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Dynamic regulation of growth hormone gene transcription.

Dunham, Lee

[Thesis]. Manchester, UK: The University of Manchester; 2016.

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Abstract

Many genes demonstrate highly dynamic pulsatile expression, with characteristic bursts of activity. Dynamic expression of the human prolactin (hPrl) gene in pituitary cells has previously been investigated identifying key temporal characteristics, influenced by the process of chromatin remodelling. Earlier work on the related pituitary human growth hormone (hGH) proximal promoter (-496/+1bp) indicated that it displayed similar dynamic behaviour. The human GH gene contains an extensive long-distance regulatory sequence, including a locus control region (-14/-32kbp) that has been shown to regulate chromatin remodelling and confer tissue-specificity of hGH expression. In this work I aimed to study dynamic regulation of the hGH gene promoter in detail. Initially I investigated the efficiency of several methods to express the luciferase gene in a 180kb hGH genomic fragment using bacterial artificial chromosome recombineering, to allow the investigation of single cell transcription dynamics. Although a functional recombinant BAC was not finalised during the course of the work, I carried out detailed time course studies using shorter hGH-reporter constructs. Using quantitative microscopy to study live single cells, I compared the dynamic characteristics of a 5kb hPrl promoter fragment with those of -840/+1bp and -3348/+1bp hGH-luciferase promoter-reporter constructs. Whilst previous hPrl analysis utilised a binary mathematical model assuming a simplified two-state (ON/OFF) process of gene transcription, I validated and applied a novel stochastic switch model (SSM), assuming instead that transcription rate can switch between any variable states at any time. Through doing so I observed an asymmetry in transcription rate switching, suggesting an all-or-nothing activation of a single UP-switch, with a greater number of rate decreasing DOWN-switches. The -3348/+1bp construct produced double the number of DOWN-switches, whilst the -840/+1bp construct produced 1.5 DOWN-switches in a 48h period. The cycling of transcriptional activity seen by the shorter construct was modified through the addition of forskolin, activating cAMP signalling. However, significant modification of the transcriptionally inactive refractory period seen with the -3348/+1bp construct (reduced from 3h to 1.9h) required histone modification through application of trichostatin A, a HDAC inhibitor. In conclusion, different promoter elements confer different transcriptional timing and dynamics. A subtler transcriptional modelling, such as used here in the SSM, reveals new insights into the phenomena of transcriptional switching, but the mechanisms involved remain to be determined.