For the first time, a mouse model was developed that mimics one of the major characteristics of human familial hypertrophic cardiomyopathy (FHC). This model was designed by Lucie Carrier and her team at Inserm Unit 582. Familial hypertrophic cardiomyopathy (FHC) is a cardiac disease characterised by a left ventricular hypertrophy, usually asymmetric and predominantly affecting the interventricular septum (asymmetric septal hypertrophy). Added to this major phenotypic characteristic are myocardial disarray and increase in interstitial fibrosis in the cardiac tissue.

 

This pathology is transmitted in an autosomal dominant mode and most of the typical forms are associated with more than 200 different mutations in 12 genes encoding sarcomeric proteins, thus making this pathology a „sarcomeropathy.“
We have previously demonstrated the following results:

 

i) Mutations in the MYBPC3 gene encoding cardiac myosin-binding protein C (cMyBP-C), which is a myosin-biding protein, cause FHC (ref 1).

 

ii) Most of families present a mutation in the MYBPC3 gene encoding cMyBP-C (ref 2).

 

iii) Most of the MYBPC3 gene mutations disrupt the reading frame and are expected to produce C-terminal truncated proteins (ref 3)

 

iv) The truncated proteins are unstable ex vivo (ref 4) and in the myocardial tissue of FHC patients carrying a frameshift mutation (ref 5). This suggests that the “null allele” mechanism leading to cMyBP-C haploinsufficiency is likely involved in the pathogenesis of FHC related to MYBPC3 mutations.

 

Taken together, these results led us to develop a mouse model deficient in cMyBP-C and to analyse the heterozygous mice with a single functional allele as a model for haploinsufficiency. This model was developed in collaboration with Ken Chien’s team (UCSD, La Jolla, CA) by targeted transgenesis in embryonic stem cells. For this, exons 1 and 2 containing the transcription initiation site of the cMyBP-C gene were replaced by the neomycin-resistant gene. This led to the transcriptional inactivation of the gene.
The functional, morphological and molecular phenotype was analysed at different post-natal windows in both homozygous (complete deficiency) and heterozygous mice (partial deficiency). Our results demonstrate that the inactivation of one or both alleles of cardiac protein C leads to different cardiac phenotypes. The homozygous mice do not express cardiac protein C (Western-blot and immunohistochemistry) and develop eccentric left ventricular hypertrophy (hypertrophy of the walls and dilatation of the ventricular cavity) with a diminution of the ejection fraction at 3-4 months (echocardiography) and diastolic dysfunction after 9 months (hemodynamic measurements). This is associated with disarray of the myocardial tissue and increase in interstitial fibrosis (histology). The heterozygous mice (see results in the figure below) which possess only one functional allele, show a slight but significant decrease in cMyBP-C cardiac level in the heart. These mice slowly develop a moderate ventricular hypertrophy, detectable only around the age of 10-11 months. This hypertrophy is associated with a large increase in interstitial fibrosis and affects mainly the interventricular septum, as in human hypertrophic cardiomyopathy. These results show that mice deficient in cMyBP-C at the heterozygous state represent the first mouse model mimicking one of the major characteristics of human FHC, which is asymmetric left ventricular hypertrophy predominantly affecting the interventricular septum.

 

1. Bonne G, Carrier L, Bercovici J, Cruaud C, Richard P, Hainque B, Gautel M, Labeit S, James M, Beckman J, Weissenbach J, Vosberg HP, Fiszman M, Komajda M and Schwartz K. Cardiac myosin binding protein-C gene splice acceptor site mutation is associated with familial hypertrophic cardiomyopathy. Nature Genet., 1995, 11:438-440.

 

2. Richard P, Charron P, Carrier L, Ledeuil C, Cheav T, Pichereau C, Benaiche A, Isnard R, Dubourg O, Burban M, Gueffet JP, Millaire A, Desnos M, Schwartz K, Hainque B and Komajda M. Hypertrophic Cardiomyopathy. Distribution of disease genes, spectrum of mutations, and implications for molecular diagnosis strategy. Circulation, 2003, 107:2227-2232.

 

3. Carrier L, Bonne G, Bahrend E, Yu B, Richard P, Niel F, Hainque B, Cruaud C, Gary F, Labeit S, Bouhour JB, Dubourg O, Desnos M, Hagège A, Trent RJ, Komajda M, Fiszman M and Schwartz K. Organization and sequence of human cardiac myosin binding protein C gene (MYBPC3) and identification of mutations predicted to produce truncated proteins in familial hypertrophic cardiomyopathy. Circ. Res., 1997, 80:427-434.

 

4. Flavigny J, Souchet M, Sébillon P, Berrebi-Bertrand I, Hainque B, Mallet A, Bril A, Schwartz K and Carrier L. COOH-terminal truncated cardiac myosin-binding protein C mutants resulting from familial hypertrophic cardiomyopathy mutations exhibit altered expression and/or incorporation in fetal rat cardiomyocytes. J Mol Biol, 1999, 294:443-456.

 

5. Vignier N, Perrot A, Schulte HD, Richard P, Sébillon P, Schwartz K, Osterziel K and Carrier L. Cardiac myosin-binding protein C and familial hypertrophic cardiomyopathy: from mutations identification to human endomyocardial proteins analysis. Circulation, 2001, 104, (suppl.):II-1.