"The cause of Parkinson’s isn’t well understood, but the reason the drug wears off is. It’s because the brain also starts losing an enzyme known as aromatic L-amino acid decarboxylase, or AADC, that is needed to convert L-Dopa into dopamine."

https://www.technologyreview.com/s/6...-gene-therapy/

I've not heard this before, and I suspect it might be a technical writer's (mis)understanding, rather than the researcher's view.

However, the underlying premise seems sound to me, and they are getting some results. That is, as more neurons die, if you can boost (e.g. by gene therapy) the amount of AADC (the enzyme that catalyzes the conversion of levodopa to dopamine) in the brain, then this might boost the amount of dopamine produced by the remaining neurons.