Thyroid transcription factor-1 (TTF1)
Transcription factors are proteins which interact with DNA near or within the gene locus. Specific transcription factors stimulate or repress transcription of particular genes by binding to their regulatory sequences. Thyroid transcription factor-1 (TTF1) is a 38 kDa nuclear protein member of the NKx2 family of homeodomain transcription factors. TTF1 was first identified as a thyroid-specific DNA-binding activity which interacted with the rat thyroglobulin gene. Human TTF1 is a single polypeptide of 371 amino acids that is encoded by a single gene locus. The amino acid sequence of human protein displays 98% identity with that of rat and mouse with absolute conservation of the 60 amino acid homeodomain.
After being found in the follicular epithelial cells of the thyroid, TTF1 was demonstrated in the lung and certain areas of the brain. More recently it has been located at other sites, including the pituitary, parathyroid gland and the parafollicular C-cells of the thyroid.
TTF1 plays a role in regulating genes in the thyroid, lung and brain. Its molecular targets in the thyroid gland are thyroglobulin, thyroperoxidase and thyrotropin receptor. TTF1 activates transcription of the genes encoding these proteins. In the lung, TTF1 promotes the transcription of the surfactant proteins A to D, and the Clara cell secretory protein. In brain, the molecular targets of TTF1 are unknown.
Besides being a tissue-specific transcription promoter in adult organs, TTF1 has been suggested a role in morphogenesis and cytodifferentiation. It is expressed in the epithelial cells of the thyroid, lung and the ventral forebrain in early embryogenesis. Experimental studies have shown developmental defects after blocking TTF1 gene expression. It is possible that some congenital abnormalities of the thyroid, lung and brain are due to abnormal expression of TTF1. After birth, and in a normal adult organism, the expression of TTF1 is confined to follicular epithelial cells and the C-cells in the thyroid, and to the type II pneumocytes and the Clara cells in the lung.
Among thyroid neoplasms, TTF1 is seen in virtually all tumours of follicular derivation, i.e. follicular adenoma and follicular and papillary carcinoma (Fig. 1a). Also the C-cell derived medullary carcinomas are positive for TTF1 immunoreactivity in virtually all cases (lower frequencies in some studies being possibly due to technical causes). Most anaplastic thyroid carcinomas have been reported negative.
Among lung neoplasms, TTF1 is widely expressed in pulmonary adenocarcinomas (Fig. 1b), the frequency of positive tumours being 60-85%, depending on the clone used. In a comparative study of 86 primary pulmonary adenocarcinomas by Comperat et al. nuclear staining was detected in 72 cases (84%) with SPT24 and 56 cases (65%) with 8G7G3/1 (also confer Table 1). Positivity for TTF1 is generally not related to subtype, since neoplasms with acinar, papillary and bronchiolo-alveolar morphology have been reported expressing TTF1. However, mucin-producing adenocarcinomas, including mucinous bronchiolo-alveolar carcinoma, are frequently TTF1 negative (Fig 1c) while in some cases being cdx2-positive.
Extrapulmonary adenocarcinomas express TTF1 very rarely (about 1%, however, 5-10% of gastrointestinal adenocarcinomas are positive with mAb SPT24, also see Table 1).
Small cell lung carcinoma express TTF1 almost consistently, about 90% (Fig. 1d), while other pulmonary neuroendocrine tumours, i.e., typical and atypical carcinoid and large cell neuroendocrine carcinoma do so variably, according to the literature 0-95% being positive! Some of this variation may be due to differences in classification and techniques used (including Ab).
Neuroendocrine tumours at other sites are very seldom TTF1 positive (0-7%), except for small cell carcinomas (40%).
Other pulmonary lesions expressing TTF1 include
bronchial adenoma and so-called sclerosing haemangioma.
A comparative study carried out in Aalborg and Odense showing the reactivity
for TTF1 applying the mAb clones SPT24 and
8G7G3/1 on multitissue block sections comprising 169 carcinomas and
TTF1 is useful as marker for pulmonary adenocarcinoma and pulmonary neuroendocrine malignancy, including small cell lung carcinoma. Its specificity is high, and the sensitivity rather good. It can be used to differentiate primary lung adenocarcinoma from metastasis of extrapulmonary origin (the subgroup of TTF1 positive gastrointestinal adenocarcinomas usually also stain for cdx2). Differential diagnosis of mucinous lung carcinoma (usually TTF1 negative) requires other markers.
The use of TTF1 immunostaining is a valuable adjunct in differential diagnosis of pleural malignancy. Malignant mesothelioma does not express TTF1 as against peripheral pulmonary adenocarcinoma, which is usually positive; other mesothelioma markers are, of course, needed to substantiate the diagnosis. Similarly, pulmonary adenocarcinoma growing or metastasizing into pleura differs from metastasis of an extrapulmonary adenocarcinoma (exception: rare cases of metastatic thyroid carcinoma) on the basis of TTF1 positivity. Also here, confirmative or exclusive results from other stains are needed.
Neuroendocrine carcinoma in the skin can be identified as Merkel cell carcinoma if cytokeratin 20 positive and TTF1 negative, and metastatic small cell lung carcinoma if TTF1 positive and cytokeratin 20 negative.
Most of the data on TTF1 immunoreactivity are derived from studies using mAb clones 8G7G3/1 and SPT24. Successful immunohistochemistry requires heat-induced epitope retrieval, preferably in an alkaline buffer.
The NordiQC assessments have shown that SPT24 is more sensitive than 8G7G3/1, particularly regarding typical and atypical lung carcinoids, but gives an "unwanted" staining of some gastrointestinal adenocarcinomas.
mAb clone 8G7G3/1 constantly cross reacts with a cytoplasmic protein in liver cells (Fig 1d). In a recent study cytoplasmic reactivity was observed in 71% of hepatocellular carcinomas with this Ab. The cytoplasmic reactivity seems fairly specific for hepatocellular carcinoma. However, we have seen the cytoplasmic staining also in neuroendocrine tumours.
Expression of TTF1 messenger RNA has been observed in follicular epithelial cells of the thyroid gland but cytoplasmic reactivity has been observed only in isolated cases of some tumours and regarded as unspecific by some authors.
Control: Normal lung and thyroid are suitable for control tissue: The nuclear staining should be as strong as possible without significant cytoplasmic reaction. Preferentially, tissue with low antigen content should also be included such as selected cases of well-differentiated pulmonary carcinoid, provided that clone SPT24 is used.
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