PSAT228 Hereditary Hypophosphatemic Rickets: Management in Adults
Authors: Manas F N U , Mols-Kowalczewski Barbara 
Published: November 2022

Abstract

Introduction
Hereditary Hypophosphatemic Rickets (HHR) is a group of rare inherited disorders consisting of X-linked hypophosphatemic rickets (XLHR, prevalence 1 in 20,000 births), autosomal dominant hypophosphatemic rickets (ADHR), and autosomal recessive hypophosphatemic rickets (ARHR). XLHR is caused by loss of function mutation of the PHEX gene, ARHR is caused by loss of function mutation of the DMP1/ENPP1 gene and ADHR is caused by activating point mutations in FGF23. These mutations result in renal phosphate wasting, hypophosphatemia, and impaired bone mineralization. It presents in childhood as skeletal deformities, abnormal gait, skeletal pain, dental abnormalities, and stunted growth. Adults have osteomalacia-related pain, fractures, and arthritis. The diagnosis is made by findings consistent with rickets and normal serum calcium, normal 25-hydroxy vitamin D, low-normal 1,25-dihydroxy vitamin D, elevated alkaline phosphatase activity, and low phosphate levels. Urine studies show low urine calcium and decreased tubular maximum reabsorption of phosphate, confirming renal wasting of phosphate. The goal of treatment in adults is to reduce pain, prevent fractures, enhance mobility, and decrease osteomalacia. Phosphate and vitamin D supplementations are used with close monitoring as it is associated with severe adverse events. It can cause hypercalcemia leading to nephrocalcinosis, nephrolithiasis, chronic kidney disease, and hyperparathyroidism. There is no evidence that the treatment affects long-term skeletal complications. Burosumab, a monoclonal antibody that inhibits FGF23, is approved for the treatment of XLHR. No studies are available about the use of Burosumab in ADHR or ARHR.

Case Presentation
A 18-year-old male with past medical history of hypophosphatemic rickets with multiple fractures, on oral phosphate tablets and calcitriol, presented to the Endocrinology clinic to establish care. He had been following Pediatric Endocrinology in the past. His blood work showed normal calcium, low phosphate of 1.6 mg/dl (normal: 2.5-4.5 mg/dl), vitamin D 25 hydroxy level of 26 ng/ml (normal: 20-100 ng/ml), vitamin D 1,25-dihydroxy level of 25 pg/ml (normal: 30-80 pg/ml), intact parathyroid hormone level of 549 pg/ml (normal: 15-65 pg/ml), and alkaline phosphatase of 438 U/l (normal: 40-150 U/l). Urine calcium was low and urine phosphorus was elevated with phosphate/creatinine ratio more than 2000 (normal: 60-962), consistent with renal phosphate wasting. Genetic testing in the past was negative for PHEX gene mutation, so he was not a candidate for Burosumab.

Conclusion
There is limited knowledge about Hereditary Hypophosphatemic Rickets, especially ADHR and ARHR due to limited cases (less than 100 cases reported so far). The current standard of treatment of HHR is sub-optimal and associated with adverse effects. Burosumab has not been tested in ADHR or ARHR, so it is unclear if it will be beneficial in this subgroup.
Presentation: Saturday, June 11, 2022 1:00 p.m. - 3:00 p.m., Sunday, June 12, 2022 12:30 p.m. - 2:30 p.m.