A new angle on the sickling process

A large group of scientists from the USA and China have published exciting new observations about the sickling process in The Journal of Clinical Investigation, which may well have important treatment implications for patients with sickle cell disease.

The fundamental problem in sickle cell disease is the coming together of sickle haemoglobin molecules, within red blood cells, to form tactoids. These long, rigid, crystal-like structures distort the shape of the red cell, result in obstruction to blood flow or vaso-occlusion and cause anaemia, due to a shortened red cell lifespan. The process of sickling, what initiates it and what controls it, is poorly understood. In a previous blog (03/06/14: Update on Clinical Trials in Sickle Cell Disease) I discussed a drug called Aes-103, currently in clinical trial, which interferes with the sickling process by stabilising sickle haemoglobin in a high oxygen affinity state. Whether this drug will be effective in clinical practice remains to be seen; all previous attempts to use drugs to block the sickling process have failed. This current research identifies a new, previously unknown, biochemical pathway, which promotes sickling and sickle cell related tissue damage, and which may be amenable to therapeutic manipulation.

The researchers used a process called metabolomic screening to look at thousands of chemicals within red cells from patients with sickle cell disease, and then compared them with the same chemicals from individuals with normal blood. They focused on a particular chemical called sphingosine-1-phosphate (S1P), which they found was present in high concentrations in all red cells, but which was increased to very high levels in the red cells and blood of patients with sickle cell disease. What all of this S1P inside red cells is doing is not clear but, in other parts of the body, it is known to be an important chemical involved in a variety of biological processes, including inflammation, blood vessel growth, damage to the lining of blood vessels and blood clotting. Inside red cells S1P is produced by an enzyme called sphingosine kinase 1 (SPHK1); the researchers found that the activity of this enzyme is greatly increased in sickle red cells accounting for the very high levels of S1P. They were able to manipulate the levels of S1P using a new drug, which is a potent inhibitor of the enzyme SPHK1, called PF-543.

Using a strain of mice, which have been genetically engineered so that they have sickle cell disease, the scientists found that treatment of the mice with PF-543 significantly reduced levels of S1P and, to their surprise, they found that this was also associated with an improvement in the appearance and shape of the red cells in the blood of the mice. Further work showed that treatment with PF-543 resulted in reduced red cell sickling, reduced haemolysis with improved anaemia and a reduced white cell count, reflecting an anti-inflammatory effect. As a result of these changes the sickle cell mice were healthier, there was less tissue and organ damage and they survived longer, even if exposed to low oxygen levels, which would normally cause extensive, fatal sickling. The scientists confirmed these unexpected findings using a different technique, in which they de-activated, or knocked-out, the gene making the enzyme SPHK1, in chimeric mice with sickle cell disease, using a lentivirus. These knock-out mice were also healthier with less sickle cell related tissue and organ damage.

But is this relevant to patients with sickle cell disease? There is no reason to think that human sickle cell disease is any different to the condition in the mice. The researchers found that S1P and SPHK1 are also present at very high concentrations in patients with sickle cell disease and, in the test tube at least, treatment of human sickle red cells with the enzyme inhibitor, PF-543, reduced the amount of sickling which occurred if the red cells were exposed to low levels of oxygen, results similar to those found in sickle cell mice.

On the basis of these results the authors think that S1P is a key, previously unknown, chemical, which promotes sickling by acting within the red cell on sickle haemoglobin. The high levels of S1P are due to activation of the enzyme, SPHK1, probably by low levels of oxygen, which are commonly found in patients with sickle cell disease, explaining the very high levels found in the disease. Blocking the activity of this enzyme has a dramatic effect, it reduces the level of S1P and alleviates many of the consequences of red cell sickling. These observations open up the possibility of potential treatments for sickle cell disease in the future, by manipulating this new biochemical pathway. It is important now to understand how exactly S1P interacts with sickle haemoglobin to cause increased sickling and whether the enzyme inhibitor PF-543 can safely and effectively be given to patients with sickle cell.

Elevated sphingosine-1-phosphate promotes sickling and sickle cell disease progression. Journal of Clinical Investigation 2014; volume 124, pages 2750-2761.

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About rogerjamos

I am a consultant haematologist who has worked in Hackney, London, UK with patients who have sickle cell disease for many years. Knowledge is power; the hope is that this blog will empower patients by putting them in touch with contemporary research into sickle cell disease and facilitating informed discussion on the issues raised. Dr Roger Amos MA, MD, FRCPath
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2 Responses to A new angle on the sickling process

  1. Lisa Rose says:

    Hello Dr. Amos,
    Do they have any theories at all as to the role of S1P in conjunction with the sickle hemoglobin? Perhaps S1P increases the attraction between the sickle hemoglobin, promoting tactoids? Or, perhaps the increase in S1P increases the bond within the tactoids, making it more difficult to break them apart when lower levels of oxygen are present? It will be interesting to see where research takes this new finding! Also, were there any negative effects to either decreasing or knocking out the S1P in the mice? I know that the status quo is to assume any measurements outside of the norm indicate something wrong; however, my devil’s advocate mind always poses the opposite question- what if the patient with SCD is ALSO receiving some type of benefit from this increase? Are there any long term concerns with reducing or eliminating the enzyme connected to S1P?
    Lisa Rose

    • rogerjamos says:

      Hi Lisa Rose
      This is more difficult to respond to. I do not think anyone knows how S1P interacts with sickle haemoglobin to promote sickling. From the work reported in the paper though, the interaction is within the red cells and does not involve the receptor for S1P. Most of the other actions of S1P are mediated by the chemical travelling through the blood and latching onto it’s receptor, which is widely distributed in many different cell types. We will have to wait for further research before it is clear how S1P affects sickle haemoglobin.

      No side effects were reported in the paper among the mice who were treated with the enzyme inhibitor to reduce the production of S1P. However, S1P appears to have many important functions in the body including, regulating blood vessel growth and permeability, modulating the inflammatory response, promoting the growth of skin cells and, perhaps most importantly, within the immune system, governing the movement of T and B lymphocytes out of the lymph nodes and into the lymphatic system. It would be surprising therefore if blocking the synthesis of S1P did not have more widespread effects than just those on red cells. The trick as always will be to limit the effect of any intervention to the red cells alone.

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