Thyroid cancer is the most common form of solid neoplasms known to be associated with radiation exposure. However, the mechanisms of radiation-induced carcinogenesis are not well understood. The high prevalence of rearrangements of the RET gene has been recently found in post-Chernobyl papillary thyroid carcinomas and in thyroid tumors from patients exposed to therapeutic external radiation. The positions of breakpoints sites in the RET and ELEI genes identified in post-Chemobyl tumors with RETPTC3 rearrangements suggested that these two genes may be aligned across from each other in the nucleus at the time of DNA breaks (Nikiforov et al., Oncogene, 1999). Consistent with this idea, we found that one pair of RET and H4 genes (contributing to RET/PTCI rearrangement) was juxtaposed in 35 percent of inter-phase nuclei of normal thyroid cells. These data suggest that two potentially recombinogenic chromosomal loci may be contiguous to each other in the nucleus predisposing to generation of rearrangement by adjacent double-strand DNA breaks produced by ionizing radiation or other genotoxic agents. The main goal of the current proposal is to explore the role of nuclear architecture and gene proximity in generation of chromosomal rearrangements after radiation exposure. We propose to use two-color FISH and three-dimensional confocal laser-scanning microscopy to determine the frequency of physical proximity of genes, contributing to the major types of RET/PTC rearrangements, in normal human thyroid follicular cells and in other cell lineages. We will establish whether chromosomal organization with respect to these loci is cell-type specific, age-dependent, or varies with cell cycle stage. Any of these parameters could explain in part the high prevalence of thyroid cancer after irradiation, and the higher susceptibility of children. Then, we will expose cultured cells to different doses of ionizing radiation to test directly the relationship between gene proximity and the frequency of radiation-induced RET/PTC rearrangements in vitro. These studies will extend our understanding of the mechanisms of radiation-induced carcinogenesis in the thyroid gland. In addition, they are likely to have importance for a broad range of chromosomal rearrangements found in cancer.
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