Studies of bromodomain and extra-terminal domain proteins in pancreatic neuroendocrine tumours

Thesis

Pancreatic neuroendocrine tumours (PNETs) occur sporadically or within familial syndromes, such as multiple endocrine neoplasia type 1 (MEN1), in which they are the leading cause of mortality. Up to 60% of PNETs present with metastases, for which current therapies are largely non-curative, and I have therefore explored the effectiveness of epigenetic modifiers, focusing on inhibitors of the bromodomain and extra-terminal domain (BET) family of epigenetic ‘readers’ (BRD2, BRD3, BRD4 and BRDT). BET proteins contain two bromodomains (BDs), BD1 and BD2, that bind to acetylated lysine residues, upregulating the transcription of cancer-promoting genes. The BET inhibitor JQ1 (which targets BD1 and BD2 non-selectively) is reported to be efficacious in preclinical PNET models, but is limited by poor bioavailability and a short half-life. Moreover, the clinical relevance of BET proteins in human PNETs and differences between BD1 and BD2 remain underexplored. The aims of my thesis were to: 1) explore the occurrence and outcomes of MEN1-associated tumours in children and adolescents; 2) characterise BET expression in PNETs; and 3) assess the efficacy and mechanisms of non-selective (I-BET151) and selective BD1 and BD2 inhibitors (BD1i and BD2i, respectively) in preclinical PNET models.

My studies revealed that PNETs develop in one-third of children and adolescents with MEN1, ~15% with metastases. Immunohistochemistry indicated BET protein overexpression in human PNETs relative to adjacent islets. BD2i minimally affected viability, apoptosis, and cell cycle progression in vitro, whereas BD1i had moderate effects. However, combining BD1i and BD2i synergistically reduced viability, achieving effects comparable to I-BET151. Similar viability effects were observed in ex vivo neoplastic islets from pancreatic β-cell specific Men1 knockout mice. Combined BD1i and BD2i also led to broader changes in gene expression and histone acetylation than single-domain targeting, albeit to a lesser extent than with I-BET151. The cell cycle-related genes for the transcription factor E2F2 and Cytoskeleton-Associated Protein 2-Like (CKAP2L) were identified as direct BET inhibitor targets, both of which were downregulated by combined BD1i and BD2i, and whose expression correlated with progression-free survival.

In summary, this thesis highlights the clinical relevance of BET proteins in PNETs, and demonstrates that combined use of BD1i and BD2i surpasses single-domain targeting in efficacy. Finally, E2F2 and CKAP2L emerge as key BET inhibitor targets with prognostic value, suggesting their importance in PNET pathogenesis and as predictors of treatment response.

Authors: Shariq, OA

• DOI: 10.5287/ora-o0xd8mqnn
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: multiple endocrine neoplasia type 1; cancer; epigenetics; neuroendocrine tumour; bromodomain; pancreatic
• Subjects: Epigenetics; Oncology