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.     

Locating a blood pressure quantitative trait locus within 117 kb on the rat genome: substitution mapping and renal expression analysis

Previously, a blood pressure (BP) quantitative trait locus (QTL) on rat chromosome 9 (RNO9) was localized to a <2.4 cM interval using congenic strains generated by introgressing segments of RNO9 from the Dahl salt-resistant (R) rat into the background of the Dahl salt-sensitive (S) rat. Renal gene expression using Affymetrix gene chips was profiled on S and a congenic strain spanning the 2.4-cM BP QTL interval. This analysis identified 20 differentially expressed genes/expressed sequence tags. Of these, the locus with the greatest differential expression (30- to 35-fold) was regulated endocrine-specific protein 18 (Resp18), which also mapped in the 2.4-cM BP QTL interval. Additional substitution mapping located the QTL to <0.4 cM or approximately 493 kb. This newly defined QTL region still included Resp18. Nucleotide variants were identified between S and R genomic DNA of Resp18 in the coding, 5' regulatory and 3' untranslated regions. The coding sequence variation (T/C) occurs in exon 2 and predicts an amino acid change (Ile/Val) in the protein product. Resp18 was considered a differentially expressed positional candidate for the QTL. To fine-map the BP QTL, we constructed a congenic strain with a smaller introgressed region. Compared with the S rat, this strain (1) had significantly lower BP, (2) did not contain the R form of Resp18, and (3) did not retain the rather spectacular differential expression of Resp18. Together, these results demonstrate that a BP QTL independent of Resp18 exists within the newly defined 117-kb QTL region on RNO9.     

Substitution mapping of a blood pressure quantitative trait locus to a 2.73 Mb region on rat chromosome 1

OBJECTIVE: To improve the localization of a blood pressure quantitative trait locus (BP QTL) on rat chromosome (RNO) 1. METHODS: Congenic substrains were derived from the progenitor congenic strains S.LEW(D1Mco4X1) and S.LEW(D1Mco4X5) which previously localized a BP QTL (region 2) to a 17cM interval on RNO1. The newly developed congenic substrains, along with control Dahl salt-sensitive (S) rats were fed a 2% NaCl diet for 24 days before their BP was compared by both tail-cuff and radiotelemetry methods. RESULTS: By comparing BP of these congenic substrains to that of S rats, we have refined the location of the BP QTL2 region to a 2.73 Mb genomic interval that contains 19 annotated genes in the latest rat genome assembly (version 2.1). Slc9a3, the gene encoding the Na(+)/H(+) exchanger 3, originally a candidate gene in the BP QTL2 region, is excluded based on its map location. CONCLUSION: Substitution mapping was used to reduce a BP QTL on RNO1 from 17 centimorgans (cM) to approximately 1.4 cM (= 2.73 Mb). This region now contains 19 annotated rat candidate genes.     

Genetic dissection of region around the Sa gene on rat chromosome 1: evidence for multiple loci affecting blood pressure

A region with a major effect on blood pressure (BP) is located on rat chromosome 1 in the vicinity of the Sa gene, a candidate gene for BP regulation. Previously, we observed a single linkage peak for BP in this region in second filial generation rats derived from a cross of the spontaneously hypertensive rat (SHR) with the Wistar-Kyoto rat (WKY), and we have reported the isolation of the region containing the BP effect in reciprocal congenic strains (WKY.SHR-Sa) and (SHR.WKY-Sa) derived from these animals. Here, we report the further genetic dissection of this region. Two congenic substrains each were derived from WKY.SHR-Sa (WISA1 and WISA2) and SHR.WKY-Sa (SISA1 and SISA2) by backcrossing to WKY and SHR, respectively. Although there was some overlap of the introgressed regions retained in the various substrains, the segments in WISA1 and SISA1 did not overlap. Furthermore, although the Sa allele in WISA1, WISA2, and SISA2 remained donor in origin, recombination in SISA1 reverted it back to the recipient (SHR) allele. Surprisingly, all 4 substrains demonstrated a highly significant BP difference compared with that of their respective parental strain, which was of a magnitude similar to those seen in the original congenic strains. The findings strongly indicate that there are at least 2 quantitative trait loci (QTLs) affecting BP in this region of rat chromosome 1. Furthermore, the BP effect seen in SISA1 indicates that at least a proportion of the BP effect of this region of rat chromosome 1 cannot be due to the Sa gene. SISA1 contains an introgressed segment of <3 cM, and this will facilitate the physical mapping of the BP QTL(s) located within it and the identification of the susceptibility-conferring genes. Our observations serve to illustrate the complexity of QTL dissection and the care needed to interpret findings from congenic studies.     

Comprehensive congenic coverage revealing multiple blood pressure quantitative trait loci on Dahl rat chromosome 10

Chromosome mapping based on congenic strains can restrict quantitative trait loci (QTLs) for blood pressure (BP) into small intervals that are otherwise indistinguishable in linkage analysis. Also, congenic strains can be created to test a candidate gene to be a BP QTL. Taking full advantage of these features, we produced 10 congenic strains by replacing various segments of chromosome (Chr) 10 of the Dahl salt-sensitive (DSS) rat with those of the Lewis (LEW) rat. These strains were made to systematically cover an entire section of Chr 10. Three of the strains were designed to narrow the intervals that harbor previously mapped QTL1 and QTL2. Two of the strains were designed for the express purpose of testing the QTL candidacy of loci for inducible nitric oxide synthase (Nos2) and angiotensin-converting enzyme (Ace) genes. BPs of these strains were measured by telemetry and compared with those of the DSS rat. Consequently, QTL1 and QTL2 were narrowed to segments of 53.5 and 100.4 centiRays, respectively. A new QTL, QTL3, was found between QTL1 and QTL2. Both Nos2 and Ace have been disqualified as QTLs in the DSS and LEW comparison. Therefore, there are no obvious candidate genes in the segments that harbor these 3 QTLs, which represent genes previously not thought to be involved in BP regulation. These QTLs will likely have an influence on studies of human hypertension because of their homology with the human CHR 17 region in which QTLs for BP have been found.     

Two linked blood pressure quantitative trait loci on chromosome 10 defined by dahl rat congenic strains

Aquantitative trait locus (QTL) for blood pressure was previously detected on rat chromosome 10 (RNO10) by linkage analysis and confirmed by the construction of congenic strains that encompass large regions of RNO10. In the present study, the rat RNO10 blood pressure QTL was dissected by the further construction of congenic substrains. The original congenic region was shown to contain 2 blood pressure QTLs (QTL 1 and QTL 2) approximately 24 cM apart. These were localized to a <2.6-cM region between markers D10Rat27 and D10Rat24 for QTL 1 and to a <3.2-cM region between D10Rat12 and D10Mco70 for QTL 2. Comparative mapping suggests that the rat RNO10 QTL 2 could be localized very close to a blood pressure QTL described by sib-pair analysis on human chromosome 17, but this is not definitively established because of multiple and complex chromosomal rearrangements between rodents and humans.     

Dissecting quantitative trait loci into opposite blood pressure effects on Dahl rat chromosome 8 by congenic strains

OBJECTIVE: Our previous linkage analyses showed that there was likely a quantitative trait locus (QTL) for blood pressure (BP) on chromosome 8 (Chr 8) in the strain comparison between the Dahl salt-sensitive (S) and the Lewis (LEW) rats. The current work is to delineate the chromosome interval harboring this QTL by using congenic strains with different chromosome substitutions. METHODS: Two congenic strains were produced by replacing different segments of the S rats with the homologous segments of the LEW rats. A genome-wide marker screening was utilized to accelerate this process. The two strains generated are designated as C8S.L1 and C8S.L2, respectively. BPs of the rats were measured by telemetry. RESULTS: C8S.L1 showed a BP lower than that of S rats. In contrast, C8S.L2 did not have chromosome overlaps with C8S.L1, but unexpectedly, exhibited a BP-raising effect, higher than that of S rats. CONCLUSION: There are at least two QTLs present in a section of Chr 8 that possess opposite BP effects. The current congenic work reveals not only the presence of QTLs, but the complexity of QTLs on BP. The novel congenic strain with hypertension more severe than S provides a new model for studies in elucidating physiological mechanisms controlling BP.     

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.     

Combining congenic coverage with gene profiling in search of candidates for blood pressure quantitative trait loci in Dahl rats.

Chromosomes (Chr) 10 and 16 of the Dahl salt-sensitive (S) rat harbor quantitative trait loci (QTLs) for blood pressure (BP). To facilitate gene discovery of these QTLs, gene profiling based on microarrays was combined with fine QTL mapping to identify potential candidate genes that are differentially expressed. First, the region harboring the BP QTL on Chr 16 was narrowed by comparative congenic mapping. In this endeavor, a number of new chromosome markers were generated and used to physically define the chromosome interval in question. Second, in an effort to minimize the costs of gene profiling without sacrificing the chance of gene discovery, a combination congenic strain was produced by replacing one segment of Chr 10 along with one segment of Chr 16 of the hypertensive S rat by those of the normotensive Lewis (LEW) rat. Both of these regions are known to contain BP QTLs. Third, kidneys of this combination congenic strain and the S strain were employed for expression profiling studies. Finally, a comparison between the two strains yielded a number of potentially differentially expressed candidates. Six Established Sequence Tags (ESTs)/genes among them were located in Chr 10 regions and 1 was found in a Chr 16 region, and the genetic make-ups of all these regions were shown to be different between S and LEW. However, none of these ESTs/genes identified by gene profiling were located in an interval containing a QTL. Thus, the present study highlights the importance of correlating the results of gene expression profiling with fine congenic mapping.     

Isolation of a chromosome 1 region affecting blood pressure and vascular disease traits in the stroke-prone rat model

Recently, a genome-wide screen has shown a major quantitative trait locus (QTL) for a stroke-associated phenotype on rat chromosome 1 (RNO1) independent of QTL for blood pressure (BP) in the stroke-prone spontaneously hypertensive rat (SHRSP) of a Heidelberg colony. However, it remains to be elucidated whether these observations reflect the existence of different genes predisposing to each of the disorders. To address this issue, we performed comprehensive approaches in a Japanese colony, Izm, as follows. First, we undertook genome-wide searches in F1(SHRSP/IzmxWKY/Izm)xSHRSP/Izm back-cross (n=63) to pursue a causal relation between hypertension and stroke. Although the strongest linkage to BP (LOD score of 3.4) was identified on RNO1, its relevance to stroke was not supported in the F1 back-cross studied. Second, we also investigated linkage to BP in F2 progeny (n=175) involving the stroke-resistant (or normal) spontaneously hypertensive rat (SHR). In F2 studies of SHR/Izm, this locus did not appear to constitute a principal BP QTL. Third, we constructed congenic animals with detailed phenotype characterization. Transfer of a chromosomal fragment between markers Klk1 and D1Rat116 from WKY/Izm onto the SHRSP/Izm background lowered systolic BP by 20 to 80 mm Hg, prevented development of apparent stroke, and exaggerated impaired glucose tolerance. In conclusion, we have successfully isolated an RNO1 region affecting BP, stroke, and glucose tolerance in SHRSP/Izm-derived congenic rats. The size of the introgressed region is large, but our novel congenic strain should help delineate complex, genetic impairments underlying BP and associated vascular disease phenotypes.     

Accelerated congenics for mapping two blood pressure quantitative trait loci on chromosome 10 of Dahl rats.

OBJECTIVE : To localize quantitative trait loci (QTL) in an animal model that is potentially relevant to human hypertension. DESIGN AND METHODS : Four congenic strains have been constructed by replacing various segments of the Dahl salt-sensitive (S) rat by those of the Lewis (LEW) rat. A marker-assisted approach was employed to facilitate this process. When these congenic strains were established, their blood pressures (BPs) were measured by telemetry and compared with that of the S rat. Moreover, a search was conducted to find possible intermediate phenotypes linking the BP effects of the QTL and other physiological traits. RESULTS : Two BP QTL, designated as QTL1 and QTL2, have been mapped to the regions of 4.2 centiMorgans (cM) and less than 12.1 cM respectively on rat chromosome 10. The effects of both QTL correlate with cardiac, left ventricular and aortic hypertrophy. The effect of QTL1 is also associated with renal hypertrophy. CONCLUSION : The current study proved that multiple QTL exist in the region of Dahl rat chromosome 10. The identification of these QTL may help unravel the mechanisms underlying the pathogenesis of certain QTL in humans.     

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.     

Multiple quantitative trait loci for blood pressure interacting epistatically and additively on Dahl rat chromosome 2

Our previous work demonstrated 2 quantitative trait loci (QTLs), C2QTL1 and C2QTL2, for blood pressure (BP) located on chromosome (Chr) 2 of Dahl salt-sensitive (DSS) rats. However, for a lack of markers, the 2 congenic strains delineating C2QTL1 and C2QTL2 could not be separated. The position of the C2QTL1 was only inferred by comparing 2 congenic strains, one having and another lacking a BP effect. Furthermore, it was not known how adjacent QTLs would interact with one another on Chr 2. In the current investigation, first, a critical chromosome marker was developed to separate 2 C2QTLs. Second, a congenic substrain was created to cover a chromosome fragment thought to harbor C2QTL1. Finally, a series of congenic strains was produced to systematically and comprehensively cover the entire Chr 2 segment containing C2QTL2 and other regions previously untested. Consequently, a total of 3 QTLs were discovered, with C2QTL3 located between C2QTL1 and C2QTL2. C2QTL1, C2QTL2, and C2QTL3 reside in chromosome segments of 5.7 centiMorgan (cM), 3.5 cM, and 1.5 cM, respectively. C2QTL1 interacted epistatically with either C2QTL2 or C2QTL3, whereas C2QTL2 and C2QTL3 showed additive effects to each other. These results suggest that BP QTLs closely linked in a segment interact epistatically and additively to one another on Chr 2.     

Major Histocompatibility Complex in the Rat and Blood Pressure Regulation

The objective of this study was to evaluate whether the major histocompatibility complex of the rat can be related to blood pressure (BP) level and BP response to stress. Blood pressure was determined under light ether anesthesia or during moderate restraint stress in normotensive Lewis rats, in rat strains congenic with respect to their RT1 haplotype [LEW.1A (RT1^a) and LEW.1W (RT1^u)], and in their recombinant lines LEW.1AR1 (RT1^ar1), LEW.1AR2 (RT1^ar2), LEW.1WR1 (RT1^wr1), and LEW.1WR2 (RT1^wr2). Under light ether anesthesia, systolic blood pressure was similar in Lewis, LEW.1A, and LEW.1W rats. There were also no significant differences in blood pressure in LEW.1AR1 and LEW.1AR2 animals when compared with Lewis or LEW.1A rats. In contrast, BP was significantly increased in LEW.1WR2 rats. On the other hand, moderate restraint stress induced a BP increase in animals of all recombinant lines compared to the respective congenic strains.These results confirmed our previous finding in recombinant inbred strains about the significant role of RT1 complex in BP regulation. Moreover, our data indicated that BP can be influenced by interaction of individual regions of the RT1 complex on the genetic background of Lewis strain. Am J Hypertens 1996;9:675–680     

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.     

Identification of candidate genes for alcohol preference by expression profiling of congenic rat strains

Background: A highly significant quantitative trait locus (QTL) on chromosome 4 that influenced alcohol preference was identified by analyzing crosses between the iP and iNP rats. Congenic strains in which the iP chromosome 4 QTL interval was transferred to the iNP (NP.P) exhibited the expected increase in alcohol consumption compared with the iNP background strain. This study was undertaken to identify genes in the chromosome 4 QTL interval that might contribute to the differences in alcohol consumption between the alcohol‐naïve congenic and background strains. Methods: RNA from 5 brain regions from each of 6 NP.P and 6 iNP rats was labeled and analyzed separately on an Affymetrix Rat Genome 230 2.0 microarray to look for both cis‐regulated and trans‐regulated genes. Expression levels were normalized using robust multi‐chip average (RMA). Differential gene expression was validated using quantitative real‐time polymerase chain reaction. Five individual brain regions (nucleus accumbens, frontal cortex, amygdala, hippocampus, and striatum) were analyzed to detect differential expression of genes within the introgressed QTL interval, as well as genes outside that region. To increase the power to detect differentially expressed genes, combined analyses (averaging data from the 5 discrete brain regions of each animal) were also carried out. Results: Analyses within individual brain regions that focused on genes within the QTL interval detected differential expression in all 5 brain regions; a total of 35 genes were detected in at least 1 region, ranging from 6 genes in the nucleus accumbens to 22 in the frontal cortex. Analysis of the whole genome detected very few differentially expressed genes outside the QTL. Combined analysis across brain regions was more powerful. Analysis focused on the genes within the QTL interval confirmed 19 of the genes detected in individual regions and detected 15 additional genes. Whole genome analysis detected 1 differentially expressed gene outside the interval. Conclusions: Cis‐regulated candidate genes for alcohol consumption were identified using microarray profiling of gene expression differences in congenic animals carrying a QTL for alcohol preference.     

Mapping and functional characterization of rat chromosome 4 regions that regulate arthritis models and phenotypes in congenic strains

OBJECTIVE: DA rats are highly susceptible to experimental models of rheumatoid arthritis (RA). Linkage analyses in different models have identified several quantitative trait loci (QTLs) within a 70-cM region of DA rat chromosome 4 (C4). We produced congenic strains for these QTLs in order to map and characterize their impact on arthritis development. METHODS: Selective breeding was used to transfer C4 intervals from arthritis-resistant PVG.1AV1 rats onto DA rats. These congenic strains were evaluated for susceptibility to arthritis induced by intradermal injection of rat type II collagen, pristane (a well-defined synthetic adjuvant oil), mycobacteria, or squalene (an endogenous adjuvant oil used in human vaccine). RESULTS: Rats congenic for PVG.1AV1 genes in the 70-cM region were less susceptible than DA rats to collagen-induced arthritis (CIA), pristane-induced arthritis, adjuvant-induced arthritis, and squalene-induced arthritis (SIA). Experiments in subcongenic strains indicated a gene regulating arthritis in males located in a 20-cM interval overlapping the QTL Pia5. A second gene, located in a 10-cM interval harboring the QTL Oia2, attenuated SIA and CIA. The latter caused a change in anticollagen antibody isotype levels toward a pattern similar to that seen in PVG.1AV1 rats. CONCLUSION: The QTL Oia2 regulates arthritis induced both by the nonimmunogenic immunostimulant squalene and by cartilage collagen. In CIA, it also skews anticollagen isotype profiles, suggesting qualitative regulation of autoimmunity. Interestingly, the homologous human chromosome region 12p12-p13 has also been linked to RA, suggesting that genetic and functional dissection of this locus will provide clues to disease pathways that lead to joint inflammation.     

Epistasis, not numbers, regulates functions of clustered Dahl rat quantitative trait loci applicable to human hypertension

Quantitative trait loci (QTLs) for blood pressure (BP) were found on chromosome 10 of Dahl salt-sensitive rats and are potentially important to human essential hypertension. But their identities and how they influence BP together were not known. Presently, we first fine mapped existing QTLs, C10QTL1, C10QTL2, and C10QTL3, by constructing congenic strains. In the process, a new QTL, C10QTL4, was identified. Because the intervals harboring C10QTL1 and C10QTL4 contain a maximum of 16 and 10 possible genes, respectively, a limited number of specific gene targets has been identified to be QTLs residing in human homologous regions on chromosome 17. Moreover, because none of these candidates encodes a gene known to influence BP, the 2 QTLs will represent novel genes for BP regulations. Second, we used congenic strains with QTL combinations to analyze the interactions between the QTLs. Consequently, a double combination of C10QTL4 and C10QTL1 possessed the same BP as each of the 2 QTLs alone. BP of a triple combination of C10QTL4, C10QTL1, and C10QTL3 was not different from BP of the C10QTL4 and C10QTL1 double combination. These results demonstrate that C10QTL4, C10QTL1, and C10QTL3 are epistatic to one another in their BP effects. In contrast, when adding C10QTL2 into the triple formation of the 3 QTLs above to create a quadruple QTL combination, BP increased proportionately, indicating that C10QTL2 acts independently of C10QTL4, C10QTL1, and C10QTL3. The epistatic and additive interactions uncovered in the animal model will help elucidate similar interactions playing a role in human essential hypertension.