Association of nonsense mutation of dystrophin gene with disruption of sarcoglycan complex in X-linked dilated cardiomyopathy

DMD-Associated Dilated Cardiomyopathy: Genotypes, Phenotypes, and Phenocopies

BACKGROUND

Variants in the DMD gene, that encodes the cytoskeletal protein, dystrophin, cause a severe form of dilated cardiomyopathy (DCM) associated with high rates of heart failure, heart transplantation, and ventricular arrhythmias. Improved early detection of individuals at risk is needed.

METHODS

Genetic testing of 40 male probands with a potential X-linked genetic cause of primary DCM was undertaken using multi-gene panel sequencing, multiplex polymerase chain reaction, and array comparative genomic hybridization. Variant location was assessed with respect to dystrophin isoform patterns and exon usage. Telomere length was evaluated as a marker of myocardial dysfunction in left ventricular tissue and blood.

RESULTS

Four pathogenic/likely pathogenic DMD variants were found in 5 probands (5/40: 12.5%). Only one rare variant was identified by gene panel testing with 3 additional multi-exon deletion/duplications found following targeted assays for structural variants. All of the pathogenic/likely pathogenic DMD variants involved dystrophin exons that had percent spliced-in scores >90, indicating high levels of constitutive expression in the human adult heart. Fifteen DMD variant-negative probands (15/40: 37.5%) had variants in autosomal genes including TTN, BAG3, LMNA, and RBM20. Myocardial telomere length was reduced in patients with DCM irrespective of genotype. No differences in blood telomere length were observed between genotype-positive family members with/without DCM and controls.

CONCLUSIONS

Primary genetic testing using multi-gene panels has a low yield and specific assays for structural variants are required if DMD-associated cardiomyopathy is suspected. Distinguishing X-linked causes of DCM from autosomal genes that show sex differences in clinical presentation is crucial for informed family management.

A Comprehensive Outlook on Dilated Cardiomyopathy (DCM): State-Of-The-Art Developments with Special Emphasis on OMICS-Based Approaches

Dilated cardiomyopathy (DCM) remains an enigmatic cardiovascular disease (CVD) condition characterized by contractile dysfunction of the myocardium due to dilation of the ventricles. DCM is one of the major forms of CVD contributing to heart failure. Dilation of the left or both ventricles with systolic dysfunction, not explained by known causes, is a hallmark of DCM. Progression of DCM leads to heart failure. Genetic and various other factors greatly contribute to the development of DCM, but the etiology has still remained elusive in a large number of cases. A significant number of studies have been carried out to identify the genetic causes of DCM. These candidate-gene studies revealed that mutations in the genes of the fibrous, cytoskeletal, and sarcomeric proteins of cardiomyocytes result in the development of DCM. However, a significant proportion of DCM patients are idiopathic in nature. In this review, we holistically described the symptoms, causes (in adults and newborns), genetic basis, and mechanistic progression of DCM. Further, we also summarized the state-of-the-art diagnosis, available biomarkers, treatments, and ongoing clinical trials of potential drug regimens. DCM-mediated heart failure is on the rise worldwide including in India. The discovery of biomarkers with a better prognostic value is the need of the hour for better management of DCM-mediated heart failure patients. With the advent of next-generation omics-based technologies, it is now possible to probe systems-level alterations in DCM patients pertaining to the identification of novel proteomic and lipidomic biomarkers. Here, we also highlight the onset of a systems-level study in Indian DCM patients by applying state-of-the-art mass-spectrometry-based “clinical proteomics” and “clinical lipidomics”.

Essential roles of the dystrophin-glycoprotein complex in different cardiac pathologies

The dystrophin-glycoprotein complex (DGC), situated at the sarcolemma dynamically remodels during cardiac disease. This review examines DGC remodeling as a common denominator in diseases affecting heart function and health. Dystrophin and the DGC serve as broad cytoskeletal integrators that are critical for maintaining stability of muscle membranes. The presence of pathogenic variants in genes encoding proteins of the DGC can cause absence of the protein and/or alterations in other complex members leading to muscular dystrophies. Targeted studies have allowed the individual functions of affected proteins to be defined. The DGC has demonstrated its dynamic function, remodeling under a number of conditions that stress the heart. Beyond genetic causes, pathogenic processes also impinge on the DGC, causing alterations in the abundance of dystrophin and associated proteins during cardiac insult such as ischemia-reperfusion injury, mechanical unloading, and myocarditis. When considering new therapeutic strategies, it is important to assess DGC remodeling as a common factor in various heart diseases. The DGC connects the internal F-actin-based cytoskeleton to laminin-211 of the extracellular space, playing an important role in the transmission of mechanical force to the extracellular matrix. The essential functions of dystrophin and the DGC have been long recognized. DGC based therapeutic approaches have been primarily focused on muscular dystrophies, however it may be a beneficial target in a number of disorders that affect the heart. This review provides an account of what we now know, and discusses how this knowledge can benefit persistent health conditions in the clinic.

Dystrophic Cardiomyopathy: Complex Pathobiological Processes to Generate Clinical Phenotype

Duchenne muscular dystrophy (DMD), Becker muscular dystrophy (BMD), and X-linked dilated cardiomyopathy (XL-DCM) consist of a unique clinical entity, the dystrophinopathies, which are due to variable mutations in the dystrophin gene. Dilated cardiomyopathy (DCM) is a common complication of dystrophinopathies, but the onset, progression, and severity of heart disease differ among these subgroups. Extensive molecular genetic studies have been conducted to assess genotype-phenotype correlation in DMD, BMD, and XL-DCM to understand the underlying mechanisms of these diseases, but the results are not always conclusive, suggesting the involvement of complex multi-layers of pathological processes that generate the final clinical phenotype. Dystrophin protein is a part of dystrophin-glycoprotein complex (DGC) that is localized in skeletal muscles, myocardium, smooth muscles, and neuronal tissues. Diversity of cardiac phenotype in dystrophinopathies suggests multiple layers of pathogenetic mechanisms in forming dystrophic cardiomyopathy. In this review article, we review the complex molecular interactions involving the pathogenesis of dystrophic cardiomyopathy, including primary gene mutations and loss of structural integrity, secondary cellular responses, and certain epigenetic and other factors that modulate gene expressions. Involvement of epigenetic gene regulation appears to lead to specific cardiac phenotypes in dystrophic hearts.

Genetic Counseling and Screening Issues in Familial Dilated Cardiomyopathy

Idiopathic dilated cardiomyopathy (IDC), a treatable condition characterized by left ventricular dilatation and systolic dysfunction of unknown cause, has only recently been recognized to have genetic etiologies. Although familial dilated cardiomyopathy (FDC) was thought to be infrequent, it is now believed that 30-50% of cases of IDC may be familial. Echocardiographic and electrocardiographic (ECG) screening of first-degree relatives of individuals with IDC and FDC is indicated because detection and treatment are possible prior to the onset of advanced, symptomatic disease. However, such screening often creates uncertainty and anxiety surrounding the significance of the results. Furthermore, FDC demonstrates incomplete penetrance, variable expression, and significant locus and allelic heterogeneity, making genetic counseling complex. The provision of genetic counseling for IDC and FDC will require collaboration between cardiologists and genetics professionals, and may also improve the recognition of FDC, the availability of support services, and overall outcomes for patients and families.

X-Linked Dilated Cardiomyopathy: A Cardiospecific Phenotype of Dystrophinopathy

X-linked dilated cardiomyopathy (XLDCM) is a distinct phenotype of dystrophinopathy characterized by preferential cardiac involvement without any overt skeletal myopathy. XLDCM is caused by mutations of the Duchenne muscular dystrophy (DMD) gene and results in lethal heart failure in individuals between 10 and 20 years. Patients with Becker muscular dystrophy, an allelic disorder, have a milder phenotype of skeletal muscle involvement compared to Duchenne muscular dystrophy (DMD) and sometimes present with dilated cardiomyopathy. The precise relationship between mutations in the DMD gene and cardiomyopathy remain unclear. However, some hypothetical mechanisms are being considered to be associated with the presence of some several dystrophin isoforms, certain reported mutations, and an unknown dystrophin-related pathophysiological mechanism. Recent therapy for Duchenne muscular dystrophy, the severe dystrophinopathy phenotype, appears promising, but the presence of XLDCM highlights the importance of focusing on cardiomyopathy while elucidating the pathomechanism and developing treatment.

Exercise and muscular dystrophy: implications and analysis of effects on musculoskeletal and cardiovascular systems

The muscular dystrophies are a heterogeneous collection of progressive, inherited diseases of muscle weakness and degeneration. Although these diseases can vary widely in their etiology and presentation, nearly all muscular dystrophies cause exercise intolerance to some degree. Here, we focus on Duchenne muscular dystrophy (DMD), the most common form of muscular dystrophy, as a paradigm for the effects of muscle disease on exercise capacity. First described in the mid-1800s, DMD is a rapidly progressive and lethal muscular dystrophy caused by mutations in the dystrophin gene. Dystrophin is a membrane-associated cytoskeletal protein, the loss of which causes numerous cellular defects including mechanical instability of the sarcolemma, increased influx of extracellular calcium, and cell signaling defects. Here, we discuss the physiological basis for exercise intolerance in DMD, focusing on the molecular and cellular defects caused by loss of dystrophin and how these manifest as organ-level dysfunction and reduced exercise capacity. The main focus of this article is the defects present in dystrophin-deficient striated muscle. However, discussion regarding the effects of dystrophin loss on other tissues, including vascular smooth muscle is also included. Collectively, the goal of this article is to summarize the current state of knowledge regarding the mechanistic basis for exercise intolerance in DMD, which may serve as an archetype for other muscular dystrophies and diseases of muscle wasting.

Long-term preservation of cardiac structure and function after adeno-associated virus serotype 9-mediated microdystrophin gene transfer in mdx mice

Dystrophin plays an important role in muscle contraction, linking the intracellular cytoskeleton to the extracellular matrix. Mutations of the dystrophin gene leading to a complete loss of the protein cause Duchenne muscular dystrophy (DMD), frequently associated with severe cardiomyopathy. Early clinical trials in DMD using gene transfer to skeletal muscle are underway, but gene transfer to dystrophic cardiac muscle has not yet been tested in humans. The aim of this study was to develop an optimized protocol for cardiac gene therapy in the mouse model of dystrophin deficiency (mdx), using a cardiac promoter for expression of a microdystrophin (μDys) transgene packaged into an adeno-associated virus serotype 9 vector (AAV9). In this study adult mdx mice were intravenously injected with 1×10(12) genomic particles of AAV9 vectors carrying a cDNA encoding μDys under the control of either a ubiquitously active cytomegalovirus (CMV) promoter or a cardiac-specific CMV-enhanced myosin light chain (MLC0.26) promoter. After 10 months, both AAV9 vectors led to sustained μDys expression in cardiac muscle, but the MLC promoter conferred about 4-fold higher protein levels. AAV9-CMV-MLC0.26-μDys resulted in significant protection of cardiac morphology and function as assessed by histopathology, echocardiography, and left ventricular catheterization. In conclusion, we established an AAV9-mediated gene transfer approach for efficient and specific long-term μDys expression in the hearts of mdx mice, resulting in a sustained therapeutic effect. Thus, this approach might be a basis for further translation into a treatment strategy for DMD-associated cardiomyopathy.

Clinical and genetic issues in dilated cardiomyopathy: a review for genetics professionals

Dilated cardiomyopathy (DCM), usually diagnosed as idiopathic dilated cardiomyopathy (IDC), has been shown to have a familial basis in 20-35% of cases. Genetic studies in familial dilated cardiomyopathy (FDC) have shown dramatic locus heterogeneity with mutations identified in >30 mostly autosomal genes showing primarily dominant transmission. Most mutations are private missense, nonsense or short insertion/deletions. Marked allelic heterogeneity is the rule. Although to date most DCM genetics fits into a Mendelian rare variant disease paradigm, this paradigm may be incomplete with only 30-35% of FDC genetic cause identified. Despite this incomplete knowledge, we predict that DCM genetics will become increasingly relevant for genetics and cardiovascular professionals. This is because DCM causes heart failure, a national epidemic, with considerable morbidity and mortality. The fact that early, even pre-symptomatic intervention can prevent or ameliorate DCM, coupled with more cost-effective genetic testing, will drive further progress in the field. Ongoing questions include: whether sporadic (IDC) disease has a genetic basis, and if so, how it differs from familial disease; which gene-specific or genetic pathways are most relevant; and whether other genetic mechanisms (e.g., DNA structural variants, epigenetics, mitochondrial mutations and others) are operative in DCM. We suggest that such new knowledge will lead to novel approaches to the prevention and treatment of DCM.

Genetics of dilated cardiomyopathy

Dilated cardiomyopathy (DCM) is a myocardial disorder defined by ventricular chamber enlargement and systolic dysfunction. DCM can result in progressive heart failure, arrhythmias, thromboembolism, and premature death, and contributes significantly to health care costs. In many cases, DCM results from acquired factors that affect cardiomyocyte function or survival. Inherited genetic variants are also now recognized to have an important role in the etiology of DCM. Despite substantial progress over the past decade, our understanding of familial DCM remains incomplete. Current concepts of the molecular pathogenesis, clinical presentation, natural history, and management of familial DCM are outlined in this review.

Follow-up of three patients with a large in-frame deletion of exons 45-55 in the Duchenne muscular dystrophy (DMD) gene

We review the clinical status of skeletal involvement and cardiac function in three unrelated patients harboring an in-frame deletion of exons 45 to 55 in the DMD gene followed up for 2 to 7 years. Two younger patients diagnosed as having X-linked dilated cardiomyopathy (XLDCM) developed congestive heart failure without overt skeletal myopathy. Heart failure recurred after viral infection but responded well to diuretics and angiotensin-converting enzyme inhibitors. One older patient diagnosed with Becker muscular dystrophy showed limb-girdle muscular atrophy and weakness at the age of 50, but did not have any cardiac symptoms. Skeletal muscle involvement in each patient remained unchanged, and cardiac function did not worsen in any of the patients during the study. In a younger XLDCM patient, the amount and molecular weight of mutant dystrophin were equally slightly decreased in both skeletal and cardiac muscles. Immunostaining for dystrophin and dystrophin-associated proteins was slightly reduced in both skeletal and cardiac muscle, with no discernible difference between the two. The phenotype of this dystrophinopathy can manifest as XLDCM in younger patients; however, careful attention to cardiac management may result in a favorable prognosis.

Dysferlin-mediated membrane repair protects the heart from stress-induced left ventricular injury

Dilated cardiomyopathy is a life-threatening syndrome that can arise from a myriad of causes, but predisposition toward this malady is inherited in many cases. A number of inherited forms of dilated cardiomyopathy arise from mutations in genes that encode proteins involved in linking the cytoskeleton to the extracellular matrix, and disruption of this link renders the cell membrane more susceptible to injury. Membrane repair is an important cellular mechanism that animal cells have developed to survive membrane disruption. We have previously shown that dysferlin deficiency leads to defective membrane resealing in skeletal muscle and muscle necrosis; however, the function of dysferlin in the heart remains to be determined. Here, we demonstrate that dysferlin is also involved in cardiomyocyte membrane repair and that dysferlin deficiency leads to cardiomyopathy. In particular, stress exercise disturbs left ventricular function in dysferlin-null mice and increases Evans blue dye uptake in dysferlin-deficient cardiomyocytes. Furthermore, a combined deficiency of dystrophin and dysferlin leads to early onset cardiomyopathy. Our results suggest that dysferlin-mediated membrane repair is important for maintaining membrane integrity of cardiomyocytes, particularly under conditions of mechanical stress. Thus, our study establishes what we believe is a novel mechanism underlying the cardiomyopathy that results from a defective membrane repair in the absence of dysferlin.

Protein- and mRNA-based phenotype-genotype correlations in DMD/BMD with point mutations and molecular basis for BMD with nonsense and frameshift mutations in the DMD gene

Straightforward detectable Duchenne muscular dystrophy (DMD) gene rearrangements, such as deletions or duplications involving an entire exon or more, are involved in about 70% of dystrophinopathies. In the remaining 30% a variety of point mutations or "small" mutations are suspected. Due to their diversity and to the large size and complexity of the DMD gene, these point mutations are difficult to detect. To overcome this diagnostic issue, we developed and optimized a routine muscle biopsy-based diagnostic strategy. The mutation detection rate is almost as high as 100% and mutations were identified in all patients for whom the diagnosis of DMD and Becker muscular dystrophy (BMD) was clinically suspected and further supported by the detection on Western blot of quantitative and/or qualitative dystrophin protein abnormalities. Here we report a total of 124 small mutations including 11 nonsense and frameshift mutations detected in BMD patients. In addition to a comprehensive assessment of muscular phenotypes that takes into account consequences of mutations on the expression of the dystrophin mRNA and protein, we provide and discuss genomic, mRNA, and protein data that pinpoint molecular mechanisms underlying BMD phenotypes associated with nonsense and frameshift mutations.

Genetics of dilated cardiomyopathy

Dilated cardiomyopathy (DCM) is a myocardial disease characterized by dilatation and impaired systolic function of the left or both ventricles. The etiology of DCM is multifactorial, and many different clinical conditions can lead to the phenotype of DCM. During recent years it has become evident that genetic factors play an important role in the etiology and pathogenesis of idiopathic DCM. The genetics of DCM have been under intensive investigation lately, and thereby the knowledge on the genetic basis of DCM has increased rapidly. The genetic background of the disease seems to be relatively heterogeneous, and the disease-associated mutations concern mostly single families and only few affected patients. Disease-associated mutations have been detected e.g. in genes encoding sarcomere, cytoskeletal, and nuclear proteins, as well as proteins involved with regulation of Ca(2+) metabolism. The mechanisms, by which mutations eventually result in clinical heart failure, are complex and not yet totally resolved. DCM causes considerable morbidity and mortality. Better knowledge of the genetic background and disease-causing mechanisms would probably help us in focusing early treatment on right subjects and potentially also developing new treatment modalities and improving cardiac outcome in the affected patients. This review deals with DCM of genetic origin.

Familial inflammatory dilated cardiomyopathy

BACKGROUND

Systematic family screening has recently identified dilated cardiomyopathy as an inherited disorder in up to 30% of cases. Mutations in genes encoding proteins responsible for myocardial architecture have been identified, but additional pathophysiological mechanisms including inflammatory reactions have been proposed.

AIMS

Identification and characterization of familial DCM, where at least one affected family member fulfils the criteria for inflammatory DCM may lead to a better understanding of the aetiology and pathogenesis of (inflammatory) DCM.

METHODS AND RESULTS

Ten families were examined. In six families, clinical characteristics and mode of inheritance were compatible with pure fDCM, fDCM with conduction defect and autosomal recessive fDCM. In four families, (auto-)immune features were diagnosed in affected and non-affected family members.

CONCLUSIONS

Familial DCM with an inflammatory component was identified as a specific subgroup of familial DCM. In most cases, the inflammatory process seems to modify, i.e. aggravate, the "classic, cytoskeletopathic" familial DCM, but in some, especially when taking clinical and genetic aspects into account, inflammatory (auto-)immune features can be addressed as the leading pathogenetic principle. Further elucidation of these families may provide a better insight into pathophysiologic processes and may aid in the development of specific therapeutic strategies.

Myofibrillar remodeling in cardiac hypertrophy, heart failure and cardiomyopathies

BACKGROUND

A wide variety of pathological conditions have been shown to result in cardiac remodelling and myocardial dysfunction. However, the mechanisms of transition from adaptive to maladaptive alterations, as well as those for changes in cardiac performance leading to heart failure, are poorly understood.

OBSERVATIONS

Extensive studies have revealed a broad spectrum of progressive changes in subcellular structures and function, as well as in signal transduction and metabolism in the heart, among different cardiovascular disorders. The present review is focused on identifying the alterations in molecular and biochemical structure of myofibrils (myofibrillar remodelling) in hypertrophied and failing myocardium in different types of heart diseases. Numerous changes at the level of gene expression for both contractile and regulatory proteins have already been reported in failing hearts and heart diseases; these changes are potential precursors for heart failure such as cardiac hypertrophy and cardiomyopathies. Myofibrillar remodelling, as a consequence of proteolysis, oxidation, and phosphorylation of some functional groups in both contractile and regulatory proteins in hearts failing due to different etiologies, has also been described.

CONCLUSIONS

Although myofibrillar remodelling appears to be associated with cardiac dysfunction, alterations in both contractile and regulatory proteins are dependent on the type and stage of heart disease.

Inflammatory cardiomyopathy: there is a specific matrix destruction in the course of the disease

Cardiomyopathies are responsible for a high proportion of cases of congestive heart failure and sudden death, as well as for the need for transplantation. Understanding of the causes of these disorders has been sought in earnest over the past decade. We hypothesized that DCM is a disease of the cytoskeleton/sarcolemma, which affects the sarcomere. Evaluation of the sarcolemma in DCM and other forms of systolic heart failure demonstrates membrane disruption; and, secondarily, the extracellular matrix architecture is also affected. Disruption of the links from the sarcolemma to ECM at the dystrophin C-terminus and those to the sarcomere and nucleus via N-terminal dystrophin interactions could lead to a "domino effect" disruption of systolic function and development of arrhythmias. We also have suggested that dystrophin mutations play a role in idiopathic DCM in males. The T-cap/MLP/alpha-actinin/titin complex appears to stabilize Z-disc function via mechanical stretch sensing. Loss of elasticity results in the primary defect in the endogenous cardiac muscle stretch sensor machinery. The over-stretching of individual myocytes leads to activation of cell death pathways, at a time when stretch-regulated survival cues are diminished due to defective stretch sensing, leading to progression of heart failure. Genetic DCM and the acquired disorder viral myocarditis have the same clinical features including heart failure, arrhythmias, and conduction block, and also similar mechanisms of disease based on the proteins targeted. In dilated cardiomyopathy, the process of progressive ventricular dilation and changes of the shape of the ventricle to a more spherical shape, associated with changes in ventricular function and/or hypertrophy, occurs without known initiating disturbance. In those cases in which resolution of cardiac dysfunction does not occur, chronic DCM results. It has been unclear what the underlying etiology of this long-term sequela could be, but viral persistence and autoimmunity have been widely speculated.

Genetic causes of human heart failure

Factors that render patients with cardiovascular disease at high risk for heart failure remain incompletely defined. Recent insights into molecular genetic causes of myocardial diseases have highlighted the importance of single-gene defects in the pathogenesis of heart failure. Through analyses of the mechanisms by which a mutation selectively perturbs one component of cardiac physiology and triggers cell and molecular responses, studies of human gene mutations provide a window into the complex processes of cardiac remodeling and heart failure. Knowledge gleaned from these studies shows promise for defining novel therapeutic targets for genetic and acquired causes of heart failure.

Genetic causes of human heart failure

Factors that render patients with cardiovascular disease at high risk for heart failure remain incompletely defined. Recent insights into molecular genetic causes of myocardial diseases have highlighted the importance of single-gene defects in the pathogenesis of heart failure. Through analyses of the mechanisms by which a mutation selectively perturbs one component of cardiac physiology and triggers cell and molecular responses, studies of human gene mutations provide a window into the complex processes of cardiac remodeling and heart failure. Knowledge gleaned from these studies shows promise for defining novel therapeutic targets for genetic and acquired causes of heart failure.

Array lessons from the heart: focus on the genome and transcriptome of cardiomyopathies

Our understanding of the cardiovascular system has evolved through the years by extensive studies emphasizing the identification of the molecular and physiological mechanisms involved in its normal function and disease pathogenesis. Major discoveries have been made along the way. However, the majority of this work has focused on specific genes or pathways rather than integrative approaches. In cardiomyopathies alone, over 30 different loci have shown mutations with varying inheritance patterns, yet mostly coding for structural proteins. The emergence of microarrays in the early 1990s paved the way to a new era of cardiovascular research. Microarrays dramatically accelerated the rhythm of discoveries by giving us the ability to simultaneously study thousands of genes in a single experiment. In the field of cardiovascular research, microarrays are having a significant contribution, with the majority of work focusing on end-stage cardiomyopathies that lead to heart failure. Novel molecular mechanisms have been identified, known pathways are seen under new light, disease subgroups begin to emerge, and the effects of various drugs are molecularly dissected. This cross-study data comparison concludes that consistent energy metabolism gene expression changes occur across dilated, hypertrophic, and ischemic cardiomyopathies, while Ca2+ homeostasis changes are prominent in the first two cardiomyopathies, and structural gene expression changes accompany mostly the dilated form. Gene expression changes are further correlated to disease genetics. The future of microarrays in the cardiomyopathy field is discussed with an emphasis on optimum experimental design and on applications in diagnosis, prognosis, and drug discovery.

Multiple pathogenetic mechanisms in X linked dilated cardiomyopathy

X linked dilated cardiomyopathy is a familial disease that is allelic to Duchenne and Becker muscular dystrophies and caused by mutations in the dystrophin gene. In several families with X linked dilated cardiomyopathy, the pattern of expression of dystrophin mutations in cardiac muscle differs from that in skeletal muscle. A number of these mutations affect transcription and splicing of the dystrophin gene in a tissue specific manner; others may affect regions of dystrophin that are presumed to have a more important role in cardiac than in skeletal muscle. These mutations are important because they highlight the fundamental differences in processing of the dystrophin gene between skeletal and cardiac tissues, as well as differences in the functional domains more relevant for one tissue or the other. This review focuses on the major mechanisms that have been proposed to explain this disorder.

Dystrophin and mutations: one gene, several proteins, multiple phenotypes

A large and complex gene on the X chromosome encodes dystrophin. Many mutations have been described in this gene, most of which affect the expression of the muscle isoform, the best-known protein product of this locus. These mutations result in the Duchenne and Becker muscular dystrophies (DMD and BMD). However, there are several other tissue specific isoforms of dystrophin, some exclusively or predominantly expressed in the brain or the retina. Mutations affecting the correct expression of these tissue-specific isoforms have been associated with the CNS involvement common in DMD. Rare mutations also account for the allelic disorder X-linked dilated cardiomyopathy, in which dystrophin expression or function is affected mostly or exclusively in the heart. Genotype definition of the dystrophin gene in patients with dystrophinopathies has taught us much about functionally important domains of the protein itself and has provided insights into several regulatory mechanisms governing the gene expression profile. Here, we focus on current understanding of the genotype-phenotype relation for mutations in the dystrophin gene and their implications for gene functions.

The heart in human dystrophinopathies

Dystrophinopathies are due to mutations in the dystrophin gene on chromosome Xp21.1 and comprise the allelic entities Duchenne muscular dystrophy (DMD), Becker muscular dystrophy (BMD) and X-linked dilative cardiomyopathy (XLDCM). In all three entities, the heart is affected to various degrees, depending on the stage of the disease and the type of the mutation (cardiac involvement, CI). The pathoanatomic evidence of CI in dystrophinopathies is the replacement of myocardium by connective tissue or fat. In DMD/BMD, the left ventricular posterobasal and lateral walls are most extensively affected, sparing the right ventricle and the atrium. Degree and dynamics of CI vary among the three entities. In DMD/BMD, CI usually remains subclinical in the early stages of the disease. Typical initial manifestations of CI in DMD/BMD are sinus tachycardia, tall R1 in V1, prominent Q in I, aVL, V6 or in II, III, and aVF, increased QT dispersion and possibly autonomic dysfunction. Initially, echocardiography is normal or shows regional wall motion abnormalities in areas of fibrosis. With spreading of fibrosis, left ventricular dysfunction and ventricular arrhythmias additionally occur. In the final stages of the disease, systolic function may lead to heart failure and sudden death. Subclinical or clinical CI is present in about 90% of the DMD/BMD patients but is the cause of death in only 20% of the DMD and 50% of the BMD patients. XLDCM is a rapidly progressive, almost exclusively myocardial disorder, starting in teenage males as heart failure due to dilative cardiomyopathy (CMP), leading to death from intractable heart failure within 1-2 years after diagnosis. Therapy of arrhythmias and CMP in all three disorders follows the established cardiological recommendations. Due to its protective effect, ACE inhibitors are recommended already at the early stages of the disease. Beta-blockers may be an additional option if indicated.

Molecular diagnosis of myocardial disease

Myocardial disorders are major causes of morbidity and mortality, including heart failure, sudden death and the need for heart transplantation. The two most common forms of myocardial disorders, dilated cardiomyopathy and hypertrophic cardiomyopathy are paradigms of left ventricular systolic dysfunction and diastolic dysfunction. The genetics of these disorders are increasingly understood with the sarcomere playing a central role in the development of HCM and the link between sarcomere and sarcolemma being key to the development of DCM. In this review, the genetics of the myocardial diseases will be described.

Molecular Mechanisms of Inherited Cardiomyopathies

Cardiomyopathies are diseases of heart muscle that may result from a diverse array of conditions that damage the heart and other organs and impair myocardial function, including infection, ischemia, and toxins. However, they may also occur as primary diseases restricted to striated muscle. Over the past decade, the importance of inherited gene defects in the pathogenesis of primary cardiomyopathies has been recognized, with mutations in some 18 genes having been identified as causing hypertrophic cardiomyopathy (HCM) and/or dilated cardiomyopathy (DCM). Defining the role of these genes in cardiac function and the mechanisms by which mutations in these genes lead to hypertrophy, dilation, and contractile failure are major goals of ongoing research. Pathophysiological mechanisms that have been implicated in HCM and DCM include the following: defective force generation, due to mutations in sarcomeric protein genes; defective force transmission, due to mutations in cytoskeletal protein genes; myocardial energy deficits, due to mutations in ATP regulatory protein genes; and abnormal Ca2+ homeostasis, due to altered availability of Ca2+ and altered myofibrillar Ca2+ sensitivity. Improved understanding that will result from these studies should ultimately lead to new approaches for the diagnosis, prognostic stratification, and treatment of patients with heart failure.

Cardiomyopathies: from genetics to the prospect of treatment

Cardiomyopathies are defined as diseases of the myocardium associated with cardiac dysfunction ranging from lifelong symptomless forms to major health problems such as progressive heart failure, arrhythmia, thromboembolism, and sudden cardiac death. They are classified by morphological characteristics as hypertrophic (HCM), dilated (DCM), arrhythmogenic right ventricular (ARVC), and restrictive cardiomyopathy (RCM). A familial cause has been shown in 50% of patients with HCM, 35% with DCM, and 30% with ARVC. In HCM, nine genetic loci and more than 130 mutations in ten different sarcomeric genes and in the gamma 2 subunit of AMP-activated protein kinase (AMPK) have been identified, suggesting impaired force production associated with inefficient use of ATP as the crucial disease mechanism. In DCM, 16 chromosomal loci with defects of several proteins also involved in the development of skeletal myopathies have been detected. These mutated cytoskeletal and nuclear transporter proteins may alter force transmission or disrupt nuclear function, resulting in cell death. Further DCM mutations have also been identified in sarcomeric genes, which indicates that different defects of the same protein can result in either HCM or DCM. In ARVC, six genetic loci and mutations in the cardiac ryanodine receptor, which controls electromechanical coupling, and in plakoglobin and desmoglobin (molecules involved in desmosomal cell-junction integrity), have been identified. Yet, no genetic linkage has been shown in RCM. Apart from disease-causing mutations, other factors, such as environment, genetic background, and the recently identified modifier genes of the renin-angiotensin, adrenergic, and endothelin systems are likely to result in the wide variety of RCM clinical presentations. Treatment options are symptomatic and are mainly focused on treatment of heart failure and prevention of thromboembolism and sudden death. Identification of patients with high risk for major arrhythmic events is important because implantable cardioverter defibrillators can prevent sudden death. Clinical and genetic risk stratification may lead to prospective trials of primary implantation of cardioverter defibrillators in people with hereditary cardiomyopathy.

VAPSE-based analysis: a two-phased candidate gene approach for elucidating genetic predisposition to complex disorders

The extraordinary success of linkage analysis in diseases with Mendelian inheritance has not extended readily to the genetics of common complex diseases. VAPSE-based analysis is a type of candidate gene approach that represents an alternative strategy by which genetic mechanisms can be defined despite the presence of substantial genetic heterogeneity. Recent advances in mutation screening and statistical methodology have enhanced substantially the efficiency and power of this approach. The "bread and butter" of VAPSE-based analysis is genotype-to-phenotype searches in large populations with computerized medical records.