Idh2R140Q Cooperates with NHD13 to Induce Immature T Cell Leukemia By Targeting Early T Cell Progenitors or DN2 Thymocytes
Authors: Negi Vijay , Goldberg Liat , Chung Yang Jo , Onozawa Masahiro , Aplan Peter D. 
Published: November 2019

Introduction
Mutations in the isocitrate dehydrogenase 1 (IDH1) and IDH2 genes are frequently observed in patients with acute myeloid leukemia, with IDH2 mutations reported in 9- 19% of AML cases. IDH2 mutation leads to loss of normal enzymatic function and accumulation of 2-hydroxyglutarate (2-HG) through a newly gained enzyme activity. The most common IDH mutation in AML patients involves IDH2 R140.
Transgenic mice that expressed an Idh2R140Q mutant in all hematopoietic tissues did not develop leukemia, nor was there a significant difference between overall survival of Idh2R140Q mice compared to wildtype mice. These findings were consistent with those from other investigators, which also suggested that expression of mutant IDH2 was not sufficient for development of leukemia, and that collaborating mutations are necessary. A recent report of whole exome sequence (WES) from mouse models of hematopoietic malignancies identified recurrent Idh1R132H mutations in NUP98-HOXD13 (NHD13) driven AML. Given that Idh1 R132 is paralogous to IDH2 R140, we considered the possibility that an Idh2R140Q mutation would collaborate with an NHD13 transgene and promote leukemic transformation. Therefore, we generated Idh2R140Q/NHD13 transgenic mice by crossing Idh2R140Q with NHD13 mice. Idh2R140Q/NHD13 transgenic mice developed a leukemia at a median of 10 months of age that was similar to human ETP in terms of immunophenotype and additional acquired cooperative mutations in genes such as Pten, N/Kras, Ptpn11, and Sh2b3.
Results
Gene set enrichment analysis (GSEA) based on RNA-Seq data from Idh2R140Q/NHD13 ETP leukemias and non-ETP T cell leukemias showed that Idh2R140Q/NHD13 ETP gene expression profile correlated well with human ETP leukemic expression profile. An in vitro thymocyte differentiation assay using co-culture of immature double negative (DN) 1 and DN2 thymocytes from Idh2R140Q/NHD13 mice on an OP9-DL1 stromal layer demonstrated a complete block in differentiation to double positive (DP) CD4+CD8+ thymocytes. The Idh2R140Q/NHD13 DN1/2 co-cultured cells arrested at the DN2 stage of differentiation, similar to the in-vivo phenotype of Idh2R140Q/NHD13 leukemia. In addition, Idh2R140Q/NHD13 DN1/2 cells had greater proliferative potential compared to wildtype control. We further observed that Idh2R140Q/NHD13 DN cells would proliferate indefinitely on OP9-DL1 stromal cells, and that treatment of Idh2R140Q/NHD13 thymocytes with AG-221, a potent and selective mutant IDH2 inhibitor, led to a marked decrease in cell proliferation.
We developed an in vivo bone marrow transplantation (BMT) model for Idh2R140Q/NHD13 ETP leukemia to assess response to AG-221 in vivo. Primary Idh2R140Q/NHD13 ETP leukemia cells were transplanted into sub-lethally (600 cGy) irradiated recipient mice. This resulted in recipient mice that were anemic and thrombocytopenic with elevated white blood cell counts, suggesting engraftment of acute leukemia. The transplanted, secondary leukemias were consistent with the primary disease immunophenotype by flow cytometry, and tissue histology showed infiltration of blasts into the bone marrow, spleen and perivascular regions of the liver, consistent with disseminated leukemia. Blast cells were positive for both CD3 and myeloperoxidase, further highlighting an ETP phenotype. An assay for clonal T cell receptor beta rearrangement confirmed clonality of recipient leukemia cells identical to the primary leukemia cells. In vivo treatment of Idh2R140Q/NHD13 ETP recipient mice with AG221 showed a significant decrease in leukemic cell expansion compared to control mice treated by gavage with vehicle only; survival data is pending.
Conclusion
In summary, the IDH2 inhibitor data suggests that targeting the mutant IDH2 in Idh2R140Q/NHD13 leukemic cells can result in a significant decrease in leukemic burden in vivo. Furthermore, the Idh2R140Q/NHD13 primary ETP leukemia BMT mice serves as an excellent model for the study of ETP leukemia development and therapy. In this context, it is important to note that 14% of human ETP show mutations in IDH1/2, and although NUP98 translocations are rare but recurrent events in human ETP ALL, these NUP98 translocations lead to enforced expression of HOXA genes, which is a common event in ETP ALL.

Disclosures
Aplan: NIH: Patents & Royalties: royalties for the invention of NUP98-HOXD13 .