Mechanisms of DNA Damage in Breast Cancer and Inhibition of DNA Repair through PARP Inhibitors

Steven Le

Introduction. Breast cancer is the most diagnosed cancer in women and evidence in research shows that breast cancer is becoming more prevalent in developed countries. Treatments depends on the cancer state, genomic markers, patient’s age, and known mutations such as BRCA 1/2 mutations. In types of breast cancer with BRCA mutations, PARP inhibition an effective method for treatment. The BRCA and PARP genes encode for DNA damage repair mechanisms, namely homologous recombination and base-excision repair. When cancer cells with BRCA mutation are treated with PARP inhibition, the cell will undergo apoptosis as a result of unrepaired DNA damage, as it lacks the necessary homologous recombination or base-excision repair mechanisms to fix the DNA damage. However, resistance to PARP inhibition can develop. Studies have found that the protein, RAD51, mediates resistance of cancer stem cells to PARP inhibition. Additional studies have found that PARP inhibitors can be combined with Myc blockage to improve efficacy. Methods. Cancer stem stems were screened for using ALDEFLUOR, which uses flow cytometry and immunofluorescence. An exploratory PCR array of 84 key DNA repair genes was completed and RAD51, which assists in DNA double-strand break repair, was identified as having higher expression in CSCs. siMYC were used to observe differences in gene expression of both MYC and RAD51. Dinaciclib was tested in combination with niraparib to observe its efficacy in improving sensitivity to PARP inhibition. Results. Lowest cell count was observed in BRCA mutant cells lines at high concentrations of PARP inhibition, as predicted. However, cancer stem cells show the opposite effect, as highest cell counts were observed in BRCA mutant cell lines with PARP inhibition. RAD51 KD in vitro and in vivo was used to determine the effect of the RAD51 protein in resistance to PARP inhibtion. MYC was found to specifically upregulate RAD 51 expression and activity, leading to increased recovery of homologous recombination repair (HR). Dinaciclib was proven as the best pharmacological treatment to improve the efficiacy of PARP inhibitors, because of Downregulation of MYC/RAD51 expression, synergistic effect with PARP inhibitor, and increase in apoptotic cells. Conclusions. PARP inhibition in cells with BRCA mutation prevent DNA damage repair and lead to death of cancer cells. Resistance of PARP inhibitors can develop through RAD51 in cancer stem cells of BRCA mutant cell lines. MYC inhibition, namely through Dinaciclib, can decrease RAD51 expression and improve efficacy of PARP inhibitors.

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