Evidence from human myectomy samples that MYBPC3 mutations cause hypertrophic cardiomyopathy through haploinsufficiency.
BACKGROUND
Most sarcomere gene mutations that cause hypertrophic cardiomyopathy are missense alleles that encode dominant negative proteins. The potential exceptions are mutations in the MYBPC3 gene (encoding cardiac myosin-binding protein-C [MyBP-C]), which frequently encode truncated proteins.


OBJECTIVE
We sought to determine whether there was evidence of haploinsufficiency in hypertrophic cardiomyopathy caused by MYBPC3 mutations by comparing left ventricular muscle from patients undergoing surgical myectomy with samples from donor hearts.


RESULTS
MyBP-C protein and mRNA levels were quantitated using immunoblotting and RT-PCR. Nine of 37 myectomy samples had mutations in MYBPC3: 2 missense alleles (Glu258Lys, Arg502Trp) and 7 premature terminations. No specific truncated MyBP-C peptides were detected in whole muscle homogenates of hypertrophic cardiomyopathy tissue. However, the overall level of MyBP-C in myofibrils was significantly reduced (P<0.0005) in tissue containing either a truncation or missense MYBPC3 mutation: 0.76+/-0.03 compared with 1.00+/-0.05 in donor and 1.01+/-0.06 in non-MYBPC3 mutant myectomies.


CONCLUSIONS
The absence of any detectable truncated MyBP-C argues against its incorporation in the myofiber and any dominant negative effect. In contrast, the lowered relative level of full length protein in both truncation and missense MYBPC3 mutations argues strongly that haploinsufficiency is sufficient to cause the disease.
Marston
Steven
S
Department of Cardiovascular Medicine, University of Oxford, Level 6 West Wing, John Radcliffe Hospital, Oxford OX39DU, United Kingdom.
Copeland
O'Neal
O
Jacques
Adam
A
Livesey
Karen
K
Tsang
Victor
V
McKenna
William J
WJ
Jalilzadeh
Shapour
S
Carballo
Sebastian
S
Redwood
Charles
C
Watkins
Hugh
H
D000483:Alleles; D002352:Carrier Proteins / Q000235:genetics* / Q000378:metabolism; D016022:Case-Control Studies; D005838:Genotype; D006238:Haploidy; D006352:Heart Ventricles / Q000378:metabolism / Q000601:surgery*; D006801:Humans; D017379:Hypertrophy, Left Ventricular / Q000235:genetics* / Q000378:metabolism; D020125:Mutation, Missense / Q000235:genetics*; D009206:Myocardium / Q000378:metabolism; D012333:RNA, Messenger; D000099163:Myosin Binding Protein C
D002352:Carrier Proteins; D012333:RNA, Messenger; D000099163:Myosin Binding Protein C