THE ROLE OF NUCLEOTIDE EXCISION REPAIR IN LATE STAGE BREAST CANCER: A STRATEGY TO OVERCOME TREATMENT RESISTANCE
No Thumbnail Available
Treatment resistance accounts for 90% of deaths of cancer patients. It is the central problem of all cancer therapeutics. Increased DNA repair is an important factor in treatment resistance. The nucleotide excision repair (NER) pathway plays an essential role in remediating DNA damage induced by many genotoxic agents such as cisplatin. We have previously shown that advanced stage breast cancers (BC) have a significantly increased NER capacity compared to early stage BC. By using a panel of 60 BC cell lines established in our lab, we discovered a microRNA, called “miRRA” that was found to be significantly downregulated in late stage compared to early stage BC and healthy tissues. When miRRA was transfected into three late stage BC cell lines, it lowered expression of three important NER genes, XPA, XPC, and RPA3, and significantly lowered functional NER capacity. Our long-term objective is to validate this microRNA as a therapeutic that reduces NER and drug resistance, allowing for a second chance at control or cure of advanced stage BC. Our hypothesis is that miRRA will lower NER capacity in advanced BC, allowing for the application or reapplication of chemotherapy that will be more effective. This project examined the molecular mechanism by which miRRA reduced NER in late stage BC. Using a dual luciferase vector assay, the miRRA-5p strand was found to bind the target sites of NER genes XPA and XPC, while the miRRA-3p strand was found to bind the target site on the RPA3 gene. These results indicated that the impact of miRRA on the NER capacity in late stage BC was due to direct miRRA binding of at least these three NER genes. The clinical significance of miRRA was assessed by determining if miRRA would enhance the cytotoxicity of cisplatin treatment. miRRA enhanced cisplatin cytotoxicity in a dose dependent manner in three different late stage BC cell lines. At the basic science level this work is the first to show intentional modulation of NER. At the pre-clinical level this work indicates that miRRA enhances cytotoxicity of cisplatin even in drug resistant late stage triple negative and luminal type BC.