Please read this recent article, may be a good start for your question:
Michael A. Lodato, Rachel E. Rodin, Craig L. Bohrson, Michael E. Coulter, Alison R. Barton, Minseok Kwon, Maxwell A. Sherman, Carl M. Vitzthum, Lovelace J. Luquette, Chandri Yandava, Pengwei Yang, Thomas W. Chittenden, Nicole E. Hatem, Steven C. Ryu, Mollie B. Woodworth, Peter J. Park, Christopher A. Walsh. Aging and neurodegeneration are associated with increased mutations in single human neurons. Science, 2017
link to the article: http://science.sciencemag.org/content/early/2017/12/06/science.aao4426