Expression of CD97 and CD55 in human medullary thyroid carcinomas

The heterodimeric CD97 protein is a member of the EGF-TM7 family of class II seven-transmembrane (7TM) receptors of 75-90 kDa and structurally related to the secretin receptor family. CD97 is expressed on leucocytes, lymphocytes and in cells of the hematopoietic system. The precise role for CD97 is still unknown. The ubiquitously expressed CD55 (also known as decay accelerating factor, DAF) protects host cells from complement attack. In addition, CD55 is a bacterial/viral receptor and was identified as a ligand for CD97. Employing computer aided UV-laser microdissection CD97 and CD55 were investigated in C-cells of non-neoplastic thyroid specimens (n=3) and in medullary thyroid carcinomas (n=54) by multiplex RT-PCR. Frozen sections of all tissues were investigated by immunohistochemistry. All non-malignant thyroid specimens expressed CD97 mRNA weakly and were devoid of immunoreactive CD97 protein. Transcripts for CD97 were detected in all 54 MTC tissue specimens and CD97 gene activity directly correlated with the histopathological stage of the MTC. CD97 transcriptional activity was high in advanced stages of MTC such as pT3/4. pT1/2 tumors with exclusive intrathyroidal growth revealed weak CD97 expression. CD55 gene expression was significantly lower in normal C-cells than in tumor tissues and all MTC displayed strong and specific CD55 immunostaining. We did not observe a correlation between the expression of CD55 mRNA or protein, respectively, and pTNM classification. In summary, in the present study we have identified CD97 as a novel marker expressed in dedifferentiated neoplastic human thyroid C-cells. CD97 and CD55 may facilitate adhesion of C-cell carcinoma to surrounding surfaces which would result in rapid tumor cell spread.