PPT1 and retinal degeneration: The validated substrates fall into 9 classes, which, strikingly, are related to phenotypes observed in PPT1 knockout (KO) mice and CLN1 patients, including seizures, decreased synapse density, mitochondrial dysfunction, synaptic vesicle endocytic deficits, impaired long-term potentiation (LTP), and retinal degeneration [22–27].