Protective effect of apolipoprotein E type 2 allele for late onset Alzheimer disease.

Gene dosage of the apolipoprotein E (APOE) epsilon 4 allele is a major risk factor for familial Alzheimer disease (AD) of late onset (after age 60). Here we studied a large series of 115 AD case subjects and 243 controls as well as 150 affected and 197 unaffected members of 66 AD families. Our data demonstrate a protective effect of the epsilon 2 allele, in addition to the dose effect of the epsilon 4 allele in sporadic AD. Although a substantial proportion (65%) of AD is attributable to the presence of epsilon 4 alleles, risk of AD is lowest in subjects with the epsilon 2/epsilon 3 genotype, with an additional 23% of AD attributable to the absence of an epsilon 2 allele. The opposite actions of the epsilon 2 and epsilon 4 alleles further support the direct involvement of APOE in the pathogenesis of AD.

Gene dose of apolipoprotein E type 4 allele and the risk of Alzheimer's disease in late onset families.

The apolipoprotein E type 4 allele (APOE-epsilon 4) is genetically associated with the common late onset familial and sporadic forms of Alzheimer's disease (AD). Risk for AD increased from 20% to 90% and mean age at onset decreased from 84 to 68 years with increasing number of APOE-epsilon 4 alleles in 42 families with late onset AD. Thus APOE-epsilon 4 gene dose is a major risk factor for late onset AD and, in these families, homozygosity for APOE-epsilon 4 was virtually sufficient to cause AD by age 80.

Longitudinal follow-up of late-onset Alzheimer disease families.

Historically, data for genetic studies are collected at one time point. However, for diseases with late onset or with complex phenotypes, such as Alzheimer disease (AD), restricting diagnosis to a single ascertainment contact may not be sufficient. Affection status may change over time and some initial diagnoses may be inconclusive. Follow-up provides the opportunity to resolve these complications. However, to date, previous studies have not formally demonstrated that longitudinally re-contacting families is practical or productive. To update data initially collected for linkage analysis of late-onset Alzheimer disease (LOAD), we successfully re-contacted 63 of 81 (78%) multiplex families (two to 17 years after ascertainment). Clinical status changed for 73 of the 230 (32%) non-affected participants. Additionally, expanded family history identified 20 additional affected individuals to supplement the data set. Furthermore, fostering ongoing relationships with participating families helped recruit 101 affected participants into an autopsy and tissue donation program. Despite similar presentations, discordance between clinical diagnosis and neuropathologic diagnosis was observed in 28% of those with tissue diagnoses. Most of the families were successfully re-contacted, and significant refinement and supplementation of the data was achieved. We concluded that serial contact with longitudinal evaluation of families has significant implications for genetic analyses.

An autosomal genomic screen for dementia in an extended Amish family.

Apolipoprotein E (APOE) is the only universally confirmed susceptibility gene for late-onset Alzheimer disease (LOAD), although many loci are believed to modulate LOAD risk. The genetic homogeneity of isolated populations, such as the Amish, potentially provide increased power to identify LOAD susceptibility genes. Population homogeneity in these special populations may reduce the total number of susceptibility genes contributing to the complex disorder, thereby increasing the ability to identify any one susceptibility gene. Dementia in the Amish is clinically indistinguishable from LOAD in the general population. Previous studies in the Amish demonstrated a significantly decreased frequency of the APOE-4 susceptibility allele, but significant familial clustering of dementia [M.A. Pericak-Vance, C.C. Johnson, J.B. Rimmler, A.M. Saunders, L.C. Robinson, E.G. D'Hondt, C.E. Jackson, J.L. Haines, Alzheimer's disease and apolipoprotein E-4 allele in an Amish population, Ann. Neurol. 39 (1996) 700-704]. These data suggested that a genetic etiology independent of APOE may underlie the dementia observed in this population. In the present analysis, we focused on a large, multiplex, inbred Amish family (24 sampled individuals; 10 of whom are affected). We completed a genomic screen to identify novel LOAD loci (n=316 genetic markers), using both model-dependent "affecteds-only" analysis (dominant and recessive) and model-independent affected relative pair analysis. Interesting results (lod>1.5 or p<0.01) were obtained for markers on eight chromosomes (2q, 5q, 6q, 7p, 8p, 8q, 11p, 18p, 18q, and 19q). The highest overall score was a multipoint lod score of 3.1 on chromosome 11p. Most regions we identified were not previously detected by genomic screens of outbred populations and may represent population-specific susceptibilities to LOAD. These loci are currently under further investigation in a study of LOAD including additional Amish families.

Neuropathological features of frontotemporal dementia and parkinsonism linked to chromosome 17q21-22 (FTDP-17): Duke Family 1684.

Frontotemporal dementia with parkinsonism (FTDP-17) is an autosomal dominant disorder that presents clinically with dementia, extrapyramidal signs, and behavioral disturbances in mid-life and progresses to death within 5 to 10 years. Pathologically, the disorder is characterized by variable neuronal loss and gliosis in the frontal and temporal lobes, limbic structures, and the midbrain. Autopsied individuals from some kindreds display abundant neurofibrillary change while others, including a single affected individual from Duke Family 1684, lack distinctive histological features and exhibit only mild neuronal loss and gliosis in limbic structures and subcortical nuclei when examined by routine silver stain. Recently, mutations in the microtubule associated protein tau have been shown to segregate with the disease in this family and in many other affected kindreds. In order to examine the distribution of tau deposits, we performed tau immunohistochemistry, immunoblotting, and immunoelectron microscopy of tau-containing filaments. Immunohistochemistry revealed numerous tau deposits within glial cells and within neurons. Twisted ribbon-like filaments observed by immunoelectron microscopy were immunodecorated with tau AT8 antibody. Sarkosyl-insoluble tau extracted from the hippocampus and cortex migrated as 2 major bands at 64 and 68 kilodaltons and a minor band at 72 kilodaltons, which after alkaline phosphatase treatment appeared to contain mainly tau isoforms with 4 repeats. Furthermore, the ratio of soluble tau with 4 to 3 microtubule-binding repeats was increased. The role of tau mutations in this disorder is discussed in this paper.

Delayed onset of Alzheimer's disease with nonsteroidal anti-inflammatory and histamine H2 blocking drugs.

Factors that modify onset of Alzheimer's disease (AD) may be revealed by comparing environmental exposures in affected and unaffected members of discordant twin pairs or sibships. Among siblings at high risk of AD, sustained use of nonsteroidal anti-inflammatory drugs (NSAIDs) was associated with delayed onset and reduced risk of AD. After adjustment for use of NSAIDs, there was minimal effect on onset with reported history of any of three common illnesses (arthritis, diabetes, or acid-peptic disease). However, independent of exposure to NSAIDs, onset was unexpectedly delayed in those reporting extended use of histamine H2 blocking drugs. Randomized clinical trials will be needed to affirm the utility of these drugs for prevention, but the present findings may have implications for pathogenesis: because NSAIDs block the calcium-dependent postsynaptic cascade that induces excitotoxic cell death in NMDA-reactive neurons, and because histamine potentiates such events, excitotoxicity may deserve additional investigation in AD.

Complete genomic screen in late-onset familial Alzheimer's disease.

Alzheimer's disease (AD) is a complex genetic disorder. Linkage analysis has helped unravel a portion of the genetic component of AD by identifying four loci that play a role in the genetics of AD (amyloid precursor protein, presenilin 1, presenilin 2, and apolipoprotein E). These loci account for approximately 50% of the genetic etiology of AD. A total genomic screen is an efficient way to identify additional genetic effects in AD. A series of multiplex late-onset (>60 years) AD families were ascertained (NINDS-ADRDA diagnostic criteria) and sampled. A subset (n = 16) of the largest families (52 affecteds with DNA, 83 unaffecteds with DNA) were used to rapidly screen the genome (n = 280 markers) for additional major genetic effects. Critical values for regional follow-up were p < or =0.05 for SimIBD or sibpair analysis and/or a LOD score > or = 1.00. Fifteen regions warranted initial follow-up based on these criteria. An additional screening set was used (n = 38 families, 89 affecteds with DNA, 216 unaffecteds with DNA) for the follow-up analysis. These analyses revealed four regions of continued interest on chromosomes 4, 6, 12, and 20. Chromosome 12 presented the strongest results. Peak two point "affecteds only" LOD scores were 1.3, 1.6, 2.7, and 2.2 and (affected relative pair SimIBD) p values were 0.04, 0.03, 0.14, and 0.04 for D12S373, D12S1057, D12S1042, and D12S390, respectively. These markers span approximately 30 cm near the centromeric region of chromosome 12. Sibpair analysis resulted in two point Maximum Lod Score (MLS) results of 0.4, 1.2, 3.2, and 1.0 for the above markers. Multipoint MLS analysis supported these findings. Saturation mapping of all available markers in the chromosome 12 region as well as further investigation of the regions on 4, 6, and 20 is ongoing with candidate gene analysis to follow.

Tight linkage of apolipoprotein C2 to myotonic dystrophy on chromosome 19.

The cDNA and genomic probes for apolipoprotein C2 detect two restriction fragment length polymorphisms on chromosome 19. The combined estimated percentage of heterozygosity, assuming equilibrium, is approximately 75%, ie, apolipoprotein C2 is informative in 75% of matings. We have analyzed over 350 individuals in large multigenerational families for linkage of apolipoprotein C2 to myotonic muscular dystrophy. The maximum lod score was 16.29 with the maximum recombination fraction (theta) of 0.02, with 95% confidence limits for theta of 0.001 to 0.065. Thus, apolipoprotein C2 is useful in carrier detection and prenatal diagnosis with an accuracy of about 98%.

Linkage studies in Charcot-Marie-Tooth disease type 2: evidence that CMT types 1 and 2 are distinct genetic entities.

Charcot-Marie-Tooth disease (CMT), the most common inherited peripheral neuropathy, is a progressive sensorimotor neuropathy divided into types 1 and 2 based upon electrophysiologic and neuropathologic differences. The more common autosomal dominant form of CMT type 1 (hereditary motor and sensory neuropathy type I) is genetically heterogeneous, with genes located on chromosomes 1 (type 1B) or 17 (type 1A). However, no locus for CMT type 2 is known. We have performed linkage studies on three large multigenerational CMT type 2 families using probes from chromosome 1 and chromosome 17, which span their respective linkage regions. Multipoint analysis of the chromosome 17 markers excluded linkage over an area of 45 cM--15 cM proximal and 30 cM distal to the region containing CMT type 1A. Multipoint analysis of the chromosome 1 markers exclude linkage 15 cM proximal and 20 cM distal to FC-gamma-RII in the region of CMT 1B. These data indicate that CMT type 2 is genetically distinct from CMT type 1.

Apolipoprotein E, survival in Alzheimer's disease patients, and the competing risks of death and Alzheimer's disease.

The apolipoprotein E (APOE) epsilon 4 allele carries an increased risk of a patient developing Alzheimer's disease (AD) while the epsilon 2 allele carries a decreased risk. We compared survival from the onset of AD in subjects with different numbers of epsilon 4 alleles and evaluated changes in genotypic frequencies with age. Two subject groups were investigated: unrelated AD case and control subjects, and affected and unaffected members from 74 multiplex AD families. In both subject groups, survival from onset decreased with increasing onset age, was longer in women, and was unrelated to epsilon 4 gene dose. The epsilon 2/epsilon 3 genotype became more common with age (p = 0.004). The epsilon 4 allele decreased in frequency with age in all patient groups but, unexpectedly, remained unchanged in control subjects. We conclude that the progression of AD is not strongly related to epsilon 4 gene dose, that the higher prevalence of AD in women may involve the longer survival of affected women, and that AD and death are competing risks involving APOE that change over time.

Hereditary motor and sensory neuropathy, X-linked: a half century follow-up.

The existence of an X-linked sensorimotor peripheral neuropathy has been debated. We reevaluated the original family, and present data on 13 affected males and 25 obligate or probable heterozygous females, documenting the devastating nature of the disease in the men and the extremely variable degree of clinical involvement in the carriers. Use of DNA probes indicates that the gene lies in the DXYS1-p58-1 region of the X-chromosome.

Tight linkage of creatine kinase (CKMM) to myotonic dystrophy on chromosome 19.

The myotonic dystrophy (DM) gene is localized to the proximal long arm of chromosome 19. There have been reports of tight linkage to a number of chromosome 19 markers, including APOC2 and creatine kinase muscle type (CKMM), but they did not establish orientation of the 2 markers to DM. We screened several large multi-generational DM families for linkage to a series of chromosome 19 markers including CKMM. CKMM is tightly linked to DM in these data with z(theta) = 28.41; theta = 0.01. Analysis of cross-over data indicates CKMM is on the same side and closer to DM than APOC2. Thus, CKMM is a useful probe for carrier detection studies in presymptomatic individuals as well as for prenatal diagnosis.

Development of a microsatellite genetic map spanning 5q31-q33 and subsequent placement of the LGMD1A locus between D5S178 and IL9.

Limb-girdle muscular dystrophy (LGMD) is a genetically and clinically heterogeneous group of disorders. We previously localized an autosomal dominant form of the disorder (LGMD1A) to chromosome 5q22-31 by linkage analysis in a single large pedigree. After developing a microsatellite genetic map incorporating six loci in q31-33 of chromosome 5 and spanning 35 cM, we have refined the original localization. Using multipoint analysis, LGMD1A is localised to a 7 cM region between the markers IL9 and D5S178 with odds > 1000:1.

Confirmation of linkage of oculopharyngeal muscular dystrophy to chromosome 14q11.2-q13 in American families suggests the existence of a second causal mutation.

Oculopharyngeal muscular dystrophy (OPMD) is a late-onset, autosomal dominant disorder characterized by progressive ptosis, dysphagia, and extremity weakness. Linkage of OPMD to 14q11.2-q13 has been reported in a series of French-Canadian families. Tightly linked markers have been defined and haplotype analysis in these data show a single segregating disease chromosome throughout the OPMD French-Canadian families. We have ascertained and sampled five multigenerational outbred American OPMD families. Four of the five families have known French-Canadian ancestry while the fifth is of English/Scottish origin. Linkage analysis was performed using standard likelihood methods. A peak multipoint lod score of 6.30 was obtained for the marker MYH7.1 in the OPMD families. The English/ Scottish family exhibited a different chromosomal haplotype for the OPMD alleles than the families of French-Canadian origin. These data suggest this family may represent a second, possibly independent mutation in this disorder. Linkage was confirmed to chromosome 14q11.2-q13 with no evidence of genetic heterogeneity.

Confirmation of locus heterogeneity in the pure form of familial spastic paraplegia.

Familial spastic paraplegia (FSP), characterized by progressive spasticity of the lower extremities, is in its "pure" form generally of autosomal dominant inheritance pattern. Hazan et al. [Nat Genet 5:163-167, 1993] reported tight linkage of a large FSP family to the highly polymorphic microsatellite marker D14S269 with z (theta) = 8.49 at theta = 0.00 They further demonstrated evidence for locus heterogeneity when they showed that 2 FSP families were unlinked to this region. We have subsequently studied 4 FSP families (3 American, one British) and excluded the disease locus in these families for approximately 30 cM on either side of D14S269, thereby confirming evidence for locus heterogeneity within the spastic paraplegia diagnostic classification.

No association between alpha 1-antichymotrypsin and familial Alzheimer's disease.

Alzheimer's disease (AD) is the most common mid to late age-of-onset neurodegenerative disorder. AD has a strong and complex genetic etiology, and multiple genes, acting independently and/or interacting, likely affect the risk of developing AD. Several genes involved with AD already have been described, but only the APOE gene on chromosome 19q has been shown to affect the risk of the most common form of AD, occurring with onset over the age of 65. Because a substantial portion of late-onset AD is not explained by APOE, other genes affecting late-onset AD likely occur. These could act either independently or perhaps interact with APOE. alpha 1-Antichymotrypsin (ACT) is a major component of the amyloid plaques found in the brains of AD patients and may play a role in the pathophysiology of AD. It has been proposed that a specific polymorphism within the ACT gene interacts with APOE to increase the risk of developing AD. Our results do not confirm this finding.

No association between very low density lipoprotein receptor (VLDL-R) and Alzheimer disease in American Caucasians.

The very low density lipoprotein receptor gene (VLDL-R) is a receptor for apolipoprotein-epsilon (APOE)-containing lipoproteins, and thus has been suggested as a possible risk factor for Alzheimer disease (AD). Recently, Okuizumi et al. [Nature Genet, II (1995) 207-209] reported an association between the 96 bp allele at the VLDL-R locus and AD in a Japanese population. The association resulted in a two-fold increase of risk that decreased with increasing age. We have examined this association in 316 Caucasian sporadic AD patients, comparing their findings to 160 Caucasian AD spouse controls. We also investigated 53 late-onset Caucasian AD families for association and linkage. Our data failed to confirm linkage and/or association to the VLDL-R locus. Stratification by age at onset or APOE genotype also failed to show significant results.

No association of alpha1-antichymotrypsin flanking region polymorphism and Alzheimer disease risk in early- and late-onset Alzheimer disease patients.

The alpha1-antichymotrypsin (AACT)-155 allele was found elsewhere to have a significant effect on Alzheimer disease (AD) risk in individuals with at least one APOE-4 allele. We compared AACT genotypes of 284 cases of sporadic AD and 172 controls. The frequency of the AACT-155 allele did not differ significantly between cases and controls, either overall or when restricted to subjects with at least one APOE-4 allele. Logistic regression controlling for age and sex failed to show an effect due to AACT either alone or acting with APOE. There was no evidence of an interaction between APOE-4 and the AACT-155 allele to reduce age at onset. Thus, our data do not support an association of AACT-155 with risk or age at onset in AD.