LAUSR

Background: Hypertrophic cardiomyopathy (HCM) affects at least 1 in 500 people worldwide, and results in the thickening of the ventricular walls and reduced cardiac function. Mutations in MYBPC3 , encoding cardiac myosin binding protein-C, are the most common cause of HCM. Previously, a highly prevalent 25bp deletion within intron 32 of MYBPC3 was described in the South Asian population. The MYBPC3 d25bp variant is present in approximately 100 million people, and encompasses a splicing branch point predicted to result in abnormal splicing of exon 33. Thus, there is a critical need to understand the mechanism by which MYBPC3 d25bp may cause cardiomyopathy. Methods: To determine the role of the 25bp deletion in vivo , knock-in humanized mice were created in which intron 32 was replaced with the human intron 32, with or without the MYBPC3 d25bp mutation. Mice were characterized at 3- and 6-months of age by echocardiography, histological, and protein analysis. The presence of aberrant exon splicing was also determined in mice carrying the MYBPC3 d25bp variant through RT-PCR and mini-gene assays. Finally, exon trapping experiments were performed to understand the mechanism behind exon skipping. Results: Under baseline conditions, MYBPC3 d25bp displayed no changes in cardiac function or morphology as measured by echocardiography (FS (%): NTG 35.3%, WT 32.8%, Het 33.7%), heart weight to body weight ratio, or histology. While exon 33 skipping was not detected by RT-PCR, the presence of an alternative splice site within exon 33 was identified in MYBPC3 d25bp mice. However, this did not affect the protein levels of cMyBP-C. Furthermore, mini-gene experiments demonstrated that the MYBPC3 d25bp mutation significantly reduced the percentage of correctly spliced transcripts (86.2% vs. 77.5%). Conclusions: These data demonstrate that the presence of the highly prevalent 25bp deletion is not sufficient to cause disease under baseline conditions. However, it is possible that the increased levels of aberrant splicing may increase the risk for developing HCM.