Can Broken Bones Heal Themselves? Microbubbles, Sound And Gene Therapy May Be The Answer

Broken bones often take much more than casts and splints to heal: In the US alone, about 100,000 people each year experience bone breaks so drastic that they don’t mend properly.

If treated inadequately, non-healed fractures can lead to disability or even death. Bone grafts can help these “non-union” fractures heal, but the invasive procedures are an unfeasible option for ill or aging patients (or severely damaged bones). And even for healthy individuals, bone grafts come with significant risk and lengthy recovery times.

A new treatment method could help more people bypass the need for bone grafts altogether: Leveraging a therapeutic technique known as sonoporation, Cedars-Sinai researchers used stem cells, gene therapy, and ultrasound-activated “microbubbles” to stimulate bone regeneration and fracture healing.

In trials on mini-pigs with broken shinbones, a single treatment of the microbubble-powered gene therapy was successful in healing fractures within just 8 weeks.

Repairing serious bone injuries can be a challenge for even the best orthopedic surgeons, since too much bone loss can make it impossible for bones to regrow. More than 2 million bone grafting procedures are performed each year, but the grafts themselves can take 3 months or longer to heal — and that’s when they’re successful.

Whether patients undergo autografts (where bone is taken from the patient’s own body) or allografts (where it is taken from a tissue bank), bone-graft subjects always face the risk that the body will reject the new marrow. And with autografts, patients have to heal from painful, invasive procedures at both the harvesting site — usually the pelvis — as well as the injury site. Even in very healthy patients, full recovery can take around six months.

“We’re combining an engineering approach with a biological approach to advance regenerative engineering, which we believe is the future of medicine,” said Dan Gazit, PhD, DMD.

To speed up bone regeneration and healing, some doctors have begun incorporating BMP – bone morphogenetic protein – into their injury treatments. BMP helps the body’s stem cells create more bone marrow, helping fractures heal more quickly.

But existing methods for administering the protein come with drawbacks: Incorporating BMP into bone implants, for example, can lead bone tissues to break down (via resorption) or form in soft tissues where they don’t belong – likely because the implantation method administers a large BMP dose.

A more targeted approach involves delivering plasmid DNA of the bone morphogenetic protein directly into cells using “viral vectors.” (Viral vectors are viruses that have been stripped of their disease-causing genes, and engineered to carry good genes into a human cell.)

Unfortunately, viral vectors have pitfalls too; side effects like inflammation are common and, in rare instances, use of viral vectors can lead to harmful DNA mutations.

The gene therapy innovation using microbubbles — which healed 0.4-inch fractures in pigs in just eight weeks, without invasive surgery — could provide a more rapid, safer, and targeted way to stimulate fracture healing using BMP.

The microbubble treatment, which was tested in the broken shins of 18 mini-pigs, involves two separate procedures. First, researchers inserted a biodegradable, sponge-like collagen scaffold into the injury site; collagen stem cells are known to attract endogenous mesenchymal stem cells (MSCs), which are proficient at producing BMP.

Scientists waited two weeks for the next procedure, allowing the scaffolds to recruit as many MSCs as possible. At that point, they injected a mix of BMP-encoding DNA and microbubbles — microscopic gas-filled, lipid-shelled bubbles — at each injury site to facilitate the sonoporation process.

Sonoporation uses sound (usually ultrasonic frequencies) to modify the permeability of cell plasma membranes. In this instance, researchers applied the pulse of an ultrasound machine to the injury site; in doing so, they triggered the gas-filled microbubbles to inflate and deflate (or “oscillate”).

The oscillation of the microbubbles gently opened tiny pores on the stem cell membranes through which the injected BMP-encoded DNA could enter the cell. Since the pore openings self-repair and thus shut back up quickly – there was minimal risk of the injected DNA migrating elsewhere within the body.

In fact, expression of the introduced gene was undetectable in the pigs after just 10 days and the subjects showed no inflammation. More impressively, the fractured tibias in all 18 pigs healed within eight weeks of the second procedure, and the bone tissue grown inside the injury site had comparable strength to that produced by surgical bone grafts. Animals not treated with the ultrasound or microbubbles did not completely heal.

The impressive results do come with a caveat: All 18 pigs used in the study were under 1 year in age, which may have contributed to their ability to heal quickly. Younger animals (and people) tend to have far more stem cells, yet large fractures are far more common in the elderly than the young.

Bearing out how the microbubble treatment fares in older adults will be key to clinical application. Human testing of the microbubble treatment has yet to begin, but the researchers plan to start soon. The paper’s co-senior authors at Cedars-Sinai – Drs. Dan Gazit and Gadi Pelled – hope to see their innovation in clinical applications as soon as five years from now.

Regardless of how quickly the microbubble treatment arrives in US hospitals, the researchers’ work addresses an important gap in skeleton repair options today.

“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 Pelled. “It addresses a major orthopedic unmet need and offers new possibilities for clinical translation.”

The original research “In situ bone tissue engineering via ultrasound-mediated gene delivery to endogenous progenitor cells in mini-pigs” was published in the journal Science Translational Medicine. Full information is available here.