There is great excitement in the sickle cell world at the moment about, what may turn out to be, a turning point in the the long road to find a practicable cure for sickle cell. A recent paper from the United States in the Journal of Clinical Investigation describes experiments using the tools of genetic engineering to switch off the BCL11A gene. This is the gene which normally suppresses the production of foetal haemoglobin (Hb F). Successful “knockdown” of the BCL11a gene allowed unrestrained synthesis of Hb F, which reached levels in red cells more than enough to block all of the complications of sickle cell.
It has been known for a long time that Hb F prevents sickle haemoglobin (Hb S) from polymerising or “sickling” and that this, in turn, greatly reduces or prevents all the problems normally associated with sickle cell disease. There are four main lines of evidence supporting the view that Hb F is protective in sickle cell.
New born babies: In the first six months of life, babies with sickle cell are well with very few problems from their sickle cell disease. This “well window” exactly matches the period of time when they have a very high concentration of Hb F in their red cells.
Co-inheritence of sickle cell and HPFH: People who inherit a combination of sickle cell and a condition called hereditary persistence of foetal haemoglobin (HPFH), have levels of Hb F in their red cells in excess of 30%. They are essentially well with very few sickle cell related complications.
Individual level of Hb F: In people with classical sickle cell disease (Hb SS), the amount of Hb F varies from person to person from <1.0% to as much as 15%. Generally speaking, the higher an individual’s Hb F level the fewer problems they have from their sickle cell.
Hydroxycarbamide: Finally, you will all know that if you take the drug hydroxycarbamide regularly it can increase your Hb F levels to values up to about 20%. Successful treatment with hydroxycarbamide has been proven to significantly reduce the frequency of sickle cell related problems, improve anaemia and prolong life.
We are all born with large amounts of Hb F in our red cells but, during the first year of life, the gamma globin gene, which produces Hb F, is silenced and the beta globin gene, which produces adult haemoglobin (Hb A) or, if you have sickle cell disease, Hb S, is activated. For many years the Holy Grail of sickle cell research has therefore been a search for a way to reverse this “switch”, to re-activate the gamma globin gene and allow the production of sufficient amounts of Hb F to block the sickling process.
Following the discovery of hydroxycarbamide, one whole area of research concentrated on trying to identify other drugs able to promote the synthesis of Hb F which were at the same time free of serious side effects. Many drugs have come and gone over the years usually because, at the doses required to produce a useful effect, they were far too toxic. The latest, pomalidomide, was discussed in a blog on the 2nd of April this year. Another approach has been to use the tools of genetic engineering to insert additional copies of the gamma globin gene, which produces Hb F, into the patients’ stem cells.
The biochemistry underlying this “switch” from making one type of haemoglobin to the other is complex and has taken many years of painstaking research to finally work out. Many different genes are involved in the process but it became clear a few years ago that one gene in particular, called BCL11A, was critical in this switch. The protein made by the BCL11A gene is the key element in silencing the Hb F gene and also in promoting the synthesis of beta haemoglobin. The attention of researches since then has focused on trying to stop BCL11A working and whether this would mean that red cells would continue to make Hb F long term.
The first problem was that BCL11A appeared to have other functions, particularly in the brain and immune system, and the fear was that indiscriminate knockdown of BCL11A would have widespread deleterious effects as well as positive affects on Hb F production. The second problem was that the stem cells in which BCL11A had been silenced proved very difficult to grow in mice, the cells tending to die off after a few generations.
These problems seem to have been overcome by a team of researchers at The Dana Faber Institute and the Boston Children’s Cancer and Blood Disorders Centre, in Boston Massachusetts, led by Professor Samuel Orkin. They used sophisticated genetic engineering tools to construct a highly specific “molecular spanner” to inactivate BCL11A, and packaged the “spanner” within a modified Lentivirus so that it is delivered directly to blood stem cells, the precursors of all the red cells in the body. In the test tube they found that blood cells treated with this “molecular spanner” produced large amounts of Hb F, up to 80% of the total. What is more they were able to replicate these results when the cells were transplanted into mice with “sickle cell disease”. The mice also produced large amounts of Hb F and showed clear evidence of improvement in their sickle cell. In their final experiment, again in the test tube, they showed that blood cells from 4 patients with sickle cell disease, were also able to successfully incorporate the “molecular spanner” and achieved similar levels of Hb F.
Although technically very complicated, the approach used here is simple in concept. The red cells own biochemical machinery is genetically manipulated to re-activate the gamma globin gene and allow the production of high levels of Hb F. The team of researchers in Boston are hoping to begin a clinical trial of the new technique using patients with sickle cell disease in early 2017.
Lineage-specific BCL11A knockdown circumvents toxicities and reverses sickle phenotype
Christian Brendel, Swaroopa Guda, Raffaele Renella, Daniel E. Bauer, Matthew C. Canver, Young-Jo Kim, Matthew M. Heeney, Denise Klatt, Jonathan Fogel, Michael D. Milsom, Stuart H. Orkin, Richard I. Gregory and David A. Williams
The Journal of Clinical Investigation 2016. doi:10.1172/JCI87885.
Check out some other blogs:
2nd April 2016: Pomalidomide – more action on the foetal haemoglobin front
23rd May 2015: More on gene therapy
3rd December 2015: The foetal haemoglobin story just got more complicated
26th July 2014: Homing in on BCL11A
3rd November 2014: Gene therapy