Context Aggressive pituitary tumours (APTs) are characterised by unusually rapid growth and lack of response to standard treatment. About 1-2% develop metastases being classified as pituitary carcinomas (PCs). For unknown reasons, the corticotroph tumours are overrepresented amongst APTs and PCs. Mutations in the ATRX gene, regulating chromatin remodelling and telomere maintenance, have been implicated in the development of several cancer types, including neuroendocrine tumours. Objective To study ATRX protein expression and mutational status of the ATRX gene in APTs and PCs. Design We investigated ATRX protein expression by using immunohistochemistry in 30 APTs and 18 PCs, mostly of Pit-1 and T-Pit cell lineage. In tumours lacking ATRX immunolabeling, mutational status of the ATRX gene was explored. Results Nine of the 48 tumours (19%) demonstrated lack of ATRX immunolabelling with a higher proportion in patients with PCs (5/18 - 28%) than in those with APTs (4/30 – 13%). Lack of ATRX was most common in the corticotroph tumours, 7/22 (32%), vs 2/24 (8%) in the tumours of the Pit-1 lineage. Loss-of-function ATRX mutations were found in all the nine ATRX immuno-negative cases: nonsense mutations (n=4), frameshift deletions (n=4) and large deletions affecting 22-28 of the 36 exons (n=3). More than one ATRX gene defect was identified in two PCs. Conclusion ATRX mutations occur in a subset of aggressive pituitary tumours and are more common in corticotroph tumours. The findings provide a rationale for performing ATRX immunohistochemistry to identify patients at risk of developing aggressive and potentially metastatic pituitary tumours.

Pituitary neuroendocrine tumours (PitNETs) associated with paragangliomas or phaeochromocytomas are rare. SDHx variants are estimated to be associated with 0.3-1.8% of PitNETs. Only a few case reports have documented the association with MAX variants. Prolactinomas are the most common PitNETs occurring in patients with SDHx variants, followed by somatotrophinomas, clinically non-functioning tumours and corticotrophinomas. One pituitary carcinoma has been described. SDHC, SDHB and SDHA mutations are inherited in an autosomal dominant fashion and tumorigenesis seems to adhere to the Knudson’s two hit hypothesis. SDHD and SDHAF2 mutations most commonly have paternal inheritance. Immunohistochemistry for SDHB or MAX and loss of heterozygosity analysis can support assessment of pathogenicity of the variants. Metabolomics is promising in the diagnosis of SDHx-related disease. Future research should aim to further clarify the role of SDHx and MAX variants or other genes in the molecular pathogenesis of PitNETs including pseudohypoxic and kinase signalling pathways along with elucidating epigenetic mechanisms to predict tumour behaviour.