Genetic Effects of Blood Pressure Quantitative Trait Loci on Hypertension-Related Organ Damage: Evaluation Using Multiple Congenic Strains

A 3-Mbp fragment on rat chromosome 1 affects susceptibility both to stroke and kidney injury under salt loading in the stroke-prone spontaneously hypertensive rat: a genetic approach using multiple congenic strains

We have previously reported that a major quantitative trait locus (QTL) responsible for susceptibility to salt-induced stroke in the stroke-prone spontaneously hypertensive rat (SHRSP) is located in a 3-Mbp region on chromosome 1 covered by SHRSP.SHR-(D1Rat23-D1Rat213)/Izm (termed Pr1.31), a congenic strain with segments from SHRSP/Izm introduced into the stroke-resistant SHR/Izm. Here, we attempted to narrow down the candidate region on chromosome 1 further through analyses of subcongenic strains constructed for the target region. Simultaneously, salt-induced kidney injury was evaluated through the measurement of urinary albumin and the gene expression of renal tubular injury markers (Kim-1 and Clu) to explore a possible mechanism leading to the onset of stroke. All subcongenic strains examined in this study showed lower susceptibility to salt-induced stroke than SHRSP. Interestingly, Pr1.31 had the lowest stroke susceptibility when compared with newly constructed subcongenic strains harboring fragments of the congenic sequence in Pr1.31. Although Kim-1 and Clu expression after 1 week of salt loading in Pr1.31 did not differ significantly from those in SHRSP, the urinary albumin level of Pr1.31 was significantly lower than those of the other subcongenic strains and that of SHRSP. The present results indicated that, although the congenic fragment in Pr1.31 harbored the gene(s) related to salt-induced organ damages, further genetic dissection of the candidate region was difficult due to multiple QTLs suggested in this region. Further analysis using Pr1.31 will unveil genetic and pathophysiological mechanisms underlying salt-induced end organ damages in SHRSP.

Evaluation of Pathological Association between Stroke-Related QTL and Salt-Induced Renal Injury in Stroke-Prone Spontaneously Hypertensive Rat

The stroke-prone spontaneously hypertensive rat (SHRSP) suffers from severe hypertension and hypertensive organ damage such as cerebral stroke and kidney injury under salt-loading. By a quantitative trait locus (QTL) analysis between SHRSP and SHR (the stroke-resistant parental strain of SHRSP), two major QTLs for stroke susceptibility were identified on chromosomes 1 and 18 of SHRSP, which were confirmed in congenic strains constructed between SHRSP and SHR. As the progression of renal dysfunction was suggested to be one of the key factors inducing stroke in SHRSP, we examined effects of the stroke-related QTLs on kidney injury using two congenic strains harboring either of SHRSP-derived fragments of chromosomes 1 and 18 in the SHR genome. The congenic strains were challenged with 1% NaCl solution for 4 weeks; measurement of systolic blood pressure and urinary isoprostane level (a marker for oxidative stress) and evaluation of renal injury by quantification of genetic marker expression and histological examination were performed. We found that the congenic rats with SHRSP-derived fragment of chromosome 18 showed more severe renal damage with higher expression of Col1α-1 (a genetic marker for renal fibrosis) and higher urinary isoprostane level. In contrast, the fragment of chromosome 1 from SHRSP did not give such effects on SHR. Blood pressure was not greater in either of the congenic strains when compared with SHR. We concluded that the QTL region on chromosome 18 might deteriorate salt-induced renal injury in SHR through a blood pressure-independent mechanism.

Further dissection of QTLs for salt-induced stroke and identification of candidate genes in the stroke-prone spontaneously hypertensive rat

We previously revealed that two major quantitative trait loci (QTLs) for stroke latency of the stroke-prone spontaneously hypertensive rat (SHRSP) under salt-loading were located on chromosome (Chr) 1 and 18. Here, we attempted further dissection of the stroke-QTLs using multiple congenic strains between SHRSP and a stroke-resistant hypertensive rat (SHR). Cox hazard model among subcongenic strains harboring a chromosomal fragment of Chr-1 QTL region showed that the most promising region was a 2.1 Mbp fragment between D1Rat177 and D1Rat97. The QTL region on Chr 18 could not be narrowed down by the analysis, which may be due to multiple QTLs in this region. Nonsynonymous sequence variations were found in four genes (Cblc, Cxcl17, Cic, and Ceacam 19) on the 2.1 Mbp fragment of Chr-1 QTL by whole-genome sequence analysis of SHRSP/Izm and SHR/Izm. Significant changes in protein structure were predicted in CBL-C and CXCL17 using I-TASSER. Comprehensive gene expression analysis in the kidney with a cDNA microarray identified three candidate genes (LOC102548695 (Zinc finger protein 45-like, Zfp45L), Ethe1, and Cxcl17). In conclusion, we successfully narrowed down the QTL region on Chr 1, and identified six candidate genes in this region.