Introduction
Normal fluid balance requires an intact thirst mechanism and normal free water excretion by the kidneys, mediated by arginine vasopressin (AVP), also known as antidiuretic hormone (ADH). AVP exerts its antidiuretic action
via the V2 vasopressin receptor (V2R) in the basolateral membrane of renal collecting duct cells. Binding and activation of the V2R, a G-protein coupled receptor (GPCR), increases intracellular cAMP and mediates shuttling of the water channel aquaporin-2 (AQP2) from cytosolic storage vesicles to the apical membrane of collecting duct cells, resulting in increased water permeability and antidiuresis [
1]. A fraction of this AQP2 is excreted in the urine and has been found to be a useful marker of V2R activity [
2]. Water loading in a normal individual suppresses plasma AVP levels and attenuates antidiuresis as a result of decreased AQP2 shuttling to the apical membrane of collecting duct cells. Consequently, less AQP2 is shed into the urine [
3,
4].
We have previously described a novel syndrome of impaired water excretion mediated by gain-of-function mutations in the X-linked gene for V2R in two unrelated male infants who presented with irritability or seizures and hyponatremia [
5]. Their clinical and laboratory findings were consistent with the syndrome of inappropriate secretion of antidiuretic hormone (SIADH) yet their AVP levels were undetectable. We have termed this condition "nephrogenic syndrome of inappropriate antidiuresis" (NSIAD). Each patient carries a missense mutation in codon 137 of
AVPR2, which results in a change from arginine to cysteine (R137C) in one patient and to leucine (R137L) in the other [
5]. Codon 137 is part of a highly conserved DRY/H motif located at the junction of the third transmembrane domain and the second intracellular loop of class 1 GPCRs. This motif is critical for G-protein coupling [
6] and the two mutations each resulted in a constitutively active V2R [
5].
Since our initial description of NSIAD, many reports from around the world, including individual and family studies, have characterized the clinical course of this syndrome [
7‐
12]. All of these patients have the R137C V2R mutation. Here we report the clinical course of the only known patient with NSIAD caused by the more potent R137L V2R mutation [
5]. This patient demonstrated the ability to maintain eunatremia during
ad lib dietary intake. However, urine AQP2 levels were elevated and did not suppress normally during a standard water loading test, consistent with a gain-of-function mutation of V2R.
Discussion
This is the first demonstration in a patient with NSIAD caused by the R137L V2R mutation of urinary AQP2 excretion which was markedly elevated at baseline and which did not suppress normally in a standard water loading test. Our patient's baseline urine AQP2 excretion exceeded that seen in normal adults following 12 hours of water deprivation (7-24 pmol/mg creatinine; personal communication, F. Umenishi), a condition known to increase urine AQP2 excretion [
3]. These findings of increased urinary AQP2 in our patient are consistent with a gain of function mutation in V2R.
Persistent urinary excretion of AQP2 during water loading has been reported in one patient with NSIAD [
12] caused by the less potent R137C mutation [
5]. In addition, altered urinary excretion of AQP2 has been described in several human diseases of pathological AVP secretion. A decrease or increase in urinary AQP2 levels was shown to correspond with diminished or exaggerated levels of AVP, as seen in central diabetes insipidus or SIADH, respectively [
3,
4]. In normal individuals, water loading reduces antidiuresis and, as expected, urine AQP2 levels [
3,
15]. In studies of water loading in normal adults, urine AQP2 excretion at 2 hours after an oral water load of 20 mL/kg was reduced to 10% of baseline [
15]. In the absence of appropriate diuresis during water loading, NSIAD patients would be predicted to have persistently elevated urine AQP2 levels. This is demonstrated in the R137C V2R mutation patient who received 50% of a standard water load [
12] and confirmed by the results of our patient, who received the standard water load, in which urine AQP2 levels did not suppress normally, falling to only 87-98% of baseline for up to 3.5 hours after water loading.
The patient's serum AVP levels merit discussion. AVP levels were undetectable (< 1 pg/ml) on initial presentation with hyponatremia during infancy, leading us to the identification of an activating V2R mutation [
5]. Following normalization of the patient's hydration status with fluid restriction and oral urea supplementation, AVP levels were in the normal range (5.5 pg/ml). AVP levels were again in the normal range (3.1 pg/ml) 6 months following self-discontinuation of urea, associated with eunatremia and hypodipsia. Furthermore, during the water loading challenge, serum AVP levels decreased from a baseline of 2.6 pg/mL to 1.1 pg/mL, the approximate assay limit of detectability, by one hour, when the patient had developed hyponatremia and serum hypoosmolality. Water loading in a patient with R137C mutation also resulted in low but detectable serum AVP level [
10]. Thus, despite the presence of a constitutively active V2R, these results indicate appropriate regulation of AVP secretion in NSAID. Possible explanations for the low but detectable AVP levels following water loading in NSIAD include contamination with platelet-bound AVP [
19] and/or a slight vasovagal stimulus that may have exerted a non-osmotic effect that resulted in an incomplete suppression of AVP secretion.
It is noteworthy that this patient demonstrated an intact thirst mechanism and relative hypodipsia, allowing him to maintain serum sodium in the 131-139 mmol/L range under
ad lib conditions. His relative basal eunatremia is likely a consequence of a diet that is no longer exclusively liquid and is in marked contrast to the severe hyponatremia observed at initial presentation, when the infant was exclusively formula-fed. From a dietary standpoint, our results concur with a previous report in which a child with NSIAD, diagnosed retrospectively, remained eunatremic after transition to solid food and discontinuation of sodium supplementation [
7,
8,
11].
An individual with NSIAD may escape detection during infancy if the activating V2R mutation is mild and/or if overhydration sufficient to induce hyponatremia does not occur. A report by Decaux
et al. of a large pedigree of NSIAD patients caused by the R137C mutation in V2R highlights the marked variability in clinical presentation of this disorder [
9]. One affected hemizygous male was apparently asymptomatic throughout life, and was discovered only after being administered a water loading test. Similarly, two other hemizygous adult males were discovered only when their 8 week old nephew was diagnosed with the disease and genetic testing was performed on the family [
8]. One of the adult males was completely asymptomatic throughout his life and the other apparently has had generalized tonic-clonic seizures early in life of unknown etiology. In addition, several heterozygous females demonstrated inappropriate antidiuresis during water loading [
8,
10,
12]. Thus, NSIAD may be more prevalent in the general population than would have otherwise been predicted based on the relatively small number of patients diagnosed in infancy.
In conclusion, this is the first report that urinary AQP2 levels in a patient with NSIAD caused by the R137L V2R mutation are elevated at baseline and do not suppress appropriately in response to water loading. This study demonstrates that urinary AQP2 can be a useful marker of V2R activity in NSIAD. Thus, increased urinary AQP2 excretion, in the setting of euvolemic hyponatremia and low or undetectable levels of serum AVP, suggests the possibility of NSIAD. This diagnosis should be confirmed by sequencing of AVPR2.
Competing interests
The authors declare that they have no competing interests.
Authors' contributions
CCC and SMR designed the study, performed the analysis, and drafted the manuscript. MAC performed the western blot analysis. SAR and SEG participated in the design of the study. All authors read and approved the final manuscript.