Pulmonary arterial hypertension (PAH) is a progressive disease characterized by increased pulmonary arterial pressure and pulmonary vascular resistance, with a prevalence of approximately 10.6 cases per 1 million adults (1). PAH is characterized by persistent high pulmonary arterial pressure and increased pulmonary vascular resistance, which, if left untreated, may eventually lead to right heart failure and patient death. With advancements in basic research, numerous clinical randomized controlled trials, the establishment of guidelines, and the implementation of new treatment protocols, significant improvements have been achieved in the treatment of patients with pulmonary hypertension; however, the mortality rate remains high. Challenges persist, including an unclear pathogenesis, lack of early diagnostic and prognostic assessment methods, and the need for optimized treatment and management strategies.

Platelets play a key role in the pathogenesis of PAH, influencing a variety of mechanisms associated with PAH pathogenesis, including thrombosis, vasoconstriction, and remodelling, all of which are core processes in PAH pulmonary vascular pathology (2). Previous studies have shown that several platelet indices, including platelet count (PLT), plateletcrit (PCT), mean platelet volume (MPV), and platelet distribution width (PDW), are strongly associated with the onset, progression, and prognosis of PAH (3–5). Although numerous studies have identified an association between platelet indices and PAH, the findings have been inconsistent, and the causal relationship between changes in platelet indices and the development of pulmonary hypertension has been unclear owing to limitations in traditional research methodologies.

Mendelian randomization (MR) analysis, as a method similar to natural randomized clinical trials, is capable of robustly assessing the causal relationship between exposure and outcome by using genetic variants such as single nucleotide polymorphisms as instrumental variables (IVs) (6). The advantage of MR analysis is that, due to the random assignment property of alleles, it can effectively reduce the influence of confounding factors and avoid reverse causality bias, thus yielding more reliable causal effects (7, 8). Multivariate Mendelian randomization (MVMR) is an extension of the MR approach, which estimates the independent direct effect of each exposure on a single outcome by analyzing the genetic variants of multiple potentially relevant exposures, thus better controlling for correlations between them and potential confounders (9).

For rare diseases such as PAH, which are difficult to conduct large-scale clinical trials, MR studies have unique advantages, and scholars such as Steeve Provencher have called for more MR studies in the field of PAH (10). Therefore, we collected large-scale data on four typical platelet indices and PAH from the Genome-Wide Association Study (GWAS), explored the correlation between individual platelet indices and PAH using univariate MR analysis, and further estimated the independent effects of each platelet index on PAH by using the MVMR approach. The aim is to assess the potential genetic causal associations and provide a scientific basis for the early diagnosis and prognostic treatment of PAH in the future.

In this study, we investigated the potential causal relationship between four platelet indices (PLT, PCT, MPV, PDW) and PAH by Mendelian randomization. A significant causal relationship between PLT and PAH was revealed genetically using two-sample MR analysis. After correction for covariance by multivariate MR analysis, the effect of PLT on PAH remained significant, emphasizing that PLT may be an important independent risk factor for the development of PAH. When sensitivity analyses were performed, Cochran's Q test did not show significant heterogeneity in the selected IVs, and the MR-PRESSO test did not detect any outliers. In addition, neither the MR-Egger intercept test nor the MR-PRESSO test found evidence of horizontal pleiotropy, which further enhances the reliability and stability of our findings.

Our study revealed a significant causal relationship between PLT and PAH on a genetic level, showing a positive correlation with the risk of developing PAH. However, for PCT, PDW and MPV, no clear association with PAH was found at the genetic level. This finding does not completely align with some clinical observational studies.

A study conducted by Ya-Guo Zheng et al. included 82 patients with idiopathic pulmonary arterial hypertension (IPAH) and 82 age- and sex-matched controls to investigate the disease severity and prognosis of idiopathic IPAH, and demonstrated that MPV and PDW were significantly higher in IPAH patients than in age- and sex-matched controls, (11.4 ± 0.9 fl vs. 10.3 ± 0.9 fl and 14.3 ± 2.9% vs. 11. ± 9%; P < 0.001) (23). An observational study of hemostatic indices in patients with PAH showed that MPV was significantly higher in patients with PAH compared to healthy controls, but platelet counts were not significantly different (3). In a study evaluating prognostic indicators of combination therapy in patients with PAH, prognosis was analyzed from 22 variables by follow-up screening of 65 patients with IPAH, and only platelet levels were found to be associated with the risk of patient death (P < 0.01). Specifically, patients with lower pre-treatment platelet levels had a higher mortality rate than other patients (5). In addition to IPAH, PAH caused by other diseases also demonstrated some correlation with platelet parameters, a prospective study included 60 children with PAH combined with congenital heart disease and found that MPV and PCT were significantly higher in the PAH-CHD group compared to healthy controls and children with congenital heart disease without PAH, and these indices were strongly correlated with the severity of PAH (4). A study of PDW levels in patients with systemic lupus erythematosus-associated pulmonary arterial hypertension (SLE-PAH) vs. systemic lupus erythematosus simplex (SLE-non-PAH) showed that PDW levels were higher in the SLE-PAH group than in the SLE-non-PAH group (P = 0.023) (24).

These findings suggest that platelet indices may be closely associated with the pathogenesis of PAH, but the exact mechanisms remain incompletely understood. The increased number of activated platelets and the multiple cytokines they release may contribute to the pathogenesis of PAH (2). Research reports indicate a high frequency of thrombotic lesions in the histopathological classification of hypertensive pulmonary vascular disease (25). Thrombotic damage is common across various types of PAH (26), with thrombus formation closely linked to platelet activation, proliferation, and aggregation. In addition to causing thrombus formation, platelet activation triggers a series of changes in the vasculature of small pulmonary arteries, as well as in vascular endothelial cells. Specifically, activated platelets release a variety of growth factors, including PDGF and VEGF, which promote endothelial and smooth muscle cell proliferation (27). This mechanism is critical during vascular remodeling and thickening of the pulmonary vascular wall in pulmonary hypertension. in vitro experiments by Hai-Tao Yang et al. demonstrated that silencing the gene for EIF3A reduced PDGF-triggered proliferation, arrested the cell cycle, and down-regulated the expression of proliferation-associated proteins in pulmonary artery smooth muscle cells, ultimately reducing their proliferation and improving PAH (28).

Hypoxia commonly occurs in the pathogenesis of PAH. Chronic stimuli such as hypoxia, viral infections, and mechanical stress can induce an inflammatory response (29, 30). Inflammation is a key mechanism in pulmonary vascular remodeling and the pathogenesis of PAH. Platelets contain ATP, ADP, 5-HT, and growth factor- and cytokine-rich α-granules (31), which play a central role in inflammatory processes. These substances released by activated platelets exacerbate local inflammation and drive the proliferation of pulmonary artery smooth muscle cells, thereby aggravating PAH (32). It has been shown that platelets are more abundant in the lungs of hypoxic mice compared to normal mice, and that they are activated following hypoxia. Activated platelets aggregate and release vasoactive mediators, cytokines, and inflammatory factors, which exacerbate the progression of PAH. The increase in platelet counts and platelet-specific protein CD41 in lung sections from patients with advanced IPAH suggests that platelet proliferation and activation also play an important role in the progression of PAH in humans (33).

Therefore, in the development and progression of PAH, platelet activation can significantly induce changes in smooth muscle cells, endothelial cells, and extracellular matrix, as well as platelet aggregation, vasoconstriction, and remodeling of small pulmonary arteries, and other physiopathologic changes, which can alter pulmonary artery pressure levels. Increased pressure in the pulmonary arteries subsequently affects platelets, which is subsequently reflected in changes in the platelet index on clinical examination. Platelets play a key role in numerous pathophysiologic mechanisms of cardiovascular events. As indicators of platelet activation and functional status, different platelet parameters reveal changes in platelet number and function (34). These parameters are easy to measure and have been extensively utilized for the early diagnosis and prognosis of various cardiovascular and cerebrovascular diseases such as coronary heart disease (35, 36), hypertension (37, 38), heart failure (39), and stroke (40). According to existing observational studies, there is indeed an association between platelet markers and PAH, but the findings of observational studies are susceptible to confounding effects and reverse causality, and therefore, whether there is a causal relationship between changes in platelet markers and the development of PAH has not been conclusively established. MR studies have a unique advantage in exploring causal associations, and we used a well-established GWAS dataset to explore the relationship. In addition, we employed an MVMR approach to investigate the independent effects of the four platelet markers on the development of pulmonary hypertension, eliminating covariance effects to ensure result robustness. Based on our findings, the possible role of changes in platelet parameters in the development and progression of pulmonary hypertension should also be considered.

This paper also has several limitations. Firstly, our study only observed a causal relationship at the genetic level between PLT and the risk of PAH, while no causal relationship was found between PCT, MPV, PDW, and PAH, which is somewhat inconsistent with many published observational studies. Due to limitations in sample size, the number of SNPs identified with PAH as the exposure factor was too small, hence this paper did not conduct MR analysis on the effect of PAH on platelet indices nor could it verify reverse causality, Future research anticipates more comprehensive GWAS data and refined MR studies. And our GWAS datasets for platelet indices and PAH were derived from European ancestry, and results need to be confirmed in other ethnic groups. Lastly, as with all published MR studies, even with measures to identify and mitigate outlier variants, the possibility of unobserved pleiotropy affecting the results cannot be completely ruled out (41).

This MR study provides new evidence indicating that PLT is a major factor independently associated with PAH, while the relationships between PCT, MPV, PDW and PAH are not significant. These findings enhance our understanding of the role of platelets in the onset and progression of PAH, holding significant implications for clinical practice and the formulation of public health strategies.

Future studies should delve deeper into the specific pathophysiological mechanisms of platelets in the pathogenesis of PAH and look forward to more evidence based on stratified randomized controlled trials to assess the predictive value of metrics such as PLT in PAH and the efficacy of antiplatelet therapy in the PAH population.