Oct. 16, 2002 A rendezvous between the maggot and some of biology's hottest technology has given scientists some new clues about the signals that enable cancer cells to grow.
In a publication in the October issue of Developmental Cell, scientists at the University of Rochester Medical Center describe how their study of fruit-fly larvae has pinpointed a DNA sequence that largely controls when cells divide and multiply. The question of how and when cells stop and start dividing is central to all forms of cancer, where cells grow unchecked.
At the center of the study is a small blob of cells painstakingly collected from 3,000 maggots by graduate student Henri Jasper. Working with geneticist Dirk Bohmann, Ph.D., in the Department of Biomedical Genetics, Jasper spent nearly a month hunched over a microscope collecting cells from the structures that eventually form the compound eyes of a fruit fly.
The work falls into a hot area of biotechnology: the mass screening of genes to see which ones are active in certain situations and which ones are not. Such gene profiling is becoming more and more common as scientists try to make sense of the vast genetic data made possible by today's advanced laboratory techniques.
Bohmann and Jasper used a new technique, serial analysis of gene expression or SAGE, that they adapted for use with the fruit fly. The technique demands only a tiny amount of material compared to conventional gene-array technology, allowing scientists to study small groups of cells in great detail. For his maggot study, Jasper collected 800,000 cells -- just a speck of material -- from the structures that become the individual units in the compound eyes of fruit flies. Using popular current technologies, such as gene arrays, Jasper would have needed to collect about 100 times as many cells.
With SAGE, Jasper took aim at one of the most-studied organs in the history of science. As a fruit fly's eye develops while the fly is in the larval stage -- while it's a maggot -- the group of cells that will go on to form each compound eye undergoes a remarkable transformation. About every two hours, row by row of cells stop dividing and "settle down," committing to a particular fate, perhaps becoming a neuron or maybe a structural cell. The wave of change sweeps across the eye in just two days, with a shifting furrow dividing the two types of cells, until all cells have stopped dividing and have become specialized.
After collecting the cells from the maggots, Jasper used sophisticated cell-sorting technology to separate the dividing cells from cells that have stopped dividing. Then he and Bohmann measured the activity of more than 4,000 genes in each group.
Says Bohmann: "We asked: How does this group of cells differ from that group of neighboring cells? They're right next to each other, yet they are doing completely different tasks. That's the beauty of this work. We were able to look at the whole landscape of gene expression in one developing organ."
Jasper and Bohmann found that the activity of a few hundred genes differs markedly on either side of the furrow, including several genes that have been identified previously by scientists using classic genetics techniques. Then, through extensive computer analysis, they discovered a very short DNA sequence that is much more likely to be present near genes that are on when cells are dividing. The snippet of DNA was near 24 of 41 genes that were active when cells were dividing, compared to only 1 of 23 genes active when the cells had stopped dividing.
When they took the protein that recognizes this DNA sequence and expressed it in the cells that had stopped multiplying, those cells suddenly began dividing.
The DNA sequence, TATCGATA, is a known "transcription factor binding site," which serves as a flag that tells cellular machinery to turn on a nearby gene. Its crucial role activating a gene had been previously identified with a single gene, but this is the first time scientists have shown that it appears to be crucial to several genes involved in cell growth. Bohmann says more study is needed to determine if the sequence is a master switch turning growth genes on. Understanding such molecular signaling -- whether in a fruit fly, a mouse, or a person -- and how some genes control many others is vital to unraveling how cancer works, Bohmann says.
Bohmann spent years studying cancer in people, then switched to studying fruit flies. His work with maggots is the kind of basic research undergoing a boom at Rochester, which is recruiting more than 100 top scientists and their teams -- including Bohmann and Jasper, who came to Rochester last year from the European Molecular Biology Laboratory in Heidelberg, Germany -- as part of a strategic plan to expand its basic research.
"The fruit fly is a very good choice for genetic studies," Bohmann says. "Many of the proteins we're interested in that exist in people also are present in fruit flies. Biochemically, we're very much the same; the same mechanisms that control how cells divide in fruit flies control how cells divide in people. But the fruit fly is much simpler -- there are far fewer genes to worry about, and the genome is 20 times smaller than that of a person. They reproduce every 12 days. So it's much easier to get answers with the fruit fly.
"It's largely thanks to the fruit fly that we understand as much as we do about how genes cause cancer," Bohmann adds.
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