BTC1.05 Role and significance of ZBTB18-CTBP2 interaction in Glioblastoma

The Zing Finger And BTB Domain Containing 18 (ZBTB18; formerly ZNF238) is a transcriptional repressor with a crucial role in brain development and neuronal differentiation. We recently showed that ZBTB18 represses poor prognosis-associated genes and that it is primarily silenced in mesenchymal GBMs through promoter methylation. To further elucidate the mechanism of ZBTB18 function, we performed Mass Spectrometry (MS) analysis of ZBTB18 co-precipitated proteins in SNB19 cells upon ZBTB18 overexpression. Our data revealed that the corepressors CTBP1 and CTBP2 could be potential ZBTB18 interactors. CTBPs are co-repressors which are implicated in human cancer by promoting several pro-oncogenic activities including EMT, cell survival, migration and invasion. We validated the interactions between ZBTB18 and CTBP2 by co-immunoprecipitation; interestingly, ZBTB18 contains a putative CTBP2 interaction motif (VLDLS) further supporting its role as new CTBP2 interactor. Gene expression analysis followed by gene set enrichment showed that both ZBTB18 overexpression and CTBP2 silencing affect the expression of EMT-signatures, suggesting that the two proteins play an opposite role. As such, ZBTB18 binding to CTBP2 could interfere with CTBP2 function. SILAC-based quantitative mass spectrometry analysis suggests that upon ZBTB18 overexpression, CTBP2 is sequestered by CTBP2 causing the disruption of the CTBP1/2 repressive complex and consequent deregulated gene expression. Overall our data suggest a new mechanism of EMT suppression by ZBTB18 through sequestration of CTBP2 and disruption of the CTBP1/CTBP2 repressive complex. A deeper examination of the proposed mechanism of ZBTB18 function could be used to reverse the mesenchymal phenotype of GBM.