Characterization of Brca1 deficient mice.

BRCA1 is a nuclear phosphoprotein that is expressed in a cell cycle regulated manner in virtually all normal dividing cells. Inheritance of a mutated copy of the BRCA1 gene increases a woman's risk for developing breast and ovarian cancer (1-3). Since the tumors that arise in these individuals consistently fail to express the wild-type allele, BRCA1 is believed to encode a tumor suppressor. Loss of the remaining functional BRCA1 allele, therefore, is one of the steps leading to neoplastic transformation of some types of epithelial cells. The isolation of the murine homologue of the human BRCA1 gene opened up the possibility of using a powerful genetic approach to study the role of this gene in both normal development and tumor formation. This genetic approach involves in vitro manipulation of the genome of embryonic stem (ES) cells, stable tissue culture cell lines derived from mouse blastocysts. After introducing mutations into the murine homologue of the BRCA1 gene Brca1 in these cell lines, four groups have generated mouse lines carrying the same mutations (4-7). Surprisingly, mice carrying a single mutant Brca1 allele do not display the increased risk for breast tumors seen in humans carrying similar mutations. However, while loss of BRCA1 appears to be a one of the many events involved in tumorgenesis in humans, these mouse lines demonstrate that gene expression is essential for development; as homozygosity for each of the Brca1 mutations results in postimplantation embryonic lethality. The survival of Brca1 deficient embryos is extended by one or two days in the absence of p53 and p21 (7,8).