PAT Abstract, 2005

Pharmacogenomics of Arrhythmia Therapy

Goals

Abnormalities of cardiac rhythm are a major public health problem. Drugs suppress cardiac arrhythmias in some patients, yet exacerbate arrhythmias or even generate new ones - the phenomenon of proarrhythmia - in others. This variability is not confined to antiarrhythmic drugs alone: in fact, the risk of proarrhythmia related to excessive prolongation of the QT interval has been the single leading cause of drug withdrawal over the past decade. While rare monogenic arrhythmia syndromes have been recognized for decades, more recent work strongly supports the overall hypothesis tested here, that common DNA variants underlie not only variability in cardiac rhythm but also the response of normal and abnormal cardiac rhythms to drug exposure. Therefore, the major goals of studies in the Pharmacogenomics of Arrhythmia Therapy (PAT) Center are:

• polymorphism discovery and characterization of genes that are candidate modulators of cardiac rhythm;
• patient accrual for a series of studies to define genomic contributors to variable QT responses to drug therapy; and
• evaluation of variability in drug responses in atrial fibrillation.

Progress

Polymorphism discovery is currently performed by resequencing exons and flanking introns in ethnically defined panels obtained from the Coriell resource: Caucasians, African-Americans, Han Chinese in the Los Angeles area, and Mexican Americans in the Los Angeles area. When promoter regions have been identified, screening extends to these. We are also rescreening distant non-coding regions identified as conserved in cross-species comparisons and functionally important in DNA-reporter assays. The priority in this center are genes encoding cardiac ion channels or their functional subunits expressed in heart, although we recognize that polymorphisms in other genes, notably those in the adrenergic and the renin-angiotensin-aldosterone signaling systems, are excellent candidate modulators of arrhythmia phenotypes and their response to drugs. When experimental platforms are available (ion channel coding regions; regulatory regions), functional studies with variant constructs are conducted. The To date (spring 2005), PAT has deposited to PharmGKB deep rescreening data (380-760 alleles each) on ~25 ion channel and other candidate arrhythmia modulator genes. This effort will continue during the upcoming cycle. In addition, we are using the power of model organism genetics (zebrafish) to identify entirely new genes and pathways involved in modulating variable responses to antiarrhythmic drugs.

A major effort is currently devoted to generation of clinical databases that include well-defined patient phenotypes, along with DNA samples. Each database contains common information, including patient history, family history, medication history, and arrhythmia history, along with database-specific phenotypes. All databases are accumulated de novo, i.e. from patients starting a drug or newly identified with an arrhythmia rather than those followed for these conditions; thus, patient recruitment has been underway since late 2001/early 2002. A major effort since initiation of the grant has been construction of web-based data entry systems, and an Oracle database to house the accumulating phenotypic and genotypic information. Databases currently being accumulated, and their recruitment to May 31, 2005, include:

• Patients undergoing cardiac surgery: Phenotypes include the complications of hemorrhage/thrombosis and atrial fibrillation. This collection was ongoing at the initiation of this award, and currently includes 1253 patients, 32.2% of whom had post-operative atrial fibrillation.

• All patients in whom therapy with a QT prolonging antiarrhythmic drug is initiated: Clinical practice suggests that these drugs should be initiated on an inpatient basis with few exceptions. We have implemented an interface with the electronic inpatient ordering system in the Vanderbilt University pharmacy that allows identification of each patient in whom therapy with these drugs is initiated. Phenotypic information includes baseline QT interval, maximum QT interval after drug challenge, and whether drug was continued at three days and at three months. This database includes 234 subjects. Plans are underway to extend ascertainment nationally in collaboration with FDA.

• All patients starting the anticoagulant coumadin: This is a prospective evaluation of the effect of CYP2C9 genotype on the rates of bleeding and of thrombosis, and on steady state dose during coumadin. The data will be required to mount a prospective study in which patients will be randomized to standard vs genotype-guided therapy. This database currently includes 335 subjects.

• All patients with atrial fibrillation (AF) referred to the arrhythmia clinics at Vanderbilt University Hospital for management: database currently includes 539 AF subjects and 120 lone AF patients. Plans are underway to extend ascertainment to increase numbers and ethnic mix.

• Normal volunteers challenged with a QT-prolonging agent, ibutilide, and a beta-blocker, atenolol: Normal volunteers are studied on the general clinical research center, after being brought into salt balance. Ibutilide is used because the maximum extent of QT prolongation is immediately evident after intravenous challenge, thereby maximizing patient safety, streamlining the conduct of the study, and eliminating the factors related to variable drug elimination as a contributor to QT response. The outcome variables include measures of autonomic function (catecholamines, upright and supine blood pressure and heart rate), baseline electrocardiographic data, electrocardiographic data obtained during and following supine exercise in the presence and absence of ibutilide and, on a second study day, in the presence and absence of atenolol challenge. 204 subjects have completed this study.

Power calculations suggest that minimal databases for association studies between common polymorphisms and arrhythmia or drug response phenotypes will require several hundred subjects. Thus, our efforts to date have focused on polymorphism identification and on database building. As common polymorphisms are identified, genotyping assays are developed, either on the Taqman or the SNPlex platforms. The center also includes a genetic epidemiology effort that will focus not only on the role of single polymorphisms in mediating the phenotypes of interest, but also on gene-gene interactions using techniques such as Multifactor Dimensionality Reduction (MDR) developed at Vanderbilt.