The PAX5‐JAK2 translocation acts as dual‐hit mutation that promotes aggressive B‐cell leukemia via nuclear STAT5 activation

While PAX5 is an important tumor suppressor gene in B‐cell acute lymphoblastic leukemia (B‐ALL), it is also involved in oncogenic translocations coding for diverse PAX5 fusion proteins. PAX5‐JAK2 encodes a protein consisting of the PAX5 DNA‐binding region fused to the constitutively active JAK2 kinase domain. Here, we studied the oncogenic function of the PAX5‐JAK2 fusion protein in a mouse model expressing it from the endogenous Pax5 locus, resulting in inactivation of one of the two Pax5 alleles. Pax5 ^Jak2/+ mice rapidly developed an aggressive B‐ALL in the absence of another cooperating exogenous gene mutation. The DNA‐binding function and kinase activity of Pax5‐Jak2 as well as IL‐7 signaling contributed to leukemia development. Interestingly, all Pax5 ^Jak2/+ tumors lost the remaining wild‐type Pax5 allele, allowing efficient DNA‐binding of Pax5‐Jak2. While we could not find evidence for a nuclear role of Pax5‐Jak2 as an epigenetic regulator, high levels of active phosphorylated STAT5 and increased expression of STAT5 target genes were seen in Pax5 ^Jak2/+ B‐ALL tumors, implying that nuclear Pax5‐Jak2 phosphorylates STAT5. Together, these data reveal Pax5‐Jak2 as an important nuclear driver of leukemogenesis by maintaining phosphorylated STAT5 levels in the nucleus.