Scientists at Northwestern University have become the first to detect in living humans a biomarker associated with Alzheimer's disease, a development that promises early intervention when therapeutics may be most effective -- long before plaques and tangles develop in the brain and dementia sets in.
A study by the Northwestern team shows that bio-bar-code amplification (BCA) technology, which was developed at the University, can detect in human cerebrospinal fluid (CSF) miniscule amounts of a toxic protein that may be responsible for the early neurological deterioration in Alzheimer's disease. The findings will be published online the week of Jan. 31 by the Proceedings of the National Academy of Sciences (PNAS).
"We have demonstrated the first diagnostic test for the potential Alzheimer's biomarker known as an ADDL," said Chad A. Mirkin, director of the Institute for Nanotechnology at Northwestern and an author on the PNAS paper. "This protein is only five nanometers wide and present in cerebrospinal fluid at very low concentration, making it very difficult to detect. Our BCA technology, which is a million times more sensitive than any other diagnostic technology, can accurately identify ADDLs, even in CSF."
Amyloid ß-derived diffusible ligands or ADDLs (pronounced "addles") are small, soluble aggregated proteins. The clinical data strongly support a recent theory in which ADDLs accumulate at the beginning of Alzheimer's disease and block memory function by a process predicted to be reversible. William L. Klein, professor of neurobiology and physiology in Northwestern's Weinberg College of Arts and Sciences, and two colleagues reported the discovery of ADDLs in 1998.
"Detection of plaques in patients may be too late," said Klein, an author on the PNAS paper who last year demonstrated that ADDLs specifically attack and disrupt synapses, the nerve cell sites responsible for information processing and memory formation. "In the last three years, there has been a big effort in Alzheimer's research to identify and detect biomarkers in cerebrospinal fluid. We think the accumulation of ADDLs is likely to be the first biomarker in Alzheimer's disease, and now this extraordinarily powerful detection technology has changed what we think might be possible."
Klein and Mirkin, who is George B. Rathmann Professor of Chemistry, led a research team that, using BCA technology, measured the concentration of ADDLs present in the cerebrospinal fluid of 30 individuals. ADDL concentrations for the individuals who had been evaluated and determined to have Alzheimer's disease were consistently higher than the levels from the control group of healthy individuals who were not demented; the two groups were easily differentiated. The researchers next would like to develop the technology so that the test could be done using a blood or urine sample instead of cerebrospinal fluid, which is more difficult to obtain.
Since its invention in 2003, the bio-bar-code amplification technology has become a powerful analytical tool for the detection of both proteins and DNA. The extraordinary sensitivity and selectivity of the test, which also has been used to detect trace amounts of anthrax lethal factor and prostate specific antigen (PSA), could be used to target known biomarkers for a wide variety of diseases, such as HIV infection, various cancers and Creutzfeldt-Jakob disease, enabling early diagnosis that would be impossible with conventional technology.
To detect ADDLs, a magnetic microparticle and a gold nanoparticle are each outfitted with an antibody specific to the ADDL antigen. When in solution, the antibodies "recognize" and bind to the ADDL, sandwiching the protein between the two particles.
Attached to each tiny gold nanoparticle (just 30 nanometers in diameter) are hundreds to thousands of identical strands of DNA. Mirkin calls this "bar-code DNA" because they have designed it as a unique label specific to the target. After the "particle-ADDL-particle" sandwich is removed magnetically from solution, the bar-code DNA is removed from the sandwich and read using standard DNA detection methodologies.
In addition to Mirkin and Klein, other authors on the PNAS paper are lead author Dimitra G. Georganopoulou, a post-doctoral fellow; Lei Chang, a research associate in neurobiology and physiology; and C. Shad Thaxton, a graduate student, all from Northwestern University; Jwa-Min Nam, a former graduate student at Northwestern; and Elliott J. Mufson, from Rush University Medical Center, Chicago.
The research was supported by the National Science Foundation, the NSF Nanoscale Science and Engineering Center, the Department of Defense, the National Institutes of Health and the Boothroyd Foundation.