Hmm. In your studies can you identify retinal tissue? Guessing not since it's such a relatively small amount and might not have a clear marker like adipose/liver tissue.

Accumulating mutations could cause Parkinson's--some sort of inflammatory syndrome from altered metabolic pathways, you can probably explain it better than me--but the study in OP seems to be suggesting retinal degeneration is a symptom, right? Maybe retinal tissue could be a "canary in the coal mine" because it's more sensitive to these changes in metabolism? I'd be interested to know what you think.