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A new mouse strain with a specific defect in the gene associated with Wilms' tumor-the primary form of kidney cancer in children, responsible for six percent of all childhood cancers-will help scientists understand the origins of this disease and provide a new window on the very complex process of kidney development.
The new mouse strain, described in the August 27 issue of Cell, was developed by an international team led by Dr. Rudolf Jaenisch, a member of the Whitehead Institute for Biomedical Research and professor of biology at MIT.
Dr. Jaenisch and his collaborators have taken advantage of new "gene knock-out" techniques to insert a nonfunctional copy of the Wilms' tumor gene WT-1 into mouse chromosomes. Embryos carrying two copies of the nonfunctional gene fail to develop kidneys.
Dr. Jordan Kreidberg, a postdoctoral fellow in the Jaenisch laboratory at the Whitehead Institute, explains that WT-1 is a member of a class of genes known as tumor suppressor genes. Scientists believe that tumor suppressor genes work in a variety of different ways to help regulate growth and development in normal cells. Tumors occur when a mutation somehow prevents such a gene from performing its normal regulatory function.
For example, in some human families, children inherit a normal copy of the WT-1 gene from one parent and a nonfunctional copy of WT-1 from the other parent. Cancer develops in these children when some event disrupts the one normal copy of the gene in a single kidney cell, and as a result the cell grows out of control, producing a tumor.
Studies of tumor suppressor genes in animal models offer new opportunities to understand the molecular basis of growth and development in health and disease.
The kidney is a particularly attractive target because kidney development depends on very precise interactions between two different types of embryonic tissue. Efforts to study this process in cell culture have had great success with regard to cellular processes; but have provided little information about the molecular control of kidney development. The primary goal of the current project was to obtain a definitive answer to the question: What role, if any, does the WT-1 gene play in normal development?
Professor David Housman of biology, one of the co-authors of the current paper, cloned the WT-1 gene several years ago. Since then, he and his associates have identified several individuals with mutations in WT-1 who have abnormal development of the urinary tract and genitals as well as increased susceptibility to Wilms' tumor. But these patients have one normal copy of WT-1. "To truly understand the role of WT-1 in early development, we had to create a system in which we could eliminate WT-1 function altogether," Dr. Kreidberg said.
The Whitehead scientists and their collaborators used gene replacement strategies to disrupt one copy of the WT-1 gene in mouse embryos. Mice descended from these embryos appear normal, but are capable of transmitting the nonfunctional copy of the gene to their offspring. When two affected mice mate, some of the embryos inherit nonfunctional copies of WT-1 from both parents. The most striking feature of these embryos is that they do not make kidneys; kidney development stops very early in embryogenesis and the primitive pre-kidney structures disappear. The embryos also exhibit abnormal development of the gonads and some disruption of the heart and lungs. All of the affected embryos die between days 13 and 15 of gestation [Normal mouse gestation is 20 days].
"These mice provide definitive proof that WT-1 plays a key role in early kidney development,"said Dr. Jaenisch. "They will allow us to begin sorting out at the molecular level the complex interactions that must occur during mammalian development to produce normal kidneys."
The new model also will help scientists understand the role of WT-1 in cancer. The structure of the protein encoded by the WT-1 gene suggests that it is a DNA-binding protein. Evidence from tissue culture studies indicate that it has the capacity to regulate the activity of other genes.
The new mouse model will speed the identification of these target genes and also will make it possible to explore the effects of different mutations in WT-1 itself.
The Wilms' tumor knock-out represents an important milestone in the new field of transgenic science. It demonstrates the power of this technology for answering questions about human development and disease-questions that cannot be addressed through human studies or through cell culture studies in the laboratory.
A version of this article appeared in the September 8, 1993 issue of MIT Tech Talk (Volume 38, Number 5).