RT-PCR analysis for FGF23 using paraffin sections in the diagnosis of phosphaturic mesenchymal tumors with and without known tumor induced osteomalacia.

Phosphaturic mesenchymal tumors of the mixed connective tissue type (PMTMCT) are extremely rare, histologically distinctive neoplasms, which cause tumor-induced osteomalacia (TIO) in most cases through the elaboration of a phosphaturic hormone, fibroblast growth factor-23 (FGF23). Rarely, identical tumors without known TIO may be observed. We studied a large group of PMTMCT for expression of FGF23, using a novel reverse transcription polymerase chain reaction (RT-PCR) assay for FGF23 in formalin-fixed, paraffin-embedded tissues. Twenty-nine PMTMCT (17 with and 12 without TIO) and 23 non-PMTMCT (16 various mesenchymal tumors, including 5 chondromyxoid fibroma, 8 chondroblastoma, 1 hemangiopericytoma, 1 aneurysmal bone cyst, and 1 high grade sarcoma; 5 carcinomas; and 2 non-neoplastic tissues) were retrieved. Total RNA was extracted from formalin-fixed, paraffin-embedded sections for RT-PCR analysis. FGF23 was amplified using 3 sets of primers that spanned the intron/exon boundaries to amplify the 3 exons of FGF23 gene (140, 125, and 175 bp). The housekeeping gene phosphoglycerokinase (189 bp) was coamplified to check the RNA quality. Sixteen of 17 (94%) PMTMCT with TIO were FGF23-positive. Nine of 12 (75%) PMTMCT without TIO were FGF23-positive. Two chondromyxoid fibroma and 1 aneurysmal bone cyst were positive; all other non-PMTMCT were negative. We conclude that RT-PCR for FGF23 is a sensitive and specific means of confirming the diagnosis of PMTMCT both in patients with and without TIO. FGF23 gene expression was present in more than 90% of PMTMCT with known TIO, confirming the role of FGF23 in this syndrome. Rare FGF23-negative PMTMCT with known TIO likely express other phosphaturic hormones (eg, frizzled-related protein 4). Our finding of expression of FGF23 in 75% of histologically identical tumors without known TIO confirms the reproducibility of the diagnosis of PMTMCT, even in the absence of known phosphaturia.