An update on clinical presentation and responses to therapy of patients with hereditary hypophosphatemic rickets with hypercalciuria (HHRH)

Updates on renal phosphate transport

PURPOSE OF REVIEW

The kidneys control systemic phosphate balance by regulating phosphate transporters mediating the reabsorption of inorganic phosphate (Pi). At least three different Na + -driven Pi cotransporters are located in the brush border membrane (BBM) of proximal tubule cells, NaPi-IIa (SLC34A1), NaPi-IIc (SLC34A3) and PiT-2 (SLC20A2). This review will discuss novel aspects of their regulation, pharmacology, and genetics.

RECENT FINDINGS

Renal NaPi transporters are not only acutely regulated by the phosphaturic hormones parathyroid hormone (PTH) and Fibroblast Growth Factor 23 (FGF23) but possibly also by further mechanisms. A role of inositol hexakisphosphate (IP6) kinases has been found and their deletion from kidneys causes hypophosphatemia, hyperphosphaturia, and bone demineralization. Inhibitors of NaPis elicit phosphaturia and may reduce levels of PTH and FGF23 in chronic kidney disease (CKD) models. The relevance of renal NaPi transporters is highlighted by loss-of-function mutations in SLC34 transporters and analysis of patients provides new insights into diseases caused by variants. Major manifestations include nephrocalcinosis and -lithiasis, rickets, and variants may predispose to an accelerated decline in kidney function.

SUMMARY

Renal Pi transporters are regulated, may provide novel drug targets for prevention or treatment of hyperphosphatemia, and contribute to the genetic risk to develop kidney stones and CKD.

Chapter 4: Differential diagnosis of primary hyperparathyroidism

The differential diagnosis of primary hyperparathyroidism can be considered clinically, biologically and radiologically. Clinically, primary hyperparathyroidism should be suspected in case of diffuse pain, renal lithiasis, osteoporosis, repeated fracture, cognitive or psychiatric disorder, or disturbance of consciousness. Nevertheless, the differential diagnosis of primary hyperparathyroidism is mainly biological, particularly in atypical forms, which must be differentiated from hypercalcemia with hypocalciuria or non-elevated PTH on the one hand, and from normo-calcemia with elevated PTH, hypophosphatemia or hypercalciuria on the other. Any differential diagnosis must be preceded by an analysis of the factors likely to disturb phospho-calcium parameters: vitamin D deficiency (assay), renal insufficiency (eGFR measurement), malabsorption (inflammatory disease of the digestive tract, celiac disease, bariatric surgery, etc.), insufficient calcium intake (GRIO questionnaire) and iatrogenic causes (diuretics, anti-osteoporotic drugs, excessive vitamin D or calcium supplementation, lithium, corticosteroid therapy, phosphorus intake). Once these factors have been eliminated, hypercalcemia with hypocalciuria should suggest a genetic cause. Hypercalcemia with non-elevated PTH may be secondary to neoplasm, hypervitaminosis D (excessive intake, production or catabolism), immobilization or endocrine causes. Elevated PTH values without hypercalcemia must be differentiated from normo-calcemic hyperparathyroidism. High PTH levels are found in PTH-resistant patients, as well as in hypophosphatemic (especially X-linked) or hypercalciuric tubulopathies (certain rare diseases, immobilization, loop diuretics or idiopathic causes favored by a metabolic syndrome). Radiologically, brown tumor must be differentiated primarily from bone metastasis, chondrosarcoma and giant cell tumor.

Presentation and outcome in carriers of pathogenic variants in SLC34A1 and SLC34A3 encoding sodium-phosphate transporter NPT 2a and 2c

Pathogenic variants in SLC34A1 and SLC34A3 encoding sodium-phosphate transporter 2a and 2c are rare causes of phosphate wasting. Since data on presentation and outcomes are scarce, we collected clinical, biochemical and genetic data via an online questionnaire and the support of European professional organizations. One hundred thirteen patients (86% children) from 90 families and 17 countries with pathogenic or likely pathogenic variants in SLC34A1 or SLC34A3 and a median follow-up of three years were analyzed. Biallelic SLC34A1 variant carriers showed polyuria, failure to thrive, vomiting, constipation, hypercalcemia and nephrocalcinosis in infancy, while biallelic SLC34A3 carriers presented in childhood or even adulthood with rickets/osteomalacia and/or osteopenia/osteoporosis, hypophosphatemia and, less frequently, nephrocalcinosis, while the prevalences of kidney stones were comparable. Adult biallelic SLC34A3 carriers had a six-fold increase chronic kidney disease (CKD) prevalence compared to the general population. All biallelic variant carriers shared a common biochemical pattern including elevated 1,25(OH)2D and alkaline phosphatase levels, suppressed parathyroid hormone (PTH), and hypercalciuria. Heterozygous carriers showed similar but less pronounced phenotypes. In biallelic SLC34A1 carriers, an attenuation of clinical features was observed after infancy, independent of treatment. Phosphate treatment was given in 55% of patients, median duration two years, and resulted in significant reduction, although not normalization, of alkaline phosphatase and of hypercalciuria but an increase in PTH levels, while 1,25(OH)2D levels remained elevated. Thus, our study indicates that biallelic SLC34A1 and SLC34A3 carriers show distinct, albeit overlapping phenotypes, with the latter having an increased risk of CKD in adulthood. Phosphate treatment may promote kidney phosphate loss and enhance 1,25(OH)2D synthesis via increased PTH production.

Effects of SLC34A3 or SLC34A1 variants on calcium and phosphorus homeostasis

BACKGROUND

Variants in SLC34A1 and SLC34A2 genes, which encode co-transporters NaPi2a and NaPi2c, respectively, can lead to hypophosphatemia due to renal phosphate loss. This condition results in hypercalcitriolemia and hypercalciuria, leading to formation of kidney stones and nephrocalcinosis. Phenotype is highly variable. Management includes hyperhydration, dietary modifications, and/or phosphate supplementation. Thiazides and azoles may be used, but randomized studies are needed to confirm their clinical efficacy.

METHODS

We conducted a retrospective study in the pediatric nephrology unit at Grenoble University Hospital from January 2010 to December 2023. The study aimed to describe clinical and biological symptoms of patients with confirmed SLC34A1 and SLC34A3 gene variants and their outcomes.

RESULTS

A total of 11 patients (9 females) from 6 different families had variants in the SLC34A1 (5 patients) and SLC34A3 (6 patients) genes. Median age at diagnosis was 72 [1-108] months. Average follow-up duration was 8.1 ± 4.5 years. Presenting symptom was nephrocalcinosis (4 cases), followed by renal colic (3 cases). At diagnosis, 90% of patients had hypercalciuria and 45% had hypercalcitriolemia. Management included hyperhydration and dietary advice. All patients showed favorable outcomes with normal growth and school attendance. One patient with an SLC34A3 variant showed regression of nephrocalcinosis. Kidney function remained normal.

CONCLUSION

Clinical and biological manifestations of SLC34 gene variants are highly variable, even among siblings; therefore, management must be personalized. Hygienic and dietary measures (such as hyperhydration, a low sodium diet, and age-appropriate calcium intake) result in favorable outcomes in most cases. Use of azoles (e.g., fluconazole) appears to be a promising therapeutic option.

Phenotypic variability in phosphate transport disorders highlights need for individualized treatment strategies

Pathogenic variants in the SLC34A1 and SLC34A3 genes, encoding sodium-phosphate cotransporters 2a (NPT2a) and 2c (NPT2c), are linked to rare phosphate-wasting disorders. In this issue, Brunkhorst et al. explore the clinical presentations, biochemical profiles, and treatment outcomes associated with these genetic variants in 113 individuals. The study highlights distinct phenotypes, potential treatment challenges, and the need for further research to optimize therapeutic strategies and understand long-term outcomes for affected individuals.

Human genetic diseases of phosphate and pyrophosphate metabolism

In humans, physiological bone and tooth mineralization is a complex cell-mediated process. Prerequisites for proper mineralization include sufficient amounts of minerals (calcium and phosphate [Pi]) to initiate the formation and the growth of apatite crystals and adequate amounts of mineralization inhibitors, such as pyrophosphate (PPi), to prevent uncontrolled extraskeletal mineralization. In this review, we provide an overview of the genetics of human disorders of mineralization, focusing on Pi and PPi metabolism and transport diseases, as the Pi/PPi ratio is an important determinant of crystal production in vivo. Variants in genes implicated in the homeostasis of this ratio may lead to a systemic or local increased Pi/PPi ratio, either by increasing the Pi concentration or by decreasing the PPi concentration, resulting in ectopic calcifications; conversely, variants may lead to a decreased Pi/PPi ratio, resulting in defective mineralization. Owing to the implication of common pathways and, occasionally, to some extent of clinical overlap, an accurate diagnosis and understanding of the pathophysiology of these disorders may be challenging. However, precise molecular characterization of these conditions not only facilitates their diagnosis, but also helps to gather evidence regarding the pathophysiology and phenotype-genotype correlation to improve medical care and develop innovative therapeutics.

Hereditary hypophosphatemic rickets with hypercalciuria (HHRH), a complex disorder in need of precision medicine

Hereditary hypophosphatemic rickets with hypercalciuria is an autosomal recessive phosphate-wasting disorder, associated with kidney and skeletal pathologies, which is caused by pathogenic variants of SLC34A3. In this issue, Zhu et al. describe a pooled analysis of 304 individuals carrying SLC34A3 variants. Their study underscores the complexity of hereditary hypophosphatemic rickets with hypercalciuria, as kidney and bone phenotypes generally do not coexist, heterozygous carriers of SLC34A3 variants also can be affected, and the response to oral phosphate supplementation is dependent on the genetic status.