A high-coverage and high-content human ORF overexpression screen to identify pathways that can bypass p16INK4a-mediated cellular senescence

The INK4a tumour suppressor gene encoding the protein commonly known as p16 is frequently mutated, silenced or deleted in melanomas and other solid tumours. Functionally, it can induce a phenotype known as cellular senescence, which is defined as a state of permanent proliferative arrest despite the presence of mitogens. Senescent cells can be characterised by low Ki-67 antigen positivity, decreased nucleoside incorporation, enlarged morphology and increased senescence-associated β-galactosidase positivity. Although p16 is known to inhibit cyclin-dependent kinase 4/6 and thus enhance the function of the retinoblastoma tumour suppressor protein, the downstream pathways of p16-mediated senescence are not clear. Furthermore, subversion of p16-mediated senescence may have relevance to tumour progression, for example in the case of constitutively-active mutant CDK4. In order to identify potential genes and pathways that can bypass the initiation of p16-mediated senescence in an unbiased fashion, we designed a high-throughput, high-content senescence assay for screening the human ORFeome lentiviral expression library developed at ARVEC facility at the Diamantina Institute. We have established multiple parameters for senescence in a p53-mutant but pRb-wild-type melanoma cell line that is able to express inducible p16. Immunofluorescent imaging for Ki-67 antigen or de novo DNA synthesis using EdU incorporation was combined with alpha-tubuin and DNA staining using DAPI to simultaneously assess proliferation and morphology. The identification of genes whose overexpression can prevent or diminish the senescent phenotype induced by p16 may lead to the discovery of novel drug targets for anti-cancer therapy.