Cardiac pathways distinguish two epistatic modules enacting BP quantitative trait loci and candidate gene analysis

Fine mapping of epistatic genetic determinants of blood pressure on rat chromosome 5

BACKGROUND

Previously, using linkage analysis and substitution mapping, two closely-linked interactive blood pressure quantitative trait loci (QTLs), BP QTL1 and BP QTL2, were located within a 13.96 Mb region from 117894038 to 131853815 bp (RGSC 3.4 version) on rat chromosome 5 (RNO5). This was done by using a series of congenic strains consisting of genomic segments of the Dahl salt-sensitive (S) rat substituted with that of the normotensive Lewis (LEW) rat. The interactive nature of the two loci was further confirmed by the construction and characterization of a panel of S.LEW bicongenic strains and corresponding S.LEW monocongenic strains, which provided definitive evidence of epistasis (genetic interaction) between BP QTL1 (7.77 Mb) and BP QTL2 (4.18 Mb). The purpose of this work was to further map these interacting QTLs.

METHOD

A new panel of seven new S.LEW bicongenic strains was constructed and characterized for BP.

RESULTS

The data obtained from these new strains further resolved BP QTL1 from 7.77 to 2.93 Mb. Further, BP QTL2 was traceable as not being a single QTL, but a composite of at least three QTLs, LEW alleles at two of which located within 2.26 Mb and 175 kb lowered BP but the third one located within 1.31 Mb increased BP.

CONCLUSION

Lack of coding variation within any of the regions further mapped within the previous QTL2 suggests noncoding variation as likely responsible for the observed epistasis.

Towards Precision Medicine for Hypertension: A Review of Genomic, Epigenomic, and Microbiomic Effects on Blood Pressure in Experimental Rat Models and Humans

Compelling evidence for the inherited nature of essential hypertension has led to extensive research in rats and humans. Rats have served as the primary model for research on the genetics of hypertension resulting in identification of genomic regions that are causally associated with hypertension. In more recent times, genome-wide studies in humans have also begun to improve our understanding of the inheritance of polygenic forms of hypertension. Based on the chronological progression of research into the genetics of hypertension as the "structural backbone," this review catalogs and discusses the rat and human genetic elements mapped and implicated in blood pressure regulation. Furthermore, the knowledge gained from these genetic studies that provide evidence to suggest that much of the genetic influence on hypertension residing within noncoding elements of our DNA and operating through pervasive epistasis or gene-gene interactions is highlighted. Lastly, perspectives on current thinking that the more complex "triad" of the genome, epigenome, and the microbiome operating to influence the inheritance of hypertension, is documented. Overall, the collective knowledge gained from rats and humans is disappointing in the sense that major hypertension-causing genes as targets for clinical management of essential hypertension may not be a clinical reality. On the other hand, the realization that the polygenic nature of hypertension prevents any single locus from being a relevant clinical target for all humans directs future studies on the genetics of hypertension towards an individualized genomic approach.

Spontaneously Hypertensive Rat Chromosome 2 with Mutant Connexin 50 Triggers Divergent Effects on Metabolic Syndrome Components

Metabolic syndrome is a frequent condition with multifactorial aetiology. Previous studies indicated the presence of genetic determinants of metabolic syndrome components on rat chromosome 2 (RNO2) and syntenic regions of the human genome. Our aim was to further explore these findings using novel rat models. We derived the BN-Dca and BN-Lx.Dca congenic strains by introgression of a limited RNO2 region from a spontaneously hypertensive rat strain carrying a mutation in the Gja8 gene (SHR-Dca, dominant cataract) into the genomic background of Brown Norway strain and congenic strain BN-Lx, respectively. We compared morphometric, metabolic and cytokine profiles of adult male BN-Lx, BN-Dca and BN-Lx.Dca rats. We performed in silico comparison of the DNA sequences throughout RNO2 differential segments captured in the new congenic strains. Both BN-Dca and BN-Lx.Dca showed lower total triacylglycerols and cholesterol concentrations compared to BN-Lx. Fasting insulin in BN-Dca was higher than in BN-Lx.Dca and BN-Lx. Concentrations of several proinflammatory cytokines were elevated in the BN-Dca strain, including IL-1α, IL-1β, IFN-γ and MCP-1. In silico analyses revealed over 740 DNA variants between BN-Lx and SHR genomes within the differential segment of the congenic strains. We derived new congenic models that prove that a limited genomic region of SHR-Dca RNO2 significantly affects lipid levels and insulin sensitivity in a divergent fashion.

Genetics of systolic and diastolic heart failure

Heart failure accounts for a significant portion of heart diseases. Molecular mechanisms gradually emerge that participate in pathways leading to left ventricular dysfunction in common systolic heart failure (SHF) and diastolic heart failure (DHF). A human genome-wide association study (GWAS) identified two markers for SHF and no GWAS on DHF has been documented. However, genetic analyses in rat models of SHF and DHF have begun to unravel the genetic components known as quantitative trait loci (QTLs) initiating systolic and diastolic function. A QTL for systolic function was detected and the gene responsible for it is identified to be that encoding the soluble epoxide hydrolase. Diastolic function is determined by multiple QTLs and the Ccl2/monocyte chemotactic protein gene is the strongest candidate. An amelioration on diastolic dysfunction is merely transient from changing such a single QTL accompanied by a blood pressure reduction. A long-term protection can be achieved only via combining alleles of several QTLs. Thus, distinct genes in synergy are involved in physiological mechanisms durably ameliorating or reversing diastolic dysfunction. These data lay the foundation for identifying causal genes responsible for individual diastolic function QTLs and the essential combination of them to attain a permanent protection against diastolic dysfunction, and consequently will facilitate the elucidation of pathophysiological mechanisms underlying hypertensive diastolic dysfunction. Novel pathways triggering systolic and diastolic dysfunction have emerged that will likely provide new diagnostic tools, innovative therapeutic targets and strategies in reducing, curing and even reversing SHF and DHF.

Dr Lewis Kitchener Dahl, the Dahl rats, and the "inconvenient truth" about the genetics of hypertension

Lewis K. Dahl is regarded as an iconic figure in the field of hypertension research. During the 1960s and 1970s he published several seminal articles in the field that shed light on the relationship between salt and hypertension. Further, the Dahl rat models of hypertension that he developed by a selective breeding strategy are among the most widely used models for hypertension research. To this day, genetic studies using this model are ongoing in our laboratory. While Dr. Dahl is known for his contributions to the field of hypertension, very little, if any, of his personal history is documented. This article details a short biography of Dr. Lewis Dahl, the history behind the development of the Dahl rats and presents an overview of the results obtained through the genetic analysis of the Dahl rat as an experimental model to study the inheritance of hypertension.

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.

Theoretical model for gene-gene, gene-environment, and gene-sex interactions based on congenic-strain analysis of blood pressure in Dahl salt-sensitive rats

There is a significant literature describing quantitative trait loci (QTL) controlling blood pressure (BP) in the Dahl salt-sensitive (S) rat. In studies to identify the genes underlying BP QTL it has been common practice to place chromosomal segments from low BP strains on the genetic background of the S rat and then reduce the congenic segments by substitution mapping. The present work suggests a model to simulate genetic interactions found using such congenic strains. The QTL are considered to be switches that can be either in series or in parallel represented by the logic operators AND or OR, respectively. The QTL switches can be on/off switches but are also allowed specific leak properties. The QTL switches are represented by a "universal" switch consisting of two molecules binding to form a complex. Genetic inputs enter the model as allelic products of one of the binding molecules and environmental variation (including dietary salt- and sex-related differences) enters as an influence on the concentration of the other binding molecule. The pairwise interactions of QTL are very well simulated and fall into recognizable patterns. There is, however, often more than one assumed model to predict a given pattern so that all patterns do not necessarily have a unique solution. Nevertheless, the models obtained provide a framework for placing the QTL in pathways relative to one another. Moreover, based on their leak properties pairs of QTL could be identified in which one QTL may alter the properties of the other QTL.

Isolation and high-throughput sequencing of two closely linked epistatic hypertension susceptibility loci with a panel of bicongenic strains

Interactions or epistasis between genetic factors may contribute to "missing heritability." While linkage analyses detect epistasis, defining the limits of the interacting segments poses a significant challenge especially when the interactions are between loci in close proximity. The goal of the present study was to isolate two such epistatic blood pressure (BP) loci on rat chromosome 5. A panel of S.LEW bicongenic strains along with the corresponding monocongenic strains was constructed. BP of each set comprising of one bicongenic and two corresponding monocongenic strains were determined along with the parental Salt-sensitive (S) strain. Epistasis was observed in one out of four sets of congenic strains, wherein systolic blood pressures (SBP) of the two monocongenic strains S.LEW(5)x6Bx9x5a and S.LEW(5)x6Bx9x5b were comparable to that of S, but the SBP of the bicongenic strain S.LEW(5)x6Bx9x5 (157 ± 4.3 mmHg) was significantly lower than that of S (196 ± 6.8 mmHg, P < 0.001). A two-way ANOVA indicated significant interactions between the LEW alleles at the two loci. The interacting loci were 2.02 Mb apart and located within genomic segments spanning 7.77 and 4.18 Mb containing 7,360 and 2,753 candidate variants, respectively. The current study demonstrates definitive evidence for epistasis and provides genetic tools for further dissection of the isolated epistatic BP loci.