The role of GDF5 in regulating enthesopathy development in the Hyp mouse model of XLH.

ABSTRACT

X-linked hypophosphatemia (XLH) is caused by mutations in PHEX, leading to rickets and osteomalacia. Adults affected with XLH develop a mineralization of the bone-tendon attachment site (enthesis), called enthesopathy, which causes significant pain and impaired movement. Entheses in mice with XLH (Hyp) have enhanced bone morphogenetic protein (BMP) and Indian hedgehog (IHH) signaling. Treatment of Hyp mice with the BMP signaling blocker palovarotene attenuated BMP/IHH signaling in Hyp entheses, thus indicating that BMP signaling plays a pathogenic role in enthesopathy development and that IHH signaling is activated by BMP signaling in entheses. It was previously shown that mRNA expression of growth/differentiation factor 5 (Gdf5) is enhanced in Hyp entheses at P14. Thus, to determine a role for GDF5 in enthesopathy development, Gdf5 was deleted globally in Hyp mice and conditionally in Scx + cells of Hyp mice. In both murine models, BMP/IHH signaling was similarly decreased in Hyp entheses, leading to decreased enthesopathy. BMP/IHH signaling remained unaffected in WT entheses with decreased Gdf5 expression. Moreover, deletion of Gdf5 in Hyp entheses starting at P30, after enthesopathy has developed, partially reversed enthesopathy. Taken together, these results demonstrate that while GDF5 is not essential for modulating BMP/IHH signaling in WT entheses, inappropriate GDF5 activity in Scx + cells contributes to XLH enthesopathy development. As such, inhibition of GDF5 signaling may be beneficial for the treatment of XLH enthesopathy.

X-linked hypophosphatemia (XLH) is a rare bone disorder that leads to short stature and poorly mineralized bones. As adults, patients with XLH often develop a mineralization of the bone-tendon attachment site, called enthesopathy, which results in significant pain. We previously showed that Achilles bone-tendon attachment sites (entheses) in mice with XLH (Hyp) have an enthesopathy characterized by increased bone morphogenetic protein (BMP) signaling. In the current studies, we show that treating Hyp mice with the BMP signaling inhibitor palovarotene prevents enthesopathy, demonstrating that the increased BMP signaling in Hyp entheses leads to enthesopathy development. We also reported that gene expression of Gdf5, which activates BMP signaling, is enhanced in Hyp entheses. Therefore, to determine if the enhanced Gdf5 expression leads to the increased BMP signaling seen Hyp entheses, Gdf5 was deleted from Hyp mice and also deleted specifically in the entheses of Hyp mice. In both mouse models, enthesopathy development was attenuated, demonstrating that the increased Gdf5 expression in Hyp entheses plays a role in enthesopathy development. These data indicate that blocking GDF5 and BMP signaling may prevent enthesopathy in patients with XLH.