Introduction
Apparent mineralocorticoid excess (AME, OMIM #218030) is an ultrarare autosomal recessive form of monogenic hypertension. Although the biochemical and hormonal features of AME were first described in 1977 by New et al. [
1], the first causative mutation in
HSD11B2 was not discovered until 1995 in a consanguineous Iranian family with AME [
2]. Since then, only ~100 AME cases have been described clinically and genetically worldwide, and the prevalence of AME remains uncertain. However, AME is most commonly found in consanguineous families or certain ethnic groups [
3‐
5].
AME is characterized by juvenile hypertension, hypokalemia, hypernatremia, low plasma renin activity and aldosterone concentration, metabolic alkalosis, and responsiveness to spironolactone [
6]. It has a spectrum of phenotypes ranging from life-threatening hypertension in infancy to a milder form of the disease in adults [
7]. Because of the broad diversity and overlapping clinical features, precise diagnosis of AME is highly reliant on genetic evidence [
8].
AME is caused by a mutation in
HSD11B2, which has been mapped to chromosome 16q22 and consists of five exons. It encodes 11β-hydroxysteroid dehydrogenase type 2 (11βHSD2), which is a microsomal enzyme mainly expressed in mineralocorticoid target tissues, such as distal nephron [
9]. 11βHSD2 plays an important role in the peripheral inactivation of cortisol to cortisone, thus protecting the mineralocorticoid receptor (MR) from inappropriate activation by cortisol [
10].
HSD11B2 mutations lead to a deficiency in the 11βHSD2 enzyme [
11], resulting in excessive cortisol stimulating the MR and causing intense water and sodium retention, hypokalemia, and hypertension [
12].
This study aimed to identify novel compound heterozygous mutations in HSD11B2 in a Chinese family with AME. We constructed a computational model of 11βHSD2 to evaluate genotype-structure-phenotype correlations. We also conducted a systematic review to summarize the clinical features of AME patients associated with loss-of-function HSD11B2 mutations.
Discussion
In this study, we identified novel compound heterozygous mutations (c.343_348del and c.1099_1101del) in HSD11B2, which were associated with AME in a Chinese family. The proband’s symptoms show that these mutations are causative of AME, and this is supported by structural modeling and in silico analysis. Moreover, we conducted the first known systematic review to analyze genotype–phenotype correlations in AME.
Patients with AME may be either homozygous or compound heterozygous carriers of
HSD11B2 mutations [
17]. It is noteworthy that previously described mutations are mostly missense mutations clustering in exons 3, 4, and 5 [
10]. Our systematic review recorded 54 homozygous or compound heterozygous mutations, including 31 (57%) missense mutations and 42 (78%) mutations located in exons 3–5. Other genetic aberrancies including nonsense, splicing, insertion, and deletion mutations were also recorded, but at a lower rate. The identified compound heterozygous mutations were situated in exons 2 and 5, and resulted in deletions of amino acid residues causing truncated 11βHSD2.
Previous cases with two identical heterozygous mutations from different families have been described. Ferrari et al. reported a girl with a homozygous mutation (c.343_348del) in
HSD11B2, resulting in hypertension and suppressed plasma renin levels [
7]. Morineau et al. described a French boy with compound heterozygous mutations (c.431 A > T and c.1099_1101del) in
HSD11B2 causing severe hypertension, low to normal serum potassium, low plasma renin, and aldosterone levels, as well as markedly reduced 11βHSD2 activity [
10]. This study is the first report of the novel form of compound mutations (c.343_348del and c.1099_1101del) in
HSD11B2 in a Chinese teenager with AME.
Genetic mechanisms could account for AME caused by the identified compound mutations in the proband. Loss of catalytic activity and affinity for the substrate and cofactor is thought to be essential mechanisms resulting in a loss of enzymatic function and development of AME [
18]. In the kidney, the non-selective MR has a similar affinity for aldosterone and cortisol, while circulating levels of cortisol are 100- to 1000-fold higher than healthy controls [
19,
20]. 11βHSD2 not only catalyzes cortisol into inactive cortisone, but also generates the co-substrate nicotinamide adenine dinucleotide, thereby preventing cortisol-occupied MR from acting as aldosterone mimics [
21]. The two heterozygous deletions identified in this study may impair the catalytic activity of 11βHSD2, decreasing the conversion of cortisol to cortisone. In addition, mutations of
HSD11B2 residues that form the substrate or coenzyme binding pocket could eliminate enzyme activity [
22]. The homozygous mutation c.343_348del in
HSD11B2 has been identified in the cofactor binding domain with an apparent increased affinity for cortisol [
23]. According to our structural modeling assessment, we postulate that the identified compound mutations disturb the substrate and coenzyme binding pocket. Therefore, intracellular cortisol would bind and overstimulate the MR, causing renal sodium reabsorption and potassium and bicarbonate excretion, resulting in the typical clinical features of hypertension and hypokalemic metabolic alkalosis. The development of nephrocalcinosis and renal cysts may be associated with long-standing hypokalemia [
24].
Previous studies have revealed close genotype-structure-phenotype correlations in AME [
11,
18,
25]. AME can be severe or mild, mainly depending on absent or residual 11βHSD2 activity caused by
HSD11B2 mutations [
6]. Patients carrying
HSD11B2 homozygous mutations resulting in little or no 11βHSD2 activity usually present with severe phenotypes of AME in early childhood, whereas heterozygous patients with mutations resulting in partial 11βHSD2 activity may present with mild forms of AME in late adolescence or early adulthood [
10,
22]. Our systematic review showed that homozygous
HSD11B2 mutations were significantly associated with low birth weight. In the case of compound heterozygosity, the clinical phenotypes result from the combined effects of two mutations [
11]. However, the proband’s relatives carrying a single heterozygous mutation only presented mild to moderate hypertension in adulthood. This nonclassic AME may reflect haploinsufficiency of
HSD11B2 [
26,
27]. Yau et al. found that mutations causing severe AME enhance dimerization, disrupt substrate or coenzyme binding sites, or severely impair structural stability of the enzyme [
11]. Our protein modeling analysis predicted that the identified compound mutations probably impacted the substrate and coenzyme binding sites concurrently. Furthermore, the variability in clinical manifestations might be associated with epigenetics, sodium, and environmental factors [
26,
28].
Because the age of AME presentation varies and complications can be severe [
29], early diagnosis and treatment are vital to prevent end-organ damages [
30]. As a congenital disorder, AME usually presents in early childhood [
29]. Our systematic review revealed a median age at genetic diagnosis of 3.8 years (interquartile range, 1.0–10.6 years), with 25.5% of patients diagnosed within the first year of life. However, our proband presented and diagnosed with a moderate to severe form of AME in late adolescence. Severe hypertension and hypokalemic alkalosis are associated with end-organ damage in AME, especially affecting cardiovascular and nervous systems, kidneys, and retina [
3]. Variability in biochemical parameters (the ratio of cortisol to cortisone) is related to the underlying genetic defect [
31], so genetic testing is a precise and effective tool to detect
HSD11B2 mutations [
6,
32]. Hence, our study also highlights the utility of next-generation sequencing for diagnosing AME even where enzymatic characterization is unavailable.
The goal of AME treatment is to control BP and correct electrolyte disturbance [
33‐
35]. Therapeutically, it is important to reduce dietary sodium [
36]. The proband was treated with spironolactone, low-dose dexamethasone, and verapamil for various periods. Spironolactone, as a mineralocorticoid antagonist, is required to block the MR from cortisol activation [
33]. Dexamethasone relieves excessive MR activation from cortisol by the suppression of peripheral cortisol secretion [
37]. However, dexamethasone cannot correct hypertension and hypokalemia in AME patients, and long-term medication has major adverse effects [
37]. Therefore, the treatment of AME with low-dose dexamethasone (1.5–2 mg/day) should be initiated in a monitored setting [
35]. Use of a calcium channel blocker as an adjunctive treatment to control hypertension was reported to be helpful [
24]. Other medicines have also been used to treat AME, such as potassium supplementation [
6] and/or epithelial sodium channel inhibitors [
37]. Nevertheless, comprehensive and effective management of AME patients still needs further investigation with larger sample sizes and long-term follow-up times.
In conclusion, we identified novel compound heterozygous mutations in HSD11B2 in a non-consanguineous family with AME. To the best of our knowledge, this is the second reported AME case in the Chinese population. Genetic testing can provide a definitive diagnosis for AME patients, which can prevent secondary organ damage together with specific patient management. Further in vitro expression studies should evaluate the enzymatic activity of mutant 11βHSD2 proteins to verify the severity of the compound mutations.
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