Recapitulation of the pro-inflammatory signature of monocytes with ACVR1 mutation using FOP patient-derived iPSCs

Fibrodysplasia ossificans progressiva (FOP) is a rare genetic disease characterized by progressive heterotopic ossification (HO) in soft tissues due to a heterozygous mutation of the ACVR1A/ALK2 gene (FOP-ACVR1A), which erroneously transduces the BMP signal by Activin-A. Although inflammation is known to trigger HO in FOP, the role of FOP-ACVR1A on inflammatory cells remains to be elucidated. Here we investigated this issue using immortalized monocytic cell lines from FOP-iPSCs (FOP-ML) and mutation-rescued iPSCs (resFOP-ML). Without any stimulation, FOP-ML showed the pro-inflammatory signature of CD16+ monocytes with an up-regulation of INHBA gene, and treatment of resFOP-ML with Activin-A induced an expression profile consistent with FOP-ML at baseline. Treatment of FOP-ML with Activin-A further induced the inflammatory profile with the up-regulation of inflammation-associated genes, some of which were suppressed by corticosteroid. Experiments using an inhibitor for TGFβ or BMP signals demonstrated that Activin-A-induced genes, such as CD16 and CCL7, were regulated by both signals, indicating Activin-A transduced dual signals in FOP-ML. A comparison with resFOP-ML identified several down-regulated genes in FOP-ML, including LYVE-1, which is known to suppress matrix-formation in vivo. The down-regulation of LYVE-1 in HO tissues was confirmed in FOP model mice, verifying the in vitro experiments. These results indicate that FOP-ML faithfully recapitulated the phenotype of primary monocytes in FOP and its combination with resFOP-ML is useful for investigating the molecular events at the initial inflammation stage of HO in FOP. To investigate the effect of mutant ACVR1, monocytic cell lines established from fibrodysplasia ossificans progressiva patient derived induced pluripotent stem cells and those mutation corrected were stimulated by Activin A or Lipopolysaccharide