Possible involvement of bone morphogenetic protein 2 in heterotopic ossification in metastatic lesion from urothelial carcinoma of bladder.

Heterotopic bone formation caused by urothelial carcinoma is rare. The precise mechanism of heterotopic ossification is still unknown. We report a case of urothelial carcinoma with heterotopic bone formation in a metastatic site and investigate the expression of bone morphogenetic protein 2 (BMP-2) and the BMP receptor (BMPR)-Ib using immunohistochemistry. Positive staining of BMP-2 was observed in the cytoplasm of tumor cells in both bladder and psoas lesions. In addition, positive staining of BMPR-Ib was seen in osteoblast-like cells adjacent to bone formation in the psoas metastasis. The heterotopic ossification may result from the metaplasia of pluripotent stem cells into osteoblast cells induced by BMP-2 in a paracrine fashion.