Apparent Mineralocorticoid Excess: Report of Six New Cases and Extensive Personal Experience

A switch in the mechanism of hypertension in the syndrome of apparent mineralocorticoid excess

The syndrome of apparent mineralocorticoid excess arises from nonfunctional mutations in 11beta-hydroxysteroid dehydrogenase type 2 (11betaHSD2), an enzyme that inactivates cortisol and confers aldosterone specificity on the mineralocorticoid receptor. Loss of 11betaHSD2 permits glucocorticoids to activate the mineralocorticoid receptor, and the hypertension in the syndrome is presumed to arise from volume expansion secondary to renal sodium retention. An 11betaHSD2 null mouse was generated on an inbred C57BL/6J genetic background, allowing survival to adulthood. 11betaHSD2(-/-) mice had BP approximately 20 mmHg higher on average compared with wild-type mice but were volume contracted, not volume expanded as expected. Initially, impaired sodium excretion associated with increased activity of the epithelial sodium channel was observed. By 80 days of age, however, channel activity was abolished and 11betaHSD2(-/-) mice lost salt. Despite the natriuresis, hypertension remained but was not attributable to intrinsic vascular dysfunction. Instead, urinary catecholamine levels in 11betaHSD2(-/-) mice were double those in wild-type mice, and alpha1-adrenergic receptor blockade rescued the hypertensive phenotype, suggesting that vasoconstriction contributes to the sustained hypertension in this model. In summary, it is proposed that renal sodium retention remains a key event in apparent mineralocorticoid excess but that the accompanying hypertension changes from a renal to a vascular etiology over time.

Advances in genetic hypertension

PURPOSE OF REVIEW

Mendelian forms of hypertension are rare genetic disorders that cause severe hypertension. This review will explore the recently identified molecular mechanisms and pathogenesis of genetic disorders that cause hypertension in children.

RECENT FINDINGS

Hypertension is now believed to be a polygenic disorder resulting from the interaction of multiple genes and the environment. A few forms of severe hypertension have been linked to single genes. The genes responsible for these disorders have all been cloned and all participate in pathways involved in heightened renal sodium reabsorption. The increased sodium reabsorption arises in the distal nephron and leads to volume expansion and hypertension.

SUMMARY

Investigating forms of monogenic hypertension has advanced the understanding of sodium transport and volume control by the kidney. Future studies will identify novel genes, pathways and treatment targets important in the fight against primary hypertension.

Impaired protein stability of 11beta-hydroxysteroid dehydrogenase type 2: a novel mechanism of apparent mineralocorticoid excess

Apparent mineralocorticoid excess (AME) is a severe form of hypertension that is caused by impaired activity of 11beta-hydroxysteroid dehydrogenase type 2 (11beta-HSD2), which converts biologically active cortisol into inactive cortisone. Mutations in HSD11B2 result in cortisol-induced activation of mineralocorticoid receptors and cause hypertension with hypokalemia, metabolic alkalosis, and suppressed circulating renin and aldosterone concentrations. This study uncovered the first patient with AME who was described in the literature, identified the genetic defect in HSD11B2, and provided evidence for a novel mechanism of reduced 11beta-HSD2 activity. This study identified a cluster of amino acids (335 to 339) in the C-terminus of 11beta-HSD2 that are essential for protein stability. The cluster includes Tyr(338), which is mutated in the index patient, and Arg(335) and Arg(337), previously reported to be mutated in hypertensive patients. It was found that wild-type 11beta-HSD2 is a relatively stable enzyme with a half-life of 21 h, whereas that of Tyr(338)His and Arg(337)His was 3 and 4 h, respectively. Enzymatic activity of Tyr(338)His was partially retained at 26 degrees C or in the presence of the chemical chaperones glycerol and dexamethasone, indicating thermodynamic instability and misfolding. The results provide evidence that the degradation of both misfolded mutant Tyr(338)His and wild-type 11beta-HSD2 occurs through the proteasome pathway. Therefore, impaired 11beta-HSD2 protein stability rather than reduced gene expression or loss of catalytic activity seems to be responsible for the development of hypertension in some individuals with AME.