Genome-wide analysis of the hypoxic breast cancer transcriptome using next generation sequencing
Keywords: Non-coding RNAs
Next generation sequencing
lncRNAs
RNA-seq
Transcriptome
Breast Cancer

Hypoxia pathways are associated with the pathogenesis of both ischaemic and neoplastic diseases. In response to hypoxia the transcription factor hypoxia‐inducible factor (HIF) induces the expression of hundreds of genes with diverse functions. These enable cells to adapt to low oxygen availability. To date, pan-genomic analyses of these transcriptional responses have focussed on protein-coding genes and microRNAs. However, the role of other classes of non-coding RNAs, in particular lncRNAs, in the hypoxia response is largely uncharacterised. My thesis aimed at improving understanding of the transcriptional regulation of the non-coding transcriptome in hypoxia. I performed an integrated genomic analysis of both non-coding and coding transcripts by massively parallel sequencing. This was interfaced with pan-genomic analyses of DNAse hypersensitivity and HIF, H3k4me3 and RNApol2 binding in hypoxic cells. These analyses have revealed that hypoxia profoundly regulated all RNA classes. snRNAs and tRNAs are globally downregulated in hypoxia, whilst miRNAs, mRNAs and lncRNAs are both up- and downregulated with an overall trend towards slight upregulation. In addition, a significant number of previously non-annotated (and largely hypoxia upregulated) transcripts were identified, including novel intergenic transcripts and natural antisense transcripts. HIF bound close to genes for mRNAs, miRNAs and lncRNAs that were upregulated by hypoxia, but was excluded from binding at genes for RNA classes that showed global downregulation. This suggests that HIF acts as a transcriptional activator (but not repressor), of lncRNAs as well as mRNAs and miRNAs. Consistent with direct regulation by HIF, many of these hypoxia-inducible, HIF-binding lncRNAs were downregulated following HIF knockdown. Analysis of RNApol2 binding and DNAse HSS signals at HIF transcriptional target genes indicated that HIF-dependent transcriptional activation occurs through release of RNApol2 that is pre-bound to open promoters of lncRNAs as well as mRNAs. In these datasets, NEAT1 was the most hypoxia-upregulated, HIF-targeted lncRNA in MCF-7 cells and, despite binding of both HIF-1 and HIF-2 isoforms at its promoter, was selectively regulated by HIF-2 alone. Furthermore, NEAT1 was induced by hypoxia in a wide range of breast cancer cell lines and in hypoxic xenograft models. Functionally, NEAT1 is required for the assembly of nuclear paraspeckle structures. Increased nuclear paraspeckle formation was observed in hypoxia and was dependent on both NEAT1 and HIF-2. Knockdown of hypoxia-induced NEAT1 significantly reduced cell proliferation and survival and induced apoptosis. Finally, high expression of NEAT1 correlated with poor clinical outcome in a large cohort of breast cancer patients. These findings extend the role of the hypoxic transcriptional response in cancer into the spectrum of non-coding transcripts and provide new insights into molecular roles of hypoxia-regulated lncRNAs, which may provide the basis for novel therapeutic targets in the future.