Actin-related myopathy without any missense mutation in the ACTA1 gene

Identification of Chimeric RNAs in Pig Skeletal Muscle and Transcriptomic Analysis of Chimeric RNA TNNI2-ACTA1 V1

Chimeric RNA was considered a special marker of cancer. However, recent studies have demonstrated that chimeric RNAs also exist in non-cancerous cells and tissues. Here, we analyzed and predicted jointly 49 chimeric RNAs by Star-Fusion and FusionMap. One chimeric RNA, we named TNNI2-ACTA1, and its eight transcript variants were identified by reverse transcriptase–polymerase chain reaction. The overexpression of TNNI2-ACTA1 V1 inhibited the proliferation of porcine skeletal muscle satellite cells through down-regulating the mRNA expression levels of cell cycle–related genes cyclinD1. However, as parental genes, there is no such effect in the TNNI2 and ACTA1. To explore the underlying mechanism for this phenomenon, we used RNA-seq to profile the transcriptomes of PSCs with overexpression. Compared with the negative control group, 1,592 differentially expressed genes (DEGs) were upregulated and 1,077 DEGs downregulated in TNNI2 group; 1,226 DEGs were upregulated and 902 DEGs downregulated in ACTA1 group; and 13 DEGs were upregulated and 16 DEGs downregulated in TNNI2-ACTA1 V1 group, respectively. Compared with the parental gene groups, three specific genes were enriched in the TNNI2-ACTA1 V1 group (NCOA3, Radixin, and DDR2). These three genes may be the key to TNNI2-ACTA1 V1 regulating cell proliferation. Taken together, our study explores the role of chimeric RNAs in normal tissues. In addition, our study as the first research provides the foundation for the mechanism of chimeric RNAs regulating porcine skeletal muscle growth.

Clinically variable nemaline myopathy in a three-generation family caused by mutation of the skeletal muscle alpha-actin gene

We present here a Finnish nemaline myopathy family with a dominant mutation in the skeletal muscle α-actin gene, p.(Glu85Lys), segregating in three generations. The index patient, a 5-year-old boy, had the typical form of nemaline myopathy with congenital muscle weakness and motor milestones delayed but reached, while his mother never had sought medical attention for her very mild muscle weakness, and his maternal grandmother had been misdiagnosed as having myotonic dystrophy. This illustrates the clinical variability in nemaline myopathy.

Clinical and Histologic Findings in ACTA1-Related Nemaline Myopathy: Case Series and Review of the Literature

BACKGROUND

Nemaline myopathy is a rare congenital disease of skeletal muscle characterized by muscle weakness and hypotonia, as well as the diagnostic presence of nemaline rods in skeletal muscle fibers. Nemaline myopathy is genetically and phenotypically heterogeneous and, so far, mutations in 11 different genes have been associated with this disease. Dominant mutations in ACTA1 are the second most frequent genetic cause of nemaline myopathy and can lead to a variety of clinical and histologic phenotypes.

PATIENTS AND METHODS

We present a series of ACTA1-related cases from a Brazilian cohort of 23 patients with nemaline myopathy, diagnosed after Sanger sequencing the entire coding region of ACTA1, and review the literature on ACTA1-related nemaline myopathy.

RESULTS

The study confirmed ACTA1 mutations in four patients, including one with intranuclear rods, one with large intracytoplasmic aggregates, and two with nemaline intracytoplasmic rods. A repeat muscle biopsy in one patient did not show histological progression.

CONCLUSION

Despite the recognized phenotypic variability in ACTA1-related nemaline myopathy, clinical and histological presentations appear to correlate with the position of the mutation, which confirms emerging genotype/phenotype correlations and better predict the prognosis of affected patients.

Mutations and polymorphisms of the skeletal muscle alpha-actin gene (ACTA1)

The ACTA1 gene encodes skeletal muscle alpha-actin, which is the predominant actin isoform in the sarcomeric thin filaments of adult skeletal muscle, and essential, along with myosin, for muscle contraction. ACTA1 disease-causing mutations were first described in 1999, when a total of 15 mutations were known. In this article we describe 177 different disease-causing ACTA1 mutations, including 85 that have not been described before. ACTA1 mutations result in five overlapping congenital myopathies: nemaline myopathy; intranuclear rod myopathy; actin filament aggregate myopathy; congenital fiber type disproportion; and myopathy with core-like areas. Mixtures of these histopathological phenotypes may be seen in a single biopsy from one patient. Irrespective of the histopathology, the disease is frequently clinically severe, with many patients dying within the first year of life. Most mutations are dominant and most patients have de novo mutations not present in the peripheral blood DNA of either parent. Only 10% of mutations are recessive and they are genetic or functional null mutations. To aid molecular diagnosis and establishing genotype-phenotype correlations, we have developed a locus-specific database for ACTA1 variations (http://waimr.uwa.edu.au).

Actinopathies and myosinopathies

The currently recognized two forms of "anabolic" protein aggregate myopathies, that is, defects in development, maturation and final formation of respective actin and myosin filaments encompass actinopathies and myosinopathies. The former are marked by mutations in the ACTA1 gene, largely of the de novo type. Aggregates of actin filaments are deposited within muscle fibers. Early clinical onset is often congenital; most patients run a rapidly progressive course and die during their first 2 years of life. Myosinopathies or myosin storage myopathies also commence in childhood, but show a much more protracted course owing to mutations in the myosin heavy chain gene MYH7. Protein aggregation consists of granular material in muscle fibers and few, if any, filaments.

Congenital myopathies

PURPOSE OF REVIEW

The aim of this review is to provide an up-to-date personal analysis of current congenital myopathy research.

RECENT FINDINGS

In the past year novel congenital myopathies have been suggested, genes have been discovered for some of the congenital myopathies for the first time (beta-tropomyosin in cap disease and perhaps skeletal muscle alpha-actin in Zebra body myopathy), further genes have been identified for congenital myopathies where other genes had already been found (cofilin in nemaline myopathy, selenoprotein N in congenital fibre type disproportion) and recessive myosin storage myopathy was associated with homozygous mutation of slow-skeletal/beta-cardiac myosin which was already known to be mutated in dominant myosin storage myopathy. There has been further clarification of the pathobiology of the congenital myopathies, including determination of the basis of epigenetic effects: silencing of the normal allele in recessive central core disease and persistence of cardiac (fetal) alpha-actin in nemaline myopathy patients with no skeletal actin.

SUMMARY

The increased understanding of the genes and pathobiology of the congenital myopathies that is developing should ultimately lead to effective treatments.

Hereditary hypotonia, muscle weakness, failure to thrive, and cognitive delay in a large moslem kindred

Five infants of a Moslem-Arab extended family were evaluated for common and characteristic clinical findings of failure to thrive, extreme muscle weakness, severe motor delay, and moderate to severe cognitive and verbal delay. All children were below the third percentile in weight and height, and three of them had head circumference below the third percentile. Neurologic examination revealed severe hypotonia, muscle weakness, and absent deep tendon reflexes. Two children died at 2 years of age, and none of the children acquired full head control and the motor milestones of rolling and sitting. Laboratory evaluation including muscle biopsies, genetic studies, and metabolic evaluation was nondiagnostic.

Protein aggregate myopathies

Protein aggregate myopathies (PAMs) based on the morphologic phenomenon of aggregation of proteins within muscle fibers may occur in children (selenoproteinopathies, actinopathies, and myosinopathies) or adults (certain myofibrillar myopathies and myosinopathies). They may be mutation related, which includes virtually all childhood forms but certain other forms as well, or sporadic, which are largely seen in adults. Their classification as myofibrillar or desmin-related myopathies, actinopathies, or myosinopathies is based on the identification of respective mutant proteins, most of them components of the sarcomeres. Recognition of PAM requires muscle biopsy and an extensive immunohistochemical and electron microscopic workup of the biopsied muscle tissue after which molecular analysis of morphologically ascertained proteins should ensue to permit recognition of individual entities and genetic counseling of patients and families. Because pathogenetic principles in PAMs are still incompletely known, causative therapy, at this time, is not available.