The left and right ventricle of a patient with a R723G mutation of the beta-myosin heavy chain and severe hypertrophic cardiomyopathy show no differences in the expression of myosin mRNA

Background: In familial hypertrophic cardiomyopathy (FHC), asymmetric left ventricular (LV) hypertrophy has been considered to be the predominant phenotypic expression, whereas right ventricular (RV) involvement is still ambiguous. In most cases, the right ventricle remains unaffected until secondary pulmonary hypertension develops. Several FHC-causing mutations of genes encoding sarcomere-related proteins have been identified which are transmitted in an autosomal-dominant manner. Methods: We report the case of a 61 year old member of a Catalan family with a Arg723Gly missense mutation of the β-myosin heavy chain (β-MHC), that is associated with a malignant phenotype characterized by sudden cardiac death and heart failure. Because of progressive systolic LV dysfunction, the patient received a heart transplant in 2003. Results: Molecular analysis of the myocardial tissue of the explanted heart, taken from the left and right ventricle, showed a similar deviation of the ratio of mutant vs wild type mRNA of the β-MHC of 71.8 ± 5% and 68.5 ± 3%, respectively. This finding was confirmed for LV biopsies of this patient on protein level, showing a similar proportion of mutated β-myosin. But since the patient is heterozygous for the β-MHC mutation and the mutation is located in a coding region, the relative increase of the expression of the mutant allele is unexpected. It has been demonstrated before by our group for several β-MHC mutations that the relative abundance of mutated mRNA/protein correlates with the clinical severity of the disease. But since the right ventricle shows no (or only minor) manifestation in terms of hypertrophy or dysfunction, the level of mRNA and protein expression is not the only factor responsible for the development of the phenotype of FHC. Conclusions: Several mechanisms through which cardiac stresses may incite maladaptive cardiac remodeling primarily of the left ventricle that result in myocardial hypertrophy and heart failure are proposed. One of those triggers could be the enhanced work load of the left ventricle, especially if a LV outflow tract gradient is present, in contrast to the lesser demands to the right ventricle which is adapted to the low pressure system of the pulmonary circulation. Further studies are needed to confirm the results of this case, as well as functional studies involving both ventricles. (Cardiol J 2010; 17, 5: 518-522

Unequal allelic expression of wild-type and mutated β-myosin in familial hypertrophic cardiomyopathy

Familial hypertrophic cardiomyopathy (FHC) is an autosomal dominant disease, which in about 30% of the patients is caused by missense mutations in one allele of the β-myosin heavy chain (β-MHC) gene (MYH7). To address potential molecular mechanisms underlying the family-specific prognosis, we determined the relative expression of mutant versus wild-type MYH7-mRNA. We found a hitherto unknown mutation-dependent unequal expression of mutant to wild-type MYH7-mRNA, which is paralleled by similar unequal expression of β-MHC at the protein level. Relative abundance of mutated versus wild-type MYH7-mRNA was determined by a specific restriction digest approach and by real-time PCR (RT-qPCR). Fourteen samples from M. soleus and myocardium of 12 genotyped and clinically well-characterized FHC patients were analyzed. The fraction of mutated MYH7-mRNA in five patients with mutation R723G averaged to 66 and 68% of total MYH7-mRNA in soleus and myocardium, respectively. For mutations I736T, R719W and V606M, fractions of mutated MYH7-mRNA in M. soleus were 39, 57 and 29%, respectively. For all mutations, unequal abundance was similar at the protein level. Importantly, fractions of mutated transcripts were comparable among siblings, in younger relatives and unrelated carriers of the same mutation. Hence, the extent of unequal expression of mutated versus wild-type transcript and protein is characteristic for each mutation, implying cis-acting regulatory mechanisms. Bioinformatics suggest mRNA stability or splicing effectors to be affected by certain mutations. Intriguingly, we observed a correlation between disease expression and fraction of mutated mRNA and protein. This strongly suggests that mutation-specific allelic imbalance represents a new pathogenic factor for FHC