MON-710 ACVR1 Activation in Primary and iPS-Derived Human Skeletal Muscle Stem Cells Impairs Myogenic Transcriptional Signature and Function

Developing optimal strategies for skeletal muscle regeneration and repair requires a detailed understanding of how these processes are regulated. The number of primary human satellite cells that can be obtained is usually extremely low, and may be impaired in disease of impaired skeletal muscle repair. One such condition is fibrodysplasia ossificans progressiva (FOP), a progressive disease characterized by massive heterotopic ossification in skeletal muscles and aberrant skeletal muscle repair after injury. FOP patients have activating mutations in the Activin A Type I receptor (ACVR1), a bone morphogenetic protein (BMP) receptor. Our overall hypothesis is that activated ACVR1 signaling caused by the ACVR1 R206H mutation incites inappropriate activation of human muscle stem cells (satellite cells, PAX7 expressing cells), causing loss of muscle cell fate and aberrant muscle repair. Since human satellite cells are difficult to obtain from live tissue donors, and injury can trigger heterotopic ossification, we created human induced pluripotent stem cell (iPSC)-derived muscle stem cells (iMuSCs) from FOP and control iPSC lines. We found that control and FOP iPSCs can differentiate into PAX7+ cells with high efficiency. Control and FOP iMuSCs can regenerate injured mouse muscle and form new human fibers, but both showed few PAX7 cells after transplant. Single cell RNA sequencing showed cell heterogeneity, and specific subsets of PAX7+ cells. FOP iMuSCs showed a chondrogenic/osteogenic signature (e.g COL1A1, DCN, OGN) with higher p38 pathway signaling activity. Skeletal muscle samples from autopsies of patients with FOP also showed increased expression of COL1A1. Additionally, we found that primary human FOP satellite cells can engraft and regenerate injured muscle, but with lower efficiency than control satellite cells. These studies used a novel iMuSC strategy to elucidate how increased ACVR1 activity affects human satellite cells function, and compare these iMuSCs to primary human satellite cells. These approaches will be useful to identify new therapeutic targets for conditions affecting skeletal muscle, and will improve our understanding of how muscle and bone interact in development and disease pathophysiology.