Squamous differentiation is a tightly regulated, complex, multistep process which occurs in stratified squamous epithelium. Broadly speaking, the process can be considered to involve the stepwise transformation of an undifferentiated, proliferative keratinocyte into a terminally differentiated, anuclear corneocyte. The process of terminal differentiation requires diverse biochemical, transcriptional and morphological changes and hence, is regulated by a complicated molecular network. Disruption to epidermal homeostasis can result in disease or neoplasia. In fact, one of the features of squamous cell carcinoma is defective differentiation. As such, study of the normal differentiation process can be useful in elucidating processes which are disrupted in malignancies.
There is recent evidence that epigenetic mediated gene repression; specifically that which is regulated by the polycomb repressor complex 2 and DNA methyltransferases 1 and 3, is a novel regulator of squamous differentiation. There is also evidence that epigenetic processes may be disrupted in a host of solid and haematological malignancies. Head and neck squamous cell carcinoma (HNSCC), a primary malignancy of the epithelial keratinocyte, is a cancer with an approximately 55% 5 year survival rate and significant treatment-associated morbidity. Worldwide, it is one of the most prevalent cancers diagnosed, with an aetiology closely related to exposure to oral carcinogens such as tobacco products and alcohol. This thesis examines the function of several epigenetic mechanisms in the regulation of normal squamous differentiation, during defective differentiation in squamous cell carcinoma and as a potential therapeutic target in squamous cell carcinoma.
In order to investigate the role of epigenetic regulation of differentiation and potential dysregulation during neoplasia I took several approaches. Firstly, I performed a comprehensive analysis of in vitro squamous differentiation using a variety of physiologically relevant tissue culture methods and validated a model of squamous differentiation and growth arrest. Using this model, I investigated whether several well characterised epigenetic repressive ‘marks’; being methylcytosine binding protein 2 and methylation of lysine 9 and 27 of histone H3; were recruited to the promoters of genes which were repressed upon growth arrest in normal keratinocytes. Surprisingly, despite loss of repressive epigenetic marks from differentiation genes upon terminal differentiation, there was no evidence for recruitment of epigenetic repressors to proliferation associated genes which were down regulated upon growth arrest. I also investigated whether H3K27me3 mediated regulation of gene expression was dysregulated in squamous cell carcinoma cell lines and patient lesions and established that the polycomb repressor complex 2 components were highly expressed in SCC and inappropriately recruited to terminal differentiation genes. Disruption of polycomb repressor complex 2 mediated methylation of histone H3 at lysine 27 was sufficient to induce the expression of squamous differentiation genes in normal keratinocytes as well as differentiation refractory squamous cell carcinoma cell lines. Furthermore, the recently described epigenetic ‘inhibitor’, 3-Deazaneplanocin A showed selective cytotoxicity against HNSCC carcinoma cell lines in vitro, as well as some efficacy against tumour growth in a xenograft model of HNSCC in vivo.
Collectively, these data highlight the important role of epigenetic mediated gene repression in normal squamous differentiation, and provides an as yet unreported means by which terminal differentiation can be induced in differentiation refractory malignant cells. Likewise, this is the first report whereby 3-Deazaneplanocin A has been used in vivo to treat a xenograft HNSCC tumour. This data provide preliminary evidence that disruption of polycomb mediated gene repression may be a clinically significant novel therapeutic target for squamous cell carcinoma.