Hybrid gene or hybrid steroids in the detection and screening for familial hyperaldosteronism type I

Biochemical, Histopathological, and Genetic Characterization of Posture-Responsive and Unresponsive APAs

CONTEXT AND OBJECTIVE

Posture-responsive and posture-unresponsive aldosterone-producing adenomas (APAs) account for approximately 40% and 60% of APAs, respectively. Somatic gene mutations have been recently reported to exist in approximately 90% of APAs. This study was designed to characterize the biochemical, histopathologic, and genetic properties of these 2 types of APA.

METHODS

Plasma levels of aldosterone and hybrid steroids (18-oxocortisol and 18-hydroxycortisol) were measured by liquid chromatography-tandem mass spectrometry. Immunohistochemistry for CYP11B2 (aldosterone synthase) and CYP17A1 (17α-hydroxylase) and deoxyribonucleic acid sequencing (Sanger and next-generation sequencing) were performed on APA tissue collected from 23 posture-unresponsive and 17 posture-responsive APA patients.

RESULTS

Patients with posture-unresponsive APA displayed higher (P < 0.01) levels of hybrid steroids, recumbent aldosterone and cortisol, larger (P < 0.01) zona fasciculata (ZF)-like tumors with higher (P < 0.01) expression of CYP17A1 (but not of CYP11B2) than patients with posture-responsive APA (most of which were not ZF-like). Of 40 studied APAs, 37 (92.5%) were found to harbor aldosterone-driving somatic mutations (KCNJ5 = 14 [35.0%], CACNA1D = 13 [32.5%], ATP1A1 = 8 [20.0%], and ATP2B3 = 2 [5.0%]), including 5 previously unreported mutations (3 in CACNA1D and 2 in ATP1A1). Notably, 64.7% (11/17) of posture-responsive APAs carried CACNA1D mutations, whereas 56.5% (13/23) of posture-unresponsive APAs harbored KCNJ5 mutations.

CONCLUSIONS

The elevated production of hybrid steroids by posture-unresponsive APAs may relate to their ZF-like tumor cell composition, resulting in expression of CYP17A1 (in addition to somatic gene mutation-driven CYP11B2 expression), thereby allowing production of cortisol, which acts as the substrate for CYP11B2-generated hybrid steroids.

Primary Aldosteronism: Changing Definitions and New Concepts of Physiology and Pathophysiology Both Inside and Outside the Kidney

In the 60 years that have passed since the discovery of the mineralocorticoid hormone aldosterone, much has been learned about its synthesis (both adrenal and extra-adrenal), regulation (by renin-angiotensin II, potassium, adrenocorticotrophin, and other factors), and effects (on both epithelial and nonepithelial tissues). Once thought to be rare, primary aldosteronism (PA, in which aldosterone secretion by the adrenal is excessive and autonomous of its principal regulator, angiotensin II) is now known to be the most common specifically treatable and potentially curable form of hypertension, with most patients lacking the clinical feature of hypokalemia, the presence of which was previously considered to be necessary to warrant further efforts towards confirming a diagnosis of PA. This, and the appreciation that aldosterone excess leads to adverse cardiovascular, renal, central nervous, and psychological effects, that are at least partly independent of its effects on blood pressure, have had a profound influence on raising clinical and research interest in PA. Such research on patients with PA has, in turn, furthered knowledge regarding aldosterone synthesis, regulation, and effects. This review summarizes current progress in our understanding of the physiology of aldosterone, and towards defining the causes (including genetic bases), epidemiology, outcomes, and clinical approaches to diagnostic workup (including screening, diagnostic confirmation, and subtype differentiation) and treatment of PA.

Primary aldosteronism: are we diagnosing and operating on too few patients?

Many cases of potentially curable primary aldosteronism are currently likely to be diagnosed as essential hypertension unless screening tests based on suppression of renin are carried out in all hypertensive patients. More than half of the patients with primary aldosteronism detected in this way have normal circulating potassium levels, so measurement of potassium is not enough to exclude primary aldosteronism. When primary aldosteronism is diagnosed, fewer than one-third of patients are suitable for surgery as initial treatment, but this still represents a significant percentage of hypertensive patients. After excluding glucocorticoid-suppressible primary aldosteronism, adrenal venous sampling is essential to detect unilateral production of aldosterone and diagnose angiotensin-responsive aldosterone-producing adenoma. One cannot rely on the computed tomography scan. If all hypertensive patients are screened for primary aldosteronism and the workup is continued methodically in those with a positive screening test, patients with unilateral overproduction of aldosterone who potentially can be cured surgically are not denied the possibility of cure.

Treatment of familial hyperaldosteronism type I: only partial suppression of adrenocorticotropin required to correct hypertension

In familial hyperaldosteronism type I, inheritance of a hybrid 11beta-hydroxylase/aldosterone synthase gene leads to ACTH-regulated overproduction of aldosterone (causing hypertension) and of "hybrid" steroids, 18-hydroxy- and 18-oxo-cortisol. To determine whether complete suppression of the hybrid gene is necessary to normalize blood pressure, we sought evidence of persisting expression in eight patients who were rendered normotensive for 1.3-4.5 yr by glucocorticoid treatment. At the time of the study, six patients were receiving dexamethasone (0.125-0.25 mg/day) and two patients were taking prednisolone (2.5 or 5 mg/day). Urinary 18-oxo-cortisol levels during treatment demonstrated close correlation with mean "day curve" (blood collected every 2 h for 24 h) cortisol (r = 0.74), consistent with regulation by ACTH. Although urinary 18-oxo-cortisol levels were lower during than before treatment (mean 12.6 +/- 2.4 SEM vs. 35.0 +/- 5.6 nmol/mmol creatinine; P < 0.01), they remained above normal (0.8-5.2 nmol/mmol creatinine) in all eight patients. Although mean upright plasma potassium levels during treatment were higher, aldosterone levels lower, PRA levels higher, and aldosterone to PRA ratios lower than before treatment, PRA levels were uncorrected (< 13 pmol/L x min) and aldosterone to PRA ratios were uncorrected (>65) during treatment in four patients. For each of the eight patients, day curve aldosterone levels during treatment correlated more tightly with cortisol (mean r for the eight patients, 0.87 +/- 0.05 SEM) than with PRA (mean r = 0.36 +/- 0.10 SEM). Hence, control of hypertension by glucocorticoid treatment was associated, in all patients, with only partial suppression of ACTH-regulated hybrid steroid and aldosterone production. Normalization of urinary hybrid steroid levels and abolition of ACTH-regulated aldosterone production is not a requisite for hypertension control and, if used as a treatment goal, may unnecessarily increase the risk of Cushingoid side effects.

A PCR-based method of screening individuals of all ages, from neonates to the elderly, for familial hyperaldosteronism type I

AIM

Unless specifically treated (glucocorticoids in low doses), Familial Hyperaldosteronism Type I (FH-I) may result in early death from stroke. We report the successful application of a rapid, polymerase chain reaction (PCR)-based method of detecting the 'hybrid' 11 beta-hydroxylase (11 beta-OHase)/aldosterone synthase (AS) gene as a screening test for FH-I.

METHODS

'Long-PCR' was used to amplify, concurrently, a 4 kb fragment of AS gene (both primers AS-specific) and a 4 kb fragment of the hybrid gene (5' primer 11 beta-OHase-specific, 3' primer AS-specific) from DNA extracted from blood either collected locally or transported from elsewhere. Sample collection and transport were straightforward. This 4 kb fragment contains all the currently recognised hybrid gene 'crossover' points.

RESULTS

Within a single family, long-PCR identified all 21 individuals known to have FH-I. Hypertension was corrected in all 11 treated with glucocorticoids. Nine with normal blood pressure are being closely followed for development of hypertension. Long-PCR cord blood analysis excluded FH-I in three neonates born to affected individuals. Long-PCR newly identified two other affected families: (1) a female (60 years) with a personal and family history of stroke and her normotensive daughter (40 years), and (2) a female (51 years) previously treated for primary aldosteronism with amiloride, her two hypertensive sons (14 and 16 years) and her hypertensive mother (78 years). No false negative or false positive results have yet been encountered. At least seven other centres have successfully performed this test.

CONCLUSION

Long-PCR is a reliable method of screening individuals of all ages for FH-I.