Tumor-induced osteomalacia in association with PTEN-negative Cowden syndrome.

Tumor-induced osteomalacia (TIO) is a rare paraneoplastic condition in which phosphaturic mesenchymal tumors (PMTs) secrete high levels of fibroblast growth factor 23 (FGF23) into the circulation. This results in renal phosphate wasting, hypophosphatemia, muscle weakness, bone pain, and pathological fractures. Recent studies suggest that fibronectin-fibroblast growth factor receptor 1 (FN1-FGFR1) translocations may be a driver of tumorigenesis. We present a patient with TIO who also exhibited clinical findings suggestive of Cowden syndrome (CS), a rare autosomal dominant disorder characterized by numerous benign hamartomas, as well as an increased risk for multiple malignancies, such as thyroid cancer. While CS is a clinical diagnosis, most, but not all, harbor a mutation in the tumor suppressor gene PTEN. Genetic testing revealed a somatic FN1-FGFR1 translocation in the FGF23-producing tumor causing TIO; however, a germline PTEN mutation was not identified. To our knowledge, this is the first reported case of concurrent TIO and CS.

Stability and degradation of fibroblast growth factor 23 (FGF23): the effect of time and temperature and assay type.

UNLABELLED: There is growing need for a reliable assay for measuring fibroblast growth factor 23 (FGF23), a regulator of phosphorus and vitamin D. In this work, we analyze and compare the performance of three available assays, including the effect of temperature and time. This knowledge will allow for better understanding of FGF23 in the future. INTRODUCTION: Intact and C-terminal FGF23 (iFGF23 and cFGF23) concentrations are important in the diagnosis of hypo- and hyperphosphatemic diseases. The effects of temperature, storage, and specimen handling on FGF23 levels are not well known. We investigated the effects of various factors on plasma and serum measurement of FGF23 using three different assays. METHODS: Serum and plasma FGF23 were measured using three commercially available ELISA assays-two measuring iFGF23 and one measuring cFGF23. Samples from subjects with known FGF23 disorders were stored at 4, 22, and 37 °C and analyzed at different intervals up to 48 hours (h). A subset of samples underwent repeated freeze-thaw cycles, and samples frozen at -80 °C for up to 60 months were reanalyzed. The effect of adding a furin convertase inhibitor on FGF23 degradation was investigated using samples stored at 37 °C for 48 h. Intact FGF23 levels were measured from plasma samples of four different groups to test the correlation of the two assays. RESULTS: Plasma FGF23 levels were stable when stored at 4 and 22 °C for 48 h. Both plasma and serum FGF23 levels demonstrated relative stability after five freeze-thaw cycles. Long-term storage at -80 °C for 40 months induced some variability in FGF23 levels. The addition of a furin inhibitor did not affect FGF23 degradation. Intact FGF23 levels showed good correlation only at the upper limit of the assay range when comparing the two assays. CONCLUSIONS: Sample type, handling, and choice of assay are factors that affect FGF23 levels and should be considered when measuring this hormone.