Activating NOTCH1 mutations are found in 50–60% of human T-cell acute lymphoblastic leukemia (T-ALL) samples. In mouse models, these mutations generally fail to induce leukemia. This observation suggests that NOTCH1 activation must collaborate with other genetic events. Mutagenesis screens previously implicated ZMIZ1 as a possible NOTCH1 collaborator in leukemia. ZMIZ1 is a transcriptional co-activator of the Protein Inhibitor of Activated STAT (PIAS)-like family. Its role in oncogenesis is unknown. Here we show that activated NOTCH1 and ZMIZ1 collaborate to induce T-ALL in mice. ZMIZ1 and activated NOTCH1 are co-expressed in a subset of human T-ALL patients and cell lines. ZMIZ1 inhibition slowed growth and sensitized leukemic cells to corticosteroids and NOTCH inhibitors. Gene expression profiling identified C-MYC, but not other NOTCH-regulated genes, as an essential downstream target of ZMIZ1. ZMIZ1 functionally interacts with NOTCH1 to promote C-MYC transcription and activity. The mechanism does not involve the NOTCH pathway and appears to be indirect and mediated independently of canonical PIAS functions through a novel N-terminal domain. Our study demonstrates the importance of identifying genetic collaborations between parallel leukemic pathways that may be therapeutically targeted. They also raise new inquiries into potential NOTCH-ZMIZ1 collaboration in a variety of C-MYC-driven cancers.

<p>PDF file - 5445K, Figure S1. Retroviral transduction efficiency.</p>

<p>PDF file - 5445K, Figure S9. Enforced expression of C-MYC increased apoptosis.</p>

<p>PDF file - 5445K, Figure S6. ZMIZ1 does not regulate C-MYC post-transcriptionally or bind NOTCH1.</p>

<p>PDF file - 5445K, Figure S5. The C-MYC pathway is downregulated after ZMIZ1 knockdown.</p>