Frequent aberrant methylation of p16INK4a in primary rat lung tumors

The p16INK4a (p16) tumor suppressor gene is frequently inactivated by homozygous deletion or methylation of the 5' CpG island in cell lines derived from human non-small-cell lung cancers. However, the frequency of dysfunction in primary tumors appears to be significantly lower than that in cell lines. This discordance could result from the occurrence or selection of p16 dysfunction during cell culture. Alternatively, techniques commonly used to examine tumors for genetic and epigenetic alterations may not be sensitive enough to detect all dysfunctions within the heterogeneous cell population present in primary tumors. If p16 inactivation plays a central role in development of non-small-cell lung cancer, then the frequency of gene inactivation in primary tumors should parallel that observed in cell lines. The present investigation addressed this issue in primary rat lung tumors and corresponding derived cell lines. A further goal was to determine whether the aberrant p16 gene methylation seen in human tumors is a conserved event in this animal model. The rat p16 gene was cloned and sequenced, and the predicted amino acid sequence of its product found to be 62% homologous to the amino acid sequence of the human analog. Homozygous deletion accounted for loss of p16 expression in 8 of 20 cell lines, while methylation of the CpG island extending throughout exon 1 was observed in 9 of 20 cell lines. 2-Deoxy-5-azacytidine treatment of cell lines with aberrant methylation restored gene expression. The methylated phenotype seen in cell lines showed an absolute correlation with detection of methylation in primary tumors. Aberrant methylation was also detected in four of eight primary tumors in which the derived cell line contained a deletion in p16. These results substantiate the primary tumor as the origin for dysfunction of the p16 gene and implicate CpG island methylation as the major mechanism for inactivating this gene in the rat lung tumors examined. Furthermore, rat lung cancer appears to be an excellent model in which to investigate the mechanisms of de novo gene methylation and the role of p16 dysfunction in the progression of neoplasia.