Margaret Huskey

X‐linked hypophosphatemia (XLH), the most prevalent heritable renal phosphate (Pi) wasting disorder, is caused by deactivating mutations of PHEX. Consequently, circulating phosphatonin FGF23 becomes elevated and hypophosphatemia in affected children leads to rickets with skeletal deformity and reduced linear growth while affected adults suffer from osteomalacia and forms of ectopic mineralization. In 2015, we reported uniquely mild XLH in six children and four of their mothers carrying the non‐coding PHEX 3′‐UTR mutation c.*231A>G. Herein, we characterize this mild XLH variant by comparing its features in 30 individuals to 30 age‐ and sex‐matched patients with XLH but without the 3′‐UTR mutation. The “UTR” and “XLH” groups, both comprising 17 children (2‐17 years, 3 girls) and 13 adults (23‐63 years, 10 women), had mean ages of 23 years. Only 43% of the UTR group versus 90% of the XLH group had received medical treatment for their disorder, including 0% versus 85% of the females, respectively (ps< 0.0001). The UTR group was taller: mean (SD) height Z‐score (HZ) ‐1.0 (1.0) versus ‐2.0 (1.4) (p=0.0034), with significantly greater height for females [‐0.9 (0.7) versus ‐2.3 (1.4), (p=0.0050)] but not males [‐1.2 (1.1) versus ‐1.9 (1.5) (p=0.1541)], respectively. Mean (SD) “arm span Z‐score” (AZ) did not differ between the UTR ‐0.8 (1.3) versus XLH ‐1.3 (1.8) groups (p=0.2269). Consequently, the UTR group was more proportionate with a mean ∆Z (AZ – HZ) of 0.1 (0.6) versus 0.7 (1.0) (p=0.0158), respectively. Compared to the XLH group, the UTR group had significantly higher fasting serum Pi and TmP/GFR (ps≤0.0060), serum FGF23 concentrations within the reference range (p=0.0068), and similar serum alkaline phosphatase levels (p = 0.6513). UTR lumbar spine bone mineral density Z‐score was higher (p=0.0343). Thus, the 3′‐UTR variant of XLH is distinctly mild, especially in girls and women, posing challenges for its recognition and management.

Nitrogen-containing bisphosphonates (N-BPs), such as alendronate, are the most widely prescribed medications for diseases involving bone, with nearly 200 million prescriptions written annually. Recently, widespread use of N-BPs has been challenged due to the risk of rare but traumatic side effects such as atypical femoral fracture (AFFs) and osteonecrosis of the jaw (ONJ). N-BPs bind to and inhibit farnesyl diphosphate synthase (FDPS), resulting in defects in protein prenylation. Yet it remains poorly understood what other cellular factors might allow N-BPs to exert their pharmacological effects. Here, we performed genome-wide studies in cells and patients to identify the poorly characterized gene, ATRAID . Loss of ATRAID function results in selective resistance to N-BP-mediated loss of cell viability and the prevention of alendronate-mediated inhibition of prenylation. ATRAID is required for alendronate inhibition of osteoclast function, and ATRAID -deficient mice have impaired therapeutic responses to alendronate in both postmenopausal and senile (old age) osteoporosis models. Lastly, we performed exome sequencing on patients taking N-BPs that suffered ONJ or an AFF. ATRAID is one of three genes that contain rare non-synonymous coding variants in patients with ONJ or AFF that is also differentially expressed in poor outcome groups of patients treated with N-BPs. We functionally validated this patient variation in ATRAID as conferring cellular hypersensitivity to N-BPs. Our work adds key insight into the mechanistic action of N-BPs and the processes that might underlie differential responsiveness to N-BPs in people. One Sentence Summary: ATRAID is essential for responses to the commonly prescribed osteoporosis drugs nitrogen-containing bisphosphonates. Overline: BONE