Hypertrophic cardiomyopathy (HCM) is a primary myocardium disease characterized by an asymmetric increase in heart muscle mass, predominantly in the interventricular septum, hypertrophied cardiocytes, cellular and sarcomeric disarray, and fibrosis [1]. Major functional impairments are compromised relaxation and hypercontractility [2]. In most cases, the disease has a genetic cause that leads to an autosomal dominant trait being transmitted throughout families. Occasionally, a de novo variant may be responsible for the phenotype in previously unaffected families. Significant contributors to the disease are mutations in the beta-myosin heavy chain (MYH7) and the cardiac isoforms of myosin binding protein-C (MYBPC3). These two genes may account for about two-thirds of all genetically confirmed cases [3]. Most genes that possess pathogenic variants code for proteins constituting the sarcomere or contributing to controlling contraction.

The heterogeneity of inherited causes of HCM corresponds to a broad spectrum of highly variable phenotypes. Mild cases with onset of symptoms in the fourth decade of life or even later contrast with early onset, severe symptoms, and a high risk of sudden cardiac death. The underlying genetic defect can partly explain the variability in the phenotype; however, genetic modifiers and polymorphisms may modulate the functional consequences of the mutation [4]. Thus, it is frequently difficult to predict the clinical outcome if a diagnosis has been established early or before the symptomatic stage. Therefore, familial case studies are frequently useful to improve our understanding of the biological consequences of a genetic alteration. Here, we document the new likely pathogenic variant together with its clinical phenotype and discuss the functional implications that this variant could have.

We have identified a novel MYH7 variant in two patients from a nuclear family, a father and son, who were both affected by hypertrophic cardiomyopathy and cardiac failure. The mutation was a c.2141T$>$G transversion, resulting in the replacement of leucine by arginine in codon 714. The variant was found in the two patients but not in the unaffected brother of the proband. Since both patients had early onset variations of the disease, relatively fast progression of symptoms, and premature cardiac death, the cardiomyopathy course in this family can be described as “malignant”.

However, the contribution of the mutation to the phenotype cannot easily be assessed because both patients had additional conditions not generally associated with HCM. The father was suffering from recurrent pericarditis for many years with apparently serious consequences for cardiac performance. Volume overload caused by or associated with HCM and pulmonary hypertension presumably contributed to a vicious cycle resulting in end-stage failure. The son was born with a congenital defect, which led to hemodynamic consequences. To summarize the clinical evaluation, these patients were exposed not only to HCM but to confounding conditions, adding to the myocardial dysfunction caused by the myosin heavy chain mutation. However, the uncommonly severe phenotype in both patients is a solid reason to argue for a severe characterization of the 714 mutation. Sometimes, the severe disease course may be explained by more than one pathogenic variant in the family or additional variants in other genes that are not pathogenic but have some modifying effect [3, 12]. However, sequencing of the coding regions for 46 cardiomyopathy-associated genes in the proband did not find candidate variants with likely functional effects. It should be noted that the possible genetic cause for the congenital heart defect in the son was not investigated.

Exon 19 MYH7 contributes to the part of the C-terminal end of the myosin head known as the “converter domain” [13]. This region consists of a stretch of 68 amino acids (in the human beta-myosin chain residues 709 to 777) thought to serve as a socket for the adjacent (C-terminal) alpha-helical extension (or “neck”) stabilized by two light chains. It has been concluded from the model building as well as from functional assays that the “neck” is in the correct orientation and position to act as a lever (for review and references, see [14]). The converter contributes to the “power stroke” of the contractile cycle by, presumably, transducing a conformational change resulting from ATP cleavage in the globular myosin head to the alpha-helical “lever arm” extending from the C-terminus of the head. The position between the myosin head and the lever arm may function as a “relay station” between the ATP cleavage center and the lever arm. Internal elastic distortions of the converter domain may be how the converter mediates a reorientation (“swinging”) of the lever arm. That swinging contributes to the translocation of actin along myosin filaments. The function of the lever in terms of speed of actin transport promoted by myosin heads depends on the length of the neck. The addition of light chain binding sites increased the speed of actin movement in vitro, and truncations led to a decrease, as was shown by exposing actin filaments to myosin heads of Dictyostelium discoideum immobilized on a solid surface [15]. Altered functions and pleiotropic effects, including changes in ATPase activity patterns, have also been demonstrated with truncated converter regions of myosin II from Dictyostelium discoideum [16]. These experiments strongly suggest that the converter region and the adjacent lever are essential for the motor system. Recently, it has been shown that conformational change in the converter is integral to the mechanochemical coupling of myosin, particularly for the ADP release step [17].

A query to the ClinVar database [6] (accessed September 2023) provided information on more than 90 different amino acid variants in 48 positions within the converter region (71% of the converter amino acids), classified from uncertain significance to pathogenic. In the first 16 amino acids (709–726), at least one missense variant was described in each position (except 709 and 711), and altogether with p.Leu714Arg, there are 33 different amino acid variations in patients for these 18 positions (Fig. 3A, Ref. [6, 18, 19, 20]). It means that this converter region is frequently mutated in cardiomyopathies. Indeed, analysis of 2913 patients with HCM allowed us to identify significant enrichment of disease-associated variants in the converter, with earlier disease onset in the carriers of these variants [18]. According to the summary of published and own data provided in [18], among 526 patients with 25 different mutations in the converter, 407 had HCM (77%), the mean age of diagnosis was 8 years, and 97% of the variant carriers were symptomatic at the age of 35; in addition, 184 patients (37%) had either sudden cardiac death or cardiac transplantation [19]. A recent study showed that variants in the converter domain have significantly higher penetrance than variants in other MYH regions [20].

Fig. 3.

Missense substitutions in the MYH7 converter domain and localization of Leu714 in the myosin molecular structure. (A) A summary of known missense variants in amino acids 709-777 in MYH7, according to the ClinVar database [6]. The amino acid variants are classified as pathogenic (red), likely pathogenic (dark red), “conflicting interpretations of pathogenicity” (lilac), or uncertain significance (black). Leu714 (L) and the novel Arg714 variant (R) are highlighted in green. (B) Secondary structure of part of MYH7 motor domain: (1) myosin active center bound with Mg${}^{2+}$$\cdot$ ADP; (2) converter domain with highlighted Leu714; (3) essential light myosin chain. (C) Enlarged part of the converter region. Images B and C are from the RCSB PDB (RCSB.org) [21]; PDB ID 8EFE (https://doi.org/10.2210/pdb8EFE/pdb) [17], created using Mol* Viewer [22].

Some experimental studies analyzed the consequences of mutations in the converter. For instance, using isolated muscle fibers (obtained from m. soleus of an HCM patient suffering from a p.Arg719Trp mutation), increased generation of force, and fiber stiffness were demonstrated as a consequence of the Arg to Trp exchange [23]. M. soleus expresses the same beta-myosin isoform as the myocardium. Stiffness under rigorous conditions and during relaxation and force development increased by similar amounts (about 50%). Increased fiber stiffness would imply a reduced ability by the myosin molecule to switch back to the original conformation at the end of one power stroke. The authors suggested that the converter is a sub-domain in myosin within which the transient structural change is needed to drive the action of the lever arm [23]. Further studies demonstrated that the p.Arg723Gly variant has a similar effect on the fiber stiffness, whereas p.Ile736Thr has essentially no effect [24, 25]. Other investigators showed slightly decreased contractile force but 15% faster velocity for the Arg719Trp and Arg723Gly variants; however, no change was noted in these characteristics for the Gly741Arg variant. In the transgenic Drosophila model carrying the p.Arg713Glu mutation, the mutants showed no actin motility, and it was demonstrated that the mutation disrupts interdomain interaction, namely with the Glu497 residue in the myosin “relay” domain [26]. The Leu714 is close to the 712 and 719 positions, at which the former participates in the myosin interdomain interactions, and the latter leads to the reported change in fiber stiffness. It may be hypothesized that the functional consequences of the p.Leu714Arg transition are similar to the substitutions of neighboring amino acids.

Molecular modeling shows that p.Leu714 constitutes the last part of the $\beta$-sheet before $\alpha$-spiral in the converter domain (Fig. 3B,C), and the variant increases the positive charge in this region by replacing the non-charged leucine with a positive-charged arginine. The closest known variant with a similar effect is p.Gly716Arg, which has been reported in several publications. Some publications provide clinical information about patients with this variant. Indeed, the first report was in 1994, when one of three affected individuals required cardiac transplantation [27]. In one family from Korea, four cases of premature sudden death and two children with full expression of the disease were seen in a pedigree with 13 affected members, all carrying this variant with complete penetrance [28]. The variant was also associated with sudden cardiac death in an American family, and the proband received an internal cardioverter defibrillator [29]. Then, the variant was identified as a de novo mutation in an Indian family, with sudden cardiac death of the proband at the age of 21 years [30]. Another de novo mutation, p.Gly716Arg, was reported in a Chinese family, and analysis of available published data on this variant (altogether 43 patients) showed that p.Gly716Arg is characterized by complete penetrance and extremely poor prognosis, with a median survival age of 42 years and 25 cases of sudden cardiac death [31]. Therefore, it could be proposed that a charge change in this part of the converter region seems to cause “malignant” HCM. Alternatively, according to the ClinVar database, two variations in the codon 714 were recently reported, p.Leu714Ile and p.Leu714Pro [32], and both were classified as variants of uncertain significance. However, it was noted that they do not change the molecular charge. It might be proposed that clinically severe phenotypes mark the majority of converter mutations in the region at or near 714, with a high risk of life-threatening complications. Using epidemiological inference, published evidence suggests a common or at least related mechanism for these mutations. Based on this suggestion, we propose that the p.Leu714Arg mutation is responsible for a similarly severe dysfunction in the converter domain as the p.Gly716Arg mutation.

A new likely pathogenic variant in MYH7 (NM_000257.4):c.2141T$>$G leading to amino acid replacement p.Leu714Arg in the cardiac beta-myosin heavy chain has been identified in two related patients suffering from “malignant” hypertrophic cardiomyopathy. Analysis of the mutation spectrum in this gene showed that the substitution is located in the critical functional converter domain, marked by a high rate of functionally relevant variants. In most cases, mutations in the converter myosin domain lead to early-onset hypertrophic cardiomyopathy with a high risk of life-threatening complications.

Data sharing is not applicable to this article, as no datasets were generated or analyzed during the current study.

MVG designed and performed the research, analysed the data, wrote the manuscript. ENP and KVP peformed diagnostics and observation of the patients, collected clinical data, prepared and discussed clinical section of the manuscript. OAM participated conducting the experiments, and discussing the results. RRS and OSG performed NGS experiments and interpreted the results. VPP and MSN planned and designed the research, discussed and edited the manuscript. All authors contributed to editorial changes in the manuscript. All authors read and approved the final manuscript. All authors have participated sufficiently in the work and agreed to be accountable for all aspects of the work.

Informed consent was obtained from the participants of the study. The study was conducted in accordance with the Declaration of Helsinki and approved by the Local Biomedical Ethics Committee at the Research Institute of Medical Genetics, Tomsk National Research Medical Center (protocol # 10, February 15, 2021) and the Local Biomedical Ethics Committee at the Cardiology Research Institute Tomsk National Research Medical Center (protocol # 151, December 22, 2016).

The authors dedicate this article to the memory of Professor Hans-Peter Vosberg, who made a significant contribution to the genetic studies of hypertrophic cardiomyopathy and who participated in writing the first version of the manuscript.

The study was performed as a part of basic research program of the Institute of Medical Genetics, Tomsk NRMC. M.V.G. received a postdoc fellowship of Max-Planck Society (Germany).

The authors declare no conflict of interest.