Genetic analysis of inherited hypertension in the rat

Sex-specific effects of dual ET-1/ANG II receptor (Dear) variants in Dahl salt-sensitive/resistant hypertension rat model

Essential (polygenic) hypertension is a complex genetic disorder that remains a major risk factor for cardiovascular disease despite clinical advances, reiterating the need to elucidate molecular genetic mechanisms. Elucidation of susceptibility genes remains a challenge, however. Blood pressure (BP) regulatory pathways through angiotensin II (ANG II) and endothelin-1 (ET-1) receptor systems comprise a priori candidate susceptibility pathways. Here we report that the dual ET-1/ANG II receptor gene ( Dear) is structurally and functionally distinct between Dahl salt-sensitive, hypertensive (S) and salt-resistant, normotensive (R) rats. The Dahl S S44/M74 variant is identical to the previously reported Dear cDNA with equivalent affinities for both ET-1 and ANG II, in contrast to Dahl R S44P/M74T variant, which exhibits absent ANG II binding but effective ET-1 binding. The S44P substitution localizes to the ANG II-binding domain predicted by the molecular recognition theory, providing compelling support of this theory. The Dear gene maps to rat chromosome 2 and cosegregates with BP in female F2(R×S) intercross rats with highly significant linkage (LOD 3.61) accounting for 14% of BP variance, but not in male F2(R×S) intercross rats. Altogether, the data suggest the hypothesis that modification of the critical balance between ANG II and ET-1 systems through variant Dear contributes to hypertension susceptibility in female F2(R×S) intercross rats. Further investigations are necessary to corroborate genetic linkage through congenic rat studies, to investigate putative gene interactions, and to show causality by transgenesis and/or intervention. More importantly, the data reiterate the importance of sex-specific factors in hypertension susceptibility.

Mapping of genetic determinants of the sympathoneural response to stress

Activation of the sympathoadrenal system (SAS, comprising the sympathetic nervous system and the adrenal medulla) in response to stressful stimuli is an important defense mechanism as well as a contributor to several cardiovascular diseases. There is variability in the SAS response to stress, although the extent to which this is genetically regulated is unclear. Some rodent models, including the hereditary hypertriglyceridemic (hHTg) rat, are hyperresponsive to stress. We investigated whether quantitative trait loci (QTLs) that affect sympathoadrenal response to stress could be identified. Second filial generation rats ( n = 189) derived from a cross of the hHTg rat and the Brown Norway rat had plasma norepinephrine (NE) and epinephrine (Epi) levels, indices of activation of the sympathoneural and adrenal medulla components, respectively, measured in the resting state and in response to an immobilization stress. Responses were assessed early (20 min) and late (120 min) after the application of the stress. A genome scan was conducted using 153 microsatellite markers. Two QTLs (maximum peak LOD scores of 4.17 and 3.52, respectively) influencing both the early and late plasma NE response to stress were found on chromosome 10. Together, the QTLs accounted for ∼20% of the total variation in both the early and late NE responses in the F2rats. Interestingly, the QTLs had no effect on plasma Epi response to stress. These findings provide evidence for a genetic determination of the response of a specific component of the SAS response to stress. Genetically determined variation in sympathetic nervous system response to stress may contribute to cardiovascular diseases.

Substitution of chromosome 1 ameliorates L-NAME hypertension and renal disease in the fawn-hooded hypertensive rat

Linkage analysis studies previously identified genetic loci associated with proteinuria and hypertension on chromosome 1 of fawn-hooded hypertensive (FHH) rats. The present studies were performed on conscious male and female rats to evaluate the influence of transfer of chromosome 1 from the Brown Norway (BN) rat to the FHH genetic background (FHH-1BN). Rats were maintained for 2 wk on 8.0% NaCl chow with NG-nitro-L-arginine methyl ester (L-NAME) in the drinking water (12.5 mg/l) to induce hypertension and accelerate the onset of renal disease. Mean arterial blood pressure (MAP) was significantly higher in the male FHH (188 +/- 3 mmHg, n = 13) compared with the BN (121 +/- 3 mmHg, n = 8); MAP in the FHH-1(BN) was midway between the two parental strains (167 +/- 5 mmHg, n = 9). Urinary protein and albumin excretion rates in the male FHH-1(BN) (Uprot = 189 +/- 36 mg/day, Ualb = 69 +/- 16 mg/day, n = 10) were also midway between levels observed in the FHH (Uprot = 485 +/- 54 mg/day; Ualb = 206 +/- 25 mg/day, n = 13) and the BN (Uprot = 32 +/- 5 mg/day, Ualb = 5 +/- 1 mg/day, n = 8). Creatinine clearance was elevated, and the degree of glomerular damage was significantly reduced in the FHH-1BN compared with the FHH. Qualitatively similar results were obtained from female FHH, FHH-1BN, and BN rats. The present results indicate that genes contributing to l-NAME-induced hypertension and renal disease are found on chromosome 1 of the FHH rat.

Fine mapping of the hyperglycemic and obesity QTL by congenic strains suggests multiple loci on rat chromosome 14

Linkage analysis previously identified a hyperglycemic quantitative trait locus (QTL), Nidd 2/of, on rat Chromosome 14 in crosses utilizing OLETF (Otsuka Long Evans Tokushima Fatty) rat, a model for type 2 diabetes. A separate QTL study mapped an obesity QTL, Obs5, to the same chromosomal region. A congenic strain placing ca. 38 cM OLETF-derived segments containing both Nidd2/of and Obs5 on the F344 background was shown to possess mild diabetic and obese phenotypes, suggesting the presence of mutations affecting the glucose metabolism and fat accumulation. In order to localize the loci more precisely, we generated a series of deletion-subcongenic strains in which OLETF-segments were shortened from either ends. We found that there are at least two hyperglycemic QTLs within the Nidd2/of locus. We predict that they are localized towards both ends of the Nidd2/of region. In contrast, Obs5 QTL was further narrowed down to a single region of ca. 10 cM fragment.

The Visual Language of Synteny

The study of polygenic disorders such as cardiovascular and metabolic diseases requires access to vast amounts of experimental and in silico data. Where animal models of disease are being used, visualization of syntenic genome regions is one of the most important tools supporting data analysis. We define what is required to visualize synteny in terms of the data being displayed, the screen layout, and user interaction. We then describe a prototype visualization tool, SyntenyVista, which provides integrated access to quantitative trait loci, microarray, and gene datasets. We believe that SyntenyVista is a significant step towards an improved representation of comparative genomics data.

Mapping the Genetic Determinants of Hypertension, Metabolic Diseases, and Related Phenotypes in the Lyon Hypertensive Rat

The complex nature of hypertension makes identifying the pathophysiology and its genetic contributions a challenging task. One powerful approach for the genetic dissection of blood pressure regulation is studying inbred rat models of hypertension, as they provide natural allele variants but reduced heterogeneity (both genetic and etiologic). Furthermore, the detailed physiologic studies to which the rat is amenable allow for the determination of intermediate phenotypes. We have performed a total genome scan in offspring of an F2 intercross between the Lyon hypertensive (LH) and Lyon normotensive rat strains to identify linkage of anthropometric, blood pressure, renal, metabolic, and endocrine phenotypes. Quantitative trait locus (QTL) regions involved in blood pressure regulation, end-stage organ damage, body and organ weight, and lipid metabolism in the LH rat were identified on chromosomes 1, 2, 3, 5, 7, 10, 13, and 17, with 2 phenotypes associated with the metabolic syndrome identified on chromosomes 1 and 17. Regions on chromosomes 2, 13, and 17 were revealed to be important for blood pressure regulation. Regions on chromosome 17 were found to significantly contribute to both metabolic homeostasis and blood pressure regulation; 2 aggregates of a total of 23 QTLs were identified, including several "intermediate phenotypes." These intermediate phenotypes may be used as closer surrogates to the mechanisms leading to hypertension and metabolic dysfunction in the LH rat.

Consomic rat model systems for physiological genomics

A consomic rat strain is one in which an entire chromosome is introgressed into the isogenic background of another inbred strain using marker-assisted selection. The development and physiological screening of two inbred consomic rat panels on two genetic backgrounds (44 strains) is well underway. Consomic strains enable one to assign traits and quantitative trait loci (QTL) to chromosomes by surveying the panel of strains with substituted chromosomes. They enable the rapid development of congenic strains over a narrow region and enable one to perform F2 linkage studies to positionally locate QTL on a single chromosome with a fixed genetic background. These rodent model systems overcome many of the problems encountered with segregating crosses where even if linkage is found, each individual in the cross is genetically unique and the combination of genes cannot be reproduced or studied in detail. For physiologists, consomics enable studies to be performed in a replicative or longitudinal manner to elucidate in greater detail the sequential expression of genes responsible for the observed phenotypes of these animals. They often provide the best available inbred control strains for physiological comparisons with the parental strains and they enable one to assess the impact of a causal gene region in a genome by allowing comparisons of the effect of replacement of a specific chromosome on a disease susceptible or a resistant genomic background. Consomic rat strains are proving to be a unique scientific resource that can greatly extend our understanding of genes and their role in the regulation of complex function and disease.

Quantitative trait locus dissection in congenic strains of the Goto-Kakizaki rat identifies a region conserved with diabetes loci in human chromosome 1q

Genetic studies in human populations and rodent models have identified regions of human chromosome 1q21–25 and rat chromosome 2 showing evidence of significant and replicated linkage to diabetes-related phenotypes. To investigate the relationship between the human and rat diabetes loci, we fine mapped the rat locus Nidd/ gk2 linked to hyperinsulinemia in an F2 cross derived from the diabetic (type 2) Goto-Kakizaki (GK) rat and the Brown Norway (BN) control rat, and carried out its genetic and pathophysiological characterization in BN.GK congenic strains. Evidence of glucose intolerance and enhanced insulin secretion in a congenic strain allowed us to localize the underlying diabetes gene(s) in a rat chromosomal interval of ∼3–6 cM conserved with an 11-Mb region of human 1q21–23. Positional diabetes candidate genes were tested for transcriptional changes between congenics and controls and sequence variations in a panel of inbred rat strains. Congenic strains of the GK rats represent powerful novel models for accurately defining the pathophysiological impact of diabetes gene(s) at the locus Nidd/ gk2 and improving functional annotations of diabetes candidates in human 1q21–23.

Uncovering cryptic genetic variation

Cryptic genetic variation is the dark matter of biology: it is variation that is not normally seen, but that might be an essential source of physiological and evolutionary potential. It is uncovered by environmental or genetic perturbations, and is thought to modify the penetrance of common diseases, the response of livestock and crops to artificial selection and the capacity of populations to respond to the emergence of a potentially advantageous macro-mutation. We argue in this review that cryptic genetic variation is pervasive but under-appreciated, we highlight recent progress in determining the nature and identity of genes that underlie cryptic genetic effects and we outline future research directions.

Genetic linkage of albuminuria and renal injury in Dahl salt-sensitive rats on a high-salt diet: comparison with spontaneously hypertensive rats

Our aim was to study the effects of high-salt diet on the genetics of albuminuria and renal injury in the Dahl salt-sensitive (SS) rat. We compared SS with salt-resistant spontaneously hypertensive rats (SHR) and with genetically related salt-sensitive stroke-prone SHR (SHRSP). Moreover, we performed genome-wide linkage analysis to identify quantitative trait loci (QTL) contributing to salt-induced renal injury in an F2population derived from SS and SHR ( n = 230). In response to high-salt diet SS and SHRSP developed a striking increase in systolic blood pressure, urinary albumin excretion (UAE), and renal damage indices compared with SHR. Both SHRSP and SS developed severe glomerulosclerosis, whereas microangiopathy, tubulointerstitial fibrosis, and inflammation were more pronounced in SHRSP. We detected two QTL with significant linkage to UAE on rat chromosomes (RNO) 6 and 19. Comparison with the recently identified salt-independent UAE QTL in young animals revealed that the UAE QTL on RNO6 is unique to high-salt conditions, whereas RNO19 plays a significant role during both low- and high-salt conditions. Some F2animals demonstrated severe microangiopathy and tubulointerstitial injury, which exceeded the degree observed in the parental SS strain. Three loci demonstrated suggestive linkage to these phenotypes on RNO3, RNO5, and RNO20, whereas no linkage to glomerular damage was found. Further analyses at these loci indicated that the severity of renal injury was attributable to the SHR allele. Our data suggest that the SHR genetic background confers greater susceptibility for the development of microangiopathy and tubulointerstitial injury in salt-sensitive hypertension than the SS background.

Integrative genomics: in silico coupling of rat physiology and complex traits with mouse and human data

Integration of the large variety of genome maps from several organisms provides the mechanism by which physiological knowledge obtained in model systems such as the rat can be projected onto the human genome to further the research on human disease. The release of the rat genome sequence provides new information for studies using the rat model and is a key reference against which existing and new rat physiological results can be aligned. Previously, we described comparative maps of the rat, mouse, and human based on EST sequence comparisons combined with radiation hybrid maps. Here, we use new data and introduce the Integrated Genomics Environment, an extensive database of curated and integrated maps, markers, and physiological results. These results are integrated by using VCMapview, a java-based map integration and visualization tool. This unique environment allows researchers to relate results from cytogenetic, genetic, and radiation hybrid studies to the genome sequence and compare regions of interest between human, mouse, and rat. Integrating rat physiology with mouse genetics and clinical results from human by using the respective genomes provides a novel route to capitalize on comparative genomics and the strengths of model organism biology.

Application of chromosomal substitution techniques in gene-function discovery

A consomic rat strain is one in which an entire chromosome is introgressed into the isogenic background of another inbred strain using marker assisted selection. The development and initial physiologic screening of two inbred consomic rat panels on two genetic backgrounds (44 strains) is well underway. The primary uses of consomic strains are: (1) to assign traits and quantitative trait loci (QTL) to chromosomes by surveying the panel of strains with substituted chromosomes; (2) to rapidly develop congenic strains over a narrow region using several approaches described in this review and perform F2 linkage studies to positionally locate QTL in a fixed genetic background. In addition, consomic strains overcome many of the problems encountered with segregating crosses where, even if linkage is found, each individual in the cross is genetically unique and the combination of genes cannot be reproduced or studied in detail. Consomic strains provide greater statistical power to detect linkage than traditional F2 crosses because of their fixed genetic backgrounds, and can produce sufficient numbers of genetically identical rats to validate the relationship between a trait and a particular chromosome. These strains allow studies to be performed in a replicative or longitudinal manner to elucidate in greater detail the sequential changes responsible for the observed phenotypes of these animals, and they enable one to assess the impact of a causal gene region in a genome by allowing comparisons of the effect of replacement of a specific chromosome upon a disease susceptible or resistant genomic background. Consomics can be used to quickly develop multiple chromosome substitution models to investigate gene-gene interactions of complex traits or diseases. Finally, they often provide the best available inbred control strain for particular physiological comparisons with the inbred parental strains. Consomic rat strains are proving to be a unique scientific resource that greatly extends our understanding of genes and complex normal and pathological function.

Functional genomics in rodent models of hypertension

Inbred strains of rodents have been used to study mammalian physiology and pathophysiology in an attempt to understand the contribution of genes in the pathogenesis of the disease process. In this review we focus on experimental animal models to identify quantitative trait loci (QTL) and possible strategies for identifying underlying genetic determinants responsible for hypertension. Confirmation of the existence of the QTL and dissection of the implicated region can be undertaken by production of either recombinant inbred, consomic or congenic strains. Despite complex interactions and the relatively few confirmed causative genes underlying QTL, recent developments in rat genome resources and advancement in statistical and bioinformatic methods will facilitate the identification of major gene(s) responsible for complex, polygenic traits.

Polygenic Control of Idiopathic Generalized Epilepsy Phenotypes in the Genetic Absence Rats from Strasbourg (GAERS)

PURPOSE

Generalized nonconvulsive absence seizures are characterized by the occurrence of synchronous and bilateral spike-and-wave discharges (SWDs) on electroencephalographic recordings, concomitant with behavioral arrest. The GAERS (genetic absence rats from Strasbourg) strain, a well-characterized inbred model for idiopathic generalized epilepsy, spontaneously develops EEG paroxysms that resemble those of typical absence seizures. The purpose of this study was to investigate the genetic control of SWD variables by using a combination of genetic analyses and electrophysiological measurements in an experimental cross derived from GAERS and Brown Norway (BN) rats.

METHODS

SWD subphenotypes were quantified on EEG recordings performed at both 3 and 6 months in a cohort of 118 GAERS x BN F2 animals. A genome-wide scan of the F2 progenies was carried out with 146 microsatellite markers that were used to test each marker locus for evidence of genetic linkage to the SWD quantitative traits.

RESULTS

We identified three quantitative trait loci (QTLs) in chromosomes 4, 7, and 8 controlling specific SWD variables in the cross, including frequency, amplitude, and severity of SWDs. Age was a major factor influencing the detection of genetic linkage to the various components of the SWDs.

CONCLUSIONS

The identification of these QTLs demonstrates the polygenic control of SWDs in the GAERS strain. Genetic linkages to specific SWD features underline the complex mechanisms contributing to SWD development in idiopathic generalized epilepsy.

Genomic sequence and organization of the family of CNR/Pcdhα genes in rat

CNR/Pcdhalpha family proteins are known as synaptic cadherins and Reelin receptors. Here we report the complete genomic sequence and organization of the rat CNR. The rat CNR cluster encodes 15 variable and 3 constant exons. The genomic organizations of the rat, mouse, and human CNR/Pcdhalpha are orthologous. The percentage identity of the coding regions between the rat and the mouse is 93.6% on average at the nucleic acid level, and between rat and human it is 82.8%. The rat CNRs (v1-v13) also contain an RGD motif in the extracellular cadherin 1 domains and cysteine repeats that are characteristic of the transmembrane and cytoplasmic domains of CNR proteins. The number of variable exons in the rat CNR cluster is identical to that of the human. The rat CNR cluster has one more variable exon than is found in laboratory mouse strains, because in the mouse a variable exon located between v7 and v8 is divided by the insertion of a retrotransposon. This exon is not disrupted in the rat, in which it is transcribed. By in silico analysis, CNR/Pcdhalpha was also mapped to rat chromosome 18, but the orientation was opposite for the mouse CNR/Pcdhalpha gene cluster. The relative expression profiles of the rat CNRs (v1-v13) show that all the CNRs are transcribed, but there are variations in the expression ratios among the CNRs.

Functional genomics as an emerging strategy for the investigation of central mechanisms in experimental hypertension

Centrally mediated increases in sympathetic nerve activity and attenuated arterial baroreflexes contribute to the pathogenesis of hypertension. Despite the characterization of cellular and physiological mechanisms that regulate blood pressure and alterations that contribute to hypertension, the genetic and molecular basis of this pathophysiology remains poorly understood. Strategies to identify genes that contribute to central pathophysiologic mechanisms in hypertension include integrative biochemistry and physiology as well as functional genomics. This article summarizes recent progress in applying functional genomics to elucidate the genetic basis of altered central blood pressure regulatory mechanisms in hypertension. We describe approaches others and we have undertaken to investigate gene expression profiles in hypertensive models in order to identify genes that contribute to the pathogenesis of hypertension. Finally, we provide the readers a roadmap for negotiating the route from experimental findings of gene expression profiling to translating their therapeutic potential. The combination of gene expression profiling and the phenotypic characterization of in vitro and in vivo loss or gain of function experiments for candidate genes have the potential to identify genes involved in the pathogenesis of hypertension and may present novel targets for therapy.

Dietary NaCl regulates renal aminopeptidase N: relevance to hypertension in the Dahl rat

Aminopeptidase N (APN) is an abundant metallohydrolase in the brush border of kidney proximal tubule cells that degrades angiotensin III (Ang III) to angiotensin IV (Ang IV) and, along with dipeptidylaminopeptidase, degrades Ang IV. We examined the impact of a high-salt diet on renal APN activity and transcript abundance in the Sprague-Dawley and Dahl salt-sensitive (SS/Jr) rat strains. APN transcript abundance and protein abundance were approximately 2-fold greater (P<0.05; n=6) in the kidneys of Sprague-Dawley and Lewis rats ingesting 8% versus 0.3% salt diets, suggesting that increased aminopeptidase activity may contribute to decreased renal sodium uptake during adaptation to a high-salt diet. In contrast, renal APN transcript abundance and activity were the same in Dahl SS/Jr rats ingesting 8.0% versus 0.3% salt diets. The APN gene was mapped, using a radiation-hybrid panel, to known quantitative loci on chromosome 1 for blood pressure in the Dahl SS/Jr rat. The results suggest that the APN gene is a good candidate for salt-sensitivity in the Dahl SS/Jr rat.

Pyridoxine 5′-phosphate oxidase is a candidate gene responsible for hypertension in Dahl-S rats

To identify candidate genes responsible for hypertension in Dahl salt-sensitive rats (Dahl-S), an oligonucleotide microarray analysis was performed to find differentially expressed genes in kidneys of Dahl-S and Lewis rats. We obtained 101 F2 male rats from Dahl-S and Lewis rats and performed precise measurements of blood pressure (BP) and heart rate by telemetric monitoring at 14 weeks of age after 9 weeks of salt-loading. The correlation analysis between genotypes of differentially expressed genes and BP in F2 rats indicated that pyridoxine 5'-phosphate oxidase (Pnpo) and catecholamine-O-methyltransferease (Comt) showed a highly significant association with BP. However, in the case of Comt, the Dahl-S genotype correlated with low BP. Short/branched chain acyl-CoA dehydrogenase and Sah also showed a significant association with systolic blood pressure. The present study provided evidence that Pnpo is a candidate gene responsible for hypertension in Dahl-S rats.

A Major Gene Locus Links Early Onset Albuminuria with Renal Interstitial Fibrosis in the MWF Rat with Polygenetic Albuminuria

ABSTRACT. The development of renal interstitial fibrosis (RIF) represents an important step in the progression of chronic proteinuric nephropathies. The Munich Wistar Frömter (MWF) rat represents a valuable model to study the progression in proteinuric renal disease. MWF animals demonstrate a significant increase of urinary albumin excretion (UAE) and RIF compared with the spontaneously hypertensive rat (SHR) with low UAE. The aim of this study was to analyze the genetic basis and the relation between UAE and RIF by genetic linkage and quantitative trait loci (QTL) mapping analysis. The authors generated a backcross population between MWF and SHR including 215 male animals. UAE was determined in young backcross animals at 8 wk, and at 14 and 24 wk of age, respectively. RIF was evaluated by Sirius red staining of kidney sections and quantified by computer-assisted image analysis at 24 wk. Total genome scan analysis identified in total eight QTL linked to UAE and a major locus on chromosome 6. At this locus, homozygosity for the MWF allele exhibited a strong effect on UAE levels (threefold elevation) and displayed significant linkage already at 8 wk (logarithm of odds [LOD] = 4.3) with increasing significance at 14 and 24 wk (LOD = 7.8 and 10.1, respectively). In addition, this was the only QTL that was linked to the amount of RIF (P= 0.0009, LOD = 2.4). These data establish a genetic link between early onset albuminuria and progression of RIF at the QTL on RNO6. This study demonstrates the power of genetic linkage analysis for the dissection of physiologic pathways involved in renal disease progression.

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.

Quantitative trait loci with opposing blood pressure effects demonstrating epistasis on Dahl rat chromosome 3

Our previous linkage studies indicated that there might be a blood pressure (BP) quantitative trait locus (QTL) on chromosome 3 (Chr 3) contrasting between the Dahl salt-sensitive (S) strain and the Lewis (LEW) strain. To prove and then to narrow down the segment containing this QTL, five congenic strains have been generated by replacing various segments of the S rats with the homologous segments of the LEW rats. They are designated as S.L1, S.L2, S.L3, S.L4, and S.L5, respectively. S.L2, S.L3, S.L4, and S.L5 are substrains of S.L1, i.e., they contain substitutions of smaller sections within the large fragment defined by S.L1. The construction of these congenic strains was facilitated by a genome-wide marker screening process. BPs of the rats were measured by telemetry. S.L2 and S.L3 shared a fragment of Chr 3 in common and both showed a BP-lowering effect, indicating the existence of "-BP" QTL alleles from LEW compared with S. In contrast, S.L4 involves a section with no overlap with either S.L2 or S.L3, and S.L4 showed a BP significantly higher than that of S rats, indicating the presence of "+BP" QTL alleles from LEW compared with S. Interestingly, the combined effect of the -BP QTL and +BP QTL alleles was "-" in S.L1, implying that the "-" QTL is epistatic to "+" QTL.

Early onset albuminuria in Dahl rats is a polygenetic trait that is independent from salt loading

The aim of the study was to characterize the genetic basis for the early onset of increased urinary albumin excretion (UAE) observed in the salt-sensitive Dahl rat (SS). We first characterized blood pressures and UAE values in adult SS compared with the spontaneously hypertensive rat (SHR) strain. Blood pressure measurements by radiotelemetry at 14 wk demonstrated similar spontaneous hypertension in both strains on a low-sodium diet containing 0.2% NaCl by weight, whereas UAE was markedly increased in SS compared with SHR (253.07 +/- 68.39 vs. 1.65 +/- 1.09 mg/24 h, P < 0.0001). Analysis of UAE in young animals of both strains fed a low-sodium diet demonstrated that UAE is elevated in SS as early as 4 wk of age (P < 0.0001), when ultrastructural evaluation of glomeruli by electron microscopy appears still normal. At 8 wk SS demonstrated a 280-fold elevated UAE compared with SHR (P < 0.0001). Consequently, to identify quantitative trait loci (QTLs) contributing to salt-independent early manifestation of increased UAE in the SS rat, we performed genome-wide linkage and QTL mapping analysis in a young F(2) population derived from the two contrasting strains. UAE was determined in 539 F(2) animals at 8 wk. We identified seven suggestive or significant UAE QTLs on rat chromosomes (RNO) RNO2, RNO6, RNO8, RNO9, RNO10, RNO11, and RNO19, accounting together for 34% of the overall variance of UAE in this F(2) population. Thus early onset albuminuria in the SS rat is under polygenetic influence and independent from salt loading.

Genetic susceptibility to carrageenan-induced innate inflammatory response in inbred strains of rats

Rat models are useful for the genetic dissection of the biology of innate immunity. Inbred rat strains were evaluated for carrageenan-induced innate inflammatory responses. Results indicated that the genetic control of innate immune responses is polygenic and influenced by gender, and may not necessarily be consistent with the genetics of experimental arthritis. The newly identified susceptible strains, in order of decreasing susceptibility, include Dahl salt-sensitive (S), Dahl salt-resistant (R), Milan normotensive strain (MNS) and Wistar Kyoto (WKY) rats. Similarly, the newly identified relatively resistant strains, in decreasing order of resistance, include DA rats, spontaneously hypertensive rats (SHRs) and Brown Norway (BN) rats. Linkage analyses using combinations of these susceptible and resistant strains are proposed.

Genetic loci contribute to the progression of vascular and cardiac hypertrophy in salt-sensitive spontaneous hypertension

OBJECTIVE

The salt-sensitive Dahl rat and the spontaneously hypertensive rat develop comparable spontaneous hypertension on a low-salt diet, whereas only the salt-sensitive Dahl rat strain develops a striking increase in blood pressure and cardiovascular hypertrophy on a high-salt diet. We set out to identify quantitative trait loci (QTLs) contributing to the progression of salt-induced organ damage in hypertension by studying an F2 population derived from both strains.

METHODS AND RESULTS

We determined systolic blood pressure (SBP), vascular aortic hypertrophy (AH), cardiac left ventricular (LV) hypertrophy (LVH), and LV fibrosis in 230 male F2-animals on a high-salt diet. A strong correlation between AH and LVH was found (r=0.58, P<0.0001), and genome-wide QTL mapping detected suggestive or significant QTLs in overlapping chromosomal fragments for AH and LVH on chromosomes 1, 3, and 19, respectively. A significant influence of SBP on the extent of LVH and AH was evident at all QTLs, although significant linkage to SBP (together with LVH) was only found on chromosome 9. No QTLs for LV fibrosis were detected.

CONCLUSIONS

This study demonstrates a strong correlation between AH and LVH in salt-sensitive hypertension and identifies QTLs contributing to the progression of cardiovascular hypertrophy in this condition.

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

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

Where are all the blood-pressure genes?

CONTEXT

The potential for discovery of underlying genetic causes for common conditions, such as hypertension, is a major justification for the human genome project. Hopes have been raised that personalised molecular prognostic and therapeutic strategies will result from genomic studies.

STARTING POINT

The largest and most recent searches of the genome (Mark Caulfield and colleagues, Lancet 2003; 361: 2118-23; Michael Province and colleagues, Am J Hypertens 2003; 16: 144-47) have found limited evidence of genes that determine hypertension, and even less evidence for the existence of causative DNA variants (alleles) within these genes. Previous genomic studies did not reach a consensus on the likely location of blood-pressure genes. The difficulty in identifying genetic causes for common conditions might be because the causative alleles are numerous, with small individual impact, and are distributed unevenly between populations. WHERE NEXT?: Without comprehensive and reliable description of the alleles associated with hypertension, using genetics for diagnosis remains tenuous. However, the discovery of a single allele proven to be associated with control of blood pressure could lead to the discovery of relevant and novel physiological targets for the prevention and treatment of hypertension.

Essential hypertension

Hypertension is a frequent, chronic, age-related disorder, which often entails debilitating cardiovascular and renal complications. Blood pressure is usually noted in combination with other cardiovascular risk factors. Diagnosis of hypertension increasingly relies on automated techniques of blood pressure measurement. The pathophysiology of essential hypertension depends on the primary or secondary inability of the kidney to excrete sodium at a normal blood pressure. The central nervous system, endocrine factors, the large arteries, and the microcirculation also have roles in the disorder. Although monogenic forms of blood pressure dysregulation exist, hypertension mostly arises as a complex quantitative trait that is affected by varying combinations of genetic and environmental factors. Non-pharmacological strategies can reduce blood pressure. Antihypertensive drug treatment diminishes the complications of hypertension. The concept that a few major genes will provide the final clue to the pathogenesis of essential hypertension is an oversimplification that contradicts the heterogeneous nature of this disorder. Further integration of genetic, molecular, clinical, and epidemiological research could disclose subsets of patients in whom specific combinations of genetic and environmental factors raise blood pressure, and might lead to more individualised treatment.

The role of Wnk4 in polygenic hypertension: a candidate gene analysis on rat chromosome 10

Linkage analyses in experimental crosses of stroke-prone spontaneously hypertensive (SHRSP) and normotensive Wistar-Kyoto (WKY) rats have strongly suggested the presence of quantitative trait loci (QTL) influencing blood pressure and ACE levels on rat chromosome 10, which have been confirmed in multiple independent studies. Analysis of the orthologous region on human chromosome 17 also revealed significant linkage to blood pressure in several populations. Wnk4, a gene previously identified to cause pseudohypoaldosteronism type II, a rare mendelian form of arterial hypertension, is located on human chromosome 17. The hypothesis has been advanced that molecular variants of this gene might contribute to common polygenic forms of hypertension, since Wnk4 is located in a region of conserved synteny that demonstrates an overlap between quantitative trait loci for primary hypertension in humans and rats. In this report, we describe the confirmation of the blood pressure QTL on rat chromosome 10 by congenic approaches, spanning the Wnk4 locus. Comparative analysis of the complete coding sequence of Wnk4 in SHRSP and WKY strains revealed no mutation and demonstrated high conservation between rat and human proteins. Furthermore, comparison of mRNA levels in the kidney showed no differences between SHRSP and WKY. Additionally, we excluded a secondary effect of blood pressure on the transcriptional regulation of Wnk4. Our results fail to support a material contribution of Wnk4 to blood pressure regulation in this model of polygenic hypertension. Thus, Wnk4 is likely not to represent the underlying disease gene for the QTL captured in chromosome 10 congenic animals.

Microarray analysis of rat chromosome 2 congenic strains

Human essential hypertension is a complex polygenic trait with underlying genetic components that remain unknown. The stroke-prone spontaneously hypertensive rat (SHRSP) is a model of human essential hypertension, and a number of reproducible blood pressure regulation quantitative trait loci have been found to map to rat chromosome 2. The SP.WKYGla2c* congenic strain was produced by introgressing a region of rat chromosome 2 from the normotensive Wistar Kyoto (WKY) strain into the genetic background of the SHRSP. Systolic and diastolic blood pressures were significantly reduced in the SP.WKYGla2c* compared with the SHRSP parental strain (198/134+/-6.1/3.3 versus 172/120+/-3.8/3.4 mm Hg; F=15.8/8.1, P=0.0009/0.013). Genome-wide microarray expression profiling was undertaken to identify differentially expressed genes among the parental SHRSP, WKY, and congenic strain. We identified a significant reduction in expression of glutathione S-transferase mu-type 2, a gene involved in the defense against oxidative stress. Quantitative reverse transcription-polymerase chain reaction relative to a beta-actin standard confirmed the microarray results with SHRSP mRNA at 8.56 x 10(-4) +/-1.6 x 10(-4) compared with SP.WKYGla2c* 3.67 x 10(-3)+/-2.8 x 10(-4) (95% CI -3.9 x 10(-3) to -1.8 x 10(-3); P=0.0034) and WKY 4.03 x 10(-3)+/-5.1 x 10(-4); (95% CI -5.4 x 10(-3) to -8.9 x 10(-4); P=0.027). We also identified regions of conserved synteny, each containing the Gstm2 gene, on mouse chromosome 3 and human chromosome 1.

Mapping of genetic loci predisposing to hypertriglyceridaemia in the hereditary hypertriglyceridaemic rat: analysis of genetic association with related traits of the insulin resistance syndrome

AIMS/HYPOTHESIS

Hypertriglyceridaemia is an important risk factor for coronary heart disease, especially in the context of the insulin resistance syndrome where it often occurs with hypertension. The two phenotypes are also associated in the hereditary hypertriglyceridaemic (hHTg) rat. The aim of this study was to map quantitative trait loci that affect plasma triglyceride concentration in the hHTg rat and determine whether they co-localize with loci for blood pressure.

METHODS

Second filial generation progeny (n=189) from a cross of the hHTg rat with the Brown Norway rat were phenotyped for fasting plasma triglyceride, glucose and insulin concentrations, and direct unrestrained resting blood pressure. A partial genome-scan was conducted using 153 microsatellite markers that were polymorphic between the two strains.

RESULTS

A major locus (lod score 6.5) influencing plasma triglyceride concentration in a co-dominant fashion was mapped to chromosome 4 between D1Mit 5 and D1Mit17. Chromosome 8 contained multiple peaks with a lod score greater than 4.0 influencing triglyceride concentration. Importantly, none of the triglyceride loci had an effect on blood pressure. The triglyceride locus on chromosome 4 co-localized with a locus for fasting plasma insulin (lod score 4.1), although the effect on insulin concentration was in the opposite direction to that on triglyceride.

CONCLUSION/INTERPRETATION

We have mapped the major loci that affect plasma triglyceride concentration in the hHTg rat. These loci do not influence blood pressure suggesting that these commonly associated phenotypes of the insulin resistance syndrome are not be due to pleiotropic effects of the same gene(s).

Genomics and homeostasis

The Cannon lecture this year illustrates how knowledge of DNA sequences of complex living organisms is beginning to shape the landscape of physiology in the 21st century. Enormous challenges and opportunities now exist for physiologists to relate the galaxy of genes to normal and pathological functions. The first extensive genomic systems biology map for cardiovascular and renal function was completed last year as well as a new hypothesis-generating tool ("physiological profiling") that enables us to hypothesize relationships between specific genes responsible for the regulation of regulatory pathways. Techniques of chromosomal substitution (consomic and congenic rats) are beginning to confirm statistical results from linkage analysis studies, narrow the regions of genetic interest for positional cloning, and provide genetically well-defined control strains for physiological studies. Patterns of gene expression identified by microarray and mapping of expressed genes to chromosomal sites are adding to the understanding of systems physiology. The previously unimaginable goal of connecting approximately 36,000 genes to the complex functions of mammalian systems is indeed well underway.

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.

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.

Identification of blood pressure quantitative trait loci that differentiate two hypertensive strains

OBJECTIVE

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.

METHODS

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.

RESULTS

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.

CONCLUSIONS

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.

Genetic dissection of increased urinary albumin excretion in the munich wistar frömter rat

An elevated urinary albumin excretion (UAE) is a risk factor for the development of chronic nephropathy and for cardiovascular mortality. The Munich Wistar Frömter (MWF) rat represents a genetic model that develops spontaneously mild hypertension and a 142-fold increased UAE compared with normal Lewis rats at 14 wk of age. The genetic basis of UAE in male MWF was analyzed in relation to BP by using a quantitative trait (QTL) mapping strategy. F1-hybrids generated from a cross between MWF and Lewis rats showed normal UAE rates similar to Lewis. A backcross strategy including 213 animals was therefore used. To account for age-of-onset effects, UAE was determined in young backcross animals at 8 wk and in adult animals at 14 and 24 wk of age, respectively. Total genome scan analysis identified three QTL with significant linkage to UAE and one QTL with suggestive linkage to UAE. These loci showed no linkage to BP, and BP explained only 2% of the total variance of UAE. Homozygosity for the MWF allele accounted for a similar mean increase in UAE in adult backcross animals at each UAE QTL. When single gene effects were analyzed, only one UAE QTL led to a significant increase in UAE. Early onset of albuminuria and a considerable increase of UAE in adult animals required homozygosity at three loci at least. This study demonstrates the polygenetic recessive determination of increased UAE by a set of genes that are unlinked to BP.

Nicotinic receptor gene cluster on rat chromosome 8 in nociceptive and blood pressure hyperresponsiveness

Spontaneously hypertensive rats (SHR) exhibit enhanced pressor, heart rate, and nociceptive responses to spinal nicotinic agonists. This accompanies a paradoxical decrease in spinal nicotinic receptor number in SHR compared with normotensive rats. The congenic strain, SHR-Lx, with an introgressed chromosome 8 segment from the normotensive Brown-Norway-Lx strain (BN-Lx) exhibits reduced blood pressure. This segment contains a gene cluster for three nicotinic receptor subunits expressed in the nervous system. We examined the implication of this gene cluster in the enhanced responsiveness of the SHR. Pressor and nociceptive responses to spinal cytisine, a nicotinic agonist, were diminished in SHR-Lx. Moreover, with repeated administration, these responses desensitized faster in SHR-Lx and progenitor BN-Lx than in progenitor SHR/Ola. This implicates the gene cluster in both cardiovascular and nociceptive responses to spinal nicotinic agonists. Since diminished responsiveness to agonist stimulation is greater than the basal blood pressure differences between the strains and the introgressed rat chromosome maps to a quantitative trait locus in human hypertension, polymorphisms in the three nicotinic receptor genes become candidates for altered central control of blood pressure.

A missense mutation in kynurenine aminotransferase-1 in spontaneously hypertensive rats

Spontaneously hypertensive rats (SHR) are the most extensively used animal model for genetic hypertension, increased stroke damage, and insulin resistance syndromes; however, the identification of target genes has proved difficult. SHR show elevated sympathetic nerve activity, and stimulation of the central blood pressure control centers with glutamate or nicotine results in exaggerated blood pressure responses, effects that appear to be genetically determined. Kynurenic acid, a competitive glutamate antagonist and a non-competitive nicotinic antagonist, can be synthesized in the brain by the enzyme kynurenine aminotransferase-1 (KAT-1). We have previously shown that KAT-1 activity is significantly reduced in SHR compared with normotensive Wistar Kyoto rats (WKY). Here we show that KAT-1 contains a missense mutation, E61G, in all the strains of SHR examined but not in any of the WKY or outbred strains. Previous studies on F2 rats from a cross of stroke-prone SHR and WKY have shown a suggestive level of linkage between elevated blood pressure and the KAT-1 locus on chromosome 3. In addition, the mutant enzyme expressed in Escherichia coli displays altered kinetics. This mutation may explain the enhanced sensitivity to glutamate and nicotine seen in SHR that may be related to an underlying mechanism of hypertension and increased sensitivity to stroke.

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

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

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.

Difference of gene expression profiles in spontaneous hypertensive rats and Wistar-Kyoto rats from two sources

Spontaneously hypertensive rats (SHR) are a well-known animal model for hypertension. We have previously identified eleven differentially expressed genes in the kidneys between SHR/Hos and Wistar-Kyoto rats (WKY/Hos) using an oligonucleotide microarray and analyzed the correlation between these genes and hypertension. In the present study, we analyzed the differentially expressed genes in the kidneys between SHR/NCrj and WKY/NCrj obtained from an other source to clarify the common and/or specific gene expression between the different sources. Furthermore, expression changes in the representative genes were characterized by Northern blot analysis using samples prepared from a third source, the Izm strain. The comparison revealed quite different changes in the differentially expressed genes among them. Sequence analysis of one of the differentially expressed genes, cytosolic epoxide hydrolase, revealed that two haplotypes could in part explain the expression level. Our study showed the complex nature of the genetic heterogeneity between SHR and WKY from different sources.

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.

A genome scan for Loci associated with aerobic running capacity in rats

Aerobic capacity is a complex trait that defines the efficiency to use atmospheric oxygen as an electron acceptor in energy transfer. Copenhagen (COP) and DA inbred rat strains show a wide difference in a test for aerobic treadmill running and serve as contrasting genetic models for aerobic capacity. A genome scan was carried out on an F(2)(COP x DA) segregating population (n=224) to detect quantitative trait loci (QTLs) associated with aerobic running capacity. Linkage analysis revealed a significant QTL on chromosome 16 (lod score, 4.0). A suggestive linkage was found near the p-terminus of chromosome 3 (lod score, 2.2) with evidence of an interaction with another QTL on chromosome 16 (lod score, 2.9). All three QTLs showed a dominant mode of inheritance in which the presence of at least one DA allele was associated with a greater distance run. These results represent the first aerobic capacity QTLs identified in genetic models.

Fine linkage mapping of the blood pressure quantitative trait locus region on rat chromosome 1

To narrow the area known to contain the blood pressure quantitative trait locus (QTL) on rat chromosome 1, we constructed a fine linkage map covering the blood pressure OTL region on the chromosome using 22 genetic markers informative for stroke-prone spontaneously hypertensive rats of the Izumo colony (SHRSP/Izm) and Wistar-Kyoto rats of the Izumo colony (WKY/Izm). Linkage mapping was done by genotyping 626 backcrossed rats from matings between SHRSP/Izm and WKY/Izm. Nineteen genetic markers informative for the two strains were selected from public databases. Two markers were newly isolated by screening a rat genomic library. One marker was mapped using a restriction endonuclease polymorphism. The region between DlWox29 and D1Smu11 was covered with 22 informative markers placed every 0.6 cM on average. In addition, 6 physiological candidates for a hypertension gene were mapped in this region either by linkage or by radiation hybrid (RH) mapping. This information should be essential for the construction and analysis of congenic strains for this QTL region.

Reduced osmotically inactive Na storage capacity and hypertension in the Dahl model

Recent evidence suggested that Na can be stored in an osmotically inactive form. We investigated whether osmotically inactive Na storage is reduced in a rat model of salt-sensitive (SS) hypertension. SS and salt-resistant (SR) Dahl-Rapp rats as well as Sprague-Dawley (SD) rats were fed a high (8%)- or low (0.1%)-NaCl diet for 4 wk (n = 10/group). Mean arterial pressure (MAP) was measured at the end of the experiment. Wet and dry weights, water content, total body Na (TBS), and bone Na content were measured by dessication and dry ashing. MAP was higher in both Dahl strains than in SD rats. In SS rats, 8% NaCl led to Na accumulation, water retention, and hypertension due to impaired renal Na excretion. There was no dietary-induced Na retention in SR and SD rats. TBS was variable; nevertheless, TBS was significantly correlated with body water and MAP in all strains. However, the extent of Na-associated volume and MAP increases was strain specific. Osmotically inactive Na in SD rats was threefold higher than in SS and SR rats. Both SS and SR Dahl rat strains displayed reduced osmotically inactive Na storage capacity compared with SD controls. A predisposition to fluid accumulation and high blood pressure results from this alteration. Additional factors, including impaired renal Na excretion, probably contribute to hypertension in SS rats. Our results draw attention to the role of osmotically inactive Na storage.

Genetic aspects of stroke: human and experimental studies

As one of the leading causes of death within both the developed and developing world, stroke is a worldwide problem. Risk factors can be identified and controlled at the level of lifestyle changes; however, genetic components of stroke have yet to be identified. The identification of such genetic components is critical in the understanding, diagnosis, and treatment of stroke in the future. This review focuses on the genetic determinants of stroke in both human and experimental systems. Mendelian disorders, candidate genes, and twin studies provide evidence for a strong genetic component of stroke. Genome-wide scanning in both human and animal models has led to the identification of regions of the genome that contain genes for stroke susceptibility and sensitivity. Animal models of stroke allow for environmental control and genetic homogeneity, not possible within a human population, and therefore are essential for the dissection of this complex, multifactorial disorder. Future genetic and genomic strategies and their role in ultimate causative gene identification are discussed.

Angiotensin-converting enzyme 2 is an essential regulator of heart function

Cardiovascular diseases are predicted to be the most common cause of death worldwide by 2020. Here we show that angiotensin-converting enzyme 2 (ace2) maps to a defined quantitative trait locus (QTL) on the X chromosome in three different rat models of hypertension. In all hypertensive rat strains, ACE2 messenger RNA and protein expression were markedly reduced, suggesting that ace2 is a candidate gene for this QTL. Targeted disruption of ACE2 in mice results in a severe cardiac contractility defect, increased angiotensin II levels, and upregulation of hypoxia-induced genes in the heart. Genetic ablation of ACE on an ACE2 mutant background completely rescues the cardiac phenotype. But disruption of ACER, a Drosophila ACE2 homologue, results in a severe defect of heart morphogenesis. These genetic data for ACE2 show that it is an essential regulator of heart function in vivo.