Background: The heterotopic ossification of muscles, tendons, and ligaments is a common problem faced by orthopaedic surgeons. We investigated the ability of Noggin (a BMP [bone morphogenetic protein] antagonist) to inhibit heterotopic ossification.
Methods: Part 1: A retroviral vector carrying the gene encoding human Noggin was developed and used to transduce muscle-derived stem cells. Part 2: Cells transduced with BMP-4 were implanted into both hind limbs of mice along with either an equal number, twice the number, or three times the number of Noggin-expressing muscle-derived stem cells (treated limb) or with nontransduced muscle-derived stem cells (control limb). At four weeks, the mice were killed and radiographs were made to look for evidence of heterotopic ossification. Part 3: Eighty milligrams of human demineralized bone matrix was implanted into the hind limbs of SCID (severe combined immunodeficiency strain) mice along with 100,000, 500,000, or 1,000,000 Noggin-expressing muscle-derived stem cells (treated limbs) or nontransduced muscle-derived stem cells (control limbs). At eight weeks, the mice were killed and radiographs were made. Part 4: Immunocompetent mice underwent bilateral Achilles tenotomy along with the implantation of 1,000,000 Noggin-expressing muscle-derived stem cells (treated limbs) or nontransduced muscle-derived stem cells (control limbs). At ten weeks, the mice were killed and radiographs were made.
Results: Part 1: An in vitro BMP inhibition assay demonstrated that Noggin was expressed by muscle-derived stem cells at a level of 280 ng per million cells per twenty-four hours. Part 2: Three varying doses of Noggin-expressing muscle-derived stem cells inhibited the heterotopic ossification elicited by BMP-4-expressing muscle-derived stem cells. Heterotopic ossification was reduced in a dose-dependent manner by 53%, 74%, and 99%, respectively (p < 0.05). Part 3: Each of three varying doses of Noggin-expressing muscle-derived stem cells significantly inhibited the heterotopic ossification elicited by demineralized bone matrix. Heterotopic ossification was reduced by 91%, 99%, and 99%, respectively (p < 0.05). Part 4: All eleven animals that underwent Achilles tenotomy developed heterotopic ossification at the site of the injury in the control limbs. In contrast, the limbs treated with the Noggin-expressing muscle-derived stem cells had a reduction in the formation of heterotopic ossification of 83% and eight of the eleven animals had no radiographic evidence of heterotopic ossification (p < 0.05).
Conclusions: The delivery of Noggin mediated by muscle-derived stem cells can inhibit heterotopic ossification caused by BMP-4, demineralized bone matrix, and trauma in an animal model.
Clinical relevance: Gene therapy to deliver Noggin may become a powerful method to inhibit heterotopic ossification in targeted areas of the body.