A Review of Genomic, Epigenomic, and Microbiomic Effects on Blood Pressure in Experimental Rat Models and Humans
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Physiol Rev 97: 1469–1528, 2017 Published September 20, 2017; doi:10.1152/physrev.00035.2016 TOWARDS PRECISION MEDICINE FOR HYPERTENSION: A REVIEW OF GENOMIC, EPIGENOMIC, AND MICROBIOMIC EFFECTS ON BLOOD PRESSURE IN EXPERIMENTAL RAT MODELS AND HUMANS Sandosh Padmanabhan and Bina Joe Institute of Cardiovascular and Medical Sciences, College of Medical, Veterinary and Life Sciences, University of Glasgow, Glasgow, United Kingdom; and Center for Hypertension and Personalized Medicine; Department of Physiology and Pharmacology, University of Toledo College of Medicine and Life Sciences, Toledo, Ohio Padmanabhan S, Joe B. Towards Precision Medicine for Hypertension: A Review of Genomic, Epigenomic, and Microbiomic Effects on Blood Pressure in Experimental Rat Models and Humans. Physiol Rev 97: 1469–1528, 2017. Published September 20, 2017; doi:10.1152/physrev.00035.2016.—Compelling evidence for the inherited nature of essential hypertension has led to extensive research in rats and humans. LRats 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 evi- dence 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 hyperten- sion, is documented. Overall, the collective knowledge gained from rats and humans is disap- pointing 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. I. BLOOD PRESSURE AS A COMPLEX I. BLOOD PRESSURE AS A COMPLEX ... 1469 POLYGENIC TRAIT II. EVIDENCE FOR INHERITANCE 1470 III. WHY AND HOW TO STUDY THE ... 1470 IV. RATS AS MODELS TO STUDY THE ... 1471 The focus on the role of arterial blood pressure as an im- V. BEYOND HIGH-RESOLUTION ... 1484 portant reporter of cardiovascular health has deep histori- VI. IDENTIFICATION OF MAPPED BP ... 1484 cal roots dating back to the 18th century. In 1733, the VII. OTHER GENES PRIORITIZED ... 1489 British clergyman Stephen Hales is documented to have VIII. PRIORITIZING CANDIDATE ... 1490 made the first published measurement of blood pressure in IX. HAS RAT GENETICS ADVANCED THE ... 1492 several animals by inserting fine tubes into arteries and mea- X. EPISTASIS OF BP QTLs 1492 suring the height to which the column of blood went up (86, XI. GENETIC STUDIES OF HUMAN ... 1495 88, 215). In 1836, Richard Bright found an association between renal disease and left ventricular hypertrophy (23). XII. EMERGING CONCEPTS 1508 He suggested that left ventricular hypertrophy could be a XIII. LOOKING AHEAD 1511 consequence of high blood pressure. Clinical features of XIV. CONCLUDING REMARKS 1514 essential hypertension were studied by T. C. Allbutt (6), who then went on to coin the term hyperpiesis to describe 0031-9333/17 Copyright © 2017 the American Physiological Society 1469 Downloaded from www.physiology.org/journal/physrev by ${individualUser.givenNames} ${individualUser.surname} (130.209.115.082) on March 7, 2018. Copyright © 2017 American Physiological Society. All rights reserved. SANDOSH PADMANABHAN AND BINA JOE essential hypertension, but the term was not popular. Eber- applied to medicine by Archibald Garrod (110). Beginning hard Frank is believed to have replaced it with the term in 1902, Garrod had studied alkaptonuria, cystinuria, and Essentielle Hypertonie, the English translation of which is albinism in relatives of patients and found that genetics Essential Hypertension, i.e., elevated high blood pressure explained these inborn errors of metabolism as Mendelian for no known causes (189). Despite these documented ob- heritable traits, but he also speculated that genetics may servations on the very nature of essential hypertension be- play a role in common diseases (110–114). ing noticeable in humans, the clinical significance of hyper- tension was, interestingly, a discovery attributed to keen More precise evidence for genetic factors to influence BP observations of individuals from a nonscientific back- came from correlative observations of a history of hyper- ground. As early as in 1906, long before the clinical concept tension in parents of hypertensive subjects in various pop- of hypertension as a risk factor became accepted, several ulations in Europe and North America (158, 251, 333, 334, insurance companies required medical examiners to docu- 378). Additional evidence was obtained through popula- ment applicants’ blood pressures (44, 190). The extent of tion studies wherein a greater concordance of blood pres- blood pressure being a good predictor of serious illness sure was observed within families than between families from stroke, heart failure, or renal disease was first recog- (226), but such studies also raised the question of additional nized by L.I. Dublin and A. J. Lotka, who were demogra- shared nongenetic, environmental factors such as food hab- phers working for a life insurance firm in the United States its within a family that could influence blood pressure. Twin (71, 72). Soon thereafter, the medical community followed studies wherein a greater similarity of blood pressures was with evidence from the studies in populations such as the documented with monozygotic than dizygotic twins (87, Framingham study to conclude that essential hypertension 207) suggested that genetic factors strongly influenced the is a condition qualitatively distinct from normotension and incidence of hypertension. Other studies suggest that ge- that elevated blood pressure was not just in those able to netic effects on BP are evident early in life by demonstrating afford life insurance (189). There was however the question a significant sib-pair and maternal-children correlation of of whether there was a “dividing line” between normal BP in infants (133, 212) and in children between 2 and 14 yr blood pressure and hypertension. Further clarification that of age (409). However, a clarification on environmental arterial blood pressure did not have a dividing line between factors overriding the genetic factors was inferred based on normal blood pressure and hypertension was provided by studies of BP variance in a Montreal adoption study of Pickering and co-workers (273, 280, 281) who studied French-Canadian families with both adopted and natural larger populations to study the frequency distributions of children (8, 9, 19, 20, 252). The study pointed to the highest arterial blood pressure in populations and emphasized that correlations of BP within children in a shared household there is a continuous relationship between arterial pressure environment, regardless of whether they were adopted or and mortality over a full range of arterial pressure. This not. This correlation between the children was more signif- definition changed the perception of hypertension being a icant than correlations between the BP of parents and their qualitatively different feature from normal blood pressure natural children, leading to the inference that nongenetic, to a “quantitative” feature, i.e., a trait that varies continu- environmental factors contribute more than genetic factors ally from low to high in a population. The recognition of to the extent of BP; however, the identities of both genetic this feature was in itself a milestone in the understanding of and environmental factors remained enigmatic. Another hypertension, but many questions remained as to what metric that is used to measure familial resemblance of a trait caused this variation in populations. and hence its genetic component is heritability. The higher the heritability, the stronger is the correlation between phe- notype and genotype. From twins and family studies, the II. EVIDENCE FOR INHERITANCE heritability of BP ranges from 15 to 40% for the office BP and around 51–69% for night time blood pressure ob- Pickering and co-workers (125–127, 283) were among the tained from ambulatory BP monitoring (131, 192, 207). In first to recognize familial correlation of blood pressure (BP). general, low heritability estimates indicate that genetic They collected BP data from large groups of normotensive mapping would be difficult for that phenotype. and hypertensive subjects along with their relatives and plotted frequency distribution curves after adjustment for age and gender. BP of relatives of hypertensive subjects were III. WHY AND HOW TO STUDY THE significantly higher than that of the relatives of normoten- GENETICS OF BLOOD PRESSURE sives (127). There was a linear relationship between the CONTROL level of BP of individual subjects with their relatives