Abstract 116: Mybphl is a Novel Myofilament Component Implicated in Arrhythmia and Dilated Cardiomyopathy

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Dilated cardiomyopathy is under significant genetic influence. Using whole-genome sequencing, we found a premature stop variant (R255X) in the MYBPHL gene in a family with DCM and conduction system disease. A Mybphl null mouse model revealed systolic dysfunction with atrial and ventricular conduction system (VCS) abnormalities. We found MyBP-HL protein highly expressed in both human and mouse atria and in a subset of ventricular cardiomyocytes. We hypothesize that MyBP-HL regulates VCS function and loss of MyBP-HL causes ventricular arrhythmia and cardiac dysfunction. We now examined hearts and isolated VCMs from Mybphl WT, het, and null mice using immunofluorescence microscopy (IFM). Clusters of MyBP-HL+ ventricular cardiomyocytes were evident in WT hearts (~1/200,000), with only 10% as many MyBP-HL+ cardiomyocytes in het samples. In WT hearts, MyBP-HL+ cells localized in the right ventricular free wall, with significantly fewer MyBP-HL+ cells in the right ventricular free wall in het hearts. The VCS marker contactin-2 (Cntn2) was used to assess MyBP-HL co-localization in the VCS. MyBP-HL co-localized with Cntn2+ cardiomyocytes in the transition zone between the atria and ventricle near the AV node, as well as in a subset of Cntn2+ Purkinje fibers. MyBP-HL+ ventricular cardiomyocytes were also found outside the VCS, suggesting other roles for those cells in regulating ventricular function. To discern MyBP-HL myofilament localization, we used super-resolution structured illumination microscopy on atrial cardiomyocytes. MyBP-HL co-localized with cMyBP-C in the C-zone of the A-band but also occupied space in the inner portion of the A-band near the M-line. This localization was not altered in het mice; however, the ratio of MyBP-HL in the inner portion of the A-band was diminished in het mice compared to WT. This suggests that MyBP-HL may bind the light meromyosin coiled-coil tail that originates from the M-line. Following the discovery that loss of MyBP-HL is associated with DCM and arrhythmia, we identified that MyBP-HL associates with the VCS and reduced MyBP-HL levels reduce the amount of MyBP-HL positive VCMs. These data suggest that loss of MyBP-HL may impair the development and function of the VCS, resulting in arrhythmias.