"This is a very unique retrovirus," Holzschu says. "This is the first time the cyclin gene has been found in any retrovirus in humans or animals."
Human cyclin D1, a protein similar to the cyclin D observed in Holzschu's walleye studies, has been implicated in several human cancers, including cancer of the breast and esophagus.
Because cyclin serves as a reproductive control agent in cells, conventional wisdom suggests that if large amounts of cyclin are produced, cells will replicate at a higher rate. So, when researchers examined the growing tumors on the walleye in the fall, they expected to find cyclin. But instead, they found only small amounts of the mRNA that codes for cyclin. In contrast, in the spring, when the tumors were dying, the researchers found a high level of cyclin mRNA. Researchers presume that the presence of this mRNA precedes the production of cyclin.
"It's possible that the overproduction of this protein causes the cells to die," Holzschu says. "If we could figure out why these tumors in walleye are dying and determine the role cyclin is playing, we might be able to use that as a principle for killing some tumors in people."
Their studies of the role cyclin plays in tumor onset and regression is a relatively new focus for Holzschu and his collaborators at Cornell University. Lorie LaPierre, a former doctoral student at Cornell who now is a postdoctoral researcher in biomedical sciences at Ohio University, was studying walleye retroviruses when she discovered that these viruses contained the cyclin gene.