LAUSR

Pulmonary arterial hypertension (PAH) is a rare but serious disease, with an estimated prevalence ranging from 10 to 52 cases per million (1). None of the currently available therapies has the ability to cure PAH, causing a mortality rate of 51% within 7 years of diagnosis (2). PAH is characterized by progressive pulmonary arterial remodeling, causing vessel lumen narrowing and elevated pulmonary arterial pressure. As a consequence, the right heart is challenged with increased afterload, causing right heart hypertrophy, which is often rapidly followed by right heart decompensation and failure—the ultimate cause of death in PAH (3). Among various other factors, enhanced extracellular matrix (ECM) deposition and pulmonary arterial smooth muscle cell (PASMC) dysregulation are regarded as key factors responsible for pulmonary arterial remodeling (4). In PAH, PASMCs show an exaggerated proliferative behavior and are resistant to apoptosis (5). The molecular changes underlying the phenotypic alterations are, so far, only partially understood. In recent years, emerging evidence suggests a significant involvement of epigenetic changes in the pathogenesis of PAH (6). These involve alterations in the chromatin state of specific genes, which can lead to their repression or activation (6) and thus may contribute to PASMC dysregulation in PAH. In this issue of the Journal, Awada and colleagues (pp. 537–550) hypothesize that upregulation of the histone methyl transferase G9a and its partner protein GLP plays a pivotal role in pulmonary vascular remodeling by maintaining the abnormal proproliferative, apoptosis-resistant phenotype of PASMCs in PAH (7). The authors observed increased G9a and/or GLP expression in PAH-PASMCs and remodeled pulmonary arteries in patients with PAH as well as in lungs and/or pulmonary arteries of Fawn-hooded rats, monocrotaline-injected rats, and Sugen/hypoxia-challenged mice. Furthermore, they demonstrate that pharmacological inhibition of the activity of the histone methyl transferase G9a/GLP by BIX01294 or UNC0642 diminished the prosurvival and proproliferative potential of PAH-PASMCs by decreasing proliferation and ECM production while simultaneously increasing apoptosis, autophagy, mitochondrial reactive oxygen species production, and cholesterol metabolism (Figure 1A). In vivo, treatment with BIX01294 alleviated pulmonary vascular remodeling and lowered mean pulmonary arterial pressure in fawn-hooded rats. Additionally, pulmonary hemodynamics and right ventricular function were improved in BIX01294-treated Sugen/hypoxia-challenged mice (Figure 1B). Awada and colleagues conducted an elegant and comprehensive study with several strengths: The upregulated expression of G9a/GLP was demonstrated in primary PASMCs from patients with PAH, as well as in three different PAH animal models (fawn-hooded rats, monocrotaline-injected rats, and Sugen/hypoxia-challenged mice). Loss-of-function studies using a pharmacological approach in vitro (primary human PAH-PASMCs) and in vivo (fawn-hooded rats and Sugen/hypoxia mice) robustly indicate that the histone methyl transferase G9a/GLPmay represent a novel therapeutic target in PAH by interfering with signaling pathways that are dysregulated in PAH (e.g., fibrosis, ECM production, cellular response to growth factor stimulus, cholesterol metabolism, lysosome, autophagy, and ferroptosis). Although innovative and convincing, the study by Awada and colleagues raises questions for further investigation. Although the authors identify a variety of dysregulated downstream signaling pathways, the mechanism of G9a/GLP upregulation in PAH remains unresolved. Moreover, they did not test whether G9a/GLP inhibition in healthy donor PASMCs causes the same cellular dysregulation (proliferation, apoptosis, and autophagy) as compared with G9a/GLP manipulation in PAH-PASMCs. In this context, it will also be exciting to see whether G9a/GLP inhibition has the ability to return dysregulated cellular function in PAH-PASMCs to healthy baseline conditions. Additionally, future studies should address possible offtarget effects of the inhibitors used. In vitro and in vivo studies using specific gene-targeting strategies (e.g., via siRNA/gene editing with CRISPR/Cas) could further strengthen the impact of the study. Moreover, the exact contribution of G9a over GLP inhibition requires further evaluation, which could be also addressed by genetic manipulation. How does this study add to our knowledge in the field? Indeed, epigenetic alterations associated with PAH pathogenesis are currently a hot topic and are summarized in excellent reviews (8–10). Because epigenetic modifications change the chromosome structure but not the DNA sequence, extensive literature has been published in recent years dealing with the therapeutic implications of targeting epigenetic enzymes in preclinical models (11–15). Besides the exciting and encouraging findings from preclinical models, unfortunately, only a few of the drugs targeting epigenetic enzymes have reached the clinical trial stage in PAH to date. Either they failed to reproduce the promising results from preclinical models, there were severe side effects, or strategies for therapeutic delivery remain in their infancy (16). Thus, a better understanding of the altered epigenetic landscape and the manner of action of the respective drugs is needed to better connect preclinical animal studies and clinical trials. In this regard, Awada and colleagues open a new avenue by identifying with G9a/GLP a novel potential target for PAH treatment. This offers a new, optimistic outlook to the PAH community, and we are excited about future follow-up studies.