The study investigates how the loss of PTEN (phosphatase and tensin homolog) leads to resistance to NOTCH1 inhibition in T-cell acute lymphoblastic leukemia (T-ALL). NOTCH1 is a key driver in T-ALL, and γ-secretase inhibitors (GSIs) are being explored as potential therapies. However, resistance to GSIs is common, and the underlying mechanisms are not well understood. The research shows that NOTCH1 regulates PTEN expression and the PI3K-AKT signaling pathway in both normal and leukemic T cells. In a Drosophila model, NOTCH and PI3K-AKT signaling synergize in tumorigenesis, and PTEN loss is associated with resistance to NOTCH1 inhibition in human T-ALL. The study found that PTEN mutations are frequent in GSI-resistant T-ALL cell lines, and these mutations are often homozygous, leading to truncated PTEN proteins. PTEN is a tumor suppressor that inhibits the PI3K-AKT pathway, and its loss leads to constitutive activation of this pathway, which promotes cell growth and survival. This aberrant activation of PI3K-AKT contributes to resistance to NOTCH1 inhibition by GSIs. The research also shows that NOTCH1 regulates PTEN expression and the PI3K-AKT pathway in both normal thymocytes and leukemic T cells. The transcriptional downregulation of PTEN downstream of NOTCH1 leads to upregulation of the PI3K-AKT pathway, which is essential for cell growth and survival. HES1 and MYC, two transcription factors controlled by NOTCH1, are involved in regulating PTEN expression. HES1 acts as a negative regulator, while MYC acts as a positive regulator. In Drosophila, the study demonstrates that the synergistic effect of NOTCH and PI3K-AKT signaling is crucial for tumor development. The loss of PTEN in T-ALL cells leads to addiction to constitutive AKT signaling, which supports cell growth and metabolism independently of NOTCH1. This suggests that PTEN loss in T-ALL converts the normal developmental transcriptional network into a mechanism that promotes leukemic cell growth. The findings highlight the importance of PTEN and the PI3K-AKT pathway in the leukemogenic program activated by NOTCH1. They also suggest that targeting both NOTCH1 and PI3K-AKT pathways could be a promising therapeutic strategy for T-ALL. The study provides a basis for the development of new therapeutic approaches that combine NOTCH1 and PI3K-AKT inhibition to overcome resistance in T-ALL.The study investigates how the loss of PTEN (phosphatase and tensin homolog) leads to resistance to NOTCH1 inhibition in T-cell acute lymphoblastic leukemia (T-ALL). NOTCH1 is a key driver in T-ALL, and γ-secretase inhibitors (GSIs) are being explored as potential therapies. However, resistance to GSIs is common, and the underlying mechanisms are not well understood. The research shows that NOTCH1 regulates PTEN expression and the PI3K-AKT signaling pathway in both normal and leukemic T cells. In a Drosophila model, NOTCH and PI3K-AKT signaling synergize in tumorigenesis, and PTEN loss is associated with resistance to NOTCH1 inhibition in human T-ALL. The study found that PTEN mutations are frequent in GSI-resistant T-ALL cell lines, and these mutations are often homozygous, leading to truncated PTEN proteins. PTEN is a tumor suppressor that inhibits the PI3K-AKT pathway, and its loss leads to constitutive activation of this pathway, which promotes cell growth and survival. This aberrant activation of PI3K-AKT contributes to resistance to NOTCH1 inhibition by GSIs. The research also shows that NOTCH1 regulates PTEN expression and the PI3K-AKT pathway in both normal thymocytes and leukemic T cells. The transcriptional downregulation of PTEN downstream of NOTCH1 leads to upregulation of the PI3K-AKT pathway, which is essential for cell growth and survival. HES1 and MYC, two transcription factors controlled by NOTCH1, are involved in regulating PTEN expression. HES1 acts as a negative regulator, while MYC acts as a positive regulator. In Drosophila, the study demonstrates that the synergistic effect of NOTCH and PI3K-AKT signaling is crucial for tumor development. The loss of PTEN in T-ALL cells leads to addiction to constitutive AKT signaling, which supports cell growth and metabolism independently of NOTCH1. This suggests that PTEN loss in T-ALL converts the normal developmental transcriptional network into a mechanism that promotes leukemic cell growth. The findings highlight the importance of PTEN and the PI3K-AKT pathway in the leukemogenic program activated by NOTCH1. They also suggest that targeting both NOTCH1 and PI3K-AKT pathways could be a promising therapeutic strategy for T-ALL. The study provides a basis for the development of new therapeutic approaches that combine NOTCH1 and PI3K-AKT inhibition to overcome resistance in T-ALL.