Polygenic Modulation of Cardiac Dysfunction in Drosophila Assessed by High-speed Video Imaging, Motion Detection Analysis and Fluorescent Microscopy

Abstract

Hypertrophic (HCM), dilated (DCM) and restrictive (RCM) cardiomyopathies are cardiac disorders often resulting from contractile protein mutations. They are frequently dominantly inherited and are characterized by a high degree of clinical heterogeneity proposed to result from modifying genetic factors. For example, HCM patients with multiple causal mutations present a more severe phenotype compared to single-mutation carriers. We employed high-speed digital video imaging and novel motion detection software to characterize in vivo cardiac structure and performance of homozygous Drosophila mutants, quantitatively assessing cardiac diameters, contractile periodicities, fractional shortening and rhythmicity parameters. Fly hearts expressing myosin with depressed or enhanced biomechanical properties exhibited hallmarks of human DCM or RCM respectively. To determine if the Drosophila cardiac phenotypes exhibit dominant modes of inheritance we studied the effects of heterozygotic expression of the myosin mutations. Interestingly, both mutations induced dominant cardiac dilatory responses. This suggests the homozygotic RCM-like phenotype is initiated by a unique cardiac remodeling pathway not activated in the presence of a wild-type myosin gene copy. We also used live-cell imaging and fluorescent microscopy to measure normalized cardiac tube area, in order to investigate polygenic effects of specific sarcomeric mutations on the severity of cardiac phenotypes in double heterozygotes. Combining a dilation-inducing troponin I mutation with the reduced function myosin mutation resulted in a dilatory cardiac phenotype at advanced age, which was more severe than that observed in single heterozygotes. However, combining the troponin mutation with the increased function myosin mutation appeared to prevent the cardiac dilation characteristic of the single heterozygotes. This suggests molecular combinations of certain mutations may have cardioprotective effects. Thus, Drosophila may serve as an effective in vivo tool for identifying and studying genetic enhancers and suppressors of cardiac dysfunction.