Transforming and Tumorogenic Activity of JAK2 by Fusion to BCR: Molecular Mechanisms of Action of a Novel BCR-JAK2 Tyrosin-Kinase.

Abstract 4683

Chromosomal translocations in human tumors frequently produce fusion genes whose chimeric protein products play an essential role in oncogenesis. Recent reports have found a BCR-JAK2 fusion gene in cases of chronic or acute myeloid leukemia, but the protein had not been characterized. We describe a BCR-JAK2 fusion gene by fluorescence in situ hybridization and RT-PCR amplification from bone marrow at diagnosis of a patient with acute lymphoblastic leukemia. After induction therapy, real time PCR showed persistent molecular response correlating with hematological remission maintained up to present. BCR-JAK2 is a 110 KDa chimeric protein containing the BCR oligomerization domain fused to the JAK2 tyrosine-kinase domain. In vitro analysis showed that BCR-JAK2 was constitutively phosphorylated and was located to the cytoplasm. BCR-JAK2 transformed the IL-3-dependent murine hematopoietic cell line Ba/F3 into IL-3 independent growth and induced STAT5b phosphorylation and translocation into the cell nuclei. The treatment with a JAK2 inhibitor abrogated BCR-JAK2 and STAT5b phosphorylation, leading to apoptosis of transformed Ba/F3 cells. To test whether BCR-JAK2 has tumorogenic ability in vivo, we performed experiments with nude mice, in which we injected subcutaneously cells transduced with the control vector and cells expressing BCR-JAK2. Notably, we only obtained tumors in the flank injected with BCR-JAK2 expressing cells, thus confirming the tumorogenic activity of the BCR-JAK2 fusion protein. We conclude that BCR-JAK2 is a new tyrosine-kinase that induces proliferation and cell survival, which can be abrogated by JAK2 inhibitors. In vitro studies demonstrate that BCR-JAK2 displays transforming activity. Moreover, the nude mice model reveals its ability to cause tumors.