PgmNr M298: Genetic inhibition of MTOR during thymic Pre-T LBL development delays tumorigenesis and points to the IRF4-CDK6 pathway as a potential target in the treatment of T-ALL/LBL.

Authors:
B. A. Mock 1 ; J. M. Gary 1,2 ; J. K. Simmons 1 ; J. Xu 3 ; S. Zhang 1 ; N. Watson 1 ; B. Gamache 1 ; K. Zhang 1 ; A. L. Kovalchuk 4 ; A. M. Michalowski 1 ; M. Kiupel 2 ; S. Gaikwad 1 ; W. Dubois 1 ; J. Testa 1

Institutes
1) CCR, NCI, NIH, Bethesda, MD; 2) Pathobiology and Diagnostic Investigation, Michigan State University, East Lansing, MI; 3) Fox Chase Cancer Center, Philadelphia, PA; 4) NIAID, NIH, Rockville, MD, USA.

Abstract:

The PI3K/AKT/MTOR pathway is frequently activated in T-cell acute lymphoblastic leukemia/lymphoma (T-ALL/LBL). To model inhibition of this pathway in lymphoma, mice with T-lymphocyte-specific, constitutively-active AKT (Tg(Lck-Akt2*)#Test, Lck-MyrAkt2) were crossed to mice with genetically reduced MTOR expression (Mtortm1Lgm knock-down, KD). Mice with genetic reduction of MTOR had increased survival relative to wild type MTOR mice (average survival of 24 versus 14 weeks, respectively), though both groups ultimately developed thymic pre-T-cell lymphoblastic leukemia/lymphoma (pre-T LBL). A similar increase in survival was observed when MTOR wild type Lck-MyrAkt2 mice were treated for 8 weeks with the rapamycin analog, everolimus, an inhibitor of the mTORC1 complex. Transcriptional profiling of wildtype (WT) and MTOR KD thymic lymphomas identified cyclin dependent kinase, Cdk6, as one of the most down-regulated genes during tumorigenesis in the Lck-AKT, MTOR KD tumors; its expression was the same in WT and KD pre-tumor thymocytes.  The interferon regulatory transcription factor, Irf4, was identified as an upstream regulator; protein levels of IRF4 were down-modulated in both pre-tumor thymocytes and pre-T LBL cells in the KD mice, compared to levels in WT littermates. Rescue experiments have confirmed the relationship between IRF4 and CDK6. Pharmacologic inhibition of MTOR in tumor cells also decreased CDK6 protein levels, further suggesting a mechanistic relationship in this tumor type. Combination treatment with the MTOR inhibitor rapamycin and the CDK4/6 inhibitor palbociclib cooperatively decreased the overall viability and signaling downstream of drug targets in human T-ALL/LBL cell lines. Further, the combination of palbociclib and rapamycin decreased tumor size and proliferation in nude mice with Lck-Akt/MTOR WT tumor flank transplants, and significantly increased survival in an intravenous transplant model of disseminated leukemia compared to single agent treatments, suggesting the potential for this drug combination in treating T-ALL/LBL.