Transposons are snippets of DNA that are also known as jumping genes because they move around a genome. Their movements are random, but they typically insert themselves into other genes and disrupt their functions, causing a mutation. For more than 50 years scientists have been looking for a transposon that can efficiently work in mammals. Tian Xu has modified a transposon from the cabbage looper moth and uses this transposon system to mutate genes in mice and discover their functions. The process is described below.
- The piggyBac transposon, found in the cabbage looper moth, is split into two modified DNA plasmids. One, the transposon lacking the gene that expresses the jumping enzyme transposase, carries a red fluorescent protein (RFP) marker. The other is the modified transposase gene, plus a gene that causes a mouse’s coat to turn black.
- Each plasmid is injected into a different mouse embryo. As the mice grow, their appearance reflects their genetic status—mice that carry the transposon gene glow pink under ultraviolet light; black mice carry transposase, the jumping enzyme. A black mouse and pink mouse are bred.
- Some of their offspring—identified by a black coat and a pink glow—carry both the transposon and transposase and have actively jumping genes.
- These mice are bred with normal mice.
- Some of their offspring will inherit a mutation but not the jumping enzyme. Without the enzyme, the transposon remains in place, and polymerase chain reaction will reveal which gene has been disrupted. Other offspring will inherit both the mutation and the jumping enzyme and can be bred with normal mice, as in step 4, to produce mice with different mutations.
- Mutant mice are bred with normal mice to produce offspring that have the same stable mutation.
- Mice, each carrying one copy of the same mutation, are bred. The breeding of these two heterozygous mice will produce a homozygous mouse with identical copies of the same gene mutation.
- The offspring with two copies of the same mutation can be identified by a red glow because they also carry two copies of the RFP gene. If a defect results, researchers can discern the function of that gene by the consequences of the mutation. Xu’s goal is to determine the cause of single-gene diseases by doing this type of analysis for each of the 20,000 mouse genes that have not been analyzed.