Prognostic and clinicopathological significance of circulating tumor cells in osteosarcoma

Osteosarcoma is the most common form of primary malignant bone tumor, with metastasis playing an essential role in determining a patient's prospects for survival. It is essential that new and better molecular targets that respond effectively to therapies and are predictive of the risk of tumor metastasis are identified. We have therefore undertaken the present prospective study to ascertain the clinical significance of circulating tumor cells (CTCs) in osteosarcoma patients. Peripheral blood was obtained from patients both pre- and post-surgery then processed using a CanPatrol™ system, an enrichment technique allowing isolation of CTCs by virtue of their size at baseline. Multiplex RNA in situ hybridization (RNA-ISH) was subsequently conducted to characterize the CTCs based on various molecular markers including MTA1, CD45, EpCAM, CK8, CK19, Vimentin and Twist. MTA1 expression was further validated by immunohistochemistry of the tumor tissue. Besides defining a diagnosis and prognosis for osteosarcoma patients, the correlation between CTC count and their molecular and clinicopathological characteristics was found to assist in the analysis of the response of patients to neoadjuvant chemotherapy. Our results revealed that the number of CTCs was significantly higher at baseline in metastatic patients than in those whose osteosarcomas were localized. The variation was attributed to the neoadjuvant chemotherapy treatment. A cut-off value of 7 CTCs/5 mL was found to effectively distinguish patients who had either a favorable or unfavorable prognosis. Notably, the ratio of mesenchymal CTCs at baseline was found to be higher in metastatic vs. localized osteosarcoma patients. In addition, the expression of MTA1 was higher in mesenchymal CTCs than the other CTC phenotypes. Furthermore, immunohistochemical analysis demonstrated a higher expression of MTA1 in tumor tissues from metastatic osteosarcoma patients. Taken together, our findings conclusively establish that the number and molecular phenotype of CTCs are predictive of tumor metastasis and the response of patients to neoadjuvant chemotherapy.