Congenitally defective aldosterone biosynthesis in humans: the involvement of point mutations of the P-450C18 gene (CYP11B2) in CMO II deficient patients.

The gene for steroid 18-hydroxylase (P-450C18) has been recently assigned to encode corticosterone methyl oxidases Type I and Type II which were previously postulated to catalyze the final two steps in the biosynthesis of aldosterone in humans. Molecular genetic analysis of the P-450C18 gene is three patients from three different families affected with CMO II deficiency has indicated that a point mutation of CGG----TGG (181Arg----Trp) in exon 3 and one of GTG----GCG (386Val----Ala) in exon 7 occur exclusively in the gene of the patients. Analysis of PCR products by restriction enzymes (HapII and HphI) has indicated that the patients are homozygous and the unaffected parent is heterozygous for both mutations, in accordance with the established concept that CMO II deficiency is inherited in an autosomal recessive manner. These data clearly provide the molecular genetic basis for the characteristic biochemical phenotype of CMO II clinical variants.     

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.     

Aldosterone deficiency II (CMO II deficiency) is not the result of a mutation of an MspI restriction site within the CYP11B gene

Summary We report our investigations of a German family with aldosterone deficiency (CMO II deficiency). Restriction fragment length polymorphism analysis using a P450c11 probe demonstrates that aMspI restriction site mutation within the CYP11B gene cannot be the underlying cause for this defect, as has been suggested previously.     

Disorders of the aldosterone synthase and steroid 11beta-hydroxylase deficiencies

The most potent corticosteroids are 11beta-hydroxylated compounds. In humans, two cytochrome P450 isoenzymes with 11beta-hydroxylase activity, catalysing the biosynthesis of cortisol and aldosterone, are present in the adrenal cortex. CYP11B1, the gene encoding 11beta-hydroxylase (P450c11), is expressed on high levels in the zona fasciculata and is regulated by ACTH. CYP11B2, the gene encoding aldosterone synthase (P450c11Aldo), is expressed in the zona glomerulosa under primary control of the renin-angiotensin system. Aldosterone synthase has 11beta-hydroxylase activity as well as 18-hydroxylase activity and 18-oxidase activity. The substrate for CYP11B2 is 11-deoxycorticosterone, that of CYP11B1 is 11-deoxycortisol. Mutations in CYP11B1 cause congenital adrenal hyperplasia (CAH) due to 11beta-hydroxylase deficiency. This disorder is characterized by androgen excess and hypertension. Mutations in CYP11B2 cause congenital hypoaldosteronism (aldosterone synthase deficiency) which is characterized by life-threatening salt loss, failure to thrive, hyponatraemia and hyperkalaemia in early infancy. Both disorders have an autosomal recessive inheritance. Classical and nonclassical forms of 11beta-hydroxylase deficiency can be distinguished. Studies in heterozygotes for classical 11beta-hydroxylase deficiency show inconsistent results with no or only mild hormonal abnormalities (elevated plasma levels of 11-deoxycortisol after ACTH stimulation). In infants with congenital hypoaldosteronism, a comparable frequency of 18-hydroxylase deficiency (aldosterone synthase deficiency type I) and of 18-oxidase deficiency (aldosterone synthase deficiency type II) can be found. Molecular genetic studies of the CYP11B1 and CYP11B2 genes in 11beta-hydroxylase deficiency or aldosterone synthase deficiency have led to the identification of several mutations. Transfection experiments showed loss of enzyme activity in vitro. In some of the patients with 18-oxidase deficiency (aldosterone synthase deficiency type II) no mutations in the CYP11B2 gene were identified. Refined methods for steroid determination are the basis for the diagnosis of inborn errors of steroidogenesis. Molecular genetic studies are complementary; on the one hand, they have practical importance for the prenatal diagnosis of virilizing CAH forms and on the other hand, they are of theoretical importance in terms of our understanding of the functioning of cytochrome P450 enzymes. Copyrightz1999S.KargerAG, Basel     

Inhibitory effects of digoxin and ouabain on aldosterone synthesis in human adrenocortical NCI-H295 cells

The present study was to investigate the effects and action mechanisms of digoxin and ouabain on steroidogenesis in human adrenocortical NCI-H295 cells. Administration of digoxin or ouabain for 24 h decreased the basal and angiotensin II (Ang II)-stimulated release of aldosterone by NCI-H295 cells. The conversions of corticosterone (substrate of cytochrome P450 aldosterone synthase, P450c11AS) to aldosterone or deoxycortisol (substrate of cytochrome P450 11beta-hydroxylase, P450c11beta) to cortisol were reduced by digoxin or ouabain. The basal and 22-hydroxy-cholesterol (a membrane-permeable cholesterol, substrate of cytochrome P450 side-chain cleavage enzyme, P450scc)-stimulated pregnenolone release in mitochondria was inhibited by digoxin or ouabain. Digoxin or ouabain suppressed the basal and Ang II-stimulated protein expression of steroidogenic acute regulatory (StAR) protein and P450scc. Incubation of digoxin or ouabain for 24 h reduced P450c11AS mRNA expression in NCI-H295 cells. Digoxin or ouabain (10(-6) M, 24 h)-treated cells showed a lower resting intracellular Ca2+ concentration ([Ca2+]i) and an attenuated response of [Ca2+]i to Ang II. Since no significant cytotoxicity was observed at 10(-6) M digoxin or ouabain, the digoxin- or ouabain-induced decrease of aldosterone or cortisol release was not associated with cytotoxicity. These results demonstrate that digoxin or ouabain inhibits the aldosterone or cortisol release via reduction of P450c11AS or P450c11beta and P450scc activities, inhibition of StAR and P450scc protein expression, suppression of P450c11AS mRNA expression, and attenuation of Ca2+ mobilization in NCI-H295 cells.     

Determination of functional effects of mutations in the steroid 21-hydroxylase gene (CYP21) using recombinant vaccinia virus

Steroid 21-hydroxylase (P450c21) is absent or defective in more than 90% of patients with congenital adrenal hyperplasia. This disorder of cortisol biosynthesis occurs in a wide spectrum of clinical severity; specific mutations in the 21-hydroxylase gene (CYP21) have been found in association with particular clinical phenotypes. To determine the functional effects of mutations causing amino acid substitutions, normal P450c21 and three mutagenized P450c21 enzymes were expressed at high levels in cultured COS-1 cells using recombinant vaccinia virus. A single amino acid substitution (Val281----Leu) present in patients with mild "nonclassical" 21-hydroxylase deficiency resulted in an enzyme with 20-50% of normal activity. A mutation (Ile172----Asn) identified in patients with the "simple virilizing" form (poor cortisol synthesis but adequate aldosterone synthesis) resulted in an enzyme with less than 2% of normal activity. Finally, a cluster mutation (Ile-Val-Glu-Met234-238----Asn-Glu-Glu-Lys) found in a patient with severe "salt wasting" 21-hydroxylase deficiency (inadequate aldosterone synthesis) results in an enzyme with no detectable activity. These data indicate that the severity of 21-hydroxylase deficiency correlates with the degree of enzymatic compromise.     

A case of primary selective hypoaldosteronism carrying three mutations in the aldosterone synthase (Cyp11b2) gene

An infant with a clinical phenotype of early onset hypoaldosteronism has been screened for mutation analysis of the Cyp11b2 gene encoding aldosterone synthase enzyme. We have described a novel nonsense mutation in exon 3 (c.508C>T) that gave rise to a shorter protein (Q170X) and two known concurrent missense mutations (c.594A>C in exon 3 and c.1157T>C in exon 7) that led to substitution of glutamic acid for aspartic acid at amino acid position 198 (E198D) and of valine for alanine at amino acid position 386 (V386A). The father, who carried E198D plus V386A mutations, showed a fractional sodium excretion of 1.25% that was unmodified by dietary salt restriction, suggesting a mild haploinsufficiency. We examined by in silico analysis the effect of the mutations on the secondary and tertiary structures of aldosterone synthase to explain the inefficient enzymatic activity. The Q170X mutation produced a truncated protein, which was consequently associated with a loss of catalytic activity. As predicted by JPred web system and Dock 6.3 software, the concurrent expression of E198D and V386A mutations induced a significant secondary structure rearrangement and a shift of the heme group and the 18-hydroxycorticosterone substrate from their optimal placement.     

Molecular genetic studies on the biosynthesis of aldosterone in humans

Corticosterone methyl oxidase Type I (CMO I) and II (CMO II) have been postulated to be the enzymes involved in the final two steps of aldosterone biosynthesis in humans. We have isolated human cDNAs for P450c11 and P450c18 as well as the corresponding genes, CYP11B1 and CYP11B2. Both protein products of these two genes as expressed in COS-7 cells exhibit steroid 11β-hydroxylase activity, but only P450c18, a product of CYP11B2, carried steroid 18-hydroxylase activity to form aldosterone. These results indicate that CYP11B2 encodes CMO, the actual catalytic function of which is retained by P450c18, a multifunctional enzyme. This conclusion is further supported by the finding that the P450c18 gene, CYP11B2, is mutated at several different loci in patients deficient in CMO I or II.     

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.     

Steroid 21-hydroxylase (P450c21): a new allele and spread of mutations through the pseudogene

Lesions in the gene encoding the adrenal enzyme steroid 21-hydroxylase (P450c21) result in defective adrenal cortisol synthesis, often accompanied by aldosterone deficiency. The symptoms range from severe neonatal disease to inconspicuous symptoms in adulthood depending on the nature of the mutations. The 21-hydroxylase gene is present in close proximity to a highly homologous pseudogene, and both genes show variation in copy number between individuals. For complete DNA sequence characterization, we have applied selective polymerase chain reaction amplification and direct sequencing of all full-length steroid 21-hydroxylase genes present in individuals. Using healthy individuals with only one remaining steroid 21-hydroxylase allele as normal references, a new allele was found in two siblings, in whom clinical and laboratory findings demonstrated moderate enzyme deficiency. Full-length sequencing of this allele displayed an Arg 484 to Pro codon change in exon 10, in the same position as a previously identified GG to C mutation found in a patient with severe 21 -hydroxylase deficiency. Arg 484 is located within a stretch of amino acids that are highly conserved between mammalian 21-hydroxylases. The finding of the presently reported 21-hydroxylase allele indicates that the GG to C mutation from the severely affected patient has arisen by a two-step mechanism, consisting of a G to C transversion accompanied by an adjacent G deletion. When sequencing 26 pseudogenes, both these mutations, which are not present in the pseudogenes hitherto reported, were found at low frequency together with a number of other polymorphisms. Thus, also rare mutations can spread via the pseudogene and can therefore be expected to arise independently in unrelated individuals.     

A case of primary selective hypoaldosteronism carrying three mutations in the aldosterone synthase (Cyp11b2) gene

An infant with a clinical phenotype of early onset hypoaldosteronism has been screened for mutation analysis of the Cyp11b2 gene encoding aldosterone synthase enzyme. We have described a novel nonsense mutation in exon 3 (c.508 C > T) that gave rise to a shorter protein (Q170X) and two known concurrent missense mutations (c.594A > C in exon 3 and c.1157 T > C in exon 7) that led to substitution of glutamic acid for aspartic acid at amino acid position 198 (E198D) and of valine for alanine at amino acid position 386 (V386A). The father, who carried E198D plus V386A mutations, showed a fractional sodium excretion of 1.25% that was unmodified by dietary salt restriction, suggesting a mild haploinsufficiency.     

Aldosterone synthase deficiency type I: hormonal and genetic analyses of two cases

Two female infants with clinical and biochemical findings of mineralocorticoid deficiency are presented. Both cases were diagnosed with aldosterone synthase type-I deficiency by multisteroid analysis. Two different mutations were located in the CYP11B2 gene: two homozygous amino acid substitutions (E188D and V386A) in case 1, and one heterozygous substitution (L324Q) and one heterozygous stop mutation (Y265X) in case 2.     

A missense mutation at Ile172----Asn or Arg356----Trp causes steroid 21-hydroxylase deficiency

Congenital adrenal hyperplasia (CAH) is a common recessive genetic disease caused mainly by steroid 21-hydroxylase (P450c21) deficiency. Many forms of CAH exist resulting from various mutations of the CYP21B gene. We sequenced CYP21B cDNA from a normal person and its genes from a patient with simple virilizing CAH. When comparing several CYP21B sequences, we found it was polymorphic. In the patient, a single base substitution replaced Ile172 (ATC) with Asn (AAC) in one allele while Arg356 (CGG) was converted to Trp (TGG) in the other. A normal P450c21 cDNA clone was transfected into COS-1 cells to produce 21-hydroxylase activity toward its substrates, progesterone and 17-hydroxyprogesterone. Mutants corresponding to Asn172 or Trp356 mutation were constructed by site-directed mutagenesis of the normal c21 cDNA clone. They failed to produce active enzyme toward either substrate upon transfection into COS-1 cells, demonstrating that these mutations caused CAH. Aligning sequences with other P450s, Ile172 could be located in the membrane anchoring domain and Arg356 in the substrate-binding site of P450c21. Both mutations are present in the CYP21A1P pseudogene, suggesting that they may be transferred from CYP21A1P by gene conversion events.     

Molecular and endocrine characterization of a mutation involving a recombination between the steroid 21-hydroxylase functional gene and pseudogene

The gene encoding steroid 21-hydroxylase activity, P450c21B, is located in the major histocompatibility complex (MHC) class III region, in close proximity to a highly homologous pseudogene, P450c21A. Recombinations between P450c21B and P450c21A have been shown to result in deficiency of 21-hydroxylase activity, the usual cause of congenital adrenal hyperplasia (CAH). A mutant P450c21 gene from a patient with simple virilizing CAH was identified and shown to be consistent with a recombination between P450c21A and P450c21B. Sequence analysis of the mutant gene showed the recombination site to be located between the first exon and the second intron. The mutant gene encodes a leucine instead of the normal proline at codon 31. This mutation resides on a chromosome bearing the HLA-B44 serotype. A comparison of mutation associated with HLA-B44 and that normally found with the HLA-Bw47 serotype suggests that the HLA-B44 mutations are of more ancient origin. The patient's homologous chromosome has a deletion of P450c21B. Endocrinological testing therefore allows for testing of the mutant gene in genetic isolation. Such testing demonstrated that the patient was capable of producing aldosterone and retaining sodium in response to a low-sodium diet, indicating that the mutant gene encodes an enzyme with partial 21-hydroxylase activity.     

Adaptation of aldosterone biosynthesis to sodium and potassium intake in the rat

The steroidogenic response of rat adrenal zona glomerulosa to stimulators is variable and depends on the activity of biosynthetic steps involved in the conversion of deoxycorticosterone (DOC) to aldosterone (Aldo). Corticosterone methyl oxidations (CMO) 1 and 2 are stimulated by sodium restriction and suppressed by potassium restriction. These slow alterations are accompanied by the appearance or disappearance of a specific zona glomerulosa mitochondrial protein with a molecular weight of 49,000. Induction of CMO 1 and 2 activities and the appearance of the 49 K protein can also be elicited in vitro by culture of rat zone glomerulosa cells in a medium with a high potassium concentration. The 49 K protein crossreacts with a monoclonal antibody raised against purified bovine adrenal cytochrome P-450(11 beta). The same antibody stains a protein with a molecular weight of 51,000 in rat zona fasciculata mitochondria and in zone glomerulosa mitochondria of rats in which CMO 1 and 2 activities have been suppressed by potassium restriction and sodium loading. The 51 K crossreactive protein was purified to electrophoretic homogeneity by chromatography on octyl-sepharose. In a reconstituted enzyme system, it converted DOC to corticosterone (B) and to 18-hydroxy-11-deoxycorticosterone (18-OH-DOC) but not to 18-hydroxycorticosterone (18-OH-B) or Aldo. A partially purified 49 K protein preparation from zona glomerulosa mitochondria of rats kept on a low-sodium, high-potassium regimen converted DOC to B, 18-OH-DOC, 18-OH-B and Aldo. According to these results, rat adrenal cytochrome P-450(11 beta) exists in two different forms, with both of them capable of hydroxylating DOC in either the 11 beta- of the 18-position, but with only the 49 K form capable of catalyzing CMO 1 and 2. The adaptation of aldosterone biosynthesis to sodium deficiency or potassium intake in rats is due to the appearance of the 49 K form of the enzyme in zona glomerulosa mitochondria.     

Molecular basis of 17α-hydroxylase/17,20-lyase deficiency

17α-Hydroxylase deficiency is characterized by a defect in either or both of 17α-hydroxylase and 17,20-lyase activities, based on the fact that a single polypeptide P450c17 can catalyze both reactions. The clinical manifestations of 17α-hydroxylase/17,20-lyase deficiency seem to be more heterogeneous than expected, varying from the classical type to less symptomatic forms as also observed in 21-hydroxylase deficiency. We have sequenced all eight exons of the CYP17 (P450c17) gene in DNA from several patients, reconstructed the mutations in a human P450c17 cDNA and expressed the mutant P450c17 in COS 1 cells to characterize the kinetic properties of 17α-hydroxylase and 17,20-lyase activities. The molecular bases of cases clinically reported as 17α-hydroxylase deficiency have turned out to be complete or partial combined deficiencies of 17α-hydroxylase/17,20-lyase. The elucidation of the molecular basis generally explains the patient's clinical profiles including the sexual phenotype of the external genitalia. In one case clinically reported as isolated 17,20-lyase deficiency, the molecular basis was found to be partial combined deficiency of both activities, somewhat discordant with the patient's clinical profile. Based on the results obtained so far we can predict that those 17α-hydroxylase deficient individuals having a homozygous stop codon in the CYP17 gene positioned at the amino terminal side of the P450c17 heme-binding cysteine (442) will all have the same phenotype. However those individuals having homozygous missense mutations or those who are compound heterozygotes having a missense mutation in at least one CYP17 allele will display their own unique phenotype which clinically will be subtly different from all others.     

Identification of novel mutations in HEXA gene in children affected with Tay Sachs disease from India

Tay Sachs disease (TSD) is a neurodegenerative disorder due to β-hexosaminidase A deficiency caused by mutations in the HEXA gene. The mutations leading to Tay Sachs disease in India are yet unknown. We aimed to determine mutations leading to TSD in India by complete sequencing of the HEXA gene. The clinical inclusion criteria included neuroregression, seizures, exaggerated startle reflex, macrocephaly, cherry red spot on fundus examination and spasticity. Neuroimaging criteria included thalamic hyperdensities on CT scan/T1W images of MRI of the brain. Biochemical criteria included deficiency of hexosaminidase A (less than 2% of total hexosaminidase activity for infantile patients). Total leukocyte hexosaminidase activity was assayed by 4-methylumbelliferyl-N-acetyl-β-D-glucosamine lysis and hexosaminidase A activity was assayed by heat inactivation method and 4-methylumbelliferyl-N-acetyl-β-D-glucosamine-6-sulphate lysis method. The exons and exon-intron boundaries of the HEXA gene were bidirectionally sequenced using an automated sequencer. Mutations were confirmed in parents and looked up in public databases. In silico analysis for mutations was carried out using SIFT, Polyphen2, MutationT@ster and Accelrys Discovery Studio softwares. Fifteen families were included in the study. We identified six novel missense mutations, c.340 G>A (p.E114K), c.964 G>A (p.D322N), c.964 G>T (p.D322Y), c.1178C>G (p.R393P) and c.1385A>T (p.E462V), c.1432 G>A (p.G478R) and two previously reported mutations. c.1277_1278insTATC and c.508C>T (p.R170W). The mutation p.E462V was found in six unrelated families from Gujarat indicating a founder effect. A previously known splice site mutation c.805+1 G>C and another intronic mutation c.672+30 T>G of unknown significance were also identified. Mutations could not be identified in one family. We conclude that TSD patients from Gujarat should be screened for the common mutation p.E462V.     

Diversity and function of mutations in p450 oxidoreductase in patients with Antley-Bixler syndrome and disordered steroidogenesis

P450 oxidoreductase (POR) is the obligatory flavoprotein intermediate that transfers electrons from reduced nicotinamide adenine dinucleotide phosphate (NADPH) to all microsomal cytochrome P450 enzymes. Although mouse Por gene ablation causes embryonic lethality, POR missense mutations cause disordered steroidogenesis, ambiguous genitalia, and Antley-Bixler syndrome (ABS), which has also been attributed to fibroblast growth factor receptor 2 (FGFR2) mutations. We sequenced the POR gene and FGFR2 exons 8 and 10 in 32 individuals with ABS and/or hormonal findings that suggested POR deficiency. POR and FGFR2 mutations segregated completely. Fifteen patients carried POR mutations on both alleles, 4 carried mutations on only one allele, 10 carried FGFR2 or FGFR3 mutations, and 3 patients carried no mutations. The 34 affected POR alleles included 10 with A287P (all from whites) and 7 with R457H (four Japanese, one African, two whites); 17 of the 34 alleles carried 16 "private" mutations, including 9 missense and 7 frameshift mutations. These 11 missense mutations, plus 10 others found in databases or reported elsewhere, were recreated by site-directed mutagenesis and were assessed by four assays: reduction of cytochrome c, oxidation of NADPH, support of 17alpha-hydroxylase activity, and support of 17,20 lyase using human P450c17. Assays that were based on cytochrome c, which is not a physiologic substrate for POR, correlated poorly with clinical phenotype, but assays that were based on POR's support of catalysis by P450c17--the enzyme most closely associated with the hormonal phenotype--provided an excellent genotype/phenotype correlation. Our large survey of patients with ABS shows that individuals with an ABS-like phenotype and normal steroidogenesis have FGFR mutations, whereas those with ambiguous genitalia and disordered steroidogenesis should be recognized as having a distinct new disease: POR deficiency.     

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.