Fibrodysplasia ossificans progressiva (FOP) is a rare and intractable disease characterized by extraskeletal bone formation through endochondral ossification. Patients with FOP harbor point mutations in ACVR1, a type I receptor for BMPs. Although mutated ACVR1 (FOP-ACVR1) has been shown to render hyperactivity in BMP signaling, we and others have uncovered a mechanism by which FOP-ACVR1 mistransduces BMP signaling in response to Activin-A, a molecule that normally transduces TGF-β signaling. Although Activin-A evokes enhanced chondrogenesis in vitro and heterotopic ossification (HO) in vivo, the underlying mechanisms have yet to be revealed. To this end, we developed a high-throughput screening (HTS) system using FOP patient–derived induced pluripotent stem cells (FOP-iPSCs) to identify pivotal pathways in enhanced chondrogenesis that are initiated by Activin-A. In a screen of 6,809 small-molecule compounds, we identified mTOR signaling as a critical pathway for the aberrant chondrogenesis of mesenchymal stromal cells derived from FOP-iPSCs (FOP-iMSCs). Two different HO mouse models, an FOP model mouse expressing FOP-ACVR1 and an FOP-iPSC–based HO model mouse, revealed critical roles for mTOR signaling in vivo. Moreover, we identified ENPP2, an enzyme that generates lysophosphatidic acid, as a linker of FOP-ACVR1 and mTOR signaling in chondrogenesis. These results uncovered the crucial role of the Activin-A/FOP-ACVR1/ENPP2/mTOR axis in FOP pathogenesis.
Kyosuke Hino, Kazuhiko Horigome, Megumi Nishio, Shingo Komura, Sanae Nagata, Chengzhu Zhao, Yonghui Jin, Koichi Kawakami, Yasuhiro Yamada, Akira Ohta, Junya Toguchida, Makoto Ikeya
Submitter: Benjamin Levi | firstname.lastname@example.org
Authors: Shailesh Agarwal, Yuji Mishina, and Benjamin Levi
University of Michigan
Published August 28, 2017
Fibrodysplasia ossificans progressiva (FOP) is a genetic condition caused by a hyperactivating mutation in the type I bone morphogenetic protein (BMP) receptor ACVR1 (ACVR1 R206H). Recently, Hatsell et al have shown that, in addition to BMP, Activin A serves as a ligand for ACVR1 R206H. We are pleased to see that Hino et al1 have further confirmed a relationship among Activin A, BMP receptor activity, and the mTOR axis in their recent manuscript.
In two recent publicationss, we have shown that mTOR signaling is up-regulated by increased BMP receptor activity.2,3 Using a FOP mouse model of constitutive type I BMP receptor activity (ACVR1 Q207D) and a separate traumatic HO model, we showed that rapamycin eliminates heterotopic ossification.3 Earlier this year at the annual International FOP Association (IFOPA) meeting, we subsequently showed our findings that rapamycin eliminates heterotopic ossification in the mouse model of FOP (Acvr1 R206H) generated and shared by our colleagues, Economides et al, at Regeneron.4 It should be noted that exploration of rapamycin as a therapeutic option in (mouse) HO (both in an FOP setting as well as trauma induced HO) was precipitated by the observation that inflammation is a shared prerequisite in these HO settings, and hence hypothesized that immune-suppressing agents such as rapamycin would be able to stop the initiation HO if administered prophylactically. Moreover, our work did not utilize either modeling of HO in vitro or the screening of libraries of compounds – rather it was based on observations made in patients and corresponding animal models.
In their recent study, Hino et al1 use induced pluripotent stem cells derived from FOP patients to study mTOR signaling in the setting of the FOP mutation. Using in vitro studies, they showed that “mTOR inhibitors suppress the chondrogenic induction of FOP-iMSCs triggered by Activin A” (Fig 3). We have previously shown that mTOR inhibitors may reduce chondrogenesis through their downstream effect on hypoxia-inducible factor-1a (Hif-1a)3; previous studies have shown that Hif-1a is critical for normal embryologic chondrogenesis.5 Moreover, by showing that “Rapamycin suppresses Activin-A-triggered HO in FOP-ACVR1 conditional transgenic mice,” (Fig 4) Hino et al confirm our findings presented at IFOPA and add to the impetus to study rapamycin for patients with FOP. However, further studies are clearly required to understand the mechanism by which mTOR inhibition reduces chondrogenesis. For example, it is unclear how increased BMP receptor activity leads to increased mTOR signaling. It is also unclear still whether these findings are unique to Activin A, or whether mTOR inhibition has a similar effect with any BMP receptor ligand. Answers to these questions may allow us to generate additional targeted drugs, and may even allow us to address diseases which are not caused by genetic mutations such as trauma-induced heterotopic ossification or heterotopic ossification caused by BMP implants used for bony healing in patients.
The findings presented by Hino et al are important to everyone studying FOP. We are excited to hear that these findings are being used to begin a clinical trial in Japan, which will hopefully have similar findings and be able to offer a therapeutic strategy for individuals with FOP. We will continue to follow these advances closely and commend the research group for their efforts and findings. Rapamycin is a generally well-tolerated drug used for pediatric and adult conditions. Therefore, we believe that rapamycin, or other mTOR inhibitors/rapalogs, hold great promise in treating this painful disease process while minimizing treatment-associated morbidity.
*Acknowledgements: BL laboratory collaborates with Boehringer Ingleheim on a project not presented here. BL laboratory holds a patent for HIF1 inhibition including rapalogs for prevention of heterotopic ossification which is not yet licensed.
1. Hino, K., et al. Activin-A enhances mTOR signaling to promote aberrant chondrogenesis in fibrodysplasia ossificans progressiva. The Journal of clinical investigation (2017).
2. Agarwal, S., et al. mTOR inhibition and BMP signaling act synergistically to reduce muscle fibrosis and improve myofiber regeneration. JCI Insight 1, e89805 (2016).
3. Agarwal, S., et al. Inhibition of Hif1alpha prevents both trauma-induced and genetic heterotopic ossification. Proceedings of the National Academy of Sciences of the United States of America 113, E338-347 (2016).
4. Hatsell, S.J., et al. ACVR1R206H receptor mutation causes fibrodysplasia ossificans progressiva by imparting responsiveness to activin A. Science translational medicine 7, 303ra137 (2015).
5. Provot, S., et al. Hif-1alpha regulates differentiation of limb bud mesenchyme and joint development. The Journal of cell biology 177, 451-464 (2007).