Mutations in either the essential or regulatory light chains of myosin are associated with a rare myopathy in human heart and skeletal muscle

Genotype-phenotype analysis in four families with mutations in beta-myosin heavy chain gene responsible for familial hypertrophic cardiomyopathy

Familial hypertrophic cardiomyopathy is a genetically heterogeneous disease in which one of the most frequently implicated gene is the gene encoding the beta-myosin heavy chain. To date, more than 40 distinct mutations have been found within this gene. In order to progress on the determination of genotype-phenotype relationship, we have screened the beta-myosin heavy chain gene for mutations in 18 probands from unrelated families. We identified the mutation implicated in the disease in four families. Two of them, the Glu930 codon deletion and the Ile263Thr mutation, are reported here for the first time. The two other mutations are the Arg723Cys mutation, that was previously described in a proband as a de novo mutation, and the Arg719Trp mutation. A poor prognosis was associated with the Glu930codon deletion (mean maximal wall thickness (MWT) = 19.5 mm +/- 5) and the Arg719Trp mutation (mean MWT = 15.3 mm +/- 7), whereas a good prognosis was associated with the Arg723Cys mutation (mean MWT = 20.1 mm +/- 7). The combination of clinical and genetic characteristics of each family member suggests that prognosis is related neither to the degree of left ventricular wall thickness nor to a change in the net electrical charge of the protein. Additional family studies are needed to confirm these findings and to contribute to stratify the prognosis according to the mutation involved.

A rapid protocol for cardiac troponin T gene mutation detection in familial hypertrophic cardiomyopathy

Mutations in the human cardiac troponin T gene (TNNT2) are associated with familial hypertrophic cardiomyopathy (FHC) linked to chromosome 1q3 (CMH2). Mutation analyses of TNNT2 have been restricted to RNA-based screening methods because only the TNNT2 cDNA sequence was known. We characterized the genomic structure of 15 TNNT2 exons spliced into the adult isoform. A protocol for rapid mutation detection based on direct sequencing of large PCR-amplified genomic DNA fragments revealed a known TNNT2 mutation (Phe110Ile) in one of 30 FHC probands. Three polymorphic short tandem repeat elements (D1S477, D1S2622, and D1S1723), useful for FHC pedigree analyses at CMH2, were shown to be physically tightly linked to TNNT2.

Effects of shaker-1 mutations on myosin-VIIa protein and mRNA expression

Numerous mammalian diseases have been found to be due to mutations in components of the actin cytoskeleton. Recently, mutations in the gene for an unconventional myosin, myosin-VIIa, were found to be the basis for the deafness and vestibular dysfunction observed in shaker-1 (sh1) mice and for a human deafness-blindness syndrome, Usher syndrome type 1B. Seven alleles of sh1 mice were analyzed to assess the affects of different myosin-VIIa mutations on both gene expression and tissue function. Myosin-VIIa is expressed in the inner ear and the retina, as well as the kidney, lung, and testis. Northern blot analysis indicated that myosin-VIIa mRNA expression, size, and stability were unaffected in the seven sh1 alleles. Immunoblot analysis showed that all seven alleles expressed some full-length myosin-VIIa protein. The range of expression, however, ran from sh1 [original], which expressed wild-type levels of protein, to two strains, sh1(4494SB) and sh1(4626SB), which expressed less than 1% of the normal level of myosin-VIIa protein. For the three alleles of sh1 that have been characterized and that have mutations in the motor domain, sh1 [original], sh1(816SB) and sh1(6J), the level of protein expression observed in these sh1 alleles correlated well with the predicted effects of the mutations on motor function. No change in retinal or testicular structure was observed at the light microscopic level during the life span of the seven sh1 alleles. Myosin-VIIa protein, when detectable, was observed to locate properly in the sh1 mice. On the basis of these results, we propose that the mutations in myosin-VIIa in the sh1 alleles leads to both motor dysfunction and to a protein destabilization phenotype.

Myocardial perfusion and sympathetic innervation in patients with hypertrophic cardiomyopathy

OBJECTIVES

This study assessed left ventricular myocardial perfusion and sympathetic innervation and function in hypertrophied and nonhypertrophied myocardial regions of patients with hypertrophic cardiomyopathy (HCM).

BACKGROUND

Patients with HCM often have clinical findings consistent with increased cardiac sympathetic outflow. Little is known about the status of sympathetic innervation specifically in hypertrophic regions.

METHODS

We conducted positron emission tomographic (PET) scanning using the perfusion imaging agent 13N-ammonia (13NH3) and the sympathoneuronal imaging agent 6-[18F]-fluorodopamine (18F-FDA) in 8 patients with HCM and 15 normal volunteers. Positron emission tomographic data corrected for attenuation and the partial volume effect were analyzed using the region-of-interest technique.

RESULTS

Myocardial 13NH3-derived radioactivity was similar in hypertrophied and nonhypertrophied regions of patients with HCM and in normal volunteers. At all time points, the 18F:13N ratio was lower in hypertrophied than in nonhypertrophied regions of HCM patients and in the septum of normal volunteers (p = 0.001). Trends in 18F-FDA-derived radioactivity over time were normal in both hypertrophied and nonhypertrophied myocardium.

CONCLUSIONS

The results are consistent with decreased neuronal uptake of catecholamines in hypertrophied but not in nonhypertrophied myocardium of patients with HCM. Other aspects of cardiac sympathoneural function seem normal. Decreased neuronal uptake could reflect local relative hypoinnervation, decreased numbers of neuronal uptake sites, or metabolic limitations on cell membrane transport. By enhancing norepinephrine delivery to adrenoceptors for a given amount of sympathetic nerve traffic, decreased neuronal uptake can explain major clinical features of HCM.

Single-strand conformation polymorphism analysis on the delta-sarcoglycan gene in Japanese patients with hypertrophic cardiomyopathy

To elucidate the etiology of hypertrophic cardiomyopathy (HC) in humans, we analyzed the delta-sarcoglycan gene (SG), which is reported to be the causal gene for HC in the Syrian hamster BIO14.6. We performed polymerase chain reaction (PCR) single-strand conformation polymorphism (SSCP) and nucleotide sequence analyses on the delta-SG in 102 patients with HC. SSCP was detected in exon 2 of the gene, but not in the other exons. The direct sequencing analysis of exon 2 revealed a C-->T substitution at nucleotide residue 84 (TAC-->TAT) with no amino acid alteration (Tyr-->Tyr). There were no significant differences in allele frequencies of C/T between the patients with HC and the control group. Patients with HC were classified into 4 subgroups: obstructive HC, nonobstructive HC, apical HC, and familial HC. The allele frequency of C/T polymorphism in each of these groups was compared with that of the control group. The obstructive HC group showed a significantly greater frequency of the allele T than in the control group (31.6% vs 15.1%, RR = 2.6, p = 0.023). No other significant differences were observed. Thus, amino acid alteration in delta-SG may not be a common cause of HC in Japanese patients.

The genomics of cardiovascular disorders: therapeutic implications

Cardiovascular disease (CVD) is a complicated series of disorders that result from the interaction between genetic predisposing mechanisms and environmental factors. Over the last few years substantial progress has been made in defining the molecular basis of several genetically transmitted non-atherosclerotic CVD such as hypertrophic and dilated cardiomyopathies, long-QT syndrome and essential hypertension. This review represents a summary of the current knowledge about the major gene polymorphisms found to be associated with these CVDs. Moreover, we will discuss how the discovery of disease-associated genes will greatly enhance the ability to formulate advanced diagnoses, to define prophylactic therapeutic strategies to prevent or reduce the progression of the disease and, finally, to proceed to the development of new drugs tailored for the specific cellular or molecular functions altered as consequence of the predisposing genes.

A newly created splice donor site in exon 25 of the MyBP-C gene is responsible for inherited hypertrophic cardiomyopathy with incomplete disease penetrance

BACKGROUND

Hypertrophic cardiomyopathy is a myocardial disorder resulting from inherited sarcomeric dysfunction. We report a mutation in the myosin-binding protein-C (MyBP-C) gene, its clinical consequences in a large family, and myocardial tissue findings that may provide insight into the mechanism of disease.

METHODS AND RESULTS

History and clinical status (examination, ECG, and echocardiography) were assessed in 49 members of a multigeneration family. Linkage analysis implicated the MyBP-C gene on chromosome 11. Myocardial mRNA, genomic MyBP-C DNA, and the myocardial proteins of patients and healthy relatives were analyzed. A single guanine nucleotide insertion in exon 25 of the MyBP-C gene resulted in the loss of 40 bases in abnormally processed mRNA. A 30-kDa truncation at the C-terminus of the protein was predicted, but a polypeptide of the expected size ( approximately 95 kDa) was not detected by immunoblot testing. The disease phenotype in this family was characterized in detail: only 10 of 27 gene carriers fulfilled diagnostic criteria. Five carriers showed borderline hypertrophic cardiomyopathy, and 12 carriers were asymptomatic, with normal ECG and echocardiograms. The age of onset in symptomatic patients was late (29 to 68 years). In 2 patients, outflow obstruction required surgery. Two family members experienced premature sudden cardiac death, but survival at 50 years was 95%.

CONCLUSIONS

Penetrance of this mutation was incomplete and age-dependent. The large number of asymptomatic carriers and the good prognosis support the interpretation of benign disease.

Temporal repolarization lability in hypertrophic cardiomyopathy caused by beta-myosin heavy-chain gene mutations

BACKGROUND

Certain genetic mutations associated with hypertrophic cardiomyopathy (HCM) carry an increased risk of sudden death. QT variability identifies patients at a high risk for sudden death from ventricular arrhythmias. We tested whether patients with HCM caused by beta-myosin heavy-chain (beta-MHC) gene mutations exhibit labile ventricular repolarization using beat-to-beat QT variability analysis.

METHODS AND RESULTS

We measured the QT variability index and heart rate-QT interval coherence from Holter monitor recordings in 36 patients with HCM caused by known beta-MHC gene mutations and in 26 age- and sex-matched controls. There were 7 distinct beta-MHC gene mutations in these 36 patients; 9 patients had HCM caused by the malignant Arg(403)Gln mutation and 8 patients had HCM caused by the more benign Leu(908)Val mutation. The QT variability index was higher in HCM patients than in controls (-1.24+/-0.17 versus -1. 58+/-0.38, P<0.01), and the greatest abnormality was detected in patients with the Arg(403)Gln mutation (-0.99+/-0.49 versus -1. 46+/-0.43 in controls, P<0.05). In keeping with this finding, coherence was lower for the entire HCM group than for controls (P<0. 001). Coherence was also significantly lower in patients with the Arg(403)Gln mutation compared with controls (P<0.05).

CONCLUSIONS

These findings suggest that (1) patients with HCM caused by beta-MHC gene mutations exhibit labile repolarization quantified by QT variability analysis and, hence, may be more at risk for sudden death from ventricular arrhythmias, and (2) indices of QT variability may be particularly abnormal in patients with beta-MHC gene mutations that are associated with a poor prognosis.

The effect of removing the N-terminal extension of the Drosophila myosin regulatory light chain upon flight ability and the contractile dynamics of indirect flight muscle

The Drosophila myosin regulatory light chain (DMLC2) is homologous to MLC2s of vertebrate organisms, except for the presence of a unique 46-amino acid N-terminal extension. To study the role of the DMLC2 N-terminal extension in Drosophila flight muscle, we constructed a truncated form of the Dmlc2 gene lacking amino acids 2-46 (Dmlc2(Delta2-46)). The mutant gene was expressed in vivo, with no wild-type Dmlc2 gene expression, via P-element-mediated germline transformation. Expression of the truncated DMLC2 rescues the recessive lethality and dominant flightless phenotype of the Dmlc2 null, with no discernible effect on indirect flight muscle (IFM) sarcomere assembly. Homozygous Dmlc2(Delta2-46) flies have reduced IFM dynamic stiffness and elastic modulus at the frequency of maximum power output. The viscous modulus, a measure of the fly's ability to perform oscillatory work, was not significantly affected in Dmlc2(Delta2-46) IFM. In vivo flight performance measurements of Dmlc2(Delta2-46) flies using a visual closed-loop flight arena show deficits in maximum metabolic power (P(*)(CO(2))), mechanical power (P(*)(mech)), and flight force. However, mutant flies were capable of generating flight force levels comparable to body weight, thus enabling them to fly, albeit with diminished performance. The reduction in elastic modulus in Dmlc2(Delta2-46) skinned fibers is consistent with the N-terminal extension being a link between the thick and thin filaments that is parallel to the cross-bridges. Removal of this parallel link causes an unfavorable shift in the resonant properties of the flight system, thus leading to attenuated flight performance.

Natural history of hypertrophic cardiomyopathy

Hypertrophic cardiomyopathy is a disease of the cardiac sarcomere and is the most common inherited cardiovascular disorder affecting up to 1 in 500 people in the general population. The disease is typified by variable clinical penetrance and heterogeneous clinical expression, resulting in a wide range of clinical manifestations. Most patients have few if any symptoms and a relatively benign clinical course. A minority are at risk of serious complications including ventricular arrhythmia, sudden death, thromboembolism, congestive cardiac failure, heart block, and infective endocarditis. This article reviews the natural history of the disease, with particular emphasis on lessons learned from recent genetic studies.

Molecular genetics of hypertrophic cardiomyopathy

Hypertrophic cardiomyopathy (HCM), a serious and often tragic disorder, is characterized by hypertrophy of the interventricular septum and left ventricular wall, hypercontractile systolic function with diastolic dysfunction, and in some cases, left ventricular outflow tract obstruction. On histopathologic examination, myofiber disarray is common. The genes for familial cases of hypertrophic cardiomyopathy are known to encode members of the sarcomere and to date nine genes have been identified (beta-myosin heavy chain, alpha-tropomyosin, cardiac troponin T, troponin I, myosin binding protein-C, regulatory myosin light chain, essential myosin light chain, cardiac actin, and titin) for this genetically and clinically heterogeneous disease. In this review the genetic basis of HCM is discussed.

Deletion in the cardiac troponin I gene in a family from northern Sweden with hypertrophic cardiomyopathy

The cardiac troponin I gene has been described to be associated with hypertrophic cardiomyopathy. Until now, mutations in this gene have been found only in the Japanese population. We now present the first non-Japanese family, from northern Sweden, with a mutation in the cardiac troponin I gene. Clinical diagnose was based on echocardiography, with a maximum left ventricular wall thickness of >13 mm, or major electrocardiographic abnormalities, excluding subjects with other known causes of cardiac hypertrophy. Mutation screening was performed with a single-strand conformation polymorphism analysis and identification of mutation by direct DNA sequencing. We have identified a 33-bp deletion in exon 8 encompassing the stop codon. Nine individuals in three generations were tested, and four were carriers of this deletion. The mother was genetically affected and died of heart failure aged 90. Echocardiography at 71 years of age revealed no hypertrophy, but the electrocardiogram showed signs of left ventricular hypertrophy. Her two sons, also genetically affected, had left ventricular hypertrophy, with maximum wall thickness of 15 and 16 mm, respectively. One daughter and four grandchildren were clinically unaffected, but one of them, a 27-year-old woman with maximum wall thickness of 8 mm and normal electrocardiogram, was found to be genetically affected. In conclusion, we describe a non-Japanese family in which hypertrophic cardiomyopathy is due to a genetic defect in the cardiac troponin I gene. This mutation is a deletion of 33 bp in the last exon, whereas the previously described mutations in this gene are single nucleotide changes and a single codon deletion. The deletion of the C-terminal part of the cardiac troponin I protein, seems in this particular family to be associated with a mild phenotypic expression of familial hypertrophic cardiomyopathy.

Altered regulation of cardiac muscle contraction by troponin T mutations that cause familial hypertrophic cardiomyopathy

To study the effect of troponin (Tn) T mutations that cause familial hypertrophic cardiomyopathy (FHC) on cardiac muscle contraction, wild-type, and the following recombinant human cardiac TnT mutants were cloned and expressed: I79N, R92Q, F110I, E163K, R278C, and intron 16(G(1) --> A) (In16). These TnT FHC mutants were reconstituted into skinned cardiac muscle preparations and characterized for their effect on maximal steady state force activation, inhibition, and the Ca(2+) sensitivity of force development. Troponin complexes containing these mutants were tested for their ability to regulate actin-tropomyosin(Tm)-activated myosin-ATPase activity. TnT(R278C) and TnT(F110I) reconstituted preparations demonstrated dramatically increased Ca(2+) sensitivity of force development, while those with TnT(R92Q) and TnT(I79N) showed a moderate increase. The deletion mutant, TnT(In16), significantly decreased both the activation and the inhibition of force, and substantially decreased the activation and the inhibition of actin-Tm-activated myosin-ATPase activity. ATPase activation was also impaired by TnT(F110I), while its inhibition was reduced by TnT(R278C). The TnT(E163K) mutation had the smallest effect on the Ca(2+) sensitivity of force; however, it produced an elevated activation of the ATPase activity in reconstituted thin filaments. These observed changes in the Ca(2+) regulation of force development caused by these mutations would likely cause altered contractility and contribute to the development of FHC.

Vertebrate isoforms of actin capping protein beta have distinct functions In vivo

Actin capping protein (CP) binds barbed ends of actin filaments to regulate actin assembly. CP is an alpha/beta heterodimer. Vertebrates have conserved isoforms of each subunit. Muscle cells contain two beta isoforms. beta1 is at the Z-line; beta2 is at the intercalated disc and cell periphery in general. To investigate the functions of the isoforms, we replaced one isoform with another using expression in hearts of transgenic mice. Mice expressing beta2 had a severe phenotype with juvenile lethality. Myofibril architecture was severely disrupted. The beta2 did not localize to the Z-line. Therefore, beta1 has a distinct function that includes interactions at the Z-line. Mice expressing beta1 showed altered morphology of the intercalated disc, without the lethality or myofibril disruption of the beta2-expressing mice. The in vivo function of CP is presumed to involve binding barbed ends of actin filaments. To test this hypothesis, we expressed a beta1 mutant that poorly binds actin. These mice showed both myofibril disruption and intercalated disc remodeling, as predicted. Therefore, CPbeta1 and CPbeta2 each have a distinct function that cannot be provided by the other isoform. CPbeta1 attaches actin filaments to the Z-line, and CPbeta2 organizes the actin at the intercalated discs.

A new mutation of the cardiac troponin T gene causing familial hypertrophic cardiomyopathy without left ventricular hypertrophy

AIM

To screen for a mutation of the cardiac troponin T gene in two families where there had been sudden deaths without an increase in left ventricular mass but with myocardial disarray suggesting hypertrophic cardiomyopathy.

METHODS

DNA from affected individuals from both families was used to screen the cardiac troponin T gene on an exon by exon basis. Mutation screening was achieved by polymerase chain reaction and direct sequencing. Where appropriate, a mutation was confirmed by restriction digest.

RESULTS

A novel missense mutation of exon 9 was found in the affected individuals of one of the families. This mutation at amino acid 94 resulted in the substitution of arginine for leucine and was not found in 100 normal control samples. A mutation of the cardiac troponin T gene was excluded in the second family.

CONCLUSIONS

A mutation of the gene for the sarcomeric protein cardiac troponin T can cause familial hypertrophic cardiomyopathy with marked myocyte disarray and frequent premature sudden death in the absence of myocardial hypertrophy at clinical or macroscopic level.

The cardiac troponin I gene is not associated with hypertrophic cardiomyopathy in patients from eastern Finland

Defects in seven genes encoding sarcomere proteins have been shown to cause hypertrophic cardiomyopathy. To date, only one study reporting defects in the cardiac troponin I gene associated with hypertrophic cardiomyopathy has been published, and the proportion of hypertrophic cardiomyopathy cases caused by defects in this gene is unknown. Therefore, the authors screened 37 unrelated Finnish patients with hypertrophic cardiomyopathy for variants in the cardiac troponin I gene. Exons 1-8 of the troponin I gene were screened with the polymerase chain reaction single-strand conformation polymorphism (PCR-SSCP) method. Five different variants (four intron variants and one silent exon variant) were found. Most variants were also present in control samples and none of the variants co-segregated with the disease in families. The results of the present study indicate that defects in the cardiac troponin I gene do not cause hypertrophic cardiomyopathy in patients from Eastern Finland.

Inherited disorders of sarcomeric proteins

The most important advances in sarcomeric protein diseases continue to be the identification of mutated genes responsible for human diseases. These have recently included those that encode skeletal muscle alpha-actin in autosomal dominant and autosomal recessive nemaline myopathy, nebulin and slow alpha-tropomyosin in autosomal recessive nemaline myopathy, and desmin and alpha B-crystallin in desminopathies.

Structural analysis of the titin gene in hypertrophic cardiomyopathy: identification of a novel disease gene

Hypertrophic cardiomyopathy (HCM) is characterized by ventricular hypertrophy accompanied by myofibrillar disarrays. Molecular genetic analyses have revealed that mutations in 8 different genes cause HCM. Mutations in these disease genes, however, could be found in about half of HCM patients, suggesting that there are other unknown disease gene(s). Because the known disease genes encode sarcomeric proteins expressed in the cardiac muscle, we searched for a disease-associated mutation in the titin gene in 82 HCM patients who had no mutation in the known disease genes. A G to T transversion in codon 740, from CGC to CTC, replacing Arginine with Leucine was found in a patient. This mutation was not found in more than 500 normal chromosomes and increased the binding affinity of titin to alpha-actitin in the yeast two-hybrid assay. These observations suggest that the titin mutation may cause HCM in this patient via altered affinity to alpha-actinin.

The frequency of occurrence of anti-cardiac receptor autoantibodies and their correlation with clinical manifestation in patients with hypertrophic cardiomyopathy

The purpose of this study was to investigate the frequency of occurrence of autoantibodies against G-protein coupled cardiovascular receptors and their relation to the clinical manifestation of hypertrophic cardiomyopathy (HCM). Autoantibodies against beta1-receptors, Muscarin-2-receptors, Angiotensin-II-receptor subtype 1 and alpha1-receptors were determined with ELISA in 52 patients with HCM (37 male, 15 female, mean age 55 +/- 15 years) and 40 healthy, age and sex matched controls. The clinical characterization of the HCM-patients included ECG, 24-h Holter, and echocardiography. The results showed that there is no significant difference in the frequency of a single autoantibody between HCM-patients and controls. However, if the number of patients who have autoantibodies against beta1-receptors and/or Muscarin-2-receptors were counted together, there are significantly more autoantibodies in HCM compared to controls (11 vs. 2, p = 0.035). Analysis of clinical data from this pooled group of patients showed that in patients with autoantibodies, heart rate variability (HRV), ultra low frequency (ULF) and very low frequency (VLF) were decreased (HRV by 20%, ULF by 50%, and VLF by 46%, p < 0.008) whereas the QTc-interval was increased by 8% (p < 0.02 each). The ratio of septal to posterior wall thickness was increased by 23% (p = 0.05), and the preejection period was prolonged by 46% in patients with autoantibodies (p < 0.001). These results suggest that the existence of these autoantibodies could be associated with an advanced stage or a severe manifestation of HCM.

Functional changes in troponin T by a splice donor site mutation that causes hypertrophic cardiomyopathy

A splice donor site mutation in intron 15 of the cardiac troponin T (TnT) gene has been shown to cause familial hypertrophic cardiomyopathy (HCM). In this study, two truncated human cardiac TnTs expected to be produced by this mutation were expressed in Escherichia coli and partially (50-55%) exchanged into rabbit permeabilized cardiac muscle fibers. The fibers into which a short truncated TnT, which lacked the COOH-terminal 21 amino acids because of the replacement of 28 amino acids with 7 novel residues, had been exchanged generated a Ca(2+)-activated maximum force that was slightly, but statistically significantly, lower than that generated by fibers into which wild-type TnT had been exchanged when troponin I (TnI) was phosphorylated by cAMP-dependent protein kinase. A long truncated TnT simply lacking the COOH-terminal 14 amino acids had no significant effect on the maximum force-generating capability in the fibers with either phosphorylated or dephosphorylated TnI. Both these two truncated TnTs conferred a lower cooperativity and a higher Ca(2+) sensitivity on the Ca(2+)-activated force generation than did wild-type TnT, independent of the phosphorylation of TnI by cAMP-dependent protein kinase. The results demonstrate that the splice donor site mutation in the cardiac TnT gene impairs the regulatory function of the TnT molecule, leading to an increase in the Ca(2+) sensitivity, and a decrease in the cooperativity, of cardiac muscle contraction, which might be involved in the pathogenesis of HCM.

Familial hypertrophic cardiomyopathy

Familial hypertrophic cardiomyopathy (FHC) is a cardiomyopathy that occurs in 0.2% of the general population. It is characterized by asymmetrical hypertrophy of the ventricle, predominantly the intraventricular septum. FHC is caused by genetic mutations in several of the sarcomeric proteins, such as myosin heavy chain, troponin T, troponin I, alpha-tropomyosin, essential and regulatory light chains of myosin, and the cardiac myosin-binding protein C. FHC is genetically heterogeneous, and, therefore, it is associated with a very diverse clinical presentation in terms of altered cardiac structure and clinical manifestations. The most severe manifestation is sudden death. The purpose of this article is to provide the reader with new insights into the genetic mutations that give rise to FHC and to discuss risk factors that are associated with severe hypertrophy and sudden death in this population.

Double heterozygosity for mutations in the β-myosin heavy chain and in the cardiac myosin binding protein C genes in a family with hypertrophic cardiomyopathy

Familial hypertrophic cardiomyopathy is a genetically heterogeneous autosomal dominant disease, caused by mutations in several sarcomeric protein genes. So far, seven genes have been shown to be associated with the disease with the β-myosin heavy chain (MYH7) and the cardiac myosin binding protein C (MYBPC3) genes being the most frequently involved.

We performed electrocardiography (ECG) and echocardiography in 15 subjects with hypertrophic cardiomyopathy from a French Caribbean family. Genetic analyses were performed on genomic DNA by haplotype analysis with microsatellite markers at each locus involved and mutation screening by single strand conformation polymorphism analysis. Based on ECG and echocardiography, eight subjects were affected and presented a classical phenotype of hypertrophic cardiomyopathy. Two new mutations cosegregating with the disease were found, one located in the MYH7 gene exon 15 (Glu483Lys) and the other in the MYBPC3 gene exon 30 (Glu1096 termination codon). Four affected subjects carried the MYH7 gene mutation, two the MYBPC3 gene mutation, and two were doubly heterozygous for the two mutations. The doubly heterozygous patients exhibited marked left ventricular hypertrophy, which was significantly greater than in the other affected subjects.

We report for the first time the simultaneous presence of two pathological mutations in two different genes in the context of familial hypertrophic cardiomyopathy. This double heterozygosity is not lethal but is associated with a more severe phenotype.

Familial hypertrophic cardiomyopathy in maine coon cats: an animal model of human disease

BACKGROUND

A naturally occurring animal model of familial hypertrophic cardiomyopathy (FHCM) is lacking. We identified a family of Maine coon cats with HCM and developed a colony to determine mode of inheritance, phenotypic expression, and natural history of the disease.

METHODS AND RESULTS

A proband was identified, and related cats were bred to produce a colony. Affected and unaffected cats were bred to determine the mode of inheritance. Echocardiography was used to identify affected offspring and determine phenotypic expression. Echocardiograms were repeated serially to determine the natural history of the disease. Of 22 offspring from breeding affected to unaffected cats, 12 (55%) were affected. When affected cats were bred to affected cats, 4 (45%) of the 9 were affected, 2 (22%) unaffected, and 3 (33%) stillborn. Findings were consistent with an autosomal dominant mode of inheritance with 100% penetrance, with the stillborns representing lethal homozygotes that died in utero. Affected cats usually did not have phenotypic evidence of HCM before 6 months of age, developed HCM during adolescence, and developed severe HCM during young adulthood. Papillary muscle hypertrophy that produced midcavitary obstruction and systolic anterior motion of the mitral valve was the most consistent manifestation of HCM. Cats died suddenly (n=5) or of heart failure (n=3). Histopathology of the myocardium revealed myocardial fiber disarray, intramural coronary arteriosclerosis, and interstitial fibrosis.

CONCLUSIONS

HCM in this family of Maine coon cats closely resembles the human form of FHCM and should prove a valuable tool for studying the gross, cellular, and molecular pathophysiology of the disease.

Familial hypertrophic cardiomyopathy and atrial fibrillation caused by Arg663His beta-cardiac myosin heavy chain mutation

More than 40 different beta-cardiac myosin heavy chain (beta-MHC) missense mutations have been identified that cause familial hypertrophic cardiomyopathy (FHC). Some of these are recognized to have important clinical manifestations, such as an increased incidence of sudden death. We report that the beta-MHC missense mutation Arg663His causes predominant cardiac morphology and atrial fibrillation. Longitudinal clinical evaluations were performed in a kindred with FHC. The nucleotide sequence of the beta-MHC gene was analyzed to define the causal mutation. A missense mutation in the beta-MHC gene, Arg663His, was identified in 24 individuals. Clinical studies demonstrated modest left ventricular hypertrophy in affected individuals, predominantly localized in the proximal segment of the interventricular septum, which increased (average = 40 +/- 8%) during 7 years of follow-up. Results showed that 47% of Arg663His adults (age > 16 years) with ventricular hypertrophy developed atrial fibrillation, significantly more (p <0.001) than observed in ungenotyped FHC populations. Survival of affected individuals remained near normal. The beta-MHC missense mutation Arg663His causes a characteristic pattern of ventricular hypertrophy. Arg663His individuals have a markedly higher prevalence of atrial fibrillation, compared with a population with ungenotyped hypertrophic cardiomyopathy. The demonstration of phenotype as a direct consequence of genotype further extends the utility of molecular data in clinical medicine. Early identification of Arg663His individuals has the potential to minimize the serious sequelae of this arrhythmia in this FHC group.

An Integrated View of Insect Flight Muscle: Genes, Motor Molecules, and Motion

Substituting an alanine for serine in the regulatory subunit of the motor protein myosin dramatically alters Drosophila's flight ability. Power output, at all levels of the flight system, is reduced. This example of deciphering a protein's function by producing malfunctions illustrates the broadening use of molecular genetics in integrative biology.

Cardiac myosin heavy chains lacking the light chain binding domain cause hypertrophic cardiomyopathy in mice

Myosin is a chemomechanical motor that converts chemical energy into the mechanical work of muscle contraction. More than 40 missense mutations in the cardiac myosin heavy chain (MHC) gene and several mutations in the two myosin light chains cause a dominantly inherited heart disease called familial hypertrophic cardiomyopathy. Very little is known about the biochemical defects in these alleles and how the mutations lead to disease. Because removal of the light chain binding domain in the lever arm of MHC should alter myosin's force transmission but not its catalytic function, we tested the hypothesis that such a mutant MHC would act as a dominant mutation in cardiac muscle. Hearts from transgenic mice expressing this mutant myosin are asymmetrically hypertrophied, with increases in mass primarily restricted to the cardiac anterior wall. Histological examination demonstrates marked cellular hypertrophy, myocyte disorganization, small vessel coronary disease, and severe valvular pathology that included thickening and plaque formation. Skinned myocytes and multicellular preparations from transgenic hearts exhibited decreased Ca2+ sensitivity of tension and decreased relaxation rates after flash photolysis of diazo 2. These experiments demonstrate that alterations in myosin force transmission are sufficient to trigger the development of hypertrophic cardiomyopathy.

Regulatory light chain mutations affect myosin motor function and kinetics

The actin-based motor protein myosin II plays a critical role in many cellular processes in both muscle and non-muscle cells. Targeted disruption of the Dictyostelium regulatory light chain (RLC) caused defects in cytokinesis and multicellular morphogenesis. In contrast, a myosin heavy chain mutant lacking the RLC binding site, and therefore bound RLC, showed normal cytokinesis and development. One interpretation of these apparently contradictory results is that the phenotypic defects in the RLC null mutant results from mislocalization of myosin caused by aggregation of RLC null myosin. To distinguish this from the alternative explanation that the RLC can directly influence myosin activity, we expressed three RLC point mutations (E12T, G18K and N94A) in a Dictyostelium RLC null mutant. The position of these mutations corresponds to the position of mutations that have been shown to result in familial hypertrophic cardiomyopathy in humans. Analysis of purified Dictyostelium myosin showed that while these mutations did not affect binding of the RLC to the MHC, its phosphorylation by myosin light chain kinase or regulation of its activity by phosphorylation, they resulted in decreased myosin function. All three mutants showed impaired cytokinesis in suspension, and one produced defective fruiting bodies with short stalks and decreased spore formation. The abnormal myosin localization seen in the RLC null mutant was restored to wild-type localization by expression of all three RLC mutants. Although two of the mutant myosins had wild-type actin-activated ATPase, they produced in vitro motility rates half that of wild type. N94A myosin showed a fivefold decrease in actin-ATPase and a similar decrease in the rate at which it moved actin in vitro. These results indicate that the RLC can play a direct role in determining the force transmission and kinetic properties of myosin.

Dominant hereditary inclusion-body myopathy gene (IBM3) maps to chromosome region 17p13.1

We recently described an autosomal dominant inclusion-body myopathy characterized by congenital joint contractures, external ophthalmoplegia, and predominantly proximal muscle weakness. A whole-genome scan, performed with 161 polymorphic markers and with DNA from 40 members of one family, indicated strong linkage for markers on chromosome 17p. After analyses with additional markers in the region and with DNA from eight additional family members, a maximum LOD score (Zmax) was detected for marker D17S1303 (Zmax=7.38; recombination fraction (theta)=0). Haplotype analyses showed that the locus (Genome Database locus name: IBM3) is flanked distally by marker D17S945 and proximally by marker D17S969. The positions of cytogenetically localized flanking markers suggest that the location of the IBM3 gene is in chromosome region 17p13.1. Radiation hybrid mapping showed that IBM3 is located in a 2-Mb chromosomal region and that the myosin heavy-chain (MHC) gene cluster, consisting of at least six genes, co-localizes to the same region. This localization raises the possibility that one of the MHC genes clustered in this region may be involved in this disorder.

Genetic basis of cardiomyopathy

The molecular basis of cardiac growth and development is a fundamental question that has intrigued many investigators in cardiovascular research. Adult cardiomyocytes are terminally differentiated and lose their ability to proliferate shortly after birth; however, in response to injury, myocytes have the capacity to synthesize new DNA and exhibit plasticity by a compensatory growth response, as is shown by re-expression of the fetal isoforms of many muscle-specific genes, which is characteristic of the proliferative response. The long-term effects of these compensatory responses may lead to the development and progression of diseases such as hypertrophic cardiomyopathy and dilated cardiomyopathy, because of a single point mutation. This concept has engaged scientists to investigate human models to explore the molecular basis of hypertrophy or dilation of the myocardium.

A post-transcriptional compensatory pathway in heterozygous ventricular myosin light chain 2-deficient mice results in lack of gene dosage effect during normal cardiac growth or hypertrophy

Our previous study of homozygous mutants of the ventricular specific isoform of myosin light chain 2 (mlc-2v) demonstrated that mlc-2v plays an essential role in murine heart development (Chen, J., Kubalak, S. W., Minamisawa, S., Price, R. L., Becker, K. D., Hickey, R., Ross, J., Jr., and Chien, K. R. (1998) J. Biol. Chem. 273, 1252-1256). As gene dosage of some myofibrillar proteins can affect muscle function, we have analyzed heterozygous mutants in depth. Ventricles of heterozygous mutants displayed a 50% reduction in mlc-2v mRNA, yet expressed normal levels of protein both under basal conditions and following induction of cardiac hypertrophy by aortic constriction. Heterozygous mutants exhibited cardiac function comparable to that of wild-type littermate controls both prior to and following aortic constriction. There were no significant differences in contractility and responses to calcium between wild-type and heterozygous unloaded cardiomyocytes. We conclude that heterozygous mutants show neither a molecular nor a physiological cardiac phenotype either at base line or following hypertrophic stimuli. These results suggest that post-transcriptional compensatory mechanisms play a major role in maintaining the level of MLC-2v protein in murine hearts. In addition, as our mlc-2v knockout mutants were created by a knock-in of Cre recombinase into the endogenous mlc-2v locus, this study demonstrates that heterozygous mlc-2v cre knock-in mice are appropriate for ventricular specific gene targeting.

Altered kinetics of contraction of mouse atrial myocytes expressing ventricular myosin regulatory light chain

To investigate the role of myosin regulatory light chain isoforms as a determinant of the kinetics of cardiac contraction, unloaded shortening velocity was determined by the slack-test method in skinned wild-type murine atrial cells and transgenic cells expressing ventricular regulatory light chain (MLC2v). Transgenic mice were generated using a 4.5-kb fragment of the murine alpha-myosin heavy chain promoter to drive high levels of MLC2v expression in the atrium. Velocity of unloaded shortening was determined at 15 degrees C in maximally activating Ca2+ solution (pCa 4.5) containing (in mmol/l) 7 EGTA, 1 free Mg2+, 4 MgATP, 14.5 creatine phosphate, and 20 imidazole (ionic strength 180 mmol/l, pH 7.0). Compared with the wild type (n = 10), the unloaded shortening velocity of MLC2v-expressing transgenic murine atrial cells (n = 10) was significantly greater (3.88 +/- 1.19 vs. 2.51 +/- 1.08 muscle lengths/s, P < 0.05). These results provide evidence that myosin light chain 2 regulates cross-bridge cycling rate. The faster rate of cycling in the presence of MLC2v suggests that the MLC2v isoform may contribute to the greater power-generating capabilities of the ventricle compared with the atrium.

Pediatric myocardial disease

Cardiomyopathies are diseases of the heart muscles. This article reviews the causes, clinical presentation, diagnosis, management, and long-term outcomes of dilated and hypertrophic cardiomyopathy.

Ca2+ sensitization and potentiation of the maximum level of myofibrillar ATPase activity caused by mutations of troponin T found in familial hypertrophic cardiomyopathy

Human wild-type cardiac troponin T, I, C and five troponin T mutants (I79N, R92Q, F110I, E244D, and R278C) causing familial hypertrophic cardiomyopathy were expressed in Escherichia coli, and then were purified and incorporated into rabbit cardiac myofibrils using a troponin exchange technique. The Ca2+-sensitive ATPase activity of these myofibrillar preparations was measured in order to examine the functional consequences of these troponin mutations. An I79N troponin T mutation was found to cause a definite increase in Ca2+ sensitivity of the myofibrillar ATPase activity without inducing any significant change in the maximum level of ATPase activity. A detailed analysis indicated the inhibitory action of troponin I to be impaired by the I79N troponin T mutation. Two more troponin T mutations (R92Q and R278C) were also found to have a Ca2+-sensitizing effect without inducing any change in maximum ATPase activity. Two other troponin T mutations (F110I and E244D) had no Ca2+-sensitizing effects on the ATPase activity, but remarkably potentiated the maximum level of ATPase activity. These findings indicate that hypertrophic cardiomyopathy-linked troponin T mutations have at least two different effects on the Ca2+-sensitive ATPase activity, Ca2+-sensitization and potentiation of the maximum level of the ATPase activity.

Molecular heterogeneity in very-long-chain acyl-CoA dehydrogenase deficiency causing pediatric cardiomyopathy and sudden death

BACKGROUND

Genetic defects are being increasingly recognized in the etiology of primary cardiomyopathy (CM). Very-long-chain acyl-CoA dehydrogenase (VLCAD) catalyzes the first step in the beta-oxidation spiral of fatty acid metabolism, the crucial pathway for cardiac energy production.

METHODS AND RESULTS

We studied 37 patients with CM, nonketotic hypoglycemia and hepatic dysfunction, skeletal myopathy, or sudden death in infancy with hepatic steatosis, features suggestive of fatty acid oxidation disorders. Single-stranded conformational variance was used to screen genomic DNA. DNA sequencing and mutational analysis revealed 21 different mutations on the VLCAD gene in 18 patients. Of the mutations, 80% were associated with CM. Severe CM in infancy was recognized in most patients (67%) at presentation. Hepatic dysfunction was common (33%). RNA blot analysis and VLCAD enzyme assays showed a severe reduction in VLCAD mRNA in patients with frame-shift or splice-site mutations and absent or severe reduction in enzyme activity in all.

CONCLUSIONS

Infantile CM is the most common clinical phenotype of VLCAD deficiency. Mutations in the human VLCAD gene are heterogeneous. Although mortality at presentation is high, both the metabolic disorder and cardiomyopathy are reversible.

Altered crossbridge kinetics in the alphaMHC403/+ mouse model of familial hypertrophic cardiomyopathy

A mutation in the cardiac beta-myosin heavy chain, Arg403Gln (R403Q), causes a severe form of familial hypertrophic cardiomyopathy (FHC) in humans. We used small-amplitude (0.25%) length-perturbation analysis to examine the mechanical properties of skinned left ventricular papillary muscle strips from mouse hearts bearing the R403Q mutation in the alpha-myosin heavy chain (alphaMHC403/+). Myofibrillar disarray with variable penetrance occurred in the left ventricular free wall of the alphaMHC403/+ hearts. In resting strips (pCa 8), dynamic stiffness was approximately 40% greater than in wild-type strips, consistent with elevated diastolic stiffness reported for murine hearts with FHC. At pCa 6 (submaximal activation), strip isometric tension was approximately 3 times higher than for wild-type strips, whereas at pCa 5 (maximal activation), tension was marginally lower. At submaximal calcium activation the characteristic frequencies of the work-producing (b) and work-absorbing (c) steps of the crossbridge were less in alphaMHC403/+ strips than in wild-type strips (b=11+/-1 versus 15+/-1 Hz; c= 58+/-3 versus 66+/-3 Hz; 27 degrees C). At maximal calcium activation, strip oscillatory power was reduced (0. 53+/-0.25 versus 1.03+/-0.18 mW/mm3; 27 degrees C), which is partly attributable to the reduced frequency b, at which crossbridge work is maximum. The results are consistent with the hypothesis that the R403Q mutation reduces the strong binding affinity of myosin for actin. Myosin heads may accumulate in a preforce state that promotes cooperative activation of the thin filament at submaximal calcium but blunts maximal tension and oscillatory power output at maximal calcium. The calcium-dependent effect of the mutation (whether facilitating or debilitating), together with a variable degree of fibrosis and myofibrillar disorder, may contribute to the diversity of clinical symptoms observed in murine FHC.

Genetic factors in preterm delivery

Tens of thousands of children deliver before they are full term each year. Although many social, environmental, and medical risk factors have been suggested, the etiology of a large percentage of preterm labor cases is still unknown. It has been noted for many years that preterm delivery is a condition that runs in families. Evidence concerning its aggregation among families, the recurrent nature of preterm labor, and its differing prevalence between races has led to the suggestion of a genetic cause for preterm delivery. There have been few formal studies to investigate this hypothesis. We suggest that modern molecular biology approaches can reveal the part that genes play in preterm delivery.

The stretch-activation response may be critical to the proper functioning of the mammalian heart

The "stretch-activation" response is essential to the generation of the oscillatory power required for the beating of insect wings. It has been conjectured but not previously shown that a stretch-activation response contributes to the performance of a beating heart. Here, we generated transgenic mice that express a human mutant myosin essential light chain derived from a family with an inherited cardiac hypertrophy. These mice faithfully replicate the cardiac disease of the patients with this mutant allele. They provide the opportunity to study the stretch-activation response before the hearts are distorted by the hypertrophic process. Studies disclose a mismatch between the physiologic heart rate and resonant frequency of the cardiac papillary muscles expressing the mutant essential light chain. This discordance reduces oscillatory power at frequencies that correspond to physiologic heart-rates and is followed by subsequent hypertrophy. It appears, therefore, that the stretch-activation response, first described in insect flight muscle, may play a role in the mammalian heart, and its further study may suggest a new way to modulate human cardiac function.

Genetic and molecular basis of cardiac arrhythmias: impact on clinical management parts I and II

Genetic approaches have succeeded in defining the molecular basis of an increasing array of heart diseases, such as hypertrophic cardiomyopathy and the long-QT syndromes, associated with serious arrhythmias. Importantly, the way in which this new knowledge can be applied to managing patients and to the development of syndrome-specific antiarrhythmic strategies is evolving rapidly because of these recent advances. In addition, the extent to which new knowledge represents a purely research tool versus the extent to which it can be applied clinically is also evolving. The present article represents a consensus report of a meeting of the European Working Group on Arrhythmias. The current state of the art of the molecular and genetic basis of inherited arrhythmias is first reviewed, followed by practical advice on the role of genetic testing in these and other syndromes and the way in which new findings have influenced current understanding of the molecular and biophysical basis of arrhythmogenesis.

Cardiac manifestations of congenital fiber-type disproportion myopathy

Cardiac involvement has not been a reported feature of congenital fiber-type disproportion myopathy. We describe two children, aged 13 years and 1 year, respectively, who presented with serious cardiac symptomatology in conjunction with congenital fiber-type disproportion. One child developed dilated cardiomyopathy and medically intractable congestive heart failure necessitating cardiac transplantation at the age of 13 years. The second (unrelated) child developed atrial fibrillation with rapid atrioventricular conduction requiring treatment with digoxin. Skeletal muscle biopsy findings in both children showed congenital fiber-type disproportion with no evidence of a structural, dystrophic, or metabolic myopathy. Adenosine triphosphatase (ATPase) reacted sections showed type I hypotrophy with a predominance of type I fibers, confirmed by histogram analysis. Examination of the heart from patient 1 at the time of transplantation confirmed dilated cardiomyopathy with hypertrophic myocardiocytes. Although cardiomyopathy is commonly associated with other childhood myopathies, to our knowledge it has not been a feature in reported cases of congenital fiber-type disproportion. We recommend close cardiac assessment, with annual electrocardiograms, of children with congenital fiber-type disproportion.

Functional characterization of Dictyostelium discoideum mutant myosins equivalent to human familial hypertrophic cardiomyopathy

Familial hypertrophic cardiomyopathy (FHC) is caused by missence mutations in beta-myosin heavy chain or other various sarcomeric proteins. To elucidate the functional impact of FHC mutations in myosin heavy chain, we generated Dictyostelium discoideum myosin II mutants equivalent to human FHC mutations by site-directed mutagenesis, and characterized their molecular-basis motor function. The current mutants, i.e. R397Q, F506C, G575R, A699R, K703Q and K703W are equivalent to R403Q, F513C, G584R, G716R, R719Q and R719W FHC mutants respectively. We measured the molecular-basis force and the sliding velocity generated by these myosin mutants. The measurement revealed that the A699R, K703Q and K703W myosins exhibited the lowest level of force with their preserved actin-activated MgATPase activity. F506C mutant showed the least impairment of the motile and enzymatic activities. The motor function of R397Q and G575R myosins were classified as intermediate. These results suggest that ELC binding domain might be important for force production.

The molecular biology and pathophysiology of hypertrophic cardiomyopathy due to mutations in the beta myosin heavy chains and the essential and regulatory light chains

Hypertrophic cardiomyopathy (HCM) is perhaps the most common cause of inherited sudden death in otherwise healthy young individuals. There are presently seven known genes in which mutations have been shown to cause the disease. The first identified disease gene was beta myosin heavy chain (BMHC). Our laboratory has identified 32 distinct BMHC gene mutations in 62 kindreds after screening representatives of over 400 kindreds. Virtually all but one of approximately 50 known mutations are restricted to the head or head-rod junction region of the molecule. We have used the mutant alleles of the BMHC gene to demonstrate that both mutant message and protein is present in the skeletal muscle of patients with HCM. Muscle biopsies from patients with identified BMHC mutations show abnormal histology. Isolated myosin and skinned fibers from these patients have abnormal mechanical properties. The BMHC gene mutations are clustered in 4 regions of the myosin head. Because one of these regions is adjacent to the ELC, we scanned HCM patient DNA for mutations in either the ELC or RLC. Linkage analysis showed that a unique mutation in the ELC caused a rare phenotype of HCM in one family. Other mutations in either light chain were also associated with the same rare phenotype in other families. Through several lines of reasoning we hypothesized that the light chain mutations interfere with the stretch-activation response of papillary muscle and adjacent ventricular tissue. This property is critical to oscillatory power output of insect flight muscle. We conjectured that this property is also exploited by portions of the heart to increase power output. In order to test this hypothesis we constructed transgenic mouse lines expressing either the human normal or mutant ELC. The cardiac morphology and mechanical properties of the transgenic mouse papillary muscle is now being studied.

In vivo short-term expression of a hypertrophic cardiomyopathy mutation in adult rabbit myocardium: myofibrillar incorporation without early disarray

Cardiac myocyte disarray is the pathological hallmark of hypertrophic cardiomyopathy (HCM), a disease of sarcomeric proteins. Mutations in the cardiac troponin T (cTnT), a major gene responsible for HCM, are associated with severe myocyte disarray. To study the pathogenesis of cardiac myocyte disarray, we expressed normal and mutant cTnT proteins in the myocardium of adult rabbits via direct intramyocardial injection of recombinant adenoviruses. Aliquots of 1010 plaque-forming units of normal (Ad/CMV/cTnT-Arg92) and mutant (Ad/CMV/cTnT-Gln92) recombinant viruses or a control vector (Ad/DeltaE) virus were mixed with equal aliquots of a reporter virus (Ad/CMV/Lac-Z) and co-injected into the myocardium of adult rabbits (n = 12). One week following gene transfer, thin myocardial sections were obtained and analyzed for beta-galactosidase, messenger RNA (mRNA) and protein expression, hematoxylin and eosin, Masson's trichrome, immunofluorescence staining, and electron microscopy. The efficiency of gene transfer varied from 2% to 60% of the cells in an area approximately 2.5 mm in length. Northern blotting confirmed expression of the transgenes into mRNA. Immunoblotting of the myofibrillar protein extracts and indirect immunofluorescence staining confirmed expression and incorporation of the transgene proteins into myofibrils. Expression of the mutant cTnT was up to 18% of the endogenous. Light and electron microscopic studies showed normal cardiac myocyte and sarcomere structures. Thus, despite incorporation of the mutant cTnT-Gln92, stable myofibrillar formation and sarcomere assembly proceeded in vivo. The absence of myocyte and sarcomere disarray may reflect the duration, or the level of expression, or the extent of myofibrillar incorporation of the mutant cTnT-Gln92, as well as the site and timing of expression of the transgenes, and interspecies variation in the pathogenesis of HCM.

Early expression of a malignant phenotype of familial hypertrophic cardiomyopathy associated with a Gly716Arg myosin heavy chain mutation in a Korean family

The clinical course and prognosis of familial hypertrophic cardiomyopathy (HCM) are different according to the type of mutation in the genes for sarcomere proteins. It has been disputed that a mutation, which occurs at a functionally important region in the sarcomere proteins, may increase the penetrance and expressivity of the disease. We searched for a causative mutation in an HCM family, which is characterized by early expression of clinical phenotype, high incidence of sudden death at young ages, and progressive heart failure in adults. Among the 32 family members in 4 generations, 13 were affected; 4 died suddenly before age 16, 2 children have already had full expression of the cardiac hypertrophy, and other adults have either progressive heart failure or poor left ventricular systolic functions. PCR-SSCP (polymerase chain reaction-single strand confirmation polymorphism) analysis of genomic DNAs isolated from peripheral blood leukocytes of the family members identified a Gly716Arg mutation in the cardiac beta-myosin heavy chain gene, which was cosegregated with the clinical phenotype. The mutation is localized near a functionally important site of the myosin heavy chain, the 2 active thiols, which contribute to the adenosine triphosphatase activity of myosin S1. This family provides further evidence that the mutation, which occurs at a functionally important site of the myosin heavy chain, is associated with the high penetrance and early expression of HCM.