Essential hypertension is a complex trait influenced by multiple genes known as quantitative trait loci (QTLs) for blood pressure (BP). It is not clear, however, what roles these QTLs play in maintaining normotension. Insights gained toward the maintenance of normotension will shed light on how hypertension can result from a deficiency or malfunctioning of this maintenance. Currently, congenic strains were systematically constructed using Dahl salt-sensitive (DSS) and Lewis (LEW) rats not only to define QTLs (i.e. in DSS background), but also to ascertain effects of the same QTLs in preserving normotension (i.e. in LEW background), a first such study. Results showed that although LEW alleles for two QTLs on Chromosome (Chr) 18 lowered BP on the DSS background, their BP-increasing DSS alleles failed to influence BP in the LEW background. To further prove that the LEW background is resistant and the DSS background is susceptible to the effects of QTLs, BP-increasing alleles of a QTL on Chr 2 were introgressed into the DSS background, and its BP-decreasing alleles into the LEW background. Indeed, there was no BP-decreasing effect on the LEW background while demonstrating a BP-increasing effect on the DSS background. Thus, a genetic regulation of BP QTLs in the LEW genome inhibits BP changes by nullifying the effects of BP-altering QTLs. In comparison, the DSS genome must have lost the buffering capacity for stabilizing BP. The current work presents good evidence that a lack of regulation for functions of BP QTLs is a potential underlying cause of hypertension.

Abstract-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.

Abstract-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. ver since the revelation of a quantitative trait locus (QTL) for blood pressure (BP) on chromosome 2 (Chr 2) of Dahl salt-sensitive (DSS) rats, 1,2 Chr 2 seems to play a role in the development of hypertension in several of the hypertensive strains. [3][4][5][6][7][8][9][10][11][12][13] In our initial work, 2 BP QTLs designated C2QTL1 and C2QTL2 were localized to regions on Chr 2 of the DSS rat. 5,6 C2QTL1 was found between the markers D2Rat303 and D2Rat166, 6 and C2QTL2 was found between the markers D2Rat166 and D2Rat131. 5 However, at the time, the position of C2QTL1 was solely inferred from comparing 2 overlapping congenic strains, one having and the other lacking a BP effect. 6 It was uncertain whether this deduction was valid in localizing a QTL for a polygenic trait. Another issue was that C2QTL1 defined by S.M1 6 and C2QTL2 defined by S.M5 and S.M6 5 shared a chromosome region of ambiguity. As a result, it could not be ruled out that there might be just 1 QTL instead of 2 in the Chr 2 region in question.Subsequent to our original work on Chr 2 QTL localizations, 1,2,5,6,14 another group found several QTLs situated adjacent to one another in 1 Chr 2 segment of DSS rats. 7 However, it was not clear how these QTLs could act with reference to one another.Based on these observations, 2 questions were addressed: are there truly multiple BP QTLs in a Chr 2 segment of the DSS rat? If there are, how do they interact with reference to one another in determining BP? Materials and Methods AnimalsCongenic strains, S.M, S.M1, S.M2, S.M5, and S.M6 and DSS strains are the same as used previously 5,6 (Figure 1). Constructions of Congenic SubstrainsS.M, S.M1, S.M5, and S.M6 5,6 were used to derive congenic substrains. The basic design was to systematically and as completely as possible cover the entire Chr 2 region of interest....

These conflicting results called into question the validity of localization of a QTL by linkage followed by the use of a congenic strain made with an incomplete chromosome coverage. To resolve this issue, we constructed five new congenic strains, designated C17S.L1 to C17S.L5, that completely spanned the Ϯ2 LOD confidence interval supposedly containing the QTL. Each congenic strain was made by replacing a segment of the DSS rat by that of the normotensive Lewis (LEW) rat. The only section to be LL homozygous is the region on Chr 17 specified in a congenic strain, as evidenced by a total genome scan. The results showed that BPs of C17S.L1 and C17S.L2 were lower (P Ͻ 0.04) than that of DSS rats. In contrast, BPs of C17S.L3, C17S.L4, and C17S.L5 were not different (P Ͼ 0.6) from that of DSS rats. Consequently, a BP QTL must be located in an interval of ϳ15 cM shared between C17S.L1 and C17S.L2 and unique to them both, as opposed to C17S.L3, C17S.L4, and C17S.L5. The present study illustrates the importance of thorough chromosome coverage, the necessity for a genome-wide screening, and the use of "negative" controls in physically mapping a QTL by congenic strains.Dahl salt-sensitive rat; normotensive Lewis rat; congenic strain; congenic substrain THE PRESENCE OF A BLOOD PRESSURE (BP) quantitative trait locus (QTL) on rat chromosome 17 (Chr 17) was originally hinted on by linkage analyses of an F2 population derived from a cross between the Dahl salt-sensitive (DSS) and Lewis (LEW) rats (6). The maximum LOD score supporting this localization was 2.9, which constitutes a suggestive linkage (6). Subsequently, another group of investigators using a different model of salt-sensitive hypertension, the Sabra strain, independently localized a QTL by linkage to a similar region, but with a higher LOD score of 3.43 (27). Employing recombinant inbred strains derived from crosses between the spontaneously hypertensive (SHR) and Brown Norway (BN) rats (22), a separate group of investigators did not find linkage to BP, but they found linkage to the left ventricular mass in a region on Chr 17 distant from the interval containing the BP QTL reported in DSS and Sabra rats (6, 27). Recently, researchers studying cardiac hypertrophy associated with pulmonary hypertension (28) showed that a QTL for right ventricular mass was localized to a Chr 17 region close to the BP QTL in DSS (6) and Sabra (27) strains. Finally, linkage analysis based on the Lyon rats also showed that a QTL on Chr 17 was involved in controlling metabolic homeostasis and BP (3).Nevertheless, the BP QTL localization (6) was questioned because a congenic strain made by replacing a segment of the DSS chromosome with that of LEW did not show any BP effect (12). Due to a lack of markers at the time, the entire Ϯ2 LOD support interval harboring the QTL was not replaced in that study (12). Consequently, the validity of the QTL could not be verified.For the convenience of presentation and discussion, the congenic strain previously made (12) is designated as congenic 1. Beca...