February 18, 2000

Gene’s rolel in promoting cellular harmony revealed

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Earl Ruley, Ph.D.

Gene's rolel in promoting cellular harmony revealed

Keeping a cell's genomic instruction booklet in mint condition is one role played by a gene called Fus, Vanderbilt University Medical Center scientists report in this month's Nature Genetics. The findings are important to ongoing efforts aimed at understanding the normal processes that go awry when a cell becomes cancerous.

"We know that cancer is a mutation-driven process," said H. Earl Ruley, Ph.D., Ingram Professor of Cancer Research and professor of Microbiology and Immunology. "In particular, mutations that affect the ability of a cell to maintain its DNA in a normal non-mutated form tend to be associated with the development of cancer."

Ruley and colleagues found that mice without the gene Fus had high levels of chromosomal instability, a hallmark of cancer. In addition, the mice had deficiencies in the development and function of white blood cells called B-lymphocytes, and they died shortly after birth.

The Fus gene was originally identified as a gene that is translocated (mutated) in a type of cancer known as liposarcoma. Translocations of Fus have also been linked to some forms of acute myeloid leukemia, but prior to the current studies, very little was known about the normal function of the Fus gene.

Ruley and Geoffrey G. Hicks, Ph.D., now a senior investigator at the Manitoba Institute of Cell Biology, generated the Fus knockout mice as part of a larger effort called "tagged sequence mutagenesis," a strategy they developed to produce a library of gene mutations that can be introduced into mice. One mutation in their library is in the Fus gene; they have over 500 more.

Ruley and Hicks were interested in Fus because it is a protein that binds to RNA, the copy of DNA that gets decoded to make proteins. Although there are many "RNA binding proteins," their functions are mostly unknown.

"It's a big black box," Ruley said. "The cell is expending a great deal of effort to make proteins that bind to RNA, and we don't know very much about what many of them are doing. The fact that some of them are directly involved in malignant change says that this is likely to be an important area of cell regulation."

Fus is the first RNA binding protein to have a potential role in genome maintenance, Ruley said. He speculated that Fus might participate in DNA repair or in chromosomal segregation during cell division. Future studies will probe how Fus functions to keep chromosomes in order.

A role for Fus in B cell development and function was also unexpected. Nagendra Singh, Ph.D., postdoctoral fellow, Luc Van Kaer, Ph.D., associate professor, and Eugene M. Oltz, Ph.D., assistant professor, in the department of Microbiology and Immunology, analyzed Fus-deficient B cells. They found that B cells did not develop normally in the Fus knockout mice.

Because the Fus knockout mice die shortly after birth and have poorly developed B cells, the investigators could not study B cell function in the knockout mice. To get around this problem, they replaced the blood-producing bone marrow in normal mice with cells from Fus knockout embryos. This technique produces so-called "chimeric" mice that have blood cells with the mutation, but that are otherwise normal.

"This allowed us to focus on the mutation just in hematopoietic (blood) cells, independent of any effect the mutation might have on the animal as a whole," Ruley said. The investigators found that Fus-deficient B cells did not replicate their DNA in response to several compounds that mimic immune response-provoking signals.

"We were able to do these immunological studies because we are part of a larger community that focuses on immunogenetics–using genetic approaches to understand how the immune system works," Ruley said. The immunogenetics program, based in the department of Microbiology and Immunology, is one of several recent initiatives in mammalian genetics at Vanderbilt.

The studies were supported by the National Institutes of Health, the Robert J. Kleberg, Jr. and Helen C. Kleberg Foundation, and Cancer Care Manitoba.