ASH 2024 | Non-genetic adaptation to BTK inhibitors in CLL

I had the privilege to discuss what we think is a non-genetic adaptation to BTK inhibitors in chronic lymphocytic leukemia. In this project, what we ask is whether CLL cells apply some rewiring of cellular signaling in response to BTK inhibition to sustain their survival. We primarily focused on the PI3K-AKT signaling pathway because it has been shown that in mature normal B-cells, this is the pathway that can sustain the survival of cells that lack B-cell receptors after its genetic ablation. And indeed also in chronic lymphocytic leukemia when we sampled a patient sample before and during ibrutinib treatment about 70 percent of patients have a paradoxical activation of AKT, ao they have AKT levels higher than before therapy in terms of phospho-AKT and then we engaged to understand what’s the molecular mechanism behind that. We performed RNA sequencing in samples obtained before and about one month on ibrutinib therapy and this revealed quite a lot of changes in the PI3K-AKT signaling pathway but the one that we found most interesting is the induction of an mRNA for protein called Rictor and Rictor is a protein that is required for the assembly of mTORC2 complex and this complex directly phosphorylates AKT on serine 473 and that’s the same phosphorylation that we saw being induced during ibrutinib treatment. 

To next understand what is the mechanism behind Rictor induction, we went back to the RNA sequencing data and we found a transcription factor called FOXO1 that subsequently turns out from cut-and-run experiments similar to ChIP-seq experiment that it binds to promote the region of Rictor and it directly induces its transcription. So what we think is happening during ibrutinib treatment in CLL is that FOXO1 gets induced, it transcriptionally activates Rictor, that helps the assembly of mTORC2 complex and this activates the phosphorylation of AKT. Clinically this is also associated with a longer lasting lymphocytosis in these patients, so the more the axis is activated the more the cells can probably survive in the peripheral blood of these patients. We were able to show that the cells on ibrutinib are highly dependent on AKT and also on FOXO1. There is actually a very good FOXO inhibitor available that is being developed for treatment of diabetes because FOXO1 also regulates glucose metabolism and this inhibitor is able to very efficiently block Rictor expression and phosphorylation of AKT in CLL and also in a PDX model with a CLL-derived cell line MEK1, it’s able to limit the growth of the tumor. 

So what we think exists in a large proportion of CLL patients during the early weeks and months of ibrutinib therapy is this adaptation that leads to activation of AKT and that sustains survival of CLL cells. We don’t think that it’s a true resistance because it mainly supports the survival, but It’s not sufficient to drive the cells into proliferation, yet I think it’s an interesting mechanism that can later allow for development of true resistance via mutations such as mutations in BTK or PLC gamma.

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