Apolipoprotein E genotype does not affect age at onset in patients with chromosome 14 encoded Alzheimer's disease

APOE and other loci affect age-at-onset in Alzheimer's disease families with PS2 mutation

Several kindreds of Volga German (VG) ancestry have a single PS2 mutation that causes an autosomal dominant form of Alzheimer's disease (AD). These families show a wide range in age-at-onset, which suggests the existence of modifying factors other than the PS2 mutation. To examine evidence for a genetic basis of variation in onset age, we performed a Bayesian oligogenic segregation and linkage analysis on nine VG families confirmed to have at least one affected PS2 carrier. This analysis simultaneously estimated the effects of APOE and PS2 and the number and effects of additional loci affecting AD age-at-onset. In addition, a family effect accounted for shared environmental effects. This analysis approach has the advantage of full use of the complete pedigree structure, as well as use of information on unsampled individuals with phenotypic data. These analyses provide evidence that APOE plays a small, but significant, role in modifying the age-at-onset in these VG families. The effects estimated for the APOE epsilon3 and epsilon4 genotypes were consistent with those estimated in previous analysis of late-onset AD families, with evidence for a dose-dependent relationship between number of epsilon4 alleles and age-at-onset. We estimated an approximately 83% posterior probability of at least one modifier locus in addition to APOE, and that the fraction of the variance in age-at-onset attributable to PS2, APOE, other loci, and family effects is approximately 70, approximately 2, approximately 6.5, and approximately 8.5%, respectively. These results provide evidence that APOE and other loci modify onset in AD caused by PS2 mutation.

Genetic factors in the genesis of Alzheimer's disease

At least four different genes have been identified that are associated with inherited susceptibiltiy to Alzheimer's disease. Some of the genes are highly penetrant (PS1, PS2, beta APP); the other, APOE, is a weaker susceptibility factor. Several additional genes are suspected to exist but have not yet been cloned.

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.

The genetics of Alzheimer's disease

Alzheimer's disease (AD) is the major cause of dementia in the U.K. The clinical diagnosis of the specific disease resulting in dementia is unreliable and thus a definitive diagnosis of AD is best made in conjunction with post-mortem findings of amyloid plaques and neurofibrillary tangles. Alzheimer's disease is neuropathologically indistinguishable in the young and old, but has been divided arbitrarily into early- and late-onset disease using age cut-offs of 60 or 65 years. Twin and family studies suggest that genetic factors play a major role in its aetiology. This review considers the three loci which have been shown to be associated with early-onset AD: amyloid precursor protein, presenilin (PS)-1 and PS-2. Mutations in these genes seem to be associated with overproduction of the 42-amino acid form of beta-amyloid, suggesting that this may be a central pathological process in AD. The impact of the different apo E alleles on the risks for late- and early-onset AD is discussed and compared with other dementing conditions. Recent analyses suggest that there are likely to be other genes besides apo E which impact on late-onset AD risk. The possible roles in AD of the mitochondrial mutation at position 4336, the PS intron 8 polymorphism, and variants in the alpha 1-antichymotrypsin and VLDL-receptor genes, are considered.

The presenilin genes: a new gene family involved in Alzheimer disease pathology

A positional cloning approach has led to the identification of two closely related genes, the presenilins (PS), for autosomal dominant presenile Alzheimer disease (AD): PS-1 at 14q24.3 and PS-2 at 1q31-q42. The PS-1 gene was identified by direct cDNA selection of yeast artificial chromosomes containing the candidate chromosomal region. Subsequently, the PS-2 gene was identified due to its high sequence homology with PS-1 and its location within the candidate region defined by linkage studies. To date, 30 different missense mutations and one in-frame splice site mutation were described in PS-1, while only two missense mutations were detected in PS-2, suggesting that PS-1 mutations are more frequently involved in familial presenile AD. The PS transcripts encode novel proteins that resemble integral transmembrane proteins of roughly 450 amino acids and at least seven transmembrane domains. The genomic organization of the PS genes is very similar showing that full length PS-1 and PS-2 are encoded by 10 exons. However, different alternative splicing patterns have been observed for PS-1 and PS-2 indicating that the corresponding proteins (ps-1 and ps-2) may have similar but not identical biological functions.