Background: In B-cell acute lymphoblastic leukemia (B-ALL), RAS-pathway activating mutations (e.g. NRAS, KRAS, BRAF) are found in ~40% of cases (Irving et al., 2014; Jerchel et al., 2018; Zhang et al., 2011). Evidence suggests that RAS-pathway mutations in B-ALL subclones are selected for during therapy and are associated with relapse, which has a poor prognosis and remains one of the leading causes of death in children with malignancy (Irving et al., 2014; Jerchel et al., 2018). Beyond its role as a driver of B-ALL, ERK-activation can induce expression of PRDM1, which functions as a tumor suppressor and triggers negative B-cell selection and cell death (Yasuda et al., 2011; Hug et al., 2014; Setz et al., 2018). During B-cell development, PRDM1 and the proto-oncogene BCL6 are reciprocal antagonists (Shaffer et al., 2002). Here, we examined the mechanism by which oncogenic RAS-ERK signaling in B-ALL avoids negative selection and cell death to promote leukemogenesis.

Results: Mimicking survival signals from pre-B cell receptor (pre-BCR), constitutive Erk activation upon oncogenic RAS (NRASG12D) expression in mouse B-cell precursors massively induced the mRNA (~390-fold) and protein (~50-fold) levels of the proto-oncogene Bcl6. Highlighting an essential role of Erk-signaling in upregulation of Bcl6 expression downstream of oncogenic RAS, pharmacological Erk-inhibition markedly reduced NRASG12D-mediated Bcl6 induction. Both the pre-BCR linker molecule Blnk and the surrogate light chains (SLCs) of the pre-BCR are required for Erk-mediated induction of Bcl6. Genetic ablation of Blnk or the SLC component λ5 abrogated RAS-mediated Bcl6-induction. Hence, oncogenic expression of Bcl6 in RAS-driven B-ALL cells depended on structural elements of the pre-BCR.

Additionally, ERK-activation in patient-derived B-ALL cells had the same effect on BCL6 expression. Pharmacological reactivation of ERK (BCI-215) markedly induced BCL6 expression, whereas small molecule inhibition of ERK-activity (trametinib) reduced BCL6 levels in patient-derived B-ALL cells. Consistent with BCL6-transactivation downstream of ERK-activation, ChIP-seq analysis revealed binding of ERK-dependent transcription factors (CREB1, ELK1, EGR1, JUND and C-JUN) to the BCL6 promoter in human B cells. Altogether, our findings illuminated a critical role of ERK-signaling in positive regulation of BCL6 expression.

Using a genetic model for Cre-mediated deletion of Bcl6 in NRASG12D B-ALL cells, we found that Bcl6 was required for leukemia-initiation in transplant recipient mice (P=0.007). Patient-derived RAS-driven B-ALL cells were highly sensitive to pharmacological inhibition of BCL6 using peptide (RI-BPI) and small molecule (FX1) inhibitors. Furthermore, pharmacological inhibition of BCL6 delayed onset of fatal disease and prolonged survival of transplant recipient mice bearing patient-derived RAS-driven B-ALL cells (P=0.009). Taken together, our findings demonstrated that suppressing BCL6 function is a promising option for targeted therapeutics in RAS-driven B-ALL.

Mechanistically, genetic ablation or pharmacological inhibition of BCL6 increased PRDM1 expression. Gene expression analysis revealed that ectopic expression of Prdm1 in B-cell precursors promoted gene expression programs of apoptosis and cell cycle arrest (GSE111692). While Bcl6-deletion resulted in cell death of B-ALL cells, this was largely reversed by shRNA-mediated silencing of Prdm1. Altogether, our results showed that compromised leukemogenesis was a result of aberrant PRDM1 expression in BCL6-deficient RAS-driven B-ALL cells.

Conclusions: We previously investigated targeted engagement of negative selection in B-ALL and showed that hyperactivation of SYK or AKT functionally mimics autoreactive pre-BCR signaling and results in negative selection and cell death (Chen et al., 2015; Shojaee et al., 2016). Here, we reinforced this concept by providing a mechanistic basis on how negative selection is downregulated in RAS-driven B-ALL and how it can be reactivated for therapeutic benefit - namely, BCL6 curbs ERK-mediated induction of PRDM1 expression to promote leukemogenesis and this mechanism can be exploited as synthetic lethality in the treatment of this disease.

No relevant conflicts of interest to declare.

Author notes

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Asterisk with author names denotes non-ASH members.

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