An A-to-C substitution involving the translation initiation codon in a patient with myophosphorylase deficiency (McArdle's disease)

Molecular analysis of myophosphorylase deficiency in Dutch patients with McArdle's disease

We report on 8 Dutch patients with McArdle's disease from 6 unrelated families. Molecular analysis revealed the presence of four previously described mutations: the common R49X mutation, the IVS14+1G>A mutation and the recently reported R269X and Y84X nonsense mutations; and two new molecular defects: a missense mutation R138W in the homozygous state in two siblings, and a frameshift mutation c.1797delT. This first genetic study of patients from The Netherlands with McArdle's disease confirms that the R49X mutation is also the most common in Dutch patients, and that there is genetic heterogeneity within this population. Moreover, our data support the hypothesis that the Y84X mutation is a relatively frequent mutation in McArdle's patients with a Central European background, and expand the already crowded map of mutations within the PYGM gene responsible for McArdle's disease.

The eyeless mouse mutation (ey1) removes an alternative start codon from the Rx/rax homeobox gene

The eyeless inbred mouse strain ZRDCT has long served as a spontaneous model for human anophthalmia and the evolutionary reduction of eyes that has occurred in some naturally blind mammals. ZRDCT mice have orbits but lack eyes and optic tracts and have hypothalamic abnormalities. Segregation data suggest that a small number of interacting genes are responsible, including at least one major recessive locus, ey1. Although predicted since the 1940s, these loci were never identified. We mapped ey1 to chromosome 18 using an F2 genome scan and there found a Met10-->Leu mutation in Rx/rax, a homeobox gene that is expressed in the anterior headfold, developing retina, pineal, and hypothalamus and is translated via a leaky scanning mechanism. The mutation affects a conserved AUG codon that functions as an alternative translation initiation site and consequently reduces the abundance of Rx protein. In contrast to a targeted Rx null allele, which causes anophthalmia, central nervous system defects, and neonatal death, the hypomorphic M10L allele is fully viable.

The molecular diagnosis of metabolic myopathies

The metabolic myopathies are distinguished by extensive clinical and genetic heterogeneity within and between individual disorders. There are a number of explanations for the variability observed that go beyond single gene mutations or degrees of heteroplasmy in the case of mitochondrial DNA mutations. Some of the contributing factors include protein subunit interactions, tissue-specificity, modifying genetic factors, and environmental triggers. Advances in the molecular analysis of metabolic myopathies during the last decade have not only improved the diagnosis of individual disorders but also helped to characterize the contributing factors that make these disorders so complex.

Neonatal metabolic myopathies

The primary presentations of neuromuscular disease in the newborn period are hypotonia and weakness. Although metabolic myopathies are inherited disorders that present from birth and may present with subtle to marked neonatal hypotonia, a number of these defects are diagnosed classically in childhood, adolescence, or adulthood. Disorders of glycogen, lipid, or mitochondrial metabolism may cause three main clinical syndromes in muscle, namely, (1) progressive weakness with hypotonia (e.g., acid maltase, debrancher enzyme, and brancher enzyme deficiencies among the glycogenoses; carnitine uptake and carnitine acylcarnitine translocase defects among the fatty acid oxidation (FAO) defects; and cytochrome oxidase deficiency among the mitochondrial disorders) or (2) acute, recurrent, reversible muscle dysfunction with exercise intolerance and acute muscle breakdown or myoglobinuria (with or without cramps), e.g., phosphorylase, phosphofructokinase, and phosphoglycerate kinase among the glycogenoses and carnitine palmitoyltransferase II deficiency among the disorders of FAO or (3) both (e.g., long-chain or very long-chain acyl coenzyme A (CoA) dehydrogenase, short-chain L-3-hydroxyacyl-CoA dehydrogenase, and trifunctional protein deficiencies among the FAO defects). Episodes of exercise-induced myoglobinuria tend to present in later childhood or adolescence; however, myoglobinuria in the first year of life may occur in FAO disorders during catabolic crises precipitated by fasting or infection. The following is a survey of genetic disorders of glycogen and lipid metabolism resulting in myopathy, focusing primarily on those defects, to date, that have presented in the neonatal or early infancy period. Disorders of mitochondrial metabolism are discussed in another chapter.

Mutation analysis in myophosphorylase deficiency (McArdle's disease)

Inherited deficiency of myophosphorylase leads to glycogen storage disease type V (McArdle's disease). We performed mutation analysis in 9 patients of eight unrelated families from Germany with typical clinical presentation of myophosphorylase deficiency. Beside previously described mutations we identified four novel mutations in the myophosphorylase gene. Four patients were homozygous for a nonsense mutation Arg49Stop that has been reported to be the most common mutation in white patients. Two affected siblings were compound heterozygotes for a novel missense mutation Gly685Arg and the nonsense mutation Arg49Stop. One patient carried a novel nonsense mutation Arg575Stop and a previously identified missense mutation Gly204Ser. In another patient, we identified a novel missense mutation Gln665Glu and a single-base deletion delA in Lys753. One patient of Turkish ancestry carried a newly identified homozygous A-to-G transition (ATG to GTG) abolishing the translation initiation codon of the myophosphorylase gene. These results suggest that Arg49Stop also is the most common genetic error associated with myophosphorylase deficiency in the German population. Our findings further demonstrate molecular heterogeneity of myophosphorylase deficiency among the clinically homogeneous patients we studied.

Genetic deficiencies of the glycogen phosphorylase system

Several types of glycogen storage disease attributable to a deficiency of phosphorylase or phosphorylase kinase have been described. These diseases have been divided according to clinical symptoms, mode of inheritance, and affected tissue. However, this classification is questionable, as the clinical symptoms of these different diseases are similar, the mode of inheritance is often difficult to establish, and the biochemical assays are subject to several technical problems. A better classification would be based upon the identification of mutations in the respective disease genes. The molecular heterogeneity, however, is large, and at least 10 genes are involved. Mutations have been found in the muscle phosphorylase gene in patients with muscle phosphorylase deficiency, in the gene encoding the liver alpha subunit of phosphorylase kinase in patients with X-linked liver glycogenosis, and in the gene for the muscle alpha subunit of phosphorylase kinase in a patient with muscle phosphorylase kinase deficiency. We review here the different deficiencies of the phosphorylase system.

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.

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.

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.

McArdle's disease: molecular genetics and metabolic consequences of the phenotype

McArdle's disease is defined as a lack of functional muscle glycogen phosphorylase. Analysis of the myophosphorylase gene has demonstrated substantial heterogeneity in the mutations that cause the disease, but in almost all individuals, the molecular phenotype is the absence of the protein in skeletal muscle. Muscle glycogen phosphorylase is a major repository of vitamin B6 in the body, accounting for at least 80% of the total body pool. In McArdle's patients, this pool is therefore missing, introducing the possibility that vitamin B6 metabolism might be altered in these individuals. Preliminary data have shown that McArdle's patients show signs of a subclinical vitamin B6 deficiency, and that oral vitamin B6 supplementation can improve vitamin B6 status and enhance fatigue resistance in muscle.