Genomic copy number alterations in radiogenic breast cancer

Abstract

The aim of this study was to elucidate the underlying molecular genetic mechanisms of radiation-induced breast carcinogenesis. Younger women are more at risk of developing breast cancer than older women following radiation exposure. Circulating oestrogen levels are highest during adolescence and early adulthood and oestrogen has a known transforming effect on breast epithelial cells. One hypothesis suggests that radiation and oestrogen synergise to drive breast cell transformation. There are currently no known genetic markers of radiogenic breast cancer. In order to investigate genetic alterations associated with radiogenic breast cancer in vitro models of radiation-induced breast epithelial cell transformation was developed. The immortalised, non-transformed breast epithelial cell line MCF-10A was exposed to fractionated doses of X-rays in the presence or absence of additional oestrogen. Radiation-treated cells displayed several phenotypic changes some of which provided evidence of cell transformation, including loss of contact inhibition and change to mesenchyme cell morphology. Genomic analysis of radiation treated cells using high-density polymorphism arrays identified a gene deletion of the POU2F1 transcription factor and amplification of the c-MYC proto-oncogene. POU2F1 has a role in cellular stress response and mediates DNA damage response through interactions with BRCA1. Amplification of c-MYC has previously been identified in breast cancers of survivors of the atomic bombs during World War II. Genetic alteration in POU2F1 and c-MYC may therefore be linked to radiation-induced breast cell transformation. Changes in gene copy-number were confirmed by fluorescent in situ hybridisation (FISH) and alterations in protein expression by western analysis. Gene copy number and expression of POU2F1 and c-MYC was investigated in a cohort of radiogenic and sporadic breast cancer tissue samples by FISH analysis and immunohistochemistry. Expression of c-MYC was higher in radiation-induced breast cancers compared to sporadic breast cancers (p = 0.002), as was the mean number of copies of c-MYC (p = 0.030). Loss of expression of POU2F1 was identified in one of 18 radiation-induced breast cancers but was not observed in sporadic breast disease (0/33). In summary, a cell model of radiation-induced breast cell transformation identified POU2F1 deletion and c-MYC amplification as putative markers of transformation, which were subsequently identified in primary tissue samples, suggesting a role for these alterations in the development of radiogenic breast cancer.