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January 22, 2004

The BCR-ABL1 kinase bypasses selection for the expression of a pre-B cell receptor in pre-B acute lymphoblastic leukemia cells

By Florian Klein

The expression of the oncogenic BCR-ABL1 kinase in pre-B cell acute lymphoblastic leukemia (ALL) results from the so-called Philadelphia translocation, which represents the most frequent recurrent genetic aberration leading to ALL in adults. Comparing genome-wide gene expression profiles of BCR-ABL1⁺ pre-B ALL and normal bone marrow pre-B cells by SAGE, many genes implicated in the transduction of survival signals through the pre-B cell receptor are transcriptionally silenced in the leukemia cells. While normal pre-B cells are selected for the expression of a functional pre-B cell receptor, 9 of 12 cases of BCR-ABL1⁺ ALL cases do not harbor a functional IGH V region gene rearrangement. In these cases, we identified traces of secondary VH gene rearrangements, which may have rendered an initially productive VH region gene nonfunctional. Even BCR-ABL1⁺ ALL cells harboring a functional VH region gene are unresponsive to pre-B cell receptor engagement and exhibit autonomous oscillatory Ca2⁺ signaling activity. Conversely, leukemia subclones surviving inhibition of BCR-ABL1 by STI571 restore responsiveness to antigen receptor engagement and differentiate into immature B cells expressing Ig light chains. In the presence of BCR-ABL1 kinase activity, defective pre-B cell receptor signaling is paralleled by the expression of a truncated isoform of the pre-B cell receptor-associated linker molecule SLP65. Also in primary leukemia cells, truncated SLP65 is expressed before but not after treatment of the patients with STI571. We conclude that ablation of the BCR-ABL1 kinase activity by STI571 reconstitutes selection for leukemia cells expressing a functional (pre-) B cell receptor.

Figure:Inhibition of BCR-ABL1 restores (pre-)B cell receptor responsiveness in pre- B ALL cells


CD19⁺ B lineage cells were purified from bone marrow mononuclear cells using immunomagnetic beads. Release of Ca2⁺ from cytoplasmic stores in these cells in response to pre-B or B cell receptor engagement by anti-VpreB or a mixture of anti-kappa and anti-lambda light chain antibodies (arrows) was measured using a laser scanning microscope (A). Untreated BCR-ABL1⁺ SUP-B15 (B), BV173 and NALM1 (not shown) pre-B ALL cells and BCR-ABL1⁺ pre-B ALL cells incubated with the BCR-ABL1 kinase inhibitor STI571 for 24 hours (C) were stimulated using an anti-m chain antibody, which can crosslink both pre-B and B cell receptors. Among STI571-treated leukemia cells, Ig Mu chain⁺ cells were separated from Ig Mu chain^- cells and analyzed independently (C). In (B), 5-fold higher concentrations of the anti-Ig Mu chain antibody than in (A) and (C) were used. The data shown here refer to SUP-B15 cells. Analyzing BV173 and NALM1 cells, similar results were obtained (not shown).