Absence of the amyloid precursor protein gene mutation (APP717: Val->Ile) in 85 cases of early onset Alzheimer's disease

No founder effect in three novel Alzheimer's disease families with APP 717 Val-->Ile mutation. Clerget-darpoux. French Alzheimer's Disease Study Group

We sequenced exons 16 and 17 of the APP (amyloid precursor protein) gene in 18 unrelated French Alzheimer's disease (AD) patients. These patients had an onset before the age of 60 and belonged to families with autosomal dominant transmission of the disease. We detected the APP 717 Val-->Ile mutation in three out of 18 (16.6%) families. In these three families, all affected subjects had the APOE 3/3 genotype, but their ages of onset ranged from 38 to 60 years, indicating that factors other than the APOE genotype influence age of onset. Analysis of two polymorphic loci adjacent to the APP gene showed that at least two independent mutational events had occurred within these pedigrees, in spite of their origin in the same region of France.

Evidence for apolipoprotein E epsilon 4 association in early-onset Alzheimer's patients with late-onset relatives

Recently several reports have extended the apolipoprotein E (APOE) epsilon 4 association found in late-onset Alzheimer's disease (LOAD) patients to early-onset (EO) AD patients. We have studied this question in a large population of 119 EOAD patients (onset < or = 60 years) in which family history was carefully assessed and in 109 controls. We show that the APOE epsilon 4 allele frequency is increased only in the subset of patients who belong to families where LOAD secondary cases are present. Our sampling scheme permits us to demonstrate that for an individual, bearing at least one epsilon 4 allele increases both the risk of AD before age 60 and the probability of belonging to a family with late-onset affected subjects. Our results suggest that a subset of EOAD cases shares a common determinism with LOAD cases.

Characteristics of familial aggregation in early-onset Alzheimer's disease: evidence of subgroups

Characteristics of familial aggregation of Alzheimer's Disease were studied in 92 families ascertained through a clinically diagnosed proband with an onset below age 60 years. In each family data were systematically collected on the sibships of the proband, of his father, and of his mother. A total of 926 relatives were included and 81% of the living relatives (i.e., 251 individuals) were directly examined. The estimated cumulative risk among first degree relatives was equal to 35% by age 89 years (95% confidence interval 22 to 47%). This result does not support the hypothesis that an autosomal dominant gene, fully penetrant by age 90 years, is segregating within all these pedigrees. Despite the fact that all probands were selected for an onset before age 60 years it was shown that two types of families could be delineated with respect to age at onset among affected relatives: all secondary cases with an onset below age 60 years were contributed by a particular group of families (type 1 families), whereas all secondary cases with an onset after age 60 years were contributed by another group of families (type 2 families). Although genetic interpretation of these findings is not straightforward, they support the hypothesis of etiologic heterogeneity in the determination of early-onset Alzheimer's disease.

Allelic association at the D14S43 locus in early onset Alzheimer's disease. French Alzheimer's Disease Collaborative Study Group

The D14S43 marker is closely linked to the major gene for early onset autosomal dominant Alzheimer's disease on chromosome 14. Allelic frequencies at the D14S43 locus were compared in 113 familial and isolated cases of early onset Alzheimer's disease (< 60 years of age at onset) (EOAD) and 109 unaffected individuals of the same geographic origin. Allele 7 was significantly (P = 0.033) more frequent in type 1 EOAD patients (13.2%), defined by the presence of at least another first degree relative with EOAD, than in controls (4.1%). Since an autosomal dominant gene is probably responsible for type 1 patients, allelic association may reflect linkage disequilibrium at the D14S43 locus. This would mean that some patients share a common ancestral mutation. However, since multiple tests were carried out, this result must be interpreted with caution, and needs confirmation in an independent sample.

Molecular basis and diagnosis of neurogenetic disorders

Over the past few years, molecular neurogenetics has developed into one of the most promising and active research fields. The new discipline applies modern molecular genetic techniques to the investigation of classical neurological disorders. In the following article, a definition of neurogenetic disease is introduced, the molecular basis of four groups of neurogenetic disorders is described and recent diagnostic developments are presented. The first group of diseases is caused by trinucleotide expansions. "Expanding" trinucleotide repeats were not known to occur in any species until about three years ago. Today, disorders such as Huntington's disease, spinocerebellar ataxia type 1, fragile X mental retardation, spinobulbar muscular atrophy and myotonic dystrophy are all known to be caused by the expansion of trinucleotides. The second group is characterized by chromosomal deletions or uniparental disomies. Lissencephaly and the Miller-Dieker syndrome, Prader-Willi and Angelman syndromes and Duchenne and Becker muscular dystrophies belong to this category. The third group includes those neurogenetic disorders that are mainly caused by point mutations such as the X-linked leukodystrophies, including Pelizaeus-Merzbacher disease and adrenoleukodystrophy, Charcot-Marie-Tooth syndrome type 1, familial forms of amyotrophic lateral sclerosis, several types of craniosynostoses and some CNS tumor syndromes. Finally, Alzheimer's and Parkinson's disease are discussed as representatives of group four, i.e. genetically heterogeneous neurological disorders.