Sox4 Is Essential for Initiation and Progression of Acute Lymphoblastic Leukemia Induced by P190-BCR-ABL Transformation

Abstract 564

Sox4 is an Sry-related HMG box transcription factor that plays critical roles in development. Sox4-null mice die at embryonic day 14 due to cardiac malformation. Recent studies have suggested that SOX4 may contribute to tumorigenesis, based on the fact that SOX4 is upregulated in many human tumor types, including brain, bladder, prostate, colon, and lung cancers. In addition, increased expression of Sox4 induced by retroviral insertional mutagenesis in mice has been shown to be associated with leukemia and lymphoma. We also found that SOX4 is highly expressed in human acute lymphoblastic leukemia (ALL) cells. We therefore set out in this study to determine the function of Sox4 and its underlying mechanisms in ALL. To overcome embryonic lethality caused by conventional Sox4 inactivation, we used Sox4 conditional null mice (Sox4^fl/fl) to establish Sox4-null ALL cell lines. First, we obtained pure B220+ pro-B cells by co-culturing Sox4^fl/fl fetal liver cells and OP-9 bone marrow stromal cells in the presence of IL-7. Subsequently, pro-B cells were transformed with p190-BCR-ABL and Sox4 inactivation was induced with Cre recombinase. We found that Sox4^−/− pro-B transformed cells exhibited a significantly slower proliferation rate than Sox4^+/− pro-B cells. Notably, cell cycle analysis indicated a lower percentage of S phase cells in Sox4^−/− pro-B transformed cells and these cells yielded much fewer colonies than transformed Sox4^+/− cells in colony formation assay. We further labeled the transformed cells with a luciferase reporter and injected them intravenously into sublethally irradiated NOD/SCID mice. Bioluminescence imaging showed that mice injected with Sox4^−/− cells developed leukemia significantly more slowly and survive longer than mice injected with Sox4^+/− cells. In conclusion, our data show that Sox4 significantly contributes to initiation and progression of leukemia from p190-BCR-ABL-transformed ALL cells in the mouse and thereby strongly support the notion that SOX4 overexpression critically contributes to acute lymphoblastic leukemia development in humans.