Background On RAS And Salt Sensitivity
The renin angiotensin system (RAS) contributes to regulation of vascular tone, electrolyte and volume homeostasis and its role in blood pressure (BP) has been well established. Two of the RAS gene polymorphisms that have been extensively studied include AGT M235T and ACE insertion/deletion (I/D).
According to family and twin studies, the heritability of hypertension ranges from 24% to 50%. Essential hypertension accounts for 95% of all cases of hypertension and involves a complex interaction of multiple polymorphisms in numerous genes, with more than 60 SNPs having been reported in European populations, while monogenic hypertension is well defined with 12 causative genes.
Salt intake is a major source of sodium in the general population and is at least two times greater than recommended intake and 10 times greater than what was consumed in the past. A high salt intake increases cardiovascular morbidity and mortality and is clearly associated with hypertension. Sodium sensitivity, defined as an increase in blood pressure in response to a higher sodium intake, is heightened with age. Some individuals manifest large blood pressure changes in response to salt repletion or depletion and these individuals are termed salt sensitive. Genetic factors along with race or ethnicity, age, body mass and diet all influence a person’s salt sensitivity. Almost 50% of individuals with essential hypertension are salt sensitive.
ACE I/D & AGT M235T T>C
Angiotensinogen (AGT) and angiotensin-1-converting enzyme (ACE), encode proteins of the renin-angiotensin system (RAS).
The ACE gene codes for the angiotensin converting enzyme and is part of the renin angiotensin system which regulates blood pressure by controlling the balance of fluids and salts in the body. This angiotensin converting enzyme converts the protein angiotensin 1 into a physiologically active protein called angiotensin 11. Angiotensin 11 is a potent vasopressor and aldosterone-stimulating peptide, which causes blood vessels to constrict, thereby increasing blood pressure. It also stimulates the production of the hormone aldosterone, which triggers the absorption of salt and water by the kidneys. This increased amount of fluid in the body also increases blood pressure. The angiotensin-converting enzyme can also cleave other proteins, including bradykinin. Bradykinin causes blood vessels to widen (dilate), which decreases blood pressure. Cleavage by the angiotensin-converting enzyme inactivates bradykinin, helping to increase blood pressure.
AGT is the natural substrate of RAS produced in the liver. The AGT gene provides instructions for making the protein angiotensinogen, which is part of the renin angiotensin system. AGT interacts with renin to produce angiotensin 1, the prohormone of angiotensin 11. Genetic variations in the AGT gene modify the plasma concentration of AGT and may be implicated in the pathogenesis of hypertension, coronary heart disease and myocardial infarction.
ACE I/D (ACE rs4341): SNP Summary
ACE is encoded by a 21kb, 26 exon gene located on chromosome 17 at q23. ACE insertion/deletion (I/D) polymorphism is the insertion or deletion (I/D) (or presence/ absence) of a 287-base pair in intron 16 of the ACE gene.
An insertion/deletion polymorphism of the ACE gene predicts approximately 40-50% of the inter-individual variability in serum ACE levels. DD homozygotes have been shown to demonstrate the highest activity of ACE, II homozygotes the lowest and ID heterozygotes in-between.
The ACE I/D gene polymorphism is most commonly associated with blood pressure and has been found to be affected by gender and race/ethnicity.
Studies have found that carriers of the DD allele have a higher incidence of hypertension compared to carriers of the II allele with classical studies having found that ACE activity level and susceptibility to hypertension follows the order DD>ID>II.
Although these DD carriers have higher incidence of hypertension, patients with essential hypertension who are homozygous for the insertion allele of the ACE gene (II) have been found to be more salt sensitive and hence see a greater spike in blood pressure in response to a higher sodium intake.
Dietary sodium/potassium balance is the most important factor affecting the Renin Angiotensin Aldosterone System (RAAS) and is mainly influenced by dietary salt intake. Reports suggest that ACE polymorphisms interact with dietary salt intake and subsequently effect the regulation of blood pressure. In DD carriers, the conversion of aldosterone to angiotensin 1 is blunted by a low sodium intake.
AGT M235T (rs699 T>C): SNP Summary
The gene that encodes angiotensinogen (AGT) is found on chromosome 1q42 to 43. The AGT M235T SNP leads to a tyrosine for cytosine substitution in the second exon of angiotensinogen gene, which results in the substitution of threonine for methionine at amino acid position 235 (M235T) in the translated protein. AGT T235 represents the mutant allele and M235 represents the wild type.
The variant Threonine (C) allele of the AGT M235T SNP has been associated with elevated levels of angiotensinogen, with individual’s homozygote for 235 TT (CC) having between 10% to 20% more plasma angiotensinogen than 235 MM (TT) individuals. Case control association studies of M235T polymorphism in essential hypertension have found conflicting results with Nigerian, French, Han Chinese and Malaysian populations finding linkage or association while others have not. There is a general agreement, however, that the T allele of angiotensinogen M235T gene is associated with increased plasma angiotensinogen levels and an increased risk of developing hypertension.
ACE I/D And Risk Of Hypertension
Studies have found a greater prevalence of ID and DD genotypes (presence of D alleles) amongst patients with hypertension. The presence of the D allele is associated with higher plasma concentrations of ACE, resulting in increased peripheral vascular resistance, loss of arterial elasticity and elevated blood pressure. Most studies have found associations between the ACE D allele and hypertension; however, some studies have not confirmed this association. This may be due to differences in methodologies, ethnicity reasons or regional aspects.
Heidari et al. (2014) found that patients carrying the DD genotype for ACE gene, had higher blood pressure lowering responses when treated with ACE inhibitors, enalapril and lisinopril than those carrying ID and II genotypes, suggesting that the D allele may be a possible genetic marker for essential hypertension among Malay male subjects.
Pacholczyk et al. carried out a study to investigate the association of I/D polymorphism with essential hypertension in a Polish population and to analyse the distribution of ACE gene polymorphisms in patients with hypertension. To examine the age dependence in the association of ACE DD genotype with hypertension, the frequency of ACE I/D polymorphism and allele distribution were investigated in two groups; a ‘young’ group with subjects <55 years of age and an ‘elderly’ group >55 years of age. To examine the sex specific association of hypertension, the genotypes of ACE I/D polymorphisms allele distribution were compared separately between men and women. To assess the potential modifying effects of body mass index on association between ACE I/D and risk of hypertension, the frequencies of genotypes of ACE I/D polymorphism and alleles distribution were examined separately in patients with normal body weight (BMI<25.0kg/m2) and in overweight or obese patients (BMI>25.0kg/m2). Data concluded that the ACE D allele might be a risk factor for hypertension in the Polish population, where a significant association between ACE I/D polymorphism and prevalence of hypertension was seen in the whole group. Apart from DD genotype, male sex, raised levels of triglycerides, total cholesterol, overweight and obesity and disorders in carbohydrate metabolism were significant risk factors for hypertension.
Frequency of DD genotype and D allele was associated with hypertension in the group of patients <55 years but not in the group >55 years. In males, the presence of at least one D allele increased risk of hypertension, whereas in females, association was in the presence of the homozygous DD genotype. Association with the D allele and prevalence of hypertension was seen in overweight/obese patients but not in those with normal body mass.
Bawazier et al. conducted a cross sectional study to evaluate the association between ACE I/D gene polymorphism and hypertension in a Yogykarta population. ACE ID and DD genotype were found to have a significant relationship with hypertension in the study population. A significant association was also found between BMI and hypertension.
Yako et al. conducted a systematic review to explore the genetic association studies of essential hypertension in African populations. Thirty-eight studies were included in the review and, in total, 46 polymorphisms in 33 genes were investigated for their association with prevalent hypertension and/or blood pressure. The polymorphisms included in the meta-analysis were those in the genes of the renin-angiotensin-aldosterone system (RAAS) and included angiotensin converting enzyme (ACE) I/D, angiotensinogen (AGT) M235T and angiotensin 11 type 1 receptor (AGTR1) 166A>C. The authors concluded that based on their systematic review, there is insufficient evidence on specific polymorphisms that pose the risk of hypertension in the African population, hence the need for further studies.
ACE I/D And Sodium Sensitivity
Freire et al. conducted a study to analyse the relationship between ACE I/D gene polymorphisms, environmental/behavioural factors and hypertension in community dwelling elderly individuals. 234 elderly participants from Bahia, Brazil participated in this study. The mean prevalence of hypertension was 59.3%. Hypertension was defined by self reporting the condition and confirmed by treatment with antihypertensive drugs. The main study finding was that ACE D allele carriers were not significantly associated with hypertension but carriers of the II genotype showed a greater prevalence of high estimated daily sodium intake, resulting in them being more prone to hypertension. Elderly patients with hypertension with the ACE II genotype showed poorer blood pressure control compared to D allele carriers (ID and DD) but had a higher sodium intake, with a mean estimated daily sodium intake of 2,57g (minimum of 0.15 and maximum 8.64g) compared to a mean intake of 1.67g in in those carrying the D allele (minimum of 0.09 and max 11.20g). The data from the study indicated that elderly patients carrying the II genotype of the ACE I/D gene polymorphism have a predisposition to sodium sensitivity and possibly poorer BP control and they should maintain strict control on their sodium intake, even if they are on taking antihypertensive medication.
The mechanism behind ACE II genotype carriers having a higher blood pressure response to salt intake may be that since sodium sensitive patients exhibit greater suppression of renin-angiotensin aldosterone system when exposed to sodium intake, it is plausible that II genotype carriers, who have lower levels of plasmatic ACE, may have a reduced suppressive response after a plasmatic sodium increase (due to lower levels of plasmatic ACE), indicating that there may be inefficiencies of an important regulatory mechanism of the renin-angiotensin aldosterone system when they are exposed to a high sodium diet.
In conclusion, interactions between II genotype and high sodium diet (>2g/day) increased the predisposition of hypertension in elderly patients and tended to impair BP control in elderly patients with raised BP being pharmacologically treated. Results from this study suggested that pharmacologically treated patients with hypertension with II genotype have poor BP control when following a high sodium diet.
Another study by Poch et al. also found the I allele of ACE I/D polymorphism to be significantly associated to salt-sensitive hypertension. They analysed the association between salt sensitivity in essential hypertension and 8 genetic polymorphisms in 6 genes (including ACE I/D and AGT M235T) of the renin angiotensin aldosterone system. Seventy-one patients with essential hypertension were classified as either salt sensitive (49%/ 35 patients) or salt resistant (51%/ 36 patients). Of the 6 candidate genes for salt sensitive hypertension, only 2 of them, ACE I/D and 11BHSD2 G534A polymorphisms, showed an association with BP response to a high salt diet.
Patients homozygous for the insertion allele of the ACE gene (II) had a significantly higher increase in blood pressure with high salt intake than patients who were homozygous for the deletion allele (DD). Heterozygous individuals (ID) showed an intermediate response. II and ID patients also had a higher prevalence of salt sensitive hypertension (68 and 59% respectively) compared to DD hypertensives (19%).
Giner et al. conducted a study to analyse the relationship between salt sensitivity and 3 polymorphisms commonly reported i.e. ACE I/D, AGT M23T and A1166C of the AT1 receptor gene. Fifty patients with essential hypertension were classified as salt sensitive or salt resistant, depending on their blood pressure response (significant increase or decrease) after a high salt intake. No relation was found between AGT M235T polymorphism or AT1 receptor A1166C polymorphism and BP response to increased salt intake, however the increase in BP as a result of high salt intake was significantly different depending on ACE I/D genotype. Individuals homozygous for the insertion allele (II) saw a much higher increase in 24-hour BP (9.8+-8.1mm Hg for systolic BP and 5.2+-4.2mm Hg for diastolic BP) than those who were homozygous for the deletion allele (DD) (1.2+-5.9mm Hg for systolic BP and -o.2+-4.2mmHg for diastolic BP). Heterozygote patients (ID) exhibited an intermediate response (5.0+-7.3 mm Hg for systolic BP and 2.4+-6.1mm Hg for diastolic BP). The authors concluded that salt sensitive essential hypertension is associated with insertion/deletion polymorphism of the ACE gene, which strengthens the hypothesis that salt sensitivity is partially determined by genetic predisposition.
AGT M235T And Risk Of Hypertension
AGT M235T gene polymorphism has been associated with essential hypertension with a hypertensive group of patients having 3x the frequency of the TT (CC) gene variation for AGT than normotensive controls, in a group of 1166 adults studied.
Ni et al. conducted a meta- analysis in which 22 studies were selected to investigate the relationship between AGT M235T polymorphism and risk of pre-eclampsia. They found that the TT (CC) genotype of AGT M235T polymorphism was associated with increased risk of pre-eclampsia and, in a subgroup based on ethnicity, increased risks were found in Caucasians but not in Hispanics or Black Africans. Possible reasons for this are discussed in the article. Authors concluded that TT (CC) genotype (vs MM (TT) and MT (CT) may play critical roles in the development of pre-eclampsia.
Srivastava et al. found that individuals with the T (C) allele of the AGT M235T SNP were strongly associated with essential hypertension in Northern Indians and their findings also provided direct evidence of a higher predisposition of Asian Indians to essential hypertension. Their findings suggested that angiotensinogen M235T polymorphisms may help in preventing the intergenotypic variations in blood pressure response to ACE inhibitor, Enalapril, in patients with essential hypertension in Northern India.
Zafarmand et al. studied a randomly selected sample of 1736 middle aged women who participated in a prospective cohort study of 17 357 Dutch women. After certain exclusions, 429 women with and 921 women without a history of elevated blood pressure during pregnancy remained for further analyses. Individuals with the TT genotype (CC) were more likely to have a history of raised BP during pregnancy than those with the MM genotype (TT). There was an increased risk in the heterozygote group, but with no clinical significance. Authors concluded that Caucasian Dutch women, aged 49-70 years, demonstrated that the presence of the T allele of the M235T polymorphism in AGT was associated with self-reported hypertensive disorders during pregnancy.
AGT And Salt Sensitivity
In a large cross-sectional study in a large free-living population, Norat et al. found that people with the TT (CC) genotype of AGT M235T polymorphism showed the greatest response to intervention compared to people with MM (TT) or MT (CT) genotype and that in populations with a low sodium exposure, genotype effects are not likely to be seen. In this large study of 11384 men and women, interactions between systolic blood pressure (SBP) and diastolic blood pressure (DBP), angiotensinogen genotype and urinary excretion of sodium and potassium were examined. Subjects with the AGT M235T homozygous MM (TT) genotype saw the weakest associations between sodium and blood pressure. Women homozygous for TT (CC) genotype had higher SBP and DBP than those with MT (CT) or MM (TT) genotypes. Blood pressure did not vary in men.
Associations between sodium and blood pressure were weakest in subjects with MM (TT) homozygous AGT M235T genotype, with the regression coefficient for systolic blood pressure associated with each unit of sodium for MT and TT genotypes approximately double that for MM homozygotes. Associations between sodium and blood pressure were most evident in the top quintile of sodium intake but no differences in SBP were seen in the lowest quintile of intake, indicating that the genotype effects in the population where not likely to be seen at low sodium intakes. The positive association between sodium and BP was stronger with SBP that with DBP. Overall, subjects with the TT (CC) genotype of AGT M235T showed the greatest response to the reduced salt intervention.
There are mixed results around the ACE I/D gene polymorphism and the AGT M235T gene polymorphism and salt sensitivity.
Overall, studies indicate individuals who are homozygous DD for ACE I/D are at a higher risk for having raised blood pressure, however individuals who are homozygous II genotype for ACE I/D, have been found to be salt sensitive i.e. they are likely to see an increase in blood pressure with a high salt intake compared to those carrying the DD genotype. Individuals carrying the ID genotype may also experience raised blood pressure in response to salt intake. Even though some studies did not find this association, given the available evidence, it would be advisable for individuals carrying the insertion allele (II or ID) to reduce their salt intake to avoid spikes in blood pressure and for overall management of blood pressure.
It should be noted that gender and ethnicity have both been found to affect risk of hypertension and/or sodium sensitivity for this SNP.
With regard to the AGT M235T SNP, studies have also shown mixed results, with some studies finding no association between this SNP and risk of blood pressure/ salt sensitivity, however some studies have found evidence that points towards the M235 TT (CC) genotype being a risk factor for hypertension. Others found that individuals carrying the TT (CC) genotype who have a high salt intake, may see a good blood pressure response to a reduced sodium intake. It would therefore be advisable that individuals with the AGT TT (CC) genotype reduce their sodium intake if they are consuming a high salt diet.
Please note that M235T variation is reported as AGT T>C on DNA reports, with the wild type being CC, heterozygote CT and homozygote TT. Many studies report on the amino acid change and refer to the variations as TT, MT or MM. TT links to CC, MT to CT and MM to TT. Check on this reporting method when reading articles to avoid confusion.
Daily sodium intake influences the relationship between angiotensin-converting enzyme gene insertion/ deletion polymorphism and hypertension in older adults.
Freire et al, 2017.