Phosphaturic mesenchymal
tumors of the mixed connective tissue type (PMT) are very rare
tumors of bone and soft tissues. Most patients with PMT have long-standing
osteomalacia secondary to production of
fibroblast growth factor 23 (FGF23), a
hormone that inhibits
phosphate reuptake within the renal proximal tubule. Previously, we have reported the detection of FGF23
mRNA in PMT by reverse transcription polymerase chain reaction (PCR); however, the low specificity and risk for nontumoral tissue contamination inherent in PCR-based methodology limit its clinical utility. We evaluated RNAscope as a semiquantitative method of in situ FGF23
mRNA detection in the diagnosis of PMT. Twenty-five
PMTs (median 52 y, range 5 to 73 y) occurred in patients with
tumor-induced osteomalacia (TIO), manifesting as masses (mean 3.9 cm, range 1.4 to 12 cm) in various bones and soft tissues. FGF23
mRNA was positive in 96% (22/23) informative cases of PMT: 16 cases scored 3+; 5 scored
as 2+; 1 scored as 1+. Among these cases, FGF23
mRNA was detected in 3 malignant
PMTs along with their
metastases. Forty control cases included aneurysmal
bone cyst (N=4), chondromyxoid
fibroma (N=8), high-grade
osteosarcomas (N=8), and (nonfamilial)
tumoral calcinosis, as well as miscellaneous cartilage-forming
tumors or osteoid-forming
tumors and soft tissue
tumors. All control cases were negative for FGF23
mRNA in the lesional cells. One aneurysmal
bone cyst had rare FGF23
mRNA-expressing osteocytes clustered around remodeled bone. One ovarian serous
carcinoma in a patient with disseminated disease, elevated serum FGF23, and TIO was negative for FGF23
mRNA in the neoplastic cells. We conclude that RNAscope is a highly sensitive and specific, semiquantitative in situ hybridization method of FGF23
mRNA detection applicable to
formalin-fixed,
paraffin-embedded tissues. Detection of FGF23 expression is a valuable diagnostic adjunct, especially in patients with occult TIO. Compared with reverse transcription PCR, this method preserves tissue morphology and reduces "false positives" related to detection of endogenous FGF23
mRNA expression by osteocytes.