Pediatr Nephrol (2011) 26:2133–2142 disease and classic Bartter syndrome. Unlike FHHNC, however, these conditions frequently present with hypoka- lemia, metabolic alkalosis, and hypocalciuria, which are distinguishable from FHHNC [34]. Presentation and clinical course The classic form of FHHNC (OMIM #248250) most commonly presents with kidney stones, convulsions, carpo- pedal spasm, polydipsia, polyuria, rickets, and recurrent urinary tract infections [24]. A subset of patients with the syndrome of FHHNC present in addition with ocular abnormalities including macular colobomata, nystagmus, severe myopia, corneal calcifications, and chorioretinitis (OMIM #248190, which turns out to be caused by claudin- 19 mutations, see below) [24, 31, 35]. Since the primary defect is related to impaired tubular reabsorption of Mg, patients almost always have a high fractional excretion of urinary Mg, while serum Mg is inappropriately low [24–31]. Family members with heterozygous mutation may have only hypercalciuria and nephrolithiasis without having hypomag- nesemia, suggesting a milder phenotype [31]. Nephrocalci- nosis, which is believed due to increased urinary Ca excretion, Mg deficiency and urinary acidification disturban- ces, has been shown to be associated with the progression rate of renal insufficiency [24–31]. The pathogenesis of the renal failure remains unexplained. It was believed that the nephrocalcinosis and resultant tubulointerstitial nephropathy was also the cause of chronic renal failure. Unfortunately, neither Mg supplement nor thiazide diuretics normalize serum Mg levels or urinary Ca excretion, or delay the progression of the disease [27, 31, 33]. Kidney transplantation, however, corrects the abnor- malities of renal Mg and Ca handling [31, 34]. Etiology of FHHNC It is now known that mutations in the claudin-16 gene [36], which encodes the tight junction protein, claudin-16 (previously known as paracellin-1), and in the claudin-19 gene [37] are responsible for FHHNC (Fig. 2). This is supported by the phenotype in mice with knockdown of these two genes. Hou et al. [38] used lentiviral transgenesis of shRNA to knock down claudin-16 expression by >99% in mouse kidneys. Claudin-16 knockdown mice had hypomagnesemia with fourfold increase in the fractional excretions of both Ca and Mg. Likewise claudin-19 knockdown mice clearly demonstrated a higher fraction excretion of Mg and Ca and the development of neph- rocalcinosis, compared to wild-type mice [39]. The fact that knockdown of claudin-16 and claudin-19 reproduce the FHHNC phenotype confirms that the human disease is due to loss-of-function mutations in these genes. Interestingly, 2137 the clinical presentation of patients with mutations of claudin-16 and claudin-19 is different: only those with mutations of claudin-19 present with ocular abnormalities. This is in part because only claudin-19 is expressed in the tight junction of the retina [37]. Hypotheses regarding the pathogenesis of FHHNC Rodriguez-Soriano et al. first proposed that FHHNC might be due to a defect in tubular reabsorption in the thick ascending limb (TAL) of loop of Henle [40]. This was based on the magnitude of the observed increase in fractional excretion of magnesium, which could only be accounted for by a defect in the TAL, and on the fact that linked transport of Ca and Mg is characteristic of the TAL. Blanchard et al. [41] subsequently demonstrated that FHHNC patients are unable to further increase their fractional excretion of Mg and Ca in response to the loop diuretic, furosemide, while having a preserved natriuretic response, thus confirming that there is a selective defect in divalent cation reabsorption in the TAL [41]. In the TAL, NaCl is actively reabsorbed against an uphill concentration gradient via a transcellular pathway involving the apical Na-K-2Cl cotransporter, NKCC2, and basolateral Na-K-ATPase. Ca and Mg reabsorption occurs by passive diffusion via the paracellular pathway, which is driven by a lumen-positive voltage. This transepithelial voltage is generated by two mechanisms: (a) Na, K, and Cl reabsorption through NKCC2 occurs simultaneously with recycling of K back into the tubule lumen via the apical K channel, ROMK; (b) NaCl that has been reabsorbed actively by the transcellular pathway establishes a concen- tration gradient of NaCl that favors back-diffusion into the lumen. Because the paracellular pathway is more permeable to Na than Cl (Na-to-Cl permeability ratio, PNa/PCl, has been found to vary from 2 to 6 [42, 43, 44]), this generates a dilution potential that is lumen-positive. Two independent hypotheses have been advanced as to how mutations in the TAL tight junction proteins, claudin- 16 and -19, can impair TAL Ca and Mg reabsorption. The first hypothesis is that claudin-16 and/or -19 might function as divalent cation-selective paracellular pores, so that in FHHNC the paracellular Ca and Mg permeability of the TALH would be expected to be impaired. However, the in vitro studies of claudin-16 permeability properties have yielded conflicting results, and the idea that claudin-16 by itself forms a Mg pore is not well supported by the available evidence [17, 45, 46]. Likewise, when claudin-19 is expressed alone or together with claudin-16, it does not seem to reconstitute a functioning Mg pore [39]. The second hypothesis, put forward by Goodenough, Hou and colleagues, is that claudin-16 and -19 function synergis- tically to form highly Na-selective paracellular pores. Over-