DNA fragile site breakage as a measure of chemical exposure and predictor of individual susceptibility to form oncogenic rearrangements.

Chromosomal rearrangements induced by non-radiation causes contribution to the majority of oncogenic fusions found in cancer. Treatment of human thyroid cells with fragile site-inducing laboratory chemicals can cause preferential DNA breakage at the RET gene and generate the RET/PTC1 rearrangement, a common driver mutation in papillary thyroid carcinomas (PTC). Here, we demonstrate that treatment with non-cytotoxic levels of environmental chemicals (benzene and diethylnitrosamine) or chemotherapeutic agents (etoposide and doxorubicin) generates significant DNA breakage within RET at levels similar to those generated by fragile site-inducing laboratory chemicals. This suggests that chronic exposure to these chemicals plays a role in the formation of non-radiation associated RET/PTC rearrangements. We also investigated whether the sensitivity of the fragile RET region could predict the likelihood of rearrangement formation using normal thyroid tissues from patients with and without RET/PTC rearrangements. We found that normal cells of patients with thyroid cancer driven by RET/PTC rearrangements have significantly higher blunt-ended, double-stranded DNA breaks at RET than those of patients without RET/PTC rearrangements. This sensitivity of a cancer driver gene suggests for the first time that a DNA breakage test at the RET region could be utilized to evaluate susceptibility to RET/PTC formation. Further, the significant increase of blunt-ended, double-stranded DNA breaks, but not other types of DNA breaks, in normal cells from patients with RET/PTC-driven tumors suggests that blunt-ended double-stranded DNA breaks are a preferred substrate for rearrangement formation, and implicate involvement of the non-homologous end joining pathway in the formation of RET/PTC rearrangements.