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.

Importance and challenge of making an early diagnosis in LMNA-related muscular dystrophy

OBJECTIVE

To identify the most useful clinical and histologic markers that facilitate early diagnosis in LMNA-related muscular dystrophy and to assess the usefulness of Western blotting (WB) for lamin A/C.

METHODS

We analyzed the clinical and histologic features and WB results of all patients with laminopathies diagnosed in a research-based diagnostic service over 8 years.

RESULTS

Although patients with congenital muscular dystrophy (MDCL) (n = 5) and Emery-Dreifuss muscular dystrophy (EDMD) (n = 5) had distinctive early clinical features, the lack of a suggestive clinical phenotype significantly delayed diagnosis in 2 of 3 patients with limb-girdle muscular dystrophy (LGMD) (n = 3). In addition, 6 of 20 muscle biopsy samples were considered nondystrophic, which contributed to delays in diagnosis in some patients. Neck extensor involvement (weakness or contractures) was the most consistent clinical sign, present in all patients. Reduced lamin A/C levels on WB were seen in 5 of 9 patients with laminopathies.

CONCLUSION

Clinical features provide the best clues for diagnosing MDCL and EDMD early in the disease, and we urge clinicians to become familiar with those phenotypes. WB for lamin A/C may contribute to diagnosis but requires technical expertise, and results are normal in many individuals with LMNA mutations. Because of the survival benefit of early diagnosis and treatment, we recommend that LMNA gene sequencing be performed in all patients with undiagnosed congenital muscular dystrophy and neck extensor weakness, all patients with genetically undiagnosed LGMD, and those with suggestive clinical signs and nonspecific histologic abnormalities.