👤 Julia Münch

Hypertrophic cardiomyopathy (HCM) is an inherited cardiomyopathy often caused by pathogenic variants in MYBPC3 and MYH7, encoding myosin-binding protein C3 and myosin heavy chain 7, respectively. These variants can cause increased actin-myosin crossbridge cycling, resulting in ventricular hypercontractility, but mice lacking Mybpc3 exhibited reduced left ventricular ejection time (LVET) as a sign of systolic dysfunction. In this study, we tested whether LVET is specifically altered in patients carrying MYBPC3 variants by retrospective echocardiographic analysis in two genotype-defined HCM cohorts. LVET was measured by echocardiography and adjusted for heart rate [LVET index (LVETI)] in 166 patients. Variant carriers were stratified for the presence (LVH+) or absence of left ventricular hypertrophy with septal thickness of ≥13 mm (LVH-). Multivariate analysis of variance (MANOVA) was used to identify differences in LVETI between variant carriers and controls with LVETI as the dependent variable, adjusted for sex, age, left ventricular ejection fraction (LVEF), interventricular septal diameter in diastole (IVSd), diastolic dysfunction, left ventricular outflow tract (LVOT) gradient at rest and medication history as confounders. In a total of 166 patients carrying MYBPC3 or MYH7 pathogenic variants (38 ± 3 years, 45% female), we compared the discovery cohort (40 MYBPC3 and 31 MYH7) and the validation cohort ('Valsartan in Attenuating Disease Evolution in Early Sarcomeric HCM'; 54 MYBPC3 and 41 MYH7) with 44 healthy controls. LVETI was lower in MYBPC3 and higher in MYH7 LVH+ patients than in controls in the discovery, validation and pooled cohorts (pooled: MYBPC3 381 ± 19 ms vs. MYH7 437 ± 38 ms, P < 0.001; MYBPC3 vs. controls 411 ± 15 ms, P < 0.001; and MYH7 vs. controls, P < 0.001). Similar findings were seen in LVH- (pooled: MYBPC3 380 ± 16 ms vs. MYH7 437 ± 39 ms, P < 0.001; MYBPC3 vs. controls, P < 0.001). While MYH7 variants were all missense as expected, 87% of the MYBPC3 variants were truncating (including nonsense variants, out-of-frame deletion and splice site variants) and 13% were non-truncating (missense and in-frame deletion). LVETI did not differ between the groups and was significantly lower than the control in both. The data suggest that variants in MYBPC3 and MYH7 result in distinct biophysical consequences, which can be detected by measuring LVETI in patients. The findings may have implications for potential genotype-specific differences in response to therapies targeting sarcomere function. Show less

MYBPC3 is the most frequently affected gene in hypertrophic cardiomyopathy (HCM), which is an autosomal-dominant cardiac disease caused by mutations in sarcomeric proteins. Bi-allelic truncating MYBPC3 mutations are associated with severe forms of neonatal cardiomyopathy. We reprogrammed skin fibroblasts from a HCM patient carrying a heterozygous MYBPC3 truncating mutation into human induced pluripotent stem cells (iPSC) and used CRISPR/Cas9 to generate bi-allelic MYBPC3 truncating mutation and isogenic control hiPSC lines. All lines expressed pluripotency markers, had normal karyotype and differentiated into endoderm, ectoderm and cardiomyocytes in vitro. This set of three lines provides a useful tool to study HCM pathomechanisms. Show less

Gene therapy is a promising option for severe forms of genetic diseases. We previously provided evidence for the feasibility of trans-splicing, exon skipping, and gene replacement in a mouse model of hypertrophic cardiomyopathy (HCM) carrying a mutation in MYBPC3, encoding cardiac myosin-binding protein C (cMyBP-C). Here we used human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) from an HCM patient carrying a heterozygous c.1358-1359insC MYBPC3 mutation and from a healthy donor. HCM hiPSC-CMs exhibited ∼50% lower MYBPC3 mRNA and cMyBP-C protein levels than control, no truncated cMyBP-C, larger cell size, and altered gene expression, thus reproducing human HCM features. We evaluated RNA trans-splicing and gene replacement after transducing hiPSC-CMs with adeno-associated virus. trans-splicing with 5' or 3' pre-trans-splicing molecules represented ∼1% of total MYBPC3 transcripts in healthy hiPSC-CMs. In contrast, gene replacement with the full-length MYBPC3 cDNA resulted in ∼2.5-fold higher MYBPC3 mRNA levels in HCM and control hiPSC-CMs. This restored the cMyBP-C level to 81% of the control level, suppressed hypertrophy, and partially restored gene expression to control level in HCM cells. This study provides evidence for (1) the feasibility of trans-splicing, although with low efficiency, and (2) efficient gene replacement in hiPSC-CMs with a MYBPC3 mutation. Show less

Hypertrophic cardiomyopathy (HCM) is an autosomal-dominant disease with mutations in genes encoding sarcomeric proteins. Previous findings suggest deregulation of the ubiquitin proteasome system (UPS) in HCM in humans and in a mouse model of HCM (Mybpc3-targeted knock-in (KI) mice). In this study we investigated transcript levels of several muscle-specific E3 ubiquitin ligases in KI mice and aimed at identifying novel protein targets. Out of 9 muscle-specific E3 ligases, Asb2β was found with the lowest mRNA level in KI compared to wild-type (WT) mice. After adenoviral-mediated Asb2β transduction of WT neonatal mouse cardiomyocytes with either a WT or inactive Asb2β mutant, desmin was identified as a new target of Asb2β by mass spectrometry, co-immunoprecipitation and immunoblotting. Immunofluorescence analysis revealed a co-localization of desmin with Asb2β at the Z-disk of the sarcomere. Knock-down of Asb2β in cardiomyocytes resulted in higher desmin protein levels. Furthermore, desmin levels were higher in ventricular samples of HCM mice and patients than controls. This study identifies desmin as a new Asb2β target for proteasomal degradation in cardiomyocytes and suggests that accumulation of desmin could contribute to UPS impairment in HCM mice and patients. Show less