While Dr. Ho, assistant professor of biochemistry and molecular biology at Jefferson Medical College of Thomas Jefferson University and Jefferson's Kimmel Cancer Center in Philadelphia, and former Jefferson colleague Steven Farber, Ph.D., and Michael Pack, Ph.D., reported the findings in Science in 2001, one huge obstacle remained: identifying a gene behind the condition.
Now, Dr. Ho, Dr. Farber, now at the Carnegie Institution of Washington, and Dr. Pack at the University of Pennsylvania School of Medicine, have found a gene, which they dubbed fat free. Reporting in the April issue of the journal Cell Metabolism, the team explains that disrupting the gene interferes with the ability to absorb lipids through the intestine. These fish die when they are about a one and half weeks old because of this defect, even though they look normal and swallow properly.
The scientists found problems in mutant zebrafish bile duct and pancreatic cells that help with lipid digestion, in addition to defects in the cells that line the intestine, where fat and cholesterol absorption take place. Specifically, they found abnormalities in the Golgi apparatus, which holds newly made or recycled proteins that help with fat metabolism and transport.
The scientists used a strategy called positional cloning both to locate fat free in the zebrafish genome and to determine its sequence. They found that the gene shares 75 percent of its sequence with a human gene called ANG2 (Another New Gene 2), and also shares parts of its sequence with a gene called COG8, which is known to affect the Golgi apparatus. A change in only one base--one "letter" in the DNA code--results in the lethal mutation in zebrafish.
"The implication is that we can now attempt to screen drugs and look to see if anything can rescue this defect and increase intestinal lipid absorption," notes Dr. Ho. "We can try to find associated genes, proteins and other partners that are involved in this complex, as well as some of the mechanisms involved. The gene is well conserved across species and no one has discovered its function as yet, which makes it very exciting."
"The gene seems to be some sort of regulator that affects trafficking of lipids of cells through the gut," says Dr. Farber. "The next step is to try to understand mechanistically how the protein functions and what other genes it works with. Once we understand that, then we can potentially design drugs. A number of genes that regulate lipid metabolism have yet to be determined, and there's much to learn about how animals process lipids."
In earlier work at Jefferson, reported in Science, the research team designed special fat molecules called "optical reporters" that glow when they are cut up by an enzyme in the intestine, enabling them to watch, biochemically speaking, lipid processing in transparent zebrafish embryos.
They created random genetic mutations in zebrafish by exposing males to a chemical agent, then breed families harboring the resulting mutations. They then fed the fluorescent molecule to resulting zebrafish embryos carrying various mutations and watched it light up in the digestive tract, liver and eventually the gallbladder, examining the pattern of fluorescence. The scientists subsequently screened for alterations in lipid processing.
One major advantage of the zebrafish model is that it allows scientists to do "forward genetics." In this case, researchers look for a change in function, such as lipid metabolism, and then figure out what causes this effect. In reverse genetics, in contrast, researchers "knock down/out" a known gene and watch what effect it has on an organism.