Characterisation of thyroid structure and function in the PBF knockout mouse

Dr Vicki Smith, MRC Research Fellow

School of Clinical and Experimental Medicine, University of Birmingham

Introduction

We are interested in the study of a protein known as PBF that is upregulated in thyroid cancer. In mice, high levels of PBF result in greatly enlarged thyroid glands. PBF also prevents two proteins that are important for thyroid hormone production from functioning properly. One of these proteins is crucial for the treatment of thyroid cancer with radioiodine.

Final Report

PBF is a protein that is present in the thyroid and is upregulated in both benign and malignant thyroid disease. We have shown that PBF can downregulate two proteins that are important for normal thyroid function, namely NIS and MCT8. NIS takes up iodide from the blood into the thyroid and is therefore important for thyroid hormone synthesis, and MCT8 secretes thyroid hormone from the thyroid into the blood. NIS is also important for radioiodine uptake and hence increased PBF protein can impact on critical treatment of thyroid cancers and their metastases.

Multinodular goitres contain increased levels of PBF, and mice that have high levels of PBF in the thyroid develop large goitres. Thyroid cancers with higher levels of PBF are more likely to recur and become metastatic, and are associated with a reduced survival rate. However, the way in which PBF affects thyroid growth and cancer development is not fully understood.

These studies indicate that PBF is an important regulator of the thyroid and to understand more about how it does this, we aim to characterise a mouse which does not have the PBF protein, a PBF knockout mouse. Our main objectives are to analyse the effect of not having PBF on the growth and development of the thyroid and on how the thyroid functions.

Our initial studies were very promising, with the successful production of chimeric mice showing a high level of contribution from the cells containing the PBF deletion. However, genetic screening of subsequent generations of mice revealed that cells with PBF deletion were not being passed from generation to generation. We therefore discovered that PBF is likely to be important in the process of reproduction, making it difficult to obtain a model in which we could study the thyroid.

To overcome this we therefore switched to creating a mouse in which PBF is only deleted in thyroid cells. We have done this by manipulating cells grown in vitro (in a dish in the laboratory) so that they will produce the PBF protein normally until we trigger the deletion specifically in the thyroid. It was possible that the deletion of PBF in non-thyroid cells may have also altered thyroid growth and function (perhaps through altered hormone secretion). This new model, which is currently under evaluation, will therefore now show us more specifically what effect losing PBF has on the thyroid gland.

Alongside this work, we have continued to intensively investigate the mechanism by which PBF inhibits radioiodine uptake via the NIS transporter. Importantly, we have recently shown that we can use a drug to restore iodide uptake following repression of the NIS protein by PBF in thyroid cells. Our future research will involve the use of both the thyroid-specific knockout mouse and drug treatment in models of thyroid cancer and goitre. This will enable us to understand more about how PBF contributes to thyroid disease and assess PBF as a therapeutic target to improve radioiodine treatment of thyroid cancer.