A sequential series of mutational events is necessary for the development of leukemia. The misexpression of TAL1, a basic helix-loop-helix (bHLH) transcription factor, is the most common mutation in T cell acute lymphoblastic leukemia (T-ALL). Tal1 transgenic mice develop leukemia with a long latency and incomplete penetrance indicating additional mutations are necessary to develop disease. To investigate additional mutational events that potentially contribute to TAL1-expressing T-ALL patients, we sought to identify cooperating mutations in Tal1 transgenic mice. Clinical studies implicated the loss of the INK4a/ARF locus, which encodes two tumor suppressors, p16^INK4a and p14^ARF, in the majority of T-ALL patients. We demonstrated disease acceleration in tal1/ink4a/arf+/-, tal1/pl6^ink4a+/- and tal1/p19^arf+/- mice, thereby providing genetic evidence that Tal1 cooperates with loss of either p16^Ink4a or p19^Arf in leukemogenesis. The cooperation of Tal1 with the loss of or p16^Ink4a or p19^Arf, is consistent with our observation that Tal1 alters cell cycle regulation in leukemia by promoting S phase induction and apoptosis in vivo.

An additional mutational event common in tal1 tumors is activation of the Notch1 signaling pathway. We provide evidence that the majority of tal1 tumors express increased levels of Notch1, and exhibit activating notch1 mutations. Additionally, tal1 tumors display sensitivity to the pharmacologic inhibition of γ-secretase activity in vitro, indicating that γ-secretase inhibitors may prove an efficacious treatment for TAL1-expressing T-ALL patients. Furthermore, we developed a doxycycline-regulated Notch^IC T-ALL cell line, which will allow the identification of important Notch1^IC target genes in leukemogenesis.