We generally understand the term “dehydration” to mean that our body is short of water. It is a potentially dangerous condition, particularly in sickle cell, and normally the amount of water in the body is very carefully controlled and regulated. The body’s natural fluid balance is determined by how much fluid you drink and how much fluid you lose, mainly through passing urine and sweating. If you are becoming dehydrated you will feel thirsty, prompting you to drink more, and your kidneys will reduce the amount of urine they produce, helping to conserve the body’s water stocks.
Just as the whole body can become dehydrated it is possible for different compartments in the body to become dehydrated at different times. One of those “compartments” are the red cells circulating in the blood, and what happens to them is of especial interest in sickle cell. The reason for this is because the rate at which sickle haemoglobin “sickles”, causing the red cell to change shape, is exquisitely dependent on the concentration of sickle haemoglobin inside the red cell. If the red cells become dehydrated, the concentration of sickle haemoglobin increases and as a result the rate of sickling increases dramatically. Sickled red cells and “dense cells”, which are those red cells permanently damaged by repeated sickling, are the cause of all the problems in sickle cell disease. Prevent sickling and the symptoms of sickle cell should be greatly reduced.
Making sure that red cells stay well hydrated should therefore be an effective way of reducing the amount of sickling which occurs. It turns out that the amount of water inside red cells is just as carefully controlled and regulated as the amount of water in the body as a whole. The red cells absorb the water they need from the bloodstream, but there are also three pathways or routes by which water and salts can be lost from the red cells. If it was possible to block or inhibit these pathways, then the hope is that water would be retained inside the red cells, keeping them maximally hydrated at all times, and minimising the symptoms of sickle cell. To this end many drugs which affect these pathways have been tried out in patients with sickle cell disease.
The three pathways by which water is lost from red cells are called: the P-sickle Pathway, the Gardos Channel and the K-Cl Co-transport Pathway.
The P-sickle pathway seems to be activated by the process of sickling itself and there was some hope that this pathway may be blocked by a drug called dipyridamole, which is already used in patients who have had a stroke or heart attack. Unfortunately, further investigation did not show any benefit from taking this drug.
The Gardos Channel is activated by calcium resulting in the loss of potassium and water from the red cell. In the test tube the pathway can be blocked by a group of drugs, which are also used to treat fungal infections. They include, clotrimazole and senicapoc. Despite initial high hopes neither of these have proven to be beneficial when tried out on patients.
The K-Cl Co-transporter pathway results in the loss of potassium (K) and chloride (Cl) from red cells together with water. It is the main pathway determining the hydration status of the red cell and so would be the most important one to try and block. It is inhibited by magnesium, which led doctors to test whether magnesium pidolate, given to patients might limit symptoms, but again the trials proved very disappointing.
Over the years many other drugs, whose mechanism of action is less well understood, have been tried to see whether they had any beneficial effects on red cell hydration and sickle cell symptoms. They include; piracetam, zinc sulphate, sodium chromoglycate and diltiazem. None of these have stood the test of time after testing in patients.
We are left then with a situation where we know that blocking water loss and maintaining red cell hydration is a potential way we can interfere with the sickling process, but none of the drugs yet tried have proved to be effective. Rather than wasting time and money continuing to test large numbers of different drugs, it would perhaps be more effective to try and understand how these pathways work in more detail. Then, in time, it might be possible to be more selective about the drugs used or indeed attempt to develop new drugs designed specifically to interfere with these pathways. This is precisely what a team of Australian investigators, led by Dr Fiona Brown from Monash University in Melbourne have done. They used a strain of mice, which had been genetically engineered to have sickle cell disease, to dissect the workings of the K-Cl co-transporter pathway.
They found that the K-Cl co-transporter pathway is controlled by four genes, called KCC1 to KCC4. In sickle cell disease mice the activity of the co-transporter is set at a higher level than normal actually potentiating the risk of red cell dehydration. The investigators identified a mutation in one of the genes, KCC1, which resulted in increased activity of the co-transporter. When this mutation was bred into the strain of mice with sickle cell disease the presence of the mutation resulted in a much more severe type of sickle cell, many of the affected mice having serious complications and dying at a very young age.
Dr Brown and her team clearly established that the K-Cl co-transporter was a very important determinant of sickle cell severity. Increased activity of the pathway resulted in a significant worsening of the disease, so presumably reducing the activity of the pathway should result in a similar improvement in symptoms. What is more, the identification of the four genes controlling the co-transporter opens up a whole new area for targeted drug therapy. Variation in the activity of the co-transporter, as a result of additional genetic influences, may help to explain some of the great variability in the severity of sickle cell disease seen between different patients.
Activation of the erythroid K-Cl cotransporter Kcc1 enhances sickle cell disease pathology in a humanized mouse model. Fiona C. Brown, Ashlee J. Conway, Loretta Cerruti, Janelle E. Collinge, Catriona McLean, James S. Wiley, Ben T. Kile, Stephen M. Jane and David J. Curtis. Blood, December 2015, volume 126, pages 2863-70