The role of the Janus-faced transcription factor PAX5-JAK2 in acute lymphoblastic leukemia

Abstract

Key Points• PAX5-JAK2 is the first nuclear DNA-binding JAK2 fusion protein with kinase activity.• JAK2 inhibitors block the kinase activity of PAX5-JAK2.PAX5-JAK2 has recently been identified as a novel recurrent fusion gene in B-cell precursor acute lymphoblastic leukemia, but the function of the encoded chimeric proteinhas not yet been characterized in detail. Herein we show that the PAX5-JAK2 chimera, which consists of the DNA-binding paired domain of PAX5 and the active kinase domain of JAK2, is a nuclear protein that has the ability to bind to wild-type PAX5 target loci. Moreover, our data provide compelling evidence that PAX5-JAK2 functions as a nuclear catalytically active kinase that autophosphorylates and in turn phosphorylates and activates downstream signal transducers and activators of transcription (STATs) in an apparently noncanonical mode. The chimeric protein also enables cytokine-independent growth of Ba/F3 cells and therefore possesses transforming potential. Importantly, the kinase activity of PAX5-JAK2 can be efficiently blocked by JAK2 inhibitors, rendering it a potential target for therapeutic intervention. Together, our data show that PAX5-JAK2 simultaneously deregulates the PAX5 downstream transcriptional program and activates the Janus kinase-STAT signaling cascade and thus, by interfering with these two important pathways, may promote leukemogenesis. (Blood. 2015;125(8):1282-1291 IntroductionThe fusion protein PAX5-JAK2 has been recurrently detected in B-cell precursor acute lymphoblastic leukemia (BCP-ALL).1-4 Both fusion partner proteins play key roles in hematopoiesis, and somatic mutations in their encoding genes have been found in different hematologic neoplasms. 5-7The paired box transcription factor PAX5, a master regulator of B-cell commitment and maintenance, 6 is a frequent target of genetic alterations in BCP-ALL. 5,8 In ;2% to 3% of the cases, structural rearrangements result in the expression of PAX5 in-frame fusion genes. 1,2,4,5,[8][9][10] PAX5 fusion partners comprise a heterogeneous group of genes encoding transcription factors, structural proteins, kinases, and genes with thus far unknown functions. 1,2,8,9,[11][12][13] Regardless of the functional and structural diversity of the fusion partners, a unique feature of PAX5 fusions is the retention of the PAX5 DNA-binding domain, conferring nuclear localization and the ability to occupy PAX5 target sites. 14 Generally, it is hypothesized that PAX5 fusions act as aberrant transcription factors antagonizing wild-type PAX5 function in a dominant negative mode. 1,8,9,11,[15][16][17][18] However, in a recent study, we have shown that a subset of the PAX5 fusion proteins may have a cellular context-dependent activation potential, indicating that some PAX5 fusions may also activate target genes, thus arguing against their simplified trans-dominant negative function. 14 Janus kinase 2 (JAK2) belongs to a family of nonreceptor tyrosine kinases and is involved in signal transduction from many cytokine and growth hormone receptors ...

Redefining ALL classification: toward detecting high-risk ALL and implementing precision medicine

“…Kinase fusions and sequence mutations confer cytokine-independent proliferation in mouse pre-B-cell lines that is abrogated by tyrosine kinase inhibitors (TKIs), ABL1 inhibitors such as imatinib and dasatinib for ABL-class fusions, and JAK2 inhibitors for JAK-STAT-activating alterations. 14,22,53 Human Ph-like leukemic cells show similar pathway activation and sensitivity to TKI in vitro and in xenograft models. There are several reports of Ph-like ALL with ABL-class rearrangements poorly responsive to chemotherapy with profound TKI responses, 22,54 which have led to prospective precision medicine trials of TKI therapy in Ph-like ALL.…”

Transposon-mediated generation of BCR-ABL1-expressing transgenic cell lines for unbiased sensitivity testing of tyrosine kinase inhibitors

“…The most commonly used approach to TKI sensitivity testing is based on exploitation of the murine Ba/F3 cell line carrying mutant BCR-ABL1 constructs introduced by LV-mediated transduction [20]. Here we present an alternative technique, the transposon-based SB system, which offers advantages with regard to efficacy, speed, and safety [32-34, 40]. Nevertheless, we have demonstrated that both technical approaches can lead to multiple insertions of BCR-ABL1 gene constructs which result in artificially elevated IC 50 values for the TKIs tested.…”

“…In contrast to lentiviruses, there is no preference for coding or non-coding regions [30]. The SB system is a fast, simple and safe procedure for stable gene transfer facilitating efficient transfection even of large constructs [31-34]. However, similar to LV transduction, the SB system is also prone to generating multiple insertions in the genome which may be favored during the selection of engineered cells in culture [30, 35].…”

Dysregulation of BCL-2 family proteins by leukemia fusion genes

“…The combination of JAK2 inhibitors, such as ruxolitinib or AZD1480, with BH3-mimetics enhanced the limited efficacy of JAK inhibitors as single agents (60,61). Waibel et al (62) used an E-TEL-JAK2 (ETV6-JAK2) mouse model of T-ALL to show that up-regulation of BCL-2/BCL-X L and down-regulation of BIM expression promote leukemic cell survival.…”