TPM3 deletions cause a hypercontractile congenital muscle stiffness phenotype

OBJECTIVE

Mutations in TPM3, encoding Tpm3.12, cause a clinically and histopathologically diverse group of myopathies characterized by muscle weakness. We report two patients with novel de novo Tpm3.12 single glutamic acid deletions at positions ΔE218 and ΔE224, resulting in a significant hypercontractile phenotype with congenital muscle stiffness, rather than weakness, and respiratory failure in one patient.

METHODS

The effect of the Tpm3.12 deletions on the contractile properties in dissected patient myofibers was measured. We used quantitative in vitro motility assay to measure Ca(2+) sensitivity of thin filaments reconstituted with recombinant Tpm3.12 ΔE218 and ΔE224.

RESULTS

Contractility studies on permeabilized myofibers demonstrated reduced maximal active tension from both patients with increased Ca(2+) sensitivity and altered cross-bridge cycling kinetics in ΔE224 fibers. In vitro motility studies showed a two-fold increase in Ca(2+) sensitivity of the fraction of filaments motile and the filament sliding velocity concentrations for both mutations.

INTERPRETATION

These data indicate that Tpm3.12 deletions ΔE218 and ΔE224 result in increased Ca(2+) sensitivity of the troponin-tropomyosin complex, resulting in abnormally active interaction of the actin and myosin complex. Both mutations are located in the charged motifs of the actin-binding residues of tropomyosin 3, thus disrupting the electrostatic interactions that facilitate accurate tropomyosin binding with actin necessary to prevent the on-state. The mutations destabilize the off-state and result in excessively sensitized excitation-contraction coupling of the contractile apparatus. This work expands the phenotypic spectrum of TPM3-related disease and provides insights into the pathophysiological mechanisms of the actin-tropomyosin complex.

A and B utrophin in human muscle and sarcolemmal A-utrophin associated with tumours

Utrophin is an autosomal homologue of dystrophin, abnormal expression of which is responsible for X-linked Duchenne and Becker muscular dystrophy. In normal mature muscle utrophin is confined to blood vessels, nerves and myotendinous and neuromuscular junctions. When dystrophin is absent utrophin is abundant on the sarcolemma. This has raised the possibility that up-regulation of utrophin may be of therapeutic benefit. Two full-length transcripts of utrophin, A and B, have been identified, which are regulated by alternatively spliced 5' promoters. In dystrophic mouse muscle, the A isoform is present on the sarcolemma, whereas the B form is confined to blood vessels. We show here using immunohistochemistry and human isoform-specific antibodies that A- and B-utrophin localisation is the same in human muscle. The A isoform is present on the sarcolemma of foetal human muscle fibres, regenerating fibres, fibres deficient in dystrophin and on blood vessels and neuromuscular junctions. B-utrophin is only detected on blood vessels. We also show that muscle adjacent to some soft tissue tumours shows increased sarcolemmal utrophin-A, showing that utrophin and dystrophin can simultaneously localise to the sarcolemma and raising the possibility that factor(s) from the tumour cells or accompanying inflammatory cells may have a role in regulating utrophin.

Congenital nemaline myopathy due to ACTA1-gene mutation and carnitine insufficiency: a case report

A premature boy with a congenital form of nemaline myopathy due to mutation in the ACTA1-gene showed decreased carnitine levels in the eighth week of life. After sufficient oral carnitine substitution he improved gradually. In the first 15 months of life he made good progress; he reached full head control, learned to sit unsupported and was able to raise objects. At that time the carnitine levels were normal without substitution. Nemaline myopathy is clinically and genetically heterogenous. The pathogenesis of the muscle weakness is poorly understood. Disturbances of carnitine metabolism in this group of patients as one possibility are conceivable. Further investigations of carnitine metabolism in patients with nemaline myopathy may shed light on the pathogenesis of this entity.

Characterization of human muscle type cofilin (CFL2) in normal and regenerating muscle

Cofilins are actin binding proteins and regulate actin assembly in vivo. Numerous cofilin homologues have been characterized in various organisms including mammals. In mice, a ubiquitously expressed cofilin (CFL1) and a skeletal muscle specific cofilin (CFL2) have been described. In the present study, we identified and characterized a human CFL2 gene localized on chromosome 14, with high homology to murine CFL2. Furthermore, we provide evidence for differentially spliced CFL2 transcripts (CFL2a and CFL2b). CFL2b is expressed predominantly in human skeletal muscle and heart, while CFL2a is expressed in various tissues. Genetic defects of CFL2 were excluded for one human muscle disorder, the chromosome 14 linked distal myopathy MPD1, and shown to be only possible to be a rare cause of another, nemaline myopathy. In a mouse model of mechanically induced muscle damage the changes of cofilin expression were monitored during the first 10 days of regeneration, with dephosphorylated CFL2 being the major isoform at later stages of muscle regeneration. A similar predominance of dephosphorylated CFL2 was observed in chronically regenerating dystrophin-deficient muscles of Duchenne muscular dystrophy patients. Therefore, the CFL2 isoform may play an important role in normal muscle function and muscle regeneration.

Mild phenotype of nemaline myopathy with sleep hypoventilation due to a mutation in the skeletal muscle alpha-actin (ACTA1) gene

Nemaline myopathy is a clinically and genetically heterogeneous condition. The clinical spectrum ranges from severe cases with antenatal or neonatal onset and early death to late onset cases with only slow progression. Three genes are known to cause nemaline myopathy: the genes for nebulin (NEB) on chromosome 2q22, slow alpha-tropomyosin (TPM3) on chromosome 1q21 and skeletal muscle alpha-actin (ACTA1) on chromosome 1q42. We present a 39-year-old lady with a mild form of nemaline myopathy, whom we have followed over a period of 25 years. She presented at the age of 7 years with symptoms of mild axial and proximal muscle weakness. The overall course was essentially static, but at 36 years, she went into life-threatening respiratory failure, for which she is currently treated with night-time ventilation. Muscle biopsies at 12, 17 and 39 years of age showed typical nemaline rods, particularly in type 1 fibres. Areas with unevenness of oxidative stain were present in the second and third biopsies. The presence of rods and core-like areas was confirmed on electron microscopy. There was no detectable alteration in actin expression immunocytochemically. A dominant missense mutation in the skeletal muscle alpha-actin gene (ACTA1) was found. This case illustrates the clinical and genetic heterogeneity of nemaline myopathy, and one phenotype of the wide spectrum of severity caused by mutations in the skeletal muscle alpha-actin (ACTA1) gene. In addition, it shows the diversity of pathological features that can occur in congenital myopathies due to mutations in the same gene.

Severe gamma-sarcoglycanopathy caused by a novel missense mutation and a large deletion

We report two siblings with a relatively severe limb-girdle muscular dystrophy. The elder sister presented at 8 years of age with inability to climb and abnormal gait. At 12 years she was barely ambulant. Her sister followed a similar course. Serum creatine kinase was 8500-10000 IU (N 25-200) in the elder sister and 17000-19000 IU in the younger sister. Muscle biopsy of the elder sister at 8 years showed chronic myopathic changes with loss of muscle fibres, active necrosis and regeneration. Immunocytochemistry demonstrated normal spectrin and dystrophin, reduced alpha-sarcoglycan and absent gamma-sarcoglycan--indicating a gamma-sarcoglycanopathy. Haplotype analysis for the markers D13S115, D13S232, D13S292, D13S787, D13S1243 and D13S283 internal to and flanking the gamma-sarcoglycan gene showed the affected sisters shared haplotypes, indicating it was possible they were suffering from a gamma-sarcoglycanopathy. Non-inheritance of paternal alleles for D13S232, D13S292 and D13S1243 suggested the inheritance of a deletion, which was confirmed by FISH, using a genomic probe from the gamma-sarcoglycan gene. The gamma-sarcoglycan cDNA was amplified by reverse transcriptase PCR from the muscle biopsy of the elder sister and sequenced. A missense mutation changing codon 69 from GGC glycine to CGC arginine was identified. HhaI digestion of exon 3 genomic PCR products showed the two affected sisters were hemizygous for the mutation, while the mother and grandmother were heterozygotes. The mutation, identified by SSCP analysis, was not observed in 116 unrelated, unaffected individuals. Previously, only two other missense mutations, the Cys283Tyr missense mutation in Gypsies and the Leu193Ser mutation in a Dutch family, have been described in the gamma-sarcoglycan gene. The fact that the affected individuals in the current and Gypsy families are gamma-sarcoglycan negative may indicate that codons 69 and 283 are important in gamma-sarcoglycan function.

Mutations in the skeletal muscle alpha-actin gene in patients with actin myopathy and nemaline myopathy

Muscle contraction results from the force generated between the thin filament protein actin and the thick filament protein myosin, which causes the thick and thin muscle filaments to slide past each other. There are skeletal muscle, cardiac muscle, smooth muscle and non-muscle isoforms of both actin and myosin. Inherited diseases in humans have been associated with defects in cardiac actin (dilated cardiomyopathy and hypertrophic cardiomyopathy), cardiac myosin (hypertrophic cardiomyopathy) and non-muscle myosin (deafness). Here we report that mutations in the human skeletal muscle alpha-actin gene (ACTA1) are associated with two different muscle diseases, 'congenital myopathy with excess of thin myofilaments' (actin myopathy) and nemaline myopathy. Both diseases are characterized by structural abnormalities of the muscle fibres and variable degrees of muscle weakness. We have identified 15 different missense mutations resulting in 14 different amino acid changes. The missense mutations in ACTA1 are distributed throughout all six coding exons, and some involve known functional domains of actin. Approximately half of the patients died within their first year, but two female patients have survived into their thirties and have children. We identified dominant mutations in all but 1 of 14 families, with the missense mutations being single and heterozygous. The only family showing dominant inheritance comprised a 33-year-old affected mother and her two affected and two unaffected children. In another family, the clinically unaffected father is a somatic mosaic for the mutation seen in both of his affected children. We identified recessive mutations in one family in which the two affected siblings had heterozygous mutations in two different exons, one paternally and the other maternally inherited. We also identified de novo mutations in seven sporadic probands for which it was possible to analyse parental DNA.

Novel mutation in the myelin protein zero gene in a family with intermediate hereditary motor and sensory neuropathy

OBJECTIVES

To determine the molecular basis for autosomal dominant intermediate hereditary motor and sensory neuropathy (HMSN) in a four generation family. The gene defects in families with intermediate HMSN are not known, but it has been suggested that most have X linked HMSN.

METHODS

All participating family members were examined clinically. Genomic DNA was obtained from 10 affected and seven unaffected members. Linkage analysis for the known HMSN loci was first performed. Mutations in the peripheral myelin protein zero gene (PMP0) were sought in two affected members, using one unaffected member for comparison, by amplification of the six exons of the gene followed by single strand conformation polymorphism (SSCP) analysis, dideoxy fingerprinting (ddF), and sequencing. Subsequently, the mutation was screened for in all affected and unaffected members in the family using Alu I digestion and in 100 unrelated control subjects using "snap back" SSCP analysis. Sequencing of cDNA from a sural nerve biopsy from an affected member was also performed.

RESULTS

The clinical phenotype was of variable severity, with motor nerve conduction velocities in the intermediate range. Linkage to PMP0 was demonstrated. Analysis of genomic DNA and cDNA for PMP0 identified a novel codon 35 GAC to TAC mutation. The mutation produces an inferred amino acid change of aspartate to tyrosine at codon six of the processed protein (Asp6Tyr) in the extracellular domain and was present in all affected family members but not in 100 unrelated controls.

CONCLUSIONS

The present findings further extend the range of phenotypes associated with PMP0 mutations and indicate that families with "intermediate" HMSN need not necessarily be X-linked as previously suggested.