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. 74(5), 655 - 668.(2013).