A Fan-w. Boston University School of Medicine, Boston, MA
The discovery of an association between APOE genotype and risk of Alzheimer disease (AD) in 1993 and the subsequent confirmation of thi5 finding in a large number of diverse populations revolutionized the thinking about the genetic basis of the disorder. Until that time, it was widely held that pathogenic mutations cause AD only in rare families segregating a form of the disorder as an autosomal dominant trait manifesting before the age of 65 years. Genetic modeling studies suggest that AD risk is governed by a combination of multiple susceptibility genes and other factors. It is estimated that APOE ~4 accounts for approximately 50% of the genetic variance for AD in Caucasian populations, but much less in other groups including African Americans. Several studies including a large meta analysis of pooled primary data indicate that the effect of ~4 varies with age and sex. For the most pan, initial reports of association between AD and other candidate susceptibility genes have not been replicated satisfactorily in independent samples. Disagreement across studies can be attributed to several factors namely diagnostic misclassification (particularly among controls), genetic heterogeneity, confounding or interaction with other factors, and population stratification between patient and control samples. Recent studies suggest that the effect of APOE genotype may be modified by polymorphisms located in the APOE promoter region. Two biologically plausible candidates, a-2.macroglobuhn and LRP receptor genes, map to a region of chromosome 12 showing linkage in multiplex families of late-onset AD, however they appear to have at most a weak connection to inherited susceptibility to AD. Hypothesis- driven approaches have focused attention on genes involved in oxidative metabolism and the vascular system. At present the most promising of these candidates is the angiotensin converting enzyme (ACE) gene. Evidence from diverse populations implicates an insertiondeletion polymorphism in ACE as a modulator of disease risk, although there is disagreement across studies about the pattern of association. These studies highlight the importance of replicating gene associations in independent samples regardless of the statistical significance obtatned from any given ample.
A FULL 1622) DISEASE.
Michael Owen. University of Wales Collqe Kingdom
of Medicine, Cardi& CFl4 4XN United
M. .I. Owen’. F. WavrantDeVrieze’. R. Crooke2, P.G. Kehor’. W. S. Wu-‘, A. My&. 1. Fenton’, F. Rice’. A. Wood’, P. Holmmw’, S. Lowstone’, N. Tunstal14, J. PerezTu?, M. Hutton’, S. Shears-‘, K. Rorhl.‘, .I.Booth.‘, J. Williams’, J.Hardy2, A. Goatc’. I) Neuropsychiatric Genetics Unit, Tenovus Building. University of Wales College of Medicine, Heath Park, Cardifi CF4 4XN, UK: Dept of Pswhologicul Medicine, University of’ Wales College of Medicine, Cardlfi S Clam, Wales; 2) Birdsall Building, Muyo Clinic. 4500 San Pablo Road, Jacksonville, FL32084. USA; 3) Department of Psych&y and Genetics, Wushington University School of Medicine, 490 Childrens Place, St Louis, MO 63110. USA: 4) Institute of Psychiatry, De Crespigny Park, Denmark Hill, London SES BAF, UK. We have genotyped 514 affected sibling pairs (ASPS) with probable/definite Alzheimers disease (AD) according to NINCDS-ADRDA diagnostic criteria with onsets above 65 years. The genome scan comprised two stages. In the first 292 ASPS were genotyped using 237 microsatellite markers separated by an average distance of 16.3cM. 16 peaks with a multipoint lad score (MLS) greater than I either in the whole sample, the e4 positive or negative subgroups were observed on chromosomes I (two peaks) 2, 5, 6, 9 (two peaks), IO (two peaks), 12, 13, 14, 19, 21 and X (two peaks). Simulation studies revealed that these findings exceeded those expected by chance, although many are likely to be false positives. The highest lod scores on chromosomes I (MLS 2.67), 9 (MLS 2.38), IO (MLS 2.27) and 19 (MLS 1.79) fuliil Lander and Kmglyak F definition of suggestive linkage In stage two we genotyped the I6 regions showing an MLS of greater than I in an independent sample of 222 ASPS. using the same markers as in stage one. Of these regions two now yield lod scores in excess of 3 and 4 give lads between 2 and 3. These data will be presented in detail.
E and Alzheimer’s
Virtually all individuals with DS have neuropathological changes consistent with a diagnosis of AD by the time they reach 40 years of age, including deposition of P-amyloid in diffuse and neuritic plaques. While most adults with DS will develop dementia by the end of the seventh decade, the average age at onset is between 50 and 55 years, with a range from 38 to 70. Hence there is a IO-30 year discordance between the presence of neuropathology and the onset of clinical dementia. The neuropathological manifestations of AD in DS have been attributed to tripication and overexpression of the gene for beta-amyloid precursor protein (APP), located on chromosome 21, but the factors influencing age at onset of dementia are unresolved. It ha\ been proposed that clinical dementia is initiated by the transition from diffuse to neuritic plaques. Studies of factors that may influence age at onset of AD by increasing P-amyloid load or accelerating the accumulation of fibrillar aggregates of amyloid p protein and neutitic plaques will be reviewed. Factors that increase or reflect the rate of P-amyloid aggregation and deposition, such as the apolipoprotein E (APOE)-~4 allele, amyloid p protein species ApI-42, male gender. and estrogen deficiency in women-indicated by age at onset of menopause- are associated with earlier onset of dementia in DS. In contrast, the presence of an APOEallele has consistently been associated with improved survival and reduced risk for AD. Atypical karyotypes leading to reduced APP dose (e.g., mosaicism for trisomy 2l), normal APP gene number (e.g.,in partial trisomy 21 )or unusal allelic variabiltty at locus D2lSl I promote longevity without evidence of cogmtive decline. Thew findings point to the importance of amyloid a protein in the development of AD.
E and Alzheimer’s
BIOLOGY AND BIOCHEMISTRY
Karl H. Weisgraber, Gladstone Institute oj’Neumlogiru1 Diseuse, San Francicm
A protein’s structue and biophysical properties are intimately related to function. The common isoforms of apolipoprotein (ape) E have differential effects in Alzheimer’a disease (AD) and on neutite outgrowth in cell culture. To gain insight into the basis for these effects, we have examined the structure and biophysical properties of the isoforms. X-ray crystallography and site-directed mutagenesis studies showed that in apoE4, which has arginine at position 112, Arg-61 in the amino-terminal domain interacts with Glu-255 in the carboxyl-terminal domain. This domain interaction does not occur in apoE or apoE (both have cyst&e at 112) because Arg-61 ia in a different conformation. We hypothesize that domain interaction contributes to the increased risk of AD associated with apoE4. Several species, including the mouse. contain equivalents of Arg-I I2 and Glu-255, but lack the critical Arg-6lrequired for domain interaction in human apoE4, suggesting that introducing an Arg-61 codon into the Apoe gene. might “humanize” mouse apoE to mimic apoE4. We have generated targeted Arg-61 knock-in mice, which are now being characterized at the biochemical, morphological. and behavioral level, particularly with respect to alterations observed rn AD. ApoE isofonns also differ in their stability and lipid binding properties. ApoE is the most stable and apoE% the least stable, whereas apoE is the best lipid binder, suggesting the isoforms differ in their ability to transport lipids. Denaturation studies revealed that apoE forms a stable folding intermediate, which is a prominent species at low pH. whereas apoE and apoE do so to a lesser extent. Since folding intermediates have been implicated in membrane translocation and cellular processing of proteins, it is possible that apoE is differentially processed by cells and that some may escape lysosomal degradation and tramlocate into the cytoplasm following internalim.tion. Domain interaction and differences in hpid binding or cellular processing may underlie the differential effect of the apoE isoforma on neurite outgrowth and AD.
ROLE OF APOE NEURODEGENERATION
David M Holtzman. Anne M Fagan, Washington University, St. Louis. MO; Kelly R. Bales, Eli Lil/y and Company, Indianapoli.~, IN: Ronald DrMattm. Washington University. St. Louis, MO: Michael C Irizmy, Mass General Hospital, Boston, MA; Karen H Ashe, Universi~ of Minnesota, Minneapolis, MN; Bradley T Hymn, Mass General Hospital, Boston, MA: Steven M. Paul, Eli Lilly and Company, Indianapolis, IN
Down syndrome(DS), defined cytogenetically by trisomy 21, is the most common chromosomal disorder associated with mental retardation. Older indivdiuals with DS show age-related changes in health and functional capacities suggestive of premature or accelerated aging. The most extensively studied aspect of aging in this population is the unique association between DS and Alzheimer’s disease(AD), and it has been suggested that DS may serve as a model for the study of pathogenic factors in AD.
The ~4 allele of apolipoprotein E (apoE) is an important genetic risk factor for Alzheimer’s disease (AD) as well as cerebral amyloid angiopathy (CAA). Increasing evidence suggests that one of the major reasons for this association is related to the ability of apoE to interact with the amylaid-! (AP) peptide and influence its concentration and structure. To determine the effect of apoE on A!3 deposition and AD as well as CAA-related pathology, we have analyzed 2 different amyloid precursor protein (APP) transgenic mice, APPsw and APPy7”‘, expressing human apoE isoforms, mouse apoE, or no apoE (apoE -/-). In both APPsw and APP”“” mice, [email protected]
deposition was attenuated but still occurred in the absence of apoE. In
Nicolr Schupf NYS Institute fh- Basic Research, Staten Islund, NY