Expression of thyroid-specific proteins such as thyroglobulin is controlled by several transcription factors. Thyroid transcription factor (TTF) 2 is the most recently characterized and has a temporal expression during thyroid embryogenesis suggesting a role in initiating differentiation. Studies on other thyroid-specific genes have shown a gradual loss in differentiation as a tumour progresses, although a good marker to distinguish follicular adenoma from follicular carcinoma is still needed. There is no information on TTF-2 expression in varying thyroid pathologies. Given its central role in thyroid differentiation, reverse transcriptase–polymerase chain reaction (PCR) was used to investigate this in both archival and fresh thyroid tissues and fine-needle aspirates.
Thyroid tissue was obtained with informed consent, snap frozen, and total RNA and messenger RNA (mRNA) was extracted. Since TTF-2 has no introns in the coding region, it was necessary to use mRNA. The RNA was reverse transcribed using oligo(dT) and MMV reverse transcriptase. The quality of the complementary DNA (cDNA) was verified by PCR using intron–exon boundary spanning primers for the phosphoglucokinase (PGK) housekeeping gene; a 250-base pair (bp) product is derived from cDNA but a 650-bp product from amplification of contaminant genomic DNA. PCR amplification of TTF-2 used two sets of primers; A + D spans the DNA-binding region and gives a product of 460 bp while primers P + E generate a product of 370 bp in the carboxyl end of the coding region. Products were analysed by agarose gel electrophoresis and compared with a 100-bp ladder.
A single band of 250 bp was obtained in all 33 thyroid tissue samples but in only three of eight fine-needle aspirates, after 40 cycles, using the PGK primers. Appropriate size products were obtained, after 30 cycles, invariably with both sets of TTF-2 primers, in all 18 benign thyroid nodules of varying pathology, including all five follicular adenomas, and in five of 15 malignant thyroid nodules. Four of five papillary carcinomas and one of seven follicular carcinomas expressed TTF-2; neither of two anaplastic carcinomas and one other carcinoma did not. One of six benign and one of two malignant thyroid nodules expressed TTF-2 on fine-needle aspiration cytology.
The results suggest loss of TTF-2 expression in poorly differentiated thyroid tumours and that TTF-2 may be helpful in distinguishing follicular adenoma from follicular carcinoma. © 2000 British Journal of Surgery Society Ltd