BACH2 promotes Lineage-Specific Fate Decisions in BCR-ABL1-Driven Leukemia

Background and Hypothesis: CML and Ph⁺ ALL are both driven by the oncogenic BCR-ABL1 tyrosine kinase and CML can progress into lymphoid blast crisis (LBC), which is clinically and biologically indistinguishable from Ph⁺ ALL. We hypothesize that the stark differences in phenotype and clinical outcomes between CML and Ph⁺ ALL/LBC are due to lineage-specific transcription factors. Our group recently identified BACH2 as a B cell-specific transcription factor that mediates negative selection at the pre-B cell receptor checkpoint and functions as a tumor suppressor in Ph⁺ ALL and LBC (Swaminathan et al., Nature Med 2013). Here we report the surprising finding that BACH2 mediates lineage-specific fate decisions in BCR-ABL1 driven leukemia: while a potent tumor suppressor in B cell lineage in Ph⁺ ALL and LBC, BACH2 is required for survival and self-renewal of myeloid lineage CML cells.

Results: A gene expression analysis in 99 patients with CML (Radich et al, 2006) revealed that patients with higher blast counts showed significantly higher gene expression values of BACH2 (P=8.88 x 10^-9), suggesting that BACH2 mRNA levels increase with CML progression. To determine the mechanistic role of BACH2 in the progression of CML, we studied genetic deletion of BACH2 in a CML mouse model. To this end, we transformed Lin^-kit⁺Sca-1⁺(LSK) cells from Bach2^+/+ and Bach2^-/- bone marrow with BCR-ABL1. We compared these cells alongside with Bach2^+/+ and Bach2^-/- pre-B cells that were transformed with BCR-ABL1 as a model for Ph⁺ ALL and LBC. Bach2^+/+ and Bach2^-/- CML-like and Ph⁺ ALL/LBC-like leukemia cells were then tested in a series of functional experiments to test their ability to initiate fatal leukemia in transplant recipients. While deletion of BACH2 accelerated B cell lineage leukemia (Ph⁺ ALL/LBC-like) in transplant recipients, we noted the opposite outcome in transplant experiments with myeloid lineage CML cells: All mice receiving Bach2^+/+BCR-ABL1-transformed LSK cells developed CML-like disease within 60 days whereas recipients of Bach2^-/- cells did not develop CML-like disease (Bach2^+/+ vs Bach2^-/-, median survival time= 33 days vs undefined; P=0.001). Additionally, we observed that deletion of Bach2 loss reduced both the colony formation ability (P=0.0059) and the S phase proliferation potential (P=0.0075) of the CML progenitors. These results were in contrast to those observed in Ph⁺Bach2^+/+ and Bach2^-/- ALLs. Reconstitution of Bach2 in Bach2^-/- CML cells rescued its colony forming ability (P= 0.023). These findings were confirmed in primary human CML cells from two patients with CML in chronic phase: Inducible overexpression of Bach2 in patient-derived CML cells conferred a selective proliferative advantage of BACH2 overexpressing cells over time, in comparison to empty vector controls. Bach2 triggered a survival program in human and mouse CML cells, in contrast to B cell lineage Ph⁺ ALL and LBC cells. Interestingly, Bach2 did not provide a selective growth advantage to untransformed myeloid and multi-lineage progenitor cells. This demonstrates that Bach2 drives lineage-specific fate decisions and cooperates with the transforming BCR-ABL1 oncogene. Utilizing an in vitro lineage-switch assay overexpressing CEBPα we characterized the opposing roles of BACH2 in Ph+ ALL and CML. Cell viability and colony formation assays demonstrated that absence of Bach2 results in increased survival and proliferation of Ph⁺ ALL cells. In striking contrast, however, CEBPα-induced myeloid reprogramming induced cell cycle arrest and suppressed colony formation in Bach2-deficient cells (P=0.0001).

Conclusion: While BACH2 is primarily expressed in the B cell lineage, functioning as a tumor suppressor in Ph⁺ ALL, we report here the surprising finding that BACH2 is required for survival and self-renewal of myeloid lineage CML cells. BACH2 mediates lineage-specific fate decisions in BCR-ABL1 driven leukemia and represents one example of how conversion of lineage identity (e.g. from CML to LBC) may impact specific requirement for malignant transformation. For instance, deletions of BACH2 at 6q15 are common in CML at the time of LBC progression but are not found in CML chronic phase or myeloid blast crisis.