Autosomal Dominant Hypophosphatemic Rickets: A Case Report and Review of the Literature

Autosomal dominant hypophosphatemic rickets (ADHR) is an extremely rare form of genetic rickets caused by mutations in the fibroblast growth factor 23 gene. ADHR is characterized by hypophosphatemia secondary to isolated renal phosphate wasting. Only a few cases of ADHR have been reported in the literature to date. We describe the case of a 17-month-old girl who presented with severe failure to thrive (length: −4.08 standard deviation (SD), weight: −2.2 SD) and hypotonia. Hypophosphatemia, decreased tubular phosphate reabsorption (69%), and rachitic lesions were found. Genetic analysis showed the heterozygous variant c.536G>A (NM_020638.3:c.536G>A) in exon 3 of the FGF23 gene, leading to the diagnosis of ADHR. She was treated with phosphate salts and oral alfacalcidol. After 4 years of treatment, at 5 years of age, the patient’s ADHR resolved spontaneously. Considering the lack of knowledge regarding ADHR, we reviewed the literature to describe the features of this rare and poorly understood disease. Eleven ADHR pediatric cases have been described thus far, with cases tending to be more common in females than males. Similar to the general population, two groups of patients with ADHR can be described depending on the mutations present: patients with an R179 and R176 mutation have early-onset of disease and higher frequency of rickets, and a milder and late-onset of disease, respectively. Symptoms and disease severity may fluctuate. Spontaneous remission may occur during the pediatric age.

Digenic Heterozygous Mutations in SLC34A3 and SLC34A1 Cause Dominant Hypophosphatemic Rickets with Hypercalciuria

Context Hypophosphatemia and metabolic bone disease are associated with hereditary hypophosphatemic rickets with hypercalciuria (HHRH) due to biallelic mutations of SLC34A3 encoding the NPT2C sodium-phosphate cotransporter and nephrolithiasis/osteoporosis, hypophosphatemic 1 (NPHLOP1) due to monoallelic mutations in SLC34A1 encoding the NPT2A sodium-phosphate cotransporter. Objective To identify a genetic cause of apparent dominant transmission of HHRH. Design and Setting Retrospective and prospective analysis of clinical and molecular characteristics of patients studied in 2 academic medical centers. Methods We recruited 4 affected and 3 unaffected members of a 4-generation family in which the proband presented with apparent HHRH. We performed clinical examinations, biochemical and radiological analyses, and molecular studies of genomic DNA. Results The proband and her affected sister and mother carried pathogenic heterozygous mutations in 2 related genes, SLC34A1 (exon 13, c.1535G>A; p.R512H) and SLC34A3 (exon 13, c.1561dupC; L521Pfs*72). The proband and her affected sister inherited both gene mutations from their mother, while their clinically less affected brother, father, and paternal grandmother carried only the SLC34A3 mutation. Renal phosphate-wasting exhibited both a gene dosage–effect and an age-dependent attenuation of severity. Conclusions We describe a kindred with autosomal dominant hypophosphatemic rickets in which whole exome analysis identified digenic heterozygous mutations in SLC34A1 and SLC34A3. Subjects with both mutations were more severely affected than subjects carrying only one mutation. These findings highlight the challenges of assigning causality to plausible genetic variants in the next generation sequencing era.

SAT-056 Autosomal Dominant Hypophosphatemic Rickets in Premature Twins Resolved with Iron Supplementation

Introduction Autosomal dominant hypophosphatemic rickets (ADHR) is a condition with variable phenotype in terms of age of presentation, severity, and possible resolution. ADHR is caused by mutations of FGF23, preventing its cleavage, producing high levels of FGF23, which leads to renal phosphate wasting. Studies in mice and adult humans, have shown a correlation between low iron levels and increased FGF23 levels. To our knowledge, three pediatric patients with ADHR resolved with iron supplementation have been reported in the literature. Clinical case We report on identical twins born at 28 weeks and 5 days by cesarean section due to premature rupture of membranes with complicated pregnancy due to twin-to-twin transfusion syndrome. Birth weights were 780 grams (2nd percentile) for twin A, 1,200 grams (50th percentile) for twin B. Hypophosphatemia was documented starting at 2 weeks of life and during the first 6 of months of life, with phosphorus levels between 2.9-3.9 mg/dL for twin A and between 2.4-5.1 mg/dL for twin B. During their NICU admission phosphorus had a positive relationship with the hemoglobin level, which was more severe on twin A. Both were treated with calcitriol and a low dose of phosphorus starting on their 2nd month of life. At 6 months of age, both had persistent hypophosphatemia, more prominently in twin A (2.7mg/dL) with high alkaline phosphatase (1,209 IU/L) and high FGF23 (343 RU/dL). At that time his hemoglobin was 9.8 g/dL and his hematocrit was 29.5%. Both were started on Polyvisol with iron. At 14 months of life phosphorus and calcium were within normal limits, therefore calcitriol and phosphate were discontinued. At 15 months of age their hemoglobin, hematocrit, iron level, and TIBC levels were normal for both twins. Phosphorus was 4.6 and 4.3 mg/dL, alkaline phosphatase reduced significantly to 819 and 413IU/L, and FGF23 normalized to 100 RU/dL and 32 RU/dL on twin A and B respectively. Upon physical examination at 15 months of age, twin A was at the 0.02% for length and weight/length at the 31%; twin B was at the 5% for length and weight/length at the 50%. Both twins had high arched palates. Twin A had craniosynostosis, left renal agenesis, bilateral epicanthal folds, overlapping 2nd toes, and clinodactyly of the fifth digits. Note the donor kid (twin A) had a more severe presentation. Genetic testing showed heterozygous mutation c536G>a (p.Arg179GLN) in the FGF23 gene. This is the same mutation previously reported to be related with ADHR resolved with iron supplementation Conclusion The study of patients with hypophosphatemia and hypophosphaturia should include evaluation of iron status (ferritin, TIBC). Treating iron deficiency on these patients might normalize phosphate levels. This would avoid cumbersome treatment with phosphate and calcitriol. Laboratory values Phosphorus:4-8 mg/dL Alkaline Phosphatase: 130-317 IU/L FGF23: 44-215 RU/dL

Autosomal Dominant Hypophosphatemic Rickets Presenting in a Phenotypically Normal Adult Female

We describe a presentation of Autosomal Dominant Hypophosphatemic Rickets (ADHR) in a 22-year-old female with normal pubertal growth and development and a negative family history in first-degree relatives. The patient presented with a 2-year history of upper and lower extremity proximal muscle pain and weakness and bilateral femoral neck and pubic bone insufficiency fractures. She had a normal serum calcium but a low phosphate as well as 25-hydroxyvitamin D (25(OH)D) levels leading initially to a diagnosis of osteomalacia. Urine phosphate reabsorption was low confirming a phosphate wasting disorder. She had an elevated Fibroblast Growth Factor 23 (FGF23) level. After Tumor-Induced Osteomalacia was ruled out by extensive imaging, she was sent for genetic testing for hereditary rickets which showed a previously reported missense variant in FGF23. Subsequently, she found out that her father’s maternal aunt and grandfather had ‘bone disorder’ and were wheelchair-bound in adulthood. After replenishment of vitamin D, treatment with calcitriol and phosphate leads to complete resolution of patient’s symptoms and laboratory abnormalities.