We have developed a xenograft model of FSHD in which we generate mature FSHD muscle tissue in the hindlimbs of immunodeficient mice. These xenografts contain mature skeletal muscle tissue, as indicated by the presence of striations and the near absence of embryonic myosin heavy chain. They are also innervated and include satellite cells capable of generating new muscle. Crucially, they reproduce key features of FSHD: (i) they express DUX4 and several of its downstream gene products at much higher levels than xenografts made from unaffected muscle cells: (ii) they show the same level of hypomethylation as the biopsies from which the cells that generate the graft were originally obtained. (iii) they express one of the gene products, SLC34A2 as both mRNA and protein, at ~10-fold higher levels in FSHD grafts than in controls, and at ~10-fold higher levels in biopsies of FSHD muscles than in healthy controls. Based on the latter observation, we are studying SLC34A2 protein as a possible biomarker for FSHD. SLC34A2 is a Na+-Pi cotransporter that is normally expressed in epithelial tissues but not in muscle. We detect it in FSHD muscle, both in biopsies and in myotubes in culture, at ~10-fold higher levels than in controls. These levels decrease ~2-fold when myotubes are exposed to 3 different inhibitors of p38 kinase, ralemetinib, pamapimod and losmapimod, the drug currently in Fulcrum’s clinical trial. We also detect higher levels of SLC34A2 in immunoblots of engrafted FSHD muscles than in control grafts, as well as in the sera of mice carrying FSHD grafts. Consistent with this result, we find ~30% more SLC34A2 protein in the sera from FSHD patients compared to controls. In recent experiments, using an antibody to the exposed extracellular domain of SLC34A2, tagged with IR-647 and injected into FSHD xenografts, we observed a subset of human fibers that were labeled in situ. If confirmed, these results open the prospect of tracking disease progression and the efficacy of different experimental therapies for FSHD in the same individuals over time, without the need for muscle biopsies.
We gratefully acknowledge support from the NIH (NS R21 101169), Friends of FSH Research, the FSHD Society, Amis FSH, and the Maryland Industrial Partnerships program.