PARP inhibitors can be used to induce synthetic lethality in cells with bi-allelic BRCA1 and BRCA2 mutations. However the effect of PARP inhibitors in combination with radiation on cells with mono-allelic mutations of BRCA1 and BRCA2 is unknown. We have examined the cell survival response of lymphoblastoid cells derived from normal individuals and those derived from carriers of BRCA1 and BRCA2 mutations, following exposure to ionising radiation and the PARP inhibitor Olaparib.Two lymphoblastoid cell lines from normal individuals and three with mono-allelic mutations in BRCA1 and BRCA2 were exposed to increasing doses of gamma radiation either alone or in combination with 5 µM Olaparib. Cell survival was measured using the MTT assay.Exposure to increasing doses of gamma radiation caused a reduction in cell survival of all cell types. The combined exposure to gamma radiation and 5 µM Olaparib did not enhance cell kill in normal or BRCA2 heterozygote lymphoblastoid cells but significantly enhanced cell kill in cells derived from BRCA1 carriers (P = 0.02). The treatment of cancer patients carrying mutations in the BRCA1 gene with radiotherapy and the PARP inhibitor Olaparib may significantly enhance radiation induced normal tissue toxicity in these patients.Citation: Bourton EC, Foster HA, Plowman PN, Harvey AJ, Parris CN (2013) Hypersensitivity of BRCA1 Heterozygote Lymphoblastoid Cells to Gamma Radiation and PARP Inhibitors.