Analysis of mitochondrial DNA (mtDNA) in the cerebrospinal fluid has found that both symptomatic Alzheimer’s patients and asymptomatic patients at risk of Alzheimer’s showed a significant decrease in levels of circulating cell-free mtDNA in the CSF. Patients with frontotemporal dementia did not display this.
Moreover, this potential biomarker occurred at least a decade before signs of dementia manifested, preceding the appearance of amyloid-beta and tau — suggesting not simply that it might be used as a very early sign of developing Alzheimer’s, but that the pathological process of Alzheimer's disease starts earlier than previously thought.
 . Low cerebrospinal fluid concentration of mitochondrial DNA in preclinical Alzheimer disease. Annals of Neurology [Internet]. 2013 ;74(5):655 - 668. Available from: http://onlinelibrary.wiley.com/doi/10.1002/ana.23955/abstract