"Chloride-shunt" syndrome: an overlooked cause of renal hypercalciuria

A patient with pseudohypoaldosteronism type II complicated by congenital hypopituitarism carrying a KLHL3 mutation

Pseudohypoaldosteronism type II (PHA II) is a renal tubular disease that causes hyperkalemia, hypertension, and metabolic acidosis. Mutations in four genes (WNK4, WNK1, KLHL3, and CUL3) are known to cause PHA II. We report a patient with PHA II carrying a KLHL3 mutation, who also had congenital hypopituitarism. The patient, a 3-yr-old boy, experienced loss of consciousness at age 10 mo. He exhibited growth failure, hypertension, hyperkalemia, and metabolic acidosis. We diagnosed him as having PHA II because he had low plasma renin activity with normal plasma aldosterone level and a low transtubular potassium gradient. Further investigations revealed defective secretion of GH and gonadotropins and anterior pituitary gland hypoplasia. Genetic analyses revealed a previously known heterozygous KLHL3 mutation (p.Leu387Pro), but no mutation was detected in 27 genes associated with congenital hypopituitarism. He was treated with sodium restriction and recombinant human GH, which normalized growth velocity. This is the first report of a molecularly confirmed patient with PHA II complicated by congenital hypopituitarism. We speculate that both GH deficiency and metabolic acidosis contributed to growth failure. Endocrinological investigations will help to individualize the treatment of patients with PHA II presenting with growth failure.

Familial or genetic primary aldosteronism and Gordon syndrome

Salt-sensitive forms of hypertension have received considerable renewed attention in recent years. This article focuses on 2 main forms of salt-sensitive hypertension (familial or genetic primary aldosteronism [PA] and Gordon syndrome) and the current state of knowledge regarding their genetic bases. The glucocorticoid-remediable form of familial PA (familial hyperaldosteronism type I) is dealt with only briefly because it is covered in depth elsewhere.

Hereditary tubular transport disorders: implications for renal handling of Ca2+ and Mg2+

The kidney plays an important role in maintaining the systemic Ca2+ and Mg2+ balance. Thus the renal reabsorptive capacity of these cations can be amended to adapt to disturbances in plasma Ca2+ and Mg2+ concentrations. The reabsorption of Ca2+ and Mg2+ is driven by transport of other electrolytes, sometimes through selective channels and often supported by hormonal stimuli. It is, therefore, not surprising that monogenic disorders affecting such renal processes may impose a shift in, or even completely blunt, the reabsorptive capacity of these divalent cations within the kidney. Accordingly, in Dent's disease, a disorder with defective proximal tubular transport, hypercalciuria is frequently observed. Dysfunctional thick ascending limb transport in Bartter's syndrome, familial hypomagnesaemia with hypercalciuria and nephrocalcinosis, and diseases associated with Ca2+-sensing receptor defects, markedly change tubular transport of Ca2+ and Mg2+. In the distal convolutions, several proteins involved in Mg2+ transport have been identified [TRPM6 (transient receptor potential melastatin 6), proEGF (pro-epidermal growth factor) and FXYD2 (Na+/K+-ATPase gamma-subunit)]. In addition, conditions such as Gitelman's syndrome, distal renal tubular acidosis and pseudohypoaldosteronism type II, as well as a mitochondrial defect associated with hypomagnesaemia, all change the renal handling of divalent cations. These hereditary disorders have, in many cases, substantially increased our understanding of the complex transport processes in the kidney and their contribution to the regulation of overall Ca2+ and Mg2+ balance.

WNK4-mediated regulation of renal ion transport proteins

Point mutations in WNK4 [for With No K (lysine)], a serine-threonine kinase that is expressed in the distal nephron of the kidney, are linked to familial hyperkalemic hypertension (FHH). The imbalanced electrolyte homeostasis in FHH has led to studies toward an understanding of WNK4-mediated regulation of ion transport proteins in the kidney. A growing number of ion transport proteins for Na(+), K(+), Ca(2+), and Cl(-), including ion channels and transporters in the transcellular pathway and claudins in the paracellular pathway, are shown to be regulated by WNK4 from studies using models ranging from Xenopus laevis oocytes to transgenic and knockin mice. WNK4 regulates these transport proteins in different directions and by different cellular mechanisms. The common theme of WNK4-mediated regulation is to alter the abundance of ion transport proteins at the plasma membrane, with the exception of claudins, which are phosphorylated in the presence of WNK4. The regulation of WNK4 can be blocked by the full-length WNK1, whose action is in turn antagonized by a kidney-specific WNK1 variant lacking the kinase domain. In addition, WNK4 also activates stress-related serine-threonine kinases to regulate members of the SLC12 family members of cation-chloride cotransporters. In many cases, the FHH-causing mutants of WNK4 exhibit differences from wild-type WNK4 in regulating ion transport proteins. These regulations well explain the clinical features of FHH and provide insights into the multilayered regulation of ion transport processes in the distal nephron.

Toward a comprehensive molecular model of active calcium reabsorption

The fine tuning of Ca(2+) excretion in the kidney takes place in the distal nephron, which consists of the distal convoluted tubule, connecting tubule, and initial portion of the cortical collecting duct. In these segments, Ca(2+) is reabsorbed through an active transcellular pathway. The apical influx of Ca(2+) into the distal renal cell is presumably the rate-limiting step in this process, and its molecular identity has remained obscure so far. The recently discovered epithelial Ca(2+) channel (ECaC) exhibits the expected properties for being the gatekeeper in transcellular Ca(2+) reabsorption. The characteristics and potential physiological role of ECaC will be discussed in this review. Our knowledge of the mechanisms involved in the regulation of transcellular Ca(2+) transport has advanced rapidly since the development of cell models originating from distal tubular cells. Studies using these models indicate that hormones including arginine vasopressin, PGE(2), adenosine, ATP, and atrial natriuretic peptide should be considered as calciotropic hormones controlling renal Ca(2+) handling. Evidence is now beginning to emerge that the stimulating calciotropic hormones utilize new cAMP-independent pathways to stimulate Ca(2+) reabsorption. These new findings allow the development of a comprehensive and detailed model of the process of transcellular calcium transport in the kidney whereby the individual contribution of the participating transporters can now be fully appreciated.

[Hypercalciuria: etiologies and treatment]

Hypercalciuria is a rare biological symptom with multiple possible etiologies in children. Normal calcium excretion rate in children is defined as lower than 4 mg/kg per day, significantly higher values being observed in infants. When using urinary calcium: creatinine ratio, normal values are below 0.22 mg/mg in children, and below 0.6 to 0.8 mg/mg in infants. In our experience half patients with hypercalciuria have idiopathic hypercalciuria. Idiopathic hypercalciuria can be hereditary with a dominant autosomal mode of inheritance. Its pathophysiology is unclear, increased calcium intestinal absorption and impaired renal tubular calcium reabsorption being the two main underlying anomalies. Patients with hypercalciuria should be informed about the risk of urolithiasis and its possible prevention by a high water intake. In those patients with nephrocalcinosis or recurrent episodes of lithiasis, hydrochlorothiazide can be effective in reducing hypercalciuria. However, adverse effects of hydrochlorothiazide on serum lipids have been recently reported and make this treatment questionable in the long term.

Transtubular potassium concentration gradient: a useful test to estimate renal aldosterone bio-activity in infants and children

The present investigation was designed to validate the usefulness of transtubular potassium (K) concentration gradient (TTKG) as an indicator of aldosterone bio-activity in infants and children. TTKG was calculated by the formula: [K]urine: (urine/plasma)osmolality/[K]venous blood. We compared this index with fractional K excretion (FEK) and urine K concentration to urine sodium (Na) concentration ratio (UK/UNa) in 473 normal children aged 1 month-15 years. Values of TTKG followed a non-gaussian distribution (median, 6.3; 3rd centile, 4.1; 97th centile, 13.4). TTKG in infants (n = 108; median, 7.8) was significantly higher than in children (n = 365; median, 6.0). TTKG correlated directly with FEK and UK/UNa. Indices of K excretion were also assessed in 13 patients with hypo- and pseudohypoaldosteronism. TTKG values varied between 1.6 and 4.1 and were all below the 3rd percentile established for the age of the subject. We conclude that calculation of TTKG is an easy and sensitive method for the evaluation of mineralocorticoid action in distal and collecting tubules.