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BCL6 is a known proto-oncogene that is often translocated in diffuse large B-cell lymphoma (DLBCL) (9). In response to TKI treatment, BCR-ABL1 acute lymphoblastic leukaemia (ALL) cells upregulate BCL6 protein [...], Tyrosine kinase inhibitors (TKIs) are widely used to treat patients with leukaemia driven by BCR-ABL1 (ref. 1) and other oncogenic tyrosine kinases (2,3). Recent efforts have focused on developing more potent TKIs that also inhibit mutant tyrosine kinases (4,5). However, even effective TKIs typically fail to eradicate leukaemia-initiating cells (LICs) (6-8), 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.

Author(s): Zhengshan Chen; Seyedmehdi Shojaee; Maike Buchner; Huimin Geng; Jae Woong Lee; Lars Klemm; Bjrn Titz; Thomas G. Graeber; Eugene Park; Ying Xim Tan; Anne Satterthwaite; Elisabeth Paietta; Stephen P. [...]

Author(s): Zhengshan Chen [1]; Seyedmehdi Shojaee [1]; Maike Buchner [1]; Huimin Geng [1]; Jae Woong Lee [1]; Lars Klemm [1]; Björn Titz [2]; Thomas G. Graeber [2]; Eugene Park [1]; [...]

Interferon-induced transmembrane protein 3 (IFITM3) has previously been identified as an endosomal protein that blocks viral infection 1-3 . Here we studied clinical cohorts of patients with B cell leukaemia and lymphoma, and identified IFITM3 as a strong predictor of poor outcome. In normal resting B cells, IFITM3 was minimally expressed and mainly localized in endosomes. However, engagement of the B cell receptor (BCR) induced both expression of IFITM3 and phosphorylation of this protein at Tyr20, which resulted in the accumulation of IFITM3 at the cell surface. In B cell leukaemia, oncogenic kinases phosphorylate IFITM3 at Tyr20, which causes constitutive localization of this protein at the plasma membrane. In a mouse model, Ifitm3 -/- naive B cells developed in normal numbers; however, the formation of germinal centres and the production of antigen-specific antibodies were compromised. Oncogenes that induce the development of leukaemia and lymphoma did not transform Ifitm3 -/- B cells. Conversely, the phosphomimetic IFITM3(Y20E) mutant induced oncogenic PI3K signalling and initiated the transformation of premalignant B cells. Mechanistic experiments revealed that IFITM3 functions as a PIP3 scaffold and central amplifier of PI3K signalling. The amplification of PI3K signals depends on IFITM3 using two lysine residues (Lys83 and Lys104) in its conserved intracellular loop as a scaffold for the accumulation of PIP3. In Ifitm3 -/- B cells, lipid rafts were depleted of PIP3, which resulted in the defective expression of over 60 lipid-raft-associated surface receptors, and impaired BCR signalling and cellular adhesion. We conclude that the phosphorylation of IFITM3 that occurs after B cells encounter antigen induces a dynamic switch from antiviral effector functions in endosomes to a PI3K amplification loop at the cell surface. IFITM3-dependent amplification of PI3K signalling, which in part acts downstream of the BCR, is critical for the rapid expansion of B cells with high affinity to antigen. In addition, multiple oncogenes depend on IFITM3 to assemble PIP3-dependent signalling complexes and amplify PI3K signalling for malignant transformation.

DNA and histone modifications have notable effects on gene expression 1 . Being the most prevalent internal modification in mRNA, the N 6 -methyladenosine (m 6 A) mRNA modification is as an important post-transcriptional mechanism of gene regulation 2-4 and has crucial roles in various normal and pathological processes 5-12 . However, it is unclear how m 6 A is specifically and dynamically deposited in the transcriptome. Here we report that histone H3 trimethylation at Lys36 (H3K36me3), a marker for transcription elongation, guides m 6 A deposition globally. We show that m 6 A modifications are enriched in the vicinity of H3K36me3 peaks, and are reduced globally when cellular H3K36me3 is depleted. Mechanistically, H3K36me3 is recognized and bound directly by METTL14, a crucial component of the m 6 A methyltransferase complex (MTC), which in turn facilitates the binding of the m 6 A MTC to adjacent RNA polymerase II, thereby delivering the m 6 A MTC to actively transcribed nascent RNAs to deposit m 6 A co-transcriptionally. In mouse embryonic stem cells, phenocopying METTL14 knockdown, H3K36me3 depletion also markedly reduces m 6 A abundance transcriptome-wide and in pluripotency transcripts, resulting in increased cell stemness. Collectively, our studies reveal the important roles of H3K36me3 and METTL14 in determining specific and dynamic deposition of m 6 A in mRNA, and uncover another layer of gene expression regulation that involves crosstalk between histone modification and RNA methylation.