A rare case of hyporeninemic hypertension: Answers

Answers

1. What is the differential diagnosis for monogenic causes of hyporeninemic hypertension?

All the conditions listed below result in volume expansion and subsequent suppression of renin secretion.

1. Liddle’s Syndrome: autosomal dominant inheritance, gain-of-function variants of SCNN1B or SCNN1G, encoding the beta and gamma subunits of the epithelial Na⁺ channel ENaC, respectively. This results in less channel ubiquitination, higher cell surface abundance of ENaC in the connecting tubule and collecting ducts, and more Na⁺ and water absorption [1, 2].

2. Gordon Syndrome (Pseudohypoaldosteronism type II): autosomal dominant inheritance, WNK1, WNK4, CUL1, and KLHL1 (KLHL1 is also inherited in an autosomal recessive manner). Clinical and laboratory characteristics include hyperkalemia, metabolic acidosis, and hypercalciuria. Large deletions within the first intron of the WNK1 gene result in increased WNK1 expression which stimulates NCC. KLHL3 and CUL3 are part of an ubiquitin-protein ligase complex which degrades WNKs. Loss-of-function variants in KLHL3 and CUL3 cause an increased abundance of WNK4 which stimulates NCC [3–5]. Gain-of-function variants in WNK4 also increase NCC activity [5].

3. Apparent mineralocorticoid excess (AME): autosomal recessive inheritance, due to inactivating variants of HSD11B2 encoding the 11β-hydroxysteroid dehydrogenase type 2 enzyme. Under normal circumstances both aldosterone and cortisol bind to mineralocorticoid receptors (MR) but cortisol is 100-fold more abundant in the circulation than aldosterone [6]. This enzyme converts cortisol to the inactive hormone cortisone and thereby protects the MR from cortisol action. Inactivating mutations causes an excess of cortisol to bind to MR and MR stimulation in the collecting ducts leading to aldosterone-like effects [7].

4. Hypertension exacerbated by pregnancy: autosomal dominant inheritance, gain-of-function variants of MR resulting in higher sensitivity to non-mineralocorticoid steroids causing enhanced Na⁺ absorption in the distal nephron. The name of this disorder is misleading since the phenotype is not limited to pregnancy or females [8].

5. Glucocorticoid remediable aldosteronism (GRA) or familial hyperaldosteronism type I: autosomal dominant inheritance, due to an unequal crossover (non-allelic homologous recombination) between the genes encoding 11β-hydroxylase and aldosterone synthase. This creates a hybrid fusion gene containing an ACTH-responsive promoter and an aldosterone synthase coding region, which responds to ACTH [9].

6. Congenital adrenal hyperplasia (CAH): autosomal recessive, caused either by inactivating mutations of 11β-hydroxylase (CYP11B1) or 17α-hydroxylase (CYP17) [10, 11]. This leads to accumulation of steroid precursors including deoxycorticosterone which activate mineralocorticoid receptor.

7. Other forms of familial hyperaldosteronism: autosomal dominant, gain-of-function variants in one of several genes, CACNA1H, CACNA1D, CLCN2, and KCNJ5 [12–15]. These genes are expressed in the adrenal gland and result in increased aldosterone production.

2. What other diagnostic studies should be considered?

To evaluate for AME the ratio of urinary tetrahydrocortisol plus allotetrahydrocortisol to tetrahydrocortisone may be helpful. GRA is confirmed by identifying the gene fusion event. Plasma and urine steroid profiles can lead to the diagnosis of CAH. A diagnosis can also be established by genetic screening either with a disease focused assay or a more comprehensive screen with broad-based kidney gene panel or exome sequencing [16].

Given the high degree of suspicion for a monogenetic cause of her hyporeninemic hypertension, testing was performed with a targeted genetic kidney panel (KidneySeq™ version 2, Iowa Institute of Human Genetics, University of Iowa) in June 2017. The genetic panel did not detect any pathogenic or likely pathogenic variants in the genes causing Liddle’s Syndrome, Gordon Syndrome, or AME. A specific CYP11B1/CYP11B2 chimeric gene fusion test to evaluate for GRA was also negative. Six months later, a newer version of KidneySeq (KidneySeq™ version 3) that included three additional genes for monogenic forms of hypertension (CACNA1H, CYP11B1, KCNJ5) detected a heterozygous in-frame deletion in the gene KCNJ5 (c.433–438delGAGACC, p.Glu147_Thr148del), which encodes the inwardly rectifying potassium channel Kir3.4 [17]. The two amino acid deletion is in close proximity to the selectivity filter in Kir3.4 and the identified variant has not been reported by the Exome Aggregation Consortium (ExAC) database (http://exac.broadinstitute.org/). Germline KCNJ5 variants cause both familial (familial hyperaldosteronism type 3) and sporadic forms of primary hyperaldosteronism [12, 18]. Somatic KCNJ5 mutations in the adrenal gland are also responsible for 40% of acquired aldosterone producing adenomas (APA) [12, 19].

3. Was there any therapeutic intervention in this child, which could result in a misleading low aldosterone level?

In pediatric nephrology it is standard to test renin and aldosterone levels in children with concern for monogenetic hypertension. Multiple factors can influence renin and aldosterone measurements including age, gender, menstrual period, diurnal rhythm, posture, medications, Na⁺ intake, serum K⁺, and extracellular fluid status [20]. Her initial aldosterone level was surprisingly low given her diagnosis of hyperaldosteronism (Table 1). However, the blood sample for renin and aldosterone testing was sent in the context of hypokalemia and intravenous fluids with a NaCl load. Both factors can result in artificially low renin and aldosterone levels. Aldosterone levels during clinic follow-up with a steady-state revealed repeatedly elevated aldosterone levels as expected with a KCNJ5 mutation (Table 1). During the initial presentation the aldosterone:renin ratio was very elevated hinting towards elevated aldosterone activity despite normal aldosterone concentration. However, there is a lack of experience in applying the aldosterone:renin ratio in children [21].

Table 1.

Outline of the patient’s characteristics from initial presentation to the last clinic visit, including systolic and diastolic blood pressures, Na+, K+, renin, aldosterone, and aldosterone/renin ratio.

	June 2017	June 2018	November 2018	June 2019	November 2019
Systolic BP (mmHg)	137	104	100	102	102
Diastolic BP (mmHg)	98	57	50	56	57
Na+ (135–145 mEq/L)	142	137	134		130
K+ (3.5–5 mEq/L)	2.6	4	3.7		4.5
Renin (1–6.5 ng/ml/hour)	0.1	44.8	64.4	33.5	66.2
Aldosterone (5–80 ng/dl)	20.5	77.9	164	112	85.6
Aldosterone:Renin Ratio	205.000	1.739	2.547	3.343	1.293

Discussion

In the granulosa cells of the adrenal glands the Kir3.4 channel moves K⁺ out of the cell and thereby contributes to hyperpolarization (Fig. 1A). Physiologically, Kir3.4 is regulated by angiotensin II and extracellular potassium. Binding of angiotensin II (AngII) to Angiotensin II type 1 (AT1) receptors inhibits Kir3.4 and causes depolarization [22]. This leads to activation of voltage-gated Ca²⁺ channels and Ca²⁺ influx, which induces aldosterone secretion (Fig. 1B). Similarly, a rise in extracellular K⁺ will also change the membrane resting potential and result in depolarization and opening of voltage-gated Ca²⁺ channels [23] (Fig. 1B). One of the voltage-gated Ca²⁺ channels is Ca_v1.3, which is encoded by the gene CACNA1D. Gain-of-function mutations in CACNA1D also cause primary hyperaldosteronism and APAs [14]. With pathogenic KCNJ5 variants the cell experiences loss of potassium inwardly rectifying current, membrane depolarization, and chronic Ca²⁺ influx via the voltage-gated Ca²⁺ channels, thus increasing intracellular Ca²⁺ levels. This induces adrenal gland zona glomerulosa proliferation and constitutive aldosterone synthesis [24, 25] (Fig. 1C). The affected two amino acids, Glu147 and Thr148, in our patient are adjacent to the critical amino acid Gly151, which is crucial to the channel’s selectivity filter [26]. Many missense variants at position 151 have been reported in patients with hyperaldosteronism and the entire amino acid motif is heavily conserved through evolution [12, 27–29]. It has been shown that mutations adjacent to the K⁺ channel selectivity filter also impair channel function, cause Na⁺ entry, chronic depolarization, and subsequently constitutive aldosterone production and cell proliferation (Fig. 1C) [12]. Therefore, we hypothesize that the in-frame deletion in our patient, which is in close proximity to the selectivity filter motif (GlyTyrGly), renders the channel nonselective for potassium [30]. The KCNJ5 gene product forms heterotetramers with Kir3.1, which is encoded by KCNJ3 and contributes to a high resting K⁺ conductance resulting in a highly negative membrane potential in adrenal zona glomerulosa cells. KCNJ5 mutations result in reduced K⁺:Na⁺ permeability ratio and thereby allow for Na⁺ entry and chronic depolarization [12]. One may also think of the Kir3.1/3.4 channel complex as an adrenal K⁺ sensor, which increases aldosterone secretion in case of a higher extracellular potassium level.

Figure 1:

A) Kir3.4 function in the membrane of zona glomerulosa cells of the adrenal glands. At baseline, a high resting K⁺ conductance with K⁺ leaving the cell via Kir3.4 results in a highly negative membrane potential. Hyperpolarization inhibits the Ca²⁺ influx via voltage-gated Ca²⁺ channels. B) Binding of AngII to Angiotensin II type 1 receptors as well as hyperkalemia inhibit Kir3.4 and result in depolarization of the membrane potential. This causes Ca²⁺ influx through voltage-gated Ca²⁺ channels, which increases intracellular Ca²⁺ levels, and thereby stimulates aldosterone biosynthesis. C) In case of a Kir3.4 mutation, ion selectivity of Kir3.4 is altered resulting in Na⁺ influx into the cell and contributing to depolarization. As seen with AngII or hyperkalemia the cell is now depolarized, chronic Ca²⁺ influx occurs and aldosterone synthesis is continuously stimulated.

The family history of our patient is notable for the lack of any known history of hypertension in dad or mom. However, pathogenic KCNJ5 variants can present as sporadic cases of hyperaldosteronism either because of limited penetrance or because the variant arose de novo [25, 31]. Only five other KCNJ5 variants presenting in children have been published so far [18, 27, 31]. Of these, two were published with KCNJ5 mutations including a p.Glu145Gln mutation (sporadic case) and a p.Ile157Ser mutation (familial case) [27, 31]. Typical characteristics for children with pathogenic KCNJ5 variants were hypokalemia, low renin, elevated aldosterone, headaches, refractory hypertension, failure to thrive, polydipsia, and polyuria (most likely from the hypokalemia) [31]. Age of onset was typically between 2 to 5 years of age. Since her hospitalization, her blood pressures have remained well controlled with amiloride, spironolactone and amlodipine. Clonidine had been discontinued. Her last ECHO did not show any left ventricular hypertrophy. However, some patients may require bilateral adrenalectomy to achieve blood pressure control. Due to the risk for development of APAs we have performed an MRI annually to evaluate for adrenal gland masses, which have remained negative so far.

Summary

Here, we report a novel in-frame deletion in KCNJ5, a gene which is a rare cause of sporadic and familial hyporeninemic hypertension. So far, this gene seems to have been overlooked in the pediatric nephrology community and it should be included in pediatric hypertension work-up in the future.