The contribution of the C-824T tyrosine hydroxylase polymorphism to the prevalence of hypertension in a South African cohort: the SABPA study

Hypertension in African Populations: Review and Computational Insights

Hypertension (HTN) is a persistent public health problem affecting approximately 1.3 billion individuals globally. Treatment-resistant hypertension (TRH) is defined as high blood pressure (BP) in a hypertensive patient that remains above goal despite use of ≥3 antihypertensive agents of different classes including a diuretic. Despite a plethora of treatment options available, only 31.0% of individuals have their HTN controlled. Interindividual genetic variability to drug response might explain this disappointing outcome because of genetic polymorphisms. Additionally, the poor knowledge of pathophysiological mechanisms underlying hypertensive disease and the long-term interaction of antihypertensive drugs with blood pressure control mechanisms further aggravates the problem. Furthermore, in Africa, there is a paucity of pharmacogenomic data on the treatment of resistant hypertension. Therefore, identification of genetic signals having the potential to predict the response of a drug for a given individual in an African population has been the subject of intensive investigation. In this review, we aim to systematically extract and discuss African evidence on the genetic variation, and pharmacogenomics towards the treatment of HTN. Furthermore, in silico methods are utilized to elucidate biological processes that will aid in identifying novel drug targets for the treatment of resistant hypertension in an African population. To provide an expanded view of genetic variants associated with the development of HTN, this study was performed using publicly available databases such as PubMed, Scopus, Web of Science, African Journal Online, PharmGKB searching for relevant papers between 1984 and 2020. A total of 2784 articles were reviewed, and only 42 studies were included following the inclusion criteria. Twenty studies reported associations with HTN and genes such as AGT (rs699), ACE (rs1799752), NOS3 (rs1799983), MTHFR (rs1801133), AGTR1 (rs5186), while twenty-two studies did not show any association within the African population. Thereafter, an in silico predictive approach was utilized to identify several genes including CLCNKB, CYPB11B2, SH2B2, STK9, and TBX5 which may act as potential drug targets because they are involved in pathways known to influence blood pressure. Next, co-expressed genes were identified as they are controlled by the same transcriptional regulatory program and may potentially be more effective as multiple drug targets in the treatment regimens for HTN. Genes belonging to the co-expressed gene cluster, ACE, AGT, AGTR1, AGTR2, and NOS3 as well as CSK and ADRG1 showed enrichment of G-protein-coupled receptor activity, the classical targets of drug discovery, which mediate cellular signaling processes. The latter is of importance, as the targeting of co-regulatory gene clusters will allow for the development of more effective HTN drug targets that could decrease the prevalence of both controlled and TRH.

Genetic factors contributing to hypertension in African-based populations: A systematic review and meta-analysis

In a systematic review, the authors explored genetic association studies of essential hypertension in African populations. Studies reporting on the association of polymorphism(s) with hypertension in African populations were included. Appropriate studies were pooled using random effects model meta-analysis, under six potential inheritance models. In all, 46 polymorphisms in 33 genes were investigated for their association with hypertension or blood pressure levels. Meta-analysis was possible for three single nucleotide polymorphisms: rs4340, rs699, and rs5186. An association was found between rs5186, rs699, and hypertension under allele contrast and homozygous codominant models (odds ratio, 1.63 [95% confidence interval, 1.04-2.54] and 4.01 [95% confidence interval, 1.17-13.80] for rs5186, respectively; and 1.80 [95% confidence interval, 1.13-2.87] for rs699). Findings were mostly robust in sensitivity analyses. According to the systematic review, there is currently insufficient evidence on the specific polymorphisms that pose the risk of hypertension in African populations. Large-scale genetic studies are warranted to better understand susceptibility polymorphisms that may be specific to African populations.

Chronic defensiveness and neuroendocrine dysfunction reflect a novel cardiac troponin T cut point: The SABPA study

BACKGROUND

Sympatho-adrenal responses are activated as an innate defense coping (DefS) mechanism during emotional stress. Whether these sympatho-adrenal responses drive cardiac troponin T (cTnT) increases are unknown. Therefore, associations between cTnT and sympatho-adrenal responses were assessed.

METHODS

A prospective bi-ethnic cohort, excluding atrial fibrillation, myocardial infarction and stroke cases, was followed for 3 years (N=342; 45.6±9.0years). We obtained serum high-sensitive cTnT and exposure measures [Coping-Strategy-Indicator, depression/Patient-Health-Questionnarie-9, 24h BP, 24h heart-rate-variability (HRV) and 24h urinary catecholamines].

RESULTS

Blacks showed moderate depression (45% vs. 16%) and 24h hypertension (67% vs. 42%) prevalence compared to Whites. A receiver-operating-characteristics cTnT cut-point 4.2ng/L predicting hypertension in Blacks was used as binary outcome measure in relation to exposure measures [AUC 0.68 (95% CI 0.60-0.76); sensitivity/specificity 63/70%; P≤0.001]. Bi-ethnic cTnT-incidence was similar (Blacks=27%, Whites=25%) with cTnT-recovery better in Blacks (9%) compared to Whites (5%), P=0.001. In cross-sectional analyses, elevated cTnT was related to DefS [OR 1.08 (95% CI 0.99-1.16); P=0.06]; 24h BP [OR 1.03-1.04 (95% CI 1.01-1.08); P≤0.02] and depressed HRV [OR 2.19 (95% CI 1.09-4.41); P=0.03] in Blacks, but not in Whites. At 3year follow-up, elevated cTnT was related to attenuated urine norepinephrine:creatinine ratio in Blacks [OR 1.46 (95% CI 1.01-2.10); P=0.04]. In Whites, a cut point of 5.6ng/L cTnT predicting hypertension was not associated with exposure measures.

CONCLUSION

Central neural control systems exemplified a brain-heart stress pathway. Desensitization of sympatho-adrenal responses occurred with initial neural- (HRV) followed by neuroendocrine dysfunction (norepinephrine:creatinine) in relation to elevated cTnT. Chronic defensiveness may thus drive the desensitization or physiological depression, reflecting ischemic heart disease risk at a novel 4.2ng/L cTnT cut-point in Blacks.

Genetic Variations of Tyrosine Hydroxylase in the Pathogenesis of Hypertension

One of the major pathophysiological features of primary hypertension is an inappropriate activation of the sympathetic nervous system, which is mediated by excessive synthesis and secretion of catecholamine into the blood. Tyrosine hydroxylase (TH), a rate-limiting enzyme in the synthesis of catecholamine, has been highlighted because genetic variations of TH could alter the activity of the sympathetic nervous system activity and subsequently contribute to the pathogenesis of hypertension. Here, we discuss the role of TH as a regulator of sympathetic activity and review several studies that investigated the relationship between genetic variations of TH and hypertension.

Defensive coping and essential amino acid markers as possible predictors for structural vascular disease in an African and Caucasian male cohort: The SABPA study

Defensive coping (DefS), oxidative stress, inflammation, and related amino acids (phenylalanine [Phe] and tyrosine [Tyr]) have been implicated in cardiovascular disease. This study assessed whether inflammation, oxidative stress, changes in essential amino acids, and altered coping strategies are correlated with subclinical vascular changes in African (n = 82) and Caucasian (n = 100) men from the Sympathetic Activity and Ambulatory Blood Pressure in Africans (SABPA) study. The Coping Strategy Indicator questionnaire identified DefS participants. Ambulatory blood pressure (BP) was monitored for 24 h, whereas carotid intima media thickness (CIMT) and cross-sectional wall area (CSWA) were determined ultrasonically. Essential amino acids were analyzed with a liquid chromatography tandem mass spectrometry method. Oxidative-inflammatory markers were measured by spectrophotometry. African men had poorer health than Caucasian men, including higher alcohol abuse, elevated BP, abdominal obesity, physical inactivity, and elevated inflammation. Phe (p < .001) and Phe/Tyr ratio (p = .006) as well as CIMT (p = .032) were higher in African men. DefS African men had higher levels of Phe (p = .002) and Phe/Tyr (p = .009) compared to DefS Caucasian men; these differences were not observed in non-DefS men. Systolic BP and inflammation (C-reactive protein) were positively associated with left (L-) CSWA, while Phe/Tyr was negatively associated with L-CSWA in DefS African men. African males presented with elevated Phe and Phe/Tyr ratio, catecholamine precursors, worsening during DefS-possibly driven by inflammation and BP contributing to structural vascular abnormalities.