Quantification of intracellular calcitonin gene transcripts in human medullary thyroid carcinoma (MTC) by hybridization

The human calcitonin gene generates 2 distinct mature mRNAs by alternative RNA processing, encoding calcitonin (CT) in thyroid C-cells or a neuropeptide (CGRP) in the brain. We evaluated quantitatively by hybridization the expression of the CT gene in tissue section of 5 MTCs (2 sporadic and 3 familial forms). The primary tumor of one MTC was compared to a brain metastasis. hybridization was carried out with tritiated cDNA probes coding for CT and CGRP mRNA. After autoradiography the number of silver grains was conunted in 400 cells by computerized analysis of digitized images and expressed as the labelling level (L.L. = grain area/cell area per day of autoradiographic exposure). This was used to calculate the relative abundance per cell of the specific messengers studied, which depends on the autoradiographic efficiency and the specific activity of the probe used. The CT mRNA content was 3.25–6.55 1010 μg equivalents in the 3 familial forms of MTC and 4.95–9.25 10 μg equivalents for the 2 sporadic forms. The levels of CT mRNA in the brain metastasis and in the primary tumor were identical (4.10 10 μg equivalents). CGRP mRNA expression was weaker in the sporadic and in the familial thyroid tumors (0.60–1.65 10 μg equivalents). The content of mRNA CGRP in the brain metastasis (0.60 10~10 /xg equivalents) was lower than that in the primary tumor (1.05 10 μg equivalents). These results suggest that most of the primary RNA transcript generated by CT gene lead to the formation of CT mRNA in MTC, and the alternative mRNA processing pathway in sporadic and familial forms does not seem to be different. No correlation was found between the amount of tumor CT mRNA and clinical course of the MTC. The latter appears to be more linked to the initial extent of the disease.