HCM-linked ∆160E cardiac troponin T mutation causes unique progressive structural and molecular ventricular remodeling in transgenic mice

Abstract

Hypertrophic cardiomyopathy (HCM) is a primary disease of cardiac muscle, and one of the most common causes of sudden cardiac death (SCD) in young people. Many mutations in cardiac troponin T (cTnT) lead to a complex form of HCM with varying degrees of ventricular hypertrophy and ~65% of all cTnT mutations occur within or flanking the elongated N-terminal TNT1 domain. Biophysical studies have predicted that distal TNT1 mutations, including Δ160E, cause disease by a novel, yet unknown mechanism as compared to N-terminal mutations. To begin to address the specific effects of this commonly observed cTnT mutation we generated two independent transgenic mouse lines carrying variant doses of the mutant transgene. Hearts from the 30% and 70% cTnT Δ160E lines demonstrated a highly unique, dose-dependent disruption in cellular and sarcomeric architecture and a highly progressive pattern of ventricular remodeling. While adult ventricular myocytes isolated from Δ160E transgenic mice exhibited dosage-independent mechanical impairments, decreased sarcoplasmic reticulum calcium load and SERCA2a calcium uptake activity, the observed decreases in calcium transients were dosage-dependent. The latter findings were concordant with measures of calcium regulatory proteins abundance and phosphorylation state. Finally, studies of whole heart physiology in the isovolumic mode demonstrated dose-dependent differences in the degree of cardiac dysfuction. We conclude that the observed clinical severity of the cTnT Δ160E mutation is caused by a combination of direct sarcomeric disruption coupled to a profound disregulation of Ca2+ homeostasis at the cellular level that results in a unique and highly progressive pattern of ventricular remodeling.

Cardiac troponin structure-function and the influence of hypertrophic cardiomyopathy associated mutations on modulation of contractility

“…Using transgenic mice, they found that cTnI P83S prolonged the isovolumetric relaxation time, impaired ejection and relaxation time, as well as blunted the β–adrenergic response and impaired myofilament cooperativity [28]. Similar to these cTnI mutations, several (but not all) HCM-associated mutations in cTnC (A8V, L29Q, A31S, C84Y and D145E) and cTnT (F110I, R92L/W/Q, R94L, A104V, R130C, Δ160E, E163R, S179F and E244D) have been demonstrated to increase Ca 2+ sensitivity of ATPase activity and/or force development [31-33, 126, 160-184]. Pinto et al .…”

“…For cTn, the effect of HCM-related mutations has been investigated using various biochemical and physiological approaches. For instance, recombinant cTn subunits containing mutations have been incorporated into reconstituted cTn complexes or thin filaments to study the effects on protein-protein interaction and ATPase activity in vitro [21-23]; adenovirus containing mutation genes have been constructed and transduced into isolated cardiomyocytes to examine effects on intact cardiomyocyte shortening and intracellular Ca 2+ handling [24]; and transgenic animal models have been developed to study the disease progression [25-33]. In general, functional studies of many of the HCM-related mutations have been shown to increase the Ca 2+ sensitivity of contractile (tension) and ATPase activity, with some exceptions for mutations which produced complex and contradictory results [23, 34].…”

Allosteric effects of cardiac troponin TNT1 mutations on actomyosin binding: A novel pathogenic mechanism for hypertrophic cardiomyopathy

“…Interestingly, these mutations show significant differences in their molecular phenotypes despite being located within the same functional domain [7,14,15]. Mutation-specific alterations in myofilament activation and myocellular Ca 2+ homeostasis in vivo differ between Δ160E and R92 mutations [14,16]. Moreover, it has been shown that N-terminal and C-terminal TNT1 mutations have distinct biophysical effects on TNT1–Tm interactions.…”

What global diastolic function is, what it is not, and how to measure it

“…If calcium reuptake is slowed, for example in ischemia or hypoxia, then the rate of relaxation is reduced (13,15). Altered deactivation of cross-bridge binding sites, for example due to phosphorylation or mutation of troponin complexes, is also associated with slow relaxation (73,81,118). In addition, cross-bridge detachment itself may modify the rate of relaxation (4,42), with slowed cross-bridge detachment, for example by mutation, leading to prolonged generation of force.…”