Variable Clinical Presentation of Children with Hereditary Hypophosphatemic Rickets with Hypercalciuria: A Case Series and Review of the Literature

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

Pathogenic variants in solute carrier family 34, member 3 (SLC34A3), the gene encoding the sodium-dependent phosphate cotransporter 2c (NPT2c), cause hereditary hypophosphatemic rickets with hypercalciuria (HHRH). Here, we report a pooled analysis of clinical and laboratory records of 304 individuals from 145 kindreds, including 20 previously unreported HHRH kindreds, in which two novel SLC34A3 pathogenic variants were identified. Compound heterozygous/homozygous carriers show above 90% penetrance for kidney and bone phenotypes. The biochemical phenotype for heterozygous carriers is intermediate with decreased serum phosphate, tubular reabsorption of phosphate (TRP (%)), fibroblast growth factor 23, and intact parathyroid hormone, but increased serum 1,25-dihydroxy vitamin D, and urine calcium excretion causing idiopathic hypercalciuria in 38%, with bone phenotypes still observed in 23% of patients. Oral phosphate supplementation is the current standard of care, which typically normalizes serum phosphate. However, although in more than half of individuals this therapy achieves correction of hypophosphatemia it fails to resolve the other outcomes. The American College of Medical Genetics and Genomics score correlated with functional analysis of frequent SLC34A3 pathogenic variants in vitro and baseline disease severity. The number of mutant alleles and baseline TRP (%) were identified as predictors for kidney and bone phenotypes, baseline TRP (%) furthermore predicted response to therapy. Certain SLC34A3/NPT2c pathogenic variants can be identified with partial responses to therapy, whereas with some overlap, others present only with kidney phenotypes and a third group present only with bone phenotypes. Thus, our report highlights important novel clinical aspects of HHRH and heterozygous carriers, raises awareness to this rare group of disorders and can be a foundation for future studies urgently needed to guide therapy of HHRH.

Hereditary Hypophosphatemic Rickets with Hypercalciuria Presenting with Enthesopathy, Renal Cysts, and High Serum c-Terminal FGF23: Single-Center Experience and Systematic Review

Hereditary hypophosphatemic rickets with hypercalciuria (HHRH) is a rare disorder of phosphate homeostasis. We describe a single-center experience of genetically proven HHRH families and perform systematic review phenotype-genotype correlation in reported biallelic probands and their monoallelic relatives. Detailed clinical, biochemical, radiological, and genetic data were retrieved from our center and a systematic review of Pub-Med and Embase databases for patients and relatives who were genetically proven. Total of nine subjects (probands:5) carrying biallelic SLC34A3 mutations (novel:2) from our center had a spectrum from rickets/osteomalacia to normal BMD, with hypophosphatemia and hypercalciuria in all. We describe the first case of genetically proven HHRH with enthesopathy. Elevated FGF23 in another patient with hypophosphatemia, iron deficiency anemia, and noncirrhotic periportal fibrosis led to initial misdiagnosis as tumoral osteomalacia. On systematic review of 58 probands (with biallelic SLC34A3 mutations; 35 males), early-onset HHRH and renal calcification were present in ~ 70% and late-onset HHRH in 10%. c.575C > T p.(Ser192Leu) variant occurred in 53% of probands without skeletal involvement. Among 110 relatives harboring monoallelic SLC34A3 mutation at median age 38 years, renal calcification, hypophosphatemia, high 1,25(OH)2D, and hypercalciuria were observed in ~30%, 22.3%, 40%, and 38.8%, respectively. Renal calcifications correlated with age but were similar across truncating and non-truncating variants. Although most relatives were asymptomatic for bone involvement, 6/12(50%) had low bone mineral density. We describe the first monocentric HHRH case series from India with varied phenotypes. In a systematic review, frequent renal calcifications and low BMD in relatives with monoallelic variants (HHRH trait) merit identification.

Mineral Metabolism in Children: Interrelation between Vitamin D and FGF23

Fibroblast growth factor 23 (FGF23) was identified at the turn of the century as the long-sought circulating phosphatonin in human pathology. Since then, several clinical and experimental studies have investigated the metabolism of FGF23 and revealed its relevant pathogenic role in various diseases. Most of these studies have been performed in adult individuals. However, the mineral metabolism of the child is, to a large extent, different from that of the adult because, in addition to bone remodeling, the child undergoes a specific process of endochondral ossification responsible for adequate mineralization of long bones’ metaphysis and growth in height. Vitamin D metabolism is known to be deeply involved in these processes. FGF23 might have an influence on bones’ growth as well as on the high and age-dependent serum phosphate concentrations found in infancy and childhood. However, the interaction between FGF23 and vitamin D in children is largely unknown. Thus, this review focuses on the following aspects of FGF23 metabolism in the pediatric age: circulating concentrations’ reference values, as well as those of other major variables involved in mineral homeostasis, and the relationship with vitamin D metabolism in the neonatal period, in vitamin D deficiency, in chronic kidney disease (CKD) and in hypophosphatemic disorders.

Genu Valgum, Fractures, and Renal Stones in a 10-year-old Girl

Rickets is a disorder of impaired bone mineralization that can arise from nutritional deficiencies and inherited conditions. We describe a 10-year-old girl presenting with genu valgum and a history of renal stones due to hereditary hypophosphatemic rickets with hypercalciuria (HHRH), a rare inherited form of rickets characterized by high 1,25 vitamin D levels, hypophosphatemia with inappropriate renal phosphate wasting, and hypercalciuria. After the diagnosis was confirmed, she began treatment with phosphorus supplementation and stopped taking vitamin D, leading to improved bone mineral density and reduction in renal symptoms. Patients with HHRH can be distinguished from those with other forms of hypophosphatemic rickets by their high 1,25 vitamin D levels in conjunction with low to normal parathyroid hormone and fibroblast growth factor 23 (FGF23) levels. Genetic testing for SLC34A3 variants provides a definitive diagnosis.

Rickets guidance: part II-management

Here, we discuss the management of different forms of rickets, including new therapeutic approaches based on recent guidelines. Management includes close monitoring of growth, the degree of leg bowing, bone pain, serum phosphate, calcium, alkaline phosphatase as a surrogate marker of osteoblast activity and thus degree of rickets, parathyroid hormone, 25-hydroxyvitamin D₃, and calciuria. An adequate calcium intake and normal 25-hydroxyvitamin D₃ levels should be assured in all patients. Children with calcipenic rickets require the supplementation or pharmacological treatment with native or active vitamin D depending on the underlying pathophysiology. Treatment of phosphopenic rickets depends on the underlying pathophysiology. Fibroblast-growth factor 23 (FGF23)-associated hypophosphatemic rickets was historically treated with frequent doses of oral phosphate salts in combination with active vitamin D, whereas tumor-induced osteomalacia (TIO) should primarily undergo tumor resection, if possible. Burosumab, a fully humanized FGF23-antibody, was recently approved for treatment of X-linked hypophosphatemia (XLH) and TIO and shown to be superior for treatment of XLH compared to conventional treatment. Forms of hypophosphatemic rickets independent of FGF23 due to genetic defects of renal tubular phosphate reabsorption are treated with oral phosphate only, since they are associated with excessive 1,25-dihydroxyvitamin D production. Finally, forms of hypophosphatemic rickets caused by Fanconi syndrome, such as nephropathic cystinosis and Dent disease require disease-specific treatment in addition to phosphate supplements and active vitamin D. Adjustment of medication should be done with consideration of treatment-associated side effects, including diarrhea, gastrointestinal discomfort, hypercalciuria, secondary hyperparathyroidism, and development of nephrocalcinosis or nephrolithiasis.