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Cardiovascular Research 31 (19%) 870-872
Cardiovascular Research
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Glucocorticoid-remediable aldosteronism
Robert G. Dluhy, Gordon H. Williams *
Department of Medicine, Harvard Medical School and Brigham and Women's Hospital, Boston, MA, USA Received 9 August 1995; accepted 12 March 1996
Keywords: Glucocorticoid remediable aldosteronism
1. Introduction
Blood pressure is known to be influenced by heritable and environmental factors. The genetic contribution is complex since multiple genes are known to participate in the regulation of blood pressure. Once blood pressure is elevated, structural change also ensues, although the degree of vascular remodeling probably relates to hormonal and genetic differences. Thus, an attempt to discover 'hypertensive genes' or mutations in genes that maintain normotension might be 'misguided' owing to the multiplicity of factors that regulate blood pressure. However, the genetic basis of a hypertensive disorder has recently been reported, indicating that the discovery of additional mutations in genes that regulate blood pressure will be likely in the future. This disorder — glucocorticoid-remediable aldosteronism (GRA) — illustrates that one successful strategy to identify ' hypertensive genes' is to subdivide the hypertensive syndrome into intermediate phenotypes. These phenotypes can then be scrutinized by searching for mutations in candidate genes that relate to the specific hypertensive phenotypes.
2. Discussion
GRA has been known since 1966 to be a form of mineralocorticoid hypertension characterized by reversal of the mineralocorticoid-excess state by the exogenous administration of a glucocorticoid, such as dexamethasone [1,2]. GRA was subsequently shown to be inherited in an autosomal fashion, establishing the genetic basis for this disorder. Clinically, the age of onset of hypertension in GRA patients is in the first two decades of life. This
* Corresponding author.
contrasts sharply with that of subjects with other causes of primary aldosteronism who are usually diagnosed during the fourth and fifth decades of life [3]. In contrast to other etiologies of primary aldosteronism, many patients with GRA are not hypokalemic and, thus, a potassium level lacks sensitivity as a screening test for this disorder. GRA had been reported worldwide, but is more common in individuals of Scottish and Irish ancestry; no cases have yet been reported in Blacks.
In GRA aldosterone secretion is positively and solely regulated by adrenocorticotropic hormone (ACTH) [4,5]. As a consequence, exogenous glucocorticoid administration profoundly suppresses aldosterone secretion in affected subjects. In addition, the exogenous administration of ACTH is associated with an exuberant and prolonged increase in aldosterone secretion and blood pressure in such subjects. In contrast, in normal individuals ACTH administration is associated with a rise and subsequent decline in aldosterone levels to basal over several days. As in other etiologies of primary aldosteronism, plasma renin levels are suppressed in GRA subjects. However, parameters of aldosterone production often overlap the normal range, consistent with one hypothesis that other unmeasured mineralocorticoids contribute to this mineralocorticoid-excess state. However, it should be recalled that aldosterone secretion in GRA is solely regulated by ACTH. Thus, there is dysregulation of mineralocorticoid secretion, since aldosterone does not have a feedback-suppressive effect: on the pituitary production of ACTH.
Two abnormal steroids — 18-hydroxycortisol (18-OHF) and 18-oxocortisol (18-OXOF) — were found by Ulick and others to be greatly overproduced in the urine in affected GRA subjects [6,7]. Like aldosterone, these 18-oxygenated Cortisol compounds were found to be positively regulated by ACTH. Moreover, these compounds
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were shown to provide a specific means of diagnosis. Although they are also modestly overproduced in patients with aldosterone-producing adenomas (but not bilateral idiopathic hyperplasia), the ratio of these compounds to aldosterone rarely exceeds one. In GRA, these 18-oxygenated Cortisol compounds are greatly overproduced both in an absolute sense and in relationship to aldosterone, with a ratio that invariably exceeds two. These abnormal steroids are hybrid compounds since they share structural features of both the aldosterone and cortisol-pro-ducing zones of the adrenal cortex. Specifically, they are hydroxylated at carbons 17 and 18, features of zona fascic-ulata and zona glomerulosa steroids, respectively. Although it was felt originally that these compounds may contribute to the mineralocorticoid-excess state in evidence in GRA, they have been found in rodents to have only weak mineralocorticoid activity [8]. However, it is possible that these hybrid compounds possess significant mineralocorticoid activity in humans.
The molecular basis of GRA was discovered by applying a candidate gene approach in a pedigree where affected and unaffected subjects were identified using as the specific biochemical phenotype these hybrid compounds. Thus, 18-OHF and TH 18-OXOF were measured in the urine of individuals in three living generations in a large GRA pedigree where a proband had been diagnosed [9]. Of 18 at-risk individuals, GRA was diagnosed in 11 additional patients. A candidate gene approach was then taken with the hypothesis that a mutation in the aldosterone gene could explain GRA. A gene expressing aldosterone synthase activity was known to be closely related to a second gene involved in adrenal steroidogenesis, steroid 11 /3-hy-droxylase [10-13]. Both genes are 95% identical in DNA sequence and have identical intron-exon structures. Both genes are located in close proximity on chromosome 8. Genetic markers were developed to the aldosterone synthase gene and segregation of these markers was compared with segregation of GRA in the pedigree described. What was found in affected subjects, but not their affected relatives, was an extra gene — a hybrid or chimeric gene
GRA Patient
Aldosterone synthase
11-OHase
^hi mm
Unequal crossing over
3' 5'_3'
a---ii mm —
-^fli mm
hi ill"
.3' 5'
Aldosterone synthase Chimaeric gene
il min —
11-OHase
Fig. 1. The chimeric gene duplication in GRA, a result of unequal crossing-over between the homologous 11 /3-hydroxylase and aldosterone synthase genes. The chimera fuses the S'-regulatory sequences of the 110-hydroxylase gene and the 3'-coding sequences of the aldosterone synthase gene. From Lifton et al. [14] by permission.
a tAldosterone
I Mineralocorticoids >
^Angiotensin II 'Na
Adrenal
I Volume
Fig. 2. The chimeric gene results in ectopic expression of aldosterone synthase activity in the cortisol-producing zona fasciculata (F) under the regulation of adrenocorticotropin (ACTH). This results in overproduction of aldosterone and other mineralocorticoids (including 18-oxygenated Cortisol compounds), which leads to suppression of the renin-angiotensin system and atrophy of the zona glomerulosa (G).
fusing sequences of the 11 /3-hydroxylase and aldosterone synthase genes [14] (Fig. 1). This gene duplication was shown to contain the 5' regulatory sequences, confirming ACTH responsiveness of 11 /3-hydroxylase, fused to more distal coding sequences of aldosterone synthase. Given the homologies of the 11/3-hydroxylase and aldosterone synthase genes, the mechanism that would likely create such a gene duplication would be a recombination event, an unequal crossing-over between these two genes.
This gene duplication appears to explain all of the known physiology and biochemistry previously reported in GRA. First, the promoter region of this chimeric gene contains regulatory sequences of 11/3-hydroxylase and would be expected to be regulated by ACTH. In addition, this chimeric gene would allow ectopic expression of aldosterone synthase and enzymatic activity in the ACTH-regulated zona fasciculata which normally only secretes Cortisol (Fig. 2). Thus, the presence of this gene duplication in the zona fasciculata would also explain the production of the C-18 oxygenated Cortisol compounds. Finally, the sole regulation of aldosterone secretion by ACTH and the suppression of aldosterone secretion by glucocorticoids is explained by this gene mutation, since the aldosterone synthase gene is abnormally regulated by ACTH promoter sequences.
In an additional study in 11 unrelated GRA pedigrees, all affected subjects had chimeric gene duplications arising from unequal crossing-over [15]. Chimeric genes were also found in a second study of 4 additional patients from unrelated pedigrees [16]. In both studies, the sites of crossing-over were variable, indicating that in different pedigrees these gene duplications arose independently and
did not descend from a single ancestral mutation. However, in all studies, the sites of crossing-over were upstream of exon 5 of aldosterone synthase, suggesting that encoded amino acids in exon 5 are essential for aldosterone synthase enzymatic functions. This hypothesis is strengthened by the construction of chimeric genes and expression of aldosterone synthase enzymatic activity in vitro [16]. In this study, aldosterone synthase enzymatic activity was retained, provided that the site of fusion between 11-hydroxylase and aldosterone synthase genes was upstream of exon 5 of aldosterone synthase.
As a result of these studies, direct genetic screening for GRA is now possible. Moreover, the presence of the gene duplication appears to be 100% sensitive and specific for diagnosing GRA and is concordant with the measurement of the abnormal steroids, TH 18-OXOF, and 18-OHF. It is recommended that patients with aldosteronism without radiographic evidence of tumors, as well as individuals with suppressed levels of PRA (especially children and young adults) should be screened for GRA. Moreover, since this is an autosomal dominant disorder, extended screening of at-risk individuals in affected families will yield many additional cases.
Treatment of this disorder is gratifying since therapy is directed. Traditionally, the suppression with glucocorticoids has been the preferred treatment. However, many subjects appear to have been overtreated with excessive glucocorticoid dosing, resulting in Cushing's syndrome. It is urged that if this modality of treatment is used, the lowest dose of a shorter-acting glucosteroid be used and patients monitored carefully for signs of glucocorticoid excess. Alternative treatments include the aldosterone antagonist, spironolactone, and amiloride, an agent which inhibits sodium reabsorption by the renal epithelial sodium channel. Long-term treatment in males with spironolactone is usually deemed unsatisfactory because of the antiandro-genic effects of this drug which cause gynecomastia, impo-tency, or both. Potassium-wasting diuretics can be used cautiously, but they may precipitate profound hypokalemia. They are probably best used in combination with potassium-sparing diuretics, with potassium levels being carefully monitored.
3. Conclusion
The genetic basis for one hypertensive syndrome has been discovered using a strategy that could be extended to essential hypertension. That is, hypertensive intermediate phenotypes can be developed using physiologic markers, such as non-modulation or insulin resistance. Heritability of such intermediate phenotypes could then be demon-
strated by pedigree screening (as in GRA) or in sib pair studies. Then a directed search of the genome could be performed for mutations, using candidate genes appropriate for each phenotype. Such a search for these gene mutations would only appear to be 'misguided' if the efforts were random, and not directed, as illustrated by the strategy used in the case of GRA.
References
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