Amino Acid Changes at Arginine 204 of Troponin I Result in Increased Calcium Sensitivity of Force Development

Small molecule drugs to improve sarcomere function in those with acquired and inherited myopathies

Muscle weakness is a hallmark of inherited or acquired myopathies. It is a major cause of functional impairment and can advance to life-threatening respiratory insufficiency. During the past decade, several small-molecule drugs that improve the contractility of skeletal muscle fibers have been developed. In this review, we provide an overview of the available literature and the mechanisms of action of small-molecule drugs that modulate the contractility of sarcomeres, the smallest contractile units in striated muscle, by acting on myosin and troponin. We also discuss their use in the treatment of skeletal myopathies. The first of three classes of drugs discussed here increase contractility by decreasing the dissociation rate of calcium from troponin and thereby sensitizing the muscle to calcium. The second two classes of drugs directly act on myosin and stimulate or inhibit the kinetics of myosin-actin interactions, which may be useful in patients with muscle weakness or stiffness.NEW & NOTEWORTHY During the past decade, several small molecule drugs that improve the contractility of skeletal muscle fibers have been developed. In this review, we provide an overview of the available literature and the mechanisms of action of small molecule drugs that modulate the contractility of sarcomeres, the smallest contractile units in striated muscle, by acting on myosin and troponin.

Clinical and functional characterization of a long survivor congenital titinopathy patient with a novel metatranscript-only titin variant

Congenital titinopathies are an emerging group of a potentially severe form of congenital myopathies caused by biallelic mutations in titin, encoding the largest existing human protein involved in the formation and stability of sarcomeres. In this study we describe a patient with a congenital myopathy characterized by multiple contractures, a rigid spine, non progressive muscular weakness, and a novel homozygous TTN pathogenic variant in a metatranscript-only exon: the c.36400A > T, p.Lys12134*. Muscle biopsies showed increased internalized nuclei, variability in fiber size, mild fibrosis, type 1 fiber predominance, and a slight increase in the number of satellite cells. RNA studies revealed the retention of intron 170 and 171 in the open reading frame, and immunoflourescence and western blot studies, a normal titin content. Single fiber functional studies showed a slight decrease in absolute maximal force and a cross-sectional area with no decreases in tension, suggesting that weakness is not sarcomere-based but due to hypotrophy. Passive properties of single fibers were not affected, but the observed increased calcium sensitivity of force generation might contribute to the contractural phenotype and rigid spine of the patient. Our findings provide evidence for a pathogenic, causative role of a metatranscript-only titin variant in a long survivor congenital titinopathy patient with distal arthrogryposis and rigid spine.

A case report of recessive restrictive cardiomyopathy caused by a novel mutation in cardiac troponin I (TNNI3)

Background

Restrictive cardiomyopathy is a rare cardiac disease, for which several genes including TNNT2, MYPN, FLNC and TNNI3 have been associated with its familial form.

Case presentation

Here we describe a female proband with a severely manifested restrictive phenotype leading to heart transplantation at the age of 41, who was found homozygous for the novel TNNI3 mutation: NM_000363.4:c.586G > C, p.(Asp196His). Her parents were third-degree cousins originating from a small village and although they were found heterozygous for the same variant they displayed no symptoms of the disease. Her older sister who was also found heterozygous was asymptomatic. Her twin sister and her brother who were homozygous for the same variant displayed a restrictive and a hypertrophic phenotype, respectively. Their children are all carriers of the mutation and remain asymptomatic until the age of 21.

Conclusion

These observations point to a recessive mode of inheritance reported for the first time for this combination of gene/disease.

A mutation of beta-tropomyosin gene in a Chinese family with distal arthrogryposis type I

BACKGROUND

Distal arthrogryposis (DA) is the most common congenital limb malformation secondary to the functional defects of joints and muscles. DA1 is one of the most commonly described forms of DA. The characteristics of DA1 include bilateral and symmetric clenched fist, overlapping fingers, camptodactyly, ulnar deviation of fingers, and positional foot deformities such as talipes equinovarus. Previous studies demonstrate that mutations of TPM2, TNNI2, TNNT3, MYH3 and MYBPC1 may contribute to DA1.

MATERIALS AND METHODS

The present study investigated 8 DA1 families/patients and 1 DA2B patient, determined sequences of TPM2, TNNI2, TNNT3, MYH3 and MYBPC1 and detected the mutation by multiple sequence alignments and bioinformatic prediction of mutation.

RESULTS

We identified a novel missense mutation of TPM2 (c.463G>A; p.A155T) in a DA1 family without genetic mutant of TNNI2, TNNT3, MYH3 and MYBPC1.

CONCLUSION

The mutation of TPM2 (c.463G>A; p.A155T) led to DA1 of the family. The identification of the mutation expands the spectrum of known TPM2 mutations, and it may contribute to novel approaches to genetic diagnosis and counseling of families with DA1.