Insulin-like growth factor mRNA binding protein 3 (IMP3) is differentially expressed in benign and malignant follicular patterned thyroid tumors.

Insulin-like growth factor mRNA binding protein 3 (IMP3) is an mRNA-binding protein that regulates transcription of insulin-like growth factor II affecting cell proliferation during embryogenesis. It is highly expressed in carcinomas of the pancreas, stomach, colon, rectum, kidneys, uterine cervix, lung, and ovary. The purpose of our study was to evaluate IMP3 expression in thyroid follicular lesions, to determine whether it has a role in differentiating among these lesions, and to understand their biological relationships. We immunostained 219 thyroid lesions selected from our surgical pathology archives including 14 hyperplastic colloid nodules (CN), 19 Hashimoto's thyroiditis (HT), two Graves disease (GD), ten Hürthle cell adenoma (HCA), 20 follicular adenoma (FA), 37 conventional papillary thyroid carcinoma (PTC), 60 follicular variant of papillary carcinoma (FVPC), 19 Hürthle cell carcinoma (HCC), 32 follicular carcinoma (FC), and six poorly differentiated/anaplastic carcinoma. Immunohistochemistry was performed on formalin-fixed sections using monoclonal antibody to IMP3. Clinicopathological data were also reviewed. In all cases, residual thyroid tissue, CN, HT, GD, HCA, and FA were completely negative for IMP3 staining. Of the 60 FVPC, 23 tumors (38%) were positive for IMP3, with 13 of these (22%) showing very strong staining (3+). Of the 32 FC, 22 tumors (69%) were positive, with seven (22%) showing very strong staining (3+). Furthermore, 33 out of 37 cases (89%) of PTC were negative for IMP3. In all four PTC cases that did stain positive, staining was weak-moderate (1-2+). Similarly, 15 out of 19 cases (79%) of HCC were negative. No significant correlation was found between pathologic tumor characteristics and IMP3 expression in differentiated follicular pattern thyroid carcinoma. With 100% specificity and 69% sensitivity for FC as compared to FA and 100% specificity for FVPC, again compared to FA, IMP3 has the potential to be diagnostically useful in differentiating malignant and benign follicular pattern thyroid lesions. This study also points to a possible common biological relationship between FC and FVPC that requires further investigation.