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.

Symptomatic Hypocalcemia due to Nutritional Vitamin D Deficiency in Three Adolescents during the COVID-19 Pandemic.

Background: Symptomatic hypocalcemia secondary to vitamin D deficiency (VDD) is rare among adolescents without underlying medical disorders, but its prevalence is higher in known risk populations. We report on three adolescent males with low nutritional intake of vitamin D and calcium and limited sun exposure who presented with hypocalcemic tetany and muscle cramps due to VDD during the COVID-19 pandemic. Case Reports. Three adolescent males (age range 14 to 16 years) presented with symptomatic hypocalcemia: paresthesia, carpopedal spasms, and muscle cramps. All reported limited dairy intake and sun exposure. Laboratory studies showed mean ionized calcium (iCa) 2.73 mg/dl (range 2.69-2.8), mean phosphorus 4.17 mg/dl (range, 3-5.4), mean parathyroid hormone (PTH) 431.67 pg/mL (range, 320-527), and mean 25-hydroxyvitamin D (25(OH)D) 7.37 ng/mL (range 5.3-10.8). All the patients presented during the COVID-19 pandemic, and one had COVID-19 infection. All were treated with oral calcium and high dose ergocalciferol. Patients 2 and 3 were also treated with intravenous calcium gluconate infusion and oral calcitriol. Conclusion: Severe VDD with symptomatic hypocalcemia can occur among adolescents without underlying medical diagnoses due to dietary and behavioral habits that limit nutritional intake and sun exposure. Risk factors of the patients may have been potentiated by pandemic-related behaviors such as more time indoors at home related to social distancing, as well as diets with limited nutrient intake. Adolescents presenting with nonspecific musculoskeletal symptoms should be screened for VDD and hypocalcemia. Appropriate treatment and preventive measures can stop immediate and long-term complications.

Long-Term Follow-up of Hypophosphatemic Bone Disease Associated With Elemental Formula Use: Sustained Correction of Bone Disease After Formula Change or Phosphate Supplementation.

In this article, we describe the long-term outcomes of children who were previously reported to have developed hypophosphatemic bone disease in association with elemental formula use. An extended chart review allowed for an updated report of 34 children with regard to severity/duration of bone disease, extent of recovery, and time to correction using radiology reports and biochemical data. After implementation of formula change and/or phosphate supplementation, we found that serum phosphorus concentration increased and serum alkaline phosphatase activity decreased in all patients, normalizing by 6.6 ± 4.0 (mean ± SD) months following diagnosis. The decrease in serum alkaline phosphatase from diagnosis to the time of correction was moderately correlated with the concurrent increase in serum phosphorus (R = 0.48, P < .05). Age at diagnosis significantly correlated with time to resolution (R = 0.51, P = .01). This study supports the earlier report that bone disease associated with hypophosphatemia during elemental formula use responds to formula change and/or phosphate supplementation.

Unexpected widespread hypophosphatemia and bone disease associated with elemental formula use in infants and children.

OBJECTIVE: Hypophosphatemia occurs with inadequate dietary intake, malabsorption, increased renal excretion, or shifts between intracellular and extracellular compartments. We noticed the common finding of amino-acid based elemental formula [EF] use in an unexpected number of cases of idiopathic hypophosphatemia occurring in infants and children evaluated for skeletal disease. We aimed to fully characterize the clinical profiles in these cases. METHODS: A retrospective chart review of children with unexplained hypophosphatemia was performed as cases accumulated from various centres in North America and Ireland. Data were analyzed to explore any relationships between feeding and biochemical or clinical features, effects of treatment, and to identify a potential mechanism. RESULTS: Fifty-one children were identified at 17 institutions with EF-associated hypophosphatemia. Most children had complex illnesses and had been solely fed Neocate® formula products for variable periods of time prior to presentation. Feeding methods varied. Hypophosphatemia was detected during evaluation of fractures or rickets. Increased alkaline phosphatase activity and appropriate renal conservation of phosphate were documented in nearly all cases. Skeletal radiographs demonstrated fractures, undermineralization, or rickets in 94% of the cases. Although the skeletal disease had often been attributed to underlying disease, most all improved with addition of supplemental phosphate or change to a different formula product. CONCLUSION: The observed biochemical profiles indicated a deficient dietary supply or severe malabsorption of phosphate, despite adequate formula composition. When transition to an alternate formula was possible, biochemical status improved shortly after introduction to the alternate formula, with eventual improvement of skeletal abnormalities. These observations strongly implicate that bioavailability of formula phosphorus may be impaired in certain clinical settings. The widespread nature of the findings lead us to strongly recommend careful monitoring of mineral metabolism in children fed EF. Transition to alternative formula use or implementation of phosphate supplementation should be performed cautiously with as severe hypocalcemia may develop.