PTTG and PBF: targets for enhancement of radioiodine uptake in thyroid tumours

Iodide uptake via the sodium iodide symporter (NIS) is reduced in many thyroid cancers, resulting in poor prognosis following treatment with 131I. The pituitary tumor transforming gene (PTTG) and its binding factor (PBF) are proto-oncogenes implicated in thyroid tumourigenesis and we previously demonstrated PTTG and PBF-induced repression of NIS in vitro. We have recently generated murine transgenic models of targeted overexpression of PBF and PTTG in the thyroid to investigate their function in vivo. Thyroid glands were harvested from 6-week old age- and gender-matched wild-type (WT), PBF and PTTG transgenic mice for mRNA analysis, immunohistochemistry or primary murine thyroid culture. Analysis by TaqMan RT-PCR showed that NIS mRNA expression was significantly reduced in PBF (0.5±0.1, P=0.0002, n=7), PTTG heterozygote (0.64-fold±0.15, P=0.01, n=5) and PTTG homozygote (0.62-fold±0.25, P=0.03, n=3) mouse thyroids. In parallel, the reduction of NIS protein expression was confirmed in PBF and PTTG transgenic mice immunohistochemically. Subsequent functional studies demonstrated repressed uptake of 125I in primary PBF (77.6±0.2% reduction, P=0.0004, n=6), PTTG heterozygote (43.4±0.7% reduction, P=0.042, n=15) and PTTG homozygote (65.4±0.9% reduction, P=0.004, n=14) mouse thyrocytes compared with WT controls. Following transfection with a PBF-specific siRNA, iodide uptake was restored in PBF thyroid cultures (2.4±0.64 fold increase, n=20, P=0.04) compared with scrambled siRNA controls, with uptake levels indistinguishable from those in WT thyroid cultures (P=0.86). Further, human primary thyroid cells transfected with PBF siRNA had elevated iodide uptake levels (2±0.07 fold, P=0.0001, n=11) compared to scrambled siRNA controls, and this correlated with increased NIS mRNA expression (4.76±0.58 fold, P=0.07, n=5).

Conclusion: PTTG and PBF potently repress NIS expression and function in vivo. Studies using PBF transgenic mice and human primary thyroid cultures emphasise PBF as a critical regulator of NIS function, and implicate PBF as a therapeutic target to improve treatment with 131I in thyroid tumours.