Mutation screening of the RYR1 gene and identification of two novel mutations in Italian malignant hyperthermia families.
Point mutations in the ryanodine receptor (RYR1) gene are associated with malignant hyperthermia, an autosomal dominant disorder triggered in susceptible people (MHS) by volatile anaesthetics and depolarising skeletal muscle relaxants. To date, 17 missense point mutations have been identified in the human RYR1 gene by screening of the cDNA obtained from muscle biopsies. Here we report single strand conformation polymorphism (SSCP) screening for nine of the most frequent RYR1 mutations using genomic DNA isolated from MHS patients. In addition, the Argl63Cys mutation was analysed by restriction enzyme digestion. We analysed 57 unrelated patients and detected seven of the known RYR1 point mutations. Furthermore, we found a new mutation, Arg2454His, segregating with the MHS phenotype in a large pedigree and a novel amino acid substitution at position 2436 in another patient, indicating a 15.8% frequency of these mutations in Italian patients. A new polymorphic site in intron 16 that causes the substitution of a G at position -7 with a C residue was identified. (+info)
Mutations in the nebulin gene associated with autosomal recessive nemaline myopathy.
The congenital nemaline myopathies are rare hereditary muscle disorders characterized by the presence in the muscle fibers of nemaline bodies consisting of proteins derived from the Z disc and thin filament. In a single large Australian family with an autosomal dominant form of nemaline myopathy, the disease is caused by a mutation in the alpha-tropomyosin gene TPM3. The typical form of nemaline myopathy is inherited as an autosomal recessive trait, the locus of which we previously assigned to chromosome 2q21.2-q22. We show here that mutations in the nebulin gene located within this region are associated with the disease. The nebulin protein is a giant protein found in the thin filaments of striated muscle. A variety of nebulin isoforms are thought to contribute to the molecular diversity of Z discs. We have studied the 3' end of the 20. 8-kb cDNA encoding the Z disc part of the 800-kDa protein and describe six disease-associated mutations in patients from five families of different ethnic origins. In two families with consanguineous parents, the patients were homozygous for point mutations. In one family with nonconsanguineous parents, the affected siblings were compound heterozygotes for two different mutations, and in two further families with one detected mutation each, haplotypes are compatible with compound heterozygosity. Immunofluorescence studies with antibodies specific to the C-terminal region of nebulin indicate that the mutations may cause protein truncation possibly associated with loss of fiber-type diversity, which may be relevant to disease pathogenesis. (+info)
A mutation in the transmembrane/luminal domain of the ryanodine receptor is associated with abnormal Ca2+ release channel function and severe central core disease.
Central core disease is a rare, nonprogressive myopathy that is characterized by hypotonia and proximal muscle weakness. In a large Mexican kindred with an unusually severe and highly penetrant form of the disorder, DNA sequencing identified an I4898T mutation in the C-terminal transmembrane/luminal region of the RyR1 protein that constitutes the skeletal muscle ryanodine receptor. All previously reported RYR1 mutations are located either in the cytoplasmic N terminus or in a central cytoplasmic region of the 5,038-aa protein. The I4898T mutation was introduced into a rabbit RYR1 cDNA and expressed in HEK-293 cells. The response of the mutant RyR1 Ca2+ channel to the agonists halothane and caffeine in a Ca2+ photometry assay was completely abolished. Coexpression of normal and mutant RYR1 cDNAs in a 1:1 ratio, however, produced RyR1 channels with normal halothane and caffeine sensitivities, but maximal levels of Ca2+ release were reduced by 67%. [3H]Ryanodine binding indicated that the heterozygous channel is activated by Ca2+ concentrations 4-fold lower than normal. Single-cell analysis of cotransfected cells showed a significantly increased resting cytoplasmic Ca2+ level and a significantly reduced luminal Ca2+ level. These data are indicative of a leaky channel, possibly caused by a reduction in the Ca2+ concentration required for channel activation. Comparison with two other coexpressed mutant/normal channels suggests that the I4898T mutation produces one of the most abnormal RyR1 channels yet investigated, and this level of abnormality is reflected in the severe and penetrant phenotype of affected central core disease individuals. (+info)
A nemaline myopathy mutation in alpha-tropomyosin causes defective regulation of striated muscle force production.
Nemaline myopathy (NM) is a rare autosomal dominant skeletal muscle myopathy characterized by severe muscle weakness and the subsequent appearance of nemaline rods within the muscle fibers. Recently, a missense mutation inTPM3, which encodes the slow skeletal alpha-tropomyosin (alphaTm), was linked to NM in a large kindred with an autosomal-dominant, childhood-onset form of the disease. We used adenoviral gene transfer to fully differentiated rat adult myocytes in vitro to determine the effects of NM mutant human alphaTm expression on striated muscle sarcomeric structure and contractile function. The mutant alphaTm was expressed and incorporated correctly into sarcomeres of adult muscle cells. The primary defect caused by expression of the mutant alphaTm was a decrease in the sensitivity of contraction to activating Ca(2+), which could help explain the hypotonia seen in NM. Interestingly, NM mutant alphaTm expression did not directly result in nemaline rod formation, which suggests that rod formation is secondary to contractile dysfunction and that load-dependent processes are likely involved in nemaline rod formation in vivo. (+info)
A novel nemaline myopathy in the Amish caused by a mutation in troponin T1.
The nemaline myopathies are characterized by weakness and eosinophilic, rodlike (nemaline) inclusions in muscle fibers. Amish nemaline myopathy is a form of nemaline myopathy common among the Old Order Amish. In the first months of life, affected infants have tremors with hypotonia and mild contractures of the shoulders and hips. Progressive worsening of the proximal contractures, weakness, and a pectus carinatum deformity develop before the children die of respiratory insufficiency, usually in the second year. The disorder has an incidence of approximately 1 in 500 among the Amish, and it is inherited in an autosomal recessive pattern. Using a genealogy database, automated pedigree software, and linkage analysis of DNA samples from four sibships, we identified an approximately 2-cM interval on chromosome 19q13.4 that was homozygous in all affected individuals. The gene for the sarcomeric thin-filament protein, slow skeletal muscle troponin T (TNNT1), maps to this interval and was sequenced. We identified a stop codon in exon 11, predicted to truncate the protein at amino acid 179, which segregates with the disease. We conclude that Amish nemaline myopathy is a distinct, heritable, myopathic disorder caused by a mutation in TNNT1. (+info)
An autosomal dominant congenital myopathy with cores and rods is associated with a neomutation in the RYR1 gene encoding the skeletal muscle ryanodine receptor.
Central core disease (CCD) and nemaline myopathy (NM) are congenital myopathies for which differential diagnosis is often based on the presence either of cores or rods. Missense mutations in the skeletal muscle ryanodine receptor gene (RYR1) have been identified in some families with CCD. Mutations in the alpha-tropomyosin and alpha-actin genes have been associated with most dominant forms of NM. Analysis of the RYR1 cDNA in a French family identified a novel Y4796C mutation that lies in the C-terminal channel-forming domain of the RyR1 protein. This mutation was linked not only to a severe and penetrant form of CCD, but also to the presence of rods in the muscle fibres and to the malignant hyperthermia susceptibility (MHS) phenotype. The Y4796C mutation was introduced into a rabbit RYR1 cDNA and expressed in HEK-293 cells. Expression of the mutant RYR1 cDNA produced channels with increased caffeine sensitivity and a significantly reduced maximal level of Ca(2+) release. Single-cell Ca(2+) analysis showed that the resting cytoplasmic level was increased by 60% in cells expressing the mutant channel. These data support the view that the rate of Ca(2+) leakage is increased in the mutant channel. The resulting chronic elevation in myoplasmic concentration is likely to be responsible for the severe expression of the disease. Haplotyping analysis indicated that the mutation arose as a neomutation in the proband. This first report of a neomutation in the RYR1 gene has strong implications for genetic linkage studies of MHS or CCD, two diseases characterized by a genetic heterogeneity. (+info)
Alteration of tropomyosin function and folding by a nemaline myopathy-causing mutation.
Mutations in the human TPM3 gene encoding gamma-tropomyosin (alpha-tropomyosin-slow) expressed in slow skeletal muscle fibers cause nemaline myopathy. Nemaline myopathy is a rare, clinically heterogeneous congenital skeletal muscle disease with associated muscle weakness, characterized by the presence of nemaline rods in muscle fibers. In one missense mutation the codon corresponding to Met-8, a highly conserved residue, is changed to Arg. Here, a rat fast alpha-tropomyosin cDNA with the Met8Arg mutation was expressed in Escherichia coli to investigate the effect of the mutation on in vitro function. The Met8Arg mutation reduces tropomyosin affinity for regulated actin 30- to 100-fold. Ca(2+)-sensitive regulatory function is retained, although activation of the actomyosin S1 ATPase in the presence of Ca(2+) is reduced. The poor activation may reflect weakened actin affinity or reduced effectiveness in switching the thin filament to the open, force-producing state. The presence of the Met8Arg mutation severely, but locally, destabilizes the tropomyosin coiled coil in a model peptide, and would be expected to impair end-to-end association between TMs on the thin filament. In muscle, the mutation may alter thin filament assembly consequent to lower actin affinity and altered binding of the N-terminus to tropomodulin at the pointed end of the filament. The mutation may also reduce force generation during activation. (+info)
A mutation in alpha-tropomyosin(slow) affects muscle strength, maturation and hypertrophy in a mouse model for nemaline myopathy.
Nemaline myopathy is a hereditary disease of skeletal muscle defined by a distinct pathology of electron-dense accumulations within the sarcomeric units called rods, muscle weakness and, in most cases, a slow oxidative (type 1) fiber predominance. We generated a transgenic mouse model to study this disorder by expressing an autosomal dominant mutant of alpha-tropomyosin(slow) previously identified in a human cohort. Rods were found in all muscles, but to varying extents which did not correlate with the amount of mutant protein present. In addition, a pathological feature not commonly associated with this disorder, cytoplasmic bodies, was found in the mouse and subsequently identified in human samples. Muscle weakness is a major feature of this disease and was examined with respect to fiber composition, degree of rod-containing fibers, fiber mechanics and fiber diameter. Hypertrophy of fast, glycolytic (type 2B) fibers was apparent at 2 months of age. Muscle weakness was apparent in mice at 5-6 months of age, mimicking the late onset observed in humans with this mutation. The late onset did not correlate with observed changes in fiber type and rod pathology. Rather, the onset of muscle weakness correlates with an age-related decrease in fiber diameter and suggests that early onset is prevented by hypertrophy of fast, glycolytic fibers. We suggest that the clinical phenotype is precipitated by a failure of the hypertrophy to persist and therefore compensate for muscle weakness. (+info)