Morphological and biochemical assessment of DNA damage and apoptosis in Down syndrome and Alzheimer disease, and effect of postmortem tissue archivalon TUNEL

We have previously shown that Alzheimer disease (AD) brain exhibits termina l deoxynucleotidyl transferase dUTP nick end-labeling (TUNEL) for DNA damag e and morphological evidence for apoptosis. Down syndrome (DS) is a neurode generative disorder that exhibits significant neuropathological parallels w ith AD. In accordance with these parallels and the need to clarify the mech anism of cell death in DS and AD, we investigated two principal issues in t he present study. First, we investigated the hypothesis that TUNEL labeling for DNA damage and morphorogical evidence for apoptosis is also present in the DS brain. All DS cases employed had a neuropathological diagnosis of A D. Analysis of these cases showed that DS brain exhibits a significant incr ease in the number of TUNEL-labeled nuclei relative to controls matched for age, Postmortem Delay, and Archival Length, and that a subset of TUNEL-pos itive nuclei exhibits apoptotic morphologies. We also report that Archival Length in 10% formalin can significantly affect TUNEL labeling in postmorte m human brain, and therefore, that Archival Length must be controlled for a s a variable in this type of study. Second, we investigated whether biochem ical evidence for the mechanism of cell death in DS and AD could be detecte d. To address this question we employed pulsed-field gel electrophoresis (P FGE) as a sensitive method to evaluate DNA integrity. Although apoptotic ol igonucleosomal laddering has not previously been observed in AD, PFGE of DN A from control, DS and AD brain in the present study revealed evidence of h igh molecular weight DNA fragmentation indicative of apoptosis. This repres ents biochemical support for an apoptotic mechanism of cell death in DS and AD. (C) 2000 Elsevier Science Inc. All rights reserved.