Gene therapy converts dead bone graft to new, living tissue

Researchers have created a way to transform the dead bone of a transplanted skeletal graft into living tissue in an experiment involving mice. The advance, which uses gene therapy to stimulate the body into treating the foreign splint as living bone, is a promising development for the thousands of cancer and trauma patients each year who suffer with fragile and failing bone grafts. The findings were posted online Feb. 13 and will appear in the March 1 issue of Nature Medicine.

The procedure, designed by a team led by Edward M. Schwarz, Ph.D., associate professor of orthopedics and of microbiology and immunology at the University of Rochester Medical Center, is intended to eventually aid people with various cancers or injuries whose treatment involves the replacement of large sections of bone.

Cancers such as osteosarcoma, one of the most common types of bone cancers, or tumors that occur adjacent to bones, often must be treated by removing the diseased section of bone and replacing it with the only alternative available - a donated section of comparable bone from a cadaver. The new splint of bone is then literally screwed into place, giving the patient most of the strength and support of the original bone. Bone, unlike any other tissue in the human body, can still perform one of its functions, structural support, even if all its cells are completely dead. A serious problem arises, however, when the bone wears over time.

"Everyday activities cause microscopic fractures in our bones," explains Schwarz. "Those fractures are normal and healthy, and our bones re-knit them constantly. But when the bone is dead, there is no healing, and those tiny fractures begin to accumulate until finally, perhaps in 10 years, the implanted section collapses, and more drastic surgery becomes necessary."

To make the transplanted bone more robust, Schwarz looked into the activity of the genes and proteins that govern its health. His team replaced sections of bones in dozens of mice, using both healthy and dead segments, then scanned the surrounding inflammatory tissue for differences in the levels of the active genes. He discovered that the genes that create two key proteins in living bone, called RANKL and VEGF, were barely expressed around the dead bone. He then modified a harmless virus to carry these genes, devised a method of freeze-drying a paste containing the virus so it could be easily handled, and painted it directly onto a bone graft during surgery.

Numerous tests in mice confirmed that the virus permeated the inflammatory tissue around the dead bone and turned on the genes. The mouse body then began to treat the implanted bone as if it were its own tissue instead of a foreign object, which would normally trigger the body to wrap the "invader" in scar tissue.

"That recognition is the key," says Schwarz. "It's at that point that the body actually begins changing the dead, foreign bone splint, into the body's own, whole, living bone."