Genetics of Primary Aldosteronism

Abstract

Primary aldosteronism is considered the commonest cause of secondary hypertension. In affected individuals, aldosterone is produced in an at least partially autonomous fashion in adrenal lesions (adenomas, [micro]nodules or diffuse hyperplasia). Over the past decade, next-generation sequencing studies have led to the insight that primary aldosteronism is largely a genetic disorder. Sporadic cases are due to somatic mutations, mostly in ion channels and pumps, and rare cases of familial hyperaldosteronism are caused by germline mutations in an overlapping set of genes. More than 90% of aldosterone-producing adenomas carry somatic mutations in K⁺ channel Kir3.4 (KCNJ5), Ca²⁺ channel Ca_V1.3 (CACNA1D), alpha-1 subunit of the Na⁺/K⁺ ATPase (ATP1A1), plasma membrane Ca²⁺ transporting ATPase 3 (ATP2B3), Ca²⁺ channel Ca_V3.2 (CACNA1H), Cl⁻ channel ClC-2 (CLCN2), β-catenin (CTNNB1), and/or G-protein subunits alpha q/11 (GNAQ/11). Mutations in some of these genes have also been identified in aldosterone-producing (micro)nodules, suggesting a disease continuum from a single cell, acquiring a somatic mutation, via a nodule to adenoma formation, and from a healthy state to subclinical to overt primary aldosteronism. Individual glands can have multiple such lesions, and they can occur on both glands in bilateral disease. Familial hyperaldosteronism, typically with early onset, is caused by germline mutations in steroid 11-beta hydroxylase/ aldosterone synthase (CYP11B1/2), CLCN2, KCNJ5, CACNA1H, and CACNA1D.

Aldosterone, the main mineralocorticoid hormone, is physiologically produced in the zona glomerulosa of the adrenal cortex. By binding to the mineralocorticoid receptor, it activates signaling cascades leading to increased renal salt and water reabsorption, as well as increased potassium and proton secretion. The production of aldosterone is normally tightly regulated. Angiotensin II (the main product of the renin-angiotensin system) and elevated serum potassium levels are the main stimuli of aldosterone production; adrenocorticotropic hormone can also temporarily increase aldosterone levels.¹ In primary aldosteronism (PA), levels of aldosterone are inappropriate for salt, volume, and/or potassium status. This excess production causes variable degrees of hypertension, possible hypokalemia, and disproportionately high levels of cardiovascular disease.² PA is considered the most important cause of secondary hypertension. An Italian study of 1672 primary care patients with hypertension, following the Endocrine Society guidelines for diagnosis, reported an overall prevalence of 5.9%, ranging from 3.9 in stage 1 hypertension to 11.8% in stage 3 hypertenson.³ Recently, using urinary aldosterone for diagnosis, even higher prevalence estimates for biochemically overt PA were reported, from 11.3% in normotension to 22.0% in stage 3 hypertension.⁴

Traditionally, several subforms of PA were distinguished: bilateral adrenal hyperplasia, also known as idiopathic hyperaldosteronism (about 60% of cases), aldosterone-producing adenomas (APAs; about 30% of cases), unilateral hyperplasia (less common), malignancy, and familial hyperaldosteronism (FH; both very rare).⁵ Recent histological and genetic studies have challenged this concept and will be discussed in this review. The diagnosis of PA is complicated, based upon the aldosterone/renin ratio as screening parameter and subsequent confirmatory testing. Of clinical importance is the distinction between unilateral and bilateral forms because unilateral forms are amenable to potentially curative surgery, whereas bilateral forms are treated with mineralocorticoid receptor antagonists.⁵

Until 2011, the molecular mechanisms underlying autonomous aldosterone production in PA were largely unknown. Discoveries made over the last decade, their potential future impact on clinical PA management and open questions in the field of PA genetics will be covered in this review.

Somatic Mutations in APAs

Because APAs are typically treated by unilateral adrenalectomy, tumor tissue is available for genetic studies. Advances in sequencing technology enabled the discovery of somatic (tumor specific) mutations in APAs. The initial study sequenced the exomes of 4 APAs and corresponding normal tissue. Two tumors carried heterozygous mutations in the KCNJ5 gene, which encodes the inward rectifier potassium channel Kir3.4. One mutation, G151R, was located within the selectivity filter of the channel, which enables K⁺ to pass through the channel and blocks the passage of Na⁺, and the second mutation, L168R, was close by. Both mutations led to abnormal sodium permeability and depolarization of the cell,⁶ which results in opening of voltage-gated calcium channels, calcium influx and increased aldosterone production.⁷ Sanger sequencing identified 6 additional somatic KCNJ5 mutations, all G151R and L168R, in 18 other APAs.⁶ Follow-up studies showed that G151R and L168R are recurrently mutated in APAs and account for >40% of APAs (Table 1). Other KCNJ5 mutations are rare.^9,13 Among rare mutations, T158A has been functionally studied, demonstrating activation of transcriptional regulators NURR1 and ATF2 downstream of calcium signaling and upstream of CYP11B2.^14,15 Further exome sequencing studies of KCNJ5 (K⁺ channel Kir3.4)-negative tumors revealed heterozygous mutations in CACNA1D, which encodes the L-type calcium channel Ca_V1.3.^16,17 CACNA1D is large, and mutations cluster much less than in KCNJ5, which is why early Sanger sequencing may have missed mutations; recent studies (albeit with rather small case numbers) show frequencies between 14 and 42% (Table 1).^9,11,18 Another frequently mutated gene with heterozygous somatic mutations is ATP1A1,^17,19 which encodes the α-1 subunit of the Na⁺/K⁺-ATPase. Somatic mutations cause abnormal permeability to Na⁺ or H⁺ and loss of pump function, leading to depolarization, aldosterone production and proliferation, likely via increased intracellular calcium and cellular acidification.^17,18,20,21 Less frequently, plasma membrane Ca²⁺ ATPase (ATP2B3) mutations cause abnormal Na⁺ and potentially Ca²⁺ permeability, along with reduced Ca²⁺ transport capacity, all resulting in elevated intracellular Ca²⁺ levels.^19,22 Heterozygous somatic gain-of-function mutations in CACNA1H, encoding the T-type calcium channel Ca_V3.2, are also infrequent.¹⁰ These mutations confer gain-of-function, with increased calcium influx, like CACNA1D mutations. Similarly rare are mutations in the gene CLCN2, encoding the ClC-2 chloride channel.^23,24

Table 1.

Somatic Mutation Frequencies in APAs From Different Ethnicities and Sexes⁸

Whereas the mutations discussed above affect ion channels or pumps and either directly or indirectly cause increased intracellular calcium levels (Figure 1), somatic mutations in CTNNB1, encoding β-catenin, point to a different mechanism. β-Catenin is part of the Wnt signaling pathway. Somatic CTNNB1 mutations are present in several tumor types,²⁵ including benign and malignant, hormone-producing and nonproducing tumors of the adrenal gland, among them APAs.^26–28 One study suggested an association of CTNNB1 mutations in APAs with pregnancy,²⁹ which, however, was doubted³⁰; such mutations were also found in males.^31,32 Interestingly, unlike the mutations described above, CTNNB1 mutations have been described to frequently cooccur with other aldosterone-driver mutations, such as a CACNA1D mutation¹¹ or mutations in the GNA11 and GNAQ genes, encoding G-protein α subunits G11 and Gq, respectively,³³ downstream of the AT1R (angiotensin 1 receptor). Mutations in GNA11 and GNAQ have only been found in conjunction with CTNNB1 mutations and are likely not sufficient for APA formation alone. Binding of angiotensin II to the AT1R results in exchange of guanosine diphophate for guanosine triphosphate, liberation of Gβγ and activation of downstream phospholipase Cβ, which then cleaves phosphatidylinositol 4,5-bisphosphate into diacyl glycerol and inositol trisphosphate. Inositol trisphosphate causes Ca²⁺ release from intracellular stores (Figure 1).¹ Somatic mutations prevent the hydrolysis of GTP that usually terminates signaling, causing constitutive activity and aldosterone production. Concurrent mutations in CTNNB1 and GNA11/Q were associated with increased expression of luteinizing hormone/choriogonadotropin receptor (LHCGR), potentially explaining an association with pregnancy.³³ In mice, expression of a Ctnnb1 gain-of-function allele to the zona glomerulosa leads to a block of transdifferentiation of zona glomerulosa cells into zona fasciculata cells, resulting in progressive hyperplastic expansion of the zona glomerulosa and increased aldosterone levels.³⁴ This observation suggests that CTNNB1 mutations may act primarily by increasing the number of aldosterone-producing cells, which may be sufficient to cause PA once tumors are large enough. Additional somatic mutations may cause increased aldosterone production on the individual cell level, aggravating the phenotype. GNA11 mutations may precede CTNNB1 mutations based on the finding of GNA11 single mutations in hyperplastic areas outside of double-mutant APAs.³³ Last, PRKACA mutations, common in cortisol-producing adenomas, have been described in 2 PA cases, however, their role in causing PA is doubtful.³⁵

Figure 1.

Somatic and germline mutations in primary aldosteronism (PA). In familial hyperaldosteronism (FH)-I, the CYP11B1/2 hybrid gene (1) is activated by adrenocorticotropic hormone (ACTH) via the MC2R (melanocortin 2 receptor) and cAMP signaling. KCNJ5 mutations (2) in aldosterone-producing adenomas (APAs) and in FH-III lead to abnormal Na⁺ influx, CLCN2 mutations (3) in APAs and in FH-II to higher Cl⁻ efflux, and ATP1A1 (4) and ATP2B3 (5) mutations in APAs to channel like-permeabilities for Na⁺, H⁺, and Ca²⁺, as well as impaired pump function. These effects cause membrane depolarization, activation of voltage-gated calcium channels, calcium influx and increased calcium signaling, stimulating CYP11B2 expression and aldosterone production. Acidification may also play a role in ATP1A1 pathophysiology (not shown). Mutations in CACNA1D (6) and CACNA1H (7) directly increase calcium permeability. GNA11 and GNAQ mutations (8) in APAs prevent termination of G-protein signaling downstream of the AT1R (angiotensin 1 receptor), leading to increased calcium release from intracellular stores. They cooccur with CTNNB1 mutations (9) that prevent β-catenin degradation with increased signaling via the TCF/LEF (T-cell factor/lymphoid enhancer factor) family. Created with BioRender.com. DAG indicates diacyl glycerol; PLC, phospholipase C; IP₃, inositol trisphosphate; and PIP₂, phosphatidylinositol 4,5-bisphosphate.

Sex and Ethnicity Aspects in APA Somatic Mutation Frequencies

Silent adrenal masses (incidentalomas, Figure 2) are common, especially in the elderly,³⁷ and routine histopathology cannot distinguish between nonproducing and aldosterone-producing lesions. The most reliable data on mutations frequencies are from recent studies that identify the culprit lesion by immunohistochemistry for aldosterone synthase (Figure 2) and perform highly sensitive panel sequencing for mutation identification. When all known genes were sequenced, mutations were found in at least 95% of APAs. As noted for KCNJ5 in earlier studies,¹³ mutation frequencies differ between sexes and ethnicities⁸ (Table 1). KCNJ5 mutations are more frequent in women, whereas CACNA1D and ATP1A1 mutations are more frequent in men. KCNJ5 mutations have the highest prevalence in Asian populations,^38–40 followed by those of European origin⁹ and African ancestry.¹¹ In contrast, CACNA1D mutations are more frequent in blacks, outnumbering KCNJ5 mutations.¹¹ Differences in diagnostic algorithms, salt or phytoestrogen ingestion cannot be excluded; however, biological differences among sexes and ethnicities, such as those in circulating hormone levels, could well underlie these effects.^8,41 Of note, female mice show a 3-fold higher adrenocortical tissue turnover than males. Female mice also use a stem cell/progenitor cell compartment in the adrenal capsule for renewal that is unused by males.⁴² Higher turnover and thus a higher probability of acquiring somatic mutations may thus underlie the higher frequency of adrenocortical tumors in women.⁴³ It is also tempting to speculate that the mutational profile of APAs depends on their cells of origin, which may differ between sexes and, possibly, ethnicities.

Figure 2.

Adrenal lesions.³⁶ Aldosterone-producing micronodules (formerly aldosterone-producing cell clusters [APCCs]) are not recognizable by hematoxylin and eosin staining but stain positive for aldosterone synthase expression. Aldosterone-producing nodules are visible on hematoxylin and eosin staining and are distinguished from aldosterone-producing adenoma (APA) by size. Multiple (micro)nodules can cooccur within a single gland. Aldosterone-producing diffuse hyperplasia shows a broad, uninterrupted strip of aldosterone synthase-positive cells. Nonproducing adenomas stain negative for aldosterone synthase. Brown color indicates aldosterone synthase positivity on immunohistochemistry. Created with BioRendor.com.

Somatic Mutations in Aldosterone-Producing (Micro)nodules and in Bilateral Disease

Small nodules of cells with increased expression of aldosterone synthase that protrude beyond the zona glomerulosa into the zona fasciculata occur in healthy individuals and PA patients. They were first described as aldosterone-producing cell clusters (APCCs) in 2010⁴⁴ and are now, according to a recently published international consensus, referred to as aldosterone-producing micronodules (Figure 2).³⁶ Using panel sequencing of APA disease genes, Nishimoto et al⁴⁵ in 2015 discovered that APCCs from healthy kidney donors carried somatic mutations in CACNA1D (6/23) and ATP1A1 (2/23).⁴⁵ Nanba et al⁴⁶ expanded on these findings by demonstrating that the APCC area increases with age in healthy kidney donors, contrary to normal zona glomerulosa CYP11B2 expression. Interestingly, in a separate clinical study, they detected an increase of the aldosterone/renin ratio with age on a high-sodium diet, whereas aldosterone stimulation on a low-sodium diet was blunted with age, suggesting that autonomous aldosterone production may increase with age due to the development of aldosterone-producing (micro)nodules, whereas normal zona glomerulosa recedes.⁴⁶ Micronodules resembling those in normal individuals were also found in adrenals from patients with unilateral PA without adenoma; they carried somatic mutations in CACNA1D, KCNJ5, ATP1A1, and ATP2B3, whereas no such mutations were found in nonnodular diffuse hyperplasia.⁴⁷ Furthermore, 15 cases with bilateral disease who, as an unusual measure, underwent unilateral adrenalectomy, showed more and larger APCCs than normotensive controls. Sequencing of aldosterone-producing lesions revealed mutations in CACNA1D in 58% of APCCs, and of KCNJ5 in 1% (in a lesion considered by the authors as micro-APA),⁴⁸ suggesting that somatic mutations in aldosterone-producing (micro)nodules may underlie bilateral disease. Interestingly, adrenal hyperplasia, including (micro)nodules, can also occur in adrenals carrying a circumscribed APA. De Sousa et al¹⁸ investigated such adrenal glands using CYP11B2 immunohistochemistry and next-generation sequencing. Sequencing of 57 APCCs adjacent to an APA revealed mutations in known APA driver genes in 15, distinct from the mutation in the APA, with different mutations among different APCCs within the same gland. Besides mutations in CACNA1D and ATP1A1, interestingly, mutations in KCNJ5, previously considered rare or absent in APCCs, but also mutations in CACNA1H, PRKACA, and CTNNB1 were observed.¹⁸ Taken together, these results suggest that somatic mutations causing autonomous aldosterone production frequently occur in human adrenal glands. They contribute to variable degrees of autonomous aldosterone secretion.

Impact of Somatic Mutations on Proliferation and Second-Hit Hypothesis

A controversy in the field has been whether mutations in aldosterone-driver genes are sufficient only to cause increased/at least partially autonomous aldosterone production or also increased proliferation. In the APCC model, a somatic mutation in a single cell is assumed to cause increased aldosterone production and proliferation, leading to an aldosterone-producing micronodule, aldosterone-producing nodule, and eventually APA. In the alternative concept of a second-hit hypothesis, genetic or environmental factors cause proliferation, with additional somatic mutations leading to APA formation.⁴⁹ Whether KCNJ5 mutations cause proliferation has been debated. The overexpression of mutant KCNJ5 in cultured cells increases lethality rather than proliferation, likely due to massive Na⁺ influx.^50,51 In vivo, however, KCNJ5-positive tumors show particularly low KCNJ5 expression.⁵² Evidence from germline KCNJ5 mutations (see below) and, specifically individuals with mosaicism for KCNJ5 mutations suggest that these mutations either increase proliferation or block transdifferentiation, leading to hyperplasia in vivo.^50,53,54 APA-associated ATPA1A1 mutations would likely be lethal if present in the germline. However, a recent study provided evidence for proliferative effects of an ATPA1A mutation in the HAC15 adrenocortical cancer cell line in vitro, an effect that was serum dependent.²¹ Evidence for the second-hit hypothesis includes an individual with bilateral macronodular adrenal hyperplasia due to familial adenomatous polyposis with APC mutation and KCNJ5-positive APA.⁵⁵ Zona glomerulosa hyperplasia but also increased nodulation and decreased vascularity and expression of stem/progenitor cell markers in areas surrounding APAs has also been suggested to support the second-hit hypothesis,^56,57 however, such lesions typically carry independent somatic mutations.¹⁸

Germline Mutations in FH

Genetic testing for FH subtypes (Table 2) is recommended in affected individuals with early onset and a positive family history of PA.⁵⁸ Familial aggregation of PA was first described in 1966 in a father and a son whose phenotype was relieved by administration of dexamethasone (glucocorticoid-remediable aldosteronism, later also FH-I).⁵⁹ This autosomal-dominant disorder is characterized by early-onset hypertension, often with a positive family history; hemorrhagic stroke and ruptured intracranial aneurysm are common in individuals without appropriate therapy.⁶⁰ Incomplete penetrance or mild forms have been reported.^61,62 In one study of 300 consecutive PA patients, 2 had FH-I. FH-I is caused by a hybrid gene that occurs due to a recombination event between CYP11B1 (11β-hydroxylase, involved in cortisol synthesis under the control of adrenocorticotropic hormone) and CYP11B2 (aldosterone synthase). This leads to ectopic expression of aldosterone synthase in the zona fasciculata under the control of adrenocorticotropic hormone and the formation of hybrid steroids 18-oxocortisol and 18-hydroxycortisol,⁶³ indicating the abnormal colocalization of enzymes involved in aldosterone and cortisol synthesis. Treatment with glucocorticoids suppresses adrenocorticotropic hormone and, sometimes in combination with mineralocorticoid receptor antagonists, normalizes aldosterone and blood pressure. Unlike other FH genes, the chimeric CYP11B1/2 gene has not been found in APAs.⁶⁴

Table 2.

FH Subforms

FH-II initially referred to all non-FH-I cases of FH.⁶⁵ An often-quoted prevalence of 6% in PA⁶⁶ is likely a considerable overestimate due to chance associations of sporadic PA cases in families. Because a heterozygous R172Q CLCN2 mutation was discovered in a family from the original report,⁶⁵ FH-II now denotes individuals with germline mutations in the CLCN2.⁶⁷ Mutations were found in 7 additional families, among them 2 de novo. FH-II patients typically had early-onset hypertension with elevated aldosterone/renin ratios, with age at diagnosis typically before age 20 years, although, as in FH-I, incomplete penetrance was reported.⁶⁷ Hypertension responded to therapy with mineralocorticoid receptor antagonists or other antihypertensive agents. Another de novo germline N-terminal mutation was reported in a patient diagnosed with PA at age 9 years.⁶⁸ The affected girl showed severe hypertension and persistent hypokalemia and responded to medical therapy. Due to a positive dexamethasone suppression test, she was initially suspected to have glucocorticoid-remediable aldosteronism, but genetic analysis for the chimeric CYP11B1/2 gene was negative. ClC-2, the chloride channel encoded by CLCN2, mediates net efflux of chloride in the zona glomerulosa.⁶⁷ CLCN2 mutations lead to increased chloride efflux, cellular depolarization, calcium influx, and aldosterone production (Figure 1).^67,68

FH-III refers to patients with heterozygous germline mutations in the KCNJ5 gene. The initially published kindred showed severe therapy-resistant PA in childhood. Hybrid steroids were even higher than in FH-I, but treatment with dexamethasone failed to normalize blood pressure or serum aldosterone.⁶⁹ Severely enlarged adrenal glands were found upon bilateral adrenalectomy, with a large zone of lipid-laden cells that expressed aldosterone synthase and enzymes involved in cortisol synthesis, explaining hybrid steroid synthesis.^69,70 Genetic analysis identified a KCNJ5 T158A mutation.⁶ Additional kindreds, often with bilateral adrenal hyperplasia, were subsequently identified, for example, patients with G151R mutation that is common as somatic mutation in APAs.⁵⁰ Interestingly, patients with another mutation, G151E, do not show massive hyperplasia and can be treated medically^50,71; this mutation appears to cause extreme Na⁺ influx that may interfere with cellular survival. This mutation does not occur in APAs, but, interestingly, has been identified in an APCC.¹⁸

FH-IV denotes patients with heterozygous germline mutations in the CACNA1H gene. One recurrent identical mutation, M1549V, was identified in 5 families with early-onset PA without other remarkable characteristics.⁷² An additional early-onset case with de novo M1549I mutation had PA and multiplex developmental disorder. The pathogenicity of additional variants is less certain.^73,74 CACNA1H mutations lead to gain of channel function, with increased calcium influx and aldosterone production.^72,75

A complex syndrome comprising PA, seizures, and neurological abnormalities was described in 2 individuals with de novo heterozygous gain-of-function CACNA1D mutations.¹⁶ Both were diagnosed with cerebral palsy. Variable associated abnormalities included transient hypoglycemia and cardiac defects. Expression of the channel in several other organs, including brain, pancreas, and heart, likely accounts for associated abnormalities; PA is variable.^76–78 Several individuals with de novo CACNA1D mutations have been diagnosed with autism spectrum disorder without major endocrine abnormalities.^79–81

Finally, any roles of ARMC5 and phosphodiesterase germline variants in PA remain to be confirmed.^82,83 ATPase mutations have not been reported in FH, suggesting incompatibility of such germline mutations with survival.

Genetic Mouse Models of PA

Several mouse models of human PA have been generated. A mouse model with transgenic expression of human CYP11B2 under the control of the human CYP11B1 promoter (resembling FH-I), shows elevated aldosterone levels and hypertension on a high-salt diet.⁸⁴ A knockin mouse (Clcn2^R180Q/+) models the commonest FH-II mutation, with normal adrenal weight and morphology, mildly elevated aldosterone levels and mildly elevated blood pressure. Intracellular Ca²⁺ oscillatory activity in the adrenal zona glomerulosa is elevated.⁸⁵ A second model carries a gain-of-function N-terminal deletion of 8 amino acids. In the homozygous state, the zona glomerulosa cells in this model are depolarized, with increased intracellular Ca²⁺ levels, elevated aldosterone and decreased renin, elevated blood pressure, hypokalemia, and moderate albuminuria.⁸⁶ Kcnj5 is not expressed in rodent adrenal glomerulosa.⁸⁷ A mouse model expressing wild type or mutant human KCNJ5 under the Akr1b7 promoter, reported only as an abstract, appears to lack adrenal hypertrophy.⁸⁸ A knockin mouse with Cacna1h^M1560V/+ mutation (FH-IV model) shows normal adrenal morphology, elevated adrenal Cyp11b2 expression and elevated blood pressure. Adrenals from these animals have elevated baseline and peak intracellular Ca²⁺ levels.⁸⁹ Last, a transgenic mouse with adrenocortical expression of a Gq-coupled designer receptor develops disorganization of adrenal zonation and hyperaldosteronism,⁹⁰ as in GNAQ mutations in APAs. Additional models with mutations in KCNK3 and KCNK9 potassium channels or cryptochrome genes have been reviewed elsewhere.⁹¹

Diagnostic and Therapeutic Advances Based on Genetic Discoveries

Of particular interest regarding clinical diagnosis are APAs with KCNJ5 mutations. Beyond their high prevalence, they are associated with early diagnosis, high aldosterone levels, large tumors,¹³ high cure rates,⁹² and lower precontrast Hounsfield units on computed tomography⁹³ due to lipid-rich fasciculata-like tumor cells.^17,93,94 Like FH-III patients, patients with KCNJ5-positive APAs have elevated concentrations of hybrid steroids,⁹⁵ allowing prediction of unilateral disease based on steroid profiling.⁹² This, together with imaging, may in the future help to bypass adrenal venous sampling in individuals with KCNJ5 mutations.

In addition, blockers of mutant KCNJ5 channels could serve as diagnostic or therapeutic tools in PA. A drop in blood pressure or aldosterone in response to short-term treatment may help to identify KCNJ5-positive tumors. Long-term therapy might lead to tumor shrinkage.⁹⁶ Sensitivity of mutant KCNJ5 channels to blockers of Na⁺ and Ca²⁺ transporting proteins such as verapamil (for G151R and L168R) and amiloride (only L168R analyzed) has been reported.⁷ In a high-throughput screen, macrolide antibiotics such as roxithromycin and clarithromycin were identified as specific blockers of both G151R and L168R, but not WT KCNJ5. Similarly, the nonantibiotic macrolide motilin receptor agonist idremcinal and synthesized macrolide derivatives without antibiotic or motilide activity specifically inhibited mutant KCNJ5 channels.⁹⁶ Macrolide compounds decrease the excessive aldosterone production associated with expression of mutant KCNJ5 channels in an aldosterone-producing cell line in vitro⁹⁶ and in APA cells carrying KCNJ5 mutations ex vivo.⁹⁷ Results of proof-of-concept studies in humans evaluating macrolides for the diagnosis of PA are pending.⁹⁸

The finding that intracellular calcium is the key signal for aldosterone also raises the question whether calcium channel blockers could be used to inhibit aldosterone production. Therapeutic use of calcium channel blockers in PA was considered long before the discovery of calcium channel mutations⁹⁹ but approved compounds target vascular calcium channels, and their antihypertensive effect is mostly aldosterone-independent.⁵ Normal aldosterone values in Cacna1h knockout mice^89,100 and the phenotype of Cacna1d knockout mice (deafness and sinoatrial node dysfunction with bradycardia and arrhythmia)¹⁰¹ argue against these channels as therapeutic targets.

Finally, the finding of a likely genetic continuum between hyperplastic lesions and APAs, combined with the availability of highly specific CYP11B2 antibodies, has led to a new histopathology consensus for the description of unilateral disease³⁶ (Figure 2).

Summary and Future Perspectives

Genetic studies, published over the last decade, have—perhaps unexpectedly—established PA as a largely genetic disorder. The initial discovery that APAs are due to somatic mutations, mostly in ion channels and pumps, was followed by the finding that similar mutations are highly prevalent in what are now called aldosterone-producing (micro)nodules, particularly in bilateral disease. There is probably a biologic continuum between somatic mutations in single cells in otherwise healthy individuals, (micro)nodule formation and eventually APA development,¹⁰² making cutoffs for diagnosis of PA somewhat arbitrary. Indeed, there is clinical evidence that subclinical PA is present in normotensive individuals and is associated with an increased risk for the development of hypertension.¹⁰³ One study, based on urinary aldosterone and oral sodium suppression tests, found a continuum of renin-independent aldosterone production, paralleling the severity of hypertension.⁴ Small lesions could be the histopathologic correlate of subclinical PA with low renin,¹⁰³ in particular in individuals with resistant hypertension, as suggested by the good response to spironolactone in the PATHWAY-2 study.¹⁰⁴

The recognition of this biological continuum also to some extent questions the distinction between unilateral and bilateral disease; hyperplastic or nodular areas are common in APA patients and can lead to contralateral recurrence.¹⁰⁵ In addition, asymmetrical aldosterone production with bilateral lesions can prevent cure after adrenalectomy.¹⁰⁶

Unanswered questions regarding the genetics and pathophysiology of PA include:

In which cell(s) does the initial somatic mutation occur—stem/progenitor cells¹⁰⁷ and/or glomerulosa cells? What are the mechanisms underlying proliferation of aldosterone-producing lesions? Are there additional mutations to discover? Additional disease genes will explain only small fractions of APAs, but it is tempting to speculate that aldosterone-producing micronodules could carry mutations without major effects on proliferation, similar to GNA11/Q mutations discussed above, and that more FH cases will be solved. Last, how these discoveries will translate into clinical application is a major unresolved question. The major challenges in PA clinical care are low rates of screening and diagnosis,¹⁰⁸ which, beyond a lack of awareness, are due to complex, expensive and often invasive screening procedures⁵ that are unavailable outside tertiary care centers. Furthermore, medical treatment options are scarce and include spironolactone, which is associated with severe side effects at higher doses, and eplerenone, which is not approved for the therapy of hypertension in many countries. Research discoveries have contributed to raised PA awareness in the academic world, but improved care will require less complicated diagnostic algorithms and new treatment options. Steroid profiling and drugs targeting mutant ion channels as discussed above are first steps. It is also conceivable that tracers could in the future recognize mutant surface proteins in imaging, but this again would likely be limited to large centers. The insight that PA, clinical or subclinical, occurs across normotension and all stages of hypertension suggests that simple screening tests, such as that for plasma renin should be performed in most hypertensive patients.

Article Information

Sources of Funding

This work was funded by the Stiftung Charité (BIH_PRO_406) and the German Research Foundation (DFG, CRC 1453 (Project-ID 431984000) and CRC 1365).

Disclosures

Rockefeller University holds a patent Compositions and methods for diagnosing and treating diseases and disorders associated with mutant KCNJ5 listing U.I. Scholl as an inventor.