Hypertension Suppression, Not a Cumulative Thrust of Quantitative Trait Loci, Predisposes Blood Pressure Homeostasis to Normotension

Shifting Paradigm from Gene Expressions to Pathways Reveals Physiological Mechanisms in Blood Pressure Control in Causation

Genetics for blood pressure (BP) in human and animals has been partitioned into two separate specialties. However, this divide is mechanistically-misleading. BP physiology is mechanistically participated by products of quantitative trait loci (QTLs). The key to unlocking its mechanistic mystery lies in the past with mammalian ancestors before humans existed. By pivoting from effects to causes, physiological mechanisms determining BP by six QTLs have been implicated. Our work relies on congenic knock-in genetics in vivo using rat models, and has reproduced the physiological outcome based on a QTL being molecularly equal to one gene. A gene dose for a QTL is irrelevant to physiological BP controls in causation. Together, QTLs join one another as a group in modularized Mendelian fashion to achieve polygenicity. Mechanistically, QTLs in the same module appear to function in a common pathway. Each is involved in a different step in the pathway toward polygenic hypertension. This work has implicated previously-concealed components of these pathways. This emerging concept is a departure from the human-centric precept that the level of QTL expressions, not physiology, would ultimately determine BP. The modularity/pathway paradigm breaks a unique conceptual ground for unravelling the physiological mechanisms of polygenic and quantitative traits like BP.

Multi-Omic Approaches to Identify Genetic Factors in Metabolic Syndrome

Metabolic syndrome (MetS) is a highly heritable disease and a major public health burden worldwide. MetS diagnosis criteria are met by the simultaneous presence of any three of the following: high triglycerides, low HDL/high LDL cholesterol, insulin resistance, hypertension, and central obesity. These diseases act synergistically in people suffering from MetS and dramatically increase risk of morbidity and mortality due to stroke and cardiovascular disease, as well as certain cancers. Each of these component features is itself a complex disease, as is MetS. As a genetically complex disease, genetic risk factors for MetS are numerous, but not very powerful individually, often requiring specific environmental stressors for the disease to manifest. When taken together, all sequence variants that contribute to MetS disease risk explain only a fraction of the heritable variance, suggesting additional, novel loci have yet to be discovered. In this article, we will give a brief overview on the genetic concepts needed to interpret genome-wide association studies (GWAS) and quantitative trait locus (QTL) data, summarize the state of the field of MetS physiological genomics, and to introduce tools and resources that can be used by the physiologist to integrate genomics into their own research on MetS and any of its component features. There is a wealth of phenotypic and molecular data in animal models and humans that can be leveraged as outlined in this article. Integrating these multi-omic QTL data for complex diseases such as MetS provides a means to unravel the pathways and mechanisms leading to complex disease and promise for novel treatments. © 2022 American Physiological Society. Compr Physiol 12:1-40, 2022.

Conserved mammalian modularity of quantitative trait loci revealed human functional orthologs in blood pressure control

Genome-wide association studies (GWAS) have routinely detected human quantitative trait loci (QTLs) for complex traits. Viewing that most GWAS single nucleotide polymorphisms (SNPs) are found in non-coding regions unrelated to the physiology of a polygenic trait of interest, a vital question to answer is whether or not any of these SNPs can functionally alter the phenotype with which it is associated. The study of blood pressure (BP) is a case in point. Conserved mechanisms in controlling BP by modularity is now unifying differing mammalian orders in that understanding mechanisms in rodents is tantamount to revealing the same in humans, while overcoming experimental limitations imposed by human studies. As a proof of principle, we used BP QTLs from Dahl salt-sensitive rats (DSS) as substitutes to capture distinct human functional orthologs. 3 DSS BP QTLs are located into distinct genome regions and correspond to several human GWAS genes. Each of the QTLs independently exerted a major impact on BP in vivo. BP was functionally changed by normotensive alleles from each of these QTLs, and yet, the human GWAS SNPs do not exist in the rat. They cannot be responsible for physiological alterations in BP caused by these QTLs. These SNPs are genome emblems for QTLs nearby, rather than being QTLs per se, since they only emerged during primate evolution after BP-regulating mechanisms have been established. We then identified specific mutated coding domains that are conserved between rodents and humans and that may implicate different steps of a common pathway or separate pathways.

Functional Captures of Multiple Human Quantitative Trait Loci Regulating Blood Pressure with the Use of Orthologs in Genetically Defined Rat Models

BACKGROUND

Most signals from human genome-wide association studies (GWAS) for blood pressure (BP) are single-nucleotide polymorphisms (SNPs). It was unknown if such SNPs can functionally affect BP. Because BP is similar between humans and rodents, unraveling basic mechanisms from rodents can reveal the same BP-modulating mechanisms in humans originating from their common ancestors while overcoming limitations in human epidemiology.

METHODS

For the first time, we used quantitative trait loci (QTLs) from Dahl salt-sensitive (DSS) rats as functional surrogates to capture human BP QTLs.

RESULTS

A total of 107 human GWAS genes may be classified into 2 common pathways of hypertension pathogeneses. Among them, 4 DSS BP QTLs correspond to 4 human GWAS genes. Each of them independently showed a major impact on BP in vivo and thus functional redundancy. BP was altered by each of these 4 QTLs, but human GWAS SNPs marking these QTLs do not exist in the rat. They cannot be responsible for physiological changes in BP caused by these QTLs and are genome signposts marking positions of the QTLs nearby, rather than being QTLs themselves. These SNPs appeared during primate evolution, independently of BP regulation. Because the functional dosage of QTLs, not their gene dose, determined hypertension pathogenesis, a role for the noncoding GWAS SNPs in BP via regulating gene expressions can be discounted.

CONCLUSIONS

The human QTLs may function in a common pathway, with each involved in a different step in the pathway leading to BP control. These results may be conceptually paradigm shifting.

Dissecting Epistatic QTL for Blood Pressure in Rats: Congenic Strains versus Heterogeneous Stocks, a Reality Check

Advances in molecular genetics have provided well-defined physical genetic maps and large numbers of genetic markers for both model organisms and humans. It is now possible to gain a fundamental understanding of the genetic architecture underlying quantitative traits, of which blood pressure (BP) is an important example. This review emphasizes analytical techniques and results obtained using the Dahl salt-sensitive (S) rat as a model of hypertension by presenting results in detail for three specific chromosomal regions harboring genetic elements of increasing complexity controlling BP. These results highlight the critical importance of genetic interactions (epistasis) on BP at all levels of structure, intragenic, intergenic, intrachromosomal, interchromosomal, and across whole genomes. In two of the three examples presented, specific DNA structural variations leading to biochemical, physiological, and pathological mechanisms are well defined. This proves the usefulness of the techniques involving interval mapping followed by substitution mapping using congenic strains. These classic techniques are compared to newer approaches using sophisticated statistical analysis on various segregating or outbred model-organism populations, which in some cases are uniquely useful in demonstrating the existence of higher-order interactions. It is speculated that hypertension as an outlier quantitative phenotype is dependent on higher-order genetic interactions. The obstacle to the identification of genetic elements and the biochemical/physiological mechanisms involved in higher-order interactions is not theoretical or technical but the lack of future resources to finish the job of identifying the individual genetic elements underlying the quantitative trait loci for BP and ascertaining their molecular functions. © 2019 American Physiological Society. Compr Physiol 9:1305-1337, 2019.

Novel Pathogenesis of Hypertension and Diastolic Dysfunction Caused by M3R (Muscarinic Cholinergic 3 Receptor) Signaling

Multiple quantitative trait loci for blood pressure (BP) are localized in humans and rodent models. Model studies have not only produced human quantitative trait loci homologues but also provided unforeseen mechanistic insights into the function modality of quantitative trait loci actions. Presently, congenic knockins, gene-specific knockout, and in vitro and in vivo function studies were used in a rat model of polygenic hypertension, DSS (Dahl salt sensitive) rats. One gene previously unknown in regulating BP was detected with 1 structural mutation(s) for each of 2 quantitative trait loci classified into 2 separate epistatic modules 1 and 3. C17QTL1 in epistatic module 2 was identified to be the gene Chrm3 encoding the M3R (muscarinic cholinergic 3 receptor), since a single function-enhancing M3R^T556M conversion correlated with elevated BP. To definitively prove that the enhanced M3R function is responsible for BP changes by the DSS alleles of C17QTL1, we generated a Chrm3 gene-specific rat knockout. We observed a reduction in BP without tachycardia in both sexes, regardless of the amount of dietary salt, and an improvement in diastolic and kidney dysfunctions. All occurred in spite of a significant reduction in M3R-dependent vasodilation. The previously seen sexual dimorphism for C17QTL1 on BP disappeared in the absence of M3R. A Chrm3-coding variation increased M3R signaling, correlating with higher BP. Removing the M3R signaling led to a decrease in BP and improvements in cardiac and renal malfunctions. A novel pathogenic pathway accounted for a portion of polygenic hypertension and has implications in applying new diagnostic and therapeutic uses against hypertension and diastolic dysfunction.

Functional Dosage of Muscarinic Cholinergic Receptor 3 Signalling, Not the Gene Dose, Determines Its Hypertension Pathogenesis

BACKGROUND

Multiple quantitative trait loci for blood pressure (BP) have been localized throughout human and rodent genomes. Few of them have been functionally identified especially in humans, and little is known about their pathogenic directionality when identified. We focused on Chrm3 encoding the muscarinic cholinergic receptor 3 (M3R) as the causal gene for C17QTL1 in the Dahl salt-sensitive rat model.

METHODS AND RESULTS

Congenic knock-ins, gene-specific knockout, and ex vivo and in vivo function studies were applied in the Dahl salt-sensitive rat model of polygenic hypertension. A Chrm3 missense T1667C mutation in the last intracellular domain functionally correlated with a rise in BP increased the M3R signalling and resensitization, and adrenal epinephrogenesis. Gene targeting that abolished the M3R function without affecting any of noncoding Chrm3 variants caused a decrease in BP, indicating that the M3R-mediated signalling promotes hypertension. In contrast, removing 8 amino acids from the M3R first extracellular loop had no effect on BP.

CONCLUSIONS

The M3R-specialized signalling constitutes a new pathway of hypertension pathogenesis within the context of a polygenic and quantitative trait. Increased epinephrine in the circulation and secreted from the adrenal glands are suggestive of a molecular mechanism partially mediating M3R to promote hypertension. The structure-function relationships for various M3R domains in their effects on BP pave the way for identifying missense mutations that impact functions on BP as potential diagnostic targets.

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.

Retinoblastoma-associated protein 140 as a candidate for a novel etiological gene to hypertension

Gene discovery in animal models may lead to the revelation of therapeutic targets for essential hypertension as well as mechanistic insights into blood pressure (BP) regulation. Our aim was to identify a disease-causing gene for a component of polygenic hypertension contrasting inbred hypertensive Dahl salt-sensitive (DSS) and normotensive Lewis rats. The chromosome segment harboring a quantitative trait locus (QTL), C16QTL, was first isolated from the rat genome via congenic strains. A candidate gene responsible for C16QTL causing a BP difference between DSS and Lewis rats was then identified using molecular analyses combining our independently-conducted total genome and gene-specific sequencings. The retinoblastoma-associated protein 140 (Rap140)/family with sequence similarity 208 member A (Fam208a) is the only candidate gene supported to be C16QTL among three genes in genome block 1 present in the C16QTL-residing interval. A mode of its actions could be to influence the expressions of genes that are downstream in a pathway potentially leading to BP regulation such as that encoding the solute carrier family 7 (cationic amino acid transporter, y+ system) member 12 (Slc7a12), which is specifically expressed in kidneys. Thus, Rap140/Fam208a probably encoding a transcription factor is the strongest candidate for a novel BP QTL that acts via a putative Rap140/Fam208a-Slc7a12-BP pathway. These data implicate a premier physiological role for Rap140/Fam208 beyond development and a first biological function for the Slc7a12 protein in any organism.