David S. Geller

Molecular mechanisms of human hypertension

Supported in part by a Specialized Center of Research in Hypertension and NIH K08 awards (to A. G. G. and D. S. G.). R. P. L. is an Investigator of the Howard Hughes Medical Institute.

K+ Channel Mutations in Adrenal Aldosterone-Producing Adenomas and Hereditary Hypertension

Potassium channel mutations drive both cell growth and hormone production in an adrenal tumor that causes severe hypertension. Endocrine tumors such as aldosterone-producing adrenal adenomas (APAs), a cause of severe hypertension, feature constitutive hormone production and unrestrained cell proliferation; the mechanisms linking these events are unknown. We identify two recurrent somatic mutations in and near the selectivity filter of the potassium (K+) channel KCNJ5 that are present in 8 of 22 human APAs studied. Both produce increased sodium (Na+) conductance and cell depolarization, which in adrenal glomerulosa cells produces calcium (Ca2+) entry, the signal for aldosterone production and cell proliferation. Similarly, we identify an inherited KCNJ5 mutation that produces increased Na+ conductance in a Mendelian form of severe aldosteronism and massive bilateral adrenal hyperplasia. These findings explain pathogenesis in a subset of patients with severe hypertension and implicate loss of K+ channel selectivity in constitutive cell proliferation and hormone production.

Mutations in the mineralocorticoid receptor gene cause autosomal dominant pseudohypoaldosteronism type I.

Pseudohypoaldosteronism type I (PHA1) is characterized by neonatal renal salt wasting with dehydration, hypotension, hyperkalaemia and metabolic acidosis, despite elevated aldosterone levels. Two forms of PHA1 exist. An autosomal recessive form features severe disease with manifestations persisting into adulthood. This form is caused by loss-of-function mutations in genes encoding subunits of the amiloride-sensitive epithelial sodium channel (ENaC; refs 2,3 ). Autosomal dominant or sporadic PHA1 is a milder disease that remits with age. Among six dominant and seven sporadic PHA1 kindreds, we have found no ENaC gene mutations, implicating mutations in other genes. As ENaC activity in the kidney is regulated by the steroid hormone aldosterone acting through the mineralocorticoid receptor, we have screened the mineralocorticoid receptor gene (MLR) for variants and have identified heterozygous mutations in one sporadic and four dominant kindreds. These include two frameshift mutations (one a de novo mutation), two premature termination codons and one splice donor mutation. These mutations segregate with PHA1 and are not found in unaffected subjects. These findings demonstrate that heterozygous MLR mutations cause PHA1, underscore the important role of mineralocorticoid receptor function in regulation of salt and blood pressure homeostasis in humans and motivate further study of this gene for a potential role in blood pressure variation.

Wnk4 controls blood pressure and potassium homeostasis via regulation of mass and activity of the distal convoluted tubule.

The mechanisms that govern homeostasis of complex systems have been elusive but can be illuminated by mutations that disrupt system behavior. Mutations in the gene encoding the kinase WNK4 cause pseudohypoaldosteronism type II (PHAII), a syndrome featuring hypertension and hyperkalemia. We show that physiology in mice transgenic for genomic segments harboring wild-type (TgWnk4WT) or PHAII mutant (TgWnk4PHAII) Wnk4 is changed in opposite directions: TgWnk4PHAII mice have higher blood pressure, hyperkalemia, hypercalciuria and marked hyperplasia of the distal convoluted tubule (DCT), whereas the opposite is true in TgWnk4WT mice. Genetic deficiency for the Na-Cl cotransporter of the DCT (NCC) reverses phenotypes seen in TgWnk4PHAII mice, demonstrating that the effects of the PHAII mutation are due to altered NCC activity. These findings establish that Wnk4 is a molecular switch that regulates the balance between NaCl reabsorption and K+ secretion by altering the mass and function of the DCT through its effect on NCC.

A Novel Form of Human Mendelian Hypertension Featuring Nonglucocorticoid-Remediable Aldosteronism

CONTEXT Primary aldosteronism is a leading cause of secondary hypertension (HTN), but the mechanisms underlying the characteristic renin-independent secretion of aldosterone remain unknown in most patients. OBJECTIVES We report a new familial form of aldosteronism in a father and two daughters. All were diagnosed with severe HTN refractory to medical treatment by age 7 yr. We performed a variety of clinical, biochemical, and genetic studies to attempt to clarify the underlying molecular defect. RESULTS Biochemical studies revealed hyporeninemia, hyperaldosteronism, and very high levels of 18-oxocortisol and 18-hydroxycortisol, steroids that reflect oxidation by both steroid 17-alpha hydroxylase and aldosterone synthase. These enzymes are normally compartmentalized in the adrenal fasciculata and glomerulosa, respectively. Administration of dexamethasone failed to suppress either aldosterone or cortisol secretion; these findings distinguish this clinical syndrome from glucocorticoid-remediable aldosteronism, another autosomal dominant form of HTN, and suggest a global defect in the regulation of adrenal steroid production. Genetic studies excluded mutation at the aldosterone synthase locus, further distinguishing this disorder from glucocorticoid-remediable aldosteronism. Because of unrelenting HTN, all three subjects underwent bilateral adrenalectomy, which in each case corrected the HTN. Adrenal glands showed dramatic enlargement, with paired adrenal weights as high as 82 g. Histology revealed massive hyperplasia and cellular hypertrophy of a single cortical compartment that had features of adrenal fasciculata or a transitional zone, with an atrophic glomerulosa. CONCLUSION These findings define a new inherited form of aldosteronism and suggest that identification of the underlying defect will provide insight into normal mechanisms regulating adrenal steroid biosynthesis.

Autosomal Dominant Pseudohypoaldosteronism Type 1: Mechanisms, Evidence for Neonatal Lethality, and Phenotypic Expression in Adults

Autosomal dominant pseudohypoaldosteronism type 1 (adPHA1) is a rare condition that is characterized by renal resistance to aldosterone, with salt wasting, hyperkalemia, and metabolic acidosis. It is thought of as a mild disorder; affected childrens symptoms respond promptly to salt therapy, and treatment is not required after childhood. Mutations in the mineralocorticoid receptor gene (MR) cause adPHA1, but the long-term consequences of MR deficiency in humans are not known. Herein are described six novel adPHA1-causing MR mutations (four de novo) and evidence that haploinsufficiency of MR is sufficient to cause adPHA1. Furthermore, genotype-phenotype correlation is reported in a large adPHA1 kindred. A number of cases of neonatal mortality in infants who were at risk for adPHA1 were identified; coupled with the frequent identification of de novo mutations in affected individuals, this suggests that the seemingly benign adPHA1 may have been a fatal neonatal disorder in previous eras, preventing propagation of disease alleles. In contrast, it is shown that adult patients with adPHA1 are clinically indistinguishable from their wild-type relatives except for presumably lifelong elevation of renin, angiotensin II, and aldosterone levels. These data highlight the critical role of MR in the maintenance of salt homeostasis early in life and illuminate the sodium dependence of pathologic effects of renin and angiotensin II. They furthermore argue that nongenomic effects of aldosterone play no significant role in the long-term development of cardiovascular disease.

Met and the epidermal growth factor receptor act cooperatively to regulate final nephron number and maintain collecting duct morphology

Ureteric bud (UB) branching during kidney development determines the final number of nephrons. Although hepatocyte growth factor and its receptor Met have been shown to stimulate branching morphogenesis in explanted embryonic kidneys, loss of Met expression is lethal during early embryogenesis without obvious kidney abnormalities. Metfl/fl;HoxB7-Cre mice, which lack Met expression selectively in the UB, were generated and found to have a reduction in final nephron number. These mice have increased Egf receptor expression in both the embryonic and adult kidney, and exogenous Egf can partially rescue the branching defect seen in kidney explants. Metfl/fl;HoxB7-Cre;wa-2/wa-2 mice, which lack normal Egfr and Met signaling, exhibit small kidneys with a marked decrease in UB branching at E14.5 as well as a reduction in final glomerular number. These mice developed progressive interstitial fibrosis surrounding collecting ducts with kidney failure and death by 3-4 weeks of age. Thus, in support of previous in vitro findings, Met and the Egf receptor can act cooperatively to regulate UB branching and mediate maintenance of the normal adult collecting duct.

A Critical Role for Vascular Smooth Muscle in Acute Glucocorticoid-Induced Hypertension

Although glucocorticoid (GC)-induced hypertension has commonly been attributed to promiscuous activation of the mineralocorticoid receptor by cortisol, thereby promoting excess reabsorption of sodium and water, numerous lines of evidence indicate that this is not the only or perhaps even the primary mechanism. GC induce a number of effects on vascular smooth muscle (VSM) in vitro that may be pertinent to hypertension, but their contribution in vivo is unknown. To address this question, a mouse model with a tissue-specific knockout (KO) of the GC receptor in the VSM was created and characterized. Similar to control mice, KO mice exhibited normal baseline BP and, interestingly, showed normal circadian variation in BP. When dexamethasone was administered, however, the acute hypertensive response was markedly attenuated in KO mice, and there was a trend toward a decreased chronic hypertensive response. These data suggest that the GC receptor in VSM plays a critical role in the acute hypertensive response to GC in vivo.

Monogenic low renin hypertension

Monogenic forms of low renin hypertension can now be identified in a large and heterogeneous family of hypertensive patients with highly specific etiologies and similar clinical manifestations. These include the following well-characterized disorders: apparent mineralocorticoid excess, Liddles Syndrome, steroid 11beta-hydroxylase (11beta-OHD) and steroid 17-hydroxylase (17-OHD) deficiencies, glucocorticoid-remediable hyperaldosteronism (familial hyperaldosteronism type I), familial hyperaldosteronism type II, hypertension exacerbated by pregnancy and primary hyperaldosteronism (Conns syndrome). The successful elucidation of specific DNA mutations in most of these conditions has emphasized the role of molecular genetics in hypertension, a field in which diagnosis can now be made on proven genetic evidence. The current knowledge of these genetic markers enables practitioners to make precise diagnoses, and to initiate specific therapy, in patients with these relatively uncommon but interesting and often treatable forms of hypertension.

Glucocorticoid-induced hypertension

Glucocorticoid-induced hypertension is a common clinical problem that is poorly understood, thus rendering treatment strategies sub-optimal. This form of hypertension has been commonly thought to be mediated by excess sodium and water reabsorption by the renal mineralocorticoid receptor. However, experimental and clinical data in both humans and animal models suggest important roles for the glucocorticoid receptor as well, in both the pathogenesis and maintenance of this hypertension. The glucocorticoid receptor is widely expressed in a number of organ systems relevant to blood pressure regulation, including the kidney, the brain and the vasculature. In vitro studies in isolated kidney tissues as well as in vascular smooth muscle and vascular endothelial cells have attempted to elucidate the molecular physiology of glucocorticoid-induced hypertension, but have generally been limited by the inability to study signaling pathways in an intact organism. More recently, the power of mouse genetics has been employed to examine the tissue-specific contributions of vascular and extra-vascular tissues to this form of hypertension. Here we review recent developments in our understanding of the pathogenesis of glucocorticoid-induced hypertension.