Divergent abnormal muscle relaxation by hypertrophic cardiomyopathy and nemaline myopathy mutant tropomyosins.
ABSTRACT Mutations in tropomyosin (Tm) have been linked to distinct inherited diseases of cardiac and skeletal muscle, hypertrophic cardiomyopathy (HCM), and nemaline myopathy (NM). How HCM and NM mutations in nearly identical Tm proteins produce the vastly divergent clinical phenotypes of heightened, prolonged cardiac muscle contraction in HCM and skeletal muscle weakness in NM is currently unknown. We report here a direct comparison of the effects of HCM (A63V) and NM (M9R) mutant Tm on membrane-intact myocyte contractile function as assessed by adenoviral gene transfer to fully differentiated cardiac muscle cells. Wild-type, and mutant HCM, and mutant NM proteins were expressed at similar levels in myocytes and incorporated into sarcomeres. Interestingly, HCM mutant Tm produced significantly longer contractions by slowing relaxation, whereas NM mutant Tm produced the opposite effect of accelerated muscle relaxation. We propose slowed relaxation caused by HCM mutant Tm can directly contribute to diastolic dysfunction seen in HCM even without secondary cardiac remodeling. Conversely, hastening of relaxation by NM mutant Tm may shift the force-frequency relationship in skeletal muscle and contribute to muscle weakness seen in NM. Together, these results implicate divergent, abnormal "turning off" of muscle contraction as a cellular basis for the differential pathogenesis of mutant Tm-associated HCM and NM.
Article: Combinatorial effects of double cardiomyopathy mutant alleles in rodent myocytes: a predictive cellular model of myofilament dysregulation in disease.[show abstract] [hide abstract]
ABSTRACT: Inherited cardiomyopathy (CM) represents a diverse group of cardiac muscle diseases that present with a broad spectrum of symptoms ranging from benign to highly malignant. Contributing to this genetic complexity and clinical heterogeneity is the emergence of a cohort of patients that are double or compound heterozygotes who have inherited two different CM mutant alleles in the same or different sarcomeric gene. These patients typically have early disease onset with worse clinical outcomes. Little experimental attention has been directed towards elucidating the physiologic basis of double CM mutations at the cellular-molecular level. Here, dual gene transfer to isolated adult rat cardiac myocytes was used to determine the primary effects of co-expressing two different CM-linked mutant proteins on intact cardiac myocyte contractile physiology. Dual expression of two CM mutants, that alone moderately increase myofilament activation, tropomyosin mutant A63V and cardiac troponin mutant R146G, were shown to additively slow myocyte relaxation beyond either mutant studied in isolation. These results were qualitatively similar to a combination of moderate and strong activating CM mutant alleles alphaTmA63V and cTnI R193H, which approached a functional threshold. Interestingly, a combination of a CM myofilament deactivating mutant, troponin C G159D, together with an activating mutant, cTnIR193H, produced a hybrid phenotype that blunted the strong activating phenotype of cTnIR193H alone. This is evidence of neutralizing effects of activating/deactivating mutant alleles in combination. Taken together, this combinatorial mutant allele functional analysis lends molecular insight into disease severity and forms the foundation for a predictive model to deconstruct the myriad of possible CM double mutations in presenting patients.PLoS ONE 01/2010; 5(2):e9140. · 4.09 Impact Factor