Abstract PD09-10: DNA Damage and Repair in Mammary Gland Development
Authors: Kass EM , Helgadottir H , Moynahan ME , Jasin M 
Published: August 2014

Abstract
Numerous factors are linked to breast cancer risk including reproductive history, early exposure to radiation, age and genetic background. Events that occur during specific stages of mammary development are believed to alter the lifetime risk of breast cancer. Epidemiological studies over the past 3 decades have suggested an association between parity and decreased lifetime breast cancer risk. Studies have also shown that breast cancer risk is significantly higher in women who received radiotherapy to the chest as adolescents compared to those who received chest radiation as adults. These associations implicate development specific changes in mammary epithelium that affect the molecular mechanisms whereby cells respond to DNA damage and, importantly, are at subsequent cancer risk.
Because many of the genes linked to hereditary breast cancer are associated with DNA double-strand break (DSB) repair, we hypothesize that there are differences in overall DSB repair, homologous recombination (HR), and cell fate following damage in mammary epithelium at different stages of mammary gland development—puberty, pregnancy, lactation and post-involution. To assess these differences we are using mice containing the DR-GFP reporter targeted to chromosome 17 to study HR at chromosome breaks induced by the transient expression of I-SceI in mammary epithelial cells. DR-GFP is a widely used repair reporter in which direct repeats of two defective GFP genes are induced to recombine by I-SceI endonuclease cleavage of one of the repeats. We have observed that the efficiency of HR in primary mammary epithelial cells derived from 8-week old virgin mice is similar to that observed in other primary cells of somatic origin (approximately 0.65% of cells heterozygous for the DR-GFP reporter). Interestingly, the amount of HR is increased in mammary epithelium derived from pregnant mice compared to age-matched virgin controls (0.98 versus 0.65%). This 1.5 fold increase in HR suggests that in pregnancy, mammary epithelium is more proficient in the accurate repair of DNA DSBs. We are also assessing DNA damage signaling in different periods of mammary gland development. Preliminary analysis of histone H2AX phosphorylation following whole-body irradiation suggests more rapid kinetics of DNA damage signaling response in mammary tissue derived from pregnant as compared to age-matched virgin mice.
We have recently generated DR-GFP containing mice that express I-SceI from a randomly integrated I-SceI transgene under the control of a tetracycline-inducible promoter. We are currently using these mice to assess HR in an entirely in vivo system and are in the process of breeding this reporter line with mice carrying mutations in Brca1 or Brca2. This mouse model will be used to assess HR in vivo in the mammary tissue of these mice during different stages of mammary gland development.
This project will provide valuable insight into how developmental changes unique to mammary gland epithelium affect cellular responses to DNA damage. We hope to consider therapeutic manipulation of identified stage-specific differences to decrease breast cancer susceptibility.
Citation Information: Cancer Res 2012;72(24 Suppl):Abstract nr PD09-10.