Spontaneous Mutations of Bcor and Jak1/2 genes Lead to an Aggressive Leukemia of B-1 Progenitor B Cells

NUP98 gene fusions, generated by non-random chromosomal translocations, are associated with a wide spectrum of high risk hematologic malignancies and have been shown to alter normal hematopoietic stem and progenitor cell (HSPC) gene expression programs. A recurrent t(11;17)(p15;p13) translocation in patients with AML leads to the production of a NUP98–PHF23 (NP23) fusion gene. The consequent NP23 fusion protein retains the PHD domain, known to bind H3K4me3, and is thought to have aberrant chromatin regulation properties. We have generated a transgenic mouse model of the NUP98-PHF23 gene fusion which develops a range of hematologic malignancies, most commonly pre-T LBL and AML. However, approximately 10% of NP23 mice develop an aggressive B-1 progenitor acute lymphoblastic leukemia (pro B-1 ALL).

B-1 and B-2 lymphocytes have distinct developmental pathways and are thought to represent arms of the innate and adaptive immune systems, respectively. Mature B-2 lymphocytes predominate in the peripheral circulation, and are characterized by expression of B220; whereas B-1 lymphocytes are more prevalent in the pleural and peritoneal cavities, and do not express B220. Murine B cell malignancies typically stain positive for B220, and represent transformed B-2 cells. In the present study, NP23 progenitor ALLs displayed an immunophenotype (Lin-B220- CD19+ AA4.1+) that was identical to that of the recently described B-1 progenitor cell. All B-1 progenitor ALLs exhibited clonal rearrangements of the IgH gene locus. Specifically, these rearrangements involve favored usage of 3’ V_H regions, similar to observations with fetal B-1 progenitor cells, further supporting the notion that these are leukemias of B-1 progenitors. Using whole exome sequencing, we found acquired mutations in the BCL6 interacting corepressor (Bcor) gene in 5 out of 7 B-1 progenitor leukemias. The mutations were all frame shift or nonsense mutations, and were located within a 9 bp “hot spot” in Bcor exon 8. In addition, 4 of 7 cases had somatic mutations of Janus kinase 1 (Jak1) or 2 (Jak2), and 7/7 cases showed hyperphosphorylation of Stat3 or Stat5, consistent with the contention that the Jak1/2 mutations are activating mutations, and leading to a hypothesis that the NP23 pro B-1 ALLs which do not harbor Jak1/2 mutations may have acquired an unidentified mutation in the Jak-Stat pathway. Of note, Jak1/2 mutations have previously been identified in a subset of high-risk pediatric B-cell precursor ALL patients. The striking correlation between Bcor and Jak1/2 mutations, occurring specifically in a subset of NP23 leukemias, implies that these three mutations (NP23, Bcor, and Jak1/2) collaborate and provide the oncogenic setting for B-1 progenitor transformation.