Gene editing: getting closer to clinical application for a rare genetic disease

One of the first clinical applications of gene editing could soon be a gene therapy approach for a life-threatening genetic disease, X-chromosome-linked hyper-IgM immunodeficiency (X-HIGM), according to a new study published in EMBO Molecular Medicine by a group of researchers of the San Raffaele Telethon Institute for Gene Therapy (SR-Tiget) in Milan.

The group is led by Luigi Naldini, director of the Institute and full professor at Vita-Salute San Raffaele University in Milan, and by Pietro Genovese, project leader in Naldini's group who, after this study, was recruited as Assistant Professor at Harvard Medical School in Boston.

Thanks to collaboration with the spinoff Genespire, these findings pave the way for the clinical application of gene-editing for the treatment of X-HIGM.


Immunodeficiency with hyper-IgM

X-HIGM is a rare and life-threatening immunodeficiency caused by mutations in the CD40LG gene, which codes for a receptor on the surface of a specific subtype of T-cells.

“The receptor is responsible for the interaction between these cells and other cells of the immune system involved in the production of antibodies (B cells) or in direct defense against external agents,” explains Valentina Vavassori, first author of the study, together with Elisabetta Mercuri.

As a result, patients with the disease develop recurrent opportunistic infections, both bacterial and viral, and have an increased risk of developing tumors and autoimmune diseases, with an average life expectancy generally below 30 years.

To date, the first line of treatment is replacement therapy with periodic injections of antibodies, while the only definitive treatment is transplantation of hematopoietic stem cells from bone marrow or umbilical cord, from a compatible donor.

Due to the recurrent infections they suffer from, however, transplantation can also be particularly critical for these patients, as well as being available only to those with a compatible donor and carrying the risk of graft-versus-host disease.


Gene editing with Crispr-Cas9 as a therapeutic option

For this reason, Naldini's group proposed a therapeutic approach that could be more effective than the previous ones: correcting the patient's own cells. However, to do that you need the gene editing technology Crispr-Cas9 and not the usual “conventional” gene therapy, which works by adding a functional copy of the gene via viral vectors.

In the case of X-HIGM, in fact, it is not enough to simply restore the function of the gene, but it is also necessary to ensure that it is very finely regulated to avoid the risk of higher than normal expression, which could lead to the development of leukemia and lymphoma.

This is why the researchers chose gene-editing Cripr-Cas9 technology, which allows the defective gene to be corrected on spot, while keeping it under the control of its physiological regulatory mechanisms. “Thanks to the Crispr-Cas9 approach, we were able to cut the DNA at a precise point of the defective gene and replace the sequence with the correct one,” continues Pietro Genovese, pointing out that the system can correct around 95% of the mutations responsible for the disease.


From laboratory research to clinical studies

The experiments were conducted on patients samples and with animal models of the disease. In both cases, the researchers assessed the effects of gene editing in hematopoietic stem cells (the progenitors of all blood cells) and in T cells (mature cells that are easier to manipulate and may offer a higher safety profile).

“We have observed that with both types of cells there is restoration of antibody production and, in animal models, there is a protection against clinically relevant infections,” concludes Naldini.

According to the researchers, these observations indicate that modified T-cells could provide an immediate and substantial benefit to immunodeficiency patients with hyper-IgM. Even if their long-term efficacy has to be assessed in clinical trials, the therapy could function also as a bridge therapy to donor hematopoietic stem cell transplantation.

The work provides the experimental basis and the scientific rationale for moving towards clinical trials of gene editing based gene therapy, which will be carried out in collaboration with the startup Genespire, founded in March 2020 by SR-Tiget researchers with Fondazione Telethon and Ospedale San Raffaele and funded by Sofinnova Partners.

The same gene editing approach could be used for other diseases that require very fine tuning of the genes involved, such as other primary immunodeficiencies.