Come with me on a flight of fantasy for a minute and entertain a radical idea. Suppose that there is, for lack of a better term, a "metabolic pattern" built into our DNA that produces both PD and diabetes (and perhaps other conditions as well) when certain conditions exist.

I ran across an article in Science Daily. I am going to take some major liberties and completely recycle the article by substituting PD for diabetes:
"<PD> is caused by an inability of the <neurons> in the <substantia nigra> to produce enough of the <neurotransmitter dopamine> to meet the body's needs.

Central to this is a loss of <neuronal> function and mass as a result of <dopamine> resistance (the inability of cells in the body to respond appropriately to <dopamine>).

New insight into how <dopamine> resistance leads to loss of <neuronal> mass has now been provided by studies .....

In the study, in the absence of the protein <"X">, the symptoms of <PD> improved in two mouse models of the disease....

<"X"> is a protein that is involved in promoting the death of a cell that is under stress because it is producing more <dopamine> than it is able to handle.

The authors therefore propose that <dopamine> resistance causes <neurons> to make more <dopamine> than they can handle, such that the stress signaling pathways that activate <"X"> are initiated and the <neurons> die....

Mere musings, but does dopamine resistance exist?