Gene-Editing Technique Shows Promise for CTLA-4 Insufficiency

Researchers in London have successfully repaired a genetic defect in human cells and mice that causes regulatory T cells and effector T cells to function abnormally, leading to severe autoimmunity. Known as CTLA-4 insufficiency, the immune systems of individuals carrying the mutation attack their own tissues and organs, including their blood cells.

The condition also 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, a type of blood cancer.

In human cells, the authors used CRISPR/Cas system to target the faulty gene in T cells taken from patients with CTLA-4 insufficiency and repair the errors. This restored the levels of CTLA-4 in the cells to those seen in healthy T cells. They were also able to improve symptoms of the disease in mice with CTLA-4 insufficiency by giving them injections of gene-edited, i.e., corrected, T cells.

“It’s really exciting to think about taking this treatment forward to patients,” says co-senior author, Professor Claire Booth, Mahboubian Professor of gene therapy and pediatric immunology at University College London (UCL) Great Ormond Street Institute of Child Health. “If we can improve their symptoms and reduce their risk of getting lymphoproliferative disease this will be a major step forward. This particular paper is important because we are using the newest gene editing techniques to precisely correct these T cells, which is a new approach in inborn errors of immunity.”

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.

Currently, the standard treatment for CTLA-4 insufficiency is a bone marrow transplant to replace the stem cells responsible for producing T cells. But transplants are risky and require high doses of chemotherapy and many weeks in hospital. Older patients with CTLA-4 insufficiency are typically not well enough to tolerate the transplant procedure.

“Our approach has many positive aspects,” Booth adds. “By correcting the patient’s T cells, we think it can improve many of the symptoms of the disease, at the same time as being 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.”

The UCL gene-editing approach uses the Nobel-prize winning gene editing technology CRISPR/Cas9 to target and snip the faulty CTLA-4 gene in two. Then a corrected sequence of DNA is delivered to the cell using a modified virus. This is then pasted over the faulty part of the gene using a cellular DNA repair mechanism known as homology-directed repair.

This allowed the researchers to preserve important sequences within the CTLA-4 gene that allow it to be switched on and off by the cell only when needed.

“Genes that play critical roles in controlling immune responses are not switched on all the time and are very tightly regulated,” says co-senior author, Professor Emma Morris, a professor of clinical cell and gene therapy and Director of UCL’s Division of Infection and Immunity. “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.”

Though rare, the UCL team believes the principle could work for other inherited diseases. “It’s a way of correcting genetic mutations that could potentially be applicable for other diseases,” Morris adds. “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 findings were published recently in the journal Science Translational Medicine.