Wilhelm Mortier

Mutations in CAV3 cause mechanical hyperirritability of skeletal muscle in rippling muscle disease

Hereditary rippling muscle disease (RMD) is an autosomal dominant human disorder characterized by mechanically triggered contractions of skeletal muscle. Genome-wide linkage analysis has identified an RMD locus on chromosome 3p25. We found missense mutations in positional candidate CAV3 (encoding caveolin 3; ref. 5) in all five families analyzed. Mutations in CAV3 have also been described in limb-girdle muscular dystrophy type 1C (LGMD1C; refs. 6,7), demonstrating the allelism of dystrophic and non-dystrophic muscle diseases.

A common mutation (ε1267delG) in congenital myasthenic patients of Gypsy ethnic origin

Objective: Mutation analysis of the acetylcholine receptor (AChR) e subunit gene in patients with sporadic or autosomal recessive congenital myasthenic syndromes (CMS). Background: The nicotinic AChR of skeletal muscle is a neurotransmitter-gated ion channel that mediates synaptic transmission at the vertebrate neuromuscular junction. Mutations in its gene may cause congenital myasthenic syndromes. A recently described mutation in exon 12 of the AChR e subunit (e1267delG) disrupts the cytoplasmic loop and the fourth transmembrane region (M4) of the AChR e subunit. Methods: Forty-three CMS patients from 35 nonrelated families were clinically classified as sporadic cases of CMS (group III according to European Neuromuscular Centre consensus) and were analyzed for e1267delG by PCR amplification and sequence analysis. Results: The authors report the complete genomic sequence and organization of the gene coding for the e subunit of the human AChR (accession number AF105999). Homozygous e1267delG was identified in 13 CMS patients from 11 independent families. All e1267delG families were of Gypsy or southeastern European origin. Genotype analysis indicated that they derive from a common ancestor (founder) causing CMS in the southeastern European Gypsy population. Phenotype analysis revealed a uniform pattern of clinical features including bilateral ptosis and mild to moderate fatigable weakness of ocular, facial, bulbar, and limb muscles. Conclusions: The mutation e1267delG might be frequent in European congenital myasthenic syndrome patients of Gypsy ethnic origin. In general, patients (e1267delG) were characterized by the onset of symptoms in early infancy, the presence of ophthalmoparesis, positive response to anticholinesterase treatment, and the benign natural course of the disease.

Rapsyn N88K is a frequent cause of congenital myasthenic syndromes in European patients

Background: Mutations in various genes of the neuromuscular junction may cause congenital myasthenic syndromes (CMS). Most mutations identified to date affect the ε-subunit gene of the acetylcholine receptor (AChR), leading to end-plate AChR deficiency. Recently, three different mutations in the RAPSN gene have been identified in four CMS patients with AChR deficiency. Objective: To perform mutation analysis of the RAPSN gene in patients with sporadic or autosomal recessive CMS. Methods: One hundred twenty CMS patients from 110 unrelated families were analyzed for the RAPSN mutation N88K by restriction fragment length polymorphism and sequence analysis. Results: In 12 CMS patients from 10 independent families, RAPSN N88K was identified either homozygous or heteroallelic to another missense mutation. Symptoms usually started perinatally or in the first years of life. However, one patient did not show any myasthenic symptoms before the third decade. Clinical symptoms typically included bilateral ptosis, weakness of facial, bulbar, and limb muscles, and a favorable response to anticholinesterase treatment. Crisis-like exacerbations with respiratory insufficiency provoked by stress, fever, or infections in early childhood were frequent. All RAPSN N88K families originate from Central or Western European countries. Genotype analysis indicated that they derive from a common ancestor (founder). Conclusions: The RAPSN mutation N88K is a frequent cause of rapsyn-related CMS in European patients. In general, patients (RAPSN N88K) were characterized by mild to moderate myasthenic symptoms with favorable response to anticholinesterase treatment. However, severity and onset of symptoms may vary to a great extent.

A sporadic case of rippling muscle disease caused by a de novo caveolin-3 mutation

Objective: To determine the cause of sporadic rippling muscle disease (RMD) in a 24-year-old patient. Background: RMD is a rare myopathy characterized by percussion-induced rapid muscle contractions (PIRC), muscle mounding, and rippling waves. We have recently found that autosomal dominant RMD is caused by mutations in the caveolin-3 gene (CAV3) on chromosome 3p25. Possibly, increased activity of neuronal nitric oxide synthase (nNOS) contributes to the clinical characteristics of increased mechanical muscle hyperexcitability. Methods: Clinical examination, mutational analysis, and immunohistochemistry of muscle tissue were performed in a patient with sporadic RMD. Results: The authors observed a de novo CAV3 missense mutation Arg26Gln. Immunohistochemistry showed reduced caveolin-3 surface expression in a muscle biopsy. In addition, the authors found normal sarcolemmal nNOS expression and a reduced expression of α-dystroglycan in muscle fibers. Conclusions: These data confirm that RMD is caused by CAV3 mutations. Moreover, there is evidence that CAV3 mutations may also be found in patients without a positive family history of RMD.

Phenotypic variability in rippling muscle disease

Objective: To characterize the phenotype of hereditary rippling muscle disease (RMD) and to report the results of genetic linkage studies. Background: RMD is a rare autosomal-dominant inherited muscle disorder. Individuals complain of muscle stiffness, exercise-induced muscle pain, and cramp-like sensations. The characteristic feature of RMD is increased mechanical muscle irritability, which is electrically silent in electromyographic examinations. Methods: Forty-six individuals from two unrelated German kindreds with RMD were examined. Linkage analysis to the RMD locus on chromosome 1q41-q43 was performed. Results: In kindred A, 15 individuals from four generations, and in kindred B, four individuals from three generations had clinical features of RMD. The most consistent clinical findings were percussion-induced rapid muscle contractions (PIRCs) and muscle mounding, which were present in all 19 affected individuals. Only 12 individuals exhibited muscle rippling, indicating that rippling is not always present in RMD. Twelve of 19 individuals had muscle-related complaints, primarily exertional cramps and stiffness. The mean age at the onset of complaints was 22 years (range, 5 to 54 years). Seven of 19 individuals showed only mechanical-induced muscle irritability but did not have muscular symptoms. Genetic analysis excluded linkage to the RMD locus on chromosome 1q4 in both kindreds. Conclusions: The phenotype of RMD is variable but generalized PIRCs are the most obvious and reliable clinical feature of RMD. Diagnostic criteria of RMD should include generalized PIRCs in addition to muscle mounding, rippling, and creatine kinase elevation.

Congenital myasthenic syndrome due to a novel missense mutation in the gene encoding choline acetyltransferase

Congenital myasthenic syndromes are caused by different genetic defects affecting proteins expressed at the neuromuscular junction. Recently, the first molecular genetic defect resulting in a presynaptic congenital myasthenic syndrome has been reported: Recessive loss-of-function mutations in CHAT, the gene encoding choline acetyltransferase, were described in five congenital myasthenic syndrome families. In this study, we investigated three patients from two independent Turkish kinships. Clinically, all patients presented with moderate myasthenic symptoms including ptosis and muscle weakness with increased fatigability. Multiple episodes of sudden apnea were reported for all patients. One child suffering from a second, unrelated disorder, i.e. hepatocellular carcinoma, showed a severe myasthenic phenotype, requiring permanent ventilation. Genetically, we identified a novel missense mutation (I336T) in the CHAT gene homozygously in all three patients. Haplotype analysis revealed that the mutant allele cosegregates with the clinical phenotype in both families (maximum combined two-point LOD-score of 2.46 for D10S1793). In summary, we confirm that CHAT mutations are responsible for a clinically distinct form of congenital myasthenic syndrome, characterized by episodic apnea. Infections and stress may lead to a life-threatening failure of neuromuscular transmission in congenital myasthenic syndrome with episodic apnea. The observation of the same mutation (I336T) in two independent Turkish kinships may suggest a common origin, i.e. founder.

A G468-T AMPD1 mutant allele contributes to the high incidence of myoadenylate deaminase deficiency in the Caucasian population

Myoadenylate deaminase deficiency is the most common metabolic disorder of skeletal muscle in the Caucasian population, affecting approximately 2% of all individuals. Although most deficient subjects are asymptomatic, some suffer from exercise-induced myalgia suggesting a causal relationship between a lack of enzyme activity and muscle function. In addition, carriers of this derangement in purine nucleotide catabolism may have an adaptive advantage related to clinical outcome in heart disease. The molecular basis of myoadenylate deaminase deficiency in Caucasians has been attributed to a single mutant allele characterized by double C to T transitions at nucleotides +34 and +143 in mRNA encoded by the AMPD1 gene. Polymerase chain reaction-based strategies have been developed to specifically identify this common mutant allele and are considered highly sensitive. Consequently, some laboratories preferentially use this technique over other available diagnostic tests for myoadenylate deaminase deficiency. We previously identified a G468-T mutation in one symptomatic patient who was only heterozygous for the common AMPD1 mutant allele. In this report, nine additional individuals with this compound heterozygous genotype are revealed in a survey of 48 patients with documented deficiency of skeletal muscle adenosine monophosphate deaminase and exercise-induced myalgia. Western blot analysis of leftover biopsy material from one of these individuals does not detect any immunoreactive myoadenylate deaminase polypeptide. Baculoviral expression of the G468-T mutant allele produces a Q156H substitution enzyme exhibiting labile catalytic activity. These combined results demonstrate that the G468-T transversion is dysfunctional and further indicate that AMPD1 alleles harboring this mutation contribute to the high incidence of partial and complete myoadenylate deaminase deficiency in the Caucasian population. Consequently, genetic tests for abnormal AMPD1 expression designed to diagnose patients with metabolic myopathy, and to evaluate genetic markers for clinical outcome in heart disease should not be based solely on the detection of a single mutant allele.

X-linked myotubular myopathy in a female infant caused by a new MTM1 gene mutation

X-linked myotubular myopathy usually affects male infants with a severe phenotype leading to early death or survival with severe handicaps. Female carriers have been reported manifesting in childhood with slowly progressive muscle weakness only. The authors describe a now 5-year-old girl with prenatal/neonatal onset of an X-linked myotubular myopathy due to a 605delT mutation in the myotubularin gene.

Severe mucous membrane involvement in epidermolysis bullosa simplex with muscular dystrophy due to a novel plectin gene mutation

Epidermolysis bullosa simplex with muscular dystrophy (OMIM 226670) is an autosomal recessive disorder caused by mutations of the human plectin gene on chromosome 8q24. Here, we report a 3-year-old girl, offspring of a consanguineous Lebanese family, who presented with skin blistering and recurrent episodes of severe respiratory distress necessitating tracheotomy at the age of 2 years. Repeated examination did not provide any evidence of muscle involvement. Indirect immunofluorescence analysis of a diagnostic skin biopsy with four different domain specific plectin antibodies showed a complete absence of plectin staining. Mutation analysis revealed a novel homozygous single guanine insertion mutation (5588insG/5588insG) residing in the N-terminal part of exon 31 of the plectin gene. Conclusion: the complete lack of protein expression, which may be attributed to a nonsense-mediated plectin mRNA decay, is likely to cause muscular dystrophy and other multisystem involvement later in life.

A newly identified chromosomal microdeletion of the rapsyn gene causes a congenital myasthenic syndrome

The objective is mutation analysis of the RAPSN gene in a patient with sporadic congenital myasthenic syndrome (CMS). Mutations in various genes encoding proteins expressed at the neuromuscular junction may cause CMS. Most mutations affect the epsilon subunit gene of the acetylcholine receptor (AChR) leading to endplate AChR deficiency. Recently, mutations in the RAPSN gene have been identified in several CMS patients with AChR deficiency. In most patients, RAPSN N88K was identified, either homozygously or heteroallelic to a second missense mutation. A sporadic CMS patient from Germany was analyzed for RAPSN mutations by RFLP, long-range PCR and sequence analysis. Clinically, the patient presents with an early onset CMS, associated with arthrogryposis multiplex congenita, recurrent episodes of respiratory insufficiency provoked by infections, and a moderate general weakness, responsive to anticholinesterase treatment. The mutation RAPSN N88K was found heterozygously to a large deletion of about 4.5 kb disrupting the RAPSN gene. Interestingly, an Alu-mediated unequal homologous recombination may have caused the deletion. We hypothesize that numerous interspersed Alu elements may predispose the RAPSN locus for genetic rearrangements.