The nuclear muscular dystrophies

Cardiovascular Involvement in Pediatric Laminopathies. Report of Six Patients and Literature Revision

Lamin A/C (LMNA) encodes for two nuclear intermediate filament proteins. Mutations in LMNA cause a highly heterogeneous group of diseases predominantly leading to muscular or cardiac disease, lipodystrophy syndromes, peripheral neuropathy, and accelerated aging disorders. Cardiac involvement includes progressive arrhythmias (brady/tachyarrhythmias, sudden cardiac death). Furthermore, cardiomyocyte damage often progresses into dilated cardiomyopathy (DCM), rarely described in the pediatric age group. Neuromuscular manifestations are even rarer in children. We report on six pediatric patients with LMNA mutations: patient 1 was operated on for aortic coarctation, non-compact left ventricle, atrial fibrillation (AF) preceding the diagnosis of DCM; patient 2 was operated on for ventricular septal defect (VSD), developed after years malignant arrhythmias preceding the progression to DCM (left ventricular non-compaction with LV dysfunction); patient 3 had ectopic atrial tachycardia as first manifestation of a DCM; patients 4 and 5 had no major arrhythmic events but only dilated ascending aorta, mildly dilated LV with mild hypertrabeculation of the lateral wall and a normally functioning but dilated left ventricle, respectively; patient 6 showed aortic coarctation, supraventricular tachycardia. Paroxysmal AF occurred in patients 1, 2, and 3 (50% of cases). Our series highlight the coexistence of congenital heart defects (CHDs) and aortic involvement with laminopathies in four of our patients: consisting of aortic coarctation (two patients), aortic root dilatation (one patient), and VSD (one patient). Aortic changes in laminopathies have been reported only once in an adult patient. This is the first report in the pediatric setting, and no associations with CHD have been previously described.

Skeletal Muscle Laminopathies: A Review of Clinical and Molecular Features

LMNA-related disorders are caused by mutations in the LMNA gene, which encodes for the nuclear envelope proteins, lamin A and C, via alternative splicing. Laminopathies are associated with a wide range of disease phenotypes, including neuromuscular, cardiac, metabolic disorders and premature aging syndromes. The most frequent diseases associated with mutations in the LMNA gene are characterized by skeletal and cardiac muscle involvement. This review will focus on genetics and clinical features of laminopathies affecting primarily skeletal muscle. Although only symptomatic treatment is available for these patients, many achievements have been made in clarifying the pathogenesis and improving the management of these diseases.

Cardiac effects of the c.1583 C→G LMNA mutation in two families with Emery-Dreifuss muscular dystrophy

The present study aimed to examine and analyze cardiac involvement in two Emery-Dreifuss muscular dystrophy (EDMD) pedigrees caused by the c.1583 C→G mutation of the lamin A/C gene (LMNA). The clinical and genetic characteristics of members of two families with EDMD were evaluated by performing neurological examinations, skeletal muscle biopsies, cardiac evaluations, including electrocardiography, 24 h Holter, ultrasound cardiography and ⁹⁹Tc^M-MIBI-gated myocardiac perfusion imaging, and genomic DNA sequencing. Family history investigations revealed an autosomal dominant transmission pattern of the disease in Family 1 and a sporadic case in Family 2. The three affected patients exhibited typical clinical features of EDMD, including joint contractures, muscle weakness and cardiac involvement. Muscle histopathological investigation revealed dystrophic features. In addition, each affected individual exhibited either cardiac arrhythmia, which was evident as sinus tachycardia, atrial flutter or complete atrioventricular inhibition. Cardiac imaging revealed dilated cardiomyopathy in two of the individuals, one of whom was presented with heart failure. The second patient presented with no significant abnormalities in cardiac structure or function. The three affected individuals exhibited a heterozygous missense mutation in the LMNA gene (c.1583 C→G), which caused a T528R amino acid change in the LMNA protein. In conclusion, the present study identified three patients with EDMD, exhibiting the same dominant LMNA mutation and presenting with a spectrum of severe cardiac abnormalities, including cardiac conduction system defects, cardiomyopathy and heart failure. As LMNA mutations have been associated with at least six clinical disorders, including EDMD, the results of the present study provide additional mutational and functional data, which may assist in further establishing LMNA mutational variation and disease pathogenesis.

Abnormal proliferation and spontaneous differentiation of myoblasts from a symptomatic female carrier of X-linked Emery-Dreifuss muscular dystrophy

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X-linked female presenting with EDMD1 not explained by uneven X-inactivation.

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First EDMD blood phenotype with highly lobulated lymphocytes in EDMD1 patient.

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Found high incidence of spontaneous differentiation in cultured patient myoblasts.

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Faster proliferation of emerin-null than emerin-positive EDMD1 patient myoblasts.

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Loss of satellite cells from the above might explain EDMD pathology.

Emery–Dreifuss muscular dystrophy (EDMD) is a neuromuscular disease characterized by early contractures, slowly progressive muscular weakness and life-threatening cardiac arrhythmia that can develop into cardiomyopathy. In X-linked EDMD (EDMD1), female carriers are usually unaffected. Here we present a clinical description and in vitro characterization of a mildly affected EDMD1 female carrying the heterozygous EMD mutation c.174_175delTT; p.Y59* that yields loss of protein. Muscle tissue sections and cultured patient myoblasts exhibited a mixed population of emerin-positive and -negative cells; thus uneven X-inactivation was excluded as causative. Patient blood cells were predominantly emerin-positive, but considerable nuclear lobulation was observed in non-granulocyte cells – a novel phenotype in EDMD. Both emerin-positive and emerin-negative myoblasts exhibited spontaneous differentiation in tissue culture, though emerin-negative myoblasts were more proliferative than emerin-positive cells. The preferential proliferation of emerin-negative myoblasts together with the high rate of spontaneous differentiation in both populations suggests that loss of functional satellite cells might be one underlying mechanism for disease pathology. This could also account for the slowly developing muscle phenotype.

Skin and sural nerve biopsies: ultrastructural findings in the first genetically confirmed cases of CADASIL in Serbia

Cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy (CADASIL) is an inherited vascular disorder caused by Notch3 gene mutations. The main histopathological hallmark is granular osmiophilic material (GOM) deposited in the close vicinity of vascular smooth muscle cells (VSMCs). The authors report the first 7 ultrastructurally and genetically confirmed cases of CADASIL in Serbia. Samples of skin and sural nerve were investigated by transmission electron microscopy. GOM deposits were observed around degenerated VSMCs in all the skin biopsies examined. Sural nerve biopsies revealed severe alterations of nerve fibers, endoneurial blood vessels with GOM deposits, endoneurial fibroblasts, and perineurial myofibroblasts. Total genomic DNA was extracted from peripheral blood leukocytes, and exons 2-6 of the Notch3 gene were amplified by PCR and subsequently sequenced. Four different mutations in exons 2 (Cys65Tyr), 3 (Gly89Cys and Arg90Cys), and 6 (Ala319Cys), which determine the CADASIL disease, were detected among all described patients. A novel missense mutation Gly89Cys involving exon 3 was detected. Due to the difficulties in the determination of the Notch3 mutations, these data suggest that electron microscopic analysis for GOMs in dermal vessel wall provides a rapid and reliable screening method for this disease.

Basic Mechanisms Mediating Cardiomyopathy and Heart Failure in Aging

Biological aging represents the major risk factor for the development of heart failure (HF), malignancies, and neurodegenerative diseases. While risk factors such as lifestyle patterns, genetic traits, blood lipid levels, and diabetes can contribute to its development, advancing age remains the most determinant predictor of cardiac disease. Several parameters of left ventricular function may be affected with aging, including increased duration of systole, decreased sympathetic stimulation, and increased left ventricle ejection time, while compliance decreases. In addition, changes in cardiac phenotype with diastolic dysfunction, reduced contractility, left ventricular hypertrophy, and HF, all increase in incidence with age. Given the limited capacity that the heart has for regeneration, reversing or slowing the progression of these abnormalities poses a major challenge. In this chapter, we present a discussion on the molecular and cellular mechanisms involved in the pathogenesis of cardiomyopathies and HF in aging and the potential involvement of specific genes identified as primary mediators of these diseases.

The laminopathies: the functional architecture of the nucleus and its contribution to disease

Most inherited diseases are associated with mutations in a specific gene. Often, mutations in two or more different genes result in diseases with a similar phenotype. Rarely do different mutations in the same gene result in a multitude of seemingly different and unrelated diseases. Mutations in the Lamin A gene (LMNA), which encodes largely ubiquitously expressed nuclear proteins (A-type lamins), are associated with at least eight different diseases, collectively called the laminopathies. Studies examining how different tissue-specific diseases arise from unique LMNA mutations are providing unanticipated insights into the structural organization of the nucleus, and how disruption of this organization relates to novel mechanisms of disease.

Molecular genetics of autosomal-recessive axonal Charcot-Marie-Tooth neuropathies

Autosomal-recessive forms of Charcot-Marie-Tooth (ARCMT) account for less than 10% of the families with CMT. On the other hand, in countries with a high prevalence of consanguinity this mode of inheritance accounts, likely, for the vast majority of CMT phenotypes. Like dominant forms, autosomal-recessive forms are generally subdivided into demyelinating forms (autosomal-recessive CMT1: ARCMT1 or CMT4) and axonal forms (ARCMT2). Until now, demyelinating ARCMT were more extensively studied at the genetic level than the axonal forms. Although the latter are undoubtedly the rarest forms among the heterogeneous group of CMT, three distinct forms have been genetically mapped and recent studies in the past 4 yr provided evidence that their respective causing genes have been characterized. Indeed, gene defects in encoding A-type lamins (LMNA), encoding Ganglioside-induced Differentiation-Associated Protein-1 (GDAP1) and encoding the mediator of RNA polymerase II transcription, subunit 25 homolog (MED25) have been identified in ARCMT2 subtypes. Given the clinical, electrophysiological and histological heterogeneity of CMT2, it is likely that unreported forms of ARCMT2, related to novel genes, remain to be discovered, leading to an even more complex classification. However, our goal in this review is to provide the reader with a clear view on the known genes and mechanisms involved in ARCMT2 and their associated phenotypes.

Diagnostic immunohistochemistry in neuromuscular disorders

Most neuromuscular disorders display only non-specific myopathological features in routine histological preparations. However, a number of proteins, including sarcolemmal, sarcomeric, and nuclear proteins as well as enzymes with defects responsible for neuromuscular disorders, have been identified during the past two decades, allowing a more specific and firm diagnosis of muscle diseases. Identification of protein defects relies predominantly on immunohistochemical preparations and on Western blot analysis. While immunohistochemistry is very useful in identifying abnormal expression of primary protein abnormalities in recessive conditions, it is less helpful in detecting primary defects in dominantly inherited disorders. Abnormal immunohistochemical expression patterns can be confirmed by Western blot analysis which may also be informative in dominant disorders, although its role has yet to be established. Besides identification of specific protein defects, immunohistochemistry is also helpful in the differentiation of inflammatory myopathies by subtyping cellular infiltrates and demonstrating up-regulation of subtle immunological parameters such as cell adhesion molecules. The role of immunohistochemistry in denervating disorders, however, remains controversial in the absence of a reliable marker of muscle fibre denervation. Nevertheless, as well as the diagnostic value of immunocytochemical analysis it may also widen understanding of muscle fibre pathology as well as help in the development of therapeutic strategies.

Gene expression in spermiogenesis

Germ cells convey parental genes to the next generation, and only germ cells perform meiosis, which is a mechanism that preserves the parental genes. The fusion of the products of germ cell meiosis, the haploid sperm and egg, creates the next generation. Sperm are the haploid germ cells that contribute genes to the egg. In preparation for this, the haploid round spermatids produced by meiosis undergo drastic morphological changes to become sperm. During this process of spermiogenesis, the nuclear form of the haploid germ cell takes shape, the mitochondria are rearranged in a specific manner, the flagellum develops and the acrosome forms. Spermatogenesis is supported by precise and orderly regulation of gene expression during the changes in chromatin structure, when protamine replaces histone. In this report, we summarize the molecular mechanisms involved in spermiogenesis.

Molecular diagnosis of inheritable neuromuscular disorders. Part II: Application of genetic testing in neuromuscular disease

Molecular genetic advances have led to refinements in the classification of inherited neuromuscular disease, and to methods of molecular testing useful for diagnosis and management of selected patients. Testing should be performed as targeted studies, sometimes sequentially, but not as wasteful panels of multiple genetic tests performed simultaneously. Accurate diagnosis through molecular testing is available for the vast majority of patients with inherited neuropathies, resulting from mutations in three genes (PMP22, MPZ, and GJB1); the most common types of muscular dystrophies (Duchenne and Becker, facioscapulohumeral, and myotonic dystrophies); the inherited motor neuron disorders (spinal muscular atrophy, Kennedy's disease, and SOD1 related amyotrophic lateral sclerosis); and many other neuromuscular disorders. The role of potential multiple genetic influences on the development of acquired neuromuscular diseases is an increasingly active area of research.

Emerin binding to Btf, a death-promoting transcriptional repressor, is disrupted by a missense mutation that causes Emery-Dreifuss muscular dystrophy

Loss of functional emerin, a nuclear membrane protein, causes X-linked recessive Emery-Dreifuss muscular dystrophy. In a yeast two-hybrid screen, we found that emerin interacts with Btf, a death-promoting transcriptional repressor, which is expressed at high levels in skeletal muscle. Biochemical analysis showed that emerin binds Btf with an equilibrium affinity (KD) of 100 nm. Using a collection of 21 clustered alanine-substitution mutations in emerin, the residues required for binding to Btf mapped to two regions of emerin that flank its lamin-binding domain. Two disease-causing mutations in emerin, S54F and Delta95-99, disrupted binding to Btf. The Delta95-99 mutation was relatively uninformative, as this mutation also disrupts emerin binding to lamin A and a different transcription repressor named germ cell-less (GCL). In striking contrast, emerin mutant S54F, which binds normally to barrier-to-autointegration factor, lamin A and GCL, selectively disrupted emerin binding to Btf. We localized endogenous Btf in HeLa cells by indirect immunoflurorescence using affinity-purified antibodies against Btf. In nonapoptotic HeLa cells Btf was found in dot-like structures throughout the nuclear interior. However, within 3 h after treating cells with Fas antibody to induce apoptosis, the distribution of Btf changed, and Btf concentrated in a distinct zone near the nuclear envelope. These results suggest that Btf localization is regulated by apoptotic signals, and that loss of emerin binding to Btf may be relevant to muscle wasting in Emery-Dreifuss muscular dystrophy.

The laminopathies: nuclear structure meets disease

Most inherited diseases are associated with mutations in a specific gene. Sometimes, mutations in two or more different genes result in diseases with a similar phenotype. Rarely do different mutations in the same gene result in a multitude of seemingly different and unrelated diseases. In the past three years, different mutations in LMNA, the gene encoding the A-type lamins, have been shown to be associated with at least six different diseases. These diseases and at least two others caused by mutations in other proteins associated with the nuclear lamina are collectively called the laminopathies. How different tissue-specific diseases arise from unique mutations in the LMNA gene, encoding almost ubiquitously expressed nuclear proteins, are providing tantalizing insights into the structural organization of the nucleus, its relation to nuclear function in different tissues and the involvement of the nuclear envelope in the development of disease.