A Candidate Hypertension Gene

Linkage analyses in experimental crosses of hypertensive and normotensive rats have strongly suggested the presence of a quantitative trait locus (QTL) influencing blood pressure on rat chromosome 1, at or near the Sa gene. To confirm the presence of such a locus and move toward identification of the causative gene, we have developed, through targeted breeding over 10 generations using an Sa gene polymorphism to select breeders at each generation, 2 congenic strains, 1 containing a segment of spontaneously hypertensive rat (SHR) chromosome 1 in a Wistar-Kyoto rat (WKY) genetic background (WKY.SHR-Sa), and the other a segment of WKY chromosome 1 in an SHR background (SHR.WKY-Sa). WKY.SHR-Sa contains at least approximately 26 cM of SHR chromosome 1, between markers mD7mit206 and D1Mit2 (and including the SHR allele of the Sa gene), and SHR.WKY-Sa carries at least approximately 15 cM of WKY chromosome 1, between mD7mit206 and D1Wox34 (and including the WKY allele of the Sa gene). Blood pressure of WKY.SHR-Sa rats measured at 16, 20, and 25 weeks of age was significantly higher than that of WKY, whereas blood pressure of SHR.WKY-Sa rats was significantly lower than that of SHR. At 25 weeks, the mean differences in systolic and diastolic blood pressure between WKY.SHR-Sa and WKY were +11.5 mm Hg (P=0.001) and +11.6 mm Hg mm Hg (P<0.001), respectively. The corresponding differences between SHR.WKy-Sa and SHR were -11.3 mm Hg (P=0.002) and -9.1 mm Hg (P=0.005), respectively. The differences represent about one fifth of the blood pressure difference between SHR and WKY. Renal Sa mRNA levels in the congenic strains reflected their Sa allele with a high level in WKY. SHR-Sa and a low level in SHR.WKY-Sa, consistent with previous data suggesting that the level of Sa expression is primarily determined by cis-acting elements in or near the Sa gene. Our results show that we have successfully isolated a major rat chromosome 1 blood pressure QTL located in the vicinity of the Sa gene in reciprocal congenic strains derived from SHR and WKY. The strains can now be used to further define the region containing the QTL and also to characterize intermediary mechanisms through which the QTL influences blood pressure. In addition, comparison of the regions introgressed in our congenic strains with the location of the peak LOD score for chromosome 1 blood pressure QTL in second filial generation progeny derived from our SHRxWKY cross suggests that there may be at least 1 further QTL influencing blood pressure on this rat chromosome.

Previously we reported that there is a blood pressure quantitative trait locus (QTL) on rat Chromosome (Chr) 7 seen when comparing Dahl salt-sensitive (S) rats and Dahl salt-resistant (R) rats. Evidence was also presented that this QTL was due to genetic variants in the adrenal steroidogenic enzyme 11beta-hydroxylase ( Cyp11b1). A series of congenic strains supported this contention. In the present work we have constructed a final congenic substrain that retains a blood pressure effect and that has an introgressed congenic segment which includes Cyp11b1 and is < 177 kb in size. None of the other genes in the congenic region (eight known genes) have known biological functions for influencing blood pressure. We believe that we have reached the limit of resolution for congenic analysis of a QTL in a rodent animal model, and we conclude that Cyp11b1 causes the observed QTL on rat Chr 7 in Dahl rats.

To describe genetic loci that differentiate blood pressures in two genetically hypertensive strains, the Dahl salt-sensitive (S) rat and the Albino Surgery (AS) rat. A genome scan was performed using 222 genetic markers on an F2 population derived from two hypertensive strains, S and AS. The F2 rats were fed 8% NaCl for 5 weeks before blood pressure measurements were taken. Three blood pressure quantitative trait loci (QTL) were detected, one on each of rat chromosomes (RNO) 2, 4 and 8. The QTL on RNO4, unlike those on RNO2 and RNO8, was not detected in any of the previous seven linkage analyses reported with the S rat as one of the parental strains. Interactions between genetic loci throughout the genome were sought and interactions involving RNO4 with RNO8 and RNO4 with RNO14 were found. Including the new RNO4 locus identified in the present study, 16 distinct regions of the S rat genome have been demonstrated, by linkage analyses, to harbour loci that control blood pressure in the S rat. Increased blood pressure in two hypertensive strains, S and AS, is differentially regulated by genetic factors present on RNOs 2, 4 and 8. Therefore, of the 16 distinct genomic regions known to harbour blood pressure QTL in S rats, 13 are likely to contain blood pressure alleles that function similarly in the S rat and the AS rat, whereas three regions differentiate the two strains.

Two of the most widely used strains for mapping quantitative trait loci (QTLs) for blood pressure (BP) are the Dahl Salt‐sensitive (S) rat and the Spontaneously Hypertensive Rat (SHR). To map a previously identified BP QTL that differentiate these two strains, we continued substitution mapping studies on a BP QTL on chromosome 9 identified by a linkage study. A total of 11 new congenic substrains were developed and characterized. BP and heart weights of two of these substrains with introgressed segments of 2.05Mb and 807.3Kb were both significantly (p&lt;0.001) lower than that of the S by 56 ± 6.61 and 41 ± 8.55 mm Hg (p&lt;0.001)), respectively. The mRNA and protein levels of Tmeff2 were lower in the congenic substrain spanning 2.05Mb. Further, because Tmeff2 was the only protein‐coding candidate gene within the substrain spanning 807.3kb, which also had a BP effect, it was prioritized as a candidate gene. Tmeff2 is known to be upregulated in patients from multiple cohorts with cardiac hypertrophy. Because Tmeff2 is also known to affect cell proliferation, we assessed the proliferation rates of cells obtained from S and the congenic substrains with BP effect. Proliferation of cardiac fibroblasts was significantly slower in the congenic compared with S (p&lt;0.05). Taken together, these results suggest that variants within the 807.3Kb congenic segment that alter the expression of Tmeff2 can be prioritized as candidate biomarkers controlling blood pressure and associated cardiac hypertrophy in both rats and in humans.

A genetic map for rat chromosome 1 was constructed using 66 microsatellite markers typed on either or both of two populations derived from inbred Dahl salt-sensitive (S) rats: F2(LEW x S) n = 151, and F2(WKY x S) n = 159. These populations had been raised on a high salt (8% NaCl) diet. Systolic blood pressure and heart weight were found to be genetically linked to two separate regions on rat chromosome 1 in the F2(LEW x S) population. One region was centered around the anonymous SA locus and accounted for 24 mmHg of blood pressure. The other region was 55 cM from the SA locus centered around a cluster of cytochromes P450 loci, and accounted for 30 mmHg of blood pressure. Since blood pressure and heart weight were highly correlated these same regions were also linked to heart weight. These results were cross-specific as linkage of these chromosome 1 regions to blood pressure and heart weight was not observed in several other F2 populations derived by crossing S and other normotensive control strains. This is presumably due to different alleles and/or different genetic backgrounds in the various populations. The SA region of chromosome 1 was found to influence body weight in F2(LEW x S) rats. Combining the present data with our previously published data on the F2(LEW x S) population showed that four separate quantitative trait loci with additive effects accounted for 106 mmHg and 38% of the total variance of blood pressure and for 506 mg and 34% of the total variance of heart wt.

Linkage studies suggested that a quantitative trait locus (QTL) for blood pressure (BP) was present in a region on chromosome 17 (Chr 17) of Dahl salt-sensitive (DSS) rats. A subsequent congenic strain targeting this QTL, however, could not confirm it. These conflicting results called into question the validity of localization of a QTL by linkage followed by the use of a congenic strain made with an incomplete chromosome coverage. To resolve this issue, we constructed five new congenic strains, designated C17S.L1 to C17S.L5, that completely spanned the +/-2 LOD confidence interval supposedly containing the QTL. Each congenic strain was made by replacing a segment of the DSS rat by that of the normotensive Lewis (LEW) rat. The only section to be LL homozygous is the region on Chr 17 specified in a congenic strain, as evidenced by a total genome scan. The results showed that BPs of C17S.L1 and C17S.L2 were lower (P < 0.04) than that of DSS rats. In contrast, BPs of C17S.L3, C17S.L4, and C17S.L5 were not different (P > 0.6) from that of DSS rats. Consequently, a BP QTL must be located in an interval of approximately 15 cM shared between C17S.L1 and C17S.L2 and unique to them both, as opposed to C17S.L3, C17S.L4, and C17S.L5. The present study illustrates the importance of thorough chromosome coverage, the necessity for a genome-wide screening, and the use of "negative" controls in physically mapping a QTL by congenic strains.

High-Resolution Mapping of the Blood Pressure QTL on Chromosome 7 Using Dahl Rat Congenic Strains

Our purposes were to develop a linkage map for rat Chromosome (Chr) 10, using chromosome-sorted DNA, and to construct congenic strains to localize blood pressure quantitative trait loci (QTL) on Chr 10 with the map. The linkage mapping panel consisted of three F2 populations totaling 418 rats. Thirty-two new and 29 known microsatellite markers were placed on the map, which spanned 88.9 centiMorgans (cM). The average distance between markers was 1.46 cM. No markers were separated by more than 6.8 cM. Four congenic strains were constructed by introgressing various segments of Chr 10 from the Milan normotensive strain (MNS) onto the background of the Dahl salt-sensitive (S) strain. A blood pressure QTL with a strong effect on blood pressure (35-42 mm Hg) when expressed on the S background was localized to a 31-cM region between D10Mco6 and D10Mcol. The region does not include the locus for inducible nitric oxide synthase (Nos2), which had been considered to be a candidate locus for the QTL.

We have used inbred and congenic rat strains in F2 segregation studies to discover epistasis in a polygenic model of hypertension. Previously, we have found evidence that the presence of a blood pressure quantitative trait locus (QTL) on chromosome 1 is conditional upon the allele status of chromosome 10. To prove the existence of an epistatic interaction we have analyzed congenic strains for chromosome 1 and 10 carrying high blood pressure QTL alleles from the spontaneously hypertensive rat on a normotensive background of the Wistar–Kyoto (WKY) rat. Additionally, a double congenic strain was developed with both chromosome 1 and 10 high blood pressure QTL alleles on the WKY background. Analysis of variance for blood pressure phenotypes as determined by radiotelemetry showed a significant effect for chromosome 10 but not chromosome 1 QTL alleles and demonstrated a significant interaction between the two loci (P<0.05). The interaction accounted for 5 mmHg of blood pressure. Thus, the identification of epistasis is critical to the understanding of the quantitative nature of blood pressure genetics.

1. The aim of the present study was to scan chromosome 1 in the hypertensive 'inherited stress-induced arterial hypertension' (ISIAH) rat strain for the quantitative trait loci (QTL) that control basal and stress-induced arterial blood pressure (ABP) levels and weight traits. 2. Two F(2) populations of 3-4- and 6-month-old male rats derived from a cross between the normotensive Wistar albino Glaxo (WAG) and hypertensive ISIAH rats were used in the search for the QTL. To identify the QTL for blood pressure (basal and under stress) and weight traits (bodyweight, as well as the weight of the adrenals, kidney and heart), 12 polymorphic markers covering a span of 234.6 Mb on chromosome 1 were analysed. 3. In 3-4-month-old rats, QTL were found for bodyweight in the vicinity of the D1Rat76 marker (230.6 Mb; P = 0.0019; logarithm of odds (LOD) score 3.23) and for relative kidney weight in the vicinity of the D1Rat117 marker (219.3 Mb; P = 0.000992; LOD score 3.41). No QTL for blood pressure were detected on chromosome 1 in the 3-4-month-old population. 4. In 6-month-old rats, a QTL for basal ABP in the region spanning 168.0-250.4 Mb, with two peaks around the markers D1Rat168 (204.8 Mb; P = 0.00087; LOD score 3.42) and D1Rat76 (P = 0.0006; LOD score 3.34), was described. A novel QTL was found in the D1Rat54-D1Rat168 region for stress-induced blood pressure (P = 0.0014; LOD score 3.08). 5. The results provide support for the existence of age-dependent differences in the genetic control of ABP and weight traits. Chromosome 1 was characterized by four QTL: for bodyweight and relative kidney weight in 3-4-month-old F(2) (ISIAH yen WAG) rats and basal ABP and ABP under emotional (restraint) stress conditions in 6-month-old F(2) rats. The QTL for stress-induced ABP seems to be novel and specific to the ISIAH rat strain.

The Antihypertensive and Lipid-Lowering treatment to prevent Heart Attack Trial (ALLHAT) compared in >30 000 high-risk hypertensive patients the effects on coronary heart disease of 3 treatment strategies: (1) based on the diuretic chlorthalidone, (2) the calcium-channel blocker (CCB) amlodipine, and (3) the angiotensin converting-enzyme (ACE) inhibitor lisinopril, respectively.1 Sponsored by the National Heart, Lung, and Blood Institute, ALLHAT stands out because no differences occurred in the incidence of the primary end point that consisted of the combination of fatal coronary heart disease and acute myocardial infarction.1 Not surprisingly, the attention of the ALLHAT consortium shifted to secondary end points, such as stroke, or to loosely defined2 components of secondary end points, such as heart failure. At the end of the line, the ALLHAT investigators based their main conclusions on events that, at the initiation of the trial, they regarded as “soft data that will at best confirm or supplement the primary endpoint.”3 More importantly, what was not identical in the 3 treatment groups was the on-treatment blood pressure despite vigorous attempts to titrate and combine the study medications to achieve a blood pressure of <140 mm Hg systolic and 90 mm Hg diastolic.1 These salient features of ALLHAT should be kept in mind whenever one attempts to interpret the findings of this landmark trial. In this issue of Hypertension , Leenen et al4 published a post hoc analysis, in which they made a direct comparison of cardiovascular and other outcomes among the 18 102 ALLHAT participants randomly assigned to amlodipine or lisinopril. In line with previous reports,1 the incidence of the primary coronary end point and total and cardiovascular mortality were similar in both groups. However, the patients randomly assigned to lisinopril experienced higher risks of stroke, combined cardiovascular disease, gastrointestinal bleeding, …

Background: In a previous genome wide linkage study we identified blood pressure quantitative trait loci (QTL) mapping to rat chromosome 2 and chromosome 3 in an F2 cross derived from SHRSP and WKY strains. We also identified a significant interaction between loci on chromosomes 2 and 3 using Pseudomarker (v 0.9) statistical framework. The aim of this study was to generate a double congenic strain to confirm chromosome 3 QTL and investigate the interaction between loci on the implicated chromosomes. Methods : A marker-assisted breeding strategy was used to generate the SP.WKYGla2a/3a double congenic strain (D2Rat13-D2Rat157, D3Mgh16-D3Wox28), using SHRSP as recipient and WKY as the donor strain. Haemodynamic measurements were carried out using radiotelemetry over a 5-week baseline period followed by 3 weeks of salt-loading (1%). Results : Radiotelemetry data for systolic blood pressure and pulse pressure are illustrated in figure a + b respectively. Systolic blood pressure was significantly reduced in the SP.WKYGa2a/3a strain (n=7) compared to SHRSP (n=13) (p = 0.0001, F = 97.77, repeated measures ANOVA). Pulse pressure was also significantly reduced compared to SHRSP (p=0.0001, F=34.17) and achieved levels comparable to WKY (n=10). Pulse pressure diurnal variation observed in SHRSP during salt-loading was abolished in the double congenic strain (AUC; p=0.0001, F=13.56). Conclusions : Almost complete reversal of SHRSP hypertensive haemodynamic profiles has been achieved in the SP.WKYGla2a/3a double congenic strain. This strain will allow interrogation of complex gene-gene, gene-environment interactions contributing to salt-sensitive hypertension in the SHRSP.

Systemic arterial hypertension affects 72 million US adults and an additional hundreds of millions of persons worldwide.1,2 Most of these are candidates for pharmacological treatment to reduce risk of cardiovascular disease (CVD) events, based primarily on a very large body of epidemiological and intervention research in humans. Because of this high prevalence and the cardiovascular consequences of untreated or inadequately treated hypertension, the selection of drugs for initial and continuing, long-term treatment has large public health and economic implications. Fortunately, such decisions and the expert recommendations that seek to guide them can call on evidence from 4 decades of randomized multicenter clinical trials evaluating effects of treatment on clinical CVD. We summarize that evidence in this article, in approximate chronological order, and we comment on the related treatment guidelines. We close with some of the major clinical questions yet to be resolved. Response by Messerli et al p 2705 Before the current era beginning in the early 1990s of emphasis on positive-control trials, which directly compare different drug regimens, there were 3 decades of trials that compared an active regimen with placebo or, in a few cases, “usual care.” For most of this period, the mainstay of treatment was generally a thiazide-type diuretic (hereinafter called thiazides) or, to a lesser extent, a β-adrenergic blocker (termed β-blockers). With few exceptions, these trials, especially those with high statistical power and thiazide-based regimens, showed benefit for CVD outcomes.3–8 This evidence, which provided a basis for recommending thiazides or β-blockers as first-step drugs in most editions of US guidelines through 1997,9 needs to continue to be given due weight in practice and practice guidelines. The largest and most consistent benefits from the earlier trials were for stroke and (where reported) heart failure. Benefits for coronary heart disease (CHD) were less clear, …

Analysis of Recent Papers in Hypertension. Jan Basile, MD, Section Editor

physiological genomics ( PG ) publishes, on a quarterly basis, brief editorial perspectives entitled “Physiological genomics in PG and beyond.” These editorial perspectives provide brief, accessible highlights of the studies published in PG in the most recent quarter, and relate those studies to