ETP-ALL is a subtype of T-cell ALL that is characterised by very immature markers so it’s probably resembling the very first T-cell that is produced in the body, a very immature cell. That’s the cell that gets to the thymus to give rise to all the more differentiated T-cells. That T-cell is characterised by specific cell surface markers that make it different or that make it clearly distinct from the more differentiated forms of T-ALL. In particular it has more what we call stem cell markers like CD34 and it may also have some myeloid markers still because that cell has just decided to become a T-cell or almost decided to become a T-cell but may still differentiate to myeloid cells as well. So it’s a little bit in-between the myeloid and the T-cells.

These markers itself could be targets for, for example, immunotherapy but, of course, they also are present on the normal ETP cells that are present in the body so that may get a bit dangerous to use immunotherapy. But these ETP cells also have specific mutations that make them vulnerable to specific kinase inhibitors or other small molecule inhibitors and that may be the way to go to inhibit these ETP-ALL samples.

The IL-7 receptor pathway is very important for normal T-cell development. Interleukin-7 is the ligand that binds to that receptor and that keeps the cells alive, the early T-cells that enter the thymus, it keeps these cells alive and makes them proliferate. These ETP-ALL cells hijack that pathway actually, they become completely independent from the need of interleukin-7 so they activate that pathway by themselves, by mutations, and these mutations can be in the interleukin-7 receptor itself or further downstream in that signalling pathway in JAK1 or in JAK3 or even in STAT5 which is the transcription factor at the bottom of that pathway that will regulate a lot of genes in those cells.

So in ETP-ALL Notch mutations are not very frequent, they’re a bit less frequent than in the other T-ALL samples, or other T-ALL cases, but still Notch mutations do occur and so they remain a valid target there. There have been a number of inhibitors that have been developed in the past years, not directly specifically targeting Notch but targeting the gamma secretase complex and it’s a complex that is needed to activate Notch. So if you can inhibit that complex you can also inhibit the Notch protein and also the mutant Notch. So there’s clearly interest in further developing such inhibitors. Unfortunately they have also some side effects, some severe gastrointestinal toxicity that is associated with treatment with these gamma secretase inhibitors but I’m sure that they will find a way to overcome that toxicity so then those inhibitors may become valid as well.

The BCL-2 inhibitor is the other one you mentioned. BCL-2 is a protein that keeps the cells alive, that prevents apoptosis and cell death and is probably closely linked to the interleukin-7 JAK-STAT pathway because BCL-2 is a target of STAT5. So if the interleukin-7 receptor pathway is activated, BCL-2 expression goes up and that keeps the cells alive because if a cell decides to proliferate it doesn’t want to die so it keeps itself alive. Targeting BCL-2 is possible, there have been a number of inhibitors that have been developed, they are currently being tested in chronic lymphocytic leukaemia and seem to be having very good effects there. These inhibitors are there and could be also potentially used in T-cell ALL or ETP-ALL so that’s definitely a promising target as well and is independent from the other mutations because it’s a general anti-apoptotic protein.

Could you tell me about XP-01?

That’s a slightly different route. This XP-01, or X protein 01, is a protein that is present in the nucleus and will shuttle proteins from the nucleus to the cytosol. In normal cells about 2-300 proteins use XP-01 to be shuttled from the nucleus to the cytosol, for example p53 or retinoblastoma-1 protein are known as important proteins that need to be shuttled out of the nucleus using this XP-01. There are very specific inhibitors for XP-01 that have been developed, initially with the aim to use them as HIV inhibitors because also the HIV viral protein uses XP-01 to be shuttled out of the nucleus. These inhibitors have not been taken further for anti-HIV products but one has looked around and has found by accident, actually, that these inhibitors seem to be very toxic for cancer cells and much less for normal cells. So it’s not understood at the moment how exactly they work; they definitely inhibit XP-01 and inhibit protein shuttling from the nucleus to the cytosol but why they are really toxic to cancer cells is not known. But that is again a very important route also for therapy since these inhibitors could potentially be used in all types of leukaemia, independent of any specific mutations that need to be present. If you want to target the cell with a JAK inhibitor you need to have a JAK mutation or an interleukin-7 mutation while the XP-01 inhibitors seem to be generally toxic to all types of leukaemia. So that’s a promising agent but we do need to understand a bit better how they actually lead to cell death in the leukaemia cells.

So typically to mimic what happens in a patient we use xenograft mouse models where we can grow the leukaemia cells from a patient in an immune deficient mouse. That mimics quite well the situation in vivo in the patient but is, of course, a mouse environment. So I just want to mention that there is some risk that what we see in the mouse and in our experimental approaches is not completely translatable to a human patient. So there’s always some danger there that if we see a very good activity of a drug in such experimental systems that later on when we move to a patient setting that that drug is not giving the hoped results. So there are some limitations of that system that we have to realise that it’s only a model system and it’s not completely reflecting the true situation.