In situ hybridization and immunohistochemistry study of thyroid peroxidase expression in thyroid tumors

Malignant thyroid tumors reportedly exhibit an anomaly in thyroid peroxidase (TPO) resulting in a lower affinity for monoclonal antibody 47 (mAb 47) in immunohistochemistry studies. The purpose of the present study was to compare TPO immunostaining in normal, benign, and malignant thyroid tissue with expression of mRNA sequences in four exons of the molecule including the epitope of mAb 47. TPO immunostaining was performed using mAb 47 and a polyclonal antibody (pAb). Messenger RNA expression was investigated by in situ hybridization using probes specific for mRNA sequences in exons 2, 12 (epitope of mAb), 15, and 17. As expected, pAb immunostaining was significantly positive on all benign tumors and 50% of carcinomas. With mAb 47, little or no immunostaining was observed in 16 of 17 carcinomas while significantly positive immunostaining was found in normal tissue and benign tumors. In situ hybridization showed a decrease and heterogeneity in the expression of all mRNA sequences in carcinomas as compared to normal tissue and benign tumors. Unlike the other three probes, the probe specific for exon 12 hybridized strongly with benign tumors but poorly with most carcinomas. Poor hybridization was usually correlated with defective mAb 47 immunostaining. These results confirm that TPO is expressed in thyroid carcinomas but in smaller amounts than in normal tissue and benign tumors. In malignant tumors, qualitative changes in TPO may also impede mAb 47 immunostaining. In situ hybridization showed a concomitant decrease in the corresponding TPO mRNA sequence. These changes could be due to abnormalities in the maturation of TPO mRNA leading to a different splicing variant.