Alleles of the spontaneously hypertensive rat decrease blood pressure at loci on chromosomes 4 and 13

Towards Precision Medicine for Hypertension: A Review of Genomic, Epigenomic, and Microbiomic Effects on Blood Pressure in Experimental Rat Models and Humans

Compelling evidence for the inherited nature of essential hypertension has led to extensive research in rats and humans. Rats have served as the primary model for research on the genetics of hypertension resulting in identification of genomic regions that are causally associated with hypertension. In more recent times, genome-wide studies in humans have also begun to improve our understanding of the inheritance of polygenic forms of hypertension. Based on the chronological progression of research into the genetics of hypertension as the "structural backbone," this review catalogs and discusses the rat and human genetic elements mapped and implicated in blood pressure regulation. Furthermore, the knowledge gained from these genetic studies that provide evidence to suggest that much of the genetic influence on hypertension residing within noncoding elements of our DNA and operating through pervasive epistasis or gene-gene interactions is highlighted. Lastly, perspectives on current thinking that the more complex "triad" of the genome, epigenome, and the microbiome operating to influence the inheritance of hypertension, is documented. Overall, the collective knowledge gained from rats and humans is disappointing in the sense that major hypertension-causing genes as targets for clinical management of essential hypertension may not be a clinical reality. On the other hand, the realization that the polygenic nature of hypertension prevents any single locus from being a relevant clinical target for all humans directs future studies on the genetics of hypertension towards an individualized genomic approach.

Expression of alpha-synuclein is increased in the hippocampus of rats with high levels of innate anxiety

A genomic region neighboring the alpha-synuclein gene, on rat chromosome 4, has been associated with anxiety- and alcohol-related behaviors in different rat strains. In this study, we have investigated potential molecular and physiological links between alpha-synuclein and the behavioral differences observed between Lewis (LEW) and Spontaneously Hypertensive (SHR) inbred rats, a genetic model of anxiety. As expected, LEW rats appeared more fearful than SHR rats in three anxiety models: open field, elevated plus maze and light/dark box. Moreover, LEW rats displayed a higher preference for alcohol and consumed higher quantities of alcohol than SHR rats. alpha-Synuclein mRNA and protein concentrations were higher in the hippocampus, but not the hypothalamus of LEW rats. This result inversely correlated with differences in dopamine turnover in the hippocampus of LEW and SHR rats, supporting the hypothesis that alpha-synuclein is important in the downregulation of dopamine neurotransmission. A novel single nucleotide polymorphism was identified in the 3'-untranslated region (3'-UTR) of the alpha-synuclein cDNA between these two rat strains. Plasmid constructs based on the LEW 3'-UTR sequence displayed increased expression of a reporter gene in transiently transfected PC12 cells, in accordance with in-vivo findings, suggesting that this nucleotide exchange might participate in the differential expression of alpha-synuclein between LEW and SHR rats. These results are consistent with a novel role for alpha-synuclein in modulating rat anxiety-like behaviors, possibly through dopaminergic mechanisms. Since the behavioral and genetic differences between these two strains are the product of independent evolutionary histories, the possibility that polymorphisms in the alpha-synuclein gene may be associated with vulnerability to anxiety-related disorders in humans requires further investigation.

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

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

Chromosomal mapping of the genetic basis of hypertension and renal disease in FHH rats

This study examined the genetic basis for hypertension and renal disease phenotypes in Fawn Hooded hypertensive (FHH) rats using chromosome substitution strains (consomic rats) in which each of the 20 autosomes as well as the X and Y chromosomes were transferred from the normal Brown Norway (BN) rat onto the FHH genetic background. Male and female rats of each of the parental and consomic strains were maintained for 2 wk on high-salt (8.0% NaCl) chow with N(G)-nitro-l-arginine methyl ester (l-NAME) in the drinking water (12.5 mg/l) to induce hypertension and renal disease. Mean arterial blood pressure (MAP) was significantly higher (by over 60 mmHg) in the male FHH compared with BN rats. Urinary protein and albumin excretion rates were increased by 15- and 40-fold, respectively, in the male FHH compared with the BN. Plasma renin activity was 10-fold higher in the FHH than the BN. Similar significant differences were observed between the female FHH and BN, but the degree of hypertension and proteinuria was of a lesser magnitude. Substitution of chromosome 20 from the BN to the FHH attenuated the development of l-NAME-induced hypertension, normalized plasma renin activity, and decreased plasma creatinine in male rats. In female rats, substitution of chromosome 15 decreased MAP and urinary protein excretion. Urinary excretion of albumin in males was decreased by substitution of chromosomes 1, 15, 16, and 18 from the BN into the FHH genetic background. The present data indicate that genes that can modify l-NAME-induced hypertension and proteinuria are on chromosomes 1, 15, 16, 18, and 20.

Evaluation of insulin resistance linkage to rat chromosome 4 in SHR of a Japanese colony

The spontaneously hypertensive rat (SHR) is a model of human insulin resistance syndrome. Quantitative trait loci for cellular defects in glucose and fatty acid metabolism have been mapped to an overlapping region of rat chromosome (RNO) RNO4 in SHR of the National Institute of Health colony, where a deletion in the Cd36 gene has been implicated as the causative mutation of insulin resistance. The present study has examined the potential presence of RNO4 linkage to a series of metabolic phenotypes in F(2) progeny derived from SHR of a Japanese colony (SHR/Izm) without the Cd36 mutation. Our data demonstrate that 'major' insulin resistance gene(s) are unlikely to exist on RNO4 in SHR/Izm and in vitro phenotypes measured in isolated adipocytes do not cosegregate in the F(2) population studied. Thus, it seems to be difficult to explain the underlying genetic mechanisms of insulin resistance by a single major gene on RNO4.

Quantitative trait locus analysis of serum insulin, triglyceride, total cholesterol and phospholipid levels in the (SM/J x A/J)F2 mice

Quantitative trait locus (QTL) analysis of serum insulin, triglyceride, total cholesterol and phospholipid levels at 10 weeks of age was performed in 321 F2 offspring from SM/J and A/J mice. Interval mapping revealed a total of 22 suggestive QTLs affecting the four traits: two insulin QTLs on Chromosomes (Chrs) 6 and 8; six triglyceride QTLs on Chrs 4, 8, 9, 11, 12 and 19; six total-cholesterol QTLs on Chrs 1, 3, 4, 14, 17 and 19; and eight phospholipid QTLs on Chrs 2, 3, 4, 6, 8, 10 and 19. Gender influenced the expression of eight of the suggestive QTLs. The total-cholesterol QTLs on Chrs 4, 14 and 17, the triglyceride QTL on Chr 9 and the phospholipid QTL on Chr 4 were specific to females. The phospholipid QTLs on Chrs 2 and 6 and the insulin QTL on Chr 8 were specific to males. In addition, common QTLs involved in the regulation of some of the traits were identified. The female-specific QTL on Chr 4 appeared to be involved in the regulation of total cholesterol and phospholipid levels. The QTL on Chr 8 affected insulin and phospholipid levels, whereas the Chr 19 QTL was common to the three lipid parameters.

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.

Quantitative trait loci for body weight in the intercross between SM/J and A/J mice

We performed a genome-wide quantitative trait locus (QTL) analysis of body weight at 10 weeks of age in a population of 321 intercross offspring from SM/J and A/J mice, progenitor strains of SMXA recombinant inbred strains. Interval mapping revealed two significant QTLs, Bwq3 (body weight QTL3) and Bwq4, on Chromosomes (Chrs) 8 and 18 respectively, and five suggestive QTLs on Chrs 2, 6, 7, 15 and 19. Bwq3 and Bwq4 explained 6% of the phenotypic variance. The SM/J alleles at both QTLs increased body weight, though the SM/J mouse was smaller than the A/J mouse. On the other hand, four of the five suggestive QTLs detected had male-specific effects on body weight and the remainder was female-specific. These suggestive QTLs explained 5-6% of the phenotypic variance and all the SM/J alleles decreased body weight.

Genetic analysis of inherited hypertension in the rat

Blood pressure is a quantitative trait that has a strong genetic component in humans and rats. Several selectively bred strains of rats with divergent blood pressures serve as an animal model for genetic dissection of the causes of inherited hypertension. The goal is to identify the genetic loci controlling blood pressure, i.e., the so-called quantitative trait loci (QTL). The theoretical basis for such genetic dissection and recent progress in understanding genetic hypertension are reviewed. The usual paradigm is to produce segregating populations derived from a hypertensive and normotensive strain and to seek linkage of blood pressure to genetic markers using recently developed statistical techniques for QTL analysis. This has yielded candidate QTL regions on almost every rat chromosome, and also some interactions between QTL have been defined. These statistically defined QTL regions are much too large to practice positional cloning to identify the genes involved. Most investigators are, therefore, fine mapping the QTL using congenic strains to substitute small segments of chromosome from one strain into another. Although impressive progress has been made, this process is slow due to the extensive breeding that is required. At this point, no blood pressure QTL have met stringent criteria for identification, but this should be an attainable goal given the recently developed genomic resources for the rat. Similar experiments are ongoing to look for genes that influence cardiac hypertrophy, stroke, and renal failure and that are independent of the genes for hypertension.

Genome scan for blood pressure loci in mice

Hypertension is a complex trait of unknown cause in humans. Mice of the inbred strain BPH/2 serve as a rodent model of human hypertension and display elevated blood pressure compared with the hypotensive strain BPL/1. An F2 intercross of BPH/2 and BPL/1 and 2 backcrosses of BPL/1 with Mus spretus were used to perform interval linkage mapping for systolic blood pressure in a genome scan. Significant linkage was observed in the F2s on chromosome 10 (logarithm of the odds score [LOD]=4.9) and on chromosome 13 in the M spretus backcross (LOD=3.3), with additional suggestive LODs on chromosomes 2, 6, 8, and 18. In addition, several suggestive linkages were observed for phenotypes associated with human hypertension. Our study is the first reported genome-wide linkage scan for blood pressure genes in the mouse.

Blood pressure, heart rate and motor activity in 6 inbred rat strains and wild rats (Rattus norvegicus): a comparative study

Inbreeding for many generations under optimal environmental conditions may have favoured the survival of alleles for blood pressure increase in phenotypically normotensive rat strains. To prove this hypothesis we measured telemetrically systolic (SBP) and diastolic blood pressure (DBP), heart rate (HR) and motor activity (MA) in 6 inbred rat strains (BB, BN, LEW, DA, F344, WKY) and wild rats most probably possessing all of the alleles for normotension. For the first time it is shown that systolic blood pressure can significantly differ between normotensive inbred rat strains and that most probably some inbred rat strains will be characterised by a systolic blood pressure found in their progenitors, the wild rats. In addition, the typical night activity of rodents was not seen in 2 inbred rat strains. All findings together may be interpreted in the sense that most, if not all inbred rats strains have more or less disturbances in blood pressure, HR and/or MA and that there is most probably no "healthy" inbred rat strain available so that wild rats may be an alternative for crossing studies dissecting hypertension in particular and diseases in general.

Genetic background determines the extent of atherosclerosis in ApoE-deficient mice

Two strains of ApoE-deficient mice were found to have markedly different plasma lipoprotein profiles and susceptibility to atherosclerosis when fed either a low-fat chow or a high-fat Western-type diet. FVB/NJ ApoE-deficient (FVB E0) mice had higher total cholesterol, HDL cholesterol, ApoA1, and ApoA2 levels when compared with C57BL/6J ApoE-deficient (C57 E0) mice. At 16 weeks of age, mean aortic root atherosclerotic lesion area was 7- to 9-fold higher in chow diet-fed C57 E0 mice and 3.5-fold higher in Western diet-fed C57 E0 mice compared with FVB E0 mice fed similar diets. Lesion area in chow diet-fed first-generation mice from a strain intercross was intermediate in size compared with parental values. The distribution of the lesion area in 150 chow diet-fed second-generation progeny spanned the range of the lesion area in both parental strains. There were no correlations between total cholesterol, non-HDL cholesterol, HDL cholesterol, ApoA1, ApoA2, ApoJ, or anti-cardiolipin antibodies and lesion area in the second-generation progeny. Thus, a genomic approach may succeed in identifying the genes responsible for the variation in atherosclerosis susceptibility in these 2 strains of ApoE-deficient mice, which could not be explained by measured plasma parameters.

Use of animal models to search for candidate genes associated with essential hypertension

The use of inbred genetically hypertensive animal models enables the dissection of the underlying complex genetic traits into its individual components, and thus the elucidation and characterization of causative genes and gene variants. In addition, genetically hypertensive animal models will also be useful for the investigation of genetic characteristics that influence the effectiveness of antihypertensive therapy with specific pharmacologic agents. This report will discuss three different strategies that have recently been used for the identification of candidate gene loci or candidate genes for hypertension. The possibility to transfer of genetic data derived in animal models to human hypertension will also be considered.