Early-Onset Hypertrophic Cardiomyopathy Mutations Significantly Increase the Velocity, Force, and Actin-Activated ATPase Activity of Human β-Cardiac Myosin

Abstract

Hypertrophic cardiomyopathy (HCM) is a heritable cardiovascular disorder that affects 1 in 500 people. A significant percentage of HCM is attributed to mutations in β-cardiac myosin, the motor protein that powers ventricular contraction. This study reports how two early-onset HCM mutations, D239N and H251N, affect the molecular biomechanics of human β-cardiac myosin. We observed significant increases (20%-90%) in actin gliding velocity, intrinsic force, and ATPase activity in comparison to wild-type myosin. Moreover, for H251N, we found significantly lower binding affinity between the S1 and S2 domains of myosin, suggesting that this mutation may further increase hyper-contractility by releasing active motors. Unlike previous HCM mutations studied at the molecular level using human β-cardiac myosin, early-onset HCM mutations lead to significantly larger changes in the fundamental biomechanical parameters and show clear hyper-contractility.

The myosin mesa and the basis of hypercontractility caused by hypertrophic cardiomyopathy mutations

“…4b). In a separate study 25 , the early-onset HCM mutation H251N, which is next to R453C and R249Q in the myosin mesa in our working model (Fig. 4a), has also been found to substantially weaken the binding of sS1 to S2 (green curve in Fig.…”

“…This parameter may be key in fine-tuned regulation of the heart 30–33 , and, as previously hypothesized 25,28,29,34–37 , may be pivotal in determining the hypercontractility known to arise from HCM mutations in human β-cardiac myosin.…”

“…For two human β-cardiac S1 HCM mutations, D239N and H251N (early onset, pathologically severe), intrinsic force, velocity (as measured by in vitro motility assays), and ATPase activity are 30–90% higher than those in wild-type (WT) S1 (ref. 25) (Table 1), thus easily accounting for clinically observed hypercontractility. However, human β-cardiac S1 HCM mutations that typically result in adult-onset disease, such as R403Q 26 and R453C 27 , show less severe effects in vitro , as compared with previously described results in a mouse α-cardiac myosin model 23 , with the largest observed change being an ~50% increase in F intrinsic compared with that of WT 27 (Table 1).…”

“…In the folded-back IHM structure, the blocked head may also be in structural contact with the proximal S2 (refs. 1,2,25,40,43,44 ). The IHM has also been observed in other classes of muscle myosin, including striated muscle myosins 41 , and in images of intact thick filaments isolated from a variety of muscle types 1–6 .…”

Impact of the Myosin Modulator Mavacamten on Force Generation and Cross‐Bridge Behavior in a Murine Model of Hypercontractility

“…This observation emphasizes the importance of understanding the mechanisms that underlie cardiac hypercontractility caused by MHC and cMyBPC mutations, and how these functional defects can be reversed or normalized using genetic or pharmacological approaches 12. Missense mutations in human cardiac β‐MHC result in variable effects on contractile function including reduced13, 14 or enhanced intrinsic force generation15, 16; decreased14, 16 or enhanced15 myosin ATPase activity; and accelerated13, 15, 17 or slowed16 actin sliding velocities. Importantly, recent data show that HCM‐causing mutations in β‐MHC that cause hypercontractility weaken myosin's S1‐S2 intradomain interactions, thus effectively increasing the total number of myosin heads that can interact with actin during systole,11 thereby chronically elevating left ventricular ejection fraction 5, 18.…”