Linkage of 11 beta-hydroxylase mutations with altered steroid biosynthesis and blood pressure in the Dahl rat

Metabolites and Hypertension: Insights into Hypertension as a Metabolic Disorder: 2019 Harriet Dustan Award

For over 100 years, essential hypertension has been researched from different perspectives ranging from genetics, physiology, and immunology to more recent ones encompassing microbiology (microbiota) as a previously underappreciated field of study contributing to the cause of hypertension. Each field of study in isolation has uniquely contributed to a variety of underlying mechanisms of blood pressure regulation. Even so, clinical management of essential hypertension has remained somewhat static. We, therefore, asked if there are any converging lines of evidence from these individual fields that could be amenable for a better clinical prognosis. Accordingly, here we present converging evidence which support the view that metabolic dysfunction underlies essential hypertension.

The hypotensive effect of salt substitutes in stage 2 hypertension: a systematic review and meta-analysis

Background

Hypertension (HTN) is a ubiquitous risk factor for numerous non-communicable diseases, including cardiovascular disease and stroke. There are currently no wholly effective pharmacological therapies for subjects with HTN. However, salt substitutes have emerged as a potential therapy for the treatment of HTN. The aim of the present study was to assess the effect of salt substitutes on reducing systolic blood pressure (SBP) and diastolic BP (DBP), following a meta-analysis of randomized controlled trials.

Methods

Studies were found via systematic searches of the Pubmed/Medline, Scopus, Ovid, Google Scholar and Cochrane library. Ten studies, comprised of 11 trials and 1119 participants, were included in the meta-analysis.

Results

Pooled weighted mean differences showed significant reductions of SBP (WMD − 8.87 mmHg; 95% CI − 11.19, − 6.55, p < 0.001) and DBP (WMD − 4.04 mmHg; 95% CI − 5.70, − 2.39) with no statistically significant heterogeneity between the 11 included comparisons of SBPs and DBPs. The stratified analysis of trials based on the mean age of participants showed a significant reduction in the mean difference of SBP in both adults (< 65 years old) and elderly (≥65 years old). However, the DBP-lowering effect of salt substitutes was only observed in adult patients (WMD − 4.22 mmHg; 95% CI − 7.85, − 0.58), but not in the elderly subjects.

Conclusions

These findings suggest that salt-substitution strategies could be used for lowering SBP and DBP in patients with stage 2 HTN; providing a nutritional platform for the treatment, amelioration, and prevention of HTN.

Dissecting Epistatic QTL for Blood Pressure in Rats: Congenic Strains versus Heterogeneous Stocks, a Reality Check

Advances in molecular genetics have provided well-defined physical genetic maps and large numbers of genetic markers for both model organisms and humans. It is now possible to gain a fundamental understanding of the genetic architecture underlying quantitative traits, of which blood pressure (BP) is an important example. This review emphasizes analytical techniques and results obtained using the Dahl salt-sensitive (S) rat as a model of hypertension by presenting results in detail for three specific chromosomal regions harboring genetic elements of increasing complexity controlling BP. These results highlight the critical importance of genetic interactions (epistasis) on BP at all levels of structure, intragenic, intergenic, intrachromosomal, interchromosomal, and across whole genomes. In two of the three examples presented, specific DNA structural variations leading to biochemical, physiological, and pathological mechanisms are well defined. This proves the usefulness of the techniques involving interval mapping followed by substitution mapping using congenic strains. These classic techniques are compared to newer approaches using sophisticated statistical analysis on various segregating or outbred model-organism populations, which in some cases are uniquely useful in demonstrating the existence of higher-order interactions. It is speculated that hypertension as an outlier quantitative phenotype is dependent on higher-order genetic interactions. The obstacle to the identification of genetic elements and the biochemical/physiological mechanisms involved in higher-order interactions is not theoretical or technical but the lack of future resources to finish the job of identifying the individual genetic elements underlying the quantitative trait loci for BP and ascertaining their molecular functions. © 2019 American Physiological Society. Compr Physiol 9:1305-1337, 2019.

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.

Genetics of hypertension: discoveries from the bench to human populations

Hypertension is a complex trait that is influenced by both heritable and environmental factors. The search for genes accounting for the susceptibility to hypertension has driven parallel efforts in human research and in research using experimental animals in controlled environmental settings. Evidence from rodent models of genetic hypertension and human Mendelian forms of hypertension and hypotension have yielded mechanistic insights into the pathways that are perturbed in blood pressure homeostasis, most of which converge at the level of renal sodium reabsorption. However, the bridging of evidence from these very diverse approaches to identify mechanisms underlying hypertension susceptibility and the translation of these findings to human populations and public health remain a challenge. Furthermore, findings from genome-wide association studies still require functional validation in experimental models. In this review, we highlight results and implications from key studies in experimental and clinical hypertension to date.

Role of Rac1 GTPase in salt-sensitive hypertension

PURPOSE OF REVIEW

The aldosterone/mineralocorticoid receptor system plays an important role in the long-term blood pressure control through Na homeostasis. Its overactivation has been implicated in salt-sensitive hypertension. Excessive salt intake augments the function of mineralocorticoid receptor, despite lowering circulating aldosterone levels, but the mechanism had long been elusive. Recently, Rac1, a member of Rho family small GTP-binding proteins, has emerged as a novel ligand-independent modulator of mineralocorticoid receptor activity. In this review, the roles of Rac1 in the pathogenesis of salt-sensitive hypertension and kidney injury have been summarized.

RECENT FINDINGS

Genetic engineering studies have highlighted the new aspects of Rac1 and its regulators in salt-sensitive hypertension and cardiac and renal disease. New evidence shows the essential roles of Rac1 in salt-evoked paradoxical mineralocorticoid receptor activation observed in salt-sensitive models and in renal tubular Na reabsorption through reduced nicotinamide-adenine dinucleotide phosphate oxidase-mediated oxidative stress or direct regulation of Na transporters.

SUMMARY

The emerging concept of 'ligand-independent aberrant mineralocorticoid receptor activation by Rac1' in the pathogenesis of salt-sensitive hypertension and kidney injury has been reviewed. Rac inhibition, in addition to mineralocorticoid receptor blockade and salt restriction, would be a new promising strategy for the treatment of salt-sensitive hypertension.

Female-specific hypertension loci on rat chromosome 13

A 3.7-Mb region of rat chromosome 13 (45.2-49.0 Mb) affects blood pressure (BP) in females only, indicating the presence of sex-specific BP loci in close proximity to the Renin locus. In the present study, we used a series of Dahl salt-sensitive/Mcwi-13 Brown Norway congenic rat strains to further resolve BP loci within this region. We identified 3 BP loci affecting female rats only, of which the 2 smaller loci (line9BP3 and line9BP4) were functionally characterized by sequence and expression analysis. Compared with SS (SS/HsdMcwiCrl), the presence of a 591-kb region of BN (BN/NHsdMcwi) chromosome 13 (line9BP3) significantly lowered BP by 21 mm Hg on an 8% NaCl diet (153 ± 7 versus 174 ± 5 mm Hg; P<0.001). Unexpectedly, the addition of 23 kb of Brown Norway chromosome 13 (line9BP4) completely erased the female-specific BP protection on 8% NaCl diet, suggesting that BN hypertensive allele(s) reside in this region. The congenic interval of the protective line 9F strain contains 3 genes (Optc, Prelp, and Fmod), and the hypertensive line 9E contains 1 additional gene (Btg2). Sequence analysis of the 2 BP loci revealed a total of 282 intergenic variants, with no coding variants. Analysis of gene expression by quantitative real-time polymerase chain reaction revealed strain- and sex-specific differences in Prelp, Fmod, and Btg2 expression, implicating these as novel candidate genes for female-specific hypertension.

Theoretical model for gene-gene, gene-environment, and gene-sex interactions based on congenic-strain analysis of blood pressure in Dahl salt-sensitive rats

There is a significant literature describing quantitative trait loci (QTL) controlling blood pressure (BP) in the Dahl salt-sensitive (S) rat. In studies to identify the genes underlying BP QTL it has been common practice to place chromosomal segments from low BP strains on the genetic background of the S rat and then reduce the congenic segments by substitution mapping. The present work suggests a model to simulate genetic interactions found using such congenic strains. The QTL are considered to be switches that can be either in series or in parallel represented by the logic operators AND or OR, respectively. The QTL switches can be on/off switches but are also allowed specific leak properties. The QTL switches are represented by a "universal" switch consisting of two molecules binding to form a complex. Genetic inputs enter the model as allelic products of one of the binding molecules and environmental variation (including dietary salt- and sex-related differences) enters as an influence on the concentration of the other binding molecule. The pairwise interactions of QTL are very well simulated and fall into recognizable patterns. There is, however, often more than one assumed model to predict a given pattern so that all patterns do not necessarily have a unique solution. Nevertheless, the models obtained provide a framework for placing the QTL in pathways relative to one another. Moreover, based on their leak properties pairs of QTL could be identified in which one QTL may alter the properties of the other QTL.

Refined mapping of blood pressure quantitative trait loci using congenic strains developed from two genetically hypertensive rat models

Previously linkage and substitution mapping were conducted between the Dahl Salt-sensitive (S) rat and the Spontaneously Hypertensive Rat (SHR) to address the hypothesis that genetic contributions to blood pressure (BP) in two genetically hypertensive rat strains are different. Among the BP quantitative trait loci (QTLs) detected, two are located on chromosome 9 within large genomic segments. The goal of the current study was to develop new iterations of congenic substrains, to further resolve both of these BP QTLs on chromosome 9 as independent congenic segments. A total of 10 new congenic substrains were developed and characterized. The newly developed congenic substrains S.SHR(9)x8Ax11A and S.SHR(9)x10Ax1, with introgressed segments of 2.05 and 6.14 Mb, represented the shortest genomic segments. Both of these congenic substrains, S.SHR(9)x8Ax11A and S.SHR(9)x10Ax1 lowered BP of the S rat by 56 mm Hg (P<0.001) and 15 mm Hg (P<0.039), respectively. The BP measurements were corroborated by radiotelemetry. Urinary protein excretion was significantly lowered by SHR alleles within S.SHR(9)x10Ax1 but not by S.SHR(9)x8Ax11A. The shorter of the two congenic segments, 2.05 Mb was further characterized and found to contain a single differentially expressed protein-coding gene, Tomoregulin-2 (Tmeff2). The protein expression of Tmeff2 was higher in the S rat compared with S.SHR(9)x8Ax11A, which also had lower cardiac hypertrophy as measured by echocardiography. Tmeff2 is known to be upregulated in patients from multiple cohorts with cardiac hypertrophy. Taken together, Tmeff2 can be prioritized as a candidate gene for hypertension and associated cardiac hypertrophy in both rats and in humans.

Proteome analysis in cardiovascular pathophysiology using Dahl rat model

Dahl salt-sensitive (DS) and salt-resistant (DR) inbred rat strains represent a well established animal model for cardiovascular research. Upon prolonged administration of high-salt-containing diet, DS rats develop systemic hypertension, and as a consequence they develop left ventricular hypertrophy, followed by heart failure. The aim of this work was to explore whether this animal model is suitable to identify biomarkers that characterize defined stages of cardiac pathophysiological conditions. The work had to be performed in two stages: in the first part proteomic differences that are attributable to the two separate rat lines (DS and DR) had to be established, and in the second part the process of development of heart failure due to feeding the rats with high-salt-containing diet has to be monitored. This work describes the results of the first stage, with the outcome of protein expression profiles of left ventricular tissues of DS and DR rats kept under low salt diet. Substantial extent of quantitative and qualitative expression differences between both strains of Dahl rats in heart tissue was detected. Using Principal Component Analysis, Linear Discriminant Analysis and other statistical means we have established sets of differentially expressed proteins, candidates for further molecular analysis of the heart failure mechanisms.

Physiological and pathophysiological applications of sensitive ELISA methods for urinary deoxycorticosterone and corticosterone in rodents

Deoxycorticosterone (DOC: a weak mineralocorticoid) is the precursor to corticosterone (B: the major glucocorticoid in rodents) and aldosterone (the major mineralocorticoid). The genes Cyp11b1 and Cyp11b2 that encode the enzymes responsible for DOC to B (11beta-hydroxylase) and DOC to aldosterone (aldosterone synthase) conversions are located on the same chromosome. The aim of this study was to develop sensitive and specific ELISA methods to quantify urinary DOC and B concentrations to assess the physiological and genetic control of the Cyp11b1/b2 locus. Antibodies raised in rabbits against DOC and B and horse radish peroxidase-goat anti-rabbit IgG enzyme tracer were used to develop the assays. Urine samples collected from mice held in metabolic cages were extracted with dichloromethane and reconstituted in assay buffer. The assays were validated for specificity, sensitivity, parallelism, accuracy and imprecision. Cross-reactivities with major interfering steroids were minimal: DOC assay (progesterone=0.735% and corticosterone=0.045%), and for B assay (aldosterone=0.14%, 11-dehydro-B=0.006%, cortisol=0.016% and DOC=0.04%) and minimum detection limit for DOC ELISA was 2.2 pg/mL (6.6 pmol/L), and for B ELISA was 6.2 pg/mL (17.9 pmol/L). The validity of urinary DOC and B ELISAs was confirmed by the excellent correlation between the results obtained before and after solvent extraction and HPLC (DOC ELISA: Y=1.092X-0.054, R(2)=0.988; B ELISA: Y=1.047X-0.226, R(2)=0.996). Accuracy studies, parallelism and imprecision data were determined and all found to be satisfactory. The methods were used in a series of metabolic cage studies which demonstrated that (i) females produce more DOC and corticosterone than males; (ii) DOC and corticosterone respond to ACTH treatment but not dietary sodium restriction; (iii) DOC:B ratios in Cyp11b1 null mice were >200-fold greater than wild type.

Aldosterone synthesis in the brain contributes to Dahl salt-sensitive rat hypertension

The enzymes required for aldosterone synthesis from cholesterol are expressed in rat and human brains. The hypertension of Dahl salt-sensitive (SS) rats is mitigated by the intracerebroventricular (i.c.v.) infusion of antagonists of the mineralocorticoid receptor (MR) and downstream effectors of mineralocorticoid action, as well as ablations of brain areas that also abrogate mineralocorticoid-salt excess hypertension in normotensive rats. We used real time RT-PCR to measure mRNA of aldosterone synthase and 11beta-hydroxylase, the requisite enzymes for the last step in the synthesis of aldosterone and corticosterone, respectively, MR and the determinants of MR ligand specificity, 11beta-hydroxysteroid dehydrogenase types 1 and 2 (11beta-HSD1&2) and hexose-6-phosphate dehydrogenase (H6PDH). A combination of extraction and ELISA was used to measure aldosterone concentrations in tissue and urine of SS and Sprague-Dawley (SD) rats. Aldosterone synthase mRNA expression was higher in the brains and lower in the adrenal glands of SS compared with SD rats. The amounts of mRNA for MR, 11beta-hydroxylase, 11beta-HSD1&2 and H6PD were similar. Aldosterone concentrations were greater in brains of SS than SD rats, yet, in keeping with the literature, the circulating and total aldosterone production of aldosterone in SS rats were not. The selective inhibitor of aldosterone synthase, FAD286, was infused i.c.v. or subcutaneously in a cross-over blood pressure study in hypertensive SS rats further challenged by a high-salt diet. The i.c.v. infusion of FAD286, at a dose that had no effect systemically, significantly and reversibly lowered blood pressure in SS rats. Aldosterone synthesis in brains of SS rats is greater than in SD rats and is important in the genesis of their salt-sensitive hypertension.

Altered corticosteroid biosynthesis in essential hypertension: A digenic phenomenon

Aldosterone plays an important role in electrolyte and blood pressure homeostasis. Our studies have focused on the role of aldosterone in essential hypertension. We have shown that plasma aldosterone and ARR are heritable characteristics and that aldosterone concentrations in older subjects are inversely correlated with birthweight and positively correlated with blood pressure. Aldosterone levels are also associated with polymorphic variation in the CYP11B2 gene, which encodes aldosterone synthase, the enzyme responsible for aldosterone production. Interestingly, CYP11B2 polymorphisms are also associated with less efficient activity of 11beta-hydroxylase, encoded by the neighbouring, highly homologous CYP11B1 gene. We propose that a digenic effect leads to increased aldosterone production, with inefficient 11beta-hydroxylation causing a long-term increase in ACTH drive to the adrenal gland and enhanced expression of CYP11B2, thereby resulting in chronically raised aldosterone secretion in response to factors such as angiotensin II and potassium. In susceptible subjects this is likely, over many years, to result in hypertension with relative aldosterone excess.

Epistatic genetic determinants of blood pressure and mortality in a salt-sensitive hypertension model

Although genetic determinants protecting against the development of elevated blood pressure (BP) are well investigated, less is known regarding their impact on longevity. We concomitantly assessed genomic regions of rat chromosomes 3 and 7 (RNO3 and RNO7) carrying genetic determinants of BP without known epistasis, for their independent and combinatorial effects on BP and the presence of genetic determinants of survival using Dahl salt-sensitive (S) strains carrying congenic segments from Dahl salt-resistant (R) rats. Although congenic and bicongenic S.R strains carried independent BP quantitative trait loci within the RNO3 and RNO7 congenic regions, only the RNO3 allele(s) independently affected survival. The bicongenic S.R strain showed epistasis between R-rat RNO3 and RNO7 alleles for BP under salt-loading conditions, with less-than-additive effects observed on a 2% NaCl diet and greater-than-additive effects observed after prolonged feeding on a 4% NaCl diet. These RNO3 and RNO7 congenic region alleles had more-than-additive effects on survival. Increased survival of bicongenic compared with RNO3 congenic rats was attributable, in part, to maintaining lower BP despite chronic exposure to an increased dietary salt (4% NaCl) intake, with both strains showing delays in reaching highest BP. R-rat RNO3 alleles were also associated with superior systolic function, with the S.R bicongenic strain showing epistasis between R-rat RNO3 and RNO7 alleles leading to compensatory hypertrophy. Whether these alleles affect survival by additional actions within other BP-regulating tissues/organs remains unexplored. This is the first report of simultaneous detection of independent and epistatic loci dictating, in part, longevity in a hypertensive rat strain.

Insight into the genetics of hypertension, a core component of the metabolic syndrome

PURPOSE OF REVIEW

To provide insight into genetics of essential hypertension, including discussion of methods used both in human and animal experimental studies and interpretation of results.

RECENT FINDINGS

On the basis of recent progress in sequencing of human genome, detection of millions of single nucleotide polymorphism markers, determination of the extend of linkage disequilibrium (haplotypes), efficient genotyping technology, collection of DNA from thousands of rigorously phenotyped patients and controls and designing sound statistical methods, genome-wide associations studies were widely applied to analyses of common diseases including essential hypertension for the first time in 2007. Concurrently, new experimental approaches combined gene expression profiling with linkage and correlation analyses to identify quantitative trait loci underlying complex traits at the molecular level.

SUMMARY

These new approaches yielded new exciting results but also posed questions regarding data analyses, interpretation and clinical significance.

A lifetime of aldosterone excess: long-term consequences of altered regulation of aldosterone production for cardiovascular function

Up to 15% of patients with essential hypertension have inappropriate regulation of aldosterone; although only a minority have distinct adrenal tumors, recent evidence shows that mineralocorticoid receptor activation contributes to the age-related blood pressure rise and illustrates the importance of aldosterone in determining cardiovascular risk. Aldosterone also has a major role in progression and outcome of ischemic heart disease. These data highlight the need to understand better the regulation of aldosterone synthesis and its action. Aldosterone effects are mediated mainly through classical nuclear receptors that alter gene transcription. In classic epithelial target tissues, signaling mechanisms are relatively well defined. However, aldosterone has major effects in nonepithelial tissues that include increased synthesis of proinflammatory molecules and reactive oxygen species; it remains unclear how these effects are controlled and how receptor specificity is maintained. Variation in aldosterone production reflects interaction of genetic and environmental factors. Although the environmental factors are well understood, the genetic control of aldosterone synthesis is still the subject of debate. Aldosterone synthase (encoded by the CYP11B2 gene) controls conversion of deoxycorticosterone to aldosterone. Polymorphic variation in CYP11B2 is associated with increased risk of hypertension, but the molecular mechanism that accounts for this is not known. Altered 11beta-hydroxylase efficiency (conversion of deoxycortisol to cortisol) as a consequence of variation in the neighboring gene (CYP11B1) may be important in contributing to altered control of aldosterone synthesis, so that the risk of hypertension may reflect a digenic effect, a concept that is discussed further. There is evidence that a long-term increase in aldosterone production from early life is determined by an interaction of genetic and environmental factors, leading to the eventual phenotypes of aldosterone-associated hypertension and cardiovascular damage in middle age and beyond. The importance of aldosterone has generated interest in its therapeutic modulation. Disadvantages associated with spironolactone (altered libido, gynecomastia) have led to a search for alternative mineralocorticoid receptor antagonists. Of these, eplerenone has been shown to reduce cardiovascular risk after myocardial infarction. The benefits and disadvantages of this therapeutic approach are discussed.

From genetics to mechanism of disease liability

With each advance in genomic technology, new statistical methods have regularly emerged to test genetic hypotheses in complex inheritance, as evidenced throughout this book. Notwithstanding the approach used, the greatest challenge in the genetics of complex traits remains the identification of the gene(s) and the molecular variant(s) accounting for a genetic inference based on statistical testing. We take the example of quantitative trait locus (QTL) mapping for blood pressure (BP) and related phenotypes in rodents to review the current landscape. Traditional approaches to refined mapping are typically hampered by the small effect and the small proportion of the variance attached to individual QTLs. The alternative of functional screens in intact animals, whether by chemical mutagenesis or gene targeting, remains a daunting undertaking. Such limitations account for the slow progress to date of inferences from QTL to gene(s). We select a QTL for differential sodium sensitivity between two mouse inbred lines to propose an approach that can be used in relatively large genomic regions (1) by optimizing the selection of candidate genes and (2) by subjecting such genes to high-throughput functional screens. While this is still work in progress, we think it abundantly illustrates what is ahead of us in delineating genetic variation that underlie complex disease.

Phenotypic consequences of variation across the aldosterone synthase and 11-beta hydroxylase locus in a hypertensive cohort: data from the MRC BRIGHT Study

BACKGROUND

Aldosterone is an important cardiovascular hormone; 15% of hypertensive subjects have alteration in aldosterone regulation, defined by a raised ratio of aldosterone to renin (ARR). Studies of the aldosterone synthase gene (CYP11B2) have focused on a single nucleotide polymorphism in the 5'promoter region (-344 C/T). In normotensive subjects, the T allele associates with raised levels of the 11-deoxysteroids, deoxycorticosterone and 11-deoxycortisol which are substrates for 11beta-hydroxylase, encoded by the adjacent and homologous gene, CYP11B1. We have speculated that this altered 11beta-hydroxylase efficiency leads to increased ACTH drive to the adrenal gland to maintain cortisol production and reported herein the association between the -344 C/T single nucleotide polymorphism (SNP) and adrenal steroid production in subjects with essential hypertension.

METHODS

The CYP11B2-344 C/T polymorphism was genotyped and urinary excretion of adrenal steroid metabolites was measured (by GCMS) in 511 unrelated hypertensives from the Medical Research Council (MRC) British Genetics of Hypertension (BRIGHT) study.

RESULTS

Thirty-five per cent of subjects were homozygous for the -344T allele whilst 16% were CC homozygotes. There was no difference in cortisol excretion rate between the two genotype groups but the index of adrenal 11beta-hydroxylation (ratio of tetrahydrodeoxycortisol/total cortisol) was significantly higher in the TT group (P < 0.005) than in the CC group. Excretion rates of the major urinary metabolite of aldosterone (tetrahydroaldosterone) correlated strongly with the ACTH-regulated steroids, cortisol (r = 0.437, P < 0.0001) and total androgen metabolites (r = 0.4, P < 0.0001) in TT but not CC subjects.

CONCLUSIONS

Hypertensives homozygous for the -344 T allele of CYP11B2 demonstrate altered 11beta-hydroxylase efficiency (CYP11B1); this is consistent with the hypothesis of a genetically determined increase in adrenal ACTH drive in these subjects. The correlation between excretion of aldosterone and cortisol metabolites and suggests that, in TT subjects, ACTH exerts an important common regulatory influence on adrenal corticosteroid production in subjects with hypertension.

Quantitative phenotype analysis for localization and identification of disease-related genes in a complex genetic background

Results from genetic investigations of blood pressure and other variables in inbred rodent models are reviewed here to illustrate the power of quantitative approaches for the detection of linkage and the ultimate identification of the underlying genes. Different studies-involving angiotensinogen and hypertension, angiotensin I-converting enzyme and cardiovascular diseases, and other traits-are used to illustrate the possibility of similar approaches to multifactorial disorders in humans.

Molecular Genetics of Experimental Hypertension and the Metabolic Syndrome

Genetic studies of human and experimental hypertension provide a means to identify key pathways that predispose individuals to increased blood pressure and associated risk factors for cardiovascular and metabolic diseases. The pathways so identified can then serve as targets for therapeutic intervention. This article discusses genetic studies in animal models of hypertension in which specific genes have been identified that regulate blood pressure and biochemical features of the metabolic syndrome. Consistent with studies in humans with monogenic disorders of blood pressure regulation, studies in rat models have demonstrated that naturally occurring genetic variation in pathways regulating sodium chloride transport can contribute to inherited variation in blood pressure. Such studies have also indicated that naturally occurring variation in genes, such as Cd36, that regulate fatty acid metabolism and ectopic accumulation of fat and fat metabolites can influence both biochemical and hemodynamic features of the metabolic syndrome and mediate the antidiabetic effects of drugs that activate the peroxisome proliferator-activated receptor-gamma. Angiotensin II receptor blockers with the ability to selectively modulate activity of peroxisome proliferator-activated receptor-gamma and expression of genes in these fat metabolism pathways may represent useful prototypes for a new class of transcription modulating drugs aimed at treating patients with hypertension and the metabolic syndrome.

Polymorphic Variation in the 11β-Hydroxylase Gene Associates With Reduced 11-Hydroxylase Efficiency

The −344 C/T and intron 2 conversion variants in the CYP11B2 gene, encoding aldosterone synthase, have been associated with markers of impaired 11β-hydroxylase activity. We hypothesize that this association is because of variations in the adjacent 11β-hydroxylase gene (CYP11B1) and arises through linkage disequilibrium between CYP11B1 and CYP11B2. The pattern of variation across the entire CYP11B locus was determined by sequencing 26 normotensive subjects stratified by and homozygous for the −344 and intron conversion variants. Eighty-three variants associated with −344 and intron conversion were identified. Haplotype analysis revealed 4 common haplotypes, accounting for 68% of chromosomes, confirming strong linkage disequilibrium across the region. Two novel CYP11B1 polymorphisms upstream of the coding region (−1889 G/T and −1859 A/G) were identified as contributing to the common haplotypes. Given the potential for such mutations to affect transcriptional regulation of CYP11B1, these were analyzed further. A total of 512 hypertensive subjects from the British Genetics of Hypertension Study population were genotyped for these polymorphisms. A significant association was identified between the −1889 polymorphism and urinary tetrahydrodeoxycortisol/total cortisol metabolite ratio, indicating reduced 11β-hydroxylase efficiency. A similar pattern was observed for the −1859 polymorphism, but this did not achieve statistical significance. Functional studies in vitro using luciferase reporter gene constructs show that these polymorphisms significantly alter the transcriptional response of CYP11B1 to stimulation by adrenocorticotropic hormone or forskolin. This study strongly suggests that the impaired 11β-hydroxylase efficiency associated previously with the CYP11B2 −344 and intron conversion variants is because of linkage with these newly identified polymorphisms in CYP11B1.

The Monocyte Chemotactic Protein-1 Gene May Contribute to Hypertension in Dahl Salt-Sensitive Rats

In a previous study, we performed a genome-wide quantitative trait loci (QTLs) analysis for blood pressure using F2 rats derived from Dahl salt-sensitive (DS) and Lewis (LEW) rats and identified two QTLs that influenced blood pressure levels. Although we determined that one of the causative genes in the chromosome (Ch) 1 region seemed to be Klk1, we did not perform detailed analyses on the Ch10 QTL region. The purpose of the present study was to identify candidate genes that influence blood pressure in the Ch10 QTL region. Using microarray analysis, we compiled a list of the genes that are differentially expressed between the two strains and that were localized to the Ch10 QTL region. Subsequent reverse transcription-polymerase chain reaction (RT-PCR) and Northern blot analysis identified that, while the expression levels of Ccl2 mRNA were not different between the kidneys of DS and LEW rats fed a normal diet, those in DS were 10-fold higher than those in LEW under a high-salt diet. Although the promoter reporter assay failed to identify causative nucleotide changes that led to the differential expression, monocyte chemotactic protein-1 (MCP-1) release from isolated monocytes were significantly higher in DS than in LEW. Intriguingly, this Ch10 QTL for blood pressure was also a possible QTL for urinary albumin excretion. Since Ccl2 is well known to be involved in various types of renal injury, it is likely that a higher expression of Ccl2 might aggravate macrophage infiltration, which in turn could aggravate tubulointerstitial injury, and thereby accelerate salt-sensitive hypertension. Thus, Ccl2 appears to be a interesting candidate gene for salt-sensitive hypertension in DS.

Congenic strains confirm aerobic running capacity quantitative trait loci on rat chromosome 16 and identify possible intermediate phenotypes

We previously identified two inbred rat strains divergent for treadmill aerobic running capacity (ARC), the low-performing Copenhagen (COP) and the high-performing DA rats, and used an F(2)(COPxDA) population to identify ARC quantitative trait loci (QTLs) on rat chromosome 16 (RNO16) and the proximal portion of rat chromosome 3 (RNO3). Two congenic rat strains were bred to further investigate these ARC QTLs by introgressing RNO16 and the proximal portion of RNO3 from DA rats into the genetic background of COP rats and were named COP.DA(chr 16) and COP.DA(chr 3), respectively. COP.DA(chr 16) rats had significantly greater ARC compared with COP rats (696.7 +/- 38.2 m vs. 571.9 +/- 27.5 m, P = 0.03). COP.DA(chr 3) rats had increased, although not significant, ARC compared with COP rats (643.6 +/- 40.9 m vs. 571.9 +/- 27.5 m). COP.DA(chr 16) rats had significantly greater subcutaneous abdominal fat, as well as decreased fasting triglyceride levels, compared with COP rats (P < 0.05), indicating that genes responsible for strain differences in fat metabolism are also located on RNO16. While this colocalization of QTLs may be coincidental, it is also possible that these differences in energy balance may be associated with the superior running performance of COP.DA(chr 16) consomic rats.

Development of hypertension and kidney hypertrophy in transgenic mice overexpressing ARAP1 gene in the kidney

Angiotensin II regulates blood pressure via activation of the type 1 receptor. We previously identified a novel angiotensin II type 1 receptor-associated protein and demonstrated that it promotes receptor recycling to the plasma membrane. To delineate the pathophysiological function of the ARAP1 in the kidneys, we generated transgenic mice that overexpress rat ARAP1 cDNA specifically in proximal tubules and tested the hypothesis that proximal tubule-specific overexpression of ARAP1 causes hypertension. Two lines of male transgenic mice, 650 and 670, displayed kidney-specific transgene expression. Systolic blood pressure was significantly elevated by &20 to 25 mm Hg in these lines of mice at 20 weeks of age compared with their nontransgenic litter mates. Urine volume, but not water intake, was significantly decreased in both lines compared with nontransgenic controls. The kidney/body weight ratio was significantly increased in both lines compared with their nontransgenic litter mates at 12 and 20 weeks of age. In contrast, no difference was observed in the ratio of brain, spleen, heart, and testis to body weight between male transgenic and nontransgenic animals. Inhibitions of the renin-angiotensin system completely normalized the systolic blood pressure of transgenic mice. Moreover, low salt intake prevented the development of hypertension, whereas high salt intake exacerbated the increase in blood pressure in transgenic mice. Therefore, our data show that proximal tubule-specific overexpression of ARAP1 leads to hypertension, suggesting that renal ARAP1 plays an important role in the regulation of blood pressure and renal function via activation of the intrarenal renin-angiotensin system.

Increased number of aldosterone-sensitive NTS neurons in Dahl salt-sensitive rats

Dahl salt-sensitive rats develop severe hypertension during a high-sodium diet, but the basis of their salt-sensitive phenotype is not completely understood. A subset of neurons in the nucleus tractus solitarius (NTS) are uniquely sensitive to the adrenal steroid hormone aldosterone, which is critically involved in sodium homeostasis, due to their expression of the enzyme 11-beta-hydroxysteroid dehydrogenase type 2 (HSD2). The number of HSD2 neurons in the NTS was counted in prehypertensive 7-week-old Dahl salt-sensitive rats and compared with two control strains: Dahl salt-resistant and Sprague-Dawley rats. Dahl salt-sensitive rats had more HSD2 neurons than age-matched Dahl salt-resistant and Sprague-Dawley rats (24% and 21%, respectively). Cell counts were also made in spontaneously hypertensive rats (SHR) and Wistar-Kyoto (WKY) rats; the number of HSD2 neurons in both of these strains was similar to the values obtained for Sprague-Dawley rats. The increased number of HSD2-immunoreactive neurons counted in Dahl salt-sensitive rats suggests that they may have a greater number of aldosterone-sensitive NTS neurons. Alternatively, an increase in HSD2 expression in Dahl salt-sensitive rats could increase the overall immunoreactivity, permitting detection of more of these neurons. In either case, the roughly 20% increase in HSD2 neurons in the NTS of prehypertensive Dahl salt-sensitive rats is a novel factor associated with their salt-sensitive phenotype. These neurons may play a role in regulating sodium appetite, which is abnormally suppressed in Dahl salt-sensitive rats.

Klk1 as One of the Genes Contributing to Hypertension in Dahl Salt-Sensitive Rat

A genome-wide quantitative trait loci analysis for blood pressure was performed using 107 male F 2 rats derived from Dahl salt-sensitive and Lewis rats. Blood pressure was assessed by telemetry, and >400 microsatellite markers were used for genotyping. Two major quantitative trait loci for blood pressure were identified at chromosome 1 and chromosome 10. The expression levels of 366 transcripts around the chromosome 1 quantitative trait loci were assessed by RT-PCR, and we found that the Klk1 (kallikrein 1) and Ngfg (nerve growth factor gamma) mRNA levels were significantly reduced in the kidneys of Dahl salt-sensitive rats compared with those in Lewis rats. The expression levels of kallikrein 1 protein were also suppressed in Dahl salt-sensitive rats compared with those in Lewis rats. Because the kallikrein–kinin system has been shown to be involved in renal function, including salt homeostasis, it is likely that the reduced expression of Klk1 contributes to salt-sensitive hypertension in Dahl salt-sensitive rats.

Mechanisms of hypertension: the expanding role of aldosterone

Hypertension is a common disorder that affects a large heterogeneous patient population. Subgroups can be identified on the basis of their responses to hormonal and biologic stimuli. These subgroups include low-renin hypertensives and nonmodulators. Aldosterone, the principal human mineralocorticoid, is increasingly recognized as playing a significant role in cardiovascular morbidity, and its role in hypertension has recently been reevaluated with studies that suggest that increased aldosterone biosynthesis (as defined by an elevated aldosterone to renin ratio) is a key phenotype in up to 15% of individuals with hypertension. It was reported previously that a polymorphism of the gene (C to T conversion at position -344) encoding aldosterone synthase is associated with hypertension, particularly in individuals with a high ratio. However, the most consistent association with this variant is a relative impairment of adrenal 11beta-hydroxylation. This review explores the evidence for this and provides a hypothesis linking impaired 11beta-hydroxylation and hypertension with a raised aldosterone to renin ratio. It is also speculated that there is substantial overlap between this group of patients and previously identified low-renin hypertensives and nonmodulators. Thus, these groups may form a neurohormonal spectrum reflecting different stages of hypertension or indeed form sequential steps in the natural history of hypertension in genetically susceptible individuals.

Aldosterone synthase gene variation and adrenocortical response to sodium status, angiotensin II and ACTH in normal male subjects

OBJECTIVE

Aldosterone synthase, a key enzyme in the terminal steps of aldosterone synthesis, is encoded by the CYP11B2 gene. A polymorphism in the 5' coding region of this gene (-344 C/T) is associated with hypertension, particularly with elevation of the aldosterone to renin ratio. A second polymorphism (a conversion in intron 2 to resemble that of the neighbouring 11beta-hydroxylase (CYP11B1) gene) is found in close linkage dysequilibrium with the variant at -344 C/T. The mechanism by which these variants predispose to cardiovascular disease and the precise intermediate phenotype associated with them remains speculative.

DESIGN

We performed a focused physiological study in normal volunteers stratified by CYP11B2 genotype.

PATIENTS

Twenty-three subjects homozygous for the T allele and 21 homozygous for the C allele of the -344 C/T polymorphism of CYP11B2 were studied.

MEASUREMENTS

Basal and angiotensin II stimulated plasma and 24-h urinary steroid excretion during low (60 mmol/day) and high (160 mmol/day) sodium intake and plasma steroids after ACTH stimulation were measured.

RESULTS

No influence of polymorphic variation on basal or stimulated plasma cortisol or aldosterone or other plasma steroid concentrations during either dietary phase was seen. However, excretion of tetrahydro-11-deoxycortisol (the urinary metabolite of 11-deoxycortisol), which is the precursor of cortisol) was increased in TT subjects during sodium restriction, consistent with impairment of zona fasciculata 11beta-hydroxylation.

CONCLUSIONS

We conclude that this polymorphism has no major influence on normal zona glomerulosa function but is associated with a change in 11beta-hydroxylation in the zona fasciculata. The mechanism remains uncertain, but alteration of 11-deoxycortisol levels without change in cortisol suggests altered efficiency of 11beta-hydroxylation. In the long term, this may lead to a minor but chronic increase in ACTH drive to the gland, which may have consequences for steroid synthesis and predispose to the risk of cardiovascular disease.

The impact of polymorphisms in the gene encoding aldosterone synthase (CYP11B2) on steroid synthesis and blood pressure regulation

The terminal stages in the synthesis of aldosterone and cortisol are catalysed by the enzymes aldosterone synthase and 11beta-hydroxylase respectively. We have previously reported that polymorphic variation in the 5' promoter region (-344C/T) of the gene encoding aldosterone synthase (CYP11B2) is associated with increased aldosterone metabolite excretion and with hypertension associated with a raised aldosterone to renin ratio (ARR). Additionally, basal and ACTH-stimulated plasma levels of 11-deoxycortisol, the precursor of cortisol, are higher in subjects carrying the T-allelic variant. We have now identified in a family study (573 individuals from 105 extended families ascertained through a hypertensive proband) that excretion of the main metabolite of this steroid (tetrahydro-11-deoxycortisol, THS) is heritable (19.4%) and that the T-allele of CYP11B2 is more strongly associated with higher THS levels than the C-allele. Raised plasma and urinary levels of 11-deoxycortisol suggest that there is relative inefficiency of 11beta-hydroxylation in the zona fasciculata; the P450 enzyme responsible for this step is encoded by the gene CYP11B1, which is highly homologous with and adjacent to CYP11B2. The association of genetic variation in the promoter of CYP11B2 which, in the adrenal cortex, is only expressed in zona glomerulosa, and zona fasciculata 11beta-hydroxylation function is paradoxical. There may be linkage dys-equilibrium between this polymorphism and a quantitative trait locus (QTL) in CYP11B1. Chronic alteration of 11beta-hydroxylase activity may increase ACTH drive to the adrenal cortex, altering the regulation of aldosterone synthesis. This may explain, at least partly, the association between CYP11B2 polymorphisms and hypertension.

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.

Is altered adrenal steroid biosynthesis a key intermediate phenotype in hypertension?

Approximately 10% of patients with hypertension have a high ratio of aldosterone to renin, but the reason for this and the relationships among low-renin essential hypertension, elevation of the ratio, and true primary aldosteronism are unclear. We have previously reported that a polymorphism of the gene (C-to-T conversion at position -344) encoding aldosterone synthase is associated with hypertension, particularly in patients with a high ratio. However, the most consistent association with this variant is a relative impairment of adrenal 11beta-hydroxylation. In this review, we propose that altered conversion of deoxycortisol to cortisol leads to a subtle, chronic increase in adrenocortrophin drive to the adrenal cortex, with eventual development of hyperplasia. In combination with other genetic or environmental factors (such as dietary sodium intake), we suggest that this might be responsible for the long-term development of a resetting of the aldosterone response to angiotensin II, giving rise to the phenotype of hypertension with a raised ratio. In some subjects, this may progress further to true primary aldosteronism with a dominant adrenal nodule. Thus, there may be a genetically influenced continuum from hypertension with a normal ratio, through hypertension with a raised ratio, and primary aldosteronism.

Use of a panel of congenic strains to evaluate differentially expressed genes as candidate genes for blood pressure quantitative trait loci

Candidate gene(s) for multiple blood pressure (BP) quantitative trait loci (QTL) were sought by analysis of differential gene expression patterns in the kidneys of a panel of eight congenic strains, each of which carries a different low-BP QTL allele with a genetic composition that is otherwise similar to that of the hypertensive Dahl salt-sensitive (S) rat strain. First, genes differentially expressed in the kidneys of one-month-old Dahl S and salt-resistant (R) rats were identified. Then, Northern filter hybridization was used to examine the expression patterns of these genes in a panel of congenic strains. Finally, their chromosomal location was determined by radiation hybrid (RH) mapping. Seven out of 37 differentially expressed genes were mapped to congenic regions carrying BP QTLs, but only one of these genes, L-2 hydroxy acid oxidase (Hao2), showed the congenic strain-specific pattern of differential kidney gene expression predicted by its chromosomal location. This data suggests that Hao2 should be examined as a candidate gene for the rat chromosome 2 (RNO2) BP QTL.

Gene expression profiling in hypertension research: a critical perspective

Recent advances in molecular biology and technology have made it possible to monitor the expression levels of virtually all genes simultaneously. As the tools for gene expression profiling have become more widely available, the number of investigators applying this technology in hypertension research, as in other fields of biomedical research, has grown rapidly. At the same time, numerous articles have been published that discuss the technical aspects of gene profiling and its promise for advancing research on the pathogenesis and treatment of multiple clinical disorders. However, much of the research carried out with gene expression profiling has been of a correlational or descriptive nature, and the true value of this technology is unclear. Despite the initial wave of enthusiasm for gene expression profiling, its actual utility for studying multifactorial disorders like hypertension remains to be established. In this review, we offer a critical perspective on the use of gene expression profiling in hypertension research and discuss some emerging strategies for taking this technology beyond the limits of correlational and descriptive studies.

Kidney specificity of rat chromosome 1 blood pressure quantitative trait locus region

Rat chromosome 1 has a region containing loci that influence blood pressure. In the present study, we investigated whether these loci mediate their effect via the kidney. Taking advantage of the histocompatibility between a congenic strain (WKY.SHR-Sa, which contains the relevant chromosomal region from the spontaneously hypertensive rat) and its parental strain, the Wistar-Kyoto rat (WKY), we compared the effect of transplanting a kidney at 5 to 6 weeks of age from either congenic rats or WKY into bilaterally nephrectomized WKY. WKY.SHR-Sa animals and WKY with intact kidneys and with unilateral nephrectomy were studied as controls. Blood pressure was measured at 12, 16, 20, and 25 weeks of age. At all time points, blood pressure was significantly higher (by between 8 to 22 mm Hg, P<0.001) in 2-kidney WKY.SHR-Sa animals compared with WKY. This genotype-related difference was maintained in unilaterally nephrectomized rats. Most importantly, WKY that received transplants from WKY.SHR-Sa rats had significantly higher blood pressure (P<0.001 at all time points) compared with those that received transplants from other WKY. At any age, this difference was between 70% to 100% of the difference observed between the 1-kidney groups. There was no difference in plasma urea or creatinine between groups or evidence of chronic rejection in the cross-transplant group. The findings indicate that the major proportion of the blood pressure effect of loci on rat chromosome 1 is mediated through the kidney, and provide a rational basis for investigating genes located in the relevant chromosomal region and expressed in the kidney as likely candidates.

QTL associated with blood pressure, heart rate, and heart weight in CBA/CaJ and BALB/cJ mice

To better understand the genetic basis of essential hypertension, we conducted a quantitative trait locus (QTL) analysis of a population of 207 (BALB/cJ x CBA/CaJ) F(2) male mice to identify genomic regions that regulate blood pressure, heart rate, and heart weight. We identified two loci, Bpq6 (blood pressure quantitative locus 6) on chromosome 15 (Chr 15; peak, 16 cM; 95% confidence interval, 0-25 cM) and Bpq7 on Chr 7 (peak, 42 cM; 95% confidence interval, 35-50 cM) that were significantly associated with blood pressure. We also identified two loci, Hrq1 (heart rate quantitative locus 1) and Hrq2, on D2Mit304 (peak, 72 cM; 95% confidence interval 60-80 cM) and D15Mit184 (peak, 25 cM; 95% confidence interval 20-35 cM), respectively, that were significantly associated with heart rate. A significant gene-gene interaction for heart rate was found between Hrq1 and D1Mit10 (peak, 57 cM; 95% confidence interval, 45-75 cM); the latter QTL was named Hrq3. We identified a significant locus for heart weight, Hwq1 (heart weight quantitative locus 1), at D14Mit67 (peak, 38 cM; 95% confidence interval, 20-43 cM). Identification of the genes for these QTL should lead to a better understanding of the causes of essential hypertension.

Mutations in aldosterone synthase gene of Milan hypertensive rats: phenotypic consequences

Using in vitro and in vivo methods, we have demonstrated increased sensitivity of adrenocortical steroidogenesis to ACTH in Milan hypertensive (MHS) compared with normotensive (MNS) rats and have investigated whether this is caused by mutations of steroidogenic enzymes. Genes encoding aldosterone synthase (CYP11B2) and 11beta-hydroxylase (CYP11B1) in MHS and MNS have been cloned and sequenced. Nucleotide 752 (G) in exon 4 of MHS CYP11B2 differs from that of MNS (A); CYP11B1 sequences were identical. The nucleotide 752 mutation caused a Q251R substitution in the amino acid sequence of MHS CYP11B2. The phenotype of MHS CYP11B2 alleles, when expressed in COS-1 cells, differed from that of MNS alleles. The relative activities of the three reactions catalyzed by CYP11B2 (11beta-hydroxylation of deoxycorticosterone, 18-hydroxylation of corticosterone, and dehydrogenation of 18-hydroxycorticosterone) were estimated after incubation of transfected cells with [(14)C]deoxycorticosterone and analysis of radioactivity associated with deoxycorticosterone, corticosterone, 18 hydroxycorticosterone, and aldosterone. Both 11- and 18-hydroxylase activities were lower (19 and 12%, respectively; P < 0.01 and P < 0.05) in cells transfected with MHS compared with MNS alleles, whereas 18-oxidase activity was 42% higher (P < 0.01). To assess the significance of the CYP11B2 mutation in vivo, DNA from F2 hybrid MHS x MNS rats was genotyped. MHS alleles were associated with lower urine volumes in both sexes, lower ventricle weights in male rats, but no difference in systolic or diastolic blood pressures between the sexes. We conclude that a mutation in CYP11B2 may affect aldosterone secretion in MHS; however, under normal environmental circumstances, we were unable to demonstrate any influence of this mutation on blood pressure.

Kidney-specific chromosome transfer in genetic hypertension: the Dahl hypothesis revisited

BACKGROUND

A central dogma in the field of essential hypertension research is that the genetic transmission of increased blood pressure is determined solely by the genotype of the kidney. This concept is based in large part on studies in experimental rat models of spontaneous hypertension in which transplantation of a kidney from a hypertensive strain into a normotensive strain was reported to increase blood pressure, and transplantation of a kidney from a normotensive strain into a hypertensive strain was reported to decrease blood pressure. The enduring interpretation of these now classic experiments remains virtually unchanged from the view originally espoused a quarter century ago by Lewis Dahl, one of the founding fathers of the field of genetic hypertension research: "Blood pressure is determined by the genotype of the donor kidney and not the genotype of the recipient."

METHODS

To test the Dahl hypothesis, we determined the blood pressure effects of selective intrarenal versus extrarenal exchange of single chromosome regions between the spontaneously hypertensive rat (SHR) and the normotensive Brown Norway (BN) rat.

RESULTS

The replacement of a defined segment of chromosome 1 in the SHR with the corresponding chromosome region of the BN rat was sufficient to attenuate hypertension when selectively achieved either inside the kidney or outside the kidney.

CONCLUSIONS

The current finding (1) demonstrates that naturally occurring genetic variants exist that can regulate blood pressure when selectively expressed outside the kidney as well as inside the kidney, and (2) compels reconsideration of the long-held view that in essential hypertension, the genetic transmission of increased blood pressure is determined solely by the genotype of the kidney.

Quantitative trait loci for body weight, blood pressure, blood glucose, and serum lipids: linkage analysis with wild rats (Rattus norvegicus)

To study polygenetically inherited human diseases like hypertension, inbred rat strains are usually the preferred models. Because many inbred generations under optimized environmental conditions may have led to the survival of "silent" disease genes, we used a cross between one wild rat and genetically hypertensive SHR rats to analyze quantitative trait loci (QTLs) of blood pressure and related traits. The (Wild x SHR)F1 hybrids were transferred into a pathogen-free environment by wet-hysterectomy and were backcrossed onto SHR to generate first backcross hybrids (BC1). Progeny from one F1 female (n = 72) were phenotypically and genetically characterized to map QTLs. Significant, subsignificant, and suggestive evidence was found for more sex-specific than common linkage of blood pressure and most blood-pressure-related traits. Male- and female-specific regions were determined on different chromosomes for blood pressures (Chrs. 2 and 7 vs 5 and 11), body weight (Chrs. 10 vs 18), and blood glucose (Chr. 17 vs 20). A linkage in both males and females was shown for serum triglycerides on chromosomes 6 and 17, respectively, and blood glucose on chromosome 15. For serum total cholesterol, a significant linkage was found on chromosome 14 only in males. Our findings not only indicate the complex character of quantitative traits per se but also show impressively their dependence on sex, age, and strains in cosegregation analysis.

Genetic rat models of hypertension: relationship to human hypertension

Experimental models of human disease are frequently used to investigate the pathophysiology of disease as well as the mechanisms of action of therapeutics. However, as long as models have been used there have been debates about the utility of experimental models and their applicability for human disease on the phenotypic and genomic level. The recent advances in molecular genetics and genomics have provided powerful tools to study the genetics of multifactorial diseases, such as hypertension. However, studies of such diseases in humans remain challenging in part due to lack of statistical power and genetic heterogeneity within patient populations. For hypertension, various rat models have been developed and used for the identification of susceptibility loci for genetic hypertension. With the advent of "comparative genomics," the application of genetic studies to both human and animal model systems allows for a new paradigm, where comparative genomics can be used to bridge between model utility and clinical relevance. This review discusses recent approaches in genetics to facilitate gene discovery for polygenic disorders with specific focus on how comparative mapping can be used to select target regions in the human genome for large-scale association studies and linkage disequilibrium testing in clinical populations.

High-resolution mapping of the blood pressure QTL on chromosome 7 using Dahl rat congenic strains

It was previously shown using Dahl salt-sensitive (S) and salt-resistant (R) rats that a blood pressure quantitative trait locus (QTL) was present on rat chromosome 7. In the present work, this QTL was localized to a region less than 0.54 cM in size on the linkage map using a series of congenic strains. This region was contained in a single yeast artificial chromosome that was 220 kb long. This small segment still contained the primary candidate locus Cyp11b1 (11beta-hydroxylase), but the adjacent candidate genes Cyp11b2 (aldosterone synthase) and Cyp11b3 were ruled out. It is concluded that 11beta-hydroxylase, through its known genetic variants altering the production of 18-hydroxy-11-deoxy corticosterone, is very likely to account for the blood pressure QTL on chromosome 7 in the Dahl rat model of hypertension. This QTL accounts for about 23 mm Hg under the condition of 2% NaCl diet for 24 days.

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.