Identification of pathogenic gene mutations in LMNA and MYBPC3 that alter RNA splicing.

Genetic variants that cause haploinsufficiency account for many autosomal dominant (AD) disorders. Gene-based diagnosis classifies variants that alter canonical splice signals as pathogenic, but due to imperfect understanding of RNA splice signals other variants that may create or eliminate splice sites are often clinically classified as variants of unknown significance (VUS). To improve recognition of pathogenic splice-altering variants in AD disorders, we used computational tools to prioritize VUS and developed a cell-based minigene splicing assay to confirm aberrant splicing. Using this two-step procedure we evaluated all rare variants in two AD cardiomyopathy genes, lamin A/C (LMNA) and myosin binding protein C (MYBPC3). We demonstrate that 13 LMNA and 35 MYBPC3 variants identified in cardiomyopathy patients alter RNA splicing, representing a 50% increase in the numbers of established damaging splice variants in these genes. Over half of these variants are annotated as VUS by clinical diagnostic laboratories. Familial analyses of one variant, a synonymous LMNA VUS, demonstrated segregation with cardiomyopathy affection status and altered cardiac LMNA splicing. Application of this strategy should improve diagnostic accuracy and variant classification in other haploinsufficient AD disorders.

A Prospective Study of Sudden Cardiac Death among Children and Young Adults.

BACKGROUND: Sudden cardiac death among children and young adults is a devastating event. We performed a prospective, population-based, clinical and genetic study of sudden cardiac death among children and young adults. METHODS: We prospectively collected clinical, demographic, and autopsy information on all cases of sudden cardiac death among children and young adults 1 to 35 years of age in Australia and New Zealand from 2010 through 2012. In cases that had no cause identified after a comprehensive autopsy that included toxicologic and histologic studies (unexplained sudden cardiac death), at least 59 cardiac genes were analyzed for a clinically relevant cardiac gene mutation. RESULTS: A total of 490 cases of sudden cardiac death were identified. The annual incidence was 1.3 cases per 100,000 persons 1 to 35 years of age; 72% of the cases involved boys or young men. Persons 31 to 35 years of age had the highest incidence of sudden cardiac death (3.2 cases per 100,000 persons per year), and persons 16 to 20 years of age had the highest incidence of unexplained sudden cardiac death (0.8 cases per 100,000 persons per year). The most common explained causes of sudden cardiac death were coronary artery disease (24% of cases) and inherited cardiomyopathies (16% of cases). Unexplained sudden cardiac death (40% of cases) was the predominant finding among persons in all age groups, except for those 31 to 35 years of age, for whom coronary artery disease was the most common finding. Younger age and death at night were independently associated with unexplained sudden cardiac death as compared with explained sudden cardiac death. A clinically relevant cardiac gene mutation was identified in 31 of 113 cases (27%) of unexplained sudden cardiac death in which genetic testing was performed. During follow-up, a clinical diagnosis of an inherited cardiovascular disease was identified in 13% of the families in which an unexplained sudden cardiac death occurred. CONCLUSIONS: The addition of genetic testing to autopsy investigation substantially increased the identification of a possible cause of sudden cardiac death among children and young adults. (Funded by the National Health and Medical Research Council of Australia and others.).

A gene-centric strategy for identifying disease-causing rare variants in dilated cardiomyopathy.

PURPOSE: We evaluated strategies for identifying disease-causing variants in genetic testing for dilated cardiomyopathy (DCM). METHODS: Cardiomyopathy gene panel testing was performed in 532 DCM patients and 527 healthy control subjects. Rare variants in 41 genes were stratified using variant-level and gene-level characteristics. RESULTS: A majority of DCM cases and controls carried rare protein-altering cardiomyopathy gene variants. Variant-level characteristics alone had limited discriminative value. Differentiation between groups was substantially improved by addition of gene-level information that incorporated ranking of genes based on literature evidence for disease association. The odds of DCM were increased to nearly 9-fold for truncating variants or high-impact missense variants in the subset of 14 genes that had the strongest biological links to DCM (P <0.0001). For some of these genes, DCM-associated variants appeared to be clustered in key protein functional domains. Multiple rare variants were present in many family probands, however, there was generally only one "driver" pathogenic variant that cosegregated with disease. CONCLUSION: Rare variants in cardiomyopathy genes can be effectively stratified by combining variant-level and gene-level information. Prioritization of genes based on their a priori likelihood of disease causation is a key factor in identifying clinically actionable variants in cardiac genetic testing.

Exome-based analysis of cardiac arrhythmia, respiratory control, and epilepsy genes in sudden unexpected death in epilepsy.

OBJECTIVE: The leading cause of epilepsy-related premature mortality is sudden unexpected death in epilepsy (SUDEP). The cause of SUDEP remains unknown. To search for genetic risk factors in SUDEP cases, we performed an exome-based analysis of rare variants. METHODS: Demographic and clinical information of 61 SUDEP cases were collected. Exome sequencing and rare variant collapsing analysis with 2,936 control exomes were performed to test for genes enriched with damaging variants. Additionally, cardiac arrhythmia, respiratory control, and epilepsy genes were screened for variants with frequency of <0.1% and predicted to be pathogenic with multiple in silico tools. RESULTS: The 61 SUDEP cases were categorized as definite SUDEP (n = 54), probable SUDEP (n = 5), and definite SUDEP plus (n = 2). We identified de novo mutations, previously reported pathogenic mutations, or candidate pathogenic variants in 28 of 61 (46%) cases. Four SUDEP cases (7%) had mutations in common genes responsible for the cardiac arrhythmia disease, long QT syndrome (LQTS). Nine cases (15%) had candidate pathogenic variants in dominant cardiac arrhythmia genes. Fifteen cases (25%) had mutations or candidate pathogenic variants in dominant epilepsy genes. No gene reached genome-wide significance with rare variant collapsing analysis; however, DEPDC5 (p = 0.00015) and KCNH2 (p = 0.0037) were among the top 30 genes, genome-wide. INTERPRETATION: A sizeable proportion of SUDEP cases have clinically relevant mutations in cardiac arrhythmia and epilepsy genes. In cases with an LQTS gene mutation, SUDEP may occur as a result of a predictable and preventable cause. Understanding the genetic basis of SUDEP may inform cascade testing of at-risk family members.

Truncations of titin causing dilated cardiomyopathy.

BACKGROUND: Dilated cardiomyopathy and hypertrophic cardiomyopathy arise from mutations in many genes. TTN, the gene encoding the sarcomere protein titin, has been insufficiently analyzed for cardiomyopathy mutations because of its enormous size. METHODS: We analyzed TTN in 312 subjects with dilated cardiomyopathy, 231 subjects with hypertrophic cardiomyopathy, and 249 controls by using next-generation or dideoxy sequencing. We evaluated deleterious variants for cosegregation in families and assessed clinical characteristics. RESULTS: We identified 72 unique mutations (25 nonsense, 23 frameshift, 23 splicing, and 1 large tandem insertion) that altered full-length titin. Among subjects studied by means of next-generation sequencing, the frequency of TTN mutations was significantly higher among subjects with dilated cardiomyopathy (54 of 203 [27%]) than among subjects with hypertrophic cardiomyopathy (3 of 231 [1%], P=3×10(-16)) or controls (7 of 249 [3%], P=9×10(-14)). TTN mutations cosegregated with dilated cardiomyopathy in families (combined lod score, 11.1) with high (>95%) observed penetrance after the age of 40 years. Mutations associated with dilated cardiomyopathy were overrepresented in the titin A-band but were absent from the Z-disk and M-band regions of titin (P≤0.01 for all comparisons). Overall, the rates of cardiac outcomes were similar in subjects with and those without TTN mutations, but adverse events occurred earlier in male mutation carriers than in female carriers (P=4×10(-5)). CONCLUSIONS: TTN truncating mutations are a common cause of dilated cardiomyopathy, occurring in approximately 25% of familial cases of idiopathic dilated cardiomyopathy and in 18% of sporadic cases. Incorporation of sequencing approaches that detect TTN truncations into genetic testing for dilated cardiomyopathy should substantially increase test sensitivity, thereby allowing earlier diagnosis and therapeutic intervention for many patients with dilated cardiomyopathy. Defining the functional effects of TTN truncating mutations should improve our understanding of the pathophysiology of dilated cardiomyopathy. (Funded by the Howard Hughes Medical Institute and others.).

Variant Location Is a Novel Risk Factor for Individuals With Arrhythmogenic Cardiomyopathy Due to a Desmoplakin (DSP) Truncating Variant.

BACKGROUND: Truncating variants in desmoplakin (DSPtv) are an important cause of arrhythmogenic cardiomyopathy; however the genetic architecture and genotype-specific risk factors are incompletely understood. We evaluated phenotype, risk factors for ventricular arrhythmias, and underlying genetics of DSPtv cardiomyopathy. METHODS: Individuals with DSPtv and any cardiac phenotype, and their gene-positive family members were included from multiple international centers. Clinical data and family history information were collected. Event-free survival from ventricular arrhythmia was assessed. Variant location was compared between cases and controls, and literature review of reported DSPtv performed. RESULTS: There were 98 probands and 72 family members (mean age at diagnosis 43±8 years, 59% women) with a DSPtv, of which 146 were considered clinically affected. Ventricular arrhythmia (sudden cardiac arrest, sustained ventricular tachycardia, appropriate implantable cardioverter defibrillator therapy) occurred in 56 (33%) individuals. DSPtv location and proband status were independent risk factors for ventricular arrhythmia. Further, gene region was important with variants in cases (cohort n=98; Clinvar n=167) more likely to occur in the regions resulting in nonsense mediated decay of both major DSP isoforms, compared with n=124 genome aggregation database control variants (148 [83.6%] versus 29 [16.4%]; P<0.0001). CONCLUSIONS: In the largest series of individuals with DSPtv, we demonstrate that variant location is a novel risk factor for ventricular arrhythmia, can inform variant interpretation, and provide critical insights to allow for precision-based clinical management.

Contribution of Noncanonical Splice Variants to TTN Truncating Variant Cardiomyopathy.

BACKGROUND: Heterozygous TTN truncating variants cause 10% to 20% of idiopathic dilated cardiomyopathy (DCM). Although variants which disrupt canonical splice signals (ie, invariant dinucleotide of the splice donor site, invariant dinucleotide of the splice acceptor site) at exon-intron junctions are readily recognized as TTN truncating variants, the effects of other nearby sequence variations on splicing and their contribution to disease is uncertain. METHODS: Rare variants of unknown significance located in the splice regions of highly expressed TTN exons from 203 DCM cases, 3329 normal subjects, and clinical variant databases were identified. The effects of these variants on splicing were assessed using an in vitro splice assay. RESULTS: Splice-altering variants of unknown significance were enriched in DCM cases over controls and present in 2% of DCM patients (P=0.002). Application of this method to clinical variant databases demonstrated 20% of similar variants of unknown significance in TTN splice regions affect splicing. Noncanonical splice-altering variants were most frequently located at position +5 of the donor site (P=4.4×107) and position -3 of the acceptor site (P=0.002). SpliceAI, an emerging in silico prediction tool, had a high positive predictive value (86%-95%) but poor sensitivity (15%-50%) for the detection of splice-altering variants. Alternate exons spliced out of most TTN transcripts frequently lacked the consensus base at +5 donor and -3 acceptor positions. CONCLUSIONS: Noncanonical splice-altering variants in TTN explain 1-2% of DCM and offer a 10-20% increase in the diagnostic power of TTN sequencing in this disease. These data suggest rules that may improve efforts to detect splice-altering variants in other genes and may explain the low percent splicing observed for many alternate TTN exons.

Differential protein expression profiling of myocardial tissue in a mouse model of hypertrophic cardiomyopathy.

Hypertrophic cardiomyopathy (HCM) is a genetic disorder caused by mutations in genes encoding sarcomere proteins. The mechanisms involved in the development of cardiac hypertrophy and heart failure remain poorly understood. Global proteomic profiling was used to study the cardiac proteome of mice predisposed to developing HCM. Hearts from three groups of mice (n=3 hearts per group) were studied: non-transgenic (NTG) and cardiac-specific transgenic models over-expressing either the normal (TnI(WT)) or a mutant cardiac troponin I gene (Gly203Ser; TnI(G203S)). Two-dimensional gel electrophoresis (2-DE) coupled with tandem mass spectrometry was used to identify proteins. Image analysis was performed using Progenesis SameSpots. A total of 34 proteins with at least a twofold change in the TnI(G203S) mouse model were identified. Alterations were detected in components involved in energy production, Ca(2+) handling, and cardiomyocyte structure. Expression level changes in cytoskeletal and contractile proteins were well represented in the study, including the intermediate filament protein desmin, which was further investigated in two additional physiological and pathological settings, i.e., exercise treatment, and severe heart failure in a novel double-mutant TnI-203/MHC-403 model of HCM. This study highlights the potential role of tissue proteomic profiling for mapping proteins, which may be critical in cardiac dysfunction and progression to heart failure in HCM.

Severe heart failure and early mortality in a double-mutation mouse model of familial hypertrophic cardiomyopathy.

BACKGROUND: Familial hypertrophic cardiomyopathy (FHC) is characterized by genetic and clinical heterogeneity. Five percent of FHC families have 2 FHC-causing mutations, which results in earlier disease onset, increased cardiac dysfunction, and a higher incidence of sudden death events. These observations suggest a relationship between the number of gene mutations and phenotype severity in FHC. METHODS AND RESULTS: We sought to develop, characterize, and investigate the pathogenic mechanisms in a double-mutant murine model of FHC. This model (designated TnI-203/MHC-403) was generated by crossbreeding mice with the Gly203Ser cardiac troponin I (TnI-203) and Arg403Gln alpha-myosin heavy chain (MHC-403) FHC-causing mutations. The mortality rate in TnI-203/MHC-403 mice was 100% by age 21 days. At age 14 days, TnI-203/MHC-403 mice developed a significantly increased ratio of heart weight to body weight, marked interstitial myocardial fibrosis, and increased expression of atrial natriuretic factor and brain natriuretic peptide compared with nontransgenic, TnI-203, and MHC-403 littermates. By age 16 to 18 days, TnI-203/MHC-403 mice rapidly developed a severe dilated cardiomyopathy and heart failure, with inducibility of ventricular arrhythmias, which led to death by 21 days. Downregulation of mRNA levels of key regulators of Ca(2+) homeostasis in TnI-203/MHC-403 mice was observed. Increased levels of phosphorylated STAT3 were observed in TnI-203/MHC-403 mice and corresponded with the onset of disease, which suggests a possible cardioprotective response. CONCLUSIONS: TnI-203/MHC-403 double-mutant mice develop a severe cardiac phenotype characterized by heart failure and early death. The presence of 2 disease-causing mutations may predispose individuals to a greater risk of developing severe heart failure than human FHC caused by a single gene mutation.

Multiple Gene Variants in Hypertrophic Cardiomyopathy in the Era of Next-Generation Sequencing.

BACKGROUND: Multiple likely pathogenic/pathogenic (LP/P; ≥2) variants in patients with hypertrophic cardiomyopathy were described 10 years ago with a prevalence of 5%. We sought to re-examine the significance of multiple rare variants in patients with hypertrophic cardiomyopathy in the setting of comprehensive and targeted panels. METHODS AND RESULTS: Of 758 hypertrophic cardiomyopathy probands, we included 382 with ≥45 cardiomyopathy genes screened. There were 224 (59%) with ≥1 rare variant (allele frequency ≤0.02%). Variants were analyzed using varying sized gene panels to represent comprehensive or targeted testing. Based on a 45-gene panel, 127 (33%) had a LP/P variant, 139 (36%) had variants of uncertain significance, and 66 (17%) had multiple rare variants. A targeted 8-gene panel yielded 125 (32%) LP/P variants, 52 (14%) variants of uncertain significance, and 14 (4%) had multiple rare variants. No proband had 2 LP/P variants. Including affected family members (total n=412), cluster-adjusted analyses identified a phenotype effect, with younger age (odds ratio, 0.95; 95% confidence interval, 0.92-0.98; P=0.004) and family history of sudden cardiac death (odds ratio, 3.5; 95% confidence interval, 1.3-9.9; P=0.02) significantly more likely in multiple versus single variant patients when considering an 8-gene panel but not larger panels. Those with multiple variants had worse event-free survival from all-cause death, cardiac transplantation, and cardiac arrest (log-rank P=0.008). CONCLUSIONS: No proband had multiple LP/P variants in contrast to previous reports. However, multiple rare variants regardless of classification were seen in 4% and contributed to earlier disease onset and cardiac events. Our findings support a cumulative variant hypothesis in hypertrophic cardiomyopathy.

Nonfamilial Hypertrophic Cardiomyopathy: Prevalence, Natural History, and Clinical Implications.

BACKGROUND: Yield of causative variants in hypertrophic cardiomyopathy (HCM) is increased in some probands, suggesting different clinical subgroups of disease occur. We hypothesized that a negative family history and no sarcomere mutations represent a nonfamilial subgroup of HCM. We sought to determine the prevalence, natural history, and potential clinical implications of this nonfamilial subgroup of HCM. METHODS AND RESULTS: Four hundred and thirteen unrelated probands with HCM seen in a specialized HCM center between 2002 and 2015 and genetic testing performed were included in this retrospective cohort study. There were 251 (61%) probands with no reported family history of HCM, including 166 (40% of total) probands with no sarcomere mutation, that is, nonfamilial HCM. Quantified family pedigree data revealed no difference in mean number of first-degree relatives screened between nonfamilial and sarcomere-positive groups. Adjusted predictors of nonfamilial status were older age (odds ratio, 1.04; 95% confidence interval, 1.02-1.06; P=0.0001), male sex (odds ratio, 1.96; 95% confidence interval, 1.11-3.45; P=0.02), hypertension (odds ratio, 2.80; 95% confidence interval, 1.57-5.00; P=0.0005), and nonasymmetric septal morphology (odds ratio, 3.41; 95% confidence interval, 1.64-7.08; P=0.001). They had a less severe clinical course with greater event-free survival from major cardiac events (P=0.04) compared with sarcomere-positive HCM probands. Genotype prediction scores showed good performance in identifying genotype-positive patients (area under the curve, 0.71-0.75) and, in combination with pedigree characteristics, were further improved. CONCLUSIONS: Approximately 40% of HCM probands have a nonfamilial subtype, with later onset and less severe clinical course. We propose a revised clinical pathway for management, highlighting the role of genetic testing, a detailed pedigree, and refined clinical surveillance recommendations for family members.

Application of proteomics in cardiovascular medicine.

Proteomics is an emerging field that has the potential to uncover new therapeutic targets for the treatment and prevention of cardiovascular disease, as well as new diagnostic biomarkers for early disease detection. The basic strategy when carrying out proteomic analysis of cardiovascular disease is to compare the protein complements of diseased hearts or sera with controls. Any proteins that have altered expression between the two groups can be studied further for their involvement in disease pathogenesis. A number of steps need to be taken to identify changes in protein expression, including sample preparation, protein separation, imaging, and protein identification. Such studies are already underway in some cardiovascular conditions including dilated cardiomyopathy and atrial fibrillation. This review provides a summary of the techniques used in proteomic analysis and their application to cardiovascular research.

A novel heterozygous mutation in cardiac calsequestrin causes autosomal dominant catecholaminergic polymorphic ventricular tachycardia.

BACKGROUND: Catecholaminergic polymorphic ventricular tachycardia (CPVT) is a lethal inherited arrhythmia syndrome characterized by adrenergically stimulated ventricular tachycardia. Mutations in the cardiac ryanodine receptor gene (RYR2) cause an autosomal dominant form of CPVT, while mutations in the cardiac calsequestrin 2 gene (CASQ2) cause an autosomal recessive form. OBJECTIVE: The aim of this study was to clinically and genetically evaluate a large family with severe autosomal dominant CPVT. METHODS: Clinical evaluation of family members was performed, including detailed history, physical examination, electrocardiogram, exercise stress test, and autopsy review of decedents. We performed genome-wide linkage analysis in 12 family members and exome sequencing in 2 affected family members. In silico models of mouse and rabbit myocyte electrophysiology were used to predict potential disease mechanisms. RESULTS: Severe CPVT with dominant inheritance in 6 members was diagnosed in a large family with 2 sudden deaths, 2 resuscitated cardiac arrests, and multiple appropriate implantable cardioverter-defibrillator shocks. A comprehensive analysis of cardiac arrhythmia genes did not reveal a pathogenic variant. Exome sequencing identified a novel heterozygous missense variant in CASQ2 (Lys180Arg) affecting a highly conserved residue, which cosegregated with disease and was absent in unaffected family members. Genome-wide linkage analysis confirmed a single linkage peak at the CASQ2 locus (logarithm of odds ratio score 3.01; θ = 0). Computer simulations predicted that haploinsufficiency was unlikely to cause the severe CPVT phenotype and suggested a dominant negative mechanism. CONCLUSION: We show for the first time that a variant in CASQ2 causes autosomal dominant CPVT. Genetic testing in dominant CPVT should include screening for heterozygous CASQ2 variants.

Proteome map of the normal murine ventricular myocardium.

Cardiovascular disease is the leading cause of morbidity and mortality in developed countries. The underlying mechanisms involved in cardiac dysfunction and heart failure are poorly understood. In this study, 2-DE was utilised to map, for the first time, proteins of normal, nonfailing mouse ventricular tissues to form a basis for future comparative analysis of mouse models with cardiovascular disorders. Proteins were obtained from ventricles of C57BL6 mice, aged 18 wk, and separated by 2-DE. A total of 150 protein spots, corresponding to 77 distinct proteins, were identified by MALDI-TOF MS. The proteins identified in mouse ventricles covered a wide range of biological processes (e.g. cell cycle, muscle contraction and signal transduction), with the majority of proteins contributing to cardiomyocyte energetics and cell structure. This 2-D gel map of mouse myocardial proteins will be an invaluable tool in proteomic research for the detection of protein changes and identifying cardiac biomarkers of cardiovascular disease.

Abnormal cardiac response to exercise in a murine model of familial hypertrophic cardiomyopathy.

Clinical outcome in familial hypertrophic cardiomyopathy (FHC) may be influenced by modifying factors such as exercise. Transgenic mice which overexpress the human disease-causing cTnI gene mutation, Gly203Ser (designated cTnI-G203S), develop all the characteristic phenotypic features of FHC. To study the modifying effect of exercise in early disease, mice underwent swimming exercise at an early age prior to the development of the FHC phenotype. In non-transgenic and cTnI-wt mice, swimming resulted in a significant increase in left ventricular wall thickness and contractility on echocardiography, consistent with a physiological hypertrophic response to exercise. In contrast, cTnI-G203S mice showed no increase in these parameters, indicating an abnormal response to exercise. The lack of a physiological response to exercise may indicate an important novel mechanistic insight into the role of exercise in triggering adverse events in FHC.

Genes, calcium and modifying factors in hypertrophic cardiomyopathy.

1. Familial hypertrophic cardiomyopathy (FHC) is a primary disorder of the myocardium characterized by remarkable diversity in clinical presentations, ranging from no symptoms to severe heart failure and sudden cardiac death. 2. Over the past 15 years, at least 11 genes have been identified, defects of which cause FHC. Most of these genes encode proteins that comprise the basic contractile unit of the heart (i.e. the sarcomere). 3. Genetic studies are now beginning to have a major impact on the diagnosis in FHC, as well as in guiding treatment and preventative strategies. Although much is known about which genes cause disease, relatively little is known about the molecular steps leading from the gene defect to the clinical phenotype and what factors modify the expression of the mutant genes. 4. Concurrent studies in cell culture and animal models of FHC are now beginning to shed light on the signalling pathways involved in FHC and the role of both environmental and genetic modifying factors. Calcium dysregulation appears to be important in the pathogenesis of FHC. 5. Understanding these basic molecular mechanisms will ultimately improve our knowledge of the basic biology of heart muscle function and will therefore provide new avenues for diagnosis and treatment not only for FHC, but also for a range of human cardiovascular diseases.

Molecular insights from a novel cardiac troponin I mouse model of familial hypertrophic cardiomyopathy.

Gene mutations in cardiac troponin I (cTnI) account for up to 5% of genotyped families with familial hypertrophic cardiomyopathy (FHC). Little is known about how cTnI mutations cause disease. Five lines of transgenic mice were generated which overexpress the human disease-causing cTnI gene mutation, Gly203Ser (designated cTnI-G203S), in a cardiac-specific manner. Mice were compared to transgenic mice that overexpress normal cTnI (cTnI-wt) and non-transgenic littermates (NTG). cTnI-G203S mice developed all the characteristic features of FHC by age 21 weeks. Left ventricular hypertrophy was observed on echocardiography (1.25+/-0.05 mm vs. 0.86+/-0.02 mm in cTnI-wt, P<0.01), associated with a significant 4-fold increase in RNA markers of hypertrophy, ANF and BNP. Myocyte hypertrophy, myofiber disarray and interstitial fibrosis were observed in cTnI-G203S mice. Expression of the cTnI-G203S mutation in neonatal cardiomyocytes resulted in a significant increase in myocyte volume, and reduced interactions with both troponins T and C. Ca2+ cycling was abnormal in adult cardiomyocytes extracted from cTnI-G203S mice, with a prolonged decay constant in Ca2+ transients and a reduced decay constant in response to caffeine treatment. Mice with the cTnI-G203S gene mutation develop all the phenotypic features of human FHC. The cTnI-G203S mutation disrupts interactions with partner proteins, and results in intracellular Ca2+ dysregulation early in life, suggesting a pathogenic role in development of FHC.

Cardiac troponin I mutations in Australian families with hypertrophic cardiomyopathy: clinical, genetic and functional consequences.

BACKGROUND: Hypertrophic cardiomyopathy (HCM) is an autosomal dominant disorder caused by mutations in sarcomeric proteins. Cardiac troponin I (cTnI) is a key switch molecule in the sarcomere. Mutations in cTnI have been identified in <1% of genotyped HCM families. METHODS: To study the prevalence, clinical significance and functional consequences of cTnI mutations, genetic testing was performed in 120 consecutive Australian families with HCM referred to a tertiary referral centre, and results correlated with clinical phenotype. Each cTnI mutation identified was tested in a mammalian two-hybrid system to evaluate the functional effects of these mutations on troponin complex interactions. RESULTS: Disease-causing missense mutations were identified in four families (3.3%). Two mutations were located at the same codon in exon 7 (R162G, R162P), and two in exon 8 (L198P, R204H). All four mutations change amino acid residues which are highly conserved and were not found in normal populations. Follow-up family screening has identified a total of seven clinically affected members in these four families, with a further four members who carry the gene mutation but have no clinical evidence of disease. Age at clinical presentation was variable (range 15-68 years) and the mean septal wall thickness was 19.3 +/- 4.6 mm (range 7-33 mm) in clinically affected individuals, including children. In all four families, at least one member had a sudden cardiac death event, including previous cardiac arrest, indicating a more malignant form of HCM. All four mutations disrupted functional interactions with troponin C and T and this may account for the increased severity of disease in these families. CONCLUSIONS: Gene mutations in cTnI occur in Australian families with HCM with a prevalence higher than previously reported and may be associated with a clinically more malignant course, reflecting significant disruptions to troponin complex interactions.