Supposedly, thyrocyte-specific transcripts such as
thyroglobulin (Tg) and
thyroid-stimulating hormone receptor (TSH-R) were proposed to be useful for the diagnosis of circulating tumour cells in patients suffering from differentiated
thyroid carcinoma (DTC). However, several research groups reported blood-borne Tg transcripts in healthy individuals. This study determines in particular the origin of Tg
mRNA in nucleated blood cells and analyses whether other tumour-associated sequences are absent in leukocytes, but widely expressed in DTC. Therefore, expression analyses for Tg, TSH-R,
cytokeratin 19 (CK 19), human
telomerase reverse transcriptase (hTERT) and oncofoetal
fibronectin (onfFN) were carried out using cDNAs derived from (1) leukocyte fractions, (2) 18
follicular thyroid carcinomas (FTCs) and 48
papillary thyroid carcinomas (PTCs), and (3) leukocytes of two thyrocyte-depleted individuals treated for C-cell
carcinoma of the thyroid. Expression of onfFN was additionally analysed by semiquantitative RT-PCR and by quantitative fluorescence-based real-time PCR. Tg and TSH-R expression was demonstrated not only in both athyroid individuals, but in all leukocyte subgroups tested, while hTERT was absent in resting CD4+ cells and only weakly expressed in the CD8+ group. CK 19 was notable in each leukocyte population except for resting CD14(+), as well as for activated and resting CD19+ cells. All blood cell fractions proved negative for onfFN
mRNA, whereas its presence in
thyroid carcinoma was 78/98% (
FTC/PTC). Threshold cycle values were calculated at:
porphobilinogen deaminase (PBGD) = 25.95+/-0.73 (
FTC)/24.55+/-5.43 (PTC) (P = 0.2878); onfFN = 25.48+/-3.15 (
FTC)/21.44+/-3.44 (PTC) (*P = 0.0001). Finally, onfFN transcripts were detected in blood samples of six out of nine patients with known DTC
metastases, demonstrating a reliable assay functionality. We propose that real-time RT-PCR of onfFN
mRNA is superior to other markers in monitoring
minimal residual disease in DTC with regard to both assay sensitivity and specificity.