RET/PTC rearragements in Hashimoto’s thyroiditis nodules.

An association between Hashimoto thyroiditis (HT) and thyroid carcinoma, particularly papillary thyroid carcinoma (PTC), has been postulated for decades, although definitive molecular links remain elusive. The majority of PTC is characterized by alterations or rearrangements along the MAPK signaling cascade. Activation of RET/PTC oncogene has been reported in both non-neoplastic and neoplastic HT thyroid follicular cells (Tallini et al., 2006), but the issue is still controversial. We have investigated cells from HT nodules for RET rearrangement, using interphase fluorescence in situ hybridization (I-FISH). Dual-color I-FISH experiments, using home-brew breakapart DNA probe, able to detect RET disruptions were performed on 207 fine needle aspiration biopsies on unselected nodules from 168 patients and 38 corresponding surgical samples. Normal tissue nuclei cutoff value (mean ±3DS) was ≥ 3%. In addition, RET/PTC was investigated by RT-PCR. Coexistent overt autoimmune thyroid disease (AITD) was assessed by serological (antithyroid autoantibodies, ATA), clinical and echographic data. RET breakage (FISH+) was observed in 13/207 (6.3%) samples. RET/PTC was confirmed by RT-PCR (RT-PCR+) in 2/12 available FISH+ nodules (16.6%). RT-PCR+ corresponded to FISH+ value ≥ 8%. No correlation between RET rearrangement and ATA (5/92 patients ATA +; 8/115 ATA-), or between AITD (6/78 AITD+; 7/129 AITD-) was observed. Nodules with RET disruption were observed along the whole cytological spectrum, with prevalence significantly higher in suspect or malignant cytology (13.6% in 66 TIR 3-5 vs 3.1% in 129 TIR 2 nodules: p=0.005 by X2test). 7 of 13 FISH+ nodules were surgically removed and available for I-FISH. RET breakage was confirmed in 3 samples, all of which were classified as classic PTC type. According to our results, RET rearrangement does not seem to correlate with the HT phenotype. The number of FISH+ nodules within the cytological classes, increases according to the risk of malignancy. Sensitivity of breakapart I-FISH strategy, able to detect gene disruption in single nuclei, could explain the difference between I-FISH and RT-PCR results. Since in our experimental condition, RET-PTC m-RNA transcript can be detected in ≥ 8% FISH+ samples , the oncogenic potential of nodules below this value remains to be clarified, and FISH+ nodules with benign cytology should be considered for a careful follow-up.