In this study of 951 potential organ donors, we found that the GG genotype of the Ser49Gly SNP in the ϐ1 adrenergic receptor gene, and the GG genotype of the Gly16Arg SNP in the ϐ2 adrenergic receptor gene are independently associated with increased lung allograft utilization from eligible donors. We also found that a combination of three favorable (associated with higher utilization) genotypes at three SNPs, the Ser49Gly SNP and Arg 389Gly SNP both in the ϐ1 adrenergic receptor gene, and the Gly16Arg SNP in the ϐ2 adrenergic receptor gene is associated with higher rates of lung allograft utilization with increasing number of genotypes in a stepwise manner. This increased utilization rate is also associated with improved oxygenation and decreased chest radiograph infiltrates with an increasing number of these genotypes.
In the present study, as has been reported previously, better oxygenation in the donor lungs was associated with increased allograft utilization. Improved oxygenation was probably related to less pulmonary edema in donors with the favorable genotypes, as suggested by the chest radiograph scores at the end of donor management. Recent data from Ware and colleagues suggests that clinical interpretation of chest radiographs can provide a reasonable assessment of the degree of pulmonary edema in donor lungs 32
. On comparison of the radiographs obtained at the beginning of assumption of donor management to radiographs obtained at the end of donor management, we further noted that not only did patients with the genotypes associated with utilization have fewer chest radiograph infiltrates at the end of donor management, but that during donor management there was a increasing improvement in chest radiographs among donors with the increasing number of the associated genotypes. On comparison of the PF ratio obtained shortly after brain death at the beginning of donor management to the PF ratio at the end of donor management and prior to organ harvest, we observed that there was a greater improvement in the PF ratio with increasing number of favorable genotypes. It is possible that the improvement in oxygenation and the chest radiographs that occurred during donor management was a result of clearance of neurogenic edema that may have accompanied brain death.
Genetic variants in β adrenergic receptor genes have been demonstrated to have functional effects in vitro
and in vivo
studies. In vitro
studies have revealed that β adrenergic receptor SNPs are associated with functional properties including higher basal and agonist-stimulated adenylate cyclase activity, greater agonist-promoted binding 21
, and greater agonist-promoted down-regulation 20,25
. Clinical studies have demonstrated that β adrenergic receptor SNPs are associated with clinical phenotypes consistent with enhanced β adrenergic receptor stimulation. Mothers with the GG genotype of the Arg16Gly SNP in the β 2 adrenergic receptor received significantly less ephedrine than mothers with AA and AG genotypes during anesthesia for cesarean sections 43
. These agonist β adrenergic effects are also reported in situations without the use of exogenous catecholamines. In a clinical study carried out on normal human volunteers, subjects with the GG genotype of the Arg16Gly SNP in the β 2 adrenergic receptor accumulated less lung water than individuals with the AA genotype, in response to a fluid challenge with isotonic saline29
. These results suggest that these specific β adrenergic receptor SNPs are associated with clinical phenotypes compatible with their β adrenergic agonist effects.
β adrenergic stimulation increases sodium transport across the alveolar epithelium by increasing intracellular cAMP, leading to an increased probability of opening of the apical sodium channel 11
and increased basolateral sodium extrusion by increasing Na+-K+-ATPase activity 33–35
. This process can accelerate the rate of alveolar fluid clearance and reduce pulmonary edema in sheep, dogs, rats, mice and the ex vivo
human lung 11,36–38
. Importantly, these effects have been demonstrated in the normal lung 4
, the acutely injured lung 12–15
and in models of hydrostatic pulmonary edema 11,16,39
. In addition to direct effects on alveolar fluid reabsorption, β adrenergic receptor stimulation also has anti-inflammatory, endothelial and lung epithelial protective effects that may be beneficial in the brain dead organ donor 40,41,42
The strengths of the current study include the relatively large cohort of donors, careful collection of clinical data, complete ascertainment of the primary outcome and the high rates of successful genotyping. The study also has some limitations. The cohort subjects were of diverse race and ethnicity. However, there was no change in our findings when we adjusted for race, and our results were consistent in stratified analyses in all major racial and ethnic groups. In addition the association of lung allograft utilization with favorable genotypes persisted when analysis was limited to just the Caucasian race. Finally there was no difference in lung utilization by race. Another limitation may be the lack of adjustment for the multiple SNPs tested. We did not correct the individual SNPs for multiple comparisons, since our pre hoc hypothesis was based on the biological plausibility of the SNPs studied. It is therefore unlikely that our results are due to a Type 1 error alone. However, as is true for all genetic epidemiology studies, these results need to be confirmed in a validation cohort.
Even though the Arg389Gly SNP individually did not have a statistically significant association with lung allograft utilization, we included it in the combined analysis based on the previously well-characterized functional effects of this SNP and the trend towards increased lung allograft utilization in our cohort. However, in analysis excluding this SNP, we found that increased lung allograft utilization with the increasing number of favorable genotypes still persisted in the model excluding this SNP.
In conclusion, two ϐ adrenergic receptor SNPs were individually associated with increased lung allograft utilization from eligible donors, and a combination of three favorable genotypes in ϐ adrenergic receptor SNPs was associated with higher rates of lung allograft utilization. The increased utilization is probably related to increased clearance of pulmonary edema and improved oxygenation in patients with the favorable genotypes. These results reinforce the importance of β-adrenergic dependent mechanisms in clearance of pulmonary edema and suggest that β-agonists may have a role during donor management in increasing lung donor utilization by improving oxygenation and the resolution of pulmonary edema.