Reporting in the January 2002 issue of the American Journal of Human Genetics, investigators at Washington University School of Medicine in St. Louis describe a small, molecular variation in some tumor samples taken from neurofibromatosis patients.
"Neurofibromatosis is a common, inherited genetic disease that affects about one in 3,500 people," says principal investigator Nicholas O. Davidson, M.D., professor of medicine and of molecular biology and pharmacology and director of the Division of Gastroenterology at Washington University School of Medicine. "The gene responsible spans a large region of chromosome 17, but we have found that a very small change in the NF gene's messenger RNA can inactivate the final product of this gene, a protein called neurofibromin."
Neurofibromin suppresses tumor development. When it is inactivated, its tumor-suppressing ability is lost, and patients with NF develop a wide variety of tumors. Approximately 25 percent of tumors studied had evidence of a single nucleotide RNA change -- called RNA editing -- that occurs when messenger RNA is edited before leaving the nucleus of the cell. Cells that did have the change, however, seemed to be more aggressive tumors, and a substantial percentage was malignant.
Since patients inherit two copies of the NF gene, most researchers believe in a "two hit" mechanism that launches neurofibromatosis. The first hit comes when a person is born with one copy of the gene already mutated to cause the disease, but that person would actually develop clinical manifestations of NF only after the second copy of the gene was "hit." The faulty RNA editing seen by Davidson could provide that second hit.
"When a cell makes a protein," explains Davidson, "the first steps occur as the DNA encoding the gene is copied into a strand of messenger RNA (mRNA). The mRNA specifies the eventual sequence of the final protein. If a reading error or nucleotide sequence change occurs as the DNA is copied into mRNA, it can change both the amino acid sequence and the properties of the final protein. These same effects can be introduced through RNA editing."
The type of error that Davidson found in some NF tumor samples is known as C ->U RNA editing. In human cells, DNA and RNA molecules consist of strands of building blocks known as nucleotides that are described as a sequence of four letters. For RNA, the letters are ACGU. Davidson and colleagues found that when the NF gene is copied into mRNA, a C at a specific spot is replaced by a U. That changes the protein made by the gene by introducing a signal to stop translation of the mRNA. This process in effect turns off the neurofibromin tumor suppressor, making patients vulnerable to tumor formation.
Davidson and colleagues first observed such C ->U editing in an abundant gene expressed in the human gastrointestinal tract.
"A gene normally made in the small intestine, called apolipoprotein B, is absolutely required for lipid transport," Davidson says. "Human small intestinal apolipoprotein B undergoes C ->U editing as a normal part of its regulation to produce a shorter version of a protein that orchestrates fat absorption. Our study shows, however, that the same RNA editing machinery essential to normal function in the intestine can inactivate the neurofibromin tumor suppressor in some patients with neurofibromatosis."
The evidence that RNA editing occurs the same way in both the gut and in tumor tissue comes from the presence of an enzyme called apobec-1 (apoB RNA editing catalytic component #1). Usually apobec-1 is found only in the gut. But Davidson and his colleagues discovered apobec-1 in nerve sheath tumor samples taken from patients with neurofibromatosis. The finding could mean that C ->U editing also might occur in tumors elsewhere in the body as an agent that disables tumor suppressor genes. Davidson and colleagues plan to test other tumor samples for the presence of apobec-1.
"We found apobec-1 in eight of 34 neurofibromatosis tumors we tested, and many of those tumors were more aggressive than the ones that didn't show RNA editing. Now we need to look at more tumors and see whether this pathway may be involved in the formation of tumors generally, or whether those tumors that involve RNA editing really are more aggressive than other types of tumors in neurofibromatosis," he says.
Davidson also hopes to look at tumors from other diseases to see whether RNA editing might be involved in creating various types of cancer.
Debnath Mukhopadhyay, et. al., "C ->U editing of neurofibromatosis 1 mRNA occurs in tumors that express both the Type II transcript and apobec-1, the catalytic subunit of the apolipoprotein B mRNA-editing enzyme," American Journal of Human Genetics, vol. 70(1), 38-50, Jan. 2002.
This research was supported by grants from the National Institutes of Health.