What is the color red? It’s the shiny skin of a Fuji apple and the bright paint on a firetruck. It’s the hue of a hummingbird’s favorite flower and the pigment in an often worn lipstick. But when it comes down to defining it, what is red?

For people who have normal color vision, the answer is both obvious and hard to describe. It’s the color next to orange on the color wheel or the top one on the rainbow. But for those who have only seen the world in different shades of gray, colors are almost impossible to define. 

This is the case for people with achromatopsia, a recessive genetic condition in which none of their cones, the cells responsible for color vision, function properly (1). Most people with achromatopsia have a mutation in the cyclic nucleotide gated channel subunit beta 3 (CNGB3) or alpha 3 (CNGA3) gene, so researchers wondered if delivering a healthy copy of the defective gene via gene therapy would help those with achromatopsia see the world in all of its colors.

She said, ‘Wow, this looks glowing. Can you tell me what color it is?’ And then I told her it's red. 
- Ayelet McKyton, Hebrew University of Jerusalem

The results across different clinical trials have been mixed. Some people see some improvement, but others don’t. So, when Ayelet McKyton, a neuroscientist at the Hebrew University of Jerusalem, received a message from a patient whose vision hadn’t improved after receiving one of the gene therapies a few months before, she wasn’t sure what to think. In the message, the patient included a photo of a bedspread with red flowers on it. “She said, ‘Wow, this looks glowing. Can you tell me what color it is?’ And then I told her it's red,” said McKyton. 

This was the first time that one of these patients had been able to distinguish any sort of color, even if what she saw was not how those with typical color vision see red. McKyton then asked other patients who had received the same gene therapy in the clinical trial about what they saw when presented with something red.

“One told me that it’s like it’s on a different level from the screen. It’s like a different dimension,” McKyton said. 

While current gene therapies for achromatopsia don’t seem to restore color vision, they certainly do something. Vision scientists are now taking a closer look at patients who have received these gene therapies for achromatopsia to see if they can figure out the effect they have on vision, why they are more effective in some patients than others, and how to identify the patients that are most likely to respond to treatment.

Eye plus brain equals vision

To turn light into sight the eye relies on two types of photoreceptors in the retina: rods and cones. Rods are responsible for night vision, while cones work in daylight. Cones allow us to recognize faces, read a book, drive a car, and most of all, they detect colors. There are three different types of cones, which detect short, medium, and long wavelengths of light.

People with achromatopsia, however, only have rods to work with. Due to mutations in CNGB3, CNGA3, or other related genes, their cones are present but not functional. This makes it very difficult for them to see during the day.

“They're very, very sensitive to light because your rods only work in very dim light, so if you have only rod vision, you have a kind of photophobia. It's like everything is always glaring, even under fairly dim conditions,” said Bevil Conway, a neuroscientist at the National Eye Institute.

There is also a special region of the retina called the fovea that only contains cones and is responsible for high-resolution vision. This means that people with achromatopsia have somewhat grainy and low-resolution vision. Their eyes can also make uncontrolled and repetitive movements, called nystagmus, and of course, they have no color vision at all.

Vision doesn’t only rely on the eyes. “Vision is eye plus brain,” said Michael Hoffmann, a biologist at Otto-von-Guericke University Magdeburg. He explained that if the eye is a digital camera, the brain is the computer that allows humans to understand the image. The visual inputs from the rod and cone cells travel to the optic nerve, which sends the signal out of the eye and into the visual cortex in the brain where the information is processed.

Studies on related visual conditions showed that there is some plasticity when it comes to this eye-brain connection. For example, in amblyopia or lazy eye, one eye has blurrier vision than the other. But if doctors perform surgery to fix the lazy eye during childhood, the visual cortex can still develop properly, and the child will have restored vision. But if doctors wait too long to treat this, then even with surgery, the brain can’t interpret visual information from the lazy eye.

“You need to fix the eye, but the brain has to learn how to see,” McKyton explained. If something like amblyopia could be treated and lead to better vision, maybe achromatopsia could be too.

A glowing red

The 2017 approval of Luxturna, a gene therapy for a rare form of inherited blindness, sparked joy and anticipation in the world of vision science. Not only was it the first gene therapy for a genetic disease ever approved by the Food and Drug Administration, but it also gave people sight. With established animal models, a clear genetic cause, and low risk of an adverse immune response to an AAV gene delivery vector in the retina, achromatopsia seemed like a good target for gene therapy (2).

Research groups around the world initiated Phase 1/2 clinical trials to investigate the safety and efficacy of the achromatopsia gene therapies. These included trials led by researchers at the University Hospital Tübingen, two trials sponsored by the Applied Genetic Technologies Corp, and two trials led by MeiraGTx. 

Because achromatopsia is rare — one in 30,000 people has it — these trials were small, each consisting of 11 to 32 patients (3). While earlier studies had already reported that the therapies were safe for patients, the treatments’ effectiveness was less clear. One study reported small improvements in visual acuity, color vision, and contrast sensitivity in the eye treated with the gene therapy, and another, which McKyton helped lead, also showed only minimal progress in photoaversion and acuity in the treated eye (4,5).

We're not saying that they can distinguish between colors. We don't know that yet, but we know that they can see red differently than they do gray. 
- Ayelet McKyton, Hebrew University of Jerusalem

“We didn't see any improvement, but they said that something had changed,” McKyton said. “We're not saying that they can distinguish between colors. We don't know that yet, but we know that they can see red differently than they do gray.” 

She and her colleagues realized that they weren’t picking up this subtle change in the patients’ color vision because the typical clinical tests for measuring it are not well suited to pick up a small change like this. So, the researchers decided to design new tests to better understand what individuals who received the restorative achromatopsia gene therapy saw (6).

“The challenge was to listen to their reports carefully in order to design a good experiment,” McKyton added. The team needed to convert subjective descriptions into something quantifiable.

After talking with some of the people who had received the achromatopsia gene therapy, she and her team designed three different tests to better understand their new red color perception. The first test measured how the patients perceived a color’s inherent lightness.

Imagine a color image edited to look black and white. A lot of colors will still look different from one another because colors have inherent differences in lightness. For someone with full color vision, red appears to be a bright color, but because rods can’t detect the long wavelength of red light, red often looks black in grayscale images. McKyton and her team hypothesized that after the gene therapy, that red would look lighter to patients, but that was not the case for the three adult patients they studied. For their fourth and final participant, a seven-year-old child, it was a different story.

“For the child, the red did become lighter, and also the other colors became more like controls. So, the child improved more than the three adults,” said McKyton.

Once the team established how the participants perceived the lightness of different colors, they performed a color detection test. The researchers placed a red colored stripe among gray stripes of different intensities. “They will have to use the color attribute in order to find it, and then we saw that definitely, with the untreated eye, they cannot see it at all. With the treated eye, they can see it,” McKyton said. 

The team also used this method to test participants’ abilities to detect yellow and cyan. Among the four people, two detected only red; one saw red and yellow; and the last adult distinguished red, yellow, and cyan. Finally, in the last test, the researchers tested the participants’ color saliency, essentially how quickly they could identify a red circle among gray circles meant to distract the eye.

“For us, color is a very, very intense attribute. It pops out to us very fast. It doesn't matter how many distractors we have; if everything is in grayscale, and one thing is red, we will catch it immediately,” McKyton explained. The participants, however, had to search for the red circle before they finally spotted it, meaning that they have very low color saliency compared to people with normal color vision.

Conway, who was not involved in the study, was intrigued by the variability in color detection among the four participants. He reasoned that it might be due to different levels of uptake of the gene therapy by the cone cells in the retina or that the population of cones varied from person to person.

“Patients who have this achromatopsia, their cones aren't working, but their cones progressively die over the course of the patient's life,” Conway said. “They don't develop normally and then just sit there waiting for a date to the ball. They gradually, progressively get fed up and then give up the ghost.”

To better understand how the brain responds to the red glowing sensation seen by gene therapy trial participants, McKyton would like to take a closer look at their brains. “We're really, really hoping to make them feel in the MRI this feeling of glow sensation, and to see whether it happens in color-specific areas or in different areas [of the brain] because maybe the color-specific areas did not develop until now,” she said. “Maybe their color areas were never used for color. Maybe they're reassigned to something else as well, and if they are, can we reverse it after a [gene therapy delivery] surgery?”

Diving into the visual cortex and other brain areas in people with achromatopsia and those who received these gene therapies will be the next step in understanding how the visual system works in these patients as well as figuring out the best ways to potentially restore their color vision more fully.

Focusing on the fovea

When it comes to understanding how achromatopsia patients might be able to gain color vision, one of the key structures to explore is the fovea. Because the fovea is so important for high-resolution and color vision, the brain devotes a lot of resources to interpreting foveal signals in the visual cortex.

“The visual cortex organizes the information of the outer world in a systematical map,” Hoffmann explained. With achromatopsia, the visual cortex can’t receive information from the fovea because the cones there don’t work. Hoffmann and his team wondered what happens to the region of the visual cortex that would normally interpret foveal signals in achromatopsia patients. Would it get remapped to process visual signals from other parts of the retina, or would it simply sit silently in the brain with no visual processing activity?

“This is important,” Hoffman added. “If you want to switch on the activity again with gene therapy in this part, you rely on this cortex to function.”

To investigate how the part of the visual cortex that processes foveal information acts in achromatopsia, the researchers performed functional magnetic resonance imaging (fMRI) on 17 people with achromatopsia and 19 people without achromatopsia (7). To their surprise, they found that this region of the visual cortex was not remapped to convey signals from any other part of the eye. Instead, the fMRI showed that these regions were silent in people with achromatopsia.

“It looks different from the visual cortex that gets input, and interestingly, it looks like the cortex we find in people who are born blind,” Hoffman said. 

With no visual input from the fovea, the region of the visual cortex that processes information from the fovea failed to develop normally. Because of that, Hoffmann explained, “We don't know, but we would assume that this makes it more difficult for the gene therapy to improve the vision.”

That doesn’t mean that researchers should give up hope on gene therapies for achromatopsia. Rather, they may just need to treat people sooner.

The visual cortex matures during the first ten years of life, so if clinicians deliver achromatopsia gene therapies in children, they may be able to restore acuity and color vision when the brain is still capable of figuring out how to interpret those signals. This hypothesis aligns with McKyton’s finding that the gene therapy was more effective in the young achromatopsia patient than in the three adult participants in her team’s analysis (6).

Hoffmann would like to develop an anatomical MRI-based screening tool to identify patients who still have a malleable visual cortex and in whom a restorative gene therapy would be most effective.

Hoffmann hopes that what he and other vision scientists learn from studying achromatopsia will help them better understand the visual system as a whole and other related visual conditions. “One thing we would like to understand,” he added, is “how you see the world with different eyes.”

References

1. Kohl, S. et al. Achromatopsia. GeneReviews Last Update: September 20, 2018.
2. Michalakis, S., Schön, C., Becirovic, E., and Biel, M. Gene therapy for achromatopsia. J Gene Med  19, e2944 (2017).
3. Aboshiha, J. et al. The cone dysfunction syndromes. Br J Ophthalmol  100, 115-121 (2016).
4. Fischer, M.D. et al. Safety and Vision Outcomes of Subretinal Gene Therapy Targeting Cone Photoreceptors in Achromatopsia: A Nonrandomized Controlled Trial. JAMA Ophthalmol  138, 643–651 (2020).
5. McKyton, A. et al. Cortical Visual Mapping following Ocular Gene Augmentation Therapy for Achromatopsia. J Neurosci  41, 7363-7371 (2021).
6. McKyton, A. et al. Seeing color following gene augmentation therapy in achromatopsia. Current Biology  33, 3489–3494 (2023).
7. Molz, B. et al. Achromatopsia—Visual Cortex Stability and Plasticity in the Absence of Functional Cones. Invest Ophthalmol Vis Sci  64, 23 (2023).