A pioneering gene-editing technology can repair key cell defects in the immune system

A new study by scientists at University College London in human cells and mice has found that a defect in cells in a key part of the immune system can be repaired
with a groundbreaking gene-editing technique.

The researchers, published in Science Translational Medicine, may lead to new treatments for white blood cells for a rare disease called regulatory T cells, which normally help control the immune system, and effector T cells
that protect the body from repeated infections and cancer.

Patients with CTLA-4 insufficiency carry a genetic mutation that causes these T cells to function abnormally
.
This causes them to suffer severe autoimmunity, where their immune system attacks their own tissues and organs, including their blood cells
.

This condition also hinders the "memory" of their immune system, which means that patients may have difficulty fighting off recurrent infections
with the same viruses and bacteria.
In some cases, it can also cause lymphoma, which is a type of blood cancer
.

In human cells, using "cut" and "paste" gene-editing techniques, using the CRISPR/Cas system, the researchers were able to locate the faulty genes in T cells extracted from CTLA-4-deficient patients and fix the errors
.
This restores CTLA-4 levels in the cells to those found
in healthy T cells.
They were also able to improve symptoms of
the disease by injecting CTLA-4-deficient mice with gene-editing (corrected) T cells.

Co-senior author Claire Booth, Mahbubian Professor of Gene Therapy and Child Immunology at University College London's Great Ormond Street Institute of Child Health, said: "The thought of applying this therapy to patients is really exciting
.
If we can improve their symptoms and reduce their risk of lymphoproliferative disease, this will be a big step
forward.
This particular paper is important because we are using the latest gene-editing techniques to precisely correct these T cells, a new way to
address innate immune errors.
”

CTLA-4 is a protein produced by T cells that helps control the activity
of the immune system.
Most people carry two active copies of the gene responsible for producing CTLA-4, but those who have only one functional copy produce too little of this protein to adequately regulate the immune system
.

Currently, the standard treatment for CTLA-4 deficiency is a bone marrow transplant to replace the stem cells responsible for producing T cells
.
But transplantation is risky, requiring high-dose chemotherapy and several weeks
of hospitalization.
Older patients with CTLA-4 insufficiency usually do not tolerate transplantation
.

Professor Booth said: "There are many positive aspects of
our approach.
By correcting a patient's T cells, we believe it can improve many of the symptoms of the disease while being much
less toxic than a bone marrow transplant.
It's easier to collect T cells, and it's easier
to correct them.
With this approach, the patient's time in the hospital will be greatly reduced
.
”

The gene-editing method, developed by researchers at University College London, uses the Nobel Prize-winning gene-editing technology CRISPR/Cas9 to locate and cut the defective CTLA-4 gene
.
The modified DNA sequence is then delivered to the cells with
the modified virus.
It is then pasted into the wrong part of
the gene using a cellular DNA repair mechanism called homology-directed repair.

This allowed the researchers to preserve important sequences in the CTLA-4 gene — known as introns — so that they were turned on or off
by the cell only when needed.

Co-senior author Professor Emma Morris, Professor of Clinical Cell and Gene Therapy and Head of the Department of Infection and Immunology at University College London, said: "The genes that play a key role in controlling the immune response are not always activated, but are very tightly regulated
.
" The techniques we use allow us to maintain the integrity of the natural (endogenous) mechanisms that control gene expression while correcting errors in
the genes themselves.
”

The study, led by Dr Thomas Fox, Clinical Doctor of the Wellcome Trust at UCL, builds on the work of Dr.
Pietro Genovese of the Dana-Farber/Boston Children's Cancer and Blood Disorders Center in Boston, MA, who is one
of the study's authors.

Although CTLA-4 deficiencies are rare, the research team says the gene-editing therapy they developed to treat the disease could be proof of principle for their approach, which could be used to treat other diseases
.

Professor Morris added: "This is a way to correct genetic mutations and may be applicable to other diseases
.
More importantly, it allows us to correct genes that are abnormally or overactively, and it also allows us to learn more about gene expression and gene regulation
.
”