Inactivation of p16^INK4a expression in malignant mesothelioma by methylation

The molecular mechanisms of oncogenesis in mesothelioma involve the loss of negative regulators of cell growth including p16^INK4a. Absence of expression of the p16^INK4a gene product is exhibited in virtually all mesothelioma tumors and cell lines examined to date. Loss of p16^INK4a expression has also been frequently observed in more common neoplasms such as lung cancer as well. In a wide variety of these malignancies, including lung cancer, p16^INK4a expression is known to be inactivated by hypermethylation of the first exon. In a survey of ten mesothelioma cell lines, one cell line (NCI-H2596) was identified as possessing loss of p16^INK4a gene product following gene methylation. This methylation in these mesothelioma cells could be reversed, resulting in re-expression of p16^INK4a protein, following the treatment of the cells with cytidine analogs, which are known inhibitors of DNA methylation. In previous clinical trials in mesothelioma, the cytidine analog dihydro-5-azacytidine (DHAC) has been found to induce clinical responses in approximately 17% of patients with mesothelioma treated with this drug, including prolonged complete responses. In addition, we identified evidence for methylation of p16^INK4a in three of 11 resected mesothelioma tumor samples. When both cell lines and tumors are combined, inactivation of p16^INK4a gene product expression following DNA hypermethylation was found in four of 21 samples (19%). We are further exploring the clinical significance of inhibition of methylation in mesothelioma by cytidine analogs. This may provide a potential treatment target in some mesothelioma tumors by inhibition of methylation.