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Context: The only genetic locus universally accepted to be important as a risk factor for late-onset Alzheimer disease (AD) is the apolipoprotein E (APOE) locus on chromosome 19. However, this locus does not account for all the risk in late-onset disease, and a recent report has suggested a second locus on chromosome 12p11-12.Objective: To look for evidence of linkage on chromosome 12 and to test for the presence of the new locus in an independent sample of familial late-onset AD cases.Design: Retrospective cohort study. As part of a 20-centimorgan genome screen (approximately equal to 200 markers), we tested a series of 18 genetic markers on chromosome 12 and carried out multipoint, nonparametric lod score and exclusion analyses.Setting: Clinic populations in the continental United States selected from the National Institute of Mental Health AD Genetics Consortium.Patients: We selected samples for DNA analysis from affected sibling pairs, 497 subjects from 230 families with 2 or more affected individuals with probable or definite AD with onset ages older than 60 years (mean+/-SD, 75+/-6 years). Within the families, we used the 2 probable or definitely affected individuals. In families with more than 2 such cases available, we used all of them; in families with only 2 such cases in which unaffected individuals were available, we also sampled the oldest unaffected individual and used genotype data from this unaffected individual to check for nonpaternity and genotyping errors.Main Outcome Measure: Presence of linkage or locus on chromosome 12.Results: Although linkage analyses confirmed the presence of a genetic susceptibility factor at the APOE locus in these families with late-onset AD, we were unable to confirm the presence of a locus close to the marker D12S1042. A moderate lod score (1.91) was found near D12S98 close to the alpha2-macroglobulin locus in the affected pairs in which both members did not possess an APOE epsilon4 allele.Conclusions: APOE remains the only locus established to be a risk factor for late-onset AD. We were unable to confirm that a locus on chromosome 12p11-12 has a major effect on risk for late-onset AD, although an effect smaller than that for APOE cannot be excluded. [ABSTRACT FROM AUTHOR]

Molecular genetic analysis is revealing the etiologies of Alzheimer's disease (AD) and related dementias. Here we review genetic and molecular biological evidence suggesting that the peptide Aβ42 is central to the etiology of AD. Recent data also suggests that dysfunction in the cytoskeletal protein tau is on the pathway that leads to neurodegeneration and dementia. Tau is produced either indirectly, by Aβ42, or directly, in some forms of frontotemporal dementia by mutations in tau itself. These data support and refine the amyloid cascade hypothesis for AD and suggest that understanding the causes and consquences of tau dysfunction is an important priority for dementia research. [ABSTRACT FROM AUTHOR]

Comments on an article on the genetics of Alzheimer's disease (AD) in a Finnish family, previously published in the periodical 'Nature Medicine' in 1999. Link of AD to a mutation in a chromosome region; Occurrence of spastic paraparesis.