Fine-mapping and comprehensive transcript analysis reveals nonsynonymous variants within a novel 1.17 Mb blood pressure QTL region on rat chromosome 10

Dissecting Epistatic QTL for Blood Pressure in Rats: Congenic Strains versus Heterogeneous Stocks, a Reality Check

Advances in molecular genetics have provided well-defined physical genetic maps and large numbers of genetic markers for both model organisms and humans. It is now possible to gain a fundamental understanding of the genetic architecture underlying quantitative traits, of which blood pressure (BP) is an important example. This review emphasizes analytical techniques and results obtained using the Dahl salt-sensitive (S) rat as a model of hypertension by presenting results in detail for three specific chromosomal regions harboring genetic elements of increasing complexity controlling BP. These results highlight the critical importance of genetic interactions (epistasis) on BP at all levels of structure, intragenic, intergenic, intrachromosomal, interchromosomal, and across whole genomes. In two of the three examples presented, specific DNA structural variations leading to biochemical, physiological, and pathological mechanisms are well defined. This proves the usefulness of the techniques involving interval mapping followed by substitution mapping using congenic strains. These classic techniques are compared to newer approaches using sophisticated statistical analysis on various segregating or outbred model-organism populations, which in some cases are uniquely useful in demonstrating the existence of higher-order interactions. It is speculated that hypertension as an outlier quantitative phenotype is dependent on higher-order genetic interactions. The obstacle to the identification of genetic elements and the biochemical/physiological mechanisms involved in higher-order interactions is not theoretical or technical but the lack of future resources to finish the job of identifying the individual genetic elements underlying the quantitative trait loci for BP and ascertaining their molecular functions. © 2019 American Physiological Society. Compr Physiol 9:1305-1337, 2019.

Attenuation of Microbiotal Dysbiosis and Hypertension in a CRISPR/Cas9 Gene Ablation Rat Model of GPER1

G-protein-coupled estrogen receptor, Gper1, has been implicated in cardiovascular disease, but its mechanistic role in blood pressure control is poorly understood. Here, we demonstrate that genetically salt-sensitive hypertensive rats with complete genomic excision of Gper1 by a multiplexed guide RNA CRISPR (Clustered Regularly Interspaced Short Palindromic Repeats)/Cas9 (CRISPR associated proteins) approach present with lower blood pressure, which was accompanied by altered microbiota, different levels of circulating short chain fatty acids, and improved vascular relaxation. Microbiotal transplantation from hypertensive Gper1+/+ rats reversed the cardiovascular protective effect exerted by the genomic deletion of Gper1. Thus, this study reveals a role for Gper1 in promoting microbiotal alterations that contribute to cardiovascular pathology. However, the exact mechanism by which Gper1 regulates blood pressure is still unknown. Our results indicate that the function of Gper1 is contextually dependent on the microbiome, whereby, contemplation of using Gper1 as a target for therapy of cardiovascular disease requires caution.

Targeted disruption of regulated endocrine-specific protein ( Resp18) in Dahl SS/Mcw rats aggravates salt-induced hypertension and renal injury

Hypertension is a classic example of a complex polygenic trait, impacted by quantitative trait loci (QTL) containing candidate genes thought to be responsible for blood pressure (BP) control in mammals. One such mapped locus is on rat chromosome 9, wherein the proof for a positional candidate gene, regulated endocrine-specific protein-18 ( Resp18) is currently inadequate. To ascertain the status of Resp18 as a BP QTL, a custom targeted gene disruption model of Resp18 was developed on the Dahl salt-sensitive (SS) background. As a result of this zinc-finger nuclease (ZFN)-mediated disruption, a 7 bp deletion occurred within exon 3 of the Resp18 locus. Targeted disruption of Resp18 gene locus in SS rats decreases its gene expression in both heart and kidney tissues regardless of their dietary salt level. Under a high-salt dietary regimen, both systolic and diastolic BP of Resp18^mutant rats were significantly increased compared with SS rats. Resp18^mutant rats demonstrated increased renal damage, as evidenced by higher proteinuria and increased renal fibrosis compared with SS rats. Furthermore, under a high-salt diet regimen, the mean survival time of Resp18^mutant rats was significantly reduced compared with SS rats. These findings serve as evidence in support of Resp18 as a gene associated with the development of hypertension and renal disease.

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.

Positional cloning of quantitative trait nucleotides for blood pressure and cardiac QT-interval by targeted CRISPR/Cas9 editing of a novel long non-coding RNA

Multiple GWAS studies have reported strong association of cardiac QT-interval to a region on HSA17. Interestingly, a rat locus homologous to this region is also linked to QT-intervals. The high resolution positional mapping study located the rat QT-interval locus to a <42.5kb region on RNO10. This region contained no variants in protein-coding sequences, but a prominent contiguous 19bp indel polymorphism was noted within a novel predicted long non-coding RNA (lncRNA), which we named as Rffl-lnc1. To assess the candidacy of this novel lncRNA on QT-interval, targeted CRISPR/Cas9 based genome-engineering approaches were applied on the rat strains used to map this locus. Targeted disruption of the rat Rffl-lnc1 locus caused aberrant, short QT-intervals and elevated blood pressure. Further, to specifically examine the significance of the 19bp polymorphism within the Rffl-lnc1 locus, a CRISPR/Cas9 based targeted knock-in rescue model was constructed by inserting the 19bp into the strain which contained the deletion polymorphism. The knock-in alleles successfully rescued the aberrant QT-interval and blood pressure phenotypes. Further studies revealed that the 19bp polymorphism was necessary and sufficient to recapitulate the phenotypic effect of the previously mapped <42.5kb rat locus. To our knowledge, this study is the first demonstration of a combination of both CRISPR/Cas9 based targeted disruption as well as CRISPR/Cas9 based targeted knock-in rescue approaches applied for a mammalian positional cloning study, which defines the quantitative trait nucleotides (QTNs) within a rat long non-coding RNA as being important for the pleiotropic regulation of both cardiac QT-intervals and blood pressure.

Diseases of the cardiovascular system such as essential hypertension do not have a clear cause, but are known to run in families. The inheritance patterns of essential hypertension and other cardiac diseases suggest that they are not due to a single defective gene but instead are caused by multiple genetic defects that are inherited together in a patient. This complex inheritance makes it difficult to pinpoint the underlying defects. Here, we describe a panel of genetically-engineered rats, using which we have discovered a novel gene, which does not code for any protein, as a gene required for maintenance of normal blood pressure. Structural defects within this non-coding RNA cause hypertension and cardiac short-QT interval. Further, by performing genome surgery to correct the gene defect, we demonstrate the precise error in nucleotides that was inherited and caused hypertension and cardiac short-QT interval syndrome.

Deep Transcriptomic Profiling of M1 Macrophages Lacking Trpc3

In previous studies using mice with macrophage-specific loss of TRPC3 we found a significant, selective effect of TRPC3 on the biology of M1, or inflammatory macrophages. Whereas activation of some components of the unfolded protein response and the pro-apoptotic mediators CamkII and Stat1 was impaired in Trpc3-deficient M1 cells, gathering insight about other molecular signatures within macrophages that might be affected by Trpc3 expression requires an alternative approach. In the present study we conducted RNA-seq analysis to interrogate the transcriptome of M1 macrophages derived from mice with macrophage-specific loss of TRPC3 and their littermate controls. We identified 160 significantly differentially expressed genes between the two groups, of which 62 were upregulated and 98 downregulated in control vs. Trpc3-deficient M1 macrophages. Gene ontology analysis revealed enrichment in processes associated to cellular movement and lipid signaling, whereas the enriched Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways included networks for calcium signaling and cell adhesion molecules, among others. This is the first deep transcriptomic analysis of macrophages in the context of Trpc3 deficiency and the data presented constitutes a unique resource to further explore functions of TRPC3 in macrophage biology.

High-resolution mapping of a novel rat blood pressure locus on chromosome 9 to a region containing the Spp2 gene and colocalization of a QTL for bone mass

Through linkage analysis of the Dahl salt-sensitive (S) rat and the spontaneously hypertensive rat (SHR), a blood pressure (BP) quantitative trait locus (QTL) was previously located on rat chromosome 9. Subsequent substitution mapping studies of this QTL revealed multiple BP QTLs within the originally identified logarithm of odds plot by linkage analysis. The focus of this study was on a 14.39 Mb region, the distal portion of which remained unmapped in our previous studies. High-resolution substitution mapping for a BP QTL in the setting of a high-salt diet indicated that an SHR-derived congenic segment of 787.9 kb containing the gene secreted phosphoprotein-2 (Spp2) lowered BP and urinary protein excretion. A nonsynonymous G/T polymorphism in the Spp2 gene was detected between the S and S.SHR congenic rats. A survey of 45 strains showed that the T allele was rare, being detected only in some substrains of SHR and WKY. Protein modeling prediction through SWISSPROT indicated that the predicted protein product of this variant was significantly altered. Importantly, in addition to improved cardiovascular and renal function, high salt-fed congenic animals carrying the SHR T variant of Spp2 had significantly lower bone mass and altered bone microarchitecture. Total bone volume and volume of trabecular bone, cortical thickness, and degree of mineralization of cortical bone were all significantly reduced in congenic rats. Our study points to opposing effects of a congenic segment containing the prioritized candidate gene Spp2 on BP and bone mass.

Pleiotropic Effect of a High Resolution Mapped Blood Pressure QTL on Tumorigenesis

This study is focused on a translationally significant, genome-wide-association-study (GWAS) locus for cardiovascular disease (QT-interval) on human chromosome 17. We have previously validated and high resolution mapped the homologous genomic segment of this human locus to <42.5 kb on rat chromosome 10. This <42.5 kb segment in rats regulates both QT-interval and blood pressure and contains a single protein-coding gene, rififylin (Rffl). The expression of Rffl in the hearts and kidneys is differential between Dahl S and S.LEW congenic rats, which are the strains used for mapping this locus. Our previous study points to altered rate of endocytic recycling as the underlying mechanism, through which Rffl operates to control both QT-interval and blood pressure. Interestingly, Rffl also contributes to tumorigenesis by repressing caspases and tumor suppressor genes. Moreover, the expression of Methyl-CpG Binding Domain Protein 2 (Mbd2) in the hearts and kidneys is also higher in the S.LEW congenic strain than the background (control) Dahl S strain. Mbd2 can repress methylated tumor suppressor genes. These data suggest that the S.LEW congenic strain could be more susceptible to tumorigenesis. To test this hypothesis, the S and S.LEW strains were compared for susceptibility to azoxymethane-induced colon tumors. The number of colon tumors was significantly higher in the S.LEW congenic strain compared with the S rat. Transcriptomic analysis confirmed that the chemical carcinogenesis pathway was significantly up-regulated in the congenic strain. These studies provide evidence for a GWAS-validated genomic segment on rat chromosome 10 as being important for the regulation of cardiovascular function and tumorigenesis.

Multiple blood pressure loci with opposing blood pressure effects on rat chromosome 1 in a homologous region linked to hypertension on human chromosome 15

Genetic dissection of blood pressure (BP) quantitative trait loci (QTLs) in rats has facilitated the fine-mapping of regions linked to the inheritance of hypertension. The goal of the current study was to further fine-map one such genomic region on rat chromosome 1 (BPQTL1b1), the homologous region of which on human chromosome 15 harbors BP QTLs, as reported by four independent studies. Of the six substrains constructed and studied, the systolic BP of two of the congenic strains were significantly lower by 36 and 27 mm Hg than that of the salt-sensitive (S) rat (P < 0.0001, P = 0.0003, respectively). The congenic segments of these two strains overlapped between 135.12 and 138.78 Mb and contained eight genes and two predicted miRNAs. None of the annotations had variants within expressed sequences. These data taken together with the previous localization resolved QTL1b1 with a 70% improvement from the original 7.39 Mb to the current 2.247 Mb interval. Furthermore, the systolic BP of one of the congenic substrains was significantly higher by 20 mm Hg (P < 0.0001) than the BP of the S rat. The limits of this newly identified QTL with a BP increasing effect (QTL1b1a) were between 134.12 and 135.76 Mb, spanning 1.64 Mb, containing two protein-coding genes, Mctp2 and Rgma, and a predicted miRNA. There were four synonymous variants within Mctp2. These data provide evidence for two independent BP QTLs with opposing BP effects within the previously identified BP QTL1b1 region. Additionally, these findings illustrate the complexity underlying the genetic mechanisms of BP regulation, wherein inherited elements beyond protein-coding sequences or known regulatory regions could be operational.

Theoretical model for gene-gene, gene-environment, and gene-sex interactions based on congenic-strain analysis of blood pressure in Dahl salt-sensitive rats

There is a significant literature describing quantitative trait loci (QTL) controlling blood pressure (BP) in the Dahl salt-sensitive (S) rat. In studies to identify the genes underlying BP QTL it has been common practice to place chromosomal segments from low BP strains on the genetic background of the S rat and then reduce the congenic segments by substitution mapping. The present work suggests a model to simulate genetic interactions found using such congenic strains. The QTL are considered to be switches that can be either in series or in parallel represented by the logic operators AND or OR, respectively. The QTL switches can be on/off switches but are also allowed specific leak properties. The QTL switches are represented by a "universal" switch consisting of two molecules binding to form a complex. Genetic inputs enter the model as allelic products of one of the binding molecules and environmental variation (including dietary salt- and sex-related differences) enters as an influence on the concentration of the other binding molecule. The pairwise interactions of QTL are very well simulated and fall into recognizable patterns. There is, however, often more than one assumed model to predict a given pattern so that all patterns do not necessarily have a unique solution. Nevertheless, the models obtained provide a framework for placing the QTL in pathways relative to one another. Moreover, based on their leak properties pairs of QTL could be identified in which one QTL may alter the properties of the other QTL.

Isolation and high-throughput sequencing of two closely linked epistatic hypertension susceptibility loci with a panel of bicongenic strains

Interactions or epistasis between genetic factors may contribute to "missing heritability." While linkage analyses detect epistasis, defining the limits of the interacting segments poses a significant challenge especially when the interactions are between loci in close proximity. The goal of the present study was to isolate two such epistatic blood pressure (BP) loci on rat chromosome 5. A panel of S.LEW bicongenic strains along with the corresponding monocongenic strains was constructed. BP of each set comprising of one bicongenic and two corresponding monocongenic strains were determined along with the parental Salt-sensitive (S) strain. Epistasis was observed in one out of four sets of congenic strains, wherein systolic blood pressures (SBP) of the two monocongenic strains S.LEW(5)x6Bx9x5a and S.LEW(5)x6Bx9x5b were comparable to that of S, but the SBP of the bicongenic strain S.LEW(5)x6Bx9x5 (157 ± 4.3 mmHg) was significantly lower than that of S (196 ± 6.8 mmHg, P < 0.001). A two-way ANOVA indicated significant interactions between the LEW alleles at the two loci. The interacting loci were 2.02 Mb apart and located within genomic segments spanning 7.77 and 4.18 Mb containing 7,360 and 2,753 candidate variants, respectively. The current study demonstrates definitive evidence for epistasis and provides genetic tools for further dissection of the isolated epistatic BP loci.

Construction of two novel reciprocal conplastic rat strains and characterization of cardiac mitochondria

Because of the lack of appropriate animal models, the potentially causal contributions of inherited mitochondrial genomic factors to complex traits are less well studied compared with inherited nuclear genomic factors. We previously detected variations between the mitochondrial DNA (mtDNA) of the Dahl salt-sensitive (S) rat and the spontaneously hypertensive rat (SHR). Specifically, multiple variations were detected in mitochondrial genes coding for subunits of proteins essential for electron transport, in mitochondrial reactive oxygen species production, and within the D-loop region. To evaluate the effects of these mtDNA variations in the absence of the corresponding nuclear genomic factors as confounding variables, novel reciprocal strains of S and SHR were constructed and characterized. When compared with that of the S rat, the heart tissue from the S.SHR(mt) conplastic strain wherein the mtDNA of the S rat was substituted with that of the SHR had a significant increase in mtDNA copy number and decrease in mitochondrial reactive oxygen species production. A corresponding increase in aerobic treadmill running capacity and a significant increase in survival that was not related to changes in blood pressure were observed in the S.SHR(mt) rats compared with the S rat. The reciprocal SHR.S(mt) rats did not differ from the SHR in any phenotype tested, suggesting lower penetrance of the S mtDNA on the nuclear genomic background of the SHR. These novel conplastic strains serve as invaluable tools to further dissect the relationship between heart function, aerobic fitness, cardiovascular disease progression, and mortality.

Life or death? A physiogenomic approach to understand individual variation in responses to hemorrhagic shock

Severe hemorrhage due to trauma is a major cause of death throughout the world. It has often been observed that some victims are able to withstand hemorrhage better than others. For decades investigators have attempted to identify physiological mechanisms that distinguish survivors from nonsurvivors for the purpose of providing more informed therapies. As an alternative approach to address this issue, we have initiated a research program to identify genes and genetic mechanisms that contribute to this phenotype of survival time after controlled hemorrhage. From physiogenomic studies using inbred rat strains, we have demonstrated that this phenotype is a heritable quantitative trait, and is therefore a complex trait regulated by multiple genes. Our work continues to identify quantitative trait loci as well as potential epigenetic mechanisms that might influence survival time after severe hemorrhage. Our ultimate goal is to improve survival to traumatic hemorrhage and attendant shock via regulation of genetic mechanisms and to provide knowledge that will lead to genetically-informed personalized treatments.

Refined mapping of blood pressure quantitative trait loci using congenic strains developed from two genetically hypertensive rat models

Previously linkage and substitution mapping were conducted between the Dahl Salt-sensitive (S) rat and the Spontaneously Hypertensive Rat (SHR) to address the hypothesis that genetic contributions to blood pressure (BP) in two genetically hypertensive rat strains are different. Among the BP quantitative trait loci (QTLs) detected, two are located on chromosome 9 within large genomic segments. The goal of the current study was to develop new iterations of congenic substrains, to further resolve both of these BP QTLs on chromosome 9 as independent congenic segments. A total of 10 new congenic substrains were developed and characterized. The newly developed congenic substrains S.SHR(9)x8Ax11A and S.SHR(9)x10Ax1, with introgressed segments of 2.05 and 6.14 Mb, represented the shortest genomic segments. Both of these congenic substrains, S.SHR(9)x8Ax11A and S.SHR(9)x10Ax1 lowered BP of the S rat by 56 mm Hg (P<0.001) and 15 mm Hg (P<0.039), respectively. The BP measurements were corroborated by radiotelemetry. Urinary protein excretion was significantly lowered by SHR alleles within S.SHR(9)x10Ax1 but not by S.SHR(9)x8Ax11A. The shorter of the two congenic segments, 2.05 Mb was further characterized and found to contain a single differentially expressed protein-coding gene, Tomoregulin-2 (Tmeff2). The protein expression of Tmeff2 was higher in the S rat compared with S.SHR(9)x8Ax11A, which also had lower cardiac hypertrophy as measured by echocardiography. Tmeff2 is known to be upregulated in patients from multiple cohorts with cardiac hypertrophy. Taken together, Tmeff2 can be prioritized as a candidate gene for hypertension and associated cardiac hypertrophy in both rats and in humans.

Augmented rififylin is a risk factor linked to aberrant cardiomyocyte function, short-QT interval and hypertension

Using congenic strains of the Dahl salt-sensitive (S) rat introgressed with genomic segments from the normotensive Lewis rat, a blood pressure quantitative trait locus was previously mapped within 104 kb on chromosome 10. The goal of the current study was to conduct extensive phenotypic studies and to further fine-map this locus. At 14 weeks of age, the blood pressure of the congenic rats fed a low-salt diet was significantly higher by 47 mm Hg (P<0.001) compared with that of the S rat. A time-course study showed that the blood pressure effect was significant from very young ages of 50 to 52 days (13 mm Hg; P<0.01). The congenic strain implanted with electrocardiography transmitters demonstrated shorter-QT intervals and increased heart rate compared with S rats (P<0.01). The average survival of the congenic strain was shorter (134 days) compared with the S rat (175 days; P<0.0007). The critical region was narrowed to <42.5 kb containing 171 variants and a single gene, rififylin. Both the mRNA and protein levels of rififylin were significantly higher in the hearts of the congenic strain. Overexpression of rififylin is known to delay endocytic recycling. Endocytic recycling of fluorescently labeled holotransferrin from cardiomyocytes of the congenic strain was slower than that of S rats (P<0.01). Frequency of cardiomyocyte beats in the congenic strain (62±9 bpm) was significantly higher than that of the S rat (24±6 bpm; P<0.001). Taken together, our study provides evidence to suggest that early perturbations in endocytic recycling caused by the overexpression of Rffl is a novel physiological mechanism potentially underlying the development of hypertension.

Vascular responses in aortic rings of a consomic rat panel derived from the Fawn Hooded Hypertensive strain

The present experiments, utilizing the high-throughput vascular protocol of PhysGen (Program for Genomic Applications) characterized the responses of aortic rings to vasoconstrictor (phenylephrine) and vasodilator (acetylcholine, sodium nitroprusside, and reduced tissue bath Po(2)) stimuli in consomic rat strains derived from a cross between the Fawn Hooded Hypertensive rat (FHH/EurMcwi) and the Brown Norway normotensive (BN/NHsdMcwi) rat. The effects of substituting individual BN chromosomes into the FHH genetic background were determined in animals that were maintained on a low-salt (0.4% NaCl) diet or switched to a high-salt (4% NaCl) diet for 3 wk. Sex-specific differences were evaluated in male and female consomic rats on similar dietary salt intake. Multiple chromosomes affected various vascular reactivity phenotypes in the FHH × BN consomic panel, and substantial salt-dependent changes in vascular reactivity and sex-specific differences in aortic reactivity were observed in individual consomic strains. However, compared with earlier studies of consomic rats derived from a cross between the BN rat and the Dahl salt-sensitive (SS) rat, only 3-7% of the vascular phenotypes were affected in a similar manner by substituting specific BN chromosomeschromosomes into the FHH genetic background versus the SS genetic background. The findings of the present study stress the potential value of consomic rat panels in gaining insight into genetic factors influencing vascular reactivity and suggest that the chromosomes that appear to be involved in the determination of aortic ring reactivity in different rodent models of hypertension are highly strain- and sex specific.

Mitochondrial polymorphisms in rat genetic models of hypertension

Hypertension is a complex trait that has been studied extensively for genetic contributions of the nuclear genome. We examined mitochondrial genomes of the hypertensive strains: the Dahl Salt-Sensitive (S) rat, the Spontaneously Hypertensive Rat (SHR), and the Albino Surgery (AS) rat, and the relatively normotensive strains: the Dahl Salt-Resistant (R) rat, the Milan Normotensive Strain (MNS), and the Lewis rat (LEW). These strains were used previously for linkage analysis for blood pressure (BP) in our laboratory. The results provide evidence to suggest that variations in the mitochondrial genome do not account for observed differences in blood pressure between the S and R rats. However, variants were detected among the mitochondrial genomes of the various hypertensive strains, S, SHR, and AS, and also among the normotensive strains R, MNS, and LEW. A total of 115, 114, 106, 106, and 16 variations in mtDNA were observed between the comparisons S versus LEW, S versus MNS, S versus SHR, S versus AS, and SHR versus AS, respectively. Among the 13 genes coding for proteins of the electron transport chain, 8 genes had nonsynonymous variations between S, LEW, MNS, SHR, and AS. The lack of any sequence variants between the mitochondrial genomes of S and R rats provides conclusive evidence that divergence in blood pressure between these two inbred strains is exclusively programmed through their nuclear genomes. The variations detected among the various hypertensive strains provides the basis to construct conplastic strains and further evaluate the effects of these variants on hypertension and associated phenotypes.

Generation of rat "supersonic" congenic/conplastic strains using superovulation and embryo transfer

Congenic strains are routinely used for positional mapping of quantitative trait loci; while conplastic strains, derived by substitution of different mitochondrial genomes on the same nuclear genetic background of inbred rodent strains, provide a way to unambiguously isolate effects of the mitochondrial genome on complex traits. Derivation of congenic or conplastic strains using a traditional backcross breeding strategy (10 backcrosses) takes more than 3 years. There are two principal strategies to speed up this process: (1) marker-assisted derivation of "speed" congenic/conplastic strains and (2) derivation of "supersonic" congenic/conplastic strains using in each backcross generation embryos obtained from 4-week-old superovulated females; thus, each backcross generation takes only 7 weeks. Both strategies could also be combined. In the current chapter, a method for derivation of "supersonic" congenic/conplastic rat strains is described.

Use of rat genomics for investigating the metabolic syndrome

The spontaneously hypertensive rat (SHR) is the most widely used animal model of essential hypertension and accompanying metabolic disturbances. In this model, the use of whole genome sequencing and gene expression profiling techniques, linkage and correlation analyses in recombinant inbred strains, and in vitro and in vivo functional studies in congenic and transgenic lines has recently enabled molecular identification of quantitative trait loci (QTLs) relevant to the metabolic syndrome: (1) a deletion variant in Cd36 (fatty acid translocase) responsible for QTLs on chromosome 4 associated with dyslipidemia, insulin resistance and hypertension, (2) mutated Srebf1 (sterol regulatory element binding factor 1) as a QTL on chromosome 10 influencing dietary-induced changes in hepatic cholesterol levels, and (3) Ogn (osteoglycin) as a QTL on chromosome 17 associated with left ventricular hypertrophy. In addition, selective replacement of the mitochondrial genome of the SHR with the mitochondrial genome of the Brown Norway rat influenced several major metabolic risk factors for type 2 diabetes and provided evidence that spontaneous variation in the mitochondrial genome per se can promote systemic metabolic disturbances relevant to the pathogenesis of metabolic syndrome. Owing to recent progress in the development of rat genomic resources, the pace of QTL identification and discovery of new disease mechanisms can be expected to accelerate in the near future.

Epistatic genetic determinants of blood pressure and mortality in a salt-sensitive hypertension model

Although genetic determinants protecting against the development of elevated blood pressure (BP) are well investigated, less is known regarding their impact on longevity. We concomitantly assessed genomic regions of rat chromosomes 3 and 7 (RNO3 and RNO7) carrying genetic determinants of BP without known epistasis, for their independent and combinatorial effects on BP and the presence of genetic determinants of survival using Dahl salt-sensitive (S) strains carrying congenic segments from Dahl salt-resistant (R) rats. Although congenic and bicongenic S.R strains carried independent BP quantitative trait loci within the RNO3 and RNO7 congenic regions, only the RNO3 allele(s) independently affected survival. The bicongenic S.R strain showed epistasis between R-rat RNO3 and RNO7 alleles for BP under salt-loading conditions, with less-than-additive effects observed on a 2% NaCl diet and greater-than-additive effects observed after prolonged feeding on a 4% NaCl diet. These RNO3 and RNO7 congenic region alleles had more-than-additive effects on survival. Increased survival of bicongenic compared with RNO3 congenic rats was attributable, in part, to maintaining lower BP despite chronic exposure to an increased dietary salt (4% NaCl) intake, with both strains showing delays in reaching highest BP. R-rat RNO3 alleles were also associated with superior systolic function, with the S.R bicongenic strain showing epistasis between R-rat RNO3 and RNO7 alleles leading to compensatory hypertrophy. Whether these alleles affect survival by additional actions within other BP-regulating tissues/organs remains unexplored. This is the first report of simultaneous detection of independent and epistatic loci dictating, in part, longevity in a hypertensive rat strain.

Blood pressure and heart rate QTL in mice of the B6/D2 lineage: sex differences and environmental influences

A quantitative trait locus (QTL) approach was used to define the genetic architecture underlying variation in systolic blood pressure (SBP) and heart rate (HR), measured indirectly on seven occasions by the tail cuff procedure. The tests were conducted in 395 F(2) adult mice (197 males, 198 females) derived from a cross of the C57BL/6J (B6) and DBA/2J (D2) strains and in 22 BXD recombinant-inbred (RI) strains. Interval mapping of F(2) data for the first 5 days of measurement nominated one statistically significant and one suggestive QTL for SBP on chromosomes (Chr) 4 and 14, respectively, and two statistically significant QTL for HR on Chr 1 (which was specific to female mice) and Chr 5. New suggestive QTL emerged for SBP on Chr 3 (female-specific) and 8 and for HR on Chr 11 for measurements recorded several weeks after mice had undergone stressful blood sampling procedures. The two statistically significant HR QTL were confirmed by analyses of BXD RI strain means. Male and female F(2) mice did not differ in SBP or HR but RI strain analyses showed pronounced strain-by-sex interactions and a negative genetic correlation between the two measures in both sexes. Evidence for a role for mitochondrial DNA was found for both HR and SBP. QTL for HR and SBP may differ in males and females and may be sensitive to different environmental contexts.

Blood pressure and proteinuria effects of multiple quantitative trait loci on rat chromosome 9 that differentiate the spontaneously hypertensive rat from the Dahl salt-sensitive rat

A blood pressure (BP) quantitative trait locus (QTL) was previously located within 117 kb on rat chromosome 9 (RNO9) using hypertensive Dahl salt-sensitive and normotensive Dahl salt-resistant rats. An independent study between two hypertensive rat strains, the Dahl salt-sensitive rat and the spontaneously hypertensive rat (SHR), also detected a QTL encompassing this 117 kb region. Dahl salt-sensitive alleles in both of these studies were associated with increased BP. To map SHR alleles that decrease BP in the Dahl salt-sensitive rat, a panel of eight congenic strains introgressing SHR alleles onto the Dahl salt-sensitive genetic background were constructed and characterized. S.SHR(9)x3B, S.SHR(9)x3A and S.SHR(9)x2B, the congenic regions of which span a portion or all of the 1 logarithm of odds (LOD) interval identified by linkage analysis, did not significantly alter BP. However, S.SHR(9), S.SHR(9)x4A, S.SHR(9)x7A, S.SHR(9)x8A and S.SHR(9)x10A, the introgressed segments of which extend distal to the 1 LOD interval, significantly reduced BP. The shortest genomic segment, BP QTL1, to which this BP-lowering effect can be traced is the differential segment of S.SHR(9)x4A and S.SHR(9)x2B, to which an urinary protein excretion QTL also maps. However, the introgressed segment of S.SHR(9)x10A, located outside of this QTL1 region, represented a second BP QTL (BP QTL2) having no detectable effects on urinary protein excretion. In summary, the data suggest that there are multiple RNO9 alleles of the SHR that lower BP of the Dahl salt-sensitive rat with or without detectable effects on urinary protein excretion and that only one of these BP QTLs, QTL1, overlaps with the 117 kb BP QTL region identified using Dahl salt-sensitive and Dahl salt-resistant rats.

Submegabase resolution of epistatically interacting quantitative trait loci for blood pressure applicable for essential hypertension

OBJECTIVE

Although genetic mapping of quantitative trait loci for blood pressure to large chromosome segments is readily achievable, their final identification confronts formidable hurdles. Restriction of the genes lodging in one quantitative trait locus interval to experimental limitation can facilitate their positional cloning. We previously delineated several quantitative trait loci for blood pressure on chromosome 10 of Dahl salt-sensitive rats, but their chromosome delimitations were either large or not definitive.

METHODS

In this study, we systematically and comprehensively constructed congenic strains with submegabase (Mb) genome resolution and analyzed their blood pressure by telemetry.

RESULTS

Three quantitative trait loci have been conclusively delimited by three congenic strains, each independently lowering the blood pressure. Their intervals are demarcated by genomic regions between 350 and 910 kilobases (kb) in size. Two of the three quantitative trait loci share an epistatic relationship and are separated from one another by less than 170 kb. Two additional quantitative trait loci for blood pressure were also tentatively delineated and their intervals range from 520 kb to 1.75 Mb. Possible genes dwelling in each quantitative trait locus-interval number between 11 and 17. None of these genes is known to exert a functional impact on blood pressure. Work is underway to find candidate genes with mutations that could be responsible for the blood pressure effect.

CONCLUSION

Novel diagnostic, prognostic, preventive and/or therapeutic targets for essential hypertension and hypertension-associated diseases are likely to emerge from the identification of these quantitative trait loci. Potential applications of these quantitative trait loci to humans are suggested from the positive results from several association studies, demonstrating the existence of quantitative trait loci in the broad homologous regions.

Insight into the genetics of hypertension, a core component of the metabolic syndrome

PURPOSE OF REVIEW

To provide insight into genetics of essential hypertension, including discussion of methods used both in human and animal experimental studies and interpretation of results.

RECENT FINDINGS

On the basis of recent progress in sequencing of human genome, detection of millions of single nucleotide polymorphism markers, determination of the extend of linkage disequilibrium (haplotypes), efficient genotyping technology, collection of DNA from thousands of rigorously phenotyped patients and controls and designing sound statistical methods, genome-wide associations studies were widely applied to analyses of common diseases including essential hypertension for the first time in 2007. Concurrently, new experimental approaches combined gene expression profiling with linkage and correlation analyses to identify quantitative trait loci underlying complex traits at the molecular level.

SUMMARY

These new approaches yielded new exciting results but also posed questions regarding data analyses, interpretation and clinical significance.

Closely linked non-additive blood pressure quantitative trait loci

There is enough evidence through linkage and substitution mapping to indicate that rat chromosome 1 harbors multiple blood pressure (BP) quantitative trait loci (QTLs). Of these, BP QTL1b was previously reported from our laboratory using congenic strains derived by introgressing normotensive alleles from the LEW rat onto the genetic background of the hypertensive Dahl salt-sensitive (S) rat. The region spanned by QTL1b is quite large (20.92 Mb), thus requiring further mapping with improved resolution so as to facilitate systematic identification of the underlying genetic determinant(s). Using congenic strains containing the LEW rat chromosomal segments on the Dahl salt-sensitive (S) rat background, further iterations of congenic substrains were constructed and characterized. Collective data obtained from this new iteration of congenic substrains provided evidence for further fragmentation of QTL1b with improved resolution. At least two separate genetic determinants of blood pressure underlie QTL1b. These are within 7.40 Mb and 7.31 Mb and are known as the QTL1b1 region and the QTL1b2 region, respectively. A genetic interaction was detected between the two BP QTLs. Interestingly, five of the previously reported differentially expressed genes located within the newly mapped QTL1b1 region remained differentially expressed. The congenic strain S.LEW(D1Mco36-D1Mco101), which harbors the QTL1b1 region alone but not the QTL1b2 region, serves as a genetic tool for further dissection of the QTL1b1 region and validation of Nr2f2 as a positional candidate gene. Overall, this study represents an intermediary yet obligatory progression towards the identification of genetic elements controlling BP.

A twofold genetic increase of ACE expression has no effect on the development of spontaneous hypertension

BACKGROUND

To study the regulation of a naturally occurring genetic variant of high angiotensin-converting enzyme (ACE) gene (Ace in rat) expression, i.e., the Ace allele of the normotensive Wistar-Kyoto (WKY) rat, in the hypertensive background of stroke-prone spontaneously hypertensive (SHRSP) rats.

METHODS

We analyzed a congenic strain termed SHRSP.WKY-Ace derived from SHRSP in which a chromosomal fragment of rat chromosome 10 including Ace was replaced by the WKY locus. We compared blood pressures by radiotelemetry, measured plasma ACE activity, tissue ACE messenger RNA (mRNA) and enzyme activities in lung, kidney, and left ventricle (LV) of the heart in adult animals.

RESULTS

Congenic animals demonstrated a twofold increase in plasma ACE activity in comparison to SHRSP (P < 0.05) and thus similar levels to WKY. The increased tissue expression of ACE mRNA and enzyme activities in lung, kidney, and LV observed in WKY were similarly found in congenic animals when compared to SHRSP (P < 0.05, respectively). Systolic and diastolic blood pressures were not different between congenic and SHRSP animals. Analysis of renin in plasma and angiotensin peptides in LV tissues indicated the induction of compensatory mechanisms by downregulation of renin and angiotensin I (Ang I) concentrations in congenic animals.

CONCLUSIONS

We demonstrated that genetically determined high ACE expression linked to WKY Ace remains unchanged in the hypertensive background of SHRSP.WKY-Ace. Our data indicate that buffering mechanisms in the renin-angiotensin system contribute to the finding that the development of spontaneous hypertension is not affected, despite an average twofold higher expression of ACE in congenic animals.

Congenic mapping of a blood pressure QTL region on rat chromosome 10 using the Dahl salt-sensitive rat with introgressed alleles from the Milan normotensive strain

Multiple blood pressure (BP) quantitative trait loci (QTLs) are reported on rat chromosome 10 (RNO10). Of these, QTLs detected by contrasting the genome of the hypertensive Dahl salt-sensitive (S) rat with two different relatively normotensive strains, Lewis (LEW) and the Milan normotensive strain (MNS), are reported. Because the deduced QTL regions of both S vs. LEW and S vs. MNS comparisons are within large genomic segments encompassing more than 2 cM, there was a need to further localize these QTLs and determine whether the QTLs are unique to specific strain comparisons. Previously, the S.MNS QTL1 was mapped to less than 2.6 cM as a differential segment between two congenic strains. In this study, multiple congenic strains spanning the projected interval were studied. The BP effect of each strain was interpreted as the net effect of alleles introgressed within that congenic strain. The results suggest that the MNS alleles within the previously proposed differential segment (D10Rat27-D10Rat24) do not independently lower BP of the S rat. However, another congenic strain, S.MNS(10) x 9, containing introgressed MNS alleles that are outside of the previously proposed differential segment is of interest because (1) it demonstrated a BP-lowering effect, (2) it is contained within a single congenic strain and is not based on the observed effect of a differential segment, and, more importantly, (3) it overlaps with the previously identified S.LEW BP QTL region. Identification of the same QTL affecting BP in multiple rat strains will provide further support for the QTL's involvement and importance in human essential hypertension.

Congenic interval mapping of RNO10 reveals a complex cluster of closely-linked genetic determinants of blood pressure

Genetic dissection of the rat genome for identifying alleles that cause abnormalities in blood pressure (BP) resulted in the mapping of a significant number of BP quantitative trait loci (QTLs). In this study we mapped at least one such BP QTL on rat chromosome 10 (RNO10) as being within the introgressed segment of a S.LEW congenic strain S.LEWx12x2x3x8 spanning 1.34 Mb from 70,725,437 bp to 72,063,232 bp. BP of 3 congenic strains that span shorter segments of this region was additionally examined. Results obtained indicate that LEW alleles that comprise a 375-kb introgressed segment of the congenic strain S.LEWx12x2x3x5 (70,725,437 bp to 71,100,513 bp) increase BP. The magnitude of change in BP exhibited by the 2 strains, S.LEWx12x2x3x8 and S.LEWx12x2x3x5, is the net phenotypic effect of the underlying genetic determinants of BP. In this respect, the current data are superior to previous QTL localization of BP QTL1, which was hypothesized based on differential congenic segments. Genetic dissection using these 2 congenic strains as tools is expected to facilitate further dissection of the regions. Meanwhile, differential congenic segments were used to predict and thereby prioritize regions for candidate gene analysis. Using this approach, 2 distinct regions of 401 kb and 409 kb within S.LEWx12x2x3x8 and a 104 kb region within S.LEWx12x2x3x5 were prioritized for further consideration. Because all of these genetic elements are located within a 1.06-Mb region of RNO10, our study has revealed a remarkable insight into a genomic module comprising very closely-linked, opposing genetic determinants of BP.