TBX4 in Pulmonary Hypertension: Linking Lung Development to Precision Therapy

Pulmonary hypertension (PH) is a lethal cardiopulmonary disease characterized by an increased pulmonary artery pressure (1). The World Symposium on Pulmonary Hypertension classifies PH into five groups based mostly on clinical and histopathologic characteristics. There is increasing appreciation of overlapping pathogenic and molecular features across groups, such as between group 1 (pulmonary arterial hypertension [PAH]) and group 3 (hypoxic lung disease–associated PH) (2). However, the molecular underpinnings of such shared features remain mysterious. Furthermore, even though current therapies have improved quality of life and extended life expectancy for patients with PAH, a curative approach remains elusive, and targeted therapies for PH of groups 2–5 are sparse. The study of lung development has been proposed as a way to define central pathogenic processes that are shared across different groups of PH. However, the molecular and genetic processes that govern the relationship between lung parenchymal and vascular structures or their relevance to postnatal pulmonary hypertension have not been defined to date.

Mutations in TBX4 (T-box4), a major risk factor for pediatric-onset PAH, represent the second most frequent genetic driver for the development of PAH after BMPR2 mutations (3, 4). TBX4 encodes a transcription factor that controls alveolar dysplasia as well as the development of interstitial lung disease (5). Although its role in embryonic branching morphogenesis is established (6), its role in alveolarization and lung vascularization in the postnatal setting has been challenging to study because of its variable penetrance and embryonic lethality (3, 7). However, the study of TBX4 offers a particularly attractive yet relatively unexplored therapeutic opportunity given its potentially shared pathogenic roles in groups 1 and 3 PH (8, 9).

Recognizing this unmet need, in this issue of the Journal, Maldonado-Velez and colleagues (pp. 415–426) engineered a lung mesenchymal–specific Tbx4 conditional knockout mouse, thereby circumventing embryonic lethality (10) (Figure 1). With this approach, the authors sought to find how TBX4 disruption in lung tissue would affect postnatal alveolar and vascular development and predispose to the development of PH. Their work established a link between lung-specific TBX4 deficiency and progressive alveolar simplification and impaired vascularization across the postnatal stages. Importantly, this also resulted in an increase in the hemodynamic manifestations of PH, demonstrating that early disruption of Tbx4 is crucial to controlling lung development and pulmonary vascular disease. Furthermore, there was minimal vascular remodeling compared with alveolar capillary loss, suggesting that alveolar capillary remodeling is a primary upstream driver for the development of PH, mirroring human TBX4 PH, in which lung hypoplasia often precedes PAH (5). Molecular profiling by RNA sequencing further revealed multiple pathways that were dysregulated by Tbx4 deficiency, including signaling across the Wnt, BMP, and IGF pathways, all critical for alveolar–vascular coordination (11, 12), lung regeneration (13), and senescence (14) and relevant to known drivers of PH.

Figure 1.

Lung mesenchymal Tbx4 (T-box4) CKO mice exhibit alveolar simplification, vascular loss, Wnt/BMP/IGF pathway dysregulation, and increased RVSP (upper). Future priorities include defining temporal and causative relationships between parenchymal lung development and pulmonary vascular disease, further elucidating the unifying role of TBX4-dependent processes across group 1, group 3, and pediatric PH, and testing TBX4-specific therapeutic strategies to restore alveolar–vascular coordination and disease reversal (middle). Limitations to address are species-specific roles of TBX4 (human TBX4 heterozygotes vs. murine Tbx4 CKO), unresolved temporal and cell type–specific activity of TBX4 in PH, and the precise dependence of TBX4 on Wnt/BMP/IGF or alternative pathways (lower). Created with BioRender.com. BMP = bone morphogenetic protein; CKO = conditional knockout; IGF = insulin-like growth factors; PH = pulmonary hypertension; RVSP = right ventricular systolic pressure.

These findings set the stage for a number of exciting future directions (Figure 1). First, this experimental platform affords the possibility of more precisely defining the temporal and causative relationships between parenchymal lung development and pulmonary vascular disease. These findings also support a model by which bidirectional intercellular communication is likely occurring in yet undefined ways between parenchymal lung tissue and the vasculature in development and in disease. Second, this work offers a unifying explanation that TBX4 may indeed represent a crucial and shared pathogenic process between groups 1 and 3 PH as well as between pediatric PH and developmental lung disorders. Third, the links between TBX4 and Wnt, BMP, and IGF pathways may guide a new therapeutic mission to restore these axes via administration of Wnt agonists, BMP/TGF balancing agents (i.e., sotatercept), or IGFBP2 supplements and to address lung parenchymal and vascular pathologies simultaneously (15).

Current limitations to this work should be acknowledged (Figure 1). First, in humans, haploinsufficiency of TBX4 in humans appears to predispose to PH, but heterozygous Tbx4^+/− mice do not develop PH. Thus, TBX4 activity may differ across humans and mice, making it important to develop a more sophisticated system to study TBX4-specific lung and vascular phenotypes in human tissue. Advances in precision-cut lung slices and/or the use of patient-derived inducible pluripotent stems cells across engineered tissue scaffolds could be appealing. Second, although the current findings suggest that lung developmental abnormalities appear to drive PH, the temporal and cell type–specific relationships have not been fully defined, leaving the possibility that there may be other undefined pathogenic processes that promote this type of PH. Third, even though identification of the Wnt, BMP, and IGF pathways as downstream of TBX4 is a step forward, there are unanswered questions regarding the precise dependence of TBX4 on these or other pathways in the final developmental and disease phenotypes.

Nonetheless, by defining the molecular axes that tie lung and vascular pathologies together, this work is a significant advance in understanding and therapeutically targeting the root and shared causes of PH.