Genomics and pharmacogenomics of salt-sensitive hypertension Minireview

Abstract

Salt sensitivity is estimated to be present in 51% of the hypertensive and 26% of the normotensive populations. The individual blood pressure response to salt is heterogeneous and possibly related to inherited susceptibility. Although the mechanisms underlying salt sensitivity are complex and not well understood, genetics can help to determine the blood response to salt intake. So far only a few genes have been found to be associated with salt-sensitive hypertension using candidate gene association studies. The kidney is critical to overall fluid and electrolyte balance and long-term regulation of blood pressure. Thus, the pathogenesis of salt sensitivity must involve a derangement in renal NaCl handling: an inability to decrease renal sodium transport and increase sodium excretion in the face of an increase in NaCl load that could be caused by aberrant counter-regulatory natriuretic/antinatriuretic pathways. We review here the literature regarding the gene variants associated with salt-sensitive hypertension and how the presence of these gene variants influences the response to antihypertensive therapy.

The prevalence of hypertension in the adult population worldwide is more than 26% and is estimated to increase to 30% by 2025 constituting an important public-health challenge in industrialized and developing countries (1). The prevalence of hypertension varies with age, sex, and ethnicity and is also affected by behavior such as the intake of dietary sodium and potassium. Excess dietary salt intake is a predominant cause of hypertension. The increase in blood pressure with an increase in sodium intake occurs in both normotensive and hypertensive subjects and irrespective of basal blood pressure levels is predictive of increased cardiovascular events and mortality (2,3). Salt sensitivity is estimated to be present in 51% of the hypertensive and 26% of the normotensive populations (4).

The individual blood pressure response to salt is heterogeneous and possibly related to inherited susceptibility. Although the mechanisms underlying salt sensitivity are complex and not well understood, genetics can determine the blood response to salt intake (5, 6). Research in the field has concentrated on the identification of common allelic variants of candidate genes for hypertension in relation to the salt-sensitive phenotype and the pathogenic role of gene-gene interaction with the aim to distinguish salt-sensitive from salt-resistant subjects. So far, however, genome-wide association studies have identified genes that influence only 2% of blood pressure variability and have not identified genes that influence the salt sensitivity of blood pressure. Only a few genes have been found to be associated with salt-sensitive hypertension using candidate gene association studies.

The kidney is critical to overall fluid and electrolyte balance and long-term regulation of blood pressure (7). The identification of rare monogenic forms of hypertension associated with abnormalities of renal tubular sodium handling (8) highlighted the important role of renal alterations in salt-sensitive hypertension but cannot explain the high incidence of salt sensitivity, especially in normotensive humans. These observations however indicate that the pathogenesis of salt sensitivity must involve a derangement in renal NaCl handling: an inability to decrease sodium transport and increase sodium excretion in the face of an increase in NaCl load that could be caused by aberrant counter-regulatory natriuretic/antinatriuretic pathways (9). The sympathetic nervous system and the renin-angiotensin system (RAS) (10–12) are examples of antinatriuretic pathways. An important counter-regulatory natriuretic pathway is afforded by the renal autocrine/paracrine dopamine system, aberrations of which are involved in the pathogenesis of hypertension (13–15)

Genetic variants associated to salt sensitivity of blood pressure

This review updates the comprehensive reviews of the genetics of salt sensitivity by Strazzullo and Galletti in 2007 (8) and Sanada et al 2011 (5).

Table 1 lists the genetic variants reported to be associated to salt sensitivity, by themselves or in association with others, in studies replicated in one or more cohorts or controlled studies involving large number of subjects (GenSalt, HyperPATH, HapMap).

Table 1.

Genes associated with salt-sensitive essential hypertension

Candidate Gene	Associated Variant	References
Genes related to increased renal sodium transport or vascular reactivity
Angiotensinogen (AGT)	rs5051 (G6A) M235T (rs699)	8,29,54
Angiotensin-converting enzyme (ACE)	Indel polymorphism	8,29
Angiotensin II type 1 receptor (AGTR1)	rs4524238	55
Aldosterone synthase (CYP11B2)	rs1799998 (T-344C)	56,57
11β-Hydroxysteroid dehydrogenase type 2 (HSD11B2, 11βHSD2)	(rs45598932 (G-209A)	58
Lysine specific demethylase 1 (LSD-1)	rs671357 rs587168	17
Striatin (STRN)	rs2540923	18
β2 adrenergic receptor (ADRB2)	rs1042714 (C79G) rs1042713 (A46G)	59
Chloride channel, Kidney A (CLCNKA)	rs1010069 rs1805152 rs848307 rs1739843	60
Epithelial sodium channel gamma subunit1 (ENaC gamma subunit1) (SCNN1G)	rs5718 (G-173A) rs4073930 rs7404408 rs4299163 s4073291 rs5735 rs4499238	34
Neural precursor cell expressed developmentally down-regulated 4-like (NEDD4L)	rs2288774 rs4149601	45,61
Protein kinase, lysine-deficient 1 (WNK1)	rs880054	45
Serum- & Glucocorticoid Regulated Kinase (SGK1)	rs2758151 rs9402751	62,63
Ste20-related proline-alanine–rich kinase (STK39)	rs6749447 rs3754777 rs1937506 rs35929607	64,65,66,67
Sodium bicarbonate exchanger (SLC4A5)	rs7571842 rs10177833	19
Uromodulin (UMOD)	rs13333226 rs4293393	20,21
ATPase, Ca++ transporting (ATP2B1)	rs2681472	67
Cytochrome P450 3A5 (CYP3A5)	rs776746 (A6986G)	68
Guanine nucleotide binding protein, subunit 3 (GNB3)	rs1129649 (C825T)	69
Potassium dependent Na+/K+/Ca2+ exchanger type 3 NCKX3 (SLC24A3)	rs3790261	70
Sodium Calcium exchanger 1 NCX1 (SLC8A1).	rs434082	70
Endothelin converting enzyme 1 gene (ECE1)	rs213014	71
NAD(P)H oxidase p22phox (CYBA)	rs4673 (C242T)	72,73
Fibroblast growth factor 5 (FGF5)	rs16998073	67
Branched chain aminotransferase 1 (BCAT1)	rs7961152	67
α-adducin (ADD1)	rs4961 (G460T) rs17833172	42,74
Genes associated to decreased renal sodium transport or vascular reactivity
Endothelin receptor type B (EDNRB, ETB)	rs5351 (G1065A)	8,29
Nitric oxide synthase 3 (NOS3)	rs2070744 (T-786C)	75,76
Dimethylarginine dimethylaminohydrolase 1 (DDAH1)	rs11161637	17,75
cGMP-dependent protein kinase 1 (PRKG1)	rs7897633	70
Bradykinin receptor B2 (BDKRB2)	rs11847625	71
von Willebrand factor (VWF)	rs2239153	77
20-HETE synthase (CYP4A11)	T8590C	78
G protein-coupled receptor kinase 4 (GRK4)	Arg65Leu (rs296036) Ala142Val (rs1024323) Ala486Val (rs1801058)	5,19,22,24,47

Genes related to increased renal sodium transport or vascular reactivity

Renin-angiotensin aldosterone system (RAAS)

The RAAS is the most important regulator of renal sodium transport and the major system involved in the increase in renal sodium transport, especially under conditions of sodium deficit (10, 16). Only two polymorphisms of AGT, one in ACE, one in AGTR1, one in CYP11B2, and one in 11βHSD2 the enzyme, that inactivates 11-hydroxysteroid in the kidney protecting the mineralocorticoid receptor from occupation by glucocorticoids, have been reported to be associated with salt-sensitive hypertension. Polymorphisms in the lysine-specific demethylase 1 gene, an epigenetic regulator of aldosterone production were also found associated to salt sensitivity (17).

Striatin is a protein that regulates the nongenomic actions of the estrogen receptor-α and mineralocorticoid receptor. STRN rs2540923 has been reported to be associated with salt sensitivity in humans (18).

Sympathetic nervous system

Increased sympathetic activity has been demonstrated in salt- sensitive hypertension. Although several polymorphisms in the enzymes involved in the synthesis of catecholamines and their receptors have been described to be associated with essential hypertension only a haplotype of the ADRB2 has been associated with salt sensitivity.

Renal ion transport

Renal ion channels and transporters are common final pathways in the regulation of the transport of sodium and other ions. Common variants of the amiloride sensitive epithelial sodium channel gamma subunit1 (ENaCg), and several in genes encoding ENaC regulators such as NEDD4L, WNK1, and SGK1, as well as the renal chloride channel CLCNA, are associated with hypertension and salt sensitivity. Polymorphisms in the STK39 gene which encodes a serine/threonine kinase that interacts with WNK kinases and cation-chloride cotransporters, have also been identified to be associated salt sensitivity of blood pressure. Similarly, variants in the SLC4A5 gene that codes for NBCe2, an electrogenic Na+-bicarbonate co-transporter that helps maintain the homeostasis of intracellular pH by co-transporting three bicarbonate anions for each Na+ cation are also associated with salt sensitivity (19). Uromodulin also known as Tamm-Horsfall protein is expressed in the thick ascending limb of Henle and has been linked to water/electrolyte balance. A polymorphism in the uromodulin gene (rs13333226) is associated with hypertension and lower fractional excretion of sodium (20). Another polymorphism rs4293393 is associated with increased diastolic blood pressure and salt sensitivity (21)

Genes that regulate vascular smooth muscle tone

Polymorphisms of genes that influence the vascular tone through an increase in the concentration of cytoplasmic calcium (Ca2+), such as SLC24A3 and SLC8A1) or endothelial function such as ECE1 are associated with salt-sensitive hypertension.

Reactive oxygen species (ROS)

In-non-phagocytic cells ROS regulate vascular tone, cell growth, and inflammation and may be important mechanisms that contribute to the maintenance of high blood pressure initiated by other primary processes. Some polymorphisms in genes regulating the production of ROS and inflammation such as NAD(P)H oxidase p22phox (CIBA), FGF5, and BCAT1, a marker of oxidative stress are also associated with salt-sensitive hypertension.

Adducins are cytoskeletal proteins that regulate the membrane organization of spectrin-actin which is involved in the internalization and recycling of Na⁺/K⁺-ATPase. Although wild-type ADD1 has only a minimal effect on Na+K+-ATPase, some of its variants increase Na+K+-ATPase activity and therefore increase renal sodium transport, are associated with salt-sensitive hypertension.

Genes related to decreased renal sodium transport or vascular reactivity

Vasoactive peptides/substances

Endothelin mediates vasoconstriction but can cause also vasodilation by increasing NO production from endothelial cells and decrease sodium transport. ETBR in vascular smooth muscle cells mediate vasoconstriction while ETBR in endothelial cells mediate vasodilatation.

Polymorphisms in two genes related to NO synthesis, NOS3 and DDAH1, and in one involved in NO signaling, PRKG1, are associated with salt sensitivity. Bradykinin causes vasodilation though the BDKRB2. 20-HETE increases blood pressure by vasoconstriction but can also decrease blood pressure by decreasing renal sodium transport, especially at the proximal tubule and thick ascending limb of Henle. The loss-of-function in 20-HETE synthase (CYP4A11) 8590C allele is associated with salt sensitivity.

Dopaminergic system

Dopamine produced in the kidney (mainly by proximal tubules) acts in an autocrine/paracrine manner to regulate renal ion transport The inhibition of renal transport of sodium and other ions occurs in multiple segments of the nephron, including the proximal and distal convoluted tubule, thick ascending limb of Henle, and collecting ducts. The inhibitory effect of dopamine and D1-like receptor agonists on sodium transport is impaired in hypertension and the decreased function of D1R in hypertension may be related to a state of constitutive desensitization due to the presence of activating variants of GRK4, including GRK4 R65L, A142V and A486V. Several studies in different ethnic groups have shown that GRK4 gene variants are associated with human essential hypertension and salt sensitivity (22–25). Salt-sensitive hypertensive Japanese subjects carrying at least three GRK4 gene variants have an impaired natriuretic response to a dopaminergic drug and salt-sensitive hypertension can be correctly predicted in 94% of the cases. Sodium excretion was inversely related to the number of GRK4 variants in hypertensive persons, and the natriuretic response to dopaminergic stimulation was impaired in normotensive persons having GRK4 gene variants (24). The presence of the different GRK4 variants depends on ethnicity. GRK4 486V is more frequent in Asians (47%) than in Caucasians (40%), Hispanics (28%), or African Americans (19%). By contrast, GRK4 65L and GRK4 142V are more frequent in African Americans (47% and 45%, respectively) than in the other ethnic groups: Caucasians (35% and 40%), Hispanics (25% and 29%), and Asians (7% and 20%) (26). In Korean children the best combination to predict obesity as sodium intake was increased, was SLC12 A3, ACE and GRK4 A486V in boys, and CYP11β-2 and GRK4 A486V in girls (27)

A particular phenotype is caused not only by one particular gene but by gene-gene interactions which may affect the same or different physiological pathways. Thus, the susceptibility to salt-sensitive hypertension may be determined not only by specific gene variants but also by combinations of multiple gene variants. Table 2 lists reported gene-gene interactions in salt-sensitive hypertension

Table 2.

Gene-gene interactions in salt-sensitive hypertension

Gene variant	Interacting with	References
Angiotensin-converting enzyme (ACE) In/del	EDNRB 1065GG HSD11B2 G534A	8,29,79,80
β2 adrenergic receptor ADRB2 C79G ADRB2 Q27E	ADRB2 A46G TH rs2070762	25,59
Neural precursor cell expressed developmentally down-regulated 4-like (NEDD4L) rs2288774	NEDD4L rs4149601	61
Potassium dependent Na+/K+/Ca2+ exchanger type 3 (NCKX3) SLC24A3 rs3790261	Sodium Calcium exchanger 1 NCX1 SLC8A1 rs434082	70
Chloride channel, Kidney A CLCNKA rs1010069	CLCNKA rs12126269	60
Cytochrome P450 3A5 CYP3A5 rs776746 (A6986G )	ABCB1	68
α-Adducin ADD1 rs4961( G460T )	WNK1 rs1159744 and NEDD4L rs4149601 ACE DD and CYP11B2 rs1799998 (−344C)	25,80
G protein-dependent receptor kinase 4 (GRK4) GRK4 486V GRK4 486V GRK4 486V GRK4 486V GRK4 486V GRK4 65L	GRK4 142V, GRK4 65L predict salt-sensitive hypertension (94%) TH 2070762 and ADRB2 Q27E GNB3 A-350G CYP11B2*1C GKR4 142V, GNB3 and PAI-1 ACE I/D	19,24,25, 27,82,

Pharmacogenomics of salt-sensitive hypertension

The inter-individual variability and abnormal response to antihypertensive drugs may be partially accounted for genetic polymorphisms. Common gene variants may influence the response to diuretics, β-adrenergic blockers, angiotensin-converting enzyme inhibitors, and angiotensin receptor blockers. Pharmacogenomics studies the genetic basis for variation in drug response (28) with the aim of predicting whether a patient will have a good response to a drug, an adverse response to a drug, or no response at all and designing therapeutic strategies on the bases of matching the pathophysiology of the increase in blood pressure to the pharmacological action of the drug prescribed. However the literature in the field is limited, in particular regarding salt-sensitive hypertension.

Genes related to increased renal sodium transport or vascular reactivity

The AGT M235T polymorphism was associated with a decrease in blood pressure after a decrease in sodium intake. Carriers of TT or MT genotype had lower blood pressure after sodium restriction compared with carriers of the MM genotype (29,30). The ACE insertion/deletion (I/D) polymorphism, DD in men and II and ID in women (31) was associated with the systolic blood pressure response to hydrochlorothiazide in a Han Chinese population of mild/moderate with essential hypertension. In the same population, CYP11B2 -344T/C was not found to be associated to the response to the diuretic (32) Interestingly neither AGT M235T nor ACE I/D was associated with antihypertensive effects of RAS blockade (33).

Each of the ENaCγ gene variants, SCNN1G rs4073930, rs4073291, rs404408, rs5735, rs4299163, and rs4499238 had a 1.09 to 1.33 mm Hg decrease in systolic blood pressure in response to low sodium diet for 7 days (34).

NEDD4L negatively regulates NCC and the renal epithelial sodium channel (ENaC). NEDD4L rs4149601 has been associated with a 6.1 mm Hg reduction of systolic blood pressure in response to hydrochlorothiazide (35). NEDD4L rs4149601 was also associated with a 4.5 mm Hg reduction in systolic blood pressure with β-adrenergic blockade or diuretic monotherapy in another study; there was no influence on the response to the calcium channel blocker, diltiazem (36). A greater blood pressure response to hydrochlorothiazide was observed with increasing copies of the G-C haplotype of NEDD4L (for the SNPs rs4149601 and rs292449, respectively) (37).

In Finnish males, STK39 rs6749447 was associated with a lower systolic and diastolic blood pressure response to losartan than bisoprolol, hydrochlorothiazide, or amlodipine (38). In another study STK39 rs6749447, rs3754777, and rs35929607 were not associated with a blood pressure response to thiazides (39). Why STK39 rs6749447 is associated with a better blood pressure lowering effect to an AT1R blocker but not to a diuretic (38,39) cannot be explained at this time. However, angiotensin II signaling related to NCC is dependent on WNK4-SPAK (encoded by STK39) signaling (40) GNB3 C825T alone or in combination with two other polymorphisms A3882C and G5249A was associated with a blood pressure response to atenolol in women but not in men (41). ADD1 rs4961 has been reported to be associated with hypertension in several but not all ethnic groups (42). ADD1 rs4961 predicted a 14 mm Hg decrease in blood pressure in response to an ouabain antagonist in some studies (43,44) and a 4 mm Hg greater decrease in blood pressure (variant vs. wild-type) in response to one month of diuretic treatment (45). The latter response was observed in the presence of two other variants that are also associated with salt sensitivity, WNK1 rs880054, NEDD4L rs4149601 (45).

Genes related to decreased renal sodium transport or vascular reactivity

GPR83 rs3768785 was associated with a good response to the AT1R blocker, candesartan, but not to hydrochlorothiazide in whites but good response to both drugs in blacks (46).

Constitutively active variants of GRK4 (rs296036, rs1024323, rs1801058) are associated with hypertension with or without salt sensitivity (19,47,48). GRK4 rs2960306 and GRK4 rs1024323 haplotype were predictive of a failure to decrease mean arterial pressure to ≤107 mm Hg in response to β-adrenergic blockade in African-American men with early hypertensive nephrosclerosis. GRK4 rs1024323 by itself was associated with a response to β-adrenergic blockade mean arterial pressure <107 mm Hg, albeit a slow one (49). In a study in two cohorts of American population with essential hypertension without renal disease, as the number of individual GRK4 SNPs 296036 (R>L) and 1024323(A>V) increased, the blood pressure response to β-adrenergic blockade in a mixed population of black and white individuals decreased while the opposite was observed with the wild-type carriers (50). Whether or not the different results between these two studies are related to ethnicity or the presence or absence of renal disease remains to be determined. In South African blacks GRK4 R65 or GRK4A142 predicted a good blood pressure response to a decrease in salt intake for 8 weeks, while GRK465L or GRK4142V predicted a limited response to reduced salt intake (51). The presence of at least three GRK4 allele variants (65L, 142V, and 486V), relative to those with fewer than three was associated with a better response to diuretic therapy (52). Hypertensive Japanese showed an allelic and genotypic association of GRK4142V with enhanced response of blood pressure to AT1 receptor blockers. Although individuals with one or two copies of GRK4142V did not show a difference in the achievement of blood pressure goal compared to those with no GRK4 variants, they had a significantly greater decrease in systolic blood pressure in response to AT1 receptor blockers than non-carriers (53).

The slow progress in the knowledge on the genetics of salt-sensitive hypertension and the pharmacogenetics of abnormal response to antihypertensive drugs is related to the complexity of the genome regulation and the heterogeneity of hypertension. The identification of genetic variants related to blood pressure regulation may reveal new therapeutic drug targets.