Pharmacogenetics of hypersensitivity to abacavir: from PGx hypothesis to confirmation to clinical utility.

The hypersensitivity (HSR) to abacavir (ABC) pharmacogenetics (PGx) program represents the progression from an exploratory discovery to a validated biomarker. Within the program, two retrospective PGx studies were conducted to identify HIV-1 patients at increased risk for ABC HSR, a treatment-limiting and potentially life-threatening adverse event. A strong statistical association between the major histocompatibility complex allele, HLA-B*5701, and clinically diagnosed ABC HSR was identified but varied between racial populations. Subsequently, ABC skin patch testing was introduced as a research tool to supplement clinical case ascertainment. In a randomized, prospective study evaluating the clinical utility of HLA-B*5701 screening, avoidance of ABC in HLA-B*5701-positive patients significantly reduced clinically diagnosed ABC HSR and eliminated patch test-positive ABC HSR. Finally, a retrospective PGx study supports the generalizability of the association across races. Prospective HLA-B*5701 screening should greatly reduce the incidence of ABC HSR by identifying patients at high risk for ABC HSR before they are treated.

Use of pairwise marker combination and recursive partitioning in a pharmacogenetic genome-wide scan.

The objective of pharmacogenetic research is to identify a genetic marker, or a set of genetic markers, that can predict how a given person will respond to a given medicine. To search for such marker combinations that are predictive of adverse drug events, we have developed and applied two complementary methods to a pharmacogenetic study of the hypersensitivity reaction (HSR) associated with treatment with abacavir, a medicine that is used to treat HIV-infected patients. Our results show that both of these methods can be used to uncover potentially useful predictive marker combinations. The pairwise marker combination method yielded a collection of marker pairs that featured a spectrum of sensitivities and specificities. Recursive partitioning results led to the genetic delineation of multiple risk categories, including those with extremely high and extremely low risk of HSR. These methods can be readily applied in pharmacogenetic candidate gene studies as well as in genome-wide scans.

SNPing away at complex diseases: analysis of single-nucleotide polymorphisms around APOE in Alzheimer disease.

There has been great interest in the prospects of using single-nucleotide polymorphisms (SNPs) in the search for complex disease genes, and several initiatives devoted to the identification and mapping of SNPs throughout the human genome are currently underway. However, actual data investigating the use of SNPs for identification of complex disease genes are scarce. To begin to look at issues surrounding the use of SNPs in complex disease studies, we have initiated a collaborative SNP mapping study around APOE, the well-established susceptibility gene for late-onset Alzheimer disease (AD). Sixty SNPs in a 1.5-Mb region surrounding APOE were genotyped in samples of unrelated cases of AD, in controls, and in families with AD. Standard tests were conducted to look for association of SNP alleles with AD, in cases and controls. We also used family-based association analyses, including recently developed methods to look for haplotype association. Evidence of association (P

A test for linkage and association in general pedigrees: the pedigree disequilibrium test.

Family-based tests of linkage disequilibrium typically are based on nuclear-family data including affected individuals and their parents or their unaffected siblings. A limitation of such tests is that they generally are not valid tests of association when data from related nuclear families from larger pedigrees are used. Standard methods require selection of a single nuclear family from any extended pedigrees when testing for linkage disequilibrium. Often data are available for larger pedigrees, and it would be desirable to have a valid test of linkage disequilibrium that can use all potentially informative data. In this study, we present the pedigree disequilibrium test (PDT) for analysis of linkage disequilibrium in general pedigrees. The PDT can use data from related nuclear families from extended pedigrees and is valid even when there is population substructure. Using computer simulations, we demonstrated validity of the test when the asymptotic distribution is used to assess the significance, and examined statistical power. Power simulations demonstrate that, when extended pedigree data are available, substantial gains in power can be attained by use of the PDT rather than existing methods that use only a subset of the data. Furthermore, the PDT remains more powerful even when there is misclassification of unaffected individuals. Our simulations suggest that there may be advantages to using the PDT even if the data consist of independent families without extended family information. Thus, the PDT provides a general test of linkage disequilibrium that can be widely applied to different data structures.

Analysis of association at single nucleotide polymorphisms in the APOE region.

The discussion of the prospects of using a dense map of single nucleotide polymorphisms (SNPs) to identify disease genes with association analysis has been extensive. However, there is little empiric evidence to support this strategy. To begin to examine the practical issues surrounding this methodology, we identified 10 SNPs in the region immediately surrounding the apolipoprotein E locus (APOE), an established susceptibility gene for Alzheimer disease. Our goal was to examine patterns of allelic association to begin to investigate the question of whether APOE could have been identified using SNPs. Our strongest evidence of association was at the 2 SNPs immediately flanking APOE.