Factor XII: Over the past years, I have become intruiged by coagulation Factor XII. It is well known to activate in the presence of non-natural materials outside our bodies. When Factor XII activates, it causes plasma (the cell-free liquid of blood) to become solid in a process called coagulation. This helps to prevent blood loss after injury, but too much coagulation results in thrombosis which blocks blood vessels and damages tissues. Thrombosis can be the end stage of atherosclerosis (a diseased vascular wall), but is also often seen taking place in cancer. Thrombosis is a major cause of death in the world.
Coagulation: The coagulation mechanism is like an avalanche in which multiple enzymes (proteins that cleave each other), stimulate each other to become active. In the avalanche metaphor, Factor XII would play the role of the first snowball that triggers the avalanche. We use this reaction in the diagnostic lab to test whether your blood coagulation is functioning properly. For instance, we can test whether you have sufficient levels of various coagulation factors in your blood or whether anticoagulant drugs like heparin are doing their job. Factor XII has recently been implicated to cause thrombosis, but apparently is not needed in our bodies to prevent bleeding: people and mice that lack Factor XII do not bleed more than usual. This makes Factor XII an ideal therapeutic target: no bleeding would follow if you would silence it with drugs. This is the common problem with existing therapies for thrombosis. However, the vast majority of people in the world do have Factor XII in their blood. This strongly indicate that we need it for something, but apparently not for blood coagulation.
The other side of Factor XII: On several instances, I noticed that Factor XII can execute an alternative role: it can behave as an inflammatory enzyme1-3. In this alternative process, it is responsible for the formation of bradykinin; a small peptide that causes blood vessels to become leaky, attracts white blood cells and causes pain. Surprisingly, Factor XII is not causing any coagulation at that very moment. In families all over the world, mutations in Factor XII, as well in its natural inhibitor C1inh have been described that lead to Factor XII hyperactivity. This causes a disease called (hereditary) angioedema, which is hallmarked by painful and dangerous attacks of inflammatory tissue swelling because vessels become too leaky (google Angioedema for pics). Although Factor XII is in action, there is no thrombosis in this disease, indicating that Factor XII is operating as inflammatory enzyme only. It is not known how these attacks are caused; only that Factor XII and bradykinin are involved.
How to explain how Factor XII has this dual behavior? Well, for Factor XII to trigger coagulation, it needs to activate its target Factor XI. In contrast, for Factor XII to trigger inflammation, it needs to activate its target prekallikrein. These targets are similar, but not identical. So, I thought “What if there is more than one form of activated Factor XII?” – one for the activation of Factor XI (coagulation) and one for the activation of prekallikrein (inflammation). For most plasma enzymes, it is commonly presumed that there is only one activated form that does all the work. The same is thought for Factor XII. Factor XII activation requires cleavage at a single place in the molecule. This molecular on/off switch was identified already some time ago4 and is indicated with a red ring in the figure to the right (left panel). However, by staring at the details of Factor XII for quite some time, I noticed that there are two other switches that can be cleaved (red rings, right panel). This means that several forms of activated Factor XII can exist! I currently think that these sites ‘direct’ Factor XII to coagulation or inflammation. Hence, the speed and sequence of cleavages in Factor XII determines its fate in blood. The animation below shows what we think happens to Factor XII.
This animation was prepared by my friend and colleague Arjan D. Barendrecht, BSc.
This project was funded by a Veni Fellowship (91612159), provided by the Netherlands Organization for Scientific Research (NWO).
1 Maas et al. J Clin Invest. 2008 Sep;118(9):3208-18.
2 Oschatz et al. Immunity 2011 Feb 25;34(2):258-68.
3 de Maat et al. Thromb Haemost 2013 Sep;110(3):458-68.
4 Kremer Hovinga et al. Blood 1994 Aug 15;84(4):1173-81.