Congenital myopathy with fiber-type disproportion accompanied by dilated cardiomyopathy in a patient with a novel p.G48A ACTA1 mutation

Dilated cardiomyopathy-associated skeletal muscle actin (ACTA1) mutation R256H disrupts actin structure and function and causes cardiomyocyte hypocontractility

Skeletal muscle actin (ACTA1) mutations are a prevalent cause of skeletal myopathies consistent with ACTA1's high expression in skeletal muscle. Rare de novo mutations in ACTA1 associated with combined cardiac and skeletal myopathies have been reported, but ACTA1 represents only ~20% of the total actin pool in cardiomyocytes, making its role in cardiomyopathy controversial. Here we demonstrate how a mutation in an actin isoform expressed at low levels in cardiomyocytes can cause cardiomyopathy by focusing on a unique ACTA1 mutation, R256H. We previously identified this mutation in multiple family members with dilated cardiomyopathy (DCM), who had reduced systolic function without clinical skeletal myopathy. Using a battery of multiscale biophysical tools, we show that R256H has potent functional effects on ACTA1 function at the molecular scale and in human cardiomyocytes. Importantly, we demonstrate that R256H acts in a dominant manner, where the incorporation of small amounts of mutant protein into thin filaments is sufficient to disrupt molecular contractility, and that this effect is dependent on the presence of troponin and tropomyosin. To understand the structural basis of this change in regulation, we resolved a structure of R256H filaments using Cryo-EM, and we see alterations in actin's structure that have the potential to disrupt interactions with tropomyosin. Finally, we show that ACTA1R256H/+ human induced pluripotent stem cell cardiomyocytes demonstrate reduced contractility and sarcomeric disorganization. Taken together, we demonstrate that R256H has multiple effects on ACTA1 function that are sufficient to cause reduced contractility and establish a likely causative relationship between ACTA1 R256H and clinical cardiomyopathy.

Variants in ACTC1 underlie distal arthrogryposis accompanied by congenital heart defects

Contraction of the human sarcomere is the result of interactions between myosin cross-bridges and actin filaments. Pathogenic variants in genes such as MYH7, TPM1, and TNNI3 that encode parts of the cardiac sarcomere cause muscle diseases that affect the heart, such as dilated cardiomyopathy and hypertrophic cardiomyopathy. In contrast, pathogenic variants in homologous genes such as MYH2, TPM2, and TNNI2 that encode parts of the skeletal muscle sarcomere cause muscle diseases affecting skeletal muscle, such as distal arthrogryposis (DA) syndromes and skeletal myopathies. To date, there have been few reports of genes (e.g., MYH7) encoding sarcomeric proteins in which the same pathogenic variant affects skeletal and cardiac muscle. Moreover, none of the known genes underlying DA have been found to contain pathogenic variants that also cause cardiac abnormalities. We report five families with DA because of heterozygous missense variants in the gene actin, alpha, cardiac muscle 1 (ACTC1). ACTC1 encodes a highly conserved actin that binds to myosin in cardiac and skeletal muscle. Pathogenic variants in ACTC1 have been found previously to underlie atrial septal defect, dilated cardiomyopathy, hypertrophic cardiomyopathy, and left ventricular noncompaction. Our discovery delineates a new DA condition because of variants in ACTC1 and suggests that some functions of ACTC1 are shared in cardiac and skeletal muscle.

Novel p.Asp27Glu ACTA1 variant features congenital myopathy with finger flexor weakness, cardiomyopathy, and cardiac conduction defects

Pathogenic variants in the skeletal muscle α-actin 1 gene (ACTA1) cause a spectrum of myopathies with clinical and myopathological diversity. Clinical presentations occur from the prenatal period to adulthood, commonly with proximal-predominant weakness and rarely preferential distal weakness. Myopathological findings are wide-ranging, with nemaline rods being most frequent. Associated cardiomyopathy is rare and conduction defects are not reported. We describe a family with congenital myopathy with prominent finger flexor weakness and cardiomyopathy with cardiac conduction defects. The proband, a 48-year-old Caucasian male, his 73-year-old mother, 41-year-old sister, and 19-year-old nephew presented with prominent finger flexor weakness on a background of neonatal hypotonia and delayed motor milestones. All had progressive cardiomyopathy with systolic dysfunction and/or left ventricular dilation. The proband and sister had intraventricular conduction delay and left anterior fascicular block, respectively. The mother had atrial fibrillation. Muscle biopsy in the proband and sister demonstrated congenital fiber-type disproportion and rare nemaline rods in the proband. A novel dominant variant in ACTA1 (c.81C>A, p.Asp27Glu) segregated within the family. This family expands the genotypic and phenotypic spectrum of ACTA1-related myopathy, highlighting preferential finger flexor involvement with cardiomyopathy and conduction disease. We emphasize early and ongoing cardiac surveillance in ACTA1-related myopathy.

Variants in ACTC1 underlie distal arthrogryposis accompanied by congenital heart defects

Contraction of the human sarcomere is the result of interactions between myosin cross-bridges and actin filaments. Pathogenic variants in genes such as MYH7 , TPM1 , and TNNI3 that encode parts of the cardiac sarcomere cause muscle diseases that affect the heart, such as dilated cardiomyopathy and hypertrophic cardiomyopathy. In contrast, pathogenic variants in homologous genes MYH2 , TPM2 , and TNNI2 , that encode parts of the skeletal muscle sarcomere, cause muscle diseases affecting skeletal muscle, such as the distal arthrogryposis (DA) syndromes and skeletal myopathies. To date, there have been few reports of genes (e.g., MYH7 ) encoding sarcomeric proteins in which the same pathogenic variant affects both skeletal and cardiac muscle. Moreover, none of the known genes underlying DA have been found to contain mutations that also cause cardiac abnormalities. We report five families with DA due to heterozygous missense variants in the gene actin, alpha, cardiac muscle 1 ( ACTC1 ). ACTC1 encodes a highly conserved actin that binds to myosin in both cardiac and skeletal muscle. Mutations in ACTC1 have previously been found to underlie atrial septal defect, dilated cardiomyopathy, hypertrophic cardiomyopathy, and left ventricular noncompaction. Our discovery delineates a new DA condition due to mutations in ACTC1 and suggests that some functions of actin, alpha, cardiac muscle 1 are shared in cardiac and skeletal muscle.

A case of congenital fiber-type disproportion syndrome presenting dilated cardiomyopathy with ACTA1 mutation

Background

Actin, alpha, skeletal muscle 1 (ACTA1) is one of the causative genes of nemaline myopathy (NM) and congenital fiber‐type disproportion (CFTD). CFTD is characterized by type 1 fiber atrophy and distinguished from NM in the absence of rods. Eight patients with CFTD, including one patient with dilated cardiomyopathy (DCM), have previously been reported. Herein, we report the case of a 10‐year‐old boy presenting with CFTD and DCM.

Methods

We performed exome sequencing and analyzed the effect of Met327Lys mutations on cultured C2C12 muscle cells compared with that seen in the wild type (WT, ACTA1) and previously identified Asp294Val mutations associated with a severe phenotype of CFTD without cardiomyopathy.

Results

Exome sequencing revealed a de novo mutation, c.980 T > A, p.(Met327Lys), in ACTA1 (NM_001100.4). C2C12 cells transfected with the WT plasmid expressed ACTA1 in the nucleus and cytoplasm. Cells with the Asp294Val mutant showed needle‐like structures in the cytoplasm, whereas the expression of the Met327Lys mutant resulted in few aggregations but many apoptotic cells.

Conclusion

Apoptosis induced in Met327Lys‐transfected muscle cells supports the pathogenicity of the mutation and can be implicated as one of the histopathological features associated with CFTD, as in NM.

Novel ACTA1 mutation causes late-presenting nemaline myopathy with unusual dark cores

ACTA1 gene encodes the skeletal muscle alpha-actin, the core of thin filaments of the sarcomere. ACTA1 mutations are responsible of several muscle disorders including nemaline, cores, actin aggregate myopathies and fiber-type disproportion. We report clinical, muscle imaging, histopatological and genetic data of an Italian family carrying a novel ACTA1 mutation. All affected members showed a late-presenting, diffuse muscle weakness with sternocleidomastoideus and temporalis atrophy. Mild dysmorphic features were also detected. The most affected muscles by muscle MRI were rectus abdominis, gluteus minimus, vastus intermedius and both gastrocnemii. Muscle biopsy showed the presence of nemaline bodies with several unusual dark areas at Gomori Trichrome, corresponding to unstructured cores with abundant electrodense material by electron microscopy. The molecular analysis revealed missense variant c.148G>A; p.(Gly50Ser) in the exon 3 of ACTA1, segregating with affected members in the family. We performed a functional essay of fibre contractility showing a higher pCa₅₀ (a measure of the calcium sensitivity of force) of type 1 fibers compared to control subjects' type 1 muscle fibers. Our findings expand the clinico-pathological spectrum of ACTA1-related congenital myopathies and the genetic spectrum of core-rod myopathies.

Asymmetric muscle weakness due to ACTA1 mosaic mutations

Objective

To characterize 2 unrelated patients with either asymmetric or unilateral muscle weakness at the clinical, genetic, histologic, and ultrastructural level.

Methods

The patients underwent thorough clinical examination, whole-body MRI, and exome sequencing. Muscle morphology was assessed by histology and electron microscopy.

Results

Both patients presented with early-onset hypotonia, delayed motor milestones, scoliosis, and reduced pulmonary function. Patient P1 manifested unilateral muscle weakness exclusively affecting the left side of the body; the asymmetry was less pronounced in patient P2. Muscle biopsies from both patients showed nemaline rods as the main histopathologic hallmark, and MRI revealed major fatty infiltrations in selective head, proximal, and distal muscles, correlating with the degree of muscle weakness asymmetry. Exome sequencing on blood DNA from both patients identified de novo ACTA1 missense mutations in a small number of reads, suggesting mutation mosaicism. Subsequent Sanger sequencing confirmed the presence of the mutations on muscle DNA, while they were barely detectable on blood DNA.

Conclusions

De novo mutations can occur anytime during embryonic development and may result in a mosaic pattern of affected cells and tissues and lead to the development of an asymmetric clinical picture. The present study points out that mosaic mutations might not be easily detectable on leukocyte DNA and thereby escape routine genetic analysis, and possibly account for a significant number of molecularly undiagnosed patients.

Identification of key candidate genes and pathways revealing the protective effect of liraglutide on diabetic cardiac muscle by integrated bioinformatics analysis

Background

Diabetes mellitus is becoming a significant health problem with the International Diabetes Federation (IDF) expecting a startling 642 million diabetes patients by 2040. Liraglutide, a glucagon-like peptide-1 (GLP-1) analog, is reported to protect against diabetic cardiomyopathy by binding to the receptor, GLP-1R. However, the underlying mechanism has yet to be clarified. This study aimed to investigate the underlying mechanisms and the effects of liraglutide on diabetic patient's cardiac muscles.

Methods

GSE102194 genetic expression profiles were extracted from the Gene Expression Omnibus (GEO) database. The gene ontology (GO) and Kyoto Encyclopedia of Genes and Genomes pathway (KEGG) enrichment analyses were carried out. Next, Cytoscape software was used to construct the protein-protein interaction (PPI) network of the differentially expressed genes (DEGs). DEGs were mapped onto a protein-protein interaction (PPI) network that comprised 249 nodes and 776 edges.

Results

A total of 520 DEGs were discovered, including 159 down-regulated genes and 361 up-regulated genes. DEGs that were upregulated were notably enriched in biological processes (BP) such as muscle system process, muscle system process, muscle structure development and anatomical structure morphogenesis while DEGs that were downregulated were rich in detection of chemical stimulus and neurological system process. KEGG pathway analysis showed the up-regulated DEGs were enriched in adrenergic signaling for cardiomyocytes, dopaminergic synapse, and circadian entrainment, while the down-regulated DEGs were enriched for factory transduction in 249 of the 520 tested samples. The modular analysis identified 4 modules that participated in some pathways associated with cardiac muscle contraction, hypertrophic cardiomyopathy (HCM), and MAPK signaling pathway.

Conclusions

Our data showed that Glp-1 could decrease the protein expression of p38, JNK, ERK1/2, and MARS proteins induced by high glucose (22 mM, 72 h). This study highlights the potential physiological processes that take place in diabetic cardiac muscles exposed to liraglutide. Our findings elucidated the regulatory network in diabetic cardiomyopathy and might provide a novel diagnostic and therapeutic target for diabetic cardiomyopathy.