In vivo short-term expression of a hypertrophic cardiomyopathy mutation in adult rabbit myocardium: myofibrillar incorporation without early disarray

Evidence for the preventive effect of the polyunsaturated phytol side chain in tocotrienols on 17beta-estradiol epoxidation

BACKGROUND

We found that 17beta-estradiol (E2) could be activated by epoxidation to bind DNA and to inhibit nuclear RNA synthesis. Vitamin E compounds are powerful antioxidants and chain-breaking free radical scavengers. The chromanol ring in Vitamin E is believed to be involved in these reactions.

METHODS

Here, we examined the preventive effect of alpha-tocopherol, alpha-, gamma- and delta-tocotrienols on E2 activation.

RESULTS

We found that when any one of these Vitamin E compounds was mixed with E2 for epoxidation by the epoxide-forming oxidant dimethyldioxirane (DMDO), alpha-tocopherol was the least effective as compared with the tocotrienols against the formation of E2 epoxide as reflected by the loss of the ability of E2 to inhibit nuclear RNA synthesis. This conclusion was further confirmed by the binding studies of [3H] labeled E2 to DNA using either DMDO or liver microsomes activation system.

CONCLUSIONS

Since the chromanol ring is shared by both tocopherols and tocotrienols and the only difference between these two subgroups of Vitamin E is the phytol side chain, we conclude that the polyunsaturated phytol group in tocotrienols plays a key preventive role in E2 epoxidation. This is the first report showing that the polyunsaturated phytol side chain in tocotrienols is involved in an antioxidative activity and it may also have a preventive effect against the E2 epoxide induced breast cancer carcinogenesis at the initiation.

[Genetically modified animal models in cardiovascular research]

It is a basic tenet of molecular and clinical medicine that specific protein complements underlie cell and organ function. Since cellular and ultimately organ function depend upon the polypeptides that are present, it is not surprising that when function is altered changes in the protein pools occur. In the heart, numerous examples of contractile protein changes correlate with functional alterations, both during normal development and during the development of numerous pathologies. Similarly, different congenital heart diseases are characterized by certain shifts in the motor proteins. To understand these relationships, and to establish models in which the pathogenic processes can be studied longitudinally, it is necessary to direct the heart to stably synthesize, in the absence of other peliotropic changes, the candidate protein. Subsequently, one can determine if the protein's presence causes the effects directly or indirectly with the goal being to define potential therapeutic targets. By affecting the heart's protein complement in a defined manner, one has the means to establish both mechanism and the function of the different mutated proteins of protein isoforms. Gene targeting and transgenesis in the mouse provides a means to modify the mammalian genome and the cardiac motor protein complement. By directing expression of an engineered protein to the heart, one is now able to effectively remodel the cardiac protein profile and study the consequences of a single genetic manipulation at the molecular, biochemical, cytological and physiologic levels, both under normal and stress stimuli.

Manipulating the contractile apparatus: genetically defined animal models of cardiovascular disease

Within the last 10 years via gene targeting and transgenesis, numerous models of cardiovascular disease have been established and used to determine if a protein's presence or absence causes cardiovascular disease. By affecting the heart's protein complement in a defined manner, the function of the different mutated proteins or protein isoforms present in the contractile apparatus can be determined and pathogenic mechanism(s) explored. We can now remodel the cardiac protein profile and effect replacement of even the most abundant contractile proteins. Precise genetic manipulation allows exploration of the structure-function relationships which underlie cardiac function, and the consequences of defined mutations at the molecular, biochemical, cytological and physiologic levels can be determined.

Decreased left ventricular ejection fraction in transgenic mice expressing mutant cardiac troponin T-Q(92), responsible for human hypertrophic cardiomyopathy

The causality of mutant sarcomeric proteins in hypertrophic cardiomyopathy (HCM) is well established. The current emphasis is to elucidate the pathogenesis of HCM in transgenic animal models. We determined the left ventricular ejection fraction (LVEF) in transgenic mice expressing mutant cardiac troponin T (cTnT)-Q(92), known to cause HCM in humans. Transgenes were constructed by placing wild-type (R(92)) or mutant (Q(92)) full-length human cTnT cDNAs 3' into a 5.5-kb murine [alpha -myosin heavy chain (MyHC)] promoter injected into fertilized zygotes. Three wild-type and six mutant lines were produced. Transgene mRNA and proteins, detected using transgene-specific probes were expressed at high levels in all wild-type and three mutant lines. The total cTnT mRNA pool was increased by up to five-fold in transgenic mice, but the total cTnT protein remained unchanged. The mean values of LVEF, determined by(178)Ta radionuclide angiography, were 57.8+/-6% (n=4) in non-transgenic littermate (NLM), 53.3+/-10 (n=6) in wild-type and 39. 4+/-6 (n=5) in mutant transgenic mice (P=0.009). The heart/body weight ratios and the number of cells stained with terminal deoxynucleotidyl transferase (TdT)-mediated nick end-labeling were similar among the groups. Three mutant mice had myocyte disarray and excess interstitial collagen and two had normal myocardial structure despite having reduced LVEF. Thus, in vivo expression of the mutant cTnT-Q(92)protein, responsible for human HCM, impaired global cardiac systolic function in transgenic mice, which also occurred in the absence of myocyte disarray and increased interstitial collagen.

Altered regulation of cardiac muscle contraction by troponin T mutations that cause familial hypertrophic cardiomyopathy

To study the effect of troponin (Tn) T mutations that cause familial hypertrophic cardiomyopathy (FHC) on cardiac muscle contraction, wild-type, and the following recombinant human cardiac TnT mutants were cloned and expressed: I79N, R92Q, F110I, E163K, R278C, and intron 16(G(1) --> A) (In16). These TnT FHC mutants were reconstituted into skinned cardiac muscle preparations and characterized for their effect on maximal steady state force activation, inhibition, and the Ca(2+) sensitivity of force development. Troponin complexes containing these mutants were tested for their ability to regulate actin-tropomyosin(Tm)-activated myosin-ATPase activity. TnT(R278C) and TnT(F110I) reconstituted preparations demonstrated dramatically increased Ca(2+) sensitivity of force development, while those with TnT(R92Q) and TnT(I79N) showed a moderate increase. The deletion mutant, TnT(In16), significantly decreased both the activation and the inhibition of force, and substantially decreased the activation and the inhibition of actin-Tm-activated myosin-ATPase activity. ATPase activation was also impaired by TnT(F110I), while its inhibition was reduced by TnT(R278C). The TnT(E163K) mutation had the smallest effect on the Ca(2+) sensitivity of force; however, it produced an elevated activation of the ATPase activity in reconstituted thin filaments. These observed changes in the Ca(2+) regulation of force development caused by these mutations would likely cause altered contractility and contribute to the development of FHC.

A transgenic rabbit model for human hypertrophic cardiomyopathy

Certain mutations in genes for sarcomeric proteins cause hypertrophic cardiomyopathy (HCM). We have developed a transgenic rabbit model for HCM caused by a common point mutation in the beta-myosin heavy chain (MyHC) gene, R400Q. Wild-type and mutant human beta-MyHC cDNAs were cloned 3' to a 7-kb murine beta-MyHC promoter. We injected purified transgenes into fertilized zygotes to generate two lines each of the wild-type and mutant transgenic rabbits. Expression of transgene mRNA and protein were confirmed by Northern blotting and 2-dimensional gel electrophoresis followed by immunoblotting, respectively. Animals carrying the mutant transgene showed substantial myocyte disarray and a 3-fold increase in interstitial collagen expression in their myocardia. Mean septal thicknesses were comparable between rabbits carrying the wild type transgene and their nontransgenic littermates (NLMs) but were significantly increased in the mutant transgenic animals. Posterior wall thickness and left ventricular mass were also increased, but dimensions and systolic function were normal. Premature death was more common in mutant than in wild-type transgenic rabbits or in NLMs. Thus, cardiac expression of beta-MyHC-Q(403) in transgenic rabbits induced hypertrophy, myocyte and myofibrillar disarray, interstitial fibrosis, and premature death, phenotypes observed in humans patients with HCM due to beta-MyHC-Q(403).