Genetic Studies of Aspirin Responsiveness
A major GeneSTAR Research Program focus is on the role of platelets in atherothrombosis, and involves an array of genetic studies and pharmacogenomic studies. Spearheaded by Drs. Lewis Becker, Nauder Faraday, and Rehan Qayyum,, this work continues to be the centerpiece of the GeneSTAR risk mechanism work. The known and hypothesized biological cascades organized and integrated into a conceptual model by our team have driven the genetic aspects of this work. (Figure1)
Faraday N, Becker DM, Becker LC. Pharmacogenomics of platelet responsiveness to aspirin. Pharmacogenomics 2007 Oct;8(10):1413-25. Review. PMID: 17979514
Program in Gene Environment Interactions: PROGENI
Low dose aspirin (ASA) is cost effective and efficacious for the prevention and treatment of coronary heart disease (CHD). The benefit of ASA is thought to be related to the irreversible acetylation of platelet cyclo-oxygenase-1 (COX-1), resulting in a reduction in platelet aggregation and platelet-mediated inflammation. Considerable inter-individual variation exists in the effect of ASA on platelet function, and this variability may be related to genetic variations across individuals. Our first set of genetic studies characterized the inhibitory effect of ASA on agonist-induced platelet aggregation, thromboxane and ATP release, and platelet-leukocyte conjugate formation in 3200 subjects from 650 families, European and and African Americans.
Platelet function and a panel of plasma inflammatory markers (C-reactive protein, interleukin-1b, interleukin-6, monocyte chemotactic protein-1, and matrix metalloproteinase-9) were measured at baseline and after 14 days of ASA, 81 mg/day. From a list of candidate genes involved in the known biochemical pathways of platelet aggregation and platelet-mediated inflammation, genes were initially selected for genotyping of 15-20 single nucleotide polymorphisms (SNPs) per gene, based on biological importance and the presence of sufficient known SNPs in coding and/or promoter regions. After the first 1600 participants were phenotyped, a complementary genome wide scan of short tandem repeat (STR) markers and fine mapping of up to 5 regions of interest was done using SNP clusters.
Based on linkage analysis, the list of candidate genes were re-prioritized and additional genotyping was performed. ASA responsiveness is firmly established as heritable. Our primary initial findings revealed that the greatest information was coming from a variant in the PEAR-1 gene, as noted in the following publication.
Herrera-Galeano JE, Becker DM, Wilson AF, Yanek LR, Bray P, Vaidya D, Faraday N, Becker LC. A novel variant in the platelet endothelial aggregation receptor-1 gene is associated with increased platelet aggregability. Arterioscler Thromb Vasc Biol, 2008 Aug;28(8):1484-90. PMCID: PMC2739240
Genome Wide Association Studies: STAMPEED
Aggregation of activated platelets on atherosclerotic plaques initiates thromboses of the arterial system, resulting in ischemic syndromes. The propensity of platelets to aggregate in vivo is characterized by a variety of in vitro assays. We and others have demonstrated that many of these platelet function assays are moderately to highly heritable in populations at increased risk for atherosclerosis, supporting the hypothesis that genetic variations underlie individual variability in the tendency for arterial thrombosis.
Inhibition of platelets by low dose aspirin (ASA) is also a heritable trait and genetic variations may be in part responsible for responsiveness to ASA. We have extensively characterized native platelet function and platelet function after low dose ASA (81 mg/day for 14 days) in 2000 individuals from 500 2-generational families with premature coronary artery disease (60% European American, 40% African American) participating in the Genetic Study of Aspirin Responsiveness (GeneSTAR) and the Johns Hopkins Sibling and Family Heart Study. All study participants have had a 500 short tandem repeat (STR) genome scan.
The overall goal has been to identify genes that modify the function of platelets, both under “normal native” conditions, and following low dose ASA. We performed high density single nucleotide polymorphism (SNP) genotyping (1 million SNPs, using the Illumina HumanHap550 BeadChip) covering the entire genome at an average 6kb density, on the DNA samples from the GeneSTAR participants phenotyped for platelet function.
We determined genomic loci associated with quantitative platelet phenotypes prioritized for high heritability, biological interest, and linkage to STR markers, using family based association analysis, with joint modeling of linkage and association. Phenotypes of highest priority included aggregation induced by collagen, adenosine diphosphate (ADP), arachidonic acid (AA), and epinephrine in platelet rich plasma (PRP), ATP release induced by AA, and urinary levels of the prostaglandin metabolite, 11-dehydro-thromboxane B2.
We have published our findings with the Framingham Heart Study in Nature Genetics (A. Johnson, et al) , and other aspects of the work have been published and are in progress with the HAPI Heart Study of the Old Order Amish at the University of Maryland (A. Shuldiner). Work on refining the genetic findings is continuing.
Trans-Omics for Precision Medicine (TOPMed) Program
We are currently performing whole genome sequencing and collaborating with the Framingham Heart Study and the Old Order Amish Study in these efforts to identify genes related to aspirin responsiveness as part of the NHLBI TOPMed Program. Dr. Rasika Mathias is taking the TOPMed lead for GeneSTAR.
Johnson AD, Yanek LR, Chen M-H, Faraday N, Larson MG, Tofler G, Lin SJ, Kraja AT, Province MA, Yang Q, Becker DM, O’Donnell CJ, Becker LC. Genome-wide meta-analyses identifies seven loci associated with platelet aggregation in response to agonists. Nat Genet, 2010 Jul;42(7):608-13. PMCID: PMC3057573
More information about this study can be found on the Washington University Division of Statistical Genomics website, our partner in the analysis. Analyzed phenotypes are available from this site in the aggregate and the initial study is described in more detail.
Lewis C. Becker, MD
Robert L. Levy Professor, Cardiology