OREF funding helps produce new model
Flexor tendon grafting in the hand frequently results in excessive scarring, but research in animals has uncovered a novel approach that may reduce the incidence of that common postoperative complication.
The gene therapy approach uses an allograft instead of the autologous grafting technique favored in clinical practice. Early studies in a mouse model of flexor tendon reconstruction compared outcomes using a live autograft with those obtained using a freeze-dried, acellular allograft as a gene delivery vehicle.
“In the mouse, we’ve shown some very promising results,” said the researcher, Hani A. Awad, PhD, assistant professor of biomedical engineering and orthopaedics at the University of Rochester School of Medicine and Dentistry.
“We have reprogrammed the repair response of the tissue and improved the outcome,” said Dr. Awad, who received a research grant from the Orthopaedic Research and Education Foundation (OREF).
This basic science research has important clinical implications.
“Hand injuries account for about 5 percent of emergency department visits nationwide. Many hand injuries are finger lacerations involving flexor tendons,” said Dr. Awad, who once injured his own flexor tendon.
After surgical repair of the flexor tendon, excess scarring and adhesions can form, especially in zone II, from the metacarpal through the middle phalanx. This leads to impaired tendon gliding and loss of digital joint function for many patients, who will require rehabilitation and sometimes tenolysis.
As an alternative to primary repair, surgeons may transplant a tendon autograft to bridge zone II. Some researchers, including Dr. Awad, believe that an autograft exacerbates the formation of fibrotic adhesion tissues. Transforming growth factor beta-1 (TGF-ß1) induces the cells in a live autograft to proliferate rapidly and migrate out of the graft, perhaps contributing to the formation of a scar, he said. The scar tissue then adheres to the surrounding tissues.
Dr. Awad thinks it may be possible to head off adhesions by transplanting cadaver tissue that has first been freeze-dried to remove all the cells. The donor tissue is then reconstituted by rewetting it in a solution containing a pharmaceutical-grade, therapeutic growth factor.
This technique has two advantages over an autograft, according to Dr. Awad. The allograft lacks the cells that could contribute to scar formation and it has the potential for loading with growth factors that would counter the scar-inducing effects of TGF-ß1.
Thus, the allograft tissue serves a dual role, as both a “scaffold” to repair a gap defect in the tendon and a “vehicle” to deliver the targeted gene.
Studies in his mouse model have shown that the extracellular matrix in the allograft binds to and mediates the functional activity of the growth factor, he said.
Gene for tendon repair
Dr. Awad uses a previously tested recombinant adeno-associated viral vector to deliver growth differentiation factor-5 (GDF-5) in his mouse model. GDF-5 is the mouse homologue of bone morphogenetic protein 14. Other authors showed that GDF-5 induces tendon-like tissue after intramuscular implantation in rats and that deficiency of GDF-5 delays tendon healing and remodeling in mice. Therefore, Dr. Awad hypothesized that delivery of the GDF-5 gene will induce rapid remodeling, reduce formation of adhesions, and improve biomechanical and functional properties.
“We’ve studied the natural history of healing in this mouse model, and GDF-5 was expressed only late in the process,” he said. “We believe that if we expressed it earlier, it would counter the effects of TGF-ß1 and reduce the scarring and adhesions.”
Next Dr. Awad and his research team developed a test to measure resistance to joint flexion due to impaired tendon gliding. Using this test, they measured impaired flexion of the mouse joint (metatarsophalangeal joint) that corresponds to the human hand anatomy.
“Our idea is that TGF-ß1 potentiates scar formation by preventing or reducing activity of a class of molecules called matrix metalloproteinases, or MMPs,” Dr. Awad explained. “Based on observations that we have made in the lab and based on examination of the literature, we believe that GDF-5 directly competes with TGF-ß1. GDF-5 rescues the activation of MMPs, and that eventually leads to remodeling of the scar.”
To further study this hypothesis, Dr. Awad has recently been awarded a 5-year grant from the National Institute of Arthritis and Musculoskeletal and Skin Diseases at the National Institutes of Health—an accomplishment he feels he could not have achieved without OREF support.
“This advance in our understanding is, in large part, due to funding from OREF,” said Dr. Awad, whose clinical collaborators are David J. Mitten, MD, and Regis J. O’Keefe, MD, PhD.
“Funding from OREF for young investigators like myself is really important,” he said.
GDF-5 is likely just one way to target TGF-ß1, said Dr. Awad. He predicted that targeted gene therapies will be key in improving surgical interventions in the future.
“Perhaps the future of orthopaedic surgery lies in using a biologic approach that is designed to target the biologic root cause of the problem we are trying to solve,” he said.
Kathleen Louden is a contributing writer for OREF and can be reached at firstname.lastname@example.org