-Active AKT signaling triggers CLL towards Richter's transformation via over-activation of Notch1.

Richter's transformation (RT) is an aggressive lymphoma which occurs upon progression from chronic lymphocytic leukemia (CLL). Transformation has been associated with genetic aberrations in the CLL-phase involving TP53, CDKN2A, MYC, and NOTCH1, however a significant proportion of RT cases lack CLL-phase associated events. Here, we report that high levels of AKT phosphorylation occurs both in high-risk CLL patients harboring TP53 and NOTCH1 mutations as well as in RT patients. Genetic over-activation of Akt in the murine Eµ-TCL1 CLL mouse model resulted in CLL to RT with significantly reduced survival and an aggressive lymphoma phenotype. In the absence of recurrent mutations, we identified a profile of genomic aberrations intermediate between CLL and DLBCL. Multi-omics assessment by phosphoproteomic/proteomic and single-cell transcriptomic profiles of this Akt-induced murine RT revealed a S100-protein-defined subcluster of highly aggressive lymphoma cells, which developed from CLL cells, through activation of Notch via Notch ligand expressed by T cells. Constitutively active Notch1 similarly induced RT of murine CLL. We identify Akt activation as an initiator of CLL transformation towards aggressive lymphoma by inducing Notch signaling between RT cells and microenvironmental T cells.