Complete and overlapping congenics proving the existence of a quantitative trait locus for blood pressure on Dahl rat chromosome 17

Abstract

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...

Individual QTLs controlling quantitative variation in blood pressure inherited in a Mendelian mode

“…All experimental procedures were in accordance with institutional, provincial and federal regulations. DSS, LEW and MNS rats have previously been described (Palijan et al, 2003a, b;Ariyarajah et al, 2004;Moujahidine et al, 2004;Charron et al, 2005a, b;Dutil et al, 2005;Eliopoulos et al, 2005;Grondin et al, 2005). Nine congenic strains had trapped BP QTLs and were largely based on those isolated previously (Palijan et al, 2003b;Ariyarajah et al, 2004;Moujahidine et al, 2004;Charron et al, 2005a;Dutil et al, 2005;Grondin et al, 2005).…”

“…The breeding procedure and screening protocol were essentially the same as those reported previously (Palijan et al, 2003a, b;Ariyarajah et al, 2004;Charron et al, 2005b;Eliopoulos et al, 2005;Grondin et al, 2005). This new congenic strain for Chr 1 was designated as DSS.LEW-(D1Rat268-D1Chm2)/Lt (abbreviated as C1S.L1).…”

“…BP measurements BP studies of the congenic strains were essentially made in the same way as previous studies (Palijan et al, 2003a, b;Ariyarajah et al, 2004;Charron et al, 2005b;Eliopoulos et al, 2005;Grondin et al, 2005). In brief, male Garrett et al (1998) Abbreviations: Chr, chromosome; DSS, Dahl salt-sensitive strain; LEW, Lewis strain; MNS, Milan normotensive strain.…”

“…Repeated measures' analysis of variance (ANOVA), followed by the Tukey test (Palijan et al, 2003a, b;Ariyarajah et al, 2004;Charron et al, 2005b;Eliopoulos et al, 2005;Grondin et al, 2005), were used to compare the differences between homozygote 1, that is DSS in Figure 1 and homozygote 2 (either LL or MM), homozygote 1 and heterozygotes, and homozygote 2 and heterozygotes for each congenic strain harboring a QTL.…”

“…Two novel functional aspects of BP QTLs have become apparent from the analyses of congenic strains. First, each of them accounts for at least 33 mm Hg/ 88 mm Hg ¼ 38% of the total BP difference between DSS and LEW parental strains, as measured directly by telemetry (Moujahidine et al, 2004;Dutil et al, 2005;Grondin et al, 2005;Charron et al, 2005a), exhibiting the status of a major QTL. Second, each QTL appears have an individual influence on BP: no combination seems to be necessary for any one of them to affect BP, despite QTLQTL epistatic interactions (Deng and Rapp, 1992;Rapp et al, 1998;Palijan et al, 2003a;Charron et al, 2005a, b;Dutil et al, 2005).…”