The Population Reference Sample, POPRES: a resource for population, disease, and pharmacological genetics research.

Technological and scientific advances, stemming in large part from the Human Genome and HapMap projects, have made large-scale, genome-wide investigations feasible and cost effective. These advances have the potential to dramatically impact drug discovery and development by identifying genetic factors that contribute to variation in disease risk as well as drug pharmacokinetics, treatment efficacy, and adverse drug reactions. In spite of the technological advancements, successful application in biomedical research would be limited without access to suitable sample collections. To facilitate exploratory genetics research, we have assembled a DNA resource from a large number of subjects participating in multiple studies throughout the world. This growing resource was initially genotyped with a commercially available genome-wide 500,000 single-nucleotide polymorphism panel. This project includes nearly 6,000 subjects of African-American, East Asian, South Asian, Mexican, and European origin. Seven informative axes of variation identified via principal-component analysis (PCA) of these data confirm the overall integrity of the data and highlight important features of the genetic structure of diverse populations. The potential value of such extensively genotyped collections is illustrated by selection of genetically matched population controls in a genome-wide analysis of abacavir-associated hypersensitivity reaction. We find that matching based on country of origin, identity-by-state distance, and multidimensional PCA do similarly well to control the type I error rate. The genotype and demographic data from this reference sample are freely available through the NCBI database of Genotypes and Phenotypes (dbGaP).

Pharmacogenetics to predict drug-related adverse events.

Identification of reliable markers to predict drug-related adverse events (DRAEs) is an important goal of the pharmaceutical industry and others within the healthcare community. We have used genetic polymorphisms, including the most frequent source of variation (single nucleotide polymorphisms, SNPs) in the human genome, in pharmacogenetic approaches designed to predict DRAEs. Three studies exemplify the principles of using polymorphisms to identify associations in progressively larger genomic regions: polymorphic repeats within the UDP-glucuronysltransferase I (UGT1A1) gene in patients experiencing hyperbilirubinemia after administration of tranilast, an experimental drug to prevent re-stenosis following coronary revascularization; high linkage disequilibrium within the Apolipoprotein E (ApoE) gene in patients with Alzheimer Disease (AD); and the polymorphic variant HLA-B57 in patients with hypersensitivity reaction after administration of abacavir, a nucleoside reverse transcriptase inhibitor for the treatment of HIV. Together, these studies demonstrate in a stepwise manner the feasibility of using pharmacogenetic approaches to predict DRAEs.

Association of genetic variations in HLA-B region with hypersensitivity to abacavir in some, but not all, populations.

Abacavir is an effective antiretroviral drug used to treat HIV-1 infection. Approximately 5% of patients treated with abacavir develop a hypersensitivity reaction that requires discontinuation of the drug. In an initial pharmacogenetic study conducted in a predominantly White male population, multiple markers in the human leukocyte antigen (HLA)-B chromosomal region were associated with hypersensitivity to abacavir. The HLA-B*5701 association has now been confirmed in White males in a subsequent, larger study (n=293, p=4.7 x 10(-18)) and is also observed in White females (n=56, p=6.8 x 10(-6)) and Hispanics (n=104, p=2.1 x 10(-4)). HLA-B*5701 was not associated with hypersensitivity in Blacks (n=78, p=0.27). HLA-B*5701 alone lacks sufficient predictive value to identify patients at risk for hypersensitivity to abacavir across diverse patient populations. Efforts are ongoing to identify markers with sufficient sensitivity and specificity to be clinically useful. Even after a marker set is identified, appropriate clinical identification and management of hypersensitivity to abacavir must remain the cornerstone of clinical practice.

Multiplexed SNP genotyping using the Qbead system: a quantum dot-encoded microsphere-based assay.

We have developed a new method using the Qbead system for high-throughput genotyping of single nucleotide polymorphisms (SNPs). The Qbead system employs fluorescent Qdot semiconductor nanocrystals, also known as quantum dots, to encode microspheres that subsequently can be used as a platform for multiplexed assays. By combining mixtures of quantum dots with distinct emission wavelengths and intensities, unique spectral 'barcodes' are created that enable the high levels of multiplexing required for complex genetic analyses. Here, we applied the Qbead system to SNP genotyping by encoding microspheres conjugated to allele-specific oligonucleotides. After hybridization of oligonucleotides to amplicons produced by multiplexed PCR of genomic DNA, individual microspheres are analyzed by flow cytometry and each SNP is distinguished by its unique spectral barcode. Using 10 model SNPs, we validated the Qbead system as an accurate and reliable technique for multiplexed SNP genotyping. By modifying the types of probes conjugated to microspheres, the Qbead system can easily be adapted to other assay chemistries for SNP genotyping as well as to other applications such as analysis of gene expression and protein-protein interactions. With its capability for high-throughput automation, the Qbead system has the potential to be a robust and cost-effective platform for a number of applications.