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).
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.