Cellular and Genetic Causes of Idiopathic Hyperaldosteronism

Abstract

Primary aldosteronism affects ≈5% to 10% of hypertensive patients and has unilateral and bilateral forms. Most unilateral primary aldosteronism is caused by computed tomography-detectable aldosterone-producing adenomas, which express CYP11B2 (aldosterone synthase) and frequently harbor somatic mutations in aldosterone-regulating genes. The cause of the most common bilateral form of primary aldosteronism, idiopathic hyperaldosteronism (IHA), is believed to be diffuse hyperplasia of aldosterone-producing cells within the adrenal cortex. Herein, a multi-institution cohort of 15 IHA adrenals was examined with CYP11B2 immunohistochemistry and next-generation sequencing. CYP11B2 immunoreactivity in adrenal glomerulosa harboring non-nodular hyperplasia was only observed in 4/15 IHA adrenals suggesting that hyperplasia of CYP11B2-expressing cells may not be the major cause of IHA. However, the adrenal cortex of all IHA adrenals harbored at least 1 CYP11B2-positive aldosterone-producing cell cluster (APCC) or micro-aldosterone-producing adenomas. The number of APCCs per case (and individual APCC area) in IHA adrenals was significantly larger than in normotensive controls. Next-generation sequencing of DNA from 99 IHA APCCs demonstrated somatic mutations in genes encoding the L-type calcium voltage-gated channel subunit α 1-D ( CACNA1D, n=57; 58%) and potassium voltage-gated channel subfamily J-5 ( KCNJ5, n=1; 1%). These data suggest that IHA may result from not only hyperplasia but also the accumulation or enlargement of computed tomography-undetectable APCC harboring somatic aldosterone-driver gene mutations. The high prevalence of mutations in the CACNA1D L-type calcium channel provides a potential actionable therapeutic target that could complement mineralocorticoid blockade and inhibit aldosterone overproduction in some IHA patients.

Keywords: adrenal cortex; aldosterone; calcium channels; hyperaldosteronism; hypertension.

Figure 1.. Classification of PA.

Primary aldosteronism (PA) is categorized into bilateral and unilateral forms based on the source of aldosterone production evaluated by adrenal vein sampling. Computed tomography (CT)-detectable PA is predominantly caused by aldosterone-producing adenoma (APA), whereas CT-negative unilateral PA is caused by CYP11B2 (aldosterone synthase) expressing aldosterone-producing cell clusters (APCCs). CT-negative bilateral PA, the most common subtype of PA (70%), is referred to as idiopathic hyperaldosteronism (IHA). Herein, examining a rare collection of IHA adrenals and combining histology, immunohistochemistry and next generation sequencing, we defined the role of somatic mutations in IHA.

Figure 2.. Representative images of APCC in IHA adrenals.

Representative images of hematoxylin and eosin (H&E, A) and CYP11B2 immunohistochemistry (B) are shown. All IHA cases harbored at least one APCC (blue arrows), supporting APCC as the common cause of IHA. Of the 15 adrenals studied, intervening zona glomerulosa (ZG) cells were negative (C) for CYP11B2 in 11/15 cases. Intervening ZG was diffusely positive (D, black arrow) for CYP11B2 only in 4/15 cases, arguing against diffuse aldosterone overproduction as the most common cause of IHA. Scale bars in A and B are 5mm, and in panels C and D are 1mm.

Figure 3.. APCC score and area in IHA adrenals.

The median APCC (aldosterone-producing cell cluster) score (A) (6.1 vs 0 /cm², p<0.0001, two-sided Mann-Whitney test) and the median APCC area (B) (0.25 vs 0.16 mm², p<0.01, two-sided Mann-Whitney test) were significantly increased in IHA adrenals vs. previously described age-matched normotensive adrenals. IHA values were not significantly different vs. previously described CT-negative unilateral PA adrenals (unilateral APCC). The APCC score was defined as the number of APCC per section divided by adrenal cortex area to normalize section differences. APCC score (in log scale) is plotted for all samples according to the adrenal type (B).

Figure 4.. Somatic CACNA1D mutations found in IHA adrenals.

The α1 subunit of CACNA1D is shown with domains recurrently mutated in APA indicated in blue. Each line represents the location of a somatic mutation(s) detected in an APCC from an IHA case (the adjacent number indicates total number at that position in this cohort). Previously reported somatic mutations in APA are shown in red, and unreported mutations in green. In total, 77% (44/57) of the CACNA1D mutations were in the domains recurrently mutated in APA.