Exploring the Role of PARP Inhibitors in Breast Cancer
Creators: name: University Of California Office Of The President
Publisher: name: University of California Office of the President
publisherIdentifier: https://ror.org/00dmfq477
publisherIdentifierScheme: ROR
Year: 2024

description: Approximately 80% of breast cancers occur sporadically, while the remaining cases are inheritable. Mutations in the breast cancer associated genes BRCA1 and BRCA2 are responsible for a major portion of the hereditary cases. Importantly, the study of BRCA1/2’s role in normal cell chromosomal DNA repair pathways has provided great insights in how defects in these repair mechanisms underlie cancer progression. Notably, recent studies have shown that inhibitors of poly (ADP-ribose) polymerase (PARP), a key DNA repair enzyme, exhibit significantly enhanced toxicity to BRCA1- or 2-deficient cells, and clinical trials have been initiated in patients with BRCA deficiency. PARP is required for the repair of DNA single-stranded breaks, and inhibition of PARP leads to the conversion of single-strand breaks to double-strand breaks. In the absence of BRCA1 or 2, homologous recombination cannot occur and the double-strand breaks remain unrepaired leading to complex chromosomal alterations and apoptosis (programmed cell death). 
 
 
 
 
 
 
 
 Although BRCA1/2 mutations are only found in 5–10% of patients with breast cancer, studies have suggested that a significant portion of patients with sporadic breast cancer may have gene expression profiles similar to those in patients with hereditary BRCA1/2 mutations. This leads to two critical questions. First, which genes in the BRCA DNA repair pathway lead to breast cancer in a manner identical to BRCA mutations or deficiency? Second, will PARP inhibitors showing promise in treating patients with BRCA hereditary defects be applicable to a proportion of sporadic breast cancers? 
 
 
 
 
 
 
 
 We will employ a high-content genomic siRNA screen to identify genes for which loss of function increases the sensitivity of cells to PARP inhibitors. Next, the “hits” obtained from the siRNA screen will be validated, categorized and prioritized using a series of secondary assays. This will allow us to gain insight into the mechanism by which PARP inhibitors function and to choose those “hits” (i.e., DNA repair pathway genes of interest) most appropriate for further study. Our approach is to use immortalized breast epithelial cells to screen a large genomic library of siRNAs in the presence and absence of a PARP inhibitor. Using high-throughput techniques, we will analyze Rad51 focus formation and DNA content as markers for homologous recombination, cell cycle arrest and apoptosis to select our candidate genes in interest. We anticipate that the “hits” of highest interest are those which cause cell death (apoptosis) in the presence, but not absence, of the PARP inhibitor. 
 
 
 
 
 
 
 
 The use of PARP inhibitors is currently a promising avenue for therapy for BRCA-deficient patients due to a high therapeutic index, decreased toxicity of the treatment, and improved quality of life for breast cancer patients. We expect to identify additional genetic biomarkers to develop a novel “profile” of sporadic human breast cancers that would be sensitive to PARP inhibitors. Finally, these genes may also be important in the cell’s ability to maintain genome stability and as such are candidate breast cancer susceptibility genes.