PI3K-Like Protein Kinases
Checkpoint machinery protects against cancer
Prof Jiri Bartek - Danish Cancer Society Research Centre, Copenhagen, Denmark
Cells with un-repaired chromosomal breaks are the driving force of malignant transformation. Could you elaborate on this?
That’s a very critical aspect of cancer but I would say it’s not only un-repaired breaks but also mis-repaired breaks. I will explain. So, first, when there is a double strand break that means that the chromosome is basically cut and part of the chromosome may not be attached when cells divide. So basically the consequence is that when the cell divides we may lose or leave behind some parts of the chromosomes due to the unrepaired break. What it means for cancer is that quite often these lost pieces of chromosomes may contain very important genes which we call tumour suppressors. Of course we have two copies of everything so initially we have two copies of each tumour suppressor but with such unrepaired breaks and losses some tumour suppressor cells get lost and such cells get an advantage in terms of growth or less dependence on growth factor. So basically it’s the loss of tumour suppressors due to un-repaired breaks, so that’s one aspect of it.
The other one is you may get repair of the break but if the repair is not correct you may connect ends of chromosomes which normally don’t belong next to each other. What this can cause in some instances is activation of what we call the fusion oncogene. So fusion means that you connect these two bits of chromosomes because of two breaks somewhere in the genome but normally they are not connected. What happens, what are these fusion oncogenes, for example in prostate cancer almost the vast majority have some fusion oncogenes, or lymphomas. What happens, what are these fusion oncogenes, usually is it’s a transcription factor gene fused to a regulatory part of another gene. Maybe one example: in B-cell lymphomas you connect a regulatory part of immunoglobulin genes and immunoglobulins we have antibodies at high levels so this regulatory region has to really drive a lot of transcription. Then a myc oncogene, so myc oncogene you have to regulate very carefully and not to have too much myc oncogene. But if you now put this immunoglobulin regulator with the myc part you have a huge expression of myc as a result of this fusion oncogene and that drives cancer. This happens when there are what we call non-precise repair mechanisms involved. So it’s the un-repaired breaks and also the mis-repaired breaks that can result in cancer.
Can you discuss the issues of chemoresistance?
It is a big problem, of course, from the clinical point of view and patients’ point of view. Again, the roots of it are very related to genome integrity control. So, in principle you can have resistance which is there, so to say, from the very beginning. If a patient is unlucky and the tumour which should be treated, for example during the process of tumour formation, lost some gene which normally drives cells into cell death, into apoptosis after, say, radiation. But if this gene is missing in the whole tumour then these cells may not respond to radiation, they cannot die, so to say, so they survive and cause problems. But that may not be this resistance from the beginning. Of course it is a big problem but a greater problem is so-called acquired resistance and that means many patients experience that initially the tumour responds, the tumour goes away after chemotherapy or radiation but not 100% of the tumour cells. What remains there are cells which either adapt, they are different mechanisms, for example one is that they have a high level of membrane enzymes which basically pump the drugs out of the cell again and then the cancer cell is resistant because the level of the drug is very low inside. But it’s just one example. So this could be different mechanisms of this and there is a selection, it’s like a small Darwinian system, if you like. There are a few cells which survive but then they are maybe under pressure again and only those which can survive, of course they will grow and eventually they will make a large tumour again and unfortunately that tumour will not respond anymore. So this is a huge problem.
Another thing is that usually what survives and grows have stem cell properties. So it’s like a paradox, if you like, you treat patients with chemotherapy or radiation but at the same time this treatment makes more DNA damage so you make more variability in what remains and there is more variety to select from and survive and grow. So it’s a double-edged sword.
Are the surviving cells genetically different?
They are genetically different but the resistance itself may be either genetic, that you select for mutations which make them advantageous in surviving or it could be what we call epigenetic, there are some changes in the chromatin and regulation of genes, for example some genes may be methylated and that means that they are not expressed anymore. That may help if that is a tumour suppressor which you silence in this way, that can help, or apoptotic gene again, they cannot die. So it could be epigenetic or genetic but the variability is even larger when the cells are bombarded with radiation or chemotherapy and then you select for these variants.
Are there cancer predisposition syndromes that are similar to premature ageing?
What is common to both these cancer prone syndromes and the premature aging syndromes is, again, that they have an impaired, so to say, or lower degree of genome integrity control. Maybe the easiest way is to look at it at the aging syndromes because all the genes which we know that they can cause these kinds of syndromes, the products of these genes are involved in DNA repair, in maintaining the DNA integrity. So that already tells you that all the genes which we know can cause these syndromes are in this business, so to say. So what happens in terms of premature aging first? If the cells cannot repair the damage there’s more accumulation of damage with increasing age. That actually reminds me that this year, as you are aware of, the Nobel prize was given to people who work on DNA repair, including Tomas Lindahl actually, who works at IFOM, here, on the advisory board. What he discovered is that DNA is a relatively vulnerable molecule, there are a lot of changes all the time and you have to really maintain it, you have to repair and watch all the time. So in these premature aging syndromes one of these genes doesn’t work, the job of which is to maintain the genome, the mutations accumulate over age and that has at least two consequences. First, the tissue has to regenerate more rapidly because there is more damage, more loss, and so the stem cells, we only have a certain level of stem cells, or number, and if you exhaust it by twenty years that’s it. If you are OK and you exhaust it by 120 then you live much longer. So it’s the depletion of the stem cell pool is one aspect. The other one, for example, neurodegeneration, that’s one aspect of these premature aging syndromes. Neurons are cells which live very long, they are not mainly replaced, a little bit, but they are very strongly dependent on precise transcription, the control and the function of the genes. If you accumulate mutations in DNA so that really also is an obstacle for transcription and function of the genes. So it all comes to deteriorating function and number of stem cells and it goes much faster if you have these defects.
In cancer, why they are also cancer prone, is similar to what I’ve said, that because you accumulate mutations there is more likelihood that you will activate an oncogene or you could lose a tumour suppressor. So that usually goes hand in hand.
How can a checkpoint machine protect a cell?
The name is, by the way, very fitting because it reminds you in real life like some kind of control at a very sensitive point, like if you go to the airport and the security checkpoint. So we have two types of these mechanisms in the cell, one is again at the very important crossings in terms of how the cell proliferates. The cell cycle progression has several phases, one critical one is when the cell decides to replicate, double the genome, so what we call the G1 to S phase transition. There, for example, the absolutely critical checkpoint there is the so-called retinoblastoma tumour suppressor and that is a very frequent target in cancer. It’s frequently disabled in cancer so the cells cannot control really when and how quickly they go into S phase and then they make mistakes and so on. So that’s very important.
The other type of checkpoint, so let’s say that will be the border patrol checkpoint or transition checkpoint, the other type is again the quality control. Again, many examples in real life but in cells it usually, again, relates to genome stability and basically they have mechanisms which control the up-sense of mutations. If there is a genetic lesion, a break or replacement of some nucleotides, there are mechanisms, these checkpoints, which can recognise that and if the problem is bigger, it’s difficult to repair, then the checkpoint would stop the cell cycle and that would give time to cells to repair properly. So what the mission of the checkpoints is to first to delay and give time for repair and usually it succeeds so then the cells will start proliferating again, that’s the best outcome. But sometimes it’s not possible to repair and then there are two options. Again, what the checkpoints can do just to keep us from cancer, let’s say, they could either keep the cells what we call arrested forever and we call it cellular senescence. So they cannot proliferate, they are still there, so they fill the tissue, the space, but they cannot proliferate. The other one is that they can commit suicide because this mechanism, a checkpoint downstream of the checkpoint, it goes through kinase signalling and maybe p53 tumour suppressor will either block the cells or they can activate the cell death and kill the cell. So in any case the mission is to prevent at all costs the cells with mistakes in DNA would be able to propagate.
So this is why checkpoints are important. The good thing about it is that we now know the function and many cancer cells have defects in checkpoints, this helps them to become a cancer, but we can use it in treatment now. Because the cancer cells only have one or two checkpoints preserved whereas normal cells have the whole spectrum so if we use smart drugs we can make the normal cells just sit there and wait but cancer cells we can, for example, eliminate their remaining checkpoints and then they are just blind to any damage, any problems, and they just go and die because it’s too much damage. So there is also an advantage there.
Is there a risk that they could go into senescence rather than dying?
There is a risk. There is a risk, it’s happening and the problem is that the senescent cells are not totally innocent. They produce a lot of molecules, chemical molecules, and influence their neighbours and bring. So it can eventually, of course, cancer in a way again and there’s a lot of research now how to kill the senescence. My laboratory is also interested in that. It’s better to get rid of them. Also for aging maybe, one idea is maybe we can be younger if we can eliminate these senescent cells which accumulate normally with aging. So there is huge interest in this.
