Thanks jeffreyn.

It seems that variations in the copy number of the SNCA gene (and other genes) can be risk factors for both familial and sporadic PD (Copy number variability in Parkinson’s disease: assembling the puzzle through a systems biology approach).

The evidence for point mutations in the SNCA gene is less clear. The way to do this is a Genome Wide Association Study (GWAS). In this context a GWAS means recruiting a lot of people with PD and compare them with matched people without a PD Dx.

The only GWAS that I can find suggests that SNCA mutations can be risk factors but these are rare, occurring in about 2% of people with sporadic PD (Resequencing analysis of five Mendelian genes and the top genes from genome-wide association studies in Parkinson’s Disease).