We are really on a role now, using the Pfizer/BioNTech and Moderna vaccine approaches that use messenger RNA (mRNA) as springboards for a wider discussion on what RNA technologies, notably CRISPR genome editing, are poised to do for healthcare. In the recent, second installment of this series, I introduced you to what doctors call beta hemoglobinopathies. This means medical conditions caused by problems with the hemoglobin beta chain, one of the two subunit types that comprise each molecule of the most common type of adult hemoglobin. We discussed how the most advanced CRISPR gene editing therapy approach —advanced in the sense of being far along the pathway to routine clinical therapy— is a treatment that works be interfering the gene that encodes a protein that turns off the gene that makes what doctors call the hemoglobin gamma chain. Using a specially designed strip of RNA that acts like a kind of GPS system within the nucleus of a cell that is to be treated, a protein enzyme called CAS9 turns on the synthesis of gamma chain by cutting the cell’s DNA in a very specific location. Two gamma chains plus two alpha chains make up a molecule of fetal hemoglobin, the same hemoglobin that’s packed into red blood cells of your fetus when you are pregnant. Therapies are focussed on two beta hemoglobin diseases in particular: beta thalassemia and sickle cell disease. In beta thalassemia, turning on the production of fetal hemoglobin with CRISPR editing supplies gamma chain to substitute for beta chain, which is missing in these patients. In sickle cell disease, turning on the production of gamma chain produces healthy fetal hemoglobin, which offsets the problems caused by the faulty beta chain that fills the red blood cells of people with sickle cell disease. In addition to being one of the earliest CRISPR therapies to reach clinical trials, where it is showing itself to be effective against beta thalassemia and also safe, this emerging treatment also sets the stage for introducing you to the two major categories of CRISPR gene therapy: ex vivo and in vivo.
The CRISPR therapy to turn on fetal hemoglobin is an example of ex vivo therapy, meaning that the genetic modification takes in cells that have been extracted from the body of the patient and will be returned to the body after they are changed. In this case, blood stem cells are drawn and the CRISPR treatment —consisting of the CAS9 cutting protein and the RNA strip to guide to the correct place— is delivered in a laboratory setting. Different methods are available to get the CRISPR system through the cell membranes, into the stem cells. One that seems to be working well involves the use of electrical current to create gaps in the membrane that close up when the current is turned off. The system then makes its way into the cell nucleus (although mature red blood cells do not have nuclei, the stem cells that give rise to red blood cells do). Once altered through CRISPR to produce the gamma chain, and thus fetal hemoglobin, the stem cells are infused back into the patient. Thus far, clinical data have demonstrated increasing levels of fetal hemoglobin in beta thalassemia patients receiving the CRISPR treatment, resulting in clinical improvement, including a need for fewer blood transfusions over time. The treatment is also advancing for those with sickle cell disease, albeit on a later timeline compared with beta thalassemia. Given that blood cells are the problem, hemoglobin diseases lend themselves well to the ex vivo type of CRISPR gene therapy, but for other disease types the treatment needs to be in vivo, meaning that the CRISPR system is delivered into the patient him- or herself and must find its way into the cells that require treatment.
As you may imagine, in vivo treatment is more challenging, since you are putting something into a patient whose body consists of trillions of cells, whereas the treatment is needed only in very particular cells. But the same work that has advanced the mRNA vaccines that you now know in relation to COVID-19 is advancing CRISPR gene therapy, as the CRISPR system can be encapsulated within the same kind of lipid nanoparticles that are used to encapsulate the mRNA vaccines so they can travel within the body, delivering the genetic payload where it needs to go. Now, there are different types of in vivo. For certain diseases, diseases of the retina that cause blindness for example, in vivo means injection directly to the very place that needs the therapy. Such retina CRISPR treatment is advancing and we’ll discuss it in a future post, but in vivo also means systemic, that the CRISPR system is injected into an easy access point of the body, such as a vein, and must travel to somewhere else, which is really the situation when the lipid nanoparticle carriers come in handy. A CRISPR treatment in this category that already is showing promise in clinical trials is for a medical condition called TTR amyloidosis. In this rare condition, TRR, which is a protein is misfolded and in its misfolded form, called amyloid, it accumulates is tissues such as the heart, kidneys, and nervous tissue. If the data show improvement, which they seem to be doing so far, such a success is likely to accelerate in vivo systemic CRISPR therapy for a wide range of conditions.
