The story of the two English infants treated for leukemia circulated quickly on the internet.  But the stories left a few questions unanswered.  What exactly were these revolutionary genetic therapies?

Gene editing is a promising new genetic engineering technique poised to have great impacts in medicine.  The first edges of the technology are now in clinical application, and results published in Science Translational Medicine

Researchers in the UK have used these techniques to make sure that new anti-cancer therapies using genetically engineered donor T-cells, can attack cancer while not being rejected or suppressed by chemotherapy. The genetically modified therapy was delivered to two patients- both infants.
The work was published Science Translational Medicine (Qasim et al., Jan 25 2017).
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TALEN is a method of disrupting the function of a specific gene.  A nuclease (an enzyme that cuts DNA) is encoded by a virus that enters the cell.  The nuclease also comes with instructions on the gene to cut, and in the case of the T-cells two genes were disrupted to make the immunotherapy treatment more effective.

The real excitement of this work is not that genetically engineered cells worked well against leukemia. That’s been demonstrated pretty well already. This work shows that donated cells can be made into universal, off-the-shelf therapies, not requiring a personalized, autologous match. It potentially widens the applicability and immediacy of the therapy and would probably lower the cost significantly.
The story received some play in popular press, but I could not locate information on the exact genetic changes outside of the original paper, which is what I’m describing herein.
The approach was based on cells that were engineered with a relatively new technology called gene editing. In contrast to what we normally think of as genetic engineering, where a gene is added or information provided to turn a gene off, here the function of specific target genes was abolished using a technique called transcription activator-like effector nuclease (TALEN) editing.
The story begins with the genetically-modified T-cells. T-cells (T-lymphocytes) are white blood cells named for the thymus gland, the place where immature T-cells (stem T-cells) differentiate into mature T-cells (as opposed to B-cells, another type of white blood cell--which arise from bone marrow). T-cells patrol the body, looking for non-self foreign cells, identified by their antigens- proteins that cover the outside of the cells.  They generally refrain from attacking “self” cells, which is a problem in cancer. Cancer cells are “self” cells.
But in the case of leukemia, malignant white blood cells express a unique surface antigen called CD19. For several years now, T-cells have been engineered to target CD19+ cells and the therapies have been quite effective. However, they typically involve self-donating the T-cells and having them re-administered once modified to attack the B-cells.
Juno Therapeutics and Novartis have developed auto-donation treatments by using patient’s own blood cells, engineering them, and then re-infusing them. In their studies, about half of patients are permanently in remission after receiving altered versions of their own blood cells. The setback for this treatment is the expense, the time involved in harvesting the blood cells form the patient, and the effort to transport the new cells back to the patient. There is great interest in creating a universal T-cell type programmed to target CD19.
Another problem is that when the cells are administered, the body can react harshly with a process recognized as “cytokine release syndrome”, recognized from a systemic inflammatory response and high fever. It is like being hit with an amazingly strong flu, and can be fatal, especially in some fragile patients.
The Fragile Patients
The infants, 11 and 16 months old, had undergone traditional chemotherapy for leukemia that was not successful. In this case they were treated with traditional chemotherapies as well as the modified T-cells.  Thus far the therapy was successful, both of the infants are now in remission.  It cannot be definitively determined if the success is due to the chemotherapy or the T-cell treatment, although it is most likely the latter, as both infants had relapsed from previous chemo treatments.  
What is Leukemia?

Leukemia is a group of cancers arising in bone marrow and result in high numbers of underdeveloped white blood cells called blastocytes. White blood cells (leukocytes) are the part of our blood that has an immune function. There are four main types of leukemia, but there are other rare forms, these infants suffered from acute lymphoblastic leukemia (ALL). The average five-year survival rate in patients with Leukemia is 57% in the United States. The five-year survival rate for children under fifteen is  ~ 60 to 85%.

Chemotherapy, radiation therapy, targeted therapy, and bone marrow transplant have been the traditional treatment therapies.  Researchers have been working on new approaches using biotechnology.
New Therapies
As mentioned previously, researchers have identified new potential therapies based on targeting a molecular signature on the surface of the malignant B-cell.  But re-engineering the cells from each single patient takes time and is expensive. What if T-cells targeting CD19+ B cells could be available as a universal type?  They could be administered to any patient and might also be altered to decrease the cytokine response, so they’d be safer to use.
To take on this approach, two genetic modifications were made. The first changed the antigen signatures of the T-cell itself. The authors used the TALEN technique to disrupt CD52 in cells obtained from a non-compatible (non-HLA match, meaning different cell-surface signatures) donor. Patients undergoing these T-cell therapies first undergo an immunodepletion treatment to suppress numbers of existing T-cells. This is done using an antibody-based chemotherapy agent that targets the CD52 receptor. This first deletion ensures that the therapeutic cells targeting CD19 malignant B cells would not be eliminated by the treatment.
The second modification using TALEN, targeted the HLA signature of the cells that would stimulate the recipient’s immune system from attacking it.  The TALEN method introduced a deletion in a region of the T-cell receptor α chain, so that the T-cell receptor would be incomplete and evade detection from the patient’s immune system.
The therapy was not without risk. While the HLA antigens were absent from most cells in the therapy, some were still present, so there was a risk of inducing rejection along with its potential complications. Both patients showed mild symptoms, and they were successfully were treated with minor interventions. The TALEN technique works because it introduces chromosome breaks in the target gene. It is possible that off-site breaks could have occurred as well. The other risk was the cytokine response once CD19-attacking cells are introduced to the body.
The breakthrough of this work was simply that a known effective therapy was now being administered from a universal donor cell type.  A number of companies including Cellectis, Cell Medica, Fate Therapeutics, Kite Therapeutics, and Regeneron have been developing universal donor techniques that allow the bioengineered T-cells from donors other than self-donation to be possible.  This cuts down on the expense, time, and logistics, making the product more readily available to the patient.
The work by Qasim et al is important because it takes an exciting new therapy and then uses gene editing techniques to make it applicable from a universal donor. The hope is that these two treated infants live long, healthy lives, and that they will be remembered as pioneers in a revolutionary area of medicine.  

Qasim et la., Molecular remission of infant B-ALL after infusion of universal TALEN gene-edited CAR T cells, Science, Jan 25, 2017. Vol. 9, Issue 374, DOI: 10.1126/scitranslmed.aaj201