DNA Base Excision Repair (BER) and Cancer Gene Therapy: Use of the Human N-Methylpurine DNA Glycosylase (MPG) to Sensitive Breast Cancer Cells to Low Dose Chemotherapy

Abstract

Abstract : To ensure that a cell's genome is not compromised, elaborate mechanisms of DNA repair are essential for both nuclear and mitochondrial DNA. The DNA Base Excision Repair (BER) pathway is responsible for the repair of alkylation and oxidative DNA damage. The short patch BER pathway begins with the simple glycosylase N(3)-methylpurine DNA glycosylase (MPG) which is responsible for the removal of damaged bases such as N(3) -methyladenine, hypoxanthine, and 1 ,N(3)(6)-ethenoadenine from the DNA. The resulting AP site is further processed by the other members in the BER pathway resulting in the insertion of the correct nucleotide. MPG, the enzyme that initiates the repair, has been over expressed in the breast cancer cell line, MDA-MB231. A construct containing mitochondrial-targeted MPG (mito-MPG) was also made, and MPG over expression was directed to the mitochondria. Over expression of nuclear- and mitochondrial-targeted MPG dramatically sensitized MDA-MB231 breast cancer cells to methylmethane sulfonate (MMS). After MMS treatment, the number of apoptotic cells was significantly higher in the cells that over expressed nuclear- and mitochondrial-MPG compared to the vector control cells. Furthermore, the cells that over expressed mito-MPG had a significant number of apoptotic cells without drug treatment. The subsequent hypothesis was that the over expression of MPG caused more bases, undamaged and damaged, to be removed. High levels of MPG generated more AP sites, especially after MMS treatment, affecting the balance of the BER pathway.