Haemoglobin F (Hb F) is only a minor component in the blood of most adults, usually less than 1% of the total haemoglobin, but may, one day, be the basis of an effective cure for sickle cell disease. Understanding how the level of Hb F is controlled is therefore of great interest. A remarkable series of discoveries over the last ten years or so have allowed scientists to dissect out the human genome and identify the DNA sequences, or genes which, acting together, control Hb F production.
We have already discussed the genetic factors controlling the gamma-beta switch. This is the process by which the Hb F genes (gamma globin genes) are silenced after birth and the genes, which make Hb A (beta globin genes), are activated. As a result of the switch, Hb F virtually disappears from our blood within a few months of birth. The switch is operated by a series of transcription factors, or specific protein molecules, including BCLIIA, KLF1, SOX6 and FOP, which interact with the Locus Control Region (LCR), the main promoter and repressor of haemoglobin gene activity. Once the switch has been activated Hb A remains the dominant haemoglobin in our blood permanently.
But, in addition to this switch, all genes, including the Hb F genes, are also regulated in a less specific way, by what are known as epigenetic phenomena. These are changes to the way that genes function, which are also inherited, but which do not involve any alteration in the DNA itself. The two most common epigenetic mechanisms are changes to the number of methyl groups attached to DNA and modifications to the histone proteins, which are wrapped around the DNA spiral or helix. Generally speaking the fewer methyl groups there are attached to DNA the more active a gene will be. Hypomethylation of the Hb F genes therefore results in partial reactivation of the genes and increased Hb F production. This is thought to be the mechanism by which hydroxycarbamide increases the level of Hb F in a patient’s blood.
In adults the total amount of Hb F and the number of F-cells varies considerably between individuals; what determines this variation in a population? From studies of identical and non-identical twins we know that approximately 90% of this variation is genetically determined; this is the third and final level of control affecting Hb F production and is distinct from both the gamma-beta switch and changes in DNA methylation.
One of the genes involved in this we have already met, this is the BCLIIA gene on chromosome 2. The product of this gene suppresses Hb F production and, as we have seen, is implicated in the gamma-beta switch. Individuals with a specific mutation in the BCLIIA gene will only partially suppress Hb F production and will therefore have persistently high levels of Hb F into adult life. These mutations are quite common and this mechanism accounts for about 15% of the variation in Hb F levels among different individuals.
The two other main determinants of Hb F levels are two genes, one called Xmn1-HBG2 and the other HIMP. The Xmn1-HBG2 gene is found on chromosome 11, in the same group of genes as the beta and gamma globin genes. A specific mutation in Xmn1-HBG2 (a change from cytosine to thymidine at position -158) is found in approximately 30% of the population and promotes higher levels of Hb F synthesis. This specific mutation in the Xmn1-HBG2 gene accounts for between 13% and 32% of the variation in Hb F levels in different populations.
The final gene to consider, called HMIP, is found on chromosome 6 (6q 23.3). Mutations in the HIMP gene also promote higher Hb F levels and account for approximately 19% of the variation in Hb F levels. Other genes have also been related to Hb F levels, but they only seem to have a minor influence on Hb F synthesis. These genes are found on the X chromosome (Xp 22.2) and chromosome 8 (8q).
Painstaking work by biochemists and geneticists has uncovered the secrets of how Hb F production is controlled and regulated, and revealed this to be an incredibly complex and interconnected system. The challenge now is to devise ways in which this system can be safely modulated by drugs, to allow high-level synthesis of Hb F to persist, in the majority of red cells, into adult life. Such a treatment would potentially transform the lives of patients with sickle cell disease and beta thalassaemia.