Mutations of the beta myosin heavy chain gene in hypertrophic cardiomyopathy: critical functional sites determine prognosis

Abstract

Objectives: To assess patients with different types of mutations of the beta myosin heavy chain (beta MHC) gene causing hypertrophic cardiomyopathy (HCM) and to determine the prognosis of patients according to the affected functional domain of beta MHC.

Design and setting: Cohort study of subjects referred to an HCM clinic at an academic hospital.

Patients: 70 probands from the HCM clinic were screened for mutations of the beta MHC gene and 148 family members of the genotype positive probands were further assessed. The control group for the genetic studies consisted of 106 healthy subjects.

Main outcome measures: Direct DNA sequencing was used to screen 70 probands for mutations of the beta MHC gene. Family members underwent genotypic and detailed clinical, ECG, and echocardiographic assessments. The survival of genotype positive subjects was evaluated according to the type of functional domain affected by the missense mutation and according to phenotypic characteristics.

Results: A mutation of the beta MHC gene was detected in 15 of 70 probands (21%). Of 148 family members studied in these 15 families, 74 were identified with a beta MHC defect. Eleven mutations were detected, including four novel mutations: Ala196Thr, Pro211Leu, Val404Leu, and Arg870Cys. Median survival was 66 years (95% confidence interval (CI) 64 to 77 years) in all affected subjects. There was a significant difference in survival between subjects according to the affected functional domain (p = 0.02). Significant independent predictors of decreased survival were the non-conservative (that is, associated with a change in the amino acid charge) missense mutations that affected the actin binding site (hazard ratio 4.4, 95% CI 1.6 to 11.8; p = 0.003) and those that affected the rod portion of beta MHC (hazard ratio 4.8, 95% CI 1.2 to 19.4; p = 0.03). No phenotypic characteristics were associated with decreased survival or cardiovascular morbidity.

Conclusions: The type of beta MHC functional domain affected by the missense mutation is predictive of overall prognosis in HCM.

Figure 1

Diagram of the functional domains of the β myosin heavy chain (MHC) and the relation of this sarcomeric protein to actin. The 11 mutations analysed in this study were detected in the following functional domains: (A) actin binding site, (B) active site (ATP binding site), (C) essential light chain binding interface, and (D) head–rod junction. Mutations were further subclassified according to the presence (+) or absence (0) of a change in the charge of the substituted amino acid.

Figure 2

Pedigrees of four families with hypertrophic cardiomyopathy (HCM) with representative β MHC mutations. Symbols indicate sex and disease status: box, male; circle, female; arrow = proband; darkened = clinical phenotype of HCM; clear = no diagnostic features (clinical, ECG, echocardiography) of HCM; slashed, deceased; HD40, HCM related death at age ⩽ 40 years; HD60, HCM related death at age ⩽ 60 years (deaths include aborted sudden death or cardiac transplantation). Family members were tested for the specific mutation detected in the proband. Plus (+) signs indicate the presence of the genetic defect and minus (−) signs indicate the absence of the mutation. (A) Pedigree of one family with non-conservative actin binding defect, the Arg403Gln mutation. Family is characterised by five premature HCM related deaths (including one early aborted sudden death and one cardiac transplant) and significant cardiovascular morbidity (see text and table 2). (B) Pedigree of family with conservative actin binding defect, the Val404Leu missense mutation. Two premature HCM related deaths have occurred among 12 affected subjects through four generations. (C) Pedigree of family with non-conservative rod defect, the Arg870Cys defect. Three premature deaths (average age at death 52 (7) years) have occurred in this kindred. (D) Pedigree of large kindred with conservative rod defect, the Leu908Val defect. There has been one premature death among 13 affected family members through four generations.

Figure 3

The four identified novel β MHC sequences are highly conserved among other myosin molecules noted in humans and other animals. The identified sequence variants were compared with known sequences of the following myosin molecules: (a) human cardiac β MHC, (b) hamster cardiac β MHC, (c) rat cardiac β MHC, (d) human cardiac α MHC, (e) rat cardiac α MHC, (f) hamster cardiac α MHC, (g) rabbit skeletal MHC, (h) human embryonic skeletal MHC, and (i) rat embryonic skeletal MHC. The numbers above the sequences indicate the amino acid locus of β MHC.

Figure 4

Kaplan-Meier (product limit) curves for the survival of patients with β MHC mutations in the study population. There was a significant difference in survival between subjects classified according to the affected functional domain and the charge change of the encoded amino acid (p=0.02, log rank test). Survival was significantly shortened in subjects with actin binding (+) or rod (+) defects. (0) indicates conservative amino acid substitution, (+) indicates non-conservative amino acid substitution. (1) Actin binding (0) defects: (— — —); (2) actin binding (+) defects: (▴—▴); (3) active site (0) defects: (); (4) active site (+) defects: (- - - - -); (5) rod (0) defects: (x..........x); and (6) rod (+) defects (• - - - - - •).