Natural history of MELAS associated with mitochondrial DNA m.3243A>G genotype

OBJECTIVE

To describe the natural history of clinical and laboratory features associated with the m.3243A>G mitochondrial DNA point mutation. Natural history data are needed to obtain prognostic information and for clinical trial planning.

METHODS

We included 85 matrilineal relatives from 35 families with at least 2 visits in this prospective cohort study. Thirty-one were fully symptomatic with mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes (MELAS), and 54 were carrier relatives. Evaluations included standardized questionnaires (medical history and daily living functioning), physical examination, neuropsychological testing, and a battery of imaging and laboratory tests. We evaluated changes in clinical and laboratory features over time and survival. Outcomes are reported over a follow-up period of up to 10.6 years (mean 3.8 ± 2.2 years for patients and 5.5 ± 3.0 for carrier relatives).

RESULTS

Neurologic examination, neuropsychological testing, and daily living scores significantly declined in all patients with MELAS, whereas no significant deterioration occurred in carrier relatives. Cerebral MRI scores declined significantly in patients with MELAS. Magnetic resonance spectroscopy estimates of lactate in the lateral ventricles increased over time, and high lactate was associated with increased mortality. Symptom onset in childhood often was associated with worse outcome. Patients with MELAS had a greater death rate than carrier relatives.

CONCLUSIONS

Patients with MELAS carrying the m.3243A>G mutation show a measurable decline in clinical and imaging outcomes. It is hoped that these data will be helpful in anticipating the disease course and in planning clinical trials for MELAS.

Cerebral lactic acidosis correlates with neurological impairment in MELAS

OBJECTIVE

To evaluate the role of chronic cerebral lactic acidosis in mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes (MELAS).

METHODS

The authors studied 91 individuals from 34 families with MELAS and the A3243G point mutation and 15 individuals from two families with myoclonus epilepsy and ragged red fibers (MERRF) and the A8344G mutation. Subjects were divided into four groups. Paternal relatives were studied as controls (Group 1). The maternally related subjects were divided clinically into three groups: asymptomatic (no clinical evidence of neurologic disease) (Group 2), oligosymptomatic (neurologic symptoms but without the full clinical picture of MELAS or MERRF) (Group 3), and symptomatic (fulfilling MELAS or MERRF criteria) (Group 4). The authors performed a standardized neurologic examination, neuropsychological testing, MRS, and leukocyte DNA analysis in all subjects.

RESULTS

The symptomatic and oligosymptomatic MELAS subjects had significantly higher ventricular lactate than the other groups. There was a significant correlation between degree of neuropsychological and neurologic impairment and cerebral lactic acidosis as estimated by ventricular MRS lactate levels.

CONCLUSIONS

High levels of ventricular lactate, the brain spectroscopic signature of MELAS, are associated with more severe neurologic impairment.

Congenital cardiomyopathy and pulmonary hypertension: another fatal variant of cytochrome-c oxidase deficiency

Biventricular hypertrophy was noted at 24 weeks' gestation in a fetus with isolated cytochrome-c oxidase (COX) deficiency. Shock, caused by hypertrophic cardiomyopathy and severe pulmonary hypertension, led to the patient's death on day 6. His phenotype defines a new lethal variant of COX deficiency characterized by prenatal-onset cardiopulmonary pathophysiology.

Exercise-induced muscle "burning," fatigue, and hyper-CKemia: mtDNA T10010C mutation in tRNA(Gly)

A 42-year-old woman presented with myopathy and without a family history of neuromuscular disorder. Muscle biopsy showed ragged red fibers and reduced activities of mitochondrial respiratory chain enzyme complexes I, III, and IV. Analysis of mitochondrial DNA revealed a heteroplasmic T10010C mutation in the transfer RNA glycine gene.

The other human genome

In the past 13 years, a new chapter of human genetics, "mitochondrial genetics", has opened up and is becoming increasingly important in differential diagnosis. Although the clinical manifestations of disorders related to mitochondrial DNA (mtDNA) are extremely variable, recent advances in genetic testing aid in the identification of patients. Muscle morphology can give important clues for diagnosis, but histological features alone cannot define a specific disorder. Biochemical analysis may reveal a single enzyme defect, or when multiple activities are affected, suggest an mtDNA mutation. However, definitive diagnosis often requires DNA analysis and documentation of a specific mtDNA abnormality. Disorders associated with mtDNA mutations are associated with a wide variety of syndromes, and owing to the properties and characteristics of mtDNA, these are often transmitted by maternal inheritance. Although therapy for mitochondrial diseases is limited, identification of the molecular defect is important for genetic counseling.

A5814G mutation in mitochondrial DNA can cause mitochondrial myopathy and cardiomyopathy

We describe a 5-year-old child with hypertrophic cardiomyopathy, mitochondrial myopathy, and lactic acidosis. Mitochondrial DNA analysis showed a heteroplasmic A5814G point mutation in the tRNA(Cys) gene. The mutational load was extremely high (>95%) in muscle, fibroblasts, and blood. This report expands the clinical heterogeneity of the A5814G mutation, which should be considered in the differential diagnosis of hypertrophic cardiomyopathy in childhood.

Surprises of genetic engineering: a possible model of polyglucosan body disease

BACKGROUND

The authors previously reported the generation of a knockout mouse model of Pompe disease caused by the inherited deficiency of lysosomal acid alpha-glucosidase (GAA). The disorder in the knockout mice (GAA-/-) resembles the human disease closely, except that the clinical symptoms develop late relative to the lifespan of the animals. In an attempt to accelerate the course of the disease in the knockouts, the authors increased the level of cytoplasmic glycogen by overexpressing glycogen synthase (GSase) or GlutI glucose transporter.

METHODS

GAA-/- mice were crossed to transgenic mice overexpressing GSase or GlutI in skeletal muscle.

RESULTS

Both transgenics on a GAA knockout background (GS/GAA-/- and GlutI/GAA-/-) developed a severe muscle wasting disorder with an early age at onset. This finding, however, is not the major focus of the study. Unexpectedly, the mice bearing the GSase transgene, but not those bearing the GlutI transgene, accumulated structurally abnormal polysaccharide (polyglucosan) similar to that observed in patients with Lafora disease, glycogenosis type IV, and glycogenosis type VII. Ultrastructurally, the periodic acid-Schiff (PAS)-positive polysaccharide inclusions were composed of short, amorphous, irregular branching filaments indistinguishable from classic polyglucosan bodies. The authors show here that increased level of GSase in the presence of normal glycogen branching enzyme (GBE) activity leads to polyglucosan accumulation. The authors have further shown that inactivation of lysosomal acid alpha-glucosidase in the knockout mice does not contribute to the process of polyglucosan formation.

CONCLUSIONS

An imbalance between GSase and GBE activities is proposed as the mechanism involved in the production of polyglucosan bodies. The authors may have inadvertently created a "muscle polyglucosan disease" by simulating the mechanism for polyglucosan formation.

Familial cerebellar ataxia with muscle coenzyme Q10 deficiency

OBJECTIVE

To describe a clinical syndrome of cerebellar ataxia associated with muscle coenzyme Q10 (CoQ10) deficiency.

BACKGROUND

Muscle CoQ10 deficiency has been reported only in a few patients with a mitochondrial encephalomyopathy characterized by 1) recurrent myoglobinuria; 2) brain involvement (seizures, ataxia, mental retardation), and 3) ragged-red fibers and lipid storage in the muscle biopsy.

METHODS

Having found decreased CoQ10 levels in muscle from a patient with unclassified familial cerebellar ataxia, the authors measured CoQ10 in muscle biopsies from other patients in whom cerebellar ataxia could not be attributed to known genetic causes.

RESULTS

The authors found muscle CoQ10 deficiency (26 to 35% of normal) in six patients with cerebellar ataxia, pyramidal signs, and seizures. All six patients responded to CoQ10 supplementation; strength increased, ataxia improved, and seizures became less frequent.

CONCLUSIONS

Primary CoQ10 deficiency is a potentially important cause of familial ataxia and should be considered in the differential diagnosis of this condition because CoQ10 administration seems to improve the clinical picture.

A novel mitochondrial 12SrRNA point mutation in parkinsonism, deafness, and neuropathy

The objective of this study was to determine whether a mitochondrial DNA mutation and defective oxidative phosphorylation are present in a pedigree with maternally inherited sensorineural deafness, levodopa-responsive parkinsonism, and neuropathy. We sequenced the mitochondrial-encoded ribosomal RNA, cytochrome c oxidase, and transfer RNA genes by cycle sequencing. A polymerase chain reaction-based restriction enzyme assay with mismatched primers was employed to show heteroplasmy of a novel 12SrRNA mutation in the proband and to screen control subjects. Spectrophotometric mitochondrial respiratory chain assays were performed in transformed lymphoblasts from the proband and 12 normal controls. A novel, heteroplasmic, maternally inherited 12SrRNA point mutation (T1095C) was found in the pedigree. Respiratory chain enzyme analysis in cultured lymphocytes from the proband revealed a significant reduction in cytochrome c oxidase activity. Secondary structure predicts that this mutation disrupts a highly conserved loop in the small subunit ribosomal RNA, which is important in the initiation of mitochondrial protein synthesis. The mutation was not found in 270 controls of diverse ethnic origins. We conclude that this mutation is pathogenic and causes an oxidative phosphorylation defect by interfering with mitochondrial protein synthesis.

G8363A mutation in the mitochondrial DNA transfer ribonucleic acidLys gene: another cause of Leigh syndrome

We identified a G-->A transition at nt-8363 in the mitochondrial DNA transfer ribonucleic acidLys gene in blood and muscle from a 13-month-old girl who had clinical and neuroradiologic evidence of Leigh syndrome and died at age 27 months. The mutation was less abundant in the same tissues from the patient's mother, who developed myoclonus epilepsy with ragged red fibers (MERRF) in her late 20s. In both mother and daughter, muscle histochemistry showed ragged red and cytochrome c oxidase-negative fibers and biochemical analysis showed partial defects of multiple respiratory-chain enzymes. A maternal half-sister of the proband had died at 2.5 years of age from neuropathologically proven Leigh syndrome. The G8363A mutation, which previously had been associated with cardiomyopathy and hearing loss, MERRF, and multiple lipomas, also should be included in the differential diagnosis of maternally inherited Leigh syndrome.

Recurrent myoglobinuria due to a nonsense mutation in the COX I gene of mitochondrial DNA

OBJECTIVE

To elucidate the molecular basis of a mitochondrial myopathy associated with recurrent myoglobinuria and cytochrome c oxidase (COX) deficiency in muscle.

BACKGROUND

Recurrent myoglobinuria is typically seen in patients with inborn errors of carbohydrate or lipid metabolism, the main sources of energy for muscle contraction. Relatively little attention has been directed to defects of the mitochondrial respiratory chain in patients with otherwise unexplained recurrent myoglobinuria.

METHODS

Having documented COX deficiency histochemically and biochemically in the muscle biopsy from a patient with exercise-induced recurrent myoglobinuria, the authors sequenced the three mitochondrial DNA (mtDNA)-encoded COX genes, and performed restriction fragment length polymorphism analysis and single-fiber PCR.

RESULTS

The authors identified a nonsense mutation (G5920A) in the COX I gene in muscle mtDNA. The mutation was heteroplasmic and abundantly present in COX-negative fibers, but less abundant or absent in COX-positive fibers; it was not found in blood or fibroblasts from the patient or in blood samples from the patient's asymptomatic mother and sister.

CONCLUSIONS

The G5920A mutation caused COX deficiency in muscle, explaining the exercise intolerance and the low muscle capacity for oxidative phosphorylation documented by cycle ergometry. The sporadic occurrence of this mutation in muscle alone suggests that it arose de novo in myogenic stem cells after germ-layer differentiation. Mutations in mtDNA-encoded COX genes should be considered in patients with recurrent myoglobinuria.

A missense mutation in the mitochondrial cytochrome b gene in a revisited case with histiocytoid cardiomyopathy

We describe a pathogenic mutation in the mitochondrial cytochrome b gene in a patient with a multisystem disorder presenting as histiocytoid cardiomyopathy in whom a defect of ubiquinol cytochrome c oxidoreductase of the electron transport chain had been documented biochemically. The mutation, a G to A transition at nucleotide 15498, results in the substitution of glycine with aspartic acid at amino acid position 251. The mutation, which is heteroplasmic and fulfills all accepted criteria for pathogenicity, is likely to impair the function of the holoenzyme as deduced from its effects on the crystal structure of ubiquinol cytochrome c oxidoreductase. This is the first molecular defect associated with histiocytoid cardiomyopathy.

Primary LAMP-2 deficiency causes X-linked vacuolar cardiomyopathy and myopathy (Danon disease)

"Lysosomal glycogen storage disease with normal acid maltase" which was originally described by Danon et al., is characterized clinically by cardiomyopathy, myopathy and variable mental retardation. The pathological hallmark of the disease is intracytoplasmic vacuoles containing autophagic material and glycogen in skeletal and cardiac muscle cells. Sarcolemmal proteins and basal lamina are associated with the vacuolar membranes. Here we report ten unrelated patients, including one of the patients from the original case report, who have primary deficiencies of LAMP-2, a principal lysosomal membrane protein. From these results and the finding that LAMP-2-deficient mice manifest a similar vacuolar cardioskeletal myopathy, we conclude that primary LAMP-2 deficiency is the cause of Danon disease. To our knowledge this is the first example of human cardiopathy-myopathy that is caused by mutations in a lysosomal structural protein rather than an enzymatic protein.

Intragenic inversion of mtDNA: a new type of pathogenic mutation in a patient with mitochondrial myopathy

We report an unusual molecular defect in the mitochondrially encoded ND1 subunit of NADH ubiquinone oxidoreductase (complex I) in a patient with mitochondrial myopathy and isolated complex I deficiency. The mutation is an inversion of seven nucleotides within the ND1 gene, which maintains the reading frame. The inversion, which alters three highly conserved amino acids in the polypeptide, was heteroplasmic in the patient's muscle but was not detectable in blood. This is the first report of a pathogenic inversion mutation in human mtDNA.

Differential features of patients with mutations in two COX assembly genes, SURF-1 and SCO2

We screened 41 patients with undiagnosed encephalomyopathies and cytochrome c oxidase (COX) deficiency for mutations in two COX assembly genes, SURF-1 and SCO2; 6 patients had mutations in SURF-1 and 3 had mutations in SCO2. All of the mutations in SURF-1 were small-scale rearrangements (deletions/insertions); 3 patients were homozygotes and the other 3 were compound heterozygotes. All patients with SCO2 mutations were compound heterozygotes for nonsense or missense mutations. All of the patients with mutations in SURF-1 had Leigh syndrome, whereas the 3 patients with SCO2 mutations had a combination of encephalopathy and hypertrophic cardiomyopathy, and the neuropathology did not show the typical features of Leigh syndrome. In patients with SCO2 mutations, onset was earlier and the clinical course and progression to death more rapid than in patients with SURF-1 mutations. In addition, biochemical and morphological studies showed that the COX deficiency was more severe in patients with SCO2 mutations. Immunohistochemical studies suggested that SURF-1 mutations result in similarly reduced levels of mitochondrial-encoded and nuclear-encoded COX subunits, whereas SCO2 mutations affected mitochondrial-encoded subunits to a greater degree. We conclude that patients with mutations in SURF-1 and SCO2 genes have distinct phenotypes despite the common biochemical defect of COX activity.

A novel missense mutation (W797R) in the myophosphorylase gene in Spanish patients with McArdle disease

OBJECTIVE

To investigate the degree of genetic heterogeneity of myophosphorylase deficiency (McArdle disease) in Spain through molecular studies of 10 new patients.

DESIGN

The coding sequence of the entire myophosphorylase gene was sequenced in DNA extracted from muscle and blood. Restriction fragment length polymorphism analysis of polymerase chain reaction fragments was used to confirm and simplify detection of a novel mutation.

SETTING

A collaborative study between 2 university laboratories in Spain and the United States.

RESULTS

Five of the 10 patients harbored a novel missense mutation in exon 20, converting a tryptophan to an arginine (W797R). Three patients were homozygous for the "common" R49X mutation, and the remaining 2 patients were compound heterozygotes for R49X and a previously described missense mutation, G204S.

CONCLUSIONS

The W797R missense mutation is the third novel mutation to be identified among Spanish patients. Its relative frequency suggests that it should be added to the R49X mutation in the molecular screening of McArdle disease in Spain.

Two cases of prenatal analysis for the pathogenic T to G substitution at nucleotide 8993 in mitochondrial DNA

We report the outcome of two prenatal analyses for the T to G mutation at nucleotide 8993 in the mitochondrial DNA. This mutation is associated with neurogenic muscle weakness, ataxia and retinitis pigmentosa (NARP) and the neurodegenerative condition, Leigh syndrome. One prospective mother was the sister of a severely affected individual, and had previously had an unaffected child and a stillborn child. The second prospective mother had two unaffected children and two affected children. The mutation was not detected in the chorionic villus sample from one fetus nor in the amniocytes from the other fetus. Both pregnancies were continued, and the resulting children were healthy at two years and five years of age. Prenatal diagnosis of this mitochondrial DNA mutation is an option likely to be acceptable to some families to prevent the birth of a child at high risk for neurological disease.

Mitochondrial DNA mutations at nucleotide 8993 show a lack of tissue- or age-related variation

Two pathogenic mitochondrial DNA mutations, a T-to-G substitution (8993T > G) and a T-to-C substitution (8993T > C), at nucleotide 8993 have been reported. We describe 13 pedigrees with mitochondrial DNA mutations at nucleotide 8993; 10 pedigrees with the 8993T > G mutation and three with the 8993T > C mutation. Prenatal diagnosis of the nucleotide 8993 mutations is technically possible. However, there are three major concerns: (i) that there is variation in mutant loads among tissues; (ii) that the mutant load in a tissue may change over time; and (iii) that the genotype-phenotype correlation is not clearly understood. We have used the 13 pedigrees to determine specifically the extent of tissue- and age-related variation of the two mutations at nucleotide 8993 in the mitochondrial DNA. The tissue variation was investigated by analysing two or more different tissues from a total of 18 individuals. The age-related variation of the mutation was investigated by comparing the amount of both mutations in blood taken at birth and at a later age. No substantial tissue variation was found, nor was there any substantial change in the proportion of either mutation over periods of 8-23 years in the four individuals studied. In addition, we noted that two features were remarkably common in families with nucleotide 8993 mutations, namely (i) unexplained infant death (8 cases in 13 pedigrees); and (ii) de novo mutations (5 of the 10 8993T > G pedigrees).

Fatal infantile cardioencephalomyopathy with COX deficiency and mutations in SCO2, a COX assembly gene

Mammalian cytochrome c oxidase (COX) catalyses the transfer of reducing equivalents from cytochrome c to molecular oxygen and pumps protons across the inner mitochondrial membrane. Mitochondrial DNA (mtDNA) encodes three COX subunits (I-III) and nuclear DNA (nDNA) encodes ten. In addition, ancillary proteins are required for the correct assembly and function of COX (refs 2, 3, 4, 5, 6). Although pathogenic mutations in mtDNA-encoded COX subunits have been described, no mutations in the nDNA-encoded subunits have been uncovered in any mendelian-inherited COX deficiency disorder. In yeast, two related COX assembly genes, SCO1 and SCO2 (for synthesis of cytochrome c oxidase), enable subunits I and II to be incorporated into the holoprotein. Here we have identified mutations in the human homologue, SCO2, in three unrelated infants with a newly recognized fatal cardioencephalomyopathy and COX deficiency. Immunohistochemical studies implied that the enzymatic deficiency, which was most severe in cardiac and skeletal muscle, was due to the loss of mtDNA-encoded COX subunits. The clinical phenotype caused by mutations in human SCO2 differs from that caused by mutations in SURF1, the only other known COX assembly gene associated with a human disease, Leigh syndrome.

Myophosphorylase gene transfer in McArdle's disease myoblasts in vitro

McArdle's disease is due to a genetic deficiency of glycogen phosphorylase and results in a lack of glucose mobilization from glycogen during anaerobic exercise. A genetic defect in Merino sheep produces a similar picture. We constructed a first-generation adenoviral recombinant containing the full-length human phosphorylase cDNA under the control of the Rous sarcoma virus promoter. Primary myoblast cultures from phosphorylase-deficient human and sheep muscle were efficiently transduced with this vector, resulting in restoration of the phosphorylase activity. A similar correction of the genetic defect in muscles of McArdle's patients in vivo appears feasible, preferably with the use of an adeno-associated viral vector.

A novel missense mutation in the glycogen branching enzyme gene in a child with myopathy and hepatopathy

We have identified a novel missense mutation in the gene for glycogen branching enzyme (GBE 1) in a 16-month-old infant with a combination of hepatic and muscular features, an atypical clinical presentation of glycogenosis type IV (GSD IV). The patient was heterozygous for a G-to-A substitution at codon 524 (R524Q), changing an encoded arginine (CGA) to glutamine (CAA), while the GBE1 gene on the other allele was not expressed. This case broadens the spectrum of mutations in patients with GSD IV and confirms the clinical and molecular heterogeneity of this disease.

Manifesting heterozygotes in a Japanese family with a novel mutation in the muscle-specific phosphoglycerate mutase (PGAM-M) gene

Muscle-specific phosphoglycerate mutase (PGAM-M) deficiency results in a metabolic myopathy (glycogenosis type X). Three mutations in the PGAM-M gene have been described thus far, two in African-American families and one in a Caucasian family. In two of them, manifesting heterozygotes were documented. We found a new PGAM-M mutation in a Japanese family with partial PGAM deficiency: a G-to-A transition at nucleotide position 209, resulting in the substitution of a highly conserved glycine at codon 97 with aspartic acid (G97D). Two heterozygous family members for the G97D mutation presented with exercise intolerance and muscle cramps. We describe the first PGAM-M mutation in the Japanese population and confirm that heterozygous individuals can be symptomatic.

A novel mutation in the mitochondrial DNA transfer ribonucleic acidAsp gene in a child with myoclonic epilepsy and psychomotor regression

A novel A7543G mutation was found in the mitochondrial DNA transfer ribonucleic acidAsp gene in an 11-year-old girl with myoclonic seizures, developmental delay, and severe behavioral problems. Muscle histochemistry failed to show any ragged red fibers or cytochrome c oxidase-negative fibers, and muscle biochemistry showed partial cytochrome c oxidase deficiency. The mutation was heteroplasmic in muscle, fibroblasts, and blood from the patient and in blood from other affected family members, and the proportion of mutant mitochondrial DNA correlated with the severity of symptoms.

A stop-codon mutation in the human mtDNA cytochrome c oxidase I gene disrupts the functional structure of complex IV

We have identified a novel stop-codon mutation in the mtDNA of a young woman with a multisystem mitochondrial disorder. Histochemical analysis of a muscle-biopsy sample showed virtually absent cytochrome c oxidase (COX) stain, and biochemical studies confirmed an isolated reduction of COX activity. Sequence analysis of the mitochondrial-encoded COX-subunit genes identified a heteroplasmic G-->A transition at nucleotide position 6930 in the gene for subunit I (COX I). The mutation changes a glycine codon to a stop codon, resulting in a predicted loss of the last 170 amino acids (33%) of the polypeptide. The mutation was present in the patient's muscle, myoblasts, and blood and was not detected in normal or disease controls. It was not detected in mtDNA from leukocytes of the patient's mother, sister, and four maternal aunts. We studied the genetic, biochemical, and morphological characteristics of transmitochondrial cybrid cell lines, obtained by fusing of platelets from the patient with human cells lacking endogenous mtDNA (rho0 cells). There was a direct relationship between the proportion of mutant mtDNA and the biochemical defect. We also observed that the threshold for the phenotypic expression of this mutation was lower than that reported in mutations involving tRNA genes. We suggest that the G6930A mutation causes a disruption in the assembly of the respiratory-chain complex IV.

Microanalysis of cardiolipin in small biopsies including skeletal muscle from patients with mitochondrial disease

Cardiolipin is a specific mitochondrial phospholipid that is present in mammalian tissues in low concentration. To measure cardiolipin in small biopsies from patients with mitochondrial disease, we developed a new technique that can detect subnanomolar levels of well-resolved molecular species, the most abundant of which are tetralinoleoyl-cardiolipin (L(4)) and trilinoleoyl-oleoyl-cardiolipin (L(3)O). To this end, a fluorescence-labeled derivative of cardiolipin (2-[naphthyl-1'-acetyl]-cardiolipin dimethyl ester) was formed and analyzed by high performance liquid chromatography. Cardiolipin was measured in skeletal muscle biopsies from 8 patients with mitochondrial disease and in 17 control subjects. In 5 patients with mitochondrial disease, cardiolipin content was higher than normal (2. 4;-7.0 vs. 0.4;-2.2 nmol/mg protein). In 3 patients with mitochondrial disease, the L(4)/L(3)O ratio was lower than normal (2;-4 vs. 4;-6). Cardiolipin was also measured in various rat and dog muscle tissues. The L(4)/L(3)O ratio was higher in condensed "muscle" type mitochondria (heart ventricle, skeletal muscle, ratios 4;-7) than in orthodox "liver" type mitochondria (liver, smooth muscle, heart auricular appendage, H9c2 myoblasts, ratios 0.4;-3), suggesting that the L(4)/L(3)O proportion is important for cristae membrane structure. We concluded that the L(4)/L(3)O ratio is a tissue-specific variable that may change in the presence of mitochondrial disease. The new method is suitable to measure cardiolipin in muscle biopsies in order to estimate concentration of mitochondria.

The mitochondrial DNA C3303T mutation can cause cardiomyopathy and/or skeletal myopathy

OBJECTIVE

Several mutations in mitochondrial DNA have been associated with infantile cardiomyopathy, including a C3303T mutation in the mitochondrial transfer RNA(Leu(UUR)) gene. Although this mutation satisfied generally accepted criteria for pathogenicity, its causative role remained to be confirmed in more families. Our objective was to establish the frequency of the C3303T mutation and to define its clinical presentation.

STUDY DESIGN

Families with cardiomyopathy and maternal inheritance were studied by polymerase chain reaction/restriction fragment length polymorphism analysis looking for the C3303T mutation.

RESULTS

We found the C3303T mutation in 8 patients from 4 unrelated families. In one, the clinical presentation was infantile cardiomyopathy; in the second family, proximal limb and neck weakness dominated the clinical picture for the first 10 years of life, when cardiac dysfunction became apparent; in the third family, 2 individuals presented with isolated skeletal myopathy and 2 others with skeletal myopathy and cardiomyopathy; in the fourth family, one patient had fatal infantile cardiomyopathy and the other had a combination of skeletal myopathy and cardiomyopathy.

CONCLUSIONS

Our findings confirm the pathogenicity of the C3303T mutation and suggest that this mutation may not be rare. The C3303T mutation should be considered in the differential diagnosis of skeletal myopathies and cardiomyopathy, especially when onset is in infancy.

Genetic counseling and prenatal diagnosis for the mitochondrial DNA mutations at nucleotide 8993

Mitochondrial genetics is complicated by heteroplasmy, or mutant load, which may be from 1%-99%, and thus may produce a gene dosage-type effect. Limited data are available for genotype/phenotype correlations in disorders caused by mtDNA mutations; therefore, prenatal diagnosis for mtDNA mutations has been hindered by an inability to predict accurately the clinical severity expected from a mutant load measured in fetal tissue. After reviewing 44 published and 12 unpublished pedigrees, we considered the possibility of prenatal diagnosis for two common mtDNA mutations at nucleotide 8993. We related the severity of symptoms to the mutant load and predicted the clinical outcome of a given mutant load. We also used the available data to generate empirical recurrence risks for genetic counseling, which may be used in conjunction with prenatal diagnosis.

Maternally inherited hearing loss in a large kindred with a novel T7511C mutation in the mitochondrial DNA tRNA(Ser(UCN)) gene

Thirty-six of 43 maternally related members of a large African American family experienced hearing loss. A muscle biopsy specimen from the proband showed cytochrome c oxidase (COX)-deficient fibers but no ragged-red fibers; biochemical analysis showed marked reduction of COX activity. A novel T7511C point mutation in the tRNA(Ser(UCN)) gene was present in almost homoplasmic levels (>95%) in the blood of 18 of 20 family members, and was also found in lower abundance in the other two. Single-fiber PCR showed that the mutational load was greater in COX-deficient muscle fibers. The tRNA(ser(UCN)) gene may be a "hot spot" for mutations associated with maternally transmitted hearing loss.

Exercise intolerance due to a nonsense mutation in the mtDNA ND4 gene

We report the first molecular defect in an NADH-dehydrogenase gene presenting as isolated myopathy. The proband had lifelong exercise intolerance but no weakness. A muscle biopsy showed cytochrome c oxidase (COX)-positive ragged-red fibers (RRFs), and analysis of the mitochondrial enzymes revealed complex I deficiency. Sequence analysis of the mitochondrial genes encoding the seven NADH-dehydrogenase subunits showed a G-to-A transition at nucleotide 11832 in the subunit 4 (ND4) gene, which changed an encoded tryptophan to a stop codon. The mutation was heteroplasmic (54%) in muscle DNA. Defects in mitochondrially encoded complex I subunits should be added to the differential diagnosis of mitochondrial myopathies.

Infantile encephalopathy associated with the MELAS A3243G mutation

MELAS syndrome is typically characterized by normal early development and childhood-onset recurrent neurologic deficits (stroke-like episodes), seizures, short stature, lactic acidosis, and ragged red fibers on muscle biopsy specimens. It is usually, but not invariably, associated with the A3243G point mutation in the mitochondrial DNA tRNALeu(UUR) gene. We report 3 unrelated children with the A3243G mutation who presented with severe psychomotor delay in early infancy. One patient's clinical picture was more consistent with Leigh syndrome, with apneic episodes, ataxia, and bilateral striatal lesions on brain magnetic resonance imaging (MRI). The second patient had generalized seizures refractory to treatment and bilateral occipital lesions on brain MRI. The third child had atypical retinal pigmentary changes, seizures, areflexia, and cerebral atrophy on brain MRI. All patients had several atypical features in addition to early onset: absence of an acute or focal neurologic deficit, variable serum and cerebrospinal fluid lactate levels, lack of ragged red fibers in muscle biopsy specimens. The proportion of mutant mtDNA in available tissues was relatively low (range, 5% to 51% in muscle; 4% to 39% in blood). These observations further extend the phenotypic expression of the A3243G "MELAS" mutation. Our findings confirm previous observations that there is poor correlation between abundance of mutant mtDNA in peripheral tissues and neurologic phenotype. This suggests that other factors contribute to the phenotypic expression of this mutation.

Molecular characterization of McArdle's disease in two large Finnish families

We have studied two large unrelated Finnish families with myophosphorylase deficiency (McArdle's disease). In one, we identified a new nonsense mutation at codon 540 in exon 14 of the myophosphorylase gene, changing an encoded glutamic acid to a stop codon (E540X). The second family carried a splice-junction mutation at the 5' splice site of intron 14 (1844+G-->A), previously reported in one Caucasian patient and in a consanguineous Druze family. These data further enlarge the list of mutations associated with McArdle's disease and establish that McArdle's disease is genetically heterogeneous also within the Finnish population.

Polymorphic variants in the human mitochondrial cytochrome b gene

We report the polymorphic variants of the human cytochrome b gene based on sequence analysis in 32 Caucasian individuals. We found 27 variants (12 synonymous changes and 15 amino acid replacements). Of these, 15 (8 silent changes and 7 amino acid replacements) have not been previously reported. Based on restriction length polymorphism analysis of patients and their maternal relatives, we conclude that these new amino acid replacements represent maternally inherited polymorphisms. Comparative analysis of the data suggests that four different genotypes can be defined for the human cytochrome b gene.

A new mutation in the myophosphorylase gene (Asn684Tyr) in a Spanish patient with McArdle's disease

We have identified a novel missense mutation, an A-T transition at codon 684 in exon 17, changing an encoded asparagine to a tyrosine (Asn684Tyr) in a Spanish patient with typical McArdle's disease. The patient was a compound heterozygote, with a previously-described mutation (Gly204Ser) on the other allele. This report expands the molecular genetic heterogeneity in McArdle's disease, emphasizes the presence of private mutations in specific ethnic groups, and indicates that geographic origin must be considered before undertaking DNA analysis for diagnosis.

A new mitochondrial DNA mutation (A3288G) in the tRNA(Leu(UUR)) gene associated with familial myopathy

We describe a family with a maternally inherited mitochondrial myopathy and an A3288G mutation in the tRNA(Leu(UUR)) gene. The proband had muscle cramping and mild weakness while her brother had long-standing limb and respiratory muscle weakness and her daughter had elevated serum CK. The mutation, which was nearly homoplasmic in muscle and heteroplasmic in blood, affects the TpsiC loop at a conserved site and was not found in 107 controls. This report confirms the frequent association of tRNA(Leu(UUR)) mutations with respiratory muscle involvement and bolsters the concept that tRNA(Leu(UUR)) is a hotspot for mtDNA mutations.

Early onset of diabetes mellitus associated with the mitochondrial DNA T14709C point mutation: patient report and literature review

We report a family in which a mother and son were affected with diabetes mellitus and myopathy characterized by ragged red fibers and suggestive of mitochondrial disease. Mitochondrial DNA (mtDNA) analysis of DNA isolated from peripheral blood showed a T-->C point mutation at nucleotide position 14709, in the transfer RNA gene for glutamic acid. We review the association of diabetes and mtDNA mutations. This child's case is unusual because of the early onset of diabetes, which is more typical of mtDNA deletions.

A nonsense mutation in the myophosphorylase gene in a Japanese family with McArdle's disease

We identified a new mutation in the myophosphorylase gene in a Japanese family with McArdle's disease. This point mutation results in the replacement of a tryptophan at amino acid position 361 with a stop codon, the third nonsense mutation in this disorder. Our findings further expand the already wide spectrum of genetic lesions associated with McArdle's disease, and establish that molecular genetic heterogeneity is also present in the Japanese population.

A nonsense mutation (G15059A) in the cytochrome b gene in a patient with exercise intolerance and myoglobinuria

We describe a new mitochondrial DNA mutation in the cytochrome b gene in a patient presenting with progressive exercise intolerance and myoglobinuria associated with complex III deficiency in muscle. The point mutation results in the replacement of a glycine at amino acid position 190 with a stop codon. This change predicts premature termination of translation, leading to a truncated protein missing 244 amino acids at the C-terminus of cytochrome b. The mutation fulfills all the accepted criteria for pathogenicity, suggesting that this is the primary cause of the myopathy in the patient.

Maternally inherited mitochondrial cardiomyopathy associated with a C-to-T transition at nucleotide 3303 of mitochondrial DNA in the tRNA(Leu(UUR)) gene

Disorders associated with mitochondrial DNA (mtDNA) mutations are usually dominated by involvement of the nervous system and skeletal muscle (hence the term "mitochondrial encephalomyopathies"). However, considering the high dependence of the heart on oxidative metabolism, it is not surprising that myocardial dysfunction is often a prominent feature in these disorders, either as isolated cardiomyopathy or as part of a multisystem mitochondrial syndrome. We report an infant with a maternally inherited C-to-T transition at nucleotide 3303 of mtDNA in the tRNA(Leu(UUR)) gene; this is the second kindred with cardiomyopathy identified to have this mutation of mitochondrial DNA. A brief review of other mitochondrial DNA defects is also included.

The mitochondrial DNA A8344G mutation in Leigh syndrome revealed by analysis in paraffin-embedded sections: revisiting the past

In 1975, we presented the results of a study on a family with a constellation of features that included a chronic spinocerebellar syndrome, neuropathologically proven Leigh syndrome, and sudden death in infancy or childhood affecting several members over three generations. Inheritance was thought to be autosomal dominant. Twenty years later, we reinterpreted the inheritance pattern as maternal. Mitochondrial DNA (mtDNA) extracted from paraffin-embedded brain samples from the proband revealed the A8344G myoclonic epilepsy and ragged-red fiber (MERRF) mutation as the molecular basis for this multifaceted neurological syndrome. This re-evaluation of archival material is an instructive example of "medical archeopathology."

Comparative biochemical studies of ATPases in cells from patients with the T8993G or T8993C mitochondrial DNA mutations

We performed comparative biochemical studies in cultured fibroblast mitochondria from patients with the T8993G or the T8993C point mutations in the ATPase 6 gene of mitochondrial DNA. We found that ATP production was much more severely decreased in cells from patients with the T8993G mutation than in those from patients with the T8993C mutation. Kinetic studies suggest that both mutations affect only the F0 sector of the mitochondrial ATPase complex. We conclude that these two mutations, which result in the substitution of different amino acids at the same site of the ATPase, result in an enzyme with different biochemical characteristics.

Study of mitochondrial DNA depletion in muscle by single-fiber polymerase chain reaction

We studied muscle biopsies from 3 children with a mitochondrial myopathy characterized histochemically by the presence of ragged-red fibers (RRF) and various numbers of cytochrome c oxidase (COX)-negative fibers. We quantitated the absolute amounts of total mitochondrial DNA (mtDNA) in isolated normal COX-positive muscle fibers and in COX-negative RRF. There was severe mtDNA depletion in all fibers from the two most severe cases. In the third case mtDNA depletion could not be established with conventional diagnostic tools, but it was documented in single COX-negative fibers; COX-positive fibers showed the same amounts of mtDNA as fibers from aged-matched controls. Our observations indicate that mtDNA single-fiber PCR quantitation is a highly sensitive and specific method for diagnosing cases with focal mtDNA depletion. This method also allows one to correlate amounts of mtDNA with histochemical phenotypes in individual fibers from patients and age-matched controls, thereby providing important information about the functional role of residual mtDNA.

Missense mutation in the mtDNA cytochrome b gene in a patient with myopathy

A patient with progressive exercise intolerance, proximal weakness, and complex III deficiency in skeletal muscle had a missense mutation in the cytochrome b gene of mitochondrial DNA (G15762A). The mutation, which leads to the substitution of a highly conserved amino acid (G339E), was heteroplasmic (85%) in the patient's muscle and was not present in 100 individuals of different ethnic backgrounds. These data strongly suggest that this molecular defect is the primary cause of the myopathy.

A splice junction mutation in the alpha(M) gene of phosphorylase kinase in a patient with myopathy

In a 28-year-old man with myopathy and phosphorylase kinase (PhK) deficiency, we found a G-to-C substitution at the 5' end of an intron in the muscle-specific alpha-subunit gene. The mutation destroys the high-consensus GT sequences at the 5' splice junction of the intron, which causes skipping of the preceding exon. This is the second molecular genetic defect identified in the myopathic variant of PhK deficiency.

Molecular genetic analysis of McArdle's disease in Spanish patients

We analyzed leukocyte DNA of 19 patients from 12 Spanish families with McArdle's disease (myophosphorylase deficiency). In 15 patients, the enzyme defect was documented histochemically in muscle, and in four the diagnosis was based on clinical and laboratory data. Three patients were homozygous and six were heterozygous for the nonsense mutation at codon 49 (R49X). Our findings indicate that the R49X mutation, which is common in English and American patients, is also present in Spanish patients with McArdle's disease, but at a lower frequency.

Clinical manifestations of mitochondrial DNA depletion

OBJECTIVE

We studied five new patients with mitochondrial DNA (mtDNA) depletion to better define the clinical spectrum of this disorder.

BACKGROUND

mtDNA depletion has been associated with myopathy or hepatopathy, or both, in infants and young children. Involvement of the CNS and peripheral nervous system has not been clearly established.

METHODS

We reviewed the clinical course and performed morphologic, biochemical, and genetic analyses of muscle samples from five patients.

RESULTS

Age at onset ranged from 3 months to 5 years, and one patient survived until age 10 1/2 years. Two patients had laboratory and clinical features reminiscent of dystrophinopathy, two had evidence of brain involvement, and two had peripheral neuropathy. Muscle biopsy specimens in all patients showed abundant ragged-red fibers. Biochemistry showed cytochrome c oxidase deficiency in all patients tested and decreased activities of other respiratory chain complexes in some.

CONCLUSIONS

Inheritance appeared to be autosomal recessive, suggesting that mutations in nuclear DNA are responsible for mtDNA depletion. mtDNA depletion should be considered in children with mitochondrial disorders of uncertain etiology, and criteria for diagnosis are proposed.

Primary adrenal insufficiency in a child with a mitochondrial DNA deletion

Mitochondrial disorders can affect any organ system, but certain tissues, such as skeletal muscle, heart, and brain are more susceptible to oxidative phosphorylation defects because of their high energy requirements. Endocrinological manifestations, especially diabetes mellitus, are common but they rarely dominate the clinical picture. We describe a 5-year-old girl who died of primary adrenal insufficiency with a mitochondrial disease. Biochemical studies in muscle showed decreased respiratory chain enzyme activities. We detected a novel 7.0 kb mtDNA deletion in muscle form the proband, but not in her mother's white blood cells. Our findings further enlarge the spectrum of clinical presentation associated with mitochondrial DNA deletions.