Hypophosphatemic Rickets with R179W Mutation in FGFR23 Gene - A Rare But Treatable Cause of Refractory Rickets

Hereditary Rickets: A Quick Guide for the Pediatrician

With the increased discovery of genes implicated in vitamin D metabolism and the regulation of calcium and phosphate homeostasis, a growing number of genetic forms of rickets are now recognized. These are categorized into calciopenic and phosphopenic rickets. Calciopenic forms of hereditary rickets are caused by genetic mutations that alter the enzymatic activity in the vitamin D activation pathway or impair the vitamin D receptor action. Hereditary forms of phosphopenic rickets, on the other hand, are caused by genetic mutations that lead to increased expression of FGF23 hormone or that impair the absorptive capacity of phosphate at the proximal renal tubule. Due to the clinical overlap between acquired and genetic forms of rickets, identifying children with hereditary rickets can be challenging. A clear understanding of the molecular basis of hereditary forms of rickets and their associated biochemical patterns allow the health care provider to assign the correct diagnosis, avoid non-effective interventions and shorten the duration of the diagnostic journey in these children. In this mini-review, known forms of hereditary rickets listed on the Online Mendelian Inheritance in Man database are discussed. Further, a clinical approach to identify and diagnose children with hereditary forms of rickets is suggested.

Genetic and clinical profile of patients with hypophosphatemic rickets

Nutritional vitamin D deficiency is the most frequent cause of rickets followed by genetic causes, that include entities classic hypophosphatemic rickets (FGF23 related), Dent disease, Fanconi syndrome, renal tubular acidosis, and vitamin D dependent rickets. Hypophosphatemia is a feature in all these forms. The diagnosis relies on a combination of clinical, biochemical and radiological features, but genetic testing is required to confirm the diagnosis. Between May 2015 and July 2019, we screened 66 patients with hypophosphatemic rickets by whole exome sequencing (WES) referred to this center in addition to measurement of intact fibroblast growth factor 23 (FGF23) levels in serum. WES revealed 36 pathogenic and 28 likely pathogenic variants in 16 different genes (PHEX, FGF23, DMP1, ENPP1, CLCN5, CTNS, SLC2A2, GATM, SLC34A1, EHHADH, SLC4A1, ATP6V1B1, ATP6V0A4, CYP27B1, VDR and FGFR1) in 63 patients which helped differentiate various forms of hypophosphatemic rickets. Intact serum FGF23 levels were significantly higher in patients with variations in PHEX, FGF23, DMP1 or ENPP1 genes. The chief genetic causes of rickets were classic hypophosphatemic rickets with elevated FGF23 levels, distal renal tubular acidosis, and vitamin D dependent rickets. Based on the present results, we propose a customized gene panel for targeted exome sequencing, which will be useful for confirming the diagnosis in most patients with hypophosphatemic rickets.

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.