The DNA damage response (DDR) pathway plays a critical role in maintaining genomic stability and preventing cancer. Disruption of this pathway by mutations in BRCA1 or its binding partner BACH1 increases cancer incidence in patients, suggesting that these proteins are essential for prevention of tumorigenesis. BACH1 and BRCA1 are essential for establishing a DNA damage checkpoint, repair of double strand breaks by homologous recombination, and maintenance of genome stability. Although BRCA1 clinical mutations disrupt BACH1 binding, it is unclear how BRCA1 binding affects BACH1 function and ultimately cancer suppression. The objective of our proposal is to study the molecular basis of breast cancer using significantly innovative experimental tools in a highly collaborative interdisciplinary research program. Specifically, the lab of Dr. Rothenberg will lead the single-molecule experimental approaches, including assay and technique development and data analysis, while the lab of Dr. Cantor, an expert leader in BRCA1 studies and characterization of the BRCA1-BACH1 system, will provide samples and design the molecular targets. The ongoing synergy between the Rothenberg and Cantor laboratories and expertise of the PIs is ideal for the proposed studies. Together, we will develop powerful single-molecule imaging techniques capable of unraveling significant details that cannot be obtained using other methods. These techniques are conceptually groundbreaking and will provide new information on the biological questions presented here. This proposal will test the novel hypothesis that BRCA1 regulates BACH1 DNA helicase activity for DNA repair and tumor suppression. Specifically, we will examine whether BRCA1 regulates BACH1 localization and/or directly modulates its helicase activity. Importantly, our proposed research will dissect how BRCA1 binding to BACH1 is linked with DNA repair and breast cancer suppression. We will directly test whether loss of the BRCA1/BACH1 complex disrupts normal cellular DNA damage response and repair. Analyzing BACH1 patient mutations will elucidate the genetic events that drive cancer formation. Uncovering how BRCA1 binding to BACH1 is linked to tumor suppression could foster the generation of rationally designed therapies to combat disease. For example, it may be possible to recapitulate how BRCA1 regulates BACH1 by introduction of a BRCA1 mimetic. Furthermore, many efforts in cancer research are centered on switching cells from DNA crosslink-resistant to DNA crosslink-sensitive. If BRCA1 regulates BACH1 helicase activity to effect DNA damage signaling, disruption of this BRCA1/BACH1 interaction could serve as a tool to render cells sensitive to chemotherapeutic drugs. If so, these proteins will represent excellent targets for developing drugs to overcome DNA crosslink resistance in breast cancers. In fact, the reagents used in our studies may be the basis for developing such drugs.