TP63 is implicated in apoptotic dysregulation in melanoma

Abstract

Cutaneous melanoma is an aggressive malignancy accounting for 4% of skin cancers but 80% of all skin-cancer related deaths. Its incidence is rapidly rising and advanced disease is notoriously treatment-resistant. The role of apoptosis in melanoma pathogenesis and chemoresistance is poorly characterised. Mutations in TP53 occur infrequently and are not critical for tumour development, yet the TP53 apoptotic pathway is abrogated; this may alternatively result from TP53 pathway defects or from alterations in other members of the TP53 family, including the TP53 homologue, TP63. The hypothesis of this thesis was that TP63 has an anti-apoptotic role in melanoma and is responsible for mediating chemoresistance. The primary aims were to investigate the biological role of TP63 in melanoma, to explore regulation of p63 expression and to understand its role in apoptosis and dysregulation of the TP53 apoptotic pathway in melanoma. Although p63 was not expressed in primary melanocytes, upregulation of both p63 mRNA and protein was observed in melanoma cell lines and tissue samples. This is the first report of significant p63 expression in this lineage. Furthermore, aberrant cytoplasmic p63 expression significantly correlated with poor overall outcome in melanoma patients. Multiple possible mechanisms were demonstrated to regulate TP63 expression in melanoma, including epigenetic modulation, microRNA regulation of gene transcription and proteosome-dependent stability of p63 protein. In response to genotoxic stress, endogenous p63 isoforms were stabilised in both nuclear and mitochondrial subcellular compartments. Translocation of p63 to the mitochondria occurred through a co-dependent process with p53 but accumulation of wt-p53 in the nucleus was inhibited by p63. Using RNAi technology, both isoforms of p63 (TA and ΔNp63) were demonstrated to confer chemoresistance in melanoma. In addition, the truncated variant, ΔNp63, was enriched in a subset of melanomas expressing CD133, pointing to an anti-apoptotic role for p63 in putative cancer stem cells in this aggressive tumour. Taken together, these data suggest that in melanoma, p63 is an oncogene which contributes to dysregulation of wt-p53 function and has an important role in mediating chemoresistance. Ultimately, these observations may provide the rationale for novel approaches aimed at sensitising advanced melanoma to chemotherapeutic agents.