21 July 1998. The focus of this symposium was the biology and diagnostic value of a recently discovered protein, termed AD7c-neuronal thread protein (NTP), in Alzheimer's disease (AD). Dr. Suzanne de la Monte presented the discovery of AD7C-NTP. The gene/protein was isolated from an AD cDNA library (7th clone) based on similarities to pancreatic NTP. AD7C-NTP is distinct from the pancreatic protein and is primarily expressed in neurons and shows a pronounced increase in vulnerable and degenerating neurons in AD. Interestingly, the cellular distribution of AD7C-NTP is distinct from the neurofibrillary tau pathology, because it is diffusely localized within the neuronal cytoplasm and is actually reduced in neurons with extensive neurofibrillary pathology. This latter aspect argues that AD7C-NTP is an earlier marker than tau of neuronal degeneration and, in support of this, AD7C-NTP occurs in the earliest neuritic abnormality and its introduction into cells results in increased tau phosphorylation. Of interest, AD7C-NTP levels show no relationship to amyloid-β deposits.
Antibodies to AD7C-NTP, both mono- and polyclonal, show about a 2.5-fold increase in immunoreactivity in cortical tissue in AD compared to controls. In contrast, in immunoblot analysis, AD7c-NTP can only be found in AD brain, suggesting that the antibodies may be cross reacting with related brain proteins that are distinct from AD7c-NTP, but that in immunoblot and sandwich ELISA assays of body fluids, the antibodies are instead specific for AD7C-NTP. In ELISA analysis of cerebral spinal fluid (CSF) samples, 84% of AD cases showed higher levels than controls. Further, AD7C-NTP levels correlated with the Blessed dementia score (r2 = 0.6) suggesting it can serve as a surrogate marker of neurodegeneration.
Dr. Philipp Kahle presented findings showing that AD7c-NTP increases as neurodegeneration proceeds. Dr. Kahle compared the assay for tau from Innogenetics with the AD7C-NTP assay in analyzing CSF samples. He found that the values produced from either test were highly correlated, r = 0.435, p Dr. Hossein Ghanbari presented work on the specificity and sensitivity of the AD7C-NTP test. In early testing, he found that 87 percent of clinically diagnosed cases of early AD show AD7C-NTP levels over two ng/ml while only 11 percent of controls show these levels. The average values for AD is 4.6 ng/ml while in controls, it is 1.2 ng/ml. Other neurological diseases were also considered. Multiple sclerosis showed one ng/ml while Parkinson disease averages 1.8 ng/ml. In non-AD demented cases, the mean value for AD7C-NTP levels was 1.1 ng/ml, with only 6 percent above two ng/ml. While this overlap might be cause for concern, it must be kept in mind that the clinical or even pathological diagnosis of AD is only 72-90 percent specific for clinical, and 84-92 percent with an autopsy, diagnoses, and therefore a 100 percent specificity would not be expected as long as the diagnosis of AD is based on traditional clinical protocols. Significantly, levels of AD7C-NTP do not increase during aging.
More recent studies show that testing urine yields results similar to the CSF test, which would greatly increase the potential utility of the AD7C-NTP test. Dr. Ghanbari presented a concordance of 82 percent between urine and CSF levels of AD7c-NTP, a comparison of 4.2 ng/ml for CSF versus 3.3 ng/ml for urine in AD cases. Importantly, AD7C-NTP in both CSF and urine is stable under frozen storage for at least a year, thereby opening an opportunity for noninvasive assessment of biochemical abnormalities in AD.
During discussion it was brought out that AD7C-NTP may have the greatest value in diagnosing "classic or pure" AD and show the greatest fall out when other confounding factors intervene, such as multi-infarct dementia. Nonetheless, it can be argued that AD7C-NTP provides not only a defined diagnostic test but also enables the collection of better data to rigorously define "AD" heterogeneity.—George Perry
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