We reported the clinical and biological results and the evolution of 11 patients, with a female-to-male ratio of 9:2, from 6 families presenting a double heterozygous mutation in the SLC34A3 gene (2 families, 6 patients) or a heterozygous mutation in the SLC34A1 gene (4 families, 5 patients). These mutations affect the sodium-dependent cotransporters NaPi2c and NaPi2a, respectively, located in the renal proximal tubules, and are responsible, along with the PiT-2 cotransporter, for the reabsorption of most of the phosphate from the primary filtrate [7]. The median age at diagnosis was 72 [1-108] months, and the average follow-up duration was 8.1 ± 4.5 years. The most frequent mode of discovery was nephrocalcinosis (4 cases), followed by nephritic colic (3 cases). Four patients were systematically screened due to affected siblings. The involvement is solely renal for all patients except patient 8, who had growth retardation at diagnosis.
Mutations in NaPi2a and NaPi2c are characterized by disturbed calcium homeostasis secondary to high concentrations of 1,25(OH)2D due to hypophosphatemia from renal phosphate loss, leading to decreased FGF23 levels [9, 16, 17–20]. Subsequently, decreased FGF23 leads to increased 1,25(OH)2D through at least two direct mechanisms: reduced inhibition of 1-α hydroxylase and reduced stimulation of 24-hydroxylase. Elevated serum calcitriol stimulates intestinal absorption of phosphate and calcium, leading to decreased PTH and hypercalciuria, which promotes the development of nephrolithiasis and/or nephrocalcinosis [6, 16].
Depending on the type of mutation (homozygous, heterozygous, compound heterozygous, missense, etc.) and the site of the mutation, the associated phenotype is highly variable, ranging from severe conditions such as IIH (SLC34A1, CYP24A1 encoding the vitamin D 24-hydroxylase enzyme) or HHRH (SLC34A3) to milder forms characterized by renal stones and/or osteoporosis [6, 23–29]. As of June 2, 2024, 77 different SLC34A1 mutations and 127 different SLC34A3 mutations are listed in the HGMD (Human Gene Mutation Database).
HHRH is an autosomal recessive disorder linked to a mutation in the SLC34A3 gene, characterized by severe rickets, growth retardation, skeletal deformities, muscle weakness, and bone pain. Acar et al. [30] identified pathogenic mutations (PHEX and SLC34A3) in 21 Turkish patients from 15 unrelated families with hereditary hypophosphatemia, showing clinical variability. Approximately 25% of individuals with homozygous or compound heterozygous SLC34A3 mutations do not present with rickets/osteomalacia, and 50% do not have nephrolithiasis or nephrocalcinosis. In our cohort of patients with a double heterozygous SLC34A3 mutation, 67% developed nephrocalcinosis, 50% had renal calculi, and 1/3 of the patients did not exhibit renal manifestations, confirming the variability in clinical presentations, which can be observed within the same family and for the same mutation, likely influenced by the timing of diagnosis and hygienic-dietary measures.
Zhu et al. [31] analysed clinical and biological records of 304 individuals (145 families) with an SLC34A3 mutation. The biological presentation of heterozygous carriers is characterized by decreased serum phosphate, reduced tubular phosphate reabsorption, normal FGF23 and PTH levels, but an increase in serum 1,25(OH)2D, leading to idiopathic hypercalciuria in 38%. Among our 6 patients with an SLC34A3 mutation, 3 patients from the same family showed mild hypophosphatemia during follow-up, necessitating oral phosphate supplementation in two of them. Conversely, we observed low PTH, hypercalciuria, and elevated 1,25(OH)2D levels in 83%, 100%, and 67% of the patients, respectively (Fig. 1).
Regarding the SLC34A1 gene, the role of mutations in phosphate renal handling and renal function is not yet fully understood [1]. Biallelic mutations in CYP24A1 and SLC34A1 can cause severe forms of autosomal recessive IIH type 1 and 2, respectively, illustrating the links between vitamin D and phosphate metabolism. The milder form of IIH is often associated with heterozygous mutations in SLC34A1, frequently accompanied by a family history of kidney stones. However, although many IIH patients have mutations in CYP24A1 or SLC34A1, mutations in these two genes do not account for all IIH cases, indicating genetic heterogeneity [26].
Prié et al. [16] reported the cases of three patients with heterozygous missense mutations: p.Ala48Phe in one patient and p.Val147Met in the other two related patients. These cases showed low renal phosphate reabsorption (TmP/GFR < 0.7 mmol), hypercalciuria, and kidney stones. A second study described patients from a consanguineous Arab family carrying a homozygous 21 bp duplication (G154¬V160dup) in the SLC34A1 gene, who developed rickets, hypophosphatemia, hypercalciuria, and markedly elevated calcitriol levels [32]. Several other mutations have been discovered since improved access to genetics, with variable clinical and biological manifestations [33, 34]. More recently, two mutations in the SLC34A1 gene (p.Gly543Cys and p.Ile456Asn) were found in two young male patients presenting with hypophosphatemia, kidney stones, and/or osteopenia [35, 36]. In our cohort, different heterozygous mutations of SLC34A1 were identified depending on the family [c.2727_292del (p.Val91_Ala97del) for family 4 and p.Val160Ala for family 5]. Nephrocalcinosis was found in 60% of our patients. Additionally, two patients developed renal calculi associated with nephrocalcinosis in one case. Only one case of growth retardation at the time of diagnosis was identified at 3 years of age, with a favorable outcome.
IIH remains a rare disorder with variable clinical and biochemical severity characterized by elevated serum concentrations of 1,25(OH)2D and low PTH levels. These patients may also exhibit hypercalciuria and alternate between chronic phases with normal calcium but inappropriate 1,25(OH)2D concentration and low PTH, and acute phases with hypercalcemia and suppressed PTH [37]. In our patient cohort, we found mild hypophosphatemia, low PTH, hypercalciuria, increased 1,25(OH)2D, and hypercalcemia at least once during follow-up in 20%, 40%, 80%, 80%, 60%, and 20% of patients, respectively (Fig. 1).
IIH may present with mild to severe hypercalcemia during the first months of life [26], with severe cases exhibiting vomiting, dehydration, and nephrocalcinosis. Indeed, we also found that patients with an SLC34A1 mutation had an earlier onset than patients with an SLC34A3 mutation, with renal stones appearing as early as the first month of life in one patient and prenatal nephrocalcinosis detected in another. This early involvement was also reported by Molain et al. [37] in the only patient in their cohort with an SLC34A1 mutation, who had hyperechoic kidneys prenatally.
Additionally, Molain et al. [37], in their study including 185 patients with PTH levels < 20 ng/L, hypercalcemia, and/or hypercalciuria, found biallelic mutations in CYP24A1 in 20 patients, SLC34A3 in 6 patients, and SLC34A1 in one patient, mainly associated with kidney disease (stones, nephrocalcinosis) in 86% of cases, indicating a broad phenotypic overlap. The serum 25(OH)D / 24,25(OH)2D ratio helps differentiate patients with an SLC34 mutation from those with a CYP24A1 mutation, as it is significantly higher in cases of CYP24A1 mutation [37]. Therefore, in cases of unexplained phosphocalcic metabolism disorders associated with renal manifestations of nephrocalcinosis and/or kidney stones, sequencing of at least these four genes (CYP24A1, SLC34A1, SLC34A3, and SLC9A3R1) seems judicious [23, 24, 37].
Management of patients with hypercalciuria secondary to SLC34 gene mutations typically relies on hydration and dietary advice, including a low-sodium diet with normal age-appropriate calcium intake. In cases of hypophosphatemia and abnormal bone mineralization, phosphate supplementation may be administered, at least during childhood [38]. The response to oral phosphate supplementation depends on the genetic status [31]. Long-term supplementation appears to reverse clinical and biochemical abnormalities in HHRH [25].
Management of our patients relied on hydration and a low-sodium diet. Discontinuation of cholecalciferol supplementation was initiated for all patients initially and restarted if the 25(OH)D level was below 50 nmol/L, exclusively in the form of daily drops. Cholecalciferol ampoules were proscribed. During follow-up, four patients received potassium citrate due to low citraturia, and two patients received oral phosphate due to hypophosphatemia. No patient was treated with thiazides or azoles. The evolution was favorable for all patients, with no stunted growth and normal schooling. Regression of nephrocalcinosis images was noted in only one patient. Renal function remained normal in all patients. However, two patients still had hypercalciuria and increased 1,25(OH)2D.
Monitoring urinary calcium levels is critical in managing patients with nephrolithiasis/nephrocalcinosis associated with hypercalciuria and elevated 1,25(OH)2D levels. Normalizing and reducing urinary calcium can prevent and slow the progression of nephrocalcinosis/nephrolithiasis, thereby mitigating the risk of chronic kidney disease and bone complications in adulthood. However, standard interventions (low sodium intake, hydration, and potassium citrate) have limited impact on hypercalciuria [39]. As a result, alternative strategies have been explored, including the use of hydrochlorothiazide, though its clinical benefit remains uncertain due to frequent side effects such as hypokalemia, asthenia, and potential long-term dermatologic lesions [40].
Moreover, azoles, particularly ketoconazole, have been employed to inhibit 1α-hydroxylase activity, thereby reducing 1,25(OH)2D levels and calciuria in patients with vitamin D hypersensitivity [41, 42]. However, this approach is associated with the risk of potential long-term hepatic toxicity [43]. Recently, fluconazole has demonstrated efficacy in reducing calciuria in certain patients with CYP24A1 or SLC34A3 mutations [44, 45]. Consequently, a randomized trial named FLUCOLITH [39] is currently underway to evaluate the effectiveness of fluconazole in normalizing calciuria after 16 weeks of treatment.