Stem Cell Transcription Factor Sox2 Is Expressed in a Subset of Folliculo-stellate Cells of Growth Hormone-Producing Pituitary Neuroendocrine Tumours and Its Expression Shows No Association with Tumour Size or IGF1 Levels: a Clinicopathological Study of 109 Cases.

Sox2 is one of the transcription factors responsible for the maintenance of stem cell phenotype. It has been implicated as a marker of stem cells in normal pituitaries and pituitary neuroendocrine tumours. To explore the clinical significance of Sox2 expression in histological sections, we performed immunohistochemical detection of Sox2 in 113 pituitary neuroendocrine tumours from 109 patients with acromegaly. In 11 tumours, we performed double immunostaining for Sox2, annexin A1 and S100 protein. Tumours were characterised using the WHO classification system. Proliferative activity and invasion were assessed. The amount of immunoreactive cells was evaluated and correlated with tumour size and biochemical features (levels of IGF1, GH, prolactin, βTSH). Sox2 cells were identified in 35/38 normal pituitaries adjacent to the tumours. In 36 tumours (33%), ≥ 1% of the cells expressed Sox2, in 24 cases (22%), Sox2 cells comprised < 1% and 49 cases (45%) were negative. We found no significant differences between Sox2 and Sox2 groups with respect to the age, initial levels of GH, IGF1, prolactin, βTSH, tumour size, invasion, proliferative activity or histological features. We observed a positive correlation between Sox2 cell count and βTSH immunoreactive cells (r = 0.459, p < 0.001) that was further verified by multivariate analysis. Using double stain, the majority of Sox2 cells coexpressed annexin A1 (average 89%) and S100 protein (average 76.2%) and showed morphological features of folliculo-stellate cells. Sox2 cells are thus commonly present in growth hormone-producing tumours and normal pituitaries, and their amount does not have any prognostic significance. Most of these cells represent a subpopulation of folliculo-stellate cells, pointing out to their role as a possible stem cell population.