Tumorigenesis is known to result from multiple genetic changes. Although endogenous and environmental insults can damage DNA, cellular mechanisms exist to repair various forms of damage or to eliminate those cells which been irreparably damaged. Hence, the accumulation of numerous genetic changes that would lead to cancer in normal cells is extremely rare. Nevertheless, disruption of a DNA repair pathway has the potential to expedite tumorigenesis by producing a cell that is hypermutable. We have demonstrated that a major pathway for repair of chromosomal double strand breaks is homologous recombination and that the protein products of the hereditary breast cancer genes, BRCA1 and BRCA2, play a key role in this process. Phenotypic characteristics of cells that are defective in homology-directed repair include chromosome instability and extreme sensitivity to certain DNA damaging agents. We propose that the genetic instability that arises from the inability to precisely repair chromosome breaks by homologous recombination results in tumor predisposition. We will first test whether peptides that interfere with interactions between BRCA2 and RAD51 or between BRCA1 and BARD1 cause cellular phenotypes typically found in homologous recombination mutants. As part of this analysis, we will also examine cellular effects caused by the expression of dominant negative RAD51 proteins. We will then determine whether mice engineered to express these dominant negative peptides in mammary gland have abnormal gland development and, importantly, if they are prone to tumors. Two approaches will be taken to express these peptides - inducible transgenesis and retroviral transduction using a system developed by the Varmus lab. In addition, mammary gland primordia from mice that die perinatally from homologous recombination defects will be transplanted to normal fat pads to induce gland development, to determine if hyperplasia results. The inter-related aims of this project include an analysis of how defective genomic caretaker function, and thus increased genetic instability, alters the initiation and progression of mammary tumors in mice that are genetically engineered for cancer development through self-sustained growth, disrupted cell-cell homeostasis and angiogenesis dependence. We will also examine the molecular mechanisms of genetic instability that arise from homologous repair defects, including instability arising from disrupted BRCA1 or BRCA2 function. Two specific outcomes of instability will be examined in molecular assays - chromosomal translocations and LOH from mitotic recombination.