So, why is haemoglobin F so important when we are thinking about possible treatments for sickle cell disease? Well, let’s start at the beginning with haemoglobin itself and then move on to discuss haemoglobin F.
Haemoglobin is what gives blood its red colour, it is a protein molecule packed tightly inside the red blood cells, which circulate around the body in the bloodstream. The main function of haemoglobin and red blood cells is to pick up oxygen in the lungs, where it is extracted from the air we breathe, and carry it to the tissues of the body, where it is used to make energy and keep the body’s systems ticking over. The usual type of haemoglobin found in blood is called haemoglobin A (adult haemoglobin or Hb A for short). Haemoglobin S (sickle haemoglobin or Hb S) differs very slightly from Hb A in structure, as a result of which it has a tendency to crystallise within red cells which leads to all the problems we are familiar with in sickle cell disease. But, in terms of delivering oxygen to the tissues, Hb S is very similar to Hb A, and in fact, is somewhat better at this task that Hb A itself.
Haemoglobin F (foetal haemoglobin or Hb F) is also very slightly different from Hb A in structure and is the main haemoglobin found in the red blood cells of babies, as they are growing in their mother’s womb, and for a few months after birth. However, over the first 6-12 months of life Hb F gradually disappears from the blood and is replaced by Hb A, so that ultimately, in adults, only 1-2% of the total haemoglobin is Hb F, the rest is Hb A. This changeover from Hb F to Hb A is called the “gamma-beta switch”, because the gamma globin genes, which make Hb F, are switched off and the beta globin genes, which make Hb A, are switched on. Hb F is a perfectly good haemoglobin when it comes to transporting oxygen and is ideally suited for babies because it’s tendency to bind oxygen is stronger than that of Hb A, so in the womb, oxygen is effectively transferred from the mother’s Hb A to the baby’s Hb F.
Some adults have inherited rare alterations in their haemoglobin genes, which mean that the gamma-beta switch never really happens and they continue to have very high levels of Hb F throughout their lives. Critically, these individuals are perfectly well and suffer no ill effects from this – in other words it is completely possible to live a normal, healthy life with 100% Hb F.
An added advantage of Hb F, in patients with sickle cell disease, is that it does not interact with sickle haemoglobin, in fact it positively inhibits sickling. Hb F prevents the Hb S molecules from moving close to one another and thereby stops them from interacting with one another, the first step in the crystallisation process. For an individual with sickle cell disease the higher the level of Hb F, provided it is evenly distributed among all of their red cells, the fewer sickle cell problems they have.
Now, in sickle cell disease the problem lies in the beta globin genes, which normally make Hb A, but in this situation make Hb S instead. But the patient’s gamma globin genes, which make Hb F, are completly normal. Therefore, if we could reverse the gamma-beta switch, by inhibiting the damaged beta globin genes and re-activating the normal gamma globin genes, those individuals would stop making sickle haemoglobin and would replace it with Hb F. They would effectively be cured of their sickle cell disease. This is the “holy grail” of sickle cell research because it would be a treatment, which used the body’s normal genes, to bypass the problems created by the damaged beta globin genes.
Needless to say, the control of the gamma-beta switch is complicated and has taken many years to unravel. Understanding this involves some molecular biochemistry – so here goes. Only a few of our genes are active at any one time, the change from gamma to beta globin gene activity is just one example of this. The body therefore must have systems for switching genes on and off. Genes are normally regulated by other DNA sequences, called promoter regions. For the genes that control haemoglobin production the main promoter is called the LCR (the Locus Control Region). These promoters are in turn regulated by proteins called transcription factors, which bind directly to the DNA of the promoter, change the orientation of the chromosome so that the promoter region is brought into direct physical contact with the gene it regulates and increase the activity of an enzyme, RNA polymerase, which gets the gene working.
A whole series of transcription factors have been shown to be of importance in controlling the gamma globin gene, which makes Hb F, and they have rather strange names, such as BCL11A, KLF1, SOX6, and FOP etc. An important one seems to be BCL11A. Genetically engineered mice, which lack a functional gene for BCL11A, have persistently high levels of Hb F in adult life and normal mice, manipulated by a viral mediated knockdown of the BCL11A protein, also show enhanced Hb F production. “Knockdown” is a technical term meaning that the mouse is prevented from making BCL11A by toxic RNA fragments introduced into mouse cells by a virus. BCL11A therefore seems to be crucial in suppressing the gamma globin genes and inhibiting Hb F production in adult life.
These findings have recently been confirmed in human cells. If you grow human red cell precursors in a test tube and treat them with the same viral mediated knockdown of BCL11A production, then those red cell precursors increase the amount of Hb F they synthesise from 1% to more than 30% of their total haemoglobin.
Of course, this is a long way from having a safe drug to do the same thing in patients with sickle cell disease. Not least, with any drug designed to interact with DNA or change gene activity, it is very important to be sure that the actions of the drug are limited to the gene or transcription factor that you are interested in, and that there are no unexpected or undesirable side-effects on other DNA sequences or genes. But it is a start.