 |
News
from Harvard Medical, Dental, & Public Health Schools |
||
|
February 19, 1999 GENETICS Mutant Genes May Cooperate
|
|
In a pair of papers appearing this month, assistant professor of
genetics Chao-Ting Wu, Harvard junior fellow James Morris, and their
colleagues show that the promoter does play a critical role in transvection
in Drosophila. Specifically, they report that fly genes with
defects in their promoters are more likely to turn on their partners
than are non-defective versions of the same gene. The researchers
also found that they could induce transvection by making tiny mutations
in the promoter region of healthy genes. The two sets of findings
are reported in the February issues of Genetics and Genes
and Development respectively.
As for how this works, Wu and her colleagues believe the maimed promoter may cause other components of the transcriptional apparatus, specifically enhancers, to be released. Freed from the defective gene, they seek out and turn on the promoter of their nearby homolog.
In a sense, then, transvection could represent a way for faulty genes to overcome their defects which, in turn, suggests a transvection-like approach might be used to correct defects associated with disease. "One reason we're so excited is that we think a better understanding of the mechanism of homolog pairing could be used to correct disease--not by integrating homologous corrective sequences, as with some current gene therapies, but by pairing homologous gene sequences," says Wu. "For example," she asks, "if someone has a gene that doesn't make enough of a particular enzyme, could we bring in a homolog with really powerful enhancers to increase production from the gene?"
Similarly, it might be possible to turn off toxic genes by introducing homologous genes that either turn the gene off or lure away its enhancers. Wu and her colleagues plan to develop models to test these ideas. "I suspect that it is going to be a long road," she says.
For Wu, part of the appeal of studying transvection is that it has been a path not much taken. "I love mysteries and I like to be where it isn't too crowded--which also makes it a great topic for students," she says. In 1993, when Morris, then an MD/PhD student, first came to her lab, people had tantalizing but untested ideas about how transvection occurred. For example, Wu's colleague Pam Geyer, who is at the University of Iowa and is a coauthor on this month's publications, had shown that two mutant versions of a fly gene called yellow could produce pigment when paired and that the mutants had complementary defects, one in the promoter and the other in the enhancer region of the gene. But the promoter defects were huge, extending even to adjacent genetic structures.
To discover if the promoter played a critical role in transvection, Morris paired 53 different mutant genes with a homolog in which the enhancers were blocked, and which were thus nonfunctional. "The Genetics paper shows that of the 17 cases in which we get transvection, every single one that could be explained by the enhancers being released to act on the homolog had a promoter disruption," Wu says. But as with Geyer's observations, most of the promoter defects were huge.
Morris decided to create minute mutations in two critical regions of the yellow gene promoter--the TATA box and the initiator (Inr). Using a difficult technology called gene replacement, Morris created three kinds of mutant yellow genes--one with a defective TATA box, another with a defective Inr, and a third with defective Tata and Inr--and, in turn, three strains of homozygous mutant flies.
All three strains were relatively pale, suggesting the three mutant genes were not producing pigment normally. He then mated each of the three strains with another pale strain--one that was homozygous for the enhancer-blocked yellow gene. Heterozygous offspring from each of the three matches were dark, suggesting that mutant versions of the yellow gene had cooperated to make pigment. "So the experiment showed yes indeed, if you make these tiny mutations, enhancers are released to act on the homolog," says Wu (top figure).
 |
 |
| Defective genes may pair up to produce protein in two ways. Top: Genes with defective promoters may release their enhancers (E), which then turn on the homolog's promoter (P). Bottom: Genes with blocked enhancers may, upon pairing with a severely mutated gene, undergo changes in shape that bring their enhancers close enough to the promoter to turn it on. |
Transvection may allow enhancers to perform other kinds of tricks. In experiments reported in the September Proceedings of the National Academy of Sciences, Morris took flies carrying the enhancer-blocked genes and mated them with flies carrying severely mutated homologs--yellow genes lacking not only the promoter but also one of the enhancers. Their offspring produced pigment. Morris believes that in attempting to pair with the severely mutated homolog, the gene carrying the blocked enhancers underwent a conformational change which, in turn, brought its enhancers closer to its own promoter, eventually enabling them to turn it on (bottom figure).
Wu and her colleagues plan to look for the various kinds of transvection in other animals. "Maybe we'll find instances in mammals. If we do see transvection, the next step is to see if we can increase its frequency. If we can, maybe we can harness this into a therapeutic method," says Wu.
--Misia Landau