The rat renin gene: assignment to chromosome 13 and linkage to the regulation of blood pressure

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.

Congenic mapping and sequence analysis of the Renin locus

Renin was the first blood pressure (BP) quantitative trait locus mapped by linkage analysis in the rat. Subsequent BP linkage and congenic studies capturing different portions of the renin region have returned conflicting results, suggesting that multiple interdependent BP loci may be residing in the chromosome 13 BP quantitative trait locus that includes Renin. We used SS-13(BN) congenic strains to map 2 BP loci in the Renin region (chr13: 45.2-49.0 Mb). We identified a 1.1-Mb protective Brown Norway region around Renin (chr13: 46.1-47.2 Mb) that significantly decreased BP by 32 mm Hg. The Renin protective BP locus was offset by an adjacent hypertensive locus (chr13: 47.2-49.0 Mb) that significantly increased BP by 29 mm Hg. Sequence analysis of the protective and hypertensive BP loci revealed 1433 and 2063 variants between Dahl salt-sensitive/Mcwi and Brown Norway rats, respectively. To further reduce the list of candidate variants, we regenotyped an overlapping SS-13(SR) congenic strain (S/renrr) with a previously reported BP phenotype. Sequence comparison among Dahl salt-sensitive, Dahl R, and Brown Norway reduced the number of candidate variants in the 2 BP loci by 42% for further study. Combined with previous studies, these data suggest that at least 4 BP loci reside within the 30-cM chromosome 13 BP quantitative trait locus that includes Renin.

Small RNA expression and strain specificity in the rat

BACKGROUND

Digital gene expression (DGE) profiling has become an established tool to study RNA expression. Here, we provide an in-depth analysis of small RNA DGE profiles from two different rat strains (BN-Lx and SHR) from six different rat tissues (spleen, liver, brain, testis, heart, kidney). We describe the expression patterns of known and novel micro (mi)RNAs and piwi-interacting (pi)RNAs.

RESULTS

We confirmed the expression of 588 known miRNAs (54 in antisense orientation) and identified 56 miRNAs homologous to known human or mouse miRNAs, as well as 45 new rat miRNAs. Furthermore, we confirmed specific A to I editing in brain for mir-376a/b/c and identified mir-377 as a novel editing target. In accordance with earlier findings, we observed a highly tissue-specific expression pattern for all tissues analyzed. The brain was found to express the highest number of tissue-specific miRNAs, followed by testis. Notably, our experiments also revealed robust strain-specific differential miRNA expression in the liver that is caused by genetic variation between the strains. Finally, we identified two types of germline-specific piRNAs in testis, mapping either to transposons or in strand-specific clusters.

CONCLUSIONS

Taken together, the small RNA compendium described here advances the annotation of small RNAs in the rat genome. Strain and tissue-specific expression patterns furthermore provide a strong basis for studying the role of small RNAs in regulatory networks as well as biological process like physiology and neurobiology that are extensively studied in this model system.

Multiple blood pressure loci on rat chromosome 13 attenuate development of hypertension in the Dahl S hypertensive rat

Previous studies have indicated that substitution of chromosome 13 of the salt-resistant Brown Norway BN/SsNHsdMcwi (BN) rat into the genomic background of the Dahl salt-sensitive SS/JrHsdMcwi (SS) rat attenuates the development of salt-sensitive hypertension and renal damage. To identify the regions within chromosome 13 that attenuate the development of hypertension during a high-salt diet in the SS rat, we phenotyped a series of overlapping congenic lines covering chromosome 13, generated from an intercross between the consomic SS-13BN rat and the SS rat. Blood pressure was determined in chronically catheterized rats after 2 wk of high-salt diet (8% NaCl) together with microalbuminuria as an index of renal damage. Four discrete regions were identified, ranging in size from 4.5 to 16 Mbp, each of which independently provided significant protection from hypertension during high-salt diet, reducing blood pressure by 20–29 mmHg. Protection was more robust in female than male rats in some of the congenic strains, suggesting a sex interaction with some of the genes determining blood pressure during high-salt diet. Among the 23 congenic strains, several regions overlapped. When three of the “protective” regions were combined onto one broad congenic strain, no summation effect was seen, obtaining the same decrease in blood pressure as with each one independently. We conclude from these studies that there are four regions within chromosome 13 containing genes that interact epistatically and influence arterial pressure.

Strong Association of a Renin Intronic Dimorphism with Essential Hypertension

The objectives of this project were two-fold: to identify the genetic mutation that has been detected as an MboI dimorphism in intron 9 of the human renin (REN) gene and to confirm a previously reported, putative association between the REN MboI dimorphism and clinical diagnosis of essential hypertension (EHT) in a population of Gulf Arabs from the United Arab Emirates. Sequencing of the MboI dimorphic site was carried out on DNA of randomly chosen cases and controls. A retrospective case-control study was carried out in 689 unrelated subjects (326 first-time, clinically diagnosed hypertensives and 363 age- and gender-matched normotensive subjects), selected from the resident population of the Abu Dhabi Emirate. A polymerase chain reaction/MboI-RFLP based method was employed to compare genotype and allele distributions. Nucleotide sequences at the MboI site of the cut and uncut alleles were determined to be GATC and GGTC, respectively. This A>G mutation is located 10,631 base pairs (bp) 3' to the start of the REN gene, and 79 bp 3' to the end of exon 9. The genotype distributions of the REN 10631A>G dimorphism were found to be significantly different between hypertensive and normotensive subjects (x2= 42.29, df=2, p<0.001). Frequencies of A alleles were 0.54 in EHT vs. 0.37 in normotensive subjects, which is even more demarcated than what was found previously. The frequency of AA genotypes was higher in the hypertensive group than in the normotensive group (34.7% vs. 14.0%). The quantification of the association of A alleles with increased risk of EHT was assessed with corresponding odds ratios (OR), which gave the following values: OR of GG vs. AG genotypes, 1.3 (95% confidence interval [CI]: 0.90-1.88); OR of GG vs. AA, 3.75 (95% CI: 2.41-5.86). In conclusion, REN 10631A alleles are significantly associated with EHT in the Emirati population. This has now been found in two different and therefore independent sample populations from the Abu Dhabi Emirate. Moreover, this genetic effect seems to be acting in a recessive fashion. Hence, either the REN gene itself, or another gene that is in linkage disequilibrium with REN 10631A>G, is implicated in the pathogenesis of EHT in Emirati.

Genome-wide searches for blood pressure quantitative trait loci in the stroke-prone spontaneously hypertensive rat of a Japanese colony

OBJECTIVES

Although several quantitative trait loci for blood pressure have been reported in stroke-prone spontaneously hypertensive rats (SHRSP), the results are not always concordant among different crosses. To evaluate potential confounding factors in linkage analysis, we performed genome-wide screens in F2 populations derived from SHRSP and Wistar-Kyoto rats of a Japanese colony.

METHODS

Two F cohorts were independently produced: F2-1 (110 male and 110 female rats), and F2-2 (174 male and 184 female rats). Blood pressure was measured longitudinally (from 2 to 5 months of age and 1 month after salt-loading) in F2-1, while it was measured at 13 weeks of age in F2-2. Subsequent to an initial screen with 251 markers in F2-1 male progeny, 170 markers were selected and characterized in the remaining populations.

RESULTS

When 578 rats were analyzed together, markers from five chromosomal regions showed significant linkage to blood pressure at 13 weeks of age. The strongest and the most consistent linkage was found on rat chromosome 1 (a maximal log of the odds score reached 8.3). In the other regions, the degree of linkage was more prominent in either of sexes. Some evidence of age-specific and sex-specific linkage was detected in five additional regions in the F2-1 cohort. In the Japanese colony, however, there was no significant linkage to several chromosomal regions previously reported in other SHRSP colonies.

CONCLUSIONS

Our data provide solid evidence of a chromosome-1 linkage and demonstrate the importance of aging, sex, and dietary manipulation in linkage analysis. Also, the combination of parental rat strains seems to be critical when searching for blood pressure quantitative trait loci.

Lack of correlation between Mbo I restriction fragment length polymorphism of renin gene and essential hypertension in Japanese

Predisposition to essential hypertension is associated with gene polymorphisms of the renin angiotensin system (RAS). Gene polymorphisms of the angiotensinogen and angiotensin converting enzyme genes are known to be risk factors for hypertension, while few studies concerning the renin gene polymorphism have been published. In the present investigation, we carried out a case control study using a Japanese population to examine the genetic influence of the renin gene on the predisposition to hypertension. Patients (n=235) recruited from outpatients at Osaka University Hospital and diagnosed with essential hypertension or receiving long-term antihypertensive medication participated in the study. Normotensive control subjects (n=510) without a history of hypertension and without diabetes mellitus were recruited from the same population, and were sex-matched with experimental subjects. A polymorphism in intron 9 of the human renin gene was determined as the Mbo I restriction fragment length polymorphism (Mbo I-RFLP). There was no significant association between Mbo I-RFLP of the renin gene and predisposition to essential hypertension in Japanese (p>0.05, chi2=2.1). These results suggest that the Mbo I (+) allele of the renin gene does not increase the risk for hypertension in Japanese.

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.

Concordance of murine quantitative trait loci for salt-induced hypertension with rat and human loci

To investigate the genetic control of salt-induced hypertension, we performed a quantitative trait locus analysis on male mice from a reciprocal backcross between the salt-sensitive C57BL/6J and the normotensive A/J inbred mouse strains after they were provided with water containing 1% salt for 2 weeks. Genome-wide scans performed on these mice and analyzed with a combination of conventional marker-based regressions and a novel simultaneous search for pairs revealed six significant quantitative trait loci associated with salt-induced blood pressure, two of which were interacting loci. These six loci, named Bpq1-6 for blood pressure quantitative trait loci, mapped to D1Mit334, D1Mit14, D4Mit164, D5Mit31, D6Mit15, and D15Mit13. Furthermore, five of these six loci were concordant with hypertension loci in rats, and four were concordant with hypertension loci in humans, suggesting that quantitative trait loci mapping in model organisms can be used to guide the search for human blood pressure genes.

The molecular basis of hypertension

More than 50 million Americans display blood pressures outside the safe physiological range. Unfortunately for most individuals, the molecular basis of hypertension is unknown, in part because pathological elevations of blood pressure are the result of abnormal expression of multiple genes. This review identifies a number of important blood pressure regulatory genes including their loci in the human, mouse, and rat genome. Phenotypes of gene deletions and overexpression in mice are summarized. More detailed discussion of selected gene products follows, beginning with proteins involved in ion transport, specifically the epithelial sodium channel and sodium proton exchangers. Next, proteins involved in vasodilation/natriuresis are discussed with emphasis on natriuretic peptides, guanylin/uroguanylin, and nitric oxide. The renin angiotensin aldosterone system has an important role antagonizing the vasodilatory cyclic GMP system.

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.

Distinct renin isoforms generated by tissue-specific transcription initiation and alternative splicing

The aspartyl protease renin catalyzes the initial and rate-limiting step in the formation of the biologically active peptide angiotensin II. It is mainly synthesized in the kidney as a preprohormone and secreted via constitutive and regulated pathways. We identified a novel transcript of the rat renin gene, renin b, characterized by the presence of an alternative first exon (exon 1b) that is spliced to exon 2 of the known transcript, termed renin a. We demonstrated that renin b is exclusively expressed in the brain. In contrast, renin a was not expressed in the brain. Using primer extension assays, we mapped the transcriptional start site of this novel mRNA within intron 1 of the rat genomic sequence, suggesting the presence of a brain-specific promoter within intron 1. The presence of a brain-specific renin isoform is evolutionally conserved, as demonstrated by the finding of renin b isoforms in mice and humans. The predicted protein renin b lacks the prefragment as well as a significant portion of the profragment and is therefore predicted not to be a secreted protein, unlike the classically described isoform renin a. As shown by in vitro translation of full-length renin b mRNA in the presence of microsomal membranes, renin b was not targeted into the endoplasmatic reticulum and remained intracellularly in transiently transfected AtT-20 cells. These findings provide evidence for a novel pathway of intracellular angiotensin generation that occurs exclusively in the brain.

Mapping of candidate genes for hypertension by fluorescence in situ hybridization on the genome of transgenic rats and mice

Transgenic animals are new and important models for the study of candidate genes in hypertension research as well as in other fields of medicine. For detailed genetic characterization of the transgenic animals, and to account for the symptoms arising from the insertion of transgenes in the genome, it is essential to identify these insertion sites. In this study, the insertion sites of the transgenes of candidate genes for hypertension were identified by fluorescence in situ hybridization (FISH) after G-banding of the chromosomes in transgenic rats and mice. This technique combines high resolution G-banding and fluorescence in situ hybridization for the mapping of four different candidate genes in six different transgenic rats as well as three different mouse transgenic lines. The presented results will help to draw conclusions about the influence of the respective integration site on transgene expression.

Effect of renin gene transfer on blood pressure in the spontaneously hypertensive rat

To investigate whether molecular variation in the renin gene contributes to the greater blood pressure of spontaneously hypertensive rats (SHR) versus normotensive Brown Norway (BN) rats, we measured blood pressure in an SHR progenitor strain and an SHR congenic strain that are genetically identical except at the renin gene and an associated segment of chromosome 13 transferred from the BN strain. Backcross breeding and molecular selection at the renin locus were used to create the SHR congenic strain (designated SHR.BN-Ren) that carries the renin gene transferred from the normotensive BN strain. We found that transfer of the renin gene from the BN strain onto the genetic background of the SHR did not decrease blood pressure in rats fed either a normal or high-salt diet. In fact, the systolic blood pressures of the SHR congenic rats tended to be slightly greater than the systolic blood pressures of the SHR progenitor rats. However, the congenic strain exhibited lower serum high-density lipoprotein cholesterol, and greater levels of total cholesterol, very-low-density lipoprotein, and intermediate-density lipoprotein cholesterol during administration of a high-fat, high-cholesterol diet. These findings demonstrate that (1) under the environmental circumstances of the current study, the greater blood pressure of SHR versus BN rats cannot be explained by strain differences in the renin gene and (2) a quantitative trait locus affecting lipid metabolism exists on chromosome 13 within the transferred chromosome segment. The SHR.BN-Ren congenic strain may provide a useful new animal model for studying the interaction between high blood pressure and dyslipidemia in cardiovascular disease.

Genetic isolation of a chromosome 1 region affecting blood pressure in the spontaneously hypertensive rat

Recent linkage studies in the spontaneously hypertensive rat (SHR) suggest that a blood pressure regulatory gene or genes may be located on rat chromosome 1q. To investigate this possibility, we replaced a region of chromosome 1 in the SHR (defined by the markers D1Mit3 and Igf2) with the corresponding chromosome segment from the normotensive Brown-Norway (BN) strain. In male SHR congenic rats carrying the transferred BN chromosome segment, 24-hour average systolic and diastolic blood pressures were significantly lower than in male progenitor SHR. Polymerase chain reaction genotyping using 60 polymorphic microsatellite markers dispersed throughout the genome confirmed the congenic status of the new strain designated SHR.BN-D1Mit3/Igf2. These findings provide direct evidence that a blood pressure regulatory gene exists on the differential segment of chromosome 1 that is sufficient to decrease blood pressure in the SHR. The SHR.BN-D1Mit3/Igf2 congenic strain represents an important new model for fine mapping and characterization of genes on chromosome 1 involved in the pathogenesis of spontaneous hypertension.

Interval mapping and congenic strains for a blood pressure QTL on rat chromosome 13

The renin locus (Ren) on rat Chromosome (Chr) 13 had previously been shown to cosegregate with blood pressure in crosses involving Dahl salt-sensitive (S) and Dahl salt-resistant (R) rats. In the present work, interval mapping of blood pressure on Chr 13 with a large F2 (S x R), n = 233, population yielded a maximum LOD = 4.2 for linkage to blood pressure, but the quantitative trait locus (QTL) was only poorly localized to a large 35-centiMorgan (cM) segment of Chr 13. In the linkage analysis, the S-rat QTL allele (S) was associated with higher, and the R-rat QTL allele (R) with lower blood pressure, the difference between homozygotes being about 20 mm Hg. A congenic strain was made by introgressing the R-rat Ren allele into the recipient S strain. This congenic strain showed a 24 mm Hg reduction (P = 0.004) in blood pressure compared with S rats for rats fed 2% NaCl diet for 24 days; this difference was confirmed by two other independent tests. Two congenic substrains were derived from the first congenic strain with shorter R Chr 13 segments on the S background. Comparisons among these congenic strains showed that a blood pressure QTL was in the 24-cM chromosomal segment between Syt2 and D13M1Mit108. This segment does not include the renin locus, which is thus excluded from being the gene on rat Chr 13 responsible for genetic differences in blood pressure detected by linkage analysis.

Endogenous retroviral transcripts in myocytes from spontaneously hypertensive rats

The spontaneously hypertensive rat (SHR) is a well studied animal model of genetic hypertension and heart disease of unknown cause. With the use of differential display, a transcript was found in SHR myocardium that on sequence analysis was identified as an endogenous retrovirus (ERV). ERV gene expression was greater than an order of magnitude increased in adult SHR hearts relative to age-matched normotensive Wistar-Kyoto rats and was further increased in hearts from SHR with heart failure. In situ hybridization studies demonstrated that increased ERV gene expression was localized to myocardial cells. Increases in ERV transcripts in SHR suggest a possible link between inherited proviral elements and genetic hypertensive heart disease.

Genetic isolation of a region of chromosome 8 that exerts major effects on blood pressure and cardiac mass in the spontaneously hypertensive rat

The spontaneously hypertensive rat (SHR) is the most widely studied animal model of essential hypertension. Despite > 30 yr of research, the primary genetic lesions responsible for hypertension in the SHR remain undefined. In this report, we describe the construction and hemodynamic characterization of a congenic strain of SHR (SHR-Lx) that carries a defined segment of chromosome 8 from a normotensive strain of Brown-Norway rats (BN-Lx strain). Transfer of this segment of chromosome 8 from the BN-Lx strain onto the SHR background resulted in substantial reductions in systolic and diastolic blood pressure and cardiac mass. Linkage and comparative mapping studies indicate that the transferred chromosome segment contains a number of candidate genes for hypertension, including genes encoding a brain dopamine receptor and a renal epithelial potassium channel. These findings demonstrate that BP regulatory gene(s) exist within the differential chromosome segment trapped in the SHR-Lx congenic strain and that this region of chromosome 8 plays a major role in the hypertension of SHR vs. BN-Lx rats.

Cosegregation of the new region on chromosome 3 with salt-induced hypertension in female F2 progeny from stroke-prone spontaneously hypertensive and Wistar-Kyoto rats

1. We investigated candidate loci for salt-sensitive high blood pressure (BP) in F2 progeny from crossing Wistar-Kyoto and stroke-prone spontaneously hypertensive rats. 2. In female F2 progeny, systolic and diastolic BP on the 12th day and the seventh month after salt loading was strongly linked with the D3Mgh12 and D3Mgh6 loci on chromosome 3, respectively. 3. These loci were linked with BP only in female F2 progeny, not in males. 4. These results indicate that hormonal factors may influence salt sensitivity, particularly with respect to gender differences.

Effects of renin gene transfer on blood pressure and renin gene expression in a congenic strain of Dahl salt-resistant rats

To investigate whether a BP-regulatory locus exists in the vicinity of the renin locus on rat chromosome 13, we transferred this chromosome segment from the Dahl salt-sensitive (S) rat onto the genetic background of the Dahl salt-resistant (R) rat. In congenic Dahl R rats carrying the S renin gene and fed an 8% salt diet, systolic BP was significantly lower than in progenitor Dahl R rats: 127 +/- 1 mmHg versus 138 +/- 4 mmHg, respectively (P < 0.05). Moreover, the decreased BP in the congenic Dahl R strain was associated with decreased kidney renin mRNA and decreased plasma renin concentration. These findings demonstrate that the Dahl S strain carries alleles in or near the renin locus that confer lower plasma renin concentration and lower BP than the corresponding alleles in the Dahl R strain, at least when studied on the genetic background of the Dahl R rat and in the environment of a high salt diet. The occurrence of coincident reductions in kidney renin mRNA, plasma renin concentration, and BP after interstrain transfer of naturally occurring renin gene variants strongly suggests that genetically determined variation in renin gene expression can affect BP.

Some physiological outcomes of renin gene studies

1. The cloning of the renin gene has permitted studies of its physiological regulation, extrarenal expression and role in disease. 2. Marked modulation of renin mRNA concentration is seen in adrenal, heart and hypothalamus in response to sodium depletion and inhibition of AII formation, as well as in models of renal and genetic hypertension in the rat. 3. One important outcome of studies of the promoter has been the discovery of a cyclic AMP-responsive sequence. 4. Sequence variations have been detected in or near the renin gene and have been used as markers in studies of its role in cardiovascular disease aetiology. 5. In conclusion, molecular biology has, in the past decade, made a significant contribution to the understanding of renin physiology and pathophysiology.

Use of recombinant inbred strains for evaluation of intermediate phenotypes in spontaneous hypertension

1. The HXB/BXH recombinant inbred (RI) strains, derived from the spontaneously hypertensive rat (SHR) and the normotensive Brown Norway (BN.1x) rat, represent a very useful system for gene mapping and for genetic analysis of certain model diseases, such as spontaneous hypertension. 2. These RI strains were genotyped in multiple genetic polymorphisms and characterized in blood pressure and some intermediate phenotypes. 3. The analysis of RI strains has revealed that (i) a gene in the vicinity of the major histocompatibility complex (RT1) on chromosome 20, a kallikrein-related gene on chromosome 4 and the renin gene on chromosome 13 were significantly associated with blood pressure, and (ii) Na+ leak in red blood cells correlated with blood pressure whereas relative heart and kidney weights as well as platelet aggregation did not.

Hypertension in the spontaneously hypertensive rat and the sex chromosomes

We investigated the involvement of loci on the sex chromosomes in the hypertension of the spontaneously hypertensive rat (SHR) by studying male F1 and F2 generation rats derived from reciprocal crosses of SHR with Wistar-Kyoto (WKY) rats (cross 1: WKY female x SHR male; cross 2: SHR female x WKY male). At 16 weeks of age there was no significant difference in the blood pressures of F1 animals derived from the two crosses. Similarly, in the F2 generation there was no significant difference in either indirect blood pressures measured at 12, 16, or 20 weeks of age or in direct systolic and diastolic blood pressures measured at 25 weeks of age between animals derived from the two crosses maintained on a normal salt diet. In a second study, cohorts of F2 rats from the two crosses were given 1% salt in their drinking water for 10 weeks from 16 weeks of age with indirect blood pressure measurements at 16 (presalt), 18, and 20 weeks and direct blood pressure measurements at 26 weeks. Although overall these animals had significantly higher blood pressures at both 20 and 26 weeks than animals of the first study, again there was no difference in blood pressures of animals derived from the two crosses, apart from a marginally significantly higher blood pressure at 18 weeks in animals from cross 1 (with SHR grandfather). The findings indicate that the sex chromosomes of the SHR and WKY rat used in these crosses do not contain loci where alleles differentially influence blood pressure under the genetic milieu provided by the cross.(ABSTRACT TRUNCATED AT 250 WORDS)

A gene differentially expressed in the kidney of the spontaneously hypertensive rat cosegregates with increased blood pressure

The role of the kidney in initiating hypertension has been much debated. Here we demonstrate that a recently identified gene of yet unknown function, termed SA, which is differentially expressed in the kidney of the spontaneously hypertensive rat, cosegregates with an increase in blood pressure in F2 rats derived from a cross of the spontaneously hypertensive rat with normotensive Wistar-Kyoto rats, accounting for 28 and 21% of the genetic variability in systolic and diastolic blood pressures, respectively. Further, the genotype at this locus appears to determine the level of expression of the gene in the kidney. The findings provide strong evidence for a primary genetic involvement of the kidney in hypertension.

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

In Dahl salt-hypertension sensitive (S) and resistant (R) strains fed a high NaCl diet, 11 beta-hydroxylase polymorphisms cosegregate with the adrenal capacity to synthesize 18-hydroxy-11-deoxycorticosterone (18-OH-DOC) and blood pressure. The R rat carries an 11 beta-hydroxylase allele that: (i) differs from those of 12 other rat strains; (ii) is associated with a uniquely reduced capacity to synthesize 18-OH-DOC; and (iii) encodes 5 amino acid substitutions in the 11 beta-hydroxylase protein. The robust salt-resistance of the Dahl R rat may be due in part to reduced synthesis of the mineralocorticoid 18-OH-DOC stemming from mutations in the 11 beta-hydroxylase gene. 11 beta-hydroxylase, located on rat chromosome 7, is the first candidate gene identified in an animal model in which coding sequence mutations have been linked to the regulation of blood pressure.

Red cell ion transport abnormalities in experimental hypertension

The original attractive hypothesis on the important role of elevated cell Na+ concentration in the pathogenesis of hypertension stimulated a search for generalized membrane defects and ion transport abnormalities in various easily accessible cells including erythrocytes. An attempt is made here to compare this hypothesis with the data on red cell ion transport alterations that were observed in experimental hypertension over the last 15 years. Several methodological (presence of extracellular Na+ in incubation media, kinetic approach to the evaluation of transport systems) and physiological problems (potassium depletion, age-dependent changes) are discussed in more detail because they can substantially modify the results obtained. Available data suggest a possible contribution of augmented Na+ leak to the development of both genetic and salt-dependent experimental hypertension. The role of alterations in the activity of the Na(+)-K+ pump or the Na(+)-K+ cotransport system still remains unclear.

Genetic determinants of diastolic and pulse pressure map to different loci in Lyon hypertensive rats

Several genetic loci involved in blood pressure regulation have recently been localized in experimental models of hypertension, but the manner in which they influence blood pressure remains unknown. Here, we report a study of the Lyon hypertensive rat strain showing that different loci are involved in the regulation of steady-state (diastolic pressure) and pulsatile (systolic-diastolic, or pulse pressure) components of blood pressure. Significant linkage was established between diastolic blood pressure and a microsatellite marker of the renin gene (REN) on rat chromosome 13, and between pulse pressure and the carboxypeptidase B gene (CPB) on chromosome 2. These findings show that two independent loci influence different haemodynamic components of blood pressure, and that pulse pressure has a specific genetic determination.

Genetics of essential hypertension

Blood pressure is a complex quantitative trait that is determined by multiple environmental and genetic factors. Although some simple Mendelian forms of high blood pressure have been described, essential hypertension is characterized by a complex mode of inheritance. Based on recent advances in molecular biology and statistical genetics, it has become feasible to search for chromosome regions that may contain genes contributing to the pathogenesis of hypertension in humans. For example, recent linkage and association studies have raised the possibility that a blood pressure regulatory locus may exist in or near the angiotensinogen gene on chromosome 1. Detailed genetic experiments in animal models of hypertension may help to guide further clinical studies and lead to an improved understanding of gene action in the pathogenesis of essential hypertension.

Association of red blood cell sodium leak with blood pressure in recombinant inbred strains

Red blood cell Na+ content as well as ouabain-resistant Na+ and Rb+ (K+) transport (susceptible or resistant to inhibition by loop diuretics) were determined in spontaneously hypertensive rats (SHR) and normotensive Brown Norway (BN) rats the erythrocytes of which were incubated in either saline or Mg(2+)-sucrose medium. Elevated ouabain-resistant Na+ net uptake contrasted with slightly decreased red blood cell Na+ content in SHR compared with BN rats. Acceleration of furosemide- and bumetanide-sensitive Na+ fluxes contributed to enhanced ouabain-resistant Na+ influx into SHR erythrocytes in saline medium, whereas higher furosemide- or bumetanide-resistant Na+ efflux caused greater ouabain-resistant Na+ efflux in Mg(2+)-sucrose medium. Furosemide- and bumetanide-resistant Rb+ leaks were augmented in SHR erythrocytes. The association of the disclosed ion transport alterations with blood pressure was examined in 20 recombinant inbred strains derived from F2 SHR x BN hybrids. Ouabain-resistant Na+ uptake as well as furosemide- and bumetanide-resistant Na+ inward leaks (but not red blood cell Na+ content or furosemide- and bumetanide-sensitive Na+ net uptake) cosegregated with systolic and pulse pressures but not diastolic pressure of the recombinant inbred strains. In contrast, neither ouabain-resistant Na+ efflux nor any component of ouabain-resistant Rb+ uptake correlated positively with blood pressure of the recombinant inbred strains. Increased ouabain-resistant Na+ influx was compensated for by accelerated ouabain-sensitive Na+ extrusion because red blood cell Na+ content was not elevated in the hypertensive strains. Thus, high cell Na+ turnover rates might be related to genetic hypertension if an altered Na+ inward leak would be less effectively compensated for in tissues involved in cardiovascular regulation.

Hypotensive effect associated with a phospholipase C-δ1 gene mutation in the spontaneously hypertensive rat

To identify the genes responsible for blood pressure in the spontaneously hypertensive rat strain, we performed a cosegregation analysis between the genotype and blood pressure in a set of male F2 rats obtained by crossmating SHR with Wistar-Kyoto rats, a parental normotensive strain. Our investigation revealed that the phospholipase C-delta 1 polymorphism, which resulted in missense mutation, cosegregates with the lower blood pressure in SHR, and that PLC-delta 1 gene is located on chromosome 8. On the other hand, we found the lack of cosegregation between blood pressure and the nerve growth factor receptor gene, which is linked to a hypertensinogenic gene locus (denoted as BP/SP-1) on chromosome 10. We propose that PLC-delta 1 gene itself of closely linked gene on chromosome 8 is a new candidate with the hypotensive effect, and that BP-SP1 locus does not directly contribute to blood pressure elevation in original SHR.

Cosegregation of blood pressure with angiotensin converting enzyme and atrial natriuretic peptide receptor genes using Dahl salt–sensitive rats

We have evaluated the genes for angiotensin converting enzyme (ACE) and guanylyl cyclase A/atrial natriuretic peptide receptor (GCA) for genetic effects on blood pressure response to high salt diet. In F2 rats derived from Milan normotensive and Dahl salt-hypertension sensitive (S) rats, both ACE and GCA cosegregated with blood pressure, and rats that were homozygous for the S allele at both the ACE and GCA loci had inordinately high blood pressure. In F2 derived from Wistar Kyoto (WKY) and S rats, GCA revealed positive cosegregation with blood pressure, but ACE did not. We conclude that certain alleles at the GCA and ACE loci (or at loci closely linked to them) have a significant genetic impact on blood pressure response to high salt in specific rat strains.

Genetic approaches to hypertension

Essential hypertension is a complex clinical disorder in which multiple environmental and genetic factors interact to increase blood pressure. To search for chromosome regions that contain genes regulating blood pressure, some investigators have begun to conduct linkage studies in rodent models of spontaneous hypertension. Preliminary results suggest that in the rat, blood pressure regulatory genes may be located in the vicinity of the kallikrein gene family on chromosome 1, the gene for angiotensin converting enzyme on chromosome 10, the renin gene on chromosome 13, and the major histocompatibility complex on chromosome 20. Some studies have also suggested that blood pressure regulatory loci may be located on the sex chromosomes. Although comparisons between humans and animals should be made with caution, it is hoped that the identification of genes regulating blood pressure in the rat might shed light on the pathogenesis of hypertension in humans.

Assignment of rat linkage group V to chromosome 19 by single-strand conformation polymorphism analysis of somatic cell hybrids

The rat provides a number of important models of human genetic disease; however, the rat genetic map has not been extensively developed. Although most rat chromosomes carry several gene assignments, some major linkage groups (LG) remain to be mapped. To determine the chromosome location of the largest unmapped linkage group in the rat (LG V containing multiple carboxylesterase loci), we used single-strand conformation polymorphism analysis to identify the rat esterase-10 gene in a panel of rat x mouse somatic cell hybrids. We found that the carboxylesterase gene family and hence LG V are located on rat chromosome 19. We have also confirmed the assignment of the angiotensinogen gene to rat chromosome 19 and have used a large set of recombinant inbred strains to map two anonymous variable number of tandem repeat (VNTR) markers to this chromosome. The current findings bring the total number of genes assigned to rat chromosome 19 from 3 to 19 and provide further evidence of substantial homology between this chromosome and chromosome 8 in the mouse.

Molecular genetics of hypertension

During the last decades the evidence that a genetic component contributes to the development of primary hypertension has been accumulating. The identification of the genes involved in blood pressure regulation, however, is only starting to emerge. The recent advances in recombinant DNA technology provide new molecular genetic strategies in cardiovascular research. In this review we will discuss the testing of candidate genes in vivo by transgenic techniques. Furthermore, we will describe the possibilities to identify the genes implicated in primary hypertension by genetic linkage analysis using polymorphic DNA markers.

High blood pressure: Hunting the genes

High blood pressure is a disease of unknown cause. Family history of the disease indicates higher risk, but it is not known which genes are involved or how they interact with environmental influences to produce the disorder. Molecular biology offers an approach to problems that have not so far been solved by classical physiology or biochemistry. By analysing polymorphic variation in chromosome markers such as minisatellite sequences, or by restriction fragment polymorphism analysis of candidate genes, attempts are being made to link genetic variations with hypertension. In genetically hypertensive rats, hypertension is associated with a polymorphism of the renin gene and with other loci on chromosomes 10 and 18. The role of these loci in human hypertension remains to be determined. Other genes such as sodium-lithium countertransport may be involved. Environmental factors such as stress or salt intake could influence the rate or timing of expression of certain genes and thus result in hypertension.

Sequence analysis of the alpha 1 Na+,K(+)-ATPase gene in the Dahl salt-sensitive rat

In the inbred Dahl salt-sensitive rat (SS/Jr strain), it has been proposed that a T for A transversion in the DNA sequence encoding amino acid 276 in the alpha 1 subunit isoform of Na+,K(+)-ATPase may impair ion transport and contribute to the pathogenesis of hypertension. This hypothesis is of major scientific interest because it represents the first attempt to explain the pathogenesis of salt-sensitive hypertension on the basis of a specifically defined mutation at the DNA level. We devised a polymerase chain reaction technique to screen the genomic DNA of multiple SS/Jr rats for the T for A transversion reported in the complementary DNA (cDNA) encoding the alpha 1 subunit of Na+,K(+)-ATPase. When eight Dahl SS/Jr rats from Harlan Sprague Dawley Inc. were tested with the polymerase chain reaction technique, we found no evidence of this mutation in the Na+,K(+)-ATPase gene. Direct sequence analysis of the gene in three SS/Jr rats also did not show the T for A transversion. These results 1) strongly suggest that commercially available Dahl SS/Jr rats do not carry a T for A transversion in the genomic DNA sequence encoding amino acid 276 in the alpha 1 subunit isoform of Na+,K(+)-ATPase and 2) raise the possibility that the previous finding of a mutation in the cDNA of the SS/Jr rat may have been due to a reverse transcriptase error during cDNA synthesis.

Chromosomal mapping of two genetic loci associated with blood-pressure regulation in hereditary hypertensive rats

The spontaneously hypertensive rat and the stroke-prone spontaneously hypertensive rat are useful models for human hypertension. In these strains hypertension is a polygenic trait, in which both autosomal and sex-linked genes can influence blood pressure. Linkage studies in crosses between the stroke-prone spontaneously hypertensive rat and the normotensive control strain Wistar-Kyoto have led to the localization of two genes, BP/SP-1 and BP/SP-2, that contribute significantly to blood pressure variation in the F2 population. BP/SP-1 and BP/SP-2 were assigned to rat chromosomes 10 and X, respectively. Comparison of the human and rat genetic maps indicates that BP/SP-1 could reside on human chromosome 17q in a region that also contains the angiotensin I-converting enzyme gene (ACE). This encodes a key enzyme of the renin-angiotensin system, and is therefore a candidate gene in primary hypertension. A rat microsatellite marker of ACE was mapped to rat chromosome 10 within the region containing BP/SP-1.

Chromosome assignments of the genes for glucocorticoid receptor, myelin basic protein, leukocyte common antigen, and TRPM2 in the rat

We have utilized rat-mouse somatic cell hybrids to make chromosomal assignments for the glucocorticoid receptor (GR), myelin basic protein (MBP), leukocyte common antigen (LCA), and testosterone-repressed prostate message-2 (TRPM2) genes in the rat. The genes for GR and MBP both map on chromosome 18 of the rat, which corresponds to the mapping of both genes on chromosome 18 of the mouse. The gene for LCA maps on chromosome 13, which is where C4b-binding protein beta-chain (C4BPB), coagulation factor V (F5), and renin have previously been assigned. This linkage group appears to be homologous to a substantial portion of mouse chromosome 1 and human chromosome 1q. Finally, the TRPM2 gene has been assigned to rat chromosome 15.

A locus on the long arm of chromosome 1 as a possible cause of essential hypertension

1. None of the genes responsible for essential hypertension has been identified. Recent work in genetically hypertensive rats has shown linkage of blood pressure with alleles of the renin gene. Since the renin gene is a member of a conserved synteny group that in humans spans chromosome 1q21.3-32.3 and includes the gene for antithrombin III (AT3), we used linkage studies to examine the relationship between alleles of AT3 and hypertension in a family having 10 affected members. 2. From the lod score obtained at a recombination fraction of zero the odds for linkage of AT3 and hypertension in this family were calculated as 6:1 in favour of linkage. This result provides grounds for further examination of the possible role of the 1q23 locus in the aetiology of essential hypertension.

Cosegregation of blood pressure with a kallikrein gene family polymorphism

It has recently been proposed that sequence variation in the gene coding for tissue kallikrein might be involved in the pathogenesis of hypertension. However, molecular evidence of an association between a sequence alteration in the kallikrein gene family and the transmission of increased blood pressure has never been reported. In 32 recombinant inbred (RI) strains derived from the spontaneously hypertensive rat (SHR) and the normotensive Brown Norway rat (BN), we investigated whether a restriction fragment length polymorphism (RFLP) marking the kallikrein gene family cosegregated with blood pressure. In the RI strains that inherited the kallikrein RFLP from the SHR progenitor strain, the median systolic, diastolic, and mean arterial pressures were significantly greater than in the RI strains that inherited the kallikrein RFLP from the BN progenitor strain. These findings suggest that in the rat, sequence variation in the kallikrein gene family, or in closely linked genes, may have the capacity to affect blood pressure.