The researchers, who were funded by the National Institutes of Health (NIH) and the Department of Defense (DOD), also found that a drug on the market for treating certain kinds of blood cancer curbs tumor growth in a mouse model of neurofibromatosis type 1. A clinical trial of the drug is underway in people with the disease.
The results of the study on mice are published in the October 31, 2008 issue of Cell.
The study's senior investigators were Luis F. Parada, Ph.D., a neuroscientist at the University of Texas Southwestern Medical Center in Dallas, and D. Wade Clapp, M.D., a hematologist at the Indiana University School of Medicine in Indianapolis. Their research was supported by NIH's National Institute of Neurological Disorders and Stroke (NINDS), NIH's National Cancer Institute (NCI), and the Neurofibromatosis Research Program of the U.S. Army Medical Research and Material Command.
"By taking a team approach and combining their unique areas of expertise, Drs. Parada and Clapp were able to shed light on a complicated disease mechanism and to develop a potential treatment," says Jane Fountain, Ph.D., a program director with NINDS.
Neurofibromatosis type 1 is a genetic disease that affects about 1 in 3500 Americans. The nerve-associated tumors, or neurofibromas, that occur in the disease tend to grow just under the skin or at the nerve root. The latter type of tumor, called a plexiform neurofibroma, can cause disabling symptoms by compressing the nerve, the spinal cord, bones, muscles and internal organs.
The tumor forming cells within a neurofibroma may become malignant and spread to other parts of the body. There is currently no treatment to prevent neurofibroma growth.
In 1990, NIH-funded investigators discovered that neurofibromatosis type 1 is caused by loss-of-function mutations in a tumor suppressor gene, now known as NF1. People with the disease have the genetic makeup NF1+/-, meaning they have one functional copy of the gene and one non-functional or mutant copy.
Still, for many years the trigger for neurofibroma growth has been a mystery. Schwann cells, which form a protective sheath around nerve fibers, were a prime suspect. However, neurofibromas also contain nerve fibers themselves, connective tissue and mast cells, the latter of which circulate in the blood and contribute to inflammation.
Combined with previous findings, the new study suggests that the formation of plexiform neurofibromas requires two steps: complete loss of NF1 in Schwann cells (rendering them NF1-/-) and an interaction between NF1-/- Schwann cells and NF1+/- mast cells. While Schwann cells appear to be the primary tumor causing cell, mast cells appear to stimulate tumor growth by recruiting other cell types and blood vessels to the tumor.
"The mast cell inflammatory response appears to be co-opted by the tumor to enhance tumor growth," says Dr. Parada.
The researchers uncovered the role of mast cells in tumor growth through a series of technically challenging experiments. Previously, Dr. Parada had shown that mice with a targeted deletion of the NF1 gene in their Schwann cells and an NF1+/- genetic background develop plexiform neurofibromas, while mice with the same targeted deletion and an NF1+/+ genetic makeup do not develop the tumors. Drs. Parada and Clapp now show that in these non-tumorigenic mice, it is possible to induce plexiform neurofibromas by transplantation of NF1+/- bone marrow (which contains mast cells and other blood cells).
The researchers also examined the role of c-kit, a molecule that is expressed by mast cells and other cell types, and is known to become overactive in some kinds of cancer. When c-kit was genetically deleted in NF1+/- bone marrow cells prior to transplantation, the transplanted cells failed to induce neurofibromas. Meanwhile, in mice that were prone to develop neurofibromas, the cancer drug Gleevec – an inhibitor of c-kit – reduced the metabolic activity and size of the tumors.
Because Gleevec is already prescribed for chronic myelogenous leukemia and other cancers, the researchers were able to secure fast regulatory approval for a phase 2 clinical trial of the drug in children and adults with neurofibromatosis type 1.
In their report, the researchers also discuss the compassionate use of Gleevec in one young neurofibromatosis patient with an airway-compressing plexiform neurofibroma. This patient was not considered an ideal candidate for surgery given the location of the tumor and its vascular nature. A three month administration of the drug was effective in significantly shrinking the tumor without any observed side effects.
"The results in this one patient are encouraging, but future research is critical to determine any long-term benefits and risks in the majority of patients," Dr. Fountain says.
The researchers say that the complex origin of tumors in neurofibromatosis – which has thwarted therapeutic development until now – could be the chink in the disease's armor. In patients with leukemia, Gleevec directly targets the cancer-causing cells, which can become genetically resistant to the drug over time and render it ineffective.
"In patients with neurofibromatosis type 1, we are using the drug to target a non-tumorigenic cell, so we believe -NIH/National Institute of Neurological Disorders and Stroke