Beta amyloid gene duplication in Alzheimer's disease and karyotypically normal Down syndrome

The inheritance of Alzheimer's disease: a new interpretation

Ninety-one families of Alzheimer patients were studied to determine the proportion of familial cases, to obtain pedigrees for the analysis of the mode of inheritance, and to look for clinical differences between the familial and the nonfamilial cases. The diagnosis was confirmed by autopsy in 26 cases. Thirty-nine cases (43%) were familial, which is defined as more than one case in the family. Our interpretation of the pedigree data is that Alzheimer's disease is etiologically heterogeneous: it may be genetic or sporadic. In the familial type we think that the disease is inherited as an autosomal dominant, with a wide range of age of onset within a family. In one-third of these families the gene is not expressed until over age 70. No clinical differences were found between the familial and the sporadic groups.

Beyond measurement: the pattern is the thing

Perhaps cognitive and brain imaging measures have been pushed almost as far as they can go in helping diagnose or understand AD. Their value may increase with the development of better models of the disease and the addition of new types of measurements to the armamentarium (e.g., eye-tracking, olfactory, and dermatoglyphic evaluations). Together they may lead to a more precise definition of the "AD pattern."

Grafting genetically modified cells to the brain: possibilities for the future

Diagnostic and therapeutic approaches to disorders of the central nervous system (CNS) are particularly difficult to develop because of the relative inaccessibility of the mammalian brain to study and chemical treatment, the complexity and interconnectedness of CNS subsystems, and the profound and continued lack of fundamental understanding of the relationship between structure and function in the CNS. Neural grafting in the CNS has recently suggested a potential approach to CNS therapy through the selective replacement of cells lost as a result of disease or damage. Independently, studies aimed at direct genetic therapy in model systems have recently begun to suggest conceptually new approaches to the treatment of several kinds of human genetic disease, especially those caused by single-gene enzyme deficiencies. We suggest that a combination of these two approaches, namely the grafting into the CNS of genetically modified cells, may provide a new approach toward the restoration of some functions in the damaged or diseased CNS. We present evidence for the feasibility of this approach, including a description of some current techniques for mammalian cell gene transfer and CNS grafting, and several possible approaches to clinical applications.

A boy with Down's syndrome having recombinant chromosome 21 but no SOD-1 excess

A 5-year-old boy with Down's syndrome of mild phenotype is described. Chromosome studies revealed that the karyotype of the proband was 46,XY,rec(21),dup q,inv(21) (p11.2q22.1)mat, and the segment 21q22.1----21qter was trisomic. The erythrocyte superoxide dismutase-1 (SOD-1) was found to be normal, and so we conclude that SOD-1 excess is not necessarily observed in patients with Down's syndrome caused by partial 21 trisomy. It is suggested that the gene for SOD-1 is located on the more proximal segment of the sub-band 21q22.1.

beta-Amyloid gene is not present in three copies in autopsy-validated Alzheimer's disease

Recently, it has been suggested that Alzheimer's disease is associated with a duplication of the amyloid precursor protein gene localized to chromosome 21q21. In this study, a cloned DNA probe (B2.3), complementary to the sequence coding the beta-amyloid peptide, and DNA polymorphisms adjacent to this sequence were used to determine the number of copies of the beta-amyloid gene in DNA isolated from human blood and brain. Individuals with trisomy 21 (Down syndrome) who were heterozygous for the polymorphisms showed a gene-dosage effect, with one allele exhibiting twice the autoradiographic intensity as the other. Heterozygous individuals with Alzheimer's disease and controls showed equal intensities of the two allelic bands, suggesting that there are only two copies of the beta-amyloid gene in these individuals. In individuals with Alzheimer's disease and in controls who were homozygous for these polymorphisms, the number of copies of the beta-amyloid gene was determined by comparing the autoradiographic intensity of beta-amyloid alleles to that of DNA fragments detected by a reference probe. No difference was detected between these two groups.

Molecular genetics: applications to the clinical neurosciences

Application of molecular biology, by means of linkage analysis and DNA probes that demonstrate restriction fragment length polymorphisms (RFLPs), has resulted in the chromosomal localization of the genes responsible for a number of neurological disorders. Characterization of the structure and function of individual genes for these diseases is in an early stage, but information available indicates that the molecular mechanisms underlying phenotypic expression of neurological diseases encompass a wide range of genetic errors ranging from the most minor (a single-base pair mutation) to large chromosomal deletions. Linkage analysis can now be used for genetic counseling in several of these disorders.

Transgenic mice with increased Cu/Zn-superoxide dismutase activity: animal model of dosage effects in Down syndrome

Down syndrome, the phenotypic expression of human trisomy 21, is presumed to result from a 1.5-fold increase in the expression of the genes on human chromosome 21. As an approach to the development of an animal model for Down syndrome, several strains of transgenic mice that carry the human Cu/Zn-superoxide dismutase gene have been prepared. These animals express the transgene in a manner similar to that of humans, with 0.9- and 0.7-kilobase transcripts in a 1:4 ratio, and synthesize the human enzyme in an active form capable of forming human-mouse enzyme heterodimers. Cu/Zn-superoxide superoxide dismutase activity is increased from 1.6- to 6.0-fold in the brains of four transgenic strains and to an equal or lesser extent in several other tissues. These animals provide a unique system for studying the consequences of increased dosage of the Cu/Zn-superoxide dismutase gene in Down syndrome and the role of this enzyme in a variety of other pathological processes.

Automated DNA sequencing and analysis of the human genome

In the past few years, striking advances have been made in automating DNA sequence analysis. Currently, efforts are underway to automate and improve DNA purification, mapping, and data processing procedures. The predictable advances in these technologies should soon place us in a position to sequence the entire human genome. The information derived from this project will have profound implications for basic biology and clinical medicine alike.

The amyloid beta protein gene is not duplicated in brains from patients with Alzheimer's disease

Complementary DNAs (cDNAs) encoding portions of the amyloid beta protein were used to investigate possible amyloid gene duplication in sporadic Alzheimer's disease. A strategy employing two Eco RI restriction fragment length polymorphisms (RFLPs) detected by the amyloid cDNAs was used. RFLPs allow the detection of a 2:1 gene dosage in the DNA of any individual who is heterozygous for a particular RFLP. The amyloid gene regions homologous to the cDNAs used were not duplicated in the DNA from brains of individuals with sporadic Alzheimer's disease. Similar results were also obtained with a strategy employing a test for 3:2 gene dosage.

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

The progressive deposition in the human brain of amyloid filaments composed of the amyloid beta protein is a principal feature of Alzheimer's disease (AD). Densitometric analysis of Southern blots probed with a complementary DNA for the amyloid protein has been carried out to determine the relative dosage of this gene in genomic DNA of 14 patients with AD, 12 aged normal subjects, and 10 patients with trisomy 21 (Down syndrome). Whereas patients in the last group showed the expected 1.5-fold increase in dosage of this gene, none of the patients with AD had a gene dosage higher than that of the normal controls. These results do not support the hypothesis that the genetic defect in AD involves duplication of a segment of chromosome 21 containing the amyloid gene. Alternative mechanisms for the brain-specific increase in amyloid protein deposition in AD should be considered.

Absence of duplication of chromosome 21 genes in familial and sporadic Alzheimer's disease

The possibility that Alzheimer's disease (AD) is caused by overexpression or duplication of one or more genes on chromosome 21 has been raised by the observation of AD-like neuropathologic changes in individuals with Down syndrome and by the mapping of both the defect for familial AD and the amyloid beta protein gene to this autosome. Possible duplication on chromosome 21 was investigated in both familial and sporadic AD by means of restriction fragment length polymorphisms for the amyloid and SODI loci, as well as for DNA markers in the vicinity of the familial AD defect and in the critical Down syndrome region of chromosome 21. No evidence of increased DNA dosage was observed in either brain or leukocytes of patients with inherited or sporadic forms of AD. Duplication of these regions is therefore not a frequent event in either form of AD. Furthermore, no significant allelic association was detected between AD and any of the loci, including the amyloid and SODI genes, providing no support for the hypothesis that defects in these specific genes are the primary cause of AD.

Failure of familial Alzheimer's disease to segregate with the A4-amyloid gene in several European families

The gene coding for the amyloid protein, a component of neuritic plaques found in brain tissue from patients with Alzheimer's disease, has been localized to chromosome 21, and neighbouring polymorphic DNA markers segregate with Alzheimer's disease in several large families. These data, and the association of Alzheimer's disease with Down's syndrome, suggest that overproduction of the amyloid protein, or production of an abnormal variant of the protein, may be the underlying pathological change causing Alzheimer's disease. We have identified a restriction fragment length polymorphism of the A4-amyloid gene, and find recombinants in two Alzheimer's disease families between Alzheimer's disease and the A4-amyloid locus. This demonstrates that the gene for plaque core A4-amyloid cannot be the locus of a defect causing Alzheimer's disease in these families. These data indicate that alterations in the plaque core amyloid gene cannot explain the molecular pathology for all cases of Alzheimer's disease.

The genetic defect in familial Alzheimer's disease is not tightly linked to the amyloid beta-protein gene

Amyloid beta-protein (AP) is a peptide of relative molecular mass (Mr) 42,000 found in the senile plaques, cerebrovascular amyloid deposits, and neurofibrillary tangles of patients with Alzheimer's disease and Down's syndrome (trisomy 21). Recent molecular genetic evidence has indicated that AP is encoded as part of a larger protein by a gene on chromosome 21 (refs 5-7). The defect in the inherited autosomal dominant form of Alzheimer's disease, familial Alzheimer's disease (FAD), has been mapped to the same approximate region of chromosome 21 by genetic linkage to anonymous DNA markers, raising the possibility that this gene product, which could be important in the pathogenesis of Alzheimer's disease, is also the site of the inherited defect in FAD (ref. 5). We have determined the pattern of segregation of the AP gene in FAD pedigrees using restriction fragment length polymorphisms. The detection of several recombination events with FAD suggests that the AP gene is not the site of the inherited defect underlying this disorder.

Localization of amyloid beta protein messenger RNA in brains from patients with Alzheimer's disease

The distribution of cells containing messenger RNA that encodes amyloid beta protein was determined in hippocampi and in various cortical regions from cynomolgus monkeys, normal humans, and patients with Alzheimer's disease by in situ hybridization. Both 35S-labeled RNA antisense and sense probes to amyloid beta protein messenger RNA were used to ensure specific hybridization. Messenger RNA for amyloid beta protein was expressed in a subset of neurons in the prefrontal cortex from monkeys, normal humans, and patients with Alzheimer's disease. This messenger RNA was also present in the neurons of all the hippocampal fields from monkeys, normal humans and, although to a lesser extent in cornu ammonis 1, patients with Alzheimer's disease. The distribution of amyloid beta protein messenger RNA was similar to that of the neurofibrillary tangles of Alzheimer's disease in some regions, but the messenger RNA was also expressed in other neurons that are not usually involved in the pathology of Alzheimer's disease.