This model, called a conditional knockout, should help scientists unlock the mysteries of normal as well as abnormal development of these systems, opening the door to a better understanding of everything from heart defects in children to some cancers to spina bifida.
"Now we can say, ‛OK, if this gene is expressed everywhere and is in all these different tissues, can we generate a mouse that will now let us look at its role in just the heart, or just in the skin, or just during ear development and so on?'" said Dr. Simon J. Conway, developmental biologist, geneticist and principal author on the research published in a special February issue of Genesis: The Journal of Genetics and Development. The journal features 43 papers documenting the development of a variety of these conditional knockouts - rather than waiting for research results from the use of those mice - to facilitate research and collaboration.
Dr. Conway's laboratory, which focuses on heart development and congenital defects, will use the new animal model to explore the role of PAX-3, a gene that regulates other genes, in the development of the heart muscle itself as well as the two big vessels that exit the heart. Children born with a mutation of this gene are often called 'blue babies' because they have only one big vessel coming off their heart instead of a separate aorta, the major vessel that sends blood out to the body, and pulmonary trunk, which returns oxygen-poor blood to the lungs. While the anatomical problem usually can be corrected surgically, some of these children also have the potentially more lethal problem of a poorly beating heart.
PAX-3 actually has many roles in both formation and function throughout the body, and is involved in not only the heart but the brain and spinal cord, skin and hearing. It even is implicated in some cancers, including melanoma and the more rare alveolar rhabdomyosarcoma. "It's thought its abnormal expression may allow tumor cells to survive, so when control of PAX-3 is misregulated, you can have tumors," Dr. Conway said.
"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.
One of his first applications for the mouse is to study the role of PAX-3 in the muscle cells of the heart that enables it to beat. "There are very, very low levels of PAX-3 in the muscle cells, the cardiomyocytes, the actual cells that are causing the heart to beat but nobody knows exactly what they are doing," he said. "If PAX-3 is really important, then the hearts of the mutants will be abnormal." If the embryos survive, he should be able to determine what role, if any, PAX-3 plays in generating heart muscle cells.
Dr. Srinagesh Koushik, Hongmei Chen and Dr. Jian Wang. members of his laboratory team, are co-authors. Dr. Conway's research is supported by the National Institutes of Health, the American Heart Association and by the Children's Heart Program H. Victor Moore Research grant.