| Summary: | The <i>O</i>-acetylated form of GD2, almost exclusively expressed in cancerous tissues, is considered to be a promising therapeutic target for neuroectoderm-derived tumors, especially for breast cancer. Our recent data have shown that 9-<i>O</i>-acetylated GD2 (9-<i>O</i>AcGD2) is the major <i>O</i>-acetylated ganglioside species in breast cancer cells. In 2015, Baumann et al. proposed that Cas 1 domain containing 1 (CASD1), which is the only known human sialyl-<i>O</i>-acetyltransferase, plays a role in GD3 <i>O</i>-acetylation. However, the mechanisms of ganglioside <i>O</i>-acetylation remain poorly understood. The aim of this study was to determine the involvement of CASD1 in GD2 <i>O</i>-acetylation in breast cancer. The role of CASD1 in <i>O</i>AcGD2 synthesis was first demonstrated using wild type CHO and CHOΔ<i>Casd1</i> cells as cellular models. Overexpression using plasmid transfection and siRNA strategies was used to modulate CASD1 expression in SUM159PT breast cancer cell line. Our results showed that <i>O</i>AcGD2 expression was reduced in SUM159PT that was transiently depleted for CASD1 expression. Additionally, <i>O</i>AcGD2 expression was increased in SUM159PT cells transiently overexpressing CASD1. The modulation of CASD1 expression using transient transfection strategies provided interesting insights into the role of CASD1 in <i>O</i>AcGD2 and <i>O</i>AcGD3 biosynthesis, and it highlights the importance of further studies on <i>O</i>-acetylation mechanisms.
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