The First Therapeutic Use of CRISPR May Be For Retinitis Pigmentosa

With all the talk about it in the scientific community, there is probably no surprise that this is yet another article discussing CRISPR. However, this may be one of the most profound discussions around the technology as it involves what could arguably be the potential flagship use of the gene editing tool – human therapy.

Retinitis Pigmentosa is an inherited genetic disorder in which a mutation in a particular gene causes the retina of the eye to degrade, subsequently leading to blindness. Historic methods used to treat genetic disorders such as this include the use of human embryonic stem cells (hESCs), where stem cells containing the unmutated gene are transplanted into the patient. The idea behind this process is that the transplanted stem cells will grow to replace the mutated cells, and will begin transcribing the healthy gene, therefore correcting the genetic disorder. There are however two major issues surrounding the use of hESCs. First, these treatments are highly controversial due to their embryonic nature, and second, the use of foreign stem cells raises the risk of immune-mediated rejection in the patient.

A collaborative team of researchers from Columbia University Medical Center and The University of Iowa have begun using CRISPR technology to take on these problems, and have found a way to use the patients’ own pluripotent stem cells, instead of hESCs, to correct the disorder.

Together with the Cas9 endonuclease, the CRISPR technology has completely revolutionized DNA editing in the last couple years. It is now being used to create knockout animal models at a much faster rate than ever before, engineer animal cells, and even to control the expression of genes instead of replacing them. What CRISPR has yet to be successfully used for however, is gene therapy in real human patients. That is what the scientists from Columbia University Medical Center and The University of Iowa hope to change.

While a 13% success rate in correcting the mutation may sound low, with the rapid growth in gene editing technology, this preliminary success is promising for treating retinitis pigmentosa.

The study consisted of generating induced pluripotent stem cells (iPSCs) from patients’ own skin cells, and then using the CRISPR/Cas9 system to insert the wild-type gene in place of the mutated gene. Deep sequencing results showed a 13% success rate in which the mutation of a premature stop codon “TAG” was replaced by the wild-type “GAG”. The mutated gene targeted in this study is called RGPR, and its point mutation is one of the most common variants causing retinitis pigmentosa. The structure of RGPR consists of many repeats and tight-binding nucleotides, which make it a difficult gene to edit. Therefore, while a 13% success rate in correcting the mutation may sound low, with the rapid growth in gene editing technology, the researchers believe this preliminary success is promising for treating this condition even when caused by a mutation in another gene.

Following the gene replacement in the patients’ own iPSCs, the therapy would involve transplanting these healthy cells into the patients’ eyes, alike to the hESC method. While this has not been completed in humans yet, another group has reportedly used CRISPR to correct a disease-causing mutation in rats with retinitis pigmentosa. This study supports the promise of gene therapy using CRISPR, and the Columbia University Medical Center and University of Iowa team are now working to show that this method does not cause any unintended genetic modifications, and that the edited iPSC are safe for reintroduction into the patients. While it will still take some time to put this therapy into actual clinical practice, Stephen Tsang, MD, PhD, one of the study’s senior authors from Columbia University Medical Center, believes that the first therapeutic use of CRISPR could be for treating eye disease.

The use of CRISPR for gene therapy and precision medicine is promising, but does not go without controversy of its own. One of the reasons it is not yet in practice is that there are ethical issues to consider when proposing CRISPR as a gene therapy for humans.

 

By Brady Slater, B.Sc

Original research article: Precision Medicine: Genetic Repair of Retinitis Pigmentosa in Patient-Derived Stem Cells

Original article: Columbia University Medical Center Newsroom

Featured image: Fundus of patient with retinitis pigmentosa, early stage. By Christian Hamel (Retinitis pigmentosa by Christian Hamel) [CC BY 2.0], via Wikimedia Commons