnosed with hemosiderin deposits in MRI, suggesting previous cerebral microbleeds. No causative mutations were identified in candidate genes associated with amyloid diseases, but linkage to APP region could not be entirely excluded.

CONCLUSIONS

The family presents an autosomal dominant form of beta-amyloidogenic disease that resembles the Italian, Flemish, and Iowa types of AD. No amyloidogenic mutations were identified, but the role of the APP region could not be entirely excluded.

Molecular epidemiology of childhood mitochondrial encephalomyopathies in a Finnish population: sequence analysis of entire mtDNA of 17 children reveals heteroplasmic mutations in tRNAArg, tRNAGlu, and tRNALeu(UUR) genes

OBJECTIVES

Many heteroplasmic point mutations in tRNA genes of mitochondrial DNA (mtDNA) have been associated with human diseases. We recently reported on a prospective 7-year study in which we enrolled 116 consecutive children with undefined encephalomyopathy. Seventeen of them were found to have both a defect in the mitochondrial respiratory chain and abnormal ultrastructure of muscle mitochondria, suggesting a clinically probable mitochondrial encephalopathy.

METHODS

We determined the frequency of mtDNA mutations in these 17 children by analyzing the entire sequence of mtDNA by conformation-sensitive gel electrophoresis and sequencing.

RESULTS

Three heteroplasmic tRNA mutations that were considered to be pathogenic were detected. Two of the mutations were novel transitions, 10438A>G in the tRNA(Arg) gene and 14696A>G in the tRNA(Glu) gene, whereas the third one was 3243A>G, the common MELAS mutation. The mutant load was very high in the blood and skeletal muscle of the patients and markedly lower in the blood of asymptomatic maternal relatives. The 10438A>G mutation changes the nucleotide flanking the anticodon, whereas 14696A>G changes a nucleotide in the stem of the pseudouridine loop, creating a novel base pair and reducing the wobble.

CONCLUSIONS

Our results emphasize that the analysis of the entire sequence of mtDNA is worthwhile in the diagnostic evaluation of patients with clinically probable mitochondrial encephalomyopathy. The frequency of pathogenic mtDNA mutations was found to be 18% among children with biochemically and histologically defined mitochondrial disease, suggesting that the likelihood of nuclear DNA mutations in such a group is several times higher than that of mtDNA mutations.

A novel mutation of the fumarase gene in a family with autosomal recessive fumarase deficiency

Fumarase hydratase (FH) deficiency is a rare familial disorder of the tricarboxylic acid cycle which is characterized by severe neurological impairment in early childhood. Several autosomal recessive mutations in the fumarate hydratase gene have been identified as a cause of the lack of fumarase activity in affected individuals. We describe a novel mutation in nucleotide 1127A>C of the fumarase cDNA which changes glutamine 376 to proline in the vicinity of the catalytic site and explains the loss of FH function. Two homozygous carriers of this mutation suffered from severe encephalopthy and died at a young age. Molecular modeling of FH structure shows that the mutation Gln376Pro in the second half of the fumarase sequence disrupts the structure of the active site. Analysis of the FH mutation and the mutant enzyme variant described here provides an explanation for the mechanism of FH deficiency at the molecular level and paves the way for the analysis of other dysfunctional FH variants.

Ubiquinone and nicotinamide treatment of patients with the 3243A-->G mtDNA mutation

The efficacy and safety of ubiquinone (Q10) and nicotinamide were evaluated in a 6-month open-label trial in patients with the 3243A-->G mitochondrial DNA mutation. Blood lactate and pyruvate concentrations decreased, but there was little clinical improvement. Q10 and nicotinamide were well tolerated, but two patients died suddenly and unexpectedly during the trial. These deaths may have been unrelated to treatment. The unpredictable course of the disease makes evaluation of the clinical response difficult.

Childhood encephalopathies and myopathies: a prospective study in a defined population to assess the frequency of mitochondrial disorders

OBJECTIVES

To assess the frequency of mitochondrial abnormalities in muscle histology, defects in respiratory chain enzyme activities, and mutations in mitochondrial DNA (mtDNA) in children with unexplained psychomotor retardation in the population of Northern Finland.

BACKGROUND

The frequency of mitochondrial diseases among patients with childhood encephalopathies and myopathies is not known. Frequencies are difficult to estimate because the clinical presentation of these disorders is variable.

METHODS

A total of 116 consecutive patients with undefined encephalopathies and myopathies were enrolled during a 7-year period in a hospital serving as the only neurologic unit for a pediatric population of 97 609 and as the only tertiary level neurologic unit for a pediatric population of 48 873. Biochemical and morphologic investigations were performed on muscle biopsy material, including oximetric and spectrophotometric analyses of oxidative phosphorylation, histochemistry, electron microscopy, and molecular analysis of mtDNA.

RESULTS

Ultrastructural changes in the mitochondria were the most common finding in the muscle biopsies (71%). Ragged-red fibers were found in 4 cases. An oxidative phosphorylation defect was found in 26 children (28%), complex I (n = 15) and complex IV (n = 13) defects being the most common. Fifteen percent of patients (n = 17/116) with unexplained encephalomyopathy or myopathy had a probable mitochondrial disease. Common pathogenic mutations were found in the mtDNA of only 1 patient (.9%).

CONCLUSIONS

The common known mutations in mtDNA are rarely causes of childhood encephalomyopathies, which is in contrast to the considerable frequency of the common MELAS mutation observed among adults in the same geographical area. Biochemically and morphologically verified mitochondrial disorders were nevertheless common among the children, making the analysis of a muscle biopsy very important for clinical diagnostic purposes.

Epidemiology of A3243G, the mutation for mitochondrial encephalomyopathy, lactic acidosis, and strokelike episodes: prevalence of the mutation in an adult population

Mitochondrial diseases are characterized by considerable clinical variability and are most often caused by mutations in mtDNA. Because of the phenotypic variability, epidemiological studies of the frequency of these disorders have been difficult to perform. We studied the prevalence of the mtDNA mutation at nucleotide 3243 in an adult population of 245,201 individuals. This mutation is the most common molecular etiology of MELAS syndrome (mitochondrial encephalomyopathy, lactic acidosis, and strokelike episodes), one of the clinical entities among the mitochondrial disorders. Patients with diabetes mellitus, sensorineural hearing impairment, epilepsy, occipital brain infarct, ophthalmoplegia, cerebral white-matter disease, basal-ganglia calcifications, hypertrophic cardiomyopathy, or ataxia were ascertained on the basis of defined clinical criteria and family-history data. A total of 615 patients were identified, and 480 samples were examined for the mutation. The mutation was found in 11 pedigrees, and its frequency was calculated to be >=16. 3/100,000 in the adult population (95% confidence interval 11.3-21. 4/100,000). The mutation had arisen in the population at least nine times, as determined by mtDNA haplotyping. Clinical evaluation of the probands revealed a syndrome that most frequently consisted of hearing impairment, cognitive decline, and short stature. The high prevalence of the common MELAS mutation in the adult population suggests that mitochondrial disorders constitute one of the largest diagnostic categories of neurogenetic diseases.

Increase of blood NAD+ and attenuation of lactacidemia during nicotinamide treatment of a patient with the MELAS syndrome

Decreased activity of complex I (NAD:ubiquinone oxidoreductase) is the most frequent biochemical finding associated with mutation at the base pair 3243 of the mitochondrial DNA. The mutation has been previously shown to lead to a defective translation. We hypothesized that due to an imperfect assembly of complex I subunits the substrate affinity of this enzyme may be lowered and this may be counteracted by increasing the mitochondrial NAD+NADH concentration. Therefore, we studied the effect and mechanism of action of nicotinamide treatment in a MELAS patient with the base pair 3243 mutation. Nicotinamide treatment was initiated after his first stroke-like episode. The blood NAD concentration (representing the intracellular concentration in erythrocytes) increased linearly being 24-fold at 6 weeks of treatment. Blood lactate and pyruvate concentration decreased by 50% within three days and 24 h urine lactate content within 2 weeks and we observed a clinical improvement together with a decrease in the lesion volume in magnetic resonance imaging within the first month. The cellular NAD increase upon nicotinamide administration was probably universal, because it occurred in a time and dose-dependent manner in cultured fibroblasts from both the patient and the controls. Alleviation of the lactate accumulation during the nicotinamide treatment suggests that an increase in the cellular NAD+NADH concentration leads to enhancement of the oxidation of reducing equivalents. However, the Km of complex I for NADH in skeletal muscle from the patient was similar to that of controls. This may indicate that physiologically mitochondrial complex I operates at non-saturating substrate concentration, and this may explain the effect of nicotinamide treatment.

Demyelinating polyneuropathy in a patient with the tRNA(Leu)(UUR) mutation at base pair 3243 of the mitochondrial DNA

A novel feature of demyelinating polyneuropathy was observed in a patient with the tRNA(Leu(UUR)) mutation at base pair 3243 of the mitochondrial DNA. Based on electrodiagnostic examination, the polyneuropathy was defined as being of the demyelinating, mixed (motor more than sensory) type. In a 1-year follow-up we observed approximately 7% reduction in both the motor and sensory conduction velocities. The other clinical features of the proband included a mild to moderate cognitive impairment and a combined hearing loss with a moderate sensorineural component. The proportion of the mutant genome found in the muscle of the proband was 29%, but the mutation was not found in his blood. A wide variability of the clinical phenotype was observed in the family of the proband. Heteroplasmic mutation was detected in the blood of most family members. The proportion of abnormal mitochondrial DNA was highest in the proband's brother, who had clinically definite mitochondrial myopathy, encephalopathy, lactic acidosis, and stroke-like episodes (MELAS) syndrome, while the mutant genome was less frequent or absent in the subjects with less severe phenotypes and in healthy individuals. The findings on this pedigree emphasize the need for studies of complete families in the search for new clinical phenotypes of mutations in mitochondrial DNA.

Mitochondrial DNA deletions in dilated cardiomyopathy: a clinical study employing endomyocardial sampling

OBJECTIVES

The aim of this study was to assess the occurrence of the two most commonly encountered mitochondrial DNA (mtDNA) deletions in the hearts of patients with idiopathic dilated cardiomyopathy.

BACKGROUND

The mutation frequency of mtDNA is high, and sporadic cases of cardiomyopathies associated with mtDNA deletions have been described. Reports of increases in mtDNA deletions with advancing age also exist.

METHODS

We studied 15 consecutive patients with typical signs of idiopathic dilated cardiomyopathy, without a family history, together with 16 control hearts obtained at autopsy from patients who died of noncardiac causes. The patients underwent both right and left heart catheterization, during which endomyocardial biopsy samples were taken. The mtDNA in these samples and in the control hearts was analyzed by the polymerase chain reaction technique for the occurrence and proportion of 5- and 7.4-kilobase (kb) deletions (Cambridge sequence map positions from nucleotides 8469 to 13447 and 8637 to 16084, respectively).

RESULTS

The 5-kb mtDNA deletion was observed in the hearts of all of the patients with idiopathic dilated cardiomyopathy, accounting for 0.32 +/- 0.05% (mean +/- SEM) of the total mtDNA. The 7.4-kb deletion was found in 7 of the 15 patients with idiopathic dilated cardiomyopathy and comprised 0.28 +/- 0.08% of the total. The 5- and 7.4-kb deletions were detected in 12 and 9 control hearts, respectively, quantitatively similar to the patients with idiopathic dilated cardiomyopathy. A sigmoidal age dependency of the mtDNA deletions was found both in the patients with cardiomyopathy and in the control hearts, but after elimination of the confounding age variable, there was no difference between these groups.

CONCLUSIONS

Because of the similarity of the age-dependent increase in the frequency of mtDNA deletions in cardiomyopathic and control hearts, the deletions have no causal relation with idiopathic dilated cardiomyopathy. The present results confirm the notion of an increase in mtDNA deletions with advancing age and show that endomyocardial tissue sampling is a feasible method for detecting mtDNA defects in affected hearts.

Adult-onset diabetes mellitus and neurosensory hearing loss in maternal relatives of MELAS patients in a family with the tRNA(Leu(UUR)) mutation

We describe a family with three cases of "clinically incomplete mitochondrial encephalomyopathy, lactic acidosis, and strokelike episodes (MELAS) syndrome" in which heteroplasmic tRNA(Leu(UUR)) mutation at nucleotide 3243 of the mitochondrial DNA was present in three generations. The amount of mutant genome varied among tissues: it was 60% in the kidney, 72% in the cardiac muscle, and 91% in the liver of the female proband's affected brother and 63% in the kidney, 71% in the cardiac muscle, and 71% in the liver of the female proband's perinatally deceased son. The tRNA(Leu(UUR)) mutation was also carried by the siblings of the proband's affected mother. None of them had any clinical signs of MELAS syndrome. This syndrome has the new feature of being associated with adult-onset diabetes mellitus, neurosensory hearing loss, and short stature.

Kearns-Sayre syndrome case presenting a mitochondrial DNA deletion with unusual direct repeats and a rudimentary RNAase mitochondrial ribonucleotide processing target sequence

A mitochondrial DNA deletion in a case of Kearns-Sayre syndrome is described. The deletion is bracketed by direct repeats that were unusual in that one of them was located 11-13 nucleotides from the deletion seam and both were conserved, which should not occur in slip replication or illegitimate elongation. The deleted region was demarcated on the deletion side by sequences that could be predicted to form hairpin structures. The 5'-side of the deletion was flanked by a sequence homologous to a 9-nucleotide piece of the conserved sequence block II of the D-loop. This arrangement around the deletion in Kearns-Sayre syndrome bears some resemblance to the arrangement in the Pearson marrow-pancreas syndrome described by A. Rötig et al. (1991, Genomics 10: 502-504).

Mitochondrial DNA deletion diagnosed by analysis of an endomyocardial biopsy specimen from a patient with Kearns-Sayre syndrome and complete heart block

Defects of mitochondrial DNA have been found at necropsy in the myocardium of patients with Kearns-Sayre syndrome. A patient with characteristics typical of Kearns-Sayre syndrome and a complete heart block is described. Southern blot analysis showed a deletion of 3.3 kb in the mitochondrial DNA in an endomyocardial biopsy specimen and in skeletal muscle. The deletion led to the disappearance of the genes for four transfer RNAs and four subunits of complex I (NADH:ubiquinone oxidoreductase) in the mitochondrial respiratory chain. The defect could not be demonstrated in whole blood despite amplification of the mitochondrial DNA region of interest by the polymerase chain reaction technique. There can be heteroplasmy--that is, normal and abnormal mitochondrial DNA populations in one cell--in different tissues, and the degree of heteroplasmy may be crucial in the development of organ-specific symptoms. This patient raises the possibility that some tissues can be specifically enriched with mitochondria with DNA defects and emphasises the need for elective sampling of the target tissue and polymerase chain reaction technique to exclude these defects. The role of mitochondrial DNA defects in idiopathic cardiomyopathies could perhaps be studied by analysis of mitochondrial DNA from endomyocardial biopsy specimens.

Fumarase deficiency: two siblings with enlarged cerebral ventricles and polyhydramnios in utero

A family having two boys with progressive encephalomyopathy and fumaric aciduria due to fumarase deficiency is described. Both patients initially presented with polyhydramnios and enlarged cerebral ventricles in utero, with subsequent cerebral atrophy, severe developmental delay, infantile spasms, and hypsarythmia on electroencephalogram. Fumarase activity in blood mononuclear cells and in the mitochondrial and cytosolic fractions of cultured skin fibroblasts was less than 0.5% of the control mean or undetectable. The older boy died at the age of 5 years and 4 months and the younger one is now 2 years and 10 months. The unrelated parents are symptomless and the other three children in the family are clinically healthy. Fumarase activities in the blood mononuclear cells of the father, mother, sister, and two brothers were 59%, 52%, 52%, 120%, and 44% of the control mean, respectively. The results strongly support autosomal recessive inheritance of fumarase deficiency and suggest its consideration in children with congenital hydrocephalus, progressive brain atrophy, and infantile spasms.