T W Kurtz

Genetic susceptibility to hypertension-induced renal damage in the rat. Evidence based on kidney-specific genome transfer.

To test the hypothesis that genetic factors can determine susceptibility to hypertension-induced renal damage, we derived an experimental animal model in which two genetically different yet histocompatible kidneys are chronically and simultaneously exposed to the same blood pressure profile and metabolic environment within the same host. Kidneys from normotensive Brown Norway rats were transplanted into unilaterally nephrectomized spontaneously hypertensive rats (SHR-RT1.N strain) that harbor the major histocompatibility complex of the Brown Norway strain. 25 d after the induction of severe hypertension with deoxycorticosterone acetate and salt, proteinuria, impaired glomerular filtration rate, and extensive vascular and glomerular injury were observed in the Brown Norway donor kidneys, but not in the SHR-RT1.N kidneys. Control experiments demonstrated that the strain differences in kidney damage could not be attributed to effects of transplantation-induced renal injury, immunologic rejection phenomena, or preexisting strain differences in blood pressure. These studies (a) demonstrate that the kidney of the normotensive Brown Norway rat is inherently much more susceptible to hypertension-induced damage than is the kidney of the spontaneously hypertensive rat, and (b) establish the feasibility of using organ-specific genome transplants to map genes expressed in the kidney that determine susceptibility to hypertension-induced renal injury in the rat.

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.

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.

Mapping and sequence analysis of the gene encoding the beta subunit of the epithelial sodium channel in experimental models of hypertension.

Objective: To investigate whether mutations in the beta subunit of the epithelial sodium channel (Scnn1b) contribute to the pathogenesis of hypertension in the spontaneously hypertensive rat (SHR) and the Dahl salt-sensitive rat. Methods: Chromosome mapping was performed by somatic cell hybrid analysis and by linkage analysis in recombinant inbred strains derived from SHR and Brown-Norway rats. Cosegregation analysis was performed by testing for correlations between blood pressure and Scnn1b genotypes in these strains. DNA sequencing was performed on cDNAs prepared from reverse-transcribed messenger RNA derived from rat kidney. Results: The Scnnib gene was closely linked to the Sa gene on rat chromosome 1. Blood pressure correlated significantly with Scnn1b gene in the recombinant inbred strains. Analysis of near full-length Scnn1b cDNAs from SHR and Dahl rats failed to reveal any coding sequence mutations that could affect the predicted amino acid sequence of the Scnnib protein. Conclusion: The Scnn1b gene maps near the Sa gene in a region of rat chromosome 1 involved in the inherited control of blood pressure. If disordered activity of the epithelial cell sodium channel contributes to the pathogenesis of hypertension in the SHR or Dahl models, it must stem from genetic lesions in sequences that regulate Scnn1b function or in sequences important to the structure or function of the other sodium channel subunits.

Gene mapping in experimental hypertension.

In the rat, the results of genetic linkage studies by candidate gene or positional mapping approaches have suggested that DNA sequences that regulate blood pressure 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 genes may be located on the sex chromosomes. Pending the results of confirmatory studies, these experiments should be interpreted with caution. However, with confirmation of these studies, it should be possible to create a variety of new animal models that will provide excellent opportunities for investigating the molecular, biochemical, and physiologic determinants of high blood pressure. In addition, in genetic studies in humans with essential hypertension, it may be worthwhile to target chromosome regions that are homologous to those implicated in linkage studies of hypertension in rodents. By narrowing the focus on selected areas of the genome, experimental linkage studies in the rat may also be used to guide the detailed molecular approaches ultimately required to identify the specific DNA sequence alterations that give rise to increased blood pressure.

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.

Genetic isolation of a blood pressure quantitative trait locus on chromosome 2 in the spontaneously hypertensive rat.

Objectives Total genome scans of genetically segregating populations derived from the spontaneously hypertensive rat (SHR) and other rat models of hypertension have suggested the presence of quantitative trait loci (QTL) regulating blood pressure and cardiac mass on multiple chromosomes, including chromosome 2. The objective of the current study was to directly test for the presence of a blood pressure QTL on rat chromosome 2. Design A new congenic strain was derived by replacing a segment of chromosome 2 in the SHR between D2Rat171 and D2Arb24 with the corresponding chromosome segment from the normotensive Brown Norway rat. Arterial pressures were directly monitored in conscious rats by radiotelemetry. Results We found that the SHR congenic strain (SHR-2) carrying a segment of chromosome 2 from the Brown Norway rat had significantly lower systolic and diastolic blood pressures than the SHR progenitor strain. The attenuation of hypertension in the SHR-2 congenic strain versus the SHR progenitor strain was accompanied by significant amelioration of cardiac hypertrophy. Conclusions These findings demonstrate that gene(s) with major effects on blood pressure exist in the differential segment of chromosome 2 trapped within the new SHR.BN congenic strain.