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. 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, 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 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%, 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.

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.

Normal lung is recommendable as positive tissue control for TTF1. The columnar epithelial cells of the terminal bronchioles serve as a “low expressor” (LE) positive tissue control, showing a moderate to strong nuclear staining reaction. The type II pneumocytes, the Clara cells and the basal cells of the terminal bronchioles all serve as “high expressors” (HE), in which a strong nuclear staining reaction is seen. The nuclear staining in the “HE” should be as strong as possible without significant cytoplasmic reaction. Thyroid is less reliable as positive tissue control for TTF1, as the thyroid epithelial cells express very high levels of TTF1, making it difficult to evaluate the sensitivity of the protocol used. Data from previous TTF1 assessments indicate that lung carcinoids – typically expressing low amounts of TTF1 – should also be included as positive control when the initial validation of a TTF1 protocol is being established. Liver can be used as negative tissue control, but a cytoplasmic staining in the hepatocytes must be accepted when using mAb 8G7G3/1. The recommendations of the above mentioned tissue controls for IHC are concordant to the guidelines published by the International Ad Hoc Expert Committee.

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