In the present study, an uGRS and a wGRS, considering 16 SNPs associated with BP, were developed in European adolescents. To our knowledge, this is the first study performed in European adolescents considering a combination of SNPs significantly associated with BP-related traits in Caucasian youth. Other GRSs used in pediatric population studies have been developed considering BP-associated SNPs derived from GWAS, in adult populations [21, 22]. In a Finnish follow-up study of children with European ancestry, a weighted GRS was developed, based on 13 SNPs previously associated with BP; the participants with the highest score in the GRS had significant higher diastolic BP at 9 years and the effect was maintained from childhood to adulthood (OR = 1.82) [21]. Additionally, a cohort study from the Netherlands, evaluated the variance explained by an adult-based GRSs developed for different traits related to anthropometry, cardiovascular and renal function, metabolism, and inflammation in Dutch adolescents. For BP levels, the variance explained in adolescents was similar to those observed in adults [23]. In contrast, other studies, which analyzed SNPs previously associated with BP in adults, did not find the same associations in adolescents with European ancestry [21, 24], reinforcing the hypothesis that genetic influences on BP change with age [37]. The transition from infancy to adulthood comprehends a number of changes in growth and BP. These changes are determined by interactions with age, lifestyle and behavioral risk factors [38]. Adolescence is a critical period when individuals with elevated BP levels are more likely to maintain higher BP levels when entering into the adulthood period [39]. It is also the stage in which most divergences in BP trajectories are observed [38]. For this reason, it is important to know whether there is a difference between the genes that modulate BP in adults and in children. Therefore, for the development of GRSs in adolescents, the SNPs significantly associated with BP found in BP-related GWAS performed in children and adolescents should be considered [24, 25].
There are several genetic polymorphisms involved in metabolic pathways related to BP. In the present study 16 SNPs were included in the GRS. Except for rs8057044, the rest of the SNPs included in the GRS had not been associated with BP in previous studies, despite the previous association between their gene and BP [21, 22, 24, 25].
One SNP was previously associated with obesity, the rs8057044 in the FTO gene, that has also been associated with high BP in a study performed in Tunisian adults. Specifically, G carriers of rs8057044 showed an association with high DBP [40]. In the present GRSs, the rs8057044 appeared as protective SNP for the development of HTN. This may be explained by the differences in both ethnicity and age between the two studies. Paired box 5 (PAX5) genes were also associated with elevated BP in French Canadian adolescents with obesity (12–18 years old) [41]. In the mentioned study, those carriers of the rs16933812 showed higher SBP levels. Similarly, in the present study the PAX5rs62533676 had a risk role for HTN.
Other SNPs were observed to be associated with BP. For example, the CACNB2 rs12258967 was analyzed in a study conducted in Lithuanian children and adolescents, showing high odds ratio of high BP in those children with this variant [15]. In contrast, in our study, the rs12258967 was not associated with BP in European adolescents. However, another SNP, related to the CACNB2 rs76466243, was significantly associated with BP and was included in the GRS. There was only one article showing a relationship of the CACNB2 gene (O.R < 1) with diabetic retinopathy [42]. Similarly, we have observed an association between this SNP and a protective effect on the development of HTN.
Another gene related to BP and HTN is the Serine/Threonine Kinase 39 (STK39) gene. This gene regulates the Na/Cl cotransporter. Several studies conducted in adult populations have found associations between different STK39 SNPs and BP or HTN [43]. However, a meta-analysis performed in adults from Asian and European populations, which analyzed the most frequent SNPs of the STK39 gene and their association with HTN did not find conclusive results [44]. In children and adolescents the only study found, performed in Chinese children, showed no associations between the most frequent SNP of STK39 rs3754777 and the risk of HTN [45]. Nevertheless, in the GRS developed in this study, a SNP related with the STRK39 rs6433023 gene has been included. This SNP is a possible protective factor for the development of HTN, hypothesizing that maybe the presence of this SNP could increase the sodium excretion in the renal tubule, reducing the BP levels.
Another widely studied gene has been the Beta 2 adrenergic receptor (ADRβ2); it appears to play an important role in salt-sensitive hypertensive patients [46]. In the developed GRS, ADRβ2 rs17108817 was also a risk factor for the development of HTN.
In addition, genes such as Pleckstrin Homology Domain Containing Family A Member 7 (PLEKHA7) and Serine/Threonine-protein kinase (ULK4) were found associated with BP [16]. However, the SNPs included in that GWAS differ from those obtained in our GRS. In our GRS, all the SNPs associated with these two genes act as a protector factors: rs75351046, rs72865722, rs10832706 for PLEKHA7 and rs4580521, ULK4 rs4973982 for ULK4.
Interestingly, from GWAS developed in children and adolescents, new loci for BP were identified, such as Integrin Subunit Alpha 11 ITGA11 rs1563894 associated with SBP assessed in prepuberty, and rs872256 associated with BP during puberty. These loci were not found to be associated with BP in adults. In the GRS developed, two risk ITGA11 SNPs, rs17320635 and rs895135, were also associated with risk of HTN. Furthermore, in the same GWAS, a SNP (rs872256) near to the SWI/SNF-related Matrix-associated, Actin-dependent Regulator Chromatin group A 2 (SMARCA2) gene was significantly associated with SBP in puberty children [24]. We also found an association between 3 SNPs of the SMARCA2 gene and BP. Two of them, rs7048826 and rs10965093, are risk SNPs for the development of HTN, whereas rs76973157 had a protective effect.
The GRS developed in the present study showed a good ability to predict the risk of HTN in adolescents. The use of external weights is a practice commonly used in the GRSs development [47]. However, in this study, internal weights were used instead. In addition, an uGRS was also built because it is more intuitive to understand than a weighted one. The wGRS and the uGRS, adjusted by principal components analyses, sex and age, both showed an AUC above 0.8 (uGRS: 0.830, wGRS: 0.838). In the literature, an AUC around or above 0.8 is an accepted value to use for clinical diagnosis [48]. In addition, when BMI z-score and pure fructose from non-natural foods were added to the GRS, the AUC increased, reaching levels close to 0.9. Both BMI z-score and pure fructose from non-natural foods showed a significant association with risk of HTN (p < 0.001). BMI and BP are widely related factors and it is expected that they are significantly related. However, the relationship between fructose and BP is controversial [34]. A study carried out in European adolescents observed an association between diastolic BP and a high consumption of pure fructose from non-natural foods [34]. The relationship between fructose and BP could be explained through the metabolism of this monosaccharide. Fructose is the only carbohydrate that can increases the uric acid levels. This increase causes hemodynamic effects on the organism (increased oxidative stress, endothelial dysfunction and activation of the renin–angiotensin–aldosterone system) and contributes to the BP increase [49].
The present study has some limitations. First, the results should be validated in other populations with larger sample size, similar age and with different ethnic origins. This would allow to test the validity of this BP-specific GRS. Instead, the GRS was internally validated, using cross-validation. Ideally, we should have performed an external validation in an independent cohort. Unfortunately, we did not have the genetic information necessary to carry it out. Second, the HELENA is a cross-sectional study and the long-term effects related with possible cardiovascular complications are not available. Moreover, repeated BP measurements over time were not performed, as suggested by the AAP guideline; therefore, it is not possible to confirm a diagnosis for HTN in this study. In addition, lack of repeated BP measurements over time is one of the reasons why we obtain a high percentage of adolescents with HTN compared with the prevalence observed in other studies. Third, we did not have genetic information of the X chromosome where the ACE2 gene is located. Perhaps the inclusion of the polymorphisms related to this gene may vary the predictive capacity of the GRS. Fourth, the 24-h recalls assess the sodium content in foods. However, this method does not considered the source of sodium from added salt. According to a recent systematic review a meta-analysis, the monitoring of sodium intake with a 24-hour urine is the most accurate method of sodium assessment. Nevertheless, studies developed with high-quality 24-h recalls and in high-income countries showed smaller differences in sodium intake comparing the two methods [50].
In contrast, our study showed some strengths; one was the inclusion of 10 cities from different European countries in the HELENA study. Moreover, a limited number of GRSs were developed to predict the risk of HTN considering both protective and risk SNPs. In addition, the GRS developed is formed by several SNPs of genes that have been related to BP in a GWAS performed in children and adolescents, reinforcing the importance of using specific GWAS and GRS for each stage of development. In this line, all SNPs related to all genes found in the scientific literature that are related to BP were included in the analyses, with special interest in those associated with BP in children and adolescents and those previously identified in the adult population. For future research we encourage to validate the present GRS developed in other cohorts with the same genetic ancestries and characteristics in youth.
In conclusion, the GRSs developed in this study could be used as genetic tools to detect adolescents with an increased risk of HTN. Therefore, the BP- specific GRSs may have the potential to guide preventative measures for HTN in youth. Personalized lifestyle interventions, such as diet and physical activity, could target individuals at high genetic risk already from early stages in life.