PO-274 Tumour subtype-specific cells of origin of malignant mesothelioma

Introduction Malignant Mesothelioma (MM) is an aggressive malignancy of the lining of the thoracic and peritoneal cavity. The primary cause is previous asbestos exposure. The pathological diagnosis of MM is rather complex due to a lack of useful biological markers and because multiple cell types are involved in the development of MM. We aimed to investigate to what extent the tumour subtype is determined by the cell of origin rather than the somatically acquired driver mutations. Material and methods Co-deletion of the conditional tumour suppressors, NF2, p53 and Cdkn2a in freshly isolated mesothelial cells using Cre viruses allowed us to establish clonal cell lines with epithelial, sacomatoid and biphasic morphology as also observed in human MM. Cells were analysed for clinical relevant protein marker profiles, tumorigenic potential and RNA expression. Results were compared to (poly)clonal cell lines obtained from conditional mice injected intra-thoracically with lentiviruses expressing Cre-recombinase driven by tissue-specific promoters. Results and discussions Using the ex vivo approach we were able to obtain clonal cell lines that upon transplantation gave rise to the main three MM tumour epithelial, sarcomatoid and biphasic subtypes. The epithelial and sarcomatoid phenotypes observed in vitro retained in the tumours, also after serial transplantation of the cell lines. Clonal biphasic cells co-expressed both epithelial and sarcomatoid markers and external factors could skew these biphasic cells towards a more epithelial or sarcomatoid phenotype. Transplantation of clonal biphasic cells only gave rise to tumours when the cells were grafted in immune-deficient mice, this in contrast to the epithelial and sarcomatoid cell lines that effectively gave tumours in syngeneic immunocompetent recipients. Analysis of tumour cell populations and derived clonal cell lines induced by lentiviral Cre-mediated switching of the same tumour suppressors in the mesothelial lining of conditional mice showed a high level of heterogeneity that included the same tumour subtypes also obtained using the ex vivo procedure. Conclusion A set of the same genetic mutations is able to drive heterogeneous MM development suggesting that the specific phenotype of the resulting tumour depends on the specific cell-of-origin. Therefore, epigenetic differences rather than acquired mutations appear a determining factor for the subtype of MM that arises. This epigenetic state appears relative stability as interconversion of the tumour subtypes was not observed.