Osteosarcoma (OS) is the most common type of bone tumor in children and adults. However, the molecular mechanism underlying OS tumorigenesis remains unclear. Here, we report that the expression of APCDDI, a Wnt antagonist, was reduced in OS tissues and cells compared to adjacent normal tissue and osteoblast cells, respectively. Mechanistically, this was due to increased levels of methylation in the promoter region of the APCDD1 gene. Consistently, the DNA methyltransferase inhibitor 5-AZA-dC, reduced DNA methylation in the APCDD1 promoter, and restored APCDD1 expression in OS tissue and cells. Moreover, DNMT3a, but not DNMT1 or DNMT3b, was the major DNA methyltransferase that facilitated hyper-methylation of DNA in the APCDD1 promoter, thus reducing APCDD1 mRNA levels in OS tissues. Importantly, ectopic expression of APCDDI suppressed activity of the Wnt/13-Catenin signaling pathway in OS cells and inhibited their invasion and reversed their EMT-like properties, while depletion of APCDDI promoted invasion and metastasis of osteosarcoma cells in vitro and in vivo. Thus, we have provided the first evidence that APCDD1 expression is epigenetically silenced in OS, which may facilitate invasion and metastasis of OS cells. (C) 2017 Elsevier Inc. All rights reserved.