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

Richter's syndrome (RS) is an aggressive transformation of Chronic Lymphocytic Leukaemia (CLL) frequently due to TP53, CDKN2, MYC or NOTCH1 mutations. whereas a significant proportion displays no specifically acquired driver mutation. We observe constitutive AKT phosphorylation not only in high-risk CLL patients harbouring p53 and NOTCH mutations but also in numerous RS patients. Consistently, genetic over-activation of AKT within the Eµ-TCL1 CLL mouse model results in a high-grade lymphoma phenotype of Richters syndrome. Multiomics assessment of our novel mouse model revealed a S100 defined subcluster of highly proliferative lymphoma cells developing from indolent CLL-like B-cells as a consequence of sudden NOTCH activation being fueled by enhanced NOTCH ligand exposure from T-cells in the microenvironment. Our data link AKT and NOTCH signaling in patient samples, genomic alterations, phosphoproteome and single-cell transcriptome profiles. Collectively, we have identified active AKT as a causative transforming pathway of indolent CLL towards aggressive RS thus providing novel mechanistic insights into the molecular understanding of RS.