Genetic Flaw Starts Biochemical Domino Effect

Michael Stern's latest research into the formation of neurofibromatosis tumors reads something like a federal racketeering indictment, except that Stern's tracing proteins instead of laundered money, and he's looking not at offshore accounts but at biochemical paths of cause and effect.

The research, which appears in the Jan. 10 issue of the Journal of Neuroscience, seeks to find the biochemical pathway that's responsible for tumors in people with the genetic disorder neurofibromatosis. Stern built his case much like a prosecutor, compiling evidence from dozens of painstaking experiments on mutant fruit flies, each with a specific genetic flaw that testified to the power of one or more proteins involved.

Neurofibromatosis is characterized by the formation of tumors of peripheral nerve cells. Scientists know the disease is caused by defects in a gene called Nf1, but they have yet to find out precisely how the defective genes cause tumors to form.

"Our results suggest that having a defect in Nf1 begins a kind of biochemical domino effect that eventually leads to tumor growth," said Stern, professor of biochemistry and cell biology.

Stern's research group used fruit flies for several reasons: the insect's genome has been sequenced; it takes only two weeks to grow an new generation of fruit flies; and scientists know which fruit fly genes are analogous to the human genes associated with neurofibromatosis.

In preparing for its experiments, Stern's group knew that Nf1 encodes a protein called neurofibromin that inhibits the effect of a second protein called Ras, which is known to promote nerve cell growth. They also knew that a third protein called PI3K was recently reported to be hyperactivated in mice that had defective Nf1 genes, and they knew that PI3K requires the activity of a fourth protein called Akt, to carry out its tasks.

In their experiments, they created more than two dozen mutant strains of fruit flies, including varieties that were either missing the genes to make one of the four proteins or were encoded to overexpress, or make extra amounts of, one of the four. Some mutants were designed to carry more than one defective trait.

Nerves from each mutant strain were examined. By comparing the mutant strains - each with a specific defect or set of defects - they buillt a case that the absence of neurofibromin allows Ras, PI3K and Akt to work in concert to inhibit a regulatory group of proteins called 'forkhead box O,' or FOXO. FOXO proteins are key players in regulating the genes responsible for programmed cell death and DNA repair - two common culprits in cancer.

"Our results raise the possibility that neurofibroma formation in individuals with neurofibromatosis might result in part from a Ras-PI3K-Akt-dependent inhibition of FOXO," Stern said.

Stern said the project required an enormous amount of work in the lab, and it wouldn't have been possible without the dedication and motivation of research technician Willliam Lavery, the paper's first author.

"Will displayed terrific leadership on this project," Stern said.

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Stern and Lavery's co-authors include research technician Michelle Wells, postdoctoral research assistant Veronica Hall, graduate student James Yager and undergraduate Alex Rottgers.

The research is supported by the Department of Defense Neurofibromatosis Research Program.

Contact: Jade Boyd
Rice University