Identification of a Novel CYP11B2 Variant in a Family with Varying Degrees of Aldosterone Synthase Deficiency

Isolated aldosterone synthase deficiency is a rare autosomal recessive disorder caused by pathogenic variants in CYP11B2, resulting in impaired aldosterone synthesis. We report on a neonate with isolated aldosterone synthase deficiency caused by a novel homozygous CYP11B2 variant Chr8:NM_000498.3:c.400G>A p.(Gly134Arg). The patient presented shortly after birth with severe signs of aldosterone deficiency. Interestingly, segregation analysis revealed that the patient’s asymptomatic father was also homozygous for the CYP11B2 variant. Biochemical evaluation of the father indicated subclinical enzyme impairment, characterized by elevated aldosterone precursors. Apparently, this homozygous variant led to different clinical phenotypes in two affected relatives. In this manuscript we elaborate on the biochemical and genetic work-up performed and describe potential pitfalls in CYP11B2 sequencing due to its homology to CYP11B1.

Hyporeninemic hypoaldosteronism in RMND1-related mitochondrial disease

BACKGROUND

RMND1 is a nuclear gene needed for proper function of mitochondria. A pathogenic gene will cause multiple oxidative phosphorylation defects. A renal phenotype consisting of hyponatremia, hyperkalemia, and acidosis is frequently reported, previously considered to be due to aldosterone insensitivity.

METHODS

Clinical features and pathophysiology of three patients will be reported. DNA of these patients was subjected to exome screening.

RESULTS

In the first family, one pathogenic heterozygous and one highly probable heterozygous mutation were detected. In the second family, a homozygous pathogenic mutation was present. The electrolyte disbalance was not due to aldosterone insensitivity but to low plasma aldosterone concentration, a consequence of low plasma renin activity. This disbalance can be treated. In all three patients, the kidney function declined. In the first family, both children suffered from an unexplained arterial thrombosis with dire consequences.

CONCLUSIONS

Hyporeninemic hypoaldosteronism is the mechanism causing the electrolyte disbalance reported in patients with RMND1 mutations, and can be treated.

Molecular Analysis of the CYP11B2 Gene in 62 Patients with Hypoaldosteronism Due to Aldosterone Synthase Deficiency

CONTEXT

Isolated congenital hypoaldosteronism presents in early infancy with symptoms including vomiting, severe dehydration, salt wasting, and failure to thrive. The main causes of this rare autosomal recessive disorder is pathogenic variants of the CYP11B2 gene leading to aldosterone synthase deficiency.

OBJECTIVE

To investigate the presence of CYP11B2 pathogenic variants in a cohort of patients with a clinical, biochemical, and hormonal profile suggestive of aldosterone synthase deficiency.

DESIGN

Clinical and molecular study.

SETTING

Tertiary academic Children's Hospital, Center for Rare Pediatric Endocrine Diseases.

PATIENTS AND METHODS

Sixty-two patients (56 unrelated patients and 6 siblings), with hypoaldosteronism and their parents, underwent CYP11B2 gene sequencing after its selective amplification against the highly homologous CYP11B1 gene. In silico analysis of the identified novel variants was carried out to evaluate protein stability and potential pathogenicity.

RESULTS

CYP11B2 gene sequencing revealed that 62 patients carried a total of 12 different pathogenic CYP11B2 gene variants, 6 of which are novel. Importantly, 96% of the 56 patients carried the previously reported p.T185I variant either in homozygosity or in compound heterozygosity with another variant. The 6 novel variants detected were: p.M1I, p.V129M, p.R141Q, p.A165T, p.R448C, and the donor splice site variant of intron 8, c.1398 + 1G > A.

CONCLUSION

Molecular diagnosis was achieved in 62 patients with aldosterone synthase deficiency, the largest cohort thus far reported. Six novel genetic variants were identified as possibly pathogenic, extending the spectrum of reported molecular defects of the CYP11B2 gene.

Molecular genetic studies in a case series of isolated hypoaldosteronism due to biosynthesis defects or aldosterone resistance

BACKGROUND AND AIM

Hypoaldosteronism is associated with either insufficient aldosterone production or aldosterone resistance (pseudohypoaldosteronism). Patients with aldosterone defects typically present with similar symptoms and findings, which include failure to thrive, vomiting, hyponatremia, hyperkalemia and metabolic acidosis. Accurate diagnosis of these clinical conditions therefore can be challenging. Molecular genetic analyses can help to greatly clarify this complexity. The aim of this study was to obtain an overview of the clinical and genetic characteristics of patients with aldosterone defects due to biosynthesis defects or aldosterone resistance.

DESIGN AND PATIENTS

We investigated the clinical and molecular genetic features of 8 consecutive patients with a clinical picture of aldosterone defects seen in our clinics during the period of May 2015 through October 2017. We screened CYP11B2 for aldosterone synthesis defects and NR3C2 and the three EnaC subunits (SCNN1A, SCNN1B and SCNN1G) for aldosterone resistance.

RESULTS

We found 4 novel and 2 previously reported mutations in the genes CYP11B2, NR3C2, SCNN1A and SCNN1G in 9 affected individuals from 7 unrelated families.

CONCLUSION

Molecular genetic investigations can help confidently diagnose these conditions and clarify the pathogenicity of aldosterone defects. This study may expand the clinical and genetic correlations of defects in aldosterone synthesis or resistance.

Two novel mutations of the CYP11B2 gene in a Japanese patient with aldosterone deficiency type 1

Isolated hypoaldosteronism is a rare and occasionally life-threatening cause of salt wasting in infancy. A 2-month-old Japanese boy of unrelated parents was examined for failure to thrive and poor weight gain. Laboratory findings were hyponatremia, hyperkalemia, high plasma renin and low aldosterone levels. Spot urine analysis by gas chromatography-mass spectrometry (GC-MS) showed that urinary excretion of corticosterone metabolites was elevated. Whereas excretion of 18-hydroxycortricosterone metabolites was within the normal range, excretion of aldosterone metabolites was undetectable. The patient was therefore suspected to have aldosterone synthase deficiency type 1. Sequence analysis of CYP11B2, the gene encoding aldosterone synthase (CYP11B2), showed that the patient was a compound heterozygote for c.168G>A, p.W56X in exon 1 and c.1149C>T, p.R384X in exon 7. p.W56X was inherited from his mother and p.R384X was from his father. Since both alleles contain nonsense mutations, a lack of CYP11B2 activity was speculated to cause his condition. To our knowledge, this is the first Japanese patient in which the molecular basis of aldosterone synthase deficiency type 1 has been clarified. This case also indicates that spot urinary steroid analysis is useful for diagnosis.

Homozygosity for a mutation in the CYP11B2 gene in an infant with congenital corticosterone methyl oxidase deficiency type II

Isolated aldosterone synthase deficiency can be the source of life-threatening salt wasting and failure to thrive in infancy. We studied an infant with failure to thrive and persistent hyponatremia despite oral sodium supplementation. Initial analyses revealed highly elevated plasma renin but normal values of plasma aldosterone. The biochemical diagnosis of corticosterone methyl oxidase deficiency type II was established by multisteroid analysis, revealing a pathognomonic pattern with a highly elevated ratio of 18-OH-corticosterone to aldosterone. This reflects an enzymatic defect in the aldosterone synthase that is responsible for the terminal steps in the aldosterone biosynthesis. Molecular genetic analysis supported the diagnosis revealing homozygosity for a pathogenic c.554C>T (p.T185I) variation in exon 3 of the CYP11B2 gene encoding aldosterone synthase. Homozygosity for two other polymorphic variations c.504C>T (p.F168F) and c.518A>G (p.K173R) were identified as well. Treatment with fludrocortisone resulted in catch-up growth. Discontinuation of treatment at the age of 9 years was later possible without any clinical or biochemical deterioration.

CONCLUSIONS

Isolated deficiency in aldosterone biosynthesis should be considered in neonates and infants with failure to thrive and salt wasting. Normal levels of plasma aldosterone compared with highly elevated levels of plasma renin indicate an impaired aldosterone biosynthesis and suggest the disorder. Recognition of its existence is important as fludrocortisone replacement therapy effectively normalizes sodium balance and growth.

Aldosterone synthase deficiency type II with hypospadias

Aldosterone synthase deficiency (ASD) type II was diagnosed in a 3 week old boy with severe dehydration. Elevated plasma renin activity, low-normal aldosterone, increased levels for 18-OH corticosterone (18-OHB) and 18-OH-deoxycorticosterone were measured. Sequencing revealed a homozygous mutation for c554C > T in exon 3 (p.T185I) (CYP11B2). Hypospadias has so far not been reported in ASD.

Fission yeast Schizosaccharomyces pombe as a new system for the investigation of corticosterone methyloxidase deficiency-causing mutations

The aldosterone synthase, CYP11B2, catalyses the conversion of 11-deoxycorticosterone to aldosterone, a process that requires three steps: a hydroxylation at position 11β to form corticosterone, another one at position 18 to produce 18-hydroxycorticosterone, and, finally, an oxidation at position 18 to form aldosterone. Aldosterone synthase deficiency usually finds its expression in infancy as a life-threatening electrolyte imbalance, caused by mutations in the CYP11B2 gene. Therefore, in depth studies of mutations and their enzymatic activities will provide information for the diagnosis and management of hypoaldosteronism caused by CYP11B2 deficiencies. Here, we report the development of a fast and cheap whole-cell technology for the enzymatic characterisation of CYP11B2 mutations. The principle of the new system is the heterologous expression of the mutants of CYP11B2 in fission yeast (Schizosaccharomyces pombe) followed by steroid bioconversion assays for the enzymatic characterisation of the investigated mutants. The new system was validated and 10 known mutations of CYP11B2 have been investigated, two of them for the first time concerning their effect on the CYP11B2 three-step reaction. The results of the fission yeast system were in good agreement with the cell culture results presenting this new system as an alternative non radioactive method that can be applied for the enzymatic characterisation of CYP11B2 mutations.

Congenital Hyperreninemic Hypoaldosteronism in Israel: Sequence Analysis of CYP11B2 Gene

BACKGROUND/AIMS

Isolated aldosterone biosynthesis defect causing congenital hyperreninemic hypoaldosteronism with otherwise normal adrenal function usually results from aldosterone synthase deficiency. Patients present with manifestations of mineralocorticoid deficiency during the first weeks of life. The largest numbers of cases have been described in Iranian Jews, who carried concomitantly two homozygous missense mutations (R181W and V386A). In a few cases with presumed aldosterone synthase deficiency no mutations in CYP11B2 gene have been identified. We describe a molecular and endocrine evaluation of seven cases of congenital hyperreninemic hypoaldosteronism in Israel.

PATIENTS/METHODS

Two of the six Jewish patients are of Iranian origin. The parents of five other patients originated from Yemen, Syria and Morocco. One patient is a Muslim-Arab. CYP11B2's exons, exon-intron boundaries and promoter region were sequenced by multiple PCR amplifications. Gene size determination was performed either by long-range PCR or by Southern blot analysis.

RESULTS

Only two patients (Iranian Jews) carried a known homozygous R181W, V386A mutations, other two were compound heterozygotes for either the R181W or V386A and one additional novel amino acid substitution (A319V or D335G), and one patient was found to be a carrier of the two novel variations (A319V and D335G). We could not find a molecular defect in 2 patients: one was a carrier of the D335G mutation and the other had no detectable molecular change in the coding and promoter regions.

CONCLUSION

The genetic and molecular basis of congenital hyperreninemic hypoaldosteronism is more heterogeneous than previously described. The significance of amino acid substitutions identified in this study remains to be determined.

A missense mutation (GGC[435Gly]-->AGC[Ser]) in exon 8 of the CYP11B2 gene inherited in Japanese patients with congenital hypoaldosteronism

OBJECTIVES

To clarify the underlying molecular mechanism of corticosterone methyl oxidase type II (CMO II) deficiency, Japanese patients newly diagnosed with CMO II deficiency were investigated.

METHODS

We analyzed the patients' genomic DNA sequence on all 9 exons of the CYP11B2 gene. In addition, restriction fragment length polymorphism (RFLP) analysis and expression studies were performed.

RESULTS

The analysis showed that the patients homozygously retained a missense mutation, Gumacr;GC[435Gly]-->Aumacr;GC[Ser], in the CYP11B2 gene. Expression studies indicated that the steroid 18-hydroxylase/oxidase activities of the mutant enzyme were substantially reduced.

CONCLUSION

These results support the hypothesis that this mutation causes CMO II deficiency in the patients, and are in accordance with our theory that the partial loss of P-450(C18) activities causes CMO II deficiency.

Aldosterone synthase deficiency and related disorders

Aldosterone's main actions are to regulate intravascular volume and serum electrolytes by controlling sodium absorbtion and potassium excretion in the distal nephron. Inherited defects in aldosterone biosynthesis thus cause hypovolemia, hyponatremia and hyperkalemia. Defective aldosterone biosynthesis may be caused by congenital adrenal hyperplasia due to 21-hydroxylase (CYP21) deficiency, in which case cortisol biosynthesis is also affected, or as an isolated defect termed aldosterone synthase (corticosterone methyloxidase, CYP11B2) deficiency. Many mutations have been documented in each of these genes; in general enzymatic activity must be reduced to <1% of normal for aldosterone biosynthesis to be impaired. An additional form of familial hyperreninemic hypoaldosteronism has been described that is not due to mutations in CYP11B2, but its etiology remains to be elucidated.

Congenital hyperreninemic hypoaldosteronism unlinked to the aldosterone synthase (CYP11B2) gene

Isolated hyperreninemic hypoaldosteronism presenting in infancy is usually caused by mutations in the CYP11B2 gene encoding aldosterone synthase. We studied five patients in four unrelated kindreds with hyperreninemic hypoaldosteronism, in whom we were unable to find such mutations. All presented in infancy with failure to thrive, hyponatremia, hyperkalemia, markedly elevated plasma renin activity, and low or inappropriately normal aldosterone levels. All had normal cortisol levels and no signs or symptoms of congenital adrenal hyperplasia. All responded to fludrocortisone treatment. There were no mutations detected in exons or splice junctions of CYP11B2. Linkage of the disorder to CYP11B2 was studied in two unrelated consanguineous patients and in an affected sib pair. The consanguineous patients were each heterozygous for at least one of three polymorphic microsatellite markers near CYP11B2, excluding linkage to CYP11B2. However, linkage of the disease to CYP11B2 could not be excluded in the affected sib pair. Genes involved in the regulation of aldosterone biosynthesis, including those encoding angiotensinogen, angiotensin-converting enzyme, and the AT1 angiotensin II receptor were similarly excluded from linkage. These results demonstrate the existence of an inherited form of hyperreninemic hypoaldosteronism distinct from aldosterone synthase deficiency. The affected gene(s) remain to be determined.

[A study of mutation(s) of the epithelial sodium channel gene in a Liddle's syndrome family]

OBJECTIVE

To study the genetic mechanism of Liddle's syndrome in a family and help the diagnosis (specially ante-natal) and treatment of Liddle's syndrome clinically.

METHODS

(1) Physical examination and measurement of serum potassium, plasma aldosterone and renin activity respectively by biochemical and radioimmunological assays were made for the member of the family. (2) Venous blood samples were collected from the members of the family and total genomic DNA was prepared. One set of specific primers was used for direct polymerase chain reaction, for amplifying C terminus of beta-subunit of epithelial sodium channel (hbetaENaC). Polymerase chain reaction products were subjected to single-strand conformation polymorphism and direct DNA sequence analysis.

RESULTS

A new frameshift mutation (1bp, INS, 600G) of the gene of C terminus of beta-subunit of hbetaENaC was found in the Liddle's syndrome family. This mutation introduced a new stop codon at position 605 and deleted the last 34 normal amino acids from the C teminus of hbeta ENaC. Eight genetically affected subjects had severe hypertension and suppressed levels of plasma aldosterone and plasma renin activity; half of them had hypokalemia.

CONCLUSION

The frameshift mutation (1bp, INS, 600G) of hbetaENaC gene is the likely cause of Liddle's syndrome in this Chinese family.

Homozygous deletion of arginine-173 in the CYP11B2 gene in a girl with congenital hypoaldosteronism. Corticosterone methyloxidase deficiency type II

The first child of consanguineous parents presented with failure to thrive and feeding problems at age 6 weeks. Important laboratory findings were low plasma sodium and elevated potassium and renin. Salt wasting was caused by an enzymatic defect in the terminal aldosterone biosynthesis. The biochemical diagnosis of corticosterone methyloxidase (CMO) deficiency type II was established on the basis of plasma multisteroid analysis, showing a pathologic increase of 18-OH-corticosterone/aldosterone ratio. Sequence analysis of the CYP11B2 gene which encodes aldosterone synthase (P450c11Aldo), the enzyme required for the terminal steps in aldosterone biosynthesis, revealed a hitherto undescribed homozygous deletion of codon 173. CYP11B2 is polymorphic at this position, encoding arginine or lysine. Both parents were heterozygous carriers of the mutation. Amino acid residue 173 in P450c11Aldo is positioned in alpha-helix D. We presume that the secondary structure of the enzyme is changed by the single amino acid deletion. This report describes a novel mutation in the CYP11B2 gene, the third known mutation associated with CMO deficiency type II.

Functional expression of a pseudohypoaldosteronism type I mutated epithelial Na+ channel lacking the pore-forming region of its alpha subunit

The autosomal recessive form of type I pseudohypoaldosteronism (PHA-I) is an inherited salt-losing syndrome resulting from diminution-of-function mutations in the 3 subunits of the epithelial Na+ channel (ENaC). A PHA-I stop mutation (alpha(R508stop)) of the ENaC alpha subunit is predicted to lack the second transmembrane domain and the intracellular COOH-terminus, regions of the protein involved in pore function. Nonetheless, we observed a measurable Na+ current in Xenopus laevis oocytes that coexpress the beta and gamma subunits with the truncated alpha subunit. The mutant alpha was coassembled with beta and gamma subunits and was present at the cell surface at a lower density, consistent with the lower Na+ current seen in oocytes with the truncated alpha subunit. The single-channel Na+ conductance for the mutant channel was only slightly decreased, and the appearance of the macroscopic currents was delayed by 48 hours with respect to wild-type. Our data suggest novel roles for the alpha subunit in the assembly and targeting of an active channel to the cell surface, and suggest that channel pores consisting of only the beta and gamma subunits can provide significant residual activity. This activity may be sufficient to explain the absence of a severe pulmonary phenotype in patients with PHA-I.

Hypertension

The monogenic forms of human hypertension have yielded to the power of modern genetic techniques in the last several years. With the successful expression cloning of the subunits of the epithelial sodium channel, a whole era has evolved in our basic understanding of the low renin forms of human hypertension. Of note, all of these hypertensive syndromes (Liddle's syndrome, glucocorticoid-remediable aldosteronism, and the apparent mineralocorticoid excess syndrome) share an underlying dysregulation of the activity of the epithelial sodium channel in the cortical collecting tubule. Loss of function defects due to mutations in the channel subunits themselves, or in the mineralocorticoid receptor (pseudohypoaldosteronism, type I) also affect blood pressure regulation consequent to renal salt wasting and dysregulation of the epithelial sodium channel in the cortical collecting tubule.

Molecular genetic study in two patients with congenital hypoaldosteronism (types I and II) in relation to previously published hormonal studies

We performed a molecular genetic study in two patients with congenital hypoaldosteronism. An original study of these patients was published in this Journal in 1982. Both index cases, a girl (patient 1) and a boy (patient 2). presented with salt-wasting and failure to thrive in the neonatal period. Parents of patient 1 were not related, whereas the parents of patient 2 were cousins. Endocrine studies had shown a defect in 18-oxidation of 18-OH-corticosterone in patient 1 and a defect in the 18-hydroxylation of corticosterone in patient 2. Plasma aldosterone was decreased in both patients, whereas 18-OH-corticosterone was elevated in patient 1 and decreased in patient 2. Plasma corticosterone and 11-deoxycorticosterone were elevated in both patients, whereas cortisol and its precursors were in the normal range. According to the nomenclature proposed by Ulick, the defects are termed corticosterone methyl oxidase (CMO) deficiency type II in patient 1, and type I in patient 2 respectively. Genetic defects in the gene CYP11B2 encoding aldosterone synthase have been described in a few cases. In patient 1, we identified only one heterozygous amino acid substitution (V386A) in exon 7, which has no deleterious effect on the enzyme activity. In patient 2 and his older brother, we identified a homozygous single base exchange (G to T) in codon 255 (GAG), causing a premature stop codon E255X (TAG). The mutant enzyme has lost the five terminal exons containing the haem binding site, and is thus a loss of function enzyme. This is only the second report of a patient with CMO deficiency type II without a mutation in the exons and exon-intron boundaries, whereas the biochemical phenotype of the two brothers with CMO deficiency type I can be explained by the patient's genotype.

Mutation THR-185 ILE is associated with corticosterone methyl oxidase deficiency type II

Two boys presenting with infection-triggered, life-threatening salt-loss and hyperkalaemia were published in 1991 in the European Journal of Pediatrics. In both boys, the diagnosis of corticosterone methyl oxidase (CMO) deficiency type II has been established on the basis of determinations of plasma and urinary steroids. We had the opportunity to perform a molecular genetic study in one of the two boys. This boy had an elevated plasma 18-hydroxycorticosterone/aldosterone ratio which is pathognomonic for CMO deficiency type II. Sequence analysis of the CYP11B2 gene revealed a homozygous single base exchange in codon 185 of CYP11B2 causing an amino acid substitution Thr185Ile.

CONCLUSION

A Thr185Ile mutation in the CYP11B2 gene was found in a patient with CMO deficiency type II. This mutation may change the secondary structure of the enzyme leading to its decreased activity.

Genotype-phenotype analysis of a newly discovered family with Liddle's syndrome

OBJECTIVE

To investigate the clinical, biologic, and molecular abnormalities in a family with Liddle's syndrome and analyze the short- and long-term efficacies of amiloride treatment.

PATIENTS

The pedigree consisted of one affected mother and four children, of whom three suffered from early-onset and moderate-to-severe hypertension.

METHODS

In addition to the biochemical and hormonal measurements, genetic analysis of the carboxy terminus of the beta subunit of the epithelial sodium channel (beta ENaC) was conducted through single-strand conformation analysis and direct sequencing. The functional properties of the mutation were analyzed using the Xenopus expression system and compared with one mutation affecting the proline-rich sequence of the beta ENaC.

RESULTS

Mild hypokalemia and suppressed levels of plasma renin and aldosterone were observed in all affected subjects. Administration of 10 mg/day amiloride for 2 months normalized the blood pressure and plasma potassium levels of all of the affected subjects, whereas their plasma and urinary aldosterone levels remained surprisingly low. A similar pattern was observed after 11 years of follow-up, but a fivefold increase in plasma aldosterone was observed under treatment with 20 mg/day amiloride for 2 weeks. Genetic analysis of the beta ENaC revealed a deletion of 32 nucleotides that had modified the open reading frame and introduced a stop codon at position 582. Expression of this beta 579del32 mutant caused a 3.7 +/- 0.3-fold increase in the amiloride-sensitive sodium current, without modification of the unitary properties of the channel. A similar increase was elicited by one mutation affecting the carboxy terminus of the beta ENaC.

CONCLUSIONS

This new mutation leading to Liddle's syndrome highlights the importance of the carboxy terminus of the beta ENaC in the activity of the epithelial sodium channel. Small doses of amiloride are able to control the blood pressure on a long-term basis in this monogenic form of hypertension.

Abnormalities of aldosterone synthesis and action in children

Genetic defects in aldosterone biosynthesis and action affect blood pressure and electrolyte homeostasis. Aldosterone synthase deficiency, salt-wasting forms of congenital adrenal hyperplasia, and adrenal hypoplasia congenita all cause aldosterone deficiency, signs of which include hyponatremia, hyperkalemia, hypovolemia, elevated plasma renin activity, and sometimes shock and death. Conversely, the inappropriate regulation of aldosterone synthesis seen in glucocorticoid-suppressible hyperaldosteronism may cause hypokalemia, suppressed plasma renin activity, and hypertension. Similar problems occur when the normal ligand specificity of the aldosterone receptor is lost, as in the syndrome of apparent mineralocorticoid excess due to 11 beta-hydroxysteroid dehydrogenase deficiency. The effects of aldosterone are mediated largely through activation of the epithelial sodium channel, and inactivating or activating mutations of this channel leads to signs of mineralocorticoid deficiency or excess, termed pseudohypoaldosteronism and Liddle's syndrome, respectively.

Molecular biology of 11β-hydroxylase and 11β-hydroxysteroid dehydrogenase enzymes

There are two steroid 11β-hydroxylase isozymes encoded by the CYP11B1 and CYP11B2 genes on human chromosome 8q. The first is expressed at high levels in the normal adrenal gland, has 11β-hydroxylase activity and is regulated by ACTH. Mutations in the corresponding gene cause congenital adrenal hyperplasia due to 11β-hydroxylase deficiency; thus, this isozyme is required for cortisol biosynthesis. The second isozyme is expressed at low levels in the normal adrenal gland but at higher levels in aldosterone-secreting tumors, and has 11β-hydroxylase, 18-hydroxylase and 18-oxidase activities. The corresponding gene is regulated by angiotensin II, and mutations in this gene are found in persons who are unable to synthesize aldosterone due to corticosterone methyloxidase II deficiency. Thus, this isozyme is required for aldosterone biosynthesis. Cortisol and aldosterone are both effective ligands of the "mineralocorticoid" receptor in vitro, but only aldosterone is a potent mineralocorticoid in vivo. This apparent specificity occurs because 11β-hydroxysteroid dehydrogenase in the kidney converts cortisol to cortisone, which is not a ligand for the receptor. This enzyme is a "short-chain" dehydrogenase which is encoded by a single gene on human chromosome 1. It is possible that mutations in this gene cause a form of childhood hypertension called apparent mineralocorticoid excess, in which the mineralocorticoid receptor is not protected from high concentrations of cortisol.