Editorials; arrhythmias; cardiac; genetics; humans; pharmacology; stem cells
Responses to drug therapy vary from benefit to no effect to adverse effects which can be serious or occasionally fatal. Increasing evidence supports the idea that genetic variants can play a major role in this spectrum of responses. Well-studied examples in cardiovascular therapeutics include predictors of steady-state warfarin dosage, predictors of reduced efficacy among patients receiving clopidogrel for drug eluting stents, and predictors of some serious adverse drug effects. This review summarizes contemporary approaches to identifying and validating genetic predictors of variability in response to drug treatment. Approaches to incorporating this new knowledge into clinical care, and the barriers to this concept, are addressed.
Purpose of review
To survey genetic variation contributing to variable responsiveness and toxicity to important cardiovascular drugs and highlight recent developments in the field of cardiovascular pharmacogenomics and personalized medicine.
Previously recognized pharmacogenomic associations with drug efficacy have been further validated (e.g. with clopidogrel and warfarin) and shown to influence clinically important outcomes. The clinical significance of variants modulating toxicity (e.g. SLCO1B1 with simvastatin) has also been confirmed. The genetic contribution to variable efficacy and toxicity of other important classes of cardiovascular drugs, such as beta-blockers, is becoming increasingly recognized. Prospective trials testing whether the use of genomic information improves clinical care are underway. Guidance based on the most well-established pharmacogenomic findings has appeared in prescribing labeling and is in the early stages of being implemented into routine clinical care.
Clinically validated gene variants that modulate responsiveness to cardiovascular drugs continue to be discovered and validated. Early steps are underway to translate these discoveries into clinical care.
personalized medicine; pharmacogenomics; variable drug response
Genetic variants of the enzyme that metabolizes warfarin, cytochrome P-450 2C9 (CYP2C9), and of a key pharmacologic target of warfarin, vitamin K epoxide reductase (VKORC1), contribute to differences in patients’ responses to various warfarin doses, but the role of these variants during initial anticoagulation is not clear.
In 297 patients starting warfarin therapy, we assessed CYP2C9 genotypes (CYP2C9 *1, *2, and *3), VKORC1 haplotypes (designated A and non-A), clinical characteristics, response to therapy (as determined by the international normalized ratio [INR]), and bleeding events. The study outcomes were the time to the first INR within the therapeutic range, the time to the first INR of more than 4, the time above the therapeutic INR range, the INR response over time, and the warfarin dose requirement.
As compared with patients with the non-A/non-A haplotype, patients with the A/A haplotype of VKORC1 had a decreased time to the first INR within the therapeutic range (P = 0.02) and to the first INR of more than 4 (P = 0.003). In contrast, the CYP2C9 genotype was not a significant predictor of the time to the first INR within the therapeutic range (P = 0.57) but was a significant predictor of the time to the first INR of more than 4 (P = 0.03). Both the CYP2C9 genotype and VKORC1 haplotype had a significant influence on the required warfarin dose after the first 2 weeks of therapy.
Initial variability in the INR response to warfarin was more strongly associated with genetic variability in the pharmacologic target of warfarin, VKORC1, than with CYP2C9.
The design of electronic health records (EHR) to translate genomic medicine into clinical care is crucial to successful introduction of new genomic services, yet there are few published guides to implementation.
The design, implemented features, and evolution of a locally developed EHR that supports a large pharmacogenomics program at a tertiary care academic medical center was tracked over a 4-year development period.
Developers and program staff created EHR mechanisms for ordering a pharmacogenomics panel in advance of clinical need (preemptive genotyping) and in response to a specific drug indication. Genetic data from panel-based genotyping were sequestered from the EHR until drug-gene interactions (DGIs) met evidentiary standards and deemed clinically actionable. A service to translate genotype to predicted drug response phenotype populated a summary of DGIs, triggered inpatient and outpatient clinical decision support, updated laboratory records, and created gene results within online personal health records.
The design of a locally developed EHR supporting pharmacogenomics has generalizable utility. The challenge of representing genomic data in a comprehensible and clinically actionable format is discussed along with reflection on the scalability of the model to larger sets of genomic data.
Sudden cardiac death from ventricular fibrillation during acute myocardial infarction is a leading cause of total and cardiovascular mortality. To our knowledge, we here report the first genome-wide association study for this trait, conducted in a set of 972 individuals with a first acute myocardial infarction, 515 of whom had ventricular fibrillation and 457 of whom did not, from the Arrhythmia Genetics in The Netherlands (AGNES) study. The most significant association to ventricular fibrillation was found at 21q21 (rs2824292, odds ratio = 1.78, 95% CI 1.47–2.13, P = 3.3 × 10−10). The association of rs2824292 with ventricular fibrillation was replicated in an independent case-control set consisting of 146 out-of-hospital cardiac arrest individuals with myocardial infarction complicated by ventricular fibrillation and 391 individuals who survived a myocardial infarction (controls) (odds ratio = 1.49, 95% CI 1.14–1.95, P = 0.004). The closest gene to this SNP is CXADR, which encodes a viral receptor previously implicated in myocarditis and dilated cardiomyopathy and which has recently been identified as a modulator of cardiac conduction. This locus has not previously been implicated in arrhythmia susceptibility.
Single nucleotide polymorphisms (SNPs) at chromosome 4q25 (near PITX2) are strongly associated with atrial fibrillation (AF). We assessed whether a 4q25 tagging SNP (rs2200733) is associated with PR interval duration in patients with lone and typical AF and controls. Patients with lone (n=169) and typical (n=269) AF enrolled in the Vanderbilt AF registry and controls (n=1403) derived from the Vanderbilt DNA Biobank (BioVU) were studied. Carriage of the rs2200733 T allele (CT or TT genotype) was more common in lone (39%) than in typical (25%) AF patients or controls (21%, P<0.01 for both comparisons). In both AF cohorts, we observed an association between genotype and PR interval duration (median PR interval for CC, CT, and TT: 162, 178, and 176 ms for lone, P=0.038 and 166, 180, and 196 ms for typical, P=0.001). After adjustment for covariates, the association between T allele and PR prolongation persisted, with mean effect size 10.9, 12.8, and 4.4 ms for lone and typical AF patients and controls (P<0.05 for each comparison). We found that a common 4q25 AF susceptibility allele (rs2200733) is associated with PR interval prolongation in patients with lone and typical AF and controls with no AF. Given that prolonged PR interval is an established risk factor for AF, this observation, in the context of previously described functional effects of PITX2 deficiency, provides further knowledge about the pathophysiological link of 4q25 variants with AF.
Efforts to define the genetic architecture underlying variable statin response have met with limited success possibly because previous studies were limited to effect based on one-single-dose. We leveraged electronic medical records (EMRs) to extract potency (ED50) and efficacy (Emax) of statin dose-response curves and tested them for association with 144 pre-selected variants. Two large biobanks were used to construct dose-response curves for 2,026 (simvastatin) and 2,252 subjects (atorvastatin). Atorvastatin was more efficacious, more potent, and demonstrated less inter-individual variability than simvastatin. A pharmacodynamic variant emerging from randomized trials (PRDM16) was associated with Emax for both. For atorvastatin, Emax was 51.7 mg/dl in homozygous for the minor allele versus 75.0 mg/dl for those homozygous for the major allele. We also identified several loci associated with ED50. The extraction of rigorously defined traits from EMRs for pharmacogenetic studies represents a promising approach to further understand of genetic factors contributing to drug response.
While the sodium channel locus SCN10A has been implicated by genome-wide association studies as a modulator of cardiac electrophysiology, the role of its gene product Nav1.8 as a modulator of cardiac ion currents is unknown.
We determined the electrophysiological and pharmacological properties of Nav1.8 in heterologous cell systems and assessed the antiarrhythmic effect of Nav1.8 block on isolated mouse and rabbit ventricular cardiomyocytes.
Methods and results
We first demonstrated that Scn10a transcripts are identified in mouse heart and that the blocker A-803467 is highly specific for Nav1.8 current over that of Nav1.5, the canonical cardiac sodium channel encoded by SCN5A. We then showed that low concentrations of A-803467 selectively block “late” sodium current and shorten action potentials in mouse and rabbit cardiomyocytes. Exaggerated late sodium current is known to mediate arrhythmogenic early afterdepolarizations in heart, and these were similarly suppressed by low concentrations of A-803467.
SCN10A expression contributes to late sodium current in heart, and represents a new target for antiarrhythmic intervention.
SCN10A; sodium channels; heart; afterdepolarizations; arrhythmia
Variability in drug responsiveness is a sine qua non of modern therapeutics, and the contribution of genomic variation is increasingly recognized. Investigating the genomic basis for variable responses to cardiovascular therapies has been a model for pharmacogenomics in general and has established critical pathways and specific loci modulating therapeutic responses to commonly used drugs such as clopidogrel, warfarin, and statins. In addition, genomic approaches have defined mechanisms and genetic variants underlying important toxicities with these and other drugs. These findings have not only resulted in changes to the product labels but also have led to development of initial clinical guidelines that consider how to facilitate incorporating genetic information to the bedside. This review summarizes the state of knowledge in cardiovascular pharmacogenomics and considers how variants described to date might be deployed in clinical decision making.
Pharmacogenomics; Polymorphism; Genetics; Clopidogrel; Warfarin; Statin; Beta-blocker, Anti-arrhythmic agents; Cardiovascular disease; Drug responsiveness; Toxicity
Marked prolongation of the QT interval and polymorphic ventricular tachycardia following medication (drug-induced long QT syndrome, diLQTS) is a severe adverse drug reaction (ADR) that phenocopies congenital long QT syndrome (cLQTS) and one of the leading causes for drug withdrawal and relabeling. We evaluated the frequency of rare non-synonymous variants in genes contributing to the maintenance of heart rhythm in cases of diLQTS using targeted capture coupled to next generation sequencing. Eleven of 31 diLQTS subjects (36%) carried a novel missense mutation in genes with known congenital arrhythmia associations or a known cLQTS mutation. In the 26 Caucasian subjects, 23% carried a highly conserved rare variant predicted to be deleterious to protein function in these genes compared with only 2-4% in public databases (p < 0.003). We conclude that rare variation in genes responsible for congenital arrhythmia syndromes is frequent in diLQTS. Our findings demonstrate that diLQTS is a pharmacogenomic syndrome predisposed by rare genetic variants.
pharmacogenomics; sudden cardiac death; adverse drug reaction; next generation sequencing
Obese patients with atrial fibrillation (AF) are frequently managed with AF ablation. We sought to examine whether there exists a body mass index (BMI) threshold beyond which odds of experiencing a complication from AF ablation increase. All patients enrolled in the Vanderbilt AF Registry who underwent catheter-based AF ablation from May 1999 to February 2012 were included. Major complications were recorded. Morbid obesity was defined as BMI >40 kg/m2, and BMI as a continuous variable was examined in multivariable analysis. Thirty-five complications (6.8%) occurred in 512 ablations. Morbidly obese patients experienced a higher rate of complications (6/42, 14.3%) than non-morbidly obese (29/470, 6.2%) (P=0.046). Using a discrete BMI cut-off, the odds of complications increased 3.1-fold in those with morbid obesity (odds ratio [OR] 3.1, 95% Confidence Interval [CI] 1.1–8.4, P=0.03) and 2.1-fold by female gender (OR 2.1, 95% CI 1.04–4.38, P=0.04). With BMI as a continuous variable, the odds of complications increased by 5% per 1 unit increase in BMI (OR 1.05, 95% CI 1.0–1.11, P=0.05) and there was a 2.2-fold increase by female gender (OR 2.2, 95% CI 1.1–4.6, P=0.03). In conclusion, morbid obesity represents a BMI threshold above which the odds of complications with AF ablation significantly increase. The increase in complications appears to be driven primarily by events in women suggesting that morbidly obese women are a special population to consider when considering AF ablation.
Atrial Fibrillation; Ablation; Obesity; Complications; Pulmonary Vein Isolation
Interindividual variation in response to metformin, first-line therapy for type 2 diabetes, is substantial. Given that transporters are determinants of metformin pharmacokinetics, we examined the effects of promoter variants in both multidrug and toxin extrusion protein 1 (MATE1) (g.−66T→C, rs2252281) and MATE2 (g.−130G→A, rs12943590) on variation in metformin disposition and response. The pharmacokinetics and glucose-lowering effects of metformin were assessed in healthy volunteers (n = 57) receiving metformin. The renal and secretory clearances of metformin were higher (22% and 26%, respectively) in carriers of variant MATE2 who were also MATE1 reference (P < 0.05). Both MATE genotypes were associated with altered post-metformin glucose tolerance, with variant carriers of MATE1 and MATE2 having an enhanced (P < 0.01) and reduced (P < 0.05) response, respectively. Consistent with these results, patients with diabetes (n = 145) carrying the MATE1 variant showed enhanced metformin response. These findings suggest that promoter variants of MATE1 and MATE2 are important determinants of metformin disposition and response in healthy volunteers and diabetic patients.
metformin; MATE1; MATE2; genetic polymorphism; pharmacokinetics; pharmacodynamics; glucose; healthy volunteers; type II diabetic patients; HbA1c
The Vanderbilt DNA repository, BioVU, links DNA from leftover clinical blood samples to de-identified electronic medical records. After initiating adult sample collection, pediatric extension required consideration of ethical concerns specific to pediatrics and implementation of specialized DNA extraction methods. In the first year of pediatric sample collection, over 11,000 samples were included from individuals younger than 18 years. We compared the pediatric BioVU cohort to the overall Vanderbilt University Medical Center pediatric population and found similar demographic characteristics; however, the BioVU cohort has higher rates of select diseases, medication exposures, and laboratory testing, demonstrating enriched representation of severe or chronic disease. This unbalanced sample accumulation may accelerate research of some cohorts, but also may limit study of relatively benign conditions and the accrual of unaffected and unbiased control samples. BioVU represents a feasible model for pediatric DNA biobanking but involves both ethical and practical considerations specific to the pediatric population.
Pediatrics; Biological Specimen Banks
Despite a greater burden of traditional risk factors, atrial fibrillation (AF) is less common among black than whites for reasons that are unclear. We have examined race- and gender-specific influences of demographic, lifestyle, anthropometric and medical factors on AF in a large cohort of blacks and whites. Among white and black participants in the Southern Community Cohort Study age 65 and older receiving Medicare coverage from 1999–2008 (n=8,836), we ascertained diagnoses of AF (ICD-9 CM 427.3). Multivariate logistic regression was used to compute AF odds ratios (ORs) associated with participant characteristics, including histories of hypertension, diabetes, stroke and myocardial infarction/coronary artery bypass graft surgery, ascertained at cohort entry. Over an average of 5.7 years of Medicare coverage, AF was diagnosed among 1,062 participants. AF prevalence was significantly lower among blacks (11%) than whites (15%; P<.0001). ORs for AF rose with age, were higher among men, the tall and obese, and among persons with each of the comorbid conditions, but the AF deficit among blacks compared with whites persisted upon adjustment for these factors (OR=0.64, 95% CI 0.55–0.73). The patterns of AF risk were similar for blacks and whites, although associations with hypertension, diabetes and stroke were somewhat stronger among blacks. In conclusion, these findings confirm the lower prevalence of AF among blacks than whites and suggest that traditional risk factors for AF apply similarly to both groups and thus do not appear to explain the AF paradox in blacks.
Atrial fibrillation; epidemiology; risk factor; black; white
Although multiple lines of evidence suggest variable expression of the cardiac sodium channel gene SCN5A plays a role in susceptibility to arrhythmia, little is known about its transcriptional regulation.
We used in silico and in vitro experiments to identify possible non-coding sequences important for transcriptional regulation of SCN5A. The results were extended to mice in which a putative regulatory region was deleted.
Methods and Results
We identified 92 non-coding regions highly conserved (>70%) between human and mouse SCN5A orthologs. Three conserved non-coding sequences (CNS) showed significant (>5-fold) activity in luciferase assays. Further in vitro studies indicated one, CNS28 in intron 1, as potential regulatory region. Using Recombinase-Mediated Cassette Exchange (RMCE), we generated mice in which a 435 bp region encompassing CNS28 was removed. Animals homozygous for the deletion showed significant increases in SCN5A transcripts, NaV1.5 protein abundance, and sodium current measured in isolated ventricular myocytes. ECGs revealed a significantly shorter QRS (10.7±0.2ms in controls vs. 9.7±0.2ms in knockouts) indicating more rapid ventricular conduction. In vitro analysis of CNS28 identified a short 3′ segment within this region required for regulatory activity and including an E-box motif. Deletion of this segment reduced reporter activity to 3.6±0.3% of baseline in CHO cells and 16±3% in myocytes (both P<0.05), and mutation of individual sites in the E-box restored activity to 62±4% and 57±2% of baseline in CHO cells and myocytes, respectively (both P<0.05).
These findings establish that regulation of cardiac sodium channel expression modulates channel function in vivo, and identify a non-coding region underlying this regulation.
Gene Expression Regulation; Sodium Channels; Mice; Transgenic
The Kir 6.1 Katp channel is believed to play an important role in ventricular repolarization as determined from both functional and genetic studies of the potassium inwardly-rectifying channel, subfamily J, member 8 (KCNJ8)-S422L missense mutation in patients with J-wave syndromes. Although Kir6.1 is also present in atrial tissue, it is unknown whether this channel modulates atrial repolarization and hence whether the S422L mutation portends a greater risk of atrial arrhythmias. This study sought to examine whether there was an increased frequency of the KCNJ8-S422L mutation among patients with atrial fibrillation (AF) and early repolarization (ER) as a possible novel susceptibility gene for AF.
Methods and results
A total of 325 lone AF probands were identified from the Vanderbilt AF Registry, a collection of clinical data and DNA from consented, consecutively enrolled participants. The coding regions of KCNJ8 were sequenced, and the patient's presenting electrocardiogram (ECG) was reviewed by two independent physicians for ER abnormalities. The KCNJ8-S422L mutation was identified in two AF probands while no other candidate gene variants were identified in these cases. Twenty-two (7%) patients were found to have ER on the ECG, including the two probands carrying the S422L variant. In one small AF kindred, the S422L variant co-segregated with AF and ER.
The KCNJ8-S422L variant is associated with both increased AF susceptibility and ER indicating a role for Kir 6.1 Katp channel in both ventricular and atrial repolarization.
Atrial fibrillation; Early repolarization; KCNJ8; Kir6.1; Mutation
Routine integration of genotype data into drug decision-making could improve patient safety, particularly if many relevant genetic variants can be assayed simultaneously before target drug prescribing. The frequency of pharmacogenetic prescribing opportunities and the potential adverse events (AE) mitigated are unknown. We examined the frequency with which 56 medications with known outcomes influenced by variant alleles were prescribed in a cohort of 52,942 medical home patients at Vanderbilt University Medical Center. Within a five-year window, we estimated that 64.8% (95% CI: 64.4%-65.2%) of individuals were exposed to at least one medication with an established pharmacogenetic association. Using previously published results for six medications with well-characterized, severe genetically-linked AEs, we estimated that 398 events (95% CI, 225 - 583) could have been prevented with an effective preemptive genotyping program. Our results suggest that multiplexed, preemptive genotyping may represent an efficient alternative approach to current single use (“reactive”) methods and may improve safety.
During the past few years, the development of effective, empirical technologies for treatment of cardiac arrhythmias has exceeded the pace at which detailed knowledge of the underlying biology has accumulated. As a result, although some clinical arrhythmias can be cured with techniques such as catheter ablation, drug treatment and prediction of the risk of sudden death remain fairly primitive. The identification of key candidate genes for monogenic arrhythmia syndromes shows that to bring basic biology to the clinic is a powerful approach. Increasingly sophisticated experimental models and methods of measurement, including stem cell-based models of human cardiac arrhythmias, are being deployed to study how perturbations in several biologic pathways can result in an arrhythmia-prone heart. The biology of arrhythmia is largely quantifiable, which allows for systematic analysis that could transform treatment strategies that are often still empirical into management based on molecular evidence.
Variable function and expression of drug transporters have been proposed as mechanisms contributing to variable response to drug therapy. Block of the HERG channel, encoding IKr, can lead to serious arrhythmias, and a key drug-blocking site in HERG has been identified on the intracellular face of the pore. We begin to advance the hypothesis that active drug uptake enhances IKr block.
Methods and Results
Reverse transcriptase–polymerase chain reaction identified expression in the human atrium and ventricle of 14 of 31 candidate drug uptake and efflux transporters, including OCTN1 (SLC22A4), a known uptake transporter of the HERG channel blocker quinidine. In situ hybridization and immunostaining localized OCTN1 expression to cardiomyocytes. The IC50 for quinidine block of IKr in CHO cells transfected with HERG alone was significantly higher than cells transfected with HERG + OCTN1 (0.66 ± 0.15 μM versus 0.14 ± 0.06 μM [52% absolute increase in drug block; 95% confidence interval, 0.4–0.64 μM]), and this effect was further potentiated by a common genetic variant of OCTN1, L503F. In the absence of OCTN1, quinidine block could be 91% ± 5% washed out, but with the transporter, washout was incomplete (57% ± 6%). OCTN1 coexpression also facilitated HERG block by flecainide and ibutilide, but not erythromycin.
Coexpression of the organic cation transporter, OCTN1, expressed in human cardiac myocytes, intensifies quinidine-induced HERG block. These findings establish a critical hypothesis that variable drug transporter activity may be a potential risk factor for torsade de pointes.
antiarrhythmia agents; arrhythmia; pharmacology; pharmacokinetics; ion channels