PRNP and Alzheimer disease: Interestingly, Schmidt et al. (2013) showed no association between CSF PrPC and cognitive status in 114 AD patients, while Breitling et al. reported in a 5 years follow-up study of 1322 elderly Germans (aged 65+ years at baseline), an inverse association between serum PrPc and cognitive functioning (Breitling et al., 2012).