Retinal Damage May Differ Between CLN5, CLN6 Disease: Sheep Study
The layers of the retina — the part of the eye that’s damaged in Batten disease — are differently affected and show a distinct progression between naturally occurring sheep models of CLN5 and CLN6 disease, a study shows.
Notably, all retinal layers were affected in sheep with CLN6 disease. However, only the outer layers of the retina showed degeneration, and at an earlier time point, in the sheep model of CLN5 disease.
In addition, several features of these Batten types were found to be similar to those previously reported for patients. That further supports the use of these sheep models to not only better understand disease progression, but also to assess the efficacy of potential therapies, the researchers noted.
The study findings suggest that different eye-targeting therapeutic approaches are needed to treat retina damage in Batten — and that those focused on CLN5 disease will likely have to be given at an earlier stage to produce the best outcomes.
The study, “Natural history of retinal degeneration in ovine models of CLN5 and CLN6 neuronal ceroid lipofuscinoses,” was published in the journal Scientific Reports.
Batten disease, also known as neuronal ceroid lipofuscinoses (NCLs), is caused by mutations in at least 13 different genes. These mutations lead to the toxic accumulation of waste molecules inside lysosomes — the cells’ recycling compartments — mainly in brain cells, leading to nerve cell damage and death.
The result is progressive cognitive and motor decline, seizures, and vision loss — all due to damage to both the retina and the brain region that processes visual information.
The retina, a thin layer of nerve cells lining the back of the eye, senses light through its photoreceptors. It then sends vision signals to the brain through the optic nerve, which is the only connection between the eye and the brain.
“Many of the studies of retinal tissue from NCL patients or models have been done at end stage disease, therefore a detailed analysis of the onset and progression of retinal dystrophy in NCL is still lacking,” the researchers wrote.
To address this, two scientists at Lincoln University’s Faculty of Agriculture and Life Sciences, in New Zealand, analyzed the eyes of naturally occurring sheep models of two Batten types — CLN5 and CLN6 disease — at different stages of the disease.
CLN5 disease is caused by mutations in the CLN5 gene, while CLN6 disease is linked to mutations in the CLN6 gene.
Vision problems often develop in early to mid-disease in CLN5 and classical CLN6 patients. Retinal responses to light are lost between ages 7 and 10 in CLN5 disease, while vision loss occurs between 3 and 7 years in CLN6 disease.
These sheep models have been extensively used to study all aspects of Batten disease, including damage to the retina. This is because the animals “replicate the primary symptomatic and neurological profiles of NCL, including a progressive loss of vision,” the researchers wrote.
Now, the team evaluated total retinal thickness, individual retinal layer thickness, astrogliosis, and lysosomal storage burden in the eyes of affected sheep. Astrogliosis refers to a process in which the number of astrocytes — star-shaped, neuron-supporting cells — in the brain increases dramatically due to the loss of nearby neurons.
The sheep were examined at age 3 months, before symptom onset, and then at 6 months (early symptoms), 12 months (established disease), and 18 months of age (end-stage disease).
The data also were compared with those of healthy sheep at the same ages, which were used as controls.
Results showed that at 3 months of age, both models had a thicker central retina than controls, and CLN6 sheep also had a thicker peripheral retina.
This may be related “to early compensatory mechanisms or inflammatory edema [swelling] in the retina of affected animals,” the researchers wrote, adding that this “requires more investigation.”
In subsequent disease stages, retinal differences relative to controls varied between the two Batten disease models. In both models, however, significant retinal thinning was obvious at end-stage disease.
Notably, this progressive reduction in thickness was observed across all retinal layers from 12 months of age onward in the CLN6 model. Conversely, in sheep with CLN5 disease, this shrinkage was specific to the retina’s outer layers, with significant thinning being detected as early as 6 months of age in the layer containing photoreceptor cell bodies.
Both models also showed abnormally shaped astrocytes. Astrogliosis was detected throughout the retina from 6 months of age in both Batten models, contrasting with the layer-restricted astrogliosis observed in controls.
“Early and progressive increases in [astrogliosis] … has also been observed in the retina of several mouse models of NCLs,” the researchers wrote.
In addition, a toxic buildup of waste material in lysosomes was detected mainly in retinal ganglion cell (RGC) bodies in both sheep models, which is consistent with previous reports of retina damage in Batten disease patients. RGCs are the bridging neurons that connect the retinal input to the brain’s visual processing centers.
Notably, this lysosomal storage accumulation was detected earlier in CLN6 sheep than in those with CLN5 — at 6 months versus 18 months of age.
These findings generally aligned with those previously reported for people with CLN5 or CLN6 disease, the team noted.
The data “highlighted the differential vulnerability of retinal layers and the time course of retinal atrophy in two distinct models of NCL disease,” the researchers wrote. These findings will help determine “potential targets for ocular therapies and the optimal timing of these therapies for protection from retinal dysfunction and degeneration in NCL,” they added.
While all patients with these Batten types would benefit most from an “the earlier the better” approach, best outcomes in CLN5 disease may only be achieved with a very early approach, the team said. The scientists noted that retinal thinning was detected before symptom onset in the CLN5 animal model.
In addition, given that retinal shrinkage is detected in both the outer and inner layers in the sheep model of CLN6 disease, a therapeutic approach involving injections in both regions may result in better outcomes.
Moreover, these data — along with the fact that sheep eyes have a similar size and retinal structure to human eyes — “further validates the use of sheep to study the retinal component of NCL and potential [eye-targeting] therapies,” the researchers wrote.
Notably, the researchers are also using these models to test several doses and delivery routes of a combined gene therapy directed at the brain and eye.