Abstract P249: Cardiac Troponin I Pro82Ser Variant Impairs Myofilament Cooperativity, Induces Diastolic Dysfunction, and Blunts β-Adrenergic Response

Abstract

Cardiac troponin I is a key regulatory protein for muscle contraction and necessary for an adequate β-adrenergic response. The Proline 82 Serine (P82S) sequence variant of troponin I gene ( TNNI3 ) is associated with late onset hypertrophic cardiomyopathy (HCM). Our preliminary data has shown that cTnIP82S is heterozygous in 3% of African Americans and is associated with increased LV mass in hypertensive black men. We hypothesize that cTnIP82S variant could influence the hypertrophic response to hypertension and/or the malignancy of the phenotypic expression when combined with known HCM disease causing myofilament mutations. We created a transgenic cTnIP82S mouse model. Echocardiography at baseline in older Tg and NTg mice (64-65 wks) showed impaired ejection time and relaxation, Tg display longer isovolumetric relaxation time (IVRT) (Tg 29.7±0.26 vs NTg 23± 0.23 msec, n=8 vs n=7 p<0.05) and a significantly higher TEI index. In the young NTg and Tg (17 wks) there were no echocardiographic differences. However, preliminary data suggest that 1 wk after transverse aortic coarctation (TAC), diastolic dysfunction was evident in Tg, and 10 weeks after TAC LV mass was also increased compared to NTg. Skinned fibers studies showed in the Tg mice borderline lower maximal Force (F max ) and myofilament calcium sensitivity, whereas n Hill showed a marked depression (Tg 1.72±0.17 vs NTg 3.09±0.44, n=7 vs n=5, p<0.05), this effect was not due to the global myofilament phosphorylation pattern. In addition, intact twitching cardiac muscle studies in Tg mice revealed an impaired dose-dependent β-adrenergic acceleration of relaxation, evidenced by a failure to increase relaxation ([-dF/dt min]/[+dF/dt max]) and accelerate calcium transients decay after isoproterenol. Pressure-volume (PV) loop studies confirm that Tg mice fail to increase: Δ change in dP/dt max (NTg 4,928.3±7.7 vs Tg 843.3±10.7, n=4 vs n=4 p<0.05) in response to isoproterenol. TnIP82S variant is near a region of TnI-TnT interaction; this could explain its dramatic effects on myofilament cooperativity. Overall these studies suggest that the expression of cTnIP82S variant in the mice heart induces diastolic dysfunction, impairs relaxation at baseline and after β-adrenergic stimulation.