Stem cells and ultrasound restores non-growing bone fractures

A group of researchers from the Cedars-Sinai Medical Center, USA, successfully recovered severe limb fractures in laboratory animals using an innovative technique based on a combination of ultrasound, stem cells and gene therapy. If the tests show the safety and effective in humans, the pioneering method will be able to replace bone grafting with non-growing and particularly complex fractures. The results of the work are published in the journal Science Translational Medicine.

“We are just at the beginning of a revolution in orthopedics,” – said Dan Gazit, PhD, DMD, co-director of the Skeletal Regeneration and Stem Cell Therapy Program in the Department of Surgery and the Cedars-Sinai Board of Governors Regenerative Medicine Institute. “We’re combining an engineering approach with a biological approach to advance regenerative engineering, which we believe is the future of medicine.”

Every year around the world, more than 2 million bone grafts are required to treat severe injuries involving traffic accidents, war or tumor removal.

The consequence of such damage is often the lack of a sufficiently large area of bone tissue in fracture sites, which the bone cannot repair independently. Transplantation allows filling such areas with the help of pieces of other bones of the patient or donor material.

“Unfortunately, bone grafts carry disadvantages,” – said Gazit, a professor of surgery at Cedars-Sinai. “There are huge unmet needs in skeleton repair.”

One of the problems is that a sufficient amount of healthy bone material is not always available for transplantation. Surgeries to remove a bone piece, usually from the pelvis, and implant it can lead to prolonged pain and expensive, lengthy hospitalizations. Further, grafts from donors may not integrate or grow properly, causing the repair to fail.

A new technique developed by the Cedars-Sinai-led team could provide a much-needed alternative to bone grafts.

Earlier, Professor Gazit’s group had already tried to treat complex fractures without the use of grafts. Scientists filled the damaged parts of the bone with collagen, which attracted mesenchymal stem cells (MSCs).

For the differentiation of MSC into osteocytes – bone cells, the researchers worked on them with bone morphogenetic proteins (BMP), a growth factor responsible for bone formation.

However, to achieve the desired result, stem cells must be exposed to BMP for at least a week, and when the protein is injected into the fracture site, it disappears within a few hours.

At the next stage of the experiment, the Gazit’s team, using viruses, built copies of genes responsible for the synthesis of BMP, directly into the DNA of mesenchymal stem cells. However, this did not bring the desired effectiveness.

In their new work, scientists used a gene delivery method that does not require the use of viruses.

To conduct a new experiment, the researchers constructed a matrix of collagen, a protein that the body uses to create bones, and implanted it in the gap between the two sides of a fractured leg bone in laboratory animals (pigs). The matrix recruited the fractured leg’s own stem cells into the gap over a period of two weeks.

To start the process of bone restoration, scientists used DNA fragments enclosed in microbubbles with a thin lipid membrane. Microbubbles in a special solution were injected into the fracture site. Further, with the help of directional ultrasound, the researchers blew up the microbubbles, creating tiny nano-holes in the MSC, which allowed the genes to penetrate into the stem cells.

Eight weeks after the surgery the bone gap was closed and the leg fracture was healed in all the laboratory animals that received the treatment Tests showed that the resulting bone tissue was as strong as with the use of own bone grafts.

“This study is the first to demonstrate that ultrasound-mediated gene delivery to an animal’s own stem cells can effectively be used to treat nonhealing bone fractures,” – said study lead author Dr. Gadi Pelled.