DNA Replication Precedes Neuronal Cell Death in Alzheimer's Disease

Abstract

Alzheimer's disease (AD) is a devastating dementia of late life that is correlated with a region-specific neuronal cell loss. Despite progress in uncovering many of the factors that contribute to the etiology of the disease, the cause of the nerve cell death remains unknown. One promising theory is that the neurons degenerate because they reenter a lethal cell cycle. This theory receives support from immunocytochemical evidence for the reexpression of several cell cycle-related proteins. Direct proof for DNA replication, however, has been lacking. We report here the use of fluorescent in situ hybridization to examine the chromosomal complement of interphase neuronal nuclei in the adult human brain. We demonstrate that a significant fraction of the hippocampal pyramidal and basal forebrain neurons in AD have fully or partially replicated four separate genetic loci on three different chromosomes. Cells in unaffected regions of the AD brain or in the hippocampus of nondemented age-matched controls show no such anomalies. We conclude that the AD neurons complete a nearly full S phase, but because mitosis is not initiated, the cells remain tetraploid. Quantitative analysis indicates that the genetic imbalance persists for many months before the cells die, and we propose that this imbalance is the direct cause of the neuronal loss in Alzheimer's disease. Key words: cell cycle; PCNA; cyclin B; hippocampus; nucleus basalis; FISH (fluorescent in situ hybridization); neurodegenerationThe death of nerve cells during early development is a constructive part of pruning and shaping the emerging nervous system. The loss of neurons during adult life, however, is a pathological process that produces behavioral disorders and potentially the death of the organism. Recently, several laboratories have offered evidence that an attempt by the cell to reenter a mitotic cycle precedes many instances of neuronal death (Smith and Lippa, 1995;Arendt et al., 1996Arendt et al., , 1998aVincent et al., 1996Vincent et al., , 1997Vincent et al., , 1998McShea et al., 1997McShea et al., , 1999Nagy et al., 1997aNagy et al., , 1998Busser et al., 1998;Smith et al., 1999). Together, these studies suggest that the paradoxical association of the generative process of cell division with the degenerative process of cell death is found at all stages of the existence of a neuron. The prohibition against cell division begins at the earliest stages of maturation of a neuron. Hindbrain neurons in mice lacking the retinoblastoma tumor suppressor gene are unable to avoid reentering the cell cycle and die by apoptosis immediately after their emigration from the ventricular zone (Lee et al., 1994). Neuronal cell death later in development has also been linked to ectopic cell cycling. Using mutant models of target-related cell death, Herrup and Busser (1995) showed that target-deprived neurons initiated the synthesis of cell cycle enzymes [cyclin D and proliferating cell nuclear antigen (PCNA)] and incorporated bromodeoxyuridine (BrdU) into high molecular weight DN...

Alzheimer's presenilin 1 causes chromosome missegregation and aneuploidy

“…In human cells, the presenilin-induced aneuploidy includes, but is not limited to trisomy 21. We found little evidence that overexpression or mutation of presenilin caused splenocytes, neurons, or hTERT cells to duplicate their chromosomes without dividing and thus become tetrasomic, another cell cycle abnormality that has been proposed and reported to develop in certain regions of the human AD brain, where it may contribute to neuronal cell death [14,28,42,49,50].…”

“…For example, as mentioned in the introduction, significant numbers of trisomy 21 and other aneuploid cells have been found in both sporadic and familial AD patients, including individuals carrying a mutation in PS-1 or PS-2 [9,26,31,49,50]. Similarly, two polymorphisms in PS-1, one in the coding sequence and one in the promoter have been found to be associated with both an increased risk of AD and an increased incidence of Down syndrome offspring due to chromosome missegregation during meiosis [24,29].…”

“…One key function of the PS proteins is to contribute to the γ-secretase enzyme, which cleaves the amyloid precursor protein (APP) in its transmembrane region to generate the C-terminus of the Aβ peptide, the major component of the amyloid deposits in the Alzheimer brain. Most FAD mutations in PS-1 or 2 change the specificity of the γ-secretase enzyme such that there is increase the production of a highly amyloidegenic class of Aβ peptide, Aβ x-42 , compared to the more common Aβ x-40 [5,11,12,27,47] Several lines of evidence indicate that abnormalities in one or more aspects of the cell cycle may contribute to AD pathogenesis [1,9,14,26,28,30,31,32,42,49,50]. For example, many sporadic and familial AD patients, including those carrying presenilin mutations, exhibit a defect in chromosome segregation that leads to aneuploidy, including trisomy 21, in many cells throughout the body [10,26,30,31,49,50].…”

Tumorigenesis and neurodegeneration: two sides of the same coin?

“…(13) Clinical evidence for the association between unscheduled cell-cycle activity and neuronal apoptosis includes the detection, by fluorescence in situ hybridization, of replicated DNA at certain chromosomal loci in hippocampal neurons affected by Alzheimer disease. (14) In animal models, ectopic expression of oncogenes in terminally differentiated cells, including neurons, has been shown to induce apoptosis rather than proliferation. For instance, mice transgenic for the SV40 T antigen, which is normally oncogenic in cycling cells, show progressive degeneration of Purkinje neurons to which transgene expression was targeted.…”

Manipulation of Neural Precursors in situ Induction of Neurogenesis in the Neocortex of Adult Mice

“…While it seems unlikely that a neuron could maintain the elaborate neurochemical and morphologic differentiation state of a mature neuron while replicating its DNA and remodeling its nucleus and soma, it is still theoretically possible. Though it is generally accepted that other neural cells, such as astroglia, can divide, most reports suggest that any attempt by differentiated neurons to re-enter the cell cycle results in aborted cycling and ultimately, death (Yang et al 2001). Significant evidence will need to be presented to convincingly demonstrate that mature neurons in the adult brain are capable of mitosis.…”