A post-irradiation-induced replication stress promotes RET proto-oncogene breakage

Ionizing radiation generates genomic instability by promoting the accumulation of chromosomal rearrangements. The oncogenic translocation RET/PTC1 is present in more than 70% of radiation-induced thyroid cancers. Both RET and CCDC6, the genes implicated in RET/PTC1, are found within common fragile sites - chromosomal regions prone to DNA breakage during slight replication stress. Given that irradiated cells become more susceptible to genomic destabilization due to the accumulation of replication-stress-related double-strand breaks (DSBs), we explored whether RET and CCDC6 exhibit DNA breakage under replicative stress several days post-irradiation of thyroid cells.

We analyzed the dynamic of DNA replication in human thyroid epithelial cells (HThy-ori-3.1) 4 days post a 5-Gy exposure using molecular DNA combing. The DNA replication schedule was evaluated through replication-timing experiments. We implemented a ChIP-qPCR assay to determine whether the RET and CCDC6 genes break following irradiation.

Our study indicates that replicative stress, occurring several days post-irradiation in thyroid cells, primarily causes DSBs in the RET gene. We discovered that both the RET and CCDC6 genes undergo late replication in thyroid cells. However, only RET's replication rate is notably delayed after irradiation.

The findings suggest that post-irradiation in the RET gene causes a breakage in the replication fork, which could potentially invade another genomic area, including CCDC6. As a result, this could greatly contribute to the high prevalence of chromosomal RET/PTC rearrangements seen in patients exposed to external radiation.