Localization of a blood pressure QTL to a 2.4-cM interval on rat chromosome 9 using congenic strains

Abstract

A blood pressure (BP) quantitative trait locus (QTL) was previously found on rat chromosome 9 using Dahl salt-sensitive (S) and Dahl salt-resistant (R) rats. A congenic strain, S.R(chr9), constructed by introgressing an R chromosomal segment into the S background, previously proved the existence of a BP QTL in a large 34.2-cM segment of chromosome 9. In the current work congenic substrains were constructed from the progenitor congenic strain, S.R(chr9). BP and heart weight comparisons between these congenic substrains and their S control localized the BP QTL to a 4.6-cM interval. Two solute carrier (Na⁺/H⁺ exchanger) genes, Nhe2 and Nhe4, were excluded as candidates based on their map locations. A second iteration of congenic substrains was used to localize the QTL further to a 2.4-cM interval. Another solute carrier (Cl⁻/HCO3− exchanger) gene, Ae3, is in this reduced interval and was sequenced for both S and R strains, but no coding sequence variations were found. Ae3 mRNA was not differentially expressed in the kidney of congenic compared to S rats. Although the identity of the QTL remains unknown its map location has been reduced from an interval of 34.2 to 2.4 cM.

Introduction

Inbred Dahl salt-sensitive (S) and salt-resistant (R) rats provide an animal model for the genetic study of salt (NaCl)-induced hypertension [1], [2]. Previously, we found a blood pressure (BP) quantitative trait locus (QTL) with a peak lod score of 5.0 around marker D9Wox10 (Inha) on rat chromosome 9 using an F₂(S × R) population of 233 rats [3]. A congenic strain, S.R(chr9), was constructed by introgressing an R chromosomal segment from chromosome 9 into the S background. This congenic strain had a lower BP (19 mm Hg, p < 0.0001) and a lower heart weight (HW) (80 mg, p < 0.0001) than S control rats, indicating the existence of a BP QTL in the introgressed region. This introgressed region was 34.2 cM as estimated from our current linkage map. The main purpose of the present work was to localize this BP QTL further by: (1) the development of more polymorphic markers between S and R rats for chromosome 9 and (2) the systematic construction of congenic substrains containing progressively reduced introgressed R regions to localize the QTL better.

Three solute carrier genes, two members of the Na⁺/H⁺ exchanger family, Nhe2 (Slc9a2) and Nhe4 (Slc9a4), and the Cl⁻/HCO3− anion-exchanger family member 3, Ae3 (Slc4a3), were known to be on rat chromosome 9. The Nhe2 and Nhe4 genes had been placed on chromosome 9 by somatic cell hybrid analysis [4]; however, their position on the linkage map relative to the QTL was unknown. Ae3, in contrast, has a well characterized position on rat chromosome 9 by linkage analysis [3], [5], [6]. Both of these solute carrier gene families can influence acid/base regulation and intracellular pH. Metabolic acidosis and reduced intracellular pH have received wide attention in the hypertension literature as possible mediators of vascular smooth muscle function [7], [8], [9], [10], [11], [12], [13], [14], [15], [16]. Thus the relationships of these solute carrier genes to the BP QTL on rat chromosome 9 were of interest.