NOTCH1 mutated chronic lymphocytic leukemia cells are characterized by a MYC-related overexpression of nucleophosmin-1 and ribosome associated components

In chronic lymphocytic leukemia (CLL), the mechanisms controlling cell growth and proliferation in presence of NOTCH1 mutations remain largely unexplored. By performing a gene expression profile of NOTCH1 mutated (NOTCH1-mut) versus NOTCH1 wild type CLL, we identified a gene signature of NOTCH1-mut CLL characterized by upregulation of genes related to ribosome biogenesis, such as nucleophosmin1 (NPM1) and ribosomal proteins (RNPs). Activation of NOTCH1 signaling by EDTA or by co-culture with JAGGED1-expressing stromal cells increased NPM1 expression, and inhibition of NOTCH1 signaling by either NOTCH1-specific small interfering RNA (siRNA) or γ-secretase inhibitor reduced NPM1 expression. Bioinformatic analyses and in-vitro activation/inhibition of NOTCH1 signaling suggested a role of MYC as mediator of NOTCH1 effects over NPM1 and RNPs expression in NOTCH1-mut CLL. Chromatin immunoprecipitation experiments performed on NOTCH1 intracellular domain (NICD)-transfected CLL-like cells showed the direct binding of NOTCH1 to the MYC promoter, and transfection with MYC-specific siRNA reduced NPM1 expression. In turn, NPM1 determined a proliferation advantage of CLL-like cells, as demonstrated by NPM1-specific siRNA transfection. In conclusion, NOTCH1 mutations in CLL are associated with the overexpression of MYC and MYC-related genes involved in protein biosynthesis including NPM1, which are allegedly responsible for cell growth and/or proliferation advantages of NOTCH1-mut CLL.Leukemia accepted article preview online, 21 March 2017. doi:10.1038/leu.2017.90.