Cardiac myosin-binding protein C mutations and hypertrophic cardiomyopathy: haploinsufficiency, deranged phosphorylation, and cardiomyocyte dysfunction.
BACKGROUND
Mutations in the MYBPC3 gene, encoding cardiac myosin-binding protein C (cMyBP-C), are a frequent cause of familial hypertrophic cardiomyopathy. In the present study, we investigated whether protein composition and function of the sarcomere are altered in a homogeneous familial hypertrophic cardiomyopathy patient group with frameshift mutations in MYBPC3 (MYBPC3(mut)).


RESULTS
Comparisons were made between cardiac samples from MYBPC3 mutant carriers (c.2373dupG, n=7; c.2864_2865delCT, n=4) and nonfailing donors (n=13). Western blots with the use of antibodies directed against cMyBP-C did not reveal truncated cMyBP-C in MYBPC3(mut). Protein expression of cMyBP-C was significantly reduced in MYBPC3(mut) by 33+/-5%. Cardiac MyBP-C phosphorylation in MYBPC3(mut) samples was similar to the values in donor samples, whereas the phosphorylation status of cardiac troponin I was reduced by 84+/-5%, indicating divergent phosphorylation of the 2 main contractile target proteins of the beta-adrenergic pathway. Force measurements in mechanically isolated Triton-permeabilized cardiomyocytes demonstrated a decrease in maximal force per cross-sectional area of the myocytes in MYBPC3(mut) (20.2+/-2.7 kN/m(2)) compared with donor (34.5+/-1.1 kN/m(2)). Moreover, Ca(2+) sensitivity was higher in MYBPC3(mut) (pCa(50)=5.62+/-0.04) than in donor (pCa(50)=5.54+/-0.02), consistent with reduced cardiac troponin I phosphorylation. Treatment with exogenous protein kinase A, to mimic beta-adrenergic stimulation, did not correct reduced maximal force but abolished the initial difference in Ca(2+) sensitivity between MYBPC3(mut) (pCa(50)=5.46+/-0.03) and donor (pCa(50)=5.48+/-0.02).


CONCLUSIONS
Frameshift MYBPC3 mutations cause haploinsufficiency, deranged phosphorylation of contractile proteins, and reduced maximal force-generating capacity of cardiomyocytes. The enhanced Ca(2+) sensitivity in MYBPC3(mut) is due to hypophosphorylation of troponin I secondary to mutation-induced dysfunction.
van Dijk
Sabine J
SJ
Laboratory for Physiology, Institute for Cardiovascular Research, VU University Medical Center, Amsterdam, The Netherlands.
Dooijes
Dennis
D
dos Remedios
Cris
C
Michels
Michelle
M
Lamers
Jos M J
JM
Winegrad
Saul
S
Schlossarek
Saskia
S
Carrier
Lucie
L
ten Cate
Folkert J
FJ
Stienen
Ger J M
GJ
van der Velden
Jolanda
J
D000328:Adult; D000368:Aged; D001706:Biopsy; D002118:Calcium / Q000378:metabolism; D024741:Cardiomyopathy, Hypertrophic, Familial / Q000235:genetics* / Q000378:metabolism*; D002352:Carrier Proteins / Q000235:genetics* / Q000378:metabolism*; D005260:Female; D016368:Frameshift Mutation; D006239:Haplotypes; D006801:Humans; D008297:Male; D008875:Middle Aged; D009200:Myocardial Contraction / Q000502:physiology; D009206:Myocardium / Q000378:metabolism / Q000473:pathology; D032383:Myocytes, Cardiac / Q000378:metabolism / Q000473:pathology; D010766:Phosphorylation / Q000502:physiology; D012333:RNA, Messenger / Q000378:metabolism; D012518:Sarcomeres / Q000378:metabolism; D000099163:Myosin Binding Protein C
D002118:Calcium; D002352:Carrier Proteins; D012333:RNA, Messenger; D000099163:Myosin Binding Protein C