Thrombotic thrombocytopenic purpura (TTP) is very rare (6 persons per million develop it each year), but is caused by mechanisms that are also involved in heart attacks and stroke which affect many people. TTP it is Â very dangerous: patients develop attacks of microangiopathy, which means that small vessels in the kidneys, heart and brain collect blood platelets in little plugs. These tissues do not receive enough blood and slowly become damaged. The platelets in the plugs are linked to each other by long strings of von Willebrand Factor (VWF), kind of like spiderwebs. Normally, VWF is kept at a normal, healthy length by an enzyme in blood plasma, called ADAMTS13. This prevents the spontaneous clumping together of platelets. In TTP patients, VWF becomes too long because ADAMTS13 is not functioning properly. TTP patients that develop episodes of microangiopathy are in serious danger; it has been estimated that the risk of mortality is as high as 80-90% if they are not treated. And even if the patients are treated, each attack still leaves marks of damage in the form of scar tissue in affected organs.
The mortality is strongly reduced when you give persons a LOT of plasma (the liquid part of blood). It is thought that in this way, we are giving patients back the ADAMTS13 that they need. I think this is partially true. The plasma therapy has a problem: most people do make ADAMTS13, but it does not work because the patients have made antibodies against it. These antibodies neutralize their own ADAMTS13 before it can do its job. Also when we try to give patients plasma in order to restore the ADAMTS13 activity, the antibodies are causing trouble by neutralizing the therapy.
When I first heard of TTP, I was amazed by it. So dangerous and unpredictable (sorry, I am a biologist). I was explained that the attacks occur because the levels of neutralizing antibodies vary from time to time. So, sometimes there is ADAMTS13 in these patients (low antibodies; no attacks), and sometimes there is not (high antibodies; attacks may occur). But I also heard that in patients that never have ADAMTS13, either because they have high antibodies all the time or they do not make it because of a genetic defect (Upshaw-Schulman syndrome) the clinical picture is similar: attacks occur in episodes with times of remission in between. To me, this appeared unexpected: you would expect continuous microangiopathy if you never have ADAMTS13, right?
Hypothesis and Results
We started working out a possible explanation for the erratic character of this dangerous disease. What if there is another enzyme that can do the job when ADAMTS13 is not fully operational? Kind of like a reserve goalkeeper that stands up to the challenge when times are hard?
Which enzyme could it be? Under what conditions would it start cleaving VWF? You can see what we came up with in the image below. When blood vessels become obstructed by little platelet plugs, the endothelial cells that line the vessels ‘sense’ the obstruction. This may be because of a lack of oxygen, but we’ll have to investigate that in more detail later. Next, these endothelial cells become alert and a little aggressive: they place a protein on their outer surface that turns on the enzyme plasmin. Plasmin is well known as the enzyme that degrades fibrin an important polymer in blood clots and is used to treat patients with heart attacks (blood clots in the heart), venous thrombosis (blood clots e.g. in the legs) and strokes (blood clots in the brain). It is already known from good work by other research groups that plasmin can also degrade VWF under purified conditions1,2. However, it did not seem as if this extra power of plasmin was related to the disease TTP at all. So, we took a
But what can we do for the patients, now that we have this information? Well, first, we want to get rid of all obstructions that clog up the vasculature in TTP patients. Next, we want to restore the platelet counts in these patients. I forgot to mention that, because all the platelets get stuck, there are only a few left to protect TTP patients from bleeding. We showed that, by activating plasmin with therapeutic agents, we can do both these things at the same time! We cut the platelet-VWF obstructions to pieces and therefore, platelets no longer get stuck at places where they are not needed. Now, the platelets circulate freely again and are able to do their job.look and found that plasmin can cleave dangerous platelet-VWF complexes to bits3. We found out that plasmin needs to stick to VWF in a specific manner to do this job. In patients, we found that plasmin is activated exactly as expected: when the vessels become closed up by platelet plugs. This may help to explain the disease presentation of TTP; a secondary enzyme system kicks in when ADAMTS13 is not functioning properly in an effort to remove obstructions. This means that TTP patients may experience more attacks than they really know; they only become clinically noticeable when this backup system fails to get the job fully done.
For the professionals – on the use of thrombolytics in TTP:
I can imagine that this sounds strange and intuitively dangerous: we are suggesting to (in the future) give thrombolytic drugs, which have bleeding as a well-known side effect to patients that have very little platelets. But please consider it once more with these accompanying comments 1) the main clinical problem in TTP are the closed up microvessels, the bleeding is secondary to that 2) the low platelet counts are a result of consumption through incorporation, rather than depressed production or immune-mediated clearance 3) plasmin activity restores platelet counts (see3, figure 6A) , which is expect willÂ reduceÂ the risk of bleeding. So yes, I think you can give thrombolytic agents to reduce the risk of bleeding, but only in aÂ consumptive thrombocytopenia. And I hope to be able to show it. Better even, I hope that other groups jump in to validate our findings! Finally, I don’t think that the dosing of thrombolytic therapy that needs to be given in TTP is equal to the amount that is given to treat its ‘regular’ indications. Fibrin has clot protection mechanisms (TAFI, FXIII), VWF hasn’t.
This project is not funded yet- it has been a ‘voluntary’ side project. I will really need funding in order to structurally work out this concept and ‘translate’ its outcomes into the clinical setting. Somehow, it is more attractive to funding agencies (which have to choose between many interesting projects) to support the identification of something ‘new’/’cutting edge’ than to support the validation and application of a concept that has already been carefully worked out and judged by the peer-review process.Â I love the conceptual leap that we made as it is (it has been a good run so far), and I would hate to see it ‘die out’ as just another promising research project that I will have to replace by another, simply because I am dictated by the laws of conventional science to identify new ‘sources of gold’, rather than to actually dig up the goods.
1 Berkowitz et al.Â J. Clin. Invest. 1987 vol. 79 (2) pp. 524-531
2 Wohner et al.Â Thromb. Res. 2012 Apr.;129(4):e41â€“6.
3 Tersteeg et al.Â CirculationÂ 2014 Jan; 129: XX-XXX