With the increase in genomic knowledge following the discovery of genome sequencing, the idea of replacing genes came about.

Canine Gene Therapy:
One study was performed using canines to test the efficacy of gene therapy on complete achromatopsia. This was one of the first gene therapy studies, not only to be performed, but also to be successful. Viral vectors were used to inject DNA sequences into the retina of dogs with the goal of replacing the gene sequence that caused the condition. The new genotype allows for regeneration of the cells that transmit light signals to the brain. In this specific experiment, green fluorescence was added to the gene to improve the affects. This experiment regenerated the rods of the retina rather than the cones that are normally associated with colorblindness, but showed improved color vision which provides evidence that gene therapies can be successful for achromatopsia.
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Figure 4: Model of subretinal injection for achromatopsia using a viral vector.
Source: Mallen, n.d.

Primate Gene Therapy:
Another study used gene therapy on monkeys with incomplete achromatopsia. A control group with normal vision was used to compare visual acuity. Viral vectors with the L-opsin gene were injected into the retinal membrane of the monkeys. Although color vision was not completely restored, the monkeys showed improvements in the light sensitivities of their retinal rods. The researchers in this study predict that the neurological connections that were "fixed" may improve over time, naturally. The fact that there was partial success in monkeys, who have similar genomes to humans, promotes the idea that the safety and efficacy of the procedure would be similar if performed on humans. The procedure for administration of the gene was the same as the canines, which suggests the idea that the difference in genes being adjusted may be reason for the different results.

Mice Gene Therapy:
A similar procedure to the canine and primate studies was performed on mice with achromatopsia. Is this study, the S-opsin protein was the injected protein targeting the CNGA3 gene specifically. Cone regeneration and increased light sensitivity were the result. Follow up exams were done eight months later to provide evidence that the results would last. Because of the similarities in mammal's visual genes, it is hypothesized that the lasting results of these mice experiments would reflect the results of human gene therapies.The images below are from this study and show the results 12 months after treatment. Notice the increased thickness and florescence in Figure 5B and Figure 6B.

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Figure 5 A: Image of retinal space of a CNGB1 mouse with rhodopsin promoters. B: Untreated retina of test mouse (left) and the treated retina 12 months after treatment (right).
Source: CNGB1, 2013
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Figure 5 A: Image of retinal space of a CNGA3 mouse with S-opsin injection. B: Retina before treatment (top) and the retina 4 months after treatment (bottom).
Source: CNGA3, 2013

All current gene therapies for achromatopsia involve the same viral vector, subretinal injection procedure. There has been clear success from these therapies but in different proportions. It is predicted that the different results are due to the difference in proteins injected as well as whether rods or cones are being targeted. Where cones are targeted, the results are consistently successful and long lasting, as opposed to the treatment of rods which showed to be partially successful. Additionally, the individual's phenotype may play a role in how the individual reacts to gene therapy. One genotype may be associated with an ability to regenerate the neurological circuits needed for colored vision while another genotype may be unable to be changed. Further research and testing is necessary to determine the accuracy of these hypotheses. Furthermore, the specific mutated gene may play a role in the results. Because there are five known genes associated with achromatopsia, it is predicted that the different genes would need different therapies to be effective. The primate and canine studies do not specify a specific target gene while the mice study targets one of the most commonly affected genes in achromatopsia patients, CNGA3. The images above both show the significant increase in photoreceptor cells and increase in retinal thickness from targeting the CNGA3 gene.

The future of gene therapy for achromatopsia patients:
  • Successful experiments on non-human species provide evidence that the therapies can be successful in restoring color vision.
  • The United States FDA has approved drugs for the treatments and funding has been approved for further research.
  • Genomic research in general has been advancing as well through new findings and technologies making it easier to sequence genomes as well as identify mutated genes. As genomic studies improve, new theories, technologies, and research are possible results and will lead to further advancements in gene therapy.
  • Clinical trials are now being approved for testing gene therapies on human vision deficiencies.

Materials and Methods
Broader Impacts
Work Cited