Low frequency of PAX8‐PPARγ rearrangement in follicular thyroid carcinomas in Japanese patients

Paired‐box gene 8 (PAX8)‐peroxisome proliferator‐activated receptor‐γ (PPARγ) gene fusion has been identified at significant frequency in follicular thyroid carcinomas (FTCs) with cytogenetically detectable translocation t(2;3)(q13;p25). This represents a possible specific molecular marker for follicular carcinoma. In this study, we examined PAX8‐PPARγ rearrangement in 24 FTC samples from Japanese patients by reverse transcribed‐polymerase chain reaction (RT‐PCR) using two upstream PAX8 primers located in exons 7 and 8 and a downstream primer in exon 1 of PPARγ. The fusion gene was detected in only one of 24 FTCs (4%). The FTC with PAX8‐PPARγ rearrangement from a 56‐year‐old man showed a product consistent with fusion between exon 8 of PAX8 and exon 1 of PPARγ. It was confirmed by direct sequencing. This FTC was histologically encapsulated, composed of trabeculae and small follicles and had complete penetration of the capsule by tumor tissues (minimally invasive type). The frequency of the fusion gene in this study was much lower than the 29–63% noted in reports from other countries suggesting that FTCs in Japanese patients may have a special genetic background, and that the high iodine intake from a typical Japanese diet might influence the frequency of the fusion gene in FTCs.