A pharmacogenetic approach to blood pressure in Lyon hypertensive rats. A chromosome 2 locus influences the response to a calcium antagonist

Body Composition and Metabolic Changes in a Lyon Hypertensive Congenic Rat and Identification of Ercc6l2 as a Positional Candidate Gene

Central obesity is genetically complex, and its exponential increase in the last decades have made it a critical public health issue. The Lyon Hypertensive (LH) rat is a well-characterized hypertensive model that also exhibits spontaneous and profound differences in body weight and adiposity, relative to its metabolically healthy control, the Lyon Normotensive (LN) rat. The mechanisms underlying the body weight differences between these strains are not well-understood, thus a congenic model (LH17LNa) was developed where a portion of the proximal arm of LN chromosome 17 is introgressed on the LH genomic background to assess the contribution of LN alleles on obesity features. Male and female LH17LNa rats were studied, but male congenics did not significantly differ from LH in this study. Female LH17LNa rats exhibited decreases in total body growth, as well as major alterations to their body composition and adiposity. The LH17LNa female rats also showed decreases in metabolic rate, and a reduction in food intake. The increased adiposity in the female LH17LNa rats was specific to abdominal white adipose tissue, and this phenomenon was further explained by significant hypertrophy in those adipocytes, with no evidence of adipocyte hyperplasia. Sequencing of the parental strains identified a novel frameshift mutation in the candidate gene Ercc6l2, which is involved in transcription-coupled DNA repair, and is implicated in premature aging. The discovery of the significance of Ercc6l2 in the context of female-specific adipocyte biology could represent a novel role of DNA repair failure syndromes in obesity pathogenesis.

Towards Precision Medicine for Hypertension: A Review of Genomic, Epigenomic, and Microbiomic Effects on Blood Pressure in Experimental Rat Models and Humans

Compelling evidence for the inherited nature of essential hypertension has led to extensive research in rats and humans. Rats have served as the primary model for research on the genetics of hypertension resulting in identification of genomic regions that are causally associated with hypertension. In more recent times, genome-wide studies in humans have also begun to improve our understanding of the inheritance of polygenic forms of hypertension. Based on the chronological progression of research into the genetics of hypertension as the "structural backbone," this review catalogs and discusses the rat and human genetic elements mapped and implicated in blood pressure regulation. Furthermore, the knowledge gained from these genetic studies that provide evidence to suggest that much of the genetic influence on hypertension residing within noncoding elements of our DNA and operating through pervasive epistasis or gene-gene interactions is highlighted. Lastly, perspectives on current thinking that the more complex "triad" of the genome, epigenome, and the microbiome operating to influence the inheritance of hypertension, is documented. Overall, the collective knowledge gained from rats and humans is disappointing in the sense that major hypertension-causing genes as targets for clinical management of essential hypertension may not be a clinical reality. On the other hand, the realization that the polygenic nature of hypertension prevents any single locus from being a relevant clinical target for all humans directs future studies on the genetics of hypertension towards an individualized genomic approach.

Y27632, a Rho-activated kinase inhibitor, normalizes dysregulation in alpha1-adrenergic receptor-induced contraction of Lyon hypertensive rat artery smooth muscle

RhoA-activated kinase (ROK) is involved in the disorders of smooth muscle contraction found in hypertension model animals and patients. We examined whether the alpha1-adrenergic receptor agonist-induced ROK signal is perturbed in resistance small mesentery artery (SMA) of Lyon genetically hypertensive (LH) rats, using a ROK antagonist, Y27632. Smooth muscle strips of SMA and aorta were isolated from LH and Lyon normotensive (LN) rats. After Ca(2+)-depletion and pre-treatment with phenylephrine (PE), smooth muscle contraction was induced by serial additions of CaCl(2). In LH SMA Ca(2+) permeated cells to a lesser extent as compared with LN SMA, while CaCl(2)-induced contraction of LH SMA was greater than that of LN SMA, indicating a higher ratio of force to Ca(2+) in LH SMA contraction (Ca(2+) sensitization). No hyper-contraction was observed in LH aorta tissues. Treatment of LH SMA with Y27632 restored both Ca(2+) permeability and Ca(2+)-force relationship to levels seen for LN SMA. In response to PE stimulation, phosphorylation of CPI-17, a phosphorylation-dependent myosin phosphatase inhibitor protein, and MYPT1 at Thr853, the inhibitory phosphorylation site of the myosin phosphatase regulatory subunit, was increased in LN SMA, but remained unchanged in LH SMA. These results suggest that the disorder in ROK-dependent Ca(2+) permeability and Ca(2+)-force relationship is responsible for LH SMA hyper-contraction. Unlike other hypertensive models, the ROK-induced hyper-contractility of LH SMA is independent of MYPT1 and CPI-17 phosphorylation, which suggests that ROK-mediated inhibition of myosin phosphatase does not affect SMA hyper-contractility in LH SMA cells.

Severe hypertension caused by alleles from normotensive Lewis for a quantitative trait locus on chromosome 2

Pursuing fully a suggestion from linkage analysis that there might be a quantitative trait locus (QTL) for blood pressure (BP) in a chromosome (Chr) 2 region of the Dahl salt-sensitive rat (DSS), four congenic strains were made by replacing various fragments of DSS Chr 2 with those of Lewis (LEW). Consequently, a BP QTL was localized to a segment of around 3 cM or near 3 Mb on Chr 2 by comparative congenics. The BP-augmenting alleles of this QTL originated from the LEW rat, a normotensive strain compared with DSS. The dissection of a QTL with such a paradoxical effect illustrated the power of congenics in unearthing a gene hidden in the context of the whole animal system, presumably by interactions with other genes. The locus for the angiotensin II receptor AT-1B ( Agtr1b) is not supported as a candidate gene for the QTL because a congenic strain harboring it did not have an effect on BP. There are ∼19 known and unknown genes present in the QTL interval. Among them, no standout candidate genes are reputed to affect BP. Thus the QTL will likely represent a novel gene for BP regulation.

Multiple Quantitative Trait Loci for Blood Pressure Interacting Epistatically and Additively on Dahl Rat Chromosome 2

Our previous work demonstrated 2 quantitative trait loci (QTLs), C2QTL1 and C2QTL2, for blood pressure (BP) located on chromosome (Chr) 2 of Dahl salt-sensitive (DSS) rats. However, for a lack of markers, the 2 congenic strains delineating C2QTL1 and C2QTL2 could not be separated. The position of the C2QTL1 was only inferred by comparing 2 congenic strains, one having and another lacking a BP effect. Furthermore, it was not known how adjacent QTLs would interact with one another on Chr 2. In the current investigation, first, a critical chromosome marker was developed to separate 2 C2QTLs. Second, a congenic substrain was created to cover a chromosome fragment thought to harbor C2QTL1. Finally, a series of congenic strains was produced to systematically and comprehensively cover the entire Chr 2 segment containing C2QTL2 and other regions previously untested. Consequently, a total of 3 QTLs were discovered, with C2QTL3 located between C2QTL1 and C2QTL2. C2QTL1, C2QTL2, and C2QTL3 reside in chromosome segments of 5.7 centiMorgan (cM), 3.5 cM, and 1.5 cM, respectively. C2QTL1 interacted epistatically with either C2QTL2 or C2QTL3, whereas C2QTL2 and C2QTL3 showed additive effects to each other. These results suggest that BP QTLs closely linked in a segment interact epistatically and additively to one another on Chr 2.

A quantitative trait locus for aortic smooth muscle cell number acting independently of blood pressure: implicating the angiotensin receptor AT1B gene as a candidate

Vascular hyperplasia may be involved in the remodeling of vasculature. It was unknown whether there were genetic determinants for aortic smooth muscle cell number (SMCN) and, if so, whether they acted independently of those for blood pressure (BP). To unravel this issue, we utilized congenic strains previously constructed for BP studies. These strains were made by replacing various chromosome 2 segments of the Dahl salt-sensitive (S) rat with those of the Milan normotensive rat (MNS). We measured and compared SMCN in aortic cross-sectional areas and BPs of these strains. Consequently, a quantitative trait locus (QTL) for SMCN was localized to a chromosome region not containing a BP QTL, but harboring the locus for the angiotensin II receptor AT1B (Agtr1b). Agtr1b became a candidate for the SMCN QTL because 1) two significant mutations were found in the coding region between S and all congenic strains possessing the MNS alleles, and 2) contractile responses to angiotensin II were significantly and selectively reduced in congenic rats harboring the MNS alleles of the SMCN QTL compared with S rats. The current investigation presents the first line of evidence that a QTL for aortic SMCN exists, and it acts independently of QTLs for BP. The relevant congenic strains developed therein potentially provide novel mammalian models for the studies of vascular remodeling disorders.

Pharmacogenetics of Antihypertensive Drug Responses

The blood pressure (BP) response to any single antihypertensive drug is characterized by marked interindividual variation, and the known predictors of response are of limited value in identifying the optimum drug for an individual patient. Analysis of genetic variation has the potential to improve our understanding of determinants of antihypertensive drug response in order to individualize drug selection. Genetic variation can influence both pharmacokinetic and pharmacodynamic mechanisms underlying variation in drug response. Classic pharmacogenetic investigations have identified variations in single genes that have a large effect on antihypertensive drug metabolism and are inherited in a Mendelian fashion. These include a polymorphism in the CYP2D6 gene, encoding a cytochrome p450 family member involved in phase I drug metabolism, and polymorphisms in genes encoding enzymes involved in phase II drug metabolism, including N-acetyltransferase (NAT2), catechol-O-methyltransferase (COMT), and phenol sulfotransferase (P-PST, SULT1A1). Although these polymorphisms have major effects on the pharmacokinetic profiles of both commonly used antihypertensive drugs such as metoprolol (CYP2D6), and lesser used drugs such as hydralazine (NAT2), methyldopa (COMT), and minoxidil (SULT1A1), they have not been shown to influence variation in the antihypertensive effect of these drugs at conventional doses. Interest is now focused on identifying genetic polymorphisms that influence the pharmacodynamic determinants of antihypertensive response. Using a candidate gene approach, such polymorphisms have been identified in genes encoding alpha-adducin (ADD1), subunits of G-proteins (GNB3 and GNAS1), the beta(1)-adrenergic receptor (ADRB1), endothelial nitric oxide synthase (NOS3), and components of the renin-angiotensin-aldosterone system (angiotensinogen [AGT], angiotensin converting enzyme [ACE], the angiotensin type I receptor [AGTR1], and aldosterone synthase [CYP11B2]). These polymorphisms have been shown to influence the BP response to diuretics (ADD1, GNB3, NOS3, and ACE), beta-blockers (GNAS1 and ADRB1), ACE inhibitors (AGT, ACE, and AGTR1), angiotensin receptor blockers (ACE and CYP11B2), and clonidine (GNB3).An emerging consensus from these studies is that single gene effects on antihypertensive drug responses are small, and even the combined effects of all presently known polymorphisms do not account for enough variation in response to be clinically useful. New genome-wide scanning techniques may lead to the identification of genes previously unsuspected of influencing drug response. Additional requirements for pharmacogenetic approaches to become clinically useful are the characterization of the effects of haplotypes and multi-locus genotypes on drug response, and consideration of gene-by-environment interactions. Such studies will require huge sample sizes and novel statistical methods, but the theoretical and technical framework is in place to make this possible.

Genetics of blood pressure, hypertensive complications, and antihypertensive drug responses

Hypertension is the most prevalent, treatable risk factor for diseases of the heart, brain and kidneys. In this review, we discuss advances in understanding of the genetics of blood pressure regulation, the development of hypertensive complications and the pharmacodynamics of antihypertensive drug responses. Discovery of single gene mutations that cause hypertension or hypotension in humans suggests that the common final pathway for regulation of blood pressure level is via alterations in renal sodium handling. Based on a working hypothesis that common genetic variations contributing to blood pressure variation in the population may also act on this same pathway, we summarize supporting evidence emerging from linkage and selected association studies of candidate genes--including those encoding components of the renin-angiotensin-aldosterone system, the epithelial sodium channel, adrenoceptors, G protein subunits, and other cellular signaling mediators and modifiers. We proceed to distinguish ischemic target organ complications due to arteriolosclerotic changes of the microvasculature from those due to atherosclerosis involving larger conduit and capacitance arteries. Using the example of subcortical white matter ischemia of the brain, we propose that interindividual variation in the arteriolosclerotic complications is more likely than atherosclerotic complications to be related to the same genetic (and environmental) mechanisms that contribute to hypertension. We conclude by summarizing the state-of-the-art of antihypertensive pharmacogenetics, which has succeeded in rejecting the null hypothesis that genetic variation does not influence blood pressure or protective target organ responses to drug therapy. In each of the three areas covered in this review, we indicate the many remaining obstacles to the routine clinical use of genetic measurements in the diagnosis, evaluation and treatment of hypertension.

Reciprocal rat chromosome 2 congenic strains reveal contrasting blood pressure and heart rate QTL

Evidence exists implying multiple blood pressure quantitative trait loci (QTL) on rat chromosome 2. To examine this possibility, four congenic strains and nine substrains were developed with varying size chromosome segments introgressed from the spontaneously hypertensive rat (SHR/lj) and normotensive Wistar-Kyoto rat (WKY/lj) onto the reciprocal genetic background. Cardiovascular phenotyping was conducted with telemetry over extended periods during standard salt (0.7%) and high-salt (8%) diets. Our results are consistent with at least three independent pressor QTL: transfer of SHR/lj alleles to WKY/lj reveals pressor QTL within D2Rat21-D2Rat27 and D2Mgh10-D2Rat62, whereas transfer of WKY/lj D2Rat161-D2Mit8 to SHR/lj reveals a depressor locus. Our results also suggest a depressor QTL in SHR/lj located within D2Rat161-D2Mgh10. Introgressed WKY/lj segments also reveal a heart rate QTL within D2Rat40-D2Rat50 which abolished salt-induced bradycardia, dependent upon adjoining SHR/lj alleles. This study confirms the presence of multiple blood pressure QTL on chromosome 2. Taken together with our other studies, we conclude that rat chromosome 2 is rich in alleles for cardiovascular and behavioral traits and for coordinated coupling between behavior and cardiovascular responses.

Optimal blood pressure control and antihypertensive regimens in hypertensive renal disease: the potential of exploring the mechanisms of response variability

In renal patients rigorous blood pressure control is crucial to prevent renal and cardiovascular target organ damage. For renoprotection target blood pressure depends on the severity of proteinuria before treatment. For proteinuria of 1--3g/day a mean arterial pressure of 98 mmHg provides additional benefit, whereas the target should be as low as 92 mmHg if proteinuria exceeds 3g/day. The antiproteinuric effect of antihypertensive intervention predicts renoprotection; it is therefore recommended that therapy should be titrated not only on blood pressure, but also on reduction of proteinuria. All currently available classes of antihypertensives can be used to reduce blood pressure in renal patients. Interventions based on blockade of the renin-angiotensin-aldosterone system have additional antiproteinuric, and thus renoprotective, potential. Large individual differences in therapeutic benefit are common, even for interventions of proven efficacy at group level. Studies applying different classes of drugs in the same patient (rotation schedules) demonstrate that individual factors are main determinants of therapy response. Exploration of the mechanisms underlying these patient factors is important to improve treatment outcome. Analysis of genetic determinants of therapy response has great potential in this respect. However, therapy response is a complex phenotype. Thus, careful study of gene-gene and gene-environment interactions will be needed in order to turn this type of knowledge into benefit for the patient.

Rat genetics: attaching physiology and pharmacology to the genome

During the past five years, the Rat Genome Project has been rapidly gaining momentum, especially since the announcement in August 2000 of plans to sequence the rat genome. Combined with the wealth of physiological and pharmacological data for the rat, the genome sequence should facilitate the discovery of mammalian genes that underlie the physiological pathways that are involved in disease. Most importantly, this combined physiological and genomic information should also lead to the development of better pre-clinical models of human disease, which will aid in the development of new therapeutics.

Use of gene markers to guide antihypertensive therapy

Sequencing of the human genome has elevated the potential for genetic information to aid in the prevention, diagnosis, and treatment of common chronic diseases. One beneficial application of genetic information is the identification of variants that influence response to pharmaceutical agents used to lower blood pressure and prevent target organ complications of hypertension. Knowledge of genetic variants that influence blood pressure response to antihypertensive drugs may allow more individualized tailoring of antihypertensive drug therapy, and provide greater insight into the molecular mechanisms regulating blood pressure levels and causing hypertension.

Altered Na+-K+ pump activity and plasma lipids in salt-hypertensive Dahl rats: relationship to Atp1a1 gene

A genetic variant of the gene for the alpha(1)-isoform of Na(+)-K(+)-ATPase (Atp1a1) was suggested to be involved in the pathogenesis of salt hypertension in Dahl rats through altered Na(+):K(+) coupling ratio. We studied Na(+)-K(+) pump activity in erythrocytes of Dahl salt-sensitive (SS/Jr) rats in relation to plasma lipids and blood pressure (BP) and the linkage of polymorphic microsatellite marker D2Arb18 (located within intron 1 and exon 2 of Atp1a1 gene) with various phenotypes in 130 SS/Jr x SR/Jr F(2) rats. Salt-hypertensive SS/Jr rats had higher erythrocyte Na(+) content, enhanced ouabain-sensitive (OS) Na(+) and Rb(+) transport, and higher Na(+):Rb(+) coupling ratio of the Na(+)-K(+) pump. BP of F(2) hybrids correlated with erythrocyte Na(+) content, OS Na(+) extrusion, and OS Na(+):Rb(+) coupling ratio, but not with OS Rb(+) uptake. In F(2) hybrids there was a significant association indicating suggestive linkage (P < 0.005, LOD score 2.5) of an intragenic marker D2Arb18 with pulse pressure but not with mean arterial pressure or any parameter of Na(+)-K(+) pump activity (including its Na(+):Rb(+) coupling ratio). In contrast, plasma cholesterol, which was elevated in salt-hypertensive Dahl rats and which correlated with BP in F(2) hybrids, was also positively associated with OS Na(+) extrusion. The abnormal Na(+):K(+) stoichiometry of the Na(+)-K(+) pump is a consequence of elevated erythrocyte Na(+) content and suppressed OS Rb(+):K(+) exchange. In conclusion, abnormal cholesterol metabolism but not the Atp1a1 gene locus might represent an important factor for both high BP and altered Na(+)-K(+) pump function in salt-hypertensive Dahl rats.

Further chromosomal mapping of a blood pressure QTL in Dahl rats on chromosome 2 using congenic strains

Both linkage and use of congenic strains have shown that a region on rat chromosome 2 (Chr 2) of Dahl salt-sensitive rats (S) contained a quantitative trait locus (QTL) for blood pressure (BP). A congenic strain was made by replacing a segment of the S rat by the homologous region of the Milan normotensive (MNS) rat. Since the region was roughly 80 cM in size, a further reduction is required toward the final identification of the QTL. Currently, three congenic substrains were made by replacing smaller sections within the 80 cM. Each strain contains a specific region of MNS in the S genetic background. Two of the three congenic strains shared a segment in common, and both showed a BP-lowering effect. One of the three congenic strains carried a unique segment and had the same BP as S. Deducing the fragment shared in the two substrains having an effect, the BP QTL has to be present in a region of roughly 15 cM. In contrast to BP, heart rates of all the congenic rats were the same as that of the S rat. Thus BP and the heart rate are under the control of independent genetic determinants.

Antihypertensive pharmacogenetics: getting the right drug into the right patient

Pharmacogenetic investigation seeks to identify genetic factors that contribute to interpatient and interdrug variation in responses to antihypertensive drug therapy. Classical studies have characterized single gene polymorphisms of drug metabolizing enzymes that are responsible for large interindividual differences in pharmacokinetic responses to several antihypertensive drugs. Progress is being made using candidate gene and genome scanning approaches to identify and characterize many additional genes influencing pharmacodynamic mechanisms that contribute to interindividual differences in responses to antihypertensive drug therapy. Knowledge of polymorphic variation in these genes will help to predict individual patients' blood pressure responses to antihypertensive drug therapy and may also provide new insights into molecular mechanisms responsible for elevation of blood pressure.

The genomics of cardiovascular disorders: therapeutic implications

Cardiovascular disease (CVD) is a complicated series of disorders that result from the interaction between genetic predisposing mechanisms and environmental factors. Over the last few years substantial progress has been made in defining the molecular basis of several genetically transmitted non-atherosclerotic CVD such as hypertrophic and dilated cardiomyopathies, long-QT syndrome and essential hypertension. This review represents a summary of the current knowledge about the major gene polymorphisms found to be associated with these CVDs. Moreover, we will discuss how the discovery of disease-associated genes will greatly enhance the ability to formulate advanced diagnoses, to define prophylactic therapeutic strategies to prevent or reduce the progression of the disease and, finally, to proceed to the development of new drugs tailored for the specific cellular or molecular functions altered as consequence of the predisposing genes.

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.

Genes and hypertension: where we are and where we should go

Our understanding of the genetics of hypertension is incomplete. A great deal has, however, been learned about the role of several <<candidate>> genes by altering their expression in transgenic and knockout models. Crosses of inbred strains, analyzed in F2 generations, have demonstrated consistent quantitative trait loci, particularly on chromosomes 1, 2 and 10, suggesting significant contributions of some genes in distinct models of rodent hypertension. The effect of these loci has been tested in congenic strains and their interaction underlined in double congenics. The weakness of testing individual animals from F2 crosses is overcome in recombinant inbred strains. In humans, Mendelian models of hypertension have contributed to progress in our understanding of this disease, but have not yet revealed any major gene of essential hypertension. Many association as well as linkage studies of humans have provided useful though somewhat contradictory data. Our renewed effort is oriented towards the discovery of genetic determinants of environmental interaction in hypertension as well as towards the future of pharmacogenomics. This progress will be the basis of future individualized treatment and prevention.

Genetic and biochemical determinants of abnormal monovalent ion transport in primary hypertension

Data obtained during the last two decades show that spontaneously hypertensive rats, an acceptable experimental model of primary human hypertension, possess increased activity of both ubiquitous and renal cell-specific isoforms of the Na+/H+ exchanger (NHE) and Na+-K+-2Cl- cotransporter. Abnormalities of these ion transporters have been found in patients suffering from essential hypertension. Recent genetic studies demonstrate that genes encoding the beta- and gamma-subunits of ENaC, a renal cell-specific isoform of the Na+-K+-2Cl- cotransporter, and alpha3-, alpha1-, and beta2-subunits of the Na+-K+ pump are localized within quantitative trait loci (QTL) for elevated blood pressure as well as for enhanced heart-to-body weight ratio, proteinuria, phosphate excretion, and stroke latency. On the basis of the homology of genome maps, several other genes encoding these transporters, as well as the Na+/H+ exchanger and Na+-K+-2Cl- cotransporter, can be predicted in QTL related to the pathogenesis of hypertension. However, despite their location within QTL, analysis of cDNA structure did not reveal any mutation in the coding region of the above-listed transporters in primary hypertension, with the exception of G276L substitution in the alpha1-Na+-K+ pump from Dahl salt-sensitive rats and a higher occurrence of T594M mutation of beta-ENaC in the black population with essential hypertension. These results suggest that, in contrast to Mendelian forms of hypertension, the altered activity of monovalent ion transporters in primary hypertension is caused by abnormalities of systems involved in the regulation of their expression and/or function. Further analysis of QTL in F2 hybrids of normotensive and hypertensive rats and in affected sibling pairs will allow mapping of genes causing abnormalities of these regulatory pathways.

Gene-environment interactions in hypertension

Environmental factors such as stress, diet, and physical activity have long been recognized as playing an important role in the pathogenesis of essential hypertension. Individuals may vary in their response to these factors depending on differences in genes determining physiologic systems that mediate the response. In this article we discuss gene-environment interactions that contribute to the development of essential hypertension (environmental susceptibility to hypertension) and those that are involved in control of the disease (pharmacogenetics).

Use of animal models to search for candidate genes associated with essential hypertension

The use of inbred genetically hypertensive animal models enables the dissection of the underlying complex genetic traits into its individual components, and thus the elucidation and characterization of causative genes and gene variants. In addition, genetically hypertensive animal models will also be useful for the investigation of genetic characteristics that influence the effectiveness of antihypertensive therapy with specific pharmacologic agents. This report will discuss three different strategies that have recently been used for the identification of candidate gene loci or candidate genes for hypertension. The possibility to transfer of genetic data derived in animal models to human hypertension will also be considered.

Genomics and hypertension: concepts, potentials, and opportunities

We are at the beginning of a biological revolution, spurred on by the Human Genome Project and associated studies. Within the next few years, expressed sequence tags (ESTs) representing all sequences expressed in humans will be determined and their genomic positions will be defined (STSs). The discovery of all the variants in the human genome that contribute to the genetic diversity of the human population will result in the construction of dense polymorphic maps. The rapid growth of the EST, STS, and single-nucleotide polymorphism (SNP) databases, coupled with impressive technological advances, will surely have a dramatic effect on biomedical research. In this review, we will examine the recent advances in genetics and genomics and place these within the context of medical research and patient care, with an emphasis on studies in the cardiovascular system.

In search of hypertension genes in Dahl salt-sensitive rats

Quantitative trait loci in Dahl rats Genetic and crude physical mapping have yielded chromosome regions containing quantitative trait loci for blood pressure in Dahl salt-sensitive rats. So far, the molecular identities of these loci are largely unknown. Intriguing still is how these quantitative trait loci would interact with each other to achieve an overall blood pressure effect Alleles of some loci previously identified as blood pressure quantitative trait loci in other rat strains appear to be the same between Dahl salt-sensitive and salt-resistant rats. Why do Dahl salt-resistant rats have low blood pressure whereas Dahl salt-sensitive rats develop high blood pressure? Recent findings With the use of congenic strains and 'double' congenics, these issues have begun to unravel. Certain quantitative trait loci exert major blood pressure effects (>20 mmHg) and each of them can be dissected as a monogenic trait Some appear to be located close to each other in the same chromosome region. Different quantitative trait loci interact epistatically to produce their combined blood pressure effects. 'Low' blood pressure alleles of one quantitative trait locus can compensate for the 'high' blood pressure alleles of other quantitative trait loci in the Dahl salt-resistant rat By integrating fine mapping and positional cloning strategies, blood pressure quantitative trait loci are being elucidated. Work in the rat may also facilitate genetic mapping of quantitative trait loci in humans.

Genetics of experimental hypertension

Experimental models of genetic hypertension are used to develop paradigms to study human essential hypertension while removing some of the complexity inherent in the study of human subjects. Since 1991 several quantitative trait loci responsible for blood pressure regulation have been identified in various rat crosses. More recently, a series of interesting quantitative trait loci influencing cardiac hypertrophy, stroke, metabolic syndrome and renal damage has also been described. It is recognized that the identification of large chromosomal regions containing a quantitative trait locus is only a first step towards gene identification. The next step is the production of congenic strains and substrains to confirm the existence of the quantitative trait locus and to narrow down the chromosomal region of interest. Several congenic strains have already been produced, with further refinement of the methodology currently in progress. The ultimate goal is to achieve positional cloning of the causal gene, a task which has so far been elusive. There are several areas of cross-fertilization between experimental and human genetics of hypertension, with a successful transfer of two loci directly from rats to humans and with new pharmacogenetic approaches which may be utilized in both experimental and clinical settings.

Pharmacogenomics: a new approach to individual therapy of hypertension?

The individual variation in the efficacy of and tolerability to antihypertensive drugs in human essential hypertension is linked to the genetic heterogeneity of this multifactorial disease. Different approaches have been pursued in the attempt to correlate a specific responsiveness to the therapy with some phenotypic traits of the patients, such as the renin-angiotensin profile or the characteristics of cell ion transports. More recently, a genetic approach to the study of the mechanisms underlying hypertension has led to the identification of some quantitative trait loci or genes that influence blood pressure in both animal models and patients. Also, individual variation to therapy can now be studied from the genetic point of view using pharmacogenomics, that is, the study of the genes or loci which are involved in determining the responsiveness to a given drug. Only a few examples of this approach are available to date. Our group has identified a polymorphism of the genes for the cytoskeletal protein, adducin, which is linked to both rat and human hypertension, sodium sensitivity and to the pressor responsiveness to diuretic therapy. These results, together with the indication that adducin can play a functional role by modulating the cellular sodium transport, have led to the identification of a new antihypertensive compound, which could be a candidate for the selective treatment of those patients in whom alterations of the renal sodium handling are associated with specific genetic traits such as the polymorphism for adducin.