Development of an S-G2 micronucleus assay for the detection of in vitro chromosomal radiosensitivity in high-risk women subjected to early mammography screening for breast cancer

Ionizing radiation induces DNA double-strand breaks (DSB). The two major repair pathways for DSB are homologous recombination (HR), operating only in late S and G2 phase, and non-homologous end joining (NHEJ), active throughout the whole cell cycle. It is known that the micronucleus (MN) assay, based on irradiation of cells in the G0 phase of the cycle cannot unequivocally detect enhanced radiosensitivity in carriers of BRCA1 and BRCA2 mutations. This is mostly due to the fact that breast cancer susceptibility genes such as BRCA1, BRCA2 and ATM are mainly involved in the G2-phase specific HR pathway and/or in the G2/M checkpoint control. To increase sensitivity of the micronucleus assay in detecting defects in these specific pathways, we designed a modified S-G2-micronucleus assay allowing detection of defects in HR and G2/M cell cycle control. Such assay may not only serve for assessing radiosensitivity in women with a proven or presumed genetic predisposition for breast cancer, but may also be a useful tool for phenotypical diagnosis of inherited genome instability diseases. Preliminary results obtained with the S-G2 micronucleus assay in a family of a patient with variant Ataxia-Telangectasia and in a pilot group of BRCA1 +/- mutation carriers are very promising and will be presented. With the new S-G2 micronucleus assay significantly increased radiation-induced MN yields were obtained in mutation carriers compared to healthy controls. When applied to BRCA1/2 mutation carriers on a broad scale and in the frame of a large population study (in progress), the results of the novel assay may aid to establish whether benefits from early mammographic screening in asymptomatic women with a family history of breast cancer are likely to outweigh the risks linked to increased cumulative doses of radiation.