Pre-B Cell Receptor Signaling Prevents Leukemic Transformation by BCR-ABL1.

Pre-B cells within the bone marrow are destined to die unless they are rescued through survival signals from the pre-B cell receptor. Studying the configuration of the immunoglobulin heavy chain locus (IGHV) in sorted human bone marrow pre-B cells by single-cell PCR, we detected a functional IGHV allele consistent with the expression of a functional pre-B cell receptor in the vast majority of normal human pre-B cells. However, only in 10 of 44 cases of BCR-ABL1-transformed pre-B cell-derived acute lymphoblastic leukemia (ALL), we detected a functional IGHV allele. For this reason, we studied the function of the pre-B cell receptor during early B cell development and progressive transformation in a BCR-ABL1-transgenic mouse model: Interestingly, BCR-ABL1-transgenic mice that have not yet undergone leukemic transformation show almost normal pre-B cell receptor selection. In these “pre-leukemic” pre-B cells, however, expression of the BCR-ABL1-transgene is extremely low as compared to full-blown ALL, suggesting that high levels of BCR-ABL1 expression are not compatible with normal expression of the pre-B cell receptor. Consistent with our observations in human ALL, full-blown ALL clones in BCR-ABL1-transgenic mice indeed show defective pre-B cell receptor selection and the pre-B cell receptors expressed on few leukemic cells are not functional. Treatment of leukemic mice with the BCR-ABL1 kinase inhibitor AMN107, however, reinstated normal pre-B cell receptor selection and pre-B cell receptor function within seven days. These data suggest that the transforming signal through BCR-ABL1 and normal survival signals through the pre-B cell receptor are mutually exclusive. In support of this hypothesis, we found that the full-blown leukemia only comprises one to four independent clones of “crippled” pre-B cells - even though all B cell precursors in these mice carry the BCR-ABL1-transgene. To test whether functional pre-B cell receptor signaling vetoes transformation by BCR-ABL1, we transformed murine pre-B cells carrying a deletion of the SLP65 gene, which is required for functional pre-B cell receptor signaling. Unlike SLP65-wildtype pre-B cells, SLP65^−/− pre-B cells can be transformed by BCR-ABL1 at a high efficiency. Reconstitution of SLP65 using a retroviral vector, however, induced rapid cell death of BCR-ABL1-transformed pre-B cells. Next, we identified human BCR-ABL1-negative ALL cases with a functional or defective pre-B cell receptor signaling cascade. Transduction of pre-B cell receptor-deficient ALL cells resulted in rapid outgrowth while ALL cells with a functional pre-B cell receptor were not permissive to transduction with BCR-ABL1. We conclude that the pre-B cell receptor represents a potent tumor suppressor and a safeguard against BCR-ABL1-mediated transformation. Only “crippled” pre-B cells with a non-functional pre-B cell receptor are susceptible to BCR-ABL1-mediated transformation.