Myosin light chain mutations in familial hypertrophic cardiomyopathy: phenotypic presentation and frequency in Danish and South African populations

Prolonged Ca2+ and force transients in myosin RLC transgenic mouse fibers expressing malignant and benign FHC mutations

Clinical studies have revealed that mutations in the ventricular myosin regulatory light chain (RLC) lead to the development of familial hypertrophic cardiomyopathy (FHC), an autosomal dominant disease characterized by left ventricular hypertrophy, myofibrillar disarray and sudden cardiac death. While mutations in other contractile proteins have been studied widely by others, there is no report elucidating the mechanism(s) associated with FHC-linked RLC mutations. In this study, we have assessed the functional consequences of two RLC mutations, R58Q and N47K, in transgenic mice. Clinical phenotypes associated with these mutations included inter-ventricular hypertrophy, abnormal ECG findings and the R58Q mutation caused multiple cases of premature sudden cardiac death. Simultaneous measurements of the ATPase and force in transgenic skinned papillary muscle fibers from mutated versus control mice showed an increase in the Ca(2+) sensitivity of ATPase and steady-state force only in R58Q fibers. The calculated energy cost or rate of dissociation of force generating myosin cross-bridges (ATPase/force ratio) plotted as a function of activation state was the same in all groups of fibers. Both mutations caused prolonged [Ca(2+)] transients in electrically stimulated intact papillary muscles; however, the R58Q mutation also resulted in a significantly prolonged force transient. Our results suggest that the phenotypes of FHC observed in patients harboring these RLC mutations correlate with the extent of physiological changes monitored in transgenic fibers. Cardiac hypertrophy observed in patients is most likely caused by the activation of compensatory mechanisms ensuing from higher workloads due to incomplete relaxation as evidenced by prolonged [Ca(2+)] transients for both N47K and R58Q fibers. Furthermore, the poor prognosis of the R58Q patients may be associated with more severe diastolic dysfunction due to the slower off-rate of Ca(2+) from troponin C leading to longer force and [Ca(2+)] transients and increased Ca(2+) sensitivity of ATPase and force.

The E22K mutation of myosin RLC that causes familial hypertrophic cardiomyopathy increases calcium sensitivity of force and ATPase in transgenic mice

Familial hypertrophic cardiomyopathy (FHC) is an autosomal dominant disease caused by mutations in all of the major sarcomeric proteins, including the ventricular myosin regulatory light-chain (RLC). The E22K-RLC mutation has been associated with a rare variant of cardiac hypertrophy defined by mid-left ventricular obstruction due to papillary muscle hypertrophy. This mutation was later found to cause ventricular and septal hypertrophy. We have generated transgenic (Tg) mouse lines of myc-WT (wild type) and myc-E22K mutant of human ventricular RLC and have examined the functional consequences of this FHC mutation in skinned cardiac-muscle preparations. In longitudinal sections of whole mouse hearts stained with hematoxylin and eosin, the E22K-mutant hearts of 13-month-old animals showed signs of inter-ventricular septal hypertrophy and enlarged papillary muscles with no filament disarray. Echo examination did not reveal evidence of cardiac hypertrophy in Tg-E22K mice compared to Tg-WT or Non-Tg hearts. Physiological studies utilizing skinned cardiac-muscle preparations showed an increase by ΔpCa50≥0.1 in Ca2+ sensitivity of myofibrillar ATPase activity and force development in Tg-E22K mice compared with Tg-WT or Non-Tg littermates. Our results suggest that E22K-linked FHC is mediated through Ca2+-dependent events. The FHC-mediated structural perturbations in RLC that affect Ca2+ binding properties of the mutated myocardium are responsible for triggering the abnormal function of the heart that in turn might initiate a hypertrophic process and lead to heart failure.

Myosin binding protein C: structural abnormalities in familial hypertrophic cardiomyopathy

The muscle protein myosin binding protein C (MyBPC) is a large multi-domain protein whose role in the sarcomere is complex and not yet fully understood. Mutations in MyBPC are strongly associated with the heart disease familial hypertrophic cardiomyopathy (FHC) and these experiments of nature have provided some insight into the intricate workings of this protein in the heart. While some regions of the MyBPC molecule have been assigned a function in the regulation of muscle contraction, the interaction of other regions with various parts of the myosin molecule and the sarcomeric proteins, actin and titin, remain obscure. In addition, several intra-domain interactions between adjacent MyBPC molecules have been identified. Although the basic structure of the molecule (a series of immunoglobulin and fibronectin domains) has been elucidated, the assembly of MyBPC in the sarcomere is a topic for debate. By analysing the MyBPC sequence with respect to FHC-causing mutations it is possible to identify individual residues or regions of each domain that may be important either for binding or regulation. This review looks at the current literature, in concert with alignments and the structural models of MyBPC, in an attempt to understand how FHC mutations may lead to the disease state.

Genetic and phenotypic characterization of mutations in myosin-binding protein C (MYBPC3) in 81 families with familial hypertrophic cardiomyopathy: total or partial haploinsufficiency

Mutations in the MYBPC3 gene, encoding the sarcomere protein myosin-binding protein C, are among the most frequent causes of autosomal dominant familial hypertrophic cardiomyopathy (FHC). We studied the frequency, type, and pathogenetic mechanism of MYBPC3 mutations in an unselected cohort of 81 FHC families, consecutively enrolled at a tertiary referral center. Nine mutations, six of which were novel, were found in 10 (12.3%) of the families using single-strand conformation polymorphism and DNA sequencing. A frameshift mutation in exon 2 clearly suggests that haploinsufficiency is a pathogenetic mechanism in FHC. In addition, splice site mutations in exon 6 and intron 31, a deletion in exon 13, and a nonsense mutation in exon 25, all lead to premature termination codons, most likely causing loss of function and haploinsufficiency. Furthermore, there were two missense mutations (D228N and A833 T) and one in-frame deletion (DeltaLys813). A considerable intrafamilial variation in phenotypic expression of MYBPC3-based FHC was noted, and we suggest that mutations influencing stability of mRNA could play a role in the variable penetrance and expressivity of the disease, perhaps via partial haploinsuffciency.

One third of Danish hypertrophic cardiomyopathy patients have mutations in MYH7 rod region

Familial hypertrophic cardiomyopathy (FHC) is, in most cases, a disease of the sarcomere, caused by a mutation in one of 10 known sarcomere disease genes. More than 266 mutations have been identified since 1989. The FHC disease gene first characterized MYH7, encodes the cardiac beta-myosin heavy chain, and contains more than 115 of these mutations. However, in most studies, only the region encoding the globular head and the hinge region of the mature cardiac beta-myosin heavy chain have been investigated. Furthermore, most studies carries out screening for mutations in the most prevalent disease genes, and discontinues screening when an apparent disease-associated mutation has been identified. The aim of the present study was to screen for mutations in the rod region of the MYH7 gene in all probands of the cohort, regardless of the known genetic status of the proband. Three disease-causing mutations were identified in the rod region in four probands using capillary electrophoresis single-strand conformation polymorphism as a screening method. All mutations were novel: N1327K, R1712W, and E1753K. Two of the probands had already been shown to carry other FHC-associated mutations. In conclusion, we show that in the Danish cohort we find one third of all MYH7 mutations in the rod-encoding region and we find that two of the patients carrying these mutations also carry mutations in other FHC disease genes stressing the need for a complete screening of all known disease genes in FHC-patients.

A point mutation in the regulatory light chain reduces the step size of skeletal muscle myosin

Current evidence favors the theory that, when the globular motor domain of myosin attaches to actin, the light chain binding domain or "lever arm" rotates, and thereby generates movement of actin filaments. Myosin is uniquely designed for such a role in that a long alpha-helix (approximately 9 nm) extending from the C terminus of the catalytic core is stabilized by two calmodulin-like molecules, the regulatory light chain (RLC) and the essential light chain (ELC). Here, we introduce a single-point mutation into the skeletal myosin RLC, which results in a large (approximately 50%) reduction in actin filament velocity (V(actin)) without any loss in actin-activated MgATPase activity. Single-molecule analysis of myosin by optical trapping showed a comparable 2-fold reduction in unitary displacement or step size (d), without a significant change in the duration of the strongly attached state (tau(on)) after the power stroke. Assuming that V(actin) approximately d/tau(on), we can account for the change in velocity primarily by a change in the step size of the lever arm without incurring any change in the kinetic properties of the mutant myosin. These results suggest that a principal role for the many light chain isoforms in the myosin II class may be to modulate the flexural rigidity of the light chain binding domain to maximize tension development and movement during muscle contraction.

Familial Hypertrophic Cardiomyopathy-linked Alterations in Ca2+ Binding of Human Cardiac Myosin Regulatory Light Chain Affect Cardiac Muscle Contraction

The ventricular isoform of human cardiac regulatory light chain (HCRLC) has been shown to be one of the sarcomeric proteins associated with familial hypertrophic cardiomyopathy (FHC), an autosomal dominant disease characterized by left ventricular and/or septal hypertrophy, myofibrillar disarray, and sudden cardiac death. Our recent studies have demonstrated that the properties of isolated HCRLC could be significantly altered by the FHC mutations and that their detrimental effects depend upon the specific position of the missense mutation. This report reveals that the Ca(2+) sensitivity of myofibrillar ATPase activity and steady-state force development are also likely to change with the location of the specific FHC HCRLC mutation. The largest effect was seen for the two FHC mutations, N47K and R58Q, located directly in or near the single Ca(2+)-Mg(2+) binding site of HCRLC, which demonstrated no Ca(2+) binding compared with wild-type and other FHC mutants (A13T, F18L, E22K, P95A). These two mutants when reconstituted in porcine cardiac muscle preparations increased Ca(2+) sensitivity of myofibrillar ATPase activity and force development. These results suggest the importance of the intact Ca(2+) binding site of HCRLC in the regulation of cardiac muscle contraction and imply its possible role in the regulatory light chain-linked pathogenesis of FHC.

Mechanical defects of muscle fibers with myosin light chain mutants that cause cardiomyopathy

Familial hypertrophic cardiomyopathy is a disease caused by single mutations in several sarcomeric proteins, including the human myosin ventricular regulatory light chain (vRLC). The effects of four of these mutations (A13T, F18L, E22K, and P95A) in vRLC on force generation were determined as a function of Ca(2+) concentration. The endogenous RLC was removed from skinned rabbit psoas muscle fibers, and replaced with either rat wildtype vRLC or recombinant rat vRLC (G13T, F18L, E22K, and P95A). Compared to fibers with wildtype rat vRLC, the E22K mutant increased Ca sensitivity of force generation, whereas the G13T and F18L mutants decreased the Ca sensitivity, and the P95A mutant had no significant effect. None of the RLC mutants affected the maximal tension (observed at saturating Ca(2+) concentrations), except for F18L, which decreased the maximal tension to 69 +/- 10% of the wildtype value. Of the mutant RLCs, only F18L decreased the cooperativity of activation of force generation. These results suggest that the primary cause of familial hypertrophic cardiomyopathy, in some cases, is perturbation in the Ca sensitivity of force generation, in which Ca-sensitizing or Ca-desensitizing effects can lead to similar disease phenotypes.

High-throughput single strand conformation polymorphism mutation detection by automated capillary array electrophoresis: validation of the method

Capillary array electrophoresis (CAE) is a novel technique, which allows for high throughput analysis of DNA fragments. When screening for mutations in whole populations or large patient groups it is necessary to have robust and well-characterized setups for high throughput analysis. For large-scale mutation screening, we have developed procedures for single strand conformation polymorphism (SSCP) assays using CAE (CAE-SSCP) whereby we may increase both the sensitivity and the throughput compared to conventional SSCP analysis. In this study we have validated CAE-SSCP by 1) comparing detection by slab-gel based SSCP with CAE-SSCP of mutations in the MYH7, MYL2, and MYL3 genes encoding sarcomere proteins from patients suffering from hypertrophic cardiomyopathy; and 2) by constructing a series of 185 mutants having substitution mutations, as well as insertion/deletion mutations, or some combinations of these, in different sequence contexts in four exons and different positions relative to the end of the amplicon (three from the KCNQ1 gene, encoding a cardiac potassium channel, and one from the TNNI3 gene encoding cardiac troponin I). The method identified 181 out of 185 mutations (98%), and the data suggest that the position of mutation in the fragment had no effect on the sensitivity. Analysis of the specificity of the method showed that only very few mutants could not be distinguished from each other and there were no false positives.

Systematic analysis of the regulatory and essential myosin light chain genes: genetic variants and mutations in hypertrophic cardiomyopathy

Hypertrophic cardiomyopathy (HCM) can be caused by mutations in genes encoding for the ventricular myosin essential and regulatory light chains. In contrast to other HCM disease genes, only a few studies describing disease-associated mutations in the myosin light chain genes have been published. Therefore, we aimed to conduct a systematic screening for mutations in the ventricular myosin light chain genes in a group of clinically well-characterised HCM patients. Further, we assessed whether the detected mutations are associated with malignant or benign phenotype in the respective families. We analysed 186 unrelated individuals with HCM for the human ventricular myosin regulatory (MYL2) and essential light chain genes (MYL3) using polymerase chain reaction, single strand conformation polymorphism analysis and automated sequencing. We found eight single nucleotide polymorphisms in exonic and adjacent intronic regions of MYL2 and MYL3. Two MYL2 missense mutations were identified in two Caucasian families while no mutation was found in MYL3. The mutation Glu22Lys was associated with moderate septal hypertrophy, a late onset of clinical manifestation, and benign disease course and prognosis. The mutation Arg58Gln showed also moderate septal hypertrophy, but, in contrast, it was associated with an early onset of clinical manifestation and premature sudden cardiac death. In conclusion, myosin light chain mutations are a very rare cause of HCM responsible for about 1% of cases. Mutations in MYL2 could be associated with both benign and malignant HCM phenotype.