The R820W mutation in the MYBPC3 gene, associated with hypertrophic cardiomyopathy in cats, causes hypertrophic cardiomyopathy and left ventricular non-compaction in humans

Bringing into focus the central domains C3-C6 of myosin binding protein C

Myosin binding protein C (MyBPC) is a multi-domain protein with each region having a distinct functional role in muscle contraction. The central domains of MyBPC have often been overlooked due to their unclear roles. However, recent research shows promise in understanding their potential structural and regulatory functions. Understanding the central region of MyBPC is important because it may have specialized function that can be used as drug targets or for disease-specific therapies. In this review, we provide a brief overview of the evolution of our understanding of the central domains of MyBPC in regard to its domain structures, arrangement and dynamics, interaction partners, hypothesized functions, disease-causing mutations, and post-translational modifications. We highlight key research studies that have helped advance our understanding of the central region. Lastly, we discuss gaps in our current understanding and potential avenues to further research and discovery.

Classification of feline hypertrophic cardiomyopathy-associated gene variants according to the American College of Medical Genetics and Genomics guidelines

Introduction

The correct labeling of a genetic variant as pathogenic is important as breeding decisions based on incorrect DNA tests can lead to the unwarranted exclusion of animals, potentially compromising the long-term health of a population. In human medicine, the American college of Medical Genetics (ACMG) guidelines provide a framework for variant classification. This study aims to apply these guidelines to six genetic variants associated with hypertrophic cardiomyopathy (HCM) in certain cat breeds and to propose a modified criterion for variant classification.

Methods

Genetic samples were sourced from five cat breeds: Maine Coon, Sphynx, Ragdoll, Devon Rex, and British Short- and Longhair. Allele frequencies were determined, and in the subset with phenotypes available, odds ratios to determine the association with HCM were calculated. In silico evaluation followed with joint evidence and data from other publications assisting in the classification of each variant.

Results

Two variants, MYBPC3:c.91G > C [A31P] and MYBPC3:c.2453C > T [R818W], were designated as pathogenic. One variant, MYH7:c.5647G > A [E1883K], was found likely pathogenic, while the remaining three were labeled as variants of unknown significance.

Discussion

Routine genetic testing is advised solely for the MYBPC3:c.91G > C [A31P] in the Maine Coon and MYBPC3:c.2453C > T [R818W] in the Ragdoll breed. The human ACMG guidelines serve as a suitable foundational tool to ascertain which variants to include; however, refining them for application in veterinary medicine might be beneficial.

The Role of Personalized Medicine in Companion Animal Cardiology

Cardiomyopathies remain one of the most common inherited cardiac diseases in both human and veterinary patients. To date, well over 100 mutated genes are known to cause cardiomyopathies in humans with only a handful known in cats and dogs. This review highlights the need and use of personalized one-health approaches to cardiovascular case management and advancement in pharmacogenetic-based therapy in veterinary medicine. Personalized medicine holds promise in understanding the molecular basis of disease and ultimately will unlock the next generation of targeted novel pharmaceuticals and aid in the reversal of detrimental effects at a molecular level.

Overlap phenotypes of the left ventricular noncompaction and hypertrophic cardiomyopathy with complex arrhythmias and heart failure induced by the novel truncated DSC2 mutation

Background

The left ventricular noncompaction cardiomyopathy (LVNC) is a rare subtype of cardiomyopathy associated with a high risk of heart failure (HF), thromboembolism, arrhythmia, and sudden cardiac death.

Methods

The proband with overlap phenotypes of LVNC and hypertrophic cardiomyopathy (HCM) complicates atrial fibrillation (AF), ventricular tachycardia (VT), and HF due to the diffuse myocardial lesion, which were diagnosed by electrocardiogram, echocardiogram and cardiac magnetic resonance imaging. Peripheral blood was collected from the proband and his relatives. DNA was extracted from the peripheral blood of proband for high-throughput target capture sequencing. The Sanger sequence verified the variants. The protein was extracted from the skin of the proband and healthy volunteer. The expression difference of desmocollin2 was detected by Western blot.

Results

The novel heterozygous truncated mutation (p.K47Rfs*2) of the DSC2 gene encoding an important component of desmosomes was detected by targeted capture sequencing. The western blots showed that the expressing level of functional desmocollin2 protein (~ 94kd) was lower in the proband than that in the healthy volunteer, indicating that DSC2 p.K47Rfs*2 obviously reduced the functional desmocollin2 protein expression in the proband.

Conclusion

The heterozygous DSC2 p.K47Rfs*2 remarkably and abnormally reduced the functional desmocollin2 expression, which may potentially induce the overlap phenotypes of LVNC and HCM, complicating AF, VT, and HF.

The Feline Cardiomyopathies: 3. Cardiomyopathies other than HCM

Practical relevance:

Although feline hypertrophic cardiomyopathy (HCM) occurs more commonly, dilated cardiomyopathy (DCM), restrictive cardiomyopathy (RCM), arrhythmogenic right ventricular cardiomyopathy (ARVC), left ventricular noncompaction (LVNC) and cardiomyopathy – nonspecific phenotype (NCM; formerly unclassified cardiomyopathy) are all recognized in domestic cats.

Patient group:

Any adult domestic cat, of either sex and of any breed, can be affected.

Diagnostics:

The non-HCM cardiomyopathies are rarely suspected in subclinically affected cats, so most are first identified when a cat presents with signs of heart failure or systemic thromboembolic disease. The definitive clinical confirmatory test for these other feline cardiomyopathies is echocardiography.

Key findings:

‘Cardiomyopathy – nonspecific phenotype’ is a catch-all term that groups hearts with myocardial changes that either do not meet the criteria for any one type of cardiomyopathy (HCM, RCM, DCM, ARVC, LVNC) or meet the echocardiography criteria for more than one type. RCM is characterized by diastolic dysfunction due to fibrosis that results in a restrictive transmitral flow pattern on Doppler echocardiography and usually marked left or biatrial enlargement. DCM is characterized by decreased myocardial contractility and is rare in cats. When it occurs, it is seldom due to taurine deficiency. However, since taurine-deficient DCM is usually reversible, a diet history should be obtained, whole blood and plasma taurine levels should be measured and taurine should be supplemented in the diet if the diet is not commercially manufactured. ARVC should be suspected in adult cats with severe right heart enlargement and right heart failure (ascites and/or pleural effusion), especially if arrhythmia is present. Feline LVNC is rare; its significance continues to be explored. Treatment of the consequences of these cardiomyopathies (management of heart failure, thromboprophylaxis, treatment of systemic arterial thromboembolism) is the same as for HCM.

Conclusions:

While these other cardiomyopathies are less prevalent than HCM in cats, their clinical and radiographic presentation is often indistinguishable from HCM. Echocardiography is usually the only ante-mortem method to determine which type of cardiomyopathy is present. However, since treatment and prognosis are often similar for the feline cardiomyopathies, distinguishing among the cardiomyopathies is often not essential for determining appropriate therapy.

Areas of uncertainty:

The feline cardiomyopathies do not always fit into one distinct category. Interrelationships among cardiomyopathies in cats may exist and understanding these relationships in the future might provide critical insights regarding treatment and prognosis.

The Feline Cardiomyopathies: 2. Hypertrophic cardiomyopathy

Practical relevance:

Hypertrophic cardiomyopathy (HCM) is the most common form of feline cardiomyopathy observed clinically and may affect up to approximately 15% of the domestic cat population, primarily as a subclinical disease. Fortunately, severe HCM, leading to heart failure or arterial thromboembolism (ATE), only occurs in a small proportion of these cats.

Patient group:

Domestic cats of any age from 3 months upward, of either sex and of any breed, can be affected. A higher prevalence in male and domestic shorthair cats has been reported.

Diagnostics:

Subclinical feline HCM may or may not produce a heart murmur or gallop sound. Substantial left atrial enlargement can often be identified radiographically in cats with severe HCM. Biomarkers should not be relied on solely to diagnose the disease. While severe feline HCM can usually be diagnosed via echocardiography alone, feline HCM with mild to moderate left ventricular (LV) wall thickening is a diagnosis of exclusion, which means there is no definitive test for HCM in these cats and so other disorders that can cause mild to moderate LV wall thickening (eg, hyperthyroidism, systemic hypertension, acromegaly, dehydration) need to be ruled out.

Key findings:

While a genetic cause of HCM has been identified in two breeds and is suspected in another, for most cats the cause is unknown. Systolic anterior motion of the mitral valve (SAM) is the most common cause of dynamic left ventricular outflow tract obstruction (DLVOTO) and, in turn, the most common cause of a heart murmur with feline HCM. While severe DLVOTO is probably clinically significant and so should be treated, lesser degrees probably are not. Furthermore, since SAM can likely be induced in most cats with HCM, the distinction between HCM without obstruction and HCM with obstruction (HOCM) is of limited importance in cats. Diastolic dysfunction, and its consequences of abnormally increased atrial pressure leading to signs of heart failure, and sluggish atrial blood flow leading to ATE, is the primary abnormality that causes clinical signs and death in affected cats. Treatment (eg, loop diuretics) is aimed at controlling heart failure. Preventive treatment (eg, antithrombotic drugs) is aimed at reducing the risk of complications (eg, ATE).

Conclusions:

Most cats with HCM show no overt clinical signs and live a normal or near-normal life despite this disease. However, a substantial minority of cats develop overt clinical signs referable to heart failure or ATE that require treatment. For most cats with clinical signs caused by HCM, the long-term prognosis is poor to grave despite therapy.

Areas of uncertainty:

Genetic mutations (variants) that cause HCM have been identified in a few breeds, but, despite valiant efforts, the cause of HCM in the vast majority of cats remains unknown. No treatment currently exists that reverses or even slows the cardiomyopathic process in HCM, again despite valiant efforts. The search goes on.

Genetics of Cardiomyopathy: Clinical and Mechanistic Implications for Heart Failure

Genetic cardiomyopathies are an important cause of sudden cardiac death across all age groups. Genetic testing in heart failure clinics is useful for family screening and providing individual prognostic insight. Obtaining a family history of at least three generations, including the creation of a pedigree, is recommended for all patients with primary cardiomyopathy. Additionally, when appropriate, consultation with a genetic counsellor can aid in the success of a genetic evaluation. Clinical screening should be performed on all first-degree relatives of patients with genetic cardiomyopathy.

Genetics has played an important role in the understanding of different cardiomyopathies, and the field of heart failure (HF) genetics is progressing rapidly. Much research has also focused on distinguishing markers of risk in patients with cardiomyopathy using genetic testing. While these efforts currently remain incomplete, new genomic technologies and analytical strategies provide promising opportunities to further explore the genetic architecture of cardiomyopathies, afford insight into the early manifestations of cardiomyopathy, and help define the molecular pathophysiological basis for cardiac remodeling. Cardiovascular physicians should be fully aware of the utility and potential pitfalls of incorporating genetic test results into pre-emptive treatment strategies for patients in the preliminary stages of HF. Future work will need to be directed towards elucidating the biological mechanisms of both rare and common gene variants and environmental determinants of plasticity in the genotype-phenotype relationship. This future research should aim to further our ability to identify, diagnose, and treat disorders that cause HF and sudden cardiac death in young patients, as well as prioritize improving our ability to stratify the risk for these patients prior to the onset of the more severe consequences of their disease.

Two Tales of Cardiomyopathy: Underscore for One Health Initiative

Hypertrophic cardiomyopathy, a common cause of sudden cardiac death, results from mutations in the cardiac sarcomere. Although there has been much scientific exploration regarding this disease, there is still much to be elucidated. This E-challenge highlights two cases of cardiomyopathy and underscores the need for future multidisciplinary collaboration as outlined by the One Health Initiative.

Differentiation of athlete's heart and hypertrophic cardiomyopathy by the fractal dimension of left ventricular trabeculae

BACKGROUND

Differentiation between exercise induced adaptive myocardial hypertrophy (athlete's heart) and hypertrophic cardiomyopathy (HCM) is currently based on echocardiographic and cardiac magnetic resonance (CMR) criteria, but these may be insufficient in patients with subtle phenotype expression. This study aimed to assess whether left ventricular (LV) fractal pattern could permit to differentiate athlete's heart from HCM.

METHODS

We recruited retrospectively 61 elite marathon runners, 67 patients with HCM, and 33 healthy subjects. A CMR study was performed in all subjects and the LV trabeculae fractal dimension (FD) was measured in end-diastolic frames of each short-axis cine sequence. For group comparison, the ratio of maximal myocardial wall thickness (mMWT)/indexed LV end-diastolic volume (LVED) was determined.

RESULTS

As compared with athletes, patients with HCM had significantly (p < 0.001) greater FD in the LV basal (1.30 ± 0.07 vs. 1.23 ± 0.05) and apical (1.38 ± 0.06 vs. 1.30 ± 0.07) regions and in the whole heart (1.34 ± 0.05 vs. 1.27 ± 0.05). FD increased with age, left atrial area and indexed left ventricular mass (p < 0.05 for all) and correlated negatively with LV and RV end-diastolic volumes (p < 0.05 each). The addition of whole heart FD to the ratio of maximal myocardial wall thickness/indexed LVEDV lead to an improvement in the ability to discriminate HCM with a net reclassification index (NRI) of 71%.

CONCLUSIONS

The FD regional distribution of the LV trabeculae differentiates patients with athlete's heart from patients with HCM. The addition of whole heart FD to the mMWT/indexed LVEDV ratio improves the predictive capacity of the model to differentiate both entities.

Large Animal Models in Regenerative Medicine and Tissue Engineering: To Do or Not to Do

Rapid developments in Regenerative Medicine and Tissue Engineering has witnessed an increasing drive toward clinical translation of breakthrough technologies. However, the progression of promising preclinical data to achieve successful clinical market authorisation remains a bottleneck. One hurdle for progress to the clinic is the transition from small animal research to advanced preclinical studies in large animals to test safety and efficacy of products. Notwithstanding this, to draw meaningful and reliable conclusions from animal experiments it is critical that the species and disease model of choice is relevant to answer the research question as well as the clinical problem. Selecting the most appropriate animal model requires in-depth knowledge of specific species and breeds to ascertain the adequacy of the model and outcome measures that closely mirror the clinical situation. Traditional reductionist approaches in animal experiments, which often do not sufficiently reflect the studied disease, are still the norm and can result in a disconnect in outcomes observed between animal studies and clinical trials. To address these concerns a reconsideration in approach will be required. This should include a stepwise approach using in vitro and ex vivo experiments as well as in silico modeling to minimize the need for in vivo studies for screening and early development studies, followed by large animal models which more closely resemble human disease. Naturally occurring, or spontaneous diseases in large animals remain a largely untapped resource, and given the similarities in pathophysiology to humans they not only allow for studying new treatment strategies but also disease etiology and prevention. Naturally occurring disease models, particularly for longer lived large animal species, allow for studying disorders at an age when the disease is most prevalent. As these diseases are usually also a concern in the chosen veterinary species they would be beneficiaries of newly developed therapies. Improved awareness of the progress in animal models is mutually beneficial for animals, researchers, human and veterinary patients. In this overview we describe advantages and disadvantages of various animal models including domesticated and companion animals used in regenerative medicine and tissue engineering to provide an informed choice of disease-relevant animal models.

Genetics of feline hypertrophic cardiomyopathy

Hypertrophic cardiomyopathy (HCM) is characterized by an abnormal increase in myocardial mass that affects cardiac structure and function. HCM is the most common inherited cardiovascular disease in humans (0.2%) and the most common cardiovascular disease in cats (14.7%). Feline HCM phenotype is very similar to the phenotype found in humans, but the time frame for the development of the disease is significantly shorter. Similar therapeutic agents are used in its treatment and it has the same complications, such as heart failure, thromboembolism and sudden cardiac death. In contrast to humans, in whom thousands of genetic variants have been identified, genetic studies in cats have been limited to fragment analysis of two sarcomeric genes identifying two variants in MYBPC3 and one in MYH7. Two of these variants have also been associated with human disease. The high prevalence of the reported variants in non-affected cats hinders the assumption of their pathogenicity in heterozygotes. An in-depth review of the literature about genetic studies on feline HCM in comparison with the same disease in humans is presented here. The close similarity in the phenotype and genotype between cats and humans makes the cat an excellent model for the pathophysiological study of the disease and future therapeutic agents.

Cardiac Phenotypes, Genetics, and Risks in Familial Noncompaction Cardiomyopathy

BACKGROUND

There is overlap in genetic causes and cardiac features in noncompaction cardiomyopathy (NCCM), hypertrophic cardiomyopathy (HCM), and dilated cardiomyopathy (DCM).

OBJECTIVES

The goal of this study was to predict phenotype and outcome in relatives according to the clinical features and genotype of NCCM index cases.

METHODS

Retrospective DNA and cardiac screening of relatives of 113 families from 143 index patients were used to classify NCCM cases according to the cardiac phenotype. These cases were classified as isolated NCCM, NCCM with left ventricular (LV) dilation (DCM), and NCCM with LV hypertrophy (HCM).

RESULTS

In 58 (51%) families, screening identified 73 relatives with NCCM and 34 with DCM or HCM without NCCM. The yield of family screening was higher in families with a mutation (p < 0.001). Fifty-four families had a mutation. Nonpenetrance was observed in 37% of the relatives with a mutation. Index cases were more often symptomatic than affected relatives (p < 0.001). NCCM with DCM (53%) was associated with LV systolic dysfunction (p < 0.001), increased risk for major adverse cardiac events, mutations in the tail of MYH7 (p < 0.001), and DCM without NCCM in relatives (p < 0.001). Isolated NCCM (43%) was associated with a milder course, mutations in the head of MYH7, asymptomatic NCCM (42%) (p = 0.018), and isolated NCCM in relatives (p = 0.004). NCCM with HCM (4%) was associated with MYBPC3 and HCM without NCCM in relatives (p < 0.001).

CONCLUSIONS

The phenotype of relatives may be predicted according to the NCCM phenotype and the mutation of index patients. NCCM phenotypes were related to outcome. In this way, clinical and genetic features of index patients may help prediction of outcome in relatives.

Inducing Pluripotency in the Domestic Cat (Felis catus)

Domestic cats suffer from a range of inherited genetic diseases, many of which display similarities with equivalent human conditions. Developing cellular models for these inherited diseases would enable drug discovery, benefiting feline health and welfare as well as enhancing the potential of cats as relevant animal models for translation to human medicine. Advances in our understanding of these diseases at the cellular level have come from the use of induced pluripotent stem cells (iPSCs). iPSCs can differentiate into virtually any cell type and can be derived from adult somatic cells, therefore overcoming the ethical implications of destroying embryos to obtain embryonic stem cells. No studies, however, report the generation of iPSCs from domestic cats [feline iPSCs (fiPSCs)]. Feline adipose-derived fibroblasts were infected with amphotropic retrovirus containing the coding sequences for human Oct4, Sox2, Klf4, cMyc, and Nanog. Isolated iPSC clones were expanded on inactivated mouse embryonic fibroblasts in the presence of feline leukemia inhibitory factor (fLIF). Retroviral delivery of human pluripotent genes gave rise to putative fiPSC colonies within 5-7 days. These iPS-like cells required fetal bovine serum and fLIF for maintenance. Colonies were domed with refractile edges, similar to mouse iPSCs. Immunocytochemistry demonstrated positive staining for stem cell markers: alkaline phosphatase, Oct4, Sox2, Nanog, and SSEA1. Cells were negative for SSEA4. Expression of endogenous feline Nanog was confirmed by quantitative polymerase chain reaction. The cells were able to differentiate in vitro into cells representative of the three germ layers. These results confirm the first generation of induced pluripotent stem cells from domestic cats. These cells will provide valuable models to study genetic diseases and explore novel therapeutic strategies.

A feline orthologue of the human MYH7 c.5647G>A (p.(Glu1883Lys)) variant causes hypertrophic cardiomyopathy in a Domestic Shorthair cat

Hypertrophic cardiomyopathy (HCM) is the most common inherited human heart disease. The same disease has a high prevalence in cats, where it is also suspected to be inherited. More than 1500 variants in MYBPC3, MYH7 and other sarcomeric genes are associated with human HCM, while in cats, only two causative variants in MYBPC3 are currently known. Here, we describe an adult Domestic Shorthair cat with arterial thromboembolism and heart failure that was diagnosed with HCM on necropsy. Sequencing of the coding regions of MYBPC3 and MYH7 revealed 21 variants, of which the MYH7 c.5647G>A (p.(Glu1883Lys)) variant was further analysed, because its orthologous variant had already been reported in a human patient with HCM, but with limited causal evidence. This variant affects the highly conserved assembly competence domain, is predicted in silico to be damaging and was found only once in population databases. Recently, functional studies have confirmed its predicted damaging effect and a paralogous variant in MYH6 has been associated with cardiac disease in humans as well. This report of an orthologous variant in a cat with HCM and its absence in 200 additional cats provides further evidence for its disease-causing nature. As the first report of feline HCM caused by a variant in MYH7, this study also emphasises this gene as a candidate gene for future studies in cats and highlights the similarity between human and feline HCM.

Inherited cardiomyopathies in veterinary medicine

Comparative and translation medicine is of particular value within the field of inherited cardiomyopathies. Despite massive advances in understanding the functional role of mutations in human cardiomyopathies, these advances have frequently failed to translate into medical discoveries that alter patient care. One potential explanation for this failure lies in the lack of suitable translational models that adequately recapitulate human cardiovascular physiology and disease expression. The vast genetic heterogeneity that complicates human cardiomyopathy research is potentially alleviated through the study of naturally occurring large animal models of disease, where incredibly homogenous populations, like those seen in a single breed of dog or cat, may exist (Kol et al., Sci Transl Med 7:308-321, 2015; Ueda and Stern, Yale J Biol Med 90:433-448, 2017). Veterinary medicine is in a unique position to provide research resources and information that may be readily applied to human disease (Kol et al., Sci Transl Med 7:308-321, 2015). Many inherited cardiomyopathies of humans are phenotypically and genotypically similar in veterinary species and ongoing research holds promise for aiding veterinary and human patients alike (Basso et al., Circulation 109:1180-1185, 2004; Fox et al., Cardiovasc Pathol 23:28-34, 2014; Fox et al., Circulation 102:1863-1870, 2000; Kittleson et al., J Vet Cardiol 17 Suppl 1:S53-73, 2015; Ueda and Stern, Yale J Biol Med 90:433-448, 2017). This article presents the current knowledge of inherited cardiomyopathies in dogs, cats, and non-human primates, with a goal of identifying areas of translational research and future directions.

Translating emerging molecular genetic insights into clinical practice in inherited cardiomyopathies

Cardiomyopathies are primarily genetic disorders of the myocardium associated with higher risk of life-threatening cardiac arrhythmias, heart failure, and sudden cardiac death. The evolving knowledge in genomic medicine during the last decade has reshaped our understanding of cardiomyopathies as diseases of multifactorial nature and complex pathophysiology. Genetic testing in cardiomyopathies has subsequently grown from primarily a research tool into an essential clinical evaluation piece with important clinical implications for patients and their families. The purpose of this review is to provide with a contemporary insight into the implications of genetic testing in diagnosis, therapy, and prognosis of patients with inherited cardiomyopathies. Here, we summarize the contemporary knowledge on genotype-phenotype correlations in inherited cardiomyopathies and highlight the recent significant achievements in the field of translational cardiovascular genetics.

Clinical genetics and outcome of left ventricular non-compaction cardiomyopathy

Aims

In this study, we aimed to clinically and genetically characterize LVNC patients and investigate the prevalence of variants in known and novel LVNC disease genes.

Introduction

Left ventricular non-compaction cardiomyopathy (LVNC) is an increasingly recognized cause of heart failure, arrhythmia, thromboembolism, and sudden cardiac death. We sought here to dissect its genetic causes, phenotypic presentation and outcome.

Methods and results

In our registry with follow-up of in the median 61 months, we analysed 95 LVNC patients (68 unrelated index patients and 27 affected relatives; definite familial LVNC = 23.5%) by cardiac phenotyping, molecular biomarkers and exome sequencing. Cardiovascular events were significantly more frequent in LVNC patients compared with an age-matched group of patients with non-ischaemic dilated cardiomyopathy (hazard ratio = 2.481, P = 0.002). Stringent genetic classification according to ACMG guidelines revealed that TTN, LMNA, and MYBPC3 are the most prevalent disease genes (13 patients are carrying a pathogenic truncating TTN variant, odds ratio = 40.7, Confidence interval = 21.6-76.6, P < 0.0001, percent spliced in 76-100%). We also identified novel candidate genes for LVNC. For RBM20, we were able to perform detailed familial, molecular and functional studies. We show that the novel variant p.R634L in the RS domain of RBM20 co-segregates with LVNC, leading to titin mis-splicing as revealed by RNA sequencing of heart tissue in mutation carriers, protein analysis, and functional splice-reporter assays.

Conclusion

Our data demonstrate that the clinical course of symptomatic LVNC can be severe. The identified pathogenic variants and distribution of disease genes-a titin-related pathomechanism is found in every fourth patient-should be considered in genetic counselling of patients. Pathogenic variants in the nuclear proteins Lamin A/C and RBM20 were associated with worse outcome.

Genetic background of Japanese patients with pediatric hypertrophic and restrictive cardiomyopathy

Hypertrophic cardiomyopathy (HCM) and restrictive cardiomyopathy (RCM) present a high risk for sudden cardiac death in pediatric patients. The aim of this study was to identify disease-associated genetic variants in Japanese patients with pediatric HCM and RCM. We analyzed 67 cardiomyopathy-associated genes in 46 HCM and 7 RCM patients diagnosed before 16 years of age using a next-generation sequencing system. We found that 78% of HCM and 71% of RCM patients carried disease-associated genetic variants. Disease-associated genetic variants were identified in 80% of HCM patients with a family history and in 77% of HCM patients with no apparent family history (NFH). MYH7 and/or MYBPC3 variants comprised 76% of HCM-associated variants, whereas troponin complex-encoding genes comprised 75% of the RCM-associated variants. In addition, 91% of HCM patients with implantable cardioverter-defibrillators and infant cases had NFH, and the 88% of HCM patients carrying disease-associated genetic variants were males who carried MYH7 or MYBPC3 variants. Moreover, two disease-associated LAMP2, one DES and one FHOD3 variants, were identified in HCM patients. In this study, pediatric HCM and RCM patients were found to carry disease-associated genetic variants at a high rate. Most of the variants were in MYH7 or MYPBC3 for HCM and TNNT2 or TNNI3 for RCM.

The HCM-linked W792R mutation in cardiac myosin-binding protein C reduces C6 FnIII domain stability

Cardiac myosin-binding protein C (cMyBP-C) is a functional sarcomeric protein that regulates contractility in response to contractile demand, and many mutations in cMyBP-C lead to hypertrophic cardiomyopathy (HCM). To gain insight into the effects of disease-causing cMyBP-C missense mutations on contractile function, we expressed the pathogenic W792R mutation (substitution of a highly conserved tryptophan residue by an arginine residue at position 792) in mouse cardiomyocytes lacking endogenous cMyBP-C and studied the functional effects using three-dimensional engineered cardiac tissue constructs (mECTs). Based on complete conservation of tryptophan at this location in fibronectin type II (FnIII) domains, we hypothesized that the W792R mutation affects folding of the C6 FnIII domain, destabilizing the mutant protein. Adenoviral transduction of wild-type (WT) and W792R cDNA achieved equivalent mRNA transcript abundance, but not equivalent protein levels, with W792R compared with WT controls. mECTs expressing W792R demonstrated abnormal contractile kinetics compared with WT mECTs that were nearly identical to cMyBP-C-deficient mECTs. We studied whether common pathways of protein degradation were responsible for the rapid degradation of W792R cMyBP-C. Inhibition of both ubiquitin-proteasome and lysosomal degradation pathways failed to increase full-length mutant protein abundance to WT equivalence, suggesting rapid cytosolic degradation. Bacterial expression of WT and W792R protein fragments demonstrated decreased mutant stability with altered thermal denaturation and increased susceptibility to trypsin digestion. These data suggest that the W792R mutation destabilizes the C6 FnIII domain of cMyBP-C, resulting in decreased full-length protein expression. This study highlights the vulnerability of FnIII-like domains to mutations that alter domain stability and further indicates that missense mutations in cMyBP-C can cause disease through a mechanism of haploinsufficiency. NEW & NOTEWORTHY This study is one of the first to describe a disease mechanism for a missense mutation in cardiac myosin-binding protein C linked to hypertrophic cardiomyopathy. The mutation decreases stability of the fibronectin type III domain and results in substantially reduced mutant protein expression dissonant to transcript abundance.

Novel α-Actin Gene Mutation p.(Ala21Val) Causing Familial Hypertrophic Cardiomyopathy, Myocardial Noncompaction, and Transmural Crypts. Clinical-Pathologic Correlation

Background

Mutations of α‐actin gene (ACTC1) have been phenotypically related to various cardiac anomalies, including hypertrophic cardiomyopathy and dilated cardiomyopathy and left ventricular (LV) myocardial noncompaction. A novel ACTC mutation is reported as cosegregating for familial hypertrophic cardiomyopathy and LV myocardial noncompaction with transmural crypts.

Methods and Results

In an Italian family of 7 subjects, 4 aged 10 (II‐1), 14 (II‐2), 43 (I‐4) and 46 years (I‐5), presenting abnormal ECG changes, dyspnea and palpitation (II‐2, I‐4, and I‐5), and recurrent cerebral ischemic attack (I‐5), underwent 2‐dimensional echo, cardiac magnetic resonance, Holter monitoring, and next‐generation sequencing gene analysis. Patients II‐2 and I‐5 with ventricular tachycardia underwent a cardiac invasive study, including coronary with LV angiography and endomyocardial biopsy. In all the affected members, ECG showed right bundle branch block and left anterior hemiblock with age‐related prolongation of QRS duration. Two‐dimensional echo and cardiac magnetic resonance documented LV myocardial noncompaction in all and in I‐4, I‐5, and II‐2 a progressive LV hypertrophy up to 22‐mm maximal wall thickness. Coronary arteries were normal. LV angiography showed transmural crypts progressing to spongeous myocardial transformation with LV dilatation and dysfunction in the oldest subject. At histology and electron microscopy detachment of myocardiocytes were associated with cell and myofibrillar disarray and degradation of intercalated discs causing disanchorage of myofilaments to cell membrane. Next‐generation sequencing showed in affected members an unreported p.(Ala21Val) mutation of ACTC.

Conclusions

Novel p.(Ala21Val) mutation of ACTC1 causes myofibrillar and intercalated disc alteration leading to familial hypertrophic cardiomyopathy and LV myocardial noncompaction with transmural crypts.

Feline Hypertrophic Cardiomyopathy: A Spontaneous Large Animal Model of Human HCM

Hypertrophic cardiomyopathy (HCM) is a common disease in pet cats, affecting 10-15% of the pet cat population. The similarity to human HCM, the rapid progression of disease, and the defined and readily determined endpoints of feline HCM make it an excellent natural model that is genotypically and phenotypically similar to human HCM. The Maine Coon and Ragdoll cats are particularly valuable models of HCM because of myosin binding protein-C mutations and even higher disease incidence compared to the overall feline population. The cat overcomes many of the limitations of rodent HCM models, and can provide enhanced translation of information from in vitro and induced small animal models to human clinical trials. Physicians and veterinarians working together in a collaborative and interdisciplinary approach can accelerate the discovery of more effective treatments for this and other cardiovascular diseases affecting human and veterinary patients.

Investigations into the Sarcomeric Protein and Ca2+-Regulation Abnormalities Underlying Hypertrophic Cardiomyopathy in Cats (Felix catus)

Hypertrophic cardiomyopathy (HCM) is the most common single gene inherited cardiomyopathy. In cats (Felix catus) HCM is even more prevalent and affects 16% of the outbred population and up to 26% in pedigree breeds such as Maine Coon and Ragdoll. Homozygous MYBPC3 mutations have been identified in these breeds but the mutations in other cats are unknown. At the clinical and physiological level feline HCM is closely analogous to human HCM but little is known about the primary causative mechanism. Most identified HCM causing mutations are in the genes coding for proteins of the sarcomere. We therefore investigated contractile and regulatory proteins in left ventricular tissue from 25 cats, 18 diagnosed with HCM, including a Ragdoll cat with a homozygous MYBPC3 R820W, and 7 non-HCM cats in comparison with human HCM (from septal myectomy) and donor heart tissue. Myofibrillar protein expression was normal except that we observed 20-44% MyBP-C haploinsufficiency in 5 of the HCM cats. Troponin extracted from 8 HCM and 5 non-HCM cat hearts was incorporated into thin filaments and studied by in vitro motility assay. All HCM cat hearts had a higher (2.06 ± 0.13 fold) Ca²⁺-sensitivity than non-HCM cats and, in all the HCM cats, Ca²⁺-sensitivity was not modulated by troponin I phosphorylation. We were able to restore modulation of Ca²⁺-sensitivity by replacing troponin T with wild-type protein or by adding 100 μM Epigallocatechin 3-gallate (EGCG). These fundamental regulatory characteristics closely mimic those seen in human HCM indicating a common molecular mechanism that is independent of the causative mutation. Thus, the HCM cat is a potentially useful large animal model.

Increased Postnatal Cardiac Hyperplasia Precedes Cardiomyocyte Hypertrophy in a Model of Hypertrophic Cardiomyopathy

Rationale: Hypertrophic cardiomyopathy (HCM) occurs in ~0.5% of the population and is a leading cause of sudden cardiac death (SCD) in young adults. Cardiomyocyte hypertrophy has been the accepted mechanism for cardiac enlargement in HCM, but the early signaling responsible for initiating hypertrophy is poorly understood. Mutations in cardiac myosin binding protein C (MYBPC3) are among the most common HCM-causing mutations. Ablation of Mybpc3 in an HCM mouse model (cMyBP-C^−/−) rapidly leads to cardiomegaly by postnatal day (PND) 9, though hearts are indistinguishable from wild-type (WT) at birth. This model provides a unique opportunity to explore early processes involved in the dramatic postnatal transition to hypertrophy.

Methods and Results: We performed microarray analysis, echocardiography, qPCR, immunohistochemistry (IHC), and isolated cardiomyocyte measurements to characterize the perinatal cMyBP-C^−/− phenotype before and after overt hypertrophy. cMyBP-C^−/− hearts showed elevated cell cycling at PND1 that transitioned to hypertrophy by PND9. An expanded time course revealed that increased cardiomyocyte cycling was associated with elevated heart weight to body weight ratios prior to cellular hypertrophy, suggesting that cell cycling resulted in cardiomyocyte proliferation. Animals heterozygous for the cMyBP-C deletion trended in the direction of the homozygous null, yet did not show increased heart size by PND9.

Conclusions: Results indicate that altered regulation of the cell cycling pathway and elevated proliferation precedes hypertrophy in the cMyBP-C^−/− HCM model, and suggests that increased cardiomyocyte number contributes to increased heart size in cMyBP-C^−/− mice. This pre-hypertrophic period may reflect a unique time during which the commitment to HCM is determined and disease severity is influenced.

Investigations into the Sarcomeric Protein and Ca2+-Regulation Abnormalities Underlying Hypertrophic Cardiomyopathy in Cats (Felix catus)

Hypertrophic cardiomyopathy (HCM) is the most common single gene inherited cardiomyopathy. In cats (Felix catus) HCM is even more prevalent and affects 16% of the outbred population and up to 26% in pedigree breeds such as Maine Coon and Ragdoll. Homozygous MYBPC3 mutations have been identified in these breeds but the mutations in other cats are unknown. At the clinical and physiological level feline HCM is closely analogous to human HCM but little is known about the primary causative mechanism. Most identified HCM causing mutations are in the genes coding for proteins of the sarcomere. We therefore investigated contractile and regulatory proteins in left ventricular tissue from 25 cats, 18 diagnosed with HCM, including a Ragdoll cat with a homozygous MYBPC3 R820W, and 7 non-HCM cats in comparison with human HCM (from septal myectomy) and donor heart tissue. Myofibrillar protein expression was normal except that we observed 20–44% MyBP-C haploinsufficiency in 5 of the HCM cats. Troponin extracted from 8 HCM and 5 non-HCM cat hearts was incorporated into thin filaments and studied by in vitro motility assay. All HCM cat hearts had a higher (2.06 ± 0.13 fold) Ca²⁺-sensitivity than non-HCM cats and, in all the HCM cats, Ca²⁺-sensitivity was not modulated by troponin I phosphorylation. We were able to restore modulation of Ca²⁺-sensitivity by replacing troponin T with wild-type protein or by adding 100 μM Epigallocatechin 3-gallate (EGCG). These fundamental regulatory characteristics closely mimic those seen in human HCM indicating a common molecular mechanism that is independent of the causative mutation. Thus, the HCM cat is a potentially useful large animal model.

The genetic basis of hypertrophic cardiomyopathy in cats and humans

Mutations in genes that encode for muscle sarcomeric proteins have been identified in humans and two breeds of domestic cats with hypertrophic cardiomyopathy (HCM). This article reviews the history, genetics, and pathogenesis of HCM in the two species in order to give veterinarians a perspective on the genetics of HCM. Hypertrophic cardiomyopathy in people is a genetic disease that has been called a disease of the sarcomere because the preponderance of mutations identified that cause HCM are in genes that encode for sarcomeric proteins (Maron and Maron, 2013). Sarcomeres are the basic contractile units of muscle and thus sarcomeric proteins are responsible for the strength, speed, and extent of muscle contraction. In people with HCM, the two most common genes affected by HCM mutations are the myosin heavy chain gene (MYH7), the gene that encodes for the motor protein β-myosin heavy chain (the sarcomeric protein that splits ATP to generate force), and the cardiac myosin binding protein-C gene (MYBPC3), a gene that encodes for the closely related structural and regulatory protein, cardiac myosin binding protein-C (cMyBP-C). To date, the two mutations linked to HCM in domestic cats (one each in Maine Coon and Ragdoll breeds) also occur in MYBPC3 (Meurs et al., 2005, 2007). This is a review of the genetics of HCM in both humans and domestic cats that focuses on the aspects of human genetics that are germane to veterinarians and on all aspects of feline HCM genetics.

Association of the myosin binding protein C3 mutation (MYBPC3 R820W) with cardiac death in a survey of 236 Ragdoll cats

OBJECTIVES

A mutation identified in the myosin binding protein C3 gene (MYBPC3 R820W) has been associated with hypertrophic cardiomyopathy (HCM) in Ragdoll cats. Ragdolls with HCM are reported to have a poor prognosis and homozygous cats seem particularly likely to develop severe HCM, although the outcome in Ragdolls tested for the MYBPC3 mutation has not been reported. We aimed to determine the influence of genotype on survival in Ragdoll cats using a questionnaire, and hypothesized that homozygous Ragdolls had shorter lifespans and were more likely to suffer cardiac death than heterozygous or wild-type (WT) cats.

ANIMALS

251 client owned Ragdoll cats.

METHODS

A questionnaire for breeders/owners of MYBPC3 genotyped Ragdolls included items related to genotype, age, sex, current status (alive/dead), and date and circumstances of death. Death was categorized as cardiac or non-cardiac. Survival was analyzed using Kaplan-Meier curves and log rank tests.

RESULTS

Completed questionnaires were received for 236 cats (156 WT, 68 heterozygous, 12 homozygous). Median survival time for homozygous cats was 5.65 years (95%CI 0.4-10.9 years) compared to heterozygous (>16.7 years) or WT (>15.2 years). Homozygous cats were more likely to die from cardiac death (p = 0.004 vs. WT; p = 0.003 vs. heterozygous) and had significantly shorter time to cardiac death (vs. WT p < 0.001; vs. heterozygous p < 0.001).

CONCLUSIONS

Ragdoll cats homozygous for the MYBPC3 R820W mutation have a shorter survival time than WT or heterozygous cats. This suggests a mode of inheritance that follows an incomplete dominance pattern.

Myocardial infarction in a patient with left ventricular noncompaction: a case report

We describe a 73-year-old male patient with left ventricular noncompaction (LVNC) who was diagnosed with acute myocardial infarction (MI), three-vessel coronary artery disease, a fresh intraventricular thrombus, and mitral regurgitation. He was treated with full anticoagulant therapy, coronary artery bypass grafting, and mitral valve repair. This case adds to a small but growing literature showing association between LVNC and MI and/or coronary artery disease. We suggest that patients with LVNC could be considered at heightened risk for MI, and the two conditions might have a common genetic underpinning in some cases.

In the thick of it: HCM-causing mutations in myosin binding proteins of the thick filament

In the 20 years since the discovery of the first mutation linked to familial hypertrophic cardiomyopathy (HCM), an astonishing number of mutations affecting numerous sarcomeric proteins have been described. Among the most prevalent of these are mutations that affect thick filament binding proteins, including the myosin essential and regulatory light chains and cardiac myosin binding protein (cMyBP)-C. However, despite the frequency with which myosin binding proteins, especially cMyBP-C, have been linked to inherited cardiomyopathies, the functional consequences of mutations in these proteins and the mechanisms by which they cause disease are still only partly understood. The purpose of this review is to summarize the known disease-causing mutations that affect the major thick filament binding proteins and to relate these mutations to protein function. Conclusions emphasize the impact that discovery of HCM-causing mutations has had on fueling insights into the basic biology of thick filament proteins and reinforce the idea that myosin binding proteins are dynamic regulators of the activation state of the thick filament that contribute to the speed and force of myosin-driven muscle contraction. Additional work is still needed to determine the mechanisms by which individual mutations induce hypertrophic phenotypes.

Identification of a novel TPM1 mutation in a family with left ventricular noncompaction and sudden death

Left ventricular noncompaction (LVNC) is a cardiomyopathy morphologically characterized by 2-layered myocardium, numerous prominent trabeculations, and deep intertrabecular recesses communicating with the left ventricular cavity. The purpose of this study was to investigate patients with LVNC for possible disease causing mutations. We screened 4 genes (TAZ, LDB3, DTNA and TPM1) in 51 patients with LVNC for mutations by polymerase chain reaction and direct DNA sequencing. A novel missense substitution in exon 1 of TPM1 (c.109A>G: p.Lys37Glu) was identified in three affected members of a family with isolated LVNC. The substitution brings about a change in amino acid charge at a highly conserved residue and could result in aberrant mRNA splicing. This variant was not identified in 200 normal control samples. Pathologic analysis of a right ventricular myocardial specimen from the proband's maternal aunt revealed endocardial and subendocardial fibrosis with prominent elastin deposition, as well as the presence of adipose tissue between muscle layers, pathologic changes that are distinct from those seen in patients with HCM or DCM. Screening of the proband and her mother for variants in other sarcomeric protein-encoding candidate genes, MYH7, MYBPC3, TNNT2, TNNI3, ACTC, MYL2, and MYL3, did not identify any other non-synonymous variants or variants in splice donor-acceptor sequences that were potentially disease causing. We conclude TPM1 is a potential candidate disease-causing gene for isolated LVNC, especially in patients experiencing sudden death.

Feline genetics: clinical applications and genetic testing

DNA testing for domestic cat diseases and appearance traits is a rapidly growing asset for veterinary medicine. Approximately 33 genes contain 50 mutations that cause feline health problems or alterations in the cat's appearance. A variety of commercial laboratories can now perform cat genetic diagnostics, allowing both the veterinary clinician and the private owner to obtain DNA test results. DNA is easily obtained from a cat via a buccal swab with a standard cotton bud or cytological brush, allowing DNA samples to be easily sent to any laboratory in the world. The DNA test results identify carriers of the traits, predict the incidence of traits from breeding programs, and influence medical prognoses and treatments. An overall goal of identifying these genetic mutations is the correction of the defect via gene therapies and designer drug therapies. Thus, genetic testing is an effective preventative medicine and a potential ultimate cure. However, genetic diagnostic tests may still be novel for many veterinary practitioners and their application in the clinical setting needs to have the same scrutiny as any other diagnostic procedure. This article will review the genetic tests for the domestic cat, potential sources of error for genetic testing, and the pros and cons of DNA results in veterinary medicine. Highlighted are genetic tests specific to the individual cat, which are a part of the cat's internal genome.