Update on the Genetics of Primary Aldosteronism and Aldosterone-Producing Adenomas

Abstract

Purpose of the Review

Primary aldosteronism (PA) is the leading cause of secondary hypertension, accounting for over 10% of patients with high blood pressure. It is characterized by autonomous production of aldosterone from the adrenal glands leading to low-renin levels. The two most common forms arise from bilateral adrenocortical hyperplasia (BAH) and aldosterone-producing adenoma (APA). We discuss recent discoveries in the genetics of PA.

Recent Findings

Most APAs harbor variants in the KCNJ5, CACNA1D, ATP1A1, ATP2B3, and CTNNB1 genes. With the exception of β-catenin (CTNNB1), all other causative genes encode ion channels; their pathogenic variants found in PA lead to altered ion transportation, cell membrane depolarization, and consequently aldosterone overproduction. Some of these genes are found mutated in the germline state (CYP11B2, CLCN2, KCNJ5, CACNA1H, and CACNA1D), leading then to familial hyperaldosteronism, and often BAH rather than single APAs.

Summary

Several genetic defects in the germline or somatic state have been identified in PA. Understanding how these molecular abnormalities lead to excess aldosterone contributes significantly to the elucidation of the pathophysiology of low-renin hypertension. It may also lead to new and more effective therapies for this disease acting at the molecular level.

Introduction

Hypertension is one of the leading causes of cardiovascular morbidity [1]. Up to 10% of hypertension is due to abnormalities of the endocrine system [2, 3]. It is estimated that primary aldosteronism (PA) accounts for up to 6% of primary care patients with high blood pressure and up to 11% of those in specialized referral centers. Thus, PA is the most common form of secondary hypertension [3–5].

The renin-angiotensin-aldosterone system

The renin-angiotensin-aldosterone system (RAAS) is an essential regulator of blood volume and systemic vascular resistance [6]. There are three important components in this system: renin, angiotensin II, and aldosterone. Renin is an enzyme produced in the juxtaglomerular cells of the kidney and catalyzes the first step in the activation of RAAS. When intravascular volume is depleted, renin is released and carries out the conversion of angiotensinogen to angiotensin I, which then is converted to angiotensin II by the angiotensin-converting enzyme. Angiotensin II stimulates aldosterone release, which is secreted by the outermost layer of the adrenal cortex, the zona glomerulosa [6]. Hyperkalemia is the other major stimulus of aldosterone secretion and directly raises its production [7, 8]. Aldosterone binds to the mineralocorticoid receptor (MR) which is widely expressed in the kidney, including at the distal convoluted tubule, connecting tubule, and the cortical collecting duct, and increases sodium reabsorption and potassium excretion. MR actions are mediated by luminal Na⁺ channels and basolateral Na⁺/ K⁺ ATPase proteins that are thus essential for the ultimate effect of aldosterone [8].

Over the last 30 years, several genetic defects have been identified in the context of PA, as causes of bilateral adrenocortical hyperplasia (BAH) or aldosterone-producing adenoma (APA) leading to high aldosterone. In this article, we review the most recent among these findings, providing a comprehensive albeit brief overview of the genetics of PA.

Benign adrenocortical tumors producing aldosterone

Overproduction of aldosterone in the context of PA is at least in part, if not entirely autonomous of the RAAS [9]. PA was first described in 1955 by Dr. Conn who reported a patient with hypertension, polydipsia, polyuria, muscular weakness, and hypokalemic alkalosis due to a large adrenocortical adenoma [10]. Early detection of PA is vital due to the increased risk of cardiovascular and cerebrovascular complications compared to patients with primary hypertension and the same risk profiles [11, 12]. Guidelines from the Endocrine Society indicate that the diagnosis of PA in patients with hypertension should consist of screening (elevated aldosterone-renin ratio) and confirmatory testing (salt loading, fludrocortisone, or captopril administration) [9, 13]. Imaging studies, including computed tomography, are used to exclude malignancy, while adrenal venous sampling is used to differentiate unilateral (mostly APA) from bilateral aldosterone production (in BAH) [9].

The most common causes of PA include BAH in 65% and APAs in 35%, followed by less common causes including unilateral hyperplasia (2%), aldosterone-producing adrenocortical carcinoma (ACC, < 1%) and the various forms of familial hyperaldosteronism (FH; < 1%) [14, 15].

Aldosterone-producing adrenocortical adenomas

A very high percentage (~90%) of APAs harbor somatic pathogenic variants in genes that encode ion channels or transporters, including KCNJ5, ATP1A1, ATP2B3, CACNA1D, CACNA1H, CLCN2, and CTNNB1 [16–24] (Table 1).

Table 1.

Clinical characteristics of aldosterone-producing adenomas based on the affected gene

Gene	Clinical features
KCNJ5	• More common in females • More common in Asians • Tumor is larger in size • Diagnosis is made at younger age • Increased levels of aldosterone, 18-oxocortisol and 18-hydroxycortisol • Higher risk of hypertension remission after adrenalectomy
ATP1A1	• More often in males • Tumor is smaller in size
ATP2B3	• More common in males • Tumor is smaller in size
CACNA1D	• More common in males • More common in African Americans • Tumor is smaller in size
CTNNB1	• More common in females • High risk of residual hypertension after adrenalectomy

KCNJ5

KCNJ5 encodes a G-protein coupled inward rectifying K⁺ channel 4 (GIRK4) with the described variants causing altered selectivity of the channel. This leads to increased intracellular Na⁺ influx and cell depolarization which causes increased intracellular Ca²⁺ and Ca²⁺ signaling, leading to increased CYP11B2 mRNA and aldosterone hypersecretion [25]. In a whole exome sequencing study of 22 cases of APA, two recurrent heterozygous somatic variants, p.G1515 and p.L168R, in KCNJ5 were found [25]. Those two variants were later found to account for the majority of KCNJ5 defects in APAs (p.G151R 54%–79%, p.L168R 23%–44%), while the rest are quite rare (0%–4.5%) [23, 26, 27]. Subsequent studies with large number of patients showed that somatic variants in this gene are found in 40% of APAs [19, 26, 28]. Few studies have evaluated cardiovascular complications in these patients: it was found that APA patients with variants in KCNJ5 present with higher left ventricular mass index (LVMI) and higher aldosterone levels compared to those without [29]. However, LVMI improved after surgery [30]. More recently, Chang et al. confirmed the above observation [31]. The results of these studies suggest that KCNJ5 variants are associated with left heart remodeling. Defects in KCNJ5 also appear to be a useful prognostic tool of hypertension remission after adrenalectomy for APA [32].

ATP1A1

ATP1A1 encodes the alpha 1 subunit of Na⁺/ K⁺ ATPase, which transports three Na⁺ anions out of the cell in exchange for two K⁺ ions into the cell. Various somatic variants including p.L104R, c.del100_104, p.V332G, and p.G99R have been reported [17, 18, 23]. p.L104R results in loss of ion selectivity and pump activity, resulting in hydrogen ions entering the cell, depolarization of the cell membrane and autonomous aldosterone overproduction [33]. c.del100_104 causes the same alterations resulting in Na⁺ influx and cell depolarization [18]. Variants in ATP1A1 have been identified in 3%–17% of APAs [13, 15, 24, 37–39].

ATP2B3

This gene encodes the plasma membrane Ca²⁺ ATPase that moves Ca²⁺ ions out of the cytoplasm. The major pathogenic somatic variants identified are p.L425_V426del and p.V426_V427del, which cause major alterations of the Ca²⁺ binding site and thus impaired Ca²⁺ ion transport. As a result, aldosterone production is promoted through the increased Ca²⁺ ion concentration that induces cell depolarization and Na⁺ influx [34]. Variants in ATP2B3 account for 0.6%–10% of APAs [17, 23, 35–37].

CACNA1D and CACNA1H

Somatic variants in CACNA1D, that encodes the α−1d subunit of a Ca_v1.3 channel, an L-type calcium channel, were reported via exome sequencing by two different groups [16, 18]. Both studies found that those variants increased calcium permeability, which resulted in increased aldosterone production. CACNA1D somatic defects account for 21%–42% of APAs and is the second most commonly mutated gene in PA [7, 16, 18, 19]. It is important to note that L-type calcium channels are blocked by dihydropyridine Ca channel blockers, so there may be a role for these medications in inoperable APA. This is especially noteworthy as mutations in this gene seem to be common in BAH [38]

CACNA1H encodes voltage-gated T-type calcium channels; variants result in gain-of-function leading to increased intracellular Ca²⁺ concentration which triggers depolarization and aldosterone secretion [39]. Defects in CACNA1H are less frequent in APAs than in CACNA1D.

CLCN2

CLCN2 is a gene coding for the chloride channel ClC2 that is expressed widely, including in the adrenal glands, primarily in the zona glomerulosa [40]. Few variants have been reported including the p.G24D, c.64–2-74del, and p.R172G ones both in the germline and the somatic state [20, 40–42]. CLCN2 defects in PA cause a gain-of-function of the ClC2 channel, thus increasing thus chloride permeability and depolarization, and resulting in calcium influx [20, 40].

CTNNB1

APAs harbor gain-of-function variants in CTNNB1, a transcription factor that encodes β-catenin and is involved in the Wnt-β-catenin signaling pathway [16, 36, 43]. Variants have been described to cause activation of the pathway regardless of cell membrane depolarization in APA [44]. As with CLCN2 somatic mutations, these seem to be quite rare, at least in surgically removed APAs.

Additional genes described in patients with PA

Genetic variants in various genes have been reported in patients with PA. Somatic variants in PRKACA and GNAS, that both cause adrenal Cushing’s syndrome, have been found in patients with cortisol and aldosterone co-secreting adenomas [45, 46]. Their role in PA pathogenesis remains unclear as their co-localization with PA-causing variants makes their contribution to aldosterone excess uncertain. Genetic variants in ATP2B4, ARMC5, PDE2A, and PDE2B were found to be associated with BAH [47–51].

Demographic distribution of variants in APAs

Defects in KCNJ5 gene appear to be more frequent in patients of European (38%) and Asian (70%) descent, while somatic variants in CACNA1D were found to be more prevalent in African Americans [19, 28, 32, 52–56]. In addition, patients that harbor variants in KCNJ5 are more frequently females than males (56%–63% vs 22%–31%), of younger age and larger tumor size, and present with higher aldosterone levels [19, 26, 28]. In a study by Nanba et al., it was shown that up to 90%–91% of the tumors, for females, and 87%, for males, harbor somatic variants in the previously mentioned genes [35, 54]. In addition, loss of heterozygosity has been found in a number of APA cases that lacked KCNJ5 variants [25].

Familial Hyperaldosteronism

FH is inherited in an autosomal dominant manner. There are four types of FH described so far: type I to type IV (FH-1 to FH-4) (Table 2).

Table 2.

Genetic characteristics of familial hyperaldosteronism

Familial hyperaldosteronism	Gene	Clinical manifestations
Type I	CYP11B1/CYP11B2 chimeric gene	Glucocorticoid-suppressive hyperaldosteronism
Type II	CLCN2	Early-onset PA
Type III	KCNJ5	Two types: Mild PA (G151E, Y152C) Severe early-onset PA (T158A, I157S, E145Q, G151R)
Type IV	CACNA1H	Early-onset PA

PA, primary aldosteronism

Familial hyperaldosteronism type I

FH-1 was first described in 1966 when a father and his son who exhibited PA symptoms were treated after receiving glucocorticoids, and thus the syndrome is also known as glucocorticoid-remediable aldosteronism (GRA) [57, 58]. The molecular basis was discovered almost 3 decades later. FH-I is caused by a chimeric genetic defect formed by two highly homologous genes, CYP11B1, and CYP11B2, both located on chromosome 8. CYP11B1 encodes 11β-hydroxylase which catalyzed the conversion of 11-deoxycortisol to cortisol and CYP11B2 encodes aldosterone synthase that coverts deoxycorticosterone to corticosterone and 18-hydroxycorticosterone to aldosterone. The result of the fusion is a gene that combines the adrenocorticotropic hormone (ACTH)-responsive regulatory sequences of CYP11B1 with the coding sequence of CYP11B2 that leads to ectopic expression of CYP11B2 in zona fasciculata and aldosterone production regulated by ACTH [57]. It is interesting that hypertension is not present in all patients with the chimeric gene; however, even those without hypertension exhibit increased thickness of the left ventricular wall and reduced diastolic function compared to controls [59].

Familial hyperaldosteronism type II

FH-II was initially reported in 1992 in families with PA due to APA and/or BAH without response to glucocorticoids [60]. Germline variants in CLCN2 were identified recently as one of the genetic causes, in a study of a family with FH-II and 80 more probands with early-onset unexplained PA [20, 40]. Several germline variants in the same gene were reported in the probands (9.9%) [40].

Familial hyperaldosteronism type III

Germline defects in KCNJ5 result in FH-III as described in 2008 in a family (a father and his two daughters) that appeared to have a new form of PA refractory to glucocorticoids [61]. This severe form of PA included massive adrenocortical hyperplasia and production of hybrid steroids [61]. It appears that particular KCNJ5 pathogenic variants present with various phenotypes; in particular, p.G151E and p.Y152C present with mild PA without adrenal abnormalities on CT, are usually diagnosed later in young adulthood and can be controlled with MR antagonists [62–64]. Others, like p.T158A, p.I157S, p.E145Q, and p.G151R are associated with early-onset severe PA and BAH that requires adrenalectomy [25, 64, 65]. However, the genotype alone cannot define the phenotype as it can vary [66]. Furthermore, in a study by Sertedaki et al., two patients with germline heterozygous KCNJ5 variants presented with hypertension and ACTH-dependent aldosterone overproduction, but without PA [67]. They both had radiologically normal-looking adrenal glands and were tested negative for the CYP11B1/CYP11B2 chimeric gene; the findings of this study demonstrated that, once again, the clinical phenotype can vary and that these variants are involved in the pathogenesis of hypertension associated with aldosterone hypersecretion in response to ACTH stimulation [67]. KCNJ5 mosaicism has also been described recently in a case of early-onset PA with BAH [68].

Familial hyperaldosteronism type IV

FH-IV is the result of germline defects in CACNA1H. The first germline defects were reported in 2015 in five unrelated people (out of 40 that were examined) with early-onset PA (age 10) that shared the same novel heterozygous variant (p.M1549V) [39]. Family analysis suggested incomplete penetrance and de novo occurrence in two kindreds [39].

PA presenting with seizures and neurological abnormalities (PASNA syndrome)

In a study of 100 unrelated individuals with unsolved early-onset PA, de novo germline variants were identified in CACNA1D gene in two patients [16]. One (p.G403D) presented with hypertension at birth, ventricular septal defect, seizures, and cerebral palsy, while the other (p.I770M) appeared to have hypertension at age 5 years old and seizures and cerebral palsy since birth.

Conclusions

Next-generation sequencing identified frequent somatic and rarely germline variants in several genes regulating aldosterone production by the adrenal gland. Continuing work on the molecular understanding of how these defects lead to higher aldosterone and low-renin hypertension may lead to more efficient therapeutic interventions in hypertension.