Fix That PHEX Loss

Phosphorous has a critical role in multiple biological functions in the body, such as skeletal mineralization, and an imbalance of this can lead to several musculoskeletal disorders. An important regulator of renal phosphate excretion is fibroblast growth factor 23 (FGF23) which is produced by osteocytes and osteoblasts themselves thus providing a mechanism for the skeletal system to influence its own mineralization needs. PHEX is a gene that regulates FGF23 secretion therefore a loss-of-function mutation in this gene would result in elevated circulating FGF23 and phosphate depletion. This mutation has been identified as a cause for X-linked hypophosphatemia (XLH). Treatment of XLH has been limited and mainly involved phosphorous replacement in combination with 1,25(OH) vitamin D. Antiresorptive osteoporosis treatment can exacerbate the skeletal mineralization process. Here we present a patient with multiple fractures who was on denosumab treatment for presumed osteoporosis before being found to have PHEX mutation. The patient is a 64 year-old female with past medical history of bilateral hip replacement and recurrent femur fractures who was seen in clinic in 2014 due to recurrent fractures and diagnosis of osteoporosis since early 2000s. She had only tried alendronate up until that point. Due to the recurrent fractures she was switched to denosumab therapy while workup was underway for secondary causes. She was found to have low phosphorous levels and elevated FGF23 therefore genetic counseling was pursued and was recommended to check for PHEX mutation. The testing came back positive for loss-of-function mutation in PHEX, and by that point she had received 3 doses of denosumab therapy. She suffered another femoral fracture which was determined to be an atypical fracture, and therefore denosumab treatment was stopped and she was continued on phosphorous replacement as well as 1,25(OH) vitamin D replacement. Most recently her phosphorous levels have been controlled with therapy, and there is current discussion underway to try burosumab, an antibody to FGF23. During evaluation for osteoporosis, it is important to consider phosphorous roles in skeletal mineralization. If recurrent fractures are seen in a patient with low phosphorous levels, especially while they are on conventional antiresorptive osteoporosis medications, genetic testing for PHEX mutations should be considered as well as safely stopping antiresorptive medications.