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Original Studies |
Second Department of Internal Medicine (Y.T., K.F., H.M.), Department of Health Sciences (Y.T.), School of Medicine, Kanazawa University, Kanazawa 920, Japan; and Third Department of Internal Medicine (S.I., I.M.), Fukui Medical School, Fukui 91011, Japan
Address all correspondence and requests for reprints to: Yoshiyu Takeda, M.D., Second Department of Internal Medicine, School of Medicine, Kanazawa University, 131 Takara-machi, Kanazawa 920, Japan. E-mail: takeday{at}mhs.mp.kanazawa-u.ac.jp
| Abstract |
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| Introduction |
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In the adrenal cortex, aldosterone is synthesized from deoxycorticosterone (DOC) by a mitochondrial cytochrome P450 enzyme, aldosterone synthase (CYP11B2) (3). The corresponding gene is located on chromosome 8q22, adjacent to a closely related gene that encodes steroid 11ß-hydroxylase (CYP11B1), which is required for cortisol biosynthesis. Mutations in CYP11B2 can cause aldosterone deficiency (4). Conversely, GRA is caused by genetic recombination between CYP11B1 and CYP11B2 that increases expression of CYP11B2 messenger RNA (mRNA) and leads to inappropriate secretion of aldosterone (5, 6, 7). Recently, mutations in the CYP11B2 gene that increase the activity of this enzyme up to 1000-fold have been reported in genetically hypertensive rats (8). Fardella et al. (9) showed that the mutations in human CYP11B2 gene, as seen in rats, increased the enzyme activity to 4-fold. To clarify the etiology of IHA, we estimated aldosterone synthase activity and expression of CYP11B2 mRNA in peripheral mononuclear leukocytes (MNL) of patients with IHA, in comparison with patients with APA and normal controls. Then, searching for mutations in the CYP11B2 gene, as mentioned above, we looked for the chimeric CYP11B1/CYP11B2 gene in patients with IHA and APA and analyzed the coding region of the CYP11B2 gene using direct sequencing methods.
| Subjects and Methods |
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Nineteen patients with primary aldosteronism, including 10 with APA (4 male and 6 female; ages, 3562 yr) and 9 with IHA (3 males and 6 females; ages, 3260 yr) were studied, as well as control subjects (6 males and 4 females; ages, 3050 yr). All patients were diagnosed from characteristic biochemical abnormalities, including hypokalemia, suppressed PRA, and increased aldosterone production. GRA was excluded by administering 2 mg/day dexamethasone to patients for 3 days. If dexamethasone failed to suppress plasma aldosterone to the normal range and serum potassium and blood pressure did not normalize, a clinical diagnosis of primary aldosteronism was made. High concentrations of plasma aldosterone in both adrenal veins, and absence of a solitary adenoma on CT scan of the adrenal glands, confirmed a diagnosis of IHA. Other biochemical parameters, including low serum potassium concentrations (APA, 2.6 ± 0.3 mmol/L; IHA, 2.8 ± 0.4 mmol/L), suppressed PRA (APA, 0.61 ± 0.17 ng/L·sec; IHA, 0.79 ± 0.21 ng/L·sec), and high plasma aldosterone concentrations (APA, 1530 ± 290 pmol/L; IHA, 1140 ± 330 pmol/L) were similar in both groups with hyperaldosteronism.
Measurement of aldosterone synthase activity
Peripheral MNL were prepared by standard techniques (10). Aldosterone synthase activity in 106 mol/MNL was assayed by replacing the medium containing 0.5 µmol/L [14C]DOC (0.001 µCi; New England Nuclear, Tokyo, Japan) and incubating the cells for 24 h. The incubation medium was extracted with a Sep-Pak C18 cartridge (Waters Associates, Milford, CT), and chromatography was performed in a reverse-phase high-performance liquid chromatography system, as previously reported (11). The activity of aldosterone synthase was estimated as the percent conversion of the total radioactivity of [14C]DOC to [14C]aldosterone.
Competitive PCR assay of CYP11B2 mRNA
Total cellular RNA, isolated from peripheral MNL of patients with IHA, those with APA, and normal controls, was amplified by an RT-PCR, as previously described (11). Briefly, 500 ng of total RNA was incubated at 42 C for 60 min with 2.5 U of Moloney murine leukemia virus reverse transcriptase (Takara, Tokyo, Japan) in a 20-µL reaction volume containing 10 mmol/L Tris-HCl (pH 8.3), 50 mmol/L KCl, 5 mmol/L MgCl2, each deoxynucleotide triphosphate at 1 mmol/L, and 2.5 mmol/L random hexanucleotide primer (Takara). This mixture was incubated for 5 min at 99 C. The single-stranded complementary DNA (cDNA) was used for competitive PCR. The sequences of sense and antisense primers for CYP11B2 were 5'-TACAGGTTTTCCTCTACTCG-3' and 5'-AGATGCAAGAC-TAGTTAATC-3', following the sequences published by Wu et al. (12) and respectively corresponding to nucleotides 12081227 and 15031522 of their own cDNA (13). The competitive templates for CYP11B2 were made using the PCR MIMIC Construction Kit (CLONTECH Laboratories, Inc., Palo Alto, CA). After quantification, a serial dilution was used as an internal standard for competitive PCR, which was performed using 2.5 µL of the reverse-transcribed DNA, 2 µL of various concentrations of the competitive template, 0.5 µmol/L each of sense and antisense primers, and 0.5 U of Taq DNA polymerase (Perkin-Elmer Japan, Tokyo, Japan) in 50 µL of 10 mmol/L Tris-HCl (pH 8.3), 50 mmol/L KCl, and 2 mmol/L MgCl2 with each deoxynucleotide triphosphate at 0.2 mmol/L. The reactions were performed for 1 min at 94 C, 1 min at 59 C, and 2 min at 72 C for 30 cycles. Aliquots of 10 µL of amplification products were electrophoresed on a 3.0% agarose gel, which then was stained with ethidium bromide and photographed. Staining intensity was quantified by computer-assisted densitometry using the BIO-PROFIL BIO-1D system (Compak, Tokyo, Japan). Intensities of each product from cDNA and those from competitive templates were plotted as a function of the known amounts of the competitive templates. Intraassay and interassay variabilities of this competitive PCR were 11.5% and 14.8%, respectively. The concentration of CYP11B2 mRNA was expressed as attomoles per 100 nanograms of total RNA. To test the yield and efficiency of the reverse transcriptase reaction, 1 µg of total RNA was subjected to RT as above, with 5 µmol/L of radioactively labeled [32P]deoxycycidine triphosphate (New England Nuclear) added to the reaction, as previously reported (14).
Southern blotting of RT-PCR products
The RT-PCR products were electrophoresed in 10-µL aliquots on a 3% agarose gel and transferred to nylon membranes. Hybridization was performed as previously reported (15), using an oligoprobe specific for CYP11B2 (5'-GGCGCGTGGCAGAGGCAGAGATGCTGC-3') that had been end-labeled with [32P]ATP (6000 Ci/mmol, New England Nuclear) using a 5'-end oligonucleotide labeling kit.
Southern analysis of genomic DNA
Genomic DNA was extracted from peripheral blood leukocytes and digested with BamHI, fractionated by agarose gel electrophoresis, transferred to a nylon membrane, and hybridized with a 32P-deoxycycidine triphosphate-labeled exon 34 probe corresponding to 11ß-hydroxylase (5). The probe was prepared by PCR using specific primers and genomic DNA as a template. DNA samples from a patient with GRA (kindly donated by Dr. Gordon, University Department of Medicine, Greenlopes Hospital, Brisbane, Australia) and from healthy volunteers were used as controls.
Analysis of DNA using a long PCR
Genomic DNA was extracted from peripheral blood leukocytes. For each patient, the isolated DNA was subjected to two amplification reactions, as previously reported by Jonsson et al. (16). In brief, in the first reaction, the sense primer (5'-TCCTTCATCTACCTTTGGCTGGGG-3') was specific for the 5' untranslated region of the aldosterone synthase gene, whereas in the second reaction, the sense primer (5'-TCATGCACCCCCAATGAGTCCCTG-3') was specific for the untranslated region of the 11ß-hydroxylase gene. For both reactions, the antisense primer (5'-GAGTCCTCCAGCTGCCTCTCAACC-3') was specific for the intron E region of the aldosterone synthase gene. The DNA was amplified by the methods previously described. Amplification products were electrophoresed on 0.8% agarose gels, stained with ethidium bromide, and visualized under ultraviolet light.
Sequence analysis of CYP11B2
Sequence analysis of the CYP11B2 gene in patients with IHA or APA was performed by PCR amplification of exons using both intron- and exon-derived primers, as previously reported (17). PCR products were cycle-sequenced with Taq polymerase FS dye-terminator sequencing kits (Perkin-Elmer Japan) on a model 377 automated DNA sequencer (Perkin-Elmer Japan).
| Results |
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| Discussion |
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-reductase (23). Shozu et
al. (24) have reported estrogen biosynthesis in THP1 cells of
human mononuclear leukemic origin, as well as aromatase activity and
expression of CYP19 mRNA in human MNL (personal communication). These
results suggest that MNL may possess steroid biosynthetic enzyme
activities. Glucocorticoid synthesis and the expression of CYP11B1 mRNA
in blood vessels were also reported (11, 15). However, we could only
detect small a amount of CYP11B1 mRNA in MNL by performing repeated
PCR. In vascular endothelial cells, the amount of CYP11B1 mRNA is
reported to be very small, compared with that of CYP11B2 (11). The
reason for this is unclear. Increased expression of CYP11B2 mRNA in MNL of patients with IHA may only reflect increased aldosterone synthesis in adrenal tissues; we have reported increased production of aldosterone in adrenal tissue involved by IHA (25). In this study, the expression of CYP11B2 mRNA in MNL was very small, compared with that in aldosteronoma or adrenal gland. The activity of aldosterone synthase in MNL was much less than that in aldosteronoma or adrenal gland. These results may suggest that the actual potential of MNL to contribute to production of aldosterone in vivo is little.
The presence of bilateral hyperplasia in IHA suggests a secondary response to a stimulatory mechanism, rather than a primary neoplastic growth, although no novel aldosterone-stimulating substance has been identified. Recently, Shozu et al. (26) have reported that a mutation in a 5'-flanking region of the CYP19 gene caused excessive peripheral aromatase expression in a boy with gynecomastia. Several reports suggest that polymorphisms in the CYP11B2 5'-flanking region may be involved with cardiovascular diseases (27, 28), associated with adrenocortical function in males (29), or useful in genetic diagnosis of 11ß-hydroxylase gene defects (30). Further analysis of the promoter region of CYP11B2 in patients with IHA is necessary.
The etiology of primary aldosteronism has been unknown since Conn first described it in 1955. However, Lifton et al. (5) have reported the candidate gene underlying the clinical entity of GRA, which was described by Sutherland et al. (31) in 1966. Focal or diffuse hyperplasia usually is present in both the remainder of the adrenal containing the APA and the contralateral gland. Gordon et al. (1) have postulated that such histologic hyperplasia outside the adenoma suggests a genetic abnormality not limited to the adenoma cells. Pascoe et al. (7) have reported that GRA chimeric gene-transfected cells possess aldosterone synthase activity. They also demonstrated that the GRA chimeric gene was expressed at higher levels than either CYP11B1 or CYP11B2 in the adrenal cortex of GRA patients (32). Gordon et al. (33) have identified 11 families with 2 or more members who had adenomas that did not show suppression by glucocorticoids, suggesting that the genetic abnormalities may be involved more frequently in primary aldosteronism than is now appreciated. However, Carroll et al. (34) were unable to detect a GRA chimeric gene in any APAs, and the GRA gene may not be involved in APAs. Recently Jonsson et al. (16) have described the long PCR procedure, which is a relatively fast, safe, and inexpensive method for diagnosis of GRA. In the present study, we could not detect the GRA chimeric gene in patients with APA or IHA, using Southern analysis or the long PCR method. Mulatero et al. (35) also were unable to find the GRA gene in a large number of patients with primary aldosteronism. Recently, Beuschlein et al. (36) have reported the somatic, heterozygous microdeletion in CYP21B gene of APA. However, they concluded little pathophysiological significance of their findings. Nodular adrenal cortical hyperplasia is common in multiple endocrine neoplasia type I. Recently, a candidate gene for multiple endocrine neoplasia type I, menin gene, has been cloned (37). We analyzed the coding region of the menin gene in genomic DNA of IHA and tumor DNA of APA and did not find any abnormalities in both patients (data not shown).
Mutations in the coding region of CYP11B2 decrease or eliminate aldosterone synthase activity in familial hypoaldosteronism (4, 17). However, some artificial mutations of CYP11B2 have been shown to increase enzymatic activity (9). Fardella et al. (38) have reported that the Arg173 variant may be linked to low-renin hypertension in Chilean patients. However, we could not find any mutations in the coding region of CYP11B2 gene, including the Arg173 variant in patients with IHA or APA.
In conclusion, no GRA-associated chimeric gene and no mutations in the coding region of the CYP11B2 gene were found in patients with IHA or APA. However, overexpression of CYP11B2 mRNA in the MNL of patients with IHA suggests that unidentified aldosterone-stimulatory factors or abnormalities of the CYP11B2 promoter region may cause the overproduction of aldosterone characteristic of IHA.
| Footnotes |
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Received October 9, 1998.
Revised January 4, 1999.
Accepted February 1, 1999.
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