The Functional Interplay Between the t(9;22)-Associated Fusion Proteins BCR/ABL and ABL/BCR in Philadelphia Chromosome Positive Acute Lymphatic Leukemia

The successful targeting of BCR/ABL by selective ABL-kinase inhibitors (AKI) such as Imatinib, Nilotinib, or Dasatinib alone is unable to eradicate the leukemic clone in Philadelphia chromosome positive (Ph+ ) leukemia. The t(9;22)(q34;q11) is a balanced translocation. Der22 involves the BCR (breakpoint cluster region) gene locus with two principal breaks: the M-bcr, encoding for the p210^BCR/ABL and the m-bcr, encoding for the 185^BCR/ABL fusion proteins, respectively. The constitutively activated BCR/ABL kinase is responsible for the leukemic transformation through an aberrant activation of multiple signaling pathways, such as Stat, Pi3K and Ras/Erk. The der9 encodes for the reciprocal ABL/BCR fusion proteins the p40^ABL/BCR, present in 65% of patients with chronic myeloid leukemia (CML) and the p96^ABL/BCR, detectable in 100% of patients with Ph+ acute lymphatic leukemia (ALL). ABL/BCRs are oncogenes able to influence the lineage commitment of hematopoietic progenitors. Aim of this study was to further disclose the role of p96^ABL/BCR for the pathogenesis of Ph+ ALL. We co-expressed p96^ABL/BCRand p185^{BCR/ ABL} from a p2A peptide-linked multi-cistronic retroviral vector, which allows the expression of multiple proteins from a single open reading frame (ORF) to identical levels. The co- expression of p96^ABL/BCR enhanced the kinase activity and, as a consequence, the transformation potential of p185^BCR/ABL in factor dependent progenitor cells and untransformed fibroblasts. Targeting p96^ABL/BCR by RNAi inhibited growth of Ph+ ALL cell lines and primary Ph+ ALL patient-derived long-term cultures (PD-LTCs). Furthermore p96^ABL/BCR negatively influenced the response to AKI in these models as shown by an increased response to AKI when p96^ABL/BCR was down-regulated. Our in vitro and in vivo stem cell studies on murine fetal liver cells and adult HSCs revealed a functional hierarchy between p96^ABL/BCR and p185^BCR/ABL. In fact, p96^ABL/BCR strongly increased stem cell capacity in replating efficiency and colony forming unit-spleen day 12 (CFU-S12) assays, whereas p185^BCR/ABL showed no effect. In contrast co-expression of p96^ABL/BCR and p185^BCR/ABL increased significantly both serial replating potential and CFU-S12 colony formation as compared to p96^ABL/BCR alone. In a syngeneic mouse model co-expression of p96^ABL/BCR abolished the capacity of p185^BCR/ABL to induce a CML-like disease and led to the induction of ALL. Taken together our here presented data reveal an important role of p96^ABL/BCR for the pathogenesis of Ph + ALL.