5-hydroxymethylcytosine but not MTAP methylation status can stratify malignant pleural mesothelioma based on the lineage of origin

BACKGROUND: Malignant pleural mesothelioma (MPM) is an aggressive tumor with poor prognosis, mainly associated with work or environmental exposure to asbestos. MPM’s molecular profile is largerly unexplored and effective therapies are still lacking. MPM rarely harbours those somatic genetic lesions that usually characterize solid epithelial-derived tumors. On this basis, our study aims at investigating MPM epigenetic profile. METHODS: We here assessed through immunohistochemistry, FISH and methylation specific PCR, the expression of 5-hydroxymethylcytosine (5- hmC) - an epigenetic marker and an important regulator of embryonic development and carcinogenesis - and the methylation status of the promoter of the MTAP gene - encoding for an enzyme involved in the rescue process of methionine and adenine - in two relevant series of FF-PE MPM samples derived from MPM thoracoscopic biopsies. Tissue sampling was performed at diagnosis. RESULTS: Within the limitations of the study cohort, the 5-hmC immunophenotype was different among the histological MPM types analysed. In fact, 18% of the epithelial MPMs were negative, 47% weakly positive, and 35% of the cases showed an intense expression of 5-hmC. Sarcomatoid and biphasic MPMs showed intense 5-hmC expression pattern (positive and weakly positive in more than 80% of cases). Among MPM featuring epithelial lineage, none showed methylation of MTAP promoter. CONCLUSIONS: Mesothelial sarcomatoid tumors featured a methylation profile characterized by a permanent gene silencing. Epithelial MPM methylation profile was in-between that of sarcomatoid MPM and the one of epithelial-derived tumors. MTAP promoter methylation level cannot be considered a suitable biomarker of epithelial MPM arousal.