Simultaneous localization of calcitonin mRNA and peptide in a medullary thyroid carcinoma

We report the visualization of calcitonin gene expression products at the mRNA and peptide levels on the same section of a medullary thyroid carcinoma by combined in situ hybridization and immunohistochemistry. mRNA detection was accomplished by hybridization with radioactively labeled antisense RNA probes followed by autoradiography and immunohistochemically using the avidin-biotin complex method. Best results were obtained when in situ hybridization preceded immunohistochemistry, as determined by quantitative analysis of the autoradiographs. When immunohistochemistry was performed prior to in situ hybridization, the RNase inhibitor heparin had to be added to the antibodies to retain hybridizable mRNA. The intensity of the two reactions varied in individual cells, indicating a functional heterogeneity of tumor cells with regard to calcitonin mRNA content and storage of the related immunoreactive peptide. These results, in combination with elevated serum calcitonin levels, suggest significant differences in the rate of secretion of individual tumor cells. Simultaneous localization of mRNA and its peptide within the same cell may, therefore, provide further insight into gene expression and secretory activity at the single cell level.