Abstract The biology of B cell Non-Hodgkin lymphomas (NHLs) is largely influenced by (de)regulation of B cell receptor signaling (BCR sig.) and DNA damage response pathway (DDR). We and others have shown that in NHLs, miR-34a is involved in DDR, and miR-150/miR-155 are involved in BCR sig. (Mraz et al., 2009, 2014; Cui et al., 2014). To identify miRNAs involved in DDR and BCR sig. we used NGS technology (HiSeq) in chronic lymphocytic leukemia (CLL) B cells treated with BCR inhibitor or DNA damaging drugs. To investigate the DDR-related miRNAs, primary CLL cells were treated with fludarabine in vitro and paired samples (before/after treatment, n = 20) were analyzed for miRNAs’ profile. This identified 6 differentially expressed miRNAs (FDR<0.1). To investigate these miRNAs in DDR in vivo we profiled the expression of 3 out of the 6 miRNAs (miR-34a, miR-1246, miR-1248) in samples obtained from 50 CLL patients before and during the therapy containing fludarabine (FCR regiment). This confirmed up-regulation of all these miRNAs (fold change>1.5, P<0.05); according to our knowledge this is the first analysis of miRNA profiles during therapy administration in NHLs. Importantly, the miR-34a level was a significant predictor (p<0.05) of patients’ response to FCR therapy (complete response vs. others) and the progression free survival (19.9 vs. 26.4 mo.; HR: 2.29). A similar trend was observed for miR-1246, however, this was not statistically significant (P = 0.11). Additionally, low miR-34a is an independent predictor (in a multivariate analysis) of a shorter overall survival (16.7 mo. vs. not reached; P = 0.0002; HR: 3.30). This suggests that CLL cells with low levels of miR-34a fail to down-modulate genes that are crucial for DDR. Several pro-survival genes targeted by miR-34a were recently identified in various cell types (BIRC3, BCL2, FOXP1, YY1, Survivin). Some of these proteins were down-modulated in CLL B cells that up-regulate miR-34a during DDR or we have transfected with synthetic miR-34a, but not in cells that fail to induce miR-34a. Surprisingly, miR-34a, miR-1246 and miR-1248 share numerous validated and predicted targets with miR-150, a known negative regulator of BCR sig. (Mraz et al., 2014). This suggests possible convergence in the mechanism of action of DNA damaging drugs and BCR inhibitors recently approved for treatment of NHLs (such as ibrutinib). To compare the effects of FCR administration with the administration of ibrutinib, we analyzed miRNA and gene expression (HiSeq) in samples from ibrutinib treated CLL patients (n = 9) before and during the therapy. The convergence of pathways targeted by DDR and BCR inhibition through changes in miRNAs’ expression are currently being interrogated. Supported by: SoMoPro II-no. 4SGA8684; NGS-PTL (306242); EHA Fellowship award; IGA MZ CR NT11218-6/2010; CZ.1.07/2.4.00/17.0042; MUNI/A/0830/2013; MH CZ-DRO (FNBr, 65269705), CZ.1.05/1.1.00/02.0068, CZ.1.07/2.3.00/30.0009. Citation Format: Katerina Cerna, Jan Oppelt, Lenka Radova, Katerina Musilova, Vaclav Seda, Gabriela Pavlasova, Michal Jez, Nikola Tom, Filip Pardy, Jitka Malcikova, Karla Plevova, Boris Tichy, Yvona Brychtova, Michael Doubek, Martin Trbusek, Jiri Mayer, Jaroslav Koca, Raffaele Calogero, Sarka Pospisilova, Marek Mraz. MicroRNA involvement in DNA damage response and BCR signaling in malignant B cells. abstract. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 3084. doi:10.1158/1538-7445.AM2015-3084