In our discovery cohort of 1,043 organ donors managed between 2001 and 2006 we identified several significant associations between β-adrenergic receptor polymorphisms and the presence of left ventricular dysfunction after brain death. Specifically, genotypes known to be associated with increased sensitivity to circulating catecholamines were associated with a higher risk of LV dysfunction during the donor management period.
The β-adrenergic receptors are located at the cell membrane of cardiomyocytes and mediate the effects of circulating catecholamines.(18
) The β1-adrenergic receptor is the predominant β-adrenergic receptor expressed on the cardiomyocyte and is responsive to circulating epinephrine and to local norepinephrine derived from cardiac sympathetic nerves.(13
) In rodents, transgenic cardiac overexpression of β1-adrenergic receptors causes progressive cardiomyopathy and heart failure.(19
) We therefore considered the β1AR 1165C/G (Arg389Gly) and 145A/G (Ser49Gly) polymorphisms as potential risk factors for donor LV dysfunction, as they affect the sensitivity and response of β1-adrenergic receptors to circulating catecholamines,(21
) which are present at very high levels early after brain death.(5
) Similarly, the β2-adrenergic receptors are also present in human myocardium, as well as in vascular smooth muscle beds. Stimulation of β2-adrenergic receptors mediates cardiac inotropic and chronotropic effects,(24
) induces cardiomyocyte apoptosis,(25
) and causes vascular smooth muscle relaxation and vasodilation in response to sympathetic tone.(26
) We therefore evaluated the associations between the β2-adrenergic receptor polymorphisms 46A/G (Gly16Arg) and 79C/G (Gln27Glu) and cardiac injury after brain death. These polymorphisms in the β1- and β2-adrenergic receptors are well-studied and have previously been associated with altered response to sympathetic stimulation,(27
) resting heart rate,(22
) risk of coronary events,(10
) vascular reactivity,(9
) and survival in patients with heart failure.(28
The initial “catecholamine storm”(23
) that occurs after brain death is often followed by a period of functional denervation, during which there is a dominance of vagal parasympathetic or inhibitory effects.(7
) This theory is supported by the observed excessive activity of the inhibitory G protein Giα in brain-dead organ donors with LV dysfunction.(29
) These dramatic physiologic changes after brain death may mediate the relationship between the “catecholamine in
sensitive” β1AR 1165GG genotype and high dopamine requirements during the subsequent donor management period.
Our initial provocative findings were then studied in a second cohort of organ donors managed between 2007 and 2008. The original study findings did not replicate in the latter cohort, and there are several potential explanations for this discrepancy. First, as discussed previously, changes in staffing and donor management strategies may have masked the influence of βAR genetic variation on allograft function. For example, significantly fewer donors in the 2007-2008 cohort were treated with dopamine (a β-receptor agonist), and higher doses of phenylephrine (an α-receptor agonist) were used. This change in inotrope/vasopressor support during donor management may have overshadowed the role of βAR signaling on cardiac function. In addition, there was a dramatic increase in the use of thyroid hormone supplementation between the 2001-2006 cohort (14%) and the 2007-2008 cohort (54%). As thyroid hormone can increase cardiac contractility, this strategic change in donor management could have also masked the relationship seen between βAR genotype and cardiac function in our discovery cohort. Second, there were notable differences in laboratory values, hemodynamics, and allograft function, suggesting that the 2007-2008 cohort was comprised of a “sicker” donor population. This observation may account for the decrease in allograft acceptance for transplantation in the latter cohort. Third, it is possible that unrecognized or unmeasured differences between study cohorts may have accounted for lack of replication. Finally, the smaller sample size in the second cohort (364 versus 1,043 donors) impacted the power to replicate our initial findings (20-36%). Thus, it is possible that our initial findings may have been false positive results, or may represent true associations, but our study was underpowered to replicate.
Understanding the pathophysiology of LV dysfunction after brain death plays a crucial role in the graft selection process for heart transplantation. Currently, approximately 60% of available cardiac grafts are discarded due to stringent acceptance criteria,(17
) leading to a great discrepancy between the number of critically-ill patients on the waiting list compared to the number of available grafts for transplantation.(30
) While non-utilization of donor hearts is a multi-factorial problem, encompassing diverse donor characteristics and logistical issues, LV dysfunction is the most frequently cited reason for non-utilization.(8
) Left ventricular dysfunction in a cardiac donor raises the specter of irreversible cardiac injury which may lead to clinically significant graft dysfunction and graft failure in the transplant recipient. However, animal and human studies now support the hypothesis that catecholamine toxicity plays a central role in reducing myocardial contractility after brain death(29
) and that cardiac dysfunction is often reversible in organ donors.(16
) Supporting this hypothesis are our discovery cohort findings of significant associations between βAR polymorphisms that mediate myocardial catecholamine sensitivity and LV dysfunction after brain death. Similarly, many transplant centers consider an allograft to be unsuitable if inotrope requirements are high during the donor management period. Our results suggest that high inotrope requirements may be associated with βAR genotypes that confer insensitivity to circulating catecholamines.
This study has significant strengths and limitations that deserve discussion. First and foremost, this represents the largest existing research database of detailed, rigorously adjudicated clinical and genetic data on over 1,400 potential organ donors managed in the current era. Second, this study represents a unique approach to the study of genetic influences in organ transplantation. We chose to study candidate gene polymorphisms in organ donors, and their influences on allograft function. Most genetic studies to-date in organ transplantation have examined associations between recipient genetic variation and post-transplant outcomes. Finally, we studied functional βAR polymorphisms that were previously shown to be associated with adverse cardiac outcomes in the general population as a means to study the pathogenesis of cardiac dysfunction after brain death, utilizing a very unique organ donor population. Limitations of this study include non-replication of initial findings in the validation cohort, which we were unable to account for by adjusting for recognized (and measurable) differences in donor management strategies during the study period. This is further exacerbated by the fact that donors were managed at a variety of local hospitals that may have had different medical management strategies prior to assumption of donor management by CTDN staff. Furthermore, characteristics and outcomes of donors managed by CTDN may not be equivalent to donor outcomes in other regions of the country, due to nationwide variations in donor management strategies. We also recognize the possible influence of uncontrolled confounding or population admixture on our genetic analyses. Although we did see consistent results when repeating our analyses in the sub-population of Caucasian donors, subtle population substructure may still be present within this racial group. Finally, complete phenotypic data could not be obtained for every donor, due to the retrospective nature of the data collection.
In conclusion, β-adrenergic receptor polymorphisms may contribute to the development of cardiac dysfunction after brain death. While we initially identified several compelling associations between the βAR SNPs of interest and cardiac function, our findings did not replicate in the validation cohort and there are several potential explanations for these discrepant results, as described above. Additional studies are therefore needed to examine the influence of donor genetic variants on post-transplant outcomes, and to assess for interactions between donor and recipient genetic modifiers in organ transplantation.