Gene expression and its physiological control in disease and development : Studies on the human PDGF-B gene and tumour hypoxia

Strict control of gene expression is essential during development and in response to physiological stimuli. This thesis describes the functional characterisation of the gene regulatory mechanisms controlling the expression of the potent human growth factor Platelet Derived Growth Factor B gene, in a cell type specific context and in response to low oxygen tension. In addition, analysis of hypoxia in neuroblastoma indicates a role during tumour differentiation. Initally, a promoter-specific enhancer system controlling the expression of PDGF-B was characterised in placentally derived choriocarcinoma cells. The specificity of this enhancer promoter interaction was shown to be dependent on specific sequence elements identified in both the promoter and enhancer regions. It was then shown that the activity of the PDGF-B promoter is controlled via modulation of histone acetylation status in a cell type specific manner and furthermore, that one role of its enhancer could be to regulate transcription via alterations in acetylation status at the promoter. PDGF-B expression was then shown to be controlled by hypoxia in a biphasic manner in bladder carcinoma cells. An initial induction was followed by repression of transcription following chronic hypoxia. The biphasic response was shown to be dependent on glucose levels and uniquely amongst hypoxia regulated genes studied so far, PDGF-B expression was shown to be repressed by low glucose. Finally, detailed in vivo and in vitro analysis revealed that the major form of differentiation in childhood neuroblastoma is towards chromaffin-like rather than ganglionic lineages. This type of differentiation did not correlate with disease progression but was suggested to be dependent on tumour hypoxia, since chromaffin differentiation markers co-localised with markers of tumour hypoxia in both clinical samples and xenogenic tumours. In conclusion, the work presented in this thesis has identified several novel, highly specific gene regulatory mechanisms that are involved in development, the response to physiological stimuli and in disease progression