Genetic epidemiology of essential hypertension

Epidemiology of essential hypertension: the role of genetic polymorphism

Over the past 20 years, the interest of the scientific community was increasingly placed in the field of genetic epidemiology and molecular genetics of blood pressure control. This paper explores references related to essential hypertension, gene and genetic epidemiology indexed in the MedLine health science database during the period 1980-2001. A systematic literature search was performed using selected keywords, such as 'genetic', 'genome' or a combination of words. We considered the study heading and evaluated the time profile of published articles. A total number of 3116 publications was collected and analyzed. Allelic distribution for the most studied polymorphisms of the renin-angiotensin system in different world populations was reviewed and reported together with a detection of their frequency in Italy: essential hypertensive patients (n = 90), healthy unrelated subjects (n = 300). Molecular variants at angiotensinogen (M and T), angiotensin II type 1 receptor (A and C) and angiotensin-converting enzyme (D and I) genes were analyzed by amplified fragment length polymorphism. A significant association was detected by chi2 analysis for angiotensinogen and angiotensin II-type I receptor allele distribution in hypertensive patients, in accordance with previous reports. Genetic data and methods are contributing more and more to epidemiological studies of complex diseases, and their application is influenced by information availability and Genome Project results.

Association of NEDD4L Ubiquitin Ligase With Essential Hypertension

NEDD4L is a ubiquitin ligase that controls cell surface expression of kidney epithelial Na+ channels by ubiquitin-mediated endocytosis and lysosome targeting. Thus, it is a significant determinant of Na+ reabsorption in the distal nephron. The NEDD4L gene is located on human chromosome 18q21 within several blood pressure quantitative trait loci, including those for familial orthostatic hypotension, essential hypertension, pulse pressure, and systolic blood pressure response to postural challenge. Because of the importance of NEDD4L to Na+ balance, many of these studies have proposed that mutations in NEDD4L may be responsible for these blood pressure phenotypes. To test this hypothesis, we fine-mapped the NEDD4L region in 2 families with orthostatic hypotension, which we previously reported to be linked to human chromosome 18q21 but failed to implicate NEDD4L in these families. We also typed multiple NEDD4L single-nucleotide polymorphisms (SNPs) in a collection of US whites, Greek whites, and African-Americans individuals with essential hypertension. A significant association between several SNPs and hypertension was observed in all 3 populations. One of the SNPs associated in African Americans is known to result in premature truncation of the NEDD4L protein. Thus, genetic variation in NEDD4L may play a role in the development or progression of some forms of abnormal blood pressure.

Genomic map of cardiovascular phenotypes of hypertension in female Dahl S rats

Genetic linkage analyses in human populations have traditionally combined male and female progeny for determination of quantitative trait loci (QTL). In contrast, most rodent studies have focused primarily on males. This study represents an extensive female-specific linkage analysis in which 236 neuroendocrine, renal, and cardiovascular traits related to arterial pressure (BP) were determined in 99 female F2 rats derived from a cross of Dahl salt-sensitive SS/JrHsdMcwi (SS) and Brown Norway normotensive BN/SsNHsdMcwi (BN) rats. We identified 126 QTL for 96 traits on 19 of the 20 autosomal chromosomes of the female progeny. Four chromosomes (3, 6, 7, and 11) were identified as especially important in regulation of arterial pressure and renal function, since aggregates of 8–11 QTL mapped together on these chromosomes. BP QTL in this female population differed considerably from those previously found in male, other female, or mixed sex population linkage analysis studies using SS rats. Kidney weight divided by body weight was identified as an intermediate phenotype that mapped to the same region of the genome as resting diastolic blood pressure and was correlated with that same BP phenotype. Seven other phenotypes were considered as “potential intermediate phenotypes, ” which mapped to the same region of the genome as a BP QTL but were not correlated with BP. These included renal vascular responses to ANG II and ACh and indices of baroreceptor responsiveness. Secondary traits were also identified that were likely to be consequences of hypertension (correlated with BP but not mapped to a BP QTL). Seven such traits were found, notably heart rate, plasma cholesterol, and renal glomerular injury. The development of a female rat systems biology map of cardiovascular function represents the first attempt to prioritize those regions of the genome important for development of hypertension and end organ damage in female rats.

Molecular genetics of human hypertension

The year 1999 saw considerable activity in the area of hypertension-related molecular genetics. Several new monogenic hypertensive disorders, as well as a monogenic form of hypotension, were elucidated. Molecular genetics has made significant inroads in explaining basic mechanisms of magnesium homeostasis. Linkage strategies have been applied in family studies, sib-pair analyses, and twin studies. More stringent criteria for association studies have been formulated. The 11 beta-hydroxysteroid dehydrogenase gene, the prostacyclin synthase gene, genes coding for variants in G proteins, and adrenergic receptor genes have received particular attention. On the horizon are better phenotyped patient and subject collectives, expanded genotyping with the availability of a 300,000 genome-wide single-nucleotide polymorphism map, multigenic studies in the form of metabolic control analyses, and new bioinformatic strategies including neural networks.

The effect of hypertension on the response to blood loss in a rodent model

OBJECTIVE

Hypertensive patients having higher baseline peripheral resistance and sympathetic tone than normotensive patients may have aberrant responses to hemorrhage. In an attempt to further characterize this clinical observation, the authors compared the hemodynamic and metabolic responses to hemorrhage between spontaneously hypertensive rats (SHR) and normotensive rats (NTR).

METHODS

Twenty adult rats (10 NTR and 10 SHR) were anesthetized with althesin via the intraperitoneal route. Femoral arteries were cannulated by cutdown. Twelve (6 SHR and 6 NTR) rats underwent controlled catheter hemorrhage of 25% of their total blood volumes. Eight rats (4 SHR and 4 NTR) served as nonhemorrhage controls. Mean arterial pressure (MAP) and base excess (BE) were measured prehemorrhage and then every 15 minutes for the next 120 minutes. Data were reported as mean +/- standard error of the mean (SEM). Group comparisons were analyzed by ANOVA with repeated values post-hoc by Bonferroni. Statistical significance was defined by an alpha = 0.05.

RESULTS

Immediately after hemorrhage, the SHR group experienced a significantly (p < 0.001) greater drop in MAP of 70 +/- 4% in the SHR vs 40 +/- 6% in the NTR. Blood pressure in the NTR returned to control values 15 minutes after hemorrhage, but the SHR remained relatively hypotensive for the entire length of the experiment. Base excess in the SHR decreased significantly (p < 0.004) by 8.2 +/- 2 mmol/L from control values, as compared with no changes in BE for the NTR.

CONCLUSIONS

The authors observed significant differences in the response to hemorrhage between hypertensive and normotensive rats. Hypertensive rats experienced a more profound hemorrhagic shock insult than normotensives for the same degree of blood loss.