The genetics of Alzheimer's disease: a review and a discussion of the implications

'Evolutionary medicine' perspectives on Alzheimer's Disease: Review and new directions

Evolution by natural selection eliminates maladaptive traits from a species, and yet Alzheimer's Disease (AD) persists with rapidly increasing prevalence globally. This apparent paradox begs an explanation within the framework of evolutionary sciences. Here, I summarize and critique previously proposed theories to explain human susceptibility to AD, grouped into 8 distinct hypotheses based on the concepts of novel extension of the lifespan; lack of selective pressure during the post-reproductive phase; antagonistic pleiotropy; rapid brain evolution; delayed neuropathy by selection for grandmothering; novel alleles selected to delay neuropathy; by-product of selection against cardiovascular disease; and thrifty genotype. Subsequently, I describe a new hypothesis inspired by the concept of mismatched environments. Many of the factors that enhance AD risk today may have been absent or functioned differently before the modern era, potentially making AD a less common affliction for age-matched individuals before industrialization and for the majority of human history. Future research is needed to further explore whether changes in environments and lifestyles across human history moderate risk factors and susceptibility to AD.

The impact of phenotypic and genetic heterogeneity on results of genome wide association studies of complex diseases

Phenotypic misclassification (between cases) has been shown to reduce the power to detect association in genetic studies. However, it is conceivable that complex traits are heterogeneous with respect to individual genetic susceptibility and disease pathophysiology, and that the effect of heterogeneity has a larger magnitude than the effect of phenotyping errors. Although an intuitively clear concept, the effect of heterogeneity on genetic studies of common diseases has received little attention. Here we investigate the impact of phenotypic and genetic heterogeneity on the statistical power of genome wide association studies (GWAS). We first performed a study of simulated genotypic and phenotypic data. Next, we analyzed the Wellcome Trust Case-Control Consortium (WTCCC) data for diabetes mellitus (DM) type 1 (T1D) and type 2 (T2D), using varying proportions of each type of diabetes in order to examine the impact of heterogeneity on the strength and statistical significance of association previously found in the WTCCC data. In both simulated and real data, heterogeneity (presence of “non-cases”) reduced the statistical power to detect genetic association and greatly decreased the estimates of risk attributed to genetic variation. This finding was also supported by the analysis of loci validated in subsequent large-scale meta-analyses. For example, heterogeneity of 50% increases the required sample size by approximately three times. These results suggest that accurate phenotype delineation may be more important for detecting true genetic associations than increase in sample size.

Alzheimer disease: new concepts on its neurobiology and the clinical role imaging will play

Alzheimer disease (AD) is one of, if not the most, feared diseases associated with aging. The prevalence of AD increases exponentially with age after 60 years. Increasing life expectancy coupled with the absence of any approved disease-modifying therapies at present position AD as a dominant public health problem. Major advances have occurred in the development of disease biomarkers for AD in the past 2 decades. At present, the most well-developed AD biomarkers are the cerebrospinal fluid analytes amyloid-β 42 and tau and the brain imaging measures amyloid positron emission tomography (PET), fluorodeoxyglucose PET, and magnetic resonance imaging. CSF and imaging biomarkers are incorporated into revised diagnostic guidelines for AD, which have recently been updated for the first time since their original formulation in 1984. Results of recent studies suggest the possibility of an ordered evolution of AD biomarker abnormalities that can be used to stage the typical 20-30-year course of the disease. When compared with biomarkers in other areas of medicine, however, the absence of standardized quantitative metrics for AD imaging biomarkers constitutes a major deficiency. Failure to move toward a standardized system of quantitative metrics has substantially limited potential diagnostic usefulness of imaging in AD. This presents an important opportunity that, if widely embraced, could greatly expand the application of imaging to improve clinical diagnosis and the quality and efficiency of clinical trials.

Alzheimer's disease and down syndrome: from meiosis to dementia

Several molecular and clinical similarities have been detected in Alzheimer's disease (AD) and Down syndrome (DS). The most remarkable feature is abnormal accumulation of beta-amyloid in the brains of both individuals affected with AD and aging DS patients followed by dementia. In addition, AD patients exhibit dermatoglyphic patterns similar to those in DS, and late maternal age is a risk factor in both diseases. AD and DS could be related genetically because AD families exhibit a higher rate of DS cases and vice versa. Although numerous discoveries have been made in the elucidation of the etiopathogenic factors in AD and DS, little progress has been achieved in understanding the origin of the common features of the two diseases. This article reviews clinical and molecular similarities in DS and AD and also chromosome 21 studies in both diseases. A new hypothesis explaining the association between AD and DS is suggested, and this hypothesis is based on the poorly understood molecular phenomenon of aberrant meiotic recombination. Aberration in meiotic recombination has been consistently detected in chromosomal diseases including trisomy 21 and sex chromosomes. There are no studies dedicated to meiotic recombination in genetic diseases; however, evidence for disturbed recombination has been documented in several neurological diseases such as Huntington's disease, myotonic dystrophy, and fragile X syndrome. Interestingly, the rate of trisomic XXY children born to mothers transmitting fragile X mutation is higher than expected. This finding suggests that AD could be associated with DS in a similar way to which fragile X syndrome is related to trisomy of sex chromosomes. Based on analogy with fragile X syndrome, it can be predicted that AD should demonstrate aberrant meiotic recombination in chromosome 21, most likely in the region D21S1/S11-D21S16 which is linked to early onset familial AD. Based on the same rationale, different patterns of meiotic recombination in the nondisjunct chromosome 21 within DS patients grouped according to the concomitant disease are predicted.

Molecular genetics of Alzheimer's disease

Genetic factors are involved in the aetiology of Alzheimer's disease (AD) in 25-40% of the cases. In some cases AD clearly segregates as an autosomal dominant trait in families. Three genes have been identified which, when mutated, cause AD: the Abeta amyloid precursor protein gene (APP), and the presenilin-1 (PSEN1) and presenilin-2 (PSEN2) genes. Together, these mutations are responsible for 30-50% of the cases with autosomal dominant AD, and for about 5% of AD in general. In cases where the inheritance pattern is unclear and in sporadic cases the epsilon4 allele of the apolipoprotein E gene (APOE) has been identified as a major risk factor contributing to the pathogenesis of AD in about 20% of the cases. Although mutations in the known genes are a rare cause of AD they are useful for the purposes of presymptomatic diagnostics in autosomal dominant AD families that segregate these mutations. Also, the identification of these genes and mutations has been extremely important to the recent evolution in the understanding of the biology of the disease. However, other causative and risk genes are involved in AD and need to be identified in order to fully elucidate the biology of AD. This will ultimately lead to the development of effective therapies for this major disease.

Neurons and extracellular neurofibrillary tangles in the hippocampal subdivisions in early-onset familial Alzheimer's disease: a case study

We have investigated morphometrically unaffected neurons, intracellular neurofibrillary tangles (I-NFT) and extracellular neurofibrillary tangles (E-NFT) in eight subdivisions of the hippocampal cortex in two cases of early-onset familial Alzheimer's disease (FAD) and six cases of early-onset sporadic Alzheimer's disease (SAD). The hippocampal subdivisions examined included: CA4, CA3, CA2, CA1, prosubiculum, subiculum and presubiculum (PRE), parasubiculum (PARA) and entorhinal cortex (ENT). CA3, CA2 and CA1 in the FAD cases showed more severe neuronal loss and much greater E-NFT formation than in the SAD cases, while ENT in both the FAD cases showed less neuronal loss and less E-NFT formation. These data suggest that the cornu ammonis is affected more severely than the ENT in the FAD cases. These observations indicate that hippocampal pathology in the FAD cases is qualitatively as well as quantitatively different from that in sporadic cases. These results provide further evidence for pathological heterogeneity in AD, although the number of FAD cases examined is very small.

Rate of progression in familial Alzheimer's disease

The clinical course of early-onset, dominantly inherited, familial Alzheimer's disease (FAD) was contrasted with late-onset, sporadic Alzheimer's disease (AD). Eight FAD and 23 sporadic AD patients were followed over a mean of 63 months from estimated disease onset. The two groups did not differ notably in duration of symptoms from onset, global disease severity, or degree of cognitive deficits on initial evaluation. The Kaplan-Meier lifetable method was used to assess time from estimated disease onset to dependence in self-care, institutionalization, and death. A greater percentage of FAD patients became dependent in self-care and died earlier than did sporadic AD patients. The lifetable results suggest that FAD may have a more rapid course than dose late-onset sporadic AD.

Familial risks for Alzheimer disease from a population-based series

Kaplan-Meier risks estimates are calculated and compared for two consecutive series (N1 = 840, N2 = 819) of first-degree relatives of Alzheimer disease (AD) patients diagnosed as either "probable" or "autopsy-confirmed" AD. The consistency of results increases confidence in estimates and suggests consistent case ascertainment over 8 years. Lifetime risk estimates to age 88 for the combined sample (23.4% +/- 3.0%) do not approach the 50% risk compatible with an autosomal dominant model of transmission. These results support our previous finding and suggest that an autosomal dominant gene(s) is not responsible for all cases of AD.

A.E. Bennett Research Award 1993. Olfactory neuroblasts from Alzheimer donors: studies on APP processing and cell regulation

Cell lines of continuously dividing human olfactory neuroblasts can be propagated using olfactory epithelium obtained from human donors at biopsy or autopsy. The expression of neuronal proteins in these cells, such as neurofilament protein and tau protein, can be increased using a combination of factors including nerve growth factor, fibroblast growth factor, interleukin 1 and interleukin 6. These cells also express aspects of human disease. Olfactory neuroblasts generated from donors with the common, sporadic forms of Alzheimer's disease, show elevated levels of the direct precursor to beta-amyloid, the amyloid precursor protein C-terminal derivative (CTD). When treated with the lysosomal inhibitor chloroquine, immunoblots of Alzheimer olfactory neuroblasts show seven-fold higher levels of CTDs than immunoblots from age-matched control neuroblasts. The disease related increases in CTDs can be reversed by treatment with agents that increase intracellular cyclic adenosine monophosphate (cAMP), such as dibutyryl-cyclic-AMP, theophylline, and isoproterenol.

Monozygotic twins concordant for late-onset probable Alzheimer disease with suspected Alzheimer disease in four sibs

Probable Alzheimer disease (AD) is described in 79-year-old male twins with monozygosity confirmed by DNA examination. The first twin to be affected began to show signs of intellectual deterioration at age 70. In the other, onset was at age 72. Four of their living sibs (current age range = 75-92) are also suspected to have AD. The possible roles of genetic and environmental factors in the development of AD in this sibship are discussed.

Alzheimer's disease: identical phenotype of familial and non-familial cases

Ninety outpatients with Alzheimer's disease according to ICD-10 diagnostic draft criteria were studied to test the hypothesis that cases with a familial aggregation are different from cases without such an aggregation with respect to cognitive impairment. In all cases the diagnosis of Alzheimer's disease was confirmed by prospective observation within 12 months of initial evaluation. Patients were divided into two groups: one consisting of 23 patients with a familial aggregation, the other consisting of 67 patients without secondary cases among first-degree relatives. By means of t-tests differences in impairment of cognitive functions between the groups were calculated. The results did not yield statistically significant differences between the groups for any of the neuropsychologically investigated cognitive deficits. Thus the hypothesis that the presence of a familial aggregation may lead to a distinct phenotype in Alzheimer's disease was not confirmed.

Selective susceptibility of cultured striatal neurons to kainic acid

Previous work has shown a selective sensitivity of striatal D2-receptor-containing neurons (D2 cells) to kainic acid (KA). In order to see whether this phenomenon exists in cultured cells, which could provide a very accessible model for subsequent mechanistic studies, we examined striatal cultures grown for up to 30 days and labeled with either [3H]spiperone, for D2 receptors, or with [3H]-SCH23390, for D1 receptors. Analysis of the cells was performed with a digital imaging system (RAS). The cultured cells were examined for ligand binding to receptors, cell size, and susceptibility to KA. The D2 cells showed an increased mortality over D1-receptor-containing neurons (D1 cells) in the presence of KA, of a magnitude similar to in vivo loss, with larger D2 cells showing the greatest vulnerability.

A family with multiple instances of definite, probable and possible early-onset Alzheimer's disease

A family with a multigenerational history of proven or suspected early-onset Alzheimer's disease (AD) consistent with autosomal-dominant inheritance is described. To date, the pedigree comprises five generations in which there are 13 known affected individuals. The mean age of onset of cognitive deficits in those for whom data are available (n = 11) is 47.6 (s.d. 3.0) years and the mean age of death (n = 10) is 58.8 (s.d. 4.0) years. The variability in the extent and quality of available data illustrates the diagnostic difficulties encountered in ascertaining such an extended pedigree, and the need for caution in interpreting the evidence.

Steroid hormones as mediators of neural plasticity

Steroid and thyroid hormone receptors are expressed in the developing brain and persist throughout adult life. They mediate a variety of effects on the brain, ranging from developmental effects of thyroid hormone and the process of sexual differentiation to the cyclic changes during reproductive cycles in adult female animals. This review summarizes data from the author's laboratory on three topics: (1) actions of extradiol and progesterone on the ventromedial nucleus of the hypothalamus in adult female and male rats, showing both the cyclicity and the consequences of brain sexual differentiation; (2) actions of estradiol on the cholinergic neurons of the basal forebrain of the female and male rat, reflecting the plasticity of the adult cholinergic system as well as sex differences which are developmentally programmed; and (3) diverse actions of estrogens, thyroid hormone and glucocorticoids on the morphology of hippocampal neurons. The review concludes by discussing the interactions between "organizational" (i.e. developmental) effects and the "activational" effects of steroids on the mature nervous system in relation to the environmental control of brain gene expression.

Family with 22-derived marker chromosome and late-onset dementia of the Alzheimer type: I. Application of a new model for estimation of the risk of disease associated with the marker

We have identified 2 sisters with probable dementia of the Alzheimer type who have an unusual 22-derived marker chromosome with a greatly elongated short arm containing 2 well-separated nucleolus organizer regions. A marker chromosome similar in appearance is uncommon in the general population. Eleven of 24 of their biological relatives were also found to have the marker. The known pedigree of this family encompasses 6 generations in 2 of which there is evidence of 10 cases of dementia of the Alzheimer type. The average age-at-onset of dementia is 65.8 +/- 5.5 years; the average age-at-death among those apparently affected is 74.9 +/- 8.3 years. A new model for the estimation of risk was applied to the family data. Persons in this family with the marker were found to be 4 times more likely to develop dementia than those without the marker, the 95% confidence interval for this risk being 1-50. The probability that the association of dementia and the marker is due to chance alone is .05 (1 in 20).

Are there clinical and epidemiological differences between familial and non-familial Alzheimer's disease?

To determine whether there are clinical and epidemiological differences between familial and non-familial Alzheimer's disease, we studied 151 patients with the clinical diagnosis of Dementia of the Alzheimer Type (DAT). Eighty-four index patients (56%) had at least one relative with reported DAT-type memory loss; the remaining 67 did not. For comparison, patients with a positive family history were grouped into a suspected familial DAT (FDAT), and those with a negative family history were grouped into a suspected non-familial DAT (NFDAT). There were no significant differences between FDAT and NFDAT patients with respect to age of symptom onset, survival time, rate of cognitive decline, motor function, behavioral signs or symptoms, head trauma, thyroid dysfunction, maternal age, or birth order. There were no significant differences between pedigrees of FDAT and NFDAT patients in respect to the prevalence of Down's syndrome or age of mortality. To examine a more "familial" set of FDAT patients, an FDAT subgroup (F2DAT) consisting of 33 index patients with two or more affected relatives with DAT-type memory loss was compared to the 67 NFDAT patients in a similar analysis. No significant differences were found. However, NFDAT index patients had a higher reported prevalence of head trauma than those patients in the F2DAT subgroup (NFDAT 22%, F2DAT 7%, OR = 4.2, CI = 0.9-20.0, P = 0.08). Thus severe head trauma may be a more important etiological factor in the non-familial form of DAT. To better examine this tendency, future studies should be conducted utilizing a larger number of pedigrees and familial criteria which account for the number of persons at risk and the age at onset of disease.

Diagnostic and pharmacological approaches in Alzheimer's disease

Alzheimer's disease is a chronic progressive disease affecting higher intellectual functioning. The clinical diagnosis is made when the onset of illness is insidious, the course slowly progressive and all the treatable causes of dementia have been ruled out. The use of more stringent criteria has improved clinical diagnosis, but at best only 80% of patients are accurately diagnosed. Ultimately the diagnosis depends upon pathological confirmation. The neuritic plaques and neurofibrillary tangles described by Alzheimer, although not pathognomonic for the disease, continue to be the basis for pathological diagnosis. The aetiology and pathophysiology of Alzheimer's disease are presently unknown. Epidemiological studies have suggested a genetic basis for the disorder, and many biochemical studies have linked it to degeneration of central cholinergic neurons, and possibly to abnormalities of other neurotransmitter systems. A marker which would permit accurate diagnosis early in the course of disease would be of major importance to researchers and clinicians alike. No marker has been found to date, although recent research results are promising. Various pharmacological strategies have been employed in the treatment of Alzheimer's disease. More recently attempts have focused on enhancing central cholinergic transmission. Despite the well-founded rationale for these studies, results have been modest.

Twin studies of Alzheimer's disease: an approach to etiology and prevention

Epidemiologic studies of environmental factors associated with risk of Alzheimer's disease (AD) have produced inconsistent and disappointing results. By contrast, family/genetic studies and case control investigations suggest that genetic causes of AD are important. The investigation of such genetic causes remains an important aim in all forms of AD including typical, late-onset disease where linkage work is impractical. But the public health burden of AD creates an especially urgent need to identify environment risk factors, if these exist, since they will more likely be susceptible to intervention. Such environmental factors may interact with genetic susceptibility to accelerate or retard disease expression, and environmental interventions that delay onset may constitute an important strategy for prevention. All these issues may be addressed by twin studies of AD, but the few such studies to date have been limited by small samples and other methodologic difficulties. This paper reviews the rationale for twin studies of AD, and describes briefly the work in this area to date. It also discusses a number of suggestions for methodologic improvements. We conclude that the time is ripe for twin studies of AD, and that such work holds considerable potential for the investigation of etiology and, possibly, for the identification of strategies for prevention.

Physical mapping of chromosome 21 DNA markers in Alzheimer's disease region using somatic cell hybrids

From a chromosome 21 phage library, we selected 10 clones located proximal of the senile plaque amyloid precursor protein gene. Since a locus for Alzheimer's disease (AD) has been localized in the pericentromeric region of chromosome 21, the selected phage clones are potential candidate probes for genetic analysis of AD. In this study, we subcloned single-copy fragments of the selected phage clones, refined their physical localization, and examined their chromosomal distribution in relation to their position on chromosome 21. The results indicated that the phage clones are identifying nine chromosome 21 loci, which, if polymorphic, may be helpful in localizing the AD locus more precisely. Moreover, since all phage clones are located close to the centromere of chromosome 21, they can be used to determine the parental origin of nondisjunction in trisomy 21 with high reliability.

Geographical associations between aluminium in drinking water and death rates with dementia (including Alzheimer's disease), Parkinson's disease and amyotrophic lateral sclerosis in Norway

Comparisons of maps and correlation and regression analysis indicate a geographical association between aluminium (Al) in drinking water and registered death rates with dementia (coded from death certificates as the underlying or a contributory cause of death) in Norway. High levels of Al in drinking water are in most cases related to acid precipitation. In general, correlations between aggregate environmental measurements and mortality are a weak source of evidence for risk factors for disease, however, and interpretations regarding cause-and-effect relationships should therefore be made with great care. The major uncertainty probably relates to the use of registered death rates with dementia as a measure of incidence rates of Alzheimer's disease. The dementia rates are correlated with population density, and it is possible that the association between AI and dementia might be due to differences in diagnosis and reporting of dementia. Thus, the present study can only be regarded as introductory, and further epidemiological studies are needed to help elucidate the role of AI in Alzheimer's disease. The results provide little evidence of an association between AI in drinking water and Parkinson's disease or amyotrophic lateral sclerosis.

Changes in calcium homeostasis during aging and Alzheimer's disease

Several observations indirectly suggest that intracellular calcium regulation may be altered by aging and Alzheimer's disease. Thus, calcium homeostasis was examined directly in skin fibroblasts from Alzheimer's patients and compared to cells from normal young and elderly controls. Alterations in both bound and free calcium were noted; cells from Alzheimer's donors have higher levels of bound calcium but lower concentrations of free intracellular calcium when compared to cells from young and normal aged donors. These changes in calcium homeostasis may be physiologically significant, since processes that require transient elevations of intracellular free calcium, such as cell spreading, decline in the Alzheimer's cells. In summary, cultured skin fibroblasts from normal aged and Alzheimer's patients demonstrate deficits in calcium homeostasis and other metabolic processes when compared to cells from young donors.

Severe head injury hastens age of onset of Alzheimer's disease

Head trauma has been found with greater frequency in the histories of Alzheimer patients than age-matched controls in some studies, but not in others. We hypothesized that events that accelerate neuron loss, such as significant head trauma, hasten the onset of symptoms of Alzheimer's disease in persons vulnerable to the disorder. Retrospective data on 148 probable Alzheimer patients and 33 demented controls were examined. Alzheimer patients with severe head injury before the age of 65 showed onset of symptoms at an earlier age than Alzheimer patients without head trauma.

Neurotrophic agents may exacerbate the pathologic cascade of Alzheimer's disease

The thesis is advanced that Alzheimer's disease is triggered by alterations in the regulatory mechanisms governing the patterns of cytoskeletal protein expression in structurally plastic neurons in the mature nervous system. As a consequence, polypeptide species acting to stabilize the cytoskeleton are preferentially affected, and neuronal architecture becomes increasingly determined by proteins involved in labile structural states. A cascade of interdigitating pathologies is then postulated to develop characterized by nerve terminal aberrancies, subsequent extrusion of atypical polypeptide species and their conjugates, reactive gliosis, abnormal neuronal growth, and degeneration. Within this context, growth factors promote and accelerate the pathologic cascade. Based on this model, a treatment strategy is suggested that the most effective management of Alzheimer's disease, particularly during earlier stages, is to delay its projected normal onset and to control the aberrant neuronal growth that is a hallmark of the malady.

Molecular genetic analysis of early onset familial Alzheimer's disease

Today, results of molecular genetic analyses of familial Alzheimer's disease are not only confusing to those not directly involved in this particular research field but, they are also extremely troublesome to those trying to elucidate the genetic basis of this devastating neurological disorder. Dr. St. George-Hyslop has provided an excellent review of the current research state, outlining the major problems hindering the molecular geneticists in reaching their final goal, namely isolation and characterization of the gene(s) predisposing to familial Alzheimer's disease. We feel confident that with additional work and further advances, many of these obstacles will be overcome and the genetic role in AD more clearly delineated.

Familial Alzheimer's disease: progress and problems

This paper reexamines recent epidemiologic and molecular genetic studies on the genetic basis of Alzheimer's Disease (AD). Careful analysis of the available epidemiologic data strongly suggests that at least a proportion of AD results from the inheritance of an autosomal dominant gene defect. However, studies of isolated families, of concordance rates in twins, and of risk for AD in relatives of AD probands yield conflicting data. While it is likely that much of the conflict can be ascribed to methodologic differences, it remains premature to conclude that all AD is transmitted as an autosomal dominant trait. Molecular genetic techniques hold the promise of isolation and characterization of the genetic defect(s) in familial AD (FAD). Recently, chromosome 21 has been implicated as the potential site of an autosomal dominant defect in some but not necessarily all FAD pedigrees. However, the results of recent genetic epidemiologic studies suggest that progress in the molecular genetic approach to AD will be difficult.

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.

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.

Functional study of T lymphocyte responsiveness in patients with dementia of the Alzheimer type

The autologous mixed lymphocyte reaction (AMLR) was used to study T lymphocytes in a group of patients with dementia of the Alzheimer type (DAT) in order to confirm the observation that their T cell proliferation in AMLR was greater than in age-matched controls, and to investigate other pathways of T cell activation, searching for correlations between immunologic and clinical findings in DAT. The mean proliferative response in AMLR was increased in patients with DAT. No differences between patients and age-matched controls were detected when other pathways of T cell activation were investigated. The degree of response in the AMLR varied among patients with DAT. This fits with the suggestion that the disorder may be a heterogeneous syndrome.

Neurogenesis of the basal forebrain in euploid and trisomy 16 mice: an animal model for developmental disorders in Down syndrome

The neurogenesis and early histochemical differentiation of the basal forebrain in trisomy 16 fetal mice and their euploid littermates were examined by combining [3H]thymidine autoradiography with acetylcholinesterase histochemistry. Neurons of the basal forebrain were being born between embryonic day 11 and 15 in both chromosomally normal (euploid) and aneuploid mice. In euploid littermate controls, neurogenesis proceeded along a caudal to rostral gradient with the peak on embryonic day 11 for caudal portions and embryonic day 13 for rostral portions of the basal forebrain. In contrast, in trisomy 16 mice, rostral sections exhibited a peak of neurogenesis on embryonic day 11, 2 days earlier than in their euploid littermate controls. Hypocellularity of the basal forebrain region was noted in trisomy 16 mice; particularly dramatic was the reduction of the population of cells that expressed acetylcholinesterase. This reduction in cell number in the trisomics was not accompanied by a reduction in cell size or by a dramatic change in the distribution of residual neurons when compared to that of euploid littermate controls. Since trisomy 16 mice do not survive the perinatal period, we examined the pattern of acetylcholinesterase expression in normal C57B1/6J mice from embryonic day 16 to postnatal day 5 to determine the postnatal disposition of these neurons. Already at embryonic day 16, fibers staining for acetylcholinesterase penetrated the striatal anlage, in their course towards targets in the cerebral cortices. By postnatal day 5, the previously expansive distribution of basal forebrain neurons had become consolidated in a more ventral and rostral position by the extensive outgrowth of the striatal neurons, a pattern resembling that seen in adult animals.

Cerebral degenerations producing dementia: importance of neuropathologic confirmation of clinical diagnoses

Dementia is a major public health concern with our increasing elderly population and currently affects more than three million Americans at an annual cost of $50 billion. The marked overlap in symptomatology between Alzheimer's disease and other primary parenchymal degenerations makes antemortem diagnosis based on clinical assessment tentative at best, with error rates of 25% commonly reported. Accurate diagnosis is of vital importance in order to improve our understanding of these illnesses, evaluate potential therapies, and provide appropriate genetic counseling to family members. Direct neuropathologic examination at autopsy is currently the only reliable method for assuring accurate diagnosis, and should be undertaken in all demented patients. To illustrate the importance of these principles, we present three patients who were clinically diagnosed with Alzheimer's disease, and subsequently found to have other dementing illnesses by careful postmortem neuropathologic examination.

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.

Familial Alzheimer's disease

A family with familial Alzheimer's disease (FAD) inherited as an apparent autosomal dominant trait is presented. Twelve individuals (6 females; 6 males) in 4 generations were affected. The disease had its onset in the late 30's, early 40's with death by age 50. Although FAD which appears to be transmitted as an autosomal dominant trait is relatively rare, such families must be identified and carefully counselled with respect to recurrence risks for subsequent generations. In this family, there are currently 20 members of Generation IV, aged 15-37, and 9 members of Generation V, aged 1-11. The majority of these individuals appear to have a 50% risk for developing this disease.

Monozygotic twins concordant for probable Alzheimer disease and increased platelet membrane fluidity

This report describes monozygotic twins who were concordant for probable Alzheimer disease, as defined by currently-accepted clinical criteria. Monozygosity was established by blood typing. Their ages of symptomatic onset were 57 and 66 yr, and the times from onset to institutionalization were 8 and 2 yr, respectively. These results suggest that age at onset and rate of progression are clinical features that can be affected by random processes or exposure to environmental factors. The platelet membrane fluidity of both twins was abnormally increased, and the respective values were identical within experimental limits. This result is consistent with published data suggesting that increased platelet membrane fluidity is associated with a clinically distinct subtype of Alzheimer disease and that this platelet membrane characteristic may be genetically determined.

Cognitive and brain imaging measures of Alzheimer's disease

Diagnosis of Alzheimer's disease (AD) during life relies upon clinical and neurobehavioral symptoms but is presumptive without microscopic verification of neuropathology. Studies in this review observed considerable heterogeneity in AD symptoms and did not agree on how to detect the earliest symptoms. Problems exist in diagnosis. Differences in symptoms and diagnosis result from how AD is defined neurobehaviorally and on the model used for description. The studies reviewed have been considered under three basic models: A severity (staging) model; a heterogeneity (subtyping) model; and an information processing model. Differences in model intent have resulted in differences in disease description. Brain imaging measures have not invalidated models but add the neural substrate needed to examine correlation of measures within each model.

The genetic defect causing familial Alzheimer's disease maps on chromosome 21

Alzheimer's disease is a leading cause of morbidity and mortality among the elderly. Several families have been described in which Alzheimer's disease is caused by an autosomal dominant gene defect. The chromosomal location of this defective gene has been discovered by using genetic linkage to DNA markers on chromosome 21. The localization on chromosome 21 provides an explanation for the occurrence of Alzheimer's disease-like pathology in Down syndrome. Isolation and characterization of the gene at this locus may yield new insights into the nature of the defect causing familial Alzheimer's disease and possibly, into the etiology of all forms of Alzheimer's disease.