Osteosarcoma is the most common kind of bone cancer in children. Found mostly in teenagers, these tumors have a very high propensity for local invasion and distant metastasis. Nearly 1,000 new cases are diagnosed in the United States every year.

Despite treatment protocols that include intensive chemotherapy and surgical resection, osteosarcoma still recurs in more than 30 percent of treated patients, most commonly in the lung. Unfortunately, patient outcomes have not significantly improved in the last two decades.

In 2006, Bang H. Hoang, MD, received an Orthopaedic Research and Education Foundation (OREF) Research Grant that supported his study of the role that small families of proteins (Wnt inhibitors) play in the development of osteosarcoma. A 2011 OREF Career Development Grant is now providing him funding to build on that promising work.

“Although osteosarcoma is very common among teenagers, it is rare compared to other cancers,” explained Dr. Hoang, associate professor in the department of orthopaedic surgery at the University of California, Irvine. “That makes it harder to obtain research funding from organizations like the National Institutes of Health (NIH) and the National Cancer Institute, which focus on more common cancers. Meanwhile, orthopaedic tumor surgeons like me know all too well how important it is to these young patients to move research forward.”

Molecular triggers
The molecular mechanisms that contribute to osteosarcoma progression are not well understood. Before they can develop more effective therapies for osteosarcoma, researchers need a better understanding of how cancer cells release signaling molecules that cause normal tissue to trigger angiogenesis and tumor growth.

The Wnt signaling pathway is a network of proteins that affects various physiologic processes, including cell differentiation and the development of cancer. Studies suggest the Wnt pathway is an important factor in osteosarcoma progression and metastasis. Preliminary data from Dr. Hoang’s first OREF-funded study indicate that Wnt proteins regulate osteosarcoma production of a protein receptor called neuropilin-2 (NRP-2).

“Our data suggest that NRP-2 is up-regulated in several osteosarcoma cell lines and that blocking NRP-2 suppresses in vivo tumor production,” said Dr. Hoang. “Other researchers have shown that NRP-2 expression in cancer correlates with increased tumor vascularity and a poor prognosis.”

These results form the foundation for his current research project, “Targeting Neuropilin-2 for Osteosarcoma Growth and Metastasis.” Neuropilins are multifunctional receptors that induce angiogenesis and tumor progression by acting as co-receptors for vascular endothelial growth factor (VEGF). Dr. Hoang is exploring the role of NRP-2 in the angiogenesis of osteosarcoma using both in vitro and animal models.

“Our objective is to understand how NRP-2 contributes to the pathophysiology of the disease. We hope that our study will lead to therapeutic strategies that will prevent tumor progression and metastasis,” said Dr. Hoang.

The first step is proving that NRP-2 is a target of Wnt signaling in vitro, using molecular techniques first to introduce the protein into cell cultures and then to introduce the inhibitors of the protein. “If inhibitors don’t succeed in slowing tumor growth, we’ll try to knock down or block the protein using molecular techniques. What we learn will be applied in the next stage, the mouse model,” explained Dr. Hoang.

That phase of the study involves injecting human tumor cells into nude mice to induce tumor growth in the tibia. The tumors will spontaneously metastasize from the bone into the lung. “At the end of the experiments we will examine the lungs and the tumors to study the effects of blocking NRP-2,” said Dr. Hoang. “We’re hopeful that we’ll find further positive results.”

Future possibilities
If NRP-2 proves to have a significant role in tumor growth and metastasis, blocking this receptor with antibodies or chemical compounds may lead to clinical trials for a second-line therapy, supplemental to the standard multimodality therapy (chemotherapy and surgical resection).

Dr. Hoang hopes other researchers will build on his results, possibly to design antibodies that can block the protein from activating the receptor. “Another approach would be to design mimic compounds that bind to the protein and prevent it from activating the receptor,” he said. “This would require screening a long list of compounds, some of which may already be available.”

With more basic science knowledge about how osteosarcoma develops, Dr. Hoang is hopeful that additional research will help reduce tumor burden, improve limb salvage, and reduce metastatic disease. “I am optimistic that we will soon see strategies that will ultimately improve overall survival rates as well as quality of life for everyone with osteosarcoma,” he said.

Inspired by patient determination
Asked what inspired him in his work as a clinician scientist, Dr. Hoang related a story of treating a high school student who had just earned a lacrosse scholarship. The patient had an undiagnosed mass on his knee. A biopsy determined it to be a high-grade osteosarcoma. The patient asked Dr. Hoang if, with all the treatment options on the horizon, he might be able to return to playing lacrosse. “I told him, ‘most likely not. You probably won’t be able to play at that level,’” Dr. Hoang recalls.

Following chemotherapy and surgery involving implantation of a large prosthesis, the patient continued to see Dr. Hoang for annual checkups through college. The patient never mentioned athletics, and Dr. Hoang didn’t ask.

Several years later, the patient and his parents came to a reunion at the hospital for patient families. “I learned that the patient played lacrosse both for his varsity team, and at the NCAA level. He also went back to his high school and coached lacrosse,” Dr. Hoang reported. “When asked about his long leg scar, he said it was a shark bite. That tells you how determined this patient was—and is—that nothing would hold him back.”

Patients such as the lacrosse player drive Dr. Hoang to conduct research. “For people with these kinds of diseases, hope is everything. To be able to offer them hope is very satisfying to me.”

Getting this satisfaction, according to Dr. Hoang, was made possible in part by his OREF grants. “Osteosarcoma is not very high on the NIH agenda. OREF is making a statement by supporting it—that it is important for musculoskeletal research and for the patients who have these diseases. I think the support from the OREF Career Development Grant is very important in moving this osteosarcoma research forward.”

Mark Crawford is a contributing writer for OREF and can be reached at communications@oref.org