Independent genetic susceptibility to cardiac hypertrophy in inherited hypertension

Cpxm2 as a novel candidate for cardiac hypertrophy and failure in hypertension

Treatment of hypertension-mediated cardiac damage with left ventricular (LV) hypertrophy (LVH) and heart failure remains challenging. To identify novel targets, we performed comparative transcriptome analysis between genetic models derived from stroke-prone spontaneously hypertensive rats (SHRSP). Here, we identified carboxypeptidase X 2 (Cpxm2) as a genetic locus affecting LV mass. Analysis of isolated rat cardiomyocytes and cardiofibroblasts indicated Cpxm2 expression and intrinsic upregulation in genetic hypertension. Immunostaining indicated that CPXM2 associates with the t-tubule network of cardiomyocytes. The functional role of Cpxm2 was further investigated in Cpxm2-deficient (KO) and wild-type (WT) mice exposed to deoxycorticosterone acetate (DOCA). WT and KO animals developed severe and similar systolic hypertension in response to DOCA. WT mice developed severe LV damage, including increases in LV masses and diameters, impairment of LV systolic and diastolic function and reduced ejection fraction. These changes were significantly ameliorated or even normalized (i.e., ejection fraction) in KO-DOCA animals. LV transcriptome analysis showed a molecular cardiac hypertrophy/remodeling signature in WT but not KO mice with significant upregulation of 1234 transcripts, including Cpxm2, in response to DOCA. Analysis of endomyocardial biopsies from patients with cardiac hypertrophy indicated significant upregulation of CPXM2 expression. These data support further translational investigation of CPXM2.

Genetics of Cardiomyopathy: Clinical and Mechanistic Implications for Heart Failure

Genetic cardiomyopathies are an important cause of sudden cardiac death across all age groups. Genetic testing in heart failure clinics is useful for family screening and providing individual prognostic insight. Obtaining a family history of at least three generations, including the creation of a pedigree, is recommended for all patients with primary cardiomyopathy. Additionally, when appropriate, consultation with a genetic counsellor can aid in the success of a genetic evaluation. Clinical screening should be performed on all first-degree relatives of patients with genetic cardiomyopathy.

Genetics has played an important role in the understanding of different cardiomyopathies, and the field of heart failure (HF) genetics is progressing rapidly. Much research has also focused on distinguishing markers of risk in patients with cardiomyopathy using genetic testing. While these efforts currently remain incomplete, new genomic technologies and analytical strategies provide promising opportunities to further explore the genetic architecture of cardiomyopathies, afford insight into the early manifestations of cardiomyopathy, and help define the molecular pathophysiological basis for cardiac remodeling. Cardiovascular physicians should be fully aware of the utility and potential pitfalls of incorporating genetic test results into pre-emptive treatment strategies for patients in the preliminary stages of HF. Future work will need to be directed towards elucidating the biological mechanisms of both rare and common gene variants and environmental determinants of plasticity in the genotype-phenotype relationship. This future research should aim to further our ability to identify, diagnose, and treat disorders that cause HF and sudden cardiac death in young patients, as well as prioritize improving our ability to stratify the risk for these patients prior to the onset of the more severe consequences of their disease.

Cardiac mechanical efficiency is preserved in primary cardiac hypertrophy despite impaired mechanical function

Increased heart size is a major risk factor for heart failure and premature mortality. Although abnormal heart growth subsequent to hypertension often accompanies disturbances in mechano-energetics and cardiac efficiency, it remains uncertain whether hypertrophy is their primary driver. In this study, we aimed to investigate the direct association between cardiac hypertrophy and cardiac mechano-energetics using isolated left-ventricular trabeculae from a rat model of primary cardiac hypertrophy and its control. We evaluated energy expenditure (heat output) and mechanical performance (force length work production) simultaneously at a range of preloads and afterloads in a microcalorimeter, we determined energy expenditure related to cross-bridge cycling and Ca²⁺ cycling (activation heat), and we quantified energy efficiency. Rats with cardiac hypertrophy exhibited increased cardiomyocyte length and width. Their trabeculae showed mechanical impairment, evidenced by lower force production, extent and kinetics of shortening, and work output. Lower force was associated with lower energy expenditure related to Ca²⁺ cycling and to cross-bridge cycling. However, despite these changes, both mechanical and cross-bridge energy efficiency were unchanged. Our results show that cardiac hypertrophy is associated with impaired contractile performance and with preservation of energy efficiency. These findings provide direction for future investigations targeting metabolic and Ca²⁺ disturbances underlying cardiac mechanical and energetic impairment in primary cardiac hypertrophy.

Morphological and Functional Characteristics of Animal Models of Myocardial Fibrosis Induced by Pressure Overload

Myocardial fibrosis is characterized by excessive deposition of myocardial interstitial collagen, abnormal distribution, and excessive proliferation of fibroblasts. According to the researches in recent years, myocardial fibrosis, as the pathological basis of various cardiovascular diseases, has been proven to be a core determinant in ventricular remodeling. Pressure load is one of the causes of myocardial fibrosis. In experimental models of pressure-overload-induced myocardial fibrosis, significant increase in left ventricular parameters such as interventricular septal thickness and left ventricular posterior wall thickness and the decrease of ejection fraction are some of the manifestations of cardiac damage. These morphological and functional changes have a serious impact on the maintenance of physiological functions. Therefore, establishing a suitable myocardial fibrosis model is the basis of its pathogenesis research. This paper will discuss the methods of establishing myocardial fibrosis model and compare the advantages and disadvantages of the models in order to provide a strong basis for establishing a myocardial fibrosis model.

Cardiomyocyte Functional Etiology in Heart Failure With Preserved Ejection Fraction Is Distinctive-A New Preclinical Model

Background

Among the growing numbers of patients with heart failure, up to one half have heart failure with preserved ejection fraction (HFpEF). The lack of effective treatments for HFpEF is a substantial and escalating unmet clinical need—and the lack of HFpEF‐specific animal models represents a major preclinical barrier in advancing understanding of HFpEF. As established treatments for heart failure with reduced ejection fraction (HFrEF) have proven ineffective for HFpEF, the contention that the intrinsic cardiomyocyte phenotype is distinct in these 2 conditions requires consideration. Our goal was to validate and characterize a new rodent model of HFpEF, undertaking longitudinal investigations to delineate the associated cardiac and cardiomyocyte pathophysiology.

Methods and Results

The selectively inbred Hypertrophic Heart Rat (HHR) strain exhibits adult cardiac enlargement (without hypertension) and premature death (40% mortality at 50 weeks) compared to its control strain, the normal heart rat. Hypertrophy was characterized in vivo by maintained systolic parameters (ejection fraction at 85%–90% control) with marked diastolic dysfunction (increased E/E′). Surprisingly, HHR cardiomyocytes were hypercontractile, exhibiting high Ca²⁺ operational levels and markedly increased L‐type Ca²⁺ channel current. In HHR, prominent regions of reparative fibrosis in the left ventricle free wall adjacent to the interventricular septum were observed.

Conclusions

Thus, the cardiomyocyte remodeling process in the etiology of this HFpEF model contrasts dramatically with the suppressed Ca²⁺ cycling state that typifies heart failure with reduced ejection fraction. These findings may explain clinical observations, that treatments considered appropriate for heart failure with reduced ejection fraction are of little benefit for HFpEF—and suggest a basis for new therapeutic strategies.

Increased cardiac work provides a link between systemic hypertension and heart failure

The spontaneously hypertensive rat (SHR) is an established model of human hypertensive heart disease transitioning into heart failure. The study of the progression to heart failure in these animals has been limited by the lack of longitudinal data. We used MRI to quantify left ventricular mass, volume, and cardiac work in SHRs at age 3 to 21 month and compared these indices to data from Wistar-Kyoto (WKY) controls. SHR had lower ejection fraction compared with WKY at all ages, but there was no difference in cardiac output at any age. At 21 month the SHR had significantly elevated stroke work (51 ± 3 mL.mmHg SHR vs. 24 ± 2 mL.mmHg WKY; n = 8, 4; P < 0.001) and cardiac minute work (14.2 ± 1.2 L.mmHg/min SHR vs. 6.2 ± 0.8 L.mmHg/min WKY; n = 8, 4; P < 0.001) compared to control, in addition to significantly larger left ventricular mass to body mass ratio (3.61 ± 0.15 mg/g SHR vs. 2.11 ± 0.008 mg/g WKY; n = 8, 6; P < 0.001). SHRs showed impaired systolic function, but developed hypertrophy to compensate and successfully maintained cardiac output. However, this was associated with an increase in cardiac work at age 21 month, which has previously demonstrated fibrosis and cell death. The interplay between these factors may be the mechanism for progression to failure in this animal model.

Mapping and confirmation of a major left ventricular mass QTL on rat chromosome 1 by contrasting SHRSP and F344 rats

An abnormal increase in left ventricular (LV) mass, i.e., LV hypertrophy (LVH), represents an important target organ damage in arterial hypertension and has been associated with poor clinical outcome. Genetic factors are contributing to variation in LV mass in addition to blood pressure and other factors such as dietary salt intake. We set out to map quantitative trait loci (QTL) for LV mass by comparing the spontaneously hypertensive stroke-prone (SHRSP) rat with LVH and normotensive Fischer rats (F344) with contrasting low LV mass. To this end we performed a genome-wide QTL mapping analysis in 232 F2 animals derived from SHRSP and F344 exposed to high-salt (4% in chow) intake for 8 wk. We mapped one major QTL for LV mass on rat chromosome 1 (RNO1) that demonstrated strong linkage (peak logarithm of odds score 8.4) to relative LV weight (RLVW) and accounted for ∼19% of the variance of this phenotype in F2 rats. We therefore generated a consomic SHRSP-1(F344) strain in which RNO1 from F344 was introgressed into the SHRSP background. Consomic and SHRSP animals showed similar blood pressures during conventional intra-arterial measurements, while RLVW was already significantly lower (-17.7%, P<0.05) in SHRSP-1(F344) in response to a normal-salt diet; a similar significant reduction of LV mass was also observed in consomic rats after high-salt intake (P<0.05 vs. SHRSP). Thus, a major QTL on RNO1 was confirmed with significant impact on LV mass in the hypertensive background of SHRSP.

Genome-wide association study of cardiac structure and systolic function in African Americans: the Candidate Gene Association Resource (CARe) study

BACKGROUND

Using data from 4 community-based cohorts of African Americans, we tested the association between genome-wide markers (single-nucleotide polymorphisms) and cardiac phenotypes in the Candidate-gene Association Resource study.

METHODS AND RESULTS

Among 6765 African Americans, we related age, sex, height, and weight-adjusted residuals for 9 cardiac phenotypes (assessed by echocardiogram or magnetic resonance imaging) to 2.5 million single-nucleotide polymorphisms genotyped using Genome-wide Affymetrix Human SNP Array 6.0 (Affy6.0) and the remainder imputed. Within the cohort, genome-wide association analysis was conducted, followed by meta-analysis across cohorts using inverse variance weights (genome-wide significance threshold=4.0 ×10(-7)). Supplementary pathway analysis was performed. We attempted replication in 3 smaller cohorts of African ancestry and tested lookups in 1 consortium of European ancestry (EchoGEN). Across the 9 phenotypes, variants in 4 genetic loci reached genome-wide significance: rs4552931 in UBE2V2 (P=1.43×10(-7)) for left ventricular mass, rs7213314 in WIPI1 (P=1.68×10(-7)) for left ventricular internal diastolic diameter, rs1571099 in PPAPDC1A (P=2.57×10(-8)) for interventricular septal wall thickness, and rs9530176 in KLF5 (P=4.02×10(-7)) for ejection fraction. Associated variants were enriched in 3 signaling pathways involved in cardiac remodeling. None of the 4 loci replicated in cohorts of African ancestry was confirmed in lookups in EchoGEN.

CONCLUSIONS

In the largest genome-wide association study of cardiac structure and function to date in African Americans, we identified 4 genetic loci related to left ventricular mass, interventricular septal wall thickness, left ventricular internal diastolic diameter, and ejection fraction, which reached genome-wide significance. Replication results suggest that these loci may be unique to individuals of African ancestry. Additional large-scale studies are warranted for these complex phenotypes.

The effect of indapamide on development of myocardial hypertrophy and fibrosis in L-NAME-induced hypertension in rat

The aim of this study was to analyze the effect of indapamide and its combination with ACE inhibitor (captopril) and antioxidant (Provinols™) on both myocardial hypertrophy and fibrosis. Wistar rats were treated with L-NAME (40 mg/kg/day, L); L-NAME plus indapamide (1 mg/kg/day), or captopril (10 mg/kg/day), or Provinols™ (40 mg/kg/day), or combination of indapamide with captopril, and indapamide with Provinols™ for 7 weeks. Blood pressure (BP), LV hypertrophy and fibrosis were determined. The content of collagens type I and III was evaluated morphometrically after picrosirius red staining. L-NAME treatment led to increased BP, LV hypertrophy, total fibrosis and relative content of collagens without the change in collagen type I/III ratio. Indapamide and captopril decreased BP, LV hypertrophy and the collagen ratio without affecting total fibrosis, while Provinols™ reduced BP, the collagen ratio and fibrosis without affecting LV hypertrophy. The combinations decreased all the parameters. Decrease of LV hypertrophy was achieved by drugs with the best reducing effect on BP, fibrosis reduction was reached by the antioxidant treatment with only partial effect on BP. Thus, the combination of antihypertensive and antioxidant treatment may represent a powerful tool in preventing myocardial remodeling induced by hypertension.

Heritable pathologic cardiac hypertrophy in adulthood is preceded by neonatal cardiac growth restriction

The identification of genetic factors influencing cardiac growth independently of increased load is crucial to an understanding of the molecular and cellular basis of pathological cardiac hypertrophy. The central aim of this investigation was to determine how pathological hypertrophy in the adult can be linked with disturbances in cardiomyocyte growth and viability in early neonatal development. The hypertrophic heart rat (HHR) model is derived from the spontaneously hypertensive rat and exhibits marked cardiac hypertrophy, in the absence of a pressure load at maturity. Hearts were harvested from male HHR, and control strain normal heart rats (NHR), at different stages of postnatal development [neonatal (P2), 4 wk, 6 wk, 8 wk, 12 wk, 20 wk]. Isolated neonatal cardiomyocytes were prepared to evaluate cell size, number, and binucleation. At postnatal day 2, HHR hearts were considerably smaller than control NHR (4.3 +/- 0.2 vs. 5.0 +/- 0.1 mg/g, P < 0.05). Cardiac growth restriction in the neonatal HHR was associated with reduced myocyte size (length and width) and an increased proportion of binucleated cardiomyocytes. Furthermore, the number of cardiomyocytes isolated from HHR neonatal hearts was significantly less ( approximately 29%) than NHR. We also observe that growth stress in the neonate is associated with accentuated PI3K and suppressed MAPK activation, although these signaling pathways are normalized in the adult heart exhibiting established hypertrophy. Thus, using the HHR model, we identified novel molecular and cellular mechanisms involving premature exit from the cell cycle, reduced cardiomyocyte endowment, and dysregulated trophic signaling during early development, which are implicated in the etiology of heritable cardiac hypertrophy in the adult.

Change of genetic determinants of left ventricular structure in adolescence: longitudinal evidence from the Georgia cardiovascular twin study

BACKGROUND

Genetic contribution to left ventricular (LV) structure is generally recognized, but whether and how this influence varies by ethnicity or with age is unknown.

METHODS

Participants were 517 European-American (EA) and African-American (AA) twin pairs (mean age: 14.6 +/- 3.0) at visit 1 and 422 EA and AA twin pairs at follow-up 4.1 years later. Echocardiograms were obtained on both visits. Data were analyzed using the structural equation modeling software Mx.

RESULTS

Body mass index (BMI) was a strong predictor for all LV measures at both visits 1 and 2, accounting for 3.5-24.2% of the total variance. Hemodynamics explained up to 4.5% additional LV measures variance. After adjusting for BMI, LV measures showed substantial heritability (range: 21-71%). Best-fitting longitudinal models revealed considerable novel genetic effects on the interventricular septum, posterior wall-, and relative wall thickness (RWT) (but not LV internal diameter), accounting for 32-41% of the phenotypic variance at visit 2, with no significant gender and ethnic effects. There was a gender difference for LV mass index in AAs (P < 0.01), with a significant influence of novel genetic effects in males (47%), but not in females. No gender difference was seen in EAs, with 34% of the phenotypic variance at visit 2 attributable to novel genetic effects.

CONCLUSIONS

The heritability of cardiac structure and geometry was equally substantial in both AAs and EAs. Significant novel genetic influences were detected for all measures but LV inner diameter and LV mass index in AA females. Further developmental genetic studies are warranted to elucidate the nature of the emerging gene effects during the transition from adolescence to adulthood.

Cardiac mass and cardiomyocyte size are governed by different genetic loci on either autosomes or chromosome Y in recombinant inbred mice

Left ventricular hypertrophy is one of the main risk factors for cardiovascular mortality and morbidity. It has been proposed that hypertrophic stimuli act in great part by increasing the size of cardiomyocytes, and that the latter characteristic is a necessary condition to differentiate left ventricular hypertrophy from other benign forms of cardiac enlargement. To test whether the same genetic loci control the size of cardiomyocytes and left ventricular mass, we performed whole genome linkage analyses in a panel of 24 recombinant inbred AXB/BXA mouse strains. Whereas one major locus was linked to left ventricular mass in both males and females, loci linked to the size of cardiomyocytes were clearly distinct and showed sex-specific linkage. Moreover, the parental origin of chromosome Y had strong effects on the size of cardiomyocytes in male mice but did not affect left ventricular mass. In addition to showing that genetic loci that increase the size of cardiomyocytes are not necessarily linked to increased left ventricular mass, our findings have important consequences in evaluating cardiac phenotypes when performing genetic manipulations in mice, and in determining the cause of sex-specific differences when using models derived from C57BL/6J mice.

Fine-mapping and comprehensive transcript analysis reveals nonsynonymous variants within a novel 1.17 Mb blood pressure QTL region on rat chromosome 10

The presence of blood pressure (BP) quantitative trait loci (QTL) on rat chromosome 10 has been clearly demonstrated by linkage analysis and substitution mapping. Using congenic strains containing the LEW rat chromosomal segments on the Dahl salt-sensitive (S) rat background, further iterations of congenic substrains were constructed and characterized to fine-map a chromosome 10 region (QTL1) linked to blood pressure. Comparison of seven congenic substrains refined QTL1 to a 1.17 Mb segment flanked by D10Mco88 and D10Mco89, which are located at 71,513,116 and 72,684,774 bp, respectively. The newly defined QTL1, containing 18 genes, is captured in its entirety within a single congenic substrain. A thorough transcript analysis revealed that 3 of these 18 genes, Ccl5, Ddx52, and RGD1559577, had nonsynonymous allelic variations between the S rat and the LEW rat. None of the detected transcripts within the newly defined QTL1 are implicated directly in BP control in humans or model organisms. Therefore, the present work defines a novel blood pressure QTL with three potential quantitative trait nucleotides.

Distinct quantitative trait loci for kidney, cardiac, and aortic mass dissociated from and associated with blood pressure in Dahl congenic rats

Blood pressure (BP) is largely determined by quantitative trait loci (QTLs) in Dahl salt-sensitive (DSS) rats. Little is known about QTLs controlling kidney (K), cardiac (C), and aortic (A) mass (i.e. Km, Cm, and Am, respectively) of DSS rats independent of BP. Their identification can facilitate our understanding of end organ damage. In this work, 36 congenic strains were employed to define QTLs for Km, Cm, and Am either independent of or associated with BP. Five new QTLs, i.e., KmQTLs, that influence Km independent of Cm, Am, and BP were defined. Four new CakmQTLs were defined for Cm, Am, and Km independent of BP. Among them, the CakmC10QTL1 interval contained 13 genes and undefined loci, and none was known to influence the phenotypes in question, paving the way for a novel gene discovery. Among 17 individual QTLs for BP, 14 also affected Cm, Km, and Am, i.e., they are BpcakmQTLs. In contrast, one BpQTL had no effect on Cm, Am, and Kam. Therefore, BP and Cm, Am, and Km have distinct and shared genetic determinants. The discovery of individual Km and Cakm QTLs will likely facilitate the identification of mechanisms underlying renal, cardiac, and/or aortic hypertrophy independent of hypertension.

The genetic dissection of essential hypertension

QTL mapping in humans and rats has identified hundreds of blood-pressure-related phenotypes and genomic regions; the next daunting task is gene identification and validation. The development of novel rat model systems that mimic many elements of the human disease, coupled with advances in the genomic and informatic infrastructure for rats, promise to revolutionize the hunt for genes that determine susceptibility to hypertension. Furthermore, methods are evolving that should enable the identification of candidate genes in human populations. Together with the computational reconstruction of regulatory networks, these methods provide opportunities to significantly advance our understanding of the underlying aetiology of hypertension.

Heritability and major gene effects on left ventricular mass in the Chinese population: a family study

Background

Genetic components controlling for echocardiographically determined left ventricular (LV) mass are still unclear in the Chinese population.

Methods

We conducted a family study from the Chin-San community, Taiwan, and a total of 368 families, 1145 subjects, were recruited to undergo echocardiography to measure LV mass. Commingling analysis, familial correlation, and complex segregation analysis were applied to detect component distributions and the mode of inheritance.

Results

The two-component distribution model was the best-fitting model to describe the distribution of LV mass. The highest familial correlation coefficients were mother-son (0.379, P < .0001) and father-son (0.356, P < .0001). Genetic heritability (h²) of LV mass was estimated as 0.268 ± 0.061 (P < .0001); it decreased to 0.153 ± 0.052 (P = .0009) after systolic blood pressure adjustment. Major gene effects with polygenic components were the best-fitting model to explain the inheritance mode of LV mass. The estimated allele frequency of the gene was 0.089.

Conclusion

There were significant familial correlations, heritability and a major gene effect on LV mass in the population-based families.

Cardiac hypertrophy, substrate utilization and metabolic remodelling: cause or effect?

1. Metabolic remodelling in the heart occurs in response to chronically altered workload and substrate availability. Recently, the importance of the metabolic remodelling processes inherent in the hypertrophic growth response (whether primary or secondary) has been recognized. 2. Altered energy demand, shifts in substrate utilization and increased oxidative stress are observed in the hypertrophic heart. Both a shift away from carbohydrate usage (i.e. insulin resistance) and a shift to carbohydrate usage (i.e. pressure loading) are associated with disturbed cardiomyocyte Ca(2+) homeostasis and the development of cardiac hypertrophy. 3. A change in the balance of myocardial usage of fatty acid and glucose substrates must entail a degree of cellular oxidative stress. Increased throughput of any substrate will necessarily involve a regional imbalance between reactive oxygen species (ROS) production and breakdown. 4. In addition to a number of enzyme generators of ROS at various intracellular locations, the heart also contains a number of endogenous anti-oxidants, to restrict steady state ROS levels. The balance between ROS generation and their elimination by endogenous anti-oxidant mechanisms plays a critical role in preserving cardiac function; inappropriate levels of myocardial ROS likely precipitate impairment of myocardial function and abnormalities in cardiac structure. 5. Although different metabolic adaptations are associated with hypertrophic responses of contrasting aetiology, there is accumulating evidence that the joint insults of increased production of ROS and disturbed Ca(2+) handling in the cardiomyocyte comprise the primary lesion. These molecular signals operate together in a feed-forward mode and have the capacity to inflict substantial functional and structural damage on the hypertrophic myocardium.

Relative contribution of the prenatal versus postnatal period on development of hypertension and growth rate of the spontaneously hypertensive rat

1. To determine the relative roles of the prenatal and postnatal (preweaning) environment on the development of blood pressure and growth rate in the spontaneously hypertensive rat (SHR) of the Okamoto strain, we used combined embryo transfer and cross-fostering techniques between SHR and normotensive Wistar-Kyoto (WKY) rats to produce offspring whose development was examined during the first 20 weeks of life. 2. We measured litter sizes, bodyweights and tail-cuff blood pressures in offspring at 4, 8, 12 and 20 weeks of age. We also recorded heart, kidney and adrenal weights at 20 weeks of age, when the study concluded. 3. We found that both the in utero and postnatal environments provided by the SHR mother could significantly affect WKY rat offspring growth rates, but blood pressure was unaffected in this strain. In SHR offspring, the SHR maternal in utero and suckling period both contributed to the rate of blood pressure development in the SHR, but not the final blood pressure of offspring at 20 weeks of age. This effect was greater for male than female offspring. Organ weights were largely unaffected by the perinatal environment in either strain. 4. We conclude that although the SHR maternal in utero and immediate postnatal environment both contribute to the rate of blood pressure development in the SHR, they do not appear to contribute to the final blood pressure of offspring at maturity. The SHR maternal environment also alters growth rate that may, in turn, underlie these effects on SHR blood pressure development, particularly in males.

Heritability of left ventricular mass in a large cohort of twins

INTRODUCTION

Left ventricular hypertrophy is recognized as one of the most important independent predictors of adverse cardiovascular outcome. The aetiology of LVH includes a number of well-recognized causes but there is considerable interest in the genetics of cardiac muscle hypertrophy. We used a large prospective twin database in order to establish the heritability of left ventricular mass (LVM).

METHODS

Normotensive twins were prospectively recruited. Demographic data were collected. The LVM was determined using the Penn formulae derived from data collected from echocardiography.

RESULTS

A total of 376 Caucasian twin pairs (182 monozygotic and 194 dizygotic) aged 25-79 years were recruited. All subjects were normotensive with no significant differences in blood pressure (mean blood pressure: monozygotic twins, 132/83 mmHg; dizygotic twins, 131/82 mmHg) or body mass index between the monozygotic and dizygotic twins. The mean LVM for monozygotic twins was 140.9 g, compared with 140.2 g for dizygotic twins. Heritability estimates suggest that the genetic variance of LVM is 0.59 (95% confidence interval, 0.5-0.67). No common shared environmental effects were identified under this model.

CONCLUSION

Our data from the largest set of twin pairs studied to date show that LVM has a sizeable genetic basis that is probably polygenic. This result has important implications for the understanding of normal and abnormal cardiac morphology at the molecular level.

QTL mapping for traits associated with stress neuroendocrine reactivity in rats

In the present study we searched for quantitative trait loci (QTLs) that affect neuroendocrine stress responses in a 20-min restraint stress paradigm using Brown-Norway (BN) and Wistar-Kyoto-Hyperactive (WKHA) rats. These strains differed in their hypothalamic-pituitary-adrenal axis (plasma ACTH and corticosterone levels, thymus, and adrenal weights) and in their renin-angiotensin-aldosterone system reactivity (plasma renin activity, aldosterone concentration). We performed a whole-genome scan on a F2 progeny derived from a WKHA x BN intercross, which led to the identification of several QTLs linked to plasma renin activity (Sr6, Sr8, Sr11, and Sr12 on chromosomes RNO2, 3, 19, and 8, respectively), plasma aldosterone concentration (Sr7 and Sr9 on RNO2 and 5, respectively), and thymus weight (Sr10, Sr13, and Srl4 on RNO5, 10, and 16, respectively). The type 1b angiotensin II receptor gene (Agtrlb) maps within the confidence intervals of QTLs on RNO2 linked to plasma renin activity (Sr6, highly significant; LOD = 5.0) and to plasma aldosterone level (Sr7, suggestive; LOD = 2.0). In vitro studies of angiotensin II-induced release of aldosterone by adrenal glomerulosa cells revealed a lower receptor potency (log EC50 = -8.16 +/- 0.11 M) and efficiency (Emax = 453.3 +/- 25.9 pg/3 x 10(4) cells/24 h) in BN than in WKHA (log EC50 = -10.66 +/- 0.18 M; Emax = 573.1 +/- 15.3 pg/3 x 10(4) cells/24 h). Moreover, differences in Agtr1b mRNA abundance and sequence reinforce the putative role of the Agtr1b gene in the differential plasma renin stress reactivity between the two rat strains.

Cardiomyocyte function associated with hyperactivity and/or hypertension in genetic models of LV hypertrophy

We examined cardiomyocyte intracellular calcium ([Ca2+]i) dynamics and sarcomere shortening dynamics in genetic rat models of left ventricular (LV) hypertrophy associated with or without hypertension (HT) and with or without hyperactive (HA) behavior. Previous selective breeding of the spontaneously hypertensive rat (SHR) strain, which is HA and HT, with the Wistar-Kyoto (WKY) rat strain, which is not hyperactive (NA) and not hypertensive (NT), has led to two unique strains: the WKHA strain, selected for HA and NT, and the WKHT strain, selected for NA and HT. Cardiomyocytes were isolated from young adult males and females of each strain, paced at 2, 3, and 4 Hz in 1.2 mM external Ca2+ concentration at 37 degrees C, and cardiomyocyte [Ca2+]i and sarcomere dynamics were recorded simultaneously. Under these conditions, LV cardiomyocyte systolic and diastolic [Ca2+]i dynamics and diastolic sarcomere dynamics in the WKHT were significantly enhanced compared with WKY controls, suggesting an underlying LV hypertrophic response that successfully compensated for HT in the absence of HA. LV cardiomyocyte [Ca2+]i dynamics in the WKHA and SHR were strikingly similar to each other and only slightly reduced compared with WKY. LV cardiomyocyte systolic and diastolic sarcomere dynamics, on the other hand, were significantly reduced in the SHR compare with WKHA and more so in male than in female SHR. We conclude from these data that HT alone is an insufficient descriptor of the cause of LV hypertrophy and diminished LV cardiomyocyte function in the SHR rat. These data further suggest that HA (augmented by male sex) in the SHR may interact with the HT state to initiate impaired cardiomyocyte function and thereby inhibit or undermine an otherwise compensatory response that may occur with HT in the absence of HA.

Distinct QTLs are linked to cardiac left ventricular mass in a sex-specific manner in a normotensive inbred rat inter-cross

Genetic mapping of the progeny of an F(2) inter-cross between WKY and WKHA rats had previously allowed us to detect male-specific linkage between locus Cm 24 and left ventricular mass index (LVMI). By further expanding that analysis, we detected additional loci that were all linked to LVMI in a sex-specific manner despite their autosomal location. In males, we detected one additional locus (Lvm 8) on Chromosome 5 (LOD=3.4), the two loci Lvm 13 (LOD=4.5) and Lvm 9 (LOD=2.8) on Chromosome 17, and locus Lvm 10 (LOD=4.2) on Chromosome 12. The locus Lvm 13 had the same boundaries as locus Cm 26 previously reported by others using a different cross. None of these loci showed linkage to LVM in females. In contrast, we identified in females the novel locus Lvm 11 on Chromosome 15 (LOD=2.8) and locus Lvm 12 (LOD=2.7) that had the same boundaries on Chromosome 3 as locus Cm 25 detected previously by others using a cross of other normotensive strains. In prepubertal males, there were no differences in the width of cardiomyocytes from WKY and WKHA rats, but cardiomyocytes from WKHA became progressively wider than that of WKY as sexual maturation progressed. Altogether, these results provide evidence that distinct genes may influence LVMI of rats in a sex-dependent manner, maybe by involving sex-specific interactions of sex steroids with particular genes involved in the determination of LVMI and/or cardiomyocyte width.

Identification of quantitative trait loci for cardiac hypertrophy in two different strains of the spontaneously hypertensive rat

Cardiac hypertrophy and left ventricular hypertrophy are known to be substantially controlled by genetic factors. As an experimental model, we undertook genome-wide screens for cardiac mass in F2 populations bred from the stroke-prone spontaneously hypertensive rats (SHRSP) and normal spontaneously hypertensive rats (SHR) and Wistar Kyoto rats (WKY) of a Japanese colony. Two F2 cohorts were independently produced: F2(SHRSP x WKY) (110 male and 110 female rats) and F2(SHR x WKY) (151 male rats). The ratio of heart weight to body weight (Hw/Bw) was evaluated at 12 months of age in F2(SHRSP x WKY) after salt-loading for 7 months, and at around 15 weeks of age in F2(SHR x WKY) who had been fed a normal rat chow diet. Subsequent to an initial screen with 251 markers in F2(SHRSP x WKY) male progeny, 170 and 161 markers were selected and characterized in F2(SHRSP x WKY) female progeny and F2(SHR x WKY) male progeny, respectively. Markers from four chromosomal regions showed suggestive or significant linkage to Hw/Bw. The strongest and the most consistent linkage was found in the vicinity of D3Mgh16 on rat chromosome (RNO) 3 (a maximal log of the odds score reached 4.0 to 6.6 across the F2 populations studied). In the other three regions on RNO6, RNO10 and RNO13, the degree of linkage was more prominent in either males or females. These data provide solid evidence for a "principal" RNO3 quantitative trait loci regulating Hw/Bw in SHRSP and SHR, and also suggest the possible presence of sexual dimorphism in regard to genetic susceptibility for cardiac hypertrophy.

No involvement of the nerve growth factor gene locus in hypertension in spontaneously hypertensive rats

Sympathetic hyper-innervation and increased levels of nerve growth factor (NGF), an essential neurotrophic factor for sympathetic neurons, have been observed in the vascular tissues of spontaneously hypertensive rats (SHRs). Such observations have suggested that the pathogenesis of hypertension might involve a qualitative or quantitative abnormality in the NGF protein, resulting from a significant mutation in the gene's promoter or coding region. In the present study, we analyzed the nucleotide sequences of the cis-element of the NGF gene in SHRs, stroke-prone SHRs (SHRSPs), and normotensive Wistar-Kyoto (WKY) rats. The present analyses revealed some differences in the 3-kb promoter region, coding exon, and 3' untranslated region (3'UTR) for the NGF gene among those strains. However, the observed differences did not lead to changes in promoter activity or to amino acid substitution; nor did they represent a link between the 3'UTR mutation of SHRSPs and elevated blood pressure in an F2 generation produced by crossbreeding SHRSPs with WKY rats. These results suggest that the NGF gene locus is not involved in hypertension in SHR/ SHRSP strains. The present study also revealed two differences between SHRs and WKY rats, as found in cultured vascular smooth muscle cells and in mRNA prepared from each strain. First, SHRs had higher expression levels of c-fos and c-jun genes, which encode the component of the AP-1 transcription factor that activates NGF gene transcription. Second, NGF mRNAs prepared from SHRs had a longer 3'UTR than those prepared from WKY rats. Although it remains to be determined whether these events play a role in the hypertension of SHR/SHRSP strains, the present results emphasize the importance of actively searching for aberrant trans-acting factor(s) leading to the enhanced expression of the NGF gene and NGF protein in SHR/SHRSP strains.

Fine mapping of Lvm1: a quantitative trait locus controlling heart size independently of blood pressure

We have previously reported a quantitative trait locus (QTL) on rat chromosome 2 that influences heart size independently of blood pressure (Left Ventricular Mass Locus 1; Lvm1). The recent release of the rat genome sequence allowed us to retest and refine this relatively broad QTL with a view to identifying within it candidate genes worthy of structural investigation. We sought to achieve this 'fine mapping' by increasing the marker density within the interval and undertaking a linkage analysis in a previously defined population of F2 hybrids generated from inbred spontaneously hypertensive rats (SHR) of the Okamoto strain and Fischer rat (F344) progenitors. We were able to reconfirm and resolve Lvm1 from its original width of approximately 45 to 15 cM. By reference to the ENSEBL rat genome data bank, we identified within Lvm1 27 known genes, 109 predicted genes and 7 pseudogenes. Of the known genes, candidates include potential regulators of cardiac growth, a sodium channel and calcium channel as well as the fibroblast growth factor 2 gene. Located nearby the Lvm1 locus was the gene for the angiotensin Type 1B receptor. Given the evidence that the ligand for the angiotensin Type 1B receptor-angiotensin II-is a potent cardiotroph, we also consider this gene a potential candidate. The identification of the precise allelic variant(s) within Lvm1 involved in the control of pressure-independent cardiac growth awaits further molecular studies.

Single nucleotide polymorphisms predict the change in left ventricular mass in response to antihypertensive treatment

BACKGROUND

Our aim was to determine whether the change in left ventricular (LV) mass in response to antihypertensive treatment could be predicted by multivariate analysis of single nucleotide polymorphisms (SNPs) in candidate genes reflecting pathways likely to be involved in blood pressure control.

METHODS

Patients with mild to moderate primary hypertension and LV hypertrophy were randomized in a double-blind fashion to treatment with either the angiotensin II type 1 receptor antagonist irbesartan (n = 48) or the beta1 adrenoreceptor blocker atenolol (n = 49). A microarray-based minisequencing system was used for genotyping 74 SNPs in 25 genes. These genotypes were related to the change in LV mass index by echocardiography, after 12 weeks treatment as monotherapy, using stepwise multiple regression analysis.

RESULTS

The blood pressure reductions were similar and significant in both treatment groups. Two SNPs in two separate genes (the angiotensinogen T1198C polymorphism, corresponding to the M235T variant and the apolipoprotein B G10108A polymorphism) for those treated with irbesartan, and the adrenoreceptor alpha2A A1817G for those treated with atenolol, significantly predicted the change in LV mass. The predictive power of these SNPs was independent of the degree of blood pressure reduction.

CONCLUSION

SNPs in the angiotensinogen, apolipoprotein B, and the alpha2 adrenoreceptor gene predicted the change in LV mass during antihypertensive therapy. These results illustrate the potential of using microarray-based technology for SNP genotyping in predicting individual drug responses.

Cosegregation analysis of natriuretic peptide genes and blood pressure in the spontaneously hypertensive rat

1. The natriuretic peptide precursor A (Nppa) and B (Nppb) genes are candidate genes for hypertension and cardiac hypertrophy in the spontaneously hypertensive rat (SHR). The purpose of the present study was to determine the role of the Nppa and Nppb genes in the development of hypertension in the SHR. 2. A cohort (n = 162) of F2 segregating intercross animals was established between strains of hypertensive SHR and normotensive Wistar-Kyoto rats. Blood pressure and heart weight were measured in each rat at 12-16 weeks of age. Rats were genotyped using 11 informative microsatellite markers, distributed in the vicinity of the Nppa marker on rat chromosome 5 including an Nppb marker. The phenotype values were compared with genotype using the computer package mapmaker 3.0 (Whitehead Institute, Boston, MA, USA) to determine whether there was a link between the genetic variants of the natriuretic peptide family and blood pressure or cardiac hypertrophy. 3. A strong correlation was observed between the Nppa marker and blood pressure. A quantitative trait locus (QTL) for blood pressure on chromosome 5 was identified between the Nppa locus and the D5Mgh15 marker, less than 2 cM from the Nppa locus. The linkage score for the blood pressure QTL on chromosome 5 was 3.8 and the QTL accounted for 43% of the total variance of systolic blood pressure, 54% of diastolic blood pressure and 59% of mean blood pressure. No association was found between the Nppb gene and blood pressure. This is the first report of linkage between the Nppa marker and blood pressure in the rat. There was no correlation between the Nppa or Nppb genes or other markers in this region and either heart weight or left ventricular weight in F2 rats. 4. These findings suggest the existence of a blood pressure-dependent Nppa marker variant or a gene close to Nppa predisposing to spontaneous hypertension in the rat. It provides a strong foundation for further detailed genetic studies in congenic strains, which may help to narrow down the location of this gene and lead to positional cloning.

Sequence analysis of the fibroblast growth factor 2 gene from the spontaneously hypertensive and hypertrophic heart rats

We have previously reported a quantitative trait locus associated with pressure-independent cardiac hypertrophy in the spontaneously hypertensive rat (SHR) of the Okamoto strain. This locus (Lvm1; left ventricular mass locus 1) contains the gene Fgf2 that codes for the potent cardiac growth factor, Fibroblast Growth Factor 2 (FGF2). Given that FGF2 appears essential for the induction of certain forms of cardiac hypertrophy in the rat, we proposed this gene as a candidate for the cardiac enlargement seen in the SHR. Previous reports of elevated FGF2 mRNA levels in the SHR, led us to hypothesise that nucleotide sequence variations occurring in the coding regions or in putative transcriptional factor binding sites within the Fgf2 promoter might play a role in cardiac hypertrophy in this strain. Given that we have also recently derived from the SHR a rat strain that develops spontaneous cardiac hypertrophy in the absence of hypertension (the Hypertrophic Heart Rat; HHR), we also took the opportunity to examine the sequence of its Fgf2 promoter and coding region. However, extensive sequence analysis of the promoter and coding regions of the SHR and HHR Fgf2 genes failed to reveal any nucleotide variations between strains. Thus, we conclude that variations in the nucleotide sequence of the promoter and coding region of the SHR Fgf2 gene do not play a role in the cardiac hypertrophy of the SHR and HHR strains.

Myocardial hypertrophy of normotensive Wistar-Kyoto rats

In our studies with spontaneously hypertensive (SHR), Wistar-Kyoto (WKY), and Wistar rats, we observed normotensive WKY rats with cardiac hypertrophy determined by a greater left ventricular (LV) mass (LVM)-to-body weight (BW) ratio (LVM/BW) than that of normotensive Wistar rats. Thus we compared the following parameters in SHR, WKY, and Wistar rats: LVM/BW, cell capacitance as index of total surface area of the myocytes, length, width, and cross-sectional area of cardiac myocytes, LV collagen volume fraction, and myocardial stiffness. The LVM/BW of WKY (2.41 ± 0.03 mg/g, n = 41) was intermediate between SHR (2.82 ± 0.04 mg/g, n = 47) and Wistar rats (1.98 ± 0.04 mg/g, n = 28). A positive correlation between blood pressure and LVM was found in SHR, whereas no such relationship was observed in WKY or Wistar rats. Cell capacitance and cross-sectional area were not significantly different in SHR and WKY rats; these values were significantly higher than those of Wistar rats. The cell length was smaller but the width was similar in WKY compared with SHR. Papillary muscles isolated from the LV of WKY and SHR were stiffer than those from Wistar rats. Consistently, a greater level of myocardial fibrosis was detected in WKY and SHR compared with Wistar rats. These findings demonstrate blood pressure-independent cardiac hypertrophy in normotensive WKY rats.

Genomic map of cardiovascular phenotypes of hypertension in female Dahl S rats

Genetic linkage analyses in human populations have traditionally combined male and female progeny for determination of quantitative trait loci (QTL). In contrast, most rodent studies have focused primarily on males. This study represents an extensive female-specific linkage analysis in which 236 neuroendocrine, renal, and cardiovascular traits related to arterial pressure (BP) were determined in 99 female F2 rats derived from a cross of Dahl salt-sensitive SS/JrHsdMcwi (SS) and Brown Norway normotensive BN/SsNHsdMcwi (BN) rats. We identified 126 QTL for 96 traits on 19 of the 20 autosomal chromosomes of the female progeny. Four chromosomes (3, 6, 7, and 11) were identified as especially important in regulation of arterial pressure and renal function, since aggregates of 8–11 QTL mapped together on these chromosomes. BP QTL in this female population differed considerably from those previously found in male, other female, or mixed sex population linkage analysis studies using SS rats. Kidney weight divided by body weight was identified as an intermediate phenotype that mapped to the same region of the genome as resting diastolic blood pressure and was correlated with that same BP phenotype. Seven other phenotypes were considered as “potential intermediate phenotypes, ” which mapped to the same region of the genome as a BP QTL but were not correlated with BP. These included renal vascular responses to ANG II and ACh and indices of baroreceptor responsiveness. Secondary traits were also identified that were likely to be consequences of hypertension (correlated with BP but not mapped to a BP QTL). Seven such traits were found, notably heart rate, plasma cholesterol, and renal glomerular injury. The development of a female rat systems biology map of cardiovascular function represents the first attempt to prioritize those regions of the genome important for development of hypertension and end organ damage in female rats.

Exaggerated response to restraint stress in rats congenic for the chromosome 1 blood pressure quantitative trait locus

1. To understand the roles of a putative hypertension gene in the chromosome 1 quantitative trait locus (QTL) region, the response to restraint stress was studied in strains congenic for this QTL. 2. To establish congenic strains, the QTL region was introgressed from stroke-prone spontaneously hypertensive rats (SHRSP)/Izm to Wistar-Kyoto/Izm (WKY/Izm) rats by repeated backcrossing. Two congenic strains (WKYpch1.0 and WKYpch1.1) were established to cover the whole QTL region between D1Wox29 and D1Arb21 (approximately 40 cM) and a smaller region between D1Smu11 and D1Arb21 (approximately 10 cM), respectively. After telemetry probes were implanted, rats were exposed to restraint stress to investigate the blood pressure response. 3. Basal blood pressure measured by radiotelemetry differed significantly between WKY rats and WKYpch1.0 (103 +/- 10 and 116 +/- 4 mmHg, respectively; P = 0.002 by anova). When exposed to restraint stress, WKYpch1.0 showed a greater increase in blood pressre than did WKY rats. The exaggerated response in the WKYpch1.0 strain was abolished by chemical sympathectomy using guanethidine. The WKYpch1.1 rats did not differ significantly from WKY rats either in basal blood pressure or in the response to restraint stress. 4. In conclusion, a QTL for high blood pressure was successfully introgressed in the established congenic strain, WKYpch1.0. A gene (or genes) in the chromosome 1 QTL region modulates the cardiovascular responses to restraint stress in these congenic rats, probably through the sympathetic nervous system.

The hypertrophic heart rat: a new normotensive model of genetic cardiac and cardiomyocyte hypertrophy

We describe a new line of rats with inherited cardiomyocyte and ventricular hypertrophy. From a second-generation cross of spontaneously hypertensive and Fischer 344 rats, we selected for low blood pressure and either high or low echocardiographic left ventricular (LV) mass over four generations to establish the hypertrophic heart rat (HHR) and normal heart rat (NHR) lines, respectively. After 13 generations of inbreeding, HHR had significantly greater ( P < 0.0001) LV mass-to-body weight ratio (2.68 g/kg, SE 0.14) than NHR matched for age (1.94 g/kg, SE 0.02) or body weight (2.13 g/kg, SE 0.03). The isolated cardiomyocytes of HHR were significantly ( P < 0.0001) longer and wider (161 μm, SE 0.83; 35.6 μm, SE 2.9) than NHR (132 μm, SE 1.2; 29.5 μm, SE 0.35). Telemetric 24-h recordings of mean arterial pressure revealed no significant differences between HHR and NHR. The HHR offers a new model of primary cardiomyocyte hypertrophy with normal blood pressure in which to examine genotypic causes and pathogenetic mechanisms of hypertrophy and its complications.

Polymorphisms in the angiotensinogen and angiotensin II type 1 receptor gene are related to change in left ventricular mass during antihypertensive treatment: results from the Swedish Irbesartan Left Ventricular Hypertrophy Investigation versus Atenolol (SILVHIA) trial

BACKGROUND

Our aim was to determine if gene polymorphisms in the renin-angiotensin-aldosterone system (RAAS) were related to the degree of change in left ventricular hypertrophy (LVH) during antihypertensive treatment.

METHODS AND RESULTS

Patients with essential hypertension and echocardiographically diagnosed LVH were included in a double-blind study to receive treatment with either the angiotensin II type 1 receptor (AT1-receptor) antagonist irbesartan (n = 41), or the beta-1 adrenergic receptor blocker atenolol (n = 43) as monotherapy for 3 months. The angiotensinogen T174M and M235T, the angiotensin-converting enzyme I/D, the AT1-receptor A1166C and the aldosterone synthase (CYP11B2) -344 C/T polymorphisms were analysed and related to the change in left ventricular mass (LVM). Patients with the angiotensinogen 174 TM genotype treated with irbesartan responded with the greatest reduction in LVM (-23 +/- 31SD g/m2 for TM and +0.5 +/- 18 g/m2 for TT, P = 0.005), independent of blood pressure reduction. Both the angiotensinogen 235 T-allele (P = 0.02) and the AT1-receptor 1166 AC genotype responded with the greatest reduction in LVM when treated with irbesartan (-0.1 +/- 19 g/m2 for AA and -18 +/- 30 g/m2 for AC, P = 0.02), independent of blood pressure reduction. These polymorphisms were not associated with the change in LVM during treatment with atenolol.

DISCUSSION

The angiotensinogen T174M and M235T and the AT1-receptor A1166C polymorphisms were related to the change in LVH during antihypertensive treatment with an AT1-receptor antagonist; of these angiotensinogen T174M was the most powerful. This highlights the role of the RAAS for left ventricular hypertrophy and the potential of pharmacogenetics as a tool for guidance of antihypertensive therapy.

Characterization of myocardium, isolated cardiomyocytes, and blood pressure in WKHA and WKY rats

We previously reported that the left ventricular (LV) mass of Wistar-Kyoto (WKY)-derived hyperactive (WKHA) rats was higher than that of WKY rats in the absence of a difference in systolic blood pressure. To extend these earlier observations, we conducted a series of functional and morphological investigations on both strains. Analysis of tissue sections revealed that the surface of ventricular tissue from WKHA rats was higher than that of WKY rats, without any enlargement of the cavity area. Analysis of isolated adult cells showed that cell width (as well as cell volume) of ventricular cardiomyocytes was significantly higher in WKHA than WKY rats. However, LV of WKHA rats contained approximately 33% less cardiomyocytes than those from WKY rats. Mean intracellular free calcium concentration of cardiomyocytes was also higher in WKHA than WKY rats. Hemodynamic measurements revealed that the values of the maximum rates of pressure change (dP/dt) were higher in LV from WKHA rats. However, these differences were reduced (-dP/dt) or abolished (+dP/dt) when the values were normalized for both the number and mean cross-sectional area of ventricular cardiomyocytes. Mean levels of systolic and diastolic blood pressure (corresponding to the 24-h average of measurements obtained continuously in conscious unrestrained animals using radiotelemetric implants) were not different between strains. However, circadian rhythm was more evident in WKY rats, because the difference between morning and night values of systolic and diastolic blood pressure was greater (by 3 mmHg) in WKY rats. Altogether, our data validate the use of WKHA rats as models of predominantly concentric LV hypertrophy developing in the absence of increased mean levels of hemodynamic cardiac load and show that the hypertrophy phenotype is more pronounced in isolated cardiomyocytes than at the level of the whole ventricle.

Relation of hemodynamics and risk factors to ventricular-vascular interactions in the elderly: the Cardiovascular Health Study

OBJECTIVE

To investigate the interaction between left ventricular (LV) geometry, carotid structure and arterial compliance in relation to hemodynamic stimuli and risk factors (plasma cholesterol, body mass index, insulin resistance, smoking habit, age, sex and race).

DESIGN

Cross-sectional.

METHODS

Echocardiography and carotid ultrasound were performed in 2375 elderly subjects without signs or history of prevalent cardiovascular disease, diabetes or renal disease (795 men; 298 non-whites; 1215 hypertensive), from the cohort of the Cardiovascular Health Study. Arterial compliance was estimated by the prognostically validated ratio of stroke volume to pulse pressure (SV/PP) as the percent deviation (Delta%) from the value predicted by individual age, heart rate and body weight.

RESULTS

Intima-medial thickness (IMT) was higher in the presence of LV hypertrophy (LVH) in normotensive and hypertensive subjects and was greatest in the presence of concentric LVH. Maximum carotid lumen diameter (CLD) was also higher in the presence of LVH (and was greatest with eccentric LVH, in association with relatively high values for stroke volume). After adjusting for blood pressure, maximum carotid lumen diameter was directly correlated with stroke volume, and IMT to LV mass (all P < 0.001). Similarly, IMT was also related to maximum carotid lumen diameter, independently of prevalent risk factors (P < 0.001). SV/PP-Delta% was reduced in both groups with concentric LV remodeling (both P < 0.0001) or concentric LVH (both P < 0.05). Adjusting for risk factors did not affect these associations in normotensives, but made them insignificant in hypertensives. In normotensives, IMT was inversely related to SV/PP-Delta% (P < 0.001), independently of risk factors, whereas no significant relation was found in hypertensives.

CONCLUSIONS

The magnitudes of carotid intima-medial thickness and lumen diameter parallel levels of LV mass and geometry, and are directly related to stroke volume and arterial stiffness; this interaction is most evident in the presence of normal blood pressure, whereas it is affected by other cardiovascular risk factors when arterial hypertension is present.

Genetic isolation of a blood pressure quantitative trait locus on chromosome 2 in the spontaneously hypertensive rat

OBJECTIVES

Total genome scans of genetically segregating populations derived from the spontaneously hypertensive rat (SHR) and other rat models of hypertension have suggested the presence of quantitative trait loci (QTL) regulating blood pressure and cardiac mass on multiple chromosomes, including chromosome 2. The objective of the current study was to directly test for the presence of a blood pressure QTL on rat chromosome 2.

DESIGN

A new congenic strain was derived by replacing a segment of chromosome 2 in the SHR between D2Rat171 and D2Arb24 with the corresponding chromosome segment from the normotensive Brown Norway rat. Arterial pressures were directly monitored in conscious rats by radiotelemetry.

RESULTS

We found that the SHR congenic strain (SHR-2) carrying a segment of chromosome 2 from the Brown Norway rat had significantly lower systolic and diastolic blood pressures than the SHR progenitor strain. The attenuation of hypertension in the SHR-2 congenic strain versus the SHR progenitor strain was accompanied by significant amelioration of cardiac hypertrophy.

CONCLUSIONS

These findings demonstrate that gene(s) with major effects on blood pressure exist in the differential segment of chromosome 2 trapped within the new SHR.BN congenic strain.

Multiple blood pressure QTL on rat chromosome 1 defined by Dahl rat congenic strains

A series of congenic strains were constructed in which segments of chromosome (chr) 1 from Lewis (LEW) rats were introgressed into the Dahl salt-sensitive (S) strain. Three blood pressure quantitative trait loci (QTL) were defined. Two of these (QTL 1a and QTL 1b) were closely linked in the region between 1q31 and 1q35. The third blood pressure QTL (QTL region 2) was close to the centromere between 1p11 and 1q12, which includes the candidate gene Slc9a3 for sodium/hydrogen exchange. The blood pressure QTL 1a and QTL 1b defined here overlap significantly with QTL for disease phenotypes of renal failure, stroke, ventricular mass, and salt susceptibility defined in other rat strains, implying that these disease phenotypes and our blood pressure phenotype have causes in common. QTL 1b also corresponded approximately with a blood pressure QTL described on human chr 15. The QTL region 2 corresponded approximately with blood pressure QTL described on mouse chr 10 and human chr 6.

Effects of antihypertensive agents on the left ventricle: clinical implications

Hypertensive heart disease (HHD) is characterized by left ventricular hypertrophy (LVH), alterations of cardiac function, and coronary flow abnormalities. LVH is an independent cardiovascular risk factor related to cardiovascular complications in patients with hypertension. Therefore, a decrease in left ventricular mass is a therapeutic goal in these patients. The effect of the different antihypertensive agents on LVH regression has been studied in nearly 500 clinical trials. Most studies conclude that there is regression of LVH after significant decrease in blood pressure with most commonly prescribed antihypertensive agents. However, the ability to regress LVH is different between antihypertensive drug classes. ACE inhibitors and calcium channel antagonists are more potent in reducing left ventricular mass than beta-blockers, with diuretics falling in the intermediate group. Recent data suggest that angiotensin AT(1) receptor antagonists reduce left ventricular mass to a similar extent as ACE inibitors or calcium channel antagonists. Although a large number of studies have established that reversal of LVH decreases the occurrence of adverse cardiovascular events in patients with hypertension, the hypothesis that LVH regression is beneficial has not yet been conclusively proven. On the other hand, the time has come to revisit the current management of HHD simply focused on controlling blood pressure and reducing left ventricular mass. In fact, it is necessary to develop new approaches aimed to repair myocardial structure and protect myocardial perfusion and function and, in doing so, to reduce in a more effective manner, adverse risk associated with HHD. The identification of genes involved in both the process of HHD and the response to therapy may be critical for the development of these new approaches. This article will review briefly the available data on the effects of antihypertensive agents on HHD. In addition, the emerging new concepts on the pharmacology of hypertensive myocardial remodeling and the pharmacogenetic basis of the treatment of HHD will be also considered.

Variants of trophic factors and expression of cardiac hypertrophy in patients with hypertrophic cardiomyopathy

Patients with hypertrophic cardiomyopathy (HCM) exhibit variable expression of left ventricular hypertrophy (LVH), a major determinant of mortality and morbidity, which is partly due to the diversity of causal mutations, genetic background (modifier genes), and probably environmental factors. We determined association of functional variants of tumor necrosis factor (TNF)- alpha, interleukin-6 (IL6), insulin-like growth factor-2 (IGF2), transforming growth factor- beta 1 (TGFB1), and aldosterone synthase (CYP11B2) genes, all previously implicated in cardiac hypertrophy, with the severity of LVH in patients with HCM. Two-dimensional echocardiography was performed and demographic variables were recorded in 142 genetically independent patients. Indices of LVH including interventricular septal thickness (IVST), left ventricular mass index (LVMI), and LVH score were measured/calculated. TNF-alpha-308G/A, IL6-174G/C, IGF2 820G/A, TGFB1-509C/T, and CYP11B2-344T/C genotypes were determined by polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP). Genotypes were identified by the presence of specific electrophoretic patterns and their distributions were according to the Hardy-Weinberg equilibrium. Demographic variables were not significantly different among the genotypes. Subjects with the AA genotype of TNF-alpha (n=8) were approximately 13 years younger at the time of clinical diagnosis. Despite a younger age, they had a greater mean LVMI than those with the GG (n=94) or GA (n=33) genotypes (191.8+/-59.5 v 139.1+/-47.3 v 132.1+/-34.3, respectively, P=0.004). TNF-alpha-308G/A genotypes accounted for 6.0% of variability of LVMI (P=0.002). Mean IVST, LVEDD, and LVH score were not significantly different. Variants of IL6, IGF2, TGFB1, and CYP11B2 were not associated with indices of LVH. The uncommon allele of TNF-alpha-308G/A polymorphism, known to produce more TNF- alpha, was associated with greater LVMI and clinical diagnosis at a younger age in patients with HCM. Functional variants of other trophic factors, previously implicated in cardiac hypertrophy, were not associated with the indices of LVH. These results suggest that TNF-alpha is a modifier gene for HCM.

Genetic mapping of quantitative trait loci influencing left ventricular mass in rats

High blood pressure is the leading cause of left ventricular hypertrophy (LVH); however, not all hypertensive patients develop LVH. Genetic factors are important in the development of LVH. With the use of F2 male rats from spontaneously hypertensive rats and Lewis rats, we performed a study to identify the quantitative trait loci (QTL) that influence left ventricular mass (LVM). Mean arterial pressure (MAP) was measured by the direct intra-arterial method in conscious animals, and LVM was determined at 24 wk of age. QTL analysis was done using 160 microsatellite markers for a genome-wide scan. Two loci that influenced body weight-adjusted LVM with logarithm of the odds scores >3.4 were found. One locus on chromosome 17 influenced LVM independently of MAP. Another locus on chromosome 7 influenced LVM and MAP. These findings indicate not only the existence of a gene on chromosome 7 that influences LVM in a manner dependent on blood pressure but also the existence of a gene on chromosome 17 that influences LVM independently of blood pressure.

Selective genotyping with epistasis can be utilized for a major quantitative trait locus mapping in hypertension in rats

Epistasis used to be considered an obstacle in mapping quantitative trait loci (QTL) despite its significance. Numerous epistases have proved to be involved in quantitative genetics. We established a backcross model that demonstrates a major QTL for hypertension (Ht). Seventy-eight backcrossed rats (BC), derived from spontaneously hypertensive rats (SHR) and normotensive Fischer 344 rats, showed bimodal distribution of systolic blood pressure (BP) values and a phenotypic segregation ratio consistent with 1:1. In this backcross analysis, sarco(endo)plasmic reticulum Ca(2+)-dependent ATPase (Serca) II heterozygotes showed widespread bimodality in frequency distribution of BP values and obviously demonstrated Ht. First, in genome-wide screening, Mapmaker/QTL analysis mapped Ht at a locus between D1Mgh8 and D1Mit4 near Sa in all 78 BC. The peak logarithm of the odds (LOD) score reached 5.3. Second, Serca II heterozygous and homozygous BC were analyzed separately using Mapmaker/QTL. In the 35 Serca II heterozygous BC, the peak LOD score was 3.8 at the same locus whereas it did not reach statistical significance in the 43 Serca II homozygotes. Third, to map Ht efficiently, we selected 18 Serca II heterozygous BC with 9 highest and 9 lowest BP values. In these 18 BC, the peak LOD score reached 8.1. In 17 of the 18, D1Mgh8 genotypes (homo or hetero) qualitatively cosegregated with BP phenotypes (high or low) (P < 0.0001, by chi-square analysis). In conclusion, selective genotyping with epistasis can be utilized for a major QTL mapping near Sa on chromosome 1 in SHR.

Congenic rats for hypertension: how useful are they for the hunting of hypertension genes?

1. Linkage studies have revealed quantitative trait loci (QTL) for blood pressure in the rat genome using genetic hypertensive rat models. To identify the genes responsible for hypertension, the construction of congenic rats is essential. 2. To date, several congenic strains have been obtained from spontaneously hypertensive or Dahl salt-sensitive rats. The results of these studies should be interpreted according to whether the rats carry the whole QTL region or not. 3. After establishing congenic strains, three strategies are possible: (i) an orthodox positional cloning in which, using subcongenic strains, the QTL region is cut down to smaller fragments suitable for physical mapping; (ii) a positional candidate strategy in which candidate genes in the QTL regions are studied; or (iii) physiological studies in which intermediate phenotypes directly associated with the hypertension gene are explored. Several other experimental strategies are also available using congenic strains as new animal models for hypertension. 4. To make the most of advances in DNA technology, the precise evaluation of the phenotypic difference between congenic strains carrying different QTL or between a congenic and parental strain is critical.

New target regions for human hypertension via comparative genomics

Models of human disease have long been used to understand the basic pathophysiology of disease and to facilitate the discovery of new therapeutics. However, as long as models have been used there have been debates about the utility of these models and their ability to mimic clinical disease at the phenotypic level. The application of genetic studies to both humans and model systems allows for a new paradigm, whereby a novel comparative genomics strategy combined with phenotypic correlates can be used to bridge between clinical relevance and model utility. This study presents a comparative genomic map for "candidate hypertension loci in humans" based on translating QTLs between rat and human, predicting 26 chromosomal regions in the human genome that are very likely to harbor hypertension genes. The predictive power appears robust, as several of these regions have also been implicated in mouse, suggesting that these regions represent primary targets for the development of SNPs for linkage disequilibrium testing in humans and/or provide a means to select specific models for additional functional studies and the development of new therapeutics.

Genetic contributions to left ventricular hypertrophy

Left ventricular (LV) hypertrophy is a common condition that profoundly affects morbidity and mortality from cardiovascular diseases, including myocardial infarction, congestive heart failure, and stroke. Noninvasive imaging methods have greatly expanded our ability to evaluate cardiac structural and functional characteristics, and enhanced our understanding of the natural history of LV hypertrophy. The etiology of LV hypertrophy is likely due to the effects of multiple genes interacting with other genes and the environment. Although hypertension is recognized as a strong determinant of LV hypertrophy, blood pressure explains only a limited amount of the interindividual variation in LV mass. Moreover, LV hypertrophy occurs in the absence of hypertension, and in some cases precedes its development. Genes encoding proteins involved in the structure of the LV, as well as genes encoding cell signal transduction, hormones, growth factors, calcium homeostasis, and blood pressure, are likely candidates for the development of common forms of LV hypertrophy. An overview of the epidemiology and pathophysiology of LV hypertrophy and dysfunction is provided, in addition to evidence of the genetic basis for LV hypertrophy.

Genetic analysis of inherited hypertension in the rat

Blood pressure is a quantitative trait that has a strong genetic component in humans and rats. Several selectively bred strains of rats with divergent blood pressures serve as an animal model for genetic dissection of the causes of inherited hypertension. The goal is to identify the genetic loci controlling blood pressure, i.e., the so-called quantitative trait loci (QTL). The theoretical basis for such genetic dissection and recent progress in understanding genetic hypertension are reviewed. The usual paradigm is to produce segregating populations derived from a hypertensive and normotensive strain and to seek linkage of blood pressure to genetic markers using recently developed statistical techniques for QTL analysis. This has yielded candidate QTL regions on almost every rat chromosome, and also some interactions between QTL have been defined. These statistically defined QTL regions are much too large to practice positional cloning to identify the genes involved. Most investigators are, therefore, fine mapping the QTL using congenic strains to substitute small segments of chromosome from one strain into another. Although impressive progress has been made, this process is slow due to the extensive breeding that is required. At this point, no blood pressure QTL have met stringent criteria for identification, but this should be an attainable goal given the recently developed genomic resources for the rat. Similar experiments are ongoing to look for genes that influence cardiac hypertrophy, stroke, and renal failure and that are independent of the genes for hypertension.

Localization of a blood pressure QTL on rat chromosome 1 using Dahl rat congenic strains

We previously reported that markers on rat chromosome 1 are genetically linked to blood pressure in an F(2) population derived from Dahl salt hypertension-sensitive (S) and Lewis (LEW) rats. Because there was evidence for more than one blood pressure quantitative trait locus (QTL) on chromosome 1, an initial congenic strain introgressing a large 118-centimorgan (cM) segment of LEW chromosome 1 into the S background had been constructed. This initial congenic strain had a reduced blood pressure compared with S rats, proving the existence of a blood pressure QTL, but not giving a good localization of the QTL. In the present work a series of five overlapping congenic substrains were produced from the original congenic strain in order to localize a blood pressure QTL to a 25-cM region near the center of chromosome 1. The congenic substrains also ruled out the Sa locus as a blood pressure QTL in the S vs. LEW comparison because the Sa locus was contained in a congenic substrain that did not alter blood pressure.

Genetic testing, screening, and counseling issues in cardiovascular disease

Genetic risk assessment for cardiovascular disease is less advanced and less widely performed to date than it is for cancer. Yet it is no less important. Alert clinicians should "think genetically" and follow up appropriately when confronted with a client having a family history of heart disease, early heart disease themselves, a known genetic disorder in which cardiac problems may be a component, or signs and symptoms indicative of a familial component to the heart problem observed. It is important for the clinician to know how, when, and to whom referral for further genetic evaluation and counseling should be made. Genetic testing and screening in children or adolescents for conditions such as hypertrophic cardiomyopathy (HCM), familial hypercholesterolemia (FH), and long QT (LQT) syndrome, when indicated, can help to save lives through preventive treatment and therapeutic interventions. Preparticipation sports physicals are one means of providing such screening and are important to conduct properly under guidelines recommended by the American Heart Association. Genetic testing for relatives of persons already identified to have heritable cardiac conditions is becoming more and more integral to mainstream primary health care but engender controversy when testing of children is involved. Clinicians must know how to interpret the results of such tests. Appropriate genetic counseling must accompany risk assessment, genetic testing, and screening for cardiovascular disease.