Prevalence of RET/PTC rearrangements in thyroid papillary carcinomas: Effects of the detection methods and genetic heterogeneity

CONTEXT: RET/PTC rearrangements have been reported in papillary thyroid carcinomas with variable frequency in studies that used different detection methods. OBJECTIVE: Our objective was to determine the role of different detection methods and tumor genetic heterogeneity on RET/PTC detection. DESIGN: Sixty-five papillary carcinomas were analyzed for RET/PTC1 and RET/PTC3 using five detection methods: standard-sensitivity RT-PCR, high-sensitivity RT-PCR, real-time LightCycler RT-PCR, Southern blot analysis, and fluorescence in situ hybridization. RESULTS: RET/PTC rearrangements were detected by standard-sensitivity RT-PCR in 14 tumors. High-sensitivity RT-PCR detected RET/PTC in all of these and in 12 additional cases, where the levels of expression corresponded to one to five positive cells. Real-time LightCycler RT-PCR detected RET/PTC in 12 and Southern blot analysis in 11 tumors. By fluorescence in situ hybridization, 14 tumors were positive, including nine cases with 50-86% positive cells and five cases with 17-35% positive cells. Overall, nine (14%) tumors harbored clonal rearrangements, which were present in the majority of tumor cells and detected by all five methods. Five (8%) cases had subclonal rearrangements present in a smaller portion of tumor cells and detected by most methods. Twelve (18%) tumors had nonclonal RET/PTC that were detected only by high-sensitivity RT-PCR. No other mutations were found in tumors harboring clonal RET/PTC, whereas 60% of tumors with subclonal and 42% of tumors with nonclonal RET/PTC harbored additional mutations. CONCLUSIONS: Our data suggest that broad variability in the reported prevalence of RET/PTC rearrangement is at least in part a result of the use of different detection methods and tumor genetic heterogeneity