Scientists at the National Eye Institute have developed a promising gene therapy strategy for a rare disease that causes severe vision loss in childhood, a form of Leber congenital amaurosis, which is caused by dominant mutations in the CRX gene that are difficult to treat with gene therapy.
The researchers tested their approach using lab-grown retinal tissues derived from patient eye cells called retinal organoids. This approach, which involved adding copies of the normal gene under its original control mechanism, partially restored the function of the CRX gene. This was revealed in a study published yesterday on the National Eye Institute's website. Dr. Anand Swaroop, head of the NEI Neurobiology Laboratory and lead physician of the study, stated, "Our therapeutic approach, based on adding more copies of this normal gene, could address Leber congenital amaurosis caused by a variety of genetic mutations."
It is worth noting that the U.S. Food and Drug Administration approved the use of the drug Luxturna in 2017 to treat patients with Leber congenital amaurosis who have mutations in a gene called RPE65. Although welcomed as a groundbreaking advancement in gene therapy, Luxturna is ineffective against other forms of Leber congenital amaurosis, including those caused by dominant mutations in the CRX gene.
According to the researchers, the CRX gene encodes a protein also called CRX, which binds to DNA and directs retinal photoreceptors to produce light-sensitive pigments known as opsins. Without this functional CRX protein, photoreceptors lose their ability to detect light and eventually die, leading to blindness and severe vision loss. Notably, it is challenging to treat some disorders caused by genetic and hereditary issues, such as this form of Leber congenital amaurosis, with gene therapy, because adding more normal genes does not always restore gene function or repair the defect. Individuals with dominant genetic mutations still maintain one normal copy of the gene, but the mutant protein interferes with the normal protein, causing the problem.
Sometimes, rather than restoring normal function, the addition of more normal genes and their respective proteins can exacerbate the disease in unpredictable ways. To explore how increasing gene expression affects patients with this form of Leber congenital amaurosis, Dr. Swaroop's team developed retinal organoids from volunteers affected by Leber congenital amaurosis and from unaffected family members. Led by Dr. Camille Croseck, they constructed complex retinal-like tissues in several stages, starting from skin cells, resulting in the production of mature photoreceptors and other retinal cells with astonishingly accurate genetic profiles.
As expected, the patient-derived retinal organoids produced lower amounts of light-sensitive opsin pigments compared to organoids made from unaffected family members. To precisely control how much of the CRX gene would be expressed by the photoreceptors in the treatment, the team engineered the CRX promoter to be delivered with the CRX gene as part of the gene therapy. The promoter is a DNA sequence adjacent to the gene that regulates the timing and manner in which genes and proteins are expressed within the genes.
The researchers injected the engineered gene and promoter inside a virus to deliver them to the photoreceptor organoids. According to the researchers, this gene enhancement strategy successfully restored some functions of the CRX protein in the patients' retinal organoids, resulting in clear expression of light-sensitive opsin pigments.
