Distinguish mutation-specific pathogenic effects from secondary disease remodelling: an essential but often overlooked concept

This editorial refers to ‘Hypercontractile mutant of ventricular myosin essential light chain leads to disruption of sarcomeric structure and function and results in restrictive cardiomyopathy in mice’ by C. C. Yuan et al., 1124–1136. Cardiomyopathies can be classified according to their morphological changes and the three most prevalent classes are hypertrophic cardiomyopathy (HCM), dilated cardiomyopathy (DCM), and restrictive cardiomyopathy (RCM). In most cases, mutations in sarcomeric and non-sarcomeric genes are found and there is a family history of cardiomyopathy.1 However, most affected genes implicated in one of the cardiomyopathies are also implicated in other forms of cardiomyopathy.1 Therefore, it is often difficult to assess the exact pathogenicity of the identified gene mutations. A central question in cardiomyopathy research is how gene mutations cause cardiac dysfunction and lead to the observed clinical phenotype.2 Studies on the pathogenicity of variants often attribute all observed changes compared with controls to the mutation studied. However, heart failure in general leads to activation of various signalling cascades leading to secondary disease remodelling such as reduced myofibril density, induction of fibrosis, or alterations in phosphorylation of proteins and cell morphology. The secondary remodelling may be an attempt to preserve cardiac function in response to mutation-induced dysfunction or may be pathogenic and contribute to severity of the disease. Both adaptive and maladaptive secondary changes in the heart will obscure the actual effect of the mutation. Yuan et al.3 report on a mutation in the thick filament protein myosin essential light chain (E143K), which has been associated with RCM in humans. While only 55% of mutant E143K protein was expressed, mice showed RCM features similar as has been observed in human, who only show RCM symptoms when carrying a homozygous mutation. Mutations in genes encoding the myosin light chains account for ∼1–5% of all genetic cardiomyopathies and are associated with HCM, DCM, and RCM, making genotype-phenotype correlations extremely difficult. The interaction between actin and myosin is fine-tuned by the myosin light chains, which in the ventricle exist of two isoforms: the essential light chains (ELC) and the regulatory light chains (RLC).4 Studies have shown that both isoforms can be phosphorylated,4,5 and defects in light chain phosphorylation have a negative impact on the heart evidenced by severe cardiac dysfunction and the development of cardiomyopathy as outlined in the paper of Yuan and colleagues. Previous studies on ELC and RLC mutations associated with HCM and DCM showed diverse mutation effects, which were partly attributed to the different locations of the mutation, either in the N-or C-terminal domains of ELC and RLC.6 Apart from the direct mutation effect on cardiomyocyte function, a study on HCM-associated ELC mutations revealed reduced binding of mutant ELCs to myosin as possible disease mechanism.7 Yuan et al.3 observed hypercontractility and increased passive tension in muscle strips from a transgenic mouse model of RCM overexpressing mutant E143K. In order to determine if the hypercontractile cellular phenotype was truly a direct mutation effect they performed an extensive set of experiment to assess morphology, interfilament lattice spacing and the interaction between mutant myosin and actin. The combination of observations such as the higher affinity of E143K-myosin to pyrene-actin and increased actin-activated myosin ATPase activity and decreased interfilament lattice spacing in E143K mice compared with wild-type mice support that the hypercontractility and increased passive tension are direct effects of the mutation. Preserved troponin I phosphorylation ruled out that the increased contractility was due to increased calcium sensitivity caused by troponin I hypophosphorylation in response to impaired β-adrenergic receptor signalling, which is frequently observed in cardiomyopathy models.8–10 Interestingly, the E143K mutation in ELC coincided with reduced phosphorylation of the RLC, which may indicate a complex interaction between myosin light chain mutations and phosphorylation (Figure 1). While the authors stated that remodelling, through hypophosphorylation of RLC, may limit the direct mutation effect on contractility, they acknowledge that hypophosphorylation of RLC might also contribute to cardiac dysfunction since it has been associated with development of hypertrophy, fibrosis, and DCM.11 Reduced RLC phosphorylation can therefore not be considered a solely good partner in crime. In addition to the direct mutation effects on contractility in the E413K mice, secondary disease remodelling further impaired cardiac function through induction of fibrosis and metabolic reprogramming. A careful assessment of mutation-specific effects and secondary (mal) adaptive changes, as done in the study by Yuan et al.,3 may shed light on future therapeutic options. While the initial mutation-induced hypercontractility may be target for therapy at an early disease stage, hypophosphorylation and secondary remodelling including fibrosis and perturbed metabolism may be better treatment options at an advanced disease stage. Schematic representation of secondary disease remodelling (left) and changes shown by ELC E143K mutation (right). Secondary disease remodelling has been observed in various cardiomyopathies and involves reduced cTnI phosphorylation due to down regulation of the β-adrenergic receptor. Also MHC isoform switch has been observed as well as changes in ELC and RLC phosphorylation. However, Yuan et al. showed a direct pathogenic effect of the ELC E143K mutation by alterations in actin/myosin interaction. In addition, a downregulation of α-MHC has been observed. RLC phosphorylation was reduced while cTnI phosphorylation was preserved which implies that the ELC E143K mutation might interfere with RLC phosphorylation independent of changes in the β-adrenergic system. Therefore, it seems that the ELC E143K mutation causes direct pathogenic effects and induces additional secondary changes. Conflict of interest: none declared.