GRN and frontotemporal dementia: Pgrn mutations causing FTD manifest in reduced plasma, serum, and cerebrospinal fluid (CSF) PGRN protein levels compared to non-mutation carriers (Ghidoni et al., 2008; Finch et al., 2009; Sleegers et al., 2009; Nicholson et al., 2014; Meeter et al., 2016; Wilke et al., 2017).