An “exciting” new gene-editing strategy means those born with a rare inherited disease of the immune system could be treated by repairing a fault in their cells.
CTLA-4 is a protein produced by T cells that helps to control the activity of the immune system. Most people carry two working copies of the gene responsible for producing CTLA-4, but those who have only one functional copy produce too little of the protein to sufficiently regulate the immune system.
For patients with the condition, CTLA-4 insufficiency causes regulatory T cells to function abnormally, leading to severe autoimmunity. The authors explained that the condition also affects effector T cells and thereby “hampers their immune system’s ‘memory,’ ” meaning patients can “struggle to fight off recurring infections by the same viruses and bacteria.” In some cases, it can also lead to lymphomas.
Gene editing to ‘cut’ out faulty genes and ‘paste’ in ‘corrected’ ones
The research, published in Science Translational Medicine, and led by scientists from University College London, demonstrated in human cells and in mice that the cell fault can be repaired.
The scientists used “cut-and-paste” gene-editing techniques. First, they used the CRISPR/Cas9 system to target the faulty gene in human T cells taken from patients with CTLA-4 insufficiency, and then snip the faulty CTLA-4 gene in two. Then, to repair the errors a corrected sequence of DNA – delivered to the cell using a modified virus – was pasted over the faulty part of the gene using a cellular DNA repair mechanism known as homology-directed repair.
The authors explained that this allowed them to “preserve” important sequences within the CTLA-4 gene – known as the intron – that allow it to be switched on and off by the cell only when needed.
The outcome was “restored levels of CTLA-4 in the cells to those seen in healthy T cells,” the authors said.
Claire Booth, PhD, Mahboubian professor of gene therapy and pediatric immunology, UCL Great Ormond Street Institute of Child Health, and co–senior author, said that it was “really exciting” to think about taking this treatment forward to patients. “If we can improve their symptoms and reduce their risk of getting lymphoproliferative disease this will be a major step forward.”
In addition, the researchers were also able to improve symptoms of the disease in mice with CTLA-4 insufficiency by giving them injections of gene-edited T cells.
Technique may help tackle many conditions
The current standard treatment for CTLA-4 insufficiency is a bone marrow transplant to replace the stem cells responsible for producing T cells. However, “transplants are risky” and require high doses of chemotherapy and many weeks in hospital, the authors explained. “Older patients with CTLA-4 insufficiency are typically not well enough to tolerate the transplant procedure.”
Dr. Booth highlighted that the approach has many “positive aspects”. By correcting the patient’s T cells, “we think it can improve many of the symptoms of the disease”, she said, and added that this new approach is much less toxic than a bone marrow transplant. “Collecting the T cells is easier and correcting the T cells is easier. With this approach the amount of time in hospital the patients would need would be far less.”
Emma Morris, PhD, professor of clinical cell and gene therapy and director of UCL’s division of infection and immunity, and co–senior author, said: “Genes that play critical roles in controlling immune responses are not switched on all the time and are very tightly regulated. The technique we have used allows us to leave the natural (endogenous) mechanisms controlling gene expression intact, at the same time as correcting the mistake in the gene itself.”
The researchers explained that, although CTLA-4 insufficiency is rare, the gene editing therapy could be a proof of principle of their approach that could be adapted to tackle other conditions.
“It’s a way of correcting genetic mutations that could potentially be applicable for other diseases,” suggested Dr. Morris. “The bigger picture is it allows us to correct genes that are dysregulated or overactive, but also allows us to understand much more about gene expression and gene regulation.”
The study was funded by the Wellcome Trust, the Association for Moleculary Pathology, the Medical Research Council, Alzheimer’s Research UK, and the UCLH/UCL NIHR Biomedical Research Centre. Dr. Morris is a founder sharehold of Quell Therapeutics and has received honoraria from Orchard Therapeutics, GlaxoSmithKline, and AstraZeneca. Dr. Booth has performed ad hoc consulting in the past 3 years for SOBI and Novartis and educational material production for SOBI and Chiesi. A patent on the intronic gene editing approach has been filed in the UK. The other authors declared that they have no completing interests.
A version of this article first appeared on Medscape UK.