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