The distinct visual defect of CNGA3-achromatopsia patients provides a unique view of the visual system and may be seen as a controlled model for assessing the role of cone photoreceptors in visual development and function. Even more unique are the patients of the AGTC-402 trial, who underwent novel restorative treatment, allowing further examination of the potential for normalization of this congenital disease.
Initial results show that clinical improvement is limited following gene therapy. This is somewhat expected, as visually impaired subjects do not necessarily recover following restorative treatment 7. The visual system requires visual input for healthy development and maintenance8 9. Long periods of visual impairment have been shown to affect its structure, causing reduced white matter in visual pathways and altered grey matter organization in the visual cortex and thalamus 10,11. Thus, after a certain point in early life, the visual system loses most of its plasticity, and the lost functions cannot be restored following sensory recovery. It is difficult to establish to what extent this is the case in CNGA3 patients. The disorder limits perception of visual stimuli from birth, including during critical developmental stages. Yet it is important to note that these patients function independently in their daily visual activities, suggesting that a substantial amount of information is still received and processed through the unaffected rods and might be sufficient for relatively normal development of the visual system.
The achromatic visual cortex, however, is clearly altered by the abnormal input. Baseler et al. demonstrated reorganization of the visual cortex in achromatopsia patients, causing a reduction in the size of areas representing foveal input 12. Treated patients' fMRI results also show cortical deficits in the form of low computational resolution 4. When examining cortical morphology, Lowndes et al. and Molz et al. demonstrated a reduction in grey matter volume and cortical surface area in the visual cortex of achromatopsia patients. Increased thickness of the visual cortex, previously associated with total congenital blindness, was shown to be specifically localized to areas representing foveal input 13,14.
The goal of the present study was to examine the involvement of white matter in the disorder and the potential of this involvement to inhibit recovery. Since the CNGA3 gene is not expressed outside the retina 2, changes observed in fiber properties prior to treatment represent either irregular development of these fibers or a response to relatively sparse input.
In our analysis, there was no significant difference in the fiber integrity of the afferent visual pathways (optic tracts and optic radiations) between patients and healthy controls. Furthermore, no change was observed in these fibers following treatment. While it is well established that the integrity of afferent visual fibers is reduced in congenital visual disorders 8,15,16, this is usually demonstrated in impairments caused by direct nerve deafferentation, retinal damage or anatomical distortion. In cases without direct neuronal damage, the correlation is not as clear. For example, in amblyopia subjects, mean diffusivity may be increased, but FA is within the normal range 17, and in congenital achiasmia, integrity of the optic radiation remains unaltered despite anatomical abnormalities 18. In this regard, our findings of preserved fiber integrity suggest that information from the still-functioning rods in the retina, despite being diminished, suffices for the development of normal optic tracts and optic radiations.
Even when focusing our analysis on the optic radiation foveal subgroup, which represents fibers that transfer information mainly from the cone photoreceptors, integrity seemed intact. It should be noted that this foveal subgroup of fibers contains rod-derived fascicles as well, which might mask some of the integrity change of cone-representing fibers. Synaptic plasticity within the retina may also contribute to the integrity of these fibers: following cone-specific receptor death or degeneration, bipolar and ganglion cells can reorganize their neural network 19,20. Haverkamp et al. showed how cone-bipolar cells form ectopic synapses with rod receptors in a mouse model of CNGA3 21. It is reasonable to think that this process could occur in achromatopsia patients, causing foveal fascicles to receive information from rod receptors and retain normal development.
The only disease-related white matter alteration that we found was reduced fiber integrity along the occipital-callosal fibers, as reflected through both mean value and point-by-point comparisons. This effect was found when comparing pretreatment patients to controls, but not when comparing the follow-up subgroup of controls to the rest of the group. The occipital-callosal fibers transfer information between each occipital lobe and the contralateral hemisphere and play a role in many visual functions. Works examining patients with lesions around the splenium or who underwent commissurotomies suggest that these fibers’ integrity is correlated with visual acuity, object identification and complex visual tasks such as reading 22–24. In general, the fibers represent a slightly later phase of visual processing, after the initial cortical computation. In naïve patients (before treatment), the fibers' abnormal structure can be thought of as a downstream effect of the changes in the visual cortex, with the abnormal processing resulting in a less cohesive connection between occipital lobes. In other words, the achromatic visual system is not adapted for the complex visual functions facilitated by these fibers. Similarly, just as changes were seen in cortical mapping following treatment, integrity increased in the splenial fibers, promoting information transfer between occipital lobes and potentially enabling more complex visual tasks. A point-by-point comparison did show a significant reduction in fiber integrity that persisted following treatment, yet this reduction was not as striking as before, only reflected in small portions of the fiber.
The splenial fibers' integrity alterations reflected through FA values were driven by RD values; relatively high pretreatment RD values were normalized in two patients following treatment. Change in RD is considered reflective of fiber sheath myelination and the presence of support cells. An increase in RD, especially when not accompanied by AD change, is correlated with a decrease in myelination and presence of fewer cell membranes 25. Our findings therefore indicate that splenial fibers in CNGA3-achromatopsia patients differ from those of healthy individuals in the state of myelination or in the activity of myelinating cells. Changes in myelination and glial cell activity are among the mechanisms that facilitate plasticity in the adult brain 26. Myelination is suggested to occur after learning novel skills, such as piano playing or reading 27–29, and it is observed in animal models after motor training 30. It is reasonable to think that following cone reactivation, as more information is transferred through the splenial fibers, a process of increased myelination occurs, causing a reduction in the fibers’ RD.
The occipital-callosal fibers of patients also have a smaller cross section in the plane of the corpus callosum. Since theses fibers predominantly transfer visual information, their relatively small size indicates a callosum less optimized for visual information processing. This may be caused by a lack of sufficient visual input during development of the corpus callosum, as has been observed in congenitally blind individuals 8,29,31.
It is important to note the limitations due to the small sample size of the study and the inter-patient variability. To that end, we used a relative rigorous statistical threshold (p < 0.005), and we scanned healthy subjects again following a time interval to demonstrate the stability of the fibers' microstructure parameters. As CNGA3-achromatopsia is a rare disorder, and restorative treatment is only being used in initial trials, it is impossible to assess large groups of suitable subjects. Currently, statistical power greatly limits any analyses, and chance may play a considerable role in apparent findings. Therefore, results should be interpreted with caution, and the conclusions drawn from them should be taken merely as a starting point for more extensive future research. We suggest considering the current article a pilot to a larger, multicenter study.