EHA 2016
The role of AKT in chronic lymphocytic leukaemia transformation to Richter's syndrome
Dr Christian Pallasch, University Hospital of Cologne, Cologne, Germany
Richter’s syndrome is a very severe complication in indolent lymphomas, particularly in chronic lymphocytic leukaemia patients. You see in a few patients that they progress from this rather low malignant phenotype towards an aggressive lymphoma and this is a very severe clinical problem since these patients rapidly progress due to the aggressive nature of the lymphoma but also due to the disease being very refractory to our current chemotherapeutics and also targeted drugs that are not very effective in these patients. The only viable option that we have right now would be allogenic stem cell transplantation but otherwise these patients have only very limited options and we have a very urgent need for further clinical trials and novel substances and combinations in Richter’s syndrome.
What causes that progression from one state to the next?
For the time being we know from genetic investigations that have been done in Richter’s syndrome patient samples that there are a couple of mutations that occur up front, like the p53 mutation, NOTCH mutation, that has been described by several groups. But there is also a group of patients that do not have a specific genetic operation or a clear driver mutation that explains why they progress from CLL towards Richter’s transformation.
The difference in our approach is that we are not screening for genetic operations, we applied a hypothesis driven approach because we have seen that AKT plays a pivotal role in CLL. We know that it is downstream of the B-cell receptor signalling pathway and downstream of it a couple of very important regulatory mechanisms, particularly in the PI3 kinase AKT pathway, are how a cell proliferates, how a cell regulates its own homeostasis, energy levels, translation and the resistance towards programmed cell death or apoptosis. So we wondered if that specific pathway is involved in progression of CLL and we applied different approaches. The first one was that we investigated samples from Richter’s syndrome patients and looked for the activation of AKT which would be a phosphorylation at a certain amino acid residue that is really crucial for the activation of AKT. We could see in these samples that we have phosphorylation of AKT in Richter’s syndrome samples while we don’t see it to that extent in normal CLL samples.
The second approach which actually really led us to the observation that this might induce Richter’s syndrome was that tumours generated a model in a mouse where you consideratively activate AKT by tagging a specific site to this protein which directs it to the cell membrane and that actually mediates that this protein is constantly activated in cells where we express this construct. We made this as a conditional allele so we could actually activate it only in specific cells in a mouse so we crossed it to certain strains that activated those consideratively active AKT by CD19 CRE only in B-cells. That we crossed to a chronic lymphocytic leukaemia model so we could see that mice that were bearing the chronic lymphocytic leukaemia allele, TCL-1, and the consideratively active AKT, that these mice developed Richter’s syndrome. In these mice what you see is that they have a significantly reduced lifespan, that their appearance of leukaemia drastically changes. So when you look at the morphology of the cells that you observe in the bloodstream they become much bigger, they have a transformed phenotype, they look like transformed blastoid lymphocytes. In addition they have much larger spleens and in these spleens you see that the architecture is totally disrupted and they have a higher frequency of mitosis. You see very large cells, blastoid cells, and this corresponds to the phenotype of a diffuse large B-cell lymphoma and by definition this is a progression from chronic lymphocytic leukaemia towards a diffuse large B-cell lymphoma which is a Richter’s syndrome. So this is now the opportunity to model Richter’s syndrome in a mouse which we did not have for the time being. This also opens up novel perspectives to use this model as a pre-clinical platform for treatment of Richter’s syndrome.
When it comes to that treatment of Richter’s syndrome would you count then AKT to be at least a biomarker or possibly a target for targeted molecules?
Yes, that could potentially be an option for future clinical trials in Richter’s syndrome patients. So we see that a certain subset of Richter’s syndrome patients has phosphorylation of AKT when we stain it with immunohistochemistry and this particular subset of patients could be a particular target for future clinical trials when we actively would target this pathway either with upstream or directly AKT targeting compounds.
