De novo deletions of p53 gene and wild-type p53 correlate with acquired cisplatin-resistance in human osteosarcoma OST cell line. Initial p53 status is a useful determinant of chemoresistance or chemosensitivity of primary tumors, however, it remains unclear whether p53 status is a critical chemoresistant marker in tumors that acquire drug-resistance after the initiation of chemotherapy. We investigated the relationship between p53 status and the development of resistance to cisplatin in osteosarcoma cell lines.Cisplatin-sensitive human osteosarcoma OST cells and acquired cisplatin-resistant OST/R cells derived from OST cells were used. Single-strand conformation polymorphism (SSCP) analysis of exons 5 to 8, and immunohistochemistry using anti-p53 antibodies were analyzed to detect mutations of p53. Fluorescence in situ hybridization (FISH) and enzyme immunoassay (EIA) were performed to detect deletions of p53.SSCP and immunohistochemistry revealed that both cell lines had wild-type p53 gene and protein. However, in OST/R cells, genomic instability of chromosome 17 and de novo deletion of the p53 gene located in chromosome 17p were detected by FISH. The constitutive levels of wild-type p53 protein measured by EIA were significantly lower in OST/R cells than in OST cells. Furthermore, p53 induction was lost in OST/R cells after cisplatin exposure.De novo deletions of the p53 gene and wild-type p53 were associated with the acquisition of cisplatin-resistance in osteosarcoma.