Functional Role of BAALC In Leukemogenesis

Abstract 4194

Cytogenetically normal acute myeloid leukemia (CN-AML) patients with high BAALC or MN1 expression have a poor prognosis. Whereas the oncogenic function of MN1 is well established, the functional role of BAALC in hematopoiesis is not known.

We therefore compared the expression of BAALC and MN1 in 140 CN-AML patients by quantitative PCR. To further assess the impact of BAALC on leukemogenesis we used retroviral gene transfer into primary murine bone marrow cells and cells immortalized with NUP98-HOXD13 (ND13) and HOXA9. Transduced cells were assessed in vitro by colony forming assays and for their sensitivity to treatment with all-trans retinoic acid (ATRA). They were also evaluated by in vivo transplantation into lethally-irradiated mice.

In the 140 CN-AML patients analyzed, the expression of BAALC and MN1 was highly correlated (R=0.71). Retroviral overexpression of MN1 or BAALC in the Hox gene-immortalized bone marrow cells did not cause upregulation of the other gene, suggesting that these genes do not regulate each other. In murine bone marrow cells BAALC did not immortalize the cells in vitro as assessed by serial replating of transduced cells in methylcellulose assays. Transplantation of transduced cells resulted in negligible engraftment of approximately 1 percent at 4 weeks after transplantation. However, co-transduction of BAALC into NUP98-HOXD13 cells (which are very sensitive to the treatment with all-trans retinoic acid) increased the 50 percent inhibitory concentration (IC₅₀) of ATRA by 4.3-fold, suggesting a negative impact of BAALC on myeloid differentiation. We next evaluated whether the differentiation inhibiting effects of BAALC may cooperate with the self renewal-promoting effects of HOXA9 to induce leukemia in mice. Mice receiving transplants of murine bone marrow cells transduced with BAALC and HOXA9 developed myeloid leukemias with a median latency of 139.5 days that were characterized by leukocytosis, massively enlarged spleens (up to 1.02 g), anemia and thrombocytopenia. Infiltrations of myeloid cells were also found in liver, spleen, and kidney. The disease was transplantable into secondary animals. By Southern blot analysis we found one to two BAALC viral integrations per mouse, suggesting that clonal disease had developed from BAALC-transduced cells.

We demonstrate for the first time that BAALC blocks myeloid differentiation and promotes leukemogenesis when combined with the self-renewal promoting oncogene HOXA9. Due to its prognostic and functional effects BAALC may become a valuable therapeutic target in leukemia patients.