Over the past 15 years, several genes and genetic variants have been established that either cause Alzheimer disease (AD) or increase disease susceptibility. However, there is growing consensus that several additional disease genes remain to be identified. Recent screens for novel AD genes have focused on either very early onset (i.e. <= 50 years) autosomal dominant AD, or on the more common late-onset (i.e. >65 or 70 years) form of the disease. Meanwhile, only a few studies have examined patients and families with a more """"""""intermediate"""""""" onset age. Notwithstanding, this group of patients displays several desirable features which facilitate the search for novel disease genes (e.g. larger pedigrees, smaller number of phenocopies, more reliable family history, higher a priori probability of a genetic cause). In this application, we propose to identify and characterize novel early- and intermediate-onset AD genes in the NIMH Genetics Initiative AD sample, which is the largest uniformly ascertained and evaluated sample assembled for the study of AD genetics to date. In a recently completed full genome screen of this sample (Blacker, 2003), 4 of the 13 either """"""""significant"""""""" or suggestive linkage regions were found in families exhibiting an early- or mixed- onset age (on chromosomes 3p26, 14q24, 15q26, and 19q13). Currently, no funds are available for further investigation of the signals predominantly observed in early- and mixed-onset families, which comprise approximately 1/3 of the total NIMH sample (i.e. 517 subjects from 131 pedigrees). A re-screen of the genotype data in these families using more finely grained onset-age strata and extended parametric analyses confirmed and refined the above mentioned signals and revealed four additional novel early-onset linkage peaks (on 1p32, 8q24, 17q21 and 21q22). In this project, we propose to systematically follow-up all of the early- and intermediate-onset signals in the NIMH sample. Families linked to 14q24 and 21q22 will be screened for mutations in PSEN1 and APP and excluded from subsequent analyses. Linkage regions will then be refined by typing of additional microsatellite markers. In confirmed regions, we will then follow a carefully devised strategy combining mutational screening by direct sequencing (in the most strongly linked families) and association testing using single-locus and haplotype analyses in positional candidate genes. Any variants found to be strongly associated with AD will also be phenotypically and functionally characterized using a variety of statistical and experimental methods. While we have devised a strategy that integrates the best available methods to identify complex disease genes, we will routinely adjust our strategy to account for methodologic advances in this rapidly evolving field.
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