Diffuse senile plaques occur commonly in the cerebellum in Alzheimer''s disease.

Diffuse senile plaques are characterized by the presence of beta protein (beta P), also called A4 protein, in a dispersed form and the apparent lack of associated dystrophic neurites or reactive glial cells. They are the most common type of senile plaque found in the cerebral cortex in Alzheimer's disease (AD), Down's syndrome (DS), and normal aging. Here is reported the frequent presence of diffuse senile plaques in the molecular layer of cerebellar cortex in AD. Typical neuritic plaques were never detected in this location, making the cerebellar molecular cortex a useful site for the study of diffuse plaques because diffuse plaques in the cerebral cortex are intermingled with neuritic plaques. Diffuse cerebellar plaques were detected by modified Bielschowsky silver stain in 47 of 100 cases of clinically and pathologically diagnosed AD and in none of 40 aged demented and nondemented controls. They were immunolabeled by antibodies to purified AD meningeal or cortical beta P, and to a synthetic beta P but not by two antibodies to the carboxyl- and amino-termini of the beta protein precursor (beta PP), which label a subgroup of cerebral cortical plaques. This latter result suggests that the beta P deposited in the cerebellar molecular layer may be derived from a form of the beta PP from which the carboxyl and amino terminal regions of the precursor have already been cleaved. Diffuse cerebellar plaques were not recognized by antibodies to neurofilaments, tau, and PHF, all of which detect dystrophic neurites in cerebral cortical neuritic plaques. Also, no association of reactive astrocytes or microglial cells with diffuse cerebellar plaques was observed. Thus, diffuse cerebellar plaques represent multifocal deposits of noncompacted beta P that cause little or no morphologic reaction in their microenvironment.     

Inflammation occurs early during the Abeta deposition process in TgCRND8 mice

Alzheimer's disease (AD) is characterized by a progressive cognitive decline leading to dementia and involves the deposition of amyloid-beta (Abeta) peptides into senile plaques. Other neuropathological features that accompany progression of the disease include a decrease in synaptic density, neurofibrillary tangles, dystrophic neurites, inflammation, and neuronal cell loss. In this study, we report the early kinetics of brain amyloid deposition and its associated inflammation in an early onset transgenic mouse model of AD (TgCRND8) harboring the human amyloid precursor protein gene with the Indiana and Swedish mutations. Both diffuse and compact plaques were detected as early as 9-10 weeks of age. Abeta-immunoreactive (Abeta-IR) plaques (4G8-positive) appeared first in the neocortex and amygdala, then in the hippocampal formation, and lastly in the thalamus. Compact plaques (ThioS-positive) with an amyloid core were observed as early as diffuse plaques were detected, but in lower numbers. Amyloid deposition increased progressively with age. The formation of plaques was concurrent with the appearance of activated microglial cells and shortly followed by the clustering of activated astrocytes around plaques at 13-14 weeks of age. This TgCRND8 mouse model allows for a rapid, time-dependent study of the relationship between the fibrillogenic process and the inflammatory response during the brain amyloidogenic process.     

Dendrite and dendritic spine alterations in Alzheimer models

Synaptic damage and loss are factors that affect the degree of dementia experienced in Alzheimer disease (AD) patients. Multicolor DiOlistic labeling of the hippocampus has been undertaken which allows the full dendritic arbor of targeted neurons to be imaged. Using this labeling technique the neuronal morphology of two transgenic mouse lines (J20 and APP/PS1) expressing mutant forms of the Amyloid Precursor Protein (APP), at various ages, have been visualized and compared to Wild Type (WT) littermate controls. Swollen bulbous dystrophic neurites with loss of spines were apparent in the transgenic animals. Upon quantification, statistically significant reductions in the number of spines and total dendrite area was observed in both transgenic mouse lines at 11 months of age. Similar morphological abnormalities were seen in human AD hippocampal tissue both qualitatively and quantitatively. Immunohistochemistry and DiOlistic labeling was combined so that Abeta plaques were imaged in relation to the dendritic trees. No preferential localization of these abnormal dystrophic neurites was seen in regions with plaques. DiI labeled reative astrocytes were often apparent in close proximity to A beta plaques.     

Impaired conditioned taste aversion learning in APP transgenic mice

Cognition in transgenic mouse models of Alzheimer's disease (AD) has been predominantly characterized in explicit spatial orientation tasks. However, dementia in AD encompasses also implicit memory systems. In the present study a line of transgenic mice (TgCRND8) encoding a double mutated allele of the human amyloid precursor protein (APP) genes was evaluated in an implicit associative learning task of conditioned taste aversion (CTA). CTA is a form of Pavlovian classical conditioning, in which a mouse learns to avoid a novel taste of saccharine (conditioned stimulus) paired with an experimentally induced (systemic injection of lithium chloride) nausea (unconditioned stimulus). In contrast to conditioned non-Tg mice, TgCRND8 APP mice developed weaker aversion against saccharine and quickly increased its consumption in repeated tests. These results indicate that TgCRND8 mice show a significant impairment not only in explicit spatial memory, as has been previously shown [Nature 408 (2000) 979], but also in implicit memory. Control experiments confirmed that TgCRND8 and non-Tg mice had comparable taste sensitivities in response to appetitive as well as aversive tastes. The study suggests that the CTA paradigm can be a sensitive tool to evaluate deficits in implicit associative learning in APP transgenic mouse models of AD.     

Amyloid cored plaques in Tg2576 transgenic mice are characterized by giant plaques,slightly activated microglia,and the lack of paired helical filament-typed,dystrophic neurites

We examined the brains of Tg2576 transgenic mice carrying human amyloid precursor protein with the Swedish mutation and Alzheimer's disease (AD) by means of immunohistochemistry and electron microscopy to clarify the characteristics of amyloid-associated pathology in the transgenic mice. In 12- to 29-month-old Tg2576 mice, congophilic cored plaques in the neocortex and hippocampus were labeled by all of the Abeta1-, Abeta40- and 42-specific antibodies, as seen in the classical plaques in AD. However, large-sized (>50 micro m in core diameter) plaques were seen more frequently in the older mice (18-29 months) than in those with AD (approximately 20% vs 2% in total cored plaques), and Tg2576 mice contained giant plaques (>75 micro m in core diameter), which were almost never seen in the brain of those with AD. Neither thread-like structures nor peripheral coronas were observed in the cored plaques of the transgenic mice in the silver impregnations. Immunohistochemically, plaque-accompanied microglia showed a slight enlargement of the cytoplasm with consistent labeling of Mac-1 and macrosialin (murine CD68), and with partial labeling of Ia antigen and macrophage-colony stimulating factor receptor. Ultrastructurally, the microglia surrounding the extracellular amyloid fibrils in the large, cored plaques showed some organella with phagocytic activity, such as secondary lysosomal, dense bodies, but intracellular amyloid fibrils were not evident. Dystrophic neurites in the plaques of the transgenic mice contained many dense multilaminar bodies, but no paired helical filaments. Our results suggest that giant cored plaques without coronas or paired helical filament-typed, dystrophic neurites are characteristic in Tg2576 mice, and that plaque-associated microglia in transgenic mice are activated to be in phagocytic function but not sufficient enough to digest extracellularly deposited amyloid fibrils.     

P3 beta-amyloid peptide has a unique and potentially pathogenic immunohistochemical profile in Alzheimer's disease brain.

The presence of beta-amyloid in brain tissue is characteristic of Alzheimer''s disease (AD). A naturally occurring derivative of the beta-amyloid peptide, p3, possesses all of the structural determinants required for fibril assembly and neurotoxicity. p3-specific antibodies were used to examine the distribution of this peptide in brain. p3 reactivity was absent or sparse in aged non-AD brains but was prevalent in selected areas of AD brain in diffuse deposits and in a subset of dystrophic neurites. p3-reactive dystrophic neurites were found both independent in the neuropil and associated with plaques. Little or no reactivity was observed to amyloid cores in classical plaques or to amyloid in the cerebral vasculature. The exclusive appearance of p3 reactivity in AD brain plus the selective localization of p3 reactivity to abnormal structures in the temporal lobe limbic system suggests that p3 may be a contributing factor to AD pathology.     

The amyloid pathology progresses in a neurotransmitter-specific manner

Past studies using transgenic models of early-staged amyloid pathology, have suggested that the amyloid pathology progresses in a neurotransmitter-specific manner where cholinergic terminals appear most vulnerable, followed by glutamatergic terminals and finally by somewhat more resistant GABAergic terminals. To determine whether this neurotransmitter-specific progression persists at later pathological stages, presynaptic bouton densities, and the areas of occupation and localization of plaque adjacent dystrophic neurites were quantified in 18-month-old APP(K670N, M671L)+PS1(M146L) doubly transgenic mice. Quantification revealed that transgenic animals had significantly lower cholinergic, glutamatergic and GABAergic presynaptic bouton densities. Cholinergic and glutamatergic dystrophic neurites appear to be heavily influenced by fibrillar Abeta as both types displayed a decreasing area of occupation with respect to increasing plaque size. Furthermore, cholinergic dystrophic neurites reside in closer proximity to plaques than glutamatergic dystrophic neurites, while GABAergic dystrophic neurites were minimal regardless of plaque size. To investigate whether similarities exist in the human AD pathology, a monoclonal antibody (McKA1) against the human vesicular glutamate transporter 1 (VGluT1) was developed. Subsequent staining in AD brain tissue revealed the novel presence of glutamatergic dystrophic neurites, to our knowledge the first evidence of a structural glutamatergic deficit in the AD pathology.     

Use of YFP to study amyloid-beta associated neurite alterations in live brain slices

Neuritic plaques are one of the defining neuropathological features of Alzheimer’s disease (AD). These structures are composed of a buildup of fibrils of the amyloid-β (Aβ) peptide (amyloid) surrounded by activated glial cells and degenerating nerve processes (dystrophic neurites). To study neuritic plaques and possible abnormalities associated with dendrites, axons, and synaptic structures, we have developed an acute slice preparation model using PDAPP, yellow fluorescent protein (YFP) double transgenic mice (a mouse model with AD-like pathology that stably expresses YFP in a subset of neurons in the brain). With laser scanning confocal microscopy, we have imaged living brain slices from PDAPP, YFP double transgenic mice as old as 20 months and have been able to visualize axons, dendrites, dendritic spines, and dystrophic neurites for many hours. Our initial studies suggest that dystrophic axons and dendrites within neuritic plaques are fairly stable structures in the absence of exogenous perturbations. This acute slice preparation model should prove to be a useful tool to explore the pathophysiology of Aβ-related axonal, dendritic, and synaptic dysfunction.     

Increased seizure threshold and severity in young transgenic CRND8 mice

Reports suggest that Alzheimer's disease (AD) patients show a high life-time prevalence of seizure-like disorders. The transgenic CRND8 (TgCRDN8) is a mouse model of AD-like amyloid pathogenesis that expresses a double-mutant form of human amyloid precursor protein 695 (K670N/M671L and V717F). We have previously reported that post-plaque TgCRND8 mice exhibited a lower threshold to seizure with a more severe seizure type when challenged with pentylenetetrazole (PTZ) intravenously. Here, we now report that pre-plaque TgCRND8 mice also demonstrate an increased sensitivity to PTZ-induced seizures with a more severe seizure type over age-matched littermate controls. A lower threshold and more severe seizure type in TgCRND8 mice prior to and after plaque deposition suggest that this genotype difference may be due to beta-amyloid (Abeta) toxicity rather than plaque formation. Thus, the TgCRND8 mice are not only a model for Abeta production and plaque deposition, but may also be useful for AD associated seizure.     

Metallothionein-I and -III expression in animal models of Alzheimer disease

Previous studies have described altered expression of metallothioneins (MTs) in neurodegenerative diseases like multiple sclerosis (MS), Down syndrome, and Alzheimer's disease (AD). In order to gain insight into the possible role of MTs in neurodegenerative processes and especially in human diseases, the use of animal models is a valuable tool. Several transgenic mouse models of AD amyloid deposits are currently available. These models express human beta-amyloid precursor protein (AbetaPP) carrying different mutations that subsequently result in a varied pattern of beta-amyloid (Abeta) deposition within the brain. We have evaluated the expression of MT-I and MT-III mRNA by in situ hybridization in three different transgenic mice models of AD: Tg2576 (carrying AbetaPP harboring the Swedish K670N/M671L mutations), TgCRND8 (Swedish and the Indiana V717F mutations), and Tg-SwDI (Swedish and Dutch/Iowa E693Q/D694N mutations). MT-I mRNA levels were induced in all transgenic lines studied, although the pattern of induction differed between the models. In the Tg2576 mice MT-I was weakly upregulated in cells surrounding Congo Red-positive plaques in the cortex and hippocampus. A more potent induction of MT-I was observed in the cortex and hippocampus of the TgCRND8 mice, likely reflecting their higher amyloid plaques content. MT-I upregulation was also more significant in Tg-SwDI mice, especially in the subiculum and hippocampus CA1 area. Immunofluorescence stainings demonstrate that astrocytes and microglia/macrophages surrounding the plaques express MT-I&II. In general, MT-I regulation follows a similar but less potent response than glial fibrillary acidic protein (GFAP) expression. In contrast to MT-I, MT-III mRNA expression was not significantly altered in any of the models examined suggesting that the various MT isoforms may have different roles in these experimental systems, and perhaps also in human AD.     

Activity changes and marked stereotypic behavior precede Abeta pathology in TgCRND8 Alzheimer mice

Alzheimer's disease (AD) is not only characterized by cognitive decline and neuropathological changes, but also by non-cognitive behavioral symptoms like restlessness, sleep disturbance, and wandering. These symptoms are categorized in the "Behavioral and Psychological Symptoms of Dementia" (BPSD). We investigated transgenic and wildtype mice of an APP transgenic mouse model of AD (TgCRND8) with respect to 24 h activity and spontaneous home cage behavior at 30, 60, 90 and 120 days of age. At all test days, transgenic and wildtype animals differed significantly with respect to activity patterns. In addition, activity rhythms changed distinctly in transgenic mice with increasing age. Transgenic mice also clearly showed more stereotypic behavior, which correlated significantly at 90 and 120 days of age with elevated corticosterone metabolite concentrations in fecal samples. Activity patterns in TgCRND8 mice resemble altered rhythms of activity in AD patients. Stereotypic behaviors may be caused by the same mechanisms as non-cognitive behavioral symptoms of AD. Thus, it is likely that analogies to BPSD that precede Abeta pathology are found in APP-overexpressing TgCRND8 mice.     

Brain Abeta amyloidosis in APPsw mice induces accumulation of presenilin-1 and tau

APPsw transgenic mice (Tg2576) overproducing mutant amyloid beta protein precursor (betaAPP) show substantial brain Abeta amyloidosis and behavioural abnormalities. To clarify the subsequent abnormalities, the disappearance of neurons and synapses and dystrophic neurite formation with accumulated proteins including hyperphosphorylated tau were examined. Tg2576 demonstrated substantial giant core plaques and diffuse plaques. The number of neurons was significantly decreased in the areas containing the amyloid cores compared with all other areas and corresponding areas in non-transgenic littermates in sections visualized by Nissl plus Congo red double staining (p<0.001). The presynaptic protein alpha-synuclein and postsynaptic protein drebrin were also absent in the amyloid cores. betaAPP and presenilin-1 were accumulated in dystrophic neurites in and around the core plaques. Tau phosphorylated at five independent sites was detected in the dystrophic neurites in the amyloid cores. Thus, the giant core plaques replaced normal brain tissues and were associated with subsequent pathological features such as dystrophic neurites and the appearance of hyperphosphorylated tau. These findings suggest a potential role for brain Abeta amyloidosis in the induction of secondary pathological steps leading to mental disturbance in Alzheimer's disease.     

Tangle-associated neuritic clusters. A new lesion in Alzheimer's disease and aging suggests that aggregates of dystrophic neurites are not necessarily associated with beta/A4.

Abnormal (dystrophic) neurites are widespread in the brains of patients with Alzheimer's disease (AD). Subsets of these neurites cluster in intimate association with amyloid deposits, constituting classic senile plaques. Two major markers expressed by many plaque-associated neurites are the microtubule associated protein tau and chromogranin A, a soluble protein of large dense core synaptic vesicles. The authors show a new type of lesion, tangle-associated neuritic clusters (TANCs), in which abnormal neurites form dense aggregates, each centered by an extracellular (ghost) neurofibrillary tangle, rather than an amyloid deposit. Neurites in TANCs are similar to plaque neurites in shape and expression of tau and chromogranin A, and different from a second, nonaggregating subset of dystrophic neurites in AD, neuropil threads. TANCs are abundant in the hippocampus of all patients with AD; a few are found in some aged nondemented people, and in the nucleus basalis of Meynert and occasionally the neocortex of AD patients. Ultrastructurally, the core of a TANC is made up of extracellular bundles of straight filaments. This core is not recognized by antibodies to native or synthetic beta A4 peptide, the major protein of plaque amyloid, thus showing that not all neuritic clusters in AD are associated with this peptide.     

The sterol regulatory element‐binding protein 2 is dysregulated by tau alterations in Alzheimer disease

Disturbed neuronal cholesterol homeostasis has been observed in Alzheimer disease (AD) and contributes to the pathogenesis of AD. As the master switch of cholesterol biosynthesis, the sterol regulatory element‐binding protein 2 (SREBP‐2) translocates to the nucleus after cleavage/activation, but its expression and activation have not been studied in AD which is the focus of the current study. We found both a significant decrease in the nuclear translocation of N‐terminal SREBP‐2 accompanied by a significant accumulation of C‐terminal SREBP‐2 in NFT‐containing pyramidal neurons in AD. N‐terminal‐ SREBP‐2 is also found in dystrophic neurites around plaques in AD brain. Western blot confirmed a significantly reduced nuclear translocation of mature SREBP‐2 (mSREBP‐2) in AD brain. Interestingly, reduced nuclear mSREBP‐2 was only found in animal models of tauopathies such as 3XTg AD mice and P301L Tau Tg mice but not in CRND8 APP transgenic mice, suggesting that tau alterations likely are involved in the changes of mSREBP‐2 distribution and activation in AD. Altogether, our study demonstrated disturbed SREBP‐2 signaling in AD and related models, and proved for the first time that tau alterations contribute to disturbed cholesterol homeostasis in AD likely through modulation of nuclear mSREBP‐2 translocation.     

Progress toward valid transgenic mouse models for Alzheimer's disease.

A transgenic mouse model for Alzheimer's disease (AD) should mimic the age-dependent accumulation of beta-amyloid plaques, neurofibrillary tangles, neuronal cell death as well as display memory loss and behavioral deficits. Age-dependent accumulation of A beta deposits in mouse brain has been achieved in mice overexpressing mutant alleles of the amyloid precursor protein (APP). In contrast, mice bearing mutant alleles of the presenilin genes show increased production of the A beta42 peptide, but do not form amyloid deposits unless mutant alleles of APP are also overproduced. Furthermore, the onset of A beta deposition is greatly accelerated, paralleling the involvement of presenilins in early onset AD. Studies of APP and presenilin transgenic mice have shown 1) the absence of a requirement for a maturation step in dense core plaque formation, 2) evidence that beta-amyloid deposition is directed by regional factors, and 3) behavioral deficits are observed before A beta deposition. Crosses of APP transgenic mice with mice modified for known AD risk factors and "humanizing" the mouse may be necessary for complete replication of AD.     

The axonal origin of a subpopulation of dystrophic neurites in Alzheimer's disease

Dystrophic neurites are observed characteristically in Alzheimer's disease (AD). They are thought to arise from sprouting dendrites and contribute to dementia because of their abnormal growth and close association with neurofibrillary tangles and neuritic plaques. In the present study, dystrophic neurites are demonstrated in the thalamic reticular nucleus in AD in the context of a normal neural and glial architecture. They do not collocalize with somata and dendrites identified by simultaneous labeling with the microtubule-associated protein MAP2, suggesting that they are derived from axons. Throughout the brain, dystrophic neurites may well be comprised of a heterogeneous population of both dendrites and axon terminals and preterminals. While many recent studies have focused upon the dendritic origin of dystrophic neurites, these results emphasize that the interconnectivity of certain brain regions may be compromised by cytoskeletal changes occurring in neurons and their axons in AD.     

Alzheimer's amyloid precursor protein-positive degenerative neurites exist even within kuru plaques not specific to Alzheimer's disease.

To clarify the relationship between amyloid formation and amyloid precursor protein (APP), the brain sections from eight patients with Alzheimer''s disease (AD) and four with Gerstmann-Sträussler Syndrome (GSS) were investigated immunohistochemically by the double-immunostaining method. In AD, most APP-positive senile plaques belong to classical plaques or primitive plaques, whereas in diffuse plaques, APP-positive neuritic components are rarely observed. The authors documented that anti-APP-labeled degenerative neurites surrounding kuru plaques in all four GSS patients. These kuru plaques were verified by double immunostaining using anti-prion protein and anti-APP. The APP-positive structures in kuru plaques were almost identical with those seen in the classical plaques in AD. The authors concluded that APP-positive degenerative neurites are not an early event in the amyloid formation of senile plaques. It is therefore postulated that depositions of beta/A4 and prion proteins are primary events that may involve the surrounding microenvironment and result in the secondary formation of APP-positive degenerative neurites, not specific to AD.     

Postmortem studies of the effect of anti-inflammatory drugs on Alzheimer-type pathology and associated inflammation

Examining postmortem tissue is the most direct way of evaluating the effect of antemortem drug use on the pathological processes believed to be important in Alzheimer’s disease (AD). A small number of studies have recently been published in which data from human autopsy tissue and animal models provides important insight into the mechanisms by which anti-inflammatory (AI) agents may protect against AD. These indicate that certain classes of AI drugs may be capable of reducing the chronic inflammation which is consistently seen in AD brain tissue. In addition, a recent study using a transgenic mouse model of AD, suggests that AI therapy may also influence the accumulation of senile plaques and dystrophic neurites. The results of these and future postmortem studies will be invaluable in the development of optimum treatment strategies.     

Antibodies to non-beta regions of the beta-amyloid precursor protein detect a subset of senile plaques.

A central unresolved issue in Alzheimer's disease is the origin of the extracellular amyloid beta protein (A beta P) found in senile plaques and its relationship to the dystrophic neurites that intimately surround it. Here the presence and distribution within senile plaques of various epitopes of the beta-amyloid precursor protein (APP) are compared with the distribution of A beta P itself and markers for plaque neurites. Several principal findings emerge: 1) antibodies to regions of APP outside of A beta P ('APP antibodies') recognize only a subgroup of senile plaques; 2) within these plaques, APP antibodies label discrete globular and granular structures morphologically resembling neurites; 3) virtually all of the plaques labeled by APP antibodies also contain neurites reactive with antibodies to tau; 4) double labeling with anti-tau and an APP antibody shows that the neuritelike profiles stained by the APP antibody are always closely associated with tau-positive neurites within the same plaque and that a minority of profiles appear to be labeled by both antibodies; and 5) antibodies to different regions throughout APP label the same profiles within plaques, suggesting the presence of the full-length precursor. The authors conclude that only a subgroup of senile plaques contain APP epitopes and that the immunostained structures are neurites. Because many A beta P-containing plaques in neocortex, cerebellum, and striatum were found to be devoid of any APP labeling, as were vascular A beta P deposits, it is unlikely that the extracellular A beta P is principally derived from the APP found within dystrophic neurites. The immunodetection of apparently full-length APP, an axonally transported protein, in selected plaque neurites provides yet another protein marker of neuritic dystrophy, possibly indicative of an aberrant regenerative response.     

Mapping genetic modulators of amyloid plaque deposition in TgCRND8 transgenic mice

Alzheimer's disease (AD) is a complex disorder for which various in vivo models exist. The TgCRND8 mouse, transgenic for the human amyloid precursor protein, is an aggressive early onset model of brain amyloid deposition. Preliminary studies revealed that when the transgene is expressed on an A/J genetic background, these mice not only survive longer but also deposit less parenchymal amyloid-beta (Abeta) peptides as compared to those on a C57BL/6 background. We performed a genome-wide study of an F2 intercross between TgCRND8 on an A/J background and C57BL/6 mice, to identify genetic modulators of amyloid accumulation and deposition. We identified four highly significant QTLs that together account for 55% of the phenotypic variance in the number of plaques (Thioflavin S). QTLs were found on the distal part of chromosome 4 with an LOD score of 8.1 at D4Mit251, on chromosome 11 with an LOD score of 5.5 at D11Mit242, on chromosome 9 with an LOD score of 5.0 at D9Mit336 and on the proximal part of chromosome 8 with an LOD score of 4.5 at D8Mit223. A/J alleles at these loci are protective and all decreased the amount of Abeta deposition. Interestingly, the QTL on chromosome 11 is also significantly linked to the levels of brain Abeta(42) and Abeta(40). Although these QTLs do not control the levels of plasmatic Abeta, other regions on chromosomes 1 and 6 show significant linkage. Further characterization of these QTL regions may lead to the identification of genes involved in the pathogenesis of AD.