RepID promotes metastatic potential in osteosarcoma through regulation of the PRC1-GATA6 axis

Osteosarcoma is the most common primary malignant bone tumor and is characterized by a high propensity for metastasis, most frequently to the lungs and lymph nodes. Despite extensive research, the key modulators and their functional roles driving osteosarcoma metastasis remain poorly defined. Here, we identify RepID (Replication initiation determinant protein), a substrate receptor which is crucial for the chromatin recruitment of the Cullin-RING ubiquitin E3 ligase 4 (CRL4) complex, as a upstream regulator of epithelial-mesenchymal transition (EMT)-associated gene expression that promotes osteosarcoma metastasis. <i>In silico</i> analysis of the Cancer Cell Line Encyclopedia (CCLE) using the CellMinerCDB portal revealed a correlation between RepID transcript levels and EMT gene signatures in osteosarcoma. Moreover, RepID expression was positively correlated with CBX4, a component of the polycomb repressive complex 1 (PRC1) that mediates epigenetic gene silencing. Mechanistically, RepID-proficient cells exhibited increased PRC1-mediated H2AK119 ubiquitination (H2AK119Ub) and enrichment of H2AK119Ub at the GATA6 promoter, leading to transcriptional silencing of GATA6. In contrast, RepID depletion resulted in a marked upregulation of GATA6 expression due to the loss of these repressive modifications. Collectively, these findings establish RepID as a pivotal upstream regulator of osteosarcoma metastasis through modulation of the PRC1-GATA6 axis, and highlight its potential as a promising therapeutic target.