Abstract P1026: Structural And Biochemical Effects Of Missense MyBP-C Mutations On The Myofilament In Human Hypertrophic Cardiomyopathy

• Published in: Circulation research Vol. 133; no. Suppl_1
• Main Authors: McAllister, Christopher M, Jani, Vivek, Nissen, Devin L, Ma, Weikang, Kass, David A, Irving, Thomas C, Day, Sharlene
• Format: Journal Article
• Language: English
• Published: 04.08.2023
• ISSN: 0009-7330; 1524-4571
• DOI: 10.1161/res.133.suppl_1.P1026

Abstract only Significance: While the disease mechanisms of truncating MyBP-C variants in HCM have been well defined, the mechanisms by which missense MyBP-C variants cause HCM are largely unknown. Recent studies show that unlike truncating variants that push myosin into an active high energy state due to their overall reduction in MyBP-C, missense MyBP-C properly incorporates into the sarcomere without affecting protein abundance. The consequences of missense MyBP-C on myosin have not been explored and may have implications for the treatment of HCM patients with missense MyBP-C variants. Results: We performed small angle X-ray diffraction (XrD) on human HCM tissue from patients with missense MyBP-C, and healthy control hearts. When healthy tissue was moved from relaxing pCa 8 to activating pCa 5 the I 1,1 /I 1,0 equatorial intensity ratio increased (p<0.001) indicating myosin is moving away from the backbone and adopting an active conformation. However, in missense hearts I 1,1 /I 1,0 is lower than healthy hearts at pCa 8 meaning more myosin is localized to the backbone. This is contrary to other HCM mutations which are proposed to push myosin to an active conformation under relaxing conditions. I 1,1 /I 1,0 does not increase at pCa 5 (p>0.05) indicating that at activating Ca myosin remains in the inactive conformation suggesting the thick filament Ca response is impaired in missense hearts. Ongoing Experiments: We are planning to perform XrD on more samples and Mant-ATP assays to determine if the structural inactivation of myosin in missense tissue also results in myosin remaining in the low energy super relaxed state (SRX). We expect the structural state to correlate with the biochemical state meaning missense hearts will demonstrate an equivalent percentage of SRX myosin compared to control hearts. Conclusions: Our finding that the transition of myosin to a structurally active conformation does not occur in human MyBP-C missense hearts distinguishes these variants from other HCM causing mutations such as truncating MyBP-C and missense MYH7 which have been shown to shift myosin to the active disordered relaxed state. If Mant-ATP assays show myosin is maintaining SRX along with an inactive conformation this suggests MyBP-C missense mutations act through a different mechanism.