Investigating pulmonary arterial hypertension from “stem” to stern. Focus on “Identification of a common Wnt-associated genetic signature across multiple cell types in pulmonary arterial hypertension”

for decades, primary cell culture in vitro and animal models in vivo have remained the cornerstones of functional modeling and drug discovery in the context of human disease. While both have proved to be invaluable tools, they are inherently limited in their ability to replicate human pathology. Primary cells are often difficult to maintain for long periods in culture, and animal models often provide little insight into species-specific facets of disease. Induced pluripotent stem (iPS) cells have earned tremendous attention in the last decade as a novel model system for the study of human pathobiology. In 2006, seminal work by Takahashi and Yamanaka (8) revealed that the retroviral gene transfer of four transcription factors (Oct3/4, Sox2, Klf4, and c-Myc) was sufficient to induce a pluripotent state in fibroblasts from both embryonic and adult mice. These findings were later replicated using human fibroblasts and blood lymphocytes (2). The resulting cells can be grown in culture and are morphologically indistinguishable from embryonic stem cells, with nearly equivalent differentiation potential (8).

An important feature of iPS cells is the fact that they typically retain genetic information from their donors, including any genetic abnormalities that may have contributed to disease. This lends them a wide range of potential applications, from regenerative medicine to personalized drug screening (2). Notably, they provide an invaluable opportunity for the study of genetic diseases, where the accuracy of more traditional models has remained a persistent problem. Pulmonary arterial hypertension (PAH) is a complex panvasculopathy, characterized by a program of endothelial smooth muscle, and fibroblast cell dysfunction throughout the pulmonary vasculature, with contributions from the surrounding mesenchymal stroma (1). Both heritable and idiopathic forms of the disease have been linked to dysregulation of the bone morphogenetic protein receptor type II (BMPR2) signaling pathway; yet the role of BMPR2 mutation in the development of PAH is not well understood, due in part to the deficiency of available PAH disease models. In vitro knockdown of BMPR2 in culture does not fully suppress wild-type signaling (7), while human PAH tissue can be scarce and is typically available only following transplant or at autopsy, when the disease has inevitably reached a very late stage. Animal models are available, but differences in pulmonary vascular remodeling between these models and patients call into question their biological fidelity (7).

Pioneering the use of iPS cells in the investigation of PAH, a new study by West and colleagues (9) in this issue of American Journal of Physiology-Cell Physiology has readdressed the role of BMPR2 mutation in the pulmonary vasculature. Working with skin fibroblasts isolated from PAH patients, their team produced a line of PAH-specific iPS cells from which they derived endothelial cell and mesenchymal stromal cell lineages. Importantly, cells produced in this manner recapitulated the genetic background of disease in these patients, but they lacked the acquired changes in phenotype that might be seen in primary cells taken directly from the pulmonary vasculature. In doing so, this platform allowed study of any genetic abnormalities in isolation, without concern for the confounding factors in late-stage disease (9). Through expression array profiling of these iPS-derived endothelial cells and mesenchymal stromal cells, West and colleagues were able to detect dysregulation of several members of the canonical Wnt signaling pathway. The upregulation of noncanonical Wnt signaling has been linked to vasoconstriction and vascular remodeling in PAH (4), and the BMPR2 and Wnt pathways are known to interact closely in the regulation of body axis patterning in early-stage development. However, their specific relationships in the adult cardiovascular system have not been fully defined, especially in the context of heritable PAH (9).

This finding represents a novel contribution to our understanding of the genetic factors that contribute to PAH but, more importantly, showcases the power of PAH-specific modeling with this exciting new technology (Fig. 1). The field of iPS cell biology is growing rapidly in concert with technologies in human genomics and genetics, and, soon, patient-specific iPS cells could prove invaluable in unraveling the growing list of genetic abnormalities associated with PAH (5). As iPS cell techniques mature, it may even be possible that compilation of these cells from a broad cohort of individuals will become a platform by which additional genetic factors may be identified in this disease, independent of classical human disease phenotyping. As technology for iPS cell generation becomes further streamlined, generation of high volumes of these cells from broad cohorts of PAH patients may become feasible as many current protocols can be pursued without the need for retroviral gene transfer and further genomic alteration (10). In addition, the utility of iPS cells is quickly progressing toward targeted drug and personalized toxicology screening (2). Finally, by coupling an iPS cell platform with genome-editing technology, as has been attempted in other cardiovascular contexts (3), it is conceivable that causal disease mutations in PAH could be corrected in pulmonary vascular cell populations for transplantation into human patients.

Fig. 1.

Pulmonary arterial hypertension (PAH)-specific induced pluripotent stem (iPS) cells promise myriad functional applications in the realm of basic science and in translational research for treatment of patients afflicted with this disease.

However, numerous caveats may temper expectations of iPS cell technology in this disease. Because PAH spans multiple vascular cell types, it is not well modeled by any single cell population in culture (7), a limitation that may muddle interpretations of the functional importance of mutations in iPS cell-derived PAH models in cell culture. Particularly for the study of PAH, further improvement of the technology is also crucial to ensure that iPS cell-derived vascular cell types accurately recapitulate activity of pulmonary vascular cells, as most differentiation protocols to generate vascular endothelium, smooth muscle cells, or fibroblasts are not specific for the pulmonary system (6). Finally, the safety of iPS cells for transplantation into PAH patients has not been fully vetted, and given the potential for tumorigenicity, immunogenicity, and other unexpected genetic and epigenetic abnormalities, there are important safety considerations for genomic editing in iPS cells prior to human transplantation (2).

Even with these caveats, however, the advent of iPS cell technology in the study of PAH is a significant step forward. It remains to be seen if the grand potential of iPS cells can be fully realized in truly altering the way in which we model, treat, and prevent human PAH.

DISCLOSURES

No conflicts of interest, financial or otherwise, are declared by the authors.

AUTHOR CONTRIBUTIONS

S.Y.C. drafted manuscript; K.A.C. and S.Y.C. edited and revised the manuscript; K.A.C. and S.Y.C. approved the final version of the manuscript.