Author + information
- Michael H. Gollob, MD∗ ()
- ↵∗Reprint requests and correspondence:
Dr. Michael H. Gollob, Inherited Arrhythmia Program, Peter Munk Cardiac Centre, Toronto General Hospital, University of Toronto, 200 Elizabeth Street, Toronto, Ontario M5G 2C4, Canada.
A genetic basis for a variety of cardiovascular diseases has emerged over the last 25 years. More than 100 genes, with varying strengths of evidence supporting their association, have been reported to be specific causes of structural and arrhythmic heart disease. Most well-established disease-causing genes with strong impact in predicting development of cardiac phenotypes have been discovered through “classical” human genetic studies, involving large families and the linking of specific genes uniquely to affected family members, so-called genetic linkage studies. These types of studies led to the discovery of the most commonly associated genes for conditions such as hypertrophic cardiomyopathy (MYH7, MYBPC3) and long QT syndrome (KCNQ1, KCNH2, SCN5A) more than 2 decades ago. In more recent years, genetic linkage studies in families with multiple affected individual members with atrial fibrillation (AF) confirmed that a single gene may cause AF, thus providing definitive evidence that AF is indeed genetically determined in some patients (1,2).
Although classical genetic studies are among the most robust methodologies to implicate a specific gene in disease causation, this approach is limited for certain conditions because of the rarity of large families with monogenic forms of disease. Therefore, genetic researchers devised novel strategies to identify genetic contributors to a disease phenotype without the need to study large families harboring the condition. The most common alternative approach is referred to as the genome-wide association study (GWAS). In contrast to the classical approach, this methodology compares the frequency of common genetic variations (single nucleotide polymorphisms [SNPs]) in large cohorts (>1,000) of disease cases and healthy controls. Hundreds of thousands of common SNPs, typically present in at least 5% of healthy persons, are quantified in the case and control populations, and by applying stringent statistical methods and replication in independent cohorts an SNP may be concluded as a risk variant for development of the disease if its frequency in cases exceeds that in controls. Using this approach, in 2007 the deCODE genetics group from Iceland reported the first SNPs to be found more commonly in persons with AF as compared with controls without AF (3). The more robustly associated SNP was found to be present in nearly 20% of the AF cohort, whereas 12% of the controls without AF also carried the genetic variant, thereby predicting a relative risk of 1.72 for developing AF should a person of Icelandic or European descent carry the variant. Unfortunately and as expected by the design of a GWAS, the risk variants did not reside within any gene but rather within a genomic desert roughly 150,000 nucleotides distant to the nearest gene, PITX2. Thus, PITX2 was crowned a “novel gene” for AF.
For those less familiar with genomic discovery research, the declaration of PITX2 as a gene for AF should be interpreted quite differently from genes identified through classical genetic linkage studies. In long QT syndrome, for example, KCNQ1 can be stated to be a definitive gene causative for the condition, with certain rare mutations in this gene predicting a high relative risk of disease development (>60-fold). In contrast, PITX2 is a gene implicated by virtue of its near vicinity to a common SNP observed more frequently in AF cases and is best considered a potential genetic contributor to AF. Because the SNP in close proximity to PITX2 is also seen at a relatively high frequency (as opposed to being rare) in persons without AF (12%), its risk in predicting disease phenotype is quite small (odds ratio: 1.72). Despite the lower relative risk, the possibility of the presence of altered PITX2 function in provoking AF vulnerability argues for a clearer understanding of its molecular role in the heart because targeting of PITX2-mediated effects in the atria may benefit a larger proportion of patients with AF than patients with rare mutations.
In this issue of the Journal, Syeda et al. (4) provide mechanistic data translating the genomic discovery of PITX2 to a pharmacological approach that may be efficacious in managing patients with AF. Using a mouse model with >50% reduction in gene expression of the cardiac isoform of PITX2 and known vulnerability to AF induction through programmed stimulation, Syeda et al. (4) confirm the previous observation of Chinchilla et al. (5) that mice with low atrial PITX2 expression show a more depolarized resting membrane potential (RMP). In turn, Syeda et al. (4) describe the observation that this depolarizing shift in atrial RMP enhances the effectiveness of sodium channel inhibition of flecainide, which succeeded in completely suppressing inducibility of AF in these mice as compared with continued vulnerability in mice with normal expression of PITX2. These investigators confirm the relevance of PITX2-mediated depolarization of RMP by demonstrating elimination of an enhanced flecainide effect when artificially holding RMP at constant values for isolated atrial cardiomyocytes from low and normal expressed PITX2 tissue before sodium-channel current measurement. Finally, Syeda et al. (4) present preliminary data suggesting that the PITX2-mediated depolarizing shift of RMP may be secondary to PITX2 down-regulation of TASK-2, a weak inwardly rectifying potassium channel known to contribute to RMP.
The contribution of Syeda et al. (4) is commendable and highlights the translational research approach necessary to take the field of gene discovery a step closer to the ultimate goal of a pharmacogenomics-based, personalized approach to patient care and away from the “1 size fits all” approach most commonly applied to most diseases, including AF. Their data imply that knowledge of PITX2 expression status in an individual patient may direct clinical decision making to enhance drug efficacy. Their work, although highly intriguing, requires further validation because of some inherent limitations in the experimental approach used and challenges in translating the observations from the mouse model to human patients. Thus far, PITX2 mouse models have been characterized in ex vivo set-ups, outside the neurohumoral and autonomic influences that play a significant role in modulating the electrical properties of human hearts. The effect of the parasympathetic mediated acetylcholine-activated potassium current (Ik,AcH) in hyperpolarizing RMP (as opposed to depolarizing) and provoking atrial arrhythmia is well recognized, and it remains to be determined whether this physiological parameter may modify or override some of the observations derived from the mouse model. Predicting PITX2 mRNA level in patients is a conundrum because, disappointingly, the AF risk–associated SNPs that have implicated PITX2 correlate with a wide range of PITX2 mRNA levels measured from the atria of human subjects, some low others high, and as illustrated in this study and by other investigators, carrier status of these SNPs does not correlate with a gradient of PITX2 levels (6). These discrepancies may highlight the effect that dynamic fluxes of left atrial pressure and volume may have on gene expression, thereby suggesting that PITX2 expression measured at any single time point may have imitations in interpretation for an individual patient and may preclude accurate pharmacological decision making. Finally, the PITX2 mouse model reflects a loss-of-function (LOF) phenotype. Heterozygous LOF mutations in PITX2 cause a genetic condition known as Axenfeld-Rieger syndrome, which causes severe abnormalities of the eyes, teeth, and other facial features. Persons with this condition are not reported to have cardiac arrhythmias despite a clear reduction in normal PITX2 levels (7,8). Understanding these additional complexities and nuances will be required before the interesting findings of Syeda et al. (4) may be translated to a personalized or pharmacogenomics-based therapeutic modality.
Genetic research on familial forms of AF and population-based GWAS approaches continue to implicate novel genes previously unrecognized in disease pathophysiology. To date, the clinical utility of these findings has been limited to considerations of risk prediction. The ultimate value of gene discovery is the ability to tailor therapy on the basis of individual genotype or to discover a critical molecular pathway predisposing to disease that may be a highly efficacious target for drug modulation. Some investigators emphasize the common SNP–common pathway approach, the early motivation for GWAS design, as a means of finding novel physiological targets on the basis of gene discovery. The limitation of this approach may be the smaller impact of these pathways on disease vulnerability, given the high number of healthy subjects carrying at-risk SNPs and therefore a theoretically smaller impact from targeted intervention. Conversely, genes discovered through genetic linkage or familial studies typically are responsible for small proportions of subjects with AF, thus leading to a perspective that targeting pathways for rare genetic forms of disease may not be applicable to the majority of cases. This concept may be countered by the landmark findings of Nobel laureates Brown and Goldstein, when their identification of the low-density lipoprotein receptor (LDL-R) gene as a cause of familial hypercholesterolemia, a 1 in 100,000 genetic condition, ultimately led to the drug targeting of the LDL-R pathway by highly efficacious statin therapy that is beneficial even to persons without LDL-R gene–based cholesterol disorders. Researchers and biopharmaceutical companies wishing to advance drug efficacy in AF now have the advantage of targeting pathways discovered through robust gene-based research. Whether the focus should be on rare pathways, common pathways, or both is debatable. Nevertheless, gene discovery in AF has opened up exciting new avenues of research that may hopefully lead to personalized therapy and positively affect the care of our patients.
↵∗ Editorials published in the Journal of the American College of Cardiology reflect the views of the author and do not necessarily represent the views of JACC or the American College of Cardiology.
Dr. Gollob is supported by a Mid-Career Investigator Award of the Heart and Stroke Foundation of Ontario (MC7449) and The Peter Munk Research Chair in Cardiovascular Molecular Medicine (Toronto General Hospital).
- American College of Cardiology Foundation
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