Familial Alzheimer's Disease Cases Research Articles

Lewy Body Pathology in Alzheimer's Disease

Lewy bodies, the characteristic pathological lesion of substantia nigra neurons in Parkinson's disease (PD), are frequently observed to accompany the amyloid plaque and neurofibrillary tangle pathology of Alzheimer's disease (AD). However the typical anatomic distribution of Lewy bodies in AD is distinct from PD. The most common site of occurrence is the amygdala, where Lewy bodies are observed in approximately 60% of both sporadic and familial AD. Other common sites of occurrence include the periamygdaloid and entorhinal cortex, while neocortical and brainstem areas develop Lewy bodies in a lower percentage of cases. In contrast, dementia with Lewy bodies (DLB), defined by widespread neocortical and brainstem Lewy bodies but frequently accompanied by variable levels of AD-type pathology, represents the other end of a spectrum of pathology associated with dementia. The observation of Lewy bodies in familial AD cases suggests that like neurofibrillary tangles, the formation of Lewy bodies can be induced by the pathological state caused by Abeta-amyloid overproduction. The role of Lewy body formation in the dysfunction and degeneration of neurons remains unclear. The protein alpha-synuclein appears to be an important structural component of Lewy bodies, an observation spurred by the discovery of point mutations in the alpha-synuclein gene linked to rare cases of autosomal dominant PD. Further investigation of alpha-synuclein and its relationship to pathological conditions promoting Lewy body formation in AD, PD, and DLB may yield further insight into pathogenesis of these diseases.

Presenilin function in APP processing.

Familial Alzheimer's disease (FAD) is now linked to at least three genes encoding the amyloid precursor protein (APP) on chromosome 21, and presenilin 1 and 2 on chromosome 14 and 1, respectively. FAD cases in whom presenilin mutations occur are more frequent than those with APP mutations. However, altogether they only account for approximately 0.1% of all the people suffering from Alzheimer's disease (AD), and the causes of the remaining 99.9% of the sporadic form of AD or senile dementia remain unknown. Since FAD presents with the same neuropathological features as sporadic AD, i.e., cognitive impairments and the amyloid plaques and tangles in the brain, our working hypothesis is that similar molecular pathogenic mechanisms underly both sporadic and familial AD. It follows that APP and the presenilins must be key players in the disease. Detailed knowledge about the cell biology of these proteins will be a rich source of insight into the pathology of AD, but will also shed light on the fundamental neurobiology of these proteins.

Brain Expression of Presenilins in Sporadic and Early-onset, Familial Alzheimer’s Disease

Mutations in the presenilin proteins cause early-onset, familial Alzheimer's disease (FAD). We characterized the cellular localization and endoproteolysis of presenilin 2 (PS2) and presenilin 1 (PS1) in brains from 25 individuals with presenilin-mutations causing FAD, as well as neurologically normal individuals and individuals with sporadic Alzheimer's disease (AD). Amino-terminal antibodies to both presenilins predominantly decorated large neurons. Regional differences between the broad distributions of the two presenilins were greatest in the cerebellum, where most Purkinje cells showed high levels of only PS2 immunoreactivity. PS2 endoproteolysis in brain yielded multiple amino-terminal fragments similar in size to the PS1 amino-terminal fragments detected in brain. In addition, two different PS2 amino-terminal antibodies also detected a prominent 42 kDa band that may represent a novel PS2 form in human brain. Similar to PS1 findings, neither amino-terminal nor antiloop PS2 antibodies revealed substantial full-length PS2 in brain. Immunocytochemical examination of brains from individuals with the N141I PS2 mutation or eight different PS1 mutations, spanning the molecule from the second transmembrane domain to the large cytoplasmic loop domain, revealed immunodecoration of no senile plaques and only neurofibrillary tangles in the M139I PS1 mutation stained with PS1 antibodies. Overall presenilin expression and the relative abundance of full-length and amino-terminal fragments in presenilin FAD cases were similar to control cases and sporadic AD cases. Thus, accumulation of full-length protein or other gross mismetabolism of neither PS2 nor PS1 is a consequence of the FAD mutations examined.

Platelets as a peripheral district where to study pathogenetic mechanisms of Alzheimer disease: the case of amyloid precursor protein

Alzheimer disease is a progressive neurodegenerative disease, characterised by a progressive cognitive and memory decline. From a neuropathological point of view, Alzheimer disease is defined by the presence of characteristic lesions, i.e. mature senile plaques, neurofibrillary tangles (NFTs) and amyloid angiopathy. In particular, accumulation of the amyloid β-peptide in the brain parenchyma and vasculature is an invariant event in the pathogenesis of both sporadic and familial Alzheimer cases. Amyloid β-peptide originates from a larger precursor, the amyloid precursor protein (APP) ubiquitously expressed. Among the different peripheral cells expressing APP forms, platelets are particularly interesting since they show concentrations of its isoforms equivalent to those found in brain. Moreover, a number of laboratories independently described alterations in APP metabolism/concentration in platelets of Alzheimer patients when compared to control subjects matched for demographic characteristics. These observations defined the frame of our work aimed to investigate if a correlation between levels of platelet APP forms and Alzheimer disease could be detected. We have reported that patients affected by Alzheimer disease show a differential level of platelet APP forms. This observation has several implications: APP processing abnormalities, believed to be a very early change in Alzheimer disease in neuronal compartment, do occur in extraneuronal tissues, such as platelets, thus, suggesting that Alzheimer disease is a systemic disorder; further, our data strongly indicate that a differential level of platelet APP isoforms can be considered a potential peripheral marker of Alzheimer disease allowing for discrimination between Alzheimer and other types of dementia.

Genes and mechanisms involved in beta-amyloid generation and Alzheimer's disease.

Alzheimer's disease is characterized by the invariable accumulation of senile plaques that are predominantly composed of amyloid beta-peptide (Abeta). Abeta is generated by proteolytic processing of the beta-amyloid precursor protein (betaAPP) involving the combined action of beta- and gamma-secretase. Cleavage within the Abeta domain by alpha-secretase prevents Abeta generation. In some very rare cases of familial AD (FAD), mutations have been identified within the betaAPP gene. These mutations are located close to or at the cleavage sites of the secretases and pathologically effect betaAPP processing by increasing Abeta production, specifically its highly amyloidogenic 42 amino acid variant (Abeta42). Most of the mutations associated with FAD have been identified in the two presenilin (PS) genes, particularly the PS1 gene. Like the mutations identified within the betaAPP gene, mutations in PS1 and PS2 cause the increased generation of Abeta42. PS1 has been shown to be functionally involved in Notch signaling, a key process in cellular differentation, and in betaAPP processing. A gene knock out of PS1 in mice leads to an embryonic lethal phenotype similar to that of mice lacking Notch. In addition, absence of PS1 results in reduced gamma-secretase cleavage and leads to an accumulation of betaAPP C-terminal fragments and decreased amounts of Abeta. Recent work may suggest that PS1 could be the gamma-secretase itself, exhibiting the properties of a novel aspartyl protease. Mutagenesis of either of two highly conserved intramembraneous aspartate residues of PS1 leads to reduced Abeta production as observed in the PS1 knockout. A corresponding mutation in PS2 interfered with betaAPP processing and Notch signaling suggesting a functional redundancy of both presenilins. In this issue, some of the recent work on the molecular mechanisms involved in Alzheimer's disease (AD) as well as novel diagnostic approaches and risk factors for AD will be discussed. In the first article, we like to give an overview on mechanisms involved in the proteolytic generation of Amyloid beta-peptide (Abeta), the major pathological player of this devastating disease. In the second part of this article recent results will be described, which demonstrate an unexpected biological and pathological function of an AD associated gene.

Alzheimer's disease presenilin-1 exon 9 deletion and L250S mutations sensitiZe SH-SY5Y neuroblastoma cells to hyperosmotic stress-induced apoptosis

Mutations in the presenilin-1 (PS1) and presenilin-2 (PS2) genes account for the majority of early-onset familial Alzheimer's disease cases. Recent studies suggest that presenilin gene mutations predispose cells to apoptosis by mechanisms involving altered calcium homeostasis and oxidative damage. In the present study, we determined whether PS1 mutations also sensitize cells to hyperosmotic stress-induced apoptosis. For this, we established SH-SY5Y neuroblastoma cell lines stably transfected with wild-type PS1 or either the PS1 exon 9 deletion (ΔE9) or PS1 L250S mutants. Cultured cells were exposed to an overnight (17 h) serum deprivation, followed by a 30 min treatment with either 20 mM glucose, 10 nM insulin-like growth factor-1 or 20 mM glucose +10 nM insulin-like growth factor-1. Cells were then cultured for a further 3, 6 or 24 h and stained for apoptotic condensed nuclei using propidium iodide. Confirmation that cells were undergoing an active apoptotic process was achieved by labelling of DNA strand breaks using the terminal dUTP nick end labelling (TUNEL) technique. We also determined cell viability using 3-(4,5-dimethyl-thiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) reduction. Propidium iodide staining revealed that all cell lines and controls showed an increased number of apoptotic cells appearing with condensed nuclei at 24 h compared with 6 h and 3 h. High glucose-induced hyperosmotic stress resulted in significantly more apoptotic cells in the PS1 ΔE9 and PS1 L250S mutation cell lines at 24 h, compared with the wild-type PS1 lines ( P<0.001, ANOVA for both comparisons). Mean values (±S.D.) for the percentage number of apoptotic cells at 24 h following high glucose treatment were 16.1±3.5%, 26.7±5.5% and 31.0±5.7% for the wild-type PS1, PS1 ΔE9 and PS1 L250S lines, respectively. The pro-apoptotic effects of high glucose treatment were reversed by 10 nM insulin-like growth factor-1, although to a lesser extent in the mutation cell lines (5.8±2.4%, 15.2±7.3% and 13.2±2.0% for the wild-type PS1, PS1 ΔE9 ( P<0.01 for comparison with wild-type PS1) and PS1 L250S ( P<0.01 for comparison with wild-type PS1) transfected lines, respectively. TUNEL labelling of cells at 24 h following treatment gave essentially the same results pattern as obtained using propidium iodide. The percentage number of apoptotic cells with DNA strand breaks (means±S.D.) following high glucose treatment was 15.4±2.6% for the wild-type PS1, 26.8±3.2% for the PS1 ΔE9 ( P<0.001 for comparison with wild-type PS1) and 29.7±6.1% for the PS1 L250S transfected lines ( P<0.001 for comparison with wild-type PS1). The PS1 ΔE9 and PS1 L250S transfected lines also showed a higher number of apoptotic cells with DNA strand breaks at 24 h following high glucose plus insulin-like growth factor-1 treatment (11.4±2.0% and 14.3±2.8%, respectively), compared with values for the wild-type PS1 lines (8.5±2.4%). These differences were significant ( P<0.01) for the comparision of wild-type PS1 and PS1 L250S, but not PS1 ΔE9 lines. The mutation-related increases in number of apoptotic cells at 24 h following high glucose treatment were not accompanied by significant differences in cell viability at this time-point. Our results indicate that PS1 mutations predispose to hyperosmotic stress-induced apoptosis and that the anti-apoptotic effects of insulin-like growth factor-1 are compromised by these mutations. Perturbations of insulin-like growth factor-1 signalling may be involved in PS1 mutation-related apoptotic neuronal cell death in Alzheimer's disease.

The impact of different presenilin 1 andpresenilin 2 mutations on amyloid deposition, neurofibrillary changes and neuronal loss in the familial Alzheimer's disease brain: evidence for other phenotype-modifying factors.

To assess the influence of the presenilin 1 (PS1) and 2 (PS2) mutations on amyloid deposition, neurofibrillary tangle (NFT) formation and neuronal loss, we performed stereologically based counts in a high-order association cortex, the superior temporal sulcus, of 30 familial Alzheimer's disease cases carrying 10 different PS1 and PS2 mutations, 51 sporadic Alzheimer's disease cases and 33 non-demented control subjects. All the PS1 and PS2 mutations assessed in this series led to enhanced deposition of total Abeta and Abeta(x-42/43) but not Abeta(x-40) senile plaques in the superior temporal sulcus when compared with brains from sporadic Alzheimer's disease patients. Some of the PS1 mutations studied (M139V, I143F, G209V, R269H, E280A), but not others, were also associated with faster rates of NFT formation and accelerated neuronal loss in the majority of the patients who harboured them when compared with sporadic Alzheimer's disease patients. In addition, our analysis showed that dramatic quantitative differences in clinical and neuropathological features can exist even among family members with the identical PS mutation. This suggests that further individual or pedigree genetic or epigenetic factors are likely to modulate PS phenotypes strongly.

Expression of Presenilin 1 in nervous system during rat development

Alzheimer's disease (AD) is a polygenic disorder involving at least four different genes. Among them, missense mutations in the presenilins segregate with the vast majority of early onset cases of familial AD. To elucidate possible function(s) of presenilin 1 (PS1), we have studied its expression during the development of the rat nervous system. Analysis by in situ hybridization showed expression of PS1 in a variety of cell types and tissues during development, with prominent expression in the nervous system. During late embryogenesis, the ventricular zone presented the highest levels of expression, paralleling the pattern previously reported for Notch. Later, during postnatal development, we observed a peak of PS1 expression at postnatal day 10, particularly in the cerebellum and hippocampus, a time when proliferation and migration are still ongoing and synapse formation is being completed. We propose that presenilins participate in at least two different developmental processes: (1) one involved in neurogenesis and skeleton formation during embryonic development, probably involving coordinate expression with Notch, and (2) a second one in the postnatal central nervous system, perhaps involved in neuritogenic and/or synaptogenic stages, most likely playing a role in amyloid precursor protein processing and amyloid beta production.

Differential effects of transforming growth factor-βs and glial cell line-derived neurotrophic factor on gene expression of presenilin-1 in human post-mitotic neurons and astrocytes

Mutations in the presenilin-1 gene are linked to the majority of early-onset familial Alzheimer's disease cases. We have previously shown that the expression of transforming growth factor-β is altered in Alzheimer's patients, compared to controls. Here we examine presenilin-1 expression in human post-mitotic neurons (hNT cells), normal human astrocytes, and human brain tumor cell lines following treatment with three isoforms of transforming growth factor-β, or glial cell line-derived neurotrophic factor, a member of the transforming growth factor-β superfamily. As the NT2/D1 teratocarcinoma cell line is treated with retinoic acid to induce differentiation to hNT cells, presenilin-1 messenger RNA expression is dramatically increased. Furthermore, there is a 2–3-fold increase in presenilin-1 messenger RNA expression following treatment of hNT cells with growth factors and similar results are found by Western blotting and with immunohistochemical staining for presenilin-1 protein. However, treatment of normal human astrocytes with cytokines results in minimal changes in presenilin-1 messenger RNA and protein. Interestingly, the expression of presenilin-1 in human U87 MG astrocytoma and human SK-N-SH neuroblastoma cells is only increased when cells are treated with glial cell line-derived neurotrophic factor or transforming growth factor-β3. These findings suggest that endogenous presenilin-1 gene expression in human neurons can be induced by growth factors present in normal and diseased brain tissue. Cytokines may play a major role in regulating expression of presenilin-1 which may affect its biological actions in physiological and pathological conditions.

Presenilin-2 mutation and polymorphism in Japanese Alzheimer disease patients

The Asn141Ile mutation of the presenilin 2 gene is responsible for familial early-onset Alzheimer disease found in Volga-German kindreds. However, the genetic influence of presenilin 2 gene on sporadic Alzheimer disease remains unknown. In this study, the frequency of the mutation and genetic association with the presenilin 2 locus were investigated in Japanese sporadic cases. The Asn141Ile mutation was not found in 88 cases of sporadic Alzheimer disease or 13 unrelated cases of familial Alzheimer disease. Fifty cases of late onset sporadic Alzheimer disease and 50 age-matched controls indicated no association with an exon 3 polymorphism of the presenilin 2 gene. These results indicate that the presenilin 2 mutation is not a major cause of Alzheimer disease.

Alzheimer's presenilin-1 mutation potentiates inositol 1,4,5-trisphosphate-mediated calcium signaling in Xenopus oocytes.

Perturbations in intracellular Ca2+ signaling may represent one mechanism underlying Alzheimer's disease (AD). The presenilin-1 gene (PS1), associated with the majority of early onset familial AD cases, has been implicated in this signaling pathway. Here we used the Xenopus oocyte expression system to investigate in greater detail the role of PS1 in intracellular Ca2+ signaling pathways. Treatment of cells expressing wild-type PS1 with a cell surface receptor agonist to stimulate the phosphoinositide second messenger pathway evoked Ca2+-activated Cl- currents that were significantly potentiated relative to controls. To determine which elements of the signal transduction pathway are responsible for the potentiation, we used photolysis of caged inositol 1,4,5-trisphosphate (IP3) and fluorescent Ca2+ imaging to demonstrate that PS1 potentiates IP3-mediated release of Ca2+ from internal stores. We show that an AD-linked mutation produces a potentiation in Ca2+ signaling that is significantly greater than that observed for wild-type PS1 and that cannot be attributed to differences in protein expression levels. Our findings support a role for PS1 in modulating IP3-mediated Ca2+ liberation and suggest that one pathophysiological mechanism by which PS1 mutations contribute to AD neurodegeneration may involve perturbations of this function.

Age-related cognitive deficits, impaired long-term potentiation and reduction in synaptic marker density in mice lacking the β-amyloid precursor protein

Mutations in the β-amyloid precursor protein are strongly associated with some cases of familial Alzheimer's disease. The normal physiological role of β-amyloid precursor protein in the brain was evaluated in a longitudinal analysis of mice deficient in β-amyloid precursor protein. Compared with wild-type control mice the β-amyloid precursor protein-null mice develop age-dependent deficits in cognitive function and also have impairments in long-term potentiation. In addition, the brains of the β-amyloid precursor protein-null mice have marked reactive gliosis in many areas especially in the cortex and hippocampus. A sub-population of mice ( n=15) died prematurely (between three and 18 months-of-age). Analysis of another six mice from the same population that were showing weight loss and hypolocomotor activity exhibited a marked reactive gliosis as detected by immunoreactivity for glial fibrillary acidic protein and a profound loss of immunoreactivities for the presynaptic terminal vesicle marker proteins synaptophysin and synapsin and the dendritic marker, microtubule-associated protein-2 in many brain areas, but most predominantly in the cortex and hippocampus. These results suggest that normal β-amyloid precursor protein may serve an essential role in the maintenance of synaptic function during ageing. A compromise of this function of the β-amyloid precursor protein may contribute to the progression of the memory decline and the neurodegenerative changes seen in Alzheimer's Disease.

N-acetyltransferase (NAT2) genotype and susceptibility to sporadic Alzheimer??s disease

The importance of environmental aggression and individual susceptibility to develop Alzheimer's Disease (AD) has been suggested by epidemiological studies on both typical familial and sporadic AD cases. In order to elucidate functions that can influence the susceptibility to AD pathogenesis, we genotyped a group of 53 sporadic late-onset AD patients, matched control individuals and a larger randomly selected non-demented population for the N-acetyltransferase (NAT2). We determined the relative frequencies of individual allele combinations that define a broad range of acetylator phenotypes. Inter-individual variability in the cytotoxic and genotoxic responses to a wide diversity of environmental chemicals is known to result from the polymorphism of NAT2 as well as other drug-metabolizing-enzyme genes. The results presented are the first to demonstrate a significant difference in the NAT2 genotype profiles of sporadic AD patients compared with the healthy population. A lower frequency of the recessive alleles NAT2*6 (chi-squared 1 d.f. = 12.56, P < 0.0004) and NAT2*5B (chi-squared 1 d.f. = 6.72, P < 0.01) was found among the AD population compared with control individuals, which was concomitant with a significantly higher number of NAT2*4 fully active allele homozygotes and heterozygotes (chi-squared 1 d.f. = 5.69, P = 0.017). The most notable observation was the absence of NAT2*5B/NAT2*6 heterozygotes among cases while being present in 22.5% of control individuals (chi-squared 1 d.f. = 13.08, P = 0.0003). These observations indicate that NAT2 is a potential low-penetrance gene in AD pathogenesis, determining an individual susceptibility trait predisposing to this degenerative disease.

The Glu318Gly mutation of the presenilin-1 gene does not necessarily cause Alzheimer's disease.

In early-onset familial Alzheimer's disease (AD) pathogenic mutations have been found in the amyloid precursor protein (APP) gene and in the presenilin (PS)-1 and PS-2 genes. We screened for mutations in these genes in 20 patients with familial AD from the Finnish population. In addition, we sampled 41 sporadic AD patients and 59 controls to test for mutations identified in our familial AD cases. We detected an A-to-G transition in the PS-1 gene, resulting in a glutamic acid (Glu)-to-glycine (Gly) substitution at codon 318 in 2 familial and 2 sporadic AD patients. The Glu318Gly mutation has previously been reported to cause AD. We also found the Glu318Gly mutation in 4 healthy aged controls (range, 74-87 years). We thus conclude that the mutation is most likely a rare polymorphism not related to AD.

Alpha-ketoglutarate dehydrogenase in Alzheimer brains bearing the APP670/671 mutation.

Alzheimer's disease (AD) is associated with a striking reduction in the activity of the alpha-ketoglutarate dehydrogenase complex (KGDHC). The deficiency occurs in brains from AD patients of undefined etiology, and in fibroblasts from both sporadic and familial AD cases. To further assess the nature of the abnormality of KGDHC in AD, KGDHC activities and immunoreactivities were analyzed in brains from AD patients bearing the Swedish APP670/671 mutation. This gene defect causes overproduction of the amyloid beta peptide. KGDHC activities were reduced by 55 to 57% compared with control values in the mutation-bearing AD cases in the medial temporal and superior frontal cortices. The immunochemical levels of KGDHC subunits Elk (-51%) and E2k (-76%) declined, whereas E3 concentrations were unchanged. The results suggest that mitochondrial dysfunction is a part of the pathophysiological process in AD even when the primary pathogenic cause is nonmitochondrial.

No association between familial Alzheimer disease and cytochrome P450 polymorphisms.

Four different loci have been found to be involved in the development of familial Alzheimer disease (AD). The epsilon4 allele of the apolipoprotein E gene on chromosome 19 is a susceptibility factor for AD, and in a small number of AD families, dominant mutations with high penetrance are operating in genes on chromosomes 1, 14 and 21. However, the disease in many familial AD cases cannot be explained by these genes; thus, other genetic factors involved in the etiology of AD should exist. Recently, an association between the cytochrome P450 2D6B (CYP2D6B) allele and the Lewy body variant of AD was reported. In the present study, 54 unrelated Swedish familial AD patients and 56 age- and sex-matched healthy controls were studied with respect to the two genetic polymorphisms of oxidative drug metabolism, CYP2D6 and CYP2C19. No significant association was found between the defect CYP2D6A and -B or CYP2C19ml and -m2 alleles and familial AD patients, with the exception of a lower frequency of CYP2D6B in the male AD cases.

Evidence for an Alzheimer disease susceptibility locus on chromosome 12 and for further locus heterogeneity.

Alzheimer disease (AD) susceptibility genes have been identified on chromosomes 1, 14, 19, and 21, and a recent study has suggested a locus on chromosome 12. To confirm or refute the existence of a familial AD susceptibility locus on chromosome 12 in an independent sample of familial AD cases. Retrospective cohort study. DNA data for 6 chromosome 12 genetic markers were evaluated using parametric lod score and nonparametric linkage methods and linkage heterogeneity tests. The latter include the admixture test of homogeneity in the total group of families and the predivided sample test in families stratified by the presence or absence of an apolipoprotein E (APOE) epsilon4 allele among affected members. Parametric analyses were repeated assuming autosomal dominant inheritance of AD and either age- and sex-dependent penetrance or zero penetrance for the analysis of unaffected relatives. Clinical populations in the continental United States, Canada, Argentina, and Italy. Fifty-three white families composed of multiple members affected with AD, from whom DNA samples were obtained from 173 patients with AD whose conditions were diagnosed using established criteria and from 146 nondemented relatives. Presence of an APOE epsilon4 allele among affected family members. Using parametric methods, no evidence for linkage to the region spanned by the chromosome 12 markers could be detected if familial AD is assumed to arise from the same genetic locus in all 53 families. However, significant evidence for linkage was detected in the presence of locus heterogeneity using the admixture test (odds ratio, 15, 135:1). The estimated proportion of linked families within the 53 families examined varied between 0.40 and 0.65, depending on the genetic model assumed and APOE status. The precise location of the AD gene could not be determined, but includes the entire region suggested previously. Nonparametric linkage analysis confirmed linkage to chromosome 12 with the strongest evidence at D12S96 (P<.001). Our data provide independent confirmation of the existence of an AD susceptibility locus on chromosome 12 and suggest the existence of AD susceptibility genes on other chromosomes. Screening a larger set of families with additional chromosome markers will be necessary for identifying the chromosome 12 AD gene.

Localization and Possible Functions of Presenilins in Brain

Presenilin-1 (PS-1) is localized to chromosome 14 and presenilin-2 (PS-2) to chromosome 1. Mutations in these genes, primarily in PS-1, account for an estimated 60% of early onset familial Alzheimer's disease cases (FAD), while FAD cases account for about 10% of all Alzheimer's disease (AD) cases. The mutations are minor but are 100% penetrant, suggesting that the proteins have acquired a toxic gain in function. The proteins have multiple transmembrane domains and have been reported to be localized to the Golgi apparatus, endoplasmic reticulum, nuclear membranes and cell surface membranes. They are thought to have functions associated with vesicular trafficking, Notch signaling and apoptosis. PS mutants show relative increases in the amount of A beta42/43 compared with A beta40 in plasma, fibroblasts and brain, observations which have been taken as a possible mechanism of their role in AD. In brain, the mRNAs for these two genes are localized primarily in neurons, with the strongest in situ hybridization signals being observed in the hippocampus, cerebellum and cerebral cortex. In AD, signals detected in the hippocampus are weaker than those in normals, while signals in the cerebellum are comparable. Immunohistochemical localization of the proteins is also primarily in neurons, and, at least for PS-1, is reduced in AD affected areas. PS-1 is localized to granular structures which are most abundant in cell bodies and dendrites. The functions of the presenilins are not yet known, but available evidence points to pyramidal neurons as the most logical site for pathological change in AD.

Estimation of the genetic contribution of presenilin-1 and -2 mutations in a population-based study of presenile Alzheimer disease.

Two closely related genes, the presenilins ( PS ), located at chromosomes 14q24.3 and 1q42.1, have been identified for autosomal dominant Alzheimer disease (AD) with onset age below 65 years (presenile AD). We performed a systematic mutation analysis of all coding and 5'-non-coding exons of PS -1 and PS -2 in a population-based epidemiological series of 101 unrelated familial and sporadic presenile AD cases. The familial cases included 10 patients of autosomal dominant AD families sampled for linkage analysis studies. In all patients mutations in the amyloid precursor protein gene ( APP ) had previously been excluded. Four different PS -1 missense mutations were identified in six familial cases, two of which where autosomal dominant cases. Three mutations resulted in onset ages above 55 years, with one segregating in an autosomal dominant family with mean onset age 64 years (range 50-78 years). One PS -2 mutation was identified in a sporadic case with onset age 62 years. Our mutation data provided estimates for PS -1 and PS -2 mutation frequencies in presenile AD of 6 and 1% respectively. When family history was accounted for mutation frequencies for PS -1 were 9% in familial cases and 18% in autosomal dominant cases. Further, polymorphisms were detected in the promoter and the 5'-non-coding region of PS -1 and in intronic and exonic sequences of PS -2 that will be useful in genetic association studies.

Identification of a novel mutation (Leu282Arg) of the human presenilin 1 gene in Alzheimer's disease

Many different mutations, causative of Alzheimer's disease, have been found in the presenilin-1 gene (PS-1). We have developed a screening method based on denaturing gradient gel electrophoresis (DGGE), which allows the mutational analysis of the whole exon 9 of PS-1. Upon the screening of a Spanish sample of early onset familial Alzheimer disease cases, we have found a novel mutation in the PS-1 gene. The mutation (a T to G transition) results in a change of the amino acid at position 282 of the presenilin protein from leucine to arginine. This mutation is located in the hydrophobic domain number 7 (exon 9) close to the site of physiological cleavage processing. The average of onset of the affected members of this family is 43±5 years, and the average age of exitus of affected members is 56±3 years. The possibility to determine the specific pathologic mechanisms of this mutation is now open.