Abstract 4448: Notch1 inactivation contributes to PI3Ki resistance in T-ALL.
Authors: Dail Monique , Wong Jason , O'Connor Daniel , Nakitandwe Joy , Chen Shann-Ching , Lawrence Jessica , Downing James R. , Sampath Deepak , Shannon Kevin 
Published: August 2014

Abstract
Tyrosine kinases inhibitors induce genotype-specific regression in advanced human cancers. However, these responses are almost invariably transient and patients ultimately relapse due to outgrowth of drug-resistant clones. Thus, increased knowledge of the mechanisms underlying de novo and acquired resistance is essential for enhancing clinical efficacy.
T cell acute lymphoblastic leukemia (T-ALL) is an aggressive cancer for which current therapies are toxic and frequently ineffective. Mutations in genes encoding components of the Notch1 and Ras/PI3K/PTEN/Akt signaling networks are prevalent in T-ALL and frequently coexist in individual patients. Thus, inhibiting these pathways alone and in combination is a rational therapeutic strategy. Notch1 inhibition with a γ-secretase inhibitor (GSI) was recently evaluated in refractory T-ALL, with disappointing results that may have been due to dose-limiting gastrointestinal toxicity. We are not aware of clinical trials of Ras/PI3K/Akt/mTOR pathway inhibitors in T-ALL.
Retroviral insertional mutagenesis (RIM) in mice is a robust strategy for modeling both the clonal heterogeneity that exists within individual tumors and the genetic diversity found across a population of patients with the same cancer. We generated transplantable T-ALLs by performing RIM in mice expressing oncogenic KrasG12D from its endogenous genetic locus (Mx1-Cre, KrasG12D) and in wild-type (WT) littermates. These aggressive multi-step cancers are characterized by recurring viral integrations in known oncogenes such as Ikzf1 and late occurring acquired Notch1 PEST domain mutations. We have extensively investigated patterns of response and resistance to targeted inhibitors of PI3K (PI3Ki) and MEK (MEKi) in this system.
Whereas KrasWT T-ALLs respond to either PI3Ki or to a combination regimen that also include a MEKi, KrasG12D T-ALLs are largely refractory to PI3Ki, but are sensitive to the MEKi + PI3Ki combination. T-ALLs invariably relapsed despite ongoing treatment, and we verified phenotypic drug resistance in relapse leukemias by transplanting and retreating them in vivo. We unexpectedly identified multiple independent PI3Ki resistant T-ALLs with markedly reduced levels of activated Notch1, including some in which Notch1 mutations that were present at enrollment were absent at relapse. Expression of activated Notch1 increases the sensitivity of mouse and human T-ALL cells to a PI3Ki, and loss of activated Notch promotes resistance and enhances PI3K signaling. Thus, oncogenic Notch1 mutations that promote outgrowth during transformation unexpectedly “switch” to become deleterious during in vivo treatment with a PI3Ki. These data suggest that aberrant Ras/PI3K signaling plays a dominant role in T lineage leukemogenesis and also raise the unexpected possibility that administering Notch1 and PI3K inhibitors sequentially or in combination might inadvertently promote resistance.
Citation Format: Monique Dail, Jason Wong, Daniel O'Connor, Joy Nakitandwe, Shann-Ching Chen, Jessica Lawrence, James R. Downing, Deepak Sampath, Kevin Shannon. Notch1 inactivation contributes to PI3Ki resistance in T-ALL. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 4448. doi:10.1158/1538-7445.AM2013-4448