Genes and Nonmotor Symptoms in Parkinson's Disease.

Published data on genetic risk factors of nonmotor symptoms (NMS) are relatively lacking since the first mutation responsible for Parkinson's disease (PD) being reported in 1996. This chapter provides a concise summary of genetic links to common individual NMS such as cognitive impairment, depression, psychosis, olfactory dysfunction, pain, and sleep disorders. Although some genetic variants such as apolipoprotein E and glucocerebrosidase demonstrate consistent links with certain NMS, it is difficult to draw definitive conclusions. A concerted effort involving standardized protocol in multiple centers and multiethnic groups will be useful to further investigate the association. With the help of high-throughput genomic techniques, more causative genes and novel genes will be discovered in the future and this will contribute further to the understanding of genetic susceptibility of NMS in PD.