It is the GBA gene that is already involved in Gaucher's disease

It has been known for a long time that mutations in the glucocerebrosidase (GBA) gene cause Gaucher's disease, a genetic disease that is common amongst jews, on account of the inability to break down a component of the membrane of red and white blood cells.  The component accumulates in the organism, causing a number of problems, including also neurological symptoms similar to those of Parkinson's disease. 

A group of American researchers examined the GBA gene in patients included in an epidemiological study on the genetics of Parkinson's disease (the GEPD study), which included  278 patients with Parkinson's disease and 179 subjects without the disease.  They found that the frequency of GBA gene mutations was three times higher amongst patients with Parkinson's disease, especially amongst those with early (before the age of 50 years) onset forms of the disease.  The frequency of mutations was higher amongst the patients whose grandparents were all Jewish. 

One should bear in mind that the fact that GBA gene mutations are associated with early onset parkinson's disease does not mean that they cause it.     

Clark et al Neurology 2007; 69: 1270-1277