AGT genetic variation, plasma AGT, and blood pressure: An analysis of the Utah Genetic Reference Project pedigrees

BACKGROUND

Much remains unknown about the genetic factors that contribute to essential hypertension. The Utah Genetic Reference Project (UGRP) large pedigree collection provides new opportunities to study quantitative relationships between genetic variation, endophenotypes, and blood pressure.

METHODS

We analyzed the relationship between common single-nucleotide polymorphisms (SNPs) and haplotypes spanning the angiotensinogen (AGT) gene and promoter region, plasma AGT levels, and systolic (SBP) and diastolic blood pressure (DBP) in 424 individuals from 41 two-generation UGRP families.

RESULTS

Plasma AGT levels are significantly correlated among UGRP family members. Correlations are higher for males than for females. Parent-offspring correlations for plasma AGT (0.30) are higher than those for SBP (0.26) and DBP (0.17) (all P values <0.01). The additive heritability (h(2)) for plasma AGT is high (0.74) and substantially exceeds heritability estimates for SBP (0.26) and DBP (0.16) (all P values <0.01). Significant linkage (logarithm of the odds (LOD) >3) is found between six AGT SNPs and plasma AGT. A model that utilizes three AGT haplotype groups produces the best LOD score (5.1) that exceeds the best single SNP LOD score (3.8). Plasma AGT and blood pressure were not significantly correlated.

CONCLUSIONS

Plasma AGT levels demonstrate high heritability in 41 UGRP families. Locus-specific heritability estimates for AGT SNPs and haplotypes approach 67%, indicating that variation at AGT accounts for a large percentage of the heritability of plasma AGT. A three-way haplotype model outperforms single SNPs for quantitative linkage analysis to plasma AGT. In these predominantly normotensive individuals, plasma AGT did not correlate significantly with blood pressure.

A real-time, telemetric method for continuous measurement of portal pressures

The ability to longitudinally monitor portal and splanchnic pressures would greatly enhance the understanding of acute and chronic liver disease by helping to assess the immediate and long-term impact of therapeutic manipulations. However, a technique for measuring portal pressures in the ambulatory setting is not currently available. To overcome this difficulty, we utilized an approach that involved the implantation of a miniature telemetric device, equipped with a specially-designed pressure transmission catheter, into the spleen of an anesthetized mouse. Using this approach, portal pressures were measured continuously over 5 d in conscious, unrestrained animals, the availability of which will help facilitate studies of the portal circulation requiring long-term stability.

A human polymorphism affects NEDD4L subcellular targeting by leading to two isoforms that contain or lack a C2 domain

Background

Ubiquitination serves multiple cellular functions, including proteasomal degradation and the control of stability, function, and intracellular localization of a wide variety of proteins. NEDD4L is a member of the HECT class of E3 ubiquitin ligases. A defining feature of NEDD4L protein isoforms is the presence or absence of an amino-terminal C2 domain, a class of subcellular, calcium-dependent targeting domains. We previously identified a common variant in human NEDD4L that generates isoforms that contain or lack a C2 domain.

Results

To address the potential functional significance of the NEDD4L common variant on NEDD4L subcellular localization, NEDD4L isoforms that either contained or lacked a C2 domain were tagged with enhanced green fluorescent protein, transfected into Xenopus laevis kidney epithelial cells, and imaged by performing confocal microscopy on live cells. We report that the presence or absence of this C2 domain exerts differential effects on the subcellular distribution of NEDD4L, the ability of C2 containing and lacking NEDD4L isoforms to mobilize in response to a calcium stimulus, and the intracellular transport of subunits of the NEDD4L substrate, ENaC. Furthermore, the ability of the C2-containing isoform to influence β-ENaC mobilization from intracellular pools involves the NEDD4L active site for ubiquitination. We propose a model to account for the potential impact of this common genetic variant on protein function at the cellular level.

Conclusion

NEDD4L isoforms that contain or lack a C2 domain target different intracellular locations. Additionally, whereas the C2-containing NEDD4L isoform is capable of shuttling between the plasma membrane and intracellular compartments in response to calcium stimulus the C2-lacking isoform can not. The C2-containing isoform differentially affects the mobilization of ENaC subunits from intracellular pools and this trafficking step requires NEDD4L ubiquitin ligase activity. This observation suggests a new mechanism for the requirement for the PY motif in cAMP-mediated exocytosis of ENaC. We have elucidated how a common genetic variant can underlie significant functional diversity in NEDD4L at the cellular level. We propose a model that describes how that functional variation may influence blood pressure. Moreover, our observations regarding differential function of the NEDD4L isoforms may impact other aspects of physiology that involve this ubiquitin ligase.

From genetics to mechanism of disease liability

With each advance in genomic technology, new statistical methods have regularly emerged to test genetic hypotheses in complex inheritance, as evidenced throughout this book. Notwithstanding the approach used, the greatest challenge in the genetics of complex traits remains the identification of the gene(s) and the molecular variant(s) accounting for a genetic inference based on statistical testing. We take the example of quantitative trait locus (QTL) mapping for blood pressure (BP) and related phenotypes in rodents to review the current landscape. Traditional approaches to refined mapping are typically hampered by the small effect and the small proportion of the variance attached to individual QTLs. The alternative of functional screens in intact animals, whether by chemical mutagenesis or gene targeting, remains a daunting undertaking. Such limitations account for the slow progress to date of inferences from QTL to gene(s). We select a QTL for differential sodium sensitivity between two mouse inbred lines to propose an approach that can be used in relatively large genomic regions (1) by optimizing the selection of candidate genes and (2) by subjecting such genes to high-throughput functional screens. While this is still work in progress, we think it abundantly illustrates what is ahead of us in delineating genetic variation that underlie complex disease.

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.

Hypervolemia of pregnancy is not maintained in mice chronically overexpressing angiotensinogen

OBJECTIVE

Women who develop pre-eclampsia show significantly less hypervolemia of pregnancy, compared with controls. We have shown that chronically elevated angiotensinogen expression increases a woman's risk of developing pre-eclampsia. Our objective was to determine whether increased angiotensinogen expression is sufficient to cause failed hypervolemia.

STUDY DESIGN

To isolate the effects of elevated angiotensinogen expression, we studied transgenic mice with either 2 or 3 copies of the murine angiotensinogen gene. Plasma volume was measured by Evans blue dye dilution, and kidney sections were immunostained for angiotensinogen and renin.

RESULTS

Three-copy mice failed to maintain hypervolemia after midgestation (P < .01) and failed to up-regulate renin expression in the distal nephron, compared with 2-copy controls. Intrarenal angiotensinogen was up-regulated during pregnancy in both genotypes.

CONCLUSION

Chronically elevated angiotensinogen expression is sufficient to cause failed hypervolemia of pregnancy. Whether this observation is related to failed up-regulation of distal tubule renin expression requires further study.

Haplotype Association Analysis of AGT Variants with Hypertension-Related Traits: The HyperGEN Study

OBJECTIVE

Function of the renin-angiotensin system is important to human hypertension, but its genetic etiology remains elusive. We set out to examine a hypothesis that multiple genetic variants in the system act together in blood pressure regulation, via intermediate phenotypes such as blood pressure reactivity.

METHODS

A sample of 531 hypertensive cases and 417 controls was selected from the HyperGEN study. Hypertension-related traits including blood pressure responses to challenges to math test, handgrip and postural change (mathBP, gripBP, and postBP), and body mass index (BMI) were analyzed for association with 10 single nucleotide polymorphisms (SNPs) in the angiotensinogen (AGT) gene. Single-marker and haplotype analyses were performed to examine the effects of both individual and multiple variants. Multiple-trait profiling was used to assess interaction of latent intermediate factors with susceptible haplotypes.

RESULTS

In Blacks, two SNPs in exon 5 and 3'UTR showed significant association with gripBP, and two promoter SNPs were strongly associated with postBP. In Whites, only borderline association was found for 2 promoter SNPs with mathBP. Haplotype analyses in Blacks confirmed association with gripBP, and detected significant association of a haplotype to BMI (p=0.029). With the interactions modeled, haplotype associations found in Blacks remain significant, while significant associations to BMI (p=0.009) and gripSBP emerged in Whites.

CONCLUSION

Genetic variants in regulatory regions of AGT showed strong association with blood pressure reactivity. Interaction of promoter and genic SNPs in AGT revealed collective action of multiple variants on blood pressure reactivity and BMI both in Blacks and in Whites, possibly following different pathways.

Transcriptional diversity and expression of NEDD4L gene in distal nephron

The ubiquitin ligase NEDD4L participates in plasma volume and blood pressure regulation by controlling expression of the epithelial sodium channel (ENaC). Genetic impairment of EnaC-Nedd4L-Proteasome system caused a rare mendelian hereditary human hypertension, Liddle syndrome. This finding suggested that Nedd4L is playing an important role in pathogenesis for hypertensive disorders. This prompted us to test a possible involvement of NEDD4L for the development of sodium-sensitive hypertension in Dahl salt-sensitive (DS) rats and its normotensive littermate Dahl salt-resistant (DR) rats. First, we analyzed the transcriptional diversity of rat Nedd4L gene and observed several isoforms with and without calcium-dependent membrane binding (C2) domain at the N-terminal of the protein as we found in human and mouse before. Then, we analyzed the expression of rat NEDD4L in the kidney of both DS and DR under high and low sodium regimens. NEDD4L expression examined by quantitative PCR technique revealed lower expression of NEDD4L transcripts in DS rats under either diet compared to DR animals; additionally, NEDD4L expression was significantly increased with sodium loading. Using in situ hybridization experiments, rat NEDD4L was predominantly expressed in distal nephron in a manner dependent on both sodium regimen and genetic background. A similar histological distribution pattern was observed in human kidney. The expression of NEDD4L in distal nephron and its response to chronic sodium loading suggest that it participates in the functioning of this segment in sodium reabsorption. This response was impaired in genetically sodium-sensitive animals. These findings suggested that Nedd4L gene products were involved in the development of salt-sensitive hypertension.

A summary of the effects of antihypertensive medications on measured blood pressure

BACKGROUND

Epidemiologic analysis of family data on blood pressure (BP) is often compromised by the effects of antihypertensive medications. A review of numerous clinical trials that investigated the effects of BP-lowering medications is summarized here.

METHODS

Published clinical trials, including 137 clinical trials with monodrug therapies and 28 clinical trials of combination drug therapies with a total of 11,739 participants, were reviewed from PubMed. Six major classes/groups of antihypertensive medications were categorized by ethnicity, including angiotensin-converting enzyme (ACE) inhibitors, alpha1-blockers, cardioselective beta-blockers (beta1-blockers), calcium channel blockers, thiazide and thiazide-like diuretics, and loop diuretics.

RESULTS

Using sitting or supine BP, for ethnic groups combined, monodrug therapy with ACE inhibitors showed a weighted average effect of lowering the systolic and diastolic BP by 12.5/9.5 mm Hg; alpha1-blockers by 15.5/11.7 mm Hg; beta1-blockers by 14.8/12.2 mm Hg; calcium channel blockers by 15.3/10.5 mm Hg; thiazide diuretics by 15.3/9.8 mm Hg; and loop diuretics by 15.8/8.2 mm Hg. However, ACE inhibitors, alpha1-blockers, and beta1-blockers were less effective in African Americans than in non-African Americans, whereas calcium channel blockers, thiazide diuretics, and loop diuretics were more effective in African Americans than in non-African Americans. For two-drug combination therapy with ethnic groups combined, the BP-lowering effect of the second medication, when compared to its effect as monodrug therapy, was 84% and 65% for systolic and diastolic BP, respectively.

CONCLUSIONS

The BP-lowering effects reported here may be used to impute the pretreatment BP levels, which can improve the information content and hence the power of epidemiologic analysis in studies where use of antihypertensive medications is a confounding factor in the BP measurements.

Further evidence of a quantitative trait locus on chromosome 18 influencing postural change in systolic blood pressure: the Hypertension Genetic Epidemiology Network (HyperGEN) Study

BACKGROUND

Postural change in systolic blood pressure (SBP) is prospectively associated with several disease outcomes including hypertension, stroke, and coronary heart disease. The objective of this study was to characterize further a possible quantitative trait locus on chromosome 18q21 influencing SBP response to a postural challenge.

METHODS

A prior genome scan of postural SBP response in 636 subjects of white ethnicity from 285 hypertensive sibships in the Hypertension Genetic Epidemiology Network (HyperGEN) indicated suggestive evidence for linkage on chromosome 18q21. This study included a de novo set of 452 African American pedigrees from the HyperGEN study and an expanded set of 372 white pedigrees. Variance components linkage analysis of postural SBP change was conducted using microsatellite markers on chromosome 18, and association studies were performed with a common single nucleotide polymorphism (variant 13) in the gene encoding NEDD4L, a key regulator of fine sodium reabsorption in the distal nephron.

RESULTS

Combined analysis of all white and African American pedigrees yielded a LOD score of 4.25 at 80 cM on chromosome 18q21, with at least nominal evidence of linkage at this position in both white (LOD: 3.43) and African American (LOD: 1.14) subjects. Postural SBP response was associated with variant 13 of the NEDD4L in a subset of white subjects taking medications effective in treating sodium volume-dependent hypertension (alpha1-blockers, calcium channel blockers, and/or diuretics).

CONCLUSION

These data provide further evidence for a quantitative trait locus on chromosome 18q21 influencing postural change in SBP.

Significance of urinary angiotensinogen in essential hypertension as a function of plasma renin and aldosterone status

OBJECTIVE

This study was performed to test the significance of urinary angiotensinogen (UAGT) in essential hypertensive patients stratified as a function of plasma renin and aldosterone.

METHODS AND RESULTS

A sample of 248 essential hypertensives, investigated under their usual sodium diet and either off-medication or under a standardized treatment, was separated into two groups on the basis of upright plasma active renin and aldosterone medians. Patients with plasma active renin and aldosterone below medians are referred to as the low renin-aldosterone essential hypertensive group (LRA-EH). Others subjects are defined as other essential hypertensives (O-EH). Blood pressure (BP) was recorded by 24-h ambulatory monitoring. UAGT was measured by a specific enzyme-linked immunosorbent assay for total angiotensinogen. Because UAGT was markedly increased in the presence of overt proteinuria (>/= 300 mg/24 h), proteinuric patients (n = 29) were excluded from subsequent analyses. UAGT was a significant predictor of systolic and diastolic BP in LRA-EH females (P < 0.01 and P = 0.05, respectively) but not in males. By contrast, urinary sodium excretion (P < 0.001) and maintenance of treatment (P = 0.002) were significant predictors of systolic BP in males. These correlations were not observed in O-EH, whether males or females.

CONCLUSIONS

In the present study, UAGT stands as a strong predictor of BP in women with low plasma renin/aldosterone, suggesting an involvement of the tubular renin-angiotensin system in these subjects. Higher sodium intake or the need to maintain treatment may account in part for the lack of a similar relationship in males.

Expression of angiotensinogen in proximal tubule as a function of glomerular filtration rate

BACKGROUND

Proximal tubule (PT) angiotensinogen (AGT) is part of a tubular renin-angiotensin system (RAS) that participates in the regulation of sodium reabsorption along the entire nephron. Physiologic maneuvers affecting AGT expression in PT also affect systemic RAS. Here, we tested the hypothesis that PT AGT is regulated by increased glomerular filtration rate (GFR).

METHODS

Complete unilateral nephrectomy (UNX) in mice was used to induce a sustained increase in GFR in the remaining kidney. AGT expression was monitored by quantitative reverse transcription-polymerase chain reaction (RT-PCR). AGT protein in PT was investigated by semiquantitative histology. We also measured AGT concentration in plasma and in 24-hour urine by a specific enzyme-linked immunosorbent assay (ELISA).

RESULTS

Seven weeks after nephrectomy, UNX animals exhibited a 2-fold increase in tubular AGT mRNA (P <.001) compared with sham-operated control animals. The proportion of PT sections exhibiting AGT immunostaining was significantly increased at day 3 (P <.05), and remained elevated at seven weeks (UNX = 0.63 +/- 0.09, sham = 0.38 +/- 0.02, P <.01), revealing recruitment of AGT-producing cells along the PT. AGT excretion in final urine corrected for creatinine and kidney weight was also elevated by UNX at seven weeks (UNX = 209 +/- 42 pmol/mg/g, sham = 147 +/- 29 pmol/mg/g, P <.05), with no difference in plasma AGT between UNX and control animals.

CONCLUSION

These observations suggest that AGT expression in PT adapts in the long-term to changes in GFR. In the UNX model, urinary AGT excretion is also elevated as a consequence of increase in net tubular flow.

Renin and kallikrein in connecting tubule of mouse

BACKGROUND

The observation of renin expression in connecting tubule, a segment that also expresses tissue kallikrein (KLK-1), raises two questions. Are the genes expressed in the same or in different cells of connecting tubule? Does this topography support the hypothesis that KLK-1 activates prorenin or is it more likely that it affords coordinated gene regulation?

METHODS

Renin and KLK-1 were examined by immunostaining and in situ hybridization. Renin activation by KLK-1 was investigated in vitro. In vivo, excretion of prorenin and active renin was compared in mice homozygous for targeted inactivation of KLK-1 (TK(-/-)) and normal littermates (TK(+/+)).

RESULTS

Using in situ immunostaining for renin and in situ hybridization for KLK-1 mRNA, we found that connecting tubule cells expressing renin also expressed KLK-1. We confirmed in vitro activation of prorenin by KLK-1, but found no difference in the ratio of active renin to prorenin in urine of TK(-/-) and TK(+/+) animals. Compared to TK(+/+) controls, TK(-/-) mice exhibited significantly lower 24-hour excretion of prorenin (5.05 +/- 1.16 mg Ang I/hour vs. 9.39 +/- 1.96 mg Ang I/hour, P < 0.05) and active renin (1.98 +/- 0.25 mg Ang I/hour vs. 3.58 +/- 0.39 mg Ang I/hour, P < 0.05), with no difference in either urine volumes or plasma renin concentrations.

CONCLUSION

Direct interaction between renin and KLK-1, not ruled out in vitro, is not supported in vivo. By contrast, lower excretion of active renin and prorenin in TK(-/-) compared to TK(+/+) suggest coordinated regulation of the two proteins in their participation to collecting duct function.

Natural selection and population history in the human angiotensinogen gene (AGT): 736 complete AGT sequences in chromosomes from around the world

Several lines of evidence suggest that patterns of genetic variability in the human angiotensinogen gene (AGT) contribute to phenotypic variability in human hypertension. The A(-6) promoter variant of AGT is associated with higher plasma angiotensinogen levels and increased risk of essential hypertension. The geographic distribution of the A(-6) variant leads to the intriguing hypothesis that the G(-6) promoter variant has been selectively advantageous outside Africa. To test these hypotheses, we investigated the roles of population history and natural selection in shaping patterns of genetic diversity in AGT, by sequencing the entire AGT gene (14400 bp) in 736 chromosomes from Africa, Asia, and Europe. We found that the A(-6) variant is present at higher frequency in African populations than in non-African populations. Neutrality tests found no evidence of a departure from selective neutrality, when whole AGT sequences were compared. However, tests restricted to sites in the vicinity of the A(-6)G polymorphism found evidence of a selective sweep. Sliding-window analyses showed that evidence of the sweep is restricted to sites in tight linkage disequilibrium (LD) with the A(-6)G polymorphism. Further, haplotypes carrying the G(-6) variant showed elevated levels of LD, suggesting that they have risen recently to high frequency. Departures from neutral expectation in some but not all regions of AGT indicate that patterns of diversity in the gene cannot be accounted for solely by population history, which would affect all regions equally. Taken together, patterns of genetic diversity in AGT suggest that natural selection has generally favored the G(-6) variant over the A(-6) variant in non-African populations. However, important localized effects may also be present.

Common variant of human NEDD4L activates a cryptic splice site to form a frameshifted transcript

The ubiquitin ligase NEDD4L is a candidate gene for essential hypertension on both functional and genetic grounds. By targeting the epithelial sodium channel (ENaC) for degradation, NEDD4L is a significant determinant of sodium reabsorption in the distal nephron. Genetic linkage has been reported to a region of chromosome 18q harboring the gene, with phenotypes that include a rare orthostatic hypotension disorder, essential hypertension, and postural change in systolic blood pressure. A systematic search for genetic polymorphisms by resequencing exons and intron boundaries in 48 Caucasians yielded 38 variants. Among these, variant 13 is common, with either G (70%) or A (30%) as the last nucleotide of a putative exon 1. This mutation could affect the generation of a previously unrecognized splice isoform. In subsequent experiments, (1) we confirmed the presence of this putative isoform in both kidney and adrenals; (2) we established that variant 13-A leads to the systematic use of an alternative splice site, generating a transcript encoding a nonfunctional protein; and (3) we demonstrated differences in tissue-specific expression of the novel isoform relative to its previously reported counterpart. Variant 13-A precludes the formation of a transcript encoding a full-length Ca2+-dependent lipid-binding (C2) domain with very high evolutionary conservation among NEDD4L orthologs. A similar C2 domain in the paralogous NEDD4 gene plays a significant role in the transfer of its product to the apical membrane of epithelial cells. Differential function of NEDD4L isoforms could prove significant in blood pressure regulation through an effect on ENaC-dependent sodium reabsorption.

Contribution of Sp1 to initiation of transcription of angiotensinogen

Several genetic polymorphisms have been identified in the proximal promoter of angiotensinogen ( AGT). Gene titration experiments in transgenic animals have demonstrated that small increases in the basal expression of AGT can lead to elevated blood pressure. The direct proof that promoter variants of AGT can lead to elevated blood pressure will ultimately require the development of specific animal models. Before such work can be contemplated, however, a formal understanding of the mechanisms controlling transcriptional activation of AGT needs to be developed. Analysis of DNA-protein interactions in vitro and transactivation experiments in cultured cells reveal the critical role of an Sp1 binding site immediately upstream of the TATA box of AGT in both mouse and human. Both sites are required for transcription initiation in the mouse. By contrast, a minimal human AGT promoter can initiate transcription in the absence of either this Sp1 site or the TATA box, albeit at a lower level. Further analysis and consideration of these interspecific differences will be essential for the development of meaningful animal models to probe the mechanism by which AGT may predispose to human essential hypertension.

Effects of dietary sodium and genetic background on angiotensinogen and Renin in mouse

Elements of a renin-angiotensin system expressed along the entire nephron, including angiotensinogen secreted by proximal tubule and renin expressed in connecting tubule, may participate in the regulation of sodium reabsorption at multiple sites of the nephron. The response of this tubular renin-angiotensin system to stepwise changes in dietary sodium was investigated in 2 mouse strains, the sodium-sensitive inbred C57BL/6 and the sodium-resistant CD1 outbred. Plasma angiotensinogen was not affected by sodium regimen, whereas plasma renin increased 2-fold under low sodium. In both strains, the variation in urinary parameters did not parallel the changes observed in plasma. Angiotensinogen and renin excretion were significantly higher under high sodium than under low sodium. Water deprivation, by contrast, induced significant activation in the tubular expression of angiotensinogen and renin. C57BL/6 exhibited significantly higher urinary excretion of angiotensinogen than did CD1 animals under both conditions of sodium intake. The extent to which these urinary parameters reflect systemic or tubular responses to challenges of sodium homeostasis may depend on the relative contribution of sodium restriction and volume depletion.

Molecular cloning and functional analysis of a factor that binds to the proximal promoter of human angiotensinogen

A significant association has been reported between a common variant in the angiotensinogen gene (AGT), allele T235, and essential hypertension. In subsequent work, it was found that another variant, the presence of an adenine instead of a guanine 6bp upstream from the initiation site of transcription, was in absolute linkage disequilibrium with T235. The nucleotide substitution at the -6 position affected the formation of DNA-protein complexes in gel mobility shift assays and the basal transcription of AGT in transactivation experiments. We have further examined the potential impact of this polymorphism on AGT promoter function. In ultraviolet cross-linking analysis. 150- and 75-kDa proteins bound to the AGT proximal promoter. The possible involvement of factors that bind to GC-rich domains, including Sp1, Sp3, and AP2, was not supported by gel mobility shift assays. Screening an expression library with a double-stranded DNA segment centered on -6 led to the isolation of cDNA clones encoding the YB1 protein. The specificity of the interaction of YB1 with the proximal promoter of AGT was verified by Southwestern blotting and gel mobility shift assays. In cotransfection experiments, YB1 reduced basal AGT promoter activity in a dose-dependent manner. Although these observations suggest a possible role for YB1 in modulating AGT expression, this function is likely to occur in the context of complex interactions involving other nuclear factors. The work illustrates the challenge of developing a molecular understanding of the relationship between common genetic variants and conditions that are only partly caused by them.

Power and replication in case-control studies

The case-control study design, a common staple of epidemiology, is increasingly used to test for genetic association. The simplicity of the design accounts for both its appeal and its limitations. Too often, however, apparent controversy arises for lack of appreciation of basic tenets underlying statistical testing. Power and replication are two concepts most commonly ignored in evaluating such studies. We review the basic principles of statistical testing, recall simple means to calculate power, and provide numerical examples pertaining to the association between angiotensinogen and essential hypertension.

Nucleotide diversity and haplotype structure of the human angiotensinogen gene in two populations

Variation in the angiotensinogen gene, AGT, has been associated with variation in plasma angiotensinogen levels. In addition, the T235M polymorphism in the AGT product is associated with an increased risk of essential hypertension in multiple populations, making AGT a good example of a quantitative-trait locus underlying susceptibility to a common disease. To better understand genetic variation in AGT, we sequenced a 14.4-kb genomic region spanning the entire AGT and identified 44 single-nucleotide polymorphisms (SNPs). Forty-two SNPs were observed both in 88 white and in 77 Japanese unselected subjects. Six major haplotypes accounted for most of the variation in this region, indicating less allelic complexity than in many other genomic regions. Although the two populations were found to share all of the major AGT haplotypes, there were substantial differences in haplotype frequencies. Pairwise linkage disequilibrium (LD), measured by the D', r(2), and d(2) statistics, demonstrated a general pattern of decline with increasing distance, but, as expected in a small genomic region, individual LD values were highly variable. LD between T235M and each of the other 39 SNPs was assessed in order to model the usefulness of LD to detect a disease-associated mutation. Among the Japanese subjects, 13 (33%) of the SNPs had r(2) values >0.1, whereas this statistic was substantially higher for the white subjects (occurring in 35/39 [90%]). LD between a hypertension-associated promoter mutation, A-6G, and 39 SNPs was also measured. Similar results were obtained, with 33% of the SNPs showing r(2)>0.1 in the Japanese subjects and 92% of the SNPs showing r(2)>0.1 in the white subjects. This difference, which occurs despite an overall similarity in LD patterns in the two populations, reflects a much higher frequency of the M235-associated haplotype in the white sample. These results have important implications for the usefulness of LD approaches in the mapping of genes underlying susceptibility to complex diseases.

Development of genetic hypotheses in essential hypertension

Essential hypertension illustrates the formidable task presented by the identification of genetic determinants of common disease. Making an initial genetic inference may prove difficult enough; the subsequent demonstration of functional significance at various levels of biological integration may be even more challenging. We review three instances in which an initial genetic inference has led to the development of testable hypotheses pursued at increasingly higher levels of biological organization. These include the adducin, the G protein beta3 subunit, and the angiotensinogen hypotheses.

Angiotensinogen in essential hypertension: from genetics to nephrology

There is general consensus that genetic variation accounts in part for individual susceptibilities to essential hypertension. In marked contrast to classic mendelian disorders, in which genetic alterations produce a gain or loss of function, genetic determinants of essential hypertension, high blood pressure of unknown cause, are expected to be small, achieving significance through the cumulative effects of environmental exposure over the course of a lifetime. Whether and how genetic factors that contribute to common diseases can be identified remain unclear. Research on a link between angiotensinogen and essential hypertension illustrates a path that began in genetics and is now leading toward nephrology. Various challenges encountered along the way may prove to be characteristic features of genetic investigations of the pathogenesis of common diseases. The implication of a gene by statistical analysis is only the beginning of a protracted process of functional analysis at increasing levels of biologic integration. The ultimate goal is to develop an understanding of the manner in which genetic variation at a locus can affect a physiologic parameter and to extract from this inference new knowledge of significance for the prevention or treatment of disease.

From genetics to mechanism of disease liability

The molecular basis of single-gene Mendelian disorders resulting from gain or loss of function is being clarified at a rapid pace. Progress in the genetics of common disease, by contrast, has been frustratingly limited, as we discuss by reference to essential hypertension (EH). The application of standard genetic paradigms to hypertension research has yielded remarkable findings. Arterial pressure (AP) variation in laboratory rats has been correlated with various genes. Likewise, rare Mendelian hypertension syndromes are increasingly understood in molecular terms. The implications of these findings for EH have proven to be modest, however. Genetic methods have been applied to investigate directly essential hypertension in humans, with mixed results. The power of such methods to identify genetic determinants of EH has been questioned. The issues confronting the genetic analysis of EH are discussed by drawing from our ongoing work along the hypothesis that molecular variants of the angiotensinogen gene may constitute inherited predispositions to the condition. Simply establishing correlation is already a daunting task. Far more challenging yet is to establish causation for a physiological phenotype, that is, to understand the mechanism by which a genetic factor may predispose to essential hypertension. Susceptibility imparted by genetic variation, modest and quantitative, modulates response to environmental exposure over time. The product of the gene under examination may be highly pleiotropic, being involved with multiple physiological processes in multiple tissues. Finally, as physiological phenotypes are defined at the level of the entire organism, ultimate demonstration of genetic determination may require specific genetic manipulations in entire organisms.

Functional analysis of a mutation occurring between the two in-frame AUG codons of human angiotensinogen

Angiotensinogen (ANG) is the specific substrate of the renin-angiotensin system, a major participant in blood pressure control. We have identified a natural mutation at the -30 amino acid position of the angiotensinogen signal peptide, in which an arginine is replaced by a proline (R-30P). Heterozygous individuals with R-30P showed a tendency to lowered plasma angiotensinogen level (1563 ng of ANG I/ml (range 1129-1941)) compared with normal individuals in the family (1892 ng of ANG I/ml (range 1603-2072)). Human angiotensinogen mRNA has two in-phase translation initiation codons (AUG) starting upstream 39 and 66 nucleotides from the cap site. R-30P occurs in a cluster of basic residues adjacent to the first AUG codon that may affect intracellular sorting of the nascent protein. Pulse-chase experiments in transiently transfected cultured cells revealed that the R-30P mutation was associated with reduced amounts of both intra- and extracellular protein. In a cell-free system, we found that two forms of native angiotensinogen were generated by alternative initiation of translation at either AUG codon. Alteration of either the first or second AUG codons abolished the synthesis of the longer and the shorter form of native angiotensinogen, respectively. Furthermore, the rate of secretion of the shorter form was lower than that of the longer form. By transplanting angiotensinogen signal peptide onto green fluorescence protein, however, we found that both forms of the signal peptide could target green fluorescence protein, normally localized in the cytoplasm, to the secretory pathway. Although the R-30P mutation may not affect intracellular sorting of angiotensinogen in a qualitative manner, it leads to a quantitative reduction in the net secretion of mature angiotensinogen through decreased translocation or increased residence time in the endoplasmic reticulum.

Elements of a paracrine tubular renin-angiotensin system along the entire nephron

The renin-angiotensin system is a major regulator of body sodium, predominantly through the actions of intrarenal angiotensin II of unclear origin. We show that polarized epithelium of the proximal tubule synthesizes and secretes angiotensinogen at its apical side and that the protein can be detected in urine as a function of dietary sodium. Furthermore, we demonstrate that renin is expressed and secreted in a restricted nephron segment, the connecting tubule, also in a sodium-dependent fashion. A paracrine renin-angiotensin system operating along the entire nephron may contribute to long-term arterial pressure regulation by integrating distant tubular sodium-reabsorbing functions.

No linkage of the lipoprotein lipase locus to hypertension in Caucasians

OBJECTIVE

A previous study has shown significant linkage of five markers near the lipoprotein lipase locus to systolic blood pressure, but not to diastolic blood pressure, in nondiabetic members of 48 Taiwanese families selected for noninsulin-dependent diabetes. However, lipoprotein lipase markers did not appear strongly linked to systolic blood pressure in a study of Mexican-Americans using a variety of selection schemes. The objective of the current study was to test whether markers near the lipoprotein lipase gene were linked to hypertension in Caucasians.

DESIGN

To test for linkage of genetic markers in or near the lipoprotein lipase gene to hypertension in Caucasians, two sets of Caucasian hypertensive sibships were genotyped. The samples included 261 sibships (431 effective sibpairs) from four field centers of the National Heart, Lung and Blood Institute Family Heart Study and 211 sibships (282 effective sibpairs) from the Health Family Tree database in Utah.

RESULTS

Two highly polymorphic markers in or near the lipoprotein lipase gene showed no evidence of excess allele sharing in either set of hypertensive sibships. Combining the two datasets resulted in 653 and 713 effective sibpairs for the two markers, sharing 0.495 +/- 0.30 and 0.486 +/- 0.28 alleles identical by descent compared to an expected sharing of 0.50. Multipoint analysis of the two loci also did not show linkage (P = 0.95).

CONCLUSIONS

We conclude that the lipoprotein lipase locus and nearby regions do not appear to be linked to hypertension in Caucasians.

Angiotensinogen Thr235 variant is associated with abnormal physiologic change of the uterine spiral arteries in first-trimester decidua

OBJECTIVE

The angiotensinogen Thr235 polymorphism associated with preeclampsia is tightly linked to a mutation in the angiotensinogen promoter A(-6), which may lead to elevated expression in decidual spiral arteries. We hypothesize that locally elevated angiotensin II levels play a role in failed physiologic change leading to preeclampsia. Our objective was to determine whether spiral artery morphologic characteristics were different in first-trimester decidual samples from women homozygous for the angiotensinogen Thr235 allele and women homozygous for the normal angiotensinogen Met235 allele.

STUDY DESIGN

We used quantitative histologic analysis to study 1266 spiral artery cross-sections in decidual samples obtained from normal pregnancies (n = 53) terminated at 8 weeks' gestation. To define vessel characteristics before pregnancy-induced remodeling, we also examined 60 arteries in nonpregnant endometrial control samples (n = 5). We measured the aspect ratio, media area, and external diameter of each cross-section with Image-Pro plus software. Maternal angiotensinogen genotypes were determined by means of mutagenically separated polymerase chain reaction. Average spiral artery morphologic measurements were compared between genotypes with the Student t test.

RESULTS

The media area/external diameter ratio was lower in decidual samples than in endometrial samples (P <.0001), consistent with pregnancy-induced physiologic changes. Women homozygous for the angiotensinogen Thr235 allele (n = 11) had a greater area/diameter ratio than did women homozygous for the normal angiotensinogen Met235 allele (n = 11, P <.05). Samples from heterozygous women (n = 31) had intermediate values.

CONCLUSION

Our results suggest that the angiotensinogen Thr235 allele predisposes women toward abnormal physiologic change, potentially beginning the cascade of events leading to preeclampsia.

Angiotensinogen T235 expression is elevated in decidual spiral arteries

Preeclampsia is associated with a common molecular variant of angiotensinogen (Met235Thr). This variant is in tight linkage disequilibrium with a mutation in the angiotensinogen promoter, G(-6)A, which leads to elevated expression in vitro. Since angiotensin II levels could play a role in atherotic changes of the uterine spiral arteries associated with preeclampsia, we investigated angiotensinogen expression in the first trimester uterus. We localized angiotensinogen transcription in uterine decidua using in situ reverse transcription PCR. We then compared decidual T235 expression levels to M235 levels in heterozygous women using an allele-specific ligation assay and a single nucleotide primer extension assay. In human decidua, angiotensinogen is expressed only in spiral artery smooth muscle cells. Heterozygous women have significantly elevated expression of the T235 allele compared to the M235 allele (P < 0.0001). These observations suggest that elevated expression of the T235 allele in decidual spiral arteries may cause first trimester atherotic changes leading to preeclampsia.

Association of genetic variations in apolipoprotein B with hypercholesterolemia, coronary artery disease, and receptor binding of low density lipoproteins

To search for unique mutations in the apolipoprotein B (apoB) gene that disrupt the binding of LDL to its receptor and cause hypercholesterolemia, we examined more than 800 patients with high LDL cholesterol levels and/or coronary artery disease (CAD). Analysis of patient DNA by single-strand conformation polymorphism and allele-specific oligonucleotide hybridization of the sequence surrounding the putative receptor- binding domain of apoB (amino acid positions 2965 to 3534) revealed seven variations. LDL from heterozygotes with either Arg 3500 Gln or Arg 3531 Cys bound defectively with the LDL receptor in competitive binding assays. The Arg 3500 Gln substitution was statistically more prevalent in patients with hypercholesterolemia (P = 0.0003). Total cholesterol and LDL-cholesterol were significantly higher (P< 0.0004) in 34 apoB 3500 Gln carriers than in the controls. The allele encoding the Arg 3531 Cys substitution was more prevalent (0.8%) in the CAD group (P = 0.05) than in the controls. A Ser 3252 Gly variant was statistically more prevalent in the hypercholesterolemic group (P = 0.03), but LDL with this mutation had normal LDL receptor-binding activity. The other four variants identified (Leu 3350 Leu, Gln 3405 Glu, Val 3396 Met, and Ser 3455 Arg) were not associated with defective LDL-receptor binding, hypercholesterolemia, or CAD, nor were the apoB mutations associated with elevated lipid levels in family members. The surprising result that only two mutations of apoB in the receptor-binding domain (Arg 3500 Gln and Arg 3531 Cys) were associated with defective LDL binding, hypercholesterolemia, or CAD is in stark contrast with familial hypercholesterolemia, where nearly 150 mutations of the LDL receptor have been described that disrupt its function. This study strongly suggests that a limited number of mutations of apoB markedly influence LDL binding to its receptor.

A conditionally immortalized cell line from murine proximal tubule

We have developed a conditionally immortalized murine cell line with proximal tubule characteristics (tsMPT) and a background suitable for genetic manipulations. tsMPT was derived from the F1 progeny of crosses between: [1] a transgenic mouse harboring a gamma-interferon (IFN-gamma)-inducible, temperature sensitive SV40 large T antigen gene (tsA58) and [2] mice of the 129/SvEv strain, the background from which most embryonic stem (ES) cells are derived. Under permissive conditions (33 degrees C and in the presence of IFN-gamma), tsMPT cells grow rapidly as monolayers with a doubling time of 23 hours; the large T antigen can be detected by immunocytochemistry and by Western blotting. When transferred to non-permissive conditions (39 degrees C, without IFN-gamma), the cells undergo differentiation coinciding with the disappearance of the large T antigen. By electron microscopy, tsMPT cells are polarized and show microvilli at their apical surface. tsMPT cells express brush border enzymes gamma-glutamyl transpeptidase and carbonic anhydrase IV. They possess Na(+)-dependent transport systems for Pi, D-glucose and L-proline as well as an amiloride-insensitive Na(+)-H+ exchanger. Intracellular cAMP generation is stimulated by parathyroid hormone but not by arginine vasopressin. Angiotensinogen mRNA and protein are present in tsMPT with markedly higher levels at non-permissive conditions. tsMPT cells should be a useful model for investigation of the functional features of the proximal tubule epithelium in relation to cellular differentiation.

Haplotypes of angiotensinogen in essential hypertension

The M235T polymorphism of the angiotensinogen gene (AGT) has been associated with essential and pregnancy-induced hypertension. Generation of haplotypes can help to resolve whether the T235 allele itself predisposes to the development of hypertension or acts as a marker of an unknown causal molecular variant. We identified 10 diallelic polymorphisms at the AGT locus and genotyped both a series of 477 probands of hypertensive families and 364 controls, all French Caucasians, as well as a series of 92 hypertensives and 122 controls from Japan. Despite a large ethnic difference in gene frequency, a significant association of T235 with hypertension was observed both in Cancasians (.46 vs. .38, P = .004) and in Japanese (.91 vs. .76, P = .002). In both groups, the G-->A substitution located at position -6 upstream of the initial transcription site occurred at the same frequency and in complete linkage disequilibrium with the T235 allele. No other polymorphism was found to be consistently associated with hypertension. Five informative haplotypes subdividing the T235 allele were generated. Whereas two of them were associated with hypertension in Caucasians, none of these two haplotypes (H3 and H4) reached statistical significance in Japanese. The analysis of the AGT-GT repeat revealed marked linkage disequilibriums between each of the diallelic polymorphisms and some (GT)n alleles, with similar patterns in the two populations. The strong disequilibrium between M235 and (GT)16 explained the increased frequency of that particular allele in French controls compared with hypertensives (.42 vs. .36, P < .01). The haplotype combining the M235T and G-6A polymorphisms appears as the ancestral allele of the human AGT gene and as the one associated with hypertension.

A nucleotide substitution in the promoter of human angiotensinogen is associated with essential hypertension and affects basal transcription in vitro

In earlier studies, we provided statistical evidence that individual differences in the angiotensinogen gene, the precursor of the vasoactive hormone angiotensin II, constitute inherited predispositions to essential hypertension in humans. We have now identified a common variant in the proximal promoter, the presence of an adenine, instead of a guanine, 6 bp upstream from the initiation site of transcription, in significant association with the disorder. Tests of promoter activity and DNA binding studies with nuclear proteins suggest that this nucleotide substitution affects the basal transcription rate of the gene. These observations provide some biological insight about the possible mechanism of a genetic predisposition to essential hypertension; they may also have important evolutionary implications.

Associations of candidate loci angiotensinogen and angiotensin-converting enzyme with severe hypertension: The NHLBI Family Heart Study

PURPOSE

In studies conducted in several different populations, the M235T substitution in the angiotensinogen (AGT) locus has been associated with hypertension.

METHODS

A case-control study was initiated in an attempt to replicate this finding. Persons with hypertension, age- and sex-matched normotensive controls, and randomly sampled individuals were probands from the Family Heart Study of the National Heart, Lung, and Blood Institute. Subjects were recruited from the Atherosclerosis Risk in Communities study (ARIC) in North Carolina and Minneapolis, MN, and from the Framingham Heart Study in Massachusetts. Genotypes were determined for the M235T substitution in the AGT locus and for the insertion/deletion polymorphism in the angiotensin-converting enzyme (ACE) locus. Simple association tests as well as logistic regression analyses were performed.

RESULTS

The association of AGT-T235 with hypertension was replicated in the Framingham sample (odds ratio, 1.60; 95% confidence interval, 1.11-2.30), but not in the ARIC white or black subjects. However, logistic regression analysis suggested a significant association of AGT with hypertension in both the ARIC white and Framingham samples when the effects of body mass index, triglycerides, and the presence of significant coronary heart disease were controlled. These analyses further suggested that, in the ARIC data, the relationship with the AGT locus is stronger in women than men and that there may be interaction (epistasis) between homozygotes for T235 and ACE-DD in the Framingham data. While the small sample size precluded logistic regression analysis, the frequency of the T235 allele in the black random sample was much higher than in the comparable white sample.

CONCLUSIONS

These results are compatible with the presence of a genetic risk factor for hypertension in or near the angiotensinogen locus.

Associations between candidate loci angiotensin-converting enzyme and angiotensinogen with coronary heart disease and myocardial infarction: the NHLBI Family Heart Study

Angiotensin-converting enzyme (ACE) and angiotensinogen (AGT) are major components of the renin-angiotensin systems. An association between myocardial infarction (MI) and the ACE DD genotype of the insertion/deletion (ID) polymorphism in intron 16 of the ACE gene has been reported. However, other similarly designed studies have not found such an association. Angiotensin II, the product of AGT, has a direct effect on vascular tone; and a variant in the AGT gene has been found to be associated with MI in the Japanese. This case-control study was initiated to investigate whether the ACEI/D and AGT M235T polymorphisms are associated with an increased risk for coronary heart disease (CHD) and MI. Our study groups were composed of participants in the National Heart Lung Blood Institute (NHLBI) Family Heart Study (FHS) selected from three population-based studies: two Atherosclerosis Risk in Communities (ARIC) centers (Forsyth County, NC, and Minneapolis, MN), and the Framingham Heart Study. In multivariate analysis within ARIC Caucasians, a significant positive association was found between CHD (controls = 230, cases = 232) and the AGT TT genotype (P = 0.022; OR = 1.84, 1.09-3.10 95% CI). When we restricted the analysis to a low-risk group for CHD (controls = 70, cases = 35) an interaction between the ACE DD and AGT TT genotypes was significant (P = 0.025; OR = 5.02 1.22-20.6 95% CI). After further subsetting low-risk cases to those with a definite MI (controls = 74, cases = 16), we found that the associations with the ACE DD genotype was also significant (P = 0.013, OR = 3.94, 1.28-12.2 95% CI). Comparable tests in the Framingham sample failed to support an association of these markers with CHD. In conclusion, within selected groups the ACE D and AGT 235T alleles are statistically associated with CHD and MI, and there is a synergistic interaction between the two alleles. These results and those from previous studies together suggest that the association of these two loci is neither strong nor consistent and involves a complex interaction among risk factors and genotypes.

Guidelines of the National Heart, Lung, and Blood Institute Working Group on Blood Drawing, Processing, and Storage for Genetic Studies

The guidelines presented here are intended for epidemiologic investigators who wish to store blood samples for genetic studies, either by extracting DNA directly from white blood cells or from immortalized cell lines. Recommended procedures for blood drawing and for processing samples are described. Protocols for freezing and storage of both white blood cells and extracted DNA are provided. Although the extraction of DNA and immortalization of cell lines should be conducted in collaboration with an experienced laboratory, a summary of the available methods with appropriate references is given, and a method for the extraction of DNA from clotted blood is referenced. In addition, criteria for selecting study subjects for whom immortalized cell lines are preferable to merely extracting DNA are presented. Finally, the use of alternative sources of genetic material, including cheek swabs and dried blood spots, is described briefly.

Lipoprotein lipase binds to low density lipoprotein receptors and induces receptor-mediated catabolism of very low density lipoproteins in vitro

Lipoprotein lipase (LPL), the major enzyme responsible for the hydrolysis of plasma triglycerides, promotes binding and catabolism of triglyceride-rich lipoproteins by various cultured cells. Recent studies demonstrate that LPL binds to three members of the low density lipoprotein (LDL) receptor family, including the LDL receptor-related protein (LRP), GP330/LRP-2, and very low density lipoprotein (VLDL) receptors and induces receptor-mediated lipoprotein catabolism. We show here that LDL receptors also bind LPL and mediate LPL-dependent catabolism of large VLDL with Sf 100-400. Up-regulation of LDL receptors by lovastatin treatment of normal human foreskin fibroblasts (FSF cells) resulted in an increase in LPL-induced VLDL binding and catabolism to a level that was 10-15-fold greater than in LDL receptor-negative fibroblasts, despite similar LRP activity in both cell lines. This indicates that the contribution of LRP to LPL-dependent degradation of VLDL is small when LDL receptors are maximally up-regulated. Furthermore studies in LRP-deficient murine embryonic fibroblasts showed that the level of LPL-dependent degradation of VLDL was similar to that in normal murine embryonic fibroblasts. LPL also promoted the internalization of protein-free triglyceride emulsions; lovastatin-treatment resulted in 2-fold higher uptake in FSF cells, indicating that LPL itself could bind to LDL receptors. However, the lower induction of emulsion catabolism as compared with native VLDL suggests that LPL-induced catabolism via LDL receptors is only partially dependent on receptor binding by LPL and instead is primarily due to activation of apolipoproteins such as apoE. A fusion protein between glutathione S-transferase and the catalytically inactive carboxyl-terminal domain of LPL (GST-LPLC) also induced binding and catabolism of VLDL. However GST-LPLC was not as active as native LPL, indicating that lipolysis is required for a maximal LPL effect. Mutations of critical tryptophan residues in GST-LPLC that abolished binding to VLDL converted the protein to an inhibitor of lipoprotein binding to LDL receptors. In solid-phase assays using immobilized receptors, LDL receptors bound to LPL in a dose-dependent manner. Both LPL and GST-LPLC promoted binding of VLDL to LDL receptor-coated wells. These results indicate that LPL binds to LDL receptors and suggest that the carboxyl-terminal domain of LPL contributes to this interaction.

Blunted renal vascular response to angiotensin II is associated with a common variant of the angiotensinogen gene and obesity

OBJECTIVE

Recently, we reported evidence for genetic linkage between human essential hypertension and the angiotensinogen gene (AGT) and an association with a common molecular variant of this gene (methionine 235 --> threonine or T235). Other studies had led us to hypothesize that blunted renal plasma flow responses to infused angiotensin II (Ang II) when in high salt balance may reflect increased intrarenal formation of Ang II, a condition that might promote hypertension. Here we examine the relationship between AGT genotype and renal vascular response to infused Ang II.

METHODS

Hypertensive (n = 34, all off medication) and normotensive (n = 57) members of families with a history of hypertension (age 18-60 years) as well as 29 normotensive volunteers without a family history of hypertension were studied after controlled diets with 200 mequiv./day sodium. Ang II was infused at a mildly pressor dose (3 ng/kg/min) and renal plasma flow was determined by steady-state plasma para-aminohippurate concentration.

RESULTS

After correction for covariates in multivariate analyses, participants homozygous for the T235 variant had significantly diminished renal plasma flow responses to the Ang II infusion (P = 0.005). Changes in renal arterial resistance were also blunted in the T235 homozygotes. Similar results were found when analysis was restricted to normotensive participants or subdivided based on family history of hypertension. No confounding factors associated with AGT genotype that could explain these differences were found. Furthermore, obesity, which also suppressed renovascular response to Ang II, was found to interact significantly (P = 0.017) with genotype such that, among T235 homozygotes, obesity had a greater blunting effect on renal vascular response.

CONCLUSIONS

Expected renovascular response to infused Ang II was blunted in persons with the AGT TT genotype. This is the first report of an association between a specific gene variant and altered renal physiology in humans with particular relevance to essential hypertension.

Lack of linkage between the endothelial nitric oxide synthase gene and hypertension

Nitric oxide is an important vasodilator formed in many tissues, including the vascular endothelium. Because of the relationship between nitric oxide and basal vascular tone, genes regulating nitric oxide have been suggested as candidate genes involved with the development of hypertension. At least three isoforms of nitric oxide synthase have been identified. Two of the isoforms, endothelial and inducible nitric oxide synthase, may have particular importance in hypertension. The gene coding for endothelial nitric oxide synthase on chromosome 7 has been cloned. Polymorphic dinucleotide repeats within this nitric oxide synthase gene were used to test for linkage to hypertension in 259 hypertensive siblings from 112 Utah hypertensive sibships. The resulting 194 sibpairs shared 108 alleles identical by state compared to the expected 108.1 alleles shared as estimated from CEPH allele frequencies. After weighting for different sibship sizes, there was only a 3.9% excess allele sharing (P = 0.21). Allele sharing in more severe hypertensive sibpairs (either two antihypertensive medications or an unmedicated diastolic blood pressure (BP) of 100 mm Hg or higher) showed a 6% excess over expected sharing of alleles (P = 0.28). There was no difference between male and female sibpair sharing of alleles (5.2% vs 7.8%, respectively, both not significant). Therefore, there was no evidence that the gene for endothelial nitric oxide synthase was linked to hypertension in these sibpairs.

Genetic mapping of adrenergic receptor genes in humans

We have genetically mapped the genes encoding four human adrenergic receptors (ARs) of subtypes alpha 1C, alpha 2A, alpha 2B, and beta 1, which are prototypic G protein coupled receptors that mediate the physiological effects of neurotransmitters, hormones, and drugs. We placed these genes onto the Cooperative Human Linkage Center (CHLC) and Genethon framework maps, within confidence intervals with greater than 1000:1 odds. With multipoint analysis the alpha 1C gene (locus ADRA1C) mapped to the interval between NEFL and D8S283; alpha 2-C4, the gene encoding the alpha 2C AR (locus ADRA2C), mapped to the interval between D4S126 and D4S62; and the alpha 2-C10 (alpha 2A AR)/beta 1 haplotype (loci ADRA2A/ADRB1) mapped to the interval between D10S259 and D10S187. A fifth AR gene, beta 2, yielded significant LOD scores with markers on the long arm of chromosome 5; however, this locus (ADRB2) could not be mapped to any specific interval with odds of greater than 1000:1. The two AR genes that are completely linked, alpha 2-C10 and beta 1, were oriented on their shared 225-kb genomic fragment relative to the direction of transcription, with beta 1 being 5' to alpha 2-C10. The positioning of these genes on high-density framework maps allows them to be tested as candidates in a spectrum of diseases that might involve AR dysfunction.

A mutation of angiotensinogen in a patient with preeclampsia leads to altered kinetics of the renin-angiotensin system

Angiotensinogen exhibits genetic linkage to and association with essential hypertension and preeclampsia, a common hypertensive disorder of pregnancy; however, the polymorphisms detected thus far provide no functional clues. In a preeclamptic patient, we have identified a mutation leading to the replacement of leucine by phenylalanine at position 10 of mature angiotensinogen (L10F), the site of renin cleavage. Kinetic analyses of the enzymes of the renin-angiotensin system, using either model peptides or full-length substrates, show that this mutation significantly alters the reactions with both renin and angiotensin-converting enzyme. For the renin reaction on a full-length substrate, this substitution leads to a 10-fold decrease in Km (from 1.1 to 0.09 microM) and a 5-fold decrease in kcat (from 1.0 to 0.22 s-1); as a result, catalytic efficiency (kcat/Km) is increased by a factor of 2 (1.1 versus 2.4 microM-1 s-1). In the reaction of angiotensin-converting enzyme on angiotensin decapeptides, the substitution has no effect on Km (38.0 versus 30.0 microM), but increases kcat and catalytic efficiency > 2-fold (kcat = 15.0 versus 37.0 s-1; kcat/Km = 0.41 versus 1.23). The renin-angiotensin system, challenged by the profound physiological adaptations of pregnancy, is perturbed in preeclampsia; consequently, the L10F mutation may promote this condition in carrier subjects.

Angiotensin-converting enzyme gene polymorphism is associated with myocardial infarction but not with development of coronary stenosis

BACKGROUND

Although both genetic and nongenetic factors contribute to the pathogenesis of coronary artery disease, the identification of specific genetic lesions has lagged behind the identification of critical environmental risk factors. A reported association between myocardial infarction (MI) and the insertion/deletion (I/D) polymorphism of the angiotensin-converting enzyme (ACE) gene in European men suggests a critical role for this genomic region. However, the generality of this association remains to be determined. It also is not clear at what stage in disease progression the association with the ACE I/D polymorphism becomes important.

METHODS AND RESULTS

We evaluated the ACE I/D polymorphism in patients who had undergone coronary angiography (402 men and 295 women) and in 203 representative control subjects. After polymerase chain reaction amplification, genotypes were determined by agarose gel sizing and by hybridization with allele-specific oligonucleotides. After patients were categorized by the degree of coronary artery stenosis and the occurrence of an MI, the distribution of ACE I/D genotypes was evaluated by log linear analysis. Patients were genetically representative of the regional population, and patients with > 60% stenosis of their coronary arteries had the same distribution of ACE I/D genotypes as did patients with < 10% stenosis. However, among patients with stenosis, the occurrence of an MI was significantly associated with the D allele in all patients (odds ratio [OR], 1.59; P = .002) and in men alone (OR, 1.63; P = .006). The lack of significance in women (OR, 1.40; P = .263) is probably due to the fact that only 36 women in the present study had experienced an MI. Furthermore, the association between MI and the ACE I/D polymorphism was independent of blood pressure, smoking habits, and body mass index.

CONCLUSIONS

Segregation of the ACE I/D polymorphism is a pervasive genetic risk factor for MI in whites but has no evident effect on the events leading to stenosis of the coronary arteries. This suggests that risk of MI is influenced by two independent processes--atherogenesis that leads to coronary stenosis followed by conversion to MI. The renin-angiotensin system appears to confer significant risk of infarction by influencing the conversion to MI but has no apparent effect on the development of atherostenosis.

Molecular screening of the lipoprotein lipase gene in hypertriglyceridemic members of familial noninsulin-dependent diabetes mellitus families

Hypertriglyceridemia is common among individuals with noninsulin-dependent diabetes mellitus (NIDDM), and heterozygous lipoprotein lipase (LPL) mutations may result in the syndrome of familial hypertriglyceridemia and low levels of high density lipoprotein (HDL) cholesterol. To test the hypothesis that heterozygous LPL mutations predispose to the hypertriglyceridemia and low HDL cholesterol levels observed among members of familial NIDDM families, we examined 36 members and 3 unrelated spouses selected from members of 20 pedigrees for triglyceride levels exceeding the age- and sex-specific 95th percentile. Eighteen pedigree members and 2 spouses were diabetic. LPL exons 1-9 were screened by single strand conformation polymorphism analysis. Six different variants were detected in exons 2, 3, 4, 8, and 9, including 4 (exons 3, 4, and 8) silent nucleotide substitutions. A common nonsense mutation (exon 9; Ser-->Ter) was present in 2 pedigrees, and a missense mutation (exon 2; Asp-->Asn) was also present in members of 2 pedigrees. Analysis of members of these families suggested an association of the exon 2 variant with hypertriglyceridemia, although this trend was no longer significant when individuals with diabetes were excluded from the analysis. The variant enzyme was not present among 83 random control individuals, and when expressed in COS-1 cells, it was similar to the wild type with respect to specific activity, heparin binding, and heat stability. Our data suggest that coding region mutations of the LPL gene cannot account for the elevated triglyceride and low HDL levels noted in diabetic individuals and their relatives in most NIDDM pedigrees, but the exon 2 Asn variant may contribute to the hypertriglyceridemia in some families.

Cellular catabolism of normal very low density lipoproteins via the low density lipoprotein receptor-related protein/alpha 2-macroglobulin receptor is induced by the C-terminal domain of lipoprotein lipase

Lipoprotein lipase (LPL) binds to the low density lipoprotein receptor-related protein (LRP)/alpha 2-macroglobulin receptor and induces catabolism of normal human very low density lipoproteins (VLDL) via LRP in vitro. Recent studies showed that the C-terminal domain of LPL can bind LRP in solid phase assays and inhibit cellular catabolism of two LRP ligands, activated alpha 2-macroglobulin and the 39-kDa receptor-associated protein (Williams, S.E., Inoue, I., Tran, H., Fry, G. L., Pladet, M.W., Iverius, P.-H., Lalouel, J.-M., Chappell, D.A., and Strickland, D.K. (1994) J. Biol. Chem. 269, 8653-8658). The current study investigated the potential for this region of LPL to promote cellular catabolism of VLDL via LRP. A fragment comprising the C-terminal domain of LPL (designated LPLC) was expressed in bacteria and found to promote cellular binding, uptake, and degradation of normal human VLDL in a dose-dependent manner. These effects were present whether LPLC was added simultaneously with 125I-VLDL or was prebound to cell surfaces prior to the assay. Mutations involving Lys407, Trp393, Trp394, or deletion of the C-terminal 14 residues reduced the effects of LPLC. Three LRP-binding proteins, the receptor-associated protein, lactoferrin, and a polyclonal antibody against LRP, competed for 125I-VLDL degradation induced by LPLC. Heparin or heparinase treatment of cells prevented LPLC-induced 125I-VLDL catabolism. Thus, cell-surface proteoglycans play an important role in this pathway. Interestingly, either LPLC or LPL when added in excess could block LPL-induced 125I-VLDL degradation presumably by interacting directly with LRP. However, unlabeled VLDL could not prevent catabolism of 125I-labeled LPLC or LPL. These data show that cellular fates for VLDL versus LPLC or LPL are divergent. This is probably due to independent catabolism of the latter via cell-surface proteoglycans. In summary, these in vitro studies indicate that a fragment of LPL corresponding to the C-terminal domain mimics the native enzyme with respect to induction of VLDL catabolism via LRP. Because LPLC lacks the catalytic site of native LPL, these studies establish that lipase activity is not required for LRP-mediated lipoprotein catabolism.

Evidence for single gene contributions to hypertension and lipid disturbances: definition, genetics, and clinical significance

Several large family studies are reviewed to identify results suggesting single gene traits contributing to the occurrence of hypertension in humans. Segregation analysis in families has suggested major gene effects for several highly heritable traits associated with hypertension. These include recessively segregating high sodium-lithium countertransport (major gene H2 = 34%), additively segregating low urinary kallikrein excretion (major gene H2 = 51%), and recessively segregating hyperinsulinemia (major gene H2 = 33%). In some families, hypertension and metabolic abnormalities (dyslipidemia, hyperinsulinemia, and obesity) seem to be related to several candidate genes studied but not conclusively proven (LPL deficiency mutations, dense LDL subfractions, or NIDDM with hyperinsulinemia). More recently, DNA markers have identified genes promoting hypertension. Glucocorticoid-remediable aldosteronism (GRA) promotes a rare but unusual form of hypertension that is unresponsive to ordinary medications but very responsive to glucocorticoid medications. GRA has been found in hypertensive persons with a specific mutation of the 11 beta-hydroxylase gene on chromosome 8q21. Many persons with essential hypertension carry a common "susceptibility gene" at the angiotensinogen locus (chromosome 1q4) identified using linkage studies in siblings, association studies, and in studies of preeclampsia and hypertension in pregnant women. These first two well-established genetic loci promoting human hypertension represent two ends of a broad spectrum. The rare "determinant" gene for GRA by itself seems to produce severe hypertension and early strokes. The angiotensinogen (AGT) "susceptibility" gene is very common (30% of Utah Caucasians) and seems to predispose to hypertension but probably requires other genetic and environmental influences to be fully expressed.(ABSTRACT TRUNCATED AT 250 WORDS)

The carboxyl-terminal domain of lipoprotein lipase binds to the low density lipoprotein receptor-related protein/alpha 2-macroglobulin receptor (LRP) and mediates binding of normal very low density lipoproteins to LRP

Lipoprotein lipase (LPL) binds with high affinity to the low density lipoprotein receptor-related protein/alpha 2-macroglobulin receptor (LRP) and promotes binding, uptake, and degradation of normal triglyceride-rich lipoproteins in a process mediated by LRP (Chappell, D. A., Fry, G. L., Naknitx, M.A., Muhonen, L. E., Pladet, M. W., Iverius, P-H., and Strickland, D. K. (1993) J. Biol. Chem. 268, 14168-14175). To localize the portion of LPL that is responsible for interacting with LRP, fragments of LPL were expressed in bacteria. A fragment of human LPL containing the COOH-terminal domain (residues 313-448, designated LPLC) which lacks the catalytic site was able to bind to LRP. Purified LRP bound specifically to microtiter wells coated with LPL or LPLC with KD values of 2.8 and 5 nM, respectively. The effects of several mutations of LPLC were tested. Mutation of Lys407 to Ala reduced the affinity of LPLC for LRP by approximately 10-fold. Like native LPL, LPLC prevented the binding of activated alpha 2-macroglobulin and the 39-kDa receptor-associated protein to LRP and inhibited the internalization and degradation of activated alpha 2-macroglobulin and receptor-associated protein in cultured fibroblasts. LPLC also bound to 125I-labeled human normal triglyceride-rich lipoproteins and promoted their binding to purified LRP and to cultured cells. Mutation of Trp393 and Trp394 to Ala completely abolished the ability of LPLC to bind to lipoproteins, but had little effect on its interaction with LRP. These data indicate that the COOH-terminal domain of LPL may function both in binding lipoproteins and mediating their interaction with LRP.

Angiotensinogen as a risk factor for essential hypertension in Japan

A common molecular variant of angiotensinogen (AGT), the precursor of the potent vasoactive hormone angiotensin II, has been incriminated as a marker for a genetic predisposition to essential hypertension in Caucasians (Jeunemaitre, X., F. Soubrier, Y. V. Kotelevtsev, R. P. Lifton, C. S. Williams, A. Charru, S. C. Hunt, P. N. Hopkins, R. R. Williams, J. M. Lalouel, and P. Corvol. 1992. Cell. 71:169-180). We now show that the same variant, T235, is associated with essential hypertension in Japanese patients. The observation of this association in a distinct, ethnically homogeneous population further substantiates an involvement of angiotensinogen in the pathogenesis of essential hypertension and has physiological, epidemiological, and evolutionary implications.