Arf and Ink4a Are Critical Factors Determining the Cell of Origin and Therapeutic Sensitivity in Myc-Induced Mouse Lymphoid Tumor

Abstract 2448

Understanding the cell of origin of tumors is important not only for elucidating detailed mechanisms of tumorigenesis but also for characterizing the context in which tumor cells develop, both of which provide valuable information that can inform preventive therapy and therapeutic strategies in the clinical setting. However, the cell of origin and the factors determining the cell of origin remain unclear.

In this study, a mouse model of precursor-B acute lymphoblastic leukemia/lymphoma (pre-B ALL/LBL: B220⁺CD19⁺CD43^+/–IgM⁻) was established by retroviral transduction of Myc oncogenes (N-Myc or c-Myc) into mouse bone marrow mononuclear cells. To identify the cell of origin of this tumor, we fractionated N-Myc-transduced hematopoietic stem cells (HSCs), common lymphoid progenitors (CLPs), myeloid progenitors (MPs) and committed progenitor B cells, and then transplanted into recipient mice in a limiting dilution manner. As a result, HSCs showed the highest susceptibility to N-Myc-induced pre-B ALL/LBL versus CLPs and MPs. Frequencies of tumor-initiating cells originated from HSCs, CLPs and MPs were 1/184, 1/558 and 1/9286, respectively. Although N-Myc was unable to induce any tumor directly from committed progenitor B cells, N-Myc was able to induce pre-B ALL/LBL directly from those cells in the absence of Ink4a and Arf genes whose expression is normally maintained at low levels for endowing self-renewal capacity to HSCs. Furthermore, we found that N-Myc induced pre-B ALL/LBL directly from Arf deficient progenitor B cells. Since Arf was expressed significantly higher in progenitor B cells than Ink4a, Arf might play a predominant role in the determination of the cell of origin. In our mouse model, there is no significant difference between two types of Myc oncogenes, N-Myc and c-Myc, in respect to tumorigenic activities of the induced tumors, suggesting similar impacts of Myc family genes on lymphoid tumor.

Next, we analyzed chemotherapeutic sensitivities of tumor cells derived from distinct cells of origin, wild-type HSCs and Ink4a/Arf^−/− progenitor B cells. Tumor cells derived from Ink4a/Arf^−/− progenitor B cells were significantly more resistant to Ara-C treatment than those derived from wild-type HSCs in vivo and in vitro. To eradicate Ink4a/Arf^−/−-derived tumor cells, the Mdm2 inhibitor Nutlin-3 was used because Nultin-3 was assumed to be able to replace the role of Arf which inhibits Mdm2 to block p53 degradation. As a result, Nutlin-3 restored p53 and thereby induced massive apoptosis in tumor cells derived from Ink4a/Arf^−/− progenitor B cells. In contrast, Nutlin-3 did not induce apoptosis in tumor cells derived from wild-type HSCs due to p53 mutations. Furthermore, Nutlin-3 effectively induced apoptosis in human B-ALL cell lines with wild-type p53 and lacking Ink4a and Arf expression. In addition, tumor cells derived from Ink4a/Arf^−/− cells were more sensitive to Nutlin-3 treatment than normal bone marrow mononuclear cells. Therefore, Mdm2 inhibition can be a novel and promising therapeutic approach to the treatment of Ink4a/Arf deficient pre-B ALL/LBL, such as is frequently found in Ph⁺ B-ALL and relapsed B-ALL.

Collectively these findings suggest that Ink4a and Arf are critical determining factors of the cell of origin of pre-B ALL/LBL, and that tumor cells derived from distinct cells of origin showed different drug sensitivities to Ara-C and Nutlin-3, which provides a novel insight into preventive therapy and different therapeutic approaches depending on genetic background of tumor cells.