BCL6 enables Ph+ acute lymphoblastic leukaemia cells to survive BCR-ABL1 kinase inhibition.

Tyrosine kinase inhibitors (TKIs) are widely used to treat patients with leukaemia driven by BCR-ABL1 (ref. 1) and other oncogenic tyrosine kinases. Recent efforts have focused on developing more potent TKIs that also inhibit mutant tyrosine kinases. However, even effective TKIs typically fail to eradicate leukaemia-initiating cells (LICs), which often cause recurrence of leukaemia after initially successful treatment. Here we report the discovery of a novel mechanism of drug resistance, which is based on protective feedback signalling of leukaemia cells in response to treatment with TKI. We identify BCL6 as a central component of this drug-resistance pathway and demonstrate that targeted inhibition of BCL6 leads to eradication of drug-resistant and leukaemia-initiating subclones

Pre–B cell receptor–mediated cell cycle arrest in Philadelphia chromosome–positive acute lymphoblastic leukemia requires IKAROS function

B cell lineage acute lymphoblastic leukemia (ALL) arises in virtually all cases from B cell precursors that are arrested at pre–B cell receptor–dependent stages. The Philadelphia chromosome–positive (Ph+) subtype of ALL accounts for 25–30% of cases of adult ALL, has the most unfavorable clinical outcome among all ALL subtypes and is defined by the oncogenic BCR-ABL1 kinase and deletions of the IKAROS gene in >80% of cases. Here, we demonstrate that the pre–B cell receptor functions as a tumor suppressor upstream of IKAROS through induction of cell cycle arrest in Ph+ ALL cells. Pre–B cell receptor–mediated cell cycle arrest in Ph+ ALL cells critically depends on IKAROS function, and is reversed by coexpression of the dominant-negative IKAROS splice variant IK6. IKAROS also promotes tumor suppression through cooperation with downstream molecules of the pre–B cell receptor signaling pathway, even if expression of the pre–B cell receptor itself is compromised. In this case, IKAROS redirects oncogenic BCR-ABL1 tyrosine kinase signaling from SRC kinase-activation to SLP65, which functions as a critical tumor suppressor downstream of the pre–B cell receptor. These findings provide a rationale for the surprisingly high frequency of IKAROS deletions in Ph+ ALL and identify IKAROS-mediated cell cycle exit as the endpoint of an emerging pathway of pre–B cell receptor–mediated tumor suppression

BCL6 is critical for the development of a diverse primary B cell repertoire

BCL6 protects germinal center (GC) B cells against DNA damage–induced apoptosis during somatic hypermutation and class-switch recombination. Although expression of BCL6 was not found in early IL-7–dependent B cell precursors, we report that IL-7Rα–Stat5 signaling negatively regulates BCL6. Upon productive VH-DJH gene rearrangement and expression of a μ heavy chain, however, activation of pre–B cell receptor signaling strongly induces BCL6 expression, whereas IL-7Rα–Stat5 signaling is attenuated. At the transition from IL-7–dependent to –independent stages of B cell development, BCL6 is activated, reaches expression levels resembling those in GC B cells, and protects pre–B cells from DNA damage–induced apoptosis during immunoglobulin (Ig) light chain gene recombination. In the absence of BCL6, DNA breaks during Ig light chain gene rearrangement lead to excessive up-regulation of Arf and p53. As a consequence, the pool of new bone marrow immature B cells is markedly reduced in size and clonal diversity. We conclude that negative regulation of Arf by BCL6 is required for pre–B cell self-renewal and the formation of a diverse polyclonal B cell repertoire

Activation-induced cytidine deaminase acts as a mutator in BCR-ABL1-transformed acute lymphoblastic leukemia cells.

The Philadelphia chromosome (Ph) encoding the oncogenic BCR-ABL1 kinase defines a subset of acute lymphoblastic leukemia (ALL) with a particularly unfavorable prognosis. ALL cells are derived from B cell precursors in most cases and typically carry rearranged immunoglobulin heavy chain (IGH) variable (V) region genes devoid of somatic mutations. Somatic hypermutation is restricted to mature germinal center B cells and depends on activation-induced cytidine deaminase (AID). Studying AID expression in 108 cases of ALL, we detected AID mRNA in 24 of 28 Ph(+) ALLs as compared with 6 of 80 Ph(-) ALLs. Forced expression of BCR-ABL1 in Ph(-) ALL cells and inhibition of the BCR-ABL1 kinase showed that aberrant expression of AID depends on BCR-ABL1 kinase activity. Consistent with aberrant AID expression in Ph(+) ALL, IGH V region genes and BCL6 were mutated in many Ph(+) but unmutated in most Ph(-) cases. In addition, AID introduced DNA single-strand breaks within the tumor suppressor gene CDKN2B in Ph(+) ALL cells, which was sensitive to BCR-ABL1 kinase inhibition and silencing of AID expression by RNA interference. These findings identify AID as a BCR-ABL1-induced mutator in Ph(+) ALL cells, which may be relevant with respect to the particularly unfavorable prognosis of this leukemia subset

Activation-induced cytidine deaminase acts as a mutator in BCR-ABL1-transformed acute lymphoblastic leukemia cells

The Philadelphia chromosome (Ph) encoding the oncogenic BCR-ABL1 kinase defines a subset of acute lymphoblastic leukemia (ALL) with a particularly unfavorable prognosis. ALL cells are derived from B cell precursors in most cases and typically carry rearranged immunoglobulin heavy chain (IGH) variable (V) region genes devoid of somatic mutations. Somatic hypermutation is restricted to mature germinal center B cells and depends on activation-induced cytidine deaminase (AID). Studying AID expression in 108 cases of ALL, we detected AID mRNA in 24 of 28 Ph(+) ALLs as compared with 6 of 80 Ph(-) ALLs. Forced expression of BCR-ABL1 in Ph(-) ALL cells and inhibition of the BCR-ABL1 kinase showed that aberrant expression of AID depends on BCR-ABL1 kinase activity. Consistent with aberrant AID expression in Ph(+) ALL, IGH V region genes and BCL6 were mutated in many Ph(+) but unmutated in most Ph(-) cases. In addition, AID introduced DNA single-strand breaks within the tumor suppressor gene CDKN2B in Ph(+) ALL cells, which was sensitive to BCR-ABL1 kinase inhibition and silencing of AID expression by RNA interference. These findings identify AID as a BCR-ABL1-induced mutator in Ph(+) ALL cells, which may be relevant with respect to the particularly unfavorable prognosis of this leukemia subset