Many of the mediators and regulators of renal salt reabsorption have been identified from physiologic studies. The manner in which these individual elements function in the context of integrated physiology in vivo is best understood from the consequence of mutations that alter the function of individual components. In addition, unbiased genetic screens have the ability to identify previously unrecognized elements of the regulatory network. Studies of inherited defects in renal salt reabsorption in humans have identified a large number of mutations that result in increased or decreased salt reabsorption. Genes implicated encode diverse proteins including ion channels and transporters, enzymes involved in hormone production, hormone receptors, protein kinases and elements of ubiquitin ligases. Importantly, these mutations have dramatic impact on blood pressure, unequivocally establishing the key role of varied renal salt reabsorption in human blood pressure variation. In addition, this work has identified previously unrecognized physiology that orchestrates the balance between salt reabsorption and potassium secretion. These findings have impacted drug development for hypertension and public health approaches to the control of blood pressure.
blood pressure; hypertension; salt-losing nephropathy; glucocorticoid-remediable aldosteronism; aldosterone-producing adenoma; apparent mineralocorticoid excess; pseudohypoaldosteronism; Liddle Syndrome; Bartter Syndrome; Gitelman Syndrome
Approaches to Identify Genes and Mutations that Contribute to Human Disease
The regulation of blood pressure is fantastically complex, with contributions from the brain, heart, vasculature, adrenal, and kidney. In the face of such complexity, it has been very difficult to identify the rate-limiting steps in the determination of long-term blood pressure from physiologic analysis alone. In this setting, human genetics has proven highly informative, because the finding of mutations in specific genes that result in significant effects on blood pressure can establish a causal link between specific genes, their biochemical pathways, and blood pressure.
There are a number of approaches to the discovery of human disease genes. One approach is to search for rare mutations with large effects on the trait. In the most extreme form these are so-called Mendelian traits, in which the presence of rare mutations produces a distinctive trait, and the transmission of that trait can be followed through families based on the clinical features. These traits generally follow a few simple patterns of inheritance. Autosomal dominant traits are produced by the inheritance of one mutated copy of a gene. As a result, such traits are commonly transmitted from an affected parent to half of their offspring, and multigenerational pedigrees with many affected subjects can sometimes be found. Dominant mutations can either be gain-of-function – in which the mutant gene has function not present in the normal gene – or less often loss of biochemical function, with a large effect produced by a 50% reduction in gene dosage. Dominant mutations that drastically impair reproductive fitness are commonly found as de novo mutations; new mutations found in affected index cases that are absent in their biological parents. Autosomal recessive traits require the inheritance of mutations in the genes on both chromosomes. Typical recessive pedigrees show affected subjects among a single sibship in a family, with parents being clinically unaffected and one in four of their offspring being affected. The mutations that cause autosomal recessive traits typically result in loss of biochemical function. Mutations on the X-chromosome produce distinctive patterns of transmission, since males have only one copy of this chromosome. As a result, these traits are never transmitted from affected fathers to their sons, and loss-of-function traits are found far more frequently among males than females. Other patterns of Mendelian transmission are much less frequent.
The development of complete genetic maps of the human genome identified extensive variations in DNA sequence, allowing the comparison of the inheritance of every segment of every chromosome to the inheritance of the Mendelian trait in families. With sufficient numbers of informative individuals and families, the chromosomal location of disease genes can be mapped, and genes in the linked interval can be searched for mutations. The finding of independent mutations that show specificity for the trait, and which significantly segregate with the trait in pedigrees, provides evidence that a gene responsible for the trait has been identified. To date, genes and their corresponding mutations that underlie more than 3000 human disease traits have been identified by this approach. The strength of Mendelian genetics has been that the identified mutations directly identify the gene whose function is altered, and is causal to the trait. A second strength is that because the effect size is typically very large, robust inferences about the effect of implicated genes on the trait are possible.
A second general approach to genetic analysis is genetic association. This approach has historically sought to determine whether specific common variants are found with significantly different frequency among cohorts of patients contrasting for specific phenotypes. Early efforts typically used candidate gene approaches, and commonly produced false-positive results. The development of dense maps containing millions of common variants called single nucleotide polymorphisms (SNPs) across the human genome, coupled with the ability to genotype these inexpensively, has led to large-scale genome-wide association studies (GWAS). These studies have allowed careful matching of genetic backgrounds of cases and controls, as well as rigorous statistical thresholds for significance. More than 1200 robust associations of common sequence variations with disease have been established using this approach. Strengths of the approach are the ability to find effects that are common in the population. These have been highly informative in diseases for which Mendelian forms have not been found. Weaknesses are that the common variants are most often not in genes, and consequently identifying the genes whose expression might be altered can be difficult to establish. Also, effect sizes are typically very small, changing disease risk by ~20%. As a result, new biological inferences are frequently difficult to immediately discern from these studies. Nonetheless, these studies can provide leads for diseases that have not yielded productive results from Mendelian studies.
Most recently, the ability to inexpensively sequence whole genomes or whole exomes (all the exons of protein-coding genes) has provided new opportunities for disease gene discovery. Classes of Mendelian traits that were previously intractable, such as diseases caused predominantly by de novo mutations, can now be solved. Moreover, one can anticipate that searches for rare variants with moderate effect size – less than typical Mendelian traits, but much larger than GWAS signals – are likely to be discovered by sequencing large cohorts of cases and controls.
Insight into Human Blood Pressure Variation from Genetic Studies
In the past 20 years, molecular genetic studies of rare Mendelian diseases featuring extreme forms of hyper- and hypotension have greatly contributed to our understanding of renal salt handling, and its role in blood pressure regulation. Despite the complexity of blood pressure regulation, which is influenced by diverse mechanisms including the neuronal, cardiovascular, and endocrine systems, many if not all of the genes thus far identified ultimately directly or indirectly affect renal salt reabsorption. Specifically, genes whose products increase renal salt reabsorption cause hypertension, while genes diminishing renal salt reabsorption result in hypovolemia, and sometimes life-threatening hypotension. Increased salt reabsorption is accompanied by water reabsorption to maintain normal concentrations of Na + , leading to increased intravascular volume, increased venous blood return to the heart, and increased cardiac output via the Frank–Starling mechanism. Blood pressure then rises according to Ohm’s law. These findings have implicated renal salt handling as a key element of long-term blood pressure homeostasis.
Nonetheless, hemodynamic patterns among hypertensive patients, even among those with primary increases in renal salt homeostasis, show increased systemic vascular resistance (SVR) with normal cardiac output. An explanation for this has been provided by Hall and Guyton, who have shown that tissues regulate their perfusion by increasing or decreasing vascular resistance according to metabolic demand. Dogs given aldosterone initially show expanded intravascular volume and increased cardiac output, but within weeks evolve to a state of high SVR and normal cardiac output. These findings establish that one cannot infer the initiating cause of hypertension from steady-state hemodynamic profiles.
Overview of Renal Salt Homeostasis
The kidneys filter about 180 liters of plasma per day, containing ~1.5 kg of salt; ~99.5% of the filtered salt load must be reabsorbed on a typical Western diet to maintain sodium homeostasis ( Figure 36.1 ). The bulk of this reabsorption (50–60%) occurs in the proximal tubule, driven by the basolateral Na + /K + -ATPase and apical Na + /H + exchanger, as well as co-transporters that couple uptake of glucose, amino acids, and other solutes to the favorable gradient for Na + reabsorption. Approximately 30% of the filtered load is reabsorbed in the thick ascending limb of Henle (TAL) via the Na + /K + /2Cl − co-transporter NKCC2, the target of loop diuretics, and ~7–10% in the distal convoluted tubule (DCT) via the thiazide-sensitive Na-Cl co-transporter NCC. The fine-tuning of renal salt reabsorption (~2–5%) occurs in the connecting tubule (CNT) and cortical collecting duct (CCD), and is predominantly mediated by the epithelial sodium channel (ENaC), the target of the potassium-sparing diuretic amiloride. Many of the tubular channels, transporters, and regulators involved in these processes of salt reabsorption are affected by loss- and/or gain-of-function mutations that will be discussed in this chapter.
A major regulatory pathway that modulates renal salt reabsorption is the renin–angotensin–aldosterone system ( Figure 36.1 ). In response to intravascular volume depletion or reduced delivery of salt to the thick ascending limb of Henle, the juxtaglomerular apparatus of the kidney secretes the active form of the aspartyl protease renin. Active renin cleaves angiotensinogen that is produced by the liver and constitutively circulates in the blood; this cleavage produces the decapeptide angiotensin I (AI), which is then cleaved by the angiotensin-converting enzyme (ACE), resulting in the octapeptide angiotensin II (AII). Angiotensin II binds to a specific G-protein-coupled receptor in adrenal glomerulosa (the type 1 angiotensin II receptor or AT1 receptor). This binding results in the activation of signaling cascades leading to adrenal glomerulosa membrane depolarization, activation of voltage-gated calcium channels, calcium influx, and increased synthesis of the steroid hormone aldosterone. Aldosterone, an effector of this pathway, communicates a signal for increased salt reabsorption to the kidney: it binds to the mineralocorticoid receptor (MR), a nuclear hormone receptor located in cells of the DCT, CNT, and principal cells of the CCD (the so-called aldosterone-sensitive distal nephron), leading to increased salt reabsorption via ENaC, and also the NCC (see above). The renin–angiotensin–aldosterone (RAA) pathway is not only mutated in genetic disorders of salt homeostasis, but is also the target of multiple pharmacologic approaches in the treatment of hypertension, including renin inhibitors, ACE inhibitors, angiotensin receptor blockers, and aldosterone antagonists.
This chapter covers the genetic disorders that modulate blood pressure by altering renal salt reabsorption, as well as the insights into physiological mechanisms of blood pressure regulation derived from these discoveries.
Genetic Disorders Causing Hypertension
Mendelian forms of hypertension are rare among the general hypertensive population. Clues to the deduction that a patient may have an underlying Mendelian cause of hypertension generally come from the age of onset, the family history, and distinctive biochemical features. Hypertension is very uncommon in the first decade of life, and unusual before the age of 18; however, this is a typical finding among subjects with Mendelian hypertension. Consequently, Mendelian diseases should be considered in patients with early onset hypertension, and should be ruled out if there is also a family history of early onset hypertension. Other diseases that can cause hypertension in young subjects, such as structural renal defects and other causes of renal insufficiency, should be ruled out as well. Hypertension can be very severe, but this is not invariably the case. Biochemical findings that are most helpful in pointing to a diagnosis are plasma renin activity, aldosterone levels (best measured in 24 hour urine specimens), and serum/plasma electrolyte values. It is important to note that patients with aldosteronism need not have hypokalemia, and the absence of this finding should not be taken to exclude a disorder caused by increased aldosterone or increased activity of the mineralocorticoid receptor. An algorithm that is helpful in the evaluation of these patients is shown in Figure 36.2 .
Disorders of the Renin–Angiotensin–Aldosterone System
Mutations that Increase Activation of the Mineralocorticoid Receptor
A number of genes have been identified in which mutation results in hypertension due to increased activation of the mineralocorticoid receptor (MR). These include diseases caused by mutations that lead to renin-independent production of aldosterone, as well as diseases in which steroids other than aldosterone can activate MR.
Mutations Resulting in Increased Aldosterone Secretion
Glucocorticoid-Remediable Aldosteronism (Familial Hyperaldosteronism Type I)
Glucocorticoid-Remediable Aldosteronism (GRA) is an autosomal dominant disease featuring hypertension with inappropriate aldosterone secretion despite suppression of the renin–angiotensin system. Patients typically present with hypertension in the first two decades of life, and are found to have elevated aldosterone secretion despite suppressed plasma renin activity, indicating autonomous production of aldosterone. Hypertension is often severe, and affected subjects are at a markedly increased risk of cerebral hemorrhage at young ages. Hypokalemia and metabolic alkalosis are variable associated findings. Consistent with autosomal dominant transmission, the family history is usually positive, with one parent and about half of siblings and offspring having early diagnosis of hypertension, and there is a frequent history of early cerebral hemorrhage.
Unlike normal individuals, aldosterone secretion in GRA shows sustained increase with administration of the cortisol secretagogue ACTH. Moreover, affected subjects produce so-called hybrid steroids, 18-hydroxy- and 18-oxocortisol, which are present in negligible amounts in normal subjects. These hybrid steroids have hydroxylation at C-17, characteristic of metabolism by 17-alpha hydroxylase (CYP17 gene) in the adrenal fasciculata, and oxidation at C-18, characteristic of metabolism by aldosterone synthase, which is normally confined to the adrenal glomerulosa ( Figure 36.3 ). Suppression of cortisol secretion from the adrenal gland by administration of exogenous glucocorticoids also causes rapid and sustained suppression of aldosterone secretion, a finding that does not occur in normal individuals. These features suggest that aldosterone in GRA is produced in the adrenal fasciculata under the control of ACTH, rather than in adrenal glomerulosa under the control of the normal secretagogue angiotensin II (Ang II).
Analysis of linkage in a large GRA kindred demonstrated complete linkage of early hypertension to chromosome 8q, which contains the gene aldosterone synthase (CYP11B2) and the closely related gene steroid 11-β hydroxylase (CYP11B1). These two genes recently evolved from a common ancestor, and are highly similar at the DNA sequence level. Patients with GRA prove uniformly to have mutations that result from unequal crossing-over recombination between these two homologous genes ( Figure 36.4 ). This recombination event produces two mutant chromosomes, one with normal copies of aldosterone synthase and 11-β hydroxylase, and a third chimeric gene between them that fuses 5′ regulatory sequences from 11-β hydroxylase to coding sequences of aldosterone synthase. This is the mutation uniformly found in patients with GRA. The other mutant chromosome has no normal gene, and only a single hybrid gene that fuses 5′ sequences from aldosterone synthase to a coding sequence that produces 11-β hydroxylase enzymatic activity. This mutation is found in some patients with 11-β hydroxylase deficiency. These events occur in pre-meiotic germ cell development, hence only one is transmitted to a zygote. All of these recombination events seen in GRA occur upstream of exon 5, with the result that the hybrid genes in patients with GRA include the two amino acids in exons 5 and 6 that are critical for the resulting enzyme having aldosterone synthase rather than 11-β hydroxylase activity. These hybrid genes bear the 5′ regulatory elements of 11-β hydroxylase, and are consequently expressed in adrenal fasciculata under the control of ACTH, but encode aldosterone synthase enzymatic activity. As a consequence, aldosterone secretion in these patients is constitutive, driven by ACTH and the maintenance of normal cortisol levels. The renin–angiotensin system is suppressed, but this fails to turn off aldosterone production. These mutations thus account for the ectopic production of aldosterone in adrenal fasciculata and its control by ACTH, and explain the resulting hypertension.
GRA should be suspected by the finding of hypertension in young individuals with elevated aldosterone level (best measured in 24 hour urine samples), despite suppressed plasma renin activity, particularly if there is a history of early hypertension among first degree relatives. Hypokalemia and metabolic alkalosis are common but by no means invariant, and the absence of these findings should not be used to exclude the diagnosis. Molecular genetic testing by either Southern blotting or PCR provides a sensitive and specific means for establishing the diagnosis. Because of the autosomal dominant transmission, there are frequently many affected members among extended families of index cases, and case finding can be performed by sequential sampling, testing all first degree relatives of affected subjects, then all first degree relatives of positive cases in subsequent rounds. Sampling in these families is likely to prevent morbidity and mortality from uncontrolled hypertension and early cerebral hemorrhage. Because of the availability of genetic screening, determination of urinary levels of hybrid steroids or dexamethasone suppression test is no longer recommended. Of note, family screening has demonstrated that not all individuals carrying the gene fusion are hypertensive, and first degree relatives of patients with confirmed GRA should be genetically screened even in the absence of hypertension.
Treatment options for GRA include use of mineralocorticoid receptor antagonists (spironolactone or eplerenone), amiloride or triamterene (which inhibit the epithelial sodium channel that drives both hypertension and hypokalemic alkalosis in GRA). Exogenous glucocorticoids can be used to shut down aldosterone production from the adrenal fasciculata. Careful attention to dosage in children is essential to maintain normal growth and to avoid glucocorticoid side-effects. Potassium-wasting diuretics such as hydrochlorothiazide and furosemide should be used with caution, because of the risk of severe hypokalemia.
Aldosterone-Producing Adrenal Adenoma and Familial Hyperaldosteronism Due to KCNJ5 Mutations
Aldosterone-producing adenoma, also known as Conn’s syndrome, is a common cause of severe hypertension, found in about 5% of patients in hypertension referral clinics worldwide, and in about half of patients diagnosed with primary aldosteronism. Patients typically present with worsening hypertension, and are found to have elevated serum and 24 hour urinary aldosterone levels in conjunction with suppressed plasma renin activity, consistent with these tumors having renin-independent aldosterone secretion. Hypokalemia and metabolic alkalosis are frequent, but not invariant findings. The finding of an adrenal mass by CT scan with increased aldosterone levels in ipsilateral adrenal vein plasma is considered diagnostic, and removal of these tumors corrects or improves blood pressure in the large majority of patients.
Exome sequencing of four aldosterone-producing adrenal adenomas (APAs) and matched blood DNA enabled identification of somatic mutations in the tumors. The results showed a low somatic mutation rate, with only 2–5 somatic mutations per tumor. Surprisingly, the K + channel encoded by KCNJ5 was mutated in two of these tumors. Examination of this channel in 22 tumors identified somatic KCNJ5 mutations in 8 tumors, and either of the same two mutations were found in all, substituting arginine for glycine at position 151 or arginine for leucine at position 168. These two positions lie in or abut the highly conserved K + channel selectivity filter that enables these channels to allow passage of K + but not other ions through the channel pore. Electrophysiologic studies demonstrated that these mutations cause markedly increased Na + conductance of the mutant channel, sufficient to depolarize the cell.
These findings explain the pathogenesis of APA in these tumors. The normal adrenal glomerulosa cell is hyperpolarized owing to constitutively open K + channels. Angiotensin II signaling results in closure of these channels, resulting in depolarization and activation of voltage-gated calcium channels, which raises intracellular calcium. Hyperkalemia produces the same results, potentially via increased frequency of depolarizing membrane potential oscillations. Increased intracellular Ca 2+ is the acute signal for increased expression of aldosterone synthase and other rate-limiting steps in aldosterone biosynthesis. Chronic signaling provides the stimulus for increased cell replication. Thus, these single mutations can explain both the cell-autonomous aldosterone secretion and cell proliferation that are the hallmarks of these benign tumors.
Subsequent work has confirmed these findings. The study of 287 tumors by Björklund et al. found the G151R or L168R mutations in 47% of APAs, and a markedly higher prevalence of these mutations among women with APA (63%) than men (22%). Only one additional mutation (E145Q) was found in two cases. Similar findings were observed by Boulkroun et al., who found either of these two mutations in 34% of all APAs with a similar bias for female subjects. These tumors are more prevalent among younger individuals, and tend to be slightly larger compared to non-KCNJ5-mutant tumors.
In addition to these mutations causing APAs, they also account for a rare inherited form of primary aldosteronism. In 2008, Geller et al. reported a father and two daughters with a new familial form of severe early-onset hypertension due to primary aldosteronism. Hypertension in these patients was diagnosed between the ages of 4 and 7 years; it was resistant to aggressive antihypertensive therapy, including spironolactone and amiloride. There was massive adrenocortical hyperplasia. Hybrid steroids 18-oxocortisol and 18-hydroxycortisol were elevated, however, in clear contrast to patients with GRA, there was a significant increase in aldosterone levels upon dexamethasone administration, and affected subjects did not have the gene fusion characteristic of GRA. Due to unrelenting hypertension, all three subjects underwent bilateral adrenalectomy in childhood, demonstrating massive adrenal hyperplasia (with paired adrenal weights up to 81 g, normal is less than 12 g) and diffuse hyperplasia of the zona fasciculata by light microscopy (with transitional zone morphology by electron microscopy). Screening of candidate genes revealed no pathogenic mutations.
Affected members of this family proved to have a T158A mutation in KCNJ5 which also modified channel selectivity, resulting in Na + conductance. In this case, since the mutation is present in every cell, rather than acquired somatically by a single cell, every cell in the adrenal glomerulosa is receiving the signal for aldosterone production and cell proliferation, accounting for the massive adrenal hyperplasia and severe aldosteronism at young ages. This Mendelian form of disease provides strong evidence that these single mutations are sufficient for cell proliferation and constitutive aldosterone secretion.
Subsequent studies have identified additional families with early severe aldosteronism and mutations in KCNJ5. Most interestingly, these include two different mutations at the same amino acid – G151 – that result in markedly different phenotypes. One of these inherited mutations is G151R, the same mutation found as a somatic mutation in APA. Patients with inherited G151R mutations develop massive adrenocortical hyperplasia, have difficult-to-control hypertension, and virtually invariably come to bilateral adrenalectomy for control of hypertension, similar to the T158A mutation described above. The other mutation is G151E. Patients with this mutation also present with early hypertension and aldosteronism; however, they do not develop adrenal hyperplasia, are typically responsive to antihypertensive therapy, and do not come to adrenalectomy. Most interestingly, the milder human phenotype resulting from G151E is associated with a much more severe electrophysiologic phenotype. G151E results in dramatically greater Na + conductance than G151R. The consequence of this is markedly increased sodium-dependent lethality. This suggests a model in which cells expressing the G151E mutation differentiate from a stem cell pool, produce aldosterone as they are born, but die rapidly, preventing development of hyperplasia. Their continuous renewal from a stem cell pool provides a long-term source for excessive aldosterone production, and a milder hypertension than in subjects with the G151R mutation.
This phenotype of primary aldosteronism without adrenal hyperplasia due to KCNJ5 mutation is as yet unique to the G151E mutation, while massive adrenal hyperplasia requiring adrenalectomy has also been reported with another KCNJ5 mutation, I157S.
While KCNJ5 mutations unequivocally explain the pathogenesis of a large fraction of APAs and a rare form of familial hypertension, the role of the wild-type channel in human adrenal function is less clear. Its activation by dopamine, an inhibitor of aldosterone secretion, suggests that the normal role of this channel might be to hyperpolarize cells, contributing to inhibition of aldosterone secretion.
Primary aldosteronism due to mutations in aldosterone synthase and KCNJ5 both produce constitutive secretion of aldosterone and hypertension. Patients with KCNJ5 mutations tend to present in the first several years of life with severe hypertension, while those with GRA are more frequently, though not exclusively, diagnosed later in the first or second decade. Consistent with greater disease severity among KCNJ5 families and impaired reproductive fitness, no families with more than four affected members have been reported to date, while a number of large, multigenerational families with more than 20 members with GRA have been studied.
Mutations Causing Impaired Cortisol Biosynthesis and Increased Mineralocorticoids
Congenital Adrenal Hyperplasia Due to 11-β-Hydroxylase Deficiency
Congenital adrenal hyperplasia (CAH) is a recessive disease, featuring cortisol deficiency caused by inherited defects of enzymes required for cortisol biosynthesis in the adrenal gland. Affected patients have increased ACTH levels due to impaired feedback inhibition, which stimulates excessive production of steroids proximal to the impaired step in cortisol biosynthesis. Precursor steroids can have androgenic effects, and female patients may present with virilization. In the most common form of CAH (21-hydroxylase deficiency), impaired synthesis of mineralocorticoids leads to salt-wasting (see below), while other defects can result in accumulation of precursors which have mineralocorticoid effects, leading to hypertension.
The most common form of congenital adrenal hyperplasia associated with hypertension is 11-β-hydroxylase deficiency ( Figure 36.3 ). Affected females may present with ambiguous genitalia at birth or with menstrual irregularities and hirsutism in adolescence or adulthood, while male subjects may present with penile enlargement or precocious puberty. Hypertension and hypokalemia are variable associated features, and are thought to occur due to the accumulation of 11-deoxycorticosterone, a moderately potent mineralocorticoid. However, there is no clear correlation between 11-deoxycorticosterone levels, the degree of virilization and the presence of hypertension. Renin and aldosterone are typically low.
In most cases, 11-β-hydroxylase deficiency is caused by recessive point mutations that cause loss-of-function of CYP11B1, although loss-of-function mutations due to the reciprocal product of unequal crossing-over between aldosterone synthase and 11-β-hydroxylase found in GRA (see above) can also produce loss-of-function mutations (in this case there is a chromosome that only expresses 11-β hydroxylase in adrenal glomerulosa).
11-β-hydroxylase deficiency should be suspected in female infants with ambiguous genitalia and male infants with penile enlargement. In the late onset form, hirsutism and menstrual irregularities in female patients and precocious puberty in boys may be the only signs of presentation. The abnormal hormone profile, in particular the finding of elevated basal and ACTH-stimulated 11-deoxycortisol concentrations, is diagnostic. Treatment with exogenous glucocorticoids suppresses ACTH secretion and the accumulation of precursor steroids. Precursor steroids should be monitored, and glucocorticoid dosage has to be carefully adjusted to avoid growth inhibition in children and exogenous Cushing’s syndrome. Persistent hypertension may require additional treatment with aldosterone antagonists or amiloride.
Congenital Adrenal Hyperplasia Due to 17-α-Hydroxylase Deficiency
17-α-hydroxylase deficiency is a rare cause of congenital adrenal hyperplasia. Patients typically present in adolescence with lack of pubertal development. Genetic females have primary amenorrhea and do not develop secondary sexual characteristics; genetic males typically have complete pseudohermaphroditism with female external genitalia and intra-abdominal testes, although ambiguous genitalia have been reported. Hypertension is a common finding, and may be associated with hypokalemia.
17-α-hydroxylase deficiency is caused by mutations in CYP17. The encoded enzyme is expressed in adrenal gland and gonads, and has both 17-hydroxylase and 17,20-lyase activities ( Figure 36.3 ). Most patients with CYP17 mutations thus have combined deficiencies of both enzymatic functions, although isolated deficiency of 17,20-lyase activity has been reported. Hydroxylation at carbon 17 of the steroid nucleus is required for cortisol production, and in its absence normal activation of the glucocorticoid receptor is not achieved. In addition, 17,20-lyase activity is needed for the generation of androgen and estrogen precursors from cortisol precursors.
Patients with CYP17 mutations consequently have cortisol deficiency leading to elevated ACTH levels, as well as androgen and estrogen deficiency, accounting for sexual infantilism and pseudohermaphroditism. Hypertension is caused by activation of the mineralocorticoid receptor (MR) due to increased levels of 11-deoxysteroids (corticosterone, 11-deoxycorticosterone, and 18-hydroxy-deoxycorticosterone), similar to the mechanism of hypertension in 11-β-hydroxylase deficiency. As a result, renin and aldosterone are typically suppressed.
The diagnosis is based on the clinical presentation and characteristic hormone profile, and treatment relies on replacement of glucocorticoids and sex steroids, the latter starting in adolescence.
Mutations that Impair Conversion of Cortisol to Cortisone: Syndrome of Apparent Mineralocorticoid Excess
The syndrome of apparent mineralocorticoid excess (AME) is an autosomal recessive disease featuring severe hypertension presenting in the first decade of life. It is associated with suppression of both the renin–angiotensin system and aldosterone secretion. Hypokalemia and metabolic alkalosis are common associated findings. The hypertension can be mitigated with antagonists of the mineralocorticoid receptor, which suggested the presence of a new circulating mineralocorticoid; however, such a molecule was not found. Instead, a defect in cortisol metabolism was identified in affected patients, causing reduced conversion of cortisol to cortisone, with a consequently increased half-life of cortisol.
How this defect in cortisol metabolism resulted in hypertension remained unclear until the cloning and subsequent purification of the mineralocorticoid receptor (MR). In vitro , cortisol binds and activates MR as well as aldosterone does, while cortisol is normally a weak mineralocorticoid in vivo . This discrepancy is explained by the presence in many aldosterone-responsive cells of an enzyme, steroid 11-β hydroxysteroid dehydrogenase type 2 (11βHSD2), that “protects” MR from cortisol by converting cortisol to cortisone. Cortisone has negligible ability to activate MR. The finding of homozygous loss-of-function mutations in 11βHSD2 in AME provided proof of the relevance of this mechanism in vivo . These findings fully explain the pathophysiology of AME, and are confirmed by the recapitulation of AME in mice deficient for 11βHSD2.
AME should be suspected in young individuals with hypertension, suppressed PRA, and low aldosterone levels. AME shares these clinical features with Liddle syndrome (see below); however, Liddle syndrome is autosomal dominant, and affected subjects consequently commonly have one parent with early severe hypertension, and may have other relatives in earlier generations or other branches of the pedigree with similar findings. In contrast, AME is suggested by the absence of disease in earlier generations and the presence of parental consanguinity. The diagnosis of AME can typically be made by genetic testing involving sequencing of 11βHSD2. Abnormal cortisol:cortisone ratios can also establish the diagnosis.
Inhibitors of the mineralocorticoid receptor, such as spironolactone or eplerenone, or inhibitors of the epithelial sodium channel, such as amiloride or triamterene, are treatment options in AME. Eplerenone has recently been suggested for prevention of strokes even in the absence of hypertension; however, the utility of this approach is not established.
Intriguingly, exuberant ingestion of natural licorice produces a clinically similar syndrome owing to the effects of a metabolite of glycyrrhetinic acid – derived from glycyrrhizic acid in licorice – which inhibits 11βHSD2. Carbenoxolone, a drug once used for peptic/gastric ulcer disease, has similar effects.
In the presence of 11βHSD2, elevated levels of cortisol are required to achieve activation of the mineralocorticoid receptor (by exceeding the capacity of 11βHSD2 to metabolize cortisol). Hypertension can therefore occur in states of glucocorticoid excess, such as ectopic ACTH syndrome, Cushing’s disease due to a pituitary tumor, iatrogenic Cushing’s syndrome or cortisol-producing adenomas.
Mutations in the Mineralocorticoid Receptor
Hypertension Exacerbated in Pregnancy
As discussed above, binding of aldosterone to the mineralocorticoid receptor causes increased renal salt reabsorption by raising the activity of the epithelial sodium channel ENaC, which results in elevated blood pressure. Geller and colleagues identified an as-yet unique family with early onset of typically severe hypertension associated with suppressed PRA, negligible aldosterone levels, and variable hypokalemia and alkalosis; this trait segregated as an autosomal dominant trait in a large extended family. A striking clinical feature among affected women was marked exacerbation of hypertension in pregnancy, requiring early delivery for poorly controlled hypertension and marked hypokalemia. Analysis of linkage in this large kindred localized the disease gene to the segment of chromosome 4 that harbors the mineralocorticoid receptor. This gene proved to have a novel missense mutation in the ligand-binding domain, S810L, which precisely co-segregated with the disease phenotype in the kindred.
The clinical features suggested that this mutation results in an activated receptor, despite the absence of aldosterone. In vitro studies demonstrated that this is the case: the mutant receptor shows partial constitutive activity in the absence of ligand. In addition, however, the mutation altered the specificity of the receptor such that some steroids lacking 21-hydroxyl groups, which normally fail to activate MR, are now potent agonists. One of these new agonists is progesterone, a steroid whose level increases dramatically in pregnancy. Cortisone is another molecule that can more weakly activate the mutant receptor, suggesting that this cortisol metabolite may play a constitutive role. The crystal structure of the mutant mineralocorticoid receptor was solved, suggesting that progesterone-induced MR S810L activation is caused by a network of contacts at the A-ring, created by L810.
Mutations that Cause Hypertension without Activation of MR
Mineralocorticoids impart their renal effects by modulating the expression and activity of ion transporters and channels. Mutations in several genes have been identified that cause hypertension without activation of MR.
Liddle syndrome is an autosomal dominant trait featuring hypertension with variable hypokalemia and alkalosis. While these features recapitulate the phenotype of the diseases with mineralocorticoid excess discussed above, there are no circulating mineralocorticoids in patients with Liddle syndrome. Hypertension is responsive to treatment with blockers of the epithelial sodium channel ENaC (amiloride or triamterene), but not to spironolactone.
Genetic studies identified rare mutations in subunits of ENaC as the cause of Liddle syndrome ( Figures 36.5 and 36.6 ). ENaC is composed of subunits encoded by three homologous genes, alpha, beta, and gamma ; each has cytoplasmic amino and carboxy termini, and traverses the plasma membrane twice. Virtually all mutations that cause Liddle syndrome eliminate a sequence required for removal of the channel from the cell surface by clathrin-coated pit-mediated endocytosis. This internalization sequence, PPPXY (where X is variable among different ENaC subunits) is present in the carboxy terminus of each subunit. Disease causing mutations introduce premature termination codons after the second transmembrane domain and before the PPPXY sequence, or alternatively produce missense mutations at one of the cognate positions of the internalization motif. These mutations result in reduced clearance and prolonged half-life of the channel at the cell surface ( Figure 36.6 ). Nedd4 was subsequently identified as having domains that specifically bind the PPPXY domains in ENaC, and contribute to its clearance from the cell surface. This protein contains ubiquitin-ligase domains, suggesting that ubiquitination and subsequent degradation plays a role in ENaC regulation. The likely candidate for this regulation in human kidney is Nedd4-2. Together, these findings suggest that mutations in ENaC in Liddle syndrome impair binding of Nedd4 to the mutant channel, and prevent ubiquitination and degradation. In addition to the mutations deleting or altering the PY motif, a mutation in the extracellular domain of γ-ENaC has been reported as causing an increased open probability of the channel. Consistent with these genetic findings, renal transplantation can cure the hypertension of Liddle syndrome.
A mouse model of Liddle syndrome does not develop hypertension or hypokalemic alkalosis on a standard diet, despite increased ENaC activity. However, a Liddle phenotype develops when these animals are fed a high-salt diet.
Liddle syndrome should be suspected in patients with early hypertension and suppressed PRA and aldosterone levels; hypokalemia and metabolic alkalosis are supportive, but not essential findings. As discussed above, these features are shared with AME. A family history of hypertension consistent with autosomal dominant transmission, however, is suggestive of Liddle syndrome. Genetic testing is recommended. Treatment with amiloride or triamterene can correct hypertension and hypokalemia.
The Syndrome of Hypertension with Hyperkalemia (Pseudohypoaldosteronism Type II)
Aldosterone is secreted in two distinct physiologic conditions: volume depletion and hyperkalemia. Restoration of normal intravascular volume in the former condition requires increased salt and water reabsorption, while K + secretion must be increased in the latter state. How the kidney achieves these alternative results has been obscure. Classical explanations have suggested that this is a result of alterations in fluid flow and/or delivery of salt to the distal nephron. The notion that this is an incomplete explanation comes from a disease in which the kidney appears to be unable to use aldosterone to direct K + secretion, but instead constitutively activates salt reabsorption. This disease, referred to as pseudohypoaldosteronism type II (also known as syndrome of hyperkalemia with hypertension, Gordon syndrome) invariably features hyperkalemia, and has variable levels of hypertension and distal renal tubular acidosis. Aldosterone levels are normal to slightly elevated, and plasma renin activity is suppressed. Serum K + levels are virtually always greater than 5.0 mM without therapy, and can be greater than 7.0 mM. The disease was first described in 1964 by Paver and Pauline, who reported a 15-year-old boy with blood pressure 180/120 mmHg, serum potassium 7.0–8.2 mM, but normal glomerular filtration, and suppressed PRA and normal aldosterone. Since early descriptions, the disease has been recognized to be familial, initially described as an autosomal dominant trait.
Subsequent studies have demonstrated that K + cannot be corrected by supplemental aldosterone; however, both hypertension and hyperkalemia can be corrected by elimination of chloride intake or by low doses of thiazide diuretics. These findings have suggested that increased activity of the thiazide-sensitive Na-Cl co-transporter may play a role in this disease.
Analysis of genetic linkage in families followed by molecular studies identified mutations in two members of a novel family of serine-threonine kinases, PRKWNK1 and PRKWNK4, as causes of PHAII ( Figure 36.7 ). Mutations in WNK4 were novel missense mutations that predominantly clustered in a short acidic domain of unknown function in the protein. Mutations in WNK1 were large deletions of 20,000 to 40,000 base pairs in the first intron of WNK1 that caused increased expression of WNK1 mRNA. These genes were both found to be expressed in the distal convoluted tubule and collecting duct of the kidney.
At the time of this discovery, the normal and disease-related functions of WNK1 and WNK4 were entirely unknown. Much subsequent work has been done to determine the roles these WNK kinases play in physiology. It appears that both play a role in coordinating the functions of diverse electrolyte flux pathways, and that a key function in the kidney is to regulate the balance between Na-Cl reabsorption and K + secretion. The WNK kinases regulate ion transport by several mechanisms, among them phosphorylation via intermediary kinases (SPAK, OSR1, and SGK1), and regulation of protein trafficking.
Early studies addressed the role of WNK4 in the regulation of NCCT using the oocyte expression system. Co-expression of NCCT with WT-WNK4 resulted in inhibition of thiazide-sensitive Na + uptake due to reduced surface expression. This effect was completely abrogated when a mutation seen in PHA II (Q562E) or kinase-dead WNK4 was used, suggesting that NCCT inhibition is lost in patients with PHA II, causing hypertension. Subsequent studies in mammalian cells suggested that this inhibitory WNK4 effect is mediated by suppression of membrane delivery via lysosomal accumulation.
Co-expression with WNK4 also dramatically reduces ROMK activity in oocytes, by interaction with an endocytic scaffold protein, intersectin. However, in contrast to the effect on NCCT, PHA II-mutant WNK4 produces increased inhibition of ROMK, accounting for hyperkalemia in PHA II. While WT WNK4 led to a modest increase of paracellular chloride permeability in MDCK cells, this effect was greatly augmented in PHA II-mutant WNK4, which may contribute to the chloride retention observed in PHA II. In addition, WNK4 inhibited ENaC activity in oocytes, and this inhibition was relieved by PHA II mutations. The inhibitory effect of WNK4 on ENaC and ROMK appears to require phosphorylation of WNK4 by SGK1, a kinase which translates aldosterone effects in the kidney.
Similar studies as for WNK4 were performed for WNK1. While a direct effect of WNK1 on NCCT was not identified, a long isoform of WNK1 (L-WNK1) inhibits WNK4 (which inhibits NCCT, see above), and a kidney-specific isoform (KS-WNK1) inhibits L-WNK1. In addition, L-WNK1 inhibits ROMK, an effect reversed by KS-WNK1 and synergistic with, but not dependent on, the WNK4 effect on ROMK ; L-WNK1 also activates ENaC. Finally, similar to WNK4, WNK1 increases paracellular chloride flux. ATII, which is produced in volume depletion, appears to be an upstream regulator of WNK4, attenuating its inhibitory effect on NCCT and thus stimulating salt reabsorption via NCCT. In contrast, the inhibitory effect of WNK4 of ROMK is retained. These findings suggest that PHA II mutations mimic a state of volume depletion with increased angiotensin II levels.
Regulation of several additional proteins (e.g., TRP channels, SLC26A6) by WNK1 and WNK4 kinases has been reported, and studies of the related kinase WNK3 has revealed a role in regulation of NKCC, as well as KCC co-transporters.
Taken together, in vitro studies suggest a key role of the WNK kinases in a functional switch among three states governing the balance between salt reabsorption and K + secretion. In a basal equilibrium state, both NaCl reabsorption and K + secretion are inhibited by the WNK kinases. In the setting of low intravascular volume, AII signaling inhibits WNK4’s inhibition of NCC, but increases inhibition of ROMK; this effect is mimicked by PHA II mutations, accounting for constitutive NaCl reabsorption and hyperkalemia seen in affected patients. In a third state, hyperkalemia leads to increased aldosterone production by the adrenal gland and, potentially via SGK, to increased ROMK activity and increased ENaC activity, with sustained inhibition of electroneutral salt reabsorption via NCCT.
Animal models have confirmed the role of WNK1 and WNK4 in generating hyperkalemia and hypertension. A transgenic mouse model carrying an additional copy of PHA II-mutant WNK4 shows hypertension, hyperkalemia, and hypercalciuria and, in addition, increased mass of the distal convoluted tubule and increased NCCT expression. In contrast, an additional copy of WT WNK4 leads to decreased DCT mass and reduced NCCT abundance. A knockout mouse model of WNK4 displays a phenotype similar to Gitelman syndrome (see below), with increased plasma renin activity, and decreased NCC activity. Deletion of the first WNK1 intron as found in PHA II in mice leads to ubiquitous ectopic KS-WNK1 expression, as well as overexpression of L-WNK1 in the DCT, which explains increased activity of NCCT through alleviation of WNK4 inhibition.
Importantly, mutations in WNK1 and WNK4 explained only about 15% of families with PHA II. Exome sequencing of unrelated PHA II cases led to the identification of two additional genes for PHA II, which together account for nearly all of the remaining cases. These two genes are partners in a ubiquitin ligase complex. Cullin 3 (CUL3) is a scaffold that serves to assemble a complex containing a RING E-3 ubiquitin ligase and one of many BTB domain-containing proteins that target specific proteins for ubiquitination. One of these BTB domain proteins is Kelch-like 3 (KLHL3). KLHL3 has a C-terminal BTB domain, and an N-terminal kelch domain, a six-bladed propeller that is likely used to bind and target proteins for ubiquitination. Surprisingly, different families showed either autosomal dominant or autosomal recessive transmission of mutations in KLHL3. Recessive mutations are distributed throughout the protein, and include typical loss-of-function mutations such as premature termination mutations. In contrast, dominant mutations cluster at positions in the kelch domain that are clustered at parts of the domain inferred to be involved in target binding. Because these dominant mutations produce a phenotype very similar to that caused by homozygous loss-of-function mutations in KLHL3, the dominant mutations likely have a dominant-negative effect.
The mutations in CUL3 that cause PHA II all cause skipping of exon 9, resulting in an in-frame deletion of 57 amino acids between the BTB-binding and RING-binding domains. These mutations are all dominant, and nearly all appear to be de novo mutations. Because these mutations phenocopy recessive loss-of-function mutations in KLHL3, it is inferred that these CUL3 mutations selectively prevent ubiquitination at targets bound by KLHL3. While CUL3 is ubiquitously expressed, KLHL3 is selectively expressed in the distal nephron.
Comparison of the phenotypes of patients with mutations in these different genes reveals strong genotype–phenotype correlations. Patients with CUL3 mutations have the most severe hyperkalemia, are nearly always hypertensive at young ages, and commonly have acidosis. Patients with recessive and dominant KLHL3 mutations have intermediate phenotypic severity, and those with WNK1 and WNK4 mutations often have only hyperkalemia without hypertension at early ages, and do not uniformly develop hypertension at later ages. The molecular mechanisms that link mutations in KLHL3 and CUL3 to PHA II are presently unknown; however, it is highly likely that these mutations will intersect with the WNK/NCC/ROMK pathway.
Genetic Disorders Causing Renal Salt Loss
Several Mendelian disorders can cause renal salt loss, leading to signs and symptoms ranging from mild polyuria and salt craving to life-threatening hypotension and shock. A diagnostic approach to these diseases is suggested in Figure 36.8 .
Disorders of the Renin–Angiotensin–Aldosterone System
Mutations Affecting Circulating Mineralocorticoid Levels
Mutations Resulting in Congenital Hypoaldosteronism
Congenital hyper-reninemic hypoaldosteronism is a rare autosomal recessive disorder. Affected patients present in infancy with salt-wasting, leading to severe dehydration, failure to thrive and growth retardation, as well as hyponatremia and hyperkalemia. Aldosterone is low despite elevated renin. These features are a clinical mirror image of the states of primary aldosteronism described above.
Genetic studies identified mutations in the aldosterone synthase gene (CYP11B2) as the cause of congenital hypoaldosteronism. Aldosterone synthase catalyzes both 18-hydroxylation of corticosterone, and the subsequent conversion of the 18-hydroxyl group to an aldehyde ( Figure 36.3 ). Several mutations, including nonsense, missense, and frameshift mutations, affect the 18-hydroxylase activity of the enzyme; as a result, levels of 18-hydroxycorticosterone are low in these patients. In other cases, specific missense mutations can abolish 18-oxidase activity, while 18-hydroxylase activity remains intact or is only slightly reduced. These patients consequently have elevated levels of 18-hydroxycortisone.
Not all patients with hyper-reninemic hypoaldosteronism, however, have mutations at the CYP11B2 locus. Kayes-Wandover et al. described four unrelated kindreds (two consanguineous) without evidence of such mutations; linkage to the CYP11B2 locus could be excluded in the two consanguineous families, suggesting genetic heterogeneity. The underlying gene defect of this disorder, termed familial hyper-reninemic hypoaldosteronism type 2, remains to be established.
Congenital Adrenal Hyperplasia Due to Steroid 21-Hydroxylase Deficiency
Steroid 21-hydroxylase deficiency is an autosomal recessive disease. It accounts for the vast majority of the cases of congenital adrenal hyperplasia (CAH), and is one of the most common Mendelian diseases. 21-hydroxylase catalyzes the 21-hydroxylation of progesterone, and is a required step in the biosynthesis of both cortisol and aldosterone ( Figure 36.3 ).
Three subforms of the disease are recognized: patients with the salt-losing form of classic CAH are severely affected and present at birth or in infancy with cortisol and mineralocorticoid deficiency; females typically have ambiguous genitalia and virilization. In the simple virilizing form of classic CAH, high ACTH levels are sufficient to produce adequate levels of glucocorticoids and mineralocorticoids at the expense of increased adrenal androgen synthesis, resulting in virilization. Finally, patients with the late-onset or non-classic form may present in childhood, adolescence or even adulthood with premature pubarche and accelerated bone age; female patients develop hirsutism, infertility or irregular menstrual cycles.
The salt-losing form of classic CAH has an estimated incidence of ~1/23,000 live births, and is due to mutations in 21-hydroxylase (CYP21A gene) in almost all cases. Girls with this condition present as neonates with masculinization. Salt-losing adrenal crisis, with vomiting, dehydration, hypotension, hypoglycemia, and electrolyte abnormalities (hyponatremia and hyperkalemia) typically occurs in the first or second week of life, and these can be the only presenting symptoms in boys. When untreated, adrenal crisis can lead to shock, cardiac arrhythmia, and neonatal death.
CYP21A mutations causing salt-losing CAH often arise from unequal crossing-over between the functional CYP21A gene and a non-functional pseudogene located in close proximity ; the mutated enzyme typically shows complete loss-of-function. 21-hydroxylase deficiency is recognized clinically in patients with ambiguous genitalia or adrenal crisis, or in newborn screening programs; the diagnosis is based on elevated levels of 17-hydroxyprogesterone. Genetic testing is available, and can be supplemental for prenatal diagnosis, genetic counseling or ambiguous cases. Prenatal therapy with exogenous glucocorticoids can prevent or ameliorate virilization in female children, but is only recommended in a research setting due to potential side-effects. If the child is known to be at risk, treatment is started as early as the mother knows she is pregnant, and stopped if the child is found to be male or unaffected by prenatal screening. Treatment after birth includes glucocorticoid and mineralocorticoid replacement, as well as salt supplementation in infants, and potentially feminizing surgery.
Rare Causes of Salt-Wasting Congenital Adrenal Hyperplasia
Very rare causes of salt-wasting congenital adrenal hyperplasia include 3-β-HSD 2 deficiency and StAR deficiency in lipoid adrenal hyperplasia. In both diseases, severe adrenal insufficiency with deficiency of gluco- and mineralocorticoids occurs at neonatal age or in infancy; genetically male patients typically have abnormalities of sexual development at birth, with female external genitalia in StAR deficiency.
Mutations in Genes Expressed in the Kidney that Cause Salt-Wasting
Pseudohypoaldosteronism Type I
Pseudohypoaldosteronism Type I (PHA I) is a rare genetic syndrome whose features include renal salt-wasting, hyperkalemia, hyponatremia, and metabolic acidosis despite elevated renin and aldosterone levels. The disease presents as either an autosomal dominant or autosomal recessive trait. The recessive form appears to be uniformly severe, and requires life-long salt supplementation. The dominant form is clinically more variable at birth, and typically largely resolves after the first years of life with the ability to self-select a high-salt diet.
Autosomal Recessive Pseudohypoaldosteronism Type I (PHAI)
Infants with autosomal recessive PHA I frequently present with life-threatening episodes of hypovolemia, sometimes leading to neonatal death. Affected subjects have marked renal salt-wasting and striking hyperkalemia (as high as 12 mM), and renal tubular acidosis despite drastic elevations of PRA and aldosterone. There is also salt-wasting from salivary and sweat glands, as well as the colon. These patients require life-long sodium supplementation and often chronic treatment with resin-binding agents for hyperkalemia. Frequent pulmonary infections somewhat reminiscent of cystic fibrosis have been reported in some patients, and markedly increased ciliary clearance of lung water has been reported.
Genetic studies led to identification of recessive loss-of-function mutations in the alpha, beta or gamma subunits of the epithelial Na + channel (ENaC) in virtually all affected subjects. Functional studies in Xenopus oocytes confirmed loss of channel activity. In summary, these findings demonstrated that mutations in all ENaC subunits can cause PHA I. All the features of this disease are explained by loss of ENaC function. Loss of ENaC function results in dramatic salt-wasting, leading to activation of the renin–angiotensin system; however, because ENaC is absent, this fails to adequately augment salt reabsorption in the kidney. Impaired salt absorption in the colon may also contribute to disease severity. K + and H + secretion are also markedly impaired, owing to loss the lumen-negative potential resulting from loss of ENaC, causing hyperkalemia and acidosis.
These findings have been confirmed in animal studies. Mice deficient for the β- and γ-subunits show a phenotype characteristic of PHA I, with early death due to Na + -wasting and K + retention, and a PHA I-like phenotype is observed in α-ENaC knockout mice with transgenic expression of rat α-ENaC for rescue. α-ENaC knockout mice show early postnatal demise with a mean survival of 24 hours postnatally due to respiratory distress, suggesting a role of the mouse channel in pulmonary liquid clearance after birth that is apparently not observed at birth in humans but is found later in life.
Autosomal Dominant PHA I
Autosomal dominant PHA I has a milder phenotype that typically largely resolves in the early years of life. Patients may be asymptomatic or can present with renal salt-wasting and mild-to-moderate hyperkalemia and acidosis. NaCl supplementation can typically be discontinued after early childhood, as the disease remits with age. Geller et al. identified heterozygous mutations in the mineralocorticoid receptor gene in five kindreds with autosomal dominant PHA I. All mutations were inferred to be loss-of-function, inferred to cause diminished ENaC activity, with resulting salt loss. Since Na + reabsorption via ENaC is required for the lumen-negative potential that drives K + and H + secretion in the cortical collecting duct, diminished ENaC activity can cause hyperkalemic acidosis in these kindreds. The improvement with age is consistent with a decreasing requirement for aldosterone action in older children, presumably due to increasing salt intake and a self-selected high-salt diet after infancy. Adult carriers show no abnormalities of serum and urinary electrolytes or blood pressure, but consistently demonstrate markedly elevated serum aldosterone levels, consistent with homeostasis being achieved by compensating for the reduced level of MR by increased aldosterone levels.
A case of autosomal recessive PHA I due to mutations in the mineralocorticoid receptor was recently reported – a newborn with very severe PHA I who was compound heterozygous for S166X and W806X deletions. Within her family, carriers of the S166X mutation had typical autosomal dominant PHA I, while carriers of the W806X mutation were clinically asymptomatic, but had elevated renin and aldosterone levels.
A mouse model of PHA 1 shows increased renin and aldosterone, but develops normally in the heterozygous state. The homozygous knockout is lethal, but can be rescued by subcutaneous NaCl injection before weaning.
Mutations Affecting Renal Ion Channels, Transporters and their Regulators
Thick Ascending Limb of Henle
Bartter syndrome is a rare autosomal recessive disorder. Patients may present at various ages with a variety of signs and symptoms. Renal salt-wasting is the key underlying primary problem, causing polyuria and subsequent polydipsia. In severe cases, patients present antenatally with polyhydramnios (due to intrauterine polyuria), and affected subjects are often born prematurely. Renal salt-wasting, typically severe, leads to volume depletion, activation of the renin–angiotensin–aldosterone system and subsequent hypokalemic alkalosis; growth and mental development are impaired in some patients. Hypercalciuria and nephrocalcinosis are frequently found and can cause end-stage renal disease; deafness has been observed in a subset of patients.
Early physiologic studies localized defective salt reabsorption in patients with Bartter syndrome to the loop of Henle ( Figure 36.9 ), and the finding of elevated levels of prostaglandins enabled treatment with inhibitors of prostaglandin synthesis. The spectrum of clinical presentations and the underlying pathophysiology of this disorder was elucidated by the identification of the underlying genetic defects.