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Abstract: SA-PO521

Isolated Hypophosphatemia due to Renal Phosphorous Wasting in Association With ENPP1 Mutations

Session Information

Category: Fluid‚ Electrolyte‚ and Acid-Base Disorders

  • 1002 Fluid‚ Electrolyte‚ and Acid-Base Disorders: Clinical


  • Choi, Jusong, University of Florida, Gainesville, Florida, United States
  • Chamarthi, Gajapathiraju, University of Florida, Gainesville, Florida, United States
  • Ojeda Damas, Dayan, University of Florida, Gainesville, Florida, United States

Biallelic mutations in the ecto-nucleotide pyrophosphatase/phosphodiesterase 1 gene is known to cause generalized arterial calcification of infancy or autosomal-recessive hypophosphatemic rickets in humans. We report a case with renal phosphorous wasting syndrome with normal FGF-23 level in a patient with heterozygous variants on ENPP1 c.1798T>C.

Case Description

A 38-year-old Caucasian male with refractory bitemporal epilepsy diagnosed in 2012 and status post response neurostimulation implantation in 2020 was referred for evaluation of hypophosphatemia incidentally found during the periods of seizure activity... patient underwent workup for hypophosphatemia. 24 hours urine studies were conducted while the patient was on phosphorus supplement because of concerns of possible breakthrough seizures due to supplementation cessation. 24 hours urine study revealed phosphorus excretion was approximately 800mg per day , serum phosphorus at 2.61 mg/dL and serum creatinine at 1.22mg /dl with eGFR greater than 59 ml. His Fractional excretion of phosphate was greater than 10 percent on different occasions despite serum phosphorus being low at 2.5 while being on phosphorus supplements. Renal wasting was suspected and further work up was persued. Pertinent labs include FGF 23 level of 27 pg/mL, PTH of 47 pg/ml, vitamin d 25-OH of 36.13 ng/mL, and 1,25-dihydroxy of 69 pg/mL. A genetic test was performed which revealedheterozygous variants on ENPP1 c.1798T>C but of uncertain significance as it has not been reported in literature Interesting patient endorses some bone mineralization problems with daughter and is currently undergoing evaluation. His low phosphorus checked during the seizure activity was attributed to redistribution.


Heterozygous mutations in the ENPP1 leading to hypophosphatamia, mildly increased FGF23 have been reported in the literature. The Genetic associations of ARHR2 is thought to enhance the activity or levels of FGF-23 which is the key phosphaturic hormone. Our patient’s mutation sequence change replaces the neutral and polar tyrosine with the basic and polar histidine at codon 600 of the ENPP1 protein. This variant has not been reported in literatures of individuals affected with ENPP1-related conditions. Moreover, there are currently insufficient available evidence to determine the role of this variant in the disease.