Researchers from the University College London (UCL) used gene therapy to partly restore the function of the retina's cone receptors in two children, according to a press release published by the institution.
This is a promising step toward reactivating previously inactive communication pathways between the retina and the brain by utilizing the plasticity of the teenage brain.
The research employs a novel method to determine if the medication alters the brain circuits particular to the cones in children with achromatopsia, a partial or total absence of color vision.
Achromatopsia is a genetic condition affecting cone cells, one of the two kinds of photoreceptors in the eyes. Since cones are what determine color, people with achromatopsia are completely color blind, and they also have very poor vision in general and are disturbed by bright light (photophobia). Researchers have been attempting to revive the dormant cone cells, because although many of them are still present, they don't send signals to the brain.
"Our study is the first to directly confirm widespread speculation that gene therapy offered to children and adolescents can successfully activate the dormant cone photoreceptor pathways and evoke visual signals never previously experienced by these patients," said Dr. Tessa Dekker, the lead author of the study.
"We are demonstrating the potential of leveraging the plasticity of our brains, which may be particularly able to adapt to treatment effects when people are young."
The study included four children between 10 and 15 years old who took part in two clinical trials led by Professor James Bainbridge at UCL and Moorfields Eye Hospital, sponsored by MeiraGTx-Janssen Pharmaceuticals.
Both of the mentioned trials test gene therapies that target specific and different achromatopsia-related genes. Their main goal is to test the safety of the treatment apart from checking for vision improvement. The total effectiveness of the treatments has not yet been assessed, as all of their results have not yet been collated. The effectiveness of the treatments has not yet been assessed, as all of their results have not yet been compiled.
Researchers used a new mapping approach called "functional magnetic resonance imaging" (fMRI) to separate emerging post-treatment cone signals from existing rod-driven signals in patients. This also enabled the researchers to detect any changes in visual function.
They used a "silent substitution" method using pairs of lights to target certain cones or rods for stimulation. The researchers also adapted their methods to accommodate another achromatopsia symptom known as nystagmus, or "dancing eyes," and compared their results to tests that involved 28 volunteers with normal vision and nine untreated patients.
Six to 14 months after treatment, two of the four children showed strong evidence for cone-mediated signals in the brain's visual cortex coming from the treated eye. The subjects had shown no evidence of cone function on any tests before the treatment. After treatment, on the other way, their measures were quite similar to those of the research participants who were normally sighted.
Subjects of the study also took a psychophysical test of cone function, which measures the eyes' capacity to differentiate between various contrast levels. It indicated a difference in cone-supported vision in the treated eyes in the same two kids.
According to the researchers, they are unable to establish if the treatment was ineffective in the other two study subjects, whether there were treatment effects that their tests may not have detected, or whether effects were delayed.
"We are still analyzing the results from our two clinical trials to see whether this gene therapy can effectively improve everyday vision for people with achromatopsia. We hope that with positive results, and with further clinical trials, we could greatly improve the sight of people with inherited retinal diseases," said Dr. Michel Michaelides, the co-lead author of the study.
The results of the study were published in the journal Brain.
Recent advances in regenerative therapy have placed the treatment of previously incurable eye diseases within arms' reach. Achromatopsia is a severe monogenic heritable retinal disease that disrupts cone function from birth, leaving patients with complete color blindness, low acuity, photosensitivity and nystagmus. While successful gene-replacement therapy in non-primate models of achromatopsia has raised widespread hopes for clinical treatment, it was yet to be determined if and how these therapies can induce new cone function in the human brain. Using a novel multimodal approach, we demonstrate for the first time that gene therapy can successfully activate dormant cone-mediated pathways in children with achromatopsia (CNGA3- and CNGB3-associated, 10–15 years). To test this, we combined functional MRI population receptive field mapping and psychophysics with stimuli that selectively measure cone photoreceptor signaling. We measured cortical and visual cone function before and after gene therapy in four paediatric patients, evaluating treatment-related change against benchmark data from untreated patients (n = 9) and normal-sighted participants (n = 28). After treatment, two of the four children displayed strong evidence for novel cone-mediated signals in visual cortex, with a retinotopic pattern that was not present in untreated achromatopsia and which is highly unlikely to emerge by chance. Importantly, this change was paired with a significant improvement in psychophysical measures of cone-mediated visual function. These improvements were specific to the treated eye, and provide strong evidence for successful read-out and use of new cone-mediated information. These data show for the first time that gene replacement therapy in achromatopsia within the plastic period of development can awaken dormant cone-signaling pathways after years of deprivation. This reveals unprecedented neural plasticity in the developing human nervous system and offers great promise for emerging regenerative therapies.