A gain-of-function mutation in the CLCN2 chloride channel gene causes primary aldosteronism

X-chromosome variants are associated with aldosterone producing adenomas

Aldosterone-producing adenomas (APAs) are a major cause of primary aldosteronism (PA) and are characterized by constitutively producing aldosterone, which leads to hypertension. Several mutations have been identified in ion channels or ion channel-associated genes that result in APAs. To date, no studies have used a genome-wide association study (GWAS) approach to search for predisposing loci for APAs. Thus, we investigated Scandinavian APA cases (n = 35) and Swedish controls (n = 60) in a GWAS and discovered a susceptibility locus on chromosome Xq13.3 (rs2224095, OR = 7.9, 95% CI = 2.8-22.4, P = 1 × 10^-7) in a 4-Mb region that was significantly associated with APA. Direct genotyping of sentinel SNP rs2224095 in a replication cohort of APAs (n = 83) and a control group (n = 740) revealed persistently strong significance (OR = 6.1, 95% CI = 3.5-10.6, p < 0.0005). We sequenced an adjacent gene, MAGEE1, of the sentinel SNP and identified a rare variant in one APA, p.Gly327Glu, which is complementary to other mutations in our primary cohort. Expression quantitative trait loci (eQTL) were investigated on the X-chromosome, and 24 trans-eQTL were identified. Some of the genes identified by trans-eQTL point towards a novel mechanistic explanation for the association of the SNPs with APAs. In conclusion, our study provides further insights into the genetic basis of APAs.

Enhanced Ca2+ signaling, mild primary aldosteronism, and hypertension in a familial hyperaldosteronism mouse model (Cacna1h M1560V/+ )

Primary aldosteronism (increased production of the adrenal steroid hormone aldosterone) is the most common cause of secondary hypertension. We here generated a mouse model of familial hyperaldosteronism type IV with a heterozygous gain-of-function mutation in a calcium channel gene (Cacna1h^M1560V/+). Cacna1h^M1560V/+ mice have about twofold elevated aldosterone:renin ratios (a screening parameter for primary aldosteronism) and elevated blood pressure, with an overall mild phenotype. Elevated adrenal aldosterone synthase expression in Cacna1h^M1560V/+ mice is associated with increased intracellular calcium concentrations in glomerulosa cells. This model allows for the ex vivo analysis of calcium signaling in aldosterone-producing glomerulosa cells of the adrenal gland. Cacna1h^−/− mice have normal aldosterone synthase expression, with implications for the evaluation of CACNA1H as a therapeutic target.

Gain-of-function mutations in the CACNA1H gene (encoding the T-type calcium channel Ca_V3.2) cause autosomal-dominant familial hyperaldosteronism type IV (FH-IV) and early-onset hypertension in humans. We used CRISPR/Cas9 to generate Cacna1h^M1560V/+ knockin mice as a model of the most common FH-IV mutation, along with corresponding knockout mice (Cacna1h^−/−). Adrenal morphology of both Cacna1h^M1560V/+ and Cacna1h^−/− mice was normal. Cacna1h^M1560V/+ mice had elevated aldosterone:renin ratios (a screening parameter for primary aldosteronism). Their adrenal Cyp11b2 (aldosterone synthase) expression was increased and remained elevated on a high-salt diet (relative autonomy, characteristic of primary aldosteronism), but plasma aldosterone was only elevated in male animals. The systolic blood pressure of Cacna1h^M1560V/+ mice was 8 mmHg higher than in wild-type littermates and remained elevated on a high-salt diet. Cacna1h^−/− mice had elevated renal Ren1 (renin-1) expression but normal adrenal Cyp11b2 levels, suggesting that in the absence of Ca_V3.2, stimulation of the renin-angiotensin system activates alternative calcium entry pathways to maintain normal aldosterone production. On a cellular level, Cacna1h^M1560V/+ adrenal slices showed increased baseline and peak intracellular calcium concentrations in the zona glomerulosa compared to controls, but the frequency of calcium spikes did not rise. We conclude that FH-IV, on a molecular level, is caused by elevated intracellular Ca²⁺ concentrations as a signal for aldosterone production in adrenal glomerulosa cells. We demonstrate that a germline Cacna1h gain-of-function mutation is sufficient to cause mild primary aldosteronism, whereas loss of Ca_V3.2 channel function can be compensated for in a chronic setting.

Update on Genetics of Primary Aldosteronism

Primary aldosteronism (PA) is the most common form of secondary hypertension, with a prevalence of 5–10% among patients with hypertension. PA is mainly classified into two subtypes: aldosterone-producing adenoma (APA) and bilateral idiopathic hyperaldosteronism. Recent developments in genetic analysis have facilitated the discovery of mutations in KCNJ5, ATP1A1, ATP2B3, CACNA1D, CACNA1H, CLCN2, and CTNNB1 in sporadic or familial forms of PA in the last decade. These findings have greatly advanced our understanding of the mechanism of excess aldosterone synthesis, particularly in APA. Most of the causative genes encode ion channels or pumps, and their mutations lead to depolarization of the cell membrane due to impairment of ion transport. Depolarization activates voltage-gated Ca²⁺ channels and intracellular calcium signaling and promotes the transcription of aldosterone synthase, resulting in overproduction of aldosterone. In this article, we review recent findings on the genetic and molecular mechanisms of PA.

Primary aldosteronism

In 1955 Dr Jerome Conn first documented primary aldosteronism (PA). Since then, screening, diagnosis and treatment have developed, in the process both refining and complicating management. Currently, screening requires 4-6 weeks of lead-up, including major changes in antihypertensive therapy, followed by a blood draw for plasma aldosterone concentration (PAC) and plasma renin activity (PRA) or concentration (PRC). Screening is considered indicative of PA on the basis of the PAC and the aldosterone to renin ratio (ARR). This is then followed by one or more of 6 confirmatory/exclusion tests. Three things have changed. First is now incontrovertible evidence that a single spot PAC is a deeply flawed index of true aldosterone status, so that many referred patients with PA fall at the first hurdle. A valid index of aldosterone status is an integrated value, measured as urinary aldosterone excretion (UEA) over 24 h. On the basis of the UEA, the prevalence of PA appears to be 3-5 times higher than the currently accepted figure of 5-10% of hypertensives. The second is the recognition that inadequately treated PA has a cardiovascular risk profile ~threefold that of matched essential hypertensives. Third is the realization that <1% of hypertensives are ever screened for PA, who are thus in double jeopardy for the risks of untreated PA on top of those for hypertension per se. Taken together, this a major if occult public health issue; if it is to be addressed, radical changes in management are needed. Some are in screening, which needs to be simply done on all newly-presenting hypertensives; others are major simplifications of screening in established hypertension. The front-line actors need to be Internists/Primary Care Providers; the costs will be significant, but much less than those of increased morbidity/premature mortality in unrecognized PA. Possible suggestions as to how best to address this constitute the final chapter of this article.

Molecular Genetic and Genomic Alterations in Cushing's Syndrome and Primary Aldosteronism

The genetic alterations that cause the development of glucocorticoid and/or mineralocorticoid producing benign adrenocortical tumors and hyperplasias have largely been elucidated over the past two decades through advances in genomics. In benign aldosterone-producing adrenocortical tumors and hyperplasias, alteration of intracellular calcium signaling has been found to be significant in aldosterone hypersecretion, with causative defects including those in KCNJ5, ATP1A1, ATP2B3, CACNA1D, CACNA1H, and CLCN2. In benign cortisol-producing adrenocortical tumors and hyperplasias abnormal cyclic adenosine monophosphate-protein kinase A signaling has been found to play a central role in tumorigenesis, with pathogenic variants in GNAS, PRKAR1A, PRKACA, PRKACB, PDE11A, and PDE8B being implicated. The role of this signaling pathway in the development of Cushing's syndrome and adrenocortical tumors was initially discovered through the study of the underlying genetic defects causing the rare multiple endocrine neoplasia syndromes McCune-Albright syndrome and Carney complex with subsequent identification of defects in genes affecting the cyclic adenosine monophosphate-protein kinase A pathway in sporadic tumors. Additionally, germline pathogenic variants in ARMC5, a putative tumor suppressor, were found to be a cause of cortisol-producing primary bilateral macronodular adrenal hyperplasia. This review describes the genetic causes of benign cortisol- and aldosterone-producing adrenocortical tumors.

Unravelling the Genetic Basis of Primary Aldosteronism

Primary aldosteronism (PA), a condition characterized by autonomous aldosterone hypersecretion, constitutes the most common cause of secondary hypertension. Over the last decade, major breakthroughs have been made in the field of genetics underpinning PA. The advent and wide application of Next Generation Sequencing (NGS) technology led to the identification of several somatic and germline mutations associated with sporadic and familial forms of PA. Somatic mutations in ion-channel genes that participate in aldosterone biosynthesis, including KCNJ5, CACNA1D, ATP1A1, and ATP2B3, have been implicated in the development of aldosterone-producing adenomas (APAs). On the other hand, germline variants in CLCN2, KCNJ5, CACNA1H, and CACNA1D genes have been implicated in the pathogenesis of the familial forms of PA, FH-II, FH-III, and F-IV, as well as PA associated with seizures and neurological abnormalities. However, recent studies have shown that the prevalence of PA is higher than previously thought, indicating the need for an improvement of our diagnostic tools. Further research is required to recognize mild forms of PA and to investigate the underlying molecular mechanisms.

How common is primary aldosteronism?

PURPOSE OF REVIEW

Primary aldosteronism, characterized by renin-independent aldosterone secretion from one or both adrenal glands, is the most common and modifiable form of secondary hypertension. The prevalence of primary aldosteronism is increasingly recognized to be much higher than previously thought with many cases still undetected.

RECENT FINDINGS

Prior prevalence studies on primary aldosteronism have reported a wide range of estimates due to heterogeneity of both disease definitions and study populations such that it is difficult to claim a single point estimate. More recent evidence demonstrates that primary aldosteronism, as defined by conventional biochemical diagnostic criteria, is highly prevalent within populations where it is not typically considered such as mild-to-moderate hypertension, prehypertension, and even normotension. Yet, our current screening approach fails to capture many cases. Furthermore, there is mounting evidence that renin-independent aldosteronism exists as a continuum of disease that extends below the current biochemical diagnostic thresholds used to define primary aldosteronsim and has clinically relevant treatment and outcome implications for a much broader patient population. Indeed, much of what we current label as 'essential hypertension' is, in fact, renin-independent aldosterone-mediated hypertension.

SUMMARY

Primary aldosteronism and milder forms of renin-independent aldosteronism are highly prevalent, yet vastly under-recognized, in the general population.

The Genotype-Based Morphology of Aldosterone-Producing Adrenocortical Disorders and Their Association with Aging

Primary aldosteronism (PA) is the most common cause of secondary hypertension, and is associated with an increased incidence of cardiovascular events. PA itself is clinically classified into the following two types: unilateral PA, mostly composed of aldosteroneproducing adenoma (APA); and bilateral hyperaldosteronism, consisting of multiple aldosterone-producing micronodules (APMs) and aldosterone-producing diffuse hyperplasia. Histopathologically, those disorders above are all composed of compact and clear cells. The cellular morphology in the above-mentioned aldosterone-producing disorders has been recently reported to be closely correlated with patterns of somatic mutations of ion channels including KCNJ5, CACNA1D, ATP1A1, ATP2B3, and others. In addition, in non-pathological adrenal glands, APMs are frequently detected regardless of the status of the renin-angiotensin-aldosterone system (RAAS). Aldosterone-producing nodules have been also proposed as non-neoplastic nodules that can be identified by hematoxylin and eosin staining. These non-neoplastic CYP11B2-positive nodules could represent possible precursors of APAs possibly due to the presence of somatic mutations. On the other hand, aging itself also plays a pivotal role in the development of aldosterone-producing lesions. For instance, the number of APMs was also reported to increase with aging. Therefore, recent studies indicated the novel classification of PA into normotensive PA (RAAS-independent APM) and clinically overt PA.

Roles of the ClC chloride channel CLH-1 in food-associated salt chemotaxis behavior of C. elegans

The ability of animals to process dynamic sensory information facilitates foraging in an ever-changing environment. However, molecular and neural mechanisms underlying such ability remain elusive. The ClC anion channels/transporters play a pivotal role in cellular ion homeostasis across all phyla. Here, we find a ClC chloride channel is involved in salt concentration chemotaxis of Caenorhabditis elegans. Genetic screening identified two altered-function mutations of clh-1 that disrupt experience-dependent salt chemotaxis. Using genetically encoded fluorescent sensors, we demonstrate that CLH-1 contributes to regulation of intracellular anion and calcium dynamics of salt-sensing neuron, ASER. The mutant CLH-1 reduced responsiveness of ASER to salt stimuli in terms of both temporal resolution and intensity, which disrupted navigation strategies for approaching preferred salt concentrations. Furthermore, other ClC genes appeared to act redundantly in salt chemotaxis. These findings provide insights into the regulatory mechanism of neuronal responsivity by ClCs that contribute to modulation of navigation behavior.

Approaches to Gene Mutation Analysis Using Formalin-Fixed Paraffin-Embedded Adrenal Tumor Tissue From Patients With Primary Aldosteronism

Aldosterone production is physiologically under the control of circulating potassium and angiotensin II as well as adrenocorticotropic hormone and other secretagogues such as serotonin. The adrenal's capacity to produce aldosterone relies heavily on the expression of a single enzyme, aldosterone synthase (CYP11B2). This enzyme carries out the final reactions in the synthesis of aldosterone and is expressed almost solely in the adrenal zona glomerulosa. From a disease standpoint, primary aldosteronism (PA) is the most common of all adrenal disorders. PA results from renin-independent adrenal expression of CYP11B2 and production of aldosterone. The major causes of PA are adrenal aldosterone-producing adenomas (APA) and adrenal idiopathic hyperaldosteronism. Our understanding of the genetic causes of APA has significantly improved through comprehensive genetic profiling with next-generation sequencing. Whole-exome sequencing has led to the discovery of mutations in six genes that cause renin-independent aldosterone production and thus PA. To facilitate broad-based prospective and retrospective studies of APA, recent technologic advancements have allowed the determination of tumor mutation status using formalin-fixed paraffin-embedded (FFPE) tissue sections. This approach has the advantages of providing ready access to archival samples and allowing CYP11B2 immunohistochemistry-guided capture of the exact tissue responsible for inappropriate aldosterone synthesis. Herein we review the methods and approaches that facilitate the use of adrenal FFPE material for DNA capture, sequencing, and mutation determination.

Cellular basis of ClC-2 Cl- channel-related brain and testis pathologies

The ClC-2 chloride channel is expressed in the plasma membrane of almost all mammalian cells. Mutations that cause the loss of ClC-2 function lead to retinal and testicular degeneration and leukodystrophy, whereas gain-of-function mutations cause hyperaldosteronism. Leukodystrophy is also observed with a loss of GlialCAM, a cell adhesion molecule that binds to ClC-2 in glia. GlialCAM changes the localization of ClC-2 and opens the channel by altering its gating. We now used cell type-specific deletion of ClC-2 in mice to show that retinal and testicular degeneration depend on a loss of ClC-2 in retinal pigment epithelial cells and Sertoli cells, respectively, whereas leukodystrophy was fully developed only when ClC-2 was disrupted in both astrocytes and oligodendrocytes. The leukodystrophy of Glialcam-/- mice could not be rescued by crosses with Clcn2op/op mice in which a mutation mimics the "opening" of ClC-2 by GlialCAM. These data indicate that GlialCAM-induced changes in biophysical properties of ClC-2 are irrelevant for GLIALCAM-related leukodystrophy. Taken together, our findings suggest that the pathology caused by Clcn2 disruption results from disturbed extracellular ion homeostasis and identifies the cells involved in this process.

Phosphodiesterase 2A and 3B variants are associated with primary aldosteronism

Familial primary aldosteronism (PA) is rare and mostly diagnosed in early-onset hypertension (HT). However, 'sporadic' bilateral adrenal hyperplasia (BAH) is the most frequent cause of PA and remains without genetic etiology in most cases. Our aim was to investigate new genetic defects associated with BAH and PA. We performed whole-exome sequencing (paired blood and adrenal tissue) in six patients with PA caused by BAH that underwent unilateral adrenalectomy. Additionally, we conducted functional studies in adrenal hyperplastic tissue and transfected cells to confirm the pathogenicity of the identified genetic variants. Rare germline variants in phosphodiesterase 2A (PDE2A) and 3B (PDE3B) genes were identified in three patients. The PDE2A heterozygous variant (p.Ile629Val) was identified in a patient with BAH and early-onset HT at 13 years of age. Two PDE3B heterozygous variants (p.Arg217Gln and p.Gly392Val) were identified in patients with BAH and HT diagnosed at 18 and 33 years of age, respectively. A strong PDE2A staining was found in all cases of BAH in zona glomerulosa and/or micronodules (that were also positive for CYP11B2). PKA activity in frozen tissue was significantly higher in BAH from patients harboring PDE2A and PDE3B variants. PDE2A and PDE3B variants significantly reduced protein expression in mutant transfected cells compared to WT. Interestingly, PDE2A and PDE3B variants increased SGK1 and SCNN1G/ENaCg at mRNA or protein levels. In conclusion, PDE2A and PDE3B variants were associated with PA caused by BAH. These novel genetic findings expand the spectrum of genetic etiologies of PA.

Aldosterone-Regulating Receptors and Aldosterone-Driver Somatic Mutations

BACKGROUND

Somatic gene mutations that facilitate inappropriate intracellular calcium entrance have been identified in most aldosterone-producing adenomas (APAs). Studies suggest that angiotensin II and adrenocorticotropic hormone (ACTH) augment aldosterone production from APAs. Little is known, however, regarding possible variations in response to hormonal stimuli between APAs with different aldosterone-driver mutations.

OBJECTIVE

To analyze the transcript expression of type 1 angiotensin II receptors (AGTR1), ACTH receptors (MC2R), and melanocortin 2 receptor accessory protein (MRAP) in APAs with known aldosterone-driver somatic mutations.

METHODS

RNA was isolated from APAs with mutations in: KCNJ5 (n = 14), ATP1A1 (n = 14), CACNA1D (n = 14), and ATP2B3 (n = 5), and from normal adjacent adrenal tissue (n = 45). Transcript expression of MC2R, MRAP, AGTR1, aldosterone synthase (CYP11B2), 17α-hydroxylase/17,20-lyase (CYP17A1), and 11β-hydroxylase (CYP11B1) were quantified using quantitative RT-PCR and normalized to β-actin.

RESULTS

Compared to adjacent normal adrenal tissue, APAs had higher transcript levels of CYP11B2 (2,216.4 [1,112.0, 2,813.5]-fold, p < 0.001), MC2R (2.88 [2.00, 4.52]-fold, p < 0.001), and AGTR1 (1.80 [1.02, 2.80]-fold, p < 0.001]), and lower transcript levels of MRAP, CYP17A1, and CYP11B1 (0.28-0.36, p < 0.001 for all). MC2R and CYP11B2 transcripts were lower in APAs with KCNJ5 vs. other mutations (p < 0.01 for both). MC2R expression correlated positively with that of AGTR1 in APAs harboring KCNJ5 and CACNA1D mutations, and with MRAP expression in APAs harboring ATPase mutations.

CONCLUSIONS

While MC2R and AGTR1 are expressed in all APAs, differences were observed based on the underlying aldosterone-driver somatic mutations. In tandem, our findings suggest that APAs with ATPase-mutations are more responsive to ACTH than KCNJ5-mutated APAs.

Recurrence of Primary Aldosteronism 10 Years After Left Adrenalectomy for Aldosterone-Producing Adenoma: A Case Report

A 29-year-old female patient diagnosed with primary aldosteronism (PA) in 2004 underwent complete adrenalectomy for left aldosterone-producing adenoma (APA) confirmed by hematoxylin and eosin (HE) and CYP11B2 staining. Her hypokalemia was corrected, and her blood pressure (BP) normalized and maintained without medication for 10 years. In 2014, her BP became elevated again, and a recurrence of PA with an adenoma on the right adrenal gland was discovered by computed tomography scan. She underwent partial right adrenalectomy in 2018 due to unsatisfactory BP control with medication and gradually enlarging adenoma. The resected adrenal tissue contained a CYP11B2 staining positive APA. Her BP was then controlled by two drugs. Sanger sequencing of DNA extracted from tissue slices revealed that both left and right adenomas carried the same aldosterone-driver KCNJ5 gene mutation, but with different nucleotide changes. We suggest that patients who undergo adrenalectomy for APA should be followed up for life.

Development and validation of a potent and specific inhibitor for the CLC-2 chloride channel

CLC-2 is a voltage-gated chloride channel that is widely expressed in mammalian tissues. In the central nervous system, CLC-2 appears in neurons and glia. Studies to define how this channel contributes to normal and pathophysiological function in the central nervous system raise questions that remain unresolved, in part due to the absence of precise pharmacological tools for modulating CLC-2 activity. Herein, we describe the development and optimization of AK-42, a specific small-molecule inhibitor of CLC-2 with nanomolar potency (IC50 = 17 ± 1 nM). AK-42 displays unprecedented selectivity (>1,000-fold) over CLC-1, the closest CLC-2 homolog, and exhibits no off-target engagement against a panel of 61 common channels, receptors, and transporters expressed in brain tissue. Computational docking, validated by mutagenesis and kinetic studies, indicates that AK-42 binds to an extracellular vestibule above the channel pore. In electrophysiological recordings of mouse CA1 hippocampal pyramidal neurons, AK-42 acutely and reversibly inhibits CLC-2 currents; no effect on current is observed on brain slices taken from CLC-2 knockout mice. These results establish AK-42 as a powerful tool for investigating CLC-2 neurophysiology.

Adrenal Incidentaloma

An adrenal incidentaloma is now established as a common endocrine diagnosis that requires a multidisciplinary approach for effective management. The majority of patients can be reassured and discharged, but a personalized approach based upon image analysis, endocrine workup, and clinical symptoms and signs are required in every case. Adrenocortical carcinoma remains a real concern but is restricted to <2% of all cases. Functional adrenal incidentaloma lesions are commoner (but still probably <10% of total) and the greatest challenge remains the diagnosis and optimum management of autonomous cortisol secretion. Modern-day surgery has improved outcomes and novel radiological and urinary biomarkers will improve early detection and patient stratification in future years to come.

Ion Channel Function and Electrical Excitability in the Zona Glomerulosa: A Network Perspective on Aldosterone Regulation

Aldosterone excess is a pathogenic factor in many hypertensive disorders. The discovery of numerous somatic and germline mutations in ion channels in primary hyperaldosteronism underscores the importance of plasma membrane conductances in determining the activation state of zona glomerulosa (zG) cells. Electrophysiological recordings describe an electrically quiescent behavior for dispersed zG cells. Yet, emerging data indicate that in native rosette structures in situ, zG cells are electrically excitable, generating slow periodic voltage spikes and coordinated bursts of Ca²⁺ oscillations. We revisit data to understand how a multitude of conductances may underlie voltage/Ca²⁺ oscillations, recognizing that zG layer self-renewal and cell heterogeneity may complicate this task. We review recent data to understand rosette architecture and apply maxims derived from computational network modeling to understand rosette function. The challenge going forward is to uncover how the rosette orchestrates the behavior of a functional network of conditional oscillators to control zG layer performance and aldosterone secretion.

The landscape of molecular mechanism for aldosterone production in aldosterone-producing adenoma

Primary aldosteronism is the most common form of secondary hypertension with a prevalence of 5-10% in hypertensive patients. Aldosterone-producing adenoma (APA) is a subtype of primary aldosteronism, and somatic mutations in KCNJ5, ATP1A1, ATP2B3, CACNA1D, CLCN2, or CTNNB1 were identified and recognized to drive aldosterone production and/or contribute to tumorigenesis in APA. Mutations of KCNJ5, ATP1A1, ATP2B3, CACNA1D, and CLCN2 are known to activate calcium signaling, and its activation potentiate CYP11B2 (aldosterone synthesis) transcription in adrenal cells. Transcriptome analyses combined with bioinformatics using APA samples were conductive for each gene mutation mediated pivotal pathway, gene ontology, and clustering. Several important intracellular molecules in increase aldosterone production were detected by transcriptome analysis, and additional functional analyses demonstrated intracellular molecular mechanisms of aldosterone production which focused on calcium signal, CYP11B2 transcription and translation. Furthermore, DNA methylation analysis revealed that promoter region of CYP11B2 was entirely hypomethylated, but that of other steroidogenic enzymes were not in APA. Integration of transcriptome and DNA methylome analysis clarified some DNA methylation associated gene expression, and the transcripts have a role for aldosterone production. In this article, we reviewed the intracellular molecular mechanisms of aldosterone production in APA, and discussed future challenges for basic studies leading to clinical practice.

Pathogenesis and treatment of primary aldosteronism

Early diagnosis and appropriate treatment of primary aldosteronism, the most frequent cause of secondary hypertension, are crucial to prevent deleterious cardiovascular outcomes. In the past decade, the discovery of genetic abnormalities responsible for sporadic and familial forms of primary aldosteronism has improved the knowledge of the pathogenesis of this disorder. Mutations in genes encoding ion channels and pumps lead to increased cytosolic concentrations of calcium in zona glomerulosa cells, which triggers CYP11B2 expression and autonomous aldosterone production. Improved understanding of the mechanisms underlying the disease is key to improving diagnostics and to developing and implementing targeted treatments. This Review provides an update on the genetic abnormalities associated with sporadic and familial forms of primary aldosteronism, their frequency among different populations and the mechanisms explaining excessive aldosterone production and adrenal nodule development. The possible effects and uses of these findings for improving the diagnostics for primary aldosteronism are discussed. Furthermore, current treatment options of primary aldosteronism are reviewed, with particular attention to the latest studies on blood pressure and cardiovascular outcomes following medical or surgical treatment. The new perspectives regarding the use of targeted drug therapy for aldosterone-producing adenomas with specific somatic mutations are also addressed.

Genetics, prevalence, screening and confirmation of primary aldosteronism: a position statement and consensus of the Working Group on Endocrine Hypertension of The European Society of Hypertension

: Autonomous aldosterone overproduction represents the underlying condition of 5-10% of patients with arterial hypertension and carries a significant burden of mortality and morbidity. The diagnostic algorithm for primary aldosteronism is sequentially based on hormonal tests (screening and confirmation tests), followed by lateralization studies (adrenal CT scanning and adrenal venous sampling) to distinguish between unilateral and bilateral disease. Despite the recommendations of the Endocrine Society guideline, primary aldosteronism is largely underdiagnosed and undertreated with high between-centre heterogeneity. Experts from the European Society of Hypertension have critically reviewed the available literature and prepared a consensus document constituting two articles to summarize current knowledge on the epidemiology, diagnosis, treatment, and complications of primary aldosteronism.

Identification of Somatic Mutations in CLCN2 in Aldosterone-Producing Adenomas

Somatic mutations driving aldosterone production have been identified in approximately 90% of aldosterone-producing adenomas (APAs) using an aldosterone synthase (CYP11B2) immunohistochemistry (IHC)-guided DNA sequencing approach. In the present study, using CYP11B2-guided whole-exome sequencing (WES) and targeted amplicon sequencing, we detected 2 somatic variants in CLCN2 in 2 APAs that were negative for currently known aldosterone-driver mutations. The CLCN2 gene encodes the voltage-gated chloride channel ClC-2. CLCN2 germline variants have previously been shown to cause familial hyperaldosteronism type II. Somatic mutations in CLCN2 were identified in 2 of 115 APAs, resulting in a prevalence of 1.74%. One of the CLCN2 somatic mutations (c.G71A,p.G24D) was identical to a previously described germline variant causing early-onset PA, but was present only as a somatic mutation. The second CLCN2 mutation, which affects the same region of the gene, has not been reported previously (c.64-2_74del). These findings prove that WES of CYP11B2-guided mutation-negative APAs can help determine rarer genetic causes of sporadic PA.

Cellular Pathophysiology of Mutant Voltage-Dependent Ca2+ Channel CACNA1H in Primary Aldosteronism

The physiological stimulation of aldosterone production in adrenocortical glomerulosa cells by angiotensin II and high plasma K+ depends on the depolarization of the cell membrane potential and the subsequent Ca2+ influx via voltage-activated Ca2+ channels. Germline mutations of the low-voltage activated T-type Ca2+ channel CACNA1H (Cav3.2) have been found in patients with primary aldosteronism. Here, we investigated the electrophysiology and Ca2+ signaling of adrenal NCI-H295R cells overexpressing CACNA1H wildtype and mutant M1549V in order to understand how mutant CACNA1H alters adrenal cell function. Whole-cell patch-clamp measurements revealed a strong activation of mutant CACNA1H at the resting membrane potential of adrenal cells. Both the expression of wildtype and mutant CACNA1H led to a depolarized membrane potential. In addition, cells expressing mutant CACNA1H developed pronounced action potential-like membrane voltage oscillations. Ca2+ measurements showed an increased basal Ca2+ activity, an altered K+ sensitivity, and abnormal oscillating Ca2+ changes in cells with mutant CACNA1H. In addition, removal of extracellular Na+ reduced CACNA1H current, voltage oscillations, and Ca2+ levels in mutant cells, suggesting a role of the partial Na+ conductance of CACNA1H in cellular pathology. In conclusion, the pathogenesis of stimulus-independent aldosterone production in patients with CACNA1H mutations involves several factors: i) a loss of normal control of the membrane potential, ii) an increased Ca2+ influx at basal conditions, and iii) alterations in sensitivity to extracellular K+ and Na+. Finally, our findings underline the importance of CACNA1H in the control of aldosterone production and support the concept of the glomerulosa cell as an electrical oscillator.

ClC-2-like Chloride Current Alterations in a Cell Model of Spinal and Bulbar Muscular Atrophy, a Polyglutamine Disease

Spinal and bulbar muscular atrophy (SBMA) is a neuromuscular disease caused by expansions of a polyglutamine (polyQ) tract in the androgen receptor (AR) gene. SBMA is associated with the progressive loss of lower motor neurons, together with muscle weakness and atrophy. PolyQ-AR is converted to a toxic species upon binding to its natural ligands, testosterone, and dihydrotestosterone (DHT). Our previous patch-clamp studies on a motor neuron-derived cell model of SBMA showed alterations in voltage-gated ion currents. Here, we identified and characterized chloride currents most likely belonging to the chloride channel-2 (ClC-2) subfamily, which showed significantly increased amplitudes in the SBMA cells. The treatment with the pituitary adenylyl cyclase-activating polypeptide (PACAP), a neuropeptide with a proven protective effect in a mouse model of SBMA, recovered chloride channel current alterations in SBMA cells. These observations suggest that the CIC-2 currents are affected in SBMA, an alteration that may contribute and potentially determine the pathophysiology of the disease.

Molecular Basis of Primary Aldosteronism and Adrenal Cushing Syndrome

This review reports the main molecular alterations leading to development of benign cortisol- and/or aldosterone-secreting adrenal tumors. Causes of adrenal Cushing syndrome can be divided in 2 groups: multiple bilateral tumors or adenomas secreting cortisol. Bilateral causes are mainly primary pigmented nodular adrenocortical disease, most of the time due to PRKAR1A germline-inactivating mutations, and primary bilateral macronodular adrenal hyperplasia that can be caused in some rare syndromic cases by germline-inactivating mutations of MEN1, APC, and FH and of ARMC5 in isolated forms. PRKACA somatic-activating mutations are the main alterations in unilateral cortisol-producing adenomas. In primary hyperaldosteronism (PA), familial forms were identified in 1% to 5% of cases: familial hyperaldosteronism type I (FH-I) due to a chimeric CYP11B1/CYP11B2 hybrid gene, FH-II due to CLCN-2 germline mutations, FH-III due to KCNJ5 germline mutations, FH-IV due to CACNA1H germline mutations and PA, and seizures and neurological abnormalities syndrome due to CACNA1D germline mutations. Several somatic mutations have been found in aldosterone-producing adenomas in KCNJ5, ATP1A1, ATP2B3, CACNA1D, and CTNNB1 genes.

In addition to these genetic alterations, genome-wide approaches identified several new alterations in transcriptome, methylome, and miRnome studies, highlighting new pathways involved in steroid dysregulation.

Application of CRISPR/Cas9 to human-induced pluripotent stem cells: from gene editing to drug discovery

Human-induced pluripotent stem cells (hiPSCs) and CRISPR/Cas9 gene editing system represent two instruments of basic and translational research, which both allow to acquire deep insight about the molecular bases of many diseases but also to develop pharmacological research.This review is focused to draw up the latest technique of gene editing applied on hiPSCs, exploiting some of the genetic manipulation directed to the discovery of innovative therapeutic strategies. There are many expediencies provided by the use of hiPSCs, which can represent a disease model clinically relevant and predictive, with a great potential if associated to CRISPR/Cas9 technology, a gene editing tool powered by ease and precision never seen before.Here, we describe the possible applications of CRISPR/Cas9 to hiPSCs: from drug development to drug screening and from gene therapy to the induction of the immunological response to specific virus infection, such as HIV and SARS-Cov-2.

Genetic and Genomic Mechanisms of Primary Aldosteronism

Aldosterone-producing adenoma (APA) and bilateral adrenal hyperplasia are the main cause of primary aldosteronism (PA), the most frequent form of secondary hypertension. Mutations in ion channels and ATPases have been identified in APA and inherited forms of PA, highlighting the central role of calcium signaling in PA development. Different somatic mutations are also found in aldosterone-producing cell clusters in adrenal glands from healthy individuals and from patients with unilateral and bilateral PA, suggesting additional pathogenic mechanisms. Recent mouse models have also contributed to a better understanding of PA. Application of genetic screening in familial PA, development of surrogate biomarkers for somatic mutations in APA, and use of targeted treatment directed at mutated proteins may allow improved management of patients.

The Potential Role of Aldosterone-Producing Cell Clusters in Adrenal Disease

Primary aldosteronism (PA) is the most common cause of secondary hypertension. The hallmark of PA is adrenal production of aldosterone under suppressed renin conditions. PA subtypes include adrenal unilateral and bilateral hyperaldosteronism. Considerable progress has been made in defining the role for somatic gene mutations in aldosterone-producing adenomas (APA) as the primary cause of unilateral PA. This includes the use of next-generation sequencing (NGS) to define recurrent somatic mutations in APA that disrupt calcium signaling, increase aldosterone synthase (CYP11B2) expression, and aldosterone production. The use of CYP11B2 immunohistochemistry on adrenal glands from normal subjects, patients with unilateral and bilateral PA has allowed the identification of CYP11B2-positive cell foci, termed aldosterone-producing cell clusters (APCC). APCC lie beneath the adrenal capsule and like APA, many APCC harbor somatic gene mutations known to increase aldosterone production. These findings suggest that APCC may play a role in pathologic progression of PA. Herein, we provide an update on recent research directed at characterizing APCC and also discuss the unanswered questions related to the role of APCC in PA.

Immunohistochemistry of the Human Adrenal CYP11B2 in Normal Individuals and in Patients with Primary Aldosteronism

The CYP11B2 enzyme is the terminal enzyme in the biosynthesis of aldosterone. Immunohistochemistry using antibodies against CYP11B2 defines cells of the adrenal ZG that synthesize aldosterone. CYP11B2 expression is normally stimulated by angiotensin II, but becomes autonomous in primary hyperaldosteronism, in most cases driven by recently discovered somatic mutations of ion channels or pumps. Cells expressing CYP11B2 in young normal humans form a continuous band beneath the adrenal capsule; in older individuals they form discrete clusters, aldosterone-producing cell clusters (APCC), surrounded by non-aldosterone producing cells in the outer layer of the adrenal gland. Aldosterone-producing adenomas may exhibit a uniform or heterogeneous expression of CYP11B2. APCC frequently persist in the adrenal with an aldosterone-producing adenoma suggesting autonomous CYP11B2 expression in these cells as well. This was confirmed by finding known mutations that drive aldosterone production in adenomas in the APCC of clinically normal people. Unilateral aldosteronism may also be due to multiple CYP11B2-expressing nodules of various sizes or a continuous band of hyperplastic ZG cells expressing CYP11B2. Use of CYP11B2 antibodies to identify areas for sequencing has greatly facilitated the detection of aldosterone-driving mutations.

Primary Aldosteronism: Where Are We Now? Where to from Here?

The past nine years have seen major advances in establishing the etiology of unilateral primary aldosteronism, and very possibly that of bilateral hyperaldosteronism, in response to somatic mutations in aldosterone synthase expressing cells. Though there have been important advances in the management of primary aldosteronism, in small but convincing studies, they represent minor changes to current guidelines. What has been totally absent is consideration of the public health issue that primary aldosterone represents, and the public policy issues that would be involved in addressing the disorder. In his introduction to PiPA 6, Martin Reincke calculated that only one in a thousand patients in Germany with primary aldosteronism were treated appropriately, an astounding figure for any disease in the 21st century. Towards remedying this totally unacceptable public health issue, the author proposes a radical simplification and streamlining of screening for primary aldosteronism, and the management of most patients by general practitioners. The second bottle-neck in current management is that of mandatory adrenal venous sampling for all but 1-2% of patients, a costly procedure requiring rare expertise. Ideally, it should be reserved - on the basis of likelihood, enhanced imaging, or peripheral steroid profiles - for a small minority of patients with clear evidence for unilateral disease. Only when costs are minimized and roadblocks removed will primary aldosteronism be properly treated as the public health issue that it is.

CUL4-DDB1-CRBN E3 Ubiquitin Ligase Regulates Proteostasis of ClC-2 Chloride Channels: Implication for Aldosteronism and Leukodystrophy

Voltage-gated ClC-2 channels are essential for chloride homeostasis. Complete knockout of mouse ClC-2 leads to testicular degeneration and neuronal myelin vacuolation. Gain-of-function and loss-of-function mutations in the ClC-2-encoding human CLCN2 gene are linked to the genetic diseases aldosteronism and leukodystrophy, respectively. The protein homeostasis (proteostasis) mechanism of ClC-2 is currently unclear. Here, we aimed to identify the molecular mechanism of endoplasmic reticulum-associated degradation of ClC-2, and to explore the pathophysiological significance of disease-associated anomalous ClC-2 proteostasis. In both heterologous expression system and native neuronal and testicular cells, ClC-2 is subject to significant regulation by cullin-RING E3 ligase-mediated polyubiquitination and proteasomal degradation. The cullin 4 (CUL4)-damage-specific DNA binding protein 1 (DDB1)-cereblon (CRBN) E3 ubiquitin ligase co-exists in the same complex with and promotes the degradation of ClC-2 channels. The CRBN-targeting immunomodulatory drug lenalidomide and the cullin E3 ligase inhibitor MLN4924 promotes and attenuates, respectively, proteasomal degradation of ClC-2. Analyses of disease-related ClC-2 mutants reveal that aldosteronism and leukodystrophy are associated with opposite alterations in ClC-2 proteostasis. Modifying CUL4 E3 ligase activity with lenalidomide and MLN4924 ameliorates disease-associated ClC-2 proteostasis abnormality. Our results highlight the significant role and therapeutic potential of CUL4 E3 ubiquitin ligase in regulating ClC-2 proteostasis.

Adrenocortical tumorigenesis: Lessons from genetics

Advances in genomics over the past two decades have allowed for elucidation of the genetic alterations leading to the development of adrenocortical tumors and/or hyperplasias. These molecular changes were initially discovered through the study of rare familial tumor syndromes such as McCune-Albright Syndrome, Carney complex, Li-Fraumeni syndrome, and Beckwith-Wiedemann syndrome, with the identification of alterations in genes and molecular pathways that subsequently led to the discovery of aberrations in these or related genes and pathways in sporadic tumors. Genetic alterations in GNAS, PRKAR1A, PRKACA, PRKACB, PDE11A, and PDE8B, that lead to aberrant cyclic adenosine monophosphate-protein (cAMP) kinase A signaling, were found to play a major role in the development of benign cortisol-producing adrenocortical tumors and/or hyperplasias, whereas genetic defects in KCNJ5, ATP1A1, ATP2B3, CACNA1D, CACNA1H, and CLCN2 were implicated in the development of benign aldosterone-producing tumors and/or hyperplasias through modification of intracellular calcium signaling. Germline ARMC5 defects were found to cause the development of primary bilateral macronodular adrenocortical hyperplasia with glucocorticoid and/or mineralocorticoid oversecretion. Adrenocortical carcinoma was linked primarily to aberrant p53 signaling and/or Wnt-β-catenin signaling, as well as IGF2 overexpression, with frequent genetic alterations in TP53, ZNRF3, CTNNB1, and 11p15. This review focuses on the genetic underpinnings of benign cortisol- and aldosterone-producing adrenocortical tumors/hyperplasias and adrenocortical carcinoma.

Adrenocortical hyperplasia: A multifaceted disease

Adrenocortical hyperplasia may develop in different contexts. Primary adrenal hyperplasia may be secondary to primary bilateral macronodular adrenocortical hyperplasia (PBMAH) or micronodular bilateral adrenal hyperplasia (MiBAH) which may be divided in primary pigmented nodular adrenocortical disease (PPNAD) and isolated micronodular adrenocortical disease (i-MAD). Both lead to oversecretion of cortisol and potentially to Cushing's syndrome. Moreover, adrenocortical hyperplasia may be secondary to longstanding ACTH stimulation in ACTH oversecretion as in Cushing's disease, ectopic ACTH secretion or glucocorticoid resistance syndrome and congenital adrenal hyperplasia secondary to various enzymatic defects within the cortex. Finally, idiopathic bilateral adrenal hyperplasia is the most common cause of primary aldosteronism. We will discuss recent findings on the multifaceted forms of adrenocortical hyperplasia.

Classic and Nonclassic Apparent Mineralocorticoid Excess Syndrome

CONTEXT

Arterial hypertension (AHT) is one of the most frequent pathologies in the general population. Subtypes of essential hypertension characterized by low renin levels allowed the identification of 2 different clinical entities: aldosterone-mediated mineralocorticoid receptor (MR) activation and cortisol-mediated MR activation.

EVIDENCE ACQUISITION

This review is based upon a search of Pubmed and Google Scholar databases, up to August 2019, for all publications relating to endocrine hypertension, apparent mineralocorticoid excess (AME) and cortisol (F) to cortisone (E) metabolism.

EVIDENCE SYNTHESIS

The spectrum of cortisol-mediated MR activation includes the classic AME syndrome to milder (nonclassic) forms of AME, the latter with a much higher prevalence (7.1%) than classic AME but different phenotype and genotype. Nonclassic AME (NC-AME) is mainly related to partial 11βHSD2 deficiency associated with genetic variations and epigenetic modifications (first hit) and potential additive actions of endogenous or exogenous inhibitors (ie, glycyrrhetinic acid-like factors [GALFS]) and other factors (ie, age, high sodium intake) (second hit). Subjects with NC-AME are characterized by a high F/E ratio, low E levels, normal to elevated blood pressure, low plasma renin and increased urinary potassium excretion. NC-AME condition should benefit from low-sodium and potassium diet recommendations and monotherapy with MR antagonists.

CONCLUSION

NC-AME has a higher prevalence and a milder phenotypical spectrum than AME. NC-AME etiology is associated to a first hit (gene and epigene level) and an additive second hit. NC-AME subjects are candidates to be treated with MR antagonists aimed to improve blood pressure, end-organ damage, and modulate the renin levels.

Genetic, Cellular, and Molecular Heterogeneity in Adrenals With Aldosterone-Producing Adenoma

Supplemental Digital Content is available in the text.

Aldosterone-producing adenoma (APA) cause primary aldosteronism—the most frequent form of secondary hypertension. Somatic mutations in genes coding for ion channels and ATPases are found in APA and in aldosterone-producing cell clusters. We investigated the genetic, cellular, and molecular heterogeneity of different aldosterone-producing structures in adrenals with APA, to get insight into the mechanisms driving their development and to investigate their clinical and biochemical correlates. Genetic analysis of APA, aldosterone-producing cell clusters, and secondary nodules was performed in adrenal tissues from 49 patients by next-generation sequencing following CYP11B2 immunohistochemistry. Results were correlated with clinical and biochemical characteristics of patients, steroid profiles, and histological features of the tumor and adjacent adrenal cortex. Somatic mutations were identified in 93.75% of APAs. Adenoma carrying KCNJ5 mutations had more clear cells and cells expressing CYP11B1, and fewer cells expressing CYP11B2 or activated β-catenin, compared with other mutational groups. 18-hydroxycortisol and 18-oxocortisol were higher in patients carrying KCNJ5 mutations and correlated with histological features of adenoma; however, mutational status could not be predicted using steroid profiling. Heterogeneous CYP11B2 expression in KCNJ5-mutated adenoma was not associated with genetic heterogeneity. Different mutations were identified in secondary nodules expressing aldosterone synthase and in independent aldosterone-producing cell clusters from adrenals with adenoma; known KCNJ5 mutations were identified in 5 aldosterone-producing cell clusters. Genetic heterogeneity in different aldosterone-producing structures in the same adrenal suggests complex mechanisms underlying APA development.

Dynamical model of the CLC-2 ion channel reveals conformational changes associated with selectivity-filter gating

This work reports a dynamical Markov state model of CLC-2 "fast" (pore) gating, based on 600 microseconds of molecular dynamics (MD) simulation. In the starting conformation of our CLC-2 model, both outer and inner channel gates are closed. The first conformational change in our dataset involves rotation of the inner-gate backbone along residues S168-G169-I170. This change is strikingly similar to that observed in the cryo-EM structure of the bovine CLC-K channel, though the volume of the intracellular (inner) region of the ion conduction pathway is further expanded in our model. From this state (inner gate open and outer gate closed), two additional states are observed, each involving a unique rotameric flip of the outer-gate residue GLUex. Both additional states involve conformational changes that orient GLUex away from the extracellular (outer) region of the ion conduction pathway. In the first additional state, the rotameric flip of GLUex results in an open, or near-open, channel pore. The equilibrium population of this state is low (∼1%), consistent with the low open probability of CLC-2 observed experimentally in the absence of a membrane potential stimulus (0 mV). In the second additional state, GLUex rotates to occlude the channel pore. This state, which has a low equilibrium population (∼1%), is only accessible when GLUex is protonated. Together, these pathways model the opening of both an inner and outer gate within the CLC-2 selectivity filter, as a function of GLUex protonation. Collectively, our findings are consistent with published experimental analyses of CLC-2 gating and provide a high-resolution structural model to guide future investigations.

Old and new genes in primary aldosteronism

Primary aldosteronism (PA) is the most common form of secondary hypertension affecting 5%-10% of patients with arterial hypertension. In PA, high blood pressure is associated with high aldosterone and low renin levels, and often hypokalemia. In a majority of cases, autonomous aldosterone production by the adrenal gland is caused by an aldosterone producing adenoma (APA) or bilateral adrenal hyperplasia (BAH). During the last ten years, a better knowledge of the pathophysiology of PA came from the discovery of somatic and germline mutations in different genes in both sporadic and familial forms of the disease. Those genes code for ion channels and pumps, as well as proteins involved in adrenal cortex development and function. Targeted next generation sequencing following immunohistochemistry guided detection of aldosterone synthase expression allows detection of somatic mutations in up to 90% of APA, while whole exome sequencing has discovered the genetic causes of four different familial forms of PA. The identification, in BAH, of somatic mutations in aldosterone producing cell clusters open new perspectives in our understanding of the bilateral form of the disease and the development of new therapeutic approaches.

Hereditary causes of primary aldosteronism and other disorders of apparent excess mineralocorticoid activity

Secondary hypertension is a common condition with a broad differential diagnosis. Identification of the true cause of hypertension can be critical for guiding appropriate management. Here, we review hereditary conditions underlying the most common cause of secondary hypertension, primary aldosteronism, as well as other disorders impacting various levels of mineralocorticoid action. Recently, several pathogenic variants of ion channels have been described as etiologies of familial aldosteronism. Defects in steroid hormone synthesis cause hypertension in 11β-hydroxylase deficiency and 17α-hydroxylase deficiency, two types of congenital adrenal hyperplasia. Inappropriate activation of mineralocorticoid receptors underlies the syndrome of apparent mineralocorticoid excess and constitutive activation of the mineralocorticoid receptor. Finally, Liddle syndrome and pseudohypoaldosteronism type 2 are disorders impacting the function of renal sodium channels, the endpoint of mineralocorticoid action. We discuss the pathophysiology, clinical presentation, diagnosis and management of these low renin hypertension states that ultimately result in apparent excess mineralocorticoid activity.

Molecular and Electrophysiological Analyses of ATP2B4 Gene Variants in Bilateral Adrenal Hyperaldosteronism

Primary aldosteronism (PA) is the most common cause of secondary hypertension with a high prevalence among patients with resistant hypertension. Despite the recent discovery of somatic variants in aldosterone-producing adenoma (APA)-associated PA, causes for PA due to bilateral aldosterone production (bilateral hyperaldosteronism; BHA) remain unknown. Herein, we identified rare gene variants in ATP2B4, in a cohort of patients with BHA. ATP2B4 belongs to the same family of Ca-ATPases as ATP2B3, which is involved in the pathogenesis of APA. Endogenous ATP2B4 expression was characterized in adrenal tissue, and the gene variants were functionally analyzed for effects on aldosterone synthase (CYP11B2) expression, steroid production in basal and agonist-stimulated conditions, and for changes in biophysical properties of channel properties. Knockdown of ATP2B4 in HAC15 exhibited reduced angiotensin II stimulation in one of four shRNA clones. Stable HAC15 cell lines with doxycycline (dox) - inducible wild-type and variant forms of ATP2B4 - were generated, and dox-induced upregulation of ATP2B4 mRNA and protein was confirmed. However, ATP2B4 variants did not alter basal or agonist-stimulated CYP11B2 expression. Whole-cell recordings in HAC15 cells indicated robust endogenous ATP2B4 conductance in native cells but reduced conductance with overexpressed WT and variant ATP2B4. The previously defined PA-causing ATP2B3 variant served as a positive control and exhibited elevated CYP11B2 mRNA. In conclusion, while this study did not confirm a pathogenic role for ATP2B4 variants in BHA, we describe the sequencing analysis for familial and sporadic BHA and outline a template for the thorough in vitro characterization of gene variants.

A Novel Loss-of-Function Variant in the Chloride Ion Channel Gene Clcn2 Associates with Atrial Fibrillation

Atrial Fibrillation (AF) is the most common cardiac arrhythmia. Its pathogenesis is complex and poorly understood. Whole exome sequencing of Danish families with AF revealed a novel four nucleotide deletion c.1041_1044del in CLCN2 shared by affected individuals. We aimed to investigate the role of genetic variation of CLCN2 encoding the inwardly rectifying chloride channel ClC-2 as a risk factor for the development of familiar AF. The effect of the CLCN2 variant was evaluated by electrophysiological recordings on transiently transfected cells. We used quantitative PCR to assess CLCN2 mRNA expression levels in human atrial and ventricular tissue samples. The nucleotide deletion CLCN2 c.1041_1044del results in a frame-shift and premature stop codon. The truncated ClC-2 p.V347fs channel does not conduct current. Co-expression with wild-type ClC-2, imitating the heterozygote state of the patients, resulted in a 50% reduction in macroscopic current, suggesting an inability of truncated ClC-2 protein to form channel complexes with wild type channel subunits. Quantitative PCR experiments using human heart tissue from healthy donors demonstrated that CLCN2 is expressed across all four heart chambers. Our genetic and functional data points to a possible link between loss of ClC-2 function and an increased risk of developing AF.

Molecular Mechanisms of Primary Aldosteronism

Primary aldosteronism (PA) results from excess production of mineralocorticoid hormone aldosterone by the adrenal cortex. It is normally caused either by unilateral aldosterone-producing adenoma (APA) or by bilateral aldosterone excess as a result of bilateral adrenal hyperplasia. PA is the most common cause of secondary hypertension and associated morbidity and mortality. While most cases of PA are sporadic, an important insight into this debilitating disease has been derived through investigating the familial forms of the disease that affect only a minor fraction of PA patients. The advent of gene expression profiling has shed light on the genes and intracellular signaling pathways that may play a role in the pathogenesis of these tumors. The genetic basis for several forms of familial PA has been uncovered in recent years although the list is likely to expand. Recently, the work from several laboratories provided evidence for the involvement of mammalian target of rapamycin pathway and inflammatory cytokines in APAs; however, their mechanism of action in tumor development and pathophysiology remains to be understood.

Aldosterone Stimulates Its Biosynthesis Via a Novel GPER-Mediated Mechanism

CONTEXT

The G protein-coupled estrogen receptor (GPER) mediates an aldosterone secretagogue effect of 17β-estradiol in human HAC15 adrenocortical cells after estrogen receptor β blockade. Because GPER mediates mineralocorticoid receptor-independent aldosterone effects in other cell types, we hypothesized that aldosterone could modulate its own synthesis via GPER activation.

METHODS

HAC15 cells were exposed to aldosterone in the presence or absence of canrenone, a mineralocorticoid receptor antagonist, and/or of the selective GPER antagonist G36. Aldosterone synthase (CYP11B2) mRNA and protein levels changes were the study end points. Similar experiments were repeated in strips obtained ex vivo from aldosterone-producing adenoma (APA) and in GPER-silenced HAC15 cells.

RESULTS

Aldosterone markedly increased CYP11B2 mRNA and protein expression (vs untreated samples, P < 0.001) in both models by acting via GPER, because these effects were abolished by G36 (P < 0.01) and not by canrenone. GPER-silencing (P < 0.01) abolished the aldosterone-induced increase of CYP11B2, thus proving that aldosterone acts via GPER to augment the step-limiting mitochondrial enzyme (CYP11B2) of its synthesis. Angiotensin II potentiated the GPER-mediated effect of aldosterone on CYP11B2. Coimmunoprecipitation studies provided evidence for GPER-angiotensin type-1 receptor heterodimerization.

CONCLUSION

We propose that this autocrine-paracrine mechanism could enhance aldosterone biosynthesis under conditions of immediate physiological need in which the renin-angiotensin-aldosterone system is stimulated as, for example, hypovolemia. Moreover, as APA overexpresses GPER this mechanism could contribute to the aldosterone excess that occurs in primary aldosteronism in a seemingly autonomous fashion from angiotensin II.

Primary Aldosteronism Diagnosis and Management: A Clinical Approach

Primary aldosteronism used to be considered a rare cause of secondary hypertension. However, accruing evidence indicates that primary aldosteronism is more common than previously recognized. The implications of this increased prevalence are important to public health because autonomous aldosterone production contributes to cardiovascular disease and can be treated in a targeted manner. This article focuses on clinical approaches for diagnosing primary aldosteronism more frequently and earlier in its course, as well as practical treatment objectives to reduce the risk for incident cardiovascular disease.

Genetic Characteristics of Aldosterone-Producing Adenomas in Blacks

Somatic mutations have been identified in aldosterone-producing adenomas (APAs) in genes that include KCNJ5, ATP1A1, ATP2B3, and CACNA1D. Based on independent studies, there appears to be racial differences in the prevalence of somatic KCNJ5 mutations, particularly between East Asians and Europeans. Despite the high cardiovascular disease mortality of blacks, there have been no studies focusing on somatic mutations in APAs in this population. In the present study, we investigated genetic characteristics of APAs in blacks using a CYP11B2 (aldosterone synthase) immunohistochemistry-guided next-generation sequencing approach. The adrenal glands with adrenocortical adenomas from 79 black patients with primary aldosteronism were studied. Seventy-three tumors from 69 adrenal glands were confirmed to be APAs by CYP11B2 immunohistochemistry. Sixty-five of 73 APAs (89%) had somatic mutations in aldosterone-driver genes. Somatic CACNA1D mutations were the most prevalent genetic alteration (42%), followed by KCNJ5 (34%), ATP1A1 (8%), and ATP2B3 mutations (4%). CACNA1D mutations were more often observed in APAs from males than those from females (55% versus 29%, P=0.033), whereas KCNJ5 mutations were more prevalent in APAs from females compared with those from males (57% versus 13%, P<0.001). No somatic mutations in aldosterone-driver genes were identified in tumors without CYP11B2 expression. In conclusion, 89% of APAs in blacks harbor aldosterone-driving mutations, and unlike Europeans and East Asians, the most frequently mutated aldosterone-driver gene was CACNA1D. Determination of racial differences in the prevalence of aldosterone-driver gene mutations may facilitate the development of personalized medicines for patients with primary aldosteronism.

Elevated aldosterone and blood pressure in a mouse model of familial hyperaldosteronism with ClC-2 mutation

Gain-of-function mutations in the chloride channel ClC-2 were recently described as a cause of familial hyperaldosteronism type II (FH-II). Here, we report the generation of a mouse model carrying a missense mutation homologous to the most common FH-II-associated CLCN2 mutation. In these Clcn2^R180Q/+ mice, adrenal morphology is normal, but Cyp11b2 expression and plasma aldosterone levels are elevated. Male Clcn2^R180Q/+ mice have increased aldosterone:renin ratios as well as elevated blood pressure levels. The counterpart knockout model (Clcn2^−/−), in contrast, requires elevated renin levels to maintain normal aldosterone levels. Adrenal slices of Clcn2^R180Q/+ mice show increased calcium oscillatory activity. Together, our work provides a knockin mouse model with a mild form of primary aldosteronism, likely due to increased chloride efflux and depolarization. We demonstrate a role of ClC-2 in normal aldosterone production beyond the observed pathophysiology.

Mutations in the chloride channel ClC-2 have been associated with familial forms of hyperaldosteronism. Here, Schewe et al. generated a mouse model carrying the most common mutation found in patients and find it recapitulates key features of the disease, providing a unique tool for future studies on its pathogenesis.

[Primary aldosteronism : Genetics and pathology]

BACKGROUND

Primary aldosteronism, the excessive production of the steroid hormone aldosterone, is the most common cause of secondary hypertension. Common subforms include bilateral adrenal hyperplasia and aldosterone-producing adenoma.

OBJECTIVES

The goal of this review is to summarize important publications on the genetic basis of primary aldosteronism.

RESULTS

Somatic mutations in the KCNJ5, CACNA1D, ATP1A1, and ATP2B3 genes have been described as causes of aldosterone-producing adenomas. They eventually all lead to increased cellular calcium influx and aldosterone production. The mechanisms of rare CTNNB1 mutations are less defined. Correlations between mutations and different histologic characteristics as well as gender and ethnicity remain unexplained. Recent publications suggest that bilateral hyperplasia is at least partially due to so-called aldosterone-producing cell clusters, often with mutations in CACNA1D. Rare familial forms show mutations in the CYP11B2, CLCN2, KCNJ5, CACNA1H, or CACNA1D genes.

CONCLUSIONS

These results suggest that a significant fraction of primary aldosteronism is due to somatic mutations in single genes.

Genetic causes of primary aldosteronism

Primary aldosteronism is characterized by at least partially autonomous production of the adrenal steroid hormone aldosterone and is the most common cause of secondary hypertension. The most frequent subforms are idiopathic hyperaldosteronism and aldosterone-producing adenoma. Rare causes include unilateral hyperplasia, adrenocortical carcinoma and Mendelian forms (familial hyperaldosteronism). Studies conducted in the last eight years have identified somatic driver mutations in a substantial portion of aldosterone-producing adenomas, including the genes KCNJ5 (encoding inwardly rectifying potassium channel GIRK4), CACNA1D (encoding a subunit of L-type voltage-gated calcium channel Ca_V1.3), ATP1A1 (encoding a subunit of Na⁺/K⁺-ATPase), ATP2B3 (encoding a Ca²⁺-ATPase), and CTNNB1 (encoding ß-catenin). In addition, aldosterone-producing cells were recently reported to form small clusters (aldosterone-producing cell clusters) beneath the adrenal capsule. Such clusters accumulate with age and appear to be more frequent in individuals with idiopathic hyperaldosteronism. The fact that they are associated with somatic mutations implicated in aldosterone-producing adenomas also suggests a precursor function for adenomas. Rare germline variants of CYP11B2 (encoding aldosterone synthase), CLCN2 (encoding voltage-gated chloride channel ClC-2), KCNJ5, CACNA1H (encoding a subunit of T-type voltage-gated calcium channel Ca_V3.2), and CACNA1D have been reported in different subtypes of familial hyperaldosteronism. Collectively, these studies suggest that primary aldosteronism is largely due to genetic mutations in single genes, with potential implications for diagnosis and therapy.

ANO4 (Anoctamin 4) Is a Novel Marker of Zona Glomerulosa That Regulates Stimulated Aldosterone Secretion

Microarray comparison of the transcriptomes of human adrenal zona glomerulosa (ZG) and zona fasciculata found several ZG-specific genes that negatively regulate aldosterone secretion. The third and most significantly upregulated ZG-gene (19.9-fold compared with zona fasciculata, P=6.58×10-24) was ANO4, a putative Ca2+-activated chloride channel. We have investigated the role of ANO4 in human adrenal, and whether it functions like the prototype anoctamin, ANO1. We evaluated ANO4 mRNA and protein expression in human adrenal by qPCR and immunohistochemistry, compared the effects of ANO4 and ANO1 overexpression on baseline and stimulated aldosterone secretion and cell proliferation in H295R cells, and analyzed ANO4 activity as a Ca2+-activated chloride channel in comparison with other anoctamins by a fluorescence-based functional assay. The expression of ANO4 in ZG was confirmed by qPCR as 23.21-fold upregulated compared with zona fasciculata (n=18; P=4.93×10-7). Immunohistochemistry found cytoplasmic, ZG-selective expression of ANO4 (anoctamin 4) protein. ANO4 overexpression in H295R cells attenuated calcium-mediated aldosterone secretion and cell proliferation in comparison to controls. The latter effects were in a different direction to those of ANO1. The functional assay showed that, in contrast to ANO1, ANO4 expression results in low levels of calcium-dependent anion transport. In conclusion, ANO4 is one of the most highly expressed genes in ZG. It attenuates stimulated aldosterone secretion and cell proliferation. Although belonging to a family of Ca2+-activated chloride channels, it does not generate significant plasma membrane chloride channel activity.

Pathogenesis of hypertension in a mouse model for human CLCN2 related hyperaldosteronism

Human primary aldosteronism (PA) can be caused by mutations in several ion channel genes but mouse models replicating this condition are lacking. We now show that almost all known PA-associated CLCN2 mutations markedly increase ClC-2 chloride currents and generate knock-in mice expressing a constitutively open ClC-2 Cl⁻ channel as mouse model for PA. The Clcn2^op allele strongly increases the chloride conductance of zona glomerulosa cells, provoking a strong depolarization and increasing cytoplasmic Ca²⁺ concentration. Clcn2^op mice display typical features of human PA, including high serum aldosterone in the presence of low renin activity, marked hypertension and hypokalemia. These symptoms are more pronounced in homozygous Clcn2^op/op than in heterozygous Clcn2^+/op mice. This difference is attributed to the unexpected finding that only ~50 % of Clcn2^+/op zona glomerulosa cells are depolarized. By reproducing essential features of human PA, Clcn2^op mice are a valuable model to study the pathological mechanisms underlying this disease.

Mutations in the chloride channel ClC-2 have been found in primary aldosteronism (PA). Here, Göppner et al. generate transgenic mice expressing a mutant form of ClC-2 that displays increased chloride currents like patient mutations, and find it recapitulates the key pathological features of PA.