Bcl11a Causes p21Cip1 Down-Regulation and Transplantable Leukemia in Nf1-Deficient Mice.

Inactivation of the tumor suppressor gene, NF1, is associated with chronic and acute forms of myeloid diseases including juvenile myelomonocytic leukemia. Nf1-deficiency mouse models have shown some phenotypes reminiscent of those of the human disease. However, Nf1mutation in itself does not lead to leukemia. We reported the identification of 37 common proviral insertion sites, loci containing candidate genes whose mutation could cooperate with inactivation of Nf1 to result in BXH-2 AML. Among these candidate genes, Bcl11a, was able to cause leukemia/lymphoma in bone marrow transduction and transplantation assay using Nf1-deficient cells. In this report, we have found that transplantation of these leukemia cells induced secondary leukemia in syngeneic recipient mice. In patients, we previously reported that BCL11A was expressed in juvenile myelomonocytic leukemia and chronic myelomonocytic leukemia samples. Interestingly, high expression levels of BCL11A are also observed in two specific subsets of samples from a large cohort of patients with acute myelogenous leukemia. Moreover, these subsets of patients tended to have a worse outcome. Regarding the biochemical mechanisms underlying the oncogenicity of Bcl11a, we have recently found that p21Cip1 induction by doxorubicin is severely impaired in Bcl11a-expressing cells, while other cell-cycle regulatory proteins are not affected. Examination of BCL11A’s role in leukemia patients using both over-expression and shRNA knocking-down approaches is underway.

Identification of 12/15-lipoxygenase as a suppressor of myeloproliferative disease

Though Abl inhibitors are often successful therapies for the initial stages of chronic myelogenous leukemia (CML), refractory cases highlight the need for novel molecular insights. We demonstrate that mice deficient in the enzyme 12/15-lipoxygenase (12/15-LO) develop a myeloproliferative disorder (MPD) that progresses to transplantable leukemia. Although not associated with dysregulation of Abl, cells isolated from chronic stage 12/15-LO–deficient (Alox15) mice exhibit increased activation of the phosphatidylinositol 3–kinase (PI3-K) pathway, as indicated by enhanced phosphorylation of Akt. Furthermore, the transcription factor interferon consensus sequence binding protein (ICSBP) is hyperphosphorylated and displays decreased nuclear accumulation, translating into increased levels of expression of the oncoprotein Bcl-2. The ICSBP defect, exaggerated levels of Bcl-2, and prolonged leukemic cell survival associated with chronic stage Alox15 MPD are all reversible upon treatment with a PI3-K inhibitor. Remarkably, the evolution of Alox15 MPD to leukemia is associated with additional regulation of ICSBP on an RNA level, highlighting the potential usefulness of the Alox15 model for understanding the transition of CML to crisis. Finally, 12/15-LO expression suppresses the growth of a human CML–derived cell line. These data identify 12/15-LO as an important suppressor of MPD via its role as a critical upstream effector in the regulation of PI3-K–dependent ICSBP phosphorylation.

MIXED LYMPHOCYTE REACTIVITY AGAINST NORMAL CELLS BY SPLENIC LYMPHOCYTES FROM TUMOR-BEARING MICE

The establishment of an intimate connection between autoimmunity and neoplasia would require the demonstration of an experimentally induced, tumor-dependent autoimmune process. For this reason, we have studied cellular immune reactions of mice bearing a transplantable leukemia (L1210). Spleen cells from hybrid BDF1 mice bearing the L1210 tumor (BDFt) reacted vigorously in mixed lymphocyte culture with mitomycin-treated, normal spleen cells from mice of the parental strain from which the L1210 tumor was derived (DBA/2). Spleen cells from nontumor-bearing BDF1 mice reacted only weakly with these parental cells. The BDFt cells likewise did not respond when cultured with mitomycin-treated spleen cells from the other parental strain (C57B1/6). The vigorous mixed lymphocyte reaction (MLR) by BDFt cells against normal parental cells of the same strain as the tumor was not due to a double exposure of the reacting cells to histocompatibility antigens shared by tumor cells and normal parental cells. The response of cells from tumor-bearing F1 mice against normal parental cells seen in these experiments suggests the possibility of the induction of an autoimmune-like process against host lymphocytes by spleen cells from leukemic mice. Theoretically such a phenomenon would considerably reduce an animal's ability to mount an immune attack against malignant cells.