Circ Cardiovasc Genet. 2015 Aug;8(4):610-7.
Hypertension Suppression, Not a Cumulative Thrust of Quantitative Trait Loci, Predisposes Blood Pressure Homeostasis to Normotension.
- 1From the Department of Medicine, Research Centre-Centre Hospitalier de l’Université de Montréal, Université de Montréal, Montréal, Québec, Canada.
- 2From the Department of Medicine, Research Centre-Centre Hospitalier de l’Université de Montréal, Université de Montréal, Montréal, Québec, Canada. firstname.lastname@example.org.
BACKGROUND: Genetics of high blood pressure (BP) has revealed causes of hypertension. The cause of normotension, however, is poorly understood. Inbred Lewis rats sustain normotension despite a genetic push in altering BP. It was unknown whether this rigid resistance to BP changes is because of an insufficient hypertensive impact from limited alleles of quantitative trait loci (QTLs) or because of an existence of a master control superseding the combined strength of hypertensive QTL alleles.
METHODS AND RESULTS: Currently, BP-elevating QTL alleles from hypertensive Dahl salt-sensitive rats (DSS) replaced those of Lewis on chromosomes 7, 8, 10, and 17 on the Lewis background. These hypertensive QTL alleles were then merged to systematically achieve multiple combinations. Results showed that there was no quantitative correlation between BP variations and the number of hypertensive QTL alleles, and that BP was only slightly elevated from a combined force of normotensive alleles from 7 QTLs. Thus, a genetic factor aside from the known QTLs seemed to be at play in preserving normotension and act as a hypertension suppressor. A follow-up study using consecutive backcrosses from Dahl salt-sensitive rats and Lewis identified a chromosome segment where a hypertension suppressor might reside.
CONCLUSIONS: Our results provide the first evidence that normotension is not enacted via a numeric advantage of BP-lowering QTL alleles, and instead can be achieved by a particular genetic component actively suppressing hypertensive QTL alleles. The identification of this hypertension suppressor could result in formulating unique diagnostic and therapeutic targets, and above all, preventive measures against essential hypertension.
KEYWORDS: blood pressure; chromosomes; genetics; models, animal; rats
(1) Rationale: Importance of controlling hypertension in health: Hypertension occurs in about 30% of the general population and is a chronic disorder that entails a high risk for fatal cardiovascular and renal diseases as well as stroke 1. Because of it, most of the scientific and clinical research in hypertension primarily focuses on uncovering etiologies of hypertension 2 and rightly so.
Genetics of hypertension susceptible to inherited variations: Genome-wide association studies (GWAS) have statistically localized more than 30 quantitative trait loci (QTLs) for blood pressure (BP) in humans 3. Studies using inbred animal models have detected an even larger number of QTLs 4, 5. In all these cases, normotensive subjects were used as controls for their hypertensive counterparts at the other end of phenotypic spectrum in a bell curve. In finding hypertension etiologies, genetic variations associated with BP differences between hypertensives and normotensives serve as genome markers for locating BP QTLs 6. Subsequently, a function-altering mutation in a gene should lead to the identification of the BP QTL in question. By correlating the mechanism by which the gene acts in correlation with higher or lower blood pressure, one may understand how hypertension is attained via susceptibility 7.
Significance in discovering pathogenic etiologies of hypertension: When a cause of hypertension is found, it is logical to follow it up with developing therapeutic and diagnostic targets that can ameliorate conditions of hypertension. Widely-used drugs 1, although not targeting etiologies of hypertension, have proven and motivated the therapeutic usage in reducing hypertension as a means of improving cardiovascular health.
Hypertension prevention by understanding anti-hypertensive mechanisms: Antithetical to hypertension, normotension appears more frequently than hypertension in about 70% of the general population, and is largely sustained 1. Some forms of normotension are unyieldingly resistant to the hypertensive assault 8. Thus, a biological counter-measure against hypertension must innately exist in the general population beyond changes in life styles and environment. In spite of this realization, very little mindful diligence has been pursued to understanding mechanisms of achieving and maintaining normotension as well as opposing hypertension, because normotension is not clinically abnormal and thus not pharmaceutically targeted. The crux of the issue is ‘what would be the benefits from investigating the normalcy that has defied, fought and won against hypertension? How does it happen?’
A well-known case of a natural defense against a disease is the resistance to infections by the human immune deficiency virus (HIV) 9, 10. Because of this discovery, novel mechanistic insights have been gained into a protective pathway antipodal to susceptibility to infection, and consequently, anti-HIV preventions became possible.
Establishing normotension: A dearth of keen scrutiny on normotension was largely due to an unproven, and yet intuitive, assumption on the mode of its determination. Similar in mechanism but opposite in effect to hypertension, normotension has been thought to be achieved in a straightforward and computable fashion from BP QTLs, i.e. via a simple mathematical reverse of hypertension, since blood pressure is a quantitative trait 11. This numerical interpretation could extrapolatively explain why hypertensive animals while possessing fewer number of BP-lowering QTL alleles remain hypertensive, and vice versa.
However, several experimental observations called this arithmetic assumption into question.
First, certain normotensive genetic backgrounds on which BP QTLs operate do not allow blood pressure to change in the presence of a QTL, even multiple QTLs 8, 12. Second, some normotensive strains maintain their low blood pressure in defiance of the environmental onslaught such as a high salt diet 13. Finally, reciprocal renal transplantations between hypertensive and normotensive rats altered blood pressure non-mathematically 14.
Genetics of normotension resisting hypertension: Is this resistant genetic background due to an insufficient quantity of QTL alleles or to the existence of a hypertension ‘suppressor’? Epistasis and pleiotropy 11 could also be involved. To address this issue, standard QTL mapping schemes by statistics such as GWAS or total genome scan 11 can not be applied. A new genetic approach is needed that relies on experimentally manipulating the genome, resulting in functional changes in blood pressure.
(2) Content of new discovery: Our recent paper 12 documents the results of an original approach in the genetic research of normotension counteractive to hypertension. In essence, first, by systematically increasing the number of hypertensive QTL alleles in the resistant background, blood pressure was slightly augmented, but not in proportion to the quantity of hypertensive QTL alleles. Second, by gradually reducing the resistant genome in backcrosses between the susceptible and resistant genetic backgrounds, a chromosome region associated with blood pressure changes was identified. Thus, a potential hypertension ‘suppressor’ is believed to exist in the resistant genetic background, and might be pleiotropic as a suppressor and concomitantly a QTL on blood pressure. Finally, the hypertension-suppression phenomenon means no effect whatsoever on changing blood pressure and is not the same as epistasis, which refers to one QTL hiding the effect of another on the same phenotype 15, 16.
(3) Significance of the new results: First, they demonstrate, for the first time, that a cumulative impact from multiple hypertensive QTL alleles does not drive BP changes and can not overcome the power of the genome that resists the rise in blood pressure. Thus, understanding fundamental mechanisms of normotension is critically important in steering research directions and potentially generating rational clinical benefits.
Long-held and instinctively-assumed concepts on how blood pressure is determined have to be biologically tested because they may be erroneous. Together with QTL modularity in function 17 when the buffering capacity including hypertension suppression is removed, the present result further proves that blood pressure as a quantitative trait is functionally effectuated non-cumulatively.
Second, the new results identify a provisional location for a hypertension ‘suppressor’ in the normotensive genome. Consequently, an anti-hypertensive genetic locus may exist. Its molecular identification will likely result in formulating a novel diagnostic and therapeutic strategy in preventing hypertension.
The nature of this ‘suppressor’ appears to be a product that the normotensive Lewis rats possess, whereas the hypertensive Dahl salt-sensitive (DSS) rats have lost, because the Lewis genome is completely dominant in heterozygotes with DSS 12. Contrary to the genetic basis of HIV resistance, the resistant individuals lack the functional chemokine (C-C motif) receptor 5 (CCR5) 9, 10 and is recessive in heterozygotes.
Third, beyond the clinical applications, the existence of a hypertension ‘suppressor’ biologically reaffirms and reinforces the paradigm that a regulatory hierarchy plays an important role in actualizing the function modality of QTL actions 16. It is deducible that the ‘suppressor’ should stand even higher in the regulatory echelon 16 than the modularized pathways executed by QTL products, because in its presence, BP-raising QTL alleles combined from 2 modules can barely affect BP 12.
Finally, the ‘suppressor’ not only prevents hypertensive QTL alleles from raising blood pressure, but also stabilizes the base-line blood pressure in Lewis rats from dropping in the form of a homeostatic buffering capacity 8. Thus, its function is likely pleiotropic.
(4) Deductive inference versus ‘high tech’ and philosophy: First, before the molecular identity of the ‘suppressor’ is unknown, the concept of the hypertension-suppression paradox in the context of a regulatory hierarchy is built on logical deductions. How valid is it?
Currently, the genetic research on polygenic traits as a whole is dominated by a large-scaled, technology-powered, consortium-based epidemiological strategy. Overshadowed by this state-of-the-art vogue, going after fundamental questions and intellectual reasoning based on well-founded scientific results via conventional strategies seem ‘old fashioned’, out of date and place, and is even impugned by some, as ‘philosophy’ not science. Should the art of inference be supplanted by ‘high techs’ or is it irreplaceable so long as science endures? What is the difference between philosophy and logical deduction in science? One can even proclaim that exactly because of predictions based on the concepts formulated by inference, modern technologies know what and where to anticipate as concrete targets. A hypertension suppressor is one.
The most notable difference between deductive logic and philosophy is that the former is experimentally testable, whereas the latter is not.
Second, our results 12 predicated on the ‘classical’ genetics with unique and innovative twists have yielded 2 novel concepts: (a) non-cumulative actions of QTLs on blood pressure and (b) the appearance of hypertension suppression. No amount of sequencing or approaches by any other modern and large-scaled ‘high techs’ could have directly addressed these issues and generated similar results. Thus, the inferred concepts (a) and (b) are valid, indispensible, and spawn forward and testable hypotheses [see (5) below].
Finally, formulating the above 2 concepts logically follows a deductive process, which is completely different from philosophizing with no testable predictions. This inference is analogous to deduce that, since light has a finite speed, a celestial object of 1 million-light years away from the earth must take that long to travel through space. What we detect from that object at the present reflects what it was 1 million-light years ago, not at the very instant when the light was produced by it.
(5) Future research directions: Two lines of hypothesis can be put forward.
First, when the genome buffering capacity 8 including hypertension suppression 12 is disabled, modularized pathways of QTLs can change blood pressure 17. The components in each pathway, their order and regulatory relationships hinged on their epistatic hierarchies need to be elucidated 16. Some preliminary evidence suggests that a post-translational modification may be a base for a regulatory relationship between 2 BP QTLs 18.
Second, since a hypertension suppressor is so powerful that it can nullify the functionality of a QTL on blood pressure, its identification has to be performed by removing it and following blood pressure changes in vivo 16.
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Acknowledgements: This work was supported by grants from the Canadian Institutes of Health Research to AYD and a doctoral fellowship to KC (Fond de recherche en sante du Quebec).
Contact Information : Alan Y. Deng
Research Centre-Centre Hospitalier de l’Université de Montréal (CRCHUM)
Montréal, Québec H1W 4A4, Canada, Email: email@example.com