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Journal of Clinical Endocrinology & Metabolism , doi:10.1210/jc.2006-1481
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The Journal of Clinical Endocrinology & Metabolism Vol. 91, No. 12 5051-5056
Copyright © 2006 by The Endocrine Society

Association between Aldosterone Production and Variation in the 11ß-Hydroxylase (CYP11B1) Gene

Helen Imrie1, Marie Freel1, Bongani M. Mayosi, Eleanor Davies, Robert Fraser, Mary Ingram, Heather J. Cordell, Martin Farrall, Peter J. Avery, Hugh Watkins, Bernard Keavney and John M. C. Connell

Institute of Human Genetics (H.I., H.J.C., B.K.) and School of Mathematics and Statistics (P.J.A.), Newcastle University, Newcastle NE1 3BZ, United Kingdom; Department of Medicine and Therapeutics (M.F., E.D., R.F., M.I., J.M.C.C.), University of Glasgow, Glasgow G11 6NT, United Kingdom; Department of Medicine (B.M.M.), University of Cape Town, Cape Town, South Africa 7923; and Department of Cardiovascular Medicine (M.F., H.W.), University of Oxford, Oxford OX3 9DU, United Kingdom

Address all correspondence and requests for reprints to: Bernard Keavney, Institute of Human Genetics, Central Parkway, Newcastle NE1 3BZ, United Kingdom. E-mail: b.d.keavney{at}ncl.ac.uk.


   Abstract
Top
 Abstract
Introduction
 Subjects and Methods
 Results
 Discussion
 References
 
Context: Variation in the region of chromosome 8 including the genes steroid 11ß-hydroxylase (CYP11B1) and aldosterone synthase (CYP11B2) influences mineralocorticoid and glucocorticoid metabolism. However, the relative importance of polymorphisms in CYP11B1 and CYP11B2 in determining these phenotypes is unknown.

Objective: Our objective was to investigate genetic influences of the CYP11B1 and CYP11B2 genes on mineralocorticoid metabolism.

Design: We measured 24-h urinary excretion of the key metabolites of the principal mineralocorticoids, glucocorticoids and androgens secreted by the adrenal cortex. We genotyped polymorphisms spanning the CYP11B1 and CYP11B2 genes, which together capture all common variations at the locus.

Participants: Participants included 573 members of 105 British Caucasian families ascertained on a hypertensive proband.

Main Outcome Measures: We assessed heritability of urinary tetrahydroaldosterone (THAldo) excretion and association of THAldo excretion with genotype.

Results: The heritability of THAldo excretion was 52% (P < 10–6). There was significant association between THAldo and genotype at several of the CYP11B1/B2 polymorphisms. The strongest association was observed at the rs6387 (2803A/G) polymorphism in intron 3 of CYP11B1 (P = 0.0004). Association followed a codominant model with a 21% higher THAldo excretion per G allele. Genotype at rs6387 accounted for 2.1% of the total population variability of THAldo. We found significant association between THAldo excretion and urinary total androgen excretion, urinary tetrahydrodeoxycortisol level, and urinary cortisol metabolites (all P < 0.001).

Conclusions: Aldosterone synthesis is highly heritable and is affected by genotype at CYP11B1. Our findings support the hypothesis that genetically determined differences in 11-hydroxylation efficiency can have downstream effects on mineralocorticoid synthesis. Such effects may be of relevance to the development of low-renin essential hypertension.


   Introduction
Top
 Abstract
 Introduction
Subjects and Methods
 Results
 Discussion
 References
 
HYPERTENSION IS A complex phenotype that reflects several genetic influences interacting with environment. Subphenotypic stratification and measurement of biochemical and physiological intermediate phenotypes provide important ways to dissect out these genetic factors. In this regard, the role of aldosterone in the genesis of hypertension and cardiovascular risk is of considerable interest. Variation of plasma aldosterone levels within the normal population predicts a subsequent rise in blood pressure and the development of hypertension; furthermore, recent studies suggest that at least 15% of hypertensive subjects have an increased aldosterone-to-renin ratio (ARR) (1, 2, 3, 4). Although the exact interpretation of this remains unclear, it identifies a large cohort of hypertensive patients in whom the regulation of aldosterone secretion by its usual trophins is abnormally sensitive. This suggests that altered adrenal corticosteroid production is a key phenotype in a significant proportion of cases of hypertension (5). The fundamental mechanism that results in this remains unclear, but an underlying genetic predisposition is likely, and for this reason it is necessary to consider the genetic basis of regulation of adrenal aldosterone synthesis.

The terminal steps of aldosterone synthesis in the zona glomerulosa (11-hydroxylation, 18-hydroxylation, and 18-oxidation of the precursor steroid deoxycorticosterone) are catalyzed by aldosterone synthase (encoded by the gene CYP11B2). In the adjacent zona fasciculata, cortisol is synthesized by the action of 11ß-hydroxylase (encoded by CYP11B1) on the precursor steroid 11-deoxycortisol. These highly homologous genes lie in close proximity on chromosome 8 in man. Variation in these genes accounts for several monogenic syndromes (e.g. glucocorticoid remediable aldosteronism and 11ß-hydroxylase deficiency) in humans as well as experimental hereditary hypertension in certain rat models (e.g. the Dahl salt-sensitive rat) (6, 7, 8, 9). For this reason, the CYP11B1 and CYP11B2 genes are attractive candidates in studies of essential hypertension.

Previous studies in patients with hypertension associated with aldosterone excess have focused on variation within CYP11B2. Most have centered on a single-nucleotide polymorphism (SNP) in the 5'-untranslated region (C–344T) and a variant within intron 2 in which the intron is exchanged for that of the corresponding intron in CYP11B1, termed intron conversion (IC). These alleles have been associated with hypertension, particularly in subjects with an increased ARR; investigators have also reported increased plasma and urinary excretion of aldosterone in some studies. However, neither the association with hypertension nor that with aldosterone excretion has been found in all studies (10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20). A more consistently observed intermediate corticosteroid phenotype seen in association with the –344 T allele and IC alleles of CYP11B2 is increased plasma levels of deoxysteroids (deoxycorticosterone and 11-deoxycortisol) basally and in response to ACTH stimulation and a corresponding increase in excretion of their principal urinary metabolites. These data are consistent with diminished efficiency of the enzyme 11ß-hydroxylase (CYP11B1), because neither cortisol nor corticosterone levels are abnormal. A potential mechanism that we have suggested to account for these observations is that the C–344T polymorphism of CYP11B2 is in linkage disequilibrium (LD) with causal variant(s) in CYP11B1 that alter enzyme efficiency, so resulting in the phenotype of increased plasma deoxysteroid levels. The resulting increase in ACTH drive to the adrenal, primarily to maintain cortisol levels, also results in increased aldosterone production, suppression of renin, and hypertension (5).

In support of this notion, we recently demonstrated a high degree of LD across the CYP11B1/B2 locus and confirmed high heritability of plasma concentrations of 11-deoxycortisol and excretion rate of its urinary metabolite, tetrahydrodeoxycortisol (THS), that was best explained by variation in CYP11B1, a finding confirmed by others (21, 22). Here we examine the heritability of the main urinary metabolite of aldosterone, tetrahydroaldosterone (THAldo), and explore its relationship with polymorphic variation in the CYP11B1 and CYP11B2 genes in a large collection of nuclear families. We also examine the relationship between urinary THAldo and the index of 11ß-hydroxylation efficiency (ratio of THS/F) as well as with other ACTH-dependent adrenal steroids.


   Subjects and Methods
Top
 Abstract
 Introduction
 Subjects and Methods
Results
 Discussion
 References
 
A total of 248 Caucasian families, comprising 1428 individuals, were recruited in the Oxford region of the United Kingdom from 1993–1997 as previously described (23). Briefly, families were selected through a proband with essential hypertension and documented systolic and diastolic blood pressure in the top 5% of the population distribution either on multiple clinical readings or ambulatory blood pressure monitoring. Probands were screened for causes of secondary hypertension according to our institution’s usual clinical protocol, and any subjects where a secondary cause could not be ruled out were deemed ineligible. To be suitable for the study, families were required to consist of at least three siblings (including the proband) clinically assessable for blood pressure if at least one parent of the sibship was available to give blood for DNA analysis and to consist of at least four assessable siblings (including the proband) if no parent was available for DNA analysis. Qualifying sibships could be in the generation of either the proband or his/her offspring, and there was no requirement for the sibship to contain additional members affected with hypertension (although this was not an exclusion criterion). Where members of the sibship were found to have essential hypertension (using identical criteria to those applied in the probands), families were extended and the spouses and offspring of hypertensive sibs collected. Thus, the family collection includes some extended families, although most are nuclear families. In all cases, to minimize the chances of bilineal inheritance, sibships where both parents were known to be hypertensive were ineligible for study. Blood pressure was measured using ambulatory monitoring for a period of 24 h (A&D TM2421; Takeda Medical, Japan) in all subjects willing to undergo monitoring. A full clinical history was taken; anthropometric measurements including height, weight, waist, and hip measurement were made; and blood was drawn (with the subject having been in a sitting position for 5 min before the blood draw) into a variety of anticoagulants for plasma and DNA analysis. Blood was not drawn at a particular time of day, although most families were visited in their homes in the evenings. From 1998–2000, families were recontacted and asked to reattend for additional phenotyping; at this visit, 12-lead electrocardiograms, echocardiograms, ultrasound measurement of carotid intima-medial thickness, and 24-h urine collections for steroid metabolites were performed (24). Urine was collected without preservative and aliquots stored at –20 C until analyzed. The study received ethical clearance from the Central Oxford Research Ethics Committee, and informed consent was obtained from all participants.

Urinary steroid analyses

The 24-h excretion rates of THS, tetrahydrocortisol (THF), allo-THF (aTHF), tetrahydrocortisone (THE), THAldo, dehydroepiandrosterone (DHA), aetiocholanolone (Aetio), and androsterone (Andro) were determined by gas chromatography-mass spectrometry using the method of Shackleton with minor modifications, as previously described (22, 25).

Genotyping

DNA was extracted using standard methods. Five SNPs that span the CYP11B2 gene were genotyped: rs1799998 in the promoter region (also known in previous publications as C–344T or SF-1), rs4539 in the third exon (also known as A2718G or K173R), rs4538 in the sixth intron (also known as A4555C), rs28930074 in the seventh exon (also known as T4991C), and rs3097 in the 3'-untranslated region (also known as G5942A; numbering for these SNPs is with respect to the A of the initial ATG codon). We also typed the biallelic IC polymorphism located in the second intron of the CYP11B2 gene, which corresponds to a substitution of a portion of the second intron of CYP11B2 with that of CYP11B1 and thus has alleles C (converted) and NC (unconverted). Three SNPs spanning the CYP11B1 gene were typed: rs 6410 (G225A) in the first exon, rs6387 (A2803G) in the third intron, and rs5316 (C4855T) in the eighth exon. Genotyping was carried out by PCR amplification and digestion by restriction enzymes as previously described (22). Genotyping was carried out blinded to the phenotypic information, and controls of known genotype were included in each genotyping run. Mendelian inheritance within families was confirmed using the PedCheck program (26). Ten percent of the samples were genotyped in duplicate, with an estimated genotyping error rate of less than 1%.

Statistical analysis

Exploratory analyses to test for normality of the phenotypic data and transform data where appropriate, to establish significant covariates, to identify and remove outliers, and to explore relationships between urinary steroid phenotypes were performed using MINITAB. Heritability of the urinary steroid phenotypes was calculated using MERLIN (27). Identity-by-descent vectors for each marker were calculated in MERLIN, and quantitative trait association analysis for individual polymorphisms was performed by a variance-components approach using the Quantitative Transmission Disequilibrium Test (QTDT) software (28). The distribution of THAldo departed significantly from normal, and despite a variety of transformations being applied to the data, none was identified that achieved normality on a formal statistical test. Departure from normality can significantly affect the results of variance-components analysis. To confirm our results, we therefore carried out linear regression, regressing trait on genotype indicator variables (a two degrees of freedom test) while allowing for correlation within families through use of an empirical Huber-White information sandwich variance estimator. The significance of the observed tests was then evaluated through use of a permutation procedure whereby in each permutation replicate, the mean trait value was permuted across families, and then the individual-specific departures from the family mean trait value were permuted within families. Ten thousand permutations of the phenotypic data were carried out for those variants that showed conventional statistical significance in the QTDT analyses, and 1000 permutations were carried out for those not showing significance, using STATA statistical software.


   Results
Top
 Abstract
 Introduction
 Subjects and Methods
 Results
Discussion
 References
 
Twenty-four-hour urine collections were available in 573 individuals from 105 families. The characteristics of the family members are shown in Table 1Go. Urinary cortisol metabolite excretion rate (total F) was calculated as the sum of THF, aTHF, and THE excreted per 24 h. Similarly, total androgen excretion rate was calculated as the sum of DHA, Aetio, and Andro. The ratio of THS/total F was used as an indicator of 11ß-hydroxylase efficiency. All of the steroid phenotypes required transformation to achieve an approximately normal distribution. After adjustment for age and sex, the heritability of the square-root transformed urinary THAldo excretion was 52%, of the log-transformed urinary total androgens was 30%, and of the log-transformed ratio of THS/total F was 50% (all P < 10–6). There was no association between any of the urinary steroid phenotypes and hypertension status; therefore, no ascertainment correction was applied in the subsequent genetic analysis.


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  		TABLE 1. Characteristics of 573 members of 105 families studied

 
The nine diallelic polymorphisms typed at the CYP11B1/B2 locus are shown on the map in Fig. 1Go. Their minor allele frequencies within the population are shown in Table 2Go. Genotyping was successful for more than 95% of available samples for all polymorphisms, and all genotypes were in Hardy-Weinberg equilibrium at the 5% level. QTDT analysis showed no significant evidence for stratification at any marker, so we used the total (both within-family and between-family) association model for all analyses. There was no significant association between genotypes and either log-transformed sum of urinary androgen excretion or the square-root transformed sum of urinary cortisol metabolites (data not shown). However, THAldo excretion was strongly associated with genotype at the rs6387 SNP in intron 3 of CYP11B1 (A2803G) in both the QTDT analyses and in the regression analyses involving simulation of 10,000 replicate datasets (P = 0.0004 in QTDT total association model; P = 0.0058 in regression model; Table 2Go). There was borderline significant association between THAldo excretion and genotype at both the rs6410 SNP in exon 1 of CYP11B1 (P = 0.014 in QTDT model; P = 0.0311 in regression model; Table 2Go), and the CYP11B2 IC polymorphism (P = 0.007 in QTDT model; P = 0.0386 in regression model; Table 2Go); both of these SNPs are in strong LD with the A2803G SNP. The association appeared to follow a codominant model; urinary THAldo was 21% higher per G allele at the most strongly associated CYP11B1 A2803G SNP (Fig. 2Go). Genotype at this SNP accounted for 2.1% of the population variability in THAldo excretion.


Figure 1
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  		FIG. 1. Representation of the CYP11B2 and CYP11B1 genes illustrating typed polymorphisms. Exons are numbered.

 

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  		TABLE 2. Association of log-transformed THAldo excretion rate with genotyped polymorphisms

 

Figure 2
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  		FIG. 2. Interval plot (mean and 95% CI) of sex-adjusted residuals of the square root transformed 24-h urinary THAldo levels and genotype at the most strongly associated polymorphism typed, rs6387 in intron 3 of CYP11B1. P value for significant association = 0.0004.

 
We performed subsidiary analyses to rule out the possibility that the association between CYP11B1 variants and urinary aldosterone excretion that we observed was due to confounding either by the presence of hypertension in 40% of our study participants or by antihypertensive therapy in those people. There was no association between genotype at rs6387 and hypertension status ({chi}2 = 1.46; P = 0.482), making confounding by hypertension unlikely. One hundred sixty participants (30%) were taking one or more antihypertensive agents. There was no significant association between THAldo excretion and current therapy with any of the four most commonly prescribed classes of antihypertensive agent: diuretics (n = 63 participants), ß-blockers (n = 52), angiotensin I-converting enzyme inhibitors or angiotensin receptor blockers (n = 64), and calcium antagonists (n = 37). Nor was there any association between genotypes and therapy with any of these antihypertensive drug classes. When analyses were restricted only to those participants free of medications, there remained significant association between the CYP11B1 A2803G polymorphism and THAldo excretion, although as expected with the exclusion of about one third of the data, the statistical significance level was less extreme (P = 0.02). These results suggest that confounding by antihypertensive therapy is unlikely to explain our results. Exogenous estrogens could potentially influence THAldo excretion; however, in analyses restricted to women participants, there was no significant association between treatment with either the oral contraceptive pill or with hormone replacement therapy and THAldo levels.

As previously shown by our group and others, there was strong evidence for association between a measure of 11-hydroxylation (here, the log-transformed urinary THS/total F ratio) and several of the genotypes, the strongest association being found with rs6410 in exon 1 of CYP11B1 (ß = 0.16; P = 8 x 10–9; r2 = 6.4%). Relationships between aldosterone excretion and ACTH-dependent variables were also explored. THAldo excretion was highly significantly associated with total cortisol metabolite excretion rate (r = 0.34; P < 0.001) and with total androgen excretion rate (r = 0.175; P < 0.001). These correlations were unaffected by the presence of hypertension or by genotype at any of the polymorphisms.


   Discussion
Top
 Abstract
 Introduction
 Subjects and Methods
 Results
 Discussion
References
 
Major genetic effects on the function of the CYP11B1/B2 locus (such as inactivating mutations and unequal crossing-over events) cause substantial derangements in salt and water handling, and consequently in blood pressure regulation, in relatively rare Mendelian families (7). The role of common polymorphisms of this locus in essential hypertension is much less clear; whereas some studies have shown an association of the rs1799998 SNP 5' to CYP11B2 with hypertension status, other studies have failed to replicate the finding (10, 12, 13, 14, 15, 16, 17, 18, 20). In this study, we have explored the effect of polymorphisms spanning the locus on intermediate phenotypes of adrenal steroid metabolism in a large family-based study. We have shown that urinary excretion of the principal metabolite of aldosterone (THAldo) is strongly heritable, confirming in a much larger family study an earlier observation that we made in a small adult twin study (29). Genetic analyses indicate that a proportion of this heritability can be explained by genotypes at polymorphisms of the CYP11B1 and CYP11B2 loci.

The strongest association was observed for the CYP11B1 rs6387 polymorphism in intron 3; association at this polymorphism was stronger than that observed at polymorphisms of the CYP11B2 locus, including the rs1799998 polymorphism that has been the subject of most previous studies. We had previously shown strong evidence for association of urinary THS excretion (a measure of 11-hydroxylation) with genotypes at CYP11B1 (22). In the present work, we confirmed strong association of these genotypes with urinary THS/total F, a more direct measure of 11-hydroxylation efficiency than THS alone. Our data therefore strongly support the existence of quantitative trait loci at CYP11B1 that directly influence both the efficiency of 11-hydroxylation and aldosterone excretion. The size of both genetic effects is relatively small, and the effect on 11-hydroxylation is approximately three times the size of the effect on aldosterone excretion (r2 = 6.4 and 2.1% respectively). There was weaker evidence of association between aldosterone excretion and the IC polymorphism of CYP11B2; we have previously shown strong LD between the rs6387 and IC polymorphisms, which likely accounts for that observation.

The fact that the strongest association with THAldo variability was CYP11B1, the gene encoding 11-hydroxylase, seems, initially, paradoxical because this is not expressed in zona glomerulosa. However, we previously suggested that CYP11B1 polymorphisms might act, at least in part, to regulate aldosterone synthesis via their effect on zona fasciculata function, with feedback mediated via ACTH (5). The significant associations we found between aldosterone excretion and excretion of zona fasciculata steroids (total cortisol, THS, and total androgens) that are ACTH dependent lends additional support to this notion. One possibility is that a reduction in 11ß-hydroxylation efficiency caused by genetic variants in CYP11B1 leads to a mild, lifelong increase in ACTH drive to the adrenal gland to maintain cortisol production, resulting in a slight increase in its precursor, 11-deoxycortisol. Such a rise in ACTH and 11-deoxycortisol could result in enhanced synthesis of aldosterone in response to its usual trophins (angiotensin II and potassium), although the mechanisms for this are currently unclear. We hypothesize that in some susceptible individuals, such a chronic small enhancement of aldosterone production may lead to a more extreme phenotype of aldosterone excess presenting clinically as hypertension with a raised ARR. Although this hypothesis remains speculative, the genetic effects we have described, if lifelong, do appear to represent a mechanism whereby the relationship between aldosterone, renin, potassium, and ACTH could be reset. Aldosterone is metabolized by 5{alpha}- and 5ß-reductase, and it is possible that genetic variation in reductase activity could also affect THAldo excretion rate. Additional studies will be required to investigate this possibility; the present findings make no assumption about any such separate influence.

Several previous studies have examined the association between genotypes at CYP11B2 and aldosterone synthesis, most having focused on the rs1799998 (C–344T) polymorphism. Although the results of these studies have been discrepant, the majority of the available data suggests that the T allele (which is in LD with the G allele at rs6387 in CYP11B1) is associated with higher aldosterone synthesis (15, 18, 20). This is the first study to provide data on CYP11B1 genotypes and aldosterone synthesis, and it suggests that the previously observed association with the rs1799998 polymorphism could arise through LD between rs1799998 and a causative variant (or variants) located in CYP11B1. We observed borderline significant association between the T allele of rs1799998 and aldosterone synthesis, but this was considerably weaker than that observed with the CYP11B1 rs6387 variant and was not robust to simulation of 10,000 replicate datasets. Tanahashi et al. (30) recently studied CYP11B2 expression levels in patients with aldosterone-producing adenomas and found association between transcript levels and genotype at both the rs4539 and the rs1799998 polymorphisms of CYP11B2 (there is strong LD between these polymorphisms); they also showed that the arginine-encoding alleles of rs1799998 were transcribed at higher levels than those encoding lysine. Although that experiment provides additional confidence that genetic variation at this locus directly influences CYP11B2 expression, it does not rule out a primary role for variants in CYP11B1 that are in LD with rs4539 and rs1799998 in CYP11B2. More data on mRNA levels of CYP11B2 among individuals with different genotypes at the CYP11B1 polymorphisms we have typed would be of major interest.

Strengths of the present study include its size, its family-based design, which eliminates the concern regarding population stratification as a source of spurious false positives, and the genotyping of polymorphisms previously shown to capture all the common genetic variation at this locus. Also, we have focused on urinary corticosteroid metabolites, which are accepted to be a more robust measure of adrenal steroids than plasma measurements and less influenced by short-term environmental influences such as posture or time of day. Ideally, when assessing mineralocorticoid activity, sodium intake should be regulated, and subjects should not be taking drugs known to affect the renin-angiotensin-aldosterone axis. It was not possible to regulate sodium intake or discontinue medications in the present study, so these are acknowledged limitations that may have led to an underestimate of the size of the genetic effect present (although less than one third of patients were on antihypertensive therapy). Our study ascertained on hypertensive probands; in theory, this could limit the generalizability of our findings, but over 60% of our population was nonhypertensive, and our findings remain significant when only the nonhypertensive subgroup is considered. Moreover, any concern regarding generalizability needs to be balanced against the known large loss in power to detect genetic effects on quantitative phenotypes when families not selected for extreme values of the trait of interest are studied. There was no association between urinary steroid phenotypes and either hypertension status or specific antihypertensive therapies in this study. This provides additional confidence that the inclusion of hypertensive individuals in this study did not result in bias that might impair our capacity to accurately determine genetic effects on those steroid phenotypes. On the other hand, the lack of association between urinary steroid phenotypes and hypertension indicates that additional environmental and genetic factors will need to be identified in those people carrying putative risk alleles to confirm that these genetically determined differences in steroid metabolism are causal factors in the etiology of hypertension.

In summary, these data provide novel evidence that urinary aldosterone excretion is highly heritable and is associated with variation in the CYP11B1 gene. Additional studies will be necessary to conclusively identify the causative variant(s) at this locus. Because LD in Caucasian populations is strong in this region, multiple polymorphisms (as here seen for rs6387 and IC) may show evidence for association as the mapping resolution increases, and distinguishing causative variants may be difficult. As we have previously shown, studies in populations with less extensive LD (such as African-origin populations) may be particularly helpful in a trans-ethnic fine-mapping approach to identification of the causative variant in this situation (31).


   Footnotes
 
The study was supported by the British Heart Foundation, Wellcome Trust, and UK Medical Research Council. B.M.M. was a Nuffield Medical Fellow while conducting this work.

Disclosure statement: There are no relationships or conflicts to disclose for any author.

First Published Online September 19, 2006

1 H.I. and M.F. provided equal contributions. Back

Abbreviations: Aetio, Aetiocholanolone; Andro, androsterone; ARR, aldosterone-to-renin ratio; aTHF, allo-THF; DHA, dehydroepiandrosterone; IC, intron conversion; LD, linkage disequilibrium; QTDT, Quantitative Transmission Disequilibrium Test; SNP, single-nucleotide polymorphism; THAldo, tetrahydroaldosterone; THE, tetrahydrocortisone; THF, tetrahydrocortisol; THS, tetrahydrodeoxycortisol.

Received July 11, 2006.

Accepted September 13, 2006.


   References
Top
 Abstract
 Introduction
 Subjects and Methods
 Results
 Discussion
 References
 

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