"We already have a mouse that has no PAX-3 but unfortunately those die very early in utero because PAX-3 is required for so many things. Transcription factors (such as PAX-3) by their very nature control large pathways of other genes," Dr. Conway said. "There are lots of different pathways that interact. Other genes do specific things but transcription factors are at the top of the pathway; they control many downstream genes. So one way to try and narrow it down is to make a specific mutation at a specific time in a specific tissue."
So the Augusta researchers have developed a mouse with which they can selectively eliminate the gene from being expressed in a single organ. They've made this conditional knockout through careful breeding and genetic engineering.
The engineering includes inserting DNA recognition signals called loxP that flank the PAX-3 gene throughout a mouse's DNA. They then breed that mouse with another mouse - called a CRE mouse. CRE is a bacterial enzyme whose natural function is to recognize loxP and remove it; about 200 different CRE lines are expressed in different parts of the mouse - within everything from the heart to the skin. "Whenever CRE finds this sequence of loxP in the DNA, it looks for another one and removes whatever is in between," Dr. Conway said, effectively and selectively eliminating the PAX-3 gene from that area in the offspring. "So if someone wants to look at the role of PAX-3 in development in the skin (where mutations can lead to severe pigmentation problems), they can take our mouse and breed it with a mouse that has a skin-specific CRE and look at the effects of mutating the gene in only the skin.