Linkage mapping for hypertension susceptibility genes

The discovery of hypertension: evolving views on the role of the kidneys, and current hot topics

Primary hypertension is increasingly common and is associated with significant morbidity. Here, we review the history of its discovery and rise during the last century with an emphasis on studies trying to identify its cause. Early studies identified a defect in sodium excretion by the kidney as being central to the pathogenesis. Recent studies have focused on a variety of genetic, congenital (fetal programming), and acquired mechanisms for causing the defect in natriuresis. Certain risk factors are apparent, including genetic polymorphisms that regulate sodium excretion, a congenital reduction in nephron number, obesity and hyperleptinemia, an elevated sympathetic nervous system, diet (salt and fructose), and metabolic (hyperuricemia) mechanisms. The kidney shows evidence for renal arteriolar vasoconstriction, an intrarenal inflammatory response, local oxidative stress, and intrarenal activation of the renin-angiotensin system. Recent studies suggest that intrarenal T cells have an important role in causing hypertension to be persistent, likely due to the induction of a local autoimmune response to neoantigens such as heat shock protein 70 and protein aggregates formed by isoketals resulting from lipid peroxidation. Salt retention due to impairment in pressure-diuresis leads to the release of cardiotonic steroids and central nervous system effects that cause systemic vasoconstriction and a rise in blood pressure. Some recent studies suggest that salt may increase blood pressure not simply by effects on extracellular volume but rather as a consequence of hyperosmolarity. These new insights could lead to new approaches for the prevention and treatment of this important disease.

Genetic determinants of hypertension: an update

Hypertension represents a global public health burden. In addition to the rarer Mendelian forms of hypertension, classic genetic studies have documented a significant heritable component to the most common form, essential hypertension (EH). Extensive efforts are under way to elucidate the genetic basis of this disease. Recently, a new form of Mendelian hypertension has been identified, pharmacogenetic association studies in hypertensive patients have identified novel gene-by-drug interactions, and the first genome-wide association studies of EH have been published. New findings in consomic and congenic rat models also offer new clues to the genetic architecture of this complex phenotype. In this review, the authors summarize and evaluate the most recent findings related to hypertension gene identification.

A review of the genetics of essential hypertension

PURPOSE OF REVIEW

Essential hypertension affects more than 20% of the adult population, and has a multifactorial origin arising from an interaction between susceptibility genes and environmental factors. Several strategies have been used to identify hypertension susceptibility genes. This review highlights recent efforts in genetic dissection of essential hypertension.

RECENT FINDINGS

Recently, further chromosomal regions harboring blood pressure loci have emerged in genome-wide linkage studies. Findings from a new systematic two-dimensional genome scan are presented, as well as sex-specific loci linked to hypertension in inbred rodent models. Many case-control association studies have been carried out, but results so far have been equivocal. This review discusses some interesting studies combining linkage and association strategies using gene-gene interactions, and studies the use of haplotypes instead of SNPs. Two novel hypertension susceptibility genes are presented, and a short summary on new insights into genes of the renin-angiotensin and adrenergic systems is given.

SUMMARY

To date, linkage and association studies have not been convincing. Genome-wide association studies may prove to be an effective approach to the problems posed by complex traits. Combined with candidate gene approaches, it is hoped this strategy will yield convincing evidence for genes associated with essential hypertension.

The genetic dissection of essential hypertension

QTL mapping in humans and rats has identified hundreds of blood-pressure-related phenotypes and genomic regions; the next daunting task is gene identification and validation. The development of novel rat model systems that mimic many elements of the human disease, coupled with advances in the genomic and informatic infrastructure for rats, promise to revolutionize the hunt for genes that determine susceptibility to hypertension. Furthermore, methods are evolving that should enable the identification of candidate genes in human populations. Together with the computational reconstruction of regulatory networks, these methods provide opportunities to significantly advance our understanding of the underlying aetiology of hypertension.

Interactive effects of common beta2-adrenoceptor haplotypes and age on susceptibility to hypertension and receptor function

Few studies have examined to what extent genetic variants of the beta2-adrenoceptor (ADRB2) are involved in the development of hypertension with age, although beta2-adrenergic receptor responsiveness declines in older subjects. To investigate this, 10 common single-nucleotide polymorphisms (SNPs) in the promoter and coding regions of the ADRB2 gene were genotyped in an unrelated population consisting of 2 ethnic groups: European American (EA; n=610) and African American (AA; n=420). ADRB2 haplotypes were estimated by expectation maximization (EM) algorithm-based methods. In the general population for EAs and AAs, the variants of the ADRB2 gene, including the individual SNPs and their haplotypes, were not associated with hypertension. However, there was a significant interaction between age and one of the common haplotypes (haplotype 1) in EAs (P=0.01). Haplotype 1 was associated with protection against hypertension in young (< or =50 years of age) but not in old (>50 years of age) EAs (odds ratio, 0.5; 95% confidence interval, 0.27 to 0.91; P=0.02). This age-specific effect was further supported by the observations that young subjects carrying > or =1 copy of haplotype 1 had significantly lower diastolic blood pressure and nearly 2-fold higher ADRB2 binding density than the noncarriers (P<0.05). With aging, their ADRB2 numbers decreased to the level of the noncarriers, along with increased body mass index (7%; P<0.05) and decreased heart rate (7%; P<0.001). Our study suggests that age is an important modifier for the effects of ADRB2 polymorphisms on ADRB2 function and the development of hypertension.

Association analysis of beta(2)-adrenergic receptor polymorphisms with hypertension in Japanese

Significant evidence has been provided for the pathophysiological involvement of the beta(2)-adrenergic receptor (ADRB2) in hypertension. Among ADRB2 polymorphisms identified to date, 2 amino acid substitutions, Arg16Gly and Gln27Glu, and a promoter variant, T-47C, are considered functionally important. In particular, Arg16Gly was shown to be associated with hypertension in black and white subjects. To investigate the relevance of ADRB2 polymorphisms to hypertension, we undertook an extensive association study in a Japanese population. An association was tested in 2 ways. First, a case-control study was conducted in 842 hypertensive and 633 normotensive subjects. In addition to the overall comparison between case and control groups, each was stratified by body mass index and compared with an independent panel of 525 diabetic subjects. Second, ANOVA and multivariate analyses were performed to test the significance of an association between ADRB2 genotype and the level of blood pressure within the entire population except for 395 subjects who had been under treatment for hypertension. Although no significant association was observed for Arg16Gly and T-47C, 2 analytical methods indicated a marginal association (P:=0.01 to 0.04) between the Glu27 variant and lower blood pressure levels. Given such a normotensive propensity, the odds ratio for Glu27 versus Gln27 allele frequencies was estimated to be 0.74, with a wide confidence interval (95% CI, 0.55 to 0.99) reflecting the low Glu27-allele frequency (6% to 8%) in Japanese. There were no apparent confounding influences of obesity and diabetes on the postulated association. Our data suggest that 3 ADRB2 polymorphisms tested are unlikely to confer principal genetic susceptibility for hypertension in the Japanese population. However, further investigation is warranted to clarify the relevance of ADRB2 polymorphisms to blood pressure regulation.

Comprehensive analysis of the renin-angiotensin gene polymorphisms with relation to hypertension in the Japanese

BACKGROUND

Components of the renin-angiotensin (R-A) system have been repeatedly investigated as candidate genes for essential hypertension. In particular, suggestive or significant association has been detected in some studies for the angiotensinogen M235T, angiotensin I-converting enzyme I/D, angiotensin II type 1 receptor A1166C, and aldosterone synthase C-344T polymorphisms, although the results remain inconclusive.

OBJECTIVE AND METHODS

To evaluate the importance of these candidate genes for hypertension, we undertook an extensive association study in the Japanese. This case-control study was conducted in a total of 1476 individuals using the four R-A gene polymorphisms. In the assessment of genotyping data, 843 hypertensive subjects were divided into three case subgroups according to severity of hypertension, while 633 normotensive subjects divided into two control subgroups by the age of enrollment. Each subgroup was further divided by sex. Subsequently, the presence of synergy (or gene-gene interaction) was evaluated among four R-A gene polymorphisms.

RESULTS

No significant association was observed between the individual R-A gene polymorphisms and hypertension status in our case-control study. The results were almost unchanged when severity of hypertension, sex-specificity, and synergy were taken into account

CONCLUSIONS

Despite a relatively large number of subjects, we did not find significant evidence for disease association in the Japanese population. Given confounding factors in the case-control strategy, the lack of association does not exclude the relevance of the R-A genes to hypertension. Further investigation needs to be performed in large-scale populations, where the use of not only hypertension status, but also 'intermediate' phenotypes would be useful.