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Group of researchers aim to validate both approaches in proof-of-concept study
Though no gene therapies are approved for the treatment of LCA16, a study helped clarify that the disorder is a nonsense mutation and may be treated with breakthrough therapies.
Reviewed by Bikash Pattnaik, PhD
Genetic abnormalities on the KCNJ13 gene are known to cause Leber’s congenital amaurosis (LCA16). This particular gene encodes the Kir7.l protein. A proof-of-concept study1 has shown read-through therapy and gene augmentation can each rescue Kir7.l channel function in induced pluripotent stem cell (iPSC)-retinal pigment epithelial (RPE) cells derived from affected individuals.
Though gene therapy is, itself, a simple concept, over- and under-expression can have negative effects, which only further emphasizes the need for optimum dosages.
“Our cell model showed that both treatments can restore the retinal cells to proper function,” said principal investigator Bikash Pattnaik, PhD, assistant professor in the departments of pediatrics and ophthalmology and visual sciences, University of Wisconsin (UW)-Madison. “This gives us hope for the value of precision medicine.”
LCA is a rare disorder-it affects 2 to 3 children per 100,000. Mutations in both photoreceptors and the RPE can cause LCA, Dr. Pattnaik said. A mutated KCNJ13 gene directly impacts the ability of the ion channel to function properly, thereby inhibiting the ability of the photoreceptor cells to encode visual stimuli.
“We started this research to determine how the RPE gene defect causes blindness,” Dr. Patnaik said. “This is a monogenic disease-meaning there is only one gene defect. Second, KCNJ13 is an ion channel, which means for it to function you need the protein component."
Because of how proteins work, developing (or researching) gene therapy on ion channels is more difficult, he said, because of the lack of or low level of expression.
“Our study demonstrates stem cell technology can be used to overcome those barriers and analyze the pathophysiology of LCA16,” he said.
The UW group first created a “disease in the dish” model to test two possible gene therapy approaches to reverse the damaged KCNJ13 gene. In the first approach, skin cells were removed from two people in the same family (one with LCA, the other with no symptoms but one copy of the mutation). The skin cells were “re-engineered” back to an undifferentiated state and analyzed.
Both types of cells appeared normal in structure. But, when they matured, the cells from the LCA-affected person lacked the expression of the protein needed for the ion channel to develop and function.
The team then tried to “rescue” the deficient ion channel through an approach known as “readthrough therapy.” When the group was creating patient-derived iPSC cells, “we hoped that would address the question about what the protein was doing, what the ion channel was doing in these cells,” he said.
“We wanted to know if this was the only defect, or if there could be something else. That’s where this became interesting.” The group used a lentivirus to try to re-establish a proper ion channel. Lentiviruses are known to have a long incubation period and can handle a larger capacity for a bigger gene, and it transfuses more efficiently than either the adenovirus or the associated-adenovirus (AAV) vectors, Dr. Pattnaik said.
“A lentivirus will get almost 80 to 90% transduction, whereas the AAV vectors are closer to 50%,” he said. Initial results with an AAV vector showed the iPSCs would not transfuse, which led the group to the lentivirus vector approach. Both the lentivirus and AAV vectors transfused in mice models, leaving the option open for future studies.
“Our study determined that the major pathogenic alteration affecting the RPE cells is the lack of functional Kir7.1 channel. We were also able to establish a genotype-phenotype (normal) correlation in an asymptomatic carrier individual.
Use of carrier (control) hiPSC-RPE, from within the family, also accounts for modifier gene functions, epigenetic changes, and environmental contributions,” the study authors noted. While only a portion of the cells completely recovered, the finding suggests that the approach shows promise for future treatment, Dr. Pattnaik said.
The FDA will not require animal studies for this approach, Dr. Pattnaik noted, because others have already set the precedent that stem cell therapy is efficacious. Though no gene therapies are approved for the treatment of LCA16, this study helped to clarify the disorder as a nonsense mutation (and may be treated with breakthrough therapy, which has been approved), he said.
“We can go directly to the patient with this approach,” he said. Though this current paper did not address efficacy in animal models, the group has shown in preclinical data that their approach is working.
Bikash Pattnaik, PhD
Dr. Pattnaik did not disclose any relevant financial disclosures.
1. Shahi PW, Hermans D, Sinha D, et al. Gene augmentation and read-through rescue channelopathy in an iPSC-RPE model of congenital blindness. https://doi.org/10.1101/485847