Ludwig Thierfelder

Mutations in the Genes for Cardiac Troponin T and α-Tropomyosin in Hypertrophic Cardiomyopathy

BACKGROUND Familial hypertrophic cardiomyopathy can be caused by mutations in the genes for beta cardiac myosin heavy chain, alpha-tropomyosin, or cardiac troponin T. It is not known how often the disease is caused by mutations in the tropomyosin and troponin genes, and the associated clinical phenotypes have not been carefully studied. METHODS Linkage between polymorphisms of the alpha-tropomyosin gene or the cardiac troponin T gene and hypertrophic cardiomyopathy was assessed in 27 families. In addition, 100 probands were screened for mutations in the alpha-tropomyosin gene, and 26 were screened for mutations in the cardiac troponin T gene. Life expectancy, the incidence of sudden death, and the extent of left ventricular hypertrophy were compared in patients with different mutations. RESULTS Genetic analyses identified only one alpha-tropomyosin mutation, identical to one previously described. Five novel mutations in cardiac troponin were identified, as well as a further example of a previously described mutation. The clinical phenotype of four troponin T mutations in seven unrelated families was similar and was characterized by a poor prognosis (life expectancy, approximately 35 years) and a high incidence of sudden death. The mean (+/- SD) maximal thickness of the left ventricular wall in subjects with cardiac troponin T mutations (16.7 +/- 5.5 mm) was significantly less than that in subjects with beta cardiac myosin heavy-chain mutations (23.7 +/- 7.7 mm, P < 0.001). CONCLUSIONS Mutations in alpha-tropomyosin are a rare cause of familial hypertrophic cardiomyopathy, accounting for approximately 3 percent of cases. Mutations in cardiac troponin T account for approximately 15 percent of cases of familial hypertrophic cardiomyopathy in this referral-center population. These mutations are characterized by relatively mild and sometimes subclinical hypertrophy but a high incidence of sudden death. Genetic testing may therefore be especially important in this group.

Mutations in the desmosomal protein plakophilin-2 are common in arrhythmogenic right ventricular cardiomyopathy

Arrhythmogenic right ventricular cardiomyopathy (ARVC) is associated with fibrofatty replacement of cardiac myocytes, ventricular tachyarrhythmias and sudden cardiac death. In 32 of 120 unrelated individuals with ARVC, we identified heterozygous mutations in PKP2, which encodes plakophilin-2, an essential armadillo-repeat protein of the cardiac desmosome. In two kindreds with ARVC, disease was incompletely penetrant in most carriers of PKP2 mutations.

Mutations of TTN , encoding the giant muscle filament titin, cause familial dilated cardiomyopathy

Congestive heart failure (CHF) can result from various disease states with inadequate cardiac output. CHF due to dilated cardiomyopathy (DCM) is a familial disease in 20–30% of cases and is associated with mutations in genes encoding cytoskeletal, contractile or inner–nuclear membrane proteins. We show that mutations in the gene encoding giant-muscle filament titin (TTN) cause autosomal dominant DCM linked to chromosome 2q31 (CMD1G; MIM 604145). Titin molecules extend from sarcomeric Z-discs to M-lines, provide an extensible scaffold for the contractile machinery and are crucial for myofibrillar elasticity and integrity. In a large DCM kindred, a segregating 2-bp insertion mutation in TTN exon 326 causes a frameshift, truncating A-band titin. The truncated protein of approximately 2 mD is expressed in skeletal muscle, but western blot studies with epitope-specific anti-titin antibodies suggest that the mutant protein is truncated to a 1.14-mD subfragment by site-specific cleavage. In another large family with DCM linked to CMD1G, a TTN missense mutation (Trp930Arg) is predicted to disrupt a highly conserved hydrophobic core sequence of an immunoglobulin fold located in the Z-disc–I-band transition zone. The identification of TTN mutations in individuals with CMD1G should provide further insights into the pathogenesis of familial forms of CHF and myofibrillar titin turnover.

Arrhythmogenic right ventricular cardiomyopathy type 5 is a fully penetrant, lethal arrhythmic disorder caused by a missense mutation in the TMEM43 gene.

Autosomal-dominant arrhythmogenic right ventricular cardiomyopathy/dysplasia (ARVC/D) causes sudden cardiac death and is characterized by clinical and genetic heterogeneity. Fifteen unrelated ARVC families with a disease-associated haplotype on chromosome 3p (ARVD5) were ascertained from a genetically isolated population. Identification of key recombination events reduced the disease region to a 2.36 Mb interval containing 20 annotated genes. Bidirectional resequencing showed one rare variant in transmembrane protein 43 (TMEM43 1073C-->T, S358L), was carried on all recombinant ARVD5 ancestral haplotypes from affected subjects and not found in population controls. The mutation occurs in a highly conserved transmembrane domain of TMEM43 and is predicted to be deleterious. Clinical outcomes in 257 affected and 151 unaffected subjects were compared, and penetrance was determined. We concluded that ARVC at locus ARVD5 is a lethal, fully penetrant, sex-influenced morbid disorder. Median life expectancy was 41 years in affected males compared to 71 years in affected females (relative risk 6.8, 95% CI 1.3-10.9). Heart failure was a late manifestation in survivors. Although little is known about the function of the TMEM43 gene, it contains a response element for PPAR gamma (an adipogenic transcription factor), which may explain the fibrofatty replacement of the myocardium, a characteristic pathological finding in ARVC.

Mutations in Sarcomere Protein Genes in Left Ventricular Noncompaction

Background— Left ventricular noncompaction constitutes a primary cardiomyopathy characterized by a severely thickened, 2-layered myocardium, numerous prominent trabeculations, and deep intertrabecular recesses. The genetic basis of this cardiomyopathy is still largely unresolved. We speculated that mutations in sarcomere protein genes known to cause hypertrophic cardiomyopathy and dilated cardiomyopathy may be associated with left ventricular noncompaction. Methods and Results— Mutational analysis in a cohort of 63 unrelated adult probands with left ventricular noncompaction and no other congenital heart anomalies was performed by denaturing high-performance liquid chromatography analysis and direct DNA sequencing of 6 genes encoding sarcomere proteins. Heterozygous mutations were identified in 11 of 63 samples in genes encoding &bgr;-myosin heavy chain (MYH7), &agr;-cardiac actin (ACTC), and cardiac troponin T (TNNT2). Nine distinct mutations, 7 of them in MYH7, 1 in ACTC, and 1 in TNNT2, were found. Clinical evaluations demonstrated familial disease in 6 of 11 probands with sarcomere gene mutations. MYH7 mutations segregated with the disease in 4 autosomal dominant LVNC kindreds. Six of the MYH7 mutations were novel, and 1 encodes a splice-site mutation, a relatively unique finding for MYH7 mutations. Modified residues in &bgr;-myosin heavy chain were located mainly within the ATP binding site. Conclusions— We conclude that left ventricular noncompaction is within the diverse spectrum of cardiac morphologies triggered by sarcomere protein gene defects. Our findings support the hypothesis that there is a shared molecular etiology of different cardiomyopathic phenotypes.

RBM20 , a gene for hereditary cardiomyopathy, regulates titin splicing

Alternative splicing has a major role in cardiac adaptive responses, as exemplified by the isoform switch of the sarcomeric protein titin, which adjusts ventricular filling. By positional cloning using a previously characterized rat strain with altered titin mRNA splicing, we identified a loss-of-function mutation in the gene encoding RNA binding motif protein 20 (Rbm20) as the underlying cause of pathological titin isoform expression. The phenotype of Rbm20-deficient rats resembled the pathology seen in individuals with dilated cardiomyopathy caused by RBM20 mutations. Deep sequencing of the human and rat cardiac transcriptome revealed an RBM20-dependent regulation of alternative splicing. In addition to titin (TTN), we identified a set of 30 genes with conserved splicing regulation between humans and rats. This network is enriched for genes that have previously been linked to cardiomyopathy, ion homeostasis and sarcomere biology. Our studies emphasize the key role of post-transcriptional regulation in cardiac function and provide mechanistic insights into the pathogenesis of human heart failure.

Mutant Desmocollin-2 Causes Arrhythmogenic Right Ventricular Cardiomyopathy

Arrhythmogenic right ventricular cardiomyopathy (ARVC) is a genetically heterogeneous heart-muscle disorder characterized by progressive fibrofatty replacement of right ventricular myocardium and an increased risk of sudden cardiac death. Mutations in desmosomal proteins that cause ARVC have been previously described; therefore, we investigated 88 unrelated patients with the disorder for mutations in human desmosomal cadherin desmocollin-2 (DSC2). We identified a heterozygous splice-acceptor-site mutation in intron 5 (c.631-2A-->G) of the DSC2 gene, which led to the use of a cryptic splice-acceptor site and the creation of a downstream premature termination codon. Quantitative analysis of cardiac DSC2 expression in patient specimens revealed a marked reduction in the abundance of the mutant transcript. Morpholino knockdown in zebrafish embryos revealed a requirement for dsc2 in the establishment of the normal myocardial structure and function, with reduced desmosomal plaque area, loss of the desmosome extracellular electron-dense midlines, and associated myocardial contractility defects. These data identify DSC2 mutations as a cause of ARVC in humans and demonstrate that physiologic levels of DSC2 are crucial for normal cardiac desmosome formation, early cardiac morphogenesis, and cardiac function.

Isolated noncompaction of the left ventricular myocardium in the adult is an autosomal dominant disorder in the majority of patients

Isolated noncompaction of the ventricular myocardium (INVM, MIM 300183 and 604169) is a congenital unclassified cardiomyopathy with numerous prominent trabeculations and deep intertrabecular recesses in a hypertrophied and hypokinetic myocardium. Mutations in the G4.5 gene result in a wide spectrum of severe infantile X‐linked cardiomyopathic phenotypes including Barth syndrome with dilated cardiomyopathy and INVM. Molecular genetic analysis of INVM has only been performed in pediatric patients. Although adult INVM patients show similar cardiac abnormalities, the influence of genetic factors, especially of mutations in G4.5, is unknown. We analyzed 25 adult INVM patients for the presence of mutations in the G4.5 gene and performed a pedigree analysis of probands. Mutations were not found in the coding sequence or splice sites of G4.5. Systematic analysis of relatives from seven of nine probands showed multiple affected members consistent with an autosomal dominant pattern of inheritance in the majority of cases. We conclude that INVM in the adult is an autosomal dominant disorder rarely caused by mutations in G4.5 and therefore genetically distinct from infantile X‐linked cases.

Sporadic hypertrophic cardiomyopathy due to de novo myosin mutations.

Hypertrophic cardiomyopathy occurs as an autosomal dominant familial disorder or as a sporadic disease without familial involvement. While missense mutations in the beta cardiac myosin heavy chain (MHC) gene account for approximately half of all cases of familial hypertrophic cardiomyopathy, the molecular causes of sporadic hypertrophic cardiomyopathy are unknown. To determine whether beta cardiac MHC mutations are also associated with sporadic disease, we screened this gene in seven individuals with sporadic hypertrophic cardiomyopathy. Mutations in the beta cardiac MHC genes were identified in two probands with sporadic disease. In that their parents were neither clinically nor genetically affected, we conclude that mutations in each proband arose de novo. Transmission of the mutation and disease to an offspring occurred in one pedigree, predicting that these are germline mutations. The demonstration of hypertrophic cardiomyopathy arising within a pedigree coincident with the appearance of a de novo mutation provides compelling genetic evidence that beta cardiac MHC mutations cause this disease. We suggest that de novo mutations account for some instances of sporadic hypertrophic cardiomyopathy and that these mutations can be transmitted to children. The clinical benefits of defining mutations responsible for familial hypertrophic cardiomyopathy should also be available to some patients with sporadic disease.

Novel Gene Locus for Autosomal Dominant Left Ventricular Noncompaction Maps to Chromosome 11p15

Background—Left ventricular noncompaction (LVNC) is a congenital unclassified cardiomyopathy with numerous prominent trabeculations and deep intertrabecular recesses in a hypertrophied and hypokinetic myocardium. It has been reported to occur in isolation or in association with congenital heart disease. Mutations in the X-linked G4.5 gene are responsible for cases of isolated LVNC in male infants, but G4.5 mutations were not found in patients with clinical onset of disease in adulthood. In addition, several families with LVNC and an autosomal dominant pattern of inheritance suggest genetic heterogeneity. Methods and Results—We performed a genome-wide linkage analysis in a family with autosomal dominant LVNC and show that a locus containing the LVNC disease gene maps to chromosome 11p15. A peak 2-point logarithm of odds score of 5.06 was obtained with marker D11S902 at &thetas;=0. Haplotype analysis defined a critical interval of 6.4 centimorgan between D11S1794 and D11S928 corresponding to a physical distance of 6.8 megabases. No disease-causing mutation was identified in 2 prime positional candidate genes, muscle LIM protein (MLP) and SOX6. Conclusions—We have mapped a locus for autosomal dominant LVNC to a 6.8-megabase region on human chromosome 11p15. Identification of the disease gene will allow genetic screening and provide fundamental insight into the understanding of myocardial morphogenesis.