Robin Williams had Lewey Body dementia, not PD. LB has some similar muscular control issues, but also dementia (as you might imagine from the name).
That said, there's a good bit of evidence that a number of dementias and PD are mitochondrial in nature. All of us eucaryotes (everything larger than a single celled organism) are reliant on mitochondria - which have a separate set of genetic material. Some of the proteins they need are made via DNA in the nucleus and some are made with their own DNA. The mitochondria in each cell are 'populations' in the sense that they reproduce like bacteria - but they have to do that as a dance with the nucleus as they need those proteins. Everything I've read includes the phrase "not fully understood". It's pretty wild, one protein handing off electrons to the next via what appears to be quantum entanglement - where alignment - and so protein shape - is critical to the angstrom (a couple of O2 atom diameters). And here's the thing: nuclear DNA is pretty well protected from change: mitochondrial is *not*, and if those proteins change...
Anyway, as mitochondrial populations (one for each cell) decrease, a complex molecule called 'Cytochrome C' (which those proteins mentioned above use to transport electron donors) also decreases. There's been some work showing that methyline blue - and the photons from trans-cranial infrared(!) - can stand in as electron donors, helping to re-establish those populations. As I said, it's pretty wild. This is happening in all tissues but is more evident (and more critical) in the brain as those cells use a *lot* of energy.
The whole area is super interesting - and may explain how complex life arose at all.
On a mobile, so no links to studies. Sorry if this seems disjointed: I'm typing into a little window... :-(