Genetic Studies of Chromosome 14 Linked Familial AD

The principal objective of the current proposal is to precisely define the chromosomal location of the gene associated with early onset Familial Alzheimer Disease (termed the FAD3 gene) which we have mapped to chromosome 14 in a previous proposal to the AHAF. The cloning and characterization of the FAD3 gene will likely provide powerful new, and otherwise unattainable insights into the biochemistry of AD which will complement and extend the knowledge already accrued from the identification of two other genes associated with inherited susceptibility to AD (APP and ApoE). Because all of the currently known candidate genes on chr 14 (eg cFOS, TGFB3, etc) have been excluded as the site of the FAD3 mutation by studies performed during the past two years under funding provided by AHAF, it will now be necessary to attempt to clone the FAD3 gene based simply upon its chromosomal location. However, an essential prerequisite for such a positional cloning strategy is a very precise localization of the FAD3 gene on chr 14 relative to a set of genetic landmarks which closely flank the disease gene. In the current research proposal we will acquire additional pedigrees segregating the FAD3 gene and we will generate additional genetic markers including markers in the 5′ end (beginning) of genes. These resources will then be applied using two different genetic paradigms in order to refme the genetic position of the FAD3 gene. The first application of these resources will be in traditional recombinational mapping methods which will benefit from these reagents by allowing an increased number of genetic events (meioses) carrying the FAD3 gene defect to be tested with more informative genetic markers in order to better define the position of the FAD3 gene relative to this larger set of genetically and physically mapped genetic markers. The second application of these resources will be to attempt to define a cluster of markers at which all affected subjects of the same ethnic origin display the same alleles. The latter phenomenon, known as linkage disequilibrium or allelic association, reflects the inheritance of a small region surrounding the disease gene on the remote ancestral “founder” chromosome on which the original mutation first occurred in the ethnic group under investigation. The discovery of a small region of chr 14 showing linkage disequilibrium/allelic association has the potential to dramatically narrow the region of interest surrounding the FAD3 gene, and has been a powerful strategy which has been successfully applied to clone other disease genes.

In addition to the goal of cloning the chr 14 FAD3 gene, we have also generated preliminary evidence which indicates the existence of at least one further FAD gene on another chromosome. Like the FAD3 gene, cloning and characterization of these other genes will shed new light on the mechanisms which lead to AD. In this proposal we will initiate attempts to define the chromosomal location of these other genes using conventional genetic linkage methods. We will initially test the hypotheses that other genes on chromosomes other than chr 14, 19, or 21 but functionally related to these genes may be involved. If these candidate genes can be excluded we will then systematically search the remainder of the genome using anonymous markers for each chromosome.