Gene dosage of the amyloid beta precursor protein in Alzheimer's disease

Slowed synthesis of DNA and methionine is a pathogenetic mechanism common to dementia in Down's syndrome, AIDS and Alzheimer's disease?

This is a presentation of the hypothesis of a pathogenetic mechanism common to the dementia seen in Alzheimer's disease (AD), Down's Syndrome (DS) and the acquired immunodeficiency syndrome (AIDS). As there is experimental evidence of defective DNA repair capacity in AD and DS, unrepaired damage to DNA occurs in these diseases and may lead to complete breakdown of cellular function and ultimate cell death. Cobalamin and folate are coordinated in a vulnerable key position in the synthesis of DNA and S-adenosylmethionine (SAM). Cobalamin/folate deficiency, a significant feature in senile dementia of Alzheimer type and in AIDS-related dementia complex, will result in concomitant slowed synthesis of DNA and SAM. The enzyme cystathionine-beta-synthetase (CBS) has been localized to the chromosome band 21q22.3. Owing to gene dosage, CBS activity is increased in trisomy 21. As a consequence, cobalamin/folate metabolism is inhibited, which leads to slowing of DNA and SAM synthesis in DS patients. Amyloidosis is a hallmark of AD and DS brain neuropathology and recent experimental findings support the view that amyloid or amyloid precursors stimulate DNA synthesis, which is in agreement with the hypothesis presented in this paper. In summary, demented patients with cobalamin/folate deficiency, trisomy 21 and human immunodeficiency virus (HIV) infection display a simultaneous downregulation of DNA and SAM synthesis, which may indicate a pathway common to the dementia seen in AD, DS and AIDS.

Evidence for a membrane defect in Alzheimer disease brain

To determine whether neurodegeneration in Alzheimer disease brain is associated with degradation of structural cell membrane molecules, we measured tissue levels of the major membrane phospholipids and their metabolites in three cortical areas from postmortem brains of Alzheimer disease patients and matched controls. Among phospholipids, there was a significant (P less than 0.05) decrease in phosphatidylcholine and phosphatidylethanolamine. There were significant (P less than 0.05) decreases in the initial phospholipid precursors choline and ethanolamine and increases in the phospholipid deacylation product glycerophosphocholine. The ratios of glycerophosphocholine to choline and glycerophosphoethanolamine to ethanolamine were significantly increased in all examined Alzheimer disease brain regions. The activity of the glycerophosphocholine-degrading enzyme glycerophosphocholine choline-phosphodiesterase was normal in Alzheimer disease brain. There was a near stoichiometric relationship between the decrease in phospholipids and the increase of phospholipid catabolites. These data are consistent with increased membrane phospholipid degradation in Alzheimer disease brain. Similar phospholipid abnormalities were not detected in brains of patients with Huntington disease, Parkinson disease, or Down syndrome. We conclude that the phospholipid abnormalities described here are not an epiphenomenon of neurodegeneration and that they may be specific for the pathomechanism of Alzheimer disease.

Beta A4 amyloid protein and its precursor in Alzheimer's disease

The beta A4 amyloid protein is now understood to play a pivotal role in the development of Alzheimer's disease. This protein is generated by the abnormal processing of the amyloid protein precursor, a large membrane glycoprotein. Insights into the mechanisms of this abnormal processing will give information relevant to the design of new therapeutic strategies for Alzheimer's disease.

Genetic linkage studies of human neurodegenerative disorders

Recombinant DNA technology has the ability to delineate the causes of several neurodegenerative disorders. Genetic linkage studies have been used successfully to localize gene defects and it is likely that in the near future the exact loci will be determined.

A novel species-specific RNA related to alternatively spliced amyloid precursor protein mRNAs

Using an S1 nuclease protection assay, we have identified a novel "variant" Amyloid Precursor Protein (APP) RNA in human brain which is 3-6-fold more abundant than APP-770, but less abundant than APP-751 or APP-695. This variant, referred to as amyloid precursor-related protein 365 (APRP-365), is not detected in mouse and rat brain RNAs. A 1.6 kilo-basepair cDNA clone corresponding to this variant APP RNA predicts the existence of a 365 amino acid protein that is similar to the amino-terminal end of APP-770 but lacks the beta-amyloid peptide and any hydrophobic transmembrane spanning domains. In a modified polymerase chain reaction (PCR), we used amplification of reverse transcribed mRNA to confirm and extend our S1 observations. Together, the features of APRP-365 suggest that the human variant is a soluble protein containing a Kunitz protease inhibitor domain.

Beta-amyloid protein in Alzheimer's disease

Beta-amyloid protein, a 42-43 amino acid polypeptide, accumulates abnormally in senile plaques and the cerebral vasculature in Alzheimer's disease. This polypeptide is derived from a membrane-associated precursor which has several isoforms expressed in many tissues. The precursor protein is processed constitutively within the beta-amyloid domain, leading to the release of the large N-terminal portion into the extracellular medium. beta-amyloid protein may be toxic to certain neuronal cell types and its early deposition may be an important event in the pathogenesis of Alzheimer's disease.

Characterization of beta-amyloid precursor proteins with or without the protease-inhibitor domain using anti-peptide antibodies

Alternative splicing of the transcript encoding the beta-amyloid precursor protein (BAPP) of Alzheimer's disease produces multiple mRNA species. Translation of these mRNAs predicts protein products of 770, 751, and 695 amino acids. The difference arises from the inclusion in BAPP-770/751 of a 56-residue insert region which is homologous to Kunitz-type protease inhibitors. We have prepared and affinity-purified anti-peptide antibodies that react specifically with either BAPP-770/751 (insert-specific) or BAPP-695 (junction-specific). A detectable level of the mRNA corresponding to the BAPP-770/751 protein was found in all cell lines tested. Immunoprecipitation of 35S-labeled proteins from these cell lines showed them to contain one or two Mr 105,000 bands reactive with the insert-specific serum, i-291. In contrast, only cos-7 cells and the human neuroblastoma cell line, IMR-32, contained mRNA species that encode the BAPP-695 protein, as shown by Northern analysis with a junction-spanning oligonucleotide probe. A band of Mr 95,000 was immunoprecipitated specifically from these two cell lines using the junction-specific serum, J-284. Indirect immunofluorescence labeling of cells corroborated these findings. All cells reacted with the insert-specific antibodies, i-291 and i-324. Only cos-7 and IMR-32 cells reacted with the junction-specific antibody, J-284. These results demonstrate the usefulness of anti-peptide antibodies for the differential detection of the BAPP-695 and BAPP-770/751 proteins.

Alpha 1-antichymotrypsin is associated solely with amyloid deposits containing the beta-protein. Amyloid and cell localization of alpha 1-antichymotrypsin

Our recent studies demonstrated that alpha 1-antichymotrypsin (ACT), a serine protease inhibitor, was associated with the beta-protein in the brain amyloid deposits of Alzheimer's disease, aged human controls and aged monkeys, suggesting a role for the inhibitor in the amyloid deposition. In the present study we used immunohistochemistry to test for the presence of ACT in the amyloid deposits which contain, as their major component, a protein different from the beta-protein. ACT was not found in the amyloid deposits in primary or secondary amyloidosis, familial and amyloidotic polyneuropathy or Creutzfeldt-Jakob disease (non-beta-protein amyloidoses), but was found (together with beta-protein) in Alzheimer's disease, Down's syndrome, normal aging, and hereditary cerebral hemorrhage with amyloidosis of Dutch origin. These results suggest a specific association of ACT with beta-protein amyloid. We next examined the distribution of the inhibitor in normal human brain and in various human neuropathological states in order to identify cells that express this protein during brain degeneration. In addition to its association with amyloid, ACT immunoreactivity was also located in astrocytes near areas of neuronal or tissue loss, in a few neurons and pericytes and in the epithelium of the choroid plexus.

Deposition of amyloid (A4) protein within the brains of persons with dementing disorders other than Alzheimer's disease and Down's syndrome

Deposition of amyloid (A4) protein was assessed in the cerebral cortex of 26 patients dying with various neurodegenerative disorders, other than Alzheimer's disease. Amyloid deposits were (variably) present in 2/3 (66%) elderly (i.e. over 65 years of age) patients with progressive supranuclear palsy, 4/7 (57%) with Parkinson's disease, 2/5 (40%) with Huntington's chorea and in both elderly patients with frontal lobe dementia but were only rarely seen in any patient before this age. The A4 protein deposits were nearly always of a diffuse type with only an occasional 'cored' neuritic plaque being present. Amyloid deposition in elderly persons may thus relate more to certain aspects of ageing and genetics than to AD, per se. Only in this latter condition are the cerebral cortical amyloid deposits widely associated with a neuritic change and a neurofibrillary degeneration of nerve cells.

Expression of beta amyloid protein precursor mRNAs: recognition of a novel alternatively spliced form and quantitation in Alzheimer's disease using PCR

We have analyzed alternatively spliced beta amyloid protein precursor (beta APP) mRNAs by using the polymerase chain reaction to amplify beta APP cDNAs produced by reverse transcription. With this approach the three previously characterized beta APP mRNAs (beta APP695, beta APP751, and beta APP770) are readily detected and compared in RNA samples extracted from specimens as small as a single cryostat section. We show that the results obtained with this method are not affected by partial RNA degradation and use it to identify a novel alternatively spliced beta APP714 mRNA that is present at low abundance in each of the many human brain regions, peripheral tissues, and cell lines that we have examined; demonstrate that nonneuronal cells in the adult human brain and meninges produce appreciable beta APP695, beta APP751, and beta APP770 mRNA; and identify changes in beta APP gene expression in the AD brain and meninges that may contribute to amyloid deposition.

Amyloidosis of the nervous system

Various types of amyloid fibril deposits occur in the nervous system with unique clinical characteristics and pathogeneses. Genetic mutations cause the familial amyloidotic polyneuropathies and acquired polyneuropathies occurring particularly in patients suffering from hypernephromas and myelomas also result from the production of abnormal proteins. Amyloid fibril deposits in cerebral plaques and vessels consisting of beta-protein are seen in acquired and familial Alzheimer's disease and in Down's syndrome individuals over 40 years of age. This amyloid fibril deposition could result from a mutational, transcriptional or post-translational alteration in these pathologic processes with most evidence supporting the latter. Other diseases including hereditary cerebral hemorrhage of the Dutch type and Batten's disease involve beta-amyloid deposition. The features of the familial and transmissible forms of the spongiform encephalopathies are associated with the prion protein which comprises the amyloid fibril deposits in these conditions. This wide variety of nervous system disorders having amyloid deposits as their primary or subsidiary characteristic make studies of these conditions intriguing models for research workers in clinical, pathologic and molecular biologic fields.

The amyloid proteins of Alzheimer's disease as potential targets for drug therapy

Two amyloid proteins accumulate in Alzheimer's disease. These proteins, beta amyloid protein and paired helical filament protein, are present in the hallmark lesions of Alzheimer's disease, neuritic plaques and neurofibrillary tangles. Although the amino acid sequences of these two proteins are likely to be different, they nevertheless share certain physical characteristics which define each as belonging to a common class of proteins, amyloid proteins. Since these proteins are probably important in the pathology of Alzheimer's disease, drugs that prevent their accumulation should have therapeutic utility. Based on the amyloidoses associated with other diseases, three mechanisms for amyloid formation have emerged. These mechanisms form a framework for studying Alzheimer amyloids and designing interventions. One mechanism involves posttranslational events which render a normal protein amyloidogenic. Proteolysis, phosphorylation, glycosylation, and transglutamination may be relevant posttranslational events in Alzheimer's disease. If more conclusive evidence can be generated suggesting that these events are involved in the abnormal formation of amyloid in Alzheimer's disease, then these events will become viable targets for drug therapy. Another mechanism for amyloid formation results from expression of an abnormal gene which, in the case of familial Alzheimer's disease, may be an important etiological component. A third mechanism involves the accumulation of a normal protein to a threshold concentration that spontaneously forms amyloid. An effective therapeutic approach for these last two mechanisms could likely include pharmacological manipulation of gene expression.

Alzheimer's disease amyloid precursor protein is present in senile plaques and cerebrospinal fluid: immunohistochemical and biochemical characterization

The amyloid fibrils deposited in cerebral vessel walls and senile plaques in Alzheimer's disease are polymeric forms of a 4 kDa fragment produced by proteolysis of a putative precursor protein (APP). Using antibodies to several fragments of the deduced precursor, we were able to demonstrate the presence of APP in senile plaques, brain extracts and cerebrospinal fluid. Membrane-associated APP is detected as a group of 105-135 kDa proteins while soluble APP is predominantly 105 kDa, does not react with an anti C-terminal antibody, and is 10 kDa shorter than the membrane-bound APP. Amino terminal sequence of the tissue 105 kDa protein indicates that APP begins at residue 18 of the cDNA sequence. These findings imply that i) two forms of APP are detected: membrane-bound and secreted, and ii) APP can be processed in situ.

Critical role of the D21S55 region on chromosome 21 in the pathogenesis of Down syndrome

The duplication of a specific region of chromosome 21 could be responsible for the main features of Down syndrome. To define and localize this region, we analyzed at the molecular level the DNA of two patients with partial duplication of chromosome 21. These patients belong to two groups of Down syndrome patients characterized by different partial trisomies 21: (i) duplication of the long arm, proximal to 21q22.2, and (ii) duplication of the end of the chromosome, distal to 21q22.2 We assessed the copy number of five chromosome 21 sequences (SOD1, D21S17, D21S55, ETS2, and D21S15) and found that D21S55 was duplicated in both cases. By means of pulsed-field gel analysis and with the knowledge of regional mapping of the probes D21S17, D21S55 and ETS2, we estimated the size of the common duplicated region to be between 400 and 3000 kilobases. This region, localized on the proximal part of 21q22.3, is suspected to contain genes the overexpression of which is crucial in the pathogenesis of Down syndrome.

Decreased DNA repair in familial Alzheimer's disease

Alterations in the capacity of a cell to repair DNA lesions play an important role in a number of human diseases. We and others have demonstrated defective DNA repair of alkylation damage in cells from patients with Alzheimer's disease. It has been hypothesized that this defect is related to the cause of Alzheimer's disease and results in the accumulation of lesions in the central nervous system neurons. One prediction of this hypothesis is that in dominantly inherited Alzheimer's disease, the repair defect will be present in half of the offspring of affected patients long before they develop symptoms of the disease. In order to test the hypothesis that decreased DNA repair is responsible for familial Alzheimer's disease and their at-risk offspring we have studied DNA repair in these individuals after exposure of lymphoblasts to alkylating agents. Our results indicate that cell lines from affected patients repair significantly less damage in 3 h than cell lines from healthy controls. A small number of at-risk individuals were also studied and some of these had lower levels of repair, although more cell lines from individuals in this group must be studied. These findings provide further support for defective DNA repair playing a role in the pathogenesis of Alzheimer's disease.

Amyloid A4 protein and its precursor in Down's syndrome and Alzheimer's disease

In patients with Alzheimer's disease, amyloid fibrils that are aggregates of A4 protein subunits are deposited in the brain. A similar process occurs at an earlier age in persons with Down's syndrome. To investigate the deposition of amyloid in these diseases, we used a radioimmunoassay to measure levels of the amyloid precursor (PreA4) in the serum of 17 patients with Down's syndrome, 15 patients with Alzheimer's disease, and 33 normal elderly controls. The mean (+/- SD) concentration of serum PreA4 was increased 1.5-fold in patients with Down's syndrome (2.49 +/- 1.13 nmol per liter) as compared with that in controls (1.68 +/- 0.49 nmol per liter; P less than 0.007); the levels in patients with Alzheimer's disease were similar to those in controls (1.83 +/- 0.78; P less than 0.98). We also found that the concentration of PreA4 in the brain tissue of two adults with Down's syndrome (100 and 190 pmol per gram) was higher than that in the brain tissue of either 26 patients with Alzheimer's disease (64.4 +/- 17.3 pmol per gram) or 17 elderly controls with neurologic disease (68.5 +/- 26.3 pmol per gram). Immunocytochemical studies of brain tissue from 26 patients with Down's syndrome showed that the deposition of A4 protein amyloid began in these patients approximately 50 years earlier than it began in 127 normal aging subjects studied previously, although the rate of deposition was the same. We conclude that, since the gene for PreA4 is on the long arm of chromosome 21, which is present in triplicate in Down's syndrome, overexpression of this gene may lead to increased levels of PreA4 and amyloid deposition in Down's syndrome. However, since increased levels of PreA4 are not present in Alzheimer's disease, additional factors must account for the amyloid deposition in that disorder.

Distinguishing Alzheimer's disease from other dementias. Questionnaire responses of close relatives and autopsy results

Epidemiologic study of Alzheimer's disease by family history requires that Alzheimer's be distinguished from other dementias. Identification of demented family members is usually based on recall by relatives. This study examines the validity of the classification of Alzheimer's disease and other dementias based on relatives' reports. Close relatives of autopsy-confirmed dementia cases were asked to complete a questionnaire describing the patient's symptoms. These informants were familiar with the patient's disease and involved in his/her care prior to death. The questionnaire included the DSM-III criteria for Primary Degenerative Dementia and the Hachinski Ischemic Scale. A diagnosis derived from the close relatives' responses was compared to the neuropathologic diagnosis for thirty-six cases: 20 Alzheimer's disease, 9 mixed Alzheimer's disease, and 7 non-Alzheimer's disease dementias. The diagnosis of Primary Degenerative Dementia derived from questionnaire responses had a sensitivity of 0.93 and specificity of 0.43 for pathologic Alzheimer's disease. Few vascular dementias were included in the series, thereby precluding the study of so-called multi-infarct dementia. Hachinski scores based on relatives' responses classified 40% of pathologically pure Alzheimer's disease cases as multi-infarct dementia (HIS greater than 7). Thus, using these elevated Hachinski scores to rule out Alzheimer's disease would cause substantial misclassification. Diagnosis based on questionnaire Primary Degenerative Dementia criteria was quite sensitive but relatively nonspecific. When attempting to obtain a complete family history or pedigree that describes the occurrence of Alzheimer's disease and other dementias in all family members, the questionnaire approach should be supplemented with additional information from medical records, physicians, and other relatives.

Altered expression of genes for amyloid and cytoskeletal proteins in Alzheimer cortex

Recent studies have indicated a normal gene dose for the amyloid precursor protein (APP) in Alzheimer's disease (AD). These findings leave open the possibility that elevated levels of messenger RNA (mRNA) for this protein may contribute to the pathogenesis of AD. Using Northern analysis, we compared the levels of mRNA for the APP and 3 cytoskeletal proteins in parietal cortex of 6 brains having marked AD-type degeneration with the levels of these mRNAs in 6 control samples. The cytoskeletal mRNAs studied were those for the human neurofilament 68-kDa subunit (HNFL), for alpha-tubulin, and for glial fibrillary acidic protein (GFAP). A ribonuclease (RNase) protection assay was also used to compare AD and control HNFL mRNA levels. The mRNAs for APP, HNFL, and alpha-tubulin were diminished in AD cortex. The decrement for APP mRNA was less than that for HNFL or alpha-tubulin. The message for GFAP in AD cortex showed no loss. The findings support a general deficit in neuronal mRNAs, including that for APP. They do not exclude the possibility of elevated levels of the message for the APP in small neuronal subsets, in subcortical neurons projecting to cortex, or as a generalized phenomenon in earlier stages of the disease.

Molecular genetic approaches to Alzheimer's disease

Alzheimer's disease (AD) has emerged in the past decade as a major public health problem. Epidemiological and neuropathological studies have revealed AD to be a very frequent disease associated with aging. Already the fourth leading cause of death in the USA and consuming a major component of health care costs, AD will take on even greater importance with the continuous growth of the elderly population. A concerted effort has been made in recent years to attack AD using an arsenal of powerful molecular biological techniques, concentrating on two areas: the characterization of proteins implicated in the pathogenesis of AD and of the genes that encode them; and the use of genetic linkage to approach the primary defect in a familial form of AD (FAD). This review attempts to summarize and interpret the recent molecular, genetic and biochemical findings concerning the pathogenesis of AD.

Amyloid beta protein enhances the survival of hippocampal neurons in vitro

The beta-amyloid protein is progressively deposited in Alzheimer's disease as vascular amyloid and as the amyloid cores of neuritic plaques. Contrary to its metabolically inert appearance, this peptide may have biological activity. To evaluate this possibility, a peptide ligand homologous to the first 28 residues of the beta-amyloid protein (beta 1-28) was tested in cultures of hippocampal pyramidal neurons for neurotrophic or neurotoxic effects. The beta 1-28 appeared to have neurotrophic activity because it enhanced neuronal survival under the culture conditions examined. This finding may help elucidate the sequence of events leading to plaque formation and neuronal damage in Alzheimer's disease.

SOD-1 activity and platelet membrane fluidity in Alzheimer's disease

An early-onset, familial form of Alzheimer's disease (AD) has been reported to be linked to a locus on the long arm of chromosome 21 (21q21). Furthermore, duplications in the vicinity of this locus involving the beta-amyloid gene and the proto-oncogene ets-2 have been reported in association with AD. The structural gene for Cu,Zn superoxide dismutase, SOD-1, is located between the beta-amyloid gene and ets-2. For this reason and because SOD-1 is a plausible candidate for a gene that might influence the fluidity of cellular membranes, we determined whether or not the subtype of AD with increased platelet membrane fluidity was associated with an increase in Cu,Zn superoxide dismutase activity.

Genetics, Alzheimer's disease and senile dementia

Genetic factors in the aetiology of Alzheimer's disease (AD), are now being intensively investigated. The homogeneity of AD is under investigation. There are a few large kindreds with early onset of AD in whom transmission appears to be typically autosomal dominant, and 65% or more of the remaining cases at any age may have genetic aetiology. Both multifactorial and autosomal dominant inheritance with age-dependent expression have been proposed, but the late onset and death of unaffected relatives from competing causes make it difficult to choose between them. Lifetime risk gives the best estimate of incidence in family studies, but clinical and pathological criteria are not clear enough for confident diagnosis of AD in late old age. A role for external factors is indicated by twin studies, and the role of aluminium is currently under investigation. Molecular genetics promises to resolve many questions. The clinicians' role will be to provide well documented families for interdisciplinary research and to help in clarifying diagnosis in late old age.

The neurobiology of Alzheimer's disease

The defining histological characteristics of Alzheimer's disease (AD) are neurofibrillary tangles and neuritic plaques, although neither is pathognomonic for this disorder. The distribution of AD histopathology suggests selective neuronal vulnerability, with specific cell populations affected within discrete regions of the cerebral hemispheres and within certain subcortical and brain-stem nuclear areas. At the ultrastructural level, tangles and plaque neurites contain paired helical filaments whose composition is unknown but may include altered cytoskeletal elements. Amyloid, deposited in plaque cores and often focally present within the cerebral vasculature, contains a polypeptide ("beta-protein," or "beta-amyloid") encoded by a chromosome 21 gene. At least in occasional families, AD has been linked to a separate chromosome 21 locus, but different underlying genetic factors may operate in other cases. Inorganic substances, including aluminum and silicon, are reported to co-localize within tangle-bearing neurons and plaque cores. Specific environmental agents have not been confirmed to be pathogenetically important, however, but may eventually prove to exert a permissive, facilitatory, or even causative role in many AD patients.

The pathology of the amyloid A4 precursor of Alzheimer's disease

The A4 amyloid protein is the major subunit present in the amyloid of Alzheimer's disease. It is derived by proteolytic cleavage from a larger precursor (PreA4) which is a neuronal membrane glycoprotein. Whereas in Down's syndrome, over-expression of the gene coding for PreA4 is likely to be responsible for the premature development of cerebral amyloidosis, a similar mechanism is yet to be demonstrated in Alzheimer's disease.

The deposition of amyloid proteins in the aging mammalian brain: implications for Alzheimer's disease

Dysfunction and loss of neurons and their processes in Alzheimer's disease is accompanied by the progressive accumulation of intraneuronal and extracellular proteinaceous filaments. Currently available evidence indicates that the principal confirmed constituent of the intraneuronal paired helical filaments (PHF) is the microtubule-associated protein, tau. Whether other neuronal proteins contribute to the PHF remains unresolved. The amyloid filaments found in cerebral and meningeal microvessels and in the centers of senile plaques appear so far to be protein chemically and immunochemically distinct from the intraneuronal PHF. The native precursor of the beta-amyloid protein comprising these amyloid filaments has now been identified and characterized in brain, non-neural tissues and cDNA-transfected cells of several species, including humans. It occurs as a heterogeneous group of approximately 105-135 kD membrane-associated proteins. cDNA-transfected cells reproduce certain beta-amyloid precursor fragment patterns observed in human brain tissue, including a favored and stable 11 kd fragment containing the carboxyl terminus and presumably the beta-amyloid region. Circulating forms of the precursor have been specifically detected in human cerebrospinal fluid. Understanding the processing of the beta-amyloid precursor protein and the origin of beta-amyloid deposits should provide insights into a potentially seminal feature of cortical degeneration in Alzheimer's disease.

Molecular genetics of Alzheimer's disease and the amyloid beta peptide precursor gene

Alzheimer's disease is a neurodegenerative disorder characterized by global cognitive decline. An autopsy of the Alzheimer patient's brain reveals two major neuropathological lesions: neurofibrillary tangles, and amyloid deposits in the form of senile plaques and cerebrovascular accumulations. While tangles appear to be a universal hallmark of dying neurons in several neurodegenerative diseases, amyloid plaques occur in only three conditions including Alzheimer's disease, Down syndrome, and to a limited extent, normal aging. The frequency of senile plaques appears to correlate well with the degree of dementia in the Alzheimer's patient. It remains unclear, however, whether amyloid formation represents one of the final stages of a long neuropathological process in the brain, or initially participates in promoting neuronal dysfunction. To address this question, we have isolated the gene encoding the precursor of the principle component of the plaque, the amyloid beta peptide. We have mapped this gene to chromosome 21, the same chromosome in which we have detected linkage between anonymous DNA markers and the familial form of Alzheimer's disease. Employing direct genetic linkage analysis, we have shown that the amyloid gene and the familial Alzheimer's disease gene represent two separate and distinct genetic loci. Here we present further information on the location of the familial Alzheimer's disease gene on chromosome 21. We also discuss the recent discovery of an alternate form of the amyloid beta peptide precursor gene which encodes a serine protease inhibitor in the Kunitz family. The presence of a protease inhibitor domain within the amyloid beta peptide precursor, itself, has profound implications for its possible role in the process of amyloid formation.(ABSTRACT TRUNCATED AT 250 WORDS)

Three types of amyloid protein precursor mRNA in human brain: their differential expression in Alzheimer's disease

Three types of amyloid protein precursor (APP) mRNA, produced by alternative splicing, were detected by Northern blotting in human brains, both control and Alzheimer's disease. These mRNAs encode APP695 consisting of 695 amino acids, APP751 harboring a 56 amino acid insert homologous to a Kunitz-type trypsin inhibitor inside APP695, and APP770 containing an additional 19 amino acid insert. Another possible APP mRNA which encodes "APP714" containing a 19 amino acid insert was not found in brain samples tested. Quantitative analysis revealed that, although the relative expression levels of the three mRNAs were variable among individuals, there was no remarkable change in expression of APP695 and APP751 mRNAs in Alzheimer's disease compared with control, but that APP770 mRNA level was elevated significantly in Alzheimer's disease.

Protein chemical and immunocytochemical studies of meningovascular beta-amyloid protein in Alzheimer's disease and normal aging

As a comparison to previous analyses of purified amyloid plaque cores from Alzheimer's disease (AD) brain, we performed protein chemical and immunocytochemical studies on amyloid filaments extracted from meningeal blood vessels of patients with Alzheimer's disease. Results were compared with those obtained from identically prepared fractions of aged normals without cerebral amyloid angiopathy or other microscopic findings of AD. The amyloid isolation method of Glenner and Wong was modified, including an extraction with sodium dodecyl sulfate (SDS). Gel electrophoresis of purified amyloid from AD meninges yielded bands centered at 4.2 kDa. Sequencing of the HPLC-purified amyloid protein from AD meninges confirmed the published beta-protein sequence for residues 1-30 and 35-40, with the exception of glutamic acid rather than glutamine at position 11. N-terminal heterogeneity was not prominent. No sequence beyond residue 40 was obtained. Proteins of similar but not identical mol. wt. were present in HPLC-purified fractions of normal meninges; neither the beta-protein sequence nor any other interpretable sequence was detected in such fractions. Two antisera raised against the purified AD meningovascular amyloid protein identified the 4.2 kDa band on Western blots of AD preparations; no protein band in this region was labeled in control preparations. The 4.2 kDa band in AD meningeal preparations was also lableled by an antiserum to synthetic beta-peptide but not by an antiserum to the carboxyl terminus of the beta-protein precursor. Both the AD meningovascular amyloid antisera selectively labeled amyloid in cortical and meningeal vessels and plaque cores; tangles, plaque neurites, and cells of normal CNS and numerous non-neural tissues were unstained. The antisera also labeled the occasional deposits of vascular amyloid and less frequent plaque core amyloid found in some aged individuals without AD. We conclude that (1) the meningovascular amyloid beta-protein of AD, whose sequence has been confirmed and extended to residue 40, was not immunocytochemically detectable in neurofibrillary tangles; (2) beta-protein could not be detected in meningeal preparations from aged controls who lack light microscopically visible meningovascular amyloid; and (3) the vascular and plaque core amyloid present in aged normals is antigenically cross-reactive with AD meningovascular amyloid.

Beta-protein deposition: a pathogenetic link between Alzheimer's disease and cerebral amyloid angiopathies

Cerebral amyloid angiopathy (CAA) refers to a group of hereditary (hereditary cerebral hemorrhage with amyloidosis, HCHWA and sporadic (SCAA) disorders characterized by amyloid fibril deposition restricted to the leptomeningeal and cortical vasculature leading to recurrent hemorrhagic and/or ischemic accidents. On clinical and biochemical grounds, two forms of HCHWA can be distinguished. The amyloid subunit of the HCHWA of Icelandic origin is related to Cystatin C, while amyloid from patients of Dutch origin (HCHWA-D) is related to the beta-protein (or A4), the main component of vascular and plaque core amyloid in Alzheimer's disease (AD) and Down's syndrome (DS) [corrected]. SCAA is an increasingly recognized cause of stroke in normotensive individual amounting to 5-10% of all cerebrovascular accidents. We now report the isolation and partial amino acid sequence of the amyloid subunit from a case of SCAA and a new case of HCHWA-D. The recognition that a heterogeneous group of diseases are linked by similar pathological and chemical features suggests that diversity of etiological factors may promote a common pathogenetic mechanism leading to amyloid-beta (A beta) deposition, and open new ways of research in AD and CAA as they are related to dementia and stroke.

Human ETS2 gene on chromosome 21 is not rearranged in Alzheimer disease

The human ETS2 gene, a member of the ETS gene family, with sequence homology with the retroviral ets sequence of the avian erythroblastosis retrovirus E26 is located on chromosome 21. Molecular genetic analysis of Down syndrome (DS) patients with partial trisomy 21 allowed us to reinforce the supposition that ETS2 may be a gene of the minimal DS genetic region. It was originally proposed that a duplication of a portion of the DS region represents the genetic basis of Alzheimer disease, a condition associated also with DS. No evidence of either rearrangements or duplications of ETS2 could be detected in DNA from fibroblasts and brain tissue of Alzheimer disease patients with either the sporadic or the familiar form of the disease. Thus, an altered ETS2 gene dosage does not seem to be a genetic cause or component of Alzheimer disease.

Beta-amyloid precursor protein of Alzheimer disease occurs as 110- to 135-kilodalton membrane-associated proteins in neural and nonneural tissues

Progressive cerebral deposition of extracellular filaments composed of the beta-amyloid protein (beta AP) is a constant feature of Alzheimer disease (AD). Since the gene on chromosome 21 encoding the beta AP precursor (beta APP) is not known to be altered in AD, transcriptional or posttranslational changes may underlie accelerated beta AP deposition. Using two antibodies to the predicted carboxyl terminus of beta APP, we have identified the native beta APP in brain and nonneural human tissues as a 110- to 135-kDa protein complex that is insoluble in buffer and found in various membrane-rich subcellular fractions. These proteins are relatively uniformly distributed in adult brain, abundant in fetal brain, and detected in nonneural tissues that contain beta APP mRNA. Similarly sized proteins occur in rat, cow, and monkey brain and in cultured human HL-60 and HeLa cells; the precise patterns in the 110- to 135-kDa range are heterogeneous among various tissues and cell lines. Confirmation that the immunodetected tissue proteins are forms of beta APP was obtained when mammalian cells transfected with a full-length beta APP cDNA showed selectively augmented expression of 110- to 135-kDa proteins and specific immunocytochemical staining. Unexpectedly, the antibodies to the carboxyl terminus of beta APP labeled amyloid-containing senile plaques in AD brain. We conclude that the highly conserved beta APP molecule occurs in mammalian tissues as a heterogeneous group of membrane-associated proteins of approximately 120 kDa. Detection of the nonamyloidogenic carboxyl terminus within plaques suggests that proteolytic processing of the beta APP into insoluble filaments occurs locally in cortical regions that develop beta-amyloid deposits with age.

Absence of mutation in the beta-amyloid cDNAs cloned from the brains of three patients with sporadic Alzheimer's disease

Using an oligonucleotide probe, we isolated cDNA clones corresponding to the precursor of the beta-amyloid peptide (BAP) from brain libraries of 3 patients with sporadic Alzheimer's disease (AD). DNA sequencing showed that the largest cDNA clone encompasses 83% of the open reading frame proposed by Kang et al. to encode the BAP precursor (APP). cDNA clones from each of the 3 AD brain libraries were identical to the sequence of the APP-cDNAs cloned from normal adult human and fetal brain. An antisense-radiolabeled RNA copy of one of the AD clones detected a pattern of 3 gene transcripts measuring 3.5, 3.2 and 1.6 kilobases (kb) in both normal and AD brain RNAs. These data suggest that there are no mutations in or about the 42 amino acid (aa) sequence of BAP and that the accumulation of amyloid consistently found in AD may result from altered post-translational processing of APP.

Multiple forms of beta-amyloid peptide precursor RNAs in a single cell type

The longest open reading frames (ORFs) of three different cDNAs ([10, 12, 18, 26], and this report) contain the exact 42 amino acid (aa) sequence of the beta-amyloid peptide (BAP) which is selectively deposited in Alzheimer's diseased (AD) brains. Each of the three cDNAs for the putative amyloid peptide precursor (APP) has been cloned from a different cell type. Using an HL 60 library, we have cloned two of these three APP cDNAs from a single cell type. The sequences of the HL 60 cDNAs are identical to the APP 751 and to the APP 770 forms of APP cDNAs. Northern blots show that oligonucleotide probes drawn from unique regions of the APP 751 and APP 770 cDNAs both hybridize to 4.0 Kilobase (Kb) and to 1.6 Kb APP RNAs from HL 60 cells. In human adult brain, an oligonucleotide probe drawn from the unique region of the APP 751 cDNA hybridizes to a 3.5 Kb APP RNA. However, a DNA probe drawn from the BAP region, which is common to the 695, 751, and 770 forms of APP cDNAs, hybridizes to 3.5, 3.2 and 1.6 Kb APP RNAs. Taken together, these results show that at least two forms of APP RNAs can exist within a single cell type and that the diversity of possible APP RNAs and complexity of their expression may have been underestimated. Thus, in addition to identifying the cells which produce BAP, a new challenge consists of determining which form of forms of APP RNAs and hence APP proteins are associated with BAP deposition in AD and Down syndrome (DS).

A new A4 amyloid mRNA contains a domain homologous to serine proteinase inhibitors

The amyloid proteins isolated from neuritic plaques and the cerebrovasculature of Alzheimer's disease are self-aggregating moieties termed A4 protein and beta-protein, respectively. A putative A4 amyloid precursor (herein termed A4(695] has been characterized by analysis of a human brain complementary DNA. We report here the sequence of a closely related amyloid cDNA, A4(751), distinguished from A4(695) by the presence of a 168 base-pair (bp) sequence which adds 57 amino acids to, and removes one residue from, the predicted A4(695) protein. The peptide predicted from this insert is very similar to the Kunitz family of serine proteinase inhibitors. The two A4-specific messenger RNAs are differentially expressed: in a limited survey, A4(751) mRNA appears to be ubiquitous, whereas A4(695) mRNA has a restricted pattern of expression which includes cells from neuronal tissue. These data may have significant implications for understanding amyloid deposition in Alzheimer's disease.