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

Catch-up Growth and Discontinuation of Fludrocortisone Treatment in Aldosterone Synthase Deficiency

BACKGROUND

Aldosterone synthase deficiency (ASD) caused by mutations in the CYP11B2 gene is characterized by isolated mineralocorticoid deficiency. Data are scarce regarding clinical and biochemical outcomes of the disease in the follow-up.

OBJECTIVE

Assessment of the growth and steroid profiles of patients with ASD at the time of diagnosis and after discontinuation of treatment.

DESIGN AND METHOD

Children with clinical diagnosis of ASD were included in a multicenter study. Growth and treatment characteristics were recorded. Plasma adrenal steroids were measured using liquid chromatography-mass spectrometry. Genetic diagnosis was confirmed by CYP11B2 gene sequencing and in silico analyses.

RESULTS

Sixteen patients from 12 families were included (8 females; median age at presentation: 3.1 months, range: 0.4 to 8.1). The most common symptom was poor weight gain (56.3%). Median age of onset of fludrocortisone treatment was 3.6 months (range: 0.9 to 8.3). Catch-up growth was achieved at median 2 months (range: 0.5 to 14.5) after treatment. Fludrocortisone could be stopped in 5 patients at a median age of 6.0 years (range: 2.2 to 7.6). Plasma steroid profiles revealed reduced aldosterone synthase activity both at diagnosis and after discontinuation of treatment compared to age-matched controls. We identified 6 novel (p.Y195H, c.1200 + 1G > A, p.F130L, p.E198del, c.1122-18G > A, p.I339_E343del) and 4 previously described CYP11B2 variants. The most common variant (40%) was p.T185I.

CONCLUSIONS

Fludrocortisone treatment is associated with a rapid catch-up growth and control of electrolyte imbalances in ASD. Decreased mineralocorticoid requirement over time can be explained by the development of physiological adaptation mechanisms rather than improved aldosterone synthase activity. As complete biochemical remission cannot be achieved, a long-term surveillance of these patients is required.

Spatially restricted substrate-binding site of cortisol-synthesizing CYP11B1 limits multiple hydroxylations and hinders aldosterone synthesis

Human cytochromes P450_11β (CYP11B1) and P450_aldo (CYP11B2) are monooxygenases that synthesize cortisol through steroid 11β-hydroxylation and aldosterone through a three-step process comprising 11β-hydroxylation and two 18-hydroxylations, respectively. CYP11B1 also catalyzes 18-monohydroxylation and 11β,18-dihydroxylation. To study the molecular basis of such catalytic divergence of the two enzymes, we examined a CYP11B1 mutant (Mt-CYP11B1) with amino acid replacements on the distal surface by determining the catalytic activities and crystal structure in the metyrapone-bound form at 1.4-Å resolution. Mt-CY11B1 retained both 11β-hydroxylase and 18-hydroxylase activities of the wild type (Wt-CYP11B1) but lacked 11β,18-dihydroxylase activity. Comparisons of the crystal structure of Mt-CYP11B1 to those of Wt-CYP11B1 and CYP11B2 that were already reported show that the mutation reduced the innermost space putatively surrounding the C3 side of substrate 11-deoxycorticosterone (DOC) bound to Wt-CYP11B1, while the corresponding space in CYP11B2 is enlarged markedly and accessible to bulk water through a channel. Molecular dynamics simulations of their DOC-bound forms supported the above findings and revealed that the enlarged space of CYP11B2 had a hydrogen bonding network involving water molecules that position DOC. Thus, upon positioning 11β-hydroxysteroid for 18-hydroxylation in their substrate-binding sites, steric hindrance could occur more strongly in Mt-CYP11B1 than in Wt-CYP11B1 but less in CYP11B2. Our investigation employing Mt-CYP11B1 sheds light on the divergence in structure and function between CYP11B1 and CYP11B2 and suggests that CYP11B1 with spatially-restricted substrate-binding site serves as 11β-hydroxylase, while CYP11B2 with spatially-extended substrate-binding site successively processes additional 18-hydroxylations to produce aldosterone.

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CYP11B1 and CYP11B2 catalyze steroid hydroxylation for syntheses of cortisol and aldosterone, respectively.

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Structural basis for their differences in ability of multiple hydroxylations remains unclear.

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A CYP11B1 mutant generated is characterized by monohydroxylase activities.

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X-ray crystallography and molecular dynamics simulation reveal spatial restriction in substrate-binding site of the mutant.

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Spatial dimension of the substrate-binding sites is crucial for differential production of gluco- and mineralocorticoids.

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.

Aldosterone deficiency with a hormone profile mimicking pseudohypoaldosteronism

Background Aldosterone deficiency (hypoaldosteronism) or aldosterone resistance (pseudohypoaldosteronism) both result in defective aldosterone activity. Case presentation A 42-day-old man presented with failure to thrive, hyponatremia, high urine sodium output, severe hyperkalemia and high plasma renin activity and aldosterone levels. NR3C2, SCNN1A, B and G sequencing showed no variants. Exclusive sodium supplementation resulted in clinical stabilization and growth normalization. His younger sibling had similar clinical and laboratory features, except for low-normal aldosterone. Both patients showed compound heterozygous mutations in CYP11B2 (c.C554T/2802pbE1-E2del). The younger patient needed transient fludrocortisone treatment and higher sodium supplementation, recuperating his weight and a normal growth velocity, although below his brother's and target height (c.10th vs. c.50th). Conclusions On a suggestive clinical picture, high aldosterone plasma levels in early infancy do not rule out aldosterone insufficiency and might mislead differential diagnosis with pseudohypoaldosteronism. Therapeutic requests and growth impairment in hypoaldosteronism vary even with a common genetic background.

Analysis of novel heterozygous mutations in the CYP11B2 gene causing congenital aldosterone synthase deficiency and literature review

Aldosterone synthase deficiency (ASD) is a rare autosomal recessive disorder characterized by severe hyperkalemia, salt loss, vomiting, severe dehydration and failure to thrive. ASD is a life-threatening electrolyte imbalance in infants resulting from mutations in CYP11B2. We described ASD in a Chinese male infant with vomiting, poor feeding and failure to thrive. He was mildly dehydrated, with a weight of 6 kg (-3.45 SDS) and length of 67 cm (-3.10 SDS). Laboratory tests showed hyponatremia (119 mmol/L), serum potassium 5.4 mmol/L, low plasma aldosterone and plasma renin activity (PRA) levels. Next-generation sequencing of his DNA revealed compound heterozygous mutations in CYP11B2, a known variant c.1391_1393delTGC (p.Leu464del, rs776404064) and a novel variant c.1294delA (p.Arg432Glyfs*37). The HEK-293T expression system was used to investigate the variants, demonstrating negligible aldosterone synthesis compared with WT CYP11B2. The patient started fludrocortisone and subsequently gained 3.2 kg of weight and normalized serum sodium (137 mmol/L). We further reviewed reported cases of ASD, summarizing clinical features and CYP11B2 mutations; missense and nonsense mutations are most frequent. Fludrocortisone treatment is essential for ASD, and the need for mineralocorticoid replacement wanes with age; eventually, therapy can be discontinued for many affected children. Our study broadens the ASD phenotypic spectrum and shows the efficiency of next-generation sequencing for patients with atypical clinical manifestations.

The Aldosterone/Renin Ratio as a Diagnostic Tool for the Diagnosis of Primary Hypoaldosteronism in Newborns and Infants

BACKGROUND/AIMS

Primary hypoaldosteronism is a rare inborn disorder with life-threatening symptoms in newborns and infants due to an aldosterone synthase defect. Diagnosis is often difficult as the plasma aldosterone concentration (PAC) can remain within the normal range and thus lead to misinterpretation and delayed initiation of life-saving therapy. We aimed to test the eligibility of the PAC/plasma renin concentration (PRC) ratio as a tool for the diagnosis of primary hypoaldosteronism in newborns and infants. Meth ods: Data of 9 patients aged 15 days to 12 months at the time of diagnosis were collected. The diagnosis of primary hypoaldosteronism was based on clinical and laboratory findings over a period of 12 years in 3 different centers in Switzerland. To enable a valid comparison, the values of PAC and PRC were correlated to reference methods.

RESULTS

In 6 patients, the PAC/PRC ratio could be determined and showed constantly decreased values <1 (pmol/l)/(mU/l). In 2 patients, renin was noted as plasma renin activity (PRA). PAC/PRA ratios were also clearly decreased. The diagnosis was subsequently genetically confirmed in 8 patients.

CONCLUSION

A PAC/PRC ratio <1 pmol/mU and a PAC/PRA ratio <28 (pmol/l)/(ng/ml × h) are reliable tools to identify primary hypoaldosteronism in newborns and infants and help to diagnose this life-threatening disease faster.

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 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.

Modulation of aldosterone and cortisol synthesis on the molecular level

CYP11B1 and the closely related CYP11B2 are involved in the production of adrenal steroid hormones. Although in human their primary structure is 93% identical they are involved in the biosynthesis of functionally diverse products, such as glucocorticoids and mineralocorticoids, respectively. In contrast, bovine CYP11B1 combines both activities in one single enzyme. The CYP11B family belongs to class I cytochromes P450 that have been described in bacteria and mitochondria and receive their electrons from a low molecular weight iron sulphur protein which is reduced by a NADPH-dependent FAD-containing reductase. In this review, we summarise the current knowledge on the modulation of aldosterone and cortisol synthesis by transcriptional regulation, on the molecular level as consequence of mutations found in patients suffering from steroid hormone-related diseases as well as introduced by site-directed mutagenesis and as consequence of protein-protein interaction with both CYP11A1 and the natural redox partner adrenodoxin.

A compound heterozygote case of type II aldosterone synthase deficiency

An infant with failure to thrive, persistent hyponatremia and episodic vomiting and diarrhea was admitted to hospital at 9 months of age, and the diagnosis of type II aldosterone synthase deficiency was confirmed by plasma and urinary steroid determinations. The entire coding sequence of the aldosterone synthase gene (CYP11B2) was determined (both strands) in the affected infant, an unaffected sibling, and both parents. An exon 3 mutation (C554T, leading to amino acid T185I) was found in the father and both siblings, and an exon 9 mutation (A1492G, leading to T498A) was found in the affected infant and the mother. Expression of the mutant sequences in COS cells showed steroidogenic patterns typical of aldosterone synthase type II deficiency, including very low levels of aldosterone synthesis (< or =0.5% of wild-type enzyme) consistent with the low aldosterone levels in the patient's plasma. Both mutations in this compound heterozygote localize to the beta 3-sheet in the cytochrome P450 enzyme structure, as does the previously characterized R181W mutation. This region of the enzyme is not part of the putative structural core, but mutations to this region suggest that it is important for conferring the unique ability of aldosterone synthase to catalyze efficient oxygenation of the C(18) carbon of steroid substrates.

Disorders of mineralocorticoid synthesis

Abnormalities of mineralocorticoid synthesis and/or metabolism profoundly affect the regulation of electrolyte and water balance and of blood pressure. Characteristic changes in extracellular potassium, sodium and hydrogen ion concentrations are usually diagnostic. Serious deficiency may be acquired, for example in Addison's disease, or inherited. In most of the inherited syndromes, the precise molecular changes in specific steroidogenic enzymes have been identified. Mineralocorticoid excess may be caused by aldosterone or 11-deoxycorticosterone by inadequate conversion of cortisol to cortisone by 11beta-hydroxysteroid dehydrogenase type 2 in target tissues (see Chapter 4), by glucocorticoid receptor deficiency or by constitutive activation of renal sodium channels. Changes in electrolyte balance and renin as well as the abnormal pattern of corticosteroid metabolism are usually diagnostic. Where these abnormalities are inherited (e.g. 11beta- or l7alpha-hydroxylase deficiencies, glucocorticoid remediable hyperaldosteronism (GRA), receptor defects, Liddle's syndrome), the molecular basis is again usually known and, in some cases, may provide the simplest diagnostic tests. Primary aldosteronism, although readily identifiable, presents problems of differential diagnosis, important because optimal treatment is different for each variant. Moreover, the mechanisms by which the variants develop are poorly understood. Finally, a significant proportion of patients with essential hypertension show characteristics of mild mineralocorticoid excess, for example low renin levels. Is this relevant to pathophysiology and, if so, is the effect induced via classic mechanisms of action or through newly discovered direct actions on the brain, heart and blood vessels? These questions are the subject of current research.