Pulmonary arterial hypertension (PAH) is a severe disease in which there is a progressive increase in pulmonary vascular resistance while the left heart pressure remains normal. Although the pathogenesis of the disease is not understood, the research that has taken place over recent years has revealed some of the cellular pathways involved in the development of characteristic PAH lesions. In 2000, one of the most important findings came about when it was revealed that the genetic alteration responsible for the hereditary form was situated in the gene that codifies BMPR2, one of the bone morphogenetic protein receptors.1,2 Since then, there has been an intense search for possible genes related with vascular regulation. Serotonin plays a crucial role, and it also has an important stimulating effect on the proliferation of smooth muscle cells of the pulmonary vasculature.3 Some years ago, an increase was reported in the plasma concentration of serotonin in 16 patients with idiopathic PAH.4 This may occur in two manners: uniting with specific receptors of the cell membrane or entering into the cytoplasm thanks to the action of the serotonin transporter protein (SERT). This latter mechanism of action is responsible for the proliferative effects.5

The SERT gene is located in chromosome 17 and has 14 exons totaling 31 kilobases. The promoter region of the gene may have two variants, one long (L) and another short (S), situated at a distance of one kilobase from the transcription initiation sites, which consist of the insertion (L forms) or the deletion (S forms) of 44 pairs of bases. The presence of the L allele is associated with a transcriptional activity between 2 and 3 times higher than that of the S allele. In patients with chronic obstructive pulmonary disease, the LL allele correlated with higher levels of pulmonary arterial pressure.6 There is a certain disparity of the results in the small number of studies performed on the distribution of the SERT genotypes in patients with PAH compared with the general population, as well as its impact in the manner of presentation and evolution of the disease. One study seems to show an increase in the LL types.7 The objective of our study was to know whether the patients with PAH could have a higher prevalence of this LL genotype and if it is related with the severity of the disease.

The study population consisted of 50 consecutive patients diagnosed with PAH, both idiopathic as well as secondary, and 50 healthy controls (25 women and 25 men) randomly selected from the Blood Transfusion Center of Galicia. The diagnosis of PAH was based on right catheterization with a mean pulmonary arterial pressure (PAPm) ≥25mmHg and a wedge pressure less than 15mmHg without specific treatment. In all cases, the protocol for standard management was followed in accordance with the recommendations of the ERS/ESC.8 The patients were stable at the time of the catheterization. The patients as well as the controls were informed in detail about the study, and they gave their written consent to participate. The local ethics committee approved the study.

Out of the 50 patients included in the study, in 49 patients, it was possible to correctly analyze polymorphism. Of these, 14 corresponded with idiopathic PAH, 13 PAH associated with connective tissue pathologies, 14 chronic thromboembolic disease, 4 cases were related with congenital cardiopathy, 2 were associated with human immunodeficiency virus, one case PAH with portal hypertension and another with chronic obstructive pulmonary disease (Table 1). Mean age at the time of diagnosis was 56±16 (range, 25–78 years); there were 30 women and 19 men.

The distribution of SERT genotypes did not vary between the control group and patients (LL: 20% vs 30%, P=.54) (Table 2). We also did not observe any differences between the three predominating types of PAH (P=.3) (Table 3). The most frequent genotype was LS (54% of patients, 56% of controls), and although within the group of patients a greater frequency of LL genotype was observed in men than in women (25% vs 14.8%), statistical significance was not reached (P=.33).

There were no differences in the age at diagnosis (LL: 57±13 vs 57±12, P=.56). A tendency was observed toward presenting higher PAPm levels in patients with the LL genotype, although without reaching statistical significance (49±5mmHg vs 42±9mmHg, P=.07). In 34 patients, the follow-up period lasted at least one year. In 18 cases, there was a good response to treatment. The distribution of LL or non-LL types did not vary according to whether they were responders or not (P=.8). Only 3 deaths occurred during the follow-up period, all of them patients with non-LL genotypes.

In our series of patients, the distribution of SERT gene alleles was similar to that found in healthy controls. There were no observed differences between the three most frequent types of PAH or between sexes. Moreover, the presence of LL was not associated with a different behavior of the disease in general, despite a certain tendency toward having a higher level of PAPm, a parameter which in the majority of studies does not correlate with survival.10,11 The presence of one type or another of SERT genotypes also did not influence the mid-term therapeutic response.

There are several studies that seem to demonstrate a very important role of serotonin in the pathogeny of PAH.12 Some years ago, the “serotonin hypothesis of PAH” was described because the mechanism of action of some appetite suppressant drugs (such as aminorex and dexfenfluramine) linked to the appearance of this disease inhibited the neuronal recaptation of serotonin and, therefore, greater secretion and stimulation of its receptors.13 Serotonin stimulates the proliferation of smooth muscle cells and fibroblasts of the pulmonary vessels and, when cultured with it, these cells extracted from patients with PAH proliferate with much greater intensity than those of healthy controls.14 The serotonin pathway is complex and is influenced by its synthesis, receptors and the SERT protein.15 Some years ago, a discovery was made of the existence of a variant in the promoter region of the SERT gene, with a long allele that increased the transcription compared with the other shorter one. An initial study showed a greater prevalence of the LL genotype in patients with PAH.7 Nevertheless, other studies with larger patient numbers did not find differences with the general population. A French-American study including 166 patients with hereditary PAH and 83 with idiopathic PAH did not observe differences in the distribution of the three SERT genotypes compared with 125 healthy controls. The LS type, as in our patients, was the most frequent. The only relevant finding was the appearance of the disease at a younger age in the cases of hereditary PAH if they were carriers of the LL genotype.16 In those who presented idiopathic PAH, the age at onset was similar, something which also happened in our series. The results did not vary when we analyzed the presence of mutations in the type 2 receptor of the bone morphogenetic proteins (BMPR2). In our study, only 3 patients of the 9 in whom we studied this gene were carriers of a mutation; therefore, we could not make comparisons. There were no survival differences depending on the SERT genotype. In our series, we were only able to analyze the response to treatment in 34 patients (70%) because in the remainder the follow-up was less than one year, with no differences found between genotypes. As only three deaths occurred, it was not possible to make a survival analysis depending on the SERT genotype. Another study by Machado et al.17 on a wide patient base that included familiar, idiopathic and associated types also found no differences in the distribution of the SERT genotypes. In this case, the age at onset of the disease in the familiar forms did not show differences with the non-familiar ones, which contrasts with the former study. As in our case, a comparison by sexes also did not present different distribution of the SERT genotypes. It is possible that in some specific types of PAH the SERT protein may have a more relevant role. One previously commented article done in France and the United Kingdom, which included 103 patients diagnosed with severe chronic obstructive pulmonary disease (mean FEV1 37% predicted), found no differences with the controls in the distribution of the SERT genotypes. However, the most important finding was that the disease was more severe in those who carried the LL type than the LS or SS types (PAPm 34±3, 23±1 and 22±1mmHg, respectively, P<.01).6 In part, this corresponds with our results, although in this case without reaching statistical significance. In this study, the expression of messenger RNA for serotonin in the muscle cells of the pulmonary arteries was more than double in those patients with LL types than in those who had any other type and, at the same time, if subjected to hypoxia, a powerful stimulant of serotonin, the LL types had 5 times more expression than the normoxic SS types. All these data support the important role that serotonin and its action pathways may have in the genesis of the underlying lesions in PAH.

The contribution of the LL genotype of SERT to favoring/aggravating PAH is difficult to quantify. It is very likely that several genetic factors are involved in the development of the disease, and quite probably linked with environmental factors. It is not easy to design studies about possible candidate genes that take into account the majority of the exogenic variables that could influence the appearance and the evolution of PAH. This, together with the low prevalence of this disease, which makes it difficult to include a high number of patients, can lead to significant biases with results that are difficult to interpret. For these reasons, we believe it is very important to continue to carry out genetic studies in order to establish risk profiles and open new therapeutic pathways. With the data available, it does not seem that this polymorphism in the SERT gene plays a relevant role in the appearance of PAH. Perhaps the LL types could be an aggravating factor of the disease in some types of patients, especially if associated with hypoxia, which should be explored.

One of the most important limitations of our study is the small number of patients. As we have already commented, PAH is a disease with a low prevalence (estimated at about 15 cases per million inhabitants18), which makes it difficult to bring together an extensive sample. It is possible that the results would be different if the number of patients were greater, but the coincidence of our findings with those of the most recent publications confer them greater credibility. Compared with the other larger series analyzing the samples of different countries with difficulty for obtaining data in an important number of cases, in our study we had all the clinical information made available to us.

In conclusion, despite the evidence that confer the SERT gene (especially its LL type) an important role in the action of serotonin as a possible candidate for increasing the susceptibility for developing PAH, we have not been able to demonstrate a greater number of patients who are carriers for this allele compared with the general population, nor was there a clear association with disease severity.

This study has been completed thanks to a research grant from Actelion.

The authors declare having no conflict of interests.