Using linkage and association to identify and model genetic effects: summary of GAW15 Group 4

A composite-likelihood approach for identifying polymorphisms that are potentially directly associated with disease

If a linkage signal can be fully accounted for by the association of a particular polymorphism with the disease, this polymorphism may be the sole causal variant in the region. On the other hand, if the linkage signal exceeds that explained by the association, different or additional directly associated loci must exist in the region. Several methods have been proposed for testing the hypothesis that association with a particular candidate single-nucleotide polymorphism (SNP) can explain an observed linkage signal. When several candidate SNPs exist, all of the existing methods test the hypothesis for each candidate SNP separately, by fitting the appropriate model for each individual candidate SNP. Here we propose a method that combines analyses of two or more candidate SNPs using a composite-likelihood approach. We use simulations to demonstrate that the proposed method can lead to substantial power increases over the earlier single SNP analyses.

The neuronal sortilin-related receptor gene SORL1 and late-onset Alzheimer's disease

Recent studies indicate that two clusters of single nucleotide polymorphisms in the neuronal sortilin-related receptor gene (SORL1) are causally associated with late-onset Alzheimer's disease (AD). At the cellular level, SORL1 is thought to be involved in intracellular trafficking of amyloid precursor protein. When this gene is suppressed, toxic amyloid beta production is increased, and high levels of amyloid betaare associated with a higher AD risk. Extending the cellular findings, gene expression studies show that SORL1 is differentially expressed in AD patients compared with controls. Furthermore, several genetic studies have identified allelic and haplotypic SORL1 variants associated with late-onset AD, and these variants confer small to modest risk of AD. Taken together, the evidence for SORL1 as a causative gene is compelling. However, putative variants have not yet been identified. Further research is necessary to determine its utility as a diagnostic marker of AD or as a target for new therapeutic approaches.