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NEWS ALERT
Harvard Medical School Office of Public Affairs
Researchers help transgenes fit in
FINDINGS AND RELEVANCE:
A new discovery promises to improve the quality of organisms that contain genes manufactured in a lab. Often times, an organism’s endogenous DNA unexpectedly switches off these transgenes. Working in flies, researchers sandwiched transgenes between protective stretches of DNA, thus insulating the foreign genes from their surroundings and ensuring optimal expression in every tissue.
Dr. Norbert Perrimon
Working in the lab of Harvard Medical School Professor Norbert Perrimon, postdoctoral researcher Michele Markstein sandwiched transgenes between protective stretches of DNA before inserting them into fruit fly cells. From the cells, she grew flies that expressed the transgenes optimally in every tissue tested. Her method appears in the April issue of Nature Genetics.
 Postdoctoral researcher Michele Markstein acted as a kind of "caseworker" for transgenes, creating the perfect environment for them in fruit flies.Self-portrait by Michele Markstein |
“Classically transgenes are randomly integrated into the DNA of the host cell, often landing in areas where the surrounding sequence prevents them from being optimally expressed,” explains Markstein. “We’ve insulated these transgenes from their surroundings, so they’re expressed at high levels, regardless of their integration spot along the chromosome.”
“Using Michele’s method, we can create quality transgenic flies on the first try,” adds Perrimon, who is also an investigator with the Howard Hughes Medical Institute.
Initially, Markstein acted as a kind of “caseworker” for the transgenes. She searched for a perfect home in the DNA of the host cells, a location where the transgenes would always be nurtured, rather than a random residence where they might be silenced.
“Michele tried about 20 integration sites, but none of them proved optimal in every tissue of the fly,” explains Perrimon. If the transgene worked in muscle, for example, it might be silent in the brain.
Markstein decided to take a different approach. Seeking to safeguard the transgenes from trouble in their new homes, she borrowed a trick from the gypsy retrovirus, which infects fruit flies. This pathogen uses insulator sequences to protect its own genetic code from the DNA of the fly. Markstein took these insulator sequences and added them to her transgenes before unleashing them on the fly cells. Animals that grew from the cells contained high levels of the transgene product in every tissue tested.
“In a sense, the transgene travels with the perfect environment, enabling it to function optimally wherever it lands,” says Markstein.
According to Perrimon, the new method could be applied to other species, though labs must first identify an appropriate insulator sequence, as the gypsy retrovirus is specific to flies. In the short term, the method will advance Perrimon’s goal of creating a massive library of transgenic fruit flies for use in experiments.
Each line of flies will contain a single transgene that disrupts the expression of an endogenous gene through a method called RNA interference. The library will eventually contain approximately 15,000 different lines, one for each gene in the fly. Researchers will use these animals to test various hypotheses about the roles of specific genes.
“The library should be complete in three to four years,” says Perrimon. “These fruit flies will allow us to probe developmental and physiological processes at a systems level in organisms.”
Labs can even control when and where each transgene is expressed. Perrimon’s team engineered the transgenes to be under control of a system called Gal4/UAS. Scientists choose Gal4 lines of flies that allow them to activate the transgene in some parts of the body while keeping it silent in others.
“This tool will help us explore how organs communicate with each other at specific points in time,” says Perrimon.
Written by Alyssa Kneller
CITATION:
Nature Genetics. April 2008, Vol. 40 Issue 4: 476-483.
“Exploiting position effects and the gypsy retrovirus insulator to engineer precisely expressed transgenes.”
Michele Markstein (1), Chrysoula Pitsouli (1), Christians Villalta (1), Susan E Celniker (2), and Norbert Perrimon (1)
CONTACTS:
Alyssa Kneller (Harvard Medical School)
public_affairs@hms.harvard.edu
617.432.0442
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