Differential expression of dipeptidyl peptidase IV (CD26) and thyroid peroxidase in neoplastic thyroid tissues

This paper reports the results of research to examine the possibility of using the differential expression of two enzymes, dipeptidyl peptidase IV (DPPIV/CD26, EC: 3.4.14.5) and thyroid peroxidase (TPO, EC: 1.11.1.7), as histochemical markers histopathologically to diagnose thyroid carcinomas. The research is based on previous reports that DPPIV/CD26 is overexpressed in differentiated thyroid carcinoma tissues, and that TPO activity is very low in thyroid carcinoma tissues. Differential expression of the two enzymes in 32 thyroid tissues of various thyroid diseases was studied by Northern blot analysis and histochemical analysis. On Northern blot analyses, all 14 differentiated thyroid carcinomas (11 papillary carcinomas and 3 follicular carcinomas) overexpressed DPPIV/CD26 mRNA, whereas all 17 benign thyroid tissues (4 normal thyroid tissues, 4 Graves' diseases, 2 adenomatous goiters and 7 follicular adenomas) showed faint mRNA expression of DPPIV/CD26. All 17 benign thyroid tissues expressed high levels of TPO mRNA, whereas all 11 papillary carcinomas strongly underexpressed TPO mRNA. Histochemically, all 17 benign tissues were DPPIV/CD26 negative and strongly TPO positive, while all 11 papillary carcinomas were strongly DPPIV/CD26 positive and TPO negative. Two of 3 follicular carcinomas were histochemically positive for the two enzymes. A medullary carcinoma did not show any mRNA expression of either enzyme. These results suggest that the differential expression of these two enzymes can be applied to study the thyroid tumorigenesis.