Artificial mutations in P450c11AS (aldosterone synthase) can increase enzymatic activity: a model for low-renin hypertension?

Hedgehog signaling and steroidogenesis

Since its discovery nearly 30 years ago, the Hedgehog (Hh) signaling pathway has been shown to be pivotal in many developmental and pathophysiological processes in several steroidogenic tissues, including the testis, ovary, adrenal cortex, and placenta. New evidence links the evolutionarily conserved Hh pathway to the steroidogenic organs, demonstrating how Hh signaling can influence their development and homeostasis and can act in concert with steroids to mediate physiological functions. In this review, we highlight the role of the components of the Hh signaling pathway in steroidogenesis of endocrine tissues.

The molecular biology, biochemistry, and physiology of human steroidogenesis and its disorders

Steroidogenesis entails processes by which cholesterol is converted to biologically active steroid hormones. Whereas most endocrine texts discuss adrenal, ovarian, testicular, placental, and other steroidogenic processes in a gland-specific fashion, steroidogenesis is better understood as a single process that is repeated in each gland with cell-type-specific variations on a single theme. Thus, understanding steroidogenesis is rooted in an understanding of the biochemistry of the various steroidogenic enzymes and cofactors and the genes that encode them. The first and rate-limiting step in steroidogenesis is the conversion of cholesterol to pregnenolone by a single enzyme, P450scc (CYP11A1), but this enzymatically complex step is subject to multiple regulatory mechanisms, yielding finely tuned quantitative regulation. Qualitative regulation determining the type of steroid to be produced is mediated by many enzymes and cofactors. Steroidogenic enzymes fall into two groups: cytochrome P450 enzymes and hydroxysteroid dehydrogenases. A cytochrome P450 may be either type 1 (in mitochondria) or type 2 (in endoplasmic reticulum), and a hydroxysteroid dehydrogenase may belong to either the aldo-keto reductase or short-chain dehydrogenase/reductase families. The activities of these enzymes are modulated by posttranslational modifications and by cofactors, especially electron-donating redox partners. The elucidation of the precise roles of these various enzymes and cofactors has been greatly facilitated by identifying the genetic bases of rare disorders of steroidogenesis. Some enzymes not principally involved in steroidogenesis may also catalyze extraglandular steroidogenesis, modulating the phenotype expected to result from some mutations. Understanding steroidogenesis is of fundamental importance to understanding disorders of sexual differentiation, reproduction, fertility, hypertension, obesity, and physiological homeostasis.

Effects of genetic variation in the aldosterone synthase (CYP11B2) gene on enzyme function

OBJECTIVE

Evidence suggests that high levels of aldosterone lead to hypertension and increased risk of cardiovascular disease. Around 15% of patients with essential hypertension have a raised aldosterone to renin ratio (ARR) suggesting that aldosterone production is inappropriately high in relation to its principal agonist angiotensin II. This may be due to increased activity of aldosterone synthase caused by genetic variation in the CYP11B2 gene. We screened the coding region of human CYP11B2 for genetic variants and tested their effects on function in vitro.

PROTOCOL

Normotensive subjects (n = 69) were screened for sequence variants in the coding region of CYP11B2 by single-stranded conformation polymorphism (SSCP) analysis and sequencing. The effects of nonsynonymous variants on enzyme activity were assessed in JEG-3 cells transiently transfected with wild-type or variant expression plasmids. The conversion of the substrate 11-deoxycorticosterone (DOC) to corticosterone (B) and aldosterone was measured.

RESULTS

Twenty variants were detected in CYP11B2 and eight analysed functionally (Arg87Gly, Asn281Thr, Gly288Ser, Lys296Asn, Asp335Asn, Gln404Arg, Ala414Pro and His439Tyr). Corticosterone synthesis was unaltered and aldosterone synthesis reduced in variant Arg87Gly; Asn281Thr increased corticosterone and decreased aldosterone production; Gly288Ser increased corticosterone production and abolished aldosterone production; Lys296Asn reduced both corticosterone and aldosterone production; Asp335Asn increased corticosterone synthesis but did not affect aldosterone production. Variants Gln404Arg, Ala414Pro and His439Tyr showed increases in both corticosterone and aldosterone synthesis compared to the wild-type.

CONCLUSION

The study confirms the genetic variability of the CYP11B2 gene and provides us with additional valuable structure-function information.

Mutations in aldosterone synthase gene of Milan hypertensive rats: phenotypic consequences

Using in vitro and in vivo methods, we have demonstrated increased sensitivity of adrenocortical steroidogenesis to ACTH in Milan hypertensive (MHS) compared with normotensive (MNS) rats and have investigated whether this is caused by mutations of steroidogenic enzymes. Genes encoding aldosterone synthase (CYP11B2) and 11beta-hydroxylase (CYP11B1) in MHS and MNS have been cloned and sequenced. Nucleotide 752 (G) in exon 4 of MHS CYP11B2 differs from that of MNS (A); CYP11B1 sequences were identical. The nucleotide 752 mutation caused a Q251R substitution in the amino acid sequence of MHS CYP11B2. The phenotype of MHS CYP11B2 alleles, when expressed in COS-1 cells, differed from that of MNS alleles. The relative activities of the three reactions catalyzed by CYP11B2 (11beta-hydroxylation of deoxycorticosterone, 18-hydroxylation of corticosterone, and dehydrogenation of 18-hydroxycorticosterone) were estimated after incubation of transfected cells with [(14)C]deoxycorticosterone and analysis of radioactivity associated with deoxycorticosterone, corticosterone, 18 hydroxycorticosterone, and aldosterone. Both 11- and 18-hydroxylase activities were lower (19 and 12%, respectively; P < 0.01 and P < 0.05) in cells transfected with MHS compared with MNS alleles, whereas 18-oxidase activity was 42% higher (P < 0.01). To assess the significance of the CYP11B2 mutation in vivo, DNA from F2 hybrid MHS x MNS rats was genotyped. MHS alleles were associated with lower urine volumes in both sexes, lower ventricle weights in male rats, but no difference in systolic or diastolic blood pressures between the sexes. We conclude that a mutation in CYP11B2 may affect aldosterone secretion in MHS; however, under normal environmental circumstances, we were unable to demonstrate any influence of this mutation on blood pressure.

The effect of amino-acid substitutions I112P, D147E and K152N in CYP11B2 on the catalytic activities of the enzyme

By replacing specific amino acids at positions 112, 147 and 152 of the human aldosterone synthase (CYP11B2) with the corresponding residues from human, mouse or rat 11beta-hydroxylase (CYP11B1), we have been able to investigate whether these residues belong to structural determinants of individual enzymatic activities. When incubated with 11-deoxycorticosterone (DOC), the 11beta-hydroxylation activity of the mutants was most effectively increased by combining D147E and I112P (sixfold increase). The two substitutions displayed an additive effect. The same tendency can be observed when using 11-deoxycortisol as a substrate, although the effect is less pronounced. The second step of the CYP11B2-dependent DOC conversion, the 18-hydroxylation activity, was not as strongly increased as the 11beta-hydroxylation potential. Activity was unaffected by D147E, whereas the single mutant I112P displayed the most pronounced activation (70% enhancement), thus causing different increasing effects on the first two enzymatic reaction steps. A slightly enhanced aldosterone synthesis from DOC could be measured due to increased levels of the intermediates. However, the 18-oxidation activity of all the mutants, except for I112S and D147E, was slightly reduced. The strongly enhanced 18-hydroxycorticosterone and aldosterone formation observed in the mutants provides important information on a possible role of such amino-acid replacements in the development of essential hypertension. Furthermore, the results indicate the possibility of a differential as well as independent modification of CYP11B2 reaction steps. The combination of functional data and computer modelling of CYP11B2 suggests an indirect involvement of residue 147 in the regulation of CYP11B isoform specific substrate conversion due to its location on the protein surface. In addition, the results indicate the functional significance of amino-acid 112 in the putative substrate access channel of human CYP11B2. Thus, we present the first example of substrate recognition and conversion being attributed to the N-terminal part of human CYP11B2.

A novel genetic locus for low renin hypertension: familial hyperaldosteronism type II maps to chromosome 7 (7p22)

Familial hyperaldosteronism type II (FH-II) is caused by adrenocortical hyperplasia or aldosteronoma or both and is frequently transmitted in an autosomal dominant fashion. Unlike FH type I (FH-I), which results from fusion of the CYP11B1 and CYP11B2 genes, hyperaldosteronism in FH-II is not glucocorticoid remediable. A large family with FH-II was used for a genome wide search and its members were evaluated by measuring the aldosterone:renin ratio. In those with an increased ratio, FH-II was confirmed by fludrocortisone suppression testing. After excluding most of the genome, genetic linkage was identified with a maximum two point lod score of 3.26 at theta=0, between FH-II in this family and the polymorphic markers D7S511, D7S517, and GATA24F03 on chromosome 7, a region that corresponds to cytogenetic band 7p22. This is the first identified locus for FH-II; its molecular elucidation may provide further insight into the aetiology of primary aldosteronism.

Amino acid residue 147 of human aldosterone synthase and 11beta-hydroxylase plays a key role in 11beta-hydroxylation

A number of amino acids differ between aldosterone synthase and 11beta-hydroxylase. To assess their importance in determining the different functional specificities, we substituted aldosterone synthase-specific (aspartate D147, isoleucine I248, glutamine Q43, and threonine T493) with 11beta-hydroxylase-specific amino acids (glutamate E147, threonine T248, arginine R43, and methionine M493), respectively. I248T, Q43R, and T493M had no effect on steroid production compared to wild-type aldosterone synthase. However, CYP11B2-D147E caused a significant increase in corticosterone production and a smaller increase in aldosterone production from 11-deoxycorticosterone (DOC). This appeared to be predominantly due to an increase in the 11beta-hydroxylation of DOC to corticosterone mediated by a decrease in Km, which was 1.4 micromol/L for the mutant compared with 5 micromol/L for the wild-type enzyme. CYP11B2-D147E had no effect on the conversion of 11-deoxycortisol to cortisol. The reverse construct (CYP11B1-E147D), substituting the 11beta-hydroxylase residue with the aldosterone synthase equivalent, decreased the conversion of DOC to corticosterone, which was mediated by an increase in Km that was 7.5 micromol/L for the mutant compared with 2.5 micromol/L for the wild-type enzyme. Again, the conversion of 11-deoxycortisol to cortisol was unimpaired. Thus, amino acid 147 is involved in the transformation of the 17-deoxysubstrate, but not the 17alpha-hydroxysubstrate. The results demonstrate that a conservative change in amino acid, even at some linear distance from known active centers, can significantly affect enzyme substrate affinity and subsequent steroid hormone production.

Genetic analysis of aldosterone synthase in patients with idiopathic hyperaldosteronism

Idiopathic hyperaldosteronism (IHA) is characterized by hypertension with excessive production of aldosterone, potassium loss, and suppression of the renin-angiotensin system. We compared activity of aldosterone synthase and expression of CYP11B2 messenger RNA (mRNA) in mononuclear leukocytes (MNL) from patients with IHA to findings in leukocytes from patients with aldosterone-producing adenoma and normal controls. Aldosterone synthase activity was estimated from conversion of [14C]deoxycorticosterone to [14C]aldosterone. Levels of CYP11B2 mRNA were determined by competitive PCR. In the same subjects, we sought the chimeric CYP11B1/CYP11B2 that is candidate gene for glucocorticoid-remediable hyperaldosteronism. Southern blot analysis and a long PCR method were used to detect the chimeric gene. Direct sequencing of the CYP11B2 also was performed. No chimeric genes or mutations in the coding region of the CYP11B2 were found in genomic DNA from these patients. However, both aldosterone synthase activity and CYP11B2 mRNA expression were greater in mononuclear leukocytes of patients with IHA than those of patients with aldosterone-producing adenoma or controls. These results suggest that regulatory factors of the CYP11B2 gene, e.g. unidentified aldosterone-stimulating substances or abnormalities in the promoter region of the CYP11B2 gene in patients with IHA resulting in oversecretion, may cause overexpression of mRNA of CYP11B2.

Inherited forms of mineralocorticoid hypertension

Aldosterone, the most important mineralocorticoid, regulates electrolyte excretion and intravascular volume mainly through its effects on renal distal convoluted tubules and cortical collecting ducts. Excess secretion of aldosterone or other mineralocorticoids or abnormal sensitivity to mineralocorticoids may result in hypertension, suppressed plasma renin activity, and hypokalemia. Such conditions often have a genetic basis, and studies of these conditions have provided valuable insights into the normal and abnormal physiology of mineralocorticoid action. Deficiencies of steroid 11 beta-hydroxylase or 17 alpha-hydroxylase are types of congenital adrenal hyperplasia, the autosomal recessive inability to synthesize cortisol. These two defects often cause hypertension because of overproduction of cortisol precursors that are, or are metabolized to, mineralocorticoid agonists. These disorders result from mutations in the CYP11B1 and CYP17 genes encoding the corresponding enzymes. Glucocorticoid-suppressible hyperaldosteronism is an autosomal dominant form of hypertension in which aldosterone secretion is abnormally regulated by corticotropin. It is caused by recombinations between linked genes encoding closely related isozymes, 11 beta-hydroxylase (CYP11B1) and aldosterone synthase (CYP11B2), generating a dysregulated chimeric gene with aldosterone synthase activity. Apparent mineralocorticoid excess is a loss of functional ligand specificity of the mineralocorticoid receptor caused by a deficiency of the kidney isozyme of 11 beta-hydroxysteroid dehydrogenase, an enzyme that normally metabolizes cortisol to cortisone to prevent cortisol from occupying the receptor. This autosomal recessive form of severe hypertension results from mutations in the HSD11K (HSD11B2) gene.

Changed ratios of glucocorticoids/mineralocorticoids caused by point mutations in the putative I-helix regions of CYP11B1 and CYP11B2

Computer modelling and site-directed mutagenesis were employed to investigate the structural basis for the regioselectivity of steroid hydroxylation and to determine whether point mutations of CYP11B1 and CYP11B2 can result in changes in the ratio of glucocorticoids/mineralocorticoids. Single replacement of CYP11B2 residues for CYP11B1-specific residues at positions 296, 301, 302, 320, and 335, belonging to the putative I-helix, gave rise to slightly increased 11 beta-hydroxylase activities. Replacement of 3 amino acids of CYP11B2 by the corresponding residues in CYP11B1 was sufficient to increase cortisol formation from about 5% to 85% of the CYP11B1 wild type level. The aldosterone synthase activities of the mutants were decreased to varying degrees indicating that point mutations at positions 296, 301, 302, and 335 could potentially cause hypoaldosteronism. Replacement of Val-320 of CYP11B1 by alanine, the corresponding residue found in CYP11B2, led to production of aldosterone by this mutant enzyme. This observation suggests that glucocorticoid-remediable hyperaldosteronism could also be due to point mutations located in the CYP11B1 sequence.

Molecular variants in the P450c11AS gene as determinants of aldosterone synthase activity in the Dahl rat model of hypertension

Dahl salt-sensitive (S) and salt-resistant (R) rats are widely used to study genetic determinants of salt-sensitive hypertension. Differences in blood pressure under a high sodium diet in these two strains may be due to differences in the synthesis of 18-OH-11-deoxycorticosterone (18-OH DOC). This difference in 18-OH-DOC synthesis is due to mutations in the Dahl R rat's gene for P450c11 beta (11 beta-hydroxylase), an adrenal enzyme involved in the synthesis of both corticosterone and 18-OH DOC from 11-deoxycorticosterone. Aldosterone/renin ratios in plasma and in the adrenals are greater in Dahl S than R rats, suggesting an altered physiologic relationship between the renin-angiotensin and aldosterone systems between these strains. We demonstrate that the mRNA for P450c11AS, (aldosterone synthase), an enzyme required for aldosterone synthesis, is identical in the Dahl S rat and in normotensive Sprague-Dawley rats, but that P450c11AS mRNA from the Dahl R rat contains 7 mutations that result in two amino acid substitutions. These two changes result in a form of P450c11AS that has a greater apparent Vmax and lower apparent Km, resulting in an enzyme that catalyzes the conversion of 11-deoxycorticosterone to aldosterone at a greater rate in Dahl R rats than the P450c11AS in Dahl S rats or Sprague-Dawley rats. Although plasma and adrenal renin are lower in Dahl S versus R rats, the regulation of P450c11AS mRNA expression in rats fed a low and high salt diet are identical in these strains. The current findings may explain both the reduced aldosterone concentrations and increased aldosterone/renin ratios previously reported in the Dahl S versus Dahl R rat.