Inhibition of NOTCH signaling by gamma secretase inhibitor engages the RB pathway and elicits cell cycle exit in T-cell acute lymphoblastic leukemia cells

NOTCH signaling is deregulated in the majority of T-cell acute lymphoblastic leukemias (T-ALL) as a result of activating mutations in NOTCHl. Gamma secretase inhibitors (GSI) block proteolytic activation of NOTCH receptors and may provide a targeted therapy for T-ALL. We have investigated the mechanisms of GSI sensitivity across a panel of T-ALL cell lines, yielding an approach for patient stratification based on pathway activity and also providing a rational combination strategy for enhanced response to GSI. Whereas the NOTCHl mutation status does not serve as a predictor of GSI sensitivity, a gene expression signature of NOTCH pathway activity does correlate with response, and may be useful in the selection of patients more likely to respond to GSI. Further-more, inhibition of the NOTCH pathway activity signature correlates with the induction of the cyclin-dependent kinase inhibitors CDKN2D (pl9,NK4d) and CDKNlB (p27KiI"), leading to derepression of RB and subsequent exit from the cell cycle. Consistent with this evidence of cell cycle exit, short-term exposure of GSI resulted in sustained molecular and pheno-typic effects after withdrawal of the compound. Combination treatment with GSI and a small molecule inhibitor of CDK4 produced synergistic growth inhibition, providing evidence that GSI engagement of the CDK4/RB pathway is an important mechanism of GSI action and supports further investigationof this combination for improved efficacy in treating T-ALL.