Bartter syndrome type 3: an unusual cause of nephrolithiasis

Giacomo Colussi, Maria Elisabetta De Ferrari, Silvana Tedeschi1, Silvia Prandoni1, Marie Louise Syrén1 and Giovanni Civati

Renal Unit, Niguarda-Ca' Granda Hospital 1 Laboratory of Molecular Genetics ICP Hospital Milan Italy

Sir,

Recent achievements in genetic research have greatly advanced our knowledge of Bartter-like syndromes at the molecular level [1]. Genotype–phenotype correlations have linked the so called ‘antenatal’ Bartter syndrome to mutations in either the bumetanide-sensitive Na-K-2Cl co-transporter or the ROMK channel (gene symbols SLC12A1 and KCNJ1, respectively; the terms Bartter syndrome type 1 and 2 (BS1 and BS2), are also in use), and the clinically milder Gitelman disease (or hypocalciuric variant of Bartter syndrome) to mutations of the thiazide-sensitive Na/Cl electroneutral co-transporter (gene symbol SLC12A3). Less well defined is the phenotype associated with mutations of a fourth protein, the basolateral membrane chloride channel, CLC-Kb (gene symbol CLCNKB; also called BS type 3, BS3). When first recognized, BS3 appeared to differ from BS1 and BS2 mainly by the absence of hypercalciuria/nephrocalcinosis [2]. As new cases were described, BS3 manifested a highly variable phenotype—ranging from severe ‘antenatal’ cases with polyhydramnios, neonatal dehydration, and hypercalciuria to almost asymptomatic patients diagnosed during adolescence [3].

We describe here a case of BS3 whose diagnosis was made in adult life, and whose presenting symptom, i.e. renal colic due to nephrolithiasis had so far been undescribed.

Case. G.S. was a 28-year-old (when first seen) male patient who had performed his military service. His past medical history was unremarkable, except for hypokalaemia, which had been detected prior to a minor surgery at the age of 25, and had not been investigated further. He had had nocturia (one to two times per night) since childhood. He came to our attention because of a recent occurrence of left-sided renal colic, resolved in the spontaneous passage of a lost stone. A renal echography after the passage of the stone showed no residual stones or nephrocalcinosis. He was 190 cm tall, weighted 102 kg and his blood pressure was 120/80 mmHg. His main biochemical parameters are shown in Table 1Go. He had normocalcaemic hypercalciuria and slight-degree normouricaemic hyperuricosuria. He also had hypokalaemia, with inappropriate urine K loss, and moderate metabolic alkalosis. When measured, both plasma aldosterone and renin levels were increased, which ruled out ‘normotensive’ primary hyperaldosteronism. A urine screen for diuretics also was negative. To rule out primary or secondary tubulopathy, the patient underwent dynamic evaluation of tubular function according to our protocols [1,4]: free water clearances were performed during maximal water-induced diuresis and furosemide administration. In this context, solute reabsorption in Henle's loop may be evaluated as the furosemide-induced increase of Na/Cl excretion, and solute reabsorption in distal convoluted tubule (DCT) as the free water clearance during maximal furosemide effect. Timed electrolyte clearances also were evaluated before and after a standard oral dose of 50 mg hydrochlorothiazide (HCT test) [4]. In this context, the maximal increase of Na/Cl excretion after the diuretic dose is an indication of solute reabsorption by the thiazide-sensitive co-transporter of the DCT. These clearance studies showed a significant defect of solute reabsorption in Henle's loop (48.5% of post-proximal delivery, n.v. 63–83), with normally high reabsorption in DCT (5.4% of filtered load, n.v. 2.2–5.4). Thus, this patient had the full biochemical and functional features of Bartter syndrome, despite the rather ‘benign’ clinical phenotype. Maximal urine concentration capacity also was evaluated after a 16-h water deprivation and the parenteral administration of a standard dose of desmopressin (dDAVP 4 µg i.m.). Urine osmolality after water deprivation was 942 mmosm/kg H2O (n.v. >800), which did not further increase after dDAVP administration (899 mosm/kg H2O).


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Table 1.  Basal biochemical data in the patient

 
A genetic screening was performed in peripheral leukocytes for germinal mutations of SLC12A3, SLC12A1, KCNJ1, which were free of significant mutations and CLCNK, which showed an inactivating mutation. Mutational analysis was performed by SSCP technique on all exons of CLCNKB gene, including exon-flanking regions. Specific primers were generated according to data obtained from Simon [2] for exons 2–19 and Konrad [3] for exon 1. Automated DNA sequencing was performed on abnormally migrating fragments on SSCP. A preliminary deletion screening was carried out by long-range PCR (LR–PCR), which amplifies fragments spanning over more exons. Patient G.S. showed the absence of an LR–PCR product of fragments 6–9. Single exon amplifications were performed for exons 6–9: there was a homozygous deletion of the entire exon 6 wherever intron 6 is present. Sequencing of exon 5 resulted in a normal pattern. The deletion leads to a stop codon after amino acid 322, truncating protein transcription in the extracellular domain between transmembrane domains 4 and 5. Even though a heterologous expression of the mutated gene was not performed, such a large deletion of the protein is expected to induce loss of function.

Comment. Our patient is an unusual, possibly extreme case of Bartter syndrome. His diagnosis remained unsuspected until adult life, he had a very mild clinical phoenotype, and he presented with a still-undescribed symptom in Bartter syndrome, i.e. nephrolithiasis. Being 28 years old at the time of diagnosis, he is by far the oldest patient so far described with proven Bartter syndrome. His age and paucisymptomaticity would have suggested Gitelman disease, but hypercalciuria and functional studies, showing abnormal Henle's loop function, suggested Bartter syndrome. He had a large deletion of the CLCNKB gene, which is expected to result in amputation of a large portion of the protein and, consequently, complete inactivation of channel function; yet, its clinical consequences were mild. Thus, our patient confirms other reports showing that no correlation between a specific type of CLCNKB gene mutation (i.e. large genomic deletions/protein truncation, amino acid substitution, or splicing abnormalities) and a particular phenotype or clinical severity [3]. This raises the question of whether the variability in the clinical severity of CLCNKB mutations might depend, more than on the residual channel function, on the presence or absence of other chloride transport systems, which compensate for the lost function of the channel.

Despite the frequent occurrence of hypercalciuria and nephrocalcinosis, nephrolithiasis has not been reported in Bartter syndrome. As detailed biochemical data in these patients, other than Ca excretion, are not found in the literature, the reasons remain speculative. Polyuria secondary to the urine concentration defect is at least one explanation. Our patient had normal urine concentration capacity, as shown by the water-deprivation test; this might easily lead to urine over-saturation for specific crystal systems, whose ion constituents in urine would happen to be increased. He had at least three known biochemical risk factors for Ca nephrolithiasis, i.e. hypercalciuria, marginal hyperuricosuria, and hyperoxaluria [5]. While hyperuricosuria and hyperoxaluria might result from dietary habits, hypercalciuria was the most severe abnormality, clearly dependent on the tubular defect. In general terms, it can be viewed as a specific model of ‘renal’ hypercalciuria. As for ‘idiopathic’ hypercalciuria, a link might also be envisaged between urine Ca loss and hyperoxaluria, through the secondary stimulation of intestinal Ca absorption favouring passive hyperabsorption of oxalate [5]. Thus, increased urine Ca excretion has to be considered the most important risk factor, both direct and indirect, for stone formation in our patient. Even though his stone could not be analysed, we think it was a calcium stone (oxalate and/or phosphate), and not a uric acid stone—at the observed urine pH of 6.9 the free uric acid concentration (57 mg/l) was far below the solubility product for uric acid of about 100 mg/l [6]. Accordingly, cystine and struvite stones also could be excluded on the basis of biochemical and clinical data. Of note is the normal urine citrate excretion, whereas hypokalaemia is one possible cause of hypocitraturia [7]. This was most likely due to the counteracting effect of metabolic alkalosis, which increases citrate excretion [8]. Thus, at least two factors (isosthenuria and normal citrate excretion) may account for the rarity of stone formation in the majority of hypercalciuric Bartter syndrome patients (of any type).

In conclusion, our case suggests that BS3 may be a benign condition, as in G.S. presenting in adult life. Hypercalciuria and clearance data should allow differentiating it from the more common Gitelman disease. Nephrolithiasis may be a novel clinical presentation. Tubular hypercalciuria should be differentiated from the much more common ‘idiopathic hypercalciuria’ of stone disease. The coexistence of hypercalciuria and hypokalaemia in a patient with stone disease should prompt the keen clinician to search for this uncommon tubular disorder.

Notes

Email: giacomo.colussi{at}iol.it Back

References

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