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

Aldosterone synthase (CYP11B2) deficiency among Palestinian infants: Three novel variants and genetic heterogeneity

Aldosterone synthase deficiency (ASD) is a rare potentially life-threatening genetic disorder that usually presents during infancy due to pathogenic variants in the CYP11B2 gene. Knowledge about CYP11B2 variants in the Arab population is scarce. Here, we present and analyze five Palestinian patients and their different novel pathogenic variants. Data on clinical presentation, electrolytes, plasma renin activity, and steroid hormone levels of five patients diagnosed with ASD were summarized. Sequencing of the CYP11B2 gene exons was followed by evolutionary conservation analysis and structural modeling of the variants. All patients were from highly consanguineous Palestinian families. The patients presented at 1-4 months of age with recurrent vomiting, poor weight gain, hyponatremia, hyperkalemia, and low aldosterone levels. Genetic analysis of the CYP11B2 gene revealed three homozygous pathogenic variants: p.Ser344Profs*9, p.G452W in two patients from an extended family, and p.Q338stop. A previously described pathogenic variant was found in one patient: p.G288S. We described four different CYP11B2 gene pathogenic variants in a relatively small population. Our findings may contribute to the future early diagnosis and therapy for patients with ASD among Arab patients who present with failure to thrive and compatible electrolyte disturbances.

Structural and functional insights into aldosterone synthase interaction with its redox partner protein adrenodoxin

Aldosterone is the major mineralocorticoid in the human body controlling blood pressure and salt homeostasis. Overproduction of aldosterone leads to primary aldosteronism, which is the most common form of secondary hypertension with limited treatment options. Production of aldosterone by cytochrome P450 11B2 (CYP11B2, aldosterone synthase) requires two reduction events with the electrons delivered by the iron/sulfur protein adrenodoxin. Very limited information is available about the structural and functional basis of adrenodoxin/CYP11B2 interaction, which impedes the development of new treatment options for primary aldosteronism. A systematic study was carried out to determine if adrenodoxin interaction with CYP11B2 might also have an allosteric component in addition to electron transfer. Indeed, local increases in adrenodoxin concentration promote binding of the substrate 11-deoxycorticosterone and the inhibitor osilodrostat (LCI699) in the active site-over 17 Å away-as well as enhance the inhibitory effect of this latter drug. The CYP11B2 structure in complex with adrenodoxin identified specific residues at the protein-protein interface interacting via five salt bridges and four hydrogen bonds. Comparisons with cholesterol-metabolizing CYP11A1 and cortisol-producing CYP11B1, which also bind adrenodoxin, revealed substantial structural differences in these regions. The structural and functional differences between different P450 interactions with adrenodoxin may provide valuable clues for an orthogonal treatment approach for primary aldosteronism by specifically targeting the interaction between CYP11B2 and adrenodoxin.

High Frequency of Variants of Candidate Genes in Black Africans with Low Renin-Resistant Hypertension

OBJECTIVES

Black subjects tend to retain salt and water, be more sensitive to aldosterone, and have suppression of plasma renin activity. Variants of the renal sodium channel (ENaC, SCNN1B) account for approximately 6% of resistant hypertension (RHT) in Blacks; other candidate genes may be important.

METHODS

Six candidate genes associated with low renin-resistant hypertension were sequenced in Black Africans from clinics in Kenya and South Africa. CYP11B2 was sequenced if the aldosterone level was high (primary aldosteronism phenotype); SCNN1B, NEDD4L, GRK4, UMOD, and NPPA genes were sequenced if the aldosterone level was low (Liddle phenotype).

RESULTS

There were 14 nonsynonymous variants (NSVs) of CYP11B2: 3 previously described and associated with alterations in aldosterone synthase production (R87G, V386A, and G435S). Out of 14, 9 variants were found in all 9 patients sequenced. There were 4 NSV of GRK4 (R65L, A116T, A142V, V486A): at least one was found in all 9 patients; 3 were previously described and associated with hypertension. There were 3 NSV of SCNN1B (R206Q, G442V, and R563Q); 2 previously described and 1 associated with hypertension. NPPA was found to have 1 NSV (V32M), not previously described and NEDD4L did not have any variants. UMOD had 3 NSV: D25G, L180V, and T585I.

CONCLUSIONS

A phenotypic approach to investigating the genetic architecture of RHT uncovered a surprisingly high yield of variants in candidate genes. These preliminary findings suggest that this novel approach may assist in understanding the genetic architecture of RHT in Blacks and explain their two fold risk of stroke.

The CYP11B subfamily

The biosynthesis of steroid hormones is dependent on P450-catalyzed reactions. In mammals, cholesterol is the common precursor of all steroid hormones, and its conversion to pregnenolone is the initial and rate-limiting step in hormone biosynthesis in steroidogenic tissues such as gonads and adrenal glands. The production of glucocorticoids and mineralocorticoids takes place in the adrenal gland and the final steps are catalyzed by 2 mitochondrial cytochromes P450, CYP11B1 (11β-hydroxylase or P45011β) and CYP11B2 (aldosterone synthase or P450aldo). The occurrence and development of these 2 enzymes in different species, their contribution to the biosynthesis of steroid hormones as well as their regulation at different levels (gene expression, cellular regulation, regulation on the level of proteins) is the topic of this chapter.

Plasma corticosteroid profiling: brief opinion of its current status in clinical diagnosis and research

Adrenal steroid biosynthesis comprises a series of dynamically interrelated, enzyme-catalyzed reactions in two separate compartments, the zona glomerulosa and the zona fasciculata/reticularis. End products (cortisol, aldosterone and androgens), together with a proportion of the intermediate compounds, appear in the circulation as a characteristic profile. Rare deficiencies of individual enzymes modify this profile in a recognizable way. Previous exhaustive profiling suggests that their diagnosis can now often be made on the basis of single-compound analyses with concomitant genetic tests. However, high-capacity liquid chromatography coupled with tandem mass spectrometry-based methods are facilitating profiling of large population samples, revealing that smaller differences in enzyme efficiency, indicated by potentially more complex corticosteroid patterns, may be related to clinical wellbeing in a much larger proportion of the population.

Genetics of hypertension: from experimental animals to humans

Essential hypertension affects 20 to 30% of the population worldwide and contributes significantly to cardiovascular mortality and morbidity. Heridability of blood pressure is around 15 to 40% but there are also substantial environmental factors affecting blood pressure variability. It is assumed that blood pressure is under the control of a large number of genes each of which has only relatively mild effects. It has therefore been difficult to discover the genes that contribute to blood pressure variation using traditional approaches including candidate gene studies and linkage studies. Animal models of hypertension, particularly in the rat, have led to the discovery of quantitative trait loci harbouring one or several hypertension related genes, but translation of these findings into human essential hypertension remains challenging. Recent development of genotyping technology made large scale genome-wide association studies possible. This approach and the study of monogenic forms of hypertension has led to the discovery of novel and robust candidate genes for human essential hypertension, many of which require functional analysis in experimental models.