An mTOR Signaling Modulator Suppressed Heterotopic Ossification of Fibrodysplasia Ossificans Progressiva

Summary: Fibrodysplasia ossificans progressiva (FOP) is a rare and intractable disorder characterized by extraskeletal bone formation through endochondral ossification. FOP patients harbor gain-of-function mutations in ACVR1 (FOP-ACVR1), a type I receptor for bone morphogenetic proteins. Despite numerous studies, no drugs have been approved for FOP. Here, we developed a high-throughput screening (HTS) system focused on the constitutive activation of FOP-ACVR1 by utilizing a chondrogenic ATDC5 cell line that stably expresses FOP-ACVR1. After HTS of 5,000 small-molecule compounds, we identified two hit compounds that are effective at suppressing the enhanced chondrogenesis of FOP patient-derived induced pluripotent stem cells (FOP-iPSCs) and suppressed the heterotopic ossification (HO) of multiple model mice, including FOP-ACVR1 transgenic mice and HO model mice utilizing FOP-iPSCs. Furthermore, we revealed that one of the hit compounds is an mTOR signaling modulator that indirectly inhibits mTOR signaling. Our results demonstrate that these hit compounds could contribute to future drug repositioning and the mechanistic analysis of mTOR signaling. : Focusing on the ligand-independent constitutive activation of mutated ACVR1 in fibrodysplasia ossificans progressiva (FOP) patients, Ikeya and colleagues have identified two hit compounds that were effective in multiple FOP model mice. One of the hit compounds, TAK 165, was an mTOR signaling modulator that indirectly regulated enhanced mTOR signaling. These findings shed light on a therapeutic strategy for FOP. Keywords: mammalian target of rapamycin (mTOR), induced pluripotent stem cell (iPSC), fibrodysplasia ossificans progressiva (FOP), endochondral ossification, heterotopic ossification, bone morphogenetic protein (BMP), transforming growth factor β (TGF-β), activin A, high-throughput screening (HTS), ACVR1