Distinct quantitative trait loci for kidney, cardiac, and aortic mass dissociated from and associated with blood pressure in Dahl congenic rats

Deep transcriptomic profiling of Dahl salt-sensitive rat kidneys with mutant form of Resp18

Expression of Regulated endocrine specific protein 18 (Resp18) is localized in numerous tissues and cell types; however, its exact cellular function is unknown. We previously showed that targeted disruption of the Resp18 locus in the Dahl SS (SS) rat (Resp18^mutant) results in higher blood pressure (BP), increased renal fibrosis, increased urinary protein excretion, and decreased mean survival time following a chronic (6 weeks) 2% high salt (HS) diet compared with the SS rat. Based on this prominent renal injury phenotype, we hypothesized that targeted disruption of Resp18 in the SS rat promotes an early onset hypertensive-signaling event through altered signatures of the renal transcriptome in response to HS. To test this hypothesis, both SS and Resp18^mutant rats were exposed to a 7-day 2% HS diet and BP was recorded by radiotelemetry. After a 7-day exposure to the HS diet, systolic BP was significantly increased in the Resp18^mutant rat compared with the SS rat throughout the circadian cycle. Therefore, we sought to investigate the renal transcriptomic response to HS in the Resp18^mutant rat. Using RNA sequencing, Resp18^mutant rats showed a differential expression of 25 renal genes, including upregulation of Ren. Upregulation of renal Ren and other differentially expressed genes were confirmed via qRT-PCR. Moreover, circulating renin activity was significantly higher in the Resp18^mutant rat compared with the WT SS rat after 7 days on HS. Collectively, these observations demonstrate that disruption of the Resp18 gene in the SS rat is associated with an altered renal transcriptomics signature as an early response to salt load.

Hypothalamic Norepinephrine Concentration and Heart Mass in Hypertensive ISIAH Rats Are Associated with a Genetic Locus on Chromosome 18

The relationship between activation of the sympathetic nervous system and cardiac hypertrophy has long been known. However, the molecular genetic basis of this association is poorly understood. Given the known role of hypothalamic norepinephrine in the activation of the sympathetic nervous system, the aim of the work was to carry out genetic mapping using Quantitative Trait Loci (QTL) analysis and determine the loci associated both with an increase in the concentration of norepinephrine in the hypothalamus and with an increase in heart mass in Inherited Stress-Induced Arterial Hypertension (ISIAH) rats simulating the stress-sensitive form of arterial hypertension. The work describes a genetic locus on chromosome 18, in which there are genes that control the development of cardiac hypertrophy associated with an increase in the concentration of norepinephrine in the hypothalamus, i.e., genes involved in enhanced sympathetic myocardial stimulation. No association of this locus with the blood pressure was found. Taking into consideration previously obtained results, it was concluded that the contribution to the development of heart hypertrophy in the ISIAH rats is controlled by different genetic loci, one of which is associated with the concentration of norepinephrine in the hypothalamus (on chromosome 18) and the other is associated with high blood pressure (on chromosome 1). Nucleotide substitutions that may be involved in the formation or absence of association with blood pressure in different rat strains are discussed.

Candidate genes in quantitative trait loci associated with absolute and relative kidney weight in rats with Inherited Stress Induced Arterial Hypertension

BACKGROUND

The kidney mass is significantly increased in hypertensive ISIAH rats with Inherited Stress Induced Arterial Hypertension as compared with normotensive WAG rats. The QTL/microarray approach was carried out to determine the positional candidate genes in the QTL for absolute and relative kidney weight.

RESULTS

Several known and predicted genes differentially expressed in ISIAH and WAG kidney were mapped to genetic loci associated with the absolute and relative kidney weight in 6-month old F2 hybrid (ISIAHxWAG) males. The knowledge-driven filtering of the list of candidates helped to suggest several positional candidate genes, which may be related to the structural and mass changes in hypertensive ISIAH kidney.

CONCLUSIONS

The further experimental validation of causative genes and detection of polymorphisms will provide opportunities to advance our understanding of the underlying nature of structural and mass changes in hypertensive ISIAH kidney.

Genetics of diastolic heart failure

Heart failure explains a large portion of heart diseases. Molecular mechanisms determining cardiac function, by inference dysfunction in heart failure, are incompletely understood, especially in the common (or congestive) systolic (SHF) and diastolic heart failure (DHF). Limited genome-wide association studies (GWASs) in humans are reported on SHF and no GWAS has been performed on DHF. Genetic analyses in a rodent model of true DHF, Dahl salt-sensitive (DSS) rats, have begun to unravel the genetic components determining diastolic function. Diastolic dysfunction of DSS rats can be ameliorated or even normalized by distinct quantitative trait loci (QTLs), designated as diastolic function/blood pressure QTLs (DF/BP QTLs), which also affect blood pressure (BP). However, an improvement in diastolic dysfunction is merely transitory from a single DF/BP QTL, despite a permanent lowering of BP. A long-term protection against diastolic dysfunction can be realized only through combining specific DF/BP QTLs. Moreover, the worsening diastolic dysfunction with age can also be reversed in a different combination of DF/BP QTLs. Thus, distinct genes in combinations must be involved in the physiological mechanisms ameliorating or reversing diastolic dysfunction. As not all the QTLs that influence BP can affect diastolic function, it is not BP reduction itself that restores diastolic function, but rather specific genes that are uniquely integrated into the pathways of blood pressure homeostasis as well as diastolic function. Thus, the elucidation of pathophysiological mechanisms causal to hypertensive diastolic dysfunction will not only provide new diagnostic tools, but also novel therapeutic targets and strategies in reducing, curing, and even reversing DHF.

Impact of salt exposure on N-acetylgalactosamine-4-sulfatase (arylsulfatase B) activity, glycosaminoglycans, kininogen, and bradykinin

N-acetylgalactosamine-4-sulfatase (Arylsulfatase B; ARSB) is the enzyme that removes sulfate groups from the N-acetylgalactosamine-4-sulfate residue at the non-reducing end of chondroitin-4-sulfate (C4S) and dermatan sulfate (DS). Previous studies demonstrated reduction in cell-bound high molecular weight kininogen in normal rat kidney (NRK) epithelial cells when chondroitin-4-sulfate content was reduced following overexpression of ARSB activity, and chondroitinase ABC produced similar decline in cell-bound kininogen. Reduction in the cell-bound kininogen was associated with increase in secreted bradykinin. In this report, we extend the in vitro findings to in vivo models, and present findings in Dahl salt-sensitive (SS) rats exposed to high (SSH) and low salt (SSL) diets. In the renal tissue of the SSH rats, ARSB activity was significantly less than in the SSL rats, and chondroitin-4-sulfate and total sulfated glycosaminoglycan content were significantly greater. Disaccharide analysis confirmed marked increase in C4S disaccharides in the renal tissue of the SSH rats. In contrast, unsulfated, hyaluronan-derived disaccharides were increased in the rats on the low salt diet. In the SSH rats, with lower ARSB activity and higher C4S levels, cell-bound, high-molecular weight kininogen was greater and urinary bradykinin was lower. ARSB activity in renal tissue and NRK cells declined when exogenous chloride concentration was increased in vitro. The impact of high chloride exposure in vivo on ARSB, chondroitin-4-sulfation, and C4S-kininogen binding provides a mechanism that links dietary salt intake with bradykinin secretion and may be a factor in blood pressure regulation.

Mapping genetic variants associated with beta-adrenergic responses in inbred mice

β-blockers and β-agonists are primarily used to treat cardiovascular diseases. Inter-individual variability in response to both drug classes is well recognized, yet the identity and relative contribution of the genetic players involved are poorly understood. This work is the first genome-wide association study (GWAS) addressing the values and susceptibility of cardiovascular-related traits to a selective β₁-blocker, Atenolol (ate), and a β-agonist, Isoproterenol (iso). The phenotypic dataset consisted of 27 highly heritable traits, each measured across 22 inbred mouse strains and four pharmacological conditions. The genotypic panel comprised 79922 informative SNPs of the mouse HapMap resource. Associations were mapped by Efficient Mixed Model Association (EMMA), a method that corrects for the population structure and genetic relatedness of the various strains. A total of 205 separate genome-wide scans were analyzed. The most significant hits include three candidate loci related to cardiac and body weight, three loci for electrocardiographic (ECG) values, two loci for the susceptibility of atrial weight index to iso, four loci for the susceptibility of systolic blood pressure (SBP) to perturbations of the β-adrenergic system, and one locus for the responsiveness of QTc (p<10⁻⁸). An additional 60 loci were suggestive for one or the other of the 27 traits, while 46 others were suggestive for one or the other drug effects (p<10⁻⁶). Most hits tagged unexpected regions, yet at least two loci for the susceptibility of SBP to β-adrenergic drugs pointed at members of the hypothalamic-pituitary-thyroid axis. Loci for cardiac-related traits were preferentially enriched in genes expressed in the heart, while 23% of the testable loci were replicated with datasets of the Mouse Phenome Database (MPD). Altogether these data and validation tests indicate that the mapped loci are relevant to the traits and responses studied.

Distinct genomic replacements from Lewis correct diastolic dysfunction, attenuate hypertension, and reduce left ventricular hypertrophy in Dahl salt-sensitive rats

BACKGROUND

Hypertension and diastolic heart failure are two common cardiovascular diseases that inflict heavy morbidity and mortality, yet relatively little is understood about their pathophysiology. The identification of quantitative trait loci for blood pressure is important in unveiling the causes of polygenic hypertension. Although Dahl salt-sensitive strain is also an excellent model for the study of diastolic heart failure, virtually nothing is known about the quantitative trait loci determining diastolic heart failure. Diastolic dysfunction often represents the onset of diastolic heart failure.

METHODS

We first characterized the cardiac phenotype of Dahl salt-sensitive strain and normotensive Lewis control rats by echocardiography to ascertain diastolic function. We then analyzed corresponding features of four newly developed and two existing congenic strains, each of which carries a specific chromosome substitution of Dahl salt-sensitive strain by its Lewis homologue and each lowering blood pressure.

RESULTS

Dahl salt-sensitive strain displayed diastolic dysfunction that was rectified in two of six congenic strains, designated as positive congenic strains, which represent the first rodent models exhibiting functional normalization of diastolic dysfunction caused by naturally occurring genetic variants. The two positive congenic strains also showed a reduction in left ventricular mass. In contrast, four of six congenic strains did not change diastolic function despite their blood pressure-lowering effects.

CONCLUSION

Genes present in the replaced chromosome segments of the two positive congenic strains are not commonly known to affect blood pressure, diastolic function or left ventricular mass. Consequently, novel prognostic, diagnostic and therapeutic strategies for hypertensive diastolic heart failure likely emerge from this work.

Detecting dominant QTL with variance component analysis in simulated pedigrees

Dominance is an important source of variation in complex traits. Here, we have carried out the first thorough investigation of quantitative trait locus (QTL) detection using variance component (VC) models extended to incorporate both additive and dominant QTL effects. Simulation results showed that the empirical distribution of the test statistic when testing for dominant QTL effects did not behave in accordance with existing theoretical expectations and varied with pedigree structure. Extensive simulations were carried out to assess accuracy of estimates, type 1 error and statistical power in two-generation human-, poultry- and pig-type pedigrees each with 1900 progeny in small-, medium- and large-sized families, respectively. The distribution of the likelihood-ratio test statistic was heavily dependent on family structure, with empirical thresholds lower for human pedigrees. Power to detect QTL was high (0.84-1.0) in pig and poultry scenarios for dominance effects accounting for >7% of phenotypic variance but much lower (0.42) in human-type pedigrees. Maternal or common environment effects can be partially confounded with dominance and must be fitted in the QTL model. Including dominance in the QTL model did not affect power to detect additive QTL effects. Also, detection of spurious dominance QTL effects only occurred when maternal effects were not included in the QTL model. When dominance effects were present in the data but not in the analysis model, this resulted in spurious detection of additive QTL or inflated estimates of additive QTL effects. The study demonstrates that dominance can be included routinely in QTL analysis of general pedigrees; however, optimal power is dependent on selection of the appropriate thresholds for pedigree structure.