Biochemical and genetic studies in a family with mitochondrial myopathy

We present a family with severe exercise intolerance, progressive proximal weakness, and lactic acidemia. Fifteen of 24 family members in five generations were affected. Since the affected males do not have offspring at this time, the family pedigree is consistent with either maternal or autosomal dominant inheritance. Muscle histochemistry showed ragged-red fibers and electron microscopy showed globular mitochondrial inclusions. Biochemical analysis showed reduced muscle activities of mitochondrial NADH-cytochrome c reductase (1 of 2 patients), succinate-cytochrome c reductase (2 patients), and cytochrome c oxidase (2 patients). For 1 patient, sequence analysis of 44% of the muscle mitochondrial DNA including all 22 transfer RNA regions showed no point mutation with pathogenic significance. Southern blot analysis showed no deletion. Six affected members of the family were treated with methylprednisolone (0.25 mg/kg) for 3 months. Muscle strength, serum lactate, and energy metabolism at rest (measured by 31P magnetic resonance spectroscopy) significantly improved with treatment.

Identification of a novel mutation in the mtDNA ND5 gene associated with MELAS

We report a novel G13513A mutation in the mitochondrial ND5 gene in a patient who had morphologically and biochemically abnormal muscle mitochondria and died at age 45 with a diagnosis of MELAS (mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes). The mutation affects an evolutionarily conserved nucleotide and was heteroplasmic in muscle, leukocytes, and several autopsy tissues, including brain. The mutation was less abundant (<5%) in leukocytes from an asymptomatic sister and was not found in over 100 controls, thus satisfying accepted criteria for pathogenicity. Our report reinforces the concept of genetic heterogeneity in MELAS and confirms that MELAS can be due to mutations in polypeptide-coding mtDNA genes.

Carnitine palmitoyltransferase II deficiency: diagnosis by molecular analysis of blood

Four missense mutations have been reported to be associated with the typical, adult form of carnitine palmitoyltransferase II (CPT II) deficiency: Three amino acid substitutions (R631C. P50H and D553N) appear to be rare, while the S113L mutation was found to be common in a group of European patients with CPT II deficiency. We analyzed genomic DNA from 20 American patients with recurrent episodes of myoglobinuria as well as DNA from 10 normal controls in order to determine the frequency of the reported missense mutations in our patient population. The three previously described rare mutations were not found in our group of patients. The S113L mutation was found in 19 of our patients: 5 patients were homozygous, 14 patients were heterozygous. Given the high frequency of this mutation in our series of patients we concluded that the clinical diagnosis of CPT II deficiency can be confirmed by a 'blood test' without resorting to a muscle biopsy.

Association of myopathy with large-scale mitochondrial DNA duplications and deletions: which is pathogenic?

We identified large-scale heteroplasmic mitochondrial DNA (mtDNA) rearrangements in a 50-year-old woman with an adult-onset progressive myopathy. The predominant mtDNA abnormality was a 21.2-kb duplicated molecule. In addition, a small population of the corresponding partially deleted 4.6-kb molecule was detected. Skeletal muscle histology revealed fibers that were negative for cytochrome c oxidase (COX) activity and had reduced mtDNA-encoded COX subunits. By single-fiber polymerase chain reaction analysis, COX-negative fibers contained a low number of wild-type or duplicated mtDNA molecules (ie, nondeleted). In situ hybridization demonstrated that the abnormal fibers contained increased amounts of mtDNA compared with normal fibers and that most of the genomes were deleted. We concluded that deleted mtDNA molecules were primarily responsible for the phenotype in this patient.

Maternally inherited encephalopathy associated with a single-base insertion in the mitochondrial tRNATrp gene

We identified a single thymidine insertion at nucleotide position 5537 (T5537i) in the mitochondrial DNA transfer RNA gene for tryptophan in a family in which the proband had a progressive neurological disorder and his brother died in infancy of Leigh syndrome. Muscle biopsy from the proband showed subsarcolemmal proliferation of mitochondria and decreased activities of oxidative metabolism enzymes, in particular complex IV. Complex IV was also severely reduced in autopsy tissues, including heart and brain tissues, from the Leigh syndrome infant. The novel T5537i mutation was very abundant in tissues from the proband and the infant (>92%) and less abundant (range, 42-89%) in blood, hair follicles, and skin fibroblasts from 4 maternal relatives, 3 of whom showed a neuropsychiatric disturbance. The mutation was not found in more than 100 control subjects. The degree of heteroplasmy in blood correlated well with the severity of the clinical presentation, suggesting specific segregation with the disease.

The mitochondrial A3243G mutation presenting as severe cardiomyopathy

A 6 year old Portuguese boy with dilated cardiomyopathy had abundant ragged red fibres in muscle (20% of total) and severe lactic acidosis. Molecular genetic analysis showed the A to G transition in the mitochondrial transfer RNALeu(UUR) gene at nt 3243 ("MELAS mutation"), which accounted for 88% and 68% of the total mtDNA in his muscle and blood, respectively. Molecular studies in blood from 16 maternal relatives identified lower percentages of the mutation only in the oligo-symptomatic mother and brother. This case reinforces the notion that cardiomyopathy can be the presenting and predominant clinical expression of the A3243G mutation.

Heterogeneous clinical presentation of the mtDNA NARP/T8993G mutation

To obtain a better molecular definition of patients with syndromic retinitis pigmentosa, we screened for mitochondrial DNA (mtDNA) alterations of the two ATPase genes and 22 tRNA-coding sequences in 10 patients whose features resembled NARP (neuropathy, ataxia, and retinitis pigmentosa) syndrome. In two patients, one of whom showed features mimicking Kearns-Sayre syndrome, we identified a heteroplasmic T8993G mutation (average 80%) in the mitochondrial ATPase 6 gene. There was no mutated mtDNA in muscle and leukocytes from the mother of one patient or in leukocytes from his brother, suggesting a rapid segregation of the mutated nucleotide. MtDNA analysis should be considered in the differential diagnosis of patients with syndromic retinitis pigmentosa.

Mitochondrial tRNA(Cys) gene mutation (A5814G): a second family with mitochondrial encephalopathy

We report an Italian family with maternally inherited encephalomyopathy including progressive external ophthalmoplegia, seizures, and neurophysiological evidence of brainstem dysfunction. Mitochondrial DNA analysis showed a heteroplasmic point mutation at position 5814 in the tRNA gene for cysteine (A5814G), previously reported in a 5-year-old girl of Portuguese origin. The mutation was very abundant (> 95%) in both muscle and blood from the proposita and was present in lower proportions (average 85 +/- 6%) in blood from three less severely affected maternal relatives. This observation confirms pathogenicity for the A5814G mutation.

Mitochondrial encephalomyopathy with coenzyme Q10 deficiency

Coenzyme Q10 (CoQ10) transfers electrons from complexes I and II of the mitochondrial respiratory chain to complex III. There is one published report of human CoQ10 deficiency describing two sisters with encephalopathy, proximal weakness, myoglobinuria, and lactic acidosis. We report a patient who had delayed motor milestones, proximal weakness, premature exertional fatigue, and episodes of exercise-induced pigmenturia. She also developed partial-complex seizures. Serum creatine kinase was approximately four times the upper limit of normal and venous lactate was mildly elevated. Skeletal muscle biopsy revealed many ragged-red fibers, cytochrome c oxidase-deficient fibers, and excess lipid. In isolated muscle mitochondria, impaired oxygen consumption was corrected by the addition of decylubiquinone. During standardized exercise, ventilatory and circulatory responses were compatible with a defect of oxidation-phosphorylation, which was confirmed by near-infrared spectroscopy analysis. Biochemical analysis of muscle extracts revealed decreased activities of complexes I+II and I+III, while CoQ10 concentration was less than 25% of normal. With a brief course of CoQ10 (150 mg daily), the patient reported subjective improvement. The triad of CNS involvement, recurrent myoglobinuria, and ragged-red fibers should alert clinicians to the possibility of CoQ10 deficiency.

Mitochondrial DNA mutations and pathogenesis

Approximately there years ago, this journal published a review on the clinical and molecular analysis of mitochondrial encephalomyopathies, with emphasis on defects in mitochondrial DNA (mtDNA). At the time, approximately 30 point mutations associated with a variety of maternally-inherited (or rarely, sporadic) disorders had been described. Since that time, almost twenty new pathogenic mtDNA point mutations have been described, and the pace of discovery of such mutations shows no signs of abating. This accumulating body of data has begun to reveal some patterns that may be relevant to pathogenesis.

Sudden infant death syndrome (SIDS) in a family with myophosphorylase deficiency

A previously healthy girl died suddenly and unexpectedly at three months of age in her sleep and an autopsy failed to reveal an adequate cause of death. As the father was known to have myophosphorylase (PPL) deficiency (McArdle's disease), we performed molecular genetic analysis of the PPL gene in autopsy muscle of the proposita. The girl was homozygous for the nonsense mutation at codon 49 most commonly associated with typical McArdle's disease. This report suggests that among children presenting as Sudden Infant Death Syndrome (SIDS) there may be cases associated with myophosphorylase deficiency.

Two large Spanish pedigrees with nonsyndromic sensorineural deafness and the mtDNA mutation at nt 1555 in the 12s rRNA gene: evidence of heteroplasmy

We describe two unrelated Spanish families with isolated sensorineural hearing loss. In both pedigrees, the deafness was transmitted maternally, which suggested a mitochondrial, DNA (mtDNA) defect. Within the same pedigree, some relatives showed aminoglycoside-induced deafness, whereas others were not exposed to aminoglycosides before the onset of hearing loss. Molecular genetic analysis in both families showed the A-to-G transition at nt 1555 (A1555G) in the mitochondrial 12S rRNA gene. In one pedigree, the mutation was homoplasmic; in the other, it was heteroplasmic. To assess the frequency of this mutation, we screened 42 patients of various ethnic backgrounds with isolated sensorineural hearing loss; none harbored the A1555G mutation. This is the first report of heteroplasmy in a family with isolated sensorineural deafness associated with the A1555G mutation.

Leigh-type neuropathology in Pearson syndrome associated with impaired ATP production and a novel mtDNA deletion

Pearson syndrome is a systemic disorder of oxidative phosphorylation in infants, predominantly affecting the bone marrow and exocrine pancreas and associated with single deletions in mitochondrial DNA (mtDNA). CNS involvement may occur in patients who survive the infantile hematopoietic disorder. We describe a Pearson syndrome patient who developed neurologic manifestations associated with the pathologic features of Leigh syndrome. Biochemical studies in muscle and skin fibroblasts showed partial deficiencies of complexes I and IV of the respiratory chain. Adenosine triphosphate production in mitochondria isolated from skin fibroblasts was reduced to 25% of controls. We detected a novel 3.6 Kb mtDNA deletion in skin fibroblasts from the proband but not in his mother's white blood cells. Leigh syndrome seems to be the common neuropathologic expression of any disorder causing severe impairment of oxidative energy production in the CNS.

Novel mutation in the mitochondrial DNA tRNA glycine gene associated with sudden unexpected death

We describe an A-to-G transition at nucleotide 10044 in the tRNA(Gly) gene of mitochondrial DNA in a sibship in which the proband died at age 8 years after a severe encephalopathy, a brother died of sudden and unexpected death, and the other six siblings had a combination of symptoms, including apparent life-threatening events and gastroesophageal reflux. This novel mutation was very abundant (> 90%) in liver and muscle of the proband and in several tissues, including blood, from his affected siblings (range 91-99%) but was less abundant in blood from the asymptomatic mother (88%) and maternal grandmother (85%). Our findings further enlarge the spectrum of clinical presentations associated with mitochondrial DNA mutations.

Mitochondrial myopathy simulating spinal muscular atrophy

A patient with a severe progressive neuromuscular disorder resembling spinal muscular atrophy is reported. The initial muscle biopsy was consistent with a denervating process. DNA analysis did not reveal deletions in exons 7 and 8 of the survival motor neuron gene. Histology, histochemistry, and biochemistry of a second muscle biopsy suggested mitochondrial myopathy accompanying the denervating features. Immunohistochemistry using anti-DNA antibodies revealed only nuclear staining in skeletal muscle, suggesting mitochondrial DNA depletion. In patients with clinical features of spinal muscular atrophy and no deletions in the survival motor neuron gene, mitochondrial DNA depletion should be considered.

Diagnosis of McArdle's disease by molecular genetic analysis of blood

We analyzed leukocyte DNA from 32 patients with suspected McArdle's disease, 24 of whom had biochemically or histochemically proven myophosphorylase deficiency. We found that 19 were homozygous for the most common mutation at codon 49, 2 were compound heterozygotes, and 1 was a manifesting heterozygote. In six patients, we could find only one mutant allele, suggesting a still unidentified mutation on the second allele. We were unable to identify any of the known mutations in four patients. Our findings indicate that the diagnosis of McArdle's disease can be established in approximately 90% of patients using DNA isolated from leukocytes, thereby avoiding muscle biopsy.

Mitochondrial DNA and RNA processing in MELAS

Mitochondrial myopathy, encephalopathy, lactic acidosis, and stroke-like episodes (MELAS), a maternally inherited disorder, is usually associated with a point mutation in mitochondrial DNA (mtDNA) at position 3,243 in the tRNA Leu(UUR) gene. To further study the pathogenesis of MELAS, we analyzed tissues from 8 MELAS-3,243 patients. Southern blot analysis showed an increase in the ratio of mtDNA to nuclear DNA in almost all tissues examined, implying that mitochondrial proliferation is ubiquitous and is not confined to ragged-red fibers in muscle. By northern blot analysis, we demonstrated increased steady-state levels of RNA 19, a polycistronic transcript corresponding to the 16S rRNA + tRNA Leu(UUR) + ND1 genes (which are contiguous in the mtDNA) in heart, kidney, and muscle. These results provide further evidence that altered mitochondrial nucleic acid metabolism may have pathogenic significance in MELAS.

Multiple mitochondrial DNA deletions in sporadic inclusion body myositis: a study of 56 patients

Inclusion body myositis, a chronic inflammatory disorder, is the most common cause of myopathy in adults over the age of 50. Diagnosis is based on clinical features and distinctive morphological findings by both light and electron microscopy. The causes of inclusion body myositis are still unknown. Ultrastructural mitochondrial changes and ragged-red fibers are common in patients with sporadic inclusion body myositis, and multiple [correction of mutiple] mitochondrial DNA (mtDNA) deletions have been reported in 3 such patients, suggesting that mtDNA mutations may have a pathogenetic role. We studied 56 patients with sporadic inclusion body myositis, using a combination of clinical, morphological, biochemical, and molecular genetic analyses to determine the frequency and the distribution of mtDNA deletions. Using the polymerase chain reaction, we found multiple mtDNA deletions in 73% of patients, compared to 40% of normal age-matched control subjects and 47% of disease control subjects. The presence of deletions correlated with morphological evidence of ragged-red, cytochrome c oxidase-negative fibers, and with defects of complexes I and IV of the electron transport chain. Although aging may account for a proportion of mtDNA deletions in patients with sporadic inclusion body myositis and control subjects, mtDNA alterations may be accelerated in sporadic inclusion body myositis.

Multiple mitochondrial DNA deletions associated with autosomal recessive ophthalmoplegia and severe cardiomyopathy

Six patients in two unrelated families from the eastern Arabian peninsula presented with childhood-onset progressive external ophthalmoplegia (PEO), mild facial and proximal limb weakness, and severe cardiomyopathy requiring cardiac transplantation. Muscle biopsies showed ragged-red and cytochrome c oxidase-negative fibers. The activities of several complexes in the electron-transport chain were decreased and Southern blot analysis showed multiple mtDNA deletions. The apparent autosomal-recessive inheritance and the association with cardiomyopathy distinguish this syndrome from autosomal-dominant PEO with multiple mtDNA deletions. The combination of autosomal-recessive PEO, cardiomyopathy, and multiple mtDNA deletions appears to be another disease due to a defect of communication between the nuclear and mitochondrial genomes.

Clinical heterogeneity associated with the mitochondrial DNA T8993C point mutation

The mitochondrial DNA (mtDNA) point mutation T8993G has been associated with maternally inherited Leigh syndrome (MILS) when very abundant (> 95%). MILS patients are usually severely affected and die in early infancy. In 1993, a novel T8993C point mutation was described in a juvenile form of Leigh syndrome (LS) characterized by a less aggressive clinical course. We describe four unrelated T8993C patients who had diverse, relatively mild, clinical manifestations. Polymerase chain reaction-restriction fragment length polymphorphism analysis showed that the heteroplasmic T8993C point mutation was very abundant in several tissues from all four patients (94.2 +/- 1.5%) but was less copious in blood from 20 maternal relatives. ATP production in mitochondria isolated from skin fibroblasts in three patients was normal, whereas in one patient it was decreased to 20-35% of controls. These findings suggest that the T8993C mutation is less severe than the more common T8993G mutation.

Maternally inherited cardiomyopathy and hearing loss associated with a novel mutation in the mitochondrial tRNA(Lys) gene (G8363A)

A novel G8363A mutation in the mtDNA tRNA(Lys) gene was associated, in two unrelated families, with a syndrome consisting of encephalomyopathy, sensorineural hearing loss, and hypertrophic cardiomyopathy. Muscle biopsies from the probands showed mitochondrial proliferation and partial defects of complexes I, III, and IV of the electron-transport chain. The G8363A mutation was very abundant (>95%) in muscle samples from the probands and was less copious in blood from 18 maternal relatives (mean 81.3% +/- 8.5%). Single-muscle-fiber analysis showed significantly higher levels of mutant genomes in cytochrome (c) oxidase-negative fibers than in cytochrome (c) oxidase-positive fibers. The mutation was not found in >200 individuals, including normal controls and patients with other mitochondrial encephalomyopathies, thus fulfilling accepted criteria for pathogenicity.

Molecular characterization of myophosphorylase deficiency in a group of patients from northern Italy

We studied a group of 14 patients from Northern Italy with myophosphorylase deficiency. The disease presented considerable clinical and biochemical heterogeneity, which was reflected at the molecular level. The clinical presentation was typical in 3 patients, mild in 7 (exercise intolerance), and severe in 4 (fixed weakness). Enzyme activity was undetectable in 10 patients, below 3% of control in 3, and 13% of control in one. Enzymatic protein was detectable immunologically only in 1 patient. Myophosphorylase mRNA was present in 8 patients, but in 7 of them it was reduced in amount. Two patients were homozygous for the common nonsense R49X mutation, 5 were heterozygous. Two missense mutations not previously observed were identified in this group of patients. The frequency of alleles with the R49X mutation was significantly lower in this group of patients than in previously reported series. Myophosphorylase deficiency is genetically heterogeneous even among patients living in a small region and with a common ethnic background.

Cloning of bovine muscle glycogen phosphorylase cDNA and identification of a mutation in cattle with myophosphorylase deficiency, an animal model for McArdle's disease

Genetic defects of myophosphorylase in humans cause a metabolic myopathy (McArdle's disease) characterized by exercise intolerance, cramps, and recurrent myoglobinuria. Recently, a breed of cattle with myophosphorylase deficiency has been identified: this is the first animal model of McArdle's disease. To define the molecular genetic error in the cattle, we cloned and sequenced the wild-type bovine myophosphorylase cDNA. Homology to human cDNA is 95.8% for the amino acid sequence, and 92.0% for the nucleotide sequence. Sequence homology to rabbit cDNA is 97.3% in amino acid, 90.8% in nucleotide. In the cDNA fragments amplified by RT-PCR from muscle RNA of the cattle with myophosphorylase deficiency, we identified a C-to-T substitution, changing an encoded arginine (CGG) to tryptophan (TGG) at codon 489. The mutant residue is adjacent to pyridoxal phosphate binding sites and to an active site residue, and the sequence around this mutation is highly conserved in different species.

Mitochondrial encephalomyopathies: what next?

In few areas of medicine has progress been more spectacular than in the field of mitochondrial diseases, especially those related to mtDNA mutations. Much remains to be done, however, and this brief review discusses the following areas of research where progress has been more limited or data are still controversial: (1) the molecular basis of respiratory-chain defects due to nuclear DNA mutations; (2) defects of mitochondrial protein importation; (3) defects of intergenomic signalling; (4) pathophysiology of mtDNA-related disorders; (5) ageing and age-related neurodegenerative diseases; (6) therapy; and (7) genetic counselling.

Comparative biochemical studies in fibroblasts from patients with different forms of Leigh syndrome

We have compared respiratory chain enzyme activities, ATP synthesis, and ATP hydrolysis in cultured fibroblast mitochondria from patients with Leigh syndrome (LS) due to: (i) cytochrome oxidase (COX) deficiency (#6); (ii) pyruvate dehydrogenase complex (PDHC) deficiency (#4); and (iii) maternally inherited LS (MILS) with the T8993G mutation in the ATPase 6 gene of mtDNA (#5). Enzyme activities were normal in patients with MILS and variably decreased in those with COX and PDHC deficiency. ATP hydrolysis was normal or mildly decreased in all three groups. In contrast, ATP synthesis was decreased in all patients but more markedly in those with MILS, and especially with pyruvate/malate as substrate. These studies show that impaired ATP production is the common feature of all three forms of LS, but it is both more severe and more specific in MILS, consistent with the genetic defect.

A novel mitochondrial DNA point mutation associated with mitochondrial encephalocardiomyopathy

A novel mitochondrial DNA (mtDNA) mutation at position nt 4320 in the tRNA(Ile) gene was associated with severe encephalopathy in a 7-month-old infant, who died of intractable hypertrophic cardiomyopathy. The mutation was present in heteroplasmic fashion (88%) in muscle and fulfills accepted criteria for pathogenicity. This is the fourth pathogenic mutation identified in this gene, which appears to be a "hotspot" for deleterious mutations affecting the heart. This report adds to the evidence of genetic heterogeneity in hypertrophic cardiomyopathies.

A new mutation associated with MELAS is located in a mitochondrial DNA polypeptide-coding gene

We report a patient with mitochondrial encephalomyopathy, lactic acidosis and stroke-like episodes (MELAS) who harbored a novel missense mutation at mtDNA position 9957 in the gene specifying subunit III of cytochrome c oxidase (COX III). This T-->C transition converted Phe-251, a highly conserved amino acid in the C-terminus of the polypeptide, to Leu. The mutation, which was not present in 107 normal controls or in 57 patients with various mitochondrial diseases, was heteroplasmic in both muscle and blood of the proband and in blood from his asymptomatic mother. These results provide evidence that the MELAS clinical phenotype can be due not only to mutations in mtDNA-encoded tRNA genes, but in polypeptide-coding genes as well.

A novel mitochondrial ATPase 6 point mutation in familial bilateral striatal necrosis

A T-to-C transition at nucleotide (nt) 9176 in the mitochondrial adenosine triphosphatase 6 (ATPase 6) gene was detected in 2 brothers with a neurological disorder resembling Leigh syndrome. The mutation was also present in the 2 other siblings and in the mother, who were asymptomatic. In the more severely affected boy (the proband), the mutation was homoplasmic in muscle, leucocytes, and fibroblasts. In leucocytes from his affected brother, 98% of mtDNA was mutant. Heteroplasmy of varying degrees was seen in leucocytes from the mother and the 2 unaffected siblings. The mutation changes a highly conserved leucine residue near the carboxyl terminus of the mitochondrial ATPase 6 subunit to proline. It could not be detected in 168 control subjects. Studies of ATP synthesis and hydrolysis in fibroblasts from the proband were normal.

A novel mtDNA point mutation in maternally inherited cardiomyopathy

A novel mtDNA mutation at position nt. 4300 in the tRNAIle gene is associated with hypertrophic cardiomyopathy inherited as a maternal trait. Interestingly, this mutation seems to cause a pure heart disease as opposed to most other mtDNA mutations, which are associated with multisystemic disorders. Hypertrophic cardiomyopathies are genetically heterogeneous, and mtDNA defects should be considered in the differential diagnosis, especially when there is evidence of maternal inheritance.

Myophosphorylase deficiency associated with rhabdomyolysis and exercise intolerance in 6 related Charolais cattle

A Charolais calf presented to the Veterinary Medical Teaching Hospital with a history of recumbency following forced exercise. The calf was unable to stand, and had severe rhabdomyolysis, dehydration, and electrolyte imbalance. Blood selenium concentrations were within normal limits. A complete absence of histochemical staining for phosphorylase was apparent in muscle biopsies. Five other animals in the herd also had exercise intolerance and had a complete absence of phosphorylase staining in muscle biopsies. Biochemical analyses confirmed a deficiency of myophosphorylase (range 0-0.3 mumol/g per minute: normals 15-27) with normal to slightly elevated muscle glycogen concentrations. Pedigrees from all affected animals showed a common ancestor on the sire's and dam's side of each phosphorylase-deficient animal, suggesting an autosomal recessive transmission. Although myophosphorylase deficiency was described in humans (McArdle's disease) over 40 years ago, these cattle represent the first animal model for this disease.

Double trouble: combined myophosphorylase and AMP deaminase deficiency in a child homozygous for nonsense mutations at both loci

A 2-yr-old boy had congenital hypotonia, limb weakness, exercise intolerance and one episode of myoglobinuria. Histochemical and biochemical analysis of muscle showed a combined defect of phosphorylase and AMP deaminase. DNA analysis showed that the child was homozygous for the mutations commonly found in both McArdle's disease and AMP deaminase deficiency. The father was heterozygous for both mutations. The mother was heterozygous for the myophosphorylase gene mutation and homozygous for the mutation in the AMP deaminase 1 gene.

Segregation patterns of a novel mutation in the mitochondrial tRNA glutamic acid gene associated with myopathy and diabetes mellitus

We have identified a novel mtDNA mutation in a 29-year-old man with myopathy and diabetes mellitus. This T-->C transition at mtDNA position 14709 alters an evolutionarily conserved nucleotide in the region specifying for the anticodon loop of the mitochondrial tRNA(Glu). The nt-14709 mutation was heteroplasmic but present at very high levels in the patient's muscle, white blood cells (WBCs), and hair follicles; lower proportions of mutated mtDNA were observed in WBCs and hair follicles of all examined maternal relatives. In the patient's muscle, abnormal fibers showed mitochondrial proliferation, severe focal defects in cytochrome c oxidase activity, and absence of cross-reacting material for mitochondrially synthesized polypeptides. These fibers had higher levels of mutated mtDNA than did surrounding "normal" fibers. Although the percentage of mutated mtDNA in WBCs from family members were distributed around the percentage observed in the mothers, the pattern was different in hair follicles, where the mutated population tended to increase in subsequent generations. PCR/RFLP analysis of single hairs showed that the intercellular variations in the percentage of mutated mtDNA differed among family members, with younger generations having a more homogeneous distribution of mutated mtDNA in different hair follicles. These results suggest that the intercellular distribution of the mutated and wild-type mtDNA populations may drift toward homogeneity in subsequent generations.

Myoclonic epilepsy with ragged-red fibers (MERRF): an immunohistochemical study of the brain

Myoclonic epilepsy with ragged-red fibers (MERRF) is a maternally inherited disorder of oxidative phosphorylation due to specific point mutations within the mitochondrial tRNA(Lys) gene. Mitochondrial dysfunction in the central nervous system (CNS) of patients with MERRF accounts for the neurological manifestations of the disease. Antibodies against subunits of complex I, III, IV and V of the respiratory chain were used to study the expression of these proteins in the frontal cortex, cerebellum and medulla from an autoptic case of MERRF. We found a selective decreased expression of subunit II of cytochrome c oxidase (COX-II) in these regions. Immunohistochemical abnormalities were more widespread than the lesions described by traditional histopathological techniques and made possible an attempt of explanation for the neurological symptoms of the patient.

High levels of mitochondrial DNA with an unstable 260-bp duplication in a patient with a mitochondrial myopathy

Other investigators reported the presence of low levels of a 260-bp heteroplasmic duplication of mitochondrial DNA in patients with mitochondrial DNA deletions and their asymptomatic mothers. In this study, we were not able to detect this polymorphism in 30 patients with mitochondrial DNA deletions, but the 260-bp duplication was detected in relatively high levels (32% in muscle) in a patient with a slowly progressive mitochondrial myopathy. The duplication was also present in cultured fibroblasts (10%) and in WBC (< 1%). Mitochondrial dysfunction in this patient was evidenced in muscle by the presence of ragged-red fibers and a partial decrease in cytochrome c oxidase activity. We also detected low levels of mitochondrial DNA harboring a triplication of the 260-bp region, indicating that this polymorphism is unstable. Taken together, our results suggest that an unstable 260-bp duplication, which includes important mitochondrial DNA cis-acting regulatory sequences, may be pathogenic per se, if present at high levels.

The molecular genetic basis of myophosphorylase deficiency (McArdle's disease)

Glycogen phosphorylase catalyzes the first step of glycogen catabolism. Hereditary defects of muscle phosphorylase lead to a myopathy characterized by exercise intolerance, cramps, and myoglobinuria (McArdle's disease). We have identified ten mutations in the myophosphorylase gene in patients with McArdle's disease. Relatively common mutations include: a nonsense mutation, CGA(Arg) to TGA at codon 49, observed in 30 of 40 American patients; deletion of a single codon 708/709, observed in 4 of 7 Japanese patients; and a missense mutation, GGC(Gly) to AGC(Ser) at codon 204, observed in 5 of 40 American patients. Apparently rare mutations include: a splice-junction mutation, G to A, at the first nt of intron 14; a deletion of G at codon 510; a mutation, ATG to CTG, in the translation initiation codon; and missense mutations, AAG(Lys) to ACG(Thr) at codon 542, CTG(Leu) to CCG(Pro) at codon 396, CTG(Leu) to CCG(Pro) at codon 291, and GAG(Glu) to AAG(Lys) at codon 654. As most mutations can be screened for using genomic DNA, patients can now be diagnosed reliably using peripheral blood cells, thus avoiding muscle biopsy. Although these findings define the wide spectrum of genetic lesions causing McArdle's disease, the clinical heterogeneity of this disorder remains to be explained.

End-tidal carbon monoxide in newborn infants: observations during the 1st week of life

Serial end-tidal carbon monoxide corrected for ambient CO (ETCOc) levels were measured in an ethnically diverse population of 87 normal newborn infants during the first 5 days of life. The results demonstrate a progressive reduction in ETCOc from 1.6 +/- 0.4 to 0.8 +/- 0.2 ppm. These levels were unrelated to ethnicity, but were inversely related to serum bilirubin levels. We conclude that ETCOc is not a useful indicator for predicting the course of transitional hyperbilirubinemia in the normal newborn infant.

Molecular genetic studies in muscle phosphoglycerate mutase (PGAM-M) deficiency

Phosphoglycerate mutase (PGAM; EC 2.7.5.3) catalyzes the interconversion of 2-phosphoglycerate and 3-phosphoglycerate in the glycolytic pathway. Hereditary muscle PGAM deficiency has been identified in 9 patients with myopathy. All patients had exercise intolerance and 6 had myoglobinuria. Seven of the 9 patients were African-Americans: 5 of them were homozygous for a nonsense mutation, TGG(Trp) to TAG at codon 78; 1 was a compound heterozygote for the nonsense mutation and a missense mutation, GAG(Glu) to GCG(Ala) at codon 89; and 1 could not be tested. The only 2 Caucasian patients, a brother and sister, were homozygous for a different missense mutation, CGG(Arg) to TGG(Trp) at codon 90. Despite the small number of patients identified, these findings indicate that there is a common mutation in African-Americans while there may be molecular genetic heterogeneity in other ethnic groups.

Phenotype-genotype correlations in skeletal muscle of patients with mtDNA deletions

Large-scale deletions of mitochondrial DNA (mtDNA) have been associated with a subgroup of mitochondrial encephalomyopathies, usually characterized by progressive external ophthalmoplegia (PEO) and mitochondrial proliferation in muscle fibers. We and others have shown that muscle from patients with mtDNA deletions have variable cytochrome c oxidase (COX) deficiency and reduction of mitochondrially-synthesized polypeptides in affected muscle fibers. The present work summarizes the phenotype-genotype correlations observed in patients' muscle. In situ hybridization revealed that, while most COX-deficient fibers had increased levels of mutant mtDNA, they almost invariably had reduced levels of normal mtDNA. PCR quantitation of both deleted and wild-type mtDNAs in normal and respiration-deficient muscle fibers from patients with the "common deletion" showed that deleted mtDNAs were present in normal fibers (31 +/- 26%), but their percentages were much higher in affected fibers (95% +/- 2%). Absolute levels of deleted mtDNA were also increased in affected fibers, whereas absolute levels of wild-type mtDNA were significantly reduced. Taken together, our results suggest that although a specific ratio between mutant and wild-type mitochondrial genomes is probably the major determinant of the respiratory chain deficiency associated with mtDNA deletions, the reduction in the absolute amounts of wild-type mtDNA may also play a significant pathogenetic role.

Molecular genetic heterogeneity of phosphoglycerate kinase (PGK) deficiency

Phosphoglycerate kinase (PGK; EC 2.7.2.3) is a glycolytic enzyme encoded by a single gene on the X chromosome and ubiquitously expressed. Hereditary PGK deficiency can cause hemolytic anemia, central nervous system dysfunction, and/or myopathy characterized by exercise intolerance, cramps, and myoglobinuria. So far, 20 PGK variants with reduced PGK activity have been identified, 8 of them in patients with myopathy. Six missense mutations and one splice-junction mutation have been identified in 7 patients, 2 of whom had myopathy. However, the biochemical and molecular bases for clinical heterogeneity in PGK deficiency remain unknown.

Heterozygotes for plasmalemmal carnitine transporter defect are at increased risk for valproic acid-associated impairment of carnitine uptake in cultured human skin fibroblasts

One of the mechanisms by which chronic valproic acid (VPA) therapy induces serum and tissue depletion of carnitine in normal controls is through inhibition of plasmalemmal carnitine uptake (Tein et al 1993). To determine the effect of VPA on proven heterozygotes for the plasmalemmal carnitine transporter defect, we studied this system in cultured human skin fibroblasts with reduced Vmax for the carnitine transporter using L-[3H]carnitine. There was en exponential dose-dependent decrease in carnitine uptake with increasing VPA concentrations and the relative inhibitory effect was the same for all three carnitine concentrations for a given cell line. Importantly, the lower the maximal velocity of carnitine uptake of the heterozygote, the lower the number of carnitine transporters and the lower the carnitine uptake per given concentration of VPA. The degree of inhibition was also directly proportional to the time of VPA preincubation up to a specific maximal saturation time. The maximal effect of VPA exposure time was reached by 10 days in the control cell line and by 3 days in the two heterozygote lines, probably reflecting earlier saturation. We conclude that patients who are heterozygous for the plasmalemmal carnitine transporter defect are at increased risk for VPA-associated serum and tissue depletion of carnitine through inhibition of plasmalemmal carnitine uptake.

Aberrant splicing in adult onset glycogen storage disease type II (GSDII): molecular identification of an IVS1 (-13T-->G) mutation in a majority of patients and a novel IVS10 (+1GT-->CT) mutation

Two newly identified splice site mutations (IVS1 -13T-->G and IVS10 +1GT-->CT) were found in a patient with adult onset of the autosomal recessive disorder glycogen storage disease type II (GSDII). The IVS1 -13T-->G transversion in the acceptor splice site was found on one allele in over two thirds of adult onset GSDII patients studied (28/41), but was not seen in 58 normal or 12 infantile onset GSDII chromosomes. Molecular analysis of cDNA from the index patient and four additional, ethnically different, individuals carrying the IVS1 -13T-->G transversion showed splicing out of the first coding exon as well as rare utilization of a cryptic splice site in the exon. An IVS10 +1GT-->CT transversion, unique to the index patient, was detected on the second chromosome. The IVS10 +1GT-->CT results in splicing out of exon 10 including part of the enzyme catalytic site. Additionally, a large deletion encompassing exon 18, previously described in four unrelated patients, was also detected in three unrelated adult GSDII patients, two of whom carried the IVS1 -13T-->G transversion. The frequency of the IVS1 splice site mutation suggests that detection of this mutation could potentially aid in the diagnosis of the phenotypically variable syndrome of adult onset GSDII. The finding that the -13T-->G mutation is a very common mutation in adult onset GSDII patients of varying ethnic and racial backgrounds, suggests that it is either an ancient mutation or confers a selective advantage. Although to our knowledge these are the first splice site mutations to be reported for GSDII, additional splice site mutations are likely and could provide the basis for later onset disease in GSDII.

Molecular genetic studies of muscle lactate dehydrogenase deficiency in white patients

We identified two new mutations in 2 white patients with muscle lactate dehydrogenase deficiency. Both patients had exercise intolerance, cramps, and recurrent myoglobinuria. One patient was homozygous for a 2-bp deletion in exon 5, resulting in a frameshift with premature termination of translation. The second patient was homozygous for a G-->A substitution at the 3' end of exon 2, leading to exon skipping and splicing of exon 1 to exon 3; the aberrantly spliced messenger RNA contains a frameshift, resulting in premature termination of translation. The present report provides evidence of molecular genetic heterogeneity in white patients with muscle lactate dehydrogenase deficiency.

Myophosphorylase deficiency: an unusually severe form with myoglobinuria

Myophosphorylase deficiency (McArdle disease) is characterized by exercise intolerance that usually starts in childhood. Severe cramps and myoglobinuria are rarely problems in children. We describe an 8-year-old boy with exercise-induced myoglobinuria; he was homozygous for the mutation most commonly encountered in patients with typical McArdle disease.

Ekbom's syndrome: lipomas, ataxia, and neuropathy with MERRF

A 66-year-old woman with hereditary deafness and multiple symmetric lipomas presented with ataxia, slight myopathy, and neuropathy. Molecular genetic analysis of mitochondrial DNA revealed the adenine to guanine transition at position 8344 in the tRNA gene for lysine that has been associated with the myoclonic epilepsy and ragged red fiber (MERRF) syndrome. The deafness was transmitted by the patient's father and may have been an unrelated autosomal defect rather than a paternally transmitted mitochondrial point mutation.