The effort to identify genes with alleles that influence susceptibility to late onset AD (LOAD) proceeds logically from linkage to association and then to identification of specific susceptibility alleles. Using plasma amyloid beta protein (Abeta42) levels as an intermediate, quantitative phenotype for LOAD, we obtained linkage at approximately 80 centimorgans (cM) on chromosome 10 (Ch10). Linkage to the same region was obtained independently in a study of affected LOAD sib-pairs. Pursuing these findings, we have now identified three Ch10 genes (VR22, PLAU, and IDE) with variants that show strong association with LOAD and/or plasma Abeta42. The proposed study focuses exclusively on IDE. There is compelling biological evidence from IDE knockout mice that the insulin degrading enzyme is normally involved in Abeta degradation, so IDE is clearly an excellent candidate gene. In our MCJ series, LOAD showed highly significant (p<10[-7]) association with the same IDE haplotypes that the Brookes group showed to be significantly (p<0.01) or highly significantly (p<10[-9]) associated with AD in multiple case-control series from Scotland and Sweden. These same haplotypes showed significant association with plasma Abeta42 in our extended LOAD series. The goal of this study is to identify the LOAD susceptibility alleles in the IDE region. In all LOAD patients from the MCJ series, we will thoroughly screen the coding and putative regulatory regions of the IDE gene and, if necessary, adjacent genes. The SNPs that we find (approximately 60 are expected from our initial screen of IDE) will be analyzed using a two stage statistical approach to identify the set of putative susceptibility alleles that show strongest, most reproducible association with LOAD and with plasma Abeta42. Strongly associating SNPs will then be evaluated for biological effects relevant to LOAD. To facilitate analysis of variants in putative regulatory regions and to increase the size of our case-control cohort, we will prepare cDNA and genomic DNA from the large number of frozen LOAD brains available through the Neuropathology Core.
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