The genetic dissection of essential hypertension

Current status of genome-wide scanning for hypertension

PURPOSE OF REVIEW

The current review focuses on new trends in genome-wide assessment of the inherited component of blood pressure variation.

RECENT FINDINGS

Systematic linkage and association analyses of a region on chromosome 1q, complemented by gene prioritization with comparative genomic evidence, revealed variants in three genes contributing to tangible increases in blood pressure. The results of one of the first two-dimensional scans were published, confirming the oligogenic epistatic nature of the genetic component of blood pressure determination. Several loci with distinct effects on blood pressure in men and women were reported, enhancing the sexually dimorphic map of complex traits. Novel approaches were applied to extract genetically and clinically distinct subsets of garden-variety hypertension, which appears to be a promising direction to take in deciphering the hypertension genetic puzzle.

SUMMARY

The current landscape of genome-wide linkage studies of hypertension is acquiring novel facets in an attempt to more appropriately grasp the genomic architecture of hypertension. The advent of genome-wide association investigations, enhanced possibilities of comparative genomics and integration with information on copy number variations and transcriptomics will most likely reshape our view of nature and the evolutionary connotations of genetic variation affecting blood pressure in the near future.

Congenic mapping of a blood pressure QTL region on rat chromosome 10 using the Dahl salt-sensitive rat with introgressed alleles from the Milan normotensive strain

Multiple blood pressure (BP) quantitative trait loci (QTLs) are reported on rat chromosome 10 (RNO10). Of these, QTLs detected by contrasting the genome of the hypertensive Dahl salt-sensitive (S) rat with two different relatively normotensive strains, Lewis (LEW) and the Milan normotensive strain (MNS), are reported. Because the deduced QTL regions of both S vs. LEW and S vs. MNS comparisons are within large genomic segments encompassing more than 2 cM, there was a need to further localize these QTLs and determine whether the QTLs are unique to specific strain comparisons. Previously, the S.MNS QTL1 was mapped to less than 2.6 cM as a differential segment between two congenic strains. In this study, multiple congenic strains spanning the projected interval were studied. The BP effect of each strain was interpreted as the net effect of alleles introgressed within that congenic strain. The results suggest that the MNS alleles within the previously proposed differential segment (D10Rat27-D10Rat24) do not independently lower BP of the S rat. However, another congenic strain, S.MNS(10) x 9, containing introgressed MNS alleles that are outside of the previously proposed differential segment is of interest because (1) it demonstrated a BP-lowering effect, (2) it is contained within a single congenic strain and is not based on the observed effect of a differential segment, and, more importantly, (3) it overlaps with the previously identified S.LEW BP QTL region. Identification of the same QTL affecting BP in multiple rat strains will provide further support for the QTL's involvement and importance in human essential hypertension.

A switch in the mechanism of hypertension in the syndrome of apparent mineralocorticoid excess

The syndrome of apparent mineralocorticoid excess arises from nonfunctional mutations in 11beta-hydroxysteroid dehydrogenase type 2 (11betaHSD2), an enzyme that inactivates cortisol and confers aldosterone specificity on the mineralocorticoid receptor. Loss of 11betaHSD2 permits glucocorticoids to activate the mineralocorticoid receptor, and the hypertension in the syndrome is presumed to arise from volume expansion secondary to renal sodium retention. An 11betaHSD2 null mouse was generated on an inbred C57BL/6J genetic background, allowing survival to adulthood. 11betaHSD2(-/-) mice had BP approximately 20 mmHg higher on average compared with wild-type mice but were volume contracted, not volume expanded as expected. Initially, impaired sodium excretion associated with increased activity of the epithelial sodium channel was observed. By 80 days of age, however, channel activity was abolished and 11betaHSD2(-/-) mice lost salt. Despite the natriuresis, hypertension remained but was not attributable to intrinsic vascular dysfunction. Instead, urinary catecholamine levels in 11betaHSD2(-/-) mice were double those in wild-type mice, and alpha1-adrenergic receptor blockade rescued the hypertensive phenotype, suggesting that vasoconstriction contributes to the sustained hypertension in this model. In summary, it is proposed that renal sodium retention remains a key event in apparent mineralocorticoid excess but that the accompanying hypertension changes from a renal to a vascular etiology over time.

Congenic interval mapping of RNO10 reveals a complex cluster of closely-linked genetic determinants of blood pressure

Genetic dissection of the rat genome for identifying alleles that cause abnormalities in blood pressure (BP) resulted in the mapping of a significant number of BP quantitative trait loci (QTLs). In this study we mapped at least one such BP QTL on rat chromosome 10 (RNO10) as being within the introgressed segment of a S.LEW congenic strain S.LEWx12x2x3x8 spanning 1.34 Mb from 70,725,437 bp to 72,063,232 bp. BP of 3 congenic strains that span shorter segments of this region was additionally examined. Results obtained indicate that LEW alleles that comprise a 375-kb introgressed segment of the congenic strain S.LEWx12x2x3x5 (70,725,437 bp to 71,100,513 bp) increase BP. The magnitude of change in BP exhibited by the 2 strains, S.LEWx12x2x3x8 and S.LEWx12x2x3x5, is the net phenotypic effect of the underlying genetic determinants of BP. In this respect, the current data are superior to previous QTL localization of BP QTL1, which was hypothesized based on differential congenic segments. Genetic dissection using these 2 congenic strains as tools is expected to facilitate further dissection of the regions. Meanwhile, differential congenic segments were used to predict and thereby prioritize regions for candidate gene analysis. Using this approach, 2 distinct regions of 401 kb and 409 kb within S.LEWx12x2x3x8 and a 104 kb region within S.LEWx12x2x3x5 were prioritized for further consideration. Because all of these genetic elements are located within a 1.06-Mb region of RNO10, our study has revealed a remarkable insight into a genomic module comprising very closely-linked, opposing genetic determinants of BP.

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.

Matrix metalloproteinase-9 polymorphism contributes to blood pressure and arterial stiffness in essential hypertension

Arterial stiffness is an independent predictor of cardiovascular events in a hypertensive population. Serum levels of matrix metalloproteinase (MMP)-9 are associated with arterial stiffness and predict cardiovascular risk. We investigated the role of MMP-9 polymorphism -1562C>T on blood pressure (BP) and arterial stiffness in a newly diagnosed hypertensive population. Untreated hypertensive patients (n=215, mean age 46+/-13 years) were studied. Supine BP, carotid-femoral pulse wave velocity (PWV) and augmentation index were assessed. Serum biochemistry and plasma MMP-9 concentrations were measured and genotyping performed following extraction of genomic DNA. BP, aortic PWV and serum MMP-9 levels were significantly higher in T-allele carriers of the -1562C>T polymorphism with a significant gene-dose effect (P<0.0001). In a stepwise regression model adjusting for known or likely determinants, the 1562C>T polymorphism emerged as an independent predictor of systolic BP (R(2)=0.25, P<0.0001), diastolic BP (R(2)=0.16, P<0.0001) and PWV (R(2)=0.47,P<0.0001). This is the first study to show the effect of MMP-9 polymorphism on BP and aortic stiffness in a hypertensive population. These results suggest that hypertensive patients carrying the T allele may be at increased risk of cardiovascular events.

Identification and chromosome mapping of loci predisposing to colorectal cancer that control Wnt/beta-catenin pathway and progression of early lesions in the rat

Sporadic colorectal cancer (CRC) is a major health concern worldwide. Epidemiologic evidence suggests a polygenic predisposition to CRC, but the genes responsible remain unknown. Here, we performed genome-wide scanning of male (ACI/SegHsd x Wistar-Furth)F2 (AWF2) rats to map susceptibility genes influencing the evolution of early colorectal lesions to adenocarcinoma following 1,2-dimethylhydrazine administration. Phenotypic analysis revealed higher incidence/multiplicity and lower size of adenomas in ACI/SegHsd (ACI) and (ACI/SegHsd x Wistar-Furth)F1 (AWF1) than Wistar-Furth (WF) rats and higher incidence/multiplicity of poorly differentiated adenocarcinomas in WF than ACI rats, with intermediate values in AWF1 rats. Linkage analysis of 138 AWF2 rats identified three loci on chromosomes 4, 15 and 18 in significant linkage with lesion multiplicity that were identified as rat Colon cancer resistance (rCcr) 1, rCcr2 and rCcr3, respectively. Seven other loci on chromosomes 5, 6, 15, 17, 18 and 20 were in suggestive linkage with adenoma/adenocarcinoma multiplicity/surface area. Six of them were identified as rCcr4-9 and a locus on chromosome 5 was identified as a susceptibility locus, rCcs1. Significant interactions between rCcr3 and rCcr6, rCcr6 and rCcr8 and rCcr5 and rCcr9, and four novel epistatic loci controlling multiplicity/size of colorectal lesions were discovered. Apc, located at rCcr3, did not show functional promoter polymorphisms. However, influence of susceptibility/resistance genes on Wnt/beta-catenin pathway was shown by defective beta-catenin inactivation in WF but not in ACI and AWF1 rat adenocarcinomas. These data indicate that inheritance of predisposition to CRC depends on interplays of several genetic factors, and suggest a possible mechanism of polygenic control of CRC progression.

Genetic susceptibility to essential hypertension: insight from angiotensinogen

Although progress in the genetics of essential hypertension may seem disappointing, it has considerable potential in defining research directions that will ultimately translate into clinical practice. The hypothesis that genetic variation at the angiotensinogen locus impacts on individual susceptibility to develop essential hypertension has motivated a substantial body of research by us and many others. We examine how analyses of the mechanisms by which variation in angiotensinogen expression may contribute to disease susceptibility and may have arisen in human populations have progressed in recent years. Although the objective of personalized medicine is still in the future, a genetic hypothesis based on human variation can uniquely empower functional genomics approaches to reach such an ultimate goal.